Commit | Line | Data |
---|---|---|
68dc0745 | 1 | =head1 NAME |
2 | ||
109f0441 | 3 | perlfaq4 - Data Manipulation |
68dc0745 | 4 | |
5 | =head1 DESCRIPTION | |
6 | ||
ae3d0b9f JH |
7 | This section of the FAQ answers questions related to manipulating |
8 | numbers, dates, strings, arrays, hashes, and miscellaneous data issues. | |
68dc0745 | 9 | |
10 | =head1 Data: Numbers | |
11 | ||
46fc3d4c | 12 | =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? |
13 | ||
d12d61cf | 14 | For the long explanation, see David Goldberg's "What Every Computer |
15 | Scientist Should Know About Floating-Point Arithmetic" | |
16 | (http://docs.sun.com/source/806-3568/ncg_goldberg.html). | |
17 | ||
ac9dac7f RGS |
18 | Internally, your computer represents floating-point numbers in binary. |
19 | Digital (as in powers of two) computers cannot store all numbers | |
20 | exactly. Some real numbers lose precision in the process. This is a | |
21 | problem with how computers store numbers and affects all computer | |
22 | languages, not just Perl. | |
46fc3d4c | 23 | |
ee891a00 | 24 | L<perlnumber> shows the gory details of number representations and |
ac9dac7f | 25 | conversions. |
49d635f9 | 26 | |
ac9dac7f | 27 | To limit the number of decimal places in your numbers, you can use the |
3bc3c5be | 28 | C<printf> or C<sprintf> function. See the L<"Floating Point |
ac9dac7f | 29 | Arithmetic"|perlop> for more details. |
49d635f9 RGS |
30 | |
31 | printf "%.2f", 10/3; | |
197aec24 | 32 | |
49d635f9 | 33 | my $number = sprintf "%.2f", 10/3; |
197aec24 | 34 | |
32969b6e BB |
35 | =head2 Why is int() broken? |
36 | ||
ac9dac7f | 37 | Your C<int()> is most probably working just fine. It's the numbers that |
32969b6e BB |
38 | aren't quite what you think. |
39 | ||
ac9dac7f | 40 | First, see the answer to "Why am I getting long decimals |
32969b6e BB |
41 | (eg, 19.9499999999999) instead of the numbers I should be getting |
42 | (eg, 19.95)?". | |
43 | ||
44 | For example, this | |
45 | ||
ac9dac7f | 46 | print int(0.6/0.2-2), "\n"; |
32969b6e BB |
47 | |
48 | will in most computers print 0, not 1, because even such simple | |
49 | numbers as 0.6 and 0.2 cannot be presented exactly by floating-point | |
50 | numbers. What you think in the above as 'three' is really more like | |
51 | 2.9999999999999995559. | |
52 | ||
68dc0745 | 53 | =head2 Why isn't my octal data interpreted correctly? |
54 | ||
109f0441 S |
55 | (contributed by brian d foy) |
56 | ||
57 | You're probably trying to convert a string to a number, which Perl only | |
58 | converts as a decimal number. When Perl converts a string to a number, it | |
59 | ignores leading spaces and zeroes, then assumes the rest of the digits | |
60 | are in base 10: | |
61 | ||
62 | my $string = '0644'; | |
63 | ||
64 | print $string + 0; # prints 644 | |
65 | ||
66 | print $string + 44; # prints 688, certainly not octal! | |
67 | ||
68 | This problem usually involves one of the Perl built-ins that has the | |
23bec515 | 69 | same name a Unix command that uses octal numbers as arguments on the |
109f0441 S |
70 | command line. In this example, C<chmod> on the command line knows that |
71 | its first argument is octal because that's what it does: | |
72 | ||
73 | %prompt> chmod 644 file | |
74 | ||
75 | If you want to use the same literal digits (644) in Perl, you have to tell | |
76 | Perl to treat them as octal numbers either by prefixing the digits with | |
77 | a C<0> or using C<oct>: | |
78 | ||
79 | chmod( 0644, $file); # right, has leading zero | |
80 | chmod( oct(644), $file ); # also correct | |
68dc0745 | 81 | |
109f0441 S |
82 | The problem comes in when you take your numbers from something that Perl |
83 | thinks is a string, such as a command line argument in C<@ARGV>: | |
68dc0745 | 84 | |
109f0441 | 85 | chmod( $ARGV[0], $file); # wrong, even if "0644" |
68dc0745 | 86 | |
109f0441 | 87 | chmod( oct($ARGV[0]), $file ); # correct, treat string as octal |
33ce146f | 88 | |
109f0441 S |
89 | You can always check the value you're using by printing it in octal |
90 | notation to ensure it matches what you think it should be. Print it | |
91 | in octal and decimal format: | |
33ce146f | 92 | |
109f0441 | 93 | printf "0%o %d", $number, $number; |
33ce146f | 94 | |
65acb1b1 | 95 | =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? |
68dc0745 | 96 | |
ac9dac7f RGS |
97 | Remember that C<int()> merely truncates toward 0. For rounding to a |
98 | certain number of digits, C<sprintf()> or C<printf()> is usually the | |
99 | easiest route. | |
92c2ed05 | 100 | |
ac9dac7f | 101 | printf("%.3f", 3.1415926535); # prints 3.142 |
68dc0745 | 102 | |
ac9dac7f RGS |
103 | The C<POSIX> module (part of the standard Perl distribution) |
104 | implements C<ceil()>, C<floor()>, and a number of other mathematical | |
105 | and trigonometric functions. | |
68dc0745 | 106 | |
ac9dac7f RGS |
107 | use POSIX; |
108 | $ceil = ceil(3.5); # 4 | |
109 | $floor = floor(3.5); # 3 | |
92c2ed05 | 110 | |
ac9dac7f RGS |
111 | In 5.000 to 5.003 perls, trigonometry was done in the C<Math::Complex> |
112 | module. With 5.004, the C<Math::Trig> module (part of the standard Perl | |
46fc3d4c | 113 | distribution) implements the trigonometric functions. Internally it |
ac9dac7f | 114 | uses the C<Math::Complex> module and some functions can break out from |
46fc3d4c | 115 | the real axis into the complex plane, for example the inverse sine of |
116 | 2. | |
68dc0745 | 117 | |
118 | Rounding in financial applications can have serious implications, and | |
119 | the rounding method used should be specified precisely. In these | |
120 | cases, it probably pays not to trust whichever system rounding is | |
121 | being used by Perl, but to instead implement the rounding function you | |
122 | need yourself. | |
123 | ||
65acb1b1 TC |
124 | To see why, notice how you'll still have an issue on half-way-point |
125 | alternation: | |
126 | ||
ac9dac7f | 127 | for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} |
65acb1b1 | 128 | |
ac9dac7f RGS |
129 | 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7 |
130 | 0.8 0.8 0.9 0.9 1.0 1.0 | |
65acb1b1 | 131 | |
ac9dac7f RGS |
132 | Don't blame Perl. It's the same as in C. IEEE says we have to do |
133 | this. Perl numbers whose absolute values are integers under 2**31 (on | |
134 | 32 bit machines) will work pretty much like mathematical integers. | |
135 | Other numbers are not guaranteed. | |
65acb1b1 | 136 | |
6f0efb17 | 137 | =head2 How do I convert between numeric representations/bases/radixes? |
68dc0745 | 138 | |
ac9dac7f RGS |
139 | As always with Perl there is more than one way to do it. Below are a |
140 | few examples of approaches to making common conversions between number | |
141 | representations. This is intended to be representational rather than | |
142 | exhaustive. | |
68dc0745 | 143 | |
ac9dac7f RGS |
144 | Some of the examples later in L<perlfaq4> use the C<Bit::Vector> |
145 | module from CPAN. The reason you might choose C<Bit::Vector> over the | |
146 | perl built in functions is that it works with numbers of ANY size, | |
147 | that it is optimized for speed on some operations, and for at least | |
148 | some programmers the notation might be familiar. | |
d92eb7b0 | 149 | |
818c4caa JH |
150 | =over 4 |
151 | ||
152 | =item How do I convert hexadecimal into decimal | |
d92eb7b0 | 153 | |
ac9dac7f | 154 | Using perl's built in conversion of C<0x> notation: |
6761e064 | 155 | |
ac9dac7f | 156 | $dec = 0xDEADBEEF; |
7207e29d | 157 | |
ac9dac7f | 158 | Using the C<hex> function: |
6761e064 | 159 | |
ac9dac7f | 160 | $dec = hex("DEADBEEF"); |
6761e064 | 161 | |
ac9dac7f | 162 | Using C<pack>: |
6761e064 | 163 | |
ac9dac7f | 164 | $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); |
6761e064 | 165 | |
ac9dac7f | 166 | Using the CPAN module C<Bit::Vector>: |
6761e064 | 167 | |
ac9dac7f RGS |
168 | use Bit::Vector; |
169 | $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); | |
170 | $dec = $vec->to_Dec(); | |
6761e064 | 171 | |
818c4caa | 172 | =item How do I convert from decimal to hexadecimal |
6761e064 | 173 | |
ac9dac7f | 174 | Using C<sprintf>: |
6761e064 | 175 | |
ac9dac7f RGS |
176 | $hex = sprintf("%X", 3735928559); # upper case A-F |
177 | $hex = sprintf("%x", 3735928559); # lower case a-f | |
6761e064 | 178 | |
ac9dac7f | 179 | Using C<unpack>: |
6761e064 | 180 | |
ac9dac7f | 181 | $hex = unpack("H*", pack("N", 3735928559)); |
6761e064 | 182 | |
ac9dac7f | 183 | Using C<Bit::Vector>: |
6761e064 | 184 | |
ac9dac7f RGS |
185 | use Bit::Vector; |
186 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
187 | $hex = $vec->to_Hex(); | |
6761e064 | 188 | |
ac9dac7f | 189 | And C<Bit::Vector> supports odd bit counts: |
6761e064 | 190 | |
ac9dac7f RGS |
191 | use Bit::Vector; |
192 | $vec = Bit::Vector->new_Dec(33, 3735928559); | |
193 | $vec->Resize(32); # suppress leading 0 if unwanted | |
194 | $hex = $vec->to_Hex(); | |
6761e064 | 195 | |
818c4caa | 196 | =item How do I convert from octal to decimal |
6761e064 JH |
197 | |
198 | Using Perl's built in conversion of numbers with leading zeros: | |
199 | ||
ac9dac7f | 200 | $dec = 033653337357; # note the leading 0! |
6761e064 | 201 | |
ac9dac7f | 202 | Using the C<oct> function: |
6761e064 | 203 | |
ac9dac7f | 204 | $dec = oct("33653337357"); |
6761e064 | 205 | |
ac9dac7f | 206 | Using C<Bit::Vector>: |
6761e064 | 207 | |
ac9dac7f RGS |
208 | use Bit::Vector; |
209 | $vec = Bit::Vector->new(32); | |
210 | $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); | |
211 | $dec = $vec->to_Dec(); | |
6761e064 | 212 | |
818c4caa | 213 | =item How do I convert from decimal to octal |
6761e064 | 214 | |
ac9dac7f | 215 | Using C<sprintf>: |
6761e064 | 216 | |
ac9dac7f | 217 | $oct = sprintf("%o", 3735928559); |
6761e064 | 218 | |
ac9dac7f | 219 | Using C<Bit::Vector>: |
6761e064 | 220 | |
ac9dac7f RGS |
221 | use Bit::Vector; |
222 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
223 | $oct = reverse join('', $vec->Chunk_List_Read(3)); | |
6761e064 | 224 | |
818c4caa | 225 | =item How do I convert from binary to decimal |
6761e064 | 226 | |
2c646907 | 227 | Perl 5.6 lets you write binary numbers directly with |
ac9dac7f | 228 | the C<0b> notation: |
2c646907 | 229 | |
ac9dac7f | 230 | $number = 0b10110110; |
6f0efb17 | 231 | |
ac9dac7f | 232 | Using C<oct>: |
6f0efb17 | 233 | |
ac9dac7f RGS |
234 | my $input = "10110110"; |
235 | $decimal = oct( "0b$input" ); | |
2c646907 | 236 | |
ac9dac7f | 237 | Using C<pack> and C<ord>: |
d92eb7b0 | 238 | |
ac9dac7f | 239 | $decimal = ord(pack('B8', '10110110')); |
68dc0745 | 240 | |
ac9dac7f | 241 | Using C<pack> and C<unpack> for larger strings: |
6761e064 | 242 | |
ac9dac7f | 243 | $int = unpack("N", pack("B32", |
6761e064 | 244 | substr("0" x 32 . "11110101011011011111011101111", -32))); |
ac9dac7f | 245 | $dec = sprintf("%d", $int); |
6761e064 | 246 | |
ac9dac7f | 247 | # substr() is used to left pad a 32 character string with zeros. |
6761e064 | 248 | |
ac9dac7f | 249 | Using C<Bit::Vector>: |
6761e064 | 250 | |
ac9dac7f RGS |
251 | $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); |
252 | $dec = $vec->to_Dec(); | |
6761e064 | 253 | |
818c4caa | 254 | =item How do I convert from decimal to binary |
6761e064 | 255 | |
ac9dac7f | 256 | Using C<sprintf> (perl 5.6+): |
4dfcc30b | 257 | |
ac9dac7f | 258 | $bin = sprintf("%b", 3735928559); |
4dfcc30b | 259 | |
ac9dac7f | 260 | Using C<unpack>: |
6761e064 | 261 | |
ac9dac7f | 262 | $bin = unpack("B*", pack("N", 3735928559)); |
6761e064 | 263 | |
ac9dac7f | 264 | Using C<Bit::Vector>: |
6761e064 | 265 | |
ac9dac7f RGS |
266 | use Bit::Vector; |
267 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
268 | $bin = $vec->to_Bin(); | |
6761e064 JH |
269 | |
270 | The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) | |
271 | are left as an exercise to the inclined reader. | |
68dc0745 | 272 | |
818c4caa | 273 | =back |
68dc0745 | 274 | |
65acb1b1 TC |
275 | =head2 Why doesn't & work the way I want it to? |
276 | ||
277 | The behavior of binary arithmetic operators depends on whether they're | |
278 | used on numbers or strings. The operators treat a string as a series | |
279 | of bits and work with that (the string C<"3"> is the bit pattern | |
280 | C<00110011>). The operators work with the binary form of a number | |
281 | (the number C<3> is treated as the bit pattern C<00000011>). | |
282 | ||
283 | So, saying C<11 & 3> performs the "and" operation on numbers (yielding | |
49d635f9 | 284 | C<3>). Saying C<"11" & "3"> performs the "and" operation on strings |
65acb1b1 TC |
285 | (yielding C<"1">). |
286 | ||
287 | Most problems with C<&> and C<|> arise because the programmer thinks | |
288 | they have a number but really it's a string. The rest arise because | |
289 | the programmer says: | |
290 | ||
ac9dac7f RGS |
291 | if ("\020\020" & "\101\101") { |
292 | # ... | |
293 | } | |
65acb1b1 TC |
294 | |
295 | but a string consisting of two null bytes (the result of C<"\020\020" | |
296 | & "\101\101">) is not a false value in Perl. You need: | |
297 | ||
ac9dac7f RGS |
298 | if ( ("\020\020" & "\101\101") !~ /[^\000]/) { |
299 | # ... | |
300 | } | |
65acb1b1 | 301 | |
68dc0745 | 302 | =head2 How do I multiply matrices? |
303 | ||
d12d61cf | 304 | Use the C<Math::Matrix> or C<Math::MatrixReal> modules (available from CPAN) |
305 | or the C<PDL> extension (also available from CPAN). | |
68dc0745 | 306 | |
307 | =head2 How do I perform an operation on a series of integers? | |
308 | ||
309 | To call a function on each element in an array, and collect the | |
310 | results, use: | |
311 | ||
ac9dac7f | 312 | @results = map { my_func($_) } @array; |
68dc0745 | 313 | |
314 | For example: | |
315 | ||
ac9dac7f | 316 | @triple = map { 3 * $_ } @single; |
68dc0745 | 317 | |
318 | To call a function on each element of an array, but ignore the | |
319 | results: | |
320 | ||
ac9dac7f RGS |
321 | foreach $iterator (@array) { |
322 | some_func($iterator); | |
323 | } | |
68dc0745 | 324 | |
325 | To call a function on each integer in a (small) range, you B<can> use: | |
326 | ||
ac9dac7f | 327 | @results = map { some_func($_) } (5 .. 25); |
68dc0745 | 328 | |
329 | but you should be aware that the C<..> operator creates an array of | |
330 | all integers in the range. This can take a lot of memory for large | |
331 | ranges. Instead use: | |
332 | ||
ac9dac7f | 333 | @results = (); |
eaffe51e | 334 | for ($i=5; $i <= 500_005; $i++) { |
ac9dac7f RGS |
335 | push(@results, some_func($i)); |
336 | } | |
68dc0745 | 337 | |
87275199 GS |
338 | This situation has been fixed in Perl5.005. Use of C<..> in a C<for> |
339 | loop will iterate over the range, without creating the entire range. | |
340 | ||
ac9dac7f RGS |
341 | for my $i (5 .. 500_005) { |
342 | push(@results, some_func($i)); | |
343 | } | |
87275199 GS |
344 | |
345 | will not create a list of 500,000 integers. | |
346 | ||
68dc0745 | 347 | =head2 How can I output Roman numerals? |
348 | ||
d12d61cf | 349 | Get the L<http://www.cpan.org/modules/by-module/Roman> module. |
68dc0745 | 350 | |
351 | =head2 Why aren't my random numbers random? | |
352 | ||
65acb1b1 TC |
353 | If you're using a version of Perl before 5.004, you must call C<srand> |
354 | once at the start of your program to seed the random number generator. | |
49d635f9 | 355 | |
5cd0b561 | 356 | BEGIN { srand() if $] < 5.004 } |
49d635f9 | 357 | |
65acb1b1 | 358 | 5.004 and later automatically call C<srand> at the beginning. Don't |
ac9dac7f RGS |
359 | call C<srand> more than once--you make your numbers less random, |
360 | rather than more. | |
92c2ed05 | 361 | |
65acb1b1 | 362 | Computers are good at being predictable and bad at being random |
06a5f41f | 363 | (despite appearances caused by bugs in your programs :-). see the |
49d635f9 | 364 | F<random> article in the "Far More Than You Ever Wanted To Know" |
d12d61cf | 365 | collection in L<http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz>, courtesy |
ac9dac7f | 366 | of Tom Phoenix, talks more about this. John von Neumann said, "Anyone |
06a5f41f | 367 | who attempts to generate random numbers by deterministic means is, of |
b432a672 | 368 | course, living in a state of sin." |
65acb1b1 TC |
369 | |
370 | If you want numbers that are more random than C<rand> with C<srand> | |
ac9dac7f | 371 | provides, you should also check out the C<Math::TrulyRandom> module from |
65acb1b1 TC |
372 | CPAN. It uses the imperfections in your system's timer to generate |
373 | random numbers, but this takes quite a while. If you want a better | |
92c2ed05 | 374 | pseudorandom generator than comes with your operating system, look at |
d12d61cf | 375 | "Numerical Recipes in C" at L<http://www.nr.com/>. |
68dc0745 | 376 | |
881bdbd4 JH |
377 | =head2 How do I get a random number between X and Y? |
378 | ||
ee891a00 | 379 | To get a random number between two values, you can use the C<rand()> |
109f0441 | 380 | built-in to get a random number between 0 and 1. From there, you shift |
ee891a00 | 381 | that into the range that you want. |
500071f4 | 382 | |
ee891a00 RGS |
383 | C<rand($x)> returns a number such that C<< 0 <= rand($x) < $x >>. Thus |
384 | what you want to have perl figure out is a random number in the range | |
385 | from 0 to the difference between your I<X> and I<Y>. | |
793f5136 | 386 | |
ee891a00 RGS |
387 | That is, to get a number between 10 and 15, inclusive, you want a |
388 | random number between 0 and 5 that you can then add to 10. | |
793f5136 | 389 | |
109f0441 | 390 | my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 ) |
793f5136 RGS |
391 | |
392 | Hence you derive the following simple function to abstract | |
393 | that. It selects a random integer between the two given | |
500071f4 RGS |
394 | integers (inclusive), For example: C<random_int_between(50,120)>. |
395 | ||
ac9dac7f | 396 | sub random_int_between { |
500071f4 RGS |
397 | my($min, $max) = @_; |
398 | # Assumes that the two arguments are integers themselves! | |
399 | return $min if $min == $max; | |
400 | ($min, $max) = ($max, $min) if $min > $max; | |
401 | return $min + int rand(1 + $max - $min); | |
402 | } | |
881bdbd4 | 403 | |
68dc0745 | 404 | =head1 Data: Dates |
405 | ||
5cd0b561 | 406 | =head2 How do I find the day or week of the year? |
68dc0745 | 407 | |
d12d61cf | 408 | The C<localtime> function returns the day of the year. Without an |
409 | argument C<localtime> uses the current time. | |
68dc0745 | 410 | |
a05e4845 | 411 | $day_of_year = (localtime)[7]; |
ffc145e8 | 412 | |
ac9dac7f | 413 | The C<POSIX> module can also format a date as the day of the year or |
5cd0b561 | 414 | week of the year. |
68dc0745 | 415 | |
5cd0b561 RGS |
416 | use POSIX qw/strftime/; |
417 | my $day_of_year = strftime "%j", localtime; | |
418 | my $week_of_year = strftime "%W", localtime; | |
419 | ||
ac9dac7f | 420 | To get the day of year for any date, use C<POSIX>'s C<mktime> to get |
d12d61cf | 421 | a time in epoch seconds for the argument to C<localtime>. |
ffc145e8 | 422 | |
ac9dac7f | 423 | use POSIX qw/mktime strftime/; |
6670e5e7 | 424 | my $week_of_year = strftime "%W", |
ac9dac7f | 425 | localtime( mktime( 0, 0, 0, 18, 11, 87 ) ); |
5cd0b561 | 426 | |
ac9dac7f | 427 | The C<Date::Calc> module provides two functions to calculate these. |
5cd0b561 RGS |
428 | |
429 | use Date::Calc; | |
430 | my $day_of_year = Day_of_Year( 1987, 12, 18 ); | |
431 | my $week_of_year = Week_of_Year( 1987, 12, 18 ); | |
ffc145e8 | 432 | |
d92eb7b0 GS |
433 | =head2 How do I find the current century or millennium? |
434 | ||
435 | Use the following simple functions: | |
436 | ||
ac9dac7f RGS |
437 | sub get_century { |
438 | return int((((localtime(shift || time))[5] + 1999))/100); | |
439 | } | |
6670e5e7 | 440 | |
ac9dac7f RGS |
441 | sub get_millennium { |
442 | return 1+int((((localtime(shift || time))[5] + 1899))/1000); | |
443 | } | |
d92eb7b0 | 444 | |
ac9dac7f RGS |
445 | On some systems, the C<POSIX> module's C<strftime()> function has been |
446 | extended in a non-standard way to use a C<%C> format, which they | |
447 | sometimes claim is the "century". It isn't, because on most such | |
448 | systems, this is only the first two digits of the four-digit year, and | |
449 | thus cannot be used to reliably determine the current century or | |
450 | millennium. | |
d92eb7b0 | 451 | |
92c2ed05 | 452 | =head2 How can I compare two dates and find the difference? |
68dc0745 | 453 | |
b68463f7 RGS |
454 | (contributed by brian d foy) |
455 | ||
ac9dac7f RGS |
456 | You could just store all your dates as a number and then subtract. |
457 | Life isn't always that simple though. If you want to work with | |
458 | formatted dates, the C<Date::Manip>, C<Date::Calc>, or C<DateTime> | |
459 | modules can help you. | |
68dc0745 | 460 | |
461 | =head2 How can I take a string and turn it into epoch seconds? | |
462 | ||
463 | If it's a regular enough string that it always has the same format, | |
92c2ed05 | 464 | you can split it up and pass the parts to C<timelocal> in the standard |
ac9dac7f RGS |
465 | C<Time::Local> module. Otherwise, you should look into the C<Date::Calc> |
466 | and C<Date::Manip> modules from CPAN. | |
68dc0745 | 467 | |
468 | =head2 How can I find the Julian Day? | |
469 | ||
7678cced RGS |
470 | (contributed by brian d foy and Dave Cross) |
471 | ||
ac9dac7f RGS |
472 | You can use the C<Time::JulianDay> module available on CPAN. Ensure |
473 | that you really want to find a Julian day, though, as many people have | |
7678cced RGS |
474 | different ideas about Julian days. See |
475 | http://www.hermetic.ch/cal_stud/jdn.htm for instance. | |
476 | ||
ac9dac7f | 477 | You can also try the C<DateTime> module, which can convert a date/time |
7678cced RGS |
478 | to a Julian Day. |
479 | ||
ac9dac7f RGS |
480 | $ perl -MDateTime -le'print DateTime->today->jd' |
481 | 2453401.5 | |
7678cced RGS |
482 | |
483 | Or the modified Julian Day | |
484 | ||
ac9dac7f RGS |
485 | $ perl -MDateTime -le'print DateTime->today->mjd' |
486 | 53401 | |
7678cced RGS |
487 | |
488 | Or even the day of the year (which is what some people think of as a | |
489 | Julian day) | |
490 | ||
ac9dac7f RGS |
491 | $ perl -MDateTime -le'print DateTime->today->doy' |
492 | 31 | |
be94a901 | 493 | |
65acb1b1 | 494 | =head2 How do I find yesterday's date? |
109f0441 S |
495 | X<date> X<yesterday> X<DateTime> X<Date::Calc> X<Time::Local> |
496 | X<daylight saving time> X<day> X<Today_and_Now> X<localtime> | |
497 | X<timelocal> | |
65acb1b1 | 498 | |
6670e5e7 | 499 | (contributed by brian d foy) |
49d635f9 | 500 | |
6670e5e7 RGS |
501 | Use one of the Date modules. The C<DateTime> module makes it simple, and |
502 | give you the same time of day, only the day before. | |
49d635f9 | 503 | |
6670e5e7 | 504 | use DateTime; |
58103a2e | 505 | |
6670e5e7 | 506 | my $yesterday = DateTime->now->subtract( days => 1 ); |
58103a2e | 507 | |
6670e5e7 | 508 | print "Yesterday was $yesterday\n"; |
49d635f9 | 509 | |
ee891a00 | 510 | You can also use the C<Date::Calc> module using its C<Today_and_Now> |
6670e5e7 | 511 | function. |
49d635f9 | 512 | |
6670e5e7 | 513 | use Date::Calc qw( Today_and_Now Add_Delta_DHMS ); |
58103a2e | 514 | |
6670e5e7 | 515 | my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 ); |
58103a2e | 516 | |
ee891a00 | 517 | print "@date_time\n"; |
58103a2e | 518 | |
6670e5e7 RGS |
519 | Most people try to use the time rather than the calendar to figure out |
520 | dates, but that assumes that days are twenty-four hours each. For | |
521 | most people, there are two days a year when they aren't: the switch to | |
522 | and from summer time throws this off. Let the modules do the work. | |
d92eb7b0 | 523 | |
109f0441 S |
524 | If you absolutely must do it yourself (or can't use one of the |
525 | modules), here's a solution using C<Time::Local>, which comes with | |
526 | Perl: | |
527 | ||
528 | # contributed by Gunnar Hjalmarsson | |
529 | use Time::Local; | |
530 | my $today = timelocal 0, 0, 12, ( localtime )[3..5]; | |
531 | my ($d, $m, $y) = ( localtime $today-86400 )[3..5]; | |
532 | printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d; | |
533 | ||
534 | In this case, you measure the day starting at noon, and subtract 24 | |
535 | hours. Even if the length of the calendar day is 23 or 25 hours, | |
536 | you'll still end up on the previous calendar day, although not at | |
537 | noon. Since you don't care about the time, the one hour difference | |
538 | doesn't matter and you end up with the previous date. | |
539 | ||
3bc3c5be | 540 | =head2 Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant? |
541 | ||
542 | (contributed by brian d foy) | |
543 | ||
23bec515 | 544 | Perl itself never had a Y2K problem, although that never stopped people |
3bc3c5be | 545 | from creating Y2K problems on their own. See the documentation for |
546 | C<localtime> for its proper use. | |
547 | ||
d12d61cf | 548 | Starting with Perl 5.11, C<localtime> and C<gmtime> can handle dates past |
3bc3c5be | 549 | 03:14:08 January 19, 2038, when a 32-bit based time would overflow. You |
550 | still might get a warning on a 32-bit C<perl>: | |
551 | ||
552 | % perl5.11.2 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )' | |
553 | Integer overflow in hexadecimal number at -e line 1. | |
554 | Wed Nov 1 19:42:39 5576711 | |
555 | ||
556 | On a 64-bit C<perl>, you can get even larger dates for those really long | |
557 | running projects: | |
558 | ||
559 | % perl5.11.2 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )' | |
560 | Thu Nov 2 00:42:39 5576711 | |
561 | ||
701f2f01 | 562 | You're still out of luck if you need to keep track of decaying protons |
3bc3c5be | 563 | though. |
5a964f20 | 564 | |
68dc0745 | 565 | =head1 Data: Strings |
566 | ||
567 | =head2 How do I validate input? | |
568 | ||
6670e5e7 RGS |
569 | (contributed by brian d foy) |
570 | ||
571 | There are many ways to ensure that values are what you expect or | |
572 | want to accept. Besides the specific examples that we cover in the | |
573 | perlfaq, you can also look at the modules with "Assert" and "Validate" | |
574 | in their names, along with other modules such as C<Regexp::Common>. | |
575 | ||
576 | Some modules have validation for particular types of input, such | |
577 | as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>, | |
578 | and C<Data::Validate::IP>. | |
68dc0745 | 579 | |
580 | =head2 How do I unescape a string? | |
581 | ||
b432a672 | 582 | It depends just what you mean by "escape". URL escapes are dealt |
92c2ed05 | 583 | with in L<perlfaq9>. Shell escapes with the backslash (C<\>) |
a6dd486b | 584 | character are removed with |
68dc0745 | 585 | |
ac9dac7f | 586 | s/\\(.)/$1/g; |
68dc0745 | 587 | |
92c2ed05 | 588 | This won't expand C<"\n"> or C<"\t"> or any other special escapes. |
68dc0745 | 589 | |
590 | =head2 How do I remove consecutive pairs of characters? | |
591 | ||
6670e5e7 RGS |
592 | (contributed by brian d foy) |
593 | ||
594 | You can use the substitution operator to find pairs of characters (or | |
595 | runs of characters) and replace them with a single instance. In this | |
596 | substitution, we find a character in C<(.)>. The memory parentheses | |
d8b950dc | 597 | store the matched character in the back-reference C<\g1> and we use |
6670e5e7 RGS |
598 | that to require that the same thing immediately follow it. We replace |
599 | that part of the string with the character in C<$1>. | |
68dc0745 | 600 | |
d8b950dc | 601 | s/(.)\g1/$1/g; |
d92eb7b0 | 602 | |
6670e5e7 RGS |
603 | We can also use the transliteration operator, C<tr///>. In this |
604 | example, the search list side of our C<tr///> contains nothing, but | |
605 | the C<c> option complements that so it contains everything. The | |
606 | replacement list also contains nothing, so the transliteration is | |
607 | almost a no-op since it won't do any replacements (or more exactly, | |
608 | replace the character with itself). However, the C<s> option squashes | |
609 | duplicated and consecutive characters in the string so a character | |
610 | does not show up next to itself | |
d92eb7b0 | 611 | |
6670e5e7 | 612 | my $str = 'Haarlem'; # in the Netherlands |
ac9dac7f | 613 | $str =~ tr///cs; # Now Harlem, like in New York |
68dc0745 | 614 | |
615 | =head2 How do I expand function calls in a string? | |
616 | ||
6670e5e7 RGS |
617 | (contributed by brian d foy) |
618 | ||
619 | This is documented in L<perlref>, and although it's not the easiest | |
620 | thing to read, it does work. In each of these examples, we call the | |
58103a2e | 621 | function inside the braces used to dereference a reference. If we |
5ae37c3f | 622 | have more than one return value, we can construct and dereference an |
6670e5e7 RGS |
623 | anonymous array. In this case, we call the function in list context. |
624 | ||
58103a2e | 625 | print "The time values are @{ [localtime] }.\n"; |
6670e5e7 RGS |
626 | |
627 | If we want to call the function in scalar context, we have to do a bit | |
628 | more work. We can really have any code we like inside the braces, so | |
629 | we simply have to end with the scalar reference, although how you do | |
e573f903 RGS |
630 | that is up to you, and you can use code inside the braces. Note that |
631 | the use of parens creates a list context, so we need C<scalar> to | |
632 | force the scalar context on the function: | |
68dc0745 | 633 | |
6670e5e7 | 634 | print "The time is ${\(scalar localtime)}.\n" |
58103a2e | 635 | |
6670e5e7 | 636 | print "The time is ${ my $x = localtime; \$x }.\n"; |
58103a2e | 637 | |
6670e5e7 RGS |
638 | If your function already returns a reference, you don't need to create |
639 | the reference yourself. | |
640 | ||
641 | sub timestamp { my $t = localtime; \$t } | |
58103a2e | 642 | |
6670e5e7 | 643 | print "The time is ${ timestamp() }.\n"; |
58103a2e RGS |
644 | |
645 | The C<Interpolation> module can also do a lot of magic for you. You can | |
646 | specify a variable name, in this case C<E>, to set up a tied hash that | |
647 | does the interpolation for you. It has several other methods to do this | |
648 | as well. | |
649 | ||
650 | use Interpolation E => 'eval'; | |
651 | print "The time values are $E{localtime()}.\n"; | |
652 | ||
653 | In most cases, it is probably easier to simply use string concatenation, | |
654 | which also forces scalar context. | |
6670e5e7 | 655 | |
ac9dac7f | 656 | print "The time is " . localtime() . ".\n"; |
68dc0745 | 657 | |
68dc0745 | 658 | =head2 How do I find matching/nesting anything? |
659 | ||
92c2ed05 GS |
660 | This isn't something that can be done in one regular expression, no |
661 | matter how complicated. To find something between two single | |
662 | characters, a pattern like C</x([^x]*)x/> will get the intervening | |
663 | bits in $1. For multiple ones, then something more like | |
ac9dac7f | 664 | C</alpha(.*?)omega/> would be needed. But none of these deals with |
6670e5e7 RGS |
665 | nested patterns. For balanced expressions using C<(>, C<{>, C<[> or |
666 | C<< < >> as delimiters, use the CPAN module Regexp::Common, or see | |
667 | L<perlre/(??{ code })>. For other cases, you'll have to write a | |
668 | parser. | |
92c2ed05 GS |
669 | |
670 | If you are serious about writing a parser, there are a number of | |
6a2af475 | 671 | modules or oddities that will make your life a lot easier. There are |
ac9dac7f RGS |
672 | the CPAN modules C<Parse::RecDescent>, C<Parse::Yapp>, and |
673 | C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8 | |
674 | the C<Text::Balanced> is part of the standard distribution. | |
68dc0745 | 675 | |
92c2ed05 GS |
676 | One simple destructive, inside-out approach that you might try is to |
677 | pull out the smallest nesting parts one at a time: | |
5a964f20 | 678 | |
ac9dac7f RGS |
679 | while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { |
680 | # do something with $1 | |
681 | } | |
5a964f20 | 682 | |
65acb1b1 TC |
683 | A more complicated and sneaky approach is to make Perl's regular |
684 | expression engine do it for you. This is courtesy Dean Inada, and | |
685 | rather has the nature of an Obfuscated Perl Contest entry, but it | |
686 | really does work: | |
687 | ||
ac9dac7f RGS |
688 | # $_ contains the string to parse |
689 | # BEGIN and END are the opening and closing markers for the | |
690 | # nested text. | |
c47ff5f1 | 691 | |
ac9dac7f RGS |
692 | @( = ('(',''); |
693 | @) = (')',''); | |
694 | ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; | |
695 | @$ = (eval{/$re/},$@!~/unmatched/i); | |
696 | print join("\n",@$[0..$#$]) if( $$[-1] ); | |
65acb1b1 | 697 | |
68dc0745 | 698 | =head2 How do I reverse a string? |
699 | ||
ac9dac7f | 700 | Use C<reverse()> in scalar context, as documented in |
68dc0745 | 701 | L<perlfunc/reverse>. |
702 | ||
ac9dac7f | 703 | $reversed = reverse $string; |
68dc0745 | 704 | |
705 | =head2 How do I expand tabs in a string? | |
706 | ||
5a964f20 | 707 | You can do it yourself: |
68dc0745 | 708 | |
ac9dac7f | 709 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; |
68dc0745 | 710 | |
ac9dac7f | 711 | Or you can just use the C<Text::Tabs> module (part of the standard Perl |
68dc0745 | 712 | distribution). |
713 | ||
ac9dac7f RGS |
714 | use Text::Tabs; |
715 | @expanded_lines = expand(@lines_with_tabs); | |
68dc0745 | 716 | |
717 | =head2 How do I reformat a paragraph? | |
718 | ||
ac9dac7f | 719 | Use C<Text::Wrap> (part of the standard Perl distribution): |
68dc0745 | 720 | |
ac9dac7f RGS |
721 | use Text::Wrap; |
722 | print wrap("\t", ' ', @paragraphs); | |
68dc0745 | 723 | |
ac9dac7f RGS |
724 | The paragraphs you give to C<Text::Wrap> should not contain embedded |
725 | newlines. C<Text::Wrap> doesn't justify the lines (flush-right). | |
46fc3d4c | 726 | |
ac9dac7f RGS |
727 | Or use the CPAN module C<Text::Autoformat>. Formatting files can be |
728 | easily done by making a shell alias, like so: | |
bc06af74 | 729 | |
ac9dac7f RGS |
730 | alias fmt="perl -i -MText::Autoformat -n0777 \ |
731 | -e 'print autoformat $_, {all=>1}' $*" | |
bc06af74 | 732 | |
ac9dac7f | 733 | See the documentation for C<Text::Autoformat> to appreciate its many |
bc06af74 JH |
734 | capabilities. |
735 | ||
49d635f9 | 736 | =head2 How can I access or change N characters of a string? |
68dc0745 | 737 | |
49d635f9 RGS |
738 | You can access the first characters of a string with substr(). |
739 | To get the first character, for example, start at position 0 | |
197aec24 | 740 | and grab the string of length 1. |
68dc0745 | 741 | |
68dc0745 | 742 | |
49d635f9 | 743 | $string = "Just another Perl Hacker"; |
ac9dac7f | 744 | $first_char = substr( $string, 0, 1 ); # 'J' |
68dc0745 | 745 | |
49d635f9 RGS |
746 | To change part of a string, you can use the optional fourth |
747 | argument which is the replacement string. | |
68dc0745 | 748 | |
ac9dac7f | 749 | substr( $string, 13, 4, "Perl 5.8.0" ); |
197aec24 | 750 | |
49d635f9 | 751 | You can also use substr() as an lvalue. |
68dc0745 | 752 | |
ac9dac7f | 753 | substr( $string, 13, 4 ) = "Perl 5.8.0"; |
197aec24 | 754 | |
68dc0745 | 755 | =head2 How do I change the Nth occurrence of something? |
756 | ||
92c2ed05 GS |
757 | You have to keep track of N yourself. For example, let's say you want |
758 | to change the fifth occurrence of C<"whoever"> or C<"whomever"> into | |
d92eb7b0 GS |
759 | C<"whosoever"> or C<"whomsoever">, case insensitively. These |
760 | all assume that $_ contains the string to be altered. | |
68dc0745 | 761 | |
ac9dac7f RGS |
762 | $count = 0; |
763 | s{((whom?)ever)}{ | |
764 | ++$count == 5 # is it the 5th? | |
765 | ? "${2}soever" # yes, swap | |
766 | : $1 # renege and leave it there | |
767 | }ige; | |
68dc0745 | 768 | |
5a964f20 TC |
769 | In the more general case, you can use the C</g> modifier in a C<while> |
770 | loop, keeping count of matches. | |
771 | ||
ac9dac7f RGS |
772 | $WANT = 3; |
773 | $count = 0; | |
774 | $_ = "One fish two fish red fish blue fish"; | |
775 | while (/(\w+)\s+fish\b/gi) { | |
776 | if (++$count == $WANT) { | |
777 | print "The third fish is a $1 one.\n"; | |
778 | } | |
779 | } | |
5a964f20 | 780 | |
92c2ed05 | 781 | That prints out: C<"The third fish is a red one."> You can also use a |
5a964f20 TC |
782 | repetition count and repeated pattern like this: |
783 | ||
ac9dac7f | 784 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; |
5a964f20 | 785 | |
68dc0745 | 786 | =head2 How can I count the number of occurrences of a substring within a string? |
787 | ||
a6dd486b | 788 | There are a number of ways, with varying efficiency. If you want a |
68dc0745 | 789 | count of a certain single character (X) within a string, you can use the |
790 | C<tr///> function like so: | |
791 | ||
ac9dac7f RGS |
792 | $string = "ThisXlineXhasXsomeXx'sXinXit"; |
793 | $count = ($string =~ tr/X//); | |
794 | print "There are $count X characters in the string"; | |
68dc0745 | 795 | |
796 | This is fine if you are just looking for a single character. However, | |
797 | if you are trying to count multiple character substrings within a | |
798 | larger string, C<tr///> won't work. What you can do is wrap a while() | |
799 | loop around a global pattern match. For example, let's count negative | |
800 | integers: | |
801 | ||
ac9dac7f RGS |
802 | $string = "-9 55 48 -2 23 -76 4 14 -44"; |
803 | while ($string =~ /-\d+/g) { $count++ } | |
804 | print "There are $count negative numbers in the string"; | |
68dc0745 | 805 | |
881bdbd4 JH |
806 | Another version uses a global match in list context, then assigns the |
807 | result to a scalar, producing a count of the number of matches. | |
808 | ||
809 | $count = () = $string =~ /-\d+/g; | |
810 | ||
109f0441 S |
811 | =head2 How do I capitalize all the words on one line? |
812 | X<Text::Autoformat> X<capitalize> X<case, title> X<case, sentence> | |
5a964f20 | 813 | |
109f0441 | 814 | (contributed by brian d foy) |
65acb1b1 | 815 | |
109f0441 S |
816 | Damian Conway's L<Text::Autoformat> handles all of the thinking |
817 | for you. | |
369b44b4 | 818 | |
ac9dac7f RGS |
819 | use Text::Autoformat; |
820 | my $x = "Dr. Strangelove or: How I Learned to Stop ". | |
821 | "Worrying and Love the Bomb"; | |
369b44b4 | 822 | |
ac9dac7f RGS |
823 | print $x, "\n"; |
824 | for my $style (qw( sentence title highlight )) { | |
825 | print autoformat($x, { case => $style }), "\n"; | |
826 | } | |
369b44b4 | 827 | |
109f0441 S |
828 | How do you want to capitalize those words? |
829 | ||
830 | FRED AND BARNEY'S LODGE # all uppercase | |
831 | Fred And Barney's Lodge # title case | |
832 | Fred and Barney's Lodge # highlight case | |
833 | ||
834 | It's not as easy a problem as it looks. How many words do you think | |
835 | are in there? Wait for it... wait for it.... If you answered 5 | |
836 | you're right. Perl words are groups of C<\w+>, but that's not what | |
837 | you want to capitalize. How is Perl supposed to know not to capitalize | |
838 | that C<s> after the apostrophe? You could try a regular expression: | |
839 | ||
840 | $string =~ s/ ( | |
841 | (^\w) #at the beginning of the line | |
842 | | # or | |
843 | (\s\w) #preceded by whitespace | |
844 | ) | |
845 | /\U$1/xg; | |
846 | ||
847 | $string =~ s/([\w']+)/\u\L$1/g; | |
848 | ||
849 | Now, what if you don't want to capitalize that "and"? Just use | |
850 | L<Text::Autoformat> and get on with the next problem. :) | |
851 | ||
49d635f9 | 852 | =head2 How can I split a [character] delimited string except when inside [character]? |
68dc0745 | 853 | |
ac9dac7f RGS |
854 | Several modules can handle this sort of parsing--C<Text::Balanced>, |
855 | C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others. | |
49d635f9 RGS |
856 | |
857 | Take the example case of trying to split a string that is | |
858 | comma-separated into its different fields. You can't use C<split(/,/)> | |
859 | because you shouldn't split if the comma is inside quotes. For | |
860 | example, take a data line like this: | |
68dc0745 | 861 | |
ac9dac7f | 862 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" |
68dc0745 | 863 | |
864 | Due to the restriction of the quotes, this is a fairly complex | |
197aec24 | 865 | problem. Thankfully, we have Jeffrey Friedl, author of |
49d635f9 | 866 | I<Mastering Regular Expressions>, to handle these for us. He |
ac9dac7f | 867 | suggests (assuming your string is contained in C<$text>): |
68dc0745 | 868 | |
ac9dac7f RGS |
869 | @new = (); |
870 | push(@new, $+) while $text =~ m{ | |
871 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes | |
872 | | ([^,]+),? | |
873 | | , | |
874 | }gx; | |
875 | push(@new, undef) if substr($text,-1,1) eq ','; | |
68dc0745 | 876 | |
46fc3d4c | 877 | If you want to represent quotation marks inside a |
878 | quotation-mark-delimited field, escape them with backslashes (eg, | |
49d635f9 | 879 | C<"like \"this\"">. |
46fc3d4c | 880 | |
ac9dac7f RGS |
881 | Alternatively, the C<Text::ParseWords> module (part of the standard |
882 | Perl distribution) lets you say: | |
68dc0745 | 883 | |
ac9dac7f RGS |
884 | use Text::ParseWords; |
885 | @new = quotewords(",", 0, $text); | |
65acb1b1 | 886 | |
68dc0745 | 887 | =head2 How do I strip blank space from the beginning/end of a string? |
888 | ||
6670e5e7 | 889 | (contributed by brian d foy) |
68dc0745 | 890 | |
6670e5e7 RGS |
891 | A substitution can do this for you. For a single line, you want to |
892 | replace all the leading or trailing whitespace with nothing. You | |
960c6898 | 893 | can do that with a pair of substitutions: |
68dc0745 | 894 | |
6670e5e7 RGS |
895 | s/^\s+//; |
896 | s/\s+$//; | |
68dc0745 | 897 | |
6670e5e7 RGS |
898 | You can also write that as a single substitution, although it turns |
899 | out the combined statement is slower than the separate ones. That | |
960c6898 | 900 | might not matter to you, though: |
68dc0745 | 901 | |
6670e5e7 | 902 | s/^\s+|\s+$//g; |
68dc0745 | 903 | |
6670e5e7 RGS |
904 | In this regular expression, the alternation matches either at the |
905 | beginning or the end of the string since the anchors have a lower | |
906 | precedence than the alternation. With the C</g> flag, the substitution | |
907 | makes all possible matches, so it gets both. Remember, the trailing | |
908 | newline matches the C<\s+>, and the C<$> anchor can match to the | |
960c6898 | 909 | absolute end of the string, so the newline disappears too. Just add |
6670e5e7 RGS |
910 | the newline to the output, which has the added benefit of preserving |
911 | "blank" (consisting entirely of whitespace) lines which the C<^\s+> | |
960c6898 | 912 | would remove all by itself: |
68dc0745 | 913 | |
960c6898 | 914 | while( <> ) { |
6670e5e7 RGS |
915 | s/^\s+|\s+$//g; |
916 | print "$_\n"; | |
917 | } | |
5a964f20 | 918 | |
960c6898 | 919 | For a multi-line string, you can apply the regular expression to each |
920 | logical line in the string by adding the C</m> flag (for | |
6670e5e7 | 921 | "multi-line"). With the C</m> flag, the C<$> matches I<before> an |
960c6898 | 922 | embedded newline, so it doesn't remove it. This pattern still removes |
923 | the newline at the end of the string: | |
6670e5e7 | 924 | |
ac9dac7f | 925 | $string =~ s/^\s+|\s+$//gm; |
6670e5e7 RGS |
926 | |
927 | Remember that lines consisting entirely of whitespace will disappear, | |
928 | since the first part of the alternation can match the entire string | |
960c6898 | 929 | and replace it with nothing. If you need to keep embedded blank lines, |
6670e5e7 | 930 | you have to do a little more work. Instead of matching any whitespace |
960c6898 | 931 | (since that includes a newline), just match the other whitespace: |
6670e5e7 RGS |
932 | |
933 | $string =~ s/^[\t\f ]+|[\t\f ]+$//mg; | |
5a964f20 | 934 | |
65acb1b1 TC |
935 | =head2 How do I pad a string with blanks or pad a number with zeroes? |
936 | ||
65acb1b1 | 937 | In the following examples, C<$pad_len> is the length to which you wish |
d92eb7b0 GS |
938 | to pad the string, C<$text> or C<$num> contains the string to be padded, |
939 | and C<$pad_char> contains the padding character. You can use a single | |
940 | character string constant instead of the C<$pad_char> variable if you | |
941 | know what it is in advance. And in the same way you can use an integer in | |
942 | place of C<$pad_len> if you know the pad length in advance. | |
65acb1b1 | 943 | |
d92eb7b0 GS |
944 | The simplest method uses the C<sprintf> function. It can pad on the left |
945 | or right with blanks and on the left with zeroes and it will not | |
946 | truncate the result. The C<pack> function can only pad strings on the | |
947 | right with blanks and it will truncate the result to a maximum length of | |
948 | C<$pad_len>. | |
65acb1b1 | 949 | |
ac9dac7f | 950 | # Left padding a string with blanks (no truncation): |
04d666b1 RGS |
951 | $padded = sprintf("%${pad_len}s", $text); |
952 | $padded = sprintf("%*s", $pad_len, $text); # same thing | |
65acb1b1 | 953 | |
ac9dac7f | 954 | # Right padding a string with blanks (no truncation): |
04d666b1 RGS |
955 | $padded = sprintf("%-${pad_len}s", $text); |
956 | $padded = sprintf("%-*s", $pad_len, $text); # same thing | |
65acb1b1 | 957 | |
ac9dac7f | 958 | # Left padding a number with 0 (no truncation): |
04d666b1 RGS |
959 | $padded = sprintf("%0${pad_len}d", $num); |
960 | $padded = sprintf("%0*d", $pad_len, $num); # same thing | |
65acb1b1 | 961 | |
ac9dac7f RGS |
962 | # Right padding a string with blanks using pack (will truncate): |
963 | $padded = pack("A$pad_len",$text); | |
65acb1b1 | 964 | |
d92eb7b0 GS |
965 | If you need to pad with a character other than blank or zero you can use |
966 | one of the following methods. They all generate a pad string with the | |
967 | C<x> operator and combine that with C<$text>. These methods do | |
968 | not truncate C<$text>. | |
65acb1b1 | 969 | |
d92eb7b0 | 970 | Left and right padding with any character, creating a new string: |
65acb1b1 | 971 | |
ac9dac7f RGS |
972 | $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; |
973 | $padded = $text . $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 974 | |
d92eb7b0 | 975 | Left and right padding with any character, modifying C<$text> directly: |
65acb1b1 | 976 | |
ac9dac7f RGS |
977 | substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); |
978 | $text .= $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 979 | |
68dc0745 | 980 | =head2 How do I extract selected columns from a string? |
981 | ||
e573f903 RGS |
982 | (contributed by brian d foy) |
983 | ||
d12d61cf | 984 | If you know the columns that contain the data, you can |
e573f903 RGS |
985 | use C<substr> to extract a single column. |
986 | ||
987 | my $column = substr( $line, $start_column, $length ); | |
988 | ||
989 | You can use C<split> if the columns are separated by whitespace or | |
990 | some other delimiter, as long as whitespace or the delimiter cannot | |
991 | appear as part of the data. | |
992 | ||
993 | my $line = ' fred barney betty '; | |
994 | my @columns = split /\s+/, $line; | |
995 | # ( '', 'fred', 'barney', 'betty' ); | |
996 | ||
997 | my $line = 'fred||barney||betty'; | |
998 | my @columns = split /\|/, $line; | |
999 | # ( 'fred', '', 'barney', '', 'betty' ); | |
1000 | ||
1001 | If you want to work with comma-separated values, don't do this since | |
1002 | that format is a bit more complicated. Use one of the modules that | |
109f0441 | 1003 | handle that format, such as C<Text::CSV>, C<Text::CSV_XS>, or |
e573f903 RGS |
1004 | C<Text::CSV_PP>. |
1005 | ||
1006 | If you want to break apart an entire line of fixed columns, you can use | |
589a5df2 | 1007 | C<unpack> with the A (ASCII) format. By using a number after the format |
e573f903 RGS |
1008 | specifier, you can denote the column width. See the C<pack> and C<unpack> |
1009 | entries in L<perlfunc> for more details. | |
1010 | ||
1011 | my @fields = unpack( $line, "A8 A8 A8 A16 A4" ); | |
1012 | ||
1013 | Note that spaces in the format argument to C<unpack> do not denote literal | |
1014 | spaces. If you have space separated data, you may want C<split> instead. | |
68dc0745 | 1015 | |
1016 | =head2 How do I find the soundex value of a string? | |
1017 | ||
7678cced RGS |
1018 | (contributed by brian d foy) |
1019 | ||
1020 | You can use the Text::Soundex module. If you want to do fuzzy or close | |
ac9dac7f RGS |
1021 | matching, you might also try the C<String::Approx>, and |
1022 | C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules. | |
68dc0745 | 1023 | |
1024 | =head2 How can I expand variables in text strings? | |
1025 | ||
e573f903 | 1026 | (contributed by brian d foy) |
5a964f20 | 1027 | |
322be77c | 1028 | If you can avoid it, don't, or if you can use a templating system, |
c195e131 RGS |
1029 | such as C<Text::Template> or C<Template> Toolkit, do that instead. You |
1030 | might even be able to get the job done with C<sprintf> or C<printf>: | |
1031 | ||
1032 | my $string = sprintf 'Say hello to %s and %s', $foo, $bar; | |
322be77c RGS |
1033 | |
1034 | However, for the one-off simple case where I don't want to pull out a | |
1035 | full templating system, I'll use a string that has two Perl scalar | |
1036 | variables in it. In this example, I want to expand C<$foo> and C<$bar> | |
c195e131 | 1037 | to their variable's values: |
e573f903 RGS |
1038 | |
1039 | my $foo = 'Fred'; | |
1040 | my $bar = 'Barney'; | |
1041 | $string = 'Say hello to $foo and $bar'; | |
1042 | ||
1043 | One way I can do this involves the substitution operator and a double | |
1044 | C</e> flag. The first C</e> evaluates C<$1> on the replacement side and | |
1045 | turns it into C<$foo>. The second /e starts with C<$foo> and replaces | |
1046 | it with its value. C<$foo>, then, turns into 'Fred', and that's finally | |
c195e131 | 1047 | what's left in the string: |
e573f903 RGS |
1048 | |
1049 | $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney' | |
322be77c | 1050 | |
e573f903 | 1051 | The C</e> will also silently ignore violations of strict, replacing |
c195e131 | 1052 | undefined variable names with the empty string. Since I'm using the |
109f0441 | 1053 | C</e> flag (twice even!), I have all of the same security problems I |
c195e131 RGS |
1054 | have with C<eval> in its string form. If there's something odd in |
1055 | C<$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then | |
1056 | I could get myself in trouble. | |
1057 | ||
1058 | To get around the security problem, I could also pull the values from | |
1059 | a hash instead of evaluating variable names. Using a single C</e>, I | |
1060 | can check the hash to ensure the value exists, and if it doesn't, I | |
1061 | can replace the missing value with a marker, in this case C<???> to | |
1062 | signal that I missed something: | |
e573f903 RGS |
1063 | |
1064 | my $string = 'This has $foo and $bar'; | |
109f0441 | 1065 | |
e573f903 RGS |
1066 | my %Replacements = ( |
1067 | foo => 'Fred', | |
ac9dac7f | 1068 | ); |
322be77c | 1069 | |
e573f903 RGS |
1070 | # $string =~ s/\$(\w+)/$Replacements{$1}/g; |
1071 | $string =~ s/\$(\w+)/ | |
1072 | exists $Replacements{$1} ? $Replacements{$1} : '???' | |
1073 | /eg; | |
322be77c | 1074 | |
e573f903 | 1075 | print $string; |
322be77c | 1076 | |
68dc0745 | 1077 | =head2 What's wrong with always quoting "$vars"? |
1078 | ||
ac9dac7f | 1079 | The problem is that those double-quotes force |
e573f903 RGS |
1080 | stringification--coercing numbers and references into strings--even |
1081 | when you don't want them to be strings. Think of it this way: | |
1082 | double-quote expansion is used to produce new strings. If you already | |
1083 | have a string, why do you need more? | |
68dc0745 | 1084 | |
1085 | If you get used to writing odd things like these: | |
1086 | ||
ac9dac7f RGS |
1087 | print "$var"; # BAD |
1088 | $new = "$old"; # BAD | |
1089 | somefunc("$var"); # BAD | |
68dc0745 | 1090 | |
1091 | You'll be in trouble. Those should (in 99.8% of the cases) be | |
1092 | the simpler and more direct: | |
1093 | ||
ac9dac7f RGS |
1094 | print $var; |
1095 | $new = $old; | |
1096 | somefunc($var); | |
68dc0745 | 1097 | |
1098 | Otherwise, besides slowing you down, you're going to break code when | |
1099 | the thing in the scalar is actually neither a string nor a number, but | |
1100 | a reference: | |
1101 | ||
ac9dac7f RGS |
1102 | func(\@array); |
1103 | sub func { | |
1104 | my $aref = shift; | |
1105 | my $oref = "$aref"; # WRONG | |
1106 | } | |
68dc0745 | 1107 | |
1108 | You can also get into subtle problems on those few operations in Perl | |
1109 | that actually do care about the difference between a string and a | |
1110 | number, such as the magical C<++> autoincrement operator or the | |
1111 | syscall() function. | |
1112 | ||
197aec24 | 1113 | Stringification also destroys arrays. |
5a964f20 | 1114 | |
ac9dac7f RGS |
1115 | @lines = `command`; |
1116 | print "@lines"; # WRONG - extra blanks | |
1117 | print @lines; # right | |
5a964f20 | 1118 | |
04d666b1 | 1119 | =head2 Why don't my E<lt>E<lt>HERE documents work? |
68dc0745 | 1120 | |
1121 | Check for these three things: | |
1122 | ||
1123 | =over 4 | |
1124 | ||
04d666b1 | 1125 | =item There must be no space after the E<lt>E<lt> part. |
68dc0745 | 1126 | |
197aec24 | 1127 | =item There (probably) should be a semicolon at the end. |
68dc0745 | 1128 | |
197aec24 | 1129 | =item You can't (easily) have any space in front of the tag. |
68dc0745 | 1130 | |
1131 | =back | |
1132 | ||
197aec24 | 1133 | If you want to indent the text in the here document, you |
5a964f20 TC |
1134 | can do this: |
1135 | ||
1136 | # all in one | |
1137 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; | |
1138 | your text | |
1139 | goes here | |
1140 | HERE_TARGET | |
1141 | ||
1142 | But the HERE_TARGET must still be flush against the margin. | |
197aec24 | 1143 | If you want that indented also, you'll have to quote |
5a964f20 TC |
1144 | in the indentation. |
1145 | ||
1146 | ($quote = <<' FINIS') =~ s/^\s+//gm; | |
1147 | ...we will have peace, when you and all your works have | |
1148 | perished--and the works of your dark master to whom you | |
1149 | would deliver us. You are a liar, Saruman, and a corrupter | |
1150 | of men's hearts. --Theoden in /usr/src/perl/taint.c | |
1151 | FINIS | |
83ded9ee | 1152 | $quote =~ s/\s+--/\n--/; |
5a964f20 TC |
1153 | |
1154 | A nice general-purpose fixer-upper function for indented here documents | |
1155 | follows. It expects to be called with a here document as its argument. | |
1156 | It looks to see whether each line begins with a common substring, and | |
a6dd486b JB |
1157 | if so, strips that substring off. Otherwise, it takes the amount of leading |
1158 | whitespace found on the first line and removes that much off each | |
5a964f20 TC |
1159 | subsequent line. |
1160 | ||
1161 | sub fix { | |
1162 | local $_ = shift; | |
a6dd486b | 1163 | my ($white, $leader); # common whitespace and common leading string |
d8b950dc | 1164 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\g1\g2?.*\n)+$/) { |
5a964f20 TC |
1165 | ($white, $leader) = ($2, quotemeta($1)); |
1166 | } else { | |
1167 | ($white, $leader) = (/^(\s+)/, ''); | |
1168 | } | |
1169 | s/^\s*?$leader(?:$white)?//gm; | |
1170 | return $_; | |
1171 | } | |
1172 | ||
c8db1d39 | 1173 | This works with leading special strings, dynamically determined: |
5a964f20 | 1174 | |
ac9dac7f | 1175 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; |
5a964f20 TC |
1176 | @@@ int |
1177 | @@@ runops() { | |
1178 | @@@ SAVEI32(runlevel); | |
1179 | @@@ runlevel++; | |
d92eb7b0 | 1180 | @@@ while ( op = (*op->op_ppaddr)() ); |
5a964f20 TC |
1181 | @@@ TAINT_NOT; |
1182 | @@@ return 0; | |
1183 | @@@ } | |
ac9dac7f | 1184 | MAIN_INTERPRETER_LOOP |
5a964f20 | 1185 | |
a6dd486b | 1186 | Or with a fixed amount of leading whitespace, with remaining |
5a964f20 TC |
1187 | indentation correctly preserved: |
1188 | ||
ac9dac7f | 1189 | $poem = fix<<EVER_ON_AND_ON; |
5a964f20 TC |
1190 | Now far ahead the Road has gone, |
1191 | And I must follow, if I can, | |
1192 | Pursuing it with eager feet, | |
1193 | Until it joins some larger way | |
1194 | Where many paths and errands meet. | |
1195 | And whither then? I cannot say. | |
1196 | --Bilbo in /usr/src/perl/pp_ctl.c | |
ac9dac7f | 1197 | EVER_ON_AND_ON |
5a964f20 | 1198 | |
68dc0745 | 1199 | =head1 Data: Arrays |
1200 | ||
65acb1b1 TC |
1201 | =head2 What is the difference between a list and an array? |
1202 | ||
8d2e243f | 1203 | (contributed by brian d foy) |
1204 | ||
1205 | A list is a fixed collection of scalars. An array is a variable that | |
1206 | holds a variable collection of scalars. An array can supply its collection | |
1207 | for list operations, so list operations also work on arrays: | |
1208 | ||
1209 | # slices | |
1210 | ( 'dog', 'cat', 'bird' )[2,3]; | |
1211 | @animals[2,3]; | |
1212 | ||
1213 | # iteration | |
1214 | foreach ( qw( dog cat bird ) ) { ... } | |
1215 | foreach ( @animals ) { ... } | |
1216 | ||
1217 | my @three = grep { length == 3 } qw( dog cat bird ); | |
1218 | my @three = grep { length == 3 } @animals; | |
d12d61cf | 1219 | |
8d2e243f | 1220 | # supply an argument list |
1221 | wash_animals( qw( dog cat bird ) ); | |
1222 | wash_animals( @animals ); | |
1223 | ||
c69ca1d4 | 1224 | Array operations, which change the scalars, rearranges them, or adds |
8d2e243f | 1225 | or subtracts some scalars, only work on arrays. These can't work on a |
1226 | list, which is fixed. Array operations include C<shift>, C<unshift>, | |
1227 | C<push>, C<pop>, and C<splice>. | |
1228 | ||
1229 | An array can also change its length: | |
1230 | ||
1231 | $#animals = 1; # truncate to two elements | |
1232 | $#animals = 10000; # pre-extend to 10,001 elements | |
1233 | ||
1234 | You can change an array element, but you can't change a list element: | |
1235 | ||
1236 | $animals[0] = 'Rottweiler'; | |
1237 | qw( dog cat bird )[0] = 'Rottweiler'; # syntax error! | |
1238 | ||
1239 | foreach ( @animals ) { | |
1240 | s/^d/fr/; # works fine | |
1241 | } | |
d12d61cf | 1242 | |
8d2e243f | 1243 | foreach ( qw( dog cat bird ) ) { |
1244 | s/^d/fr/; # Error! Modification of read only value! | |
1245 | } | |
1246 | ||
d12d61cf | 1247 | However, if the list element is itself a variable, it appears that you |
8d2e243f | 1248 | can change a list element. However, the list element is the variable, not |
1249 | the data. You're not changing the list element, but something the list | |
d12d61cf | 1250 | element refers to. The list element itself doesn't change: it's still |
8d2e243f | 1251 | the same variable. |
65acb1b1 | 1252 | |
8d2e243f | 1253 | You also have to be careful about context. You can assign an array to |
1254 | a scalar to get the number of elements in the array. This only works | |
1255 | for arrays, though: | |
1256 | ||
1257 | my $count = @animals; # only works with arrays | |
d12d61cf | 1258 | |
8d2e243f | 1259 | If you try to do the same thing with what you think is a list, you |
1260 | get a quite different result. Although it looks like you have a list | |
1261 | on the righthand side, Perl actually sees a bunch of scalars separated | |
1262 | by a comma: | |
65acb1b1 | 1263 | |
8d2e243f | 1264 | my $scalar = ( 'dog', 'cat', 'bird' ); # $scalar gets bird |
65acb1b1 | 1265 | |
8d2e243f | 1266 | Since you're assigning to a scalar, the righthand side is in scalar |
1267 | context. The comma operator (yes, it's an operator!) in scalar | |
1268 | context evaluates its lefthand side, throws away the result, and | |
1269 | evaluates it's righthand side and returns the result. In effect, | |
1270 | that list-lookalike assigns to C<$scalar> it's rightmost value. Many | |
c69ca1d4 | 1271 | people mess this up because they choose a list-lookalike whose |
8d2e243f | 1272 | last element is also the count they expect: |
1273 | ||
1274 | my $scalar = ( 1, 2, 3 ); # $scalar gets 3, accidentally | |
65acb1b1 | 1275 | |
68dc0745 | 1276 | =head2 What is the difference between $array[1] and @array[1]? |
1277 | ||
8d2e243f | 1278 | (contributed by brian d foy) |
1279 | ||
1280 | The difference is the sigil, that special character in front of the | |
1281 | array name. The C<$> sigil means "exactly one item", while the C<@> | |
1282 | sigil means "zero or more items". The C<$> gets you a single scalar, | |
1283 | while the C<@> gets you a list. | |
68dc0745 | 1284 | |
8d2e243f | 1285 | The confusion arises because people incorrectly assume that the sigil |
1286 | denotes the variable type. | |
68dc0745 | 1287 | |
8d2e243f | 1288 | The C<$array[1]> is a single-element access to the array. It's going |
1289 | to return the item in index 1 (or undef if there is no item there). | |
1290 | If you intend to get exactly one element from the array, this is the | |
1291 | form you should use. | |
68dc0745 | 1292 | |
8d2e243f | 1293 | The C<@array[1]> is an array slice, although it has only one index. |
1294 | You can pull out multiple elements simultaneously by specifying | |
1295 | additional indices as a list, like C<@array[1,4,3,0]>. | |
68dc0745 | 1296 | |
8d2e243f | 1297 | Using a slice on the lefthand side of the assignment supplies list |
d12d61cf | 1298 | context to the righthand side. This can lead to unexpected results. |
1299 | For instance, if you want to read a single line from a filehandle, | |
8d2e243f | 1300 | assigning to a scalar value is fine: |
68dc0745 | 1301 | |
8d2e243f | 1302 | $array[1] = <STDIN>; |
1303 | ||
1304 | However, in list context, the line input operator returns all of the | |
1305 | lines as a list. The first line goes into C<@array[1]> and the rest | |
1306 | of the lines mysteriously disappear: | |
1307 | ||
1308 | @array[1] = <STDIN>; # most likely not what you want | |
1309 | ||
1310 | Either the C<use warnings> pragma or the B<-w> flag will warn you when | |
1311 | you use an array slice with a single index. | |
68dc0745 | 1312 | |
d92eb7b0 | 1313 | =head2 How can I remove duplicate elements from a list or array? |
68dc0745 | 1314 | |
6670e5e7 | 1315 | (contributed by brian d foy) |
68dc0745 | 1316 | |
6670e5e7 RGS |
1317 | Use a hash. When you think the words "unique" or "duplicated", think |
1318 | "hash keys". | |
68dc0745 | 1319 | |
6670e5e7 RGS |
1320 | If you don't care about the order of the elements, you could just |
1321 | create the hash then extract the keys. It's not important how you | |
1322 | create that hash: just that you use C<keys> to get the unique | |
1323 | elements. | |
551e1d92 | 1324 | |
ac9dac7f RGS |
1325 | my %hash = map { $_, 1 } @array; |
1326 | # or a hash slice: @hash{ @array } = (); | |
1327 | # or a foreach: $hash{$_} = 1 foreach ( @array ); | |
1328 | ||
1329 | my @unique = keys %hash; | |
68dc0745 | 1330 | |
ac9dac7f RGS |
1331 | If you want to use a module, try the C<uniq> function from |
1332 | C<List::MoreUtils>. In list context it returns the unique elements, | |
1333 | preserving their order in the list. In scalar context, it returns the | |
1334 | number of unique elements. | |
1335 | ||
1336 | use List::MoreUtils qw(uniq); | |
1337 | ||
1338 | my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7 | |
1339 | my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7 | |
68dc0745 | 1340 | |
6670e5e7 RGS |
1341 | You can also go through each element and skip the ones you've seen |
1342 | before. Use a hash to keep track. The first time the loop sees an | |
1343 | element, that element has no key in C<%Seen>. The C<next> statement | |
1344 | creates the key and immediately uses its value, which is C<undef>, so | |
1345 | the loop continues to the C<push> and increments the value for that | |
1346 | key. The next time the loop sees that same element, its key exists in | |
1347 | the hash I<and> the value for that key is true (since it's not 0 or | |
ac9dac7f RGS |
1348 | C<undef>), so the next skips that iteration and the loop goes to the |
1349 | next element. | |
551e1d92 | 1350 | |
6670e5e7 RGS |
1351 | my @unique = (); |
1352 | my %seen = (); | |
68dc0745 | 1353 | |
6670e5e7 RGS |
1354 | foreach my $elem ( @array ) |
1355 | { | |
1356 | next if $seen{ $elem }++; | |
1357 | push @unique, $elem; | |
1358 | } | |
68dc0745 | 1359 | |
6670e5e7 RGS |
1360 | You can write this more briefly using a grep, which does the |
1361 | same thing. | |
68dc0745 | 1362 | |
ac9dac7f RGS |
1363 | my %seen = (); |
1364 | my @unique = grep { ! $seen{ $_ }++ } @array; | |
65acb1b1 | 1365 | |
ddbc1f16 | 1366 | =head2 How can I tell whether a certain element is contained in a list or array? |
5a964f20 | 1367 | |
109f0441 | 1368 | (portions of this answer contributed by Anno Siegel and brian d foy) |
9e72e4c6 | 1369 | |
5a964f20 TC |
1370 | Hearing the word "in" is an I<in>dication that you probably should have |
1371 | used a hash, not a list or array, to store your data. Hashes are | |
1372 | designed to answer this question quickly and efficiently. Arrays aren't. | |
68dc0745 | 1373 | |
109f0441 S |
1374 | That being said, there are several ways to approach this. In Perl 5.10 |
1375 | and later, you can use the smart match operator to check that an item is | |
1376 | contained in an array or a hash: | |
1377 | ||
1378 | use 5.010; | |
1379 | ||
1380 | if( $item ~~ @array ) | |
1381 | { | |
1382 | say "The array contains $item" | |
1383 | } | |
1384 | ||
1385 | if( $item ~~ %hash ) | |
1386 | { | |
1387 | say "The hash contains $item" | |
1388 | } | |
1389 | ||
1390 | With earlier versions of Perl, you have to do a bit more work. If you | |
5a964f20 | 1391 | are going to make this query many times over arbitrary string values, |
881bdbd4 | 1392 | the fastest way is probably to invert the original array and maintain a |
109f0441 | 1393 | hash whose keys are the first array's values: |
68dc0745 | 1394 | |
ac9dac7f RGS |
1395 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; |
1396 | %is_blue = (); | |
1397 | for (@blues) { $is_blue{$_} = 1 } | |
68dc0745 | 1398 | |
ac9dac7f RGS |
1399 | Now you can check whether C<$is_blue{$some_color}>. It might have |
1400 | been a good idea to keep the blues all in a hash in the first place. | |
68dc0745 | 1401 | |
1402 | If the values are all small integers, you could use a simple indexed | |
1403 | array. This kind of an array will take up less space: | |
1404 | ||
ac9dac7f RGS |
1405 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); |
1406 | @is_tiny_prime = (); | |
1407 | for (@primes) { $is_tiny_prime[$_] = 1 } | |
1408 | # or simply @istiny_prime[@primes] = (1) x @primes; | |
68dc0745 | 1409 | |
1410 | Now you check whether $is_tiny_prime[$some_number]. | |
1411 | ||
1412 | If the values in question are integers instead of strings, you can save | |
1413 | quite a lot of space by using bit strings instead: | |
1414 | ||
ac9dac7f RGS |
1415 | @articles = ( 1..10, 150..2000, 2017 ); |
1416 | undef $read; | |
1417 | for (@articles) { vec($read,$_,1) = 1 } | |
68dc0745 | 1418 | |
1419 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. | |
1420 | ||
9e72e4c6 RGS |
1421 | These methods guarantee fast individual tests but require a re-organization |
1422 | of the original list or array. They only pay off if you have to test | |
1423 | multiple values against the same array. | |
68dc0745 | 1424 | |
ac9dac7f | 1425 | If you are testing only once, the standard module C<List::Util> exports |
9e72e4c6 | 1426 | the function C<first> for this purpose. It works by stopping once it |
c195e131 | 1427 | finds the element. It's written in C for speed, and its Perl equivalent |
9e72e4c6 | 1428 | looks like this subroutine: |
68dc0745 | 1429 | |
9e72e4c6 RGS |
1430 | sub first (&@) { |
1431 | my $code = shift; | |
1432 | foreach (@_) { | |
1433 | return $_ if &{$code}(); | |
1434 | } | |
1435 | undef; | |
1436 | } | |
68dc0745 | 1437 | |
9e72e4c6 RGS |
1438 | If speed is of little concern, the common idiom uses grep in scalar context |
1439 | (which returns the number of items that passed its condition) to traverse the | |
1440 | entire list. This does have the benefit of telling you how many matches it | |
1441 | found, though. | |
68dc0745 | 1442 | |
9e72e4c6 | 1443 | my $is_there = grep $_ eq $whatever, @array; |
65acb1b1 | 1444 | |
9e72e4c6 RGS |
1445 | If you want to actually extract the matching elements, simply use grep in |
1446 | list context. | |
68dc0745 | 1447 | |
9e72e4c6 | 1448 | my @matches = grep $_ eq $whatever, @array; |
58103a2e | 1449 | |
68dc0745 | 1450 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? |
1451 | ||
ac9dac7f RGS |
1452 | Use a hash. Here's code to do both and more. It assumes that each |
1453 | element is unique in a given array: | |
68dc0745 | 1454 | |
ac9dac7f RGS |
1455 | @union = @intersection = @difference = (); |
1456 | %count = (); | |
1457 | foreach $element (@array1, @array2) { $count{$element}++ } | |
1458 | foreach $element (keys %count) { | |
1459 | push @union, $element; | |
1460 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; | |
1461 | } | |
68dc0745 | 1462 | |
ac9dac7f RGS |
1463 | Note that this is the I<symmetric difference>, that is, all elements |
1464 | in either A or in B but not in both. Think of it as an xor operation. | |
d92eb7b0 | 1465 | |
65acb1b1 TC |
1466 | =head2 How do I test whether two arrays or hashes are equal? |
1467 | ||
109f0441 S |
1468 | With Perl 5.10 and later, the smart match operator can give you the answer |
1469 | with the least amount of work: | |
1470 | ||
1471 | use 5.010; | |
1472 | ||
1473 | if( @array1 ~~ @array2 ) | |
1474 | { | |
1475 | say "The arrays are the same"; | |
1476 | } | |
1477 | ||
1478 | if( %hash1 ~~ %hash2 ) # doesn't check values! | |
1479 | { | |
1480 | say "The hash keys are the same"; | |
1481 | } | |
1482 | ||
ac9dac7f RGS |
1483 | The following code works for single-level arrays. It uses a |
1484 | stringwise comparison, and does not distinguish defined versus | |
1485 | undefined empty strings. Modify if you have other needs. | |
65acb1b1 | 1486 | |
ac9dac7f | 1487 | $are_equal = compare_arrays(\@frogs, \@toads); |
65acb1b1 | 1488 | |
ac9dac7f RGS |
1489 | sub compare_arrays { |
1490 | my ($first, $second) = @_; | |
1491 | no warnings; # silence spurious -w undef complaints | |
1492 | return 0 unless @$first == @$second; | |
1493 | for (my $i = 0; $i < @$first; $i++) { | |
1494 | return 0 if $first->[$i] ne $second->[$i]; | |
1495 | } | |
1496 | return 1; | |
1497 | } | |
65acb1b1 TC |
1498 | |
1499 | For multilevel structures, you may wish to use an approach more | |
ac9dac7f | 1500 | like this one. It uses the CPAN module C<FreezeThaw>: |
65acb1b1 | 1501 | |
ac9dac7f RGS |
1502 | use FreezeThaw qw(cmpStr); |
1503 | @a = @b = ( "this", "that", [ "more", "stuff" ] ); | |
65acb1b1 | 1504 | |
ac9dac7f RGS |
1505 | printf "a and b contain %s arrays\n", |
1506 | cmpStr(\@a, \@b) == 0 | |
1507 | ? "the same" | |
1508 | : "different"; | |
65acb1b1 | 1509 | |
ac9dac7f RGS |
1510 | This approach also works for comparing hashes. Here we'll demonstrate |
1511 | two different answers: | |
65acb1b1 | 1512 | |
ac9dac7f | 1513 | use FreezeThaw qw(cmpStr cmpStrHard); |
65acb1b1 | 1514 | |
ac9dac7f RGS |
1515 | %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); |
1516 | $a{EXTRA} = \%b; | |
1517 | $b{EXTRA} = \%a; | |
65acb1b1 | 1518 | |
ac9dac7f | 1519 | printf "a and b contain %s hashes\n", |
65acb1b1 TC |
1520 | cmpStr(\%a, \%b) == 0 ? "the same" : "different"; |
1521 | ||
ac9dac7f | 1522 | printf "a and b contain %s hashes\n", |
65acb1b1 TC |
1523 | cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; |
1524 | ||
1525 | ||
1526 | The first reports that both those the hashes contain the same data, | |
1527 | while the second reports that they do not. Which you prefer is left as | |
1528 | an exercise to the reader. | |
1529 | ||
68dc0745 | 1530 | =head2 How do I find the first array element for which a condition is true? |
1531 | ||
49d635f9 | 1532 | To find the first array element which satisfies a condition, you can |
ac9dac7f RGS |
1533 | use the C<first()> function in the C<List::Util> module, which comes |
1534 | with Perl 5.8. This example finds the first element that contains | |
1535 | "Perl". | |
49d635f9 RGS |
1536 | |
1537 | use List::Util qw(first); | |
197aec24 | 1538 | |
49d635f9 | 1539 | my $element = first { /Perl/ } @array; |
197aec24 | 1540 | |
ac9dac7f | 1541 | If you cannot use C<List::Util>, you can make your own loop to do the |
49d635f9 RGS |
1542 | same thing. Once you find the element, you stop the loop with last. |
1543 | ||
1544 | my $found; | |
ac9dac7f | 1545 | foreach ( @array ) { |
6670e5e7 | 1546 | if( /Perl/ ) { $found = $_; last } |
49d635f9 RGS |
1547 | } |
1548 | ||
1549 | If you want the array index, you can iterate through the indices | |
1550 | and check the array element at each index until you find one | |
1551 | that satisfies the condition. | |
1552 | ||
197aec24 | 1553 | my( $found, $index ) = ( undef, -1 ); |
ac9dac7f RGS |
1554 | for( $i = 0; $i < @array; $i++ ) { |
1555 | if( $array[$i] =~ /Perl/ ) { | |
6670e5e7 RGS |
1556 | $found = $array[$i]; |
1557 | $index = $i; | |
1558 | last; | |
1559 | } | |
1560 | } | |
68dc0745 | 1561 | |
1562 | =head2 How do I handle linked lists? | |
1563 | ||
159235ed | 1564 | (contributed by brian d foy) |
65acb1b1 | 1565 | |
159235ed | 1566 | Perl's arrays do not have a fixed size, so you don't need linked lists |
1567 | if you just want to add or remove items. You can use array operations | |
1568 | such as C<push>, C<pop>, C<shift>, C<unshift>, or C<splice> to do | |
1569 | that. | |
1570 | ||
1571 | Sometimes, however, linked lists can be useful in situations where you | |
1572 | want to "shard" an array so you have have many small arrays instead of | |
1573 | a single big array. You can keep arrays longer than Perl's largest | |
1574 | array index, lock smaller arrays separately in threaded programs, | |
1575 | reallocate less memory, or quickly insert elements in the middle of | |
1576 | the chain. | |
1577 | ||
1578 | Steve Lembark goes through the details in his YAPC::NA 2009 talk "Perly | |
84adb724 | 1579 | Linked Lists" ( http://www.slideshare.net/lembark/perly-linked-lists ), |
159235ed | 1580 | although you can just use his C<LinkedList::Single> module. |
68dc0745 | 1581 | |
1582 | =head2 How do I handle circular lists? | |
109f0441 S |
1583 | X<circular> X<array> X<Tie::Cycle> X<Array::Iterator::Circular> |
1584 | X<cycle> X<modulus> | |
68dc0745 | 1585 | |
109f0441 S |
1586 | (contributed by brian d foy) |
1587 | ||
589a5df2 | 1588 | If you want to cycle through an array endlessly, you can increment the |
109f0441 | 1589 | index modulo the number of elements in the array: |
68dc0745 | 1590 | |
109f0441 S |
1591 | my @array = qw( a b c ); |
1592 | my $i = 0; | |
1593 | ||
1594 | while( 1 ) { | |
1595 | print $array[ $i++ % @array ], "\n"; | |
1596 | last if $i > 20; | |
1597 | } | |
ac9dac7f | 1598 | |
109f0441 S |
1599 | You can also use C<Tie::Cycle> to use a scalar that always has the |
1600 | next element of the circular array: | |
ac9dac7f RGS |
1601 | |
1602 | use Tie::Cycle; | |
1603 | ||
1604 | tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ]; | |
1605 | ||
1606 | print $cycle; # FFFFFF | |
1607 | print $cycle; # 000000 | |
1608 | print $cycle; # FFFF00 | |
68dc0745 | 1609 | |
109f0441 S |
1610 | The C<Array::Iterator::Circular> creates an iterator object for |
1611 | circular arrays: | |
1612 | ||
1613 | use Array::Iterator::Circular; | |
1614 | ||
1615 | my $color_iterator = Array::Iterator::Circular->new( | |
1616 | qw(red green blue orange) | |
1617 | ); | |
1618 | ||
1619 | foreach ( 1 .. 20 ) { | |
1620 | print $color_iterator->next, "\n"; | |
1621 | } | |
1622 | ||
68dc0745 | 1623 | =head2 How do I shuffle an array randomly? |
1624 | ||
45bbf655 JH |
1625 | If you either have Perl 5.8.0 or later installed, or if you have |
1626 | Scalar-List-Utils 1.03 or later installed, you can say: | |
1627 | ||
ac9dac7f | 1628 | use List::Util 'shuffle'; |
45bbf655 JH |
1629 | |
1630 | @shuffled = shuffle(@list); | |
1631 | ||
f05bbc40 | 1632 | If not, you can use a Fisher-Yates shuffle. |
5a964f20 | 1633 | |
ac9dac7f RGS |
1634 | sub fisher_yates_shuffle { |
1635 | my $deck = shift; # $deck is a reference to an array | |
109f0441 S |
1636 | return unless @$deck; # must not be empty! |
1637 | ||
ac9dac7f RGS |
1638 | my $i = @$deck; |
1639 | while (--$i) { | |
1640 | my $j = int rand ($i+1); | |
1641 | @$deck[$i,$j] = @$deck[$j,$i]; | |
1642 | } | |
1643 | } | |
5a964f20 | 1644 | |
ac9dac7f RGS |
1645 | # shuffle my mpeg collection |
1646 | # | |
1647 | my @mpeg = <audio/*/*.mp3>; | |
1648 | fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place | |
1649 | print @mpeg; | |
5a964f20 | 1650 | |
45bbf655 | 1651 | Note that the above implementation shuffles an array in place, |
ac9dac7f | 1652 | unlike the C<List::Util::shuffle()> which takes a list and returns |
45bbf655 JH |
1653 | a new shuffled list. |
1654 | ||
d92eb7b0 | 1655 | You've probably seen shuffling algorithms that work using splice, |
a6dd486b | 1656 | randomly picking another element to swap the current element with |
68dc0745 | 1657 | |
ac9dac7f RGS |
1658 | srand; |
1659 | @new = (); | |
1660 | @old = 1 .. 10; # just a demo | |
1661 | while (@old) { | |
1662 | push(@new, splice(@old, rand @old, 1)); | |
1663 | } | |
68dc0745 | 1664 | |
ac9dac7f RGS |
1665 | This is bad because splice is already O(N), and since you do it N |
1666 | times, you just invented a quadratic algorithm; that is, O(N**2). | |
1667 | This does not scale, although Perl is so efficient that you probably | |
1668 | won't notice this until you have rather largish arrays. | |
68dc0745 | 1669 | |
1670 | =head2 How do I process/modify each element of an array? | |
1671 | ||
1672 | Use C<for>/C<foreach>: | |
1673 | ||
ac9dac7f | 1674 | for (@lines) { |
6670e5e7 RGS |
1675 | s/foo/bar/; # change that word |
1676 | tr/XZ/ZX/; # swap those letters | |
ac9dac7f | 1677 | } |
68dc0745 | 1678 | |
1679 | Here's another; let's compute spherical volumes: | |
1680 | ||
ac9dac7f | 1681 | for (@volumes = @radii) { # @volumes has changed parts |
6670e5e7 RGS |
1682 | $_ **= 3; |
1683 | $_ *= (4/3) * 3.14159; # this will be constant folded | |
ac9dac7f | 1684 | } |
197aec24 | 1685 | |
ac9dac7f | 1686 | which can also be done with C<map()> which is made to transform |
49d635f9 RGS |
1687 | one list into another: |
1688 | ||
1689 | @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii; | |
68dc0745 | 1690 | |
76817d6d JH |
1691 | If you want to do the same thing to modify the values of the |
1692 | hash, you can use the C<values> function. As of Perl 5.6 | |
1693 | the values are not copied, so if you modify $orbit (in this | |
1694 | case), you modify the value. | |
5a964f20 | 1695 | |
ac9dac7f | 1696 | for $orbit ( values %orbits ) { |
6670e5e7 | 1697 | ($orbit **= 3) *= (4/3) * 3.14159; |
ac9dac7f | 1698 | } |
818c4caa | 1699 | |
76817d6d JH |
1700 | Prior to perl 5.6 C<values> returned copies of the values, |
1701 | so older perl code often contains constructions such as | |
1702 | C<@orbits{keys %orbits}> instead of C<values %orbits> where | |
1703 | the hash is to be modified. | |
818c4caa | 1704 | |
68dc0745 | 1705 | =head2 How do I select a random element from an array? |
1706 | ||
ac9dac7f | 1707 | Use the C<rand()> function (see L<perlfunc/rand>): |
68dc0745 | 1708 | |
ac9dac7f RGS |
1709 | $index = rand @array; |
1710 | $element = $array[$index]; | |
68dc0745 | 1711 | |
793f5136 | 1712 | Or, simply: |
ac9dac7f RGS |
1713 | |
1714 | my $element = $array[ rand @array ]; | |
5a964f20 | 1715 | |
68dc0745 | 1716 | =head2 How do I permute N elements of a list? |
c69ca1d4 | 1717 | X<List::Permutor> X<permute> X<Algorithm::Loops> X<Knuth> |
c195e131 | 1718 | X<The Art of Computer Programming> X<Fischer-Krause> |
68dc0745 | 1719 | |
c195e131 | 1720 | Use the C<List::Permutor> module on CPAN. If the list is actually an |
ac9dac7f | 1721 | array, try the C<Algorithm::Permute> module (also on CPAN). It's |
c195e131 | 1722 | written in XS code and is very efficient: |
49d635f9 RGS |
1723 | |
1724 | use Algorithm::Permute; | |
c195e131 | 1725 | |
49d635f9 RGS |
1726 | my @array = 'a'..'d'; |
1727 | my $p_iterator = Algorithm::Permute->new ( \@array ); | |
c195e131 | 1728 | |
49d635f9 RGS |
1729 | while (my @perm = $p_iterator->next) { |
1730 | print "next permutation: (@perm)\n"; | |
ac9dac7f | 1731 | } |
49d635f9 | 1732 | |
197aec24 RGS |
1733 | For even faster execution, you could do: |
1734 | ||
ac9dac7f | 1735 | use Algorithm::Permute; |
c195e131 | 1736 | |
ac9dac7f | 1737 | my @array = 'a'..'d'; |
c195e131 | 1738 | |
ac9dac7f RGS |
1739 | Algorithm::Permute::permute { |
1740 | print "next permutation: (@array)\n"; | |
1741 | } @array; | |
197aec24 | 1742 | |
c195e131 RGS |
1743 | Here's a little program that generates all permutations of all the |
1744 | words on each line of input. The algorithm embodied in the | |
1745 | C<permute()> function is discussed in Volume 4 (still unpublished) of | |
1746 | Knuth's I<The Art of Computer Programming> and will work on any list: | |
49d635f9 RGS |
1747 | |
1748 | #!/usr/bin/perl -n | |
ac003c96 | 1749 | # Fischer-Krause ordered permutation generator |
49d635f9 RGS |
1750 | |
1751 | sub permute (&@) { | |
1752 | my $code = shift; | |
1753 | my @idx = 0..$#_; | |
1754 | while ( $code->(@_[@idx]) ) { | |
1755 | my $p = $#idx; | |
1756 | --$p while $idx[$p-1] > $idx[$p]; | |
1757 | my $q = $p or return; | |
1758 | push @idx, reverse splice @idx, $p; | |
1759 | ++$q while $idx[$p-1] > $idx[$q]; | |
1760 | @idx[$p-1,$q]=@idx[$q,$p-1]; | |
1761 | } | |
68dc0745 | 1762 | } |
68dc0745 | 1763 | |
c195e131 RGS |
1764 | permute { print "@_\n" } split; |
1765 | ||
1766 | The C<Algorithm::Loops> module also provides the C<NextPermute> and | |
1767 | C<NextPermuteNum> functions which efficiently find all unique permutations | |
1768 | of an array, even if it contains duplicate values, modifying it in-place: | |
1769 | if its elements are in reverse-sorted order then the array is reversed, | |
1770 | making it sorted, and it returns false; otherwise the next | |
1771 | permutation is returned. | |
1772 | ||
1773 | C<NextPermute> uses string order and C<NextPermuteNum> numeric order, so | |
1774 | you can enumerate all the permutations of C<0..9> like this: | |
1775 | ||
1776 | use Algorithm::Loops qw(NextPermuteNum); | |
109f0441 | 1777 | |
c195e131 RGS |
1778 | my @list= 0..9; |
1779 | do { print "@list\n" } while NextPermuteNum @list; | |
b8d2732a | 1780 | |
68dc0745 | 1781 | =head2 How do I sort an array by (anything)? |
1782 | ||
1783 | Supply a comparison function to sort() (described in L<perlfunc/sort>): | |
1784 | ||
ac9dac7f | 1785 | @list = sort { $a <=> $b } @list; |
68dc0745 | 1786 | |
1787 | The default sort function is cmp, string comparison, which would | |
c47ff5f1 | 1788 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is |
68dc0745 | 1789 | the numerical comparison operator. |
1790 | ||
1791 | If you have a complicated function needed to pull out the part you | |
1792 | want to sort on, then don't do it inside the sort function. Pull it | |
1793 | out first, because the sort BLOCK can be called many times for the | |
1794 | same element. Here's an example of how to pull out the first word | |
1795 | after the first number on each item, and then sort those words | |
1796 | case-insensitively. | |
1797 | ||
ac9dac7f RGS |
1798 | @idx = (); |
1799 | for (@data) { | |
1800 | ($item) = /\d+\s*(\S+)/; | |
1801 | push @idx, uc($item); | |
1802 | } | |
1803 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; | |
68dc0745 | 1804 | |
a6dd486b | 1805 | which could also be written this way, using a trick |
68dc0745 | 1806 | that's come to be known as the Schwartzian Transform: |
1807 | ||
ac9dac7f RGS |
1808 | @sorted = map { $_->[0] } |
1809 | sort { $a->[1] cmp $b->[1] } | |
1810 | map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; | |
68dc0745 | 1811 | |
1812 | If you need to sort on several fields, the following paradigm is useful. | |
1813 | ||
ac9dac7f RGS |
1814 | @sorted = sort { |
1815 | field1($a) <=> field1($b) || | |
1816 | field2($a) cmp field2($b) || | |
1817 | field3($a) cmp field3($b) | |
1818 | } @data; | |
68dc0745 | 1819 | |
1820 | This can be conveniently combined with precalculation of keys as given | |
1821 | above. | |
1822 | ||
379e39d7 | 1823 | See the F<sort> article in the "Far More Than You Ever Wanted |
49d635f9 | 1824 | To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for |
06a5f41f | 1825 | more about this approach. |
68dc0745 | 1826 | |
ac9dac7f | 1827 | See also the question later in L<perlfaq4> on sorting hashes. |
68dc0745 | 1828 | |
1829 | =head2 How do I manipulate arrays of bits? | |
1830 | ||
ac9dac7f RGS |
1831 | Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise |
1832 | operations. | |
1833 | ||
109f0441 S |
1834 | For example, you don't have to store individual bits in an array |
1835 | (which would mean that you're wasting a lot of space). To convert an | |
1836 | array of bits to a string, use C<vec()> to set the right bits. This | |
1837 | sets C<$vec> to have bit N set only if C<$ints[N]> was set: | |
ac9dac7f | 1838 | |
109f0441 | 1839 | @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... ) |
ac9dac7f | 1840 | $vec = ''; |
109f0441 S |
1841 | foreach( 0 .. $#ints ) { |
1842 | vec($vec,$_,1) = 1 if $ints[$_]; | |
1843 | } | |
ac9dac7f | 1844 | |
109f0441 S |
1845 | The string C<$vec> only takes up as many bits as it needs. For |
1846 | instance, if you had 16 entries in C<@ints>, C<$vec> only needs two | |
1847 | bytes to store them (not counting the scalar variable overhead). | |
1848 | ||
1849 | Here's how, given a vector in C<$vec>, you can get those bits into | |
1850 | your C<@ints> array: | |
ac9dac7f RGS |
1851 | |
1852 | sub bitvec_to_list { | |
1853 | my $vec = shift; | |
1854 | my @ints; | |
1855 | # Find null-byte density then select best algorithm | |
1856 | if ($vec =~ tr/\0// / length $vec > 0.95) { | |
1857 | use integer; | |
1858 | my $i; | |
1859 | ||
1860 | # This method is faster with mostly null-bytes | |
1861 | while($vec =~ /[^\0]/g ) { | |
1862 | $i = -9 + 8 * pos $vec; | |
1863 | push @ints, $i if vec($vec, ++$i, 1); | |
1864 | push @ints, $i if vec($vec, ++$i, 1); | |
1865 | push @ints, $i if vec($vec, ++$i, 1); | |
1866 | push @ints, $i if vec($vec, ++$i, 1); | |
1867 | push @ints, $i if vec($vec, ++$i, 1); | |
1868 | push @ints, $i if vec($vec, ++$i, 1); | |
1869 | push @ints, $i if vec($vec, ++$i, 1); | |
1870 | push @ints, $i if vec($vec, ++$i, 1); | |
1871 | } | |
1872 | } | |
1873 | else { | |
1874 | # This method is a fast general algorithm | |
1875 | use integer; | |
1876 | my $bits = unpack "b*", $vec; | |
1877 | push @ints, 0 if $bits =~ s/^(\d)// && $1; | |
1878 | push @ints, pos $bits while($bits =~ /1/g); | |
1879 | } | |
1880 | ||
1881 | return \@ints; | |
1882 | } | |
68dc0745 | 1883 | |
1884 | This method gets faster the more sparse the bit vector is. | |
1885 | (Courtesy of Tim Bunce and Winfried Koenig.) | |
1886 | ||
76817d6d JH |
1887 | You can make the while loop a lot shorter with this suggestion |
1888 | from Benjamin Goldberg: | |
1889 | ||
1890 | while($vec =~ /[^\0]+/g ) { | |
ac9dac7f RGS |
1891 | push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8; |
1892 | } | |
76817d6d | 1893 | |
ac9dac7f | 1894 | Or use the CPAN module C<Bit::Vector>: |
cc30d1a7 | 1895 | |
ac9dac7f RGS |
1896 | $vector = Bit::Vector->new($num_of_bits); |
1897 | $vector->Index_List_Store(@ints); | |
1898 | @ints = $vector->Index_List_Read(); | |
cc30d1a7 | 1899 | |
ac9dac7f RGS |
1900 | C<Bit::Vector> provides efficient methods for bit vector, sets of |
1901 | small integers and "big int" math. | |
cc30d1a7 JH |
1902 | |
1903 | Here's a more extensive illustration using vec(): | |
65acb1b1 | 1904 | |
ac9dac7f RGS |
1905 | # vec demo |
1906 | $vector = "\xff\x0f\xef\xfe"; | |
1907 | print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", | |
65acb1b1 | 1908 | unpack("N", $vector), "\n"; |
ac9dac7f RGS |
1909 | $is_set = vec($vector, 23, 1); |
1910 | print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; | |
65acb1b1 | 1911 | pvec($vector); |
65acb1b1 | 1912 | |
ac9dac7f RGS |
1913 | set_vec(1,1,1); |
1914 | set_vec(3,1,1); | |
1915 | set_vec(23,1,1); | |
1916 | ||
1917 | set_vec(3,1,3); | |
1918 | set_vec(3,2,3); | |
1919 | set_vec(3,4,3); | |
1920 | set_vec(3,4,7); | |
1921 | set_vec(3,8,3); | |
1922 | set_vec(3,8,7); | |
1923 | ||
1924 | set_vec(0,32,17); | |
1925 | set_vec(1,32,17); | |
1926 | ||
1927 | sub set_vec { | |
1928 | my ($offset, $width, $value) = @_; | |
1929 | my $vector = ''; | |
1930 | vec($vector, $offset, $width) = $value; | |
1931 | print "offset=$offset width=$width value=$value\n"; | |
1932 | pvec($vector); | |
1933 | } | |
65acb1b1 | 1934 | |
ac9dac7f RGS |
1935 | sub pvec { |
1936 | my $vector = shift; | |
1937 | my $bits = unpack("b*", $vector); | |
1938 | my $i = 0; | |
1939 | my $BASE = 8; | |
1940 | ||
1941 | print "vector length in bytes: ", length($vector), "\n"; | |
1942 | @bytes = unpack("A8" x length($vector), $bits); | |
1943 | print "bits are: @bytes\n\n"; | |
1944 | } | |
65acb1b1 | 1945 | |
68dc0745 | 1946 | =head2 Why does defined() return true on empty arrays and hashes? |
1947 | ||
65acb1b1 TC |
1948 | The short story is that you should probably only use defined on scalars or |
1949 | functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> | |
1950 | in the 5.004 release or later of Perl for more detail. | |
68dc0745 | 1951 | |
1952 | =head1 Data: Hashes (Associative Arrays) | |
1953 | ||
1954 | =head2 How do I process an entire hash? | |
1955 | ||
ee891a00 RGS |
1956 | (contributed by brian d foy) |
1957 | ||
1958 | There are a couple of ways that you can process an entire hash. You | |
1959 | can get a list of keys, then go through each key, or grab a one | |
1960 | key-value pair at a time. | |
68dc0745 | 1961 | |
ee891a00 RGS |
1962 | To go through all of the keys, use the C<keys> function. This extracts |
1963 | all of the keys of the hash and gives them back to you as a list. You | |
1964 | can then get the value through the particular key you're processing: | |
1965 | ||
1966 | foreach my $key ( keys %hash ) { | |
1967 | my $value = $hash{$key} | |
1968 | ... | |
ac9dac7f | 1969 | } |
68dc0745 | 1970 | |
ee891a00 | 1971 | Once you have the list of keys, you can process that list before you |
109f0441 | 1972 | process the hash elements. For instance, you can sort the keys so you |
ee891a00 RGS |
1973 | can process them in lexical order: |
1974 | ||
1975 | foreach my $key ( sort keys %hash ) { | |
1976 | my $value = $hash{$key} | |
1977 | ... | |
1978 | } | |
1979 | ||
1980 | Or, you might want to only process some of the items. If you only want | |
1981 | to deal with the keys that start with C<text:>, you can select just | |
1982 | those using C<grep>: | |
1983 | ||
1984 | foreach my $key ( grep /^text:/, keys %hash ) { | |
1985 | my $value = $hash{$key} | |
1986 | ... | |
1987 | } | |
1988 | ||
1989 | If the hash is very large, you might not want to create a long list of | |
109f0441 | 1990 | keys. To save some memory, you can grab one key-value pair at a time using |
ee891a00 RGS |
1991 | C<each()>, which returns a pair you haven't seen yet: |
1992 | ||
1993 | while( my( $key, $value ) = each( %hash ) ) { | |
1994 | ... | |
1995 | } | |
1996 | ||
1997 | The C<each> operator returns the pairs in apparently random order, so if | |
1998 | ordering matters to you, you'll have to stick with the C<keys> method. | |
1999 | ||
2000 | The C<each()> operator can be a bit tricky though. You can't add or | |
2001 | delete keys of the hash while you're using it without possibly | |
2002 | skipping or re-processing some pairs after Perl internally rehashes | |
2003 | all of the elements. Additionally, a hash has only one iterator, so if | |
2004 | you use C<keys>, C<values>, or C<each> on the same hash, you can reset | |
2005 | the iterator and mess up your processing. See the C<each> entry in | |
2006 | L<perlfunc> for more details. | |
68dc0745 | 2007 | |
109f0441 S |
2008 | =head2 How do I merge two hashes? |
2009 | X<hash> X<merge> X<slice, hash> | |
2010 | ||
2011 | (contributed by brian d foy) | |
2012 | ||
2013 | Before you decide to merge two hashes, you have to decide what to do | |
2014 | if both hashes contain keys that are the same and if you want to leave | |
2015 | the original hashes as they were. | |
2016 | ||
2017 | If you want to preserve the original hashes, copy one hash (C<%hash1>) | |
2018 | to a new hash (C<%new_hash>), then add the keys from the other hash | |
2019 | (C<%hash2> to the new hash. Checking that the key already exists in | |
2020 | C<%new_hash> gives you a chance to decide what to do with the | |
2021 | duplicates: | |
2022 | ||
2023 | my %new_hash = %hash1; # make a copy; leave %hash1 alone | |
2024 | ||
2025 | foreach my $key2 ( keys %hash2 ) | |
2026 | { | |
2027 | if( exists $new_hash{$key2} ) | |
2028 | { | |
2029 | warn "Key [$key2] is in both hashes!"; | |
2030 | # handle the duplicate (perhaps only warning) | |
2031 | ... | |
2032 | next; | |
2033 | } | |
2034 | else | |
2035 | { | |
2036 | $new_hash{$key2} = $hash2{$key2}; | |
2037 | } | |
2038 | } | |
2039 | ||
2040 | If you don't want to create a new hash, you can still use this looping | |
2041 | technique; just change the C<%new_hash> to C<%hash1>. | |
2042 | ||
2043 | foreach my $key2 ( keys %hash2 ) | |
2044 | { | |
2045 | if( exists $hash1{$key2} ) | |
2046 | { | |
2047 | warn "Key [$key2] is in both hashes!"; | |
2048 | # handle the duplicate (perhaps only warning) | |
2049 | ... | |
2050 | next; | |
2051 | } | |
2052 | else | |
2053 | { | |
2054 | $hash1{$key2} = $hash2{$key2}; | |
2055 | } | |
2056 | } | |
2057 | ||
2058 | If you don't care that one hash overwrites keys and values from the other, you | |
2059 | could just use a hash slice to add one hash to another. In this case, values | |
2060 | from C<%hash2> replace values from C<%hash1> when they have keys in common: | |
2061 | ||
2062 | @hash1{ keys %hash2 } = values %hash2; | |
2063 | ||
68dc0745 | 2064 | =head2 What happens if I add or remove keys from a hash while iterating over it? |
2065 | ||
28b41a80 | 2066 | (contributed by brian d foy) |
d92eb7b0 | 2067 | |
28b41a80 | 2068 | The easy answer is "Don't do that!" |
d92eb7b0 | 2069 | |
28b41a80 RGS |
2070 | If you iterate through the hash with each(), you can delete the key |
2071 | most recently returned without worrying about it. If you delete or add | |
2072 | other keys, the iterator may skip or double up on them since perl | |
2073 | may rearrange the hash table. See the | |
2074 | entry for C<each()> in L<perlfunc>. | |
68dc0745 | 2075 | |
2076 | =head2 How do I look up a hash element by value? | |
2077 | ||
2078 | Create a reverse hash: | |
2079 | ||
ac9dac7f RGS |
2080 | %by_value = reverse %by_key; |
2081 | $key = $by_value{$value}; | |
68dc0745 | 2082 | |
2083 | That's not particularly efficient. It would be more space-efficient | |
2084 | to use: | |
2085 | ||
ac9dac7f RGS |
2086 | while (($key, $value) = each %by_key) { |
2087 | $by_value{$value} = $key; | |
2088 | } | |
68dc0745 | 2089 | |
d92eb7b0 GS |
2090 | If your hash could have repeated values, the methods above will only find |
2091 | one of the associated keys. This may or may not worry you. If it does | |
2092 | worry you, you can always reverse the hash into a hash of arrays instead: | |
2093 | ||
ac9dac7f RGS |
2094 | while (($key, $value) = each %by_key) { |
2095 | push @{$key_list_by_value{$value}}, $key; | |
2096 | } | |
68dc0745 | 2097 | |
2098 | =head2 How can I know how many entries are in a hash? | |
2099 | ||
109f0441 S |
2100 | (contributed by brian d foy) |
2101 | ||
2102 | This is very similar to "How do I process an entire hash?", also in | |
2103 | L<perlfaq4>, but a bit simpler in the common cases. | |
2104 | ||
2105 | You can use the C<keys()> built-in function in scalar context to find out | |
2106 | have many entries you have in a hash: | |
68dc0745 | 2107 | |
109f0441 | 2108 | my $key_count = keys %hash; # must be scalar context! |
d12d61cf | 2109 | |
109f0441 | 2110 | If you want to find out how many entries have a defined value, that's |
d12d61cf | 2111 | a bit different. You have to check each value. A C<grep> is handy: |
109f0441 S |
2112 | |
2113 | my $defined_value_count = grep { defined } values %hash; | |
68dc0745 | 2114 | |
109f0441 S |
2115 | You can use that same structure to count the entries any way that |
2116 | you like. If you want the count of the keys with vowels in them, | |
2117 | you just test for that instead: | |
2118 | ||
2119 | my $vowel_count = grep { /[aeiou]/ } keys %hash; | |
d12d61cf | 2120 | |
109f0441 S |
2121 | The C<grep> in scalar context returns the count. If you want the list |
2122 | of matching items, just use it in list context instead: | |
2123 | ||
2124 | my @defined_values = grep { defined } values %hash; | |
2125 | ||
2126 | The C<keys()> function also resets the iterator, which means that you may | |
197aec24 | 2127 | see strange results if you use this between uses of other hash operators |
109f0441 | 2128 | such as C<each()>. |
68dc0745 | 2129 | |
2130 | =head2 How do I sort a hash (optionally by value instead of key)? | |
2131 | ||
a05e4845 RGS |
2132 | (contributed by brian d foy) |
2133 | ||
2134 | To sort a hash, start with the keys. In this example, we give the list of | |
2135 | keys to the sort function which then compares them ASCIIbetically (which | |
2136 | might be affected by your locale settings). The output list has the keys | |
2137 | in ASCIIbetical order. Once we have the keys, we can go through them to | |
2138 | create a report which lists the keys in ASCIIbetical order. | |
2139 | ||
2140 | my @keys = sort { $a cmp $b } keys %hash; | |
58103a2e | 2141 | |
a05e4845 RGS |
2142 | foreach my $key ( @keys ) |
2143 | { | |
109f0441 | 2144 | printf "%-20s %6d\n", $key, $hash{$key}; |
a05e4845 RGS |
2145 | } |
2146 | ||
58103a2e | 2147 | We could get more fancy in the C<sort()> block though. Instead of |
a05e4845 | 2148 | comparing the keys, we can compute a value with them and use that |
58103a2e | 2149 | value as the comparison. |
a05e4845 RGS |
2150 | |
2151 | For instance, to make our report order case-insensitive, we use | |
58103a2e | 2152 | the C<\L> sequence in a double-quoted string to make everything |
a05e4845 RGS |
2153 | lowercase. The C<sort()> block then compares the lowercased |
2154 | values to determine in which order to put the keys. | |
2155 | ||
2156 | my @keys = sort { "\L$a" cmp "\L$b" } keys %hash; | |
58103a2e | 2157 | |
a05e4845 | 2158 | Note: if the computation is expensive or the hash has many elements, |
58103a2e | 2159 | you may want to look at the Schwartzian Transform to cache the |
a05e4845 RGS |
2160 | computation results. |
2161 | ||
2162 | If we want to sort by the hash value instead, we use the hash key | |
2163 | to look it up. We still get out a list of keys, but this time they | |
2164 | are ordered by their value. | |
2165 | ||
2166 | my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash; | |
2167 | ||
2168 | From there we can get more complex. If the hash values are the same, | |
2169 | we can provide a secondary sort on the hash key. | |
2170 | ||
58103a2e RGS |
2171 | my @keys = sort { |
2172 | $hash{$a} <=> $hash{$b} | |
a05e4845 RGS |
2173 | or |
2174 | "\L$a" cmp "\L$b" | |
2175 | } keys %hash; | |
68dc0745 | 2176 | |
2177 | =head2 How can I always keep my hash sorted? | |
ac9dac7f | 2178 | X<hash tie sort DB_File Tie::IxHash> |
68dc0745 | 2179 | |
ac9dac7f RGS |
2180 | You can look into using the C<DB_File> module and C<tie()> using the |
2181 | C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory | |
2182 | Databases">. The C<Tie::IxHash> module from CPAN might also be | |
2183 | instructive. Although this does keep your hash sorted, you might not | |
2184 | like the slow down you suffer from the tie interface. Are you sure you | |
2185 | need to do this? :) | |
68dc0745 | 2186 | |
2187 | =head2 What's the difference between "delete" and "undef" with hashes? | |
2188 | ||
92993692 JH |
2189 | Hashes contain pairs of scalars: the first is the key, the |
2190 | second is the value. The key will be coerced to a string, | |
2191 | although the value can be any kind of scalar: string, | |
ac9dac7f | 2192 | number, or reference. If a key C<$key> is present in |
92993692 JH |
2193 | %hash, C<exists($hash{$key})> will return true. The value |
2194 | for a given key can be C<undef>, in which case | |
2195 | C<$hash{$key}> will be C<undef> while C<exists $hash{$key}> | |
2196 | will return true. This corresponds to (C<$key>, C<undef>) | |
2197 | being in the hash. | |
68dc0745 | 2198 | |
589a5df2 | 2199 | Pictures help... Here's the C<%hash> table: |
68dc0745 | 2200 | |
2201 | keys values | |
2202 | +------+------+ | |
2203 | | a | 3 | | |
2204 | | x | 7 | | |
2205 | | d | 0 | | |
2206 | | e | 2 | | |
2207 | +------+------+ | |
2208 | ||
2209 | And these conditions hold | |
2210 | ||
92993692 JH |
2211 | $hash{'a'} is true |
2212 | $hash{'d'} is false | |
2213 | defined $hash{'d'} is true | |
2214 | defined $hash{'a'} is true | |
e9d185f8 | 2215 | exists $hash{'a'} is true (Perl 5 only) |
92993692 | 2216 | grep ($_ eq 'a', keys %hash) is true |
68dc0745 | 2217 | |
2218 | If you now say | |
2219 | ||
92993692 | 2220 | undef $hash{'a'} |
68dc0745 | 2221 | |
2222 | your table now reads: | |
2223 | ||
2224 | ||
2225 | keys values | |
2226 | +------+------+ | |
2227 | | a | undef| | |
2228 | | x | 7 | | |
2229 | | d | 0 | | |
2230 | | e | 2 | | |
2231 | +------+------+ | |
2232 | ||
2233 | and these conditions now hold; changes in caps: | |
2234 | ||
92993692 JH |
2235 | $hash{'a'} is FALSE |
2236 | $hash{'d'} is false | |
2237 | defined $hash{'d'} is true | |
2238 | defined $hash{'a'} is FALSE | |
e9d185f8 | 2239 | exists $hash{'a'} is true (Perl 5 only) |
92993692 | 2240 | grep ($_ eq 'a', keys %hash) is true |
68dc0745 | 2241 | |
2242 | Notice the last two: you have an undef value, but a defined key! | |
2243 | ||
2244 | Now, consider this: | |
2245 | ||
92993692 | 2246 | delete $hash{'a'} |
68dc0745 | 2247 | |
2248 | your table now reads: | |
2249 | ||
2250 | keys values | |
2251 | +------+------+ | |
2252 | | x | 7 | | |
2253 | | d | 0 | | |
2254 | | e | 2 | | |
2255 | +------+------+ | |
2256 | ||
2257 | and these conditions now hold; changes in caps: | |
2258 | ||
92993692 JH |
2259 | $hash{'a'} is false |
2260 | $hash{'d'} is false | |
2261 | defined $hash{'d'} is true | |
2262 | defined $hash{'a'} is false | |
e9d185f8 | 2263 | exists $hash{'a'} is FALSE (Perl 5 only) |
92993692 | 2264 | grep ($_ eq 'a', keys %hash) is FALSE |
68dc0745 | 2265 | |
2266 | See, the whole entry is gone! | |
2267 | ||
2268 | =head2 Why don't my tied hashes make the defined/exists distinction? | |
2269 | ||
92993692 JH |
2270 | This depends on the tied hash's implementation of EXISTS(). |
2271 | For example, there isn't the concept of undef with hashes | |
2272 | that are tied to DBM* files. It also means that exists() and | |
2273 | defined() do the same thing with a DBM* file, and what they | |
2274 | end up doing is not what they do with ordinary hashes. | |
68dc0745 | 2275 | |
2276 | =head2 How do I reset an each() operation part-way through? | |
2277 | ||
fb2fe781 RGS |
2278 | (contributed by brian d foy) |
2279 | ||
2280 | You can use the C<keys> or C<values> functions to reset C<each>. To | |
2281 | simply reset the iterator used by C<each> without doing anything else, | |
2282 | use one of them in void context: | |
2283 | ||
2284 | keys %hash; # resets iterator, nothing else. | |
2285 | values %hash; # resets iterator, nothing else. | |
2286 | ||
2287 | See the documentation for C<each> in L<perlfunc>. | |
68dc0745 | 2288 | |
2289 | =head2 How can I get the unique keys from two hashes? | |
2290 | ||
d92eb7b0 GS |
2291 | First you extract the keys from the hashes into lists, then solve |
2292 | the "removing duplicates" problem described above. For example: | |
68dc0745 | 2293 | |
ac9dac7f RGS |
2294 | %seen = (); |
2295 | for $element (keys(%foo), keys(%bar)) { | |
2296 | $seen{$element}++; | |
2297 | } | |
2298 | @uniq = keys %seen; | |
68dc0745 | 2299 | |
2300 | Or more succinctly: | |
2301 | ||
ac9dac7f | 2302 | @uniq = keys %{{%foo,%bar}}; |
68dc0745 | 2303 | |
2304 | Or if you really want to save space: | |
2305 | ||
ac9dac7f RGS |
2306 | %seen = (); |
2307 | while (defined ($key = each %foo)) { | |
2308 | $seen{$key}++; | |
2309 | } | |
2310 | while (defined ($key = each %bar)) { | |
2311 | $seen{$key}++; | |
2312 | } | |
2313 | @uniq = keys %seen; | |
68dc0745 | 2314 | |
2315 | =head2 How can I store a multidimensional array in a DBM file? | |
2316 | ||
2317 | Either stringify the structure yourself (no fun), or else | |
2318 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer | |
2319 | it on top of either DB_File or GDBM_File. | |
2320 | ||
2321 | =head2 How can I make my hash remember the order I put elements into it? | |
2322 | ||
ac9dac7f | 2323 | Use the C<Tie::IxHash> from CPAN. |
68dc0745 | 2324 | |
ac9dac7f RGS |
2325 | use Tie::IxHash; |
2326 | ||
2327 | tie my %myhash, 'Tie::IxHash'; | |
2328 | ||
2329 | for (my $i=0; $i<20; $i++) { | |
2330 | $myhash{$i} = 2*$i; | |
2331 | } | |
2332 | ||
2333 | my @keys = keys %myhash; | |
2334 | # @keys = (0,1,2,3,...) | |
46fc3d4c | 2335 | |
68dc0745 | 2336 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
2337 | ||
109f0441 S |
2338 | (contributed by brian d foy) |
2339 | ||
2340 | Are you using a really old version of Perl? | |
2341 | ||
2342 | Normally, accessing a hash key's value for a nonexistent key will | |
2343 | I<not> create the key. | |
2344 | ||
2345 | my %hash = (); | |
2346 | my $value = $hash{ 'foo' }; | |
2347 | print "This won't print\n" if exists $hash{ 'foo' }; | |
2348 | ||
2349 | Passing C<$hash{ 'foo' }> to a subroutine used to be a special case, though. | |
2350 | Since you could assign directly to C<$_[0]>, Perl had to be ready to | |
2351 | make that assignment so it created the hash key ahead of time: | |
2352 | ||
2353 | my_sub( $hash{ 'foo' } ); | |
2354 | print "This will print before 5.004\n" if exists $hash{ 'foo' }; | |
68dc0745 | 2355 | |
109f0441 S |
2356 | sub my_sub { |
2357 | # $_[0] = 'bar'; # create hash key in case you do this | |
2358 | 1; | |
2359 | } | |
2360 | ||
2361 | Since Perl 5.004, however, this situation is a special case and Perl | |
2362 | creates the hash key only when you make the assignment: | |
68dc0745 | 2363 | |
109f0441 S |
2364 | my_sub( $hash{ 'foo' } ); |
2365 | print "This will print, even after 5.004\n" if exists $hash{ 'foo' }; | |
2366 | ||
2367 | sub my_sub { | |
2368 | $_[0] = 'bar'; | |
2369 | } | |
68dc0745 | 2370 | |
109f0441 S |
2371 | However, if you want the old behavior (and think carefully about that |
2372 | because it's a weird side effect), you can pass a hash slice instead. | |
2373 | Perl 5.004 didn't make this a special case: | |
68dc0745 | 2374 | |
109f0441 | 2375 | my_sub( @hash{ qw/foo/ } ); |
68dc0745 | 2376 | |
fc36a67e | 2377 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
68dc0745 | 2378 | |
65acb1b1 TC |
2379 | Usually a hash ref, perhaps like this: |
2380 | ||
ac9dac7f RGS |
2381 | $record = { |
2382 | NAME => "Jason", | |
2383 | EMPNO => 132, | |
2384 | TITLE => "deputy peon", | |
2385 | AGE => 23, | |
2386 | SALARY => 37_000, | |
2387 | PALS => [ "Norbert", "Rhys", "Phineas"], | |
2388 | }; | |
65acb1b1 | 2389 | |
ab093f19 | 2390 | References are documented in L<perlref> and L<perlreftut>. |
65acb1b1 TC |
2391 | Examples of complex data structures are given in L<perldsc> and |
2392 | L<perllol>. Examples of structures and object-oriented classes are | |
2393 | in L<perltoot>. | |
68dc0745 | 2394 | |
2395 | =head2 How can I use a reference as a hash key? | |
2396 | ||
109f0441 | 2397 | (contributed by brian d foy and Ben Morrow) |
9e72e4c6 RGS |
2398 | |
2399 | Hash keys are strings, so you can't really use a reference as the key. | |
2400 | When you try to do that, perl turns the reference into its stringified | |
ac9dac7f RGS |
2401 | form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get |
2402 | back the reference from the stringified form, at least without doing | |
109f0441 S |
2403 | some extra work on your own. |
2404 | ||
2405 | Remember that the entry in the hash will still be there even if | |
2406 | the referenced variable goes out of scope, and that it is entirely | |
2407 | possible for Perl to subsequently allocate a different variable at | |
2408 | the same address. This will mean a new variable might accidentally | |
2409 | be associated with the value for an old. | |
2410 | ||
2411 | If you have Perl 5.10 or later, and you just want to store a value | |
2412 | against the reference for lookup later, you can use the core | |
2413 | Hash::Util::Fieldhash module. This will also handle renaming the | |
2414 | keys if you use multiple threads (which causes all variables to be | |
2415 | reallocated at new addresses, changing their stringification), and | |
2416 | garbage-collecting the entries when the referenced variable goes out | |
2417 | of scope. | |
2418 | ||
2419 | If you actually need to be able to get a real reference back from | |
2420 | each hash entry, you can use the Tie::RefHash module, which does the | |
2421 | required work for you. | |
68dc0745 | 2422 | |
ebeb11a2 | 2423 | =head2 How can I check if a key exists in a multilevel hash? |
a1bbdff3 | 2424 | |
2425 | (contributed by brian d foy) | |
2426 | ||
2427 | The trick to this problem is avoiding accidental autovivification. If | |
2428 | you want to check three keys deep, you might naïvely try this: | |
2429 | ||
2430 | my %hash; | |
2431 | if( exists $hash{key1}{key2}{key3} ) { | |
2432 | ...; | |
2433 | } | |
6f1f337b | 2434 | |
a1bbdff3 | 2435 | Even though you started with a completely empty hash, after that call to |
2436 | C<exists> you've created the structure you needed to check for C<key3>: | |
2437 | ||
2438 | %hash = ( | |
2439 | 'key1' => { | |
2440 | 'key2' => {} | |
2441 | } | |
2442 | ); | |
2443 | ||
2444 | That's autovivification. You can get around this in a few ways. The | |
2445 | easiest way is to just turn it off. The lexical C<autovivification> | |
2446 | pragma is available on CPAN. Now you don't add to the hash: | |
2447 | ||
2448 | { | |
2449 | no autovivification; | |
2450 | my %hash; | |
2451 | if( exists $hash{key1}{key2}{key3} ) { | |
2452 | ...; | |
2453 | } | |
2454 | } | |
2455 | ||
2456 | The C<Data::Diver> module on CPAN can do it for you too. Its C<Dive> | |
2457 | subroutine can tell you not only if the keys exist but also get the | |
2458 | value: | |
2459 | ||
2460 | use Data::Diver qw(Dive); | |
2461 | ||
2462 | my @exists = Dive( \%hash, qw(key1 key2 key3) ); | |
2463 | if( ! @exists ) { | |
2464 | ...; # keys do not exist | |
2465 | } | |
2466 | elsif( ! defined $exists[0] ) { | |
2467 | ...; # keys exist but value is undef | |
2468 | } | |
2469 | ||
2470 | You can easily do this yourself too by checking each level of the hash | |
2471 | before you move onto the next level. This is essentially what | |
2472 | C<Data::Diver> does for you: | |
2473 | ||
2474 | if( check_hash( \%hash, qw(key1 key2 key3) ) ) { | |
2475 | ...; | |
2476 | } | |
2477 | ||
2478 | sub check_hash { | |
2479 | my( $hash, @keys ) = @_; | |
2480 | ||
2481 | return unless @keys; | |
2482 | ||
2483 | foreach my $key ( @keys ) { | |
2484 | return unless eval { exists $hash->{$key} }; | |
2485 | $hash = $hash->{$key}; | |
2486 | } | |
2487 | ||
2488 | return 1; | |
2489 | } | |
2490 | ||
68dc0745 | 2491 | =head1 Data: Misc |
2492 | ||
2493 | =head2 How do I handle binary data correctly? | |
2494 | ||
ac9dac7f | 2495 | Perl is binary clean, so it can handle binary data just fine. |
e573f903 | 2496 | On Windows or DOS, however, you have to use C<binmode> for binary |
ac9dac7f RGS |
2497 | files to avoid conversions for line endings. In general, you should |
2498 | use C<binmode> any time you want to work with binary data. | |
68dc0745 | 2499 | |
ac9dac7f | 2500 | Also see L<perlfunc/"binmode"> or L<perlopentut>. |
68dc0745 | 2501 | |
ac9dac7f | 2502 | If you're concerned about 8-bit textual data then see L<perllocale>. |
54310121 | 2503 | If you want to deal with multibyte characters, however, there are |
68dc0745 | 2504 | some gotchas. See the section on Regular Expressions. |
2505 | ||
2506 | =head2 How do I determine whether a scalar is a number/whole/integer/float? | |
2507 | ||
2508 | Assuming that you don't care about IEEE notations like "NaN" or | |
a13ded55 | 2509 | "Infinity", you probably just want to use a regular expression: |
68dc0745 | 2510 | |
a13ded55 | 2511 | use 5.010; |
2512 | ||
2513 | given( $number ) { | |
2514 | when( /\D/ ) | |
2515 | { say "\thas nondigits"; continue } | |
2516 | when( /^\d+\z/ ) | |
2517 | { say "\tis a whole number"; continue } | |
2518 | when( /^-?\d+\z/ ) | |
2519 | { say "\tis an integer"; continue } | |
2520 | when( /^[+-]?\d+\z/ ) | |
2521 | { say "\tis a +/- integer"; continue } | |
2522 | when( /^-?(?:\d+\.?|\.\d)\d*\z/ ) | |
2523 | { say "\tis a real number"; continue } | |
2524 | when( /^[+-]?(?=\.?\d)\d*\.?\d*(?:e[+-]?\d+)?\z/i) | |
2525 | { say "\tis a C float" } | |
2526 | } | |
68dc0745 | 2527 | |
f0d19b68 RGS |
2528 | There are also some commonly used modules for the task. |
2529 | L<Scalar::Util> (distributed with 5.8) provides access to perl's | |
ac9dac7f | 2530 | internal function C<looks_like_number> for determining whether a |
a13ded55 | 2531 | variable looks like a number. L<Data::Types> exports functions that |
ac9dac7f RGS |
2532 | validate data types using both the above and other regular |
2533 | expressions. Thirdly, there is C<Regexp::Common> which has regular | |
2534 | expressions to match various types of numbers. Those three modules are | |
2535 | available from the CPAN. | |
f0d19b68 RGS |
2536 | |
2537 | If you're on a POSIX system, Perl supports the C<POSIX::strtod> | |
a13ded55 | 2538 | function. Its semantics are somewhat cumbersome, so here's a |
2539 | C<getnum> wrapper function for more convenient access. This function | |
ac9dac7f | 2540 | takes a string and returns the number it found, or C<undef> for input |
a13ded55 | 2541 | that isn't a C float. The C<is_numeric> function is a front end to |
ac9dac7f RGS |
2542 | C<getnum> if you just want to say, "Is this a float?" |
2543 | ||
2544 | sub getnum { | |
2545 | use POSIX qw(strtod); | |
2546 | my $str = shift; | |
2547 | $str =~ s/^\s+//; | |
2548 | $str =~ s/\s+$//; | |
2549 | $! = 0; | |
2550 | my($num, $unparsed) = strtod($str); | |
2551 | if (($str eq '') || ($unparsed != 0) || $!) { | |
2552 | return undef; | |
2553 | } | |
2554 | else { | |
2555 | return $num; | |
2556 | } | |
2557 | } | |
5a964f20 | 2558 | |
ac9dac7f | 2559 | sub is_numeric { defined getnum($_[0]) } |
5a964f20 | 2560 | |
f0d19b68 | 2561 | Or you could check out the L<String::Scanf> module on the CPAN |
ac9dac7f RGS |
2562 | instead. The C<POSIX> module (part of the standard Perl distribution) |
2563 | provides the C<strtod> and C<strtol> for converting strings to double | |
2564 | and longs, respectively. | |
68dc0745 | 2565 | |
2566 | =head2 How do I keep persistent data across program calls? | |
2567 | ||
2568 | For some specific applications, you can use one of the DBM modules. | |
ac9dac7f RGS |
2569 | See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw> |
2570 | or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part | |
2571 | of the standard distribution. Here's one example using C<Storable>'s C<store> | |
fe854a6f | 2572 | and C<retrieve> functions: |
65acb1b1 | 2573 | |
ac9dac7f RGS |
2574 | use Storable; |
2575 | store(\%hash, "filename"); | |
65acb1b1 | 2576 | |
ac9dac7f RGS |
2577 | # later on... |
2578 | $href = retrieve("filename"); # by ref | |
2579 | %hash = %{ retrieve("filename") }; # direct to hash | |
68dc0745 | 2580 | |
2581 | =head2 How do I print out or copy a recursive data structure? | |
2582 | ||
ac9dac7f RGS |
2583 | The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great |
2584 | for printing out data structures. The C<Storable> module on CPAN (or the | |
6f82c03a EM |
2585 | 5.8 release of Perl), provides a function called C<dclone> that recursively |
2586 | copies its argument. | |
65acb1b1 | 2587 | |
ac9dac7f RGS |
2588 | use Storable qw(dclone); |
2589 | $r2 = dclone($r1); | |
68dc0745 | 2590 | |
ac9dac7f | 2591 | Where C<$r1> can be a reference to any kind of data structure you'd like. |
65acb1b1 TC |
2592 | It will be deeply copied. Because C<dclone> takes and returns references, |
2593 | you'd have to add extra punctuation if you had a hash of arrays that | |
2594 | you wanted to copy. | |
68dc0745 | 2595 | |
ac9dac7f | 2596 | %newhash = %{ dclone(\%oldhash) }; |
68dc0745 | 2597 | |
2598 | =head2 How do I define methods for every class/object? | |
2599 | ||
109f0441 S |
2600 | (contributed by Ben Morrow) |
2601 | ||
2602 | You can use the C<UNIVERSAL> class (see L<UNIVERSAL>). However, please | |
2603 | be very careful to consider the consequences of doing this: adding | |
2604 | methods to every object is very likely to have unintended | |
2605 | consequences. If possible, it would be better to have all your object | |
2606 | inherit from some common base class, or to use an object system like | |
2607 | Moose that supports roles. | |
68dc0745 | 2608 | |
2609 | =head2 How do I verify a credit card checksum? | |
2610 | ||
ac9dac7f | 2611 | Get the C<Business::CreditCard> module from CPAN. |
68dc0745 | 2612 | |
65acb1b1 TC |
2613 | =head2 How do I pack arrays of doubles or floats for XS code? |
2614 | ||
109f0441 | 2615 | The arrays.h/arrays.c code in the C<PGPLOT> module on CPAN does just this. |
65acb1b1 | 2616 | If you're doing a lot of float or double processing, consider using |
ac9dac7f | 2617 | the C<PDL> module from CPAN instead--it makes number-crunching easy. |
65acb1b1 | 2618 | |
109f0441 S |
2619 | See L<http://search.cpan.org/dist/PGPLOT> for the code. |
2620 | ||
68dc0745 | 2621 | =head1 AUTHOR AND COPYRIGHT |
2622 | ||
8d2e243f | 2623 | Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and |
7678cced | 2624 | other authors as noted. All rights reserved. |
5a964f20 | 2625 | |
5a7beb56 JH |
2626 | This documentation is free; you can redistribute it and/or modify it |
2627 | under the same terms as Perl itself. | |
5a964f20 TC |
2628 | |
2629 | Irrespective of its distribution, all code examples in this file | |
2630 | are hereby placed into the public domain. You are permitted and | |
2631 | encouraged to use this code in your own programs for fun | |
2632 | or for profit as you see fit. A simple comment in the code giving | |
2633 | credit would be courteous but is not required. |