Commit | Line | Data |
---|---|---|
68dc0745 | 1 | =head1 NAME |
2 | ||
500071f4 | 3 | perlfaq4 - Data Manipulation ($Revision: 3606 $) |
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 | ||
49d635f9 RGS |
14 | Internally, your computer represents floating-point numbers |
15 | in binary. Digital (as in powers of two) computers cannot | |
16 | store all numbers exactly. Some real numbers lose precision | |
17 | in the process. This is a problem with how computers store | |
18 | numbers and affects all computer languages, not just Perl. | |
46fc3d4c | 19 | |
49d635f9 RGS |
20 | L<perlnumber> show the gory details of number |
21 | representations and conversions. | |
22 | ||
23 | To limit the number of decimal places in your numbers, you | |
24 | can use the printf or sprintf function. See the | |
197aec24 | 25 | L<"Floating Point 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 | ||
33 | Your int() is most probably working just fine. It's the numbers that | |
34 | aren't quite what you think. | |
35 | ||
36 | First, see the above item "Why am I getting long decimals | |
37 | (eg, 19.9499999999999) instead of the numbers I should be getting | |
38 | (eg, 19.95)?". | |
39 | ||
40 | For example, this | |
41 | ||
42 | print int(0.6/0.2-2), "\n"; | |
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 | |
53 | leading "0" and hexadecimal literals must start with a leading "0x". | |
54 | If they are read in from somewhere and assigned, no automatic | |
55 | conversion takes place. You must explicitly use oct() or hex() if you | |
56 | want the values converted to decimal. oct() interprets hex ("0x350"), | |
57 | octal ("0350" or even without the leading "0", like "377") and binary | |
58 | ("0b1010") numbers, while hex() only converts hexadecimal ones, with | |
59 | or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef". | |
33ce146f | 60 | The inverse mapping from decimal to octal can be done with either the |
49d635f9 | 61 | "%o" or "%O" sprintf() formats. |
68dc0745 | 62 | |
63 | This problem shows up most often when people try using chmod(), mkdir(), | |
197aec24 | 64 | umask(), or sysopen(), which by widespread tradition typically take |
33ce146f | 65 | permissions in octal. |
68dc0745 | 66 | |
33ce146f | 67 | chmod(644, $file); # WRONG |
68dc0745 | 68 | chmod(0644, $file); # right |
69 | ||
197aec24 | 70 | Note the mistake in the first line was specifying the decimal literal |
33ce146f PP |
71 | 644, rather than the intended octal literal 0644. The problem can |
72 | be seen with: | |
73 | ||
434f7166 | 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 |
33ce146f PP |
79 | with the following digits restricted to the set 0..7. |
80 | ||
65acb1b1 | 81 | =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? |
68dc0745 | 82 | |
92c2ed05 GS |
83 | Remember that int() merely truncates toward 0. For rounding to a |
84 | certain number of digits, sprintf() or printf() is usually the easiest | |
85 | route. | |
86 | ||
87 | printf("%.3f", 3.1415926535); # prints 3.142 | |
68dc0745 | 88 | |
87275199 | 89 | The POSIX module (part of the standard Perl distribution) implements |
68dc0745 | 90 | ceil(), floor(), and a number of other mathematical and trigonometric |
91 | functions. | |
92 | ||
92c2ed05 GS |
93 | use POSIX; |
94 | $ceil = ceil(3.5); # 4 | |
95 | $floor = floor(3.5); # 3 | |
96 | ||
a6dd486b | 97 | In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex |
87275199 | 98 | module. With 5.004, the Math::Trig module (part of the standard Perl |
46fc3d4c | 99 | distribution) implements the trigonometric functions. Internally it |
100 | uses the Math::Complex module and some functions can break out from | |
101 | the real axis into the complex plane, for example the inverse sine of | |
102 | 2. | |
68dc0745 | 103 | |
104 | Rounding in financial applications can have serious implications, and | |
105 | the rounding method used should be specified precisely. In these | |
106 | cases, it probably pays not to trust whichever system rounding is | |
107 | being used by Perl, but to instead implement the rounding function you | |
108 | need yourself. | |
109 | ||
65acb1b1 TC |
110 | To see why, notice how you'll still have an issue on half-way-point |
111 | alternation: | |
112 | ||
113 | for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} | |
114 | ||
197aec24 | 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 |
65acb1b1 TC |
116 | 0.8 0.8 0.9 0.9 1.0 1.0 |
117 | ||
118 | Don't blame Perl. It's the same as in C. IEEE says we have to do this. | |
119 | Perl numbers whose absolute values are integers under 2**31 (on 32 bit | |
120 | machines) will work pretty much like mathematical integers. Other numbers | |
121 | are not guaranteed. | |
122 | ||
6f0efb17 | 123 | =head2 How do I convert between numeric representations/bases/radixes? |
68dc0745 | 124 | |
6761e064 JH |
125 | As always with Perl there is more than one way to do it. Below |
126 | are a few examples of approaches to making common conversions | |
127 | between number representations. This is intended to be representational | |
128 | rather than exhaustive. | |
68dc0745 | 129 | |
6761e064 JH |
130 | Some of the examples below use the Bit::Vector module from CPAN. |
131 | The reason you might choose Bit::Vector over the perl built in | |
132 | functions is that it works with numbers of ANY size, that it is | |
133 | optimized for speed on some operations, and for at least some | |
134 | 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 | |
6761e064 JH |
140 | Using perl's built in conversion of 0x notation: |
141 | ||
6f0efb17 | 142 | $dec = 0xDEADBEEF; |
7207e29d | 143 | |
6761e064 JH |
144 | Using the hex function: |
145 | ||
6f0efb17 | 146 | $dec = hex("DEADBEEF"); |
6761e064 JH |
147 | |
148 | Using pack: | |
149 | ||
6f0efb17 | 150 | $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); |
6761e064 JH |
151 | |
152 | Using the CPAN module Bit::Vector: | |
153 | ||
154 | use Bit::Vector; | |
155 | $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); | |
156 | $dec = $vec->to_Dec(); | |
157 | ||
818c4caa | 158 | =item How do I convert from decimal to hexadecimal |
6761e064 | 159 | |
04d666b1 | 160 | Using sprintf: |
6761e064 | 161 | |
6f0efb17 RGS |
162 | $hex = sprintf("%X", 3735928559); # upper case A-F |
163 | $hex = sprintf("%x", 3735928559); # lower case a-f | |
6761e064 | 164 | |
6f0efb17 | 165 | Using unpack: |
6761e064 JH |
166 | |
167 | $hex = unpack("H*", pack("N", 3735928559)); | |
168 | ||
6f0efb17 | 169 | Using Bit::Vector: |
6761e064 JH |
170 | |
171 | use Bit::Vector; | |
172 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
173 | $hex = $vec->to_Hex(); | |
174 | ||
175 | And Bit::Vector supports odd bit counts: | |
176 | ||
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(); | |
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 | ||
6f0efb17 | 186 | $dec = 033653337357; # note the leading 0! |
6761e064 JH |
187 | |
188 | Using the oct function: | |
189 | ||
6f0efb17 | 190 | $dec = oct("33653337357"); |
6761e064 JH |
191 | |
192 | Using Bit::Vector: | |
193 | ||
194 | use Bit::Vector; | |
195 | $vec = Bit::Vector->new(32); | |
196 | $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); | |
197 | $dec = $vec->to_Dec(); | |
198 | ||
818c4caa | 199 | =item How do I convert from decimal to octal |
6761e064 JH |
200 | |
201 | Using sprintf: | |
202 | ||
203 | $oct = sprintf("%o", 3735928559); | |
204 | ||
6f0efb17 | 205 | Using Bit::Vector: |
6761e064 JH |
206 | |
207 | use Bit::Vector; | |
208 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
209 | $oct = reverse join('', $vec->Chunk_List_Read(3)); | |
210 | ||
818c4caa | 211 | =item How do I convert from binary to decimal |
6761e064 | 212 | |
2c646907 JH |
213 | Perl 5.6 lets you write binary numbers directly with |
214 | the 0b notation: | |
215 | ||
6f0efb17 RGS |
216 | $number = 0b10110110; |
217 | ||
218 | Using oct: | |
219 | ||
220 | my $input = "10110110"; | |
221 | $decimal = oct( "0b$input" ); | |
2c646907 | 222 | |
6f0efb17 | 223 | Using pack and ord: |
d92eb7b0 GS |
224 | |
225 | $decimal = ord(pack('B8', '10110110')); | |
68dc0745 | 226 | |
6f0efb17 | 227 | Using pack and unpack for larger strings: |
6761e064 JH |
228 | |
229 | $int = unpack("N", pack("B32", | |
230 | substr("0" x 32 . "11110101011011011111011101111", -32))); | |
231 | $dec = sprintf("%d", $int); | |
232 | ||
5efd7060 | 233 | # substr() is used to left pad a 32 character string with zeros. |
6761e064 JH |
234 | |
235 | Using Bit::Vector: | |
236 | ||
237 | $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); | |
238 | $dec = $vec->to_Dec(); | |
239 | ||
818c4caa | 240 | =item How do I convert from decimal to binary |
6761e064 | 241 | |
4dfcc30b RGS |
242 | Using sprintf (perl 5.6+): |
243 | ||
244 | $bin = sprintf("%b", 3735928559); | |
245 | ||
246 | Using unpack: | |
6761e064 JH |
247 | |
248 | $bin = unpack("B*", pack("N", 3735928559)); | |
249 | ||
250 | Using Bit::Vector: | |
251 | ||
252 | use Bit::Vector; | |
253 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
254 | $bin = $vec->to_Bin(); | |
255 | ||
256 | The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) | |
257 | are left as an exercise to the inclined reader. | |
68dc0745 | 258 | |
818c4caa | 259 | =back |
68dc0745 | 260 | |
65acb1b1 TC |
261 | =head2 Why doesn't & work the way I want it to? |
262 | ||
263 | The behavior of binary arithmetic operators depends on whether they're | |
264 | used on numbers or strings. The operators treat a string as a series | |
265 | of bits and work with that (the string C<"3"> is the bit pattern | |
266 | C<00110011>). The operators work with the binary form of a number | |
267 | (the number C<3> is treated as the bit pattern C<00000011>). | |
268 | ||
269 | So, saying C<11 & 3> performs the "and" operation on numbers (yielding | |
49d635f9 | 270 | C<3>). Saying C<"11" & "3"> performs the "and" operation on strings |
65acb1b1 TC |
271 | (yielding C<"1">). |
272 | ||
273 | Most problems with C<&> and C<|> arise because the programmer thinks | |
274 | they have a number but really it's a string. The rest arise because | |
275 | the programmer says: | |
276 | ||
277 | if ("\020\020" & "\101\101") { | |
278 | # ... | |
279 | } | |
280 | ||
281 | but a string consisting of two null bytes (the result of C<"\020\020" | |
282 | & "\101\101">) is not a false value in Perl. You need: | |
283 | ||
284 | if ( ("\020\020" & "\101\101") !~ /[^\000]/) { | |
285 | # ... | |
286 | } | |
287 | ||
68dc0745 | 288 | =head2 How do I multiply matrices? |
289 | ||
290 | Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) | |
291 | or the PDL extension (also available from CPAN). | |
292 | ||
293 | =head2 How do I perform an operation on a series of integers? | |
294 | ||
295 | To call a function on each element in an array, and collect the | |
296 | results, use: | |
297 | ||
298 | @results = map { my_func($_) } @array; | |
299 | ||
300 | For example: | |
301 | ||
302 | @triple = map { 3 * $_ } @single; | |
303 | ||
304 | To call a function on each element of an array, but ignore the | |
305 | results: | |
306 | ||
307 | foreach $iterator (@array) { | |
65acb1b1 | 308 | some_func($iterator); |
68dc0745 | 309 | } |
310 | ||
311 | To call a function on each integer in a (small) range, you B<can> use: | |
312 | ||
65acb1b1 | 313 | @results = map { some_func($_) } (5 .. 25); |
68dc0745 | 314 | |
315 | but you should be aware that the C<..> operator creates an array of | |
316 | all integers in the range. This can take a lot of memory for large | |
317 | ranges. Instead use: | |
318 | ||
319 | @results = (); | |
320 | for ($i=5; $i < 500_005; $i++) { | |
65acb1b1 | 321 | push(@results, some_func($i)); |
68dc0745 | 322 | } |
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 | ||
327 | for my $i (5 .. 500_005) { | |
328 | push(@results, some_func($i)); | |
329 | } | |
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 $] < 5.004 } |
49d635f9 | 343 | |
65acb1b1 | 344 | 5.004 and later automatically call C<srand> at the beginning. Don't |
49d635f9 | 345 | call C<srand> more than once---you make your numbers less random, rather |
65acb1b1 | 346 | than more. |
92c2ed05 | 347 | |
65acb1b1 | 348 | Computers are good at being predictable and bad at being random |
06a5f41f | 349 | (despite appearances caused by bugs in your programs :-). see the |
49d635f9 RGS |
350 | F<random> article in the "Far More Than You Ever Wanted To Know" |
351 | collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of | |
b432a672 | 352 | Tom Phoenix, talks more about this. John von Neumann said, "Anyone |
06a5f41f | 353 | who attempts to generate random numbers by deterministic means is, of |
b432a672 | 354 | course, living in a state of sin." |
65acb1b1 TC |
355 | |
356 | If you want numbers that are more random than C<rand> with C<srand> | |
357 | provides, you should also check out the Math::TrulyRandom module from | |
358 | CPAN. It uses the imperfections in your system's timer to generate | |
359 | random numbers, but this takes quite a while. If you want a better | |
92c2ed05 | 360 | pseudorandom generator than comes with your operating system, look at |
b432a672 | 361 | "Numerical Recipes in C" at http://www.nr.com/ . |
68dc0745 | 362 | |
881bdbd4 JH |
363 | =head2 How do I get a random number between X and Y? |
364 | ||
500071f4 RGS |
365 | To get a random number between two values, you can use the |
366 | C<rand()> builtin to get a random number between 0 and | |
367 | ||
793f5136 RGS |
368 | C<rand($x)> returns a number such that |
369 | C<< 0 <= rand($x) < $x >>. Thus what you want to have perl | |
370 | figure out is a random number in the range from 0 to the | |
371 | difference between your I<X> and I<Y>. | |
372 | ||
373 | That is, to get a number between 10 and 15, inclusive, you | |
374 | want a random number between 0 and 5 that you can then add | |
375 | to 10. | |
376 | ||
500071f4 | 377 | my $number = 10 + int rand( 15-10+1 ); |
793f5136 RGS |
378 | |
379 | Hence you derive the following simple function to abstract | |
380 | that. It selects a random integer between the two given | |
500071f4 RGS |
381 | integers (inclusive), For example: C<random_int_between(50,120)>. |
382 | ||
383 | sub random_int_between ($$) { | |
384 | my($min, $max) = @_; | |
385 | # Assumes that the two arguments are integers themselves! | |
386 | return $min if $min == $max; | |
387 | ($min, $max) = ($max, $min) if $min > $max; | |
388 | return $min + int rand(1 + $max - $min); | |
389 | } | |
881bdbd4 | 390 | |
68dc0745 | 391 | =head1 Data: Dates |
392 | ||
5cd0b561 | 393 | =head2 How do I find the day or week of the year? |
68dc0745 | 394 | |
571e049f | 395 | The localtime function returns the day of the year. Without an |
5cd0b561 | 396 | argument localtime uses the current time. |
68dc0745 | 397 | |
a05e4845 | 398 | $day_of_year = (localtime)[7]; |
ffc145e8 | 399 | |
5cd0b561 RGS |
400 | The POSIX module can also format a date as the day of the year or |
401 | week of the year. | |
68dc0745 | 402 | |
5cd0b561 RGS |
403 | use POSIX qw/strftime/; |
404 | my $day_of_year = strftime "%j", localtime; | |
405 | my $week_of_year = strftime "%W", localtime; | |
406 | ||
407 | To get the day of year for any date, use the Time::Local module to get | |
408 | a time in epoch seconds for the argument to localtime. | |
ffc145e8 | 409 | |
5cd0b561 RGS |
410 | use POSIX qw/strftime/; |
411 | use Time::Local; | |
6670e5e7 | 412 | my $week_of_year = strftime "%W", |
5cd0b561 RGS |
413 | localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) ); |
414 | ||
b68463f7 | 415 | The Date::Calc module provides two functions to calculate these. |
5cd0b561 RGS |
416 | |
417 | use Date::Calc; | |
418 | my $day_of_year = Day_of_Year( 1987, 12, 18 ); | |
419 | my $week_of_year = Week_of_Year( 1987, 12, 18 ); | |
ffc145e8 | 420 | |
d92eb7b0 GS |
421 | =head2 How do I find the current century or millennium? |
422 | ||
423 | Use the following simple functions: | |
424 | ||
197aec24 | 425 | sub get_century { |
d92eb7b0 | 426 | return int((((localtime(shift || time))[5] + 1999))/100); |
197aec24 | 427 | } |
6670e5e7 | 428 | |
197aec24 | 429 | sub get_millennium { |
d92eb7b0 | 430 | return 1+int((((localtime(shift || time))[5] + 1899))/1000); |
197aec24 | 431 | } |
d92eb7b0 | 432 | |
49d635f9 RGS |
433 | On some systems, the POSIX module's strftime() function has |
434 | been extended in a non-standard way to use a C<%C> format, | |
435 | which they sometimes claim is the "century". It isn't, | |
436 | because on most such systems, this is only the first two | |
437 | digits of the four-digit year, and thus cannot be used to | |
438 | reliably determine the current century or millennium. | |
d92eb7b0 | 439 | |
92c2ed05 | 440 | =head2 How can I compare two dates and find the difference? |
68dc0745 | 441 | |
b68463f7 RGS |
442 | (contributed by brian d foy) |
443 | ||
444 | You could just store all your dates as a number and then subtract. Life | |
445 | isn't always that simple though. If you want to work with formatted | |
446 | dates, the Date::Manip, Date::Calc, or DateTime modules can help you. | |
447 | ||
68dc0745 | 448 | |
449 | =head2 How can I take a string and turn it into epoch seconds? | |
450 | ||
451 | If it's a regular enough string that it always has the same format, | |
92c2ed05 GS |
452 | you can split it up and pass the parts to C<timelocal> in the standard |
453 | Time::Local module. Otherwise, you should look into the Date::Calc | |
454 | and Date::Manip modules from CPAN. | |
68dc0745 | 455 | |
456 | =head2 How can I find the Julian Day? | |
457 | ||
7678cced RGS |
458 | (contributed by brian d foy and Dave Cross) |
459 | ||
460 | You can use the Time::JulianDay module available on CPAN. Ensure that | |
461 | you really want to find a Julian day, though, as many people have | |
462 | different ideas about Julian days. See | |
463 | http://www.hermetic.ch/cal_stud/jdn.htm for instance. | |
464 | ||
465 | You can also try the DateTime module, which can convert a date/time | |
466 | to a Julian Day. | |
467 | ||
468 | $ perl -MDateTime -le'print DateTime->today->jd' | |
469 | 2453401.5 | |
470 | ||
471 | Or the modified Julian Day | |
472 | ||
473 | $ perl -MDateTime -le'print DateTime->today->mjd' | |
474 | 53401 | |
475 | ||
476 | Or even the day of the year (which is what some people think of as a | |
477 | Julian day) | |
478 | ||
479 | $ perl -MDateTime -le'print DateTime->today->doy' | |
480 | 31 | |
be94a901 | 481 | |
65acb1b1 TC |
482 | =head2 How do I find yesterday's date? |
483 | ||
6670e5e7 | 484 | (contributed by brian d foy) |
49d635f9 | 485 | |
6670e5e7 RGS |
486 | Use one of the Date modules. The C<DateTime> module makes it simple, and |
487 | give you the same time of day, only the day before. | |
49d635f9 | 488 | |
6670e5e7 | 489 | use DateTime; |
58103a2e | 490 | |
6670e5e7 | 491 | my $yesterday = DateTime->now->subtract( days => 1 ); |
58103a2e | 492 | |
6670e5e7 | 493 | print "Yesterday was $yesterday\n"; |
49d635f9 | 494 | |
6670e5e7 RGS |
495 | You can also use the C<Date::Calc> module using its Today_and_Now |
496 | function. | |
49d635f9 | 497 | |
6670e5e7 | 498 | use Date::Calc qw( Today_and_Now Add_Delta_DHMS ); |
58103a2e | 499 | |
6670e5e7 | 500 | my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 ); |
58103a2e | 501 | |
6670e5e7 | 502 | print "@date\n"; |
58103a2e | 503 | |
6670e5e7 RGS |
504 | Most people try to use the time rather than the calendar to figure out |
505 | dates, but that assumes that days are twenty-four hours each. For | |
506 | most people, there are two days a year when they aren't: the switch to | |
507 | and from summer time throws this off. Let the modules do the work. | |
d92eb7b0 | 508 | |
87275199 | 509 | =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? |
68dc0745 | 510 | |
65acb1b1 TC |
511 | Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is |
512 | Y2K compliant (whatever that means). The programmers you've hired to | |
513 | use it, however, probably are not. | |
514 | ||
515 | Long answer: The question belies a true understanding of the issue. | |
516 | Perl is just as Y2K compliant as your pencil--no more, and no less. | |
517 | Can you use your pencil to write a non-Y2K-compliant memo? Of course | |
518 | you can. Is that the pencil's fault? Of course it isn't. | |
92c2ed05 | 519 | |
87275199 | 520 | The date and time functions supplied with Perl (gmtime and localtime) |
65acb1b1 TC |
521 | supply adequate information to determine the year well beyond 2000 |
522 | (2038 is when trouble strikes for 32-bit machines). The year returned | |
90fdbbb7 | 523 | by these functions when used in a list context is the year minus 1900. |
65acb1b1 TC |
524 | For years between 1910 and 1999 this I<happens> to be a 2-digit decimal |
525 | number. To avoid the year 2000 problem simply do not treat the year as | |
526 | a 2-digit number. It isn't. | |
68dc0745 | 527 | |
5a964f20 | 528 | When gmtime() and localtime() are used in scalar context they return |
68dc0745 | 529 | a timestamp string that contains a fully-expanded year. For example, |
530 | C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 | |
531 | 2001". There's no year 2000 problem here. | |
532 | ||
5a964f20 TC |
533 | That doesn't mean that Perl can't be used to create non-Y2K compliant |
534 | programs. It can. But so can your pencil. It's the fault of the user, | |
b432a672 AL |
535 | not the language. At the risk of inflaming the NRA: "Perl doesn't |
536 | break Y2K, people do." See http://www.perl.org/about/y2k.html for | |
5a964f20 TC |
537 | a longer exposition. |
538 | ||
68dc0745 | 539 | =head1 Data: Strings |
540 | ||
541 | =head2 How do I validate input? | |
542 | ||
6670e5e7 RGS |
543 | (contributed by brian d foy) |
544 | ||
545 | There are many ways to ensure that values are what you expect or | |
546 | want to accept. Besides the specific examples that we cover in the | |
547 | perlfaq, you can also look at the modules with "Assert" and "Validate" | |
548 | in their names, along with other modules such as C<Regexp::Common>. | |
549 | ||
550 | Some modules have validation for particular types of input, such | |
551 | as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>, | |
552 | and C<Data::Validate::IP>. | |
68dc0745 | 553 | |
554 | =head2 How do I unescape a string? | |
555 | ||
b432a672 | 556 | It depends just what you mean by "escape". URL escapes are dealt |
92c2ed05 | 557 | with in L<perlfaq9>. Shell escapes with the backslash (C<\>) |
a6dd486b | 558 | character are removed with |
68dc0745 | 559 | |
560 | s/\\(.)/$1/g; | |
561 | ||
92c2ed05 | 562 | This won't expand C<"\n"> or C<"\t"> or any other special escapes. |
68dc0745 | 563 | |
564 | =head2 How do I remove consecutive pairs of characters? | |
565 | ||
6670e5e7 RGS |
566 | (contributed by brian d foy) |
567 | ||
568 | You can use the substitution operator to find pairs of characters (or | |
569 | runs of characters) and replace them with a single instance. In this | |
570 | substitution, we find a character in C<(.)>. The memory parentheses | |
571 | store the matched character in the back-reference C<\1> and we use | |
572 | that to require that the same thing immediately follow it. We replace | |
573 | that part of the string with the character in C<$1>. | |
68dc0745 | 574 | |
6670e5e7 | 575 | s/(.)\1/$1/g; |
d92eb7b0 | 576 | |
6670e5e7 RGS |
577 | We can also use the transliteration operator, C<tr///>. In this |
578 | example, the search list side of our C<tr///> contains nothing, but | |
579 | the C<c> option complements that so it contains everything. The | |
580 | replacement list also contains nothing, so the transliteration is | |
581 | almost a no-op since it won't do any replacements (or more exactly, | |
582 | replace the character with itself). However, the C<s> option squashes | |
583 | duplicated and consecutive characters in the string so a character | |
584 | does not show up next to itself | |
d92eb7b0 | 585 | |
6670e5e7 RGS |
586 | my $str = 'Haarlem'; # in the Netherlands |
587 | $str =~ tr///cs; # Now Harlem, like in New York | |
68dc0745 | 588 | |
589 | =head2 How do I expand function calls in a string? | |
590 | ||
6670e5e7 RGS |
591 | (contributed by brian d foy) |
592 | ||
593 | This is documented in L<perlref>, and although it's not the easiest | |
594 | thing to read, it does work. In each of these examples, we call the | |
58103a2e | 595 | function inside the braces used to dereference a reference. If we |
d7f8936a | 596 | have a more than one return value, we can construct and dereference an |
6670e5e7 RGS |
597 | anonymous array. In this case, we call the function in list context. |
598 | ||
58103a2e | 599 | print "The time values are @{ [localtime] }.\n"; |
6670e5e7 RGS |
600 | |
601 | If we want to call the function in scalar context, we have to do a bit | |
602 | more work. We can really have any code we like inside the braces, so | |
603 | we simply have to end with the scalar reference, although how you do | |
604 | that is up to you, and you can use code inside the braces. | |
68dc0745 | 605 | |
6670e5e7 | 606 | print "The time is ${\(scalar localtime)}.\n" |
58103a2e | 607 | |
6670e5e7 | 608 | print "The time is ${ my $x = localtime; \$x }.\n"; |
58103a2e | 609 | |
6670e5e7 RGS |
610 | If your function already returns a reference, you don't need to create |
611 | the reference yourself. | |
612 | ||
613 | sub timestamp { my $t = localtime; \$t } | |
58103a2e | 614 | |
6670e5e7 | 615 | print "The time is ${ timestamp() }.\n"; |
58103a2e RGS |
616 | |
617 | The C<Interpolation> module can also do a lot of magic for you. You can | |
618 | specify a variable name, in this case C<E>, to set up a tied hash that | |
619 | does the interpolation for you. It has several other methods to do this | |
620 | as well. | |
621 | ||
622 | use Interpolation E => 'eval'; | |
623 | print "The time values are $E{localtime()}.\n"; | |
624 | ||
625 | In most cases, it is probably easier to simply use string concatenation, | |
626 | which also forces scalar context. | |
6670e5e7 RGS |
627 | |
628 | print "The time is " . localtime . ".\n"; | |
68dc0745 | 629 | |
68dc0745 | 630 | =head2 How do I find matching/nesting anything? |
631 | ||
92c2ed05 GS |
632 | This isn't something that can be done in one regular expression, no |
633 | matter how complicated. To find something between two single | |
634 | characters, a pattern like C</x([^x]*)x/> will get the intervening | |
635 | bits in $1. For multiple ones, then something more like | |
636 | C</alpha(.*?)omega/> would be needed. But none of these deals with | |
6670e5e7 RGS |
637 | nested patterns. For balanced expressions using C<(>, C<{>, C<[> or |
638 | C<< < >> as delimiters, use the CPAN module Regexp::Common, or see | |
639 | L<perlre/(??{ code })>. For other cases, you'll have to write a | |
640 | parser. | |
92c2ed05 GS |
641 | |
642 | If you are serious about writing a parser, there are a number of | |
6a2af475 GS |
643 | modules or oddities that will make your life a lot easier. There are |
644 | the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; | |
6670e5e7 RGS |
645 | and the byacc program. Starting from perl 5.8 the Text::Balanced is |
646 | part of the standard distribution. | |
68dc0745 | 647 | |
92c2ed05 GS |
648 | One simple destructive, inside-out approach that you might try is to |
649 | pull out the smallest nesting parts one at a time: | |
5a964f20 | 650 | |
d92eb7b0 | 651 | while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { |
5a964f20 | 652 | # do something with $1 |
197aec24 | 653 | } |
5a964f20 | 654 | |
65acb1b1 TC |
655 | A more complicated and sneaky approach is to make Perl's regular |
656 | expression engine do it for you. This is courtesy Dean Inada, and | |
657 | rather has the nature of an Obfuscated Perl Contest entry, but it | |
658 | really does work: | |
659 | ||
660 | # $_ contains the string to parse | |
661 | # BEGIN and END are the opening and closing markers for the | |
662 | # nested text. | |
c47ff5f1 | 663 | |
65acb1b1 TC |
664 | @( = ('(',''); |
665 | @) = (')',''); | |
666 | ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; | |
5ed30e05 | 667 | @$ = (eval{/$re/},$@!~/unmatched/i); |
65acb1b1 TC |
668 | print join("\n",@$[0..$#$]) if( $$[-1] ); |
669 | ||
68dc0745 | 670 | =head2 How do I reverse a string? |
671 | ||
5a964f20 | 672 | Use reverse() in scalar context, as documented in |
68dc0745 | 673 | L<perlfunc/reverse>. |
674 | ||
675 | $reversed = reverse $string; | |
676 | ||
677 | =head2 How do I expand tabs in a string? | |
678 | ||
5a964f20 | 679 | You can do it yourself: |
68dc0745 | 680 | |
681 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; | |
682 | ||
87275199 | 683 | Or you can just use the Text::Tabs module (part of the standard Perl |
68dc0745 | 684 | distribution). |
685 | ||
686 | use Text::Tabs; | |
687 | @expanded_lines = expand(@lines_with_tabs); | |
688 | ||
689 | =head2 How do I reformat a paragraph? | |
690 | ||
87275199 | 691 | Use Text::Wrap (part of the standard Perl distribution): |
68dc0745 | 692 | |
693 | use Text::Wrap; | |
694 | print wrap("\t", ' ', @paragraphs); | |
695 | ||
92c2ed05 | 696 | The paragraphs you give to Text::Wrap should not contain embedded |
46fc3d4c | 697 | newlines. Text::Wrap doesn't justify the lines (flush-right). |
698 | ||
bc06af74 JH |
699 | Or use the CPAN module Text::Autoformat. Formatting files can be easily |
700 | done by making a shell alias, like so: | |
701 | ||
702 | alias fmt="perl -i -MText::Autoformat -n0777 \ | |
703 | -e 'print autoformat $_, {all=>1}' $*" | |
704 | ||
705 | See the documentation for Text::Autoformat to appreciate its many | |
706 | capabilities. | |
707 | ||
49d635f9 | 708 | =head2 How can I access or change N characters of a string? |
68dc0745 | 709 | |
49d635f9 RGS |
710 | You can access the first characters of a string with substr(). |
711 | To get the first character, for example, start at position 0 | |
197aec24 | 712 | and grab the string of length 1. |
68dc0745 | 713 | |
68dc0745 | 714 | |
49d635f9 RGS |
715 | $string = "Just another Perl Hacker"; |
716 | $first_char = substr( $string, 0, 1 ); # 'J' | |
68dc0745 | 717 | |
49d635f9 RGS |
718 | To change part of a string, you can use the optional fourth |
719 | argument which is the replacement string. | |
68dc0745 | 720 | |
49d635f9 | 721 | substr( $string, 13, 4, "Perl 5.8.0" ); |
197aec24 | 722 | |
49d635f9 | 723 | You can also use substr() as an lvalue. |
68dc0745 | 724 | |
49d635f9 | 725 | substr( $string, 13, 4 ) = "Perl 5.8.0"; |
197aec24 | 726 | |
68dc0745 | 727 | =head2 How do I change the Nth occurrence of something? |
728 | ||
92c2ed05 GS |
729 | You have to keep track of N yourself. For example, let's say you want |
730 | to change the fifth occurrence of C<"whoever"> or C<"whomever"> into | |
d92eb7b0 GS |
731 | C<"whosoever"> or C<"whomsoever">, case insensitively. These |
732 | all assume that $_ contains the string to be altered. | |
68dc0745 | 733 | |
734 | $count = 0; | |
735 | s{((whom?)ever)}{ | |
736 | ++$count == 5 # is it the 5th? | |
737 | ? "${2}soever" # yes, swap | |
738 | : $1 # renege and leave it there | |
d92eb7b0 | 739 | }ige; |
68dc0745 | 740 | |
5a964f20 TC |
741 | In the more general case, you can use the C</g> modifier in a C<while> |
742 | loop, keeping count of matches. | |
743 | ||
744 | $WANT = 3; | |
745 | $count = 0; | |
d92eb7b0 | 746 | $_ = "One fish two fish red fish blue fish"; |
5a964f20 TC |
747 | while (/(\w+)\s+fish\b/gi) { |
748 | if (++$count == $WANT) { | |
749 | print "The third fish is a $1 one.\n"; | |
5a964f20 TC |
750 | } |
751 | } | |
752 | ||
92c2ed05 | 753 | That prints out: C<"The third fish is a red one."> You can also use a |
5a964f20 TC |
754 | repetition count and repeated pattern like this: |
755 | ||
756 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; | |
757 | ||
68dc0745 | 758 | =head2 How can I count the number of occurrences of a substring within a string? |
759 | ||
a6dd486b | 760 | There are a number of ways, with varying efficiency. If you want a |
68dc0745 | 761 | count of a certain single character (X) within a string, you can use the |
762 | C<tr///> function like so: | |
763 | ||
368c9434 | 764 | $string = "ThisXlineXhasXsomeXx'sXinXit"; |
68dc0745 | 765 | $count = ($string =~ tr/X//); |
d92eb7b0 | 766 | print "There are $count X characters in the string"; |
68dc0745 | 767 | |
768 | This is fine if you are just looking for a single character. However, | |
769 | if you are trying to count multiple character substrings within a | |
770 | larger string, C<tr///> won't work. What you can do is wrap a while() | |
771 | loop around a global pattern match. For example, let's count negative | |
772 | integers: | |
773 | ||
774 | $string = "-9 55 48 -2 23 -76 4 14 -44"; | |
775 | while ($string =~ /-\d+/g) { $count++ } | |
776 | print "There are $count negative numbers in the string"; | |
777 | ||
881bdbd4 JH |
778 | Another version uses a global match in list context, then assigns the |
779 | result to a scalar, producing a count of the number of matches. | |
780 | ||
781 | $count = () = $string =~ /-\d+/g; | |
782 | ||
68dc0745 | 783 | =head2 How do I capitalize all the words on one line? |
784 | ||
785 | To make the first letter of each word upper case: | |
3fe9a6f1 | 786 | |
68dc0745 | 787 | $line =~ s/\b(\w)/\U$1/g; |
788 | ||
46fc3d4c | 789 | This has the strange effect of turning "C<don't do it>" into "C<Don'T |
a6dd486b | 790 | Do It>". Sometimes you might want this. Other times you might need a |
24f1ba9b | 791 | more thorough solution (Suggested by brian d foy): |
46fc3d4c | 792 | |
793 | $string =~ s/ ( | |
794 | (^\w) #at the beginning of the line | |
795 | | # or | |
796 | (\s\w) #preceded by whitespace | |
797 | ) | |
798 | /\U$1/xg; | |
799 | $string =~ /([\w']+)/\u\L$1/g; | |
800 | ||
68dc0745 | 801 | To make the whole line upper case: |
3fe9a6f1 | 802 | |
68dc0745 | 803 | $line = uc($line); |
804 | ||
805 | To force each word to be lower case, with the first letter upper case: | |
3fe9a6f1 | 806 | |
68dc0745 | 807 | $line =~ s/(\w+)/\u\L$1/g; |
808 | ||
5a964f20 TC |
809 | You can (and probably should) enable locale awareness of those |
810 | characters by placing a C<use locale> pragma in your program. | |
92c2ed05 | 811 | See L<perllocale> for endless details on locales. |
5a964f20 | 812 | |
65acb1b1 | 813 | This is sometimes referred to as putting something into "title |
d92eb7b0 | 814 | case", but that's not quite accurate. Consider the proper |
65acb1b1 TC |
815 | capitalization of the movie I<Dr. Strangelove or: How I Learned to |
816 | Stop Worrying and Love the Bomb>, for example. | |
817 | ||
369b44b4 RGS |
818 | Damian Conway's L<Text::Autoformat> module provides some smart |
819 | case transformations: | |
820 | ||
821 | use Text::Autoformat; | |
822 | my $x = "Dr. Strangelove or: How I Learned to Stop ". | |
823 | "Worrying and Love the Bomb"; | |
824 | ||
825 | print $x, "\n"; | |
826 | for my $style (qw( sentence title highlight )) | |
827 | { | |
828 | print autoformat($x, { case => $style }), "\n"; | |
829 | } | |
830 | ||
49d635f9 | 831 | =head2 How can I split a [character] delimited string except when inside [character]? |
68dc0745 | 832 | |
49d635f9 | 833 | Several modules can handle this sort of pasing---Text::Balanced, |
7678cced | 834 | Text::CSV, Text::CSV_XS, and Text::ParseWords, among others. |
49d635f9 RGS |
835 | |
836 | Take the example case of trying to split a string that is | |
837 | comma-separated into its different fields. You can't use C<split(/,/)> | |
838 | because you shouldn't split if the comma is inside quotes. For | |
839 | example, take a data line like this: | |
68dc0745 | 840 | |
841 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" | |
842 | ||
843 | Due to the restriction of the quotes, this is a fairly complex | |
197aec24 | 844 | problem. Thankfully, we have Jeffrey Friedl, author of |
49d635f9 | 845 | I<Mastering Regular Expressions>, to handle these for us. He |
68dc0745 | 846 | suggests (assuming your string is contained in $text): |
847 | ||
848 | @new = (); | |
849 | push(@new, $+) while $text =~ m{ | |
850 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes | |
851 | | ([^,]+),? | |
852 | | , | |
853 | }gx; | |
854 | push(@new, undef) if substr($text,-1,1) eq ','; | |
855 | ||
46fc3d4c | 856 | If you want to represent quotation marks inside a |
857 | quotation-mark-delimited field, escape them with backslashes (eg, | |
49d635f9 | 858 | C<"like \"this\"">. |
46fc3d4c | 859 | |
87275199 | 860 | Alternatively, the Text::ParseWords module (part of the standard Perl |
68dc0745 | 861 | distribution) lets you say: |
862 | ||
863 | use Text::ParseWords; | |
864 | @new = quotewords(",", 0, $text); | |
865 | ||
a6dd486b | 866 | There's also a Text::CSV (Comma-Separated Values) module on CPAN. |
65acb1b1 | 867 | |
68dc0745 | 868 | =head2 How do I strip blank space from the beginning/end of a string? |
869 | ||
6670e5e7 | 870 | (contributed by brian d foy) |
68dc0745 | 871 | |
6670e5e7 RGS |
872 | A substitution can do this for you. For a single line, you want to |
873 | replace all the leading or trailing whitespace with nothing. You | |
874 | can do that with a pair of substitutions. | |
68dc0745 | 875 | |
6670e5e7 RGS |
876 | s/^\s+//; |
877 | s/\s+$//; | |
68dc0745 | 878 | |
6670e5e7 RGS |
879 | You can also write that as a single substitution, although it turns |
880 | out the combined statement is slower than the separate ones. That | |
881 | might not matter to you, though. | |
68dc0745 | 882 | |
6670e5e7 | 883 | s/^\s+|\s+$//g; |
68dc0745 | 884 | |
6670e5e7 RGS |
885 | In this regular expression, the alternation matches either at the |
886 | beginning or the end of the string since the anchors have a lower | |
887 | precedence than the alternation. With the C</g> flag, the substitution | |
888 | makes all possible matches, so it gets both. Remember, the trailing | |
889 | newline matches the C<\s+>, and the C<$> anchor can match to the | |
890 | physical end of the string, so the newline disappears too. Just add | |
891 | the newline to the output, which has the added benefit of preserving | |
892 | "blank" (consisting entirely of whitespace) lines which the C<^\s+> | |
893 | would remove all by itself. | |
68dc0745 | 894 | |
6670e5e7 RGS |
895 | while( <> ) |
896 | { | |
897 | s/^\s+|\s+$//g; | |
898 | print "$_\n"; | |
899 | } | |
5a964f20 | 900 | |
6670e5e7 RGS |
901 | For a multi-line string, you can apply the regular expression |
902 | to each logical line in the string by adding the C</m> flag (for | |
903 | "multi-line"). With the C</m> flag, the C<$> matches I<before> an | |
904 | embedded newline, so it doesn't remove it. It still removes the | |
905 | newline at the end of the string. | |
906 | ||
907 | $string =~ s/^\s+|\s+$//gm; | |
908 | ||
909 | Remember that lines consisting entirely of whitespace will disappear, | |
910 | since the first part of the alternation can match the entire string | |
911 | and replace it with nothing. If need to keep embedded blank lines, | |
912 | you have to do a little more work. Instead of matching any whitespace | |
913 | (since that includes a newline), just match the other whitespace. | |
914 | ||
915 | $string =~ s/^[\t\f ]+|[\t\f ]+$//mg; | |
5a964f20 | 916 | |
65acb1b1 TC |
917 | =head2 How do I pad a string with blanks or pad a number with zeroes? |
918 | ||
65acb1b1 | 919 | In the following examples, C<$pad_len> is the length to which you wish |
d92eb7b0 GS |
920 | to pad the string, C<$text> or C<$num> contains the string to be padded, |
921 | and C<$pad_char> contains the padding character. You can use a single | |
922 | character string constant instead of the C<$pad_char> variable if you | |
923 | know what it is in advance. And in the same way you can use an integer in | |
924 | place of C<$pad_len> if you know the pad length in advance. | |
65acb1b1 | 925 | |
d92eb7b0 GS |
926 | The simplest method uses the C<sprintf> function. It can pad on the left |
927 | or right with blanks and on the left with zeroes and it will not | |
928 | truncate the result. The C<pack> function can only pad strings on the | |
929 | right with blanks and it will truncate the result to a maximum length of | |
930 | C<$pad_len>. | |
65acb1b1 | 931 | |
d92eb7b0 | 932 | # Left padding a string with blanks (no truncation): |
04d666b1 RGS |
933 | $padded = sprintf("%${pad_len}s", $text); |
934 | $padded = sprintf("%*s", $pad_len, $text); # same thing | |
65acb1b1 | 935 | |
d92eb7b0 | 936 | # Right padding a string with blanks (no truncation): |
04d666b1 RGS |
937 | $padded = sprintf("%-${pad_len}s", $text); |
938 | $padded = sprintf("%-*s", $pad_len, $text); # same thing | |
65acb1b1 | 939 | |
197aec24 | 940 | # Left padding a number with 0 (no truncation): |
04d666b1 RGS |
941 | $padded = sprintf("%0${pad_len}d", $num); |
942 | $padded = sprintf("%0*d", $pad_len, $num); # same thing | |
65acb1b1 | 943 | |
d92eb7b0 GS |
944 | # Right padding a string with blanks using pack (will truncate): |
945 | $padded = pack("A$pad_len",$text); | |
65acb1b1 | 946 | |
d92eb7b0 GS |
947 | If you need to pad with a character other than blank or zero you can use |
948 | one of the following methods. They all generate a pad string with the | |
949 | C<x> operator and combine that with C<$text>. These methods do | |
950 | not truncate C<$text>. | |
65acb1b1 | 951 | |
d92eb7b0 | 952 | Left and right padding with any character, creating a new string: |
65acb1b1 | 953 | |
d92eb7b0 GS |
954 | $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; |
955 | $padded = $text . $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 956 | |
d92eb7b0 | 957 | Left and right padding with any character, modifying C<$text> directly: |
65acb1b1 | 958 | |
d92eb7b0 GS |
959 | substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); |
960 | $text .= $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 961 | |
68dc0745 | 962 | =head2 How do I extract selected columns from a string? |
963 | ||
964 | Use substr() or unpack(), both documented in L<perlfunc>. | |
197aec24 | 965 | If you prefer thinking in terms of columns instead of widths, |
5a964f20 TC |
966 | you can use this kind of thing: |
967 | ||
968 | # determine the unpack format needed to split Linux ps output | |
969 | # arguments are cut columns | |
970 | my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72); | |
971 | ||
197aec24 | 972 | sub cut2fmt { |
5a964f20 TC |
973 | my(@positions) = @_; |
974 | my $template = ''; | |
975 | my $lastpos = 1; | |
976 | for my $place (@positions) { | |
197aec24 | 977 | $template .= "A" . ($place - $lastpos) . " "; |
5a964f20 TC |
978 | $lastpos = $place; |
979 | } | |
980 | $template .= "A*"; | |
981 | return $template; | |
982 | } | |
68dc0745 | 983 | |
984 | =head2 How do I find the soundex value of a string? | |
985 | ||
7678cced RGS |
986 | (contributed by brian d foy) |
987 | ||
988 | You can use the Text::Soundex module. If you want to do fuzzy or close | |
989 | matching, you might also try the String::Approx, and Text::Metaphone, | |
990 | and Text::DoubleMetaphone modules. | |
68dc0745 | 991 | |
992 | =head2 How can I expand variables in text strings? | |
993 | ||
7678cced RGS |
994 | Let's assume that you have a string that contains placeholder |
995 | variables. | |
68dc0745 | 996 | |
997 | $text = 'this has a $foo in it and a $bar'; | |
5a964f20 | 998 | |
7678cced RGS |
999 | You can use a substitution with a double evaluation. The |
1000 | first /e turns C<$1> into C<$foo>, and the second /e turns | |
1001 | C<$foo> into its value. You may want to wrap this in an | |
1002 | C<eval>: if you try to get the value of an undeclared variable | |
1003 | while running under C<use strict>, you get a fatal error. | |
5a964f20 | 1004 | |
7678cced RGS |
1005 | eval { $text =~ s/(\$\w+)/$1/eeg }; |
1006 | die if $@; | |
68dc0745 | 1007 | |
5a964f20 TC |
1008 | It's probably better in the general case to treat those |
1009 | variables as entries in some special hash. For example: | |
1010 | ||
197aec24 | 1011 | %user_defs = ( |
5a964f20 TC |
1012 | foo => 23, |
1013 | bar => 19, | |
1014 | ); | |
1015 | $text =~ s/\$(\w+)/$user_defs{$1}/g; | |
68dc0745 | 1016 | |
1017 | =head2 What's wrong with always quoting "$vars"? | |
1018 | ||
a6dd486b JB |
1019 | The problem is that those double-quotes force stringification-- |
1020 | coercing numbers and references into strings--even when you | |
1021 | don't want them to be strings. Think of it this way: double-quote | |
197aec24 | 1022 | expansion is used to produce new strings. If you already |
65acb1b1 | 1023 | have a string, why do you need more? |
68dc0745 | 1024 | |
1025 | If you get used to writing odd things like these: | |
1026 | ||
1027 | print "$var"; # BAD | |
1028 | $new = "$old"; # BAD | |
1029 | somefunc("$var"); # BAD | |
1030 | ||
1031 | You'll be in trouble. Those should (in 99.8% of the cases) be | |
1032 | the simpler and more direct: | |
1033 | ||
1034 | print $var; | |
1035 | $new = $old; | |
1036 | somefunc($var); | |
1037 | ||
1038 | Otherwise, besides slowing you down, you're going to break code when | |
1039 | the thing in the scalar is actually neither a string nor a number, but | |
1040 | a reference: | |
1041 | ||
1042 | func(\@array); | |
1043 | sub func { | |
1044 | my $aref = shift; | |
1045 | my $oref = "$aref"; # WRONG | |
1046 | } | |
1047 | ||
1048 | You can also get into subtle problems on those few operations in Perl | |
1049 | that actually do care about the difference between a string and a | |
1050 | number, such as the magical C<++> autoincrement operator or the | |
1051 | syscall() function. | |
1052 | ||
197aec24 | 1053 | Stringification also destroys arrays. |
5a964f20 TC |
1054 | |
1055 | @lines = `command`; | |
1056 | print "@lines"; # WRONG - extra blanks | |
1057 | print @lines; # right | |
1058 | ||
04d666b1 | 1059 | =head2 Why don't my E<lt>E<lt>HERE documents work? |
68dc0745 | 1060 | |
1061 | Check for these three things: | |
1062 | ||
1063 | =over 4 | |
1064 | ||
04d666b1 | 1065 | =item There must be no space after the E<lt>E<lt> part. |
68dc0745 | 1066 | |
197aec24 | 1067 | =item There (probably) should be a semicolon at the end. |
68dc0745 | 1068 | |
197aec24 | 1069 | =item You can't (easily) have any space in front of the tag. |
68dc0745 | 1070 | |
1071 | =back | |
1072 | ||
197aec24 | 1073 | If you want to indent the text in the here document, you |
5a964f20 TC |
1074 | can do this: |
1075 | ||
1076 | # all in one | |
1077 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; | |
1078 | your text | |
1079 | goes here | |
1080 | HERE_TARGET | |
1081 | ||
1082 | But the HERE_TARGET must still be flush against the margin. | |
197aec24 | 1083 | If you want that indented also, you'll have to quote |
5a964f20 TC |
1084 | in the indentation. |
1085 | ||
1086 | ($quote = <<' FINIS') =~ s/^\s+//gm; | |
1087 | ...we will have peace, when you and all your works have | |
1088 | perished--and the works of your dark master to whom you | |
1089 | would deliver us. You are a liar, Saruman, and a corrupter | |
1090 | of men's hearts. --Theoden in /usr/src/perl/taint.c | |
1091 | FINIS | |
83ded9ee | 1092 | $quote =~ s/\s+--/\n--/; |
5a964f20 TC |
1093 | |
1094 | A nice general-purpose fixer-upper function for indented here documents | |
1095 | follows. It expects to be called with a here document as its argument. | |
1096 | It looks to see whether each line begins with a common substring, and | |
a6dd486b JB |
1097 | if so, strips that substring off. Otherwise, it takes the amount of leading |
1098 | whitespace found on the first line and removes that much off each | |
5a964f20 TC |
1099 | subsequent line. |
1100 | ||
1101 | sub fix { | |
1102 | local $_ = shift; | |
a6dd486b | 1103 | my ($white, $leader); # common whitespace and common leading string |
5a964f20 TC |
1104 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { |
1105 | ($white, $leader) = ($2, quotemeta($1)); | |
1106 | } else { | |
1107 | ($white, $leader) = (/^(\s+)/, ''); | |
1108 | } | |
1109 | s/^\s*?$leader(?:$white)?//gm; | |
1110 | return $_; | |
1111 | } | |
1112 | ||
c8db1d39 | 1113 | This works with leading special strings, dynamically determined: |
5a964f20 TC |
1114 | |
1115 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; | |
1116 | @@@ int | |
1117 | @@@ runops() { | |
1118 | @@@ SAVEI32(runlevel); | |
1119 | @@@ runlevel++; | |
d92eb7b0 | 1120 | @@@ while ( op = (*op->op_ppaddr)() ); |
5a964f20 TC |
1121 | @@@ TAINT_NOT; |
1122 | @@@ return 0; | |
1123 | @@@ } | |
1124 | MAIN_INTERPRETER_LOOP | |
1125 | ||
a6dd486b | 1126 | Or with a fixed amount of leading whitespace, with remaining |
5a964f20 TC |
1127 | indentation correctly preserved: |
1128 | ||
1129 | $poem = fix<<EVER_ON_AND_ON; | |
1130 | Now far ahead the Road has gone, | |
1131 | And I must follow, if I can, | |
1132 | Pursuing it with eager feet, | |
1133 | Until it joins some larger way | |
1134 | Where many paths and errands meet. | |
1135 | And whither then? I cannot say. | |
1136 | --Bilbo in /usr/src/perl/pp_ctl.c | |
1137 | EVER_ON_AND_ON | |
1138 | ||
68dc0745 | 1139 | =head1 Data: Arrays |
1140 | ||
65acb1b1 TC |
1141 | =head2 What is the difference between a list and an array? |
1142 | ||
1143 | An array has a changeable length. A list does not. An array is something | |
1144 | you can push or pop, while a list is a set of values. Some people make | |
1145 | the distinction that a list is a value while an array is a variable. | |
1146 | Subroutines are passed and return lists, you put things into list | |
1147 | context, you initialize arrays with lists, and you foreach() across | |
1148 | a list. C<@> variables are arrays, anonymous arrays are arrays, arrays | |
1149 | in scalar context behave like the number of elements in them, subroutines | |
a6dd486b | 1150 | access their arguments through the array C<@_>, and push/pop/shift only work |
65acb1b1 TC |
1151 | on arrays. |
1152 | ||
1153 | As a side note, there's no such thing as a list in scalar context. | |
1154 | When you say | |
1155 | ||
1156 | $scalar = (2, 5, 7, 9); | |
1157 | ||
d92eb7b0 GS |
1158 | you're using the comma operator in scalar context, so it uses the scalar |
1159 | comma operator. There never was a list there at all! This causes the | |
1160 | last value to be returned: 9. | |
65acb1b1 | 1161 | |
68dc0745 | 1162 | =head2 What is the difference between $array[1] and @array[1]? |
1163 | ||
a6dd486b | 1164 | The former is a scalar value; the latter an array slice, making |
68dc0745 | 1165 | it a list with one (scalar) value. You should use $ when you want a |
1166 | scalar value (most of the time) and @ when you want a list with one | |
1167 | scalar value in it (very, very rarely; nearly never, in fact). | |
1168 | ||
1169 | Sometimes it doesn't make a difference, but sometimes it does. | |
1170 | For example, compare: | |
1171 | ||
1172 | $good[0] = `some program that outputs several lines`; | |
1173 | ||
1174 | with | |
1175 | ||
1176 | @bad[0] = `same program that outputs several lines`; | |
1177 | ||
197aec24 | 1178 | The C<use warnings> pragma and the B<-w> flag will warn you about these |
9f1b1f2d | 1179 | matters. |
68dc0745 | 1180 | |
d92eb7b0 | 1181 | =head2 How can I remove duplicate elements from a list or array? |
68dc0745 | 1182 | |
6670e5e7 | 1183 | (contributed by brian d foy) |
68dc0745 | 1184 | |
6670e5e7 RGS |
1185 | Use a hash. When you think the words "unique" or "duplicated", think |
1186 | "hash keys". | |
68dc0745 | 1187 | |
6670e5e7 RGS |
1188 | If you don't care about the order of the elements, you could just |
1189 | create the hash then extract the keys. It's not important how you | |
1190 | create that hash: just that you use C<keys> to get the unique | |
1191 | elements. | |
551e1d92 | 1192 | |
6670e5e7 RGS |
1193 | my %hash = map { $_, 1 } @array; |
1194 | # or a hash slice: @hash{ @array } = (); | |
1195 | # or a foreach: $hash{$_} = 1 foreach ( @array ); | |
68dc0745 | 1196 | |
6670e5e7 | 1197 | my @unique = keys %hash; |
68dc0745 | 1198 | |
6670e5e7 RGS |
1199 | You can also go through each element and skip the ones you've seen |
1200 | before. Use a hash to keep track. The first time the loop sees an | |
1201 | element, that element has no key in C<%Seen>. The C<next> statement | |
1202 | creates the key and immediately uses its value, which is C<undef>, so | |
1203 | the loop continues to the C<push> and increments the value for that | |
1204 | key. The next time the loop sees that same element, its key exists in | |
1205 | the hash I<and> the value for that key is true (since it's not 0 or | |
1206 | undef), so the next skips that iteration and the loop goes to the next | |
1207 | element. | |
551e1d92 | 1208 | |
6670e5e7 RGS |
1209 | my @unique = (); |
1210 | my %seen = (); | |
68dc0745 | 1211 | |
6670e5e7 RGS |
1212 | foreach my $elem ( @array ) |
1213 | { | |
1214 | next if $seen{ $elem }++; | |
1215 | push @unique, $elem; | |
1216 | } | |
68dc0745 | 1217 | |
6670e5e7 RGS |
1218 | You can write this more briefly using a grep, which does the |
1219 | same thing. | |
68dc0745 | 1220 | |
6670e5e7 RGS |
1221 | my %seen = (); |
1222 | my @unique = grep { ! $seen{ $_ }++ } @array; | |
65acb1b1 | 1223 | |
ddbc1f16 | 1224 | =head2 How can I tell whether a certain element is contained in a list or array? |
5a964f20 | 1225 | |
9e72e4c6 RGS |
1226 | (portions of this answer contributed by Anno Siegel) |
1227 | ||
5a964f20 TC |
1228 | Hearing the word "in" is an I<in>dication that you probably should have |
1229 | used a hash, not a list or array, to store your data. Hashes are | |
1230 | designed to answer this question quickly and efficiently. Arrays aren't. | |
68dc0745 | 1231 | |
5a964f20 TC |
1232 | That being said, there are several ways to approach this. If you |
1233 | are going to make this query many times over arbitrary string values, | |
881bdbd4 JH |
1234 | the fastest way is probably to invert the original array and maintain a |
1235 | hash whose keys are the first array's values. | |
68dc0745 | 1236 | |
1237 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; | |
881bdbd4 | 1238 | %is_blue = (); |
68dc0745 | 1239 | for (@blues) { $is_blue{$_} = 1 } |
1240 | ||
1241 | Now you can check whether $is_blue{$some_color}. It might have been a | |
1242 | good idea to keep the blues all in a hash in the first place. | |
1243 | ||
1244 | If the values are all small integers, you could use a simple indexed | |
1245 | array. This kind of an array will take up less space: | |
1246 | ||
1247 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); | |
881bdbd4 | 1248 | @is_tiny_prime = (); |
d92eb7b0 GS |
1249 | for (@primes) { $is_tiny_prime[$_] = 1 } |
1250 | # or simply @istiny_prime[@primes] = (1) x @primes; | |
68dc0745 | 1251 | |
1252 | Now you check whether $is_tiny_prime[$some_number]. | |
1253 | ||
1254 | If the values in question are integers instead of strings, you can save | |
1255 | quite a lot of space by using bit strings instead: | |
1256 | ||
1257 | @articles = ( 1..10, 150..2000, 2017 ); | |
1258 | undef $read; | |
7b8d334a | 1259 | for (@articles) { vec($read,$_,1) = 1 } |
68dc0745 | 1260 | |
1261 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. | |
1262 | ||
9e72e4c6 RGS |
1263 | These methods guarantee fast individual tests but require a re-organization |
1264 | of the original list or array. They only pay off if you have to test | |
1265 | multiple values against the same array. | |
68dc0745 | 1266 | |
9e72e4c6 RGS |
1267 | If you are testing only once, the standard module List::Util exports |
1268 | the function C<first> for this purpose. It works by stopping once it | |
1269 | finds the element. It's written in C for speed, and its Perl equivalant | |
1270 | looks like this subroutine: | |
68dc0745 | 1271 | |
9e72e4c6 RGS |
1272 | sub first (&@) { |
1273 | my $code = shift; | |
1274 | foreach (@_) { | |
1275 | return $_ if &{$code}(); | |
1276 | } | |
1277 | undef; | |
1278 | } | |
68dc0745 | 1279 | |
9e72e4c6 RGS |
1280 | If speed is of little concern, the common idiom uses grep in scalar context |
1281 | (which returns the number of items that passed its condition) to traverse the | |
1282 | entire list. This does have the benefit of telling you how many matches it | |
1283 | found, though. | |
68dc0745 | 1284 | |
9e72e4c6 | 1285 | my $is_there = grep $_ eq $whatever, @array; |
65acb1b1 | 1286 | |
9e72e4c6 RGS |
1287 | If you want to actually extract the matching elements, simply use grep in |
1288 | list context. | |
68dc0745 | 1289 | |
9e72e4c6 | 1290 | my @matches = grep $_ eq $whatever, @array; |
58103a2e | 1291 | |
68dc0745 | 1292 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? |
1293 | ||
1294 | Use a hash. Here's code to do both and more. It assumes that | |
1295 | each element is unique in a given array: | |
1296 | ||
1297 | @union = @intersection = @difference = (); | |
1298 | %count = (); | |
1299 | foreach $element (@array1, @array2) { $count{$element}++ } | |
1300 | foreach $element (keys %count) { | |
1301 | push @union, $element; | |
1302 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; | |
1303 | } | |
1304 | ||
d92eb7b0 | 1305 | Note that this is the I<symmetric difference>, that is, all elements in |
a6dd486b | 1306 | either A or in B but not in both. Think of it as an xor operation. |
d92eb7b0 | 1307 | |
65acb1b1 TC |
1308 | =head2 How do I test whether two arrays or hashes are equal? |
1309 | ||
1310 | The following code works for single-level arrays. It uses a stringwise | |
1311 | comparison, and does not distinguish defined versus undefined empty | |
1312 | strings. Modify if you have other needs. | |
1313 | ||
1314 | $are_equal = compare_arrays(\@frogs, \@toads); | |
1315 | ||
1316 | sub compare_arrays { | |
1317 | my ($first, $second) = @_; | |
9f1b1f2d | 1318 | no warnings; # silence spurious -w undef complaints |
65acb1b1 TC |
1319 | return 0 unless @$first == @$second; |
1320 | for (my $i = 0; $i < @$first; $i++) { | |
1321 | return 0 if $first->[$i] ne $second->[$i]; | |
1322 | } | |
1323 | return 1; | |
1324 | } | |
1325 | ||
1326 | For multilevel structures, you may wish to use an approach more | |
1327 | like this one. It uses the CPAN module FreezeThaw: | |
1328 | ||
1329 | use FreezeThaw qw(cmpStr); | |
1330 | @a = @b = ( "this", "that", [ "more", "stuff" ] ); | |
1331 | ||
1332 | printf "a and b contain %s arrays\n", | |
197aec24 RGS |
1333 | cmpStr(\@a, \@b) == 0 |
1334 | ? "the same" | |
65acb1b1 TC |
1335 | : "different"; |
1336 | ||
1337 | This approach also works for comparing hashes. Here | |
1338 | we'll demonstrate two different answers: | |
1339 | ||
1340 | use FreezeThaw qw(cmpStr cmpStrHard); | |
1341 | ||
1342 | %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); | |
1343 | $a{EXTRA} = \%b; | |
197aec24 | 1344 | $b{EXTRA} = \%a; |
65acb1b1 TC |
1345 | |
1346 | printf "a and b contain %s hashes\n", | |
1347 | cmpStr(\%a, \%b) == 0 ? "the same" : "different"; | |
1348 | ||
1349 | printf "a and b contain %s hashes\n", | |
1350 | cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; | |
1351 | ||
1352 | ||
1353 | The first reports that both those the hashes contain the same data, | |
1354 | while the second reports that they do not. Which you prefer is left as | |
1355 | an exercise to the reader. | |
1356 | ||
68dc0745 | 1357 | =head2 How do I find the first array element for which a condition is true? |
1358 | ||
49d635f9 RGS |
1359 | To find the first array element which satisfies a condition, you can |
1360 | use the first() function in the List::Util module, which comes with | |
1361 | Perl 5.8. This example finds the first element that contains "Perl". | |
1362 | ||
1363 | use List::Util qw(first); | |
197aec24 | 1364 | |
49d635f9 | 1365 | my $element = first { /Perl/ } @array; |
197aec24 | 1366 | |
49d635f9 RGS |
1367 | If you cannot use List::Util, you can make your own loop to do the |
1368 | same thing. Once you find the element, you stop the loop with last. | |
1369 | ||
1370 | my $found; | |
6670e5e7 | 1371 | foreach ( @array ) |
49d635f9 | 1372 | { |
6670e5e7 | 1373 | if( /Perl/ ) { $found = $_; last } |
49d635f9 RGS |
1374 | } |
1375 | ||
1376 | If you want the array index, you can iterate through the indices | |
1377 | and check the array element at each index until you find one | |
1378 | that satisfies the condition. | |
1379 | ||
197aec24 | 1380 | my( $found, $index ) = ( undef, -1 ); |
6670e5e7 RGS |
1381 | for( $i = 0; $i < @array; $i++ ) |
1382 | { | |
1383 | if( $array[$i] =~ /Perl/ ) | |
1384 | { | |
1385 | $found = $array[$i]; | |
1386 | $index = $i; | |
1387 | last; | |
1388 | } | |
1389 | } | |
68dc0745 | 1390 | |
1391 | =head2 How do I handle linked lists? | |
1392 | ||
1393 | In general, you usually don't need a linked list in Perl, since with | |
1394 | regular arrays, you can push and pop or shift and unshift at either end, | |
5a964f20 | 1395 | or you can use splice to add and/or remove arbitrary number of elements at |
87275199 | 1396 | arbitrary points. Both pop and shift are both O(1) operations on Perl's |
5a964f20 TC |
1397 | dynamic arrays. In the absence of shifts and pops, push in general |
1398 | needs to reallocate on the order every log(N) times, and unshift will | |
1399 | need to copy pointers each time. | |
68dc0745 | 1400 | |
1401 | If you really, really wanted, you could use structures as described in | |
1402 | L<perldsc> or L<perltoot> and do just what the algorithm book tells you | |
65acb1b1 TC |
1403 | to do. For example, imagine a list node like this: |
1404 | ||
1405 | $node = { | |
1406 | VALUE => 42, | |
1407 | LINK => undef, | |
1408 | }; | |
1409 | ||
1410 | You could walk the list this way: | |
1411 | ||
1412 | print "List: "; | |
1413 | for ($node = $head; $node; $node = $node->{LINK}) { | |
1414 | print $node->{VALUE}, " "; | |
1415 | } | |
1416 | print "\n"; | |
1417 | ||
a6dd486b | 1418 | You could add to the list this way: |
65acb1b1 TC |
1419 | |
1420 | my ($head, $tail); | |
1421 | $tail = append($head, 1); # grow a new head | |
1422 | for $value ( 2 .. 10 ) { | |
1423 | $tail = append($tail, $value); | |
1424 | } | |
1425 | ||
1426 | sub append { | |
1427 | my($list, $value) = @_; | |
1428 | my $node = { VALUE => $value }; | |
1429 | if ($list) { | |
1430 | $node->{LINK} = $list->{LINK}; | |
1431 | $list->{LINK} = $node; | |
1432 | } else { | |
1433 | $_[0] = $node; # replace caller's version | |
1434 | } | |
1435 | return $node; | |
1436 | } | |
1437 | ||
1438 | But again, Perl's built-in are virtually always good enough. | |
68dc0745 | 1439 | |
1440 | =head2 How do I handle circular lists? | |
1441 | ||
1442 | Circular lists could be handled in the traditional fashion with linked | |
1443 | lists, or you could just do something like this with an array: | |
1444 | ||
1445 | unshift(@array, pop(@array)); # the last shall be first | |
1446 | push(@array, shift(@array)); # and vice versa | |
1447 | ||
1448 | =head2 How do I shuffle an array randomly? | |
1449 | ||
45bbf655 JH |
1450 | If you either have Perl 5.8.0 or later installed, or if you have |
1451 | Scalar-List-Utils 1.03 or later installed, you can say: | |
1452 | ||
f05bbc40 | 1453 | use List::Util 'shuffle'; |
45bbf655 JH |
1454 | |
1455 | @shuffled = shuffle(@list); | |
1456 | ||
f05bbc40 | 1457 | If not, you can use a Fisher-Yates shuffle. |
5a964f20 | 1458 | |
5a964f20 | 1459 | sub fisher_yates_shuffle { |
cc30d1a7 JH |
1460 | my $deck = shift; # $deck is a reference to an array |
1461 | my $i = @$deck; | |
6670e5e7 | 1462 | while (--$i) { |
5a964f20 | 1463 | my $j = int rand ($i+1); |
cc30d1a7 | 1464 | @$deck[$i,$j] = @$deck[$j,$i]; |
5a964f20 TC |
1465 | } |
1466 | } | |
1467 | ||
cc30d1a7 JH |
1468 | # shuffle my mpeg collection |
1469 | # | |
1470 | my @mpeg = <audio/*/*.mp3>; | |
1471 | fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place | |
1472 | print @mpeg; | |
5a964f20 | 1473 | |
45bbf655 JH |
1474 | Note that the above implementation shuffles an array in place, |
1475 | unlike the List::Util::shuffle() which takes a list and returns | |
1476 | a new shuffled list. | |
1477 | ||
d92eb7b0 | 1478 | You've probably seen shuffling algorithms that work using splice, |
a6dd486b | 1479 | randomly picking another element to swap the current element with |
68dc0745 | 1480 | |
1481 | srand; | |
1482 | @new = (); | |
1483 | @old = 1 .. 10; # just a demo | |
1484 | while (@old) { | |
1485 | push(@new, splice(@old, rand @old, 1)); | |
1486 | } | |
1487 | ||
5a964f20 TC |
1488 | This is bad because splice is already O(N), and since you do it N times, |
1489 | you just invented a quadratic algorithm; that is, O(N**2). This does | |
1490 | not scale, although Perl is so efficient that you probably won't notice | |
1491 | this until you have rather largish arrays. | |
68dc0745 | 1492 | |
1493 | =head2 How do I process/modify each element of an array? | |
1494 | ||
1495 | Use C<for>/C<foreach>: | |
1496 | ||
1497 | for (@lines) { | |
6670e5e7 RGS |
1498 | s/foo/bar/; # change that word |
1499 | tr/XZ/ZX/; # swap those letters | |
68dc0745 | 1500 | } |
1501 | ||
1502 | Here's another; let's compute spherical volumes: | |
1503 | ||
5a964f20 | 1504 | for (@volumes = @radii) { # @volumes has changed parts |
6670e5e7 RGS |
1505 | $_ **= 3; |
1506 | $_ *= (4/3) * 3.14159; # this will be constant folded | |
68dc0745 | 1507 | } |
197aec24 | 1508 | |
49d635f9 RGS |
1509 | which can also be done with map() which is made to transform |
1510 | one list into another: | |
1511 | ||
1512 | @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii; | |
68dc0745 | 1513 | |
76817d6d JH |
1514 | If you want to do the same thing to modify the values of the |
1515 | hash, you can use the C<values> function. As of Perl 5.6 | |
1516 | the values are not copied, so if you modify $orbit (in this | |
1517 | case), you modify the value. | |
5a964f20 | 1518 | |
76817d6d | 1519 | for $orbit ( values %orbits ) { |
6670e5e7 | 1520 | ($orbit **= 3) *= (4/3) * 3.14159; |
5a964f20 | 1521 | } |
818c4caa | 1522 | |
76817d6d JH |
1523 | Prior to perl 5.6 C<values> returned copies of the values, |
1524 | so older perl code often contains constructions such as | |
1525 | C<@orbits{keys %orbits}> instead of C<values %orbits> where | |
1526 | the hash is to be modified. | |
818c4caa | 1527 | |
68dc0745 | 1528 | =head2 How do I select a random element from an array? |
1529 | ||
1530 | Use the rand() function (see L<perlfunc/rand>): | |
1531 | ||
68dc0745 | 1532 | $index = rand @array; |
1533 | $element = $array[$index]; | |
1534 | ||
793f5136 RGS |
1535 | Or, simply: |
1536 | my $element = $array[ rand @array ]; | |
5a964f20 | 1537 | |
68dc0745 | 1538 | =head2 How do I permute N elements of a list? |
1539 | ||
49d635f9 RGS |
1540 | Use the List::Permutor module on CPAN. If the list is |
1541 | actually an array, try the Algorithm::Permute module (also | |
1542 | on CPAN). It's written in XS code and is very efficient. | |
1543 | ||
1544 | use Algorithm::Permute; | |
1545 | my @array = 'a'..'d'; | |
1546 | my $p_iterator = Algorithm::Permute->new ( \@array ); | |
1547 | while (my @perm = $p_iterator->next) { | |
1548 | print "next permutation: (@perm)\n"; | |
1549 | } | |
1550 | ||
197aec24 RGS |
1551 | For even faster execution, you could do: |
1552 | ||
1553 | use Algorithm::Permute; | |
1554 | my @array = 'a'..'d'; | |
1555 | Algorithm::Permute::permute { | |
1556 | print "next permutation: (@array)\n"; | |
1557 | } @array; | |
1558 | ||
49d635f9 RGS |
1559 | Here's a little program that generates all permutations of |
1560 | all the words on each line of input. The algorithm embodied | |
1561 | in the permute() function is discussed in Volume 4 (still | |
1562 | unpublished) of Knuth's I<The Art of Computer Programming> | |
1563 | and will work on any list: | |
1564 | ||
1565 | #!/usr/bin/perl -n | |
1566 | # Fischer-Kause ordered permutation generator | |
1567 | ||
1568 | sub permute (&@) { | |
1569 | my $code = shift; | |
1570 | my @idx = 0..$#_; | |
1571 | while ( $code->(@_[@idx]) ) { | |
1572 | my $p = $#idx; | |
1573 | --$p while $idx[$p-1] > $idx[$p]; | |
1574 | my $q = $p or return; | |
1575 | push @idx, reverse splice @idx, $p; | |
1576 | ++$q while $idx[$p-1] > $idx[$q]; | |
1577 | @idx[$p-1,$q]=@idx[$q,$p-1]; | |
1578 | } | |
68dc0745 | 1579 | } |
68dc0745 | 1580 | |
49d635f9 | 1581 | permute {print"@_\n"} split; |
b8d2732a | 1582 | |
68dc0745 | 1583 | =head2 How do I sort an array by (anything)? |
1584 | ||
1585 | Supply a comparison function to sort() (described in L<perlfunc/sort>): | |
1586 | ||
1587 | @list = sort { $a <=> $b } @list; | |
1588 | ||
1589 | The default sort function is cmp, string comparison, which would | |
c47ff5f1 | 1590 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is |
68dc0745 | 1591 | the numerical comparison operator. |
1592 | ||
1593 | If you have a complicated function needed to pull out the part you | |
1594 | want to sort on, then don't do it inside the sort function. Pull it | |
1595 | out first, because the sort BLOCK can be called many times for the | |
1596 | same element. Here's an example of how to pull out the first word | |
1597 | after the first number on each item, and then sort those words | |
1598 | case-insensitively. | |
1599 | ||
1600 | @idx = (); | |
1601 | for (@data) { | |
1602 | ($item) = /\d+\s*(\S+)/; | |
1603 | push @idx, uc($item); | |
1604 | } | |
1605 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; | |
1606 | ||
a6dd486b | 1607 | which could also be written this way, using a trick |
68dc0745 | 1608 | that's come to be known as the Schwartzian Transform: |
1609 | ||
1610 | @sorted = map { $_->[0] } | |
1611 | sort { $a->[1] cmp $b->[1] } | |
d92eb7b0 | 1612 | map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; |
68dc0745 | 1613 | |
1614 | If you need to sort on several fields, the following paradigm is useful. | |
1615 | ||
1616 | @sorted = sort { field1($a) <=> field1($b) || | |
1617 | field2($a) cmp field2($b) || | |
1618 | field3($a) cmp field3($b) | |
1619 | } @data; | |
1620 | ||
1621 | This can be conveniently combined with precalculation of keys as given | |
1622 | above. | |
1623 | ||
379e39d7 | 1624 | See the F<sort> article in the "Far More Than You Ever Wanted |
49d635f9 | 1625 | To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for |
06a5f41f | 1626 | more about this approach. |
68dc0745 | 1627 | |
1628 | See also the question below on sorting hashes. | |
1629 | ||
1630 | =head2 How do I manipulate arrays of bits? | |
1631 | ||
1632 | Use pack() and unpack(), or else vec() and the bitwise operations. | |
1633 | ||
1634 | For example, this sets $vec to have bit N set if $ints[N] was set: | |
1635 | ||
1636 | $vec = ''; | |
1637 | foreach(@ints) { vec($vec,$_,1) = 1 } | |
1638 | ||
cc30d1a7 | 1639 | Here's how, given a vector in $vec, you can |
68dc0745 | 1640 | get those bits into your @ints array: |
1641 | ||
1642 | sub bitvec_to_list { | |
1643 | my $vec = shift; | |
1644 | my @ints; | |
1645 | # Find null-byte density then select best algorithm | |
1646 | if ($vec =~ tr/\0// / length $vec > 0.95) { | |
1647 | use integer; | |
1648 | my $i; | |
1649 | # This method is faster with mostly null-bytes | |
1650 | while($vec =~ /[^\0]/g ) { | |
1651 | $i = -9 + 8 * pos $vec; | |
1652 | push @ints, $i if vec($vec, ++$i, 1); | |
1653 | push @ints, $i if vec($vec, ++$i, 1); | |
1654 | push @ints, $i if vec($vec, ++$i, 1); | |
1655 | push @ints, $i if vec($vec, ++$i, 1); | |
1656 | push @ints, $i if vec($vec, ++$i, 1); | |
1657 | push @ints, $i if vec($vec, ++$i, 1); | |
1658 | push @ints, $i if vec($vec, ++$i, 1); | |
1659 | push @ints, $i if vec($vec, ++$i, 1); | |
1660 | } | |
1661 | } else { | |
1662 | # This method is a fast general algorithm | |
1663 | use integer; | |
1664 | my $bits = unpack "b*", $vec; | |
1665 | push @ints, 0 if $bits =~ s/^(\d)// && $1; | |
1666 | push @ints, pos $bits while($bits =~ /1/g); | |
1667 | } | |
1668 | return \@ints; | |
1669 | } | |
1670 | ||
1671 | This method gets faster the more sparse the bit vector is. | |
1672 | (Courtesy of Tim Bunce and Winfried Koenig.) | |
1673 | ||
76817d6d JH |
1674 | You can make the while loop a lot shorter with this suggestion |
1675 | from Benjamin Goldberg: | |
1676 | ||
1677 | while($vec =~ /[^\0]+/g ) { | |
1678 | push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8; | |
1679 | } | |
1680 | ||
cc30d1a7 JH |
1681 | Or use the CPAN module Bit::Vector: |
1682 | ||
1683 | $vector = Bit::Vector->new($num_of_bits); | |
1684 | $vector->Index_List_Store(@ints); | |
1685 | @ints = $vector->Index_List_Read(); | |
1686 | ||
1687 | Bit::Vector provides efficient methods for bit vector, sets of small integers | |
197aec24 | 1688 | and "big int" math. |
cc30d1a7 JH |
1689 | |
1690 | Here's a more extensive illustration using vec(): | |
65acb1b1 TC |
1691 | |
1692 | # vec demo | |
1693 | $vector = "\xff\x0f\xef\xfe"; | |
197aec24 | 1694 | print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", |
65acb1b1 TC |
1695 | unpack("N", $vector), "\n"; |
1696 | $is_set = vec($vector, 23, 1); | |
1697 | print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; | |
1698 | pvec($vector); | |
1699 | ||
1700 | set_vec(1,1,1); | |
1701 | set_vec(3,1,1); | |
1702 | set_vec(23,1,1); | |
1703 | ||
1704 | set_vec(3,1,3); | |
1705 | set_vec(3,2,3); | |
1706 | set_vec(3,4,3); | |
1707 | set_vec(3,4,7); | |
1708 | set_vec(3,8,3); | |
1709 | set_vec(3,8,7); | |
1710 | ||
1711 | set_vec(0,32,17); | |
1712 | set_vec(1,32,17); | |
1713 | ||
197aec24 | 1714 | sub set_vec { |
65acb1b1 TC |
1715 | my ($offset, $width, $value) = @_; |
1716 | my $vector = ''; | |
1717 | vec($vector, $offset, $width) = $value; | |
1718 | print "offset=$offset width=$width value=$value\n"; | |
1719 | pvec($vector); | |
1720 | } | |
1721 | ||
1722 | sub pvec { | |
1723 | my $vector = shift; | |
1724 | my $bits = unpack("b*", $vector); | |
1725 | my $i = 0; | |
1726 | my $BASE = 8; | |
1727 | ||
1728 | print "vector length in bytes: ", length($vector), "\n"; | |
1729 | @bytes = unpack("A8" x length($vector), $bits); | |
1730 | print "bits are: @bytes\n\n"; | |
197aec24 | 1731 | } |
65acb1b1 | 1732 | |
68dc0745 | 1733 | =head2 Why does defined() return true on empty arrays and hashes? |
1734 | ||
65acb1b1 TC |
1735 | The short story is that you should probably only use defined on scalars or |
1736 | functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> | |
1737 | in the 5.004 release or later of Perl for more detail. | |
68dc0745 | 1738 | |
1739 | =head1 Data: Hashes (Associative Arrays) | |
1740 | ||
1741 | =head2 How do I process an entire hash? | |
1742 | ||
1743 | Use the each() function (see L<perlfunc/each>) if you don't care | |
1744 | whether it's sorted: | |
1745 | ||
5a964f20 | 1746 | while ( ($key, $value) = each %hash) { |
68dc0745 | 1747 | print "$key = $value\n"; |
1748 | } | |
1749 | ||
1750 | If you want it sorted, you'll have to use foreach() on the result of | |
1751 | sorting the keys as shown in an earlier question. | |
1752 | ||
1753 | =head2 What happens if I add or remove keys from a hash while iterating over it? | |
1754 | ||
28b41a80 | 1755 | (contributed by brian d foy) |
d92eb7b0 | 1756 | |
28b41a80 | 1757 | The easy answer is "Don't do that!" |
d92eb7b0 | 1758 | |
28b41a80 RGS |
1759 | If you iterate through the hash with each(), you can delete the key |
1760 | most recently returned without worrying about it. If you delete or add | |
1761 | other keys, the iterator may skip or double up on them since perl | |
1762 | may rearrange the hash table. See the | |
1763 | entry for C<each()> in L<perlfunc>. | |
68dc0745 | 1764 | |
1765 | =head2 How do I look up a hash element by value? | |
1766 | ||
1767 | Create a reverse hash: | |
1768 | ||
1769 | %by_value = reverse %by_key; | |
1770 | $key = $by_value{$value}; | |
1771 | ||
1772 | That's not particularly efficient. It would be more space-efficient | |
1773 | to use: | |
1774 | ||
1775 | while (($key, $value) = each %by_key) { | |
1776 | $by_value{$value} = $key; | |
1777 | } | |
1778 | ||
d92eb7b0 GS |
1779 | If your hash could have repeated values, the methods above will only find |
1780 | one of the associated keys. This may or may not worry you. If it does | |
1781 | worry you, you can always reverse the hash into a hash of arrays instead: | |
1782 | ||
1783 | while (($key, $value) = each %by_key) { | |
1784 | push @{$key_list_by_value{$value}}, $key; | |
1785 | } | |
68dc0745 | 1786 | |
1787 | =head2 How can I know how many entries are in a hash? | |
1788 | ||
1789 | If you mean how many keys, then all you have to do is | |
875e5c2f | 1790 | use the keys() function in a scalar context: |
68dc0745 | 1791 | |
875e5c2f | 1792 | $num_keys = keys %hash; |
68dc0745 | 1793 | |
197aec24 RGS |
1794 | The keys() function also resets the iterator, which means that you may |
1795 | see strange results if you use this between uses of other hash operators | |
875e5c2f | 1796 | such as each(). |
68dc0745 | 1797 | |
1798 | =head2 How do I sort a hash (optionally by value instead of key)? | |
1799 | ||
a05e4845 RGS |
1800 | (contributed by brian d foy) |
1801 | ||
1802 | To sort a hash, start with the keys. In this example, we give the list of | |
1803 | keys to the sort function which then compares them ASCIIbetically (which | |
1804 | might be affected by your locale settings). The output list has the keys | |
1805 | in ASCIIbetical order. Once we have the keys, we can go through them to | |
1806 | create a report which lists the keys in ASCIIbetical order. | |
1807 | ||
1808 | my @keys = sort { $a cmp $b } keys %hash; | |
58103a2e | 1809 | |
a05e4845 RGS |
1810 | foreach my $key ( @keys ) |
1811 | { | |
1812 | printf "%-20s %6d\n", $key, $hash{$value}; | |
1813 | } | |
1814 | ||
58103a2e | 1815 | We could get more fancy in the C<sort()> block though. Instead of |
a05e4845 | 1816 | comparing the keys, we can compute a value with them and use that |
58103a2e | 1817 | value as the comparison. |
a05e4845 RGS |
1818 | |
1819 | For instance, to make our report order case-insensitive, we use | |
58103a2e | 1820 | the C<\L> sequence in a double-quoted string to make everything |
a05e4845 RGS |
1821 | lowercase. The C<sort()> block then compares the lowercased |
1822 | values to determine in which order to put the keys. | |
1823 | ||
1824 | my @keys = sort { "\L$a" cmp "\L$b" } keys %hash; | |
58103a2e | 1825 | |
a05e4845 | 1826 | Note: if the computation is expensive or the hash has many elements, |
58103a2e | 1827 | you may want to look at the Schwartzian Transform to cache the |
a05e4845 RGS |
1828 | computation results. |
1829 | ||
1830 | If we want to sort by the hash value instead, we use the hash key | |
1831 | to look it up. We still get out a list of keys, but this time they | |
1832 | are ordered by their value. | |
1833 | ||
1834 | my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash; | |
1835 | ||
1836 | From there we can get more complex. If the hash values are the same, | |
1837 | we can provide a secondary sort on the hash key. | |
1838 | ||
58103a2e RGS |
1839 | my @keys = sort { |
1840 | $hash{$a} <=> $hash{$b} | |
a05e4845 RGS |
1841 | or |
1842 | "\L$a" cmp "\L$b" | |
1843 | } keys %hash; | |
68dc0745 | 1844 | |
1845 | =head2 How can I always keep my hash sorted? | |
1846 | ||
1847 | You can look into using the DB_File module and tie() using the | |
1848 | $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">. | |
5a964f20 | 1849 | The Tie::IxHash module from CPAN might also be instructive. |
68dc0745 | 1850 | |
1851 | =head2 What's the difference between "delete" and "undef" with hashes? | |
1852 | ||
92993692 JH |
1853 | Hashes contain pairs of scalars: the first is the key, the |
1854 | second is the value. The key will be coerced to a string, | |
1855 | although the value can be any kind of scalar: string, | |
1856 | number, or reference. If a key $key is present in | |
1857 | %hash, C<exists($hash{$key})> will return true. The value | |
1858 | for a given key can be C<undef>, in which case | |
1859 | C<$hash{$key}> will be C<undef> while C<exists $hash{$key}> | |
1860 | will return true. This corresponds to (C<$key>, C<undef>) | |
1861 | being in the hash. | |
68dc0745 | 1862 | |
92993692 | 1863 | Pictures help... here's the %hash table: |
68dc0745 | 1864 | |
1865 | keys values | |
1866 | +------+------+ | |
1867 | | a | 3 | | |
1868 | | x | 7 | | |
1869 | | d | 0 | | |
1870 | | e | 2 | | |
1871 | +------+------+ | |
1872 | ||
1873 | And these conditions hold | |
1874 | ||
92993692 JH |
1875 | $hash{'a'} is true |
1876 | $hash{'d'} is false | |
1877 | defined $hash{'d'} is true | |
1878 | defined $hash{'a'} is true | |
1879 | exists $hash{'a'} is true (Perl5 only) | |
1880 | grep ($_ eq 'a', keys %hash) is true | |
68dc0745 | 1881 | |
1882 | If you now say | |
1883 | ||
92993692 | 1884 | undef $hash{'a'} |
68dc0745 | 1885 | |
1886 | your table now reads: | |
1887 | ||
1888 | ||
1889 | keys values | |
1890 | +------+------+ | |
1891 | | a | undef| | |
1892 | | x | 7 | | |
1893 | | d | 0 | | |
1894 | | e | 2 | | |
1895 | +------+------+ | |
1896 | ||
1897 | and these conditions now hold; changes in caps: | |
1898 | ||
92993692 JH |
1899 | $hash{'a'} is FALSE |
1900 | $hash{'d'} is false | |
1901 | defined $hash{'d'} is true | |
1902 | defined $hash{'a'} is FALSE | |
1903 | exists $hash{'a'} is true (Perl5 only) | |
1904 | grep ($_ eq 'a', keys %hash) is true | |
68dc0745 | 1905 | |
1906 | Notice the last two: you have an undef value, but a defined key! | |
1907 | ||
1908 | Now, consider this: | |
1909 | ||
92993692 | 1910 | delete $hash{'a'} |
68dc0745 | 1911 | |
1912 | your table now reads: | |
1913 | ||
1914 | keys values | |
1915 | +------+------+ | |
1916 | | x | 7 | | |
1917 | | d | 0 | | |
1918 | | e | 2 | | |
1919 | +------+------+ | |
1920 | ||
1921 | and these conditions now hold; changes in caps: | |
1922 | ||
92993692 JH |
1923 | $hash{'a'} is false |
1924 | $hash{'d'} is false | |
1925 | defined $hash{'d'} is true | |
1926 | defined $hash{'a'} is false | |
1927 | exists $hash{'a'} is FALSE (Perl5 only) | |
1928 | grep ($_ eq 'a', keys %hash) is FALSE | |
68dc0745 | 1929 | |
1930 | See, the whole entry is gone! | |
1931 | ||
1932 | =head2 Why don't my tied hashes make the defined/exists distinction? | |
1933 | ||
92993692 JH |
1934 | This depends on the tied hash's implementation of EXISTS(). |
1935 | For example, there isn't the concept of undef with hashes | |
1936 | that are tied to DBM* files. It also means that exists() and | |
1937 | defined() do the same thing with a DBM* file, and what they | |
1938 | end up doing is not what they do with ordinary hashes. | |
68dc0745 | 1939 | |
1940 | =head2 How do I reset an each() operation part-way through? | |
1941 | ||
5a964f20 | 1942 | Using C<keys %hash> in scalar context returns the number of keys in |
68dc0745 | 1943 | the hash I<and> resets the iterator associated with the hash. You may |
1944 | need to do this if you use C<last> to exit a loop early so that when you | |
46fc3d4c | 1945 | re-enter it, the hash iterator has been reset. |
68dc0745 | 1946 | |
1947 | =head2 How can I get the unique keys from two hashes? | |
1948 | ||
d92eb7b0 GS |
1949 | First you extract the keys from the hashes into lists, then solve |
1950 | the "removing duplicates" problem described above. For example: | |
68dc0745 | 1951 | |
1952 | %seen = (); | |
1953 | for $element (keys(%foo), keys(%bar)) { | |
1954 | $seen{$element}++; | |
1955 | } | |
1956 | @uniq = keys %seen; | |
1957 | ||
1958 | Or more succinctly: | |
1959 | ||
1960 | @uniq = keys %{{%foo,%bar}}; | |
1961 | ||
1962 | Or if you really want to save space: | |
1963 | ||
1964 | %seen = (); | |
1965 | while (defined ($key = each %foo)) { | |
1966 | $seen{$key}++; | |
1967 | } | |
1968 | while (defined ($key = each %bar)) { | |
1969 | $seen{$key}++; | |
1970 | } | |
1971 | @uniq = keys %seen; | |
1972 | ||
1973 | =head2 How can I store a multidimensional array in a DBM file? | |
1974 | ||
1975 | Either stringify the structure yourself (no fun), or else | |
1976 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer | |
1977 | it on top of either DB_File or GDBM_File. | |
1978 | ||
1979 | =head2 How can I make my hash remember the order I put elements into it? | |
1980 | ||
1981 | Use the Tie::IxHash from CPAN. | |
1982 | ||
46fc3d4c | 1983 | use Tie::IxHash; |
5f8d77f1 | 1984 | tie my %myhash, 'Tie::IxHash'; |
49d635f9 | 1985 | for (my $i=0; $i<20; $i++) { |
46fc3d4c | 1986 | $myhash{$i} = 2*$i; |
1987 | } | |
49d635f9 | 1988 | my @keys = keys %myhash; |
46fc3d4c | 1989 | # @keys = (0,1,2,3,...) |
1990 | ||
68dc0745 | 1991 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
1992 | ||
1993 | If you say something like: | |
1994 | ||
1995 | somefunc($hash{"nonesuch key here"}); | |
1996 | ||
1997 | Then that element "autovivifies"; that is, it springs into existence | |
1998 | whether you store something there or not. That's because functions | |
1999 | get scalars passed in by reference. If somefunc() modifies C<$_[0]>, | |
2000 | it has to be ready to write it back into the caller's version. | |
2001 | ||
87275199 | 2002 | This has been fixed as of Perl5.004. |
68dc0745 | 2003 | |
2004 | Normally, merely accessing a key's value for a nonexistent key does | |
2005 | I<not> cause that key to be forever there. This is different than | |
2006 | awk's behavior. | |
2007 | ||
fc36a67e | 2008 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
68dc0745 | 2009 | |
65acb1b1 TC |
2010 | Usually a hash ref, perhaps like this: |
2011 | ||
2012 | $record = { | |
2013 | NAME => "Jason", | |
2014 | EMPNO => 132, | |
2015 | TITLE => "deputy peon", | |
2016 | AGE => 23, | |
2017 | SALARY => 37_000, | |
2018 | PALS => [ "Norbert", "Rhys", "Phineas"], | |
2019 | }; | |
2020 | ||
2021 | References are documented in L<perlref> and the upcoming L<perlreftut>. | |
2022 | Examples of complex data structures are given in L<perldsc> and | |
2023 | L<perllol>. Examples of structures and object-oriented classes are | |
2024 | in L<perltoot>. | |
68dc0745 | 2025 | |
2026 | =head2 How can I use a reference as a hash key? | |
2027 | ||
9e72e4c6 RGS |
2028 | (contributed by brian d foy) |
2029 | ||
2030 | Hash keys are strings, so you can't really use a reference as the key. | |
2031 | When you try to do that, perl turns the reference into its stringified | |
2032 | form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get back | |
2033 | the reference from the stringified form, at least without doing some | |
2034 | extra work on your own. Also remember that hash keys must be unique, but | |
2035 | two different variables can store the same reference (and those variables | |
2036 | can change later). | |
2037 | ||
2038 | The Tie::RefHash module, which is distributed with perl, might be what | |
2039 | you want. It handles that extra work. | |
68dc0745 | 2040 | |
2041 | =head1 Data: Misc | |
2042 | ||
2043 | =head2 How do I handle binary data correctly? | |
2044 | ||
2045 | Perl is binary clean, so this shouldn't be a problem. For example, | |
2046 | this works fine (assuming the files are found): | |
2047 | ||
2048 | if (`cat /vmunix` =~ /gzip/) { | |
2049 | print "Your kernel is GNU-zip enabled!\n"; | |
2050 | } | |
2051 | ||
d92eb7b0 GS |
2052 | On less elegant (read: Byzantine) systems, however, you have |
2053 | to play tedious games with "text" versus "binary" files. See | |
49d635f9 | 2054 | L<perlfunc/"binmode"> or L<perlopentut>. |
68dc0745 | 2055 | |
2056 | If you're concerned about 8-bit ASCII data, then see L<perllocale>. | |
2057 | ||
54310121 | 2058 | If you want to deal with multibyte characters, however, there are |
68dc0745 | 2059 | some gotchas. See the section on Regular Expressions. |
2060 | ||
2061 | =head2 How do I determine whether a scalar is a number/whole/integer/float? | |
2062 | ||
2063 | Assuming that you don't care about IEEE notations like "NaN" or | |
2064 | "Infinity", you probably just want to use a regular expression. | |
2065 | ||
65acb1b1 TC |
2066 | if (/\D/) { print "has nondigits\n" } |
2067 | if (/^\d+$/) { print "is a whole number\n" } | |
2068 | if (/^-?\d+$/) { print "is an integer\n" } | |
2069 | if (/^[+-]?\d+$/) { print "is a +/- integer\n" } | |
2070 | if (/^-?\d+\.?\d*$/) { print "is a real number\n" } | |
881bdbd4 | 2071 | if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" } |
65acb1b1 | 2072 | if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) |
881bdbd4 | 2073 | { print "a C float\n" } |
68dc0745 | 2074 | |
f0d19b68 RGS |
2075 | There are also some commonly used modules for the task. |
2076 | L<Scalar::Util> (distributed with 5.8) provides access to perl's | |
2077 | internal function C<looks_like_number> for determining | |
2078 | whether a variable looks like a number. L<Data::Types> | |
2079 | exports functions that validate data types using both the | |
2080 | above and other regular expressions. Thirdly, there is | |
2081 | C<Regexp::Common> which has regular expressions to match | |
2082 | various types of numbers. Those three modules are available | |
2083 | from the CPAN. | |
2084 | ||
2085 | If you're on a POSIX system, Perl supports the C<POSIX::strtod> | |
5a964f20 TC |
2086 | function. Its semantics are somewhat cumbersome, so here's a C<getnum> |
2087 | wrapper function for more convenient access. This function takes | |
2088 | a string and returns the number it found, or C<undef> for input that | |
2089 | isn't a C float. The C<is_numeric> function is a front end to C<getnum> | |
b432a672 | 2090 | if you just want to say, "Is this a float?" |
5a964f20 TC |
2091 | |
2092 | sub getnum { | |
2093 | use POSIX qw(strtod); | |
2094 | my $str = shift; | |
2095 | $str =~ s/^\s+//; | |
2096 | $str =~ s/\s+$//; | |
2097 | $! = 0; | |
2098 | my($num, $unparsed) = strtod($str); | |
2099 | if (($str eq '') || ($unparsed != 0) || $!) { | |
2100 | return undef; | |
2101 | } else { | |
2102 | return $num; | |
197aec24 RGS |
2103 | } |
2104 | } | |
5a964f20 | 2105 | |
197aec24 | 2106 | sub is_numeric { defined getnum($_[0]) } |
5a964f20 | 2107 | |
f0d19b68 | 2108 | Or you could check out the L<String::Scanf> module on the CPAN |
b5b6f210 JH |
2109 | instead. The POSIX module (part of the standard Perl distribution) provides |
2110 | the C<strtod> and C<strtol> for converting strings to double and longs, | |
6cecdcac | 2111 | respectively. |
68dc0745 | 2112 | |
2113 | =head2 How do I keep persistent data across program calls? | |
2114 | ||
2115 | For some specific applications, you can use one of the DBM modules. | |
fe854a6f AT |
2116 | See L<AnyDBM_File>. More generically, you should consult the FreezeThaw |
2117 | or Storable modules from CPAN. Starting from Perl 5.8 Storable is part | |
2118 | of the standard distribution. Here's one example using Storable's C<store> | |
2119 | and C<retrieve> functions: | |
65acb1b1 | 2120 | |
197aec24 | 2121 | use Storable; |
65acb1b1 TC |
2122 | store(\%hash, "filename"); |
2123 | ||
197aec24 | 2124 | # later on... |
65acb1b1 TC |
2125 | $href = retrieve("filename"); # by ref |
2126 | %hash = %{ retrieve("filename") }; # direct to hash | |
68dc0745 | 2127 | |
2128 | =head2 How do I print out or copy a recursive data structure? | |
2129 | ||
65acb1b1 | 2130 | The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great |
6f82c03a EM |
2131 | for printing out data structures. The Storable module on CPAN (or the |
2132 | 5.8 release of Perl), provides a function called C<dclone> that recursively | |
2133 | copies its argument. | |
65acb1b1 | 2134 | |
197aec24 | 2135 | use Storable qw(dclone); |
65acb1b1 | 2136 | $r2 = dclone($r1); |
68dc0745 | 2137 | |
65acb1b1 TC |
2138 | Where $r1 can be a reference to any kind of data structure you'd like. |
2139 | It will be deeply copied. Because C<dclone> takes and returns references, | |
2140 | you'd have to add extra punctuation if you had a hash of arrays that | |
2141 | you wanted to copy. | |
68dc0745 | 2142 | |
65acb1b1 | 2143 | %newhash = %{ dclone(\%oldhash) }; |
68dc0745 | 2144 | |
2145 | =head2 How do I define methods for every class/object? | |
2146 | ||
2147 | Use the UNIVERSAL class (see L<UNIVERSAL>). | |
2148 | ||
2149 | =head2 How do I verify a credit card checksum? | |
2150 | ||
2151 | Get the Business::CreditCard module from CPAN. | |
2152 | ||
65acb1b1 TC |
2153 | =head2 How do I pack arrays of doubles or floats for XS code? |
2154 | ||
2155 | The kgbpack.c code in the PGPLOT module on CPAN does just this. | |
2156 | If you're doing a lot of float or double processing, consider using | |
2157 | the PDL module from CPAN instead--it makes number-crunching easy. | |
2158 | ||
500071f4 RGS |
2159 | =head1 REVISION |
2160 | ||
2161 | Revision: $Revision: 3606 $ | |
2162 | ||
2163 | Date: $Date: 2006-03-06 12:05:47 +0100 (lun, 06 mar 2006) $ | |
2164 | ||
2165 | See L<perlfaq> for source control details and availability. | |
2166 | ||
68dc0745 | 2167 | =head1 AUTHOR AND COPYRIGHT |
2168 | ||
58103a2e | 2169 | Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and |
7678cced | 2170 | other authors as noted. All rights reserved. |
5a964f20 | 2171 | |
5a7beb56 JH |
2172 | This documentation is free; you can redistribute it and/or modify it |
2173 | under the same terms as Perl itself. | |
5a964f20 TC |
2174 | |
2175 | Irrespective of its distribution, all code examples in this file | |
2176 | are hereby placed into the public domain. You are permitted and | |
2177 | encouraged to use this code in your own programs for fun | |
2178 | or for profit as you see fit. A simple comment in the code giving | |
2179 | credit would be courteous but is not required. |