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68dc0745 | 1 | =head1 NAME |
2 | ||
fb2fe781 | 3 | perlfaq4 - Data Manipulation ($Revision: 10394 $) |
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 | ||
ac9dac7f RGS |
14 | Internally, your computer represents floating-point numbers in binary. |
15 | Digital (as in powers of two) computers cannot store all numbers | |
16 | exactly. Some real numbers lose precision in the process. This is a | |
17 | problem with how computers store numbers and affects all computer | |
18 | languages, not just Perl. | |
46fc3d4c | 19 | |
ee891a00 | 20 | L<perlnumber> shows the gory details of number representations and |
ac9dac7f | 21 | conversions. |
49d635f9 | 22 | |
ac9dac7f RGS |
23 | To limit the number of decimal places in your numbers, you can use the |
24 | printf or sprintf function. See the L<"Floating Point | |
25 | Arithmetic"|perlop> for more details. | |
49d635f9 RGS |
26 | |
27 | printf "%.2f", 10/3; | |
197aec24 | 28 | |
49d635f9 | 29 | my $number = sprintf "%.2f", 10/3; |
197aec24 | 30 | |
32969b6e BB |
31 | =head2 Why is int() broken? |
32 | ||
ac9dac7f | 33 | Your C<int()> is most probably working just fine. It's the numbers that |
32969b6e BB |
34 | aren't quite what you think. |
35 | ||
ac9dac7f | 36 | First, see the answer to "Why am I getting long decimals |
32969b6e BB |
37 | (eg, 19.9499999999999) instead of the numbers I should be getting |
38 | (eg, 19.95)?". | |
39 | ||
40 | For example, this | |
41 | ||
ac9dac7f | 42 | print int(0.6/0.2-2), "\n"; |
32969b6e BB |
43 | |
44 | will in most computers print 0, not 1, because even such simple | |
45 | numbers as 0.6 and 0.2 cannot be presented exactly by floating-point | |
46 | numbers. What you think in the above as 'three' is really more like | |
47 | 2.9999999999999995559. | |
48 | ||
68dc0745 | 49 | =head2 Why isn't my octal data interpreted correctly? |
50 | ||
49d635f9 RGS |
51 | Perl only understands octal and hex numbers as such when they occur as |
52 | literals in your program. Octal literals in perl must start with a | |
ac9dac7f | 53 | leading C<0> and hexadecimal literals must start with a leading C<0x>. |
49d635f9 | 54 | If they are read in from somewhere and assigned, no automatic |
ac9dac7f RGS |
55 | conversion takes place. You must explicitly use C<oct()> or C<hex()> if you |
56 | want the values converted to decimal. C<oct()> interprets hexadecimal (C<0x350>), | |
57 | octal (C<0350> or even without the leading C<0>, like C<377>) and binary | |
58 | (C<0b1010>) numbers, while C<hex()> only converts hexadecimal ones, with | |
59 | or without a leading C<0x>, such as C<0x255>, C<3A>, C<ff>, or C<deadbeef>. | |
33ce146f | 60 | The inverse mapping from decimal to octal can be done with either the |
ac9dac7f | 61 | <%o> or C<%O> C<sprintf()> formats. |
68dc0745 | 62 | |
ac9dac7f RGS |
63 | This problem shows up most often when people try using C<chmod()>, |
64 | C<mkdir()>, C<umask()>, or C<sysopen()>, which by widespread tradition | |
65 | typically take permissions in octal. | |
68dc0745 | 66 | |
ac9dac7f RGS |
67 | chmod(644, $file); # WRONG |
68 | chmod(0644, $file); # right | |
68dc0745 | 69 | |
197aec24 | 70 | Note the mistake in the first line was specifying the decimal literal |
ac9dac7f | 71 | C<644>, rather than the intended octal literal C<0644>. The problem can |
33ce146f PP |
72 | be seen with: |
73 | ||
ac9dac7f | 74 | printf("%#o",644); # prints 01204 |
33ce146f PP |
75 | |
76 | Surely you had not intended C<chmod(01204, $file);> - did you? If you | |
77 | want to use numeric literals as arguments to chmod() et al. then please | |
197aec24 | 78 | try to express them as octal constants, that is with a leading zero and |
ac9dac7f | 79 | with the following digits restricted to the set C<0..7>. |
33ce146f | 80 | |
65acb1b1 | 81 | =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? |
68dc0745 | 82 | |
ac9dac7f RGS |
83 | Remember that C<int()> merely truncates toward 0. For rounding to a |
84 | certain number of digits, C<sprintf()> or C<printf()> is usually the | |
85 | easiest route. | |
92c2ed05 | 86 | |
ac9dac7f | 87 | printf("%.3f", 3.1415926535); # prints 3.142 |
68dc0745 | 88 | |
ac9dac7f RGS |
89 | The C<POSIX> module (part of the standard Perl distribution) |
90 | implements C<ceil()>, C<floor()>, and a number of other mathematical | |
91 | and trigonometric functions. | |
68dc0745 | 92 | |
ac9dac7f RGS |
93 | use POSIX; |
94 | $ceil = ceil(3.5); # 4 | |
95 | $floor = floor(3.5); # 3 | |
92c2ed05 | 96 | |
ac9dac7f RGS |
97 | In 5.000 to 5.003 perls, trigonometry was done in the C<Math::Complex> |
98 | module. With 5.004, the C<Math::Trig> module (part of the standard Perl | |
46fc3d4c | 99 | distribution) implements the trigonometric functions. Internally it |
ac9dac7f | 100 | uses the C<Math::Complex> module and some functions can break out from |
46fc3d4c | 101 | the real axis into the complex plane, for example the inverse sine of |
102 | 2. | |
68dc0745 | 103 | |
104 | Rounding in financial applications can have serious implications, and | |
105 | the rounding method used should be specified precisely. In these | |
106 | cases, it probably pays not to trust whichever system rounding is | |
107 | being used by Perl, but to instead implement the rounding function you | |
108 | need yourself. | |
109 | ||
65acb1b1 TC |
110 | To see why, notice how you'll still have an issue on half-way-point |
111 | alternation: | |
112 | ||
ac9dac7f | 113 | for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} |
65acb1b1 | 114 | |
ac9dac7f RGS |
115 | 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 |
116 | 0.8 0.8 0.9 0.9 1.0 1.0 | |
65acb1b1 | 117 | |
ac9dac7f RGS |
118 | Don't blame Perl. It's the same as in C. IEEE says we have to do |
119 | this. Perl numbers whose absolute values are integers under 2**31 (on | |
120 | 32 bit machines) will work pretty much like mathematical integers. | |
121 | Other numbers are not guaranteed. | |
65acb1b1 | 122 | |
6f0efb17 | 123 | =head2 How do I convert between numeric representations/bases/radixes? |
68dc0745 | 124 | |
ac9dac7f RGS |
125 | As always with Perl there is more than one way to do it. Below are a |
126 | few examples of approaches to making common conversions between number | |
127 | representations. This is intended to be representational rather than | |
128 | exhaustive. | |
68dc0745 | 129 | |
ac9dac7f RGS |
130 | Some of the examples later in L<perlfaq4> use the C<Bit::Vector> |
131 | module from CPAN. The reason you might choose C<Bit::Vector> over the | |
132 | perl built in functions is that it works with numbers of ANY size, | |
133 | that it is optimized for speed on some operations, and for at least | |
134 | some programmers the notation might be familiar. | |
d92eb7b0 | 135 | |
818c4caa JH |
136 | =over 4 |
137 | ||
138 | =item How do I convert hexadecimal into decimal | |
d92eb7b0 | 139 | |
ac9dac7f | 140 | Using perl's built in conversion of C<0x> notation: |
6761e064 | 141 | |
ac9dac7f | 142 | $dec = 0xDEADBEEF; |
7207e29d | 143 | |
ac9dac7f | 144 | Using the C<hex> function: |
6761e064 | 145 | |
ac9dac7f | 146 | $dec = hex("DEADBEEF"); |
6761e064 | 147 | |
ac9dac7f | 148 | Using C<pack>: |
6761e064 | 149 | |
ac9dac7f | 150 | $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); |
6761e064 | 151 | |
ac9dac7f | 152 | Using the CPAN module C<Bit::Vector>: |
6761e064 | 153 | |
ac9dac7f RGS |
154 | use Bit::Vector; |
155 | $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); | |
156 | $dec = $vec->to_Dec(); | |
6761e064 | 157 | |
818c4caa | 158 | =item How do I convert from decimal to hexadecimal |
6761e064 | 159 | |
ac9dac7f | 160 | Using C<sprintf>: |
6761e064 | 161 | |
ac9dac7f RGS |
162 | $hex = sprintf("%X", 3735928559); # upper case A-F |
163 | $hex = sprintf("%x", 3735928559); # lower case a-f | |
6761e064 | 164 | |
ac9dac7f | 165 | Using C<unpack>: |
6761e064 | 166 | |
ac9dac7f | 167 | $hex = unpack("H*", pack("N", 3735928559)); |
6761e064 | 168 | |
ac9dac7f | 169 | Using C<Bit::Vector>: |
6761e064 | 170 | |
ac9dac7f RGS |
171 | use Bit::Vector; |
172 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
173 | $hex = $vec->to_Hex(); | |
6761e064 | 174 | |
ac9dac7f | 175 | And C<Bit::Vector> supports odd bit counts: |
6761e064 | 176 | |
ac9dac7f RGS |
177 | use Bit::Vector; |
178 | $vec = Bit::Vector->new_Dec(33, 3735928559); | |
179 | $vec->Resize(32); # suppress leading 0 if unwanted | |
180 | $hex = $vec->to_Hex(); | |
6761e064 | 181 | |
818c4caa | 182 | =item How do I convert from octal to decimal |
6761e064 JH |
183 | |
184 | Using Perl's built in conversion of numbers with leading zeros: | |
185 | ||
ac9dac7f | 186 | $dec = 033653337357; # note the leading 0! |
6761e064 | 187 | |
ac9dac7f | 188 | Using the C<oct> function: |
6761e064 | 189 | |
ac9dac7f | 190 | $dec = oct("33653337357"); |
6761e064 | 191 | |
ac9dac7f | 192 | Using C<Bit::Vector>: |
6761e064 | 193 | |
ac9dac7f RGS |
194 | use Bit::Vector; |
195 | $vec = Bit::Vector->new(32); | |
196 | $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); | |
197 | $dec = $vec->to_Dec(); | |
6761e064 | 198 | |
818c4caa | 199 | =item How do I convert from decimal to octal |
6761e064 | 200 | |
ac9dac7f | 201 | Using C<sprintf>: |
6761e064 | 202 | |
ac9dac7f | 203 | $oct = sprintf("%o", 3735928559); |
6761e064 | 204 | |
ac9dac7f | 205 | Using C<Bit::Vector>: |
6761e064 | 206 | |
ac9dac7f RGS |
207 | use Bit::Vector; |
208 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
209 | $oct = reverse join('', $vec->Chunk_List_Read(3)); | |
6761e064 | 210 | |
818c4caa | 211 | =item How do I convert from binary to decimal |
6761e064 | 212 | |
2c646907 | 213 | Perl 5.6 lets you write binary numbers directly with |
ac9dac7f | 214 | the C<0b> notation: |
2c646907 | 215 | |
ac9dac7f | 216 | $number = 0b10110110; |
6f0efb17 | 217 | |
ac9dac7f | 218 | Using C<oct>: |
6f0efb17 | 219 | |
ac9dac7f RGS |
220 | my $input = "10110110"; |
221 | $decimal = oct( "0b$input" ); | |
2c646907 | 222 | |
ac9dac7f | 223 | Using C<pack> and C<ord>: |
d92eb7b0 | 224 | |
ac9dac7f | 225 | $decimal = ord(pack('B8', '10110110')); |
68dc0745 | 226 | |
ac9dac7f | 227 | Using C<pack> and C<unpack> for larger strings: |
6761e064 | 228 | |
ac9dac7f | 229 | $int = unpack("N", pack("B32", |
6761e064 | 230 | substr("0" x 32 . "11110101011011011111011101111", -32))); |
ac9dac7f | 231 | $dec = sprintf("%d", $int); |
6761e064 | 232 | |
ac9dac7f | 233 | # substr() is used to left pad a 32 character string with zeros. |
6761e064 | 234 | |
ac9dac7f | 235 | Using C<Bit::Vector>: |
6761e064 | 236 | |
ac9dac7f RGS |
237 | $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); |
238 | $dec = $vec->to_Dec(); | |
6761e064 | 239 | |
818c4caa | 240 | =item How do I convert from decimal to binary |
6761e064 | 241 | |
ac9dac7f | 242 | Using C<sprintf> (perl 5.6+): |
4dfcc30b | 243 | |
ac9dac7f | 244 | $bin = sprintf("%b", 3735928559); |
4dfcc30b | 245 | |
ac9dac7f | 246 | Using C<unpack>: |
6761e064 | 247 | |
ac9dac7f | 248 | $bin = unpack("B*", pack("N", 3735928559)); |
6761e064 | 249 | |
ac9dac7f | 250 | Using C<Bit::Vector>: |
6761e064 | 251 | |
ac9dac7f RGS |
252 | use Bit::Vector; |
253 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
254 | $bin = $vec->to_Bin(); | |
6761e064 JH |
255 | |
256 | The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) | |
257 | are left as an exercise to the inclined reader. | |
68dc0745 | 258 | |
818c4caa | 259 | =back |
68dc0745 | 260 | |
65acb1b1 TC |
261 | =head2 Why doesn't & work the way I want it to? |
262 | ||
263 | The behavior of binary arithmetic operators depends on whether they're | |
264 | used on numbers or strings. The operators treat a string as a series | |
265 | of bits and work with that (the string C<"3"> is the bit pattern | |
266 | C<00110011>). The operators work with the binary form of a number | |
267 | (the number C<3> is treated as the bit pattern C<00000011>). | |
268 | ||
269 | So, saying C<11 & 3> performs the "and" operation on numbers (yielding | |
49d635f9 | 270 | C<3>). Saying C<"11" & "3"> performs the "and" operation on strings |
65acb1b1 TC |
271 | (yielding C<"1">). |
272 | ||
273 | Most problems with C<&> and C<|> arise because the programmer thinks | |
274 | they have a number but really it's a string. The rest arise because | |
275 | the programmer says: | |
276 | ||
ac9dac7f RGS |
277 | if ("\020\020" & "\101\101") { |
278 | # ... | |
279 | } | |
65acb1b1 TC |
280 | |
281 | but a string consisting of two null bytes (the result of C<"\020\020" | |
282 | & "\101\101">) is not a false value in Perl. You need: | |
283 | ||
ac9dac7f RGS |
284 | if ( ("\020\020" & "\101\101") !~ /[^\000]/) { |
285 | # ... | |
286 | } | |
65acb1b1 | 287 | |
68dc0745 | 288 | =head2 How do I multiply matrices? |
289 | ||
290 | Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) | |
291 | or the PDL extension (also available from CPAN). | |
292 | ||
293 | =head2 How do I perform an operation on a series of integers? | |
294 | ||
295 | To call a function on each element in an array, and collect the | |
296 | results, use: | |
297 | ||
ac9dac7f | 298 | @results = map { my_func($_) } @array; |
68dc0745 | 299 | |
300 | For example: | |
301 | ||
ac9dac7f | 302 | @triple = map { 3 * $_ } @single; |
68dc0745 | 303 | |
304 | To call a function on each element of an array, but ignore the | |
305 | results: | |
306 | ||
ac9dac7f RGS |
307 | foreach $iterator (@array) { |
308 | some_func($iterator); | |
309 | } | |
68dc0745 | 310 | |
311 | To call a function on each integer in a (small) range, you B<can> use: | |
312 | ||
ac9dac7f | 313 | @results = map { some_func($_) } (5 .. 25); |
68dc0745 | 314 | |
315 | but you should be aware that the C<..> operator creates an array of | |
316 | all integers in the range. This can take a lot of memory for large | |
317 | ranges. Instead use: | |
318 | ||
ac9dac7f RGS |
319 | @results = (); |
320 | for ($i=5; $i < 500_005; $i++) { | |
321 | push(@results, some_func($i)); | |
322 | } | |
68dc0745 | 323 | |
87275199 GS |
324 | This situation has been fixed in Perl5.005. Use of C<..> in a C<for> |
325 | loop will iterate over the range, without creating the entire range. | |
326 | ||
ac9dac7f RGS |
327 | for my $i (5 .. 500_005) { |
328 | push(@results, some_func($i)); | |
329 | } | |
87275199 GS |
330 | |
331 | will not create a list of 500,000 integers. | |
332 | ||
68dc0745 | 333 | =head2 How can I output Roman numerals? |
334 | ||
a93751fa | 335 | Get the http://www.cpan.org/modules/by-module/Roman module. |
68dc0745 | 336 | |
337 | =head2 Why aren't my random numbers random? | |
338 | ||
65acb1b1 TC |
339 | If you're using a version of Perl before 5.004, you must call C<srand> |
340 | once at the start of your program to seed the random number generator. | |
49d635f9 | 341 | |
5cd0b561 | 342 | BEGIN { srand() if $] < 5.004 } |
49d635f9 | 343 | |
65acb1b1 | 344 | 5.004 and later automatically call C<srand> at the beginning. Don't |
ac9dac7f RGS |
345 | call C<srand> more than once--you make your numbers less random, |
346 | rather than more. | |
92c2ed05 | 347 | |
65acb1b1 | 348 | Computers are good at being predictable and bad at being random |
06a5f41f | 349 | (despite appearances caused by bugs in your programs :-). see the |
49d635f9 | 350 | F<random> article in the "Far More Than You Ever Wanted To Know" |
ac9dac7f RGS |
351 | collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy |
352 | of Tom Phoenix, talks more about this. John von Neumann said, "Anyone | |
06a5f41f | 353 | who attempts to generate random numbers by deterministic means is, of |
b432a672 | 354 | course, living in a state of sin." |
65acb1b1 TC |
355 | |
356 | If you want numbers that are more random than C<rand> with C<srand> | |
ac9dac7f | 357 | provides, you should also check out the C<Math::TrulyRandom> module from |
65acb1b1 TC |
358 | CPAN. It uses the imperfections in your system's timer to generate |
359 | random numbers, but this takes quite a while. If you want a better | |
92c2ed05 | 360 | pseudorandom generator than comes with your operating system, look at |
b432a672 | 361 | "Numerical Recipes in C" at http://www.nr.com/ . |
68dc0745 | 362 | |
881bdbd4 JH |
363 | =head2 How do I get a random number between X and Y? |
364 | ||
ee891a00 RGS |
365 | To get a random number between two values, you can use the C<rand()> |
366 | builtin to get a random number between 0 and 1. From there, you shift | |
367 | that into the range that you want. | |
500071f4 | 368 | |
ee891a00 RGS |
369 | C<rand($x)> returns a number such that C<< 0 <= rand($x) < $x >>. Thus |
370 | what you want to have perl figure out is a random number in the range | |
371 | from 0 to the difference between your I<X> and I<Y>. | |
793f5136 | 372 | |
ee891a00 RGS |
373 | That is, to get a number between 10 and 15, inclusive, you want a |
374 | random number between 0 and 5 that you can then add to 10. | |
793f5136 | 375 | |
500071f4 | 376 | my $number = 10 + int rand( 15-10+1 ); |
793f5136 RGS |
377 | |
378 | Hence you derive the following simple function to abstract | |
379 | that. It selects a random integer between the two given | |
500071f4 RGS |
380 | integers (inclusive), For example: C<random_int_between(50,120)>. |
381 | ||
ac9dac7f | 382 | sub random_int_between { |
500071f4 RGS |
383 | my($min, $max) = @_; |
384 | # Assumes that the two arguments are integers themselves! | |
385 | return $min if $min == $max; | |
386 | ($min, $max) = ($max, $min) if $min > $max; | |
387 | return $min + int rand(1 + $max - $min); | |
388 | } | |
881bdbd4 | 389 | |
68dc0745 | 390 | =head1 Data: Dates |
391 | ||
5cd0b561 | 392 | =head2 How do I find the day or week of the year? |
68dc0745 | 393 | |
571e049f | 394 | The localtime function returns the day of the year. Without an |
5cd0b561 | 395 | argument localtime uses the current time. |
68dc0745 | 396 | |
a05e4845 | 397 | $day_of_year = (localtime)[7]; |
ffc145e8 | 398 | |
ac9dac7f | 399 | The C<POSIX> module can also format a date as the day of the year or |
5cd0b561 | 400 | week of the year. |
68dc0745 | 401 | |
5cd0b561 RGS |
402 | use POSIX qw/strftime/; |
403 | my $day_of_year = strftime "%j", localtime; | |
404 | my $week_of_year = strftime "%W", localtime; | |
405 | ||
ac9dac7f | 406 | To get the day of year for any date, use C<POSIX>'s C<mktime> to get |
5cd0b561 | 407 | a time in epoch seconds for the argument to localtime. |
ffc145e8 | 408 | |
ac9dac7f | 409 | use POSIX qw/mktime strftime/; |
6670e5e7 | 410 | my $week_of_year = strftime "%W", |
ac9dac7f | 411 | localtime( mktime( 0, 0, 0, 18, 11, 87 ) ); |
5cd0b561 | 412 | |
ac9dac7f | 413 | The C<Date::Calc> module provides two functions to calculate these. |
5cd0b561 RGS |
414 | |
415 | use Date::Calc; | |
416 | my $day_of_year = Day_of_Year( 1987, 12, 18 ); | |
417 | my $week_of_year = Week_of_Year( 1987, 12, 18 ); | |
ffc145e8 | 418 | |
d92eb7b0 GS |
419 | =head2 How do I find the current century or millennium? |
420 | ||
421 | Use the following simple functions: | |
422 | ||
ac9dac7f RGS |
423 | sub get_century { |
424 | return int((((localtime(shift || time))[5] + 1999))/100); | |
425 | } | |
6670e5e7 | 426 | |
ac9dac7f RGS |
427 | sub get_millennium { |
428 | return 1+int((((localtime(shift || time))[5] + 1899))/1000); | |
429 | } | |
d92eb7b0 | 430 | |
ac9dac7f RGS |
431 | On some systems, the C<POSIX> module's C<strftime()> function has been |
432 | extended in a non-standard way to use a C<%C> format, which they | |
433 | sometimes claim is the "century". It isn't, because on most such | |
434 | systems, this is only the first two digits of the four-digit year, and | |
435 | thus cannot be used to reliably determine the current century or | |
436 | millennium. | |
d92eb7b0 | 437 | |
92c2ed05 | 438 | =head2 How can I compare two dates and find the difference? |
68dc0745 | 439 | |
b68463f7 RGS |
440 | (contributed by brian d foy) |
441 | ||
ac9dac7f RGS |
442 | You could just store all your dates as a number and then subtract. |
443 | Life isn't always that simple though. If you want to work with | |
444 | formatted dates, the C<Date::Manip>, C<Date::Calc>, or C<DateTime> | |
445 | modules can help you. | |
68dc0745 | 446 | |
447 | =head2 How can I take a string and turn it into epoch seconds? | |
448 | ||
449 | If it's a regular enough string that it always has the same format, | |
92c2ed05 | 450 | you can split it up and pass the parts to C<timelocal> in the standard |
ac9dac7f RGS |
451 | C<Time::Local> module. Otherwise, you should look into the C<Date::Calc> |
452 | and C<Date::Manip> modules from CPAN. | |
68dc0745 | 453 | |
454 | =head2 How can I find the Julian Day? | |
455 | ||
7678cced RGS |
456 | (contributed by brian d foy and Dave Cross) |
457 | ||
ac9dac7f RGS |
458 | You can use the C<Time::JulianDay> module available on CPAN. Ensure |
459 | that you really want to find a Julian day, though, as many people have | |
7678cced RGS |
460 | different ideas about Julian days. See |
461 | http://www.hermetic.ch/cal_stud/jdn.htm for instance. | |
462 | ||
ac9dac7f | 463 | You can also try the C<DateTime> module, which can convert a date/time |
7678cced RGS |
464 | to a Julian Day. |
465 | ||
ac9dac7f RGS |
466 | $ perl -MDateTime -le'print DateTime->today->jd' |
467 | 2453401.5 | |
7678cced RGS |
468 | |
469 | Or the modified Julian Day | |
470 | ||
ac9dac7f RGS |
471 | $ perl -MDateTime -le'print DateTime->today->mjd' |
472 | 53401 | |
7678cced RGS |
473 | |
474 | Or even the day of the year (which is what some people think of as a | |
475 | Julian day) | |
476 | ||
ac9dac7f RGS |
477 | $ perl -MDateTime -le'print DateTime->today->doy' |
478 | 31 | |
be94a901 | 479 | |
65acb1b1 TC |
480 | =head2 How do I find yesterday's date? |
481 | ||
6670e5e7 | 482 | (contributed by brian d foy) |
49d635f9 | 483 | |
6670e5e7 RGS |
484 | Use one of the Date modules. The C<DateTime> module makes it simple, and |
485 | give you the same time of day, only the day before. | |
49d635f9 | 486 | |
6670e5e7 | 487 | use DateTime; |
58103a2e | 488 | |
6670e5e7 | 489 | my $yesterday = DateTime->now->subtract( days => 1 ); |
58103a2e | 490 | |
6670e5e7 | 491 | print "Yesterday was $yesterday\n"; |
49d635f9 | 492 | |
ee891a00 | 493 | You can also use the C<Date::Calc> module using its C<Today_and_Now> |
6670e5e7 | 494 | function. |
49d635f9 | 495 | |
6670e5e7 | 496 | use Date::Calc qw( Today_and_Now Add_Delta_DHMS ); |
58103a2e | 497 | |
6670e5e7 | 498 | my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 ); |
58103a2e | 499 | |
ee891a00 | 500 | print "@date_time\n"; |
58103a2e | 501 | |
6670e5e7 RGS |
502 | Most people try to use the time rather than the calendar to figure out |
503 | dates, but that assumes that days are twenty-four hours each. For | |
504 | most people, there are two days a year when they aren't: the switch to | |
505 | and from summer time throws this off. Let the modules do the work. | |
d92eb7b0 | 506 | |
ac9dac7f | 507 | =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? |
68dc0745 | 508 | |
65acb1b1 | 509 | Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is |
ac9dac7f | 510 | Y2K compliant (whatever that means). The programmers you've hired to |
65acb1b1 TC |
511 | use it, however, probably are not. |
512 | ||
513 | Long answer: The question belies a true understanding of the issue. | |
514 | Perl is just as Y2K compliant as your pencil--no more, and no less. | |
515 | Can you use your pencil to write a non-Y2K-compliant memo? Of course | |
516 | you can. Is that the pencil's fault? Of course it isn't. | |
92c2ed05 | 517 | |
87275199 | 518 | The date and time functions supplied with Perl (gmtime and localtime) |
dc164757 MS |
519 | supply adequate information to determine the year well beyond 2000 and |
520 | 2038. The year returned by these functions when used in a list | |
521 | context is the year minus 1900. For years between 1910 and 1999 this | |
522 | I<happens> to be a 2-digit decimal number. To avoid the year 2000 | |
523 | problem simply do not treat the year as a 2-digit number. It isn't. | |
68dc0745 | 524 | |
5a964f20 | 525 | When gmtime() and localtime() are used in scalar context they return |
68dc0745 | 526 | a timestamp string that contains a fully-expanded year. For example, |
527 | C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 | |
528 | 2001". There's no year 2000 problem here. | |
529 | ||
5a964f20 TC |
530 | That doesn't mean that Perl can't be used to create non-Y2K compliant |
531 | programs. It can. But so can your pencil. It's the fault of the user, | |
b432a672 AL |
532 | not the language. At the risk of inflaming the NRA: "Perl doesn't |
533 | break Y2K, people do." See http://www.perl.org/about/y2k.html for | |
5a964f20 TC |
534 | a longer exposition. |
535 | ||
dc164757 MS |
536 | =head2 Does Perl have a Year 2038 problem? |
537 | ||
538 | No, all of Perl's built in date and time functions and modules will | |
539 | work to about 2 billion years before and after 1970. | |
540 | ||
541 | Many systems cannot count time past the year 2038. Older versions of | |
542 | Perl were dependent on the system to do date calculation and thus | |
543 | shared their 2038 bug. | |
544 | ||
68dc0745 | 545 | =head1 Data: Strings |
546 | ||
547 | =head2 How do I validate input? | |
548 | ||
6670e5e7 RGS |
549 | (contributed by brian d foy) |
550 | ||
551 | There are many ways to ensure that values are what you expect or | |
552 | want to accept. Besides the specific examples that we cover in the | |
553 | perlfaq, you can also look at the modules with "Assert" and "Validate" | |
554 | in their names, along with other modules such as C<Regexp::Common>. | |
555 | ||
556 | Some modules have validation for particular types of input, such | |
557 | as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>, | |
558 | and C<Data::Validate::IP>. | |
68dc0745 | 559 | |
560 | =head2 How do I unescape a string? | |
561 | ||
b432a672 | 562 | It depends just what you mean by "escape". URL escapes are dealt |
92c2ed05 | 563 | with in L<perlfaq9>. Shell escapes with the backslash (C<\>) |
a6dd486b | 564 | character are removed with |
68dc0745 | 565 | |
ac9dac7f | 566 | s/\\(.)/$1/g; |
68dc0745 | 567 | |
92c2ed05 | 568 | This won't expand C<"\n"> or C<"\t"> or any other special escapes. |
68dc0745 | 569 | |
570 | =head2 How do I remove consecutive pairs of characters? | |
571 | ||
6670e5e7 RGS |
572 | (contributed by brian d foy) |
573 | ||
574 | You can use the substitution operator to find pairs of characters (or | |
575 | runs of characters) and replace them with a single instance. In this | |
576 | substitution, we find a character in C<(.)>. The memory parentheses | |
577 | store the matched character in the back-reference C<\1> and we use | |
578 | that to require that the same thing immediately follow it. We replace | |
579 | that part of the string with the character in C<$1>. | |
68dc0745 | 580 | |
ac9dac7f | 581 | s/(.)\1/$1/g; |
d92eb7b0 | 582 | |
6670e5e7 RGS |
583 | We can also use the transliteration operator, C<tr///>. In this |
584 | example, the search list side of our C<tr///> contains nothing, but | |
585 | the C<c> option complements that so it contains everything. The | |
586 | replacement list also contains nothing, so the transliteration is | |
587 | almost a no-op since it won't do any replacements (or more exactly, | |
588 | replace the character with itself). However, the C<s> option squashes | |
589 | duplicated and consecutive characters in the string so a character | |
590 | does not show up next to itself | |
d92eb7b0 | 591 | |
6670e5e7 | 592 | my $str = 'Haarlem'; # in the Netherlands |
ac9dac7f | 593 | $str =~ tr///cs; # Now Harlem, like in New York |
68dc0745 | 594 | |
595 | =head2 How do I expand function calls in a string? | |
596 | ||
6670e5e7 RGS |
597 | (contributed by brian d foy) |
598 | ||
599 | This is documented in L<perlref>, and although it's not the easiest | |
600 | thing to read, it does work. In each of these examples, we call the | |
58103a2e | 601 | function inside the braces used to dereference a reference. If we |
5ae37c3f | 602 | have more than one return value, we can construct and dereference an |
6670e5e7 RGS |
603 | anonymous array. In this case, we call the function in list context. |
604 | ||
58103a2e | 605 | print "The time values are @{ [localtime] }.\n"; |
6670e5e7 RGS |
606 | |
607 | If we want to call the function in scalar context, we have to do a bit | |
608 | more work. We can really have any code we like inside the braces, so | |
609 | we simply have to end with the scalar reference, although how you do | |
e573f903 RGS |
610 | that is up to you, and you can use code inside the braces. Note that |
611 | the use of parens creates a list context, so we need C<scalar> to | |
612 | force the scalar context on the function: | |
68dc0745 | 613 | |
6670e5e7 | 614 | print "The time is ${\(scalar localtime)}.\n" |
58103a2e | 615 | |
6670e5e7 | 616 | print "The time is ${ my $x = localtime; \$x }.\n"; |
58103a2e | 617 | |
6670e5e7 RGS |
618 | If your function already returns a reference, you don't need to create |
619 | the reference yourself. | |
620 | ||
621 | sub timestamp { my $t = localtime; \$t } | |
58103a2e | 622 | |
6670e5e7 | 623 | print "The time is ${ timestamp() }.\n"; |
58103a2e RGS |
624 | |
625 | The C<Interpolation> module can also do a lot of magic for you. You can | |
626 | specify a variable name, in this case C<E>, to set up a tied hash that | |
627 | does the interpolation for you. It has several other methods to do this | |
628 | as well. | |
629 | ||
630 | use Interpolation E => 'eval'; | |
631 | print "The time values are $E{localtime()}.\n"; | |
632 | ||
633 | In most cases, it is probably easier to simply use string concatenation, | |
634 | which also forces scalar context. | |
6670e5e7 | 635 | |
ac9dac7f | 636 | print "The time is " . localtime() . ".\n"; |
68dc0745 | 637 | |
68dc0745 | 638 | =head2 How do I find matching/nesting anything? |
639 | ||
92c2ed05 GS |
640 | This isn't something that can be done in one regular expression, no |
641 | matter how complicated. To find something between two single | |
642 | characters, a pattern like C</x([^x]*)x/> will get the intervening | |
643 | bits in $1. For multiple ones, then something more like | |
ac9dac7f | 644 | C</alpha(.*?)omega/> would be needed. But none of these deals with |
6670e5e7 RGS |
645 | nested patterns. For balanced expressions using C<(>, C<{>, C<[> or |
646 | C<< < >> as delimiters, use the CPAN module Regexp::Common, or see | |
647 | L<perlre/(??{ code })>. For other cases, you'll have to write a | |
648 | parser. | |
92c2ed05 GS |
649 | |
650 | If you are serious about writing a parser, there are a number of | |
6a2af475 | 651 | modules or oddities that will make your life a lot easier. There are |
ac9dac7f RGS |
652 | the CPAN modules C<Parse::RecDescent>, C<Parse::Yapp>, and |
653 | C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8 | |
654 | the C<Text::Balanced> is part of the standard distribution. | |
68dc0745 | 655 | |
92c2ed05 GS |
656 | One simple destructive, inside-out approach that you might try is to |
657 | pull out the smallest nesting parts one at a time: | |
5a964f20 | 658 | |
ac9dac7f RGS |
659 | while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { |
660 | # do something with $1 | |
661 | } | |
5a964f20 | 662 | |
65acb1b1 TC |
663 | A more complicated and sneaky approach is to make Perl's regular |
664 | expression engine do it for you. This is courtesy Dean Inada, and | |
665 | rather has the nature of an Obfuscated Perl Contest entry, but it | |
666 | really does work: | |
667 | ||
ac9dac7f RGS |
668 | # $_ contains the string to parse |
669 | # BEGIN and END are the opening and closing markers for the | |
670 | # nested text. | |
c47ff5f1 | 671 | |
ac9dac7f RGS |
672 | @( = ('(',''); |
673 | @) = (')',''); | |
674 | ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; | |
675 | @$ = (eval{/$re/},$@!~/unmatched/i); | |
676 | print join("\n",@$[0..$#$]) if( $$[-1] ); | |
65acb1b1 | 677 | |
68dc0745 | 678 | =head2 How do I reverse a string? |
679 | ||
ac9dac7f | 680 | Use C<reverse()> in scalar context, as documented in |
68dc0745 | 681 | L<perlfunc/reverse>. |
682 | ||
ac9dac7f | 683 | $reversed = reverse $string; |
68dc0745 | 684 | |
685 | =head2 How do I expand tabs in a string? | |
686 | ||
5a964f20 | 687 | You can do it yourself: |
68dc0745 | 688 | |
ac9dac7f | 689 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; |
68dc0745 | 690 | |
ac9dac7f | 691 | Or you can just use the C<Text::Tabs> module (part of the standard Perl |
68dc0745 | 692 | distribution). |
693 | ||
ac9dac7f RGS |
694 | use Text::Tabs; |
695 | @expanded_lines = expand(@lines_with_tabs); | |
68dc0745 | 696 | |
697 | =head2 How do I reformat a paragraph? | |
698 | ||
ac9dac7f | 699 | Use C<Text::Wrap> (part of the standard Perl distribution): |
68dc0745 | 700 | |
ac9dac7f RGS |
701 | use Text::Wrap; |
702 | print wrap("\t", ' ', @paragraphs); | |
68dc0745 | 703 | |
ac9dac7f RGS |
704 | The paragraphs you give to C<Text::Wrap> should not contain embedded |
705 | newlines. C<Text::Wrap> doesn't justify the lines (flush-right). | |
46fc3d4c | 706 | |
ac9dac7f RGS |
707 | Or use the CPAN module C<Text::Autoformat>. Formatting files can be |
708 | easily done by making a shell alias, like so: | |
bc06af74 | 709 | |
ac9dac7f RGS |
710 | alias fmt="perl -i -MText::Autoformat -n0777 \ |
711 | -e 'print autoformat $_, {all=>1}' $*" | |
bc06af74 | 712 | |
ac9dac7f | 713 | See the documentation for C<Text::Autoformat> to appreciate its many |
bc06af74 JH |
714 | capabilities. |
715 | ||
49d635f9 | 716 | =head2 How can I access or change N characters of a string? |
68dc0745 | 717 | |
49d635f9 RGS |
718 | You can access the first characters of a string with substr(). |
719 | To get the first character, for example, start at position 0 | |
197aec24 | 720 | and grab the string of length 1. |
68dc0745 | 721 | |
68dc0745 | 722 | |
49d635f9 | 723 | $string = "Just another Perl Hacker"; |
ac9dac7f | 724 | $first_char = substr( $string, 0, 1 ); # 'J' |
68dc0745 | 725 | |
49d635f9 RGS |
726 | To change part of a string, you can use the optional fourth |
727 | argument which is the replacement string. | |
68dc0745 | 728 | |
ac9dac7f | 729 | substr( $string, 13, 4, "Perl 5.8.0" ); |
197aec24 | 730 | |
49d635f9 | 731 | You can also use substr() as an lvalue. |
68dc0745 | 732 | |
ac9dac7f | 733 | substr( $string, 13, 4 ) = "Perl 5.8.0"; |
197aec24 | 734 | |
68dc0745 | 735 | =head2 How do I change the Nth occurrence of something? |
736 | ||
92c2ed05 GS |
737 | You have to keep track of N yourself. For example, let's say you want |
738 | to change the fifth occurrence of C<"whoever"> or C<"whomever"> into | |
d92eb7b0 GS |
739 | C<"whosoever"> or C<"whomsoever">, case insensitively. These |
740 | all assume that $_ contains the string to be altered. | |
68dc0745 | 741 | |
ac9dac7f RGS |
742 | $count = 0; |
743 | s{((whom?)ever)}{ | |
744 | ++$count == 5 # is it the 5th? | |
745 | ? "${2}soever" # yes, swap | |
746 | : $1 # renege and leave it there | |
747 | }ige; | |
68dc0745 | 748 | |
5a964f20 TC |
749 | In the more general case, you can use the C</g> modifier in a C<while> |
750 | loop, keeping count of matches. | |
751 | ||
ac9dac7f RGS |
752 | $WANT = 3; |
753 | $count = 0; | |
754 | $_ = "One fish two fish red fish blue fish"; | |
755 | while (/(\w+)\s+fish\b/gi) { | |
756 | if (++$count == $WANT) { | |
757 | print "The third fish is a $1 one.\n"; | |
758 | } | |
759 | } | |
5a964f20 | 760 | |
92c2ed05 | 761 | That prints out: C<"The third fish is a red one."> You can also use a |
5a964f20 TC |
762 | repetition count and repeated pattern like this: |
763 | ||
ac9dac7f | 764 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; |
5a964f20 | 765 | |
68dc0745 | 766 | =head2 How can I count the number of occurrences of a substring within a string? |
767 | ||
a6dd486b | 768 | There are a number of ways, with varying efficiency. If you want a |
68dc0745 | 769 | count of a certain single character (X) within a string, you can use the |
770 | C<tr///> function like so: | |
771 | ||
ac9dac7f RGS |
772 | $string = "ThisXlineXhasXsomeXx'sXinXit"; |
773 | $count = ($string =~ tr/X//); | |
774 | print "There are $count X characters in the string"; | |
68dc0745 | 775 | |
776 | This is fine if you are just looking for a single character. However, | |
777 | if you are trying to count multiple character substrings within a | |
778 | larger string, C<tr///> won't work. What you can do is wrap a while() | |
779 | loop around a global pattern match. For example, let's count negative | |
780 | integers: | |
781 | ||
ac9dac7f RGS |
782 | $string = "-9 55 48 -2 23 -76 4 14 -44"; |
783 | while ($string =~ /-\d+/g) { $count++ } | |
784 | print "There are $count negative numbers in the string"; | |
68dc0745 | 785 | |
881bdbd4 JH |
786 | Another version uses a global match in list context, then assigns the |
787 | result to a scalar, producing a count of the number of matches. | |
788 | ||
789 | $count = () = $string =~ /-\d+/g; | |
790 | ||
68dc0745 | 791 | =head2 How do I capitalize all the words on one line? |
792 | ||
793 | To make the first letter of each word upper case: | |
3fe9a6f1 | 794 | |
ac9dac7f | 795 | $line =~ s/\b(\w)/\U$1/g; |
68dc0745 | 796 | |
46fc3d4c | 797 | This has the strange effect of turning "C<don't do it>" into "C<Don'T |
a6dd486b | 798 | Do It>". Sometimes you might want this. Other times you might need a |
24f1ba9b | 799 | more thorough solution (Suggested by brian d foy): |
46fc3d4c | 800 | |
ac9dac7f RGS |
801 | $string =~ s/ ( |
802 | (^\w) #at the beginning of the line | |
803 | | # or | |
804 | (\s\w) #preceded by whitespace | |
805 | ) | |
806 | /\U$1/xg; | |
807 | ||
808 | $string =~ s/([\w']+)/\u\L$1/g; | |
46fc3d4c | 809 | |
68dc0745 | 810 | To make the whole line upper case: |
3fe9a6f1 | 811 | |
ac9dac7f | 812 | $line = uc($line); |
68dc0745 | 813 | |
814 | To force each word to be lower case, with the first letter upper case: | |
3fe9a6f1 | 815 | |
ac9dac7f | 816 | $line =~ s/(\w+)/\u\L$1/g; |
68dc0745 | 817 | |
5a964f20 TC |
818 | You can (and probably should) enable locale awareness of those |
819 | characters by placing a C<use locale> pragma in your program. | |
92c2ed05 | 820 | See L<perllocale> for endless details on locales. |
5a964f20 | 821 | |
65acb1b1 | 822 | This is sometimes referred to as putting something into "title |
d92eb7b0 | 823 | case", but that's not quite accurate. Consider the proper |
65acb1b1 TC |
824 | capitalization of the movie I<Dr. Strangelove or: How I Learned to |
825 | Stop Worrying and Love the Bomb>, for example. | |
826 | ||
369b44b4 RGS |
827 | Damian Conway's L<Text::Autoformat> module provides some smart |
828 | case transformations: | |
829 | ||
ac9dac7f RGS |
830 | use Text::Autoformat; |
831 | my $x = "Dr. Strangelove or: How I Learned to Stop ". | |
832 | "Worrying and Love the Bomb"; | |
369b44b4 | 833 | |
ac9dac7f RGS |
834 | print $x, "\n"; |
835 | for my $style (qw( sentence title highlight )) { | |
836 | print autoformat($x, { case => $style }), "\n"; | |
837 | } | |
369b44b4 | 838 | |
49d635f9 | 839 | =head2 How can I split a [character] delimited string except when inside [character]? |
68dc0745 | 840 | |
ac9dac7f RGS |
841 | Several modules can handle this sort of parsing--C<Text::Balanced>, |
842 | C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others. | |
49d635f9 RGS |
843 | |
844 | Take the example case of trying to split a string that is | |
845 | comma-separated into its different fields. You can't use C<split(/,/)> | |
846 | because you shouldn't split if the comma is inside quotes. For | |
847 | example, take a data line like this: | |
68dc0745 | 848 | |
ac9dac7f | 849 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" |
68dc0745 | 850 | |
851 | Due to the restriction of the quotes, this is a fairly complex | |
197aec24 | 852 | problem. Thankfully, we have Jeffrey Friedl, author of |
49d635f9 | 853 | I<Mastering Regular Expressions>, to handle these for us. He |
ac9dac7f | 854 | suggests (assuming your string is contained in C<$text>): |
68dc0745 | 855 | |
ac9dac7f RGS |
856 | @new = (); |
857 | push(@new, $+) while $text =~ m{ | |
858 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes | |
859 | | ([^,]+),? | |
860 | | , | |
861 | }gx; | |
862 | push(@new, undef) if substr($text,-1,1) eq ','; | |
68dc0745 | 863 | |
46fc3d4c | 864 | If you want to represent quotation marks inside a |
865 | quotation-mark-delimited field, escape them with backslashes (eg, | |
49d635f9 | 866 | C<"like \"this\"">. |
46fc3d4c | 867 | |
ac9dac7f RGS |
868 | Alternatively, the C<Text::ParseWords> module (part of the standard |
869 | Perl distribution) lets you say: | |
68dc0745 | 870 | |
ac9dac7f RGS |
871 | use Text::ParseWords; |
872 | @new = quotewords(",", 0, $text); | |
65acb1b1 | 873 | |
68dc0745 | 874 | =head2 How do I strip blank space from the beginning/end of a string? |
875 | ||
6670e5e7 | 876 | (contributed by brian d foy) |
68dc0745 | 877 | |
6670e5e7 RGS |
878 | A substitution can do this for you. For a single line, you want to |
879 | replace all the leading or trailing whitespace with nothing. You | |
880 | can do that with a pair of substitutions. | |
68dc0745 | 881 | |
6670e5e7 RGS |
882 | s/^\s+//; |
883 | s/\s+$//; | |
68dc0745 | 884 | |
6670e5e7 RGS |
885 | You can also write that as a single substitution, although it turns |
886 | out the combined statement is slower than the separate ones. That | |
887 | might not matter to you, though. | |
68dc0745 | 888 | |
6670e5e7 | 889 | s/^\s+|\s+$//g; |
68dc0745 | 890 | |
6670e5e7 RGS |
891 | In this regular expression, the alternation matches either at the |
892 | beginning or the end of the string since the anchors have a lower | |
893 | precedence than the alternation. With the C</g> flag, the substitution | |
894 | makes all possible matches, so it gets both. Remember, the trailing | |
895 | newline matches the C<\s+>, and the C<$> anchor can match to the | |
896 | physical end of the string, so the newline disappears too. Just add | |
897 | the newline to the output, which has the added benefit of preserving | |
898 | "blank" (consisting entirely of whitespace) lines which the C<^\s+> | |
899 | would remove all by itself. | |
68dc0745 | 900 | |
6670e5e7 RGS |
901 | while( <> ) |
902 | { | |
903 | s/^\s+|\s+$//g; | |
904 | print "$_\n"; | |
905 | } | |
5a964f20 | 906 | |
6670e5e7 RGS |
907 | For a multi-line string, you can apply the regular expression |
908 | to each logical line in the string by adding the C</m> flag (for | |
909 | "multi-line"). With the C</m> flag, the C<$> matches I<before> an | |
910 | embedded newline, so it doesn't remove it. It still removes the | |
911 | newline at the end of the string. | |
912 | ||
ac9dac7f | 913 | $string =~ s/^\s+|\s+$//gm; |
6670e5e7 RGS |
914 | |
915 | Remember that lines consisting entirely of whitespace will disappear, | |
916 | since the first part of the alternation can match the entire string | |
917 | and replace it with nothing. If need to keep embedded blank lines, | |
918 | you have to do a little more work. Instead of matching any whitespace | |
919 | (since that includes a newline), just match the other whitespace. | |
920 | ||
921 | $string =~ s/^[\t\f ]+|[\t\f ]+$//mg; | |
5a964f20 | 922 | |
65acb1b1 TC |
923 | =head2 How do I pad a string with blanks or pad a number with zeroes? |
924 | ||
65acb1b1 | 925 | In the following examples, C<$pad_len> is the length to which you wish |
d92eb7b0 GS |
926 | to pad the string, C<$text> or C<$num> contains the string to be padded, |
927 | and C<$pad_char> contains the padding character. You can use a single | |
928 | character string constant instead of the C<$pad_char> variable if you | |
929 | know what it is in advance. And in the same way you can use an integer in | |
930 | place of C<$pad_len> if you know the pad length in advance. | |
65acb1b1 | 931 | |
d92eb7b0 GS |
932 | The simplest method uses the C<sprintf> function. It can pad on the left |
933 | or right with blanks and on the left with zeroes and it will not | |
934 | truncate the result. The C<pack> function can only pad strings on the | |
935 | right with blanks and it will truncate the result to a maximum length of | |
936 | C<$pad_len>. | |
65acb1b1 | 937 | |
ac9dac7f | 938 | # Left padding a string with blanks (no truncation): |
04d666b1 RGS |
939 | $padded = sprintf("%${pad_len}s", $text); |
940 | $padded = sprintf("%*s", $pad_len, $text); # same thing | |
65acb1b1 | 941 | |
ac9dac7f | 942 | # Right padding a string with blanks (no truncation): |
04d666b1 RGS |
943 | $padded = sprintf("%-${pad_len}s", $text); |
944 | $padded = sprintf("%-*s", $pad_len, $text); # same thing | |
65acb1b1 | 945 | |
ac9dac7f | 946 | # Left padding a number with 0 (no truncation): |
04d666b1 RGS |
947 | $padded = sprintf("%0${pad_len}d", $num); |
948 | $padded = sprintf("%0*d", $pad_len, $num); # same thing | |
65acb1b1 | 949 | |
ac9dac7f RGS |
950 | # Right padding a string with blanks using pack (will truncate): |
951 | $padded = pack("A$pad_len",$text); | |
65acb1b1 | 952 | |
d92eb7b0 GS |
953 | If you need to pad with a character other than blank or zero you can use |
954 | one of the following methods. They all generate a pad string with the | |
955 | C<x> operator and combine that with C<$text>. These methods do | |
956 | not truncate C<$text>. | |
65acb1b1 | 957 | |
d92eb7b0 | 958 | Left and right padding with any character, creating a new string: |
65acb1b1 | 959 | |
ac9dac7f RGS |
960 | $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; |
961 | $padded = $text . $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 962 | |
d92eb7b0 | 963 | Left and right padding with any character, modifying C<$text> directly: |
65acb1b1 | 964 | |
ac9dac7f RGS |
965 | substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); |
966 | $text .= $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 967 | |
68dc0745 | 968 | =head2 How do I extract selected columns from a string? |
969 | ||
e573f903 RGS |
970 | (contributed by brian d foy) |
971 | ||
972 | If you know where the columns that contain the data, you can | |
973 | use C<substr> to extract a single column. | |
974 | ||
975 | my $column = substr( $line, $start_column, $length ); | |
976 | ||
977 | You can use C<split> if the columns are separated by whitespace or | |
978 | some other delimiter, as long as whitespace or the delimiter cannot | |
979 | appear as part of the data. | |
980 | ||
981 | my $line = ' fred barney betty '; | |
982 | my @columns = split /\s+/, $line; | |
983 | # ( '', 'fred', 'barney', 'betty' ); | |
984 | ||
985 | my $line = 'fred||barney||betty'; | |
986 | my @columns = split /\|/, $line; | |
987 | # ( 'fred', '', 'barney', '', 'betty' ); | |
988 | ||
989 | If you want to work with comma-separated values, don't do this since | |
990 | that format is a bit more complicated. Use one of the modules that | |
991 | handle that fornat, such as C<Text::CSV>, C<Text::CSV_XS>, or | |
992 | C<Text::CSV_PP>. | |
993 | ||
994 | If you want to break apart an entire line of fixed columns, you can use | |
995 | C<unpack> with the A (ASCII) format. by using a number after the format | |
996 | specifier, you can denote the column width. See the C<pack> and C<unpack> | |
997 | entries in L<perlfunc> for more details. | |
998 | ||
999 | my @fields = unpack( $line, "A8 A8 A8 A16 A4" ); | |
1000 | ||
1001 | Note that spaces in the format argument to C<unpack> do not denote literal | |
1002 | spaces. If you have space separated data, you may want C<split> instead. | |
68dc0745 | 1003 | |
1004 | =head2 How do I find the soundex value of a string? | |
1005 | ||
7678cced RGS |
1006 | (contributed by brian d foy) |
1007 | ||
1008 | You can use the Text::Soundex module. If you want to do fuzzy or close | |
ac9dac7f RGS |
1009 | matching, you might also try the C<String::Approx>, and |
1010 | C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules. | |
68dc0745 | 1011 | |
1012 | =head2 How can I expand variables in text strings? | |
1013 | ||
e573f903 | 1014 | (contributed by brian d foy) |
5a964f20 | 1015 | |
322be77c | 1016 | If you can avoid it, don't, or if you can use a templating system, |
c195e131 RGS |
1017 | such as C<Text::Template> or C<Template> Toolkit, do that instead. You |
1018 | might even be able to get the job done with C<sprintf> or C<printf>: | |
1019 | ||
1020 | my $string = sprintf 'Say hello to %s and %s', $foo, $bar; | |
322be77c RGS |
1021 | |
1022 | However, for the one-off simple case where I don't want to pull out a | |
1023 | full templating system, I'll use a string that has two Perl scalar | |
1024 | variables in it. In this example, I want to expand C<$foo> and C<$bar> | |
c195e131 | 1025 | to their variable's values: |
e573f903 RGS |
1026 | |
1027 | my $foo = 'Fred'; | |
1028 | my $bar = 'Barney'; | |
1029 | $string = 'Say hello to $foo and $bar'; | |
1030 | ||
1031 | One way I can do this involves the substitution operator and a double | |
1032 | C</e> flag. The first C</e> evaluates C<$1> on the replacement side and | |
1033 | turns it into C<$foo>. The second /e starts with C<$foo> and replaces | |
1034 | it with its value. C<$foo>, then, turns into 'Fred', and that's finally | |
c195e131 | 1035 | what's left in the string: |
e573f903 RGS |
1036 | |
1037 | $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney' | |
322be77c | 1038 | |
e573f903 | 1039 | The C</e> will also silently ignore violations of strict, replacing |
c195e131 RGS |
1040 | undefined variable names with the empty string. Since I'm using the |
1041 | C</e> flag (twice even!), I have all of the same security problems I | |
1042 | have with C<eval> in its string form. If there's something odd in | |
1043 | C<$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then | |
1044 | I could get myself in trouble. | |
1045 | ||
1046 | To get around the security problem, I could also pull the values from | |
1047 | a hash instead of evaluating variable names. Using a single C</e>, I | |
1048 | can check the hash to ensure the value exists, and if it doesn't, I | |
1049 | can replace the missing value with a marker, in this case C<???> to | |
1050 | signal that I missed something: | |
e573f903 RGS |
1051 | |
1052 | my $string = 'This has $foo and $bar'; | |
1053 | ||
1054 | my %Replacements = ( | |
1055 | foo => 'Fred', | |
ac9dac7f | 1056 | ); |
322be77c | 1057 | |
e573f903 RGS |
1058 | # $string =~ s/\$(\w+)/$Replacements{$1}/g; |
1059 | $string =~ s/\$(\w+)/ | |
1060 | exists $Replacements{$1} ? $Replacements{$1} : '???' | |
1061 | /eg; | |
322be77c | 1062 | |
e573f903 | 1063 | print $string; |
322be77c | 1064 | |
68dc0745 | 1065 | =head2 What's wrong with always quoting "$vars"? |
1066 | ||
ac9dac7f | 1067 | The problem is that those double-quotes force |
e573f903 RGS |
1068 | stringification--coercing numbers and references into strings--even |
1069 | when you don't want them to be strings. Think of it this way: | |
1070 | double-quote expansion is used to produce new strings. If you already | |
1071 | have a string, why do you need more? | |
68dc0745 | 1072 | |
1073 | If you get used to writing odd things like these: | |
1074 | ||
ac9dac7f RGS |
1075 | print "$var"; # BAD |
1076 | $new = "$old"; # BAD | |
1077 | somefunc("$var"); # BAD | |
68dc0745 | 1078 | |
1079 | You'll be in trouble. Those should (in 99.8% of the cases) be | |
1080 | the simpler and more direct: | |
1081 | ||
ac9dac7f RGS |
1082 | print $var; |
1083 | $new = $old; | |
1084 | somefunc($var); | |
68dc0745 | 1085 | |
1086 | Otherwise, besides slowing you down, you're going to break code when | |
1087 | the thing in the scalar is actually neither a string nor a number, but | |
1088 | a reference: | |
1089 | ||
ac9dac7f RGS |
1090 | func(\@array); |
1091 | sub func { | |
1092 | my $aref = shift; | |
1093 | my $oref = "$aref"; # WRONG | |
1094 | } | |
68dc0745 | 1095 | |
1096 | You can also get into subtle problems on those few operations in Perl | |
1097 | that actually do care about the difference between a string and a | |
1098 | number, such as the magical C<++> autoincrement operator or the | |
1099 | syscall() function. | |
1100 | ||
197aec24 | 1101 | Stringification also destroys arrays. |
5a964f20 | 1102 | |
ac9dac7f RGS |
1103 | @lines = `command`; |
1104 | print "@lines"; # WRONG - extra blanks | |
1105 | print @lines; # right | |
5a964f20 | 1106 | |
04d666b1 | 1107 | =head2 Why don't my E<lt>E<lt>HERE documents work? |
68dc0745 | 1108 | |
1109 | Check for these three things: | |
1110 | ||
1111 | =over 4 | |
1112 | ||
04d666b1 | 1113 | =item There must be no space after the E<lt>E<lt> part. |
68dc0745 | 1114 | |
197aec24 | 1115 | =item There (probably) should be a semicolon at the end. |
68dc0745 | 1116 | |
197aec24 | 1117 | =item You can't (easily) have any space in front of the tag. |
68dc0745 | 1118 | |
1119 | =back | |
1120 | ||
197aec24 | 1121 | If you want to indent the text in the here document, you |
5a964f20 TC |
1122 | can do this: |
1123 | ||
1124 | # all in one | |
1125 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; | |
1126 | your text | |
1127 | goes here | |
1128 | HERE_TARGET | |
1129 | ||
1130 | But the HERE_TARGET must still be flush against the margin. | |
197aec24 | 1131 | If you want that indented also, you'll have to quote |
5a964f20 TC |
1132 | in the indentation. |
1133 | ||
1134 | ($quote = <<' FINIS') =~ s/^\s+//gm; | |
1135 | ...we will have peace, when you and all your works have | |
1136 | perished--and the works of your dark master to whom you | |
1137 | would deliver us. You are a liar, Saruman, and a corrupter | |
1138 | of men's hearts. --Theoden in /usr/src/perl/taint.c | |
1139 | FINIS | |
83ded9ee | 1140 | $quote =~ s/\s+--/\n--/; |
5a964f20 TC |
1141 | |
1142 | A nice general-purpose fixer-upper function for indented here documents | |
1143 | follows. It expects to be called with a here document as its argument. | |
1144 | It looks to see whether each line begins with a common substring, and | |
a6dd486b JB |
1145 | if so, strips that substring off. Otherwise, it takes the amount of leading |
1146 | whitespace found on the first line and removes that much off each | |
5a964f20 TC |
1147 | subsequent line. |
1148 | ||
1149 | sub fix { | |
1150 | local $_ = shift; | |
a6dd486b | 1151 | my ($white, $leader); # common whitespace and common leading string |
5a964f20 TC |
1152 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { |
1153 | ($white, $leader) = ($2, quotemeta($1)); | |
1154 | } else { | |
1155 | ($white, $leader) = (/^(\s+)/, ''); | |
1156 | } | |
1157 | s/^\s*?$leader(?:$white)?//gm; | |
1158 | return $_; | |
1159 | } | |
1160 | ||
c8db1d39 | 1161 | This works with leading special strings, dynamically determined: |
5a964f20 | 1162 | |
ac9dac7f | 1163 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; |
5a964f20 TC |
1164 | @@@ int |
1165 | @@@ runops() { | |
1166 | @@@ SAVEI32(runlevel); | |
1167 | @@@ runlevel++; | |
d92eb7b0 | 1168 | @@@ while ( op = (*op->op_ppaddr)() ); |
5a964f20 TC |
1169 | @@@ TAINT_NOT; |
1170 | @@@ return 0; | |
1171 | @@@ } | |
ac9dac7f | 1172 | MAIN_INTERPRETER_LOOP |
5a964f20 | 1173 | |
a6dd486b | 1174 | Or with a fixed amount of leading whitespace, with remaining |
5a964f20 TC |
1175 | indentation correctly preserved: |
1176 | ||
ac9dac7f | 1177 | $poem = fix<<EVER_ON_AND_ON; |
5a964f20 TC |
1178 | Now far ahead the Road has gone, |
1179 | And I must follow, if I can, | |
1180 | Pursuing it with eager feet, | |
1181 | Until it joins some larger way | |
1182 | Where many paths and errands meet. | |
1183 | And whither then? I cannot say. | |
1184 | --Bilbo in /usr/src/perl/pp_ctl.c | |
ac9dac7f | 1185 | EVER_ON_AND_ON |
5a964f20 | 1186 | |
68dc0745 | 1187 | =head1 Data: Arrays |
1188 | ||
65acb1b1 TC |
1189 | =head2 What is the difference between a list and an array? |
1190 | ||
ac9dac7f RGS |
1191 | An array has a changeable length. A list does not. An array is |
1192 | something you can push or pop, while a list is a set of values. Some | |
1193 | people make the distinction that a list is a value while an array is a | |
1194 | variable. Subroutines are passed and return lists, you put things into | |
1195 | list context, you initialize arrays with lists, and you C<foreach()> | |
1196 | across a list. C<@> variables are arrays, anonymous arrays are | |
1197 | arrays, arrays in scalar context behave like the number of elements in | |
1198 | them, subroutines access their arguments through the array C<@_>, and | |
1199 | C<push>/C<pop>/C<shift> only work on arrays. | |
65acb1b1 TC |
1200 | |
1201 | As a side note, there's no such thing as a list in scalar context. | |
1202 | When you say | |
1203 | ||
ac9dac7f | 1204 | $scalar = (2, 5, 7, 9); |
65acb1b1 | 1205 | |
d92eb7b0 | 1206 | you're using the comma operator in scalar context, so it uses the scalar |
ac9dac7f | 1207 | comma operator. There never was a list there at all! This causes the |
d92eb7b0 | 1208 | last value to be returned: 9. |
65acb1b1 | 1209 | |
68dc0745 | 1210 | =head2 What is the difference between $array[1] and @array[1]? |
1211 | ||
a6dd486b | 1212 | The former is a scalar value; the latter an array slice, making |
68dc0745 | 1213 | it a list with one (scalar) value. You should use $ when you want a |
1214 | scalar value (most of the time) and @ when you want a list with one | |
1215 | scalar value in it (very, very rarely; nearly never, in fact). | |
1216 | ||
1217 | Sometimes it doesn't make a difference, but sometimes it does. | |
1218 | For example, compare: | |
1219 | ||
ac9dac7f | 1220 | $good[0] = `some program that outputs several lines`; |
68dc0745 | 1221 | |
1222 | with | |
1223 | ||
ac9dac7f | 1224 | @bad[0] = `same program that outputs several lines`; |
68dc0745 | 1225 | |
197aec24 | 1226 | The C<use warnings> pragma and the B<-w> flag will warn you about these |
9f1b1f2d | 1227 | matters. |
68dc0745 | 1228 | |
d92eb7b0 | 1229 | =head2 How can I remove duplicate elements from a list or array? |
68dc0745 | 1230 | |
6670e5e7 | 1231 | (contributed by brian d foy) |
68dc0745 | 1232 | |
6670e5e7 RGS |
1233 | Use a hash. When you think the words "unique" or "duplicated", think |
1234 | "hash keys". | |
68dc0745 | 1235 | |
6670e5e7 RGS |
1236 | If you don't care about the order of the elements, you could just |
1237 | create the hash then extract the keys. It's not important how you | |
1238 | create that hash: just that you use C<keys> to get the unique | |
1239 | elements. | |
551e1d92 | 1240 | |
ac9dac7f RGS |
1241 | my %hash = map { $_, 1 } @array; |
1242 | # or a hash slice: @hash{ @array } = (); | |
1243 | # or a foreach: $hash{$_} = 1 foreach ( @array ); | |
1244 | ||
1245 | my @unique = keys %hash; | |
68dc0745 | 1246 | |
ac9dac7f RGS |
1247 | If you want to use a module, try the C<uniq> function from |
1248 | C<List::MoreUtils>. In list context it returns the unique elements, | |
1249 | preserving their order in the list. In scalar context, it returns the | |
1250 | number of unique elements. | |
1251 | ||
1252 | use List::MoreUtils qw(uniq); | |
1253 | ||
1254 | my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7 | |
1255 | my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7 | |
68dc0745 | 1256 | |
6670e5e7 RGS |
1257 | You can also go through each element and skip the ones you've seen |
1258 | before. Use a hash to keep track. The first time the loop sees an | |
1259 | element, that element has no key in C<%Seen>. The C<next> statement | |
1260 | creates the key and immediately uses its value, which is C<undef>, so | |
1261 | the loop continues to the C<push> and increments the value for that | |
1262 | key. The next time the loop sees that same element, its key exists in | |
1263 | the hash I<and> the value for that key is true (since it's not 0 or | |
ac9dac7f RGS |
1264 | C<undef>), so the next skips that iteration and the loop goes to the |
1265 | next element. | |
551e1d92 | 1266 | |
6670e5e7 RGS |
1267 | my @unique = (); |
1268 | my %seen = (); | |
68dc0745 | 1269 | |
6670e5e7 RGS |
1270 | foreach my $elem ( @array ) |
1271 | { | |
1272 | next if $seen{ $elem }++; | |
1273 | push @unique, $elem; | |
1274 | } | |
68dc0745 | 1275 | |
6670e5e7 RGS |
1276 | You can write this more briefly using a grep, which does the |
1277 | same thing. | |
68dc0745 | 1278 | |
ac9dac7f RGS |
1279 | my %seen = (); |
1280 | my @unique = grep { ! $seen{ $_ }++ } @array; | |
65acb1b1 | 1281 | |
ddbc1f16 | 1282 | =head2 How can I tell whether a certain element is contained in a list or array? |
5a964f20 | 1283 | |
9e72e4c6 RGS |
1284 | (portions of this answer contributed by Anno Siegel) |
1285 | ||
5a964f20 TC |
1286 | Hearing the word "in" is an I<in>dication that you probably should have |
1287 | used a hash, not a list or array, to store your data. Hashes are | |
1288 | designed to answer this question quickly and efficiently. Arrays aren't. | |
68dc0745 | 1289 | |
5a964f20 TC |
1290 | That being said, there are several ways to approach this. If you |
1291 | are going to make this query many times over arbitrary string values, | |
881bdbd4 JH |
1292 | the fastest way is probably to invert the original array and maintain a |
1293 | hash whose keys are the first array's values. | |
68dc0745 | 1294 | |
ac9dac7f RGS |
1295 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; |
1296 | %is_blue = (); | |
1297 | for (@blues) { $is_blue{$_} = 1 } | |
68dc0745 | 1298 | |
ac9dac7f RGS |
1299 | Now you can check whether C<$is_blue{$some_color}>. It might have |
1300 | been a good idea to keep the blues all in a hash in the first place. | |
68dc0745 | 1301 | |
1302 | If the values are all small integers, you could use a simple indexed | |
1303 | array. This kind of an array will take up less space: | |
1304 | ||
ac9dac7f RGS |
1305 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); |
1306 | @is_tiny_prime = (); | |
1307 | for (@primes) { $is_tiny_prime[$_] = 1 } | |
1308 | # or simply @istiny_prime[@primes] = (1) x @primes; | |
68dc0745 | 1309 | |
1310 | Now you check whether $is_tiny_prime[$some_number]. | |
1311 | ||
1312 | If the values in question are integers instead of strings, you can save | |
1313 | quite a lot of space by using bit strings instead: | |
1314 | ||
ac9dac7f RGS |
1315 | @articles = ( 1..10, 150..2000, 2017 ); |
1316 | undef $read; | |
1317 | for (@articles) { vec($read,$_,1) = 1 } | |
68dc0745 | 1318 | |
1319 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. | |
1320 | ||
9e72e4c6 RGS |
1321 | These methods guarantee fast individual tests but require a re-organization |
1322 | of the original list or array. They only pay off if you have to test | |
1323 | multiple values against the same array. | |
68dc0745 | 1324 | |
ac9dac7f | 1325 | If you are testing only once, the standard module C<List::Util> exports |
9e72e4c6 | 1326 | the function C<first> for this purpose. It works by stopping once it |
c195e131 | 1327 | finds the element. It's written in C for speed, and its Perl equivalent |
9e72e4c6 | 1328 | looks like this subroutine: |
68dc0745 | 1329 | |
9e72e4c6 RGS |
1330 | sub first (&@) { |
1331 | my $code = shift; | |
1332 | foreach (@_) { | |
1333 | return $_ if &{$code}(); | |
1334 | } | |
1335 | undef; | |
1336 | } | |
68dc0745 | 1337 | |
9e72e4c6 RGS |
1338 | If speed is of little concern, the common idiom uses grep in scalar context |
1339 | (which returns the number of items that passed its condition) to traverse the | |
1340 | entire list. This does have the benefit of telling you how many matches it | |
1341 | found, though. | |
68dc0745 | 1342 | |
9e72e4c6 | 1343 | my $is_there = grep $_ eq $whatever, @array; |
65acb1b1 | 1344 | |
9e72e4c6 RGS |
1345 | If you want to actually extract the matching elements, simply use grep in |
1346 | list context. | |
68dc0745 | 1347 | |
9e72e4c6 | 1348 | my @matches = grep $_ eq $whatever, @array; |
58103a2e | 1349 | |
68dc0745 | 1350 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? |
1351 | ||
ac9dac7f RGS |
1352 | Use a hash. Here's code to do both and more. It assumes that each |
1353 | element is unique in a given array: | |
68dc0745 | 1354 | |
ac9dac7f RGS |
1355 | @union = @intersection = @difference = (); |
1356 | %count = (); | |
1357 | foreach $element (@array1, @array2) { $count{$element}++ } | |
1358 | foreach $element (keys %count) { | |
1359 | push @union, $element; | |
1360 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; | |
1361 | } | |
68dc0745 | 1362 | |
ac9dac7f RGS |
1363 | Note that this is the I<symmetric difference>, that is, all elements |
1364 | in either A or in B but not in both. Think of it as an xor operation. | |
d92eb7b0 | 1365 | |
65acb1b1 TC |
1366 | =head2 How do I test whether two arrays or hashes are equal? |
1367 | ||
ac9dac7f RGS |
1368 | The following code works for single-level arrays. It uses a |
1369 | stringwise comparison, and does not distinguish defined versus | |
1370 | undefined empty strings. Modify if you have other needs. | |
65acb1b1 | 1371 | |
ac9dac7f | 1372 | $are_equal = compare_arrays(\@frogs, \@toads); |
65acb1b1 | 1373 | |
ac9dac7f RGS |
1374 | sub compare_arrays { |
1375 | my ($first, $second) = @_; | |
1376 | no warnings; # silence spurious -w undef complaints | |
1377 | return 0 unless @$first == @$second; | |
1378 | for (my $i = 0; $i < @$first; $i++) { | |
1379 | return 0 if $first->[$i] ne $second->[$i]; | |
1380 | } | |
1381 | return 1; | |
1382 | } | |
65acb1b1 TC |
1383 | |
1384 | For multilevel structures, you may wish to use an approach more | |
ac9dac7f | 1385 | like this one. It uses the CPAN module C<FreezeThaw>: |
65acb1b1 | 1386 | |
ac9dac7f RGS |
1387 | use FreezeThaw qw(cmpStr); |
1388 | @a = @b = ( "this", "that", [ "more", "stuff" ] ); | |
65acb1b1 | 1389 | |
ac9dac7f RGS |
1390 | printf "a and b contain %s arrays\n", |
1391 | cmpStr(\@a, \@b) == 0 | |
1392 | ? "the same" | |
1393 | : "different"; | |
65acb1b1 | 1394 | |
ac9dac7f RGS |
1395 | This approach also works for comparing hashes. Here we'll demonstrate |
1396 | two different answers: | |
65acb1b1 | 1397 | |
ac9dac7f | 1398 | use FreezeThaw qw(cmpStr cmpStrHard); |
65acb1b1 | 1399 | |
ac9dac7f RGS |
1400 | %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); |
1401 | $a{EXTRA} = \%b; | |
1402 | $b{EXTRA} = \%a; | |
65acb1b1 | 1403 | |
ac9dac7f | 1404 | printf "a and b contain %s hashes\n", |
65acb1b1 TC |
1405 | cmpStr(\%a, \%b) == 0 ? "the same" : "different"; |
1406 | ||
ac9dac7f | 1407 | printf "a and b contain %s hashes\n", |
65acb1b1 TC |
1408 | cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; |
1409 | ||
1410 | ||
1411 | The first reports that both those the hashes contain the same data, | |
1412 | while the second reports that they do not. Which you prefer is left as | |
1413 | an exercise to the reader. | |
1414 | ||
68dc0745 | 1415 | =head2 How do I find the first array element for which a condition is true? |
1416 | ||
49d635f9 | 1417 | To find the first array element which satisfies a condition, you can |
ac9dac7f RGS |
1418 | use the C<first()> function in the C<List::Util> module, which comes |
1419 | with Perl 5.8. This example finds the first element that contains | |
1420 | "Perl". | |
49d635f9 RGS |
1421 | |
1422 | use List::Util qw(first); | |
197aec24 | 1423 | |
49d635f9 | 1424 | my $element = first { /Perl/ } @array; |
197aec24 | 1425 | |
ac9dac7f | 1426 | If you cannot use C<List::Util>, you can make your own loop to do the |
49d635f9 RGS |
1427 | same thing. Once you find the element, you stop the loop with last. |
1428 | ||
1429 | my $found; | |
ac9dac7f | 1430 | foreach ( @array ) { |
6670e5e7 | 1431 | if( /Perl/ ) { $found = $_; last } |
49d635f9 RGS |
1432 | } |
1433 | ||
1434 | If you want the array index, you can iterate through the indices | |
1435 | and check the array element at each index until you find one | |
1436 | that satisfies the condition. | |
1437 | ||
197aec24 | 1438 | my( $found, $index ) = ( undef, -1 ); |
ac9dac7f RGS |
1439 | for( $i = 0; $i < @array; $i++ ) { |
1440 | if( $array[$i] =~ /Perl/ ) { | |
6670e5e7 RGS |
1441 | $found = $array[$i]; |
1442 | $index = $i; | |
1443 | last; | |
1444 | } | |
1445 | } | |
68dc0745 | 1446 | |
1447 | =head2 How do I handle linked lists? | |
1448 | ||
1449 | In general, you usually don't need a linked list in Perl, since with | |
ac9dac7f RGS |
1450 | regular arrays, you can push and pop or shift and unshift at either |
1451 | end, or you can use splice to add and/or remove arbitrary number of | |
ac003c96 | 1452 | elements at arbitrary points. Both pop and shift are O(1) |
ac9dac7f RGS |
1453 | operations on Perl's dynamic arrays. In the absence of shifts and |
1454 | pops, push in general needs to reallocate on the order every log(N) | |
1455 | times, and unshift will need to copy pointers each time. | |
68dc0745 | 1456 | |
1457 | If you really, really wanted, you could use structures as described in | |
ac9dac7f RGS |
1458 | L<perldsc> or L<perltoot> and do just what the algorithm book tells |
1459 | you to do. For example, imagine a list node like this: | |
65acb1b1 | 1460 | |
ac9dac7f RGS |
1461 | $node = { |
1462 | VALUE => 42, | |
1463 | LINK => undef, | |
1464 | }; | |
65acb1b1 TC |
1465 | |
1466 | You could walk the list this way: | |
1467 | ||
ac9dac7f RGS |
1468 | print "List: "; |
1469 | for ($node = $head; $node; $node = $node->{LINK}) { | |
1470 | print $node->{VALUE}, " "; | |
1471 | } | |
1472 | print "\n"; | |
65acb1b1 | 1473 | |
a6dd486b | 1474 | You could add to the list this way: |
65acb1b1 | 1475 | |
ac9dac7f RGS |
1476 | my ($head, $tail); |
1477 | $tail = append($head, 1); # grow a new head | |
1478 | for $value ( 2 .. 10 ) { | |
1479 | $tail = append($tail, $value); | |
1480 | } | |
65acb1b1 | 1481 | |
ac9dac7f RGS |
1482 | sub append { |
1483 | my($list, $value) = @_; | |
1484 | my $node = { VALUE => $value }; | |
1485 | if ($list) { | |
1486 | $node->{LINK} = $list->{LINK}; | |
1487 | $list->{LINK} = $node; | |
1488 | } | |
1489 | else { | |
1490 | $_[0] = $node; # replace caller's version | |
1491 | } | |
1492 | return $node; | |
1493 | } | |
65acb1b1 TC |
1494 | |
1495 | But again, Perl's built-in are virtually always good enough. | |
68dc0745 | 1496 | |
1497 | =head2 How do I handle circular lists? | |
1498 | ||
1499 | Circular lists could be handled in the traditional fashion with linked | |
1500 | lists, or you could just do something like this with an array: | |
1501 | ||
ac9dac7f RGS |
1502 | unshift(@array, pop(@array)); # the last shall be first |
1503 | push(@array, shift(@array)); # and vice versa | |
1504 | ||
1505 | You can also use C<Tie::Cycle>: | |
1506 | ||
1507 | use Tie::Cycle; | |
1508 | ||
1509 | tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ]; | |
1510 | ||
1511 | print $cycle; # FFFFFF | |
1512 | print $cycle; # 000000 | |
1513 | print $cycle; # FFFF00 | |
68dc0745 | 1514 | |
1515 | =head2 How do I shuffle an array randomly? | |
1516 | ||
45bbf655 JH |
1517 | If you either have Perl 5.8.0 or later installed, or if you have |
1518 | Scalar-List-Utils 1.03 or later installed, you can say: | |
1519 | ||
ac9dac7f | 1520 | use List::Util 'shuffle'; |
45bbf655 JH |
1521 | |
1522 | @shuffled = shuffle(@list); | |
1523 | ||
f05bbc40 | 1524 | If not, you can use a Fisher-Yates shuffle. |
5a964f20 | 1525 | |
ac9dac7f RGS |
1526 | sub fisher_yates_shuffle { |
1527 | my $deck = shift; # $deck is a reference to an array | |
1528 | my $i = @$deck; | |
1529 | while (--$i) { | |
1530 | my $j = int rand ($i+1); | |
1531 | @$deck[$i,$j] = @$deck[$j,$i]; | |
1532 | } | |
1533 | } | |
5a964f20 | 1534 | |
ac9dac7f RGS |
1535 | # shuffle my mpeg collection |
1536 | # | |
1537 | my @mpeg = <audio/*/*.mp3>; | |
1538 | fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place | |
1539 | print @mpeg; | |
5a964f20 | 1540 | |
45bbf655 | 1541 | Note that the above implementation shuffles an array in place, |
ac9dac7f | 1542 | unlike the C<List::Util::shuffle()> which takes a list and returns |
45bbf655 JH |
1543 | a new shuffled list. |
1544 | ||
d92eb7b0 | 1545 | You've probably seen shuffling algorithms that work using splice, |
a6dd486b | 1546 | randomly picking another element to swap the current element with |
68dc0745 | 1547 | |
ac9dac7f RGS |
1548 | srand; |
1549 | @new = (); | |
1550 | @old = 1 .. 10; # just a demo | |
1551 | while (@old) { | |
1552 | push(@new, splice(@old, rand @old, 1)); | |
1553 | } | |
68dc0745 | 1554 | |
ac9dac7f RGS |
1555 | This is bad because splice is already O(N), and since you do it N |
1556 | times, you just invented a quadratic algorithm; that is, O(N**2). | |
1557 | This does not scale, although Perl is so efficient that you probably | |
1558 | won't notice this until you have rather largish arrays. | |
68dc0745 | 1559 | |
1560 | =head2 How do I process/modify each element of an array? | |
1561 | ||
1562 | Use C<for>/C<foreach>: | |
1563 | ||
ac9dac7f | 1564 | for (@lines) { |
6670e5e7 RGS |
1565 | s/foo/bar/; # change that word |
1566 | tr/XZ/ZX/; # swap those letters | |
ac9dac7f | 1567 | } |
68dc0745 | 1568 | |
1569 | Here's another; let's compute spherical volumes: | |
1570 | ||
ac9dac7f | 1571 | for (@volumes = @radii) { # @volumes has changed parts |
6670e5e7 RGS |
1572 | $_ **= 3; |
1573 | $_ *= (4/3) * 3.14159; # this will be constant folded | |
ac9dac7f | 1574 | } |
197aec24 | 1575 | |
ac9dac7f | 1576 | which can also be done with C<map()> which is made to transform |
49d635f9 RGS |
1577 | one list into another: |
1578 | ||
1579 | @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii; | |
68dc0745 | 1580 | |
76817d6d JH |
1581 | If you want to do the same thing to modify the values of the |
1582 | hash, you can use the C<values> function. As of Perl 5.6 | |
1583 | the values are not copied, so if you modify $orbit (in this | |
1584 | case), you modify the value. | |
5a964f20 | 1585 | |
ac9dac7f | 1586 | for $orbit ( values %orbits ) { |
6670e5e7 | 1587 | ($orbit **= 3) *= (4/3) * 3.14159; |
ac9dac7f | 1588 | } |
818c4caa | 1589 | |
76817d6d JH |
1590 | Prior to perl 5.6 C<values> returned copies of the values, |
1591 | so older perl code often contains constructions such as | |
1592 | C<@orbits{keys %orbits}> instead of C<values %orbits> where | |
1593 | the hash is to be modified. | |
818c4caa | 1594 | |
68dc0745 | 1595 | =head2 How do I select a random element from an array? |
1596 | ||
ac9dac7f | 1597 | Use the C<rand()> function (see L<perlfunc/rand>): |
68dc0745 | 1598 | |
ac9dac7f RGS |
1599 | $index = rand @array; |
1600 | $element = $array[$index]; | |
68dc0745 | 1601 | |
793f5136 | 1602 | Or, simply: |
ac9dac7f RGS |
1603 | |
1604 | my $element = $array[ rand @array ]; | |
5a964f20 | 1605 | |
68dc0745 | 1606 | =head2 How do I permute N elements of a list? |
c195e131 RGS |
1607 | X<List::Permuter> X<permute> X<Algorithm::Loops> X<Knuth> |
1608 | X<The Art of Computer Programming> X<Fischer-Krause> | |
68dc0745 | 1609 | |
c195e131 | 1610 | Use the C<List::Permutor> module on CPAN. If the list is actually an |
ac9dac7f | 1611 | array, try the C<Algorithm::Permute> module (also on CPAN). It's |
c195e131 | 1612 | written in XS code and is very efficient: |
49d635f9 RGS |
1613 | |
1614 | use Algorithm::Permute; | |
c195e131 | 1615 | |
49d635f9 RGS |
1616 | my @array = 'a'..'d'; |
1617 | my $p_iterator = Algorithm::Permute->new ( \@array ); | |
c195e131 | 1618 | |
49d635f9 RGS |
1619 | while (my @perm = $p_iterator->next) { |
1620 | print "next permutation: (@perm)\n"; | |
ac9dac7f | 1621 | } |
49d635f9 | 1622 | |
197aec24 RGS |
1623 | For even faster execution, you could do: |
1624 | ||
ac9dac7f | 1625 | use Algorithm::Permute; |
c195e131 | 1626 | |
ac9dac7f | 1627 | my @array = 'a'..'d'; |
c195e131 | 1628 | |
ac9dac7f RGS |
1629 | Algorithm::Permute::permute { |
1630 | print "next permutation: (@array)\n"; | |
1631 | } @array; | |
197aec24 | 1632 | |
c195e131 RGS |
1633 | Here's a little program that generates all permutations of all the |
1634 | words on each line of input. The algorithm embodied in the | |
1635 | C<permute()> function is discussed in Volume 4 (still unpublished) of | |
1636 | Knuth's I<The Art of Computer Programming> and will work on any list: | |
49d635f9 RGS |
1637 | |
1638 | #!/usr/bin/perl -n | |
ac003c96 | 1639 | # Fischer-Krause ordered permutation generator |
49d635f9 RGS |
1640 | |
1641 | sub permute (&@) { | |
1642 | my $code = shift; | |
1643 | my @idx = 0..$#_; | |
1644 | while ( $code->(@_[@idx]) ) { | |
1645 | my $p = $#idx; | |
1646 | --$p while $idx[$p-1] > $idx[$p]; | |
1647 | my $q = $p or return; | |
1648 | push @idx, reverse splice @idx, $p; | |
1649 | ++$q while $idx[$p-1] > $idx[$q]; | |
1650 | @idx[$p-1,$q]=@idx[$q,$p-1]; | |
1651 | } | |
68dc0745 | 1652 | } |
68dc0745 | 1653 | |
c195e131 RGS |
1654 | permute { print "@_\n" } split; |
1655 | ||
1656 | The C<Algorithm::Loops> module also provides the C<NextPermute> and | |
1657 | C<NextPermuteNum> functions which efficiently find all unique permutations | |
1658 | of an array, even if it contains duplicate values, modifying it in-place: | |
1659 | if its elements are in reverse-sorted order then the array is reversed, | |
1660 | making it sorted, and it returns false; otherwise the next | |
1661 | permutation is returned. | |
1662 | ||
1663 | C<NextPermute> uses string order and C<NextPermuteNum> numeric order, so | |
1664 | you can enumerate all the permutations of C<0..9> like this: | |
1665 | ||
1666 | use Algorithm::Loops qw(NextPermuteNum); | |
1667 | ||
1668 | my @list= 0..9; | |
1669 | do { print "@list\n" } while NextPermuteNum @list; | |
b8d2732a | 1670 | |
68dc0745 | 1671 | =head2 How do I sort an array by (anything)? |
1672 | ||
1673 | Supply a comparison function to sort() (described in L<perlfunc/sort>): | |
1674 | ||
ac9dac7f | 1675 | @list = sort { $a <=> $b } @list; |
68dc0745 | 1676 | |
1677 | The default sort function is cmp, string comparison, which would | |
c47ff5f1 | 1678 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is |
68dc0745 | 1679 | the numerical comparison operator. |
1680 | ||
1681 | If you have a complicated function needed to pull out the part you | |
1682 | want to sort on, then don't do it inside the sort function. Pull it | |
1683 | out first, because the sort BLOCK can be called many times for the | |
1684 | same element. Here's an example of how to pull out the first word | |
1685 | after the first number on each item, and then sort those words | |
1686 | case-insensitively. | |
1687 | ||
ac9dac7f RGS |
1688 | @idx = (); |
1689 | for (@data) { | |
1690 | ($item) = /\d+\s*(\S+)/; | |
1691 | push @idx, uc($item); | |
1692 | } | |
1693 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; | |
68dc0745 | 1694 | |
a6dd486b | 1695 | which could also be written this way, using a trick |
68dc0745 | 1696 | that's come to be known as the Schwartzian Transform: |
1697 | ||
ac9dac7f RGS |
1698 | @sorted = map { $_->[0] } |
1699 | sort { $a->[1] cmp $b->[1] } | |
1700 | map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; | |
68dc0745 | 1701 | |
1702 | If you need to sort on several fields, the following paradigm is useful. | |
1703 | ||
ac9dac7f RGS |
1704 | @sorted = sort { |
1705 | field1($a) <=> field1($b) || | |
1706 | field2($a) cmp field2($b) || | |
1707 | field3($a) cmp field3($b) | |
1708 | } @data; | |
68dc0745 | 1709 | |
1710 | This can be conveniently combined with precalculation of keys as given | |
1711 | above. | |
1712 | ||
379e39d7 | 1713 | See the F<sort> article in the "Far More Than You Ever Wanted |
49d635f9 | 1714 | To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for |
06a5f41f | 1715 | more about this approach. |
68dc0745 | 1716 | |
ac9dac7f | 1717 | See also the question later in L<perlfaq4> on sorting hashes. |
68dc0745 | 1718 | |
1719 | =head2 How do I manipulate arrays of bits? | |
1720 | ||
ac9dac7f RGS |
1721 | Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise |
1722 | operations. | |
1723 | ||
1724 | For example, this sets C<$vec> to have bit N set if C<$ints[N]> was | |
1725 | set: | |
1726 | ||
1727 | $vec = ''; | |
1728 | foreach(@ints) { vec($vec,$_,1) = 1 } | |
1729 | ||
1730 | Here's how, given a vector in C<$vec>, you can get those bits into your | |
1731 | C<@ints> array: | |
1732 | ||
1733 | sub bitvec_to_list { | |
1734 | my $vec = shift; | |
1735 | my @ints; | |
1736 | # Find null-byte density then select best algorithm | |
1737 | if ($vec =~ tr/\0// / length $vec > 0.95) { | |
1738 | use integer; | |
1739 | my $i; | |
1740 | ||
1741 | # This method is faster with mostly null-bytes | |
1742 | while($vec =~ /[^\0]/g ) { | |
1743 | $i = -9 + 8 * pos $vec; | |
1744 | push @ints, $i if vec($vec, ++$i, 1); | |
1745 | push @ints, $i if vec($vec, ++$i, 1); | |
1746 | push @ints, $i if vec($vec, ++$i, 1); | |
1747 | push @ints, $i if vec($vec, ++$i, 1); | |
1748 | push @ints, $i if vec($vec, ++$i, 1); | |
1749 | push @ints, $i if vec($vec, ++$i, 1); | |
1750 | push @ints, $i if vec($vec, ++$i, 1); | |
1751 | push @ints, $i if vec($vec, ++$i, 1); | |
1752 | } | |
1753 | } | |
1754 | else { | |
1755 | # This method is a fast general algorithm | |
1756 | use integer; | |
1757 | my $bits = unpack "b*", $vec; | |
1758 | push @ints, 0 if $bits =~ s/^(\d)// && $1; | |
1759 | push @ints, pos $bits while($bits =~ /1/g); | |
1760 | } | |
1761 | ||
1762 | return \@ints; | |
1763 | } | |
68dc0745 | 1764 | |
1765 | This method gets faster the more sparse the bit vector is. | |
1766 | (Courtesy of Tim Bunce and Winfried Koenig.) | |
1767 | ||
76817d6d JH |
1768 | You can make the while loop a lot shorter with this suggestion |
1769 | from Benjamin Goldberg: | |
1770 | ||
1771 | while($vec =~ /[^\0]+/g ) { | |
ac9dac7f RGS |
1772 | push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8; |
1773 | } | |
76817d6d | 1774 | |
ac9dac7f | 1775 | Or use the CPAN module C<Bit::Vector>: |
cc30d1a7 | 1776 | |
ac9dac7f RGS |
1777 | $vector = Bit::Vector->new($num_of_bits); |
1778 | $vector->Index_List_Store(@ints); | |
1779 | @ints = $vector->Index_List_Read(); | |
cc30d1a7 | 1780 | |
ac9dac7f RGS |
1781 | C<Bit::Vector> provides efficient methods for bit vector, sets of |
1782 | small integers and "big int" math. | |
cc30d1a7 JH |
1783 | |
1784 | Here's a more extensive illustration using vec(): | |
65acb1b1 | 1785 | |
ac9dac7f RGS |
1786 | # vec demo |
1787 | $vector = "\xff\x0f\xef\xfe"; | |
1788 | print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", | |
65acb1b1 | 1789 | unpack("N", $vector), "\n"; |
ac9dac7f RGS |
1790 | $is_set = vec($vector, 23, 1); |
1791 | print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; | |
65acb1b1 | 1792 | pvec($vector); |
65acb1b1 | 1793 | |
ac9dac7f RGS |
1794 | set_vec(1,1,1); |
1795 | set_vec(3,1,1); | |
1796 | set_vec(23,1,1); | |
1797 | ||
1798 | set_vec(3,1,3); | |
1799 | set_vec(3,2,3); | |
1800 | set_vec(3,4,3); | |
1801 | set_vec(3,4,7); | |
1802 | set_vec(3,8,3); | |
1803 | set_vec(3,8,7); | |
1804 | ||
1805 | set_vec(0,32,17); | |
1806 | set_vec(1,32,17); | |
1807 | ||
1808 | sub set_vec { | |
1809 | my ($offset, $width, $value) = @_; | |
1810 | my $vector = ''; | |
1811 | vec($vector, $offset, $width) = $value; | |
1812 | print "offset=$offset width=$width value=$value\n"; | |
1813 | pvec($vector); | |
1814 | } | |
65acb1b1 | 1815 | |
ac9dac7f RGS |
1816 | sub pvec { |
1817 | my $vector = shift; | |
1818 | my $bits = unpack("b*", $vector); | |
1819 | my $i = 0; | |
1820 | my $BASE = 8; | |
1821 | ||
1822 | print "vector length in bytes: ", length($vector), "\n"; | |
1823 | @bytes = unpack("A8" x length($vector), $bits); | |
1824 | print "bits are: @bytes\n\n"; | |
1825 | } | |
65acb1b1 | 1826 | |
68dc0745 | 1827 | =head2 Why does defined() return true on empty arrays and hashes? |
1828 | ||
65acb1b1 TC |
1829 | The short story is that you should probably only use defined on scalars or |
1830 | functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> | |
1831 | in the 5.004 release or later of Perl for more detail. | |
68dc0745 | 1832 | |
1833 | =head1 Data: Hashes (Associative Arrays) | |
1834 | ||
1835 | =head2 How do I process an entire hash? | |
1836 | ||
ee891a00 RGS |
1837 | (contributed by brian d foy) |
1838 | ||
1839 | There are a couple of ways that you can process an entire hash. You | |
1840 | can get a list of keys, then go through each key, or grab a one | |
1841 | key-value pair at a time. | |
68dc0745 | 1842 | |
ee891a00 RGS |
1843 | To go through all of the keys, use the C<keys> function. This extracts |
1844 | all of the keys of the hash and gives them back to you as a list. You | |
1845 | can then get the value through the particular key you're processing: | |
1846 | ||
1847 | foreach my $key ( keys %hash ) { | |
1848 | my $value = $hash{$key} | |
1849 | ... | |
ac9dac7f | 1850 | } |
68dc0745 | 1851 | |
ee891a00 RGS |
1852 | Once you have the list of keys, you can process that list before you |
1853 | process the hashh elements. For instance, you can sort the keys so you | |
1854 | can process them in lexical order: | |
1855 | ||
1856 | foreach my $key ( sort keys %hash ) { | |
1857 | my $value = $hash{$key} | |
1858 | ... | |
1859 | } | |
1860 | ||
1861 | Or, you might want to only process some of the items. If you only want | |
1862 | to deal with the keys that start with C<text:>, you can select just | |
1863 | those using C<grep>: | |
1864 | ||
1865 | foreach my $key ( grep /^text:/, keys %hash ) { | |
1866 | my $value = $hash{$key} | |
1867 | ... | |
1868 | } | |
1869 | ||
1870 | If the hash is very large, you might not want to create a long list of | |
1871 | keys. To save some memory, you can grab on key-value pair at a time using | |
1872 | C<each()>, which returns a pair you haven't seen yet: | |
1873 | ||
1874 | while( my( $key, $value ) = each( %hash ) ) { | |
1875 | ... | |
1876 | } | |
1877 | ||
1878 | The C<each> operator returns the pairs in apparently random order, so if | |
1879 | ordering matters to you, you'll have to stick with the C<keys> method. | |
1880 | ||
1881 | The C<each()> operator can be a bit tricky though. You can't add or | |
1882 | delete keys of the hash while you're using it without possibly | |
1883 | skipping or re-processing some pairs after Perl internally rehashes | |
1884 | all of the elements. Additionally, a hash has only one iterator, so if | |
1885 | you use C<keys>, C<values>, or C<each> on the same hash, you can reset | |
1886 | the iterator and mess up your processing. See the C<each> entry in | |
1887 | L<perlfunc> for more details. | |
68dc0745 | 1888 | |
1889 | =head2 What happens if I add or remove keys from a hash while iterating over it? | |
1890 | ||
28b41a80 | 1891 | (contributed by brian d foy) |
d92eb7b0 | 1892 | |
28b41a80 | 1893 | The easy answer is "Don't do that!" |
d92eb7b0 | 1894 | |
28b41a80 RGS |
1895 | If you iterate through the hash with each(), you can delete the key |
1896 | most recently returned without worrying about it. If you delete or add | |
1897 | other keys, the iterator may skip or double up on them since perl | |
1898 | may rearrange the hash table. See the | |
1899 | entry for C<each()> in L<perlfunc>. | |
68dc0745 | 1900 | |
1901 | =head2 How do I look up a hash element by value? | |
1902 | ||
1903 | Create a reverse hash: | |
1904 | ||
ac9dac7f RGS |
1905 | %by_value = reverse %by_key; |
1906 | $key = $by_value{$value}; | |
68dc0745 | 1907 | |
1908 | That's not particularly efficient. It would be more space-efficient | |
1909 | to use: | |
1910 | ||
ac9dac7f RGS |
1911 | while (($key, $value) = each %by_key) { |
1912 | $by_value{$value} = $key; | |
1913 | } | |
68dc0745 | 1914 | |
d92eb7b0 GS |
1915 | If your hash could have repeated values, the methods above will only find |
1916 | one of the associated keys. This may or may not worry you. If it does | |
1917 | worry you, you can always reverse the hash into a hash of arrays instead: | |
1918 | ||
ac9dac7f RGS |
1919 | while (($key, $value) = each %by_key) { |
1920 | push @{$key_list_by_value{$value}}, $key; | |
1921 | } | |
68dc0745 | 1922 | |
1923 | =head2 How can I know how many entries are in a hash? | |
1924 | ||
1925 | If you mean how many keys, then all you have to do is | |
875e5c2f | 1926 | use the keys() function in a scalar context: |
68dc0745 | 1927 | |
875e5c2f | 1928 | $num_keys = keys %hash; |
68dc0745 | 1929 | |
197aec24 RGS |
1930 | The keys() function also resets the iterator, which means that you may |
1931 | see strange results if you use this between uses of other hash operators | |
875e5c2f | 1932 | such as each(). |
68dc0745 | 1933 | |
1934 | =head2 How do I sort a hash (optionally by value instead of key)? | |
1935 | ||
a05e4845 RGS |
1936 | (contributed by brian d foy) |
1937 | ||
1938 | To sort a hash, start with the keys. In this example, we give the list of | |
1939 | keys to the sort function which then compares them ASCIIbetically (which | |
1940 | might be affected by your locale settings). The output list has the keys | |
1941 | in ASCIIbetical order. Once we have the keys, we can go through them to | |
1942 | create a report which lists the keys in ASCIIbetical order. | |
1943 | ||
1944 | my @keys = sort { $a cmp $b } keys %hash; | |
58103a2e | 1945 | |
a05e4845 RGS |
1946 | foreach my $key ( @keys ) |
1947 | { | |
1948 | printf "%-20s %6d\n", $key, $hash{$value}; | |
1949 | } | |
1950 | ||
58103a2e | 1951 | We could get more fancy in the C<sort()> block though. Instead of |
a05e4845 | 1952 | comparing the keys, we can compute a value with them and use that |
58103a2e | 1953 | value as the comparison. |
a05e4845 RGS |
1954 | |
1955 | For instance, to make our report order case-insensitive, we use | |
58103a2e | 1956 | the C<\L> sequence in a double-quoted string to make everything |
a05e4845 RGS |
1957 | lowercase. The C<sort()> block then compares the lowercased |
1958 | values to determine in which order to put the keys. | |
1959 | ||
1960 | my @keys = sort { "\L$a" cmp "\L$b" } keys %hash; | |
58103a2e | 1961 | |
a05e4845 | 1962 | Note: if the computation is expensive or the hash has many elements, |
58103a2e | 1963 | you may want to look at the Schwartzian Transform to cache the |
a05e4845 RGS |
1964 | computation results. |
1965 | ||
1966 | If we want to sort by the hash value instead, we use the hash key | |
1967 | to look it up. We still get out a list of keys, but this time they | |
1968 | are ordered by their value. | |
1969 | ||
1970 | my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash; | |
1971 | ||
1972 | From there we can get more complex. If the hash values are the same, | |
1973 | we can provide a secondary sort on the hash key. | |
1974 | ||
58103a2e RGS |
1975 | my @keys = sort { |
1976 | $hash{$a} <=> $hash{$b} | |
a05e4845 RGS |
1977 | or |
1978 | "\L$a" cmp "\L$b" | |
1979 | } keys %hash; | |
68dc0745 | 1980 | |
1981 | =head2 How can I always keep my hash sorted? | |
ac9dac7f | 1982 | X<hash tie sort DB_File Tie::IxHash> |
68dc0745 | 1983 | |
ac9dac7f RGS |
1984 | You can look into using the C<DB_File> module and C<tie()> using the |
1985 | C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory | |
1986 | Databases">. The C<Tie::IxHash> module from CPAN might also be | |
1987 | instructive. Although this does keep your hash sorted, you might not | |
1988 | like the slow down you suffer from the tie interface. Are you sure you | |
1989 | need to do this? :) | |
68dc0745 | 1990 | |
1991 | =head2 What's the difference between "delete" and "undef" with hashes? | |
1992 | ||
92993692 JH |
1993 | Hashes contain pairs of scalars: the first is the key, the |
1994 | second is the value. The key will be coerced to a string, | |
1995 | although the value can be any kind of scalar: string, | |
ac9dac7f | 1996 | number, or reference. If a key C<$key> is present in |
92993692 JH |
1997 | %hash, C<exists($hash{$key})> will return true. The value |
1998 | for a given key can be C<undef>, in which case | |
1999 | C<$hash{$key}> will be C<undef> while C<exists $hash{$key}> | |
2000 | will return true. This corresponds to (C<$key>, C<undef>) | |
2001 | being in the hash. | |
68dc0745 | 2002 | |
ac9dac7f | 2003 | Pictures help... here's the C<%hash> table: |
68dc0745 | 2004 | |
2005 | keys values | |
2006 | +------+------+ | |
2007 | | a | 3 | | |
2008 | | x | 7 | | |
2009 | | d | 0 | | |
2010 | | e | 2 | | |
2011 | +------+------+ | |
2012 | ||
2013 | And these conditions hold | |
2014 | ||
92993692 JH |
2015 | $hash{'a'} is true |
2016 | $hash{'d'} is false | |
2017 | defined $hash{'d'} is true | |
2018 | defined $hash{'a'} is true | |
e9d185f8 | 2019 | exists $hash{'a'} is true (Perl 5 only) |
92993692 | 2020 | grep ($_ eq 'a', keys %hash) is true |
68dc0745 | 2021 | |
2022 | If you now say | |
2023 | ||
92993692 | 2024 | undef $hash{'a'} |
68dc0745 | 2025 | |
2026 | your table now reads: | |
2027 | ||
2028 | ||
2029 | keys values | |
2030 | +------+------+ | |
2031 | | a | undef| | |
2032 | | x | 7 | | |
2033 | | d | 0 | | |
2034 | | e | 2 | | |
2035 | +------+------+ | |
2036 | ||
2037 | and these conditions now hold; changes in caps: | |
2038 | ||
92993692 JH |
2039 | $hash{'a'} is FALSE |
2040 | $hash{'d'} is false | |
2041 | defined $hash{'d'} is true | |
2042 | defined $hash{'a'} is FALSE | |
e9d185f8 | 2043 | exists $hash{'a'} is true (Perl 5 only) |
92993692 | 2044 | grep ($_ eq 'a', keys %hash) is true |
68dc0745 | 2045 | |
2046 | Notice the last two: you have an undef value, but a defined key! | |
2047 | ||
2048 | Now, consider this: | |
2049 | ||
92993692 | 2050 | delete $hash{'a'} |
68dc0745 | 2051 | |
2052 | your table now reads: | |
2053 | ||
2054 | keys values | |
2055 | +------+------+ | |
2056 | | x | 7 | | |
2057 | | d | 0 | | |
2058 | | e | 2 | | |
2059 | +------+------+ | |
2060 | ||
2061 | and these conditions now hold; changes in caps: | |
2062 | ||
92993692 JH |
2063 | $hash{'a'} is false |
2064 | $hash{'d'} is false | |
2065 | defined $hash{'d'} is true | |
2066 | defined $hash{'a'} is false | |
e9d185f8 | 2067 | exists $hash{'a'} is FALSE (Perl 5 only) |
92993692 | 2068 | grep ($_ eq 'a', keys %hash) is FALSE |
68dc0745 | 2069 | |
2070 | See, the whole entry is gone! | |
2071 | ||
2072 | =head2 Why don't my tied hashes make the defined/exists distinction? | |
2073 | ||
92993692 JH |
2074 | This depends on the tied hash's implementation of EXISTS(). |
2075 | For example, there isn't the concept of undef with hashes | |
2076 | that are tied to DBM* files. It also means that exists() and | |
2077 | defined() do the same thing with a DBM* file, and what they | |
2078 | end up doing is not what they do with ordinary hashes. | |
68dc0745 | 2079 | |
2080 | =head2 How do I reset an each() operation part-way through? | |
2081 | ||
fb2fe781 RGS |
2082 | (contributed by brian d foy) |
2083 | ||
2084 | You can use the C<keys> or C<values> functions to reset C<each>. To | |
2085 | simply reset the iterator used by C<each> without doing anything else, | |
2086 | use one of them in void context: | |
2087 | ||
2088 | keys %hash; # resets iterator, nothing else. | |
2089 | values %hash; # resets iterator, nothing else. | |
2090 | ||
2091 | See the documentation for C<each> in L<perlfunc>. | |
68dc0745 | 2092 | |
2093 | =head2 How can I get the unique keys from two hashes? | |
2094 | ||
d92eb7b0 GS |
2095 | First you extract the keys from the hashes into lists, then solve |
2096 | the "removing duplicates" problem described above. For example: | |
68dc0745 | 2097 | |
ac9dac7f RGS |
2098 | %seen = (); |
2099 | for $element (keys(%foo), keys(%bar)) { | |
2100 | $seen{$element}++; | |
2101 | } | |
2102 | @uniq = keys %seen; | |
68dc0745 | 2103 | |
2104 | Or more succinctly: | |
2105 | ||
ac9dac7f | 2106 | @uniq = keys %{{%foo,%bar}}; |
68dc0745 | 2107 | |
2108 | Or if you really want to save space: | |
2109 | ||
ac9dac7f RGS |
2110 | %seen = (); |
2111 | while (defined ($key = each %foo)) { | |
2112 | $seen{$key}++; | |
2113 | } | |
2114 | while (defined ($key = each %bar)) { | |
2115 | $seen{$key}++; | |
2116 | } | |
2117 | @uniq = keys %seen; | |
68dc0745 | 2118 | |
2119 | =head2 How can I store a multidimensional array in a DBM file? | |
2120 | ||
2121 | Either stringify the structure yourself (no fun), or else | |
2122 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer | |
2123 | it on top of either DB_File or GDBM_File. | |
2124 | ||
2125 | =head2 How can I make my hash remember the order I put elements into it? | |
2126 | ||
ac9dac7f | 2127 | Use the C<Tie::IxHash> from CPAN. |
68dc0745 | 2128 | |
ac9dac7f RGS |
2129 | use Tie::IxHash; |
2130 | ||
2131 | tie my %myhash, 'Tie::IxHash'; | |
2132 | ||
2133 | for (my $i=0; $i<20; $i++) { | |
2134 | $myhash{$i} = 2*$i; | |
2135 | } | |
2136 | ||
2137 | my @keys = keys %myhash; | |
2138 | # @keys = (0,1,2,3,...) | |
46fc3d4c | 2139 | |
68dc0745 | 2140 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
2141 | ||
2142 | If you say something like: | |
2143 | ||
ac9dac7f | 2144 | somefunc($hash{"nonesuch key here"}); |
68dc0745 | 2145 | |
2146 | Then that element "autovivifies"; that is, it springs into existence | |
2147 | whether you store something there or not. That's because functions | |
2148 | get scalars passed in by reference. If somefunc() modifies C<$_[0]>, | |
2149 | it has to be ready to write it back into the caller's version. | |
2150 | ||
87275199 | 2151 | This has been fixed as of Perl5.004. |
68dc0745 | 2152 | |
2153 | Normally, merely accessing a key's value for a nonexistent key does | |
2154 | I<not> cause that key to be forever there. This is different than | |
2155 | awk's behavior. | |
2156 | ||
fc36a67e | 2157 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
68dc0745 | 2158 | |
65acb1b1 TC |
2159 | Usually a hash ref, perhaps like this: |
2160 | ||
ac9dac7f RGS |
2161 | $record = { |
2162 | NAME => "Jason", | |
2163 | EMPNO => 132, | |
2164 | TITLE => "deputy peon", | |
2165 | AGE => 23, | |
2166 | SALARY => 37_000, | |
2167 | PALS => [ "Norbert", "Rhys", "Phineas"], | |
2168 | }; | |
65acb1b1 TC |
2169 | |
2170 | References are documented in L<perlref> and the upcoming L<perlreftut>. | |
2171 | Examples of complex data structures are given in L<perldsc> and | |
2172 | L<perllol>. Examples of structures and object-oriented classes are | |
2173 | in L<perltoot>. | |
68dc0745 | 2174 | |
2175 | =head2 How can I use a reference as a hash key? | |
2176 | ||
9e72e4c6 RGS |
2177 | (contributed by brian d foy) |
2178 | ||
2179 | Hash keys are strings, so you can't really use a reference as the key. | |
2180 | When you try to do that, perl turns the reference into its stringified | |
ac9dac7f RGS |
2181 | form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get |
2182 | back the reference from the stringified form, at least without doing | |
2183 | some extra work on your own. Also remember that hash keys must be | |
2184 | unique, but two different variables can store the same reference (and | |
2185 | those variables can change later). | |
9e72e4c6 | 2186 | |
ac9dac7f RGS |
2187 | The C<Tie::RefHash> module, which is distributed with perl, might be |
2188 | what you want. It handles that extra work. | |
68dc0745 | 2189 | |
2190 | =head1 Data: Misc | |
2191 | ||
2192 | =head2 How do I handle binary data correctly? | |
2193 | ||
ac9dac7f | 2194 | Perl is binary clean, so it can handle binary data just fine. |
e573f903 | 2195 | On Windows or DOS, however, you have to use C<binmode> for binary |
ac9dac7f RGS |
2196 | files to avoid conversions for line endings. In general, you should |
2197 | use C<binmode> any time you want to work with binary data. | |
68dc0745 | 2198 | |
ac9dac7f | 2199 | Also see L<perlfunc/"binmode"> or L<perlopentut>. |
68dc0745 | 2200 | |
ac9dac7f | 2201 | If you're concerned about 8-bit textual data then see L<perllocale>. |
54310121 | 2202 | If you want to deal with multibyte characters, however, there are |
68dc0745 | 2203 | some gotchas. See the section on Regular Expressions. |
2204 | ||
2205 | =head2 How do I determine whether a scalar is a number/whole/integer/float? | |
2206 | ||
2207 | Assuming that you don't care about IEEE notations like "NaN" or | |
2208 | "Infinity", you probably just want to use a regular expression. | |
2209 | ||
ac9dac7f RGS |
2210 | if (/\D/) { print "has nondigits\n" } |
2211 | if (/^\d+$/) { print "is a whole number\n" } | |
2212 | if (/^-?\d+$/) { print "is an integer\n" } | |
2213 | if (/^[+-]?\d+$/) { print "is a +/- integer\n" } | |
2214 | if (/^-?\d+\.?\d*$/) { print "is a real number\n" } | |
2215 | if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" } | |
2216 | if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) | |
881bdbd4 | 2217 | { print "a C float\n" } |
68dc0745 | 2218 | |
f0d19b68 RGS |
2219 | There are also some commonly used modules for the task. |
2220 | L<Scalar::Util> (distributed with 5.8) provides access to perl's | |
ac9dac7f RGS |
2221 | internal function C<looks_like_number> for determining whether a |
2222 | variable looks like a number. L<Data::Types> exports functions that | |
2223 | validate data types using both the above and other regular | |
2224 | expressions. Thirdly, there is C<Regexp::Common> which has regular | |
2225 | expressions to match various types of numbers. Those three modules are | |
2226 | available from the CPAN. | |
f0d19b68 RGS |
2227 | |
2228 | If you're on a POSIX system, Perl supports the C<POSIX::strtod> | |
ac9dac7f RGS |
2229 | function. Its semantics are somewhat cumbersome, so here's a |
2230 | C<getnum> wrapper function for more convenient access. This function | |
2231 | takes a string and returns the number it found, or C<undef> for input | |
2232 | that isn't a C float. The C<is_numeric> function is a front end to | |
2233 | C<getnum> if you just want to say, "Is this a float?" | |
2234 | ||
2235 | sub getnum { | |
2236 | use POSIX qw(strtod); | |
2237 | my $str = shift; | |
2238 | $str =~ s/^\s+//; | |
2239 | $str =~ s/\s+$//; | |
2240 | $! = 0; | |
2241 | my($num, $unparsed) = strtod($str); | |
2242 | if (($str eq '') || ($unparsed != 0) || $!) { | |
2243 | return undef; | |
2244 | } | |
2245 | else { | |
2246 | return $num; | |
2247 | } | |
2248 | } | |
5a964f20 | 2249 | |
ac9dac7f | 2250 | sub is_numeric { defined getnum($_[0]) } |
5a964f20 | 2251 | |
f0d19b68 | 2252 | Or you could check out the L<String::Scanf> module on the CPAN |
ac9dac7f RGS |
2253 | instead. The C<POSIX> module (part of the standard Perl distribution) |
2254 | provides the C<strtod> and C<strtol> for converting strings to double | |
2255 | and longs, respectively. | |
68dc0745 | 2256 | |
2257 | =head2 How do I keep persistent data across program calls? | |
2258 | ||
2259 | For some specific applications, you can use one of the DBM modules. | |
ac9dac7f RGS |
2260 | See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw> |
2261 | or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part | |
2262 | of the standard distribution. Here's one example using C<Storable>'s C<store> | |
fe854a6f | 2263 | and C<retrieve> functions: |
65acb1b1 | 2264 | |
ac9dac7f RGS |
2265 | use Storable; |
2266 | store(\%hash, "filename"); | |
65acb1b1 | 2267 | |
ac9dac7f RGS |
2268 | # later on... |
2269 | $href = retrieve("filename"); # by ref | |
2270 | %hash = %{ retrieve("filename") }; # direct to hash | |
68dc0745 | 2271 | |
2272 | =head2 How do I print out or copy a recursive data structure? | |
2273 | ||
ac9dac7f RGS |
2274 | The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great |
2275 | for printing out data structures. The C<Storable> module on CPAN (or the | |
6f82c03a EM |
2276 | 5.8 release of Perl), provides a function called C<dclone> that recursively |
2277 | copies its argument. | |
65acb1b1 | 2278 | |
ac9dac7f RGS |
2279 | use Storable qw(dclone); |
2280 | $r2 = dclone($r1); | |
68dc0745 | 2281 | |
ac9dac7f | 2282 | Where C<$r1> can be a reference to any kind of data structure you'd like. |
65acb1b1 TC |
2283 | It will be deeply copied. Because C<dclone> takes and returns references, |
2284 | you'd have to add extra punctuation if you had a hash of arrays that | |
2285 | you wanted to copy. | |
68dc0745 | 2286 | |
ac9dac7f | 2287 | %newhash = %{ dclone(\%oldhash) }; |
68dc0745 | 2288 | |
2289 | =head2 How do I define methods for every class/object? | |
2290 | ||
ac9dac7f | 2291 | Use the C<UNIVERSAL> class (see L<UNIVERSAL>). |
68dc0745 | 2292 | |
2293 | =head2 How do I verify a credit card checksum? | |
2294 | ||
ac9dac7f | 2295 | Get the C<Business::CreditCard> module from CPAN. |
68dc0745 | 2296 | |
65acb1b1 TC |
2297 | =head2 How do I pack arrays of doubles or floats for XS code? |
2298 | ||
ac9dac7f | 2299 | The kgbpack.c code in the C<PGPLOT> module on CPAN does just this. |
65acb1b1 | 2300 | If you're doing a lot of float or double processing, consider using |
ac9dac7f | 2301 | the C<PDL> module from CPAN instead--it makes number-crunching easy. |
65acb1b1 | 2302 | |
500071f4 RGS |
2303 | =head1 REVISION |
2304 | ||
fb2fe781 | 2305 | Revision: $Revision: 10394 $ |
500071f4 | 2306 | |
fb2fe781 | 2307 | Date: $Date: 2007-12-09 18:47:15 +0100 (Sun, 09 Dec 2007) $ |
500071f4 RGS |
2308 | |
2309 | See L<perlfaq> for source control details and availability. | |
2310 | ||
68dc0745 | 2311 | =head1 AUTHOR AND COPYRIGHT |
2312 | ||
ee891a00 | 2313 | Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and |
7678cced | 2314 | other authors as noted. All rights reserved. |
5a964f20 | 2315 | |
5a7beb56 JH |
2316 | This documentation is free; you can redistribute it and/or modify it |
2317 | under the same terms as Perl itself. | |
5a964f20 TC |
2318 | |
2319 | Irrespective of its distribution, all code examples in this file | |
2320 | are hereby placed into the public domain. You are permitted and | |
2321 | encouraged to use this code in your own programs for fun | |
2322 | or for profit as you see fit. A simple comment in the code giving | |
2323 | credit would be courteous but is not required. |