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68dc0745 | 1 | =head1 NAME |
2 | ||
58103a2e | 3 | perlfaq4 - Data Manipulation ($Revision: 1.73 $, $Date: 2005/12/31 00:54:37 $) |
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 $] < 5.004 } |
49d635f9 | 343 | |
65acb1b1 | 344 | 5.004 and later automatically call C<srand> at the beginning. Don't |
49d635f9 | 345 | call C<srand> more than once---you make your numbers less random, rather |
65acb1b1 | 346 | than more. |
92c2ed05 | 347 | |
65acb1b1 | 348 | Computers are good at being predictable and bad at being random |
06a5f41f | 349 | (despite appearances caused by bugs in your programs :-). see the |
49d635f9 RGS |
350 | F<random> article in the "Far More Than You Ever Wanted To Know" |
351 | collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of | |
b432a672 | 352 | Tom Phoenix, talks more about this. John von Neumann said, "Anyone |
06a5f41f | 353 | who attempts to generate random numbers by deterministic means is, of |
b432a672 | 354 | course, living in a state of sin." |
65acb1b1 TC |
355 | |
356 | If you want numbers that are more random than C<rand> with C<srand> | |
357 | provides, you should also check out the Math::TrulyRandom module from | |
358 | CPAN. It uses the imperfections in your system's timer to generate | |
359 | random numbers, but this takes quite a while. If you want a better | |
92c2ed05 | 360 | pseudorandom generator than comes with your operating system, look at |
b432a672 | 361 | "Numerical Recipes in C" at http://www.nr.com/ . |
68dc0745 | 362 | |
881bdbd4 JH |
363 | =head2 How do I get a random number between X and Y? |
364 | ||
793f5136 RGS |
365 | C<rand($x)> returns a number such that |
366 | C<< 0 <= rand($x) < $x >>. Thus what you want to have perl | |
367 | figure out is a random number in the range from 0 to the | |
368 | difference between your I<X> and I<Y>. | |
369 | ||
370 | That is, to get a number between 10 and 15, inclusive, you | |
371 | want a random number between 0 and 5 that you can then add | |
372 | to 10. | |
373 | ||
374 | my $number = 10 + int rand( 15-10+1 ); | |
375 | ||
376 | Hence you derive the following simple function to abstract | |
377 | that. It selects a random integer between the two given | |
378 | integers (inclusive), For example: C<random_int_in(50,120)>. | |
881bdbd4 JH |
379 | |
380 | sub random_int_in ($$) { | |
381 | my($min, $max) = @_; | |
382 | # Assumes that the two arguments are integers themselves! | |
383 | return $min if $min == $max; | |
384 | ($min, $max) = ($max, $min) if $min > $max; | |
385 | return $min + int rand(1 + $max - $min); | |
386 | } | |
387 | ||
68dc0745 | 388 | =head1 Data: Dates |
389 | ||
5cd0b561 | 390 | =head2 How do I find the day or week of the year? |
68dc0745 | 391 | |
571e049f | 392 | The localtime function returns the day of the year. Without an |
5cd0b561 | 393 | argument localtime uses the current time. |
68dc0745 | 394 | |
a05e4845 | 395 | $day_of_year = (localtime)[7]; |
ffc145e8 | 396 | |
5cd0b561 RGS |
397 | The POSIX module can also format a date as the day of the year or |
398 | week of the year. | |
68dc0745 | 399 | |
5cd0b561 RGS |
400 | use POSIX qw/strftime/; |
401 | my $day_of_year = strftime "%j", localtime; | |
402 | my $week_of_year = strftime "%W", localtime; | |
403 | ||
404 | To get the day of year for any date, use the Time::Local module to get | |
405 | a time in epoch seconds for the argument to localtime. | |
ffc145e8 | 406 | |
5cd0b561 RGS |
407 | use POSIX qw/strftime/; |
408 | use Time::Local; | |
6670e5e7 | 409 | my $week_of_year = strftime "%W", |
5cd0b561 RGS |
410 | localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) ); |
411 | ||
b68463f7 | 412 | The Date::Calc module provides two functions to calculate these. |
5cd0b561 RGS |
413 | |
414 | use Date::Calc; | |
415 | my $day_of_year = Day_of_Year( 1987, 12, 18 ); | |
416 | my $week_of_year = Week_of_Year( 1987, 12, 18 ); | |
ffc145e8 | 417 | |
d92eb7b0 GS |
418 | =head2 How do I find the current century or millennium? |
419 | ||
420 | Use the following simple functions: | |
421 | ||
197aec24 | 422 | sub get_century { |
d92eb7b0 | 423 | return int((((localtime(shift || time))[5] + 1999))/100); |
197aec24 | 424 | } |
6670e5e7 | 425 | |
197aec24 | 426 | sub get_millennium { |
d92eb7b0 | 427 | return 1+int((((localtime(shift || time))[5] + 1899))/1000); |
197aec24 | 428 | } |
d92eb7b0 | 429 | |
49d635f9 RGS |
430 | On some systems, the POSIX module's strftime() function has |
431 | been extended in a non-standard way to use a C<%C> format, | |
432 | which they sometimes claim is the "century". It isn't, | |
433 | because on most such systems, this is only the first two | |
434 | digits of the four-digit year, and thus cannot be used to | |
435 | reliably determine the current century or millennium. | |
d92eb7b0 | 436 | |
92c2ed05 | 437 | =head2 How can I compare two dates and find the difference? |
68dc0745 | 438 | |
b68463f7 RGS |
439 | (contributed by brian d foy) |
440 | ||
441 | You could just store all your dates as a number and then subtract. Life | |
442 | isn't always that simple though. If you want to work with formatted | |
443 | dates, the Date::Manip, Date::Calc, or DateTime modules can help you. | |
444 | ||
68dc0745 | 445 | |
446 | =head2 How can I take a string and turn it into epoch seconds? | |
447 | ||
448 | If it's a regular enough string that it always has the same format, | |
92c2ed05 GS |
449 | you can split it up and pass the parts to C<timelocal> in the standard |
450 | Time::Local module. Otherwise, you should look into the Date::Calc | |
451 | and Date::Manip modules from CPAN. | |
68dc0745 | 452 | |
453 | =head2 How can I find the Julian Day? | |
454 | ||
7678cced RGS |
455 | (contributed by brian d foy and Dave Cross) |
456 | ||
457 | You can use the Time::JulianDay module available on CPAN. Ensure that | |
458 | you really want to find a Julian day, though, as many people have | |
459 | different ideas about Julian days. See | |
460 | http://www.hermetic.ch/cal_stud/jdn.htm for instance. | |
461 | ||
462 | You can also try the DateTime module, which can convert a date/time | |
463 | to a Julian Day. | |
464 | ||
465 | $ perl -MDateTime -le'print DateTime->today->jd' | |
466 | 2453401.5 | |
467 | ||
468 | Or the modified Julian Day | |
469 | ||
470 | $ perl -MDateTime -le'print DateTime->today->mjd' | |
471 | 53401 | |
472 | ||
473 | Or even the day of the year (which is what some people think of as a | |
474 | Julian day) | |
475 | ||
476 | $ perl -MDateTime -le'print DateTime->today->doy' | |
477 | 31 | |
be94a901 | 478 | |
65acb1b1 TC |
479 | =head2 How do I find yesterday's date? |
480 | ||
6670e5e7 | 481 | (contributed by brian d foy) |
49d635f9 | 482 | |
6670e5e7 RGS |
483 | Use one of the Date modules. The C<DateTime> module makes it simple, and |
484 | give you the same time of day, only the day before. | |
49d635f9 | 485 | |
6670e5e7 | 486 | use DateTime; |
58103a2e | 487 | |
6670e5e7 | 488 | my $yesterday = DateTime->now->subtract( days => 1 ); |
58103a2e | 489 | |
6670e5e7 | 490 | print "Yesterday was $yesterday\n"; |
49d635f9 | 491 | |
6670e5e7 RGS |
492 | You can also use the C<Date::Calc> module using its Today_and_Now |
493 | function. | |
49d635f9 | 494 | |
6670e5e7 | 495 | use Date::Calc qw( Today_and_Now Add_Delta_DHMS ); |
58103a2e | 496 | |
6670e5e7 | 497 | my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 ); |
58103a2e | 498 | |
6670e5e7 | 499 | print "@date\n"; |
58103a2e | 500 | |
6670e5e7 RGS |
501 | Most people try to use the time rather than the calendar to figure out |
502 | dates, but that assumes that days are twenty-four hours each. For | |
503 | most people, there are two days a year when they aren't: the switch to | |
504 | and from summer time throws this off. Let the modules do the work. | |
d92eb7b0 | 505 | |
87275199 | 506 | =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? |
68dc0745 | 507 | |
65acb1b1 TC |
508 | Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is |
509 | Y2K compliant (whatever that means). The programmers you've hired to | |
510 | use it, however, probably are not. | |
511 | ||
512 | Long answer: The question belies a true understanding of the issue. | |
513 | Perl is just as Y2K compliant as your pencil--no more, and no less. | |
514 | Can you use your pencil to write a non-Y2K-compliant memo? Of course | |
515 | you can. Is that the pencil's fault? Of course it isn't. | |
92c2ed05 | 516 | |
87275199 | 517 | The date and time functions supplied with Perl (gmtime and localtime) |
65acb1b1 TC |
518 | supply adequate information to determine the year well beyond 2000 |
519 | (2038 is when trouble strikes for 32-bit machines). The year returned | |
90fdbbb7 | 520 | by these functions when used in a list context is the year minus 1900. |
65acb1b1 TC |
521 | For years between 1910 and 1999 this I<happens> to be a 2-digit decimal |
522 | number. To avoid the year 2000 problem simply do not treat the year as | |
523 | a 2-digit number. It isn't. | |
68dc0745 | 524 | |
5a964f20 | 525 | When gmtime() and localtime() are used in scalar context they return |
68dc0745 | 526 | a timestamp string that contains a fully-expanded year. For example, |
527 | C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 | |
528 | 2001". There's no year 2000 problem here. | |
529 | ||
5a964f20 TC |
530 | That doesn't mean that Perl can't be used to create non-Y2K compliant |
531 | programs. It can. But so can your pencil. It's the fault of the user, | |
b432a672 AL |
532 | not the language. At the risk of inflaming the NRA: "Perl doesn't |
533 | break Y2K, people do." See http://www.perl.org/about/y2k.html for | |
5a964f20 TC |
534 | a longer exposition. |
535 | ||
68dc0745 | 536 | =head1 Data: Strings |
537 | ||
538 | =head2 How do I validate input? | |
539 | ||
6670e5e7 RGS |
540 | (contributed by brian d foy) |
541 | ||
542 | There are many ways to ensure that values are what you expect or | |
543 | want to accept. Besides the specific examples that we cover in the | |
544 | perlfaq, you can also look at the modules with "Assert" and "Validate" | |
545 | in their names, along with other modules such as C<Regexp::Common>. | |
546 | ||
547 | Some modules have validation for particular types of input, such | |
548 | as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>, | |
549 | and C<Data::Validate::IP>. | |
68dc0745 | 550 | |
551 | =head2 How do I unescape a string? | |
552 | ||
b432a672 | 553 | It depends just what you mean by "escape". URL escapes are dealt |
92c2ed05 | 554 | with in L<perlfaq9>. Shell escapes with the backslash (C<\>) |
a6dd486b | 555 | character are removed with |
68dc0745 | 556 | |
557 | s/\\(.)/$1/g; | |
558 | ||
92c2ed05 | 559 | This won't expand C<"\n"> or C<"\t"> or any other special escapes. |
68dc0745 | 560 | |
561 | =head2 How do I remove consecutive pairs of characters? | |
562 | ||
6670e5e7 RGS |
563 | (contributed by brian d foy) |
564 | ||
565 | You can use the substitution operator to find pairs of characters (or | |
566 | runs of characters) and replace them with a single instance. In this | |
567 | substitution, we find a character in C<(.)>. The memory parentheses | |
568 | store the matched character in the back-reference C<\1> and we use | |
569 | that to require that the same thing immediately follow it. We replace | |
570 | that part of the string with the character in C<$1>. | |
68dc0745 | 571 | |
6670e5e7 | 572 | s/(.)\1/$1/g; |
d92eb7b0 | 573 | |
6670e5e7 RGS |
574 | We can also use the transliteration operator, C<tr///>. In this |
575 | example, the search list side of our C<tr///> contains nothing, but | |
576 | the C<c> option complements that so it contains everything. The | |
577 | replacement list also contains nothing, so the transliteration is | |
578 | almost a no-op since it won't do any replacements (or more exactly, | |
579 | replace the character with itself). However, the C<s> option squashes | |
580 | duplicated and consecutive characters in the string so a character | |
581 | does not show up next to itself | |
d92eb7b0 | 582 | |
6670e5e7 RGS |
583 | my $str = 'Haarlem'; # in the Netherlands |
584 | $str =~ tr///cs; # Now Harlem, like in New York | |
68dc0745 | 585 | |
586 | =head2 How do I expand function calls in a string? | |
587 | ||
6670e5e7 RGS |
588 | (contributed by brian d foy) |
589 | ||
590 | This is documented in L<perlref>, and although it's not the easiest | |
591 | thing to read, it does work. In each of these examples, we call the | |
58103a2e | 592 | function inside the braces used to dereference a reference. If we |
d7f8936a | 593 | have a more than one return value, we can construct and dereference an |
6670e5e7 RGS |
594 | anonymous array. In this case, we call the function in list context. |
595 | ||
58103a2e | 596 | print "The time values are @{ [localtime] }.\n"; |
6670e5e7 RGS |
597 | |
598 | If we want to call the function in scalar context, we have to do a bit | |
599 | more work. We can really have any code we like inside the braces, so | |
600 | we simply have to end with the scalar reference, although how you do | |
601 | that is up to you, and you can use code inside the braces. | |
68dc0745 | 602 | |
6670e5e7 | 603 | print "The time is ${\(scalar localtime)}.\n" |
58103a2e | 604 | |
6670e5e7 | 605 | print "The time is ${ my $x = localtime; \$x }.\n"; |
58103a2e | 606 | |
6670e5e7 RGS |
607 | If your function already returns a reference, you don't need to create |
608 | the reference yourself. | |
609 | ||
610 | sub timestamp { my $t = localtime; \$t } | |
58103a2e | 611 | |
6670e5e7 | 612 | print "The time is ${ timestamp() }.\n"; |
58103a2e RGS |
613 | |
614 | The C<Interpolation> module can also do a lot of magic for you. You can | |
615 | specify a variable name, in this case C<E>, to set up a tied hash that | |
616 | does the interpolation for you. It has several other methods to do this | |
617 | as well. | |
618 | ||
619 | use Interpolation E => 'eval'; | |
620 | print "The time values are $E{localtime()}.\n"; | |
621 | ||
622 | In most cases, it is probably easier to simply use string concatenation, | |
623 | which also forces scalar context. | |
6670e5e7 RGS |
624 | |
625 | print "The time is " . localtime . ".\n"; | |
68dc0745 | 626 | |
68dc0745 | 627 | =head2 How do I find matching/nesting anything? |
628 | ||
92c2ed05 GS |
629 | This isn't something that can be done in one regular expression, no |
630 | matter how complicated. To find something between two single | |
631 | characters, a pattern like C</x([^x]*)x/> will get the intervening | |
632 | bits in $1. For multiple ones, then something more like | |
633 | C</alpha(.*?)omega/> would be needed. But none of these deals with | |
6670e5e7 RGS |
634 | nested patterns. For balanced expressions using C<(>, C<{>, C<[> or |
635 | C<< < >> as delimiters, use the CPAN module Regexp::Common, or see | |
636 | L<perlre/(??{ code })>. For other cases, you'll have to write a | |
637 | parser. | |
92c2ed05 GS |
638 | |
639 | If you are serious about writing a parser, there are a number of | |
6a2af475 GS |
640 | modules or oddities that will make your life a lot easier. There are |
641 | the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; | |
6670e5e7 RGS |
642 | and the byacc program. Starting from perl 5.8 the Text::Balanced is |
643 | part of the standard distribution. | |
68dc0745 | 644 | |
92c2ed05 GS |
645 | One simple destructive, inside-out approach that you might try is to |
646 | pull out the smallest nesting parts one at a time: | |
5a964f20 | 647 | |
d92eb7b0 | 648 | while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { |
5a964f20 | 649 | # do something with $1 |
197aec24 | 650 | } |
5a964f20 | 651 | |
65acb1b1 TC |
652 | A more complicated and sneaky approach is to make Perl's regular |
653 | expression engine do it for you. This is courtesy Dean Inada, and | |
654 | rather has the nature of an Obfuscated Perl Contest entry, but it | |
655 | really does work: | |
656 | ||
657 | # $_ contains the string to parse | |
658 | # BEGIN and END are the opening and closing markers for the | |
659 | # nested text. | |
c47ff5f1 | 660 | |
65acb1b1 TC |
661 | @( = ('(',''); |
662 | @) = (')',''); | |
663 | ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; | |
5ed30e05 | 664 | @$ = (eval{/$re/},$@!~/unmatched/i); |
65acb1b1 TC |
665 | print join("\n",@$[0..$#$]) if( $$[-1] ); |
666 | ||
68dc0745 | 667 | =head2 How do I reverse a string? |
668 | ||
5a964f20 | 669 | Use reverse() in scalar context, as documented in |
68dc0745 | 670 | L<perlfunc/reverse>. |
671 | ||
672 | $reversed = reverse $string; | |
673 | ||
674 | =head2 How do I expand tabs in a string? | |
675 | ||
5a964f20 | 676 | You can do it yourself: |
68dc0745 | 677 | |
678 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; | |
679 | ||
87275199 | 680 | Or you can just use the Text::Tabs module (part of the standard Perl |
68dc0745 | 681 | distribution). |
682 | ||
683 | use Text::Tabs; | |
684 | @expanded_lines = expand(@lines_with_tabs); | |
685 | ||
686 | =head2 How do I reformat a paragraph? | |
687 | ||
87275199 | 688 | Use Text::Wrap (part of the standard Perl distribution): |
68dc0745 | 689 | |
690 | use Text::Wrap; | |
691 | print wrap("\t", ' ', @paragraphs); | |
692 | ||
92c2ed05 | 693 | The paragraphs you give to Text::Wrap should not contain embedded |
46fc3d4c | 694 | newlines. Text::Wrap doesn't justify the lines (flush-right). |
695 | ||
bc06af74 JH |
696 | Or use the CPAN module Text::Autoformat. Formatting files can be easily |
697 | done by making a shell alias, like so: | |
698 | ||
699 | alias fmt="perl -i -MText::Autoformat -n0777 \ | |
700 | -e 'print autoformat $_, {all=>1}' $*" | |
701 | ||
702 | See the documentation for Text::Autoformat to appreciate its many | |
703 | capabilities. | |
704 | ||
49d635f9 | 705 | =head2 How can I access or change N characters of a string? |
68dc0745 | 706 | |
49d635f9 RGS |
707 | You can access the first characters of a string with substr(). |
708 | To get the first character, for example, start at position 0 | |
197aec24 | 709 | and grab the string of length 1. |
68dc0745 | 710 | |
68dc0745 | 711 | |
49d635f9 RGS |
712 | $string = "Just another Perl Hacker"; |
713 | $first_char = substr( $string, 0, 1 ); # 'J' | |
68dc0745 | 714 | |
49d635f9 RGS |
715 | To change part of a string, you can use the optional fourth |
716 | argument which is the replacement string. | |
68dc0745 | 717 | |
49d635f9 | 718 | substr( $string, 13, 4, "Perl 5.8.0" ); |
197aec24 | 719 | |
49d635f9 | 720 | You can also use substr() as an lvalue. |
68dc0745 | 721 | |
49d635f9 | 722 | substr( $string, 13, 4 ) = "Perl 5.8.0"; |
197aec24 | 723 | |
68dc0745 | 724 | =head2 How do I change the Nth occurrence of something? |
725 | ||
92c2ed05 GS |
726 | You have to keep track of N yourself. For example, let's say you want |
727 | to change the fifth occurrence of C<"whoever"> or C<"whomever"> into | |
d92eb7b0 GS |
728 | C<"whosoever"> or C<"whomsoever">, case insensitively. These |
729 | all assume that $_ contains the string to be altered. | |
68dc0745 | 730 | |
731 | $count = 0; | |
732 | s{((whom?)ever)}{ | |
733 | ++$count == 5 # is it the 5th? | |
734 | ? "${2}soever" # yes, swap | |
735 | : $1 # renege and leave it there | |
d92eb7b0 | 736 | }ige; |
68dc0745 | 737 | |
5a964f20 TC |
738 | In the more general case, you can use the C</g> modifier in a C<while> |
739 | loop, keeping count of matches. | |
740 | ||
741 | $WANT = 3; | |
742 | $count = 0; | |
d92eb7b0 | 743 | $_ = "One fish two fish red fish blue fish"; |
5a964f20 TC |
744 | while (/(\w+)\s+fish\b/gi) { |
745 | if (++$count == $WANT) { | |
746 | print "The third fish is a $1 one.\n"; | |
5a964f20 TC |
747 | } |
748 | } | |
749 | ||
92c2ed05 | 750 | That prints out: C<"The third fish is a red one."> You can also use a |
5a964f20 TC |
751 | repetition count and repeated pattern like this: |
752 | ||
753 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; | |
754 | ||
68dc0745 | 755 | =head2 How can I count the number of occurrences of a substring within a string? |
756 | ||
a6dd486b | 757 | There are a number of ways, with varying efficiency. If you want a |
68dc0745 | 758 | count of a certain single character (X) within a string, you can use the |
759 | C<tr///> function like so: | |
760 | ||
368c9434 | 761 | $string = "ThisXlineXhasXsomeXx'sXinXit"; |
68dc0745 | 762 | $count = ($string =~ tr/X//); |
d92eb7b0 | 763 | print "There are $count X characters in the string"; |
68dc0745 | 764 | |
765 | This is fine if you are just looking for a single character. However, | |
766 | if you are trying to count multiple character substrings within a | |
767 | larger string, C<tr///> won't work. What you can do is wrap a while() | |
768 | loop around a global pattern match. For example, let's count negative | |
769 | integers: | |
770 | ||
771 | $string = "-9 55 48 -2 23 -76 4 14 -44"; | |
772 | while ($string =~ /-\d+/g) { $count++ } | |
773 | print "There are $count negative numbers in the string"; | |
774 | ||
881bdbd4 JH |
775 | Another version uses a global match in list context, then assigns the |
776 | result to a scalar, producing a count of the number of matches. | |
777 | ||
778 | $count = () = $string =~ /-\d+/g; | |
779 | ||
68dc0745 | 780 | =head2 How do I capitalize all the words on one line? |
781 | ||
782 | To make the first letter of each word upper case: | |
3fe9a6f1 | 783 | |
68dc0745 | 784 | $line =~ s/\b(\w)/\U$1/g; |
785 | ||
46fc3d4c | 786 | This has the strange effect of turning "C<don't do it>" into "C<Don'T |
a6dd486b | 787 | Do It>". Sometimes you might want this. Other times you might need a |
24f1ba9b | 788 | more thorough solution (Suggested by brian d foy): |
46fc3d4c | 789 | |
790 | $string =~ s/ ( | |
791 | (^\w) #at the beginning of the line | |
792 | | # or | |
793 | (\s\w) #preceded by whitespace | |
794 | ) | |
795 | /\U$1/xg; | |
796 | $string =~ /([\w']+)/\u\L$1/g; | |
797 | ||
68dc0745 | 798 | To make the whole line upper case: |
3fe9a6f1 | 799 | |
68dc0745 | 800 | $line = uc($line); |
801 | ||
802 | To force each word to be lower case, with the first letter upper case: | |
3fe9a6f1 | 803 | |
68dc0745 | 804 | $line =~ s/(\w+)/\u\L$1/g; |
805 | ||
5a964f20 TC |
806 | You can (and probably should) enable locale awareness of those |
807 | characters by placing a C<use locale> pragma in your program. | |
92c2ed05 | 808 | See L<perllocale> for endless details on locales. |
5a964f20 | 809 | |
65acb1b1 | 810 | This is sometimes referred to as putting something into "title |
d92eb7b0 | 811 | case", but that's not quite accurate. Consider the proper |
65acb1b1 TC |
812 | capitalization of the movie I<Dr. Strangelove or: How I Learned to |
813 | Stop Worrying and Love the Bomb>, for example. | |
814 | ||
369b44b4 RGS |
815 | Damian Conway's L<Text::Autoformat> module provides some smart |
816 | case transformations: | |
817 | ||
818 | use Text::Autoformat; | |
819 | my $x = "Dr. Strangelove or: How I Learned to Stop ". | |
820 | "Worrying and Love the Bomb"; | |
821 | ||
822 | print $x, "\n"; | |
823 | for my $style (qw( sentence title highlight )) | |
824 | { | |
825 | print autoformat($x, { case => $style }), "\n"; | |
826 | } | |
827 | ||
49d635f9 | 828 | =head2 How can I split a [character] delimited string except when inside [character]? |
68dc0745 | 829 | |
49d635f9 | 830 | Several modules can handle this sort of pasing---Text::Balanced, |
7678cced | 831 | Text::CSV, Text::CSV_XS, and Text::ParseWords, among others. |
49d635f9 RGS |
832 | |
833 | Take the example case of trying to split a string that is | |
834 | comma-separated into its different fields. You can't use C<split(/,/)> | |
835 | because you shouldn't split if the comma is inside quotes. For | |
836 | example, take a data line like this: | |
68dc0745 | 837 | |
838 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" | |
839 | ||
840 | Due to the restriction of the quotes, this is a fairly complex | |
197aec24 | 841 | problem. Thankfully, we have Jeffrey Friedl, author of |
49d635f9 | 842 | I<Mastering Regular Expressions>, to handle these for us. He |
68dc0745 | 843 | suggests (assuming your string is contained in $text): |
844 | ||
845 | @new = (); | |
846 | push(@new, $+) while $text =~ m{ | |
847 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes | |
848 | | ([^,]+),? | |
849 | | , | |
850 | }gx; | |
851 | push(@new, undef) if substr($text,-1,1) eq ','; | |
852 | ||
46fc3d4c | 853 | If you want to represent quotation marks inside a |
854 | quotation-mark-delimited field, escape them with backslashes (eg, | |
49d635f9 | 855 | C<"like \"this\"">. |
46fc3d4c | 856 | |
87275199 | 857 | Alternatively, the Text::ParseWords module (part of the standard Perl |
68dc0745 | 858 | distribution) lets you say: |
859 | ||
860 | use Text::ParseWords; | |
861 | @new = quotewords(",", 0, $text); | |
862 | ||
a6dd486b | 863 | There's also a Text::CSV (Comma-Separated Values) module on CPAN. |
65acb1b1 | 864 | |
68dc0745 | 865 | =head2 How do I strip blank space from the beginning/end of a string? |
866 | ||
6670e5e7 | 867 | (contributed by brian d foy) |
68dc0745 | 868 | |
6670e5e7 RGS |
869 | A substitution can do this for you. For a single line, you want to |
870 | replace all the leading or trailing whitespace with nothing. You | |
871 | can do that with a pair of substitutions. | |
68dc0745 | 872 | |
6670e5e7 RGS |
873 | s/^\s+//; |
874 | s/\s+$//; | |
68dc0745 | 875 | |
6670e5e7 RGS |
876 | You can also write that as a single substitution, although it turns |
877 | out the combined statement is slower than the separate ones. That | |
878 | might not matter to you, though. | |
68dc0745 | 879 | |
6670e5e7 | 880 | s/^\s+|\s+$//g; |
68dc0745 | 881 | |
6670e5e7 RGS |
882 | In this regular expression, the alternation matches either at the |
883 | beginning or the end of the string since the anchors have a lower | |
884 | precedence than the alternation. With the C</g> flag, the substitution | |
885 | makes all possible matches, so it gets both. Remember, the trailing | |
886 | newline matches the C<\s+>, and the C<$> anchor can match to the | |
887 | physical end of the string, so the newline disappears too. Just add | |
888 | the newline to the output, which has the added benefit of preserving | |
889 | "blank" (consisting entirely of whitespace) lines which the C<^\s+> | |
890 | would remove all by itself. | |
68dc0745 | 891 | |
6670e5e7 RGS |
892 | while( <> ) |
893 | { | |
894 | s/^\s+|\s+$//g; | |
895 | print "$_\n"; | |
896 | } | |
5a964f20 | 897 | |
6670e5e7 RGS |
898 | For a multi-line string, you can apply the regular expression |
899 | to each logical line in the string by adding the C</m> flag (for | |
900 | "multi-line"). With the C</m> flag, the C<$> matches I<before> an | |
901 | embedded newline, so it doesn't remove it. It still removes the | |
902 | newline at the end of the string. | |
903 | ||
904 | $string =~ s/^\s+|\s+$//gm; | |
905 | ||
906 | Remember that lines consisting entirely of whitespace will disappear, | |
907 | since the first part of the alternation can match the entire string | |
908 | and replace it with nothing. If need to keep embedded blank lines, | |
909 | you have to do a little more work. Instead of matching any whitespace | |
910 | (since that includes a newline), just match the other whitespace. | |
911 | ||
912 | $string =~ s/^[\t\f ]+|[\t\f ]+$//mg; | |
5a964f20 | 913 | |
65acb1b1 TC |
914 | =head2 How do I pad a string with blanks or pad a number with zeroes? |
915 | ||
65acb1b1 | 916 | In the following examples, C<$pad_len> is the length to which you wish |
d92eb7b0 GS |
917 | to pad the string, C<$text> or C<$num> contains the string to be padded, |
918 | and C<$pad_char> contains the padding character. You can use a single | |
919 | character string constant instead of the C<$pad_char> variable if you | |
920 | know what it is in advance. And in the same way you can use an integer in | |
921 | place of C<$pad_len> if you know the pad length in advance. | |
65acb1b1 | 922 | |
d92eb7b0 GS |
923 | The simplest method uses the C<sprintf> function. It can pad on the left |
924 | or right with blanks and on the left with zeroes and it will not | |
925 | truncate the result. The C<pack> function can only pad strings on the | |
926 | right with blanks and it will truncate the result to a maximum length of | |
927 | C<$pad_len>. | |
65acb1b1 | 928 | |
d92eb7b0 | 929 | # Left padding a string with blanks (no truncation): |
04d666b1 RGS |
930 | $padded = sprintf("%${pad_len}s", $text); |
931 | $padded = sprintf("%*s", $pad_len, $text); # same thing | |
65acb1b1 | 932 | |
d92eb7b0 | 933 | # Right padding a string with blanks (no truncation): |
04d666b1 RGS |
934 | $padded = sprintf("%-${pad_len}s", $text); |
935 | $padded = sprintf("%-*s", $pad_len, $text); # same thing | |
65acb1b1 | 936 | |
197aec24 | 937 | # Left padding a number with 0 (no truncation): |
04d666b1 RGS |
938 | $padded = sprintf("%0${pad_len}d", $num); |
939 | $padded = sprintf("%0*d", $pad_len, $num); # same thing | |
65acb1b1 | 940 | |
d92eb7b0 GS |
941 | # Right padding a string with blanks using pack (will truncate): |
942 | $padded = pack("A$pad_len",$text); | |
65acb1b1 | 943 | |
d92eb7b0 GS |
944 | If you need to pad with a character other than blank or zero you can use |
945 | one of the following methods. They all generate a pad string with the | |
946 | C<x> operator and combine that with C<$text>. These methods do | |
947 | not truncate C<$text>. | |
65acb1b1 | 948 | |
d92eb7b0 | 949 | Left and right padding with any character, creating a new string: |
65acb1b1 | 950 | |
d92eb7b0 GS |
951 | $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; |
952 | $padded = $text . $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 953 | |
d92eb7b0 | 954 | Left and right padding with any character, modifying C<$text> directly: |
65acb1b1 | 955 | |
d92eb7b0 GS |
956 | substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); |
957 | $text .= $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 958 | |
68dc0745 | 959 | =head2 How do I extract selected columns from a string? |
960 | ||
961 | Use substr() or unpack(), both documented in L<perlfunc>. | |
197aec24 | 962 | If you prefer thinking in terms of columns instead of widths, |
5a964f20 TC |
963 | you can use this kind of thing: |
964 | ||
965 | # determine the unpack format needed to split Linux ps output | |
966 | # arguments are cut columns | |
967 | my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72); | |
968 | ||
197aec24 | 969 | sub cut2fmt { |
5a964f20 TC |
970 | my(@positions) = @_; |
971 | my $template = ''; | |
972 | my $lastpos = 1; | |
973 | for my $place (@positions) { | |
197aec24 | 974 | $template .= "A" . ($place - $lastpos) . " "; |
5a964f20 TC |
975 | $lastpos = $place; |
976 | } | |
977 | $template .= "A*"; | |
978 | return $template; | |
979 | } | |
68dc0745 | 980 | |
981 | =head2 How do I find the soundex value of a string? | |
982 | ||
7678cced RGS |
983 | (contributed by brian d foy) |
984 | ||
985 | You can use the Text::Soundex module. If you want to do fuzzy or close | |
986 | matching, you might also try the String::Approx, and Text::Metaphone, | |
987 | and Text::DoubleMetaphone modules. | |
68dc0745 | 988 | |
989 | =head2 How can I expand variables in text strings? | |
990 | ||
7678cced RGS |
991 | Let's assume that you have a string that contains placeholder |
992 | variables. | |
68dc0745 | 993 | |
994 | $text = 'this has a $foo in it and a $bar'; | |
5a964f20 | 995 | |
7678cced RGS |
996 | You can use a substitution with a double evaluation. The |
997 | first /e turns C<$1> into C<$foo>, and the second /e turns | |
998 | C<$foo> into its value. You may want to wrap this in an | |
999 | C<eval>: if you try to get the value of an undeclared variable | |
1000 | while running under C<use strict>, you get a fatal error. | |
5a964f20 | 1001 | |
7678cced RGS |
1002 | eval { $text =~ s/(\$\w+)/$1/eeg }; |
1003 | die if $@; | |
68dc0745 | 1004 | |
5a964f20 TC |
1005 | It's probably better in the general case to treat those |
1006 | variables as entries in some special hash. For example: | |
1007 | ||
197aec24 | 1008 | %user_defs = ( |
5a964f20 TC |
1009 | foo => 23, |
1010 | bar => 19, | |
1011 | ); | |
1012 | $text =~ s/\$(\w+)/$user_defs{$1}/g; | |
68dc0745 | 1013 | |
1014 | =head2 What's wrong with always quoting "$vars"? | |
1015 | ||
a6dd486b JB |
1016 | The problem is that those double-quotes force stringification-- |
1017 | coercing numbers and references into strings--even when you | |
1018 | don't want them to be strings. Think of it this way: double-quote | |
197aec24 | 1019 | expansion is used to produce new strings. If you already |
65acb1b1 | 1020 | have a string, why do you need more? |
68dc0745 | 1021 | |
1022 | If you get used to writing odd things like these: | |
1023 | ||
1024 | print "$var"; # BAD | |
1025 | $new = "$old"; # BAD | |
1026 | somefunc("$var"); # BAD | |
1027 | ||
1028 | You'll be in trouble. Those should (in 99.8% of the cases) be | |
1029 | the simpler and more direct: | |
1030 | ||
1031 | print $var; | |
1032 | $new = $old; | |
1033 | somefunc($var); | |
1034 | ||
1035 | Otherwise, besides slowing you down, you're going to break code when | |
1036 | the thing in the scalar is actually neither a string nor a number, but | |
1037 | a reference: | |
1038 | ||
1039 | func(\@array); | |
1040 | sub func { | |
1041 | my $aref = shift; | |
1042 | my $oref = "$aref"; # WRONG | |
1043 | } | |
1044 | ||
1045 | You can also get into subtle problems on those few operations in Perl | |
1046 | that actually do care about the difference between a string and a | |
1047 | number, such as the magical C<++> autoincrement operator or the | |
1048 | syscall() function. | |
1049 | ||
197aec24 | 1050 | Stringification also destroys arrays. |
5a964f20 TC |
1051 | |
1052 | @lines = `command`; | |
1053 | print "@lines"; # WRONG - extra blanks | |
1054 | print @lines; # right | |
1055 | ||
04d666b1 | 1056 | =head2 Why don't my E<lt>E<lt>HERE documents work? |
68dc0745 | 1057 | |
1058 | Check for these three things: | |
1059 | ||
1060 | =over 4 | |
1061 | ||
04d666b1 | 1062 | =item There must be no space after the E<lt>E<lt> part. |
68dc0745 | 1063 | |
197aec24 | 1064 | =item There (probably) should be a semicolon at the end. |
68dc0745 | 1065 | |
197aec24 | 1066 | =item You can't (easily) have any space in front of the tag. |
68dc0745 | 1067 | |
1068 | =back | |
1069 | ||
197aec24 | 1070 | If you want to indent the text in the here document, you |
5a964f20 TC |
1071 | can do this: |
1072 | ||
1073 | # all in one | |
1074 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; | |
1075 | your text | |
1076 | goes here | |
1077 | HERE_TARGET | |
1078 | ||
1079 | But the HERE_TARGET must still be flush against the margin. | |
197aec24 | 1080 | If you want that indented also, you'll have to quote |
5a964f20 TC |
1081 | in the indentation. |
1082 | ||
1083 | ($quote = <<' FINIS') =~ s/^\s+//gm; | |
1084 | ...we will have peace, when you and all your works have | |
1085 | perished--and the works of your dark master to whom you | |
1086 | would deliver us. You are a liar, Saruman, and a corrupter | |
1087 | of men's hearts. --Theoden in /usr/src/perl/taint.c | |
1088 | FINIS | |
83ded9ee | 1089 | $quote =~ s/\s+--/\n--/; |
5a964f20 TC |
1090 | |
1091 | A nice general-purpose fixer-upper function for indented here documents | |
1092 | follows. It expects to be called with a here document as its argument. | |
1093 | It looks to see whether each line begins with a common substring, and | |
a6dd486b JB |
1094 | if so, strips that substring off. Otherwise, it takes the amount of leading |
1095 | whitespace found on the first line and removes that much off each | |
5a964f20 TC |
1096 | subsequent line. |
1097 | ||
1098 | sub fix { | |
1099 | local $_ = shift; | |
a6dd486b | 1100 | my ($white, $leader); # common whitespace and common leading string |
5a964f20 TC |
1101 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { |
1102 | ($white, $leader) = ($2, quotemeta($1)); | |
1103 | } else { | |
1104 | ($white, $leader) = (/^(\s+)/, ''); | |
1105 | } | |
1106 | s/^\s*?$leader(?:$white)?//gm; | |
1107 | return $_; | |
1108 | } | |
1109 | ||
c8db1d39 | 1110 | This works with leading special strings, dynamically determined: |
5a964f20 TC |
1111 | |
1112 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; | |
1113 | @@@ int | |
1114 | @@@ runops() { | |
1115 | @@@ SAVEI32(runlevel); | |
1116 | @@@ runlevel++; | |
d92eb7b0 | 1117 | @@@ while ( op = (*op->op_ppaddr)() ); |
5a964f20 TC |
1118 | @@@ TAINT_NOT; |
1119 | @@@ return 0; | |
1120 | @@@ } | |
1121 | MAIN_INTERPRETER_LOOP | |
1122 | ||
a6dd486b | 1123 | Or with a fixed amount of leading whitespace, with remaining |
5a964f20 TC |
1124 | indentation correctly preserved: |
1125 | ||
1126 | $poem = fix<<EVER_ON_AND_ON; | |
1127 | Now far ahead the Road has gone, | |
1128 | And I must follow, if I can, | |
1129 | Pursuing it with eager feet, | |
1130 | Until it joins some larger way | |
1131 | Where many paths and errands meet. | |
1132 | And whither then? I cannot say. | |
1133 | --Bilbo in /usr/src/perl/pp_ctl.c | |
1134 | EVER_ON_AND_ON | |
1135 | ||
68dc0745 | 1136 | =head1 Data: Arrays |
1137 | ||
65acb1b1 TC |
1138 | =head2 What is the difference between a list and an array? |
1139 | ||
1140 | An array has a changeable length. A list does not. An array is something | |
1141 | you can push or pop, while a list is a set of values. Some people make | |
1142 | the distinction that a list is a value while an array is a variable. | |
1143 | Subroutines are passed and return lists, you put things into list | |
1144 | context, you initialize arrays with lists, and you foreach() across | |
1145 | a list. C<@> variables are arrays, anonymous arrays are arrays, arrays | |
1146 | in scalar context behave like the number of elements in them, subroutines | |
a6dd486b | 1147 | access their arguments through the array C<@_>, and push/pop/shift only work |
65acb1b1 TC |
1148 | on arrays. |
1149 | ||
1150 | As a side note, there's no such thing as a list in scalar context. | |
1151 | When you say | |
1152 | ||
1153 | $scalar = (2, 5, 7, 9); | |
1154 | ||
d92eb7b0 GS |
1155 | you're using the comma operator in scalar context, so it uses the scalar |
1156 | comma operator. There never was a list there at all! This causes the | |
1157 | last value to be returned: 9. | |
65acb1b1 | 1158 | |
68dc0745 | 1159 | =head2 What is the difference between $array[1] and @array[1]? |
1160 | ||
a6dd486b | 1161 | The former is a scalar value; the latter an array slice, making |
68dc0745 | 1162 | it a list with one (scalar) value. You should use $ when you want a |
1163 | scalar value (most of the time) and @ when you want a list with one | |
1164 | scalar value in it (very, very rarely; nearly never, in fact). | |
1165 | ||
1166 | Sometimes it doesn't make a difference, but sometimes it does. | |
1167 | For example, compare: | |
1168 | ||
1169 | $good[0] = `some program that outputs several lines`; | |
1170 | ||
1171 | with | |
1172 | ||
1173 | @bad[0] = `same program that outputs several lines`; | |
1174 | ||
197aec24 | 1175 | The C<use warnings> pragma and the B<-w> flag will warn you about these |
9f1b1f2d | 1176 | matters. |
68dc0745 | 1177 | |
d92eb7b0 | 1178 | =head2 How can I remove duplicate elements from a list or array? |
68dc0745 | 1179 | |
6670e5e7 | 1180 | (contributed by brian d foy) |
68dc0745 | 1181 | |
6670e5e7 RGS |
1182 | Use a hash. When you think the words "unique" or "duplicated", think |
1183 | "hash keys". | |
68dc0745 | 1184 | |
6670e5e7 RGS |
1185 | If you don't care about the order of the elements, you could just |
1186 | create the hash then extract the keys. It's not important how you | |
1187 | create that hash: just that you use C<keys> to get the unique | |
1188 | elements. | |
551e1d92 | 1189 | |
6670e5e7 RGS |
1190 | my %hash = map { $_, 1 } @array; |
1191 | # or a hash slice: @hash{ @array } = (); | |
1192 | # or a foreach: $hash{$_} = 1 foreach ( @array ); | |
68dc0745 | 1193 | |
6670e5e7 | 1194 | my @unique = keys %hash; |
68dc0745 | 1195 | |
6670e5e7 RGS |
1196 | You can also go through each element and skip the ones you've seen |
1197 | before. Use a hash to keep track. The first time the loop sees an | |
1198 | element, that element has no key in C<%Seen>. The C<next> statement | |
1199 | creates the key and immediately uses its value, which is C<undef>, so | |
1200 | the loop continues to the C<push> and increments the value for that | |
1201 | key. The next time the loop sees that same element, its key exists in | |
1202 | the hash I<and> the value for that key is true (since it's not 0 or | |
1203 | undef), so the next skips that iteration and the loop goes to the next | |
1204 | element. | |
551e1d92 | 1205 | |
6670e5e7 RGS |
1206 | my @unique = (); |
1207 | my %seen = (); | |
68dc0745 | 1208 | |
6670e5e7 RGS |
1209 | foreach my $elem ( @array ) |
1210 | { | |
1211 | next if $seen{ $elem }++; | |
1212 | push @unique, $elem; | |
1213 | } | |
68dc0745 | 1214 | |
6670e5e7 RGS |
1215 | You can write this more briefly using a grep, which does the |
1216 | same thing. | |
68dc0745 | 1217 | |
6670e5e7 RGS |
1218 | my %seen = (); |
1219 | my @unique = grep { ! $seen{ $_ }++ } @array; | |
65acb1b1 | 1220 | |
ddbc1f16 | 1221 | =head2 How can I tell whether a certain element is contained in a list or array? |
5a964f20 | 1222 | |
9e72e4c6 RGS |
1223 | (portions of this answer contributed by Anno Siegel) |
1224 | ||
5a964f20 TC |
1225 | Hearing the word "in" is an I<in>dication that you probably should have |
1226 | used a hash, not a list or array, to store your data. Hashes are | |
1227 | designed to answer this question quickly and efficiently. Arrays aren't. | |
68dc0745 | 1228 | |
5a964f20 TC |
1229 | That being said, there are several ways to approach this. If you |
1230 | are going to make this query many times over arbitrary string values, | |
881bdbd4 JH |
1231 | the fastest way is probably to invert the original array and maintain a |
1232 | hash whose keys are the first array's values. | |
68dc0745 | 1233 | |
1234 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; | |
881bdbd4 | 1235 | %is_blue = (); |
68dc0745 | 1236 | for (@blues) { $is_blue{$_} = 1 } |
1237 | ||
1238 | Now you can check whether $is_blue{$some_color}. It might have been a | |
1239 | good idea to keep the blues all in a hash in the first place. | |
1240 | ||
1241 | If the values are all small integers, you could use a simple indexed | |
1242 | array. This kind of an array will take up less space: | |
1243 | ||
1244 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); | |
881bdbd4 | 1245 | @is_tiny_prime = (); |
d92eb7b0 GS |
1246 | for (@primes) { $is_tiny_prime[$_] = 1 } |
1247 | # or simply @istiny_prime[@primes] = (1) x @primes; | |
68dc0745 | 1248 | |
1249 | Now you check whether $is_tiny_prime[$some_number]. | |
1250 | ||
1251 | If the values in question are integers instead of strings, you can save | |
1252 | quite a lot of space by using bit strings instead: | |
1253 | ||
1254 | @articles = ( 1..10, 150..2000, 2017 ); | |
1255 | undef $read; | |
7b8d334a | 1256 | for (@articles) { vec($read,$_,1) = 1 } |
68dc0745 | 1257 | |
1258 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. | |
1259 | ||
9e72e4c6 RGS |
1260 | These methods guarantee fast individual tests but require a re-organization |
1261 | of the original list or array. They only pay off if you have to test | |
1262 | multiple values against the same array. | |
68dc0745 | 1263 | |
9e72e4c6 RGS |
1264 | If you are testing only once, the standard module List::Util exports |
1265 | the function C<first> for this purpose. It works by stopping once it | |
1266 | finds the element. It's written in C for speed, and its Perl equivalant | |
1267 | looks like this subroutine: | |
68dc0745 | 1268 | |
9e72e4c6 RGS |
1269 | sub first (&@) { |
1270 | my $code = shift; | |
1271 | foreach (@_) { | |
1272 | return $_ if &{$code}(); | |
1273 | } | |
1274 | undef; | |
1275 | } | |
68dc0745 | 1276 | |
9e72e4c6 RGS |
1277 | If speed is of little concern, the common idiom uses grep in scalar context |
1278 | (which returns the number of items that passed its condition) to traverse the | |
1279 | entire list. This does have the benefit of telling you how many matches it | |
1280 | found, though. | |
68dc0745 | 1281 | |
9e72e4c6 | 1282 | my $is_there = grep $_ eq $whatever, @array; |
65acb1b1 | 1283 | |
9e72e4c6 RGS |
1284 | If you want to actually extract the matching elements, simply use grep in |
1285 | list context. | |
68dc0745 | 1286 | |
9e72e4c6 | 1287 | my @matches = grep $_ eq $whatever, @array; |
58103a2e | 1288 | |
68dc0745 | 1289 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? |
1290 | ||
1291 | Use a hash. Here's code to do both and more. It assumes that | |
1292 | each element is unique in a given array: | |
1293 | ||
1294 | @union = @intersection = @difference = (); | |
1295 | %count = (); | |
1296 | foreach $element (@array1, @array2) { $count{$element}++ } | |
1297 | foreach $element (keys %count) { | |
1298 | push @union, $element; | |
1299 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; | |
1300 | } | |
1301 | ||
d92eb7b0 | 1302 | Note that this is the I<symmetric difference>, that is, all elements in |
a6dd486b | 1303 | either A or in B but not in both. Think of it as an xor operation. |
d92eb7b0 | 1304 | |
65acb1b1 TC |
1305 | =head2 How do I test whether two arrays or hashes are equal? |
1306 | ||
1307 | The following code works for single-level arrays. It uses a stringwise | |
1308 | comparison, and does not distinguish defined versus undefined empty | |
1309 | strings. Modify if you have other needs. | |
1310 | ||
1311 | $are_equal = compare_arrays(\@frogs, \@toads); | |
1312 | ||
1313 | sub compare_arrays { | |
1314 | my ($first, $second) = @_; | |
9f1b1f2d | 1315 | no warnings; # silence spurious -w undef complaints |
65acb1b1 TC |
1316 | return 0 unless @$first == @$second; |
1317 | for (my $i = 0; $i < @$first; $i++) { | |
1318 | return 0 if $first->[$i] ne $second->[$i]; | |
1319 | } | |
1320 | return 1; | |
1321 | } | |
1322 | ||
1323 | For multilevel structures, you may wish to use an approach more | |
1324 | like this one. It uses the CPAN module FreezeThaw: | |
1325 | ||
1326 | use FreezeThaw qw(cmpStr); | |
1327 | @a = @b = ( "this", "that", [ "more", "stuff" ] ); | |
1328 | ||
1329 | printf "a and b contain %s arrays\n", | |
197aec24 RGS |
1330 | cmpStr(\@a, \@b) == 0 |
1331 | ? "the same" | |
65acb1b1 TC |
1332 | : "different"; |
1333 | ||
1334 | This approach also works for comparing hashes. Here | |
1335 | we'll demonstrate two different answers: | |
1336 | ||
1337 | use FreezeThaw qw(cmpStr cmpStrHard); | |
1338 | ||
1339 | %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); | |
1340 | $a{EXTRA} = \%b; | |
197aec24 | 1341 | $b{EXTRA} = \%a; |
65acb1b1 TC |
1342 | |
1343 | printf "a and b contain %s hashes\n", | |
1344 | cmpStr(\%a, \%b) == 0 ? "the same" : "different"; | |
1345 | ||
1346 | printf "a and b contain %s hashes\n", | |
1347 | cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; | |
1348 | ||
1349 | ||
1350 | The first reports that both those the hashes contain the same data, | |
1351 | while the second reports that they do not. Which you prefer is left as | |
1352 | an exercise to the reader. | |
1353 | ||
68dc0745 | 1354 | =head2 How do I find the first array element for which a condition is true? |
1355 | ||
49d635f9 RGS |
1356 | To find the first array element which satisfies a condition, you can |
1357 | use the first() function in the List::Util module, which comes with | |
1358 | Perl 5.8. This example finds the first element that contains "Perl". | |
1359 | ||
1360 | use List::Util qw(first); | |
197aec24 | 1361 | |
49d635f9 | 1362 | my $element = first { /Perl/ } @array; |
197aec24 | 1363 | |
49d635f9 RGS |
1364 | If you cannot use List::Util, you can make your own loop to do the |
1365 | same thing. Once you find the element, you stop the loop with last. | |
1366 | ||
1367 | my $found; | |
6670e5e7 | 1368 | foreach ( @array ) |
49d635f9 | 1369 | { |
6670e5e7 | 1370 | if( /Perl/ ) { $found = $_; last } |
49d635f9 RGS |
1371 | } |
1372 | ||
1373 | If you want the array index, you can iterate through the indices | |
1374 | and check the array element at each index until you find one | |
1375 | that satisfies the condition. | |
1376 | ||
197aec24 | 1377 | my( $found, $index ) = ( undef, -1 ); |
6670e5e7 RGS |
1378 | for( $i = 0; $i < @array; $i++ ) |
1379 | { | |
1380 | if( $array[$i] =~ /Perl/ ) | |
1381 | { | |
1382 | $found = $array[$i]; | |
1383 | $index = $i; | |
1384 | last; | |
1385 | } | |
1386 | } | |
68dc0745 | 1387 | |
1388 | =head2 How do I handle linked lists? | |
1389 | ||
1390 | In general, you usually don't need a linked list in Perl, since with | |
1391 | regular arrays, you can push and pop or shift and unshift at either end, | |
5a964f20 | 1392 | or you can use splice to add and/or remove arbitrary number of elements at |
87275199 | 1393 | arbitrary points. Both pop and shift are both O(1) operations on Perl's |
5a964f20 TC |
1394 | dynamic arrays. In the absence of shifts and pops, push in general |
1395 | needs to reallocate on the order every log(N) times, and unshift will | |
1396 | need to copy pointers each time. | |
68dc0745 | 1397 | |
1398 | If you really, really wanted, you could use structures as described in | |
1399 | L<perldsc> or L<perltoot> and do just what the algorithm book tells you | |
65acb1b1 TC |
1400 | to do. For example, imagine a list node like this: |
1401 | ||
1402 | $node = { | |
1403 | VALUE => 42, | |
1404 | LINK => undef, | |
1405 | }; | |
1406 | ||
1407 | You could walk the list this way: | |
1408 | ||
1409 | print "List: "; | |
1410 | for ($node = $head; $node; $node = $node->{LINK}) { | |
1411 | print $node->{VALUE}, " "; | |
1412 | } | |
1413 | print "\n"; | |
1414 | ||
a6dd486b | 1415 | You could add to the list this way: |
65acb1b1 TC |
1416 | |
1417 | my ($head, $tail); | |
1418 | $tail = append($head, 1); # grow a new head | |
1419 | for $value ( 2 .. 10 ) { | |
1420 | $tail = append($tail, $value); | |
1421 | } | |
1422 | ||
1423 | sub append { | |
1424 | my($list, $value) = @_; | |
1425 | my $node = { VALUE => $value }; | |
1426 | if ($list) { | |
1427 | $node->{LINK} = $list->{LINK}; | |
1428 | $list->{LINK} = $node; | |
1429 | } else { | |
1430 | $_[0] = $node; # replace caller's version | |
1431 | } | |
1432 | return $node; | |
1433 | } | |
1434 | ||
1435 | But again, Perl's built-in are virtually always good enough. | |
68dc0745 | 1436 | |
1437 | =head2 How do I handle circular lists? | |
1438 | ||
1439 | Circular lists could be handled in the traditional fashion with linked | |
1440 | lists, or you could just do something like this with an array: | |
1441 | ||
1442 | unshift(@array, pop(@array)); # the last shall be first | |
1443 | push(@array, shift(@array)); # and vice versa | |
1444 | ||
1445 | =head2 How do I shuffle an array randomly? | |
1446 | ||
45bbf655 JH |
1447 | If you either have Perl 5.8.0 or later installed, or if you have |
1448 | Scalar-List-Utils 1.03 or later installed, you can say: | |
1449 | ||
f05bbc40 | 1450 | use List::Util 'shuffle'; |
45bbf655 JH |
1451 | |
1452 | @shuffled = shuffle(@list); | |
1453 | ||
f05bbc40 | 1454 | If not, you can use a Fisher-Yates shuffle. |
5a964f20 | 1455 | |
5a964f20 | 1456 | sub fisher_yates_shuffle { |
cc30d1a7 JH |
1457 | my $deck = shift; # $deck is a reference to an array |
1458 | my $i = @$deck; | |
6670e5e7 | 1459 | while (--$i) { |
5a964f20 | 1460 | my $j = int rand ($i+1); |
cc30d1a7 | 1461 | @$deck[$i,$j] = @$deck[$j,$i]; |
5a964f20 TC |
1462 | } |
1463 | } | |
1464 | ||
cc30d1a7 JH |
1465 | # shuffle my mpeg collection |
1466 | # | |
1467 | my @mpeg = <audio/*/*.mp3>; | |
1468 | fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place | |
1469 | print @mpeg; | |
5a964f20 | 1470 | |
45bbf655 JH |
1471 | Note that the above implementation shuffles an array in place, |
1472 | unlike the List::Util::shuffle() which takes a list and returns | |
1473 | a new shuffled list. | |
1474 | ||
d92eb7b0 | 1475 | You've probably seen shuffling algorithms that work using splice, |
a6dd486b | 1476 | randomly picking another element to swap the current element with |
68dc0745 | 1477 | |
1478 | srand; | |
1479 | @new = (); | |
1480 | @old = 1 .. 10; # just a demo | |
1481 | while (@old) { | |
1482 | push(@new, splice(@old, rand @old, 1)); | |
1483 | } | |
1484 | ||
5a964f20 TC |
1485 | This is bad because splice is already O(N), and since you do it N times, |
1486 | you just invented a quadratic algorithm; that is, O(N**2). This does | |
1487 | not scale, although Perl is so efficient that you probably won't notice | |
1488 | this until you have rather largish arrays. | |
68dc0745 | 1489 | |
1490 | =head2 How do I process/modify each element of an array? | |
1491 | ||
1492 | Use C<for>/C<foreach>: | |
1493 | ||
1494 | for (@lines) { | |
6670e5e7 RGS |
1495 | s/foo/bar/; # change that word |
1496 | tr/XZ/ZX/; # swap those letters | |
68dc0745 | 1497 | } |
1498 | ||
1499 | Here's another; let's compute spherical volumes: | |
1500 | ||
5a964f20 | 1501 | for (@volumes = @radii) { # @volumes has changed parts |
6670e5e7 RGS |
1502 | $_ **= 3; |
1503 | $_ *= (4/3) * 3.14159; # this will be constant folded | |
68dc0745 | 1504 | } |
197aec24 | 1505 | |
49d635f9 RGS |
1506 | which can also be done with map() which is made to transform |
1507 | one list into another: | |
1508 | ||
1509 | @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii; | |
68dc0745 | 1510 | |
76817d6d JH |
1511 | If you want to do the same thing to modify the values of the |
1512 | hash, you can use the C<values> function. As of Perl 5.6 | |
1513 | the values are not copied, so if you modify $orbit (in this | |
1514 | case), you modify the value. | |
5a964f20 | 1515 | |
76817d6d | 1516 | for $orbit ( values %orbits ) { |
6670e5e7 | 1517 | ($orbit **= 3) *= (4/3) * 3.14159; |
5a964f20 | 1518 | } |
818c4caa | 1519 | |
76817d6d JH |
1520 | Prior to perl 5.6 C<values> returned copies of the values, |
1521 | so older perl code often contains constructions such as | |
1522 | C<@orbits{keys %orbits}> instead of C<values %orbits> where | |
1523 | the hash is to be modified. | |
818c4caa | 1524 | |
68dc0745 | 1525 | =head2 How do I select a random element from an array? |
1526 | ||
1527 | Use the rand() function (see L<perlfunc/rand>): | |
1528 | ||
68dc0745 | 1529 | $index = rand @array; |
1530 | $element = $array[$index]; | |
1531 | ||
793f5136 RGS |
1532 | Or, simply: |
1533 | my $element = $array[ rand @array ]; | |
5a964f20 | 1534 | |
68dc0745 | 1535 | =head2 How do I permute N elements of a list? |
1536 | ||
49d635f9 RGS |
1537 | Use the List::Permutor module on CPAN. If the list is |
1538 | actually an array, try the Algorithm::Permute module (also | |
1539 | on CPAN). It's written in XS code and is very efficient. | |
1540 | ||
1541 | use Algorithm::Permute; | |
1542 | my @array = 'a'..'d'; | |
1543 | my $p_iterator = Algorithm::Permute->new ( \@array ); | |
1544 | while (my @perm = $p_iterator->next) { | |
1545 | print "next permutation: (@perm)\n"; | |
1546 | } | |
1547 | ||
197aec24 RGS |
1548 | For even faster execution, you could do: |
1549 | ||
1550 | use Algorithm::Permute; | |
1551 | my @array = 'a'..'d'; | |
1552 | Algorithm::Permute::permute { | |
1553 | print "next permutation: (@array)\n"; | |
1554 | } @array; | |
1555 | ||
49d635f9 RGS |
1556 | Here's a little program that generates all permutations of |
1557 | all the words on each line of input. The algorithm embodied | |
1558 | in the permute() function is discussed in Volume 4 (still | |
1559 | unpublished) of Knuth's I<The Art of Computer Programming> | |
1560 | and will work on any list: | |
1561 | ||
1562 | #!/usr/bin/perl -n | |
1563 | # Fischer-Kause ordered permutation generator | |
1564 | ||
1565 | sub permute (&@) { | |
1566 | my $code = shift; | |
1567 | my @idx = 0..$#_; | |
1568 | while ( $code->(@_[@idx]) ) { | |
1569 | my $p = $#idx; | |
1570 | --$p while $idx[$p-1] > $idx[$p]; | |
1571 | my $q = $p or return; | |
1572 | push @idx, reverse splice @idx, $p; | |
1573 | ++$q while $idx[$p-1] > $idx[$q]; | |
1574 | @idx[$p-1,$q]=@idx[$q,$p-1]; | |
1575 | } | |
68dc0745 | 1576 | } |
68dc0745 | 1577 | |
49d635f9 | 1578 | permute {print"@_\n"} split; |
b8d2732a | 1579 | |
68dc0745 | 1580 | =head2 How do I sort an array by (anything)? |
1581 | ||
1582 | Supply a comparison function to sort() (described in L<perlfunc/sort>): | |
1583 | ||
1584 | @list = sort { $a <=> $b } @list; | |
1585 | ||
1586 | The default sort function is cmp, string comparison, which would | |
c47ff5f1 | 1587 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is |
68dc0745 | 1588 | the numerical comparison operator. |
1589 | ||
1590 | If you have a complicated function needed to pull out the part you | |
1591 | want to sort on, then don't do it inside the sort function. Pull it | |
1592 | out first, because the sort BLOCK can be called many times for the | |
1593 | same element. Here's an example of how to pull out the first word | |
1594 | after the first number on each item, and then sort those words | |
1595 | case-insensitively. | |
1596 | ||
1597 | @idx = (); | |
1598 | for (@data) { | |
1599 | ($item) = /\d+\s*(\S+)/; | |
1600 | push @idx, uc($item); | |
1601 | } | |
1602 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; | |
1603 | ||
a6dd486b | 1604 | which could also be written this way, using a trick |
68dc0745 | 1605 | that's come to be known as the Schwartzian Transform: |
1606 | ||
1607 | @sorted = map { $_->[0] } | |
1608 | sort { $a->[1] cmp $b->[1] } | |
d92eb7b0 | 1609 | map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; |
68dc0745 | 1610 | |
1611 | If you need to sort on several fields, the following paradigm is useful. | |
1612 | ||
1613 | @sorted = sort { field1($a) <=> field1($b) || | |
1614 | field2($a) cmp field2($b) || | |
1615 | field3($a) cmp field3($b) | |
1616 | } @data; | |
1617 | ||
1618 | This can be conveniently combined with precalculation of keys as given | |
1619 | above. | |
1620 | ||
379e39d7 | 1621 | See the F<sort> article in the "Far More Than You Ever Wanted |
49d635f9 | 1622 | To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for |
06a5f41f | 1623 | more about this approach. |
68dc0745 | 1624 | |
1625 | See also the question below on sorting hashes. | |
1626 | ||
1627 | =head2 How do I manipulate arrays of bits? | |
1628 | ||
1629 | Use pack() and unpack(), or else vec() and the bitwise operations. | |
1630 | ||
1631 | For example, this sets $vec to have bit N set if $ints[N] was set: | |
1632 | ||
1633 | $vec = ''; | |
1634 | foreach(@ints) { vec($vec,$_,1) = 1 } | |
1635 | ||
cc30d1a7 | 1636 | Here's how, given a vector in $vec, you can |
68dc0745 | 1637 | get those bits into your @ints array: |
1638 | ||
1639 | sub bitvec_to_list { | |
1640 | my $vec = shift; | |
1641 | my @ints; | |
1642 | # Find null-byte density then select best algorithm | |
1643 | if ($vec =~ tr/\0// / length $vec > 0.95) { | |
1644 | use integer; | |
1645 | my $i; | |
1646 | # This method is faster with mostly null-bytes | |
1647 | while($vec =~ /[^\0]/g ) { | |
1648 | $i = -9 + 8 * pos $vec; | |
1649 | push @ints, $i if vec($vec, ++$i, 1); | |
1650 | push @ints, $i if vec($vec, ++$i, 1); | |
1651 | push @ints, $i if vec($vec, ++$i, 1); | |
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 | } | |
1658 | } else { | |
1659 | # This method is a fast general algorithm | |
1660 | use integer; | |
1661 | my $bits = unpack "b*", $vec; | |
1662 | push @ints, 0 if $bits =~ s/^(\d)// && $1; | |
1663 | push @ints, pos $bits while($bits =~ /1/g); | |
1664 | } | |
1665 | return \@ints; | |
1666 | } | |
1667 | ||
1668 | This method gets faster the more sparse the bit vector is. | |
1669 | (Courtesy of Tim Bunce and Winfried Koenig.) | |
1670 | ||
76817d6d JH |
1671 | You can make the while loop a lot shorter with this suggestion |
1672 | from Benjamin Goldberg: | |
1673 | ||
1674 | while($vec =~ /[^\0]+/g ) { | |
1675 | push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8; | |
1676 | } | |
1677 | ||
cc30d1a7 JH |
1678 | Or use the CPAN module Bit::Vector: |
1679 | ||
1680 | $vector = Bit::Vector->new($num_of_bits); | |
1681 | $vector->Index_List_Store(@ints); | |
1682 | @ints = $vector->Index_List_Read(); | |
1683 | ||
1684 | Bit::Vector provides efficient methods for bit vector, sets of small integers | |
197aec24 | 1685 | and "big int" math. |
cc30d1a7 JH |
1686 | |
1687 | Here's a more extensive illustration using vec(): | |
65acb1b1 TC |
1688 | |
1689 | # vec demo | |
1690 | $vector = "\xff\x0f\xef\xfe"; | |
197aec24 | 1691 | print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", |
65acb1b1 TC |
1692 | unpack("N", $vector), "\n"; |
1693 | $is_set = vec($vector, 23, 1); | |
1694 | print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; | |
1695 | pvec($vector); | |
1696 | ||
1697 | set_vec(1,1,1); | |
1698 | set_vec(3,1,1); | |
1699 | set_vec(23,1,1); | |
1700 | ||
1701 | set_vec(3,1,3); | |
1702 | set_vec(3,2,3); | |
1703 | set_vec(3,4,3); | |
1704 | set_vec(3,4,7); | |
1705 | set_vec(3,8,3); | |
1706 | set_vec(3,8,7); | |
1707 | ||
1708 | set_vec(0,32,17); | |
1709 | set_vec(1,32,17); | |
1710 | ||
197aec24 | 1711 | sub set_vec { |
65acb1b1 TC |
1712 | my ($offset, $width, $value) = @_; |
1713 | my $vector = ''; | |
1714 | vec($vector, $offset, $width) = $value; | |
1715 | print "offset=$offset width=$width value=$value\n"; | |
1716 | pvec($vector); | |
1717 | } | |
1718 | ||
1719 | sub pvec { | |
1720 | my $vector = shift; | |
1721 | my $bits = unpack("b*", $vector); | |
1722 | my $i = 0; | |
1723 | my $BASE = 8; | |
1724 | ||
1725 | print "vector length in bytes: ", length($vector), "\n"; | |
1726 | @bytes = unpack("A8" x length($vector), $bits); | |
1727 | print "bits are: @bytes\n\n"; | |
197aec24 | 1728 | } |
65acb1b1 | 1729 | |
68dc0745 | 1730 | =head2 Why does defined() return true on empty arrays and hashes? |
1731 | ||
65acb1b1 TC |
1732 | The short story is that you should probably only use defined on scalars or |
1733 | functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> | |
1734 | in the 5.004 release or later of Perl for more detail. | |
68dc0745 | 1735 | |
1736 | =head1 Data: Hashes (Associative Arrays) | |
1737 | ||
1738 | =head2 How do I process an entire hash? | |
1739 | ||
1740 | Use the each() function (see L<perlfunc/each>) if you don't care | |
1741 | whether it's sorted: | |
1742 | ||
5a964f20 | 1743 | while ( ($key, $value) = each %hash) { |
68dc0745 | 1744 | print "$key = $value\n"; |
1745 | } | |
1746 | ||
1747 | If you want it sorted, you'll have to use foreach() on the result of | |
1748 | sorting the keys as shown in an earlier question. | |
1749 | ||
1750 | =head2 What happens if I add or remove keys from a hash while iterating over it? | |
1751 | ||
28b41a80 | 1752 | (contributed by brian d foy) |
d92eb7b0 | 1753 | |
28b41a80 | 1754 | The easy answer is "Don't do that!" |
d92eb7b0 | 1755 | |
28b41a80 RGS |
1756 | If you iterate through the hash with each(), you can delete the key |
1757 | most recently returned without worrying about it. If you delete or add | |
1758 | other keys, the iterator may skip or double up on them since perl | |
1759 | may rearrange the hash table. See the | |
1760 | entry for C<each()> in L<perlfunc>. | |
68dc0745 | 1761 | |
1762 | =head2 How do I look up a hash element by value? | |
1763 | ||
1764 | Create a reverse hash: | |
1765 | ||
1766 | %by_value = reverse %by_key; | |
1767 | $key = $by_value{$value}; | |
1768 | ||
1769 | That's not particularly efficient. It would be more space-efficient | |
1770 | to use: | |
1771 | ||
1772 | while (($key, $value) = each %by_key) { | |
1773 | $by_value{$value} = $key; | |
1774 | } | |
1775 | ||
d92eb7b0 GS |
1776 | If your hash could have repeated values, the methods above will only find |
1777 | one of the associated keys. This may or may not worry you. If it does | |
1778 | worry you, you can always reverse the hash into a hash of arrays instead: | |
1779 | ||
1780 | while (($key, $value) = each %by_key) { | |
1781 | push @{$key_list_by_value{$value}}, $key; | |
1782 | } | |
68dc0745 | 1783 | |
1784 | =head2 How can I know how many entries are in a hash? | |
1785 | ||
1786 | If you mean how many keys, then all you have to do is | |
875e5c2f | 1787 | use the keys() function in a scalar context: |
68dc0745 | 1788 | |
875e5c2f | 1789 | $num_keys = keys %hash; |
68dc0745 | 1790 | |
197aec24 RGS |
1791 | The keys() function also resets the iterator, which means that you may |
1792 | see strange results if you use this between uses of other hash operators | |
875e5c2f | 1793 | such as each(). |
68dc0745 | 1794 | |
1795 | =head2 How do I sort a hash (optionally by value instead of key)? | |
1796 | ||
a05e4845 RGS |
1797 | (contributed by brian d foy) |
1798 | ||
1799 | To sort a hash, start with the keys. In this example, we give the list of | |
1800 | keys to the sort function which then compares them ASCIIbetically (which | |
1801 | might be affected by your locale settings). The output list has the keys | |
1802 | in ASCIIbetical order. Once we have the keys, we can go through them to | |
1803 | create a report which lists the keys in ASCIIbetical order. | |
1804 | ||
1805 | my @keys = sort { $a cmp $b } keys %hash; | |
58103a2e | 1806 | |
a05e4845 RGS |
1807 | foreach my $key ( @keys ) |
1808 | { | |
1809 | printf "%-20s %6d\n", $key, $hash{$value}; | |
1810 | } | |
1811 | ||
58103a2e | 1812 | We could get more fancy in the C<sort()> block though. Instead of |
a05e4845 | 1813 | comparing the keys, we can compute a value with them and use that |
58103a2e | 1814 | value as the comparison. |
a05e4845 RGS |
1815 | |
1816 | For instance, to make our report order case-insensitive, we use | |
58103a2e | 1817 | the C<\L> sequence in a double-quoted string to make everything |
a05e4845 RGS |
1818 | lowercase. The C<sort()> block then compares the lowercased |
1819 | values to determine in which order to put the keys. | |
1820 | ||
1821 | my @keys = sort { "\L$a" cmp "\L$b" } keys %hash; | |
58103a2e | 1822 | |
a05e4845 | 1823 | Note: if the computation is expensive or the hash has many elements, |
58103a2e | 1824 | you may want to look at the Schwartzian Transform to cache the |
a05e4845 RGS |
1825 | computation results. |
1826 | ||
1827 | If we want to sort by the hash value instead, we use the hash key | |
1828 | to look it up. We still get out a list of keys, but this time they | |
1829 | are ordered by their value. | |
1830 | ||
1831 | my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash; | |
1832 | ||
1833 | From there we can get more complex. If the hash values are the same, | |
1834 | we can provide a secondary sort on the hash key. | |
1835 | ||
58103a2e RGS |
1836 | my @keys = sort { |
1837 | $hash{$a} <=> $hash{$b} | |
a05e4845 RGS |
1838 | or |
1839 | "\L$a" cmp "\L$b" | |
1840 | } keys %hash; | |
68dc0745 | 1841 | |
1842 | =head2 How can I always keep my hash sorted? | |
1843 | ||
1844 | You can look into using the DB_File module and tie() using the | |
1845 | $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">. | |
5a964f20 | 1846 | The Tie::IxHash module from CPAN might also be instructive. |
68dc0745 | 1847 | |
1848 | =head2 What's the difference between "delete" and "undef" with hashes? | |
1849 | ||
92993692 JH |
1850 | Hashes contain pairs of scalars: the first is the key, the |
1851 | second is the value. The key will be coerced to a string, | |
1852 | although the value can be any kind of scalar: string, | |
1853 | number, or reference. If a key $key is present in | |
1854 | %hash, C<exists($hash{$key})> will return true. The value | |
1855 | for a given key can be C<undef>, in which case | |
1856 | C<$hash{$key}> will be C<undef> while C<exists $hash{$key}> | |
1857 | will return true. This corresponds to (C<$key>, C<undef>) | |
1858 | being in the hash. | |
68dc0745 | 1859 | |
92993692 | 1860 | Pictures help... here's the %hash table: |
68dc0745 | 1861 | |
1862 | keys values | |
1863 | +------+------+ | |
1864 | | a | 3 | | |
1865 | | x | 7 | | |
1866 | | d | 0 | | |
1867 | | e | 2 | | |
1868 | +------+------+ | |
1869 | ||
1870 | And these conditions hold | |
1871 | ||
92993692 JH |
1872 | $hash{'a'} is true |
1873 | $hash{'d'} is false | |
1874 | defined $hash{'d'} is true | |
1875 | defined $hash{'a'} is true | |
1876 | exists $hash{'a'} is true (Perl5 only) | |
1877 | grep ($_ eq 'a', keys %hash) is true | |
68dc0745 | 1878 | |
1879 | If you now say | |
1880 | ||
92993692 | 1881 | undef $hash{'a'} |
68dc0745 | 1882 | |
1883 | your table now reads: | |
1884 | ||
1885 | ||
1886 | keys values | |
1887 | +------+------+ | |
1888 | | a | undef| | |
1889 | | x | 7 | | |
1890 | | d | 0 | | |
1891 | | e | 2 | | |
1892 | +------+------+ | |
1893 | ||
1894 | and these conditions now hold; changes in caps: | |
1895 | ||
92993692 JH |
1896 | $hash{'a'} is FALSE |
1897 | $hash{'d'} is false | |
1898 | defined $hash{'d'} is true | |
1899 | defined $hash{'a'} is FALSE | |
1900 | exists $hash{'a'} is true (Perl5 only) | |
1901 | grep ($_ eq 'a', keys %hash) is true | |
68dc0745 | 1902 | |
1903 | Notice the last two: you have an undef value, but a defined key! | |
1904 | ||
1905 | Now, consider this: | |
1906 | ||
92993692 | 1907 | delete $hash{'a'} |
68dc0745 | 1908 | |
1909 | your table now reads: | |
1910 | ||
1911 | keys values | |
1912 | +------+------+ | |
1913 | | x | 7 | | |
1914 | | d | 0 | | |
1915 | | e | 2 | | |
1916 | +------+------+ | |
1917 | ||
1918 | and these conditions now hold; changes in caps: | |
1919 | ||
92993692 JH |
1920 | $hash{'a'} is false |
1921 | $hash{'d'} is false | |
1922 | defined $hash{'d'} is true | |
1923 | defined $hash{'a'} is false | |
1924 | exists $hash{'a'} is FALSE (Perl5 only) | |
1925 | grep ($_ eq 'a', keys %hash) is FALSE | |
68dc0745 | 1926 | |
1927 | See, the whole entry is gone! | |
1928 | ||
1929 | =head2 Why don't my tied hashes make the defined/exists distinction? | |
1930 | ||
92993692 JH |
1931 | This depends on the tied hash's implementation of EXISTS(). |
1932 | For example, there isn't the concept of undef with hashes | |
1933 | that are tied to DBM* files. It also means that exists() and | |
1934 | defined() do the same thing with a DBM* file, and what they | |
1935 | end up doing is not what they do with ordinary hashes. | |
68dc0745 | 1936 | |
1937 | =head2 How do I reset an each() operation part-way through? | |
1938 | ||
5a964f20 | 1939 | Using C<keys %hash> in scalar context returns the number of keys in |
68dc0745 | 1940 | the hash I<and> resets the iterator associated with the hash. You may |
1941 | need to do this if you use C<last> to exit a loop early so that when you | |
46fc3d4c | 1942 | re-enter it, the hash iterator has been reset. |
68dc0745 | 1943 | |
1944 | =head2 How can I get the unique keys from two hashes? | |
1945 | ||
d92eb7b0 GS |
1946 | First you extract the keys from the hashes into lists, then solve |
1947 | the "removing duplicates" problem described above. For example: | |
68dc0745 | 1948 | |
1949 | %seen = (); | |
1950 | for $element (keys(%foo), keys(%bar)) { | |
1951 | $seen{$element}++; | |
1952 | } | |
1953 | @uniq = keys %seen; | |
1954 | ||
1955 | Or more succinctly: | |
1956 | ||
1957 | @uniq = keys %{{%foo,%bar}}; | |
1958 | ||
1959 | Or if you really want to save space: | |
1960 | ||
1961 | %seen = (); | |
1962 | while (defined ($key = each %foo)) { | |
1963 | $seen{$key}++; | |
1964 | } | |
1965 | while (defined ($key = each %bar)) { | |
1966 | $seen{$key}++; | |
1967 | } | |
1968 | @uniq = keys %seen; | |
1969 | ||
1970 | =head2 How can I store a multidimensional array in a DBM file? | |
1971 | ||
1972 | Either stringify the structure yourself (no fun), or else | |
1973 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer | |
1974 | it on top of either DB_File or GDBM_File. | |
1975 | ||
1976 | =head2 How can I make my hash remember the order I put elements into it? | |
1977 | ||
1978 | Use the Tie::IxHash from CPAN. | |
1979 | ||
46fc3d4c | 1980 | use Tie::IxHash; |
5f8d77f1 | 1981 | tie my %myhash, 'Tie::IxHash'; |
49d635f9 | 1982 | for (my $i=0; $i<20; $i++) { |
46fc3d4c | 1983 | $myhash{$i} = 2*$i; |
1984 | } | |
49d635f9 | 1985 | my @keys = keys %myhash; |
46fc3d4c | 1986 | # @keys = (0,1,2,3,...) |
1987 | ||
68dc0745 | 1988 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
1989 | ||
1990 | If you say something like: | |
1991 | ||
1992 | somefunc($hash{"nonesuch key here"}); | |
1993 | ||
1994 | Then that element "autovivifies"; that is, it springs into existence | |
1995 | whether you store something there or not. That's because functions | |
1996 | get scalars passed in by reference. If somefunc() modifies C<$_[0]>, | |
1997 | it has to be ready to write it back into the caller's version. | |
1998 | ||
87275199 | 1999 | This has been fixed as of Perl5.004. |
68dc0745 | 2000 | |
2001 | Normally, merely accessing a key's value for a nonexistent key does | |
2002 | I<not> cause that key to be forever there. This is different than | |
2003 | awk's behavior. | |
2004 | ||
fc36a67e | 2005 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
68dc0745 | 2006 | |
65acb1b1 TC |
2007 | Usually a hash ref, perhaps like this: |
2008 | ||
2009 | $record = { | |
2010 | NAME => "Jason", | |
2011 | EMPNO => 132, | |
2012 | TITLE => "deputy peon", | |
2013 | AGE => 23, | |
2014 | SALARY => 37_000, | |
2015 | PALS => [ "Norbert", "Rhys", "Phineas"], | |
2016 | }; | |
2017 | ||
2018 | References are documented in L<perlref> and the upcoming L<perlreftut>. | |
2019 | Examples of complex data structures are given in L<perldsc> and | |
2020 | L<perllol>. Examples of structures and object-oriented classes are | |
2021 | in L<perltoot>. | |
68dc0745 | 2022 | |
2023 | =head2 How can I use a reference as a hash key? | |
2024 | ||
9e72e4c6 RGS |
2025 | (contributed by brian d foy) |
2026 | ||
2027 | Hash keys are strings, so you can't really use a reference as the key. | |
2028 | When you try to do that, perl turns the reference into its stringified | |
2029 | form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get back | |
2030 | the reference from the stringified form, at least without doing some | |
2031 | extra work on your own. Also remember that hash keys must be unique, but | |
2032 | two different variables can store the same reference (and those variables | |
2033 | can change later). | |
2034 | ||
2035 | The Tie::RefHash module, which is distributed with perl, might be what | |
2036 | you want. It handles that extra work. | |
68dc0745 | 2037 | |
2038 | =head1 Data: Misc | |
2039 | ||
2040 | =head2 How do I handle binary data correctly? | |
2041 | ||
2042 | Perl is binary clean, so this shouldn't be a problem. For example, | |
2043 | this works fine (assuming the files are found): | |
2044 | ||
2045 | if (`cat /vmunix` =~ /gzip/) { | |
2046 | print "Your kernel is GNU-zip enabled!\n"; | |
2047 | } | |
2048 | ||
d92eb7b0 GS |
2049 | On less elegant (read: Byzantine) systems, however, you have |
2050 | to play tedious games with "text" versus "binary" files. See | |
49d635f9 | 2051 | L<perlfunc/"binmode"> or L<perlopentut>. |
68dc0745 | 2052 | |
2053 | If you're concerned about 8-bit ASCII data, then see L<perllocale>. | |
2054 | ||
54310121 | 2055 | If you want to deal with multibyte characters, however, there are |
68dc0745 | 2056 | some gotchas. See the section on Regular Expressions. |
2057 | ||
2058 | =head2 How do I determine whether a scalar is a number/whole/integer/float? | |
2059 | ||
2060 | Assuming that you don't care about IEEE notations like "NaN" or | |
2061 | "Infinity", you probably just want to use a regular expression. | |
2062 | ||
65acb1b1 TC |
2063 | if (/\D/) { print "has nondigits\n" } |
2064 | if (/^\d+$/) { print "is a whole number\n" } | |
2065 | if (/^-?\d+$/) { print "is an integer\n" } | |
2066 | if (/^[+-]?\d+$/) { print "is a +/- integer\n" } | |
2067 | if (/^-?\d+\.?\d*$/) { print "is a real number\n" } | |
881bdbd4 | 2068 | if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" } |
65acb1b1 | 2069 | if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) |
881bdbd4 | 2070 | { print "a C float\n" } |
68dc0745 | 2071 | |
f0d19b68 RGS |
2072 | There are also some commonly used modules for the task. |
2073 | L<Scalar::Util> (distributed with 5.8) provides access to perl's | |
2074 | internal function C<looks_like_number> for determining | |
2075 | whether a variable looks like a number. L<Data::Types> | |
2076 | exports functions that validate data types using both the | |
2077 | above and other regular expressions. Thirdly, there is | |
2078 | C<Regexp::Common> which has regular expressions to match | |
2079 | various types of numbers. Those three modules are available | |
2080 | from the CPAN. | |
2081 | ||
2082 | If you're on a POSIX system, Perl supports the C<POSIX::strtod> | |
5a964f20 TC |
2083 | function. Its semantics are somewhat cumbersome, so here's a C<getnum> |
2084 | wrapper function for more convenient access. This function takes | |
2085 | a string and returns the number it found, or C<undef> for input that | |
2086 | isn't a C float. The C<is_numeric> function is a front end to C<getnum> | |
b432a672 | 2087 | if you just want to say, "Is this a float?" |
5a964f20 TC |
2088 | |
2089 | sub getnum { | |
2090 | use POSIX qw(strtod); | |
2091 | my $str = shift; | |
2092 | $str =~ s/^\s+//; | |
2093 | $str =~ s/\s+$//; | |
2094 | $! = 0; | |
2095 | my($num, $unparsed) = strtod($str); | |
2096 | if (($str eq '') || ($unparsed != 0) || $!) { | |
2097 | return undef; | |
2098 | } else { | |
2099 | return $num; | |
197aec24 RGS |
2100 | } |
2101 | } | |
5a964f20 | 2102 | |
197aec24 | 2103 | sub is_numeric { defined getnum($_[0]) } |
5a964f20 | 2104 | |
f0d19b68 | 2105 | Or you could check out the L<String::Scanf> module on the CPAN |
b5b6f210 JH |
2106 | instead. The POSIX module (part of the standard Perl distribution) provides |
2107 | the C<strtod> and C<strtol> for converting strings to double and longs, | |
6cecdcac | 2108 | respectively. |
68dc0745 | 2109 | |
2110 | =head2 How do I keep persistent data across program calls? | |
2111 | ||
2112 | For some specific applications, you can use one of the DBM modules. | |
fe854a6f AT |
2113 | See L<AnyDBM_File>. More generically, you should consult the FreezeThaw |
2114 | or Storable modules from CPAN. Starting from Perl 5.8 Storable is part | |
2115 | of the standard distribution. Here's one example using Storable's C<store> | |
2116 | and C<retrieve> functions: | |
65acb1b1 | 2117 | |
197aec24 | 2118 | use Storable; |
65acb1b1 TC |
2119 | store(\%hash, "filename"); |
2120 | ||
197aec24 | 2121 | # later on... |
65acb1b1 TC |
2122 | $href = retrieve("filename"); # by ref |
2123 | %hash = %{ retrieve("filename") }; # direct to hash | |
68dc0745 | 2124 | |
2125 | =head2 How do I print out or copy a recursive data structure? | |
2126 | ||
65acb1b1 | 2127 | The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great |
6f82c03a EM |
2128 | for printing out data structures. The Storable module on CPAN (or the |
2129 | 5.8 release of Perl), provides a function called C<dclone> that recursively | |
2130 | copies its argument. | |
65acb1b1 | 2131 | |
197aec24 | 2132 | use Storable qw(dclone); |
65acb1b1 | 2133 | $r2 = dclone($r1); |
68dc0745 | 2134 | |
65acb1b1 TC |
2135 | Where $r1 can be a reference to any kind of data structure you'd like. |
2136 | It will be deeply copied. Because C<dclone> takes and returns references, | |
2137 | you'd have to add extra punctuation if you had a hash of arrays that | |
2138 | you wanted to copy. | |
68dc0745 | 2139 | |
65acb1b1 | 2140 | %newhash = %{ dclone(\%oldhash) }; |
68dc0745 | 2141 | |
2142 | =head2 How do I define methods for every class/object? | |
2143 | ||
2144 | Use the UNIVERSAL class (see L<UNIVERSAL>). | |
2145 | ||
2146 | =head2 How do I verify a credit card checksum? | |
2147 | ||
2148 | Get the Business::CreditCard module from CPAN. | |
2149 | ||
65acb1b1 TC |
2150 | =head2 How do I pack arrays of doubles or floats for XS code? |
2151 | ||
2152 | The kgbpack.c code in the PGPLOT module on CPAN does just this. | |
2153 | If you're doing a lot of float or double processing, consider using | |
2154 | the PDL module from CPAN instead--it makes number-crunching easy. | |
2155 | ||
68dc0745 | 2156 | =head1 AUTHOR AND COPYRIGHT |
2157 | ||
58103a2e | 2158 | Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and |
7678cced | 2159 | other authors as noted. All rights reserved. |
5a964f20 | 2160 | |
5a7beb56 JH |
2161 | This documentation is free; you can redistribute it and/or modify it |
2162 | under the same terms as Perl itself. | |
5a964f20 TC |
2163 | |
2164 | Irrespective of its distribution, all code examples in this file | |
2165 | are hereby placed into the public domain. You are permitted and | |
2166 | encouraged to use this code in your own programs for fun | |
2167 | or for profit as you see fit. A simple comment in the code giving | |
2168 | credit would be courteous but is not required. |