Commit | Line | Data |
---|---|---|
68dc0745 | 1 | =head1 NAME |
2 | ||
0d6290d3 | 3 | perlfaq4 - Data Manipulation ($Revision: 1.25 $, $Date: 2002/05/30 07:04:25 $) |
68dc0745 | 4 | |
5 | =head1 DESCRIPTION | |
6 | ||
ae3d0b9f JH |
7 | This section of the FAQ answers questions related to manipulating |
8 | numbers, dates, strings, arrays, hashes, and miscellaneous data issues. | |
68dc0745 | 9 | |
10 | =head1 Data: Numbers | |
11 | ||
46fc3d4c | 12 | =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? |
13 | ||
5a964f20 | 14 | The infinite set that a mathematician thinks of as the real numbers can |
a6dd486b | 15 | only be approximated on a computer, since the computer only has a finite |
5a964f20 TC |
16 | number of bits to store an infinite number of, um, numbers. |
17 | ||
46fc3d4c | 18 | Internally, your computer represents floating-point numbers in binary. |
92c2ed05 GS |
19 | Floating-point numbers read in from a file or appearing as literals |
20 | in your program are converted from their decimal floating-point | |
a6dd486b | 21 | representation (eg, 19.95) to an internal binary representation. |
46fc3d4c | 22 | |
23 | However, 19.95 can't be precisely represented as a binary | |
24 | floating-point number, just like 1/3 can't be exactly represented as a | |
25 | decimal floating-point number. The computer's binary representation | |
26 | of 19.95, therefore, isn't exactly 19.95. | |
27 | ||
28 | When a floating-point number gets printed, the binary floating-point | |
29 | representation is converted back to decimal. These decimal numbers | |
30 | are displayed in either the format you specify with printf(), or the | |
a6dd486b | 31 | current output format for numbers. (See L<perlvar/"$#"> if you use |
46fc3d4c | 32 | print. C<$#> has a different default value in Perl5 than it did in |
87275199 | 33 | Perl4. Changing C<$#> yourself is deprecated.) |
46fc3d4c | 34 | |
35 | This affects B<all> computer languages that represent decimal | |
36 | floating-point numbers in binary, not just Perl. Perl provides | |
37 | arbitrary-precision decimal numbers with the Math::BigFloat module | |
38 | (part of the standard Perl distribution), but mathematical operations | |
39 | are consequently slower. | |
40 | ||
80ba158a | 41 | If precision is important, such as when dealing with money, it's good |
1affb2ee MS |
42 | to work with integers and then divide at the last possible moment. |
43 | For example, work in pennies (1995) instead of dollars and cents | |
6b927632 | 44 | (19.95) and divide by 100 at the end. |
1affb2ee | 45 | |
46fc3d4c | 46 | To get rid of the superfluous digits, just use a format (eg, |
47 | C<printf("%.2f", 19.95)>) to get the required precision. | |
65acb1b1 | 48 | See L<perlop/"Floating-point Arithmetic">. |
46fc3d4c | 49 | |
68dc0745 | 50 | =head2 Why isn't my octal data interpreted correctly? |
51 | ||
52 | Perl only understands octal and hex numbers as such when they occur | |
33ce146f PP |
53 | as literals in your program. Octal literals in perl must start with |
54 | a leading "0" and hexadecimal literals must start with a leading "0x". | |
55 | If they are read in from somewhere and assigned, no automatic | |
56 | conversion takes place. You must explicitly use oct() or hex() if you | |
57 | want the values converted to decimal. oct() interprets | |
68dc0745 | 58 | both hex ("0x350") numbers and octal ones ("0350" or even without the |
59 | leading "0", like "377"), while hex() only converts hexadecimal ones, | |
60 | with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef". | |
33ce146f PP |
61 | The inverse mapping from decimal to octal can be done with either the |
62 | "%o" or "%O" sprintf() formats. To get from decimal to hex try either | |
63 | the "%x" or the "%X" formats to sprintf(). | |
68dc0745 | 64 | |
65 | This problem shows up most often when people try using chmod(), mkdir(), | |
33ce146f PP |
66 | umask(), or sysopen(), which by widespread tradition typically take |
67 | permissions in octal. | |
68dc0745 | 68 | |
33ce146f | 69 | chmod(644, $file); # WRONG |
68dc0745 | 70 | chmod(0644, $file); # right |
71 | ||
33ce146f PP |
72 | Note the mistake in the first line was specifying the decimal literal |
73 | 644, rather than the intended octal literal 0644. The problem can | |
74 | be seen with: | |
75 | ||
434f7166 | 76 | printf("%#o",644); # prints 01204 |
33ce146f PP |
77 | |
78 | Surely you had not intended C<chmod(01204, $file);> - did you? If you | |
79 | want to use numeric literals as arguments to chmod() et al. then please | |
80 | try to express them as octal constants, that is with a leading zero and | |
81 | with the following digits restricted to the set 0..7. | |
82 | ||
65acb1b1 | 83 | =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? |
68dc0745 | 84 | |
92c2ed05 GS |
85 | Remember that int() merely truncates toward 0. For rounding to a |
86 | certain number of digits, sprintf() or printf() is usually the easiest | |
87 | route. | |
88 | ||
89 | printf("%.3f", 3.1415926535); # prints 3.142 | |
68dc0745 | 90 | |
87275199 | 91 | The POSIX module (part of the standard Perl distribution) implements |
68dc0745 | 92 | ceil(), floor(), and a number of other mathematical and trigonometric |
93 | functions. | |
94 | ||
92c2ed05 GS |
95 | use POSIX; |
96 | $ceil = ceil(3.5); # 4 | |
97 | $floor = floor(3.5); # 3 | |
98 | ||
a6dd486b | 99 | In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex |
87275199 | 100 | module. With 5.004, the Math::Trig module (part of the standard Perl |
46fc3d4c | 101 | distribution) implements the trigonometric functions. Internally it |
102 | uses the Math::Complex module and some functions can break out from | |
103 | the real axis into the complex plane, for example the inverse sine of | |
104 | 2. | |
68dc0745 | 105 | |
106 | Rounding in financial applications can have serious implications, and | |
107 | the rounding method used should be specified precisely. In these | |
108 | cases, it probably pays not to trust whichever system rounding is | |
109 | being used by Perl, but to instead implement the rounding function you | |
110 | need yourself. | |
111 | ||
65acb1b1 TC |
112 | To see why, notice how you'll still have an issue on half-way-point |
113 | alternation: | |
114 | ||
115 | for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} | |
116 | ||
117 | 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 | |
118 | 0.8 0.8 0.9 0.9 1.0 1.0 | |
119 | ||
120 | Don't blame Perl. It's the same as in C. IEEE says we have to do this. | |
121 | Perl numbers whose absolute values are integers under 2**31 (on 32 bit | |
122 | machines) will work pretty much like mathematical integers. Other numbers | |
123 | are not guaranteed. | |
124 | ||
ae3d0b9f | 125 | =head2 How do I convert between numeric representations? |
68dc0745 | 126 | |
6761e064 JH |
127 | As always with Perl there is more than one way to do it. Below |
128 | are a few examples of approaches to making common conversions | |
129 | between number representations. This is intended to be representational | |
130 | rather than exhaustive. | |
68dc0745 | 131 | |
6761e064 JH |
132 | Some of the examples below use the Bit::Vector module from CPAN. |
133 | The reason you might choose Bit::Vector over the perl built in | |
134 | functions is that it works with numbers of ANY size, that it is | |
135 | optimized for speed on some operations, and for at least some | |
136 | programmers the notation might be familiar. | |
d92eb7b0 | 137 | |
818c4caa JH |
138 | =over 4 |
139 | ||
140 | =item How do I convert hexadecimal into decimal | |
d92eb7b0 | 141 | |
6761e064 JH |
142 | Using perl's built in conversion of 0x notation: |
143 | ||
144 | $int = 0xDEADBEEF; | |
145 | $dec = sprintf("%d", $int); | |
7207e29d | 146 | |
6761e064 JH |
147 | Using the hex function: |
148 | ||
149 | $int = hex("DEADBEEF"); | |
150 | $dec = sprintf("%d", $int); | |
151 | ||
152 | Using pack: | |
153 | ||
154 | $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); | |
155 | $dec = sprintf("%d", $int); | |
156 | ||
157 | Using the CPAN module Bit::Vector: | |
158 | ||
159 | use Bit::Vector; | |
160 | $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); | |
161 | $dec = $vec->to_Dec(); | |
162 | ||
818c4caa | 163 | =item How do I convert from decimal to hexadecimal |
6761e064 JH |
164 | |
165 | Using sprint: | |
166 | ||
167 | $hex = sprintf("%X", 3735928559); | |
168 | ||
169 | Using unpack | |
170 | ||
171 | $hex = unpack("H*", pack("N", 3735928559)); | |
172 | ||
173 | Using Bit::Vector | |
174 | ||
175 | use Bit::Vector; | |
176 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
177 | $hex = $vec->to_Hex(); | |
178 | ||
179 | And Bit::Vector supports odd bit counts: | |
180 | ||
181 | use Bit::Vector; | |
182 | $vec = Bit::Vector->new_Dec(33, 3735928559); | |
183 | $vec->Resize(32); # suppress leading 0 if unwanted | |
184 | $hex = $vec->to_Hex(); | |
185 | ||
818c4caa | 186 | =item How do I convert from octal to decimal |
6761e064 JH |
187 | |
188 | Using Perl's built in conversion of numbers with leading zeros: | |
189 | ||
190 | $int = 033653337357; # note the leading 0! | |
191 | $dec = sprintf("%d", $int); | |
192 | ||
193 | Using the oct function: | |
194 | ||
195 | $int = oct("33653337357"); | |
196 | $dec = sprintf("%d", $int); | |
197 | ||
198 | Using Bit::Vector: | |
199 | ||
200 | use Bit::Vector; | |
201 | $vec = Bit::Vector->new(32); | |
202 | $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); | |
203 | $dec = $vec->to_Dec(); | |
204 | ||
818c4caa | 205 | =item How do I convert from decimal to octal |
6761e064 JH |
206 | |
207 | Using sprintf: | |
208 | ||
209 | $oct = sprintf("%o", 3735928559); | |
210 | ||
211 | Using Bit::Vector | |
212 | ||
213 | use Bit::Vector; | |
214 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
215 | $oct = reverse join('', $vec->Chunk_List_Read(3)); | |
216 | ||
818c4caa | 217 | =item How do I convert from binary to decimal |
6761e064 | 218 | |
2c646907 JH |
219 | Perl 5.6 lets you write binary numbers directly with |
220 | the 0b notation: | |
221 | ||
222 | $number = 0b10110110; | |
223 | ||
6761e064 | 224 | Using pack and ord |
d92eb7b0 GS |
225 | |
226 | $decimal = ord(pack('B8', '10110110')); | |
68dc0745 | 227 | |
6761e064 JH |
228 | Using pack and unpack for larger strings |
229 | ||
230 | $int = unpack("N", pack("B32", | |
231 | substr("0" x 32 . "11110101011011011111011101111", -32))); | |
232 | $dec = sprintf("%d", $int); | |
233 | ||
5efd7060 | 234 | # substr() is used to left pad a 32 character string with zeros. |
6761e064 JH |
235 | |
236 | Using Bit::Vector: | |
237 | ||
238 | $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); | |
239 | $dec = $vec->to_Dec(); | |
240 | ||
818c4caa | 241 | =item How do I convert from decimal to binary |
6761e064 JH |
242 | |
243 | Using unpack; | |
244 | ||
245 | $bin = unpack("B*", pack("N", 3735928559)); | |
246 | ||
247 | Using Bit::Vector: | |
248 | ||
249 | use Bit::Vector; | |
250 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
251 | $bin = $vec->to_Bin(); | |
252 | ||
253 | The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) | |
254 | are left as an exercise to the inclined reader. | |
68dc0745 | 255 | |
818c4caa | 256 | =back |
68dc0745 | 257 | |
65acb1b1 TC |
258 | =head2 Why doesn't & work the way I want it to? |
259 | ||
260 | The behavior of binary arithmetic operators depends on whether they're | |
261 | used on numbers or strings. The operators treat a string as a series | |
262 | of bits and work with that (the string C<"3"> is the bit pattern | |
263 | C<00110011>). The operators work with the binary form of a number | |
264 | (the number C<3> is treated as the bit pattern C<00000011>). | |
265 | ||
266 | So, saying C<11 & 3> performs the "and" operation on numbers (yielding | |
267 | C<1>). Saying C<"11" & "3"> performs the "and" operation on strings | |
268 | (yielding C<"1">). | |
269 | ||
270 | Most problems with C<&> and C<|> arise because the programmer thinks | |
271 | they have a number but really it's a string. The rest arise because | |
272 | the programmer says: | |
273 | ||
274 | if ("\020\020" & "\101\101") { | |
275 | # ... | |
276 | } | |
277 | ||
278 | but a string consisting of two null bytes (the result of C<"\020\020" | |
279 | & "\101\101">) is not a false value in Perl. You need: | |
280 | ||
281 | if ( ("\020\020" & "\101\101") !~ /[^\000]/) { | |
282 | # ... | |
283 | } | |
284 | ||
68dc0745 | 285 | =head2 How do I multiply matrices? |
286 | ||
287 | Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) | |
288 | or the PDL extension (also available from CPAN). | |
289 | ||
290 | =head2 How do I perform an operation on a series of integers? | |
291 | ||
292 | To call a function on each element in an array, and collect the | |
293 | results, use: | |
294 | ||
295 | @results = map { my_func($_) } @array; | |
296 | ||
297 | For example: | |
298 | ||
299 | @triple = map { 3 * $_ } @single; | |
300 | ||
301 | To call a function on each element of an array, but ignore the | |
302 | results: | |
303 | ||
304 | foreach $iterator (@array) { | |
65acb1b1 | 305 | some_func($iterator); |
68dc0745 | 306 | } |
307 | ||
308 | To call a function on each integer in a (small) range, you B<can> use: | |
309 | ||
65acb1b1 | 310 | @results = map { some_func($_) } (5 .. 25); |
68dc0745 | 311 | |
312 | but you should be aware that the C<..> operator creates an array of | |
313 | all integers in the range. This can take a lot of memory for large | |
314 | ranges. Instead use: | |
315 | ||
316 | @results = (); | |
317 | for ($i=5; $i < 500_005; $i++) { | |
65acb1b1 | 318 | push(@results, some_func($i)); |
68dc0745 | 319 | } |
320 | ||
87275199 GS |
321 | This situation has been fixed in Perl5.005. Use of C<..> in a C<for> |
322 | loop will iterate over the range, without creating the entire range. | |
323 | ||
324 | for my $i (5 .. 500_005) { | |
325 | push(@results, some_func($i)); | |
326 | } | |
327 | ||
328 | will not create a list of 500,000 integers. | |
329 | ||
68dc0745 | 330 | =head2 How can I output Roman numerals? |
331 | ||
a93751fa | 332 | Get the http://www.cpan.org/modules/by-module/Roman module. |
68dc0745 | 333 | |
334 | =head2 Why aren't my random numbers random? | |
335 | ||
65acb1b1 TC |
336 | If you're using a version of Perl before 5.004, you must call C<srand> |
337 | once at the start of your program to seed the random number generator. | |
338 | 5.004 and later automatically call C<srand> at the beginning. Don't | |
339 | call C<srand> more than once--you make your numbers less random, rather | |
340 | than more. | |
92c2ed05 | 341 | |
65acb1b1 | 342 | Computers are good at being predictable and bad at being random |
06a5f41f JH |
343 | (despite appearances caused by bugs in your programs :-). see the |
344 | F<random> artitcle in the "Far More Than You Ever Wanted To Know" | |
345 | collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz , courtesy of | |
346 | Tom Phoenix, talks more about this. John von Neumann said, ``Anyone | |
347 | who attempts to generate random numbers by deterministic means is, of | |
65acb1b1 TC |
348 | course, living in a state of sin.'' |
349 | ||
350 | If you want numbers that are more random than C<rand> with C<srand> | |
351 | provides, you should also check out the Math::TrulyRandom module from | |
352 | CPAN. It uses the imperfections in your system's timer to generate | |
353 | random numbers, but this takes quite a while. If you want a better | |
92c2ed05 | 354 | pseudorandom generator than comes with your operating system, look at |
65acb1b1 | 355 | ``Numerical Recipes in C'' at http://www.nr.com/ . |
68dc0745 | 356 | |
881bdbd4 JH |
357 | =head2 How do I get a random number between X and Y? |
358 | ||
359 | Use the following simple function. It selects a random integer between | |
360 | (and possibly including!) the two given integers, e.g., | |
361 | C<random_int_in(50,120)> | |
362 | ||
363 | sub random_int_in ($$) { | |
364 | my($min, $max) = @_; | |
365 | # Assumes that the two arguments are integers themselves! | |
366 | return $min if $min == $max; | |
367 | ($min, $max) = ($max, $min) if $min > $max; | |
368 | return $min + int rand(1 + $max - $min); | |
369 | } | |
370 | ||
68dc0745 | 371 | =head1 Data: Dates |
372 | ||
373 | =head2 How do I find the week-of-the-year/day-of-the-year? | |
374 | ||
375 | The day of the year is in the array returned by localtime() (see | |
376 | L<perlfunc/"localtime">): | |
377 | ||
378 | $day_of_year = (localtime(time()))[7]; | |
379 | ||
d92eb7b0 GS |
380 | =head2 How do I find the current century or millennium? |
381 | ||
382 | Use the following simple functions: | |
383 | ||
384 | sub get_century { | |
385 | return int((((localtime(shift || time))[5] + 1999))/100); | |
386 | } | |
387 | sub get_millennium { | |
388 | return 1+int((((localtime(shift || time))[5] + 1899))/1000); | |
389 | } | |
390 | ||
391 | On some systems, you'll find that the POSIX module's strftime() function | |
392 | has been extended in a non-standard way to use a C<%C> format, which they | |
393 | sometimes claim is the "century". It isn't, because on most such systems, | |
394 | this is only the first two digits of the four-digit year, and thus cannot | |
395 | be used to reliably determine the current century or millennium. | |
396 | ||
92c2ed05 | 397 | =head2 How can I compare two dates and find the difference? |
68dc0745 | 398 | |
92c2ed05 GS |
399 | If you're storing your dates as epoch seconds then simply subtract one |
400 | from the other. If you've got a structured date (distinct year, day, | |
d92eb7b0 GS |
401 | month, hour, minute, seconds values), then for reasons of accessibility, |
402 | simplicity, and efficiency, merely use either timelocal or timegm (from | |
403 | the Time::Local module in the standard distribution) to reduce structured | |
404 | dates to epoch seconds. However, if you don't know the precise format of | |
405 | your dates, then you should probably use either of the Date::Manip and | |
406 | Date::Calc modules from CPAN before you go hacking up your own parsing | |
407 | routine to handle arbitrary date formats. | |
68dc0745 | 408 | |
409 | =head2 How can I take a string and turn it into epoch seconds? | |
410 | ||
411 | If it's a regular enough string that it always has the same format, | |
92c2ed05 GS |
412 | you can split it up and pass the parts to C<timelocal> in the standard |
413 | Time::Local module. Otherwise, you should look into the Date::Calc | |
414 | and Date::Manip modules from CPAN. | |
68dc0745 | 415 | |
416 | =head2 How can I find the Julian Day? | |
417 | ||
2a2bf5f4 JH |
418 | Use the Time::JulianDay module (part of the Time-modules bundle |
419 | available from CPAN.) | |
d92eb7b0 | 420 | |
89435c96 MS |
421 | Before you immerse yourself too deeply in this, be sure to verify that |
422 | it is the I<Julian> Day you really want. Are you interested in a way | |
423 | of getting serial days so that you just can tell how many days they | |
424 | are apart or so that you can do also other date arithmetic? If you | |
d92eb7b0 | 425 | are interested in performing date arithmetic, this can be done using |
2a2bf5f4 | 426 | modules Date::Manip or Date::Calc. |
89435c96 MS |
427 | |
428 | There is too many details and much confusion on this issue to cover in | |
429 | this FAQ, but the term is applied (correctly) to a calendar now | |
430 | supplanted by the Gregorian Calendar, with the Julian Calendar failing | |
431 | to adjust properly for leap years on centennial years (among other | |
432 | annoyances). The term is also used (incorrectly) to mean: [1] days in | |
433 | the Gregorian Calendar; and [2] days since a particular starting time | |
434 | or `epoch', usually 1970 in the Unix world and 1980 in the | |
435 | MS-DOS/Windows world. If you find that it is not the first meaning | |
436 | that you really want, then check out the Date::Manip and Date::Calc | |
437 | modules. (Thanks to David Cassell for most of this text.) | |
be94a901 | 438 | |
65acb1b1 TC |
439 | =head2 How do I find yesterday's date? |
440 | ||
441 | The C<time()> function returns the current time in seconds since the | |
d92eb7b0 | 442 | epoch. Take twenty-four hours off that: |
65acb1b1 TC |
443 | |
444 | $yesterday = time() - ( 24 * 60 * 60 ); | |
445 | ||
446 | Then you can pass this to C<localtime()> and get the individual year, | |
447 | month, day, hour, minute, seconds values. | |
448 | ||
d92eb7b0 GS |
449 | Note very carefully that the code above assumes that your days are |
450 | twenty-four hours each. For most people, there are two days a year | |
451 | when they aren't: the switch to and from summer time throws this off. | |
452 | A solution to this issue is offered by Russ Allbery. | |
453 | ||
454 | sub yesterday { | |
455 | my $now = defined $_[0] ? $_[0] : time; | |
456 | my $then = $now - 60 * 60 * 24; | |
457 | my $ndst = (localtime $now)[8] > 0; | |
458 | my $tdst = (localtime $then)[8] > 0; | |
459 | $then - ($tdst - $ndst) * 60 * 60; | |
460 | } | |
461 | # Should give you "this time yesterday" in seconds since epoch relative to | |
462 | # the first argument or the current time if no argument is given and | |
463 | # suitable for passing to localtime or whatever else you need to do with | |
464 | # it. $ndst is whether we're currently in daylight savings time; $tdst is | |
465 | # whether the point 24 hours ago was in daylight savings time. If $tdst | |
466 | # and $ndst are the same, a boundary wasn't crossed, and the correction | |
467 | # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more | |
468 | # from yesterday's time since we gained an extra hour while going off | |
469 | # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a | |
470 | # negative hour (add an hour) to yesterday's time since we lost an hour. | |
471 | # | |
472 | # All of this is because during those days when one switches off or onto | |
473 | # DST, a "day" isn't 24 hours long; it's either 23 or 25. | |
474 | # | |
475 | # The explicit settings of $ndst and $tdst are necessary because localtime | |
476 | # only says it returns the system tm struct, and the system tm struct at | |
87275199 | 477 | # least on Solaris doesn't guarantee any particular positive value (like, |
d92eb7b0 GS |
478 | # say, 1) for isdst, just a positive value. And that value can |
479 | # potentially be negative, if DST information isn't available (this sub | |
480 | # just treats those cases like no DST). | |
481 | # | |
482 | # Note that between 2am and 3am on the day after the time zone switches | |
483 | # off daylight savings time, the exact hour of "yesterday" corresponding | |
484 | # to the current hour is not clearly defined. Note also that if used | |
485 | # between 2am and 3am the day after the change to daylight savings time, | |
486 | # the result will be between 3am and 4am of the previous day; it's | |
487 | # arguable whether this is correct. | |
488 | # | |
489 | # This sub does not attempt to deal with leap seconds (most things don't). | |
490 | # | |
491 | # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu> | |
492 | # This code is in the public domain | |
493 | ||
87275199 | 494 | =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? |
68dc0745 | 495 | |
65acb1b1 TC |
496 | Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is |
497 | Y2K compliant (whatever that means). The programmers you've hired to | |
498 | use it, however, probably are not. | |
499 | ||
500 | Long answer: The question belies a true understanding of the issue. | |
501 | Perl is just as Y2K compliant as your pencil--no more, and no less. | |
502 | Can you use your pencil to write a non-Y2K-compliant memo? Of course | |
503 | you can. Is that the pencil's fault? Of course it isn't. | |
92c2ed05 | 504 | |
87275199 | 505 | The date and time functions supplied with Perl (gmtime and localtime) |
65acb1b1 TC |
506 | supply adequate information to determine the year well beyond 2000 |
507 | (2038 is when trouble strikes for 32-bit machines). The year returned | |
90fdbbb7 | 508 | by these functions when used in a list context is the year minus 1900. |
65acb1b1 TC |
509 | For years between 1910 and 1999 this I<happens> to be a 2-digit decimal |
510 | number. To avoid the year 2000 problem simply do not treat the year as | |
511 | a 2-digit number. It isn't. | |
68dc0745 | 512 | |
5a964f20 | 513 | When gmtime() and localtime() are used in scalar context they return |
68dc0745 | 514 | a timestamp string that contains a fully-expanded year. For example, |
515 | C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 | |
516 | 2001". There's no year 2000 problem here. | |
517 | ||
5a964f20 TC |
518 | That doesn't mean that Perl can't be used to create non-Y2K compliant |
519 | programs. It can. But so can your pencil. It's the fault of the user, | |
520 | not the language. At the risk of inflaming the NRA: ``Perl doesn't | |
521 | break Y2K, people do.'' See http://language.perl.com/news/y2k.html for | |
522 | a longer exposition. | |
523 | ||
68dc0745 | 524 | =head1 Data: Strings |
525 | ||
526 | =head2 How do I validate input? | |
527 | ||
528 | The answer to this question is usually a regular expression, perhaps | |
5a964f20 | 529 | with auxiliary logic. See the more specific questions (numbers, mail |
68dc0745 | 530 | addresses, etc.) for details. |
531 | ||
532 | =head2 How do I unescape a string? | |
533 | ||
92c2ed05 GS |
534 | It depends just what you mean by ``escape''. URL escapes are dealt |
535 | with in L<perlfaq9>. Shell escapes with the backslash (C<\>) | |
a6dd486b | 536 | character are removed with |
68dc0745 | 537 | |
538 | s/\\(.)/$1/g; | |
539 | ||
92c2ed05 | 540 | This won't expand C<"\n"> or C<"\t"> or any other special escapes. |
68dc0745 | 541 | |
542 | =head2 How do I remove consecutive pairs of characters? | |
543 | ||
92c2ed05 | 544 | To turn C<"abbcccd"> into C<"abccd">: |
68dc0745 | 545 | |
d92eb7b0 GS |
546 | s/(.)\1/$1/g; # add /s to include newlines |
547 | ||
548 | Here's a solution that turns "abbcccd" to "abcd": | |
549 | ||
550 | y///cs; # y == tr, but shorter :-) | |
68dc0745 | 551 | |
552 | =head2 How do I expand function calls in a string? | |
553 | ||
554 | This is documented in L<perlref>. In general, this is fraught with | |
555 | quoting and readability problems, but it is possible. To interpolate | |
5a964f20 | 556 | a subroutine call (in list context) into a string: |
68dc0745 | 557 | |
558 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; | |
559 | ||
560 | If you prefer scalar context, similar chicanery is also useful for | |
561 | arbitrary expressions: | |
562 | ||
563 | print "That yields ${\($n + 5)} widgets\n"; | |
564 | ||
92c2ed05 GS |
565 | Version 5.004 of Perl had a bug that gave list context to the |
566 | expression in C<${...}>, but this is fixed in version 5.005. | |
567 | ||
568 | See also ``How can I expand variables in text strings?'' in this | |
569 | section of the FAQ. | |
46fc3d4c | 570 | |
68dc0745 | 571 | =head2 How do I find matching/nesting anything? |
572 | ||
92c2ed05 GS |
573 | This isn't something that can be done in one regular expression, no |
574 | matter how complicated. To find something between two single | |
575 | characters, a pattern like C</x([^x]*)x/> will get the intervening | |
576 | bits in $1. For multiple ones, then something more like | |
577 | C</alpha(.*?)omega/> would be needed. But none of these deals with | |
f0f835c2 A |
578 | nested patterns. For balanced expressions using C<(>, C<{>, C<[> |
579 | or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see | |
580 | L<perlre/(??{ code })>. For other cases, you'll have to write a parser. | |
92c2ed05 GS |
581 | |
582 | If you are serious about writing a parser, there are a number of | |
6a2af475 GS |
583 | modules or oddities that will make your life a lot easier. There are |
584 | the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; | |
83df6a1d JH |
585 | and the byacc program. Starting from perl 5.8 the Text::Balanced |
586 | is part of the standard distribution. | |
68dc0745 | 587 | |
92c2ed05 GS |
588 | One simple destructive, inside-out approach that you might try is to |
589 | pull out the smallest nesting parts one at a time: | |
5a964f20 | 590 | |
d92eb7b0 | 591 | while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { |
5a964f20 TC |
592 | # do something with $1 |
593 | } | |
594 | ||
65acb1b1 TC |
595 | A more complicated and sneaky approach is to make Perl's regular |
596 | expression engine do it for you. This is courtesy Dean Inada, and | |
597 | rather has the nature of an Obfuscated Perl Contest entry, but it | |
598 | really does work: | |
599 | ||
600 | # $_ contains the string to parse | |
601 | # BEGIN and END are the opening and closing markers for the | |
602 | # nested text. | |
c47ff5f1 | 603 | |
65acb1b1 TC |
604 | @( = ('(',''); |
605 | @) = (')',''); | |
606 | ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; | |
5ed30e05 | 607 | @$ = (eval{/$re/},$@!~/unmatched/i); |
65acb1b1 TC |
608 | print join("\n",@$[0..$#$]) if( $$[-1] ); |
609 | ||
68dc0745 | 610 | =head2 How do I reverse a string? |
611 | ||
5a964f20 | 612 | Use reverse() in scalar context, as documented in |
68dc0745 | 613 | L<perlfunc/reverse>. |
614 | ||
615 | $reversed = reverse $string; | |
616 | ||
617 | =head2 How do I expand tabs in a string? | |
618 | ||
5a964f20 | 619 | You can do it yourself: |
68dc0745 | 620 | |
621 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; | |
622 | ||
87275199 | 623 | Or you can just use the Text::Tabs module (part of the standard Perl |
68dc0745 | 624 | distribution). |
625 | ||
626 | use Text::Tabs; | |
627 | @expanded_lines = expand(@lines_with_tabs); | |
628 | ||
629 | =head2 How do I reformat a paragraph? | |
630 | ||
87275199 | 631 | Use Text::Wrap (part of the standard Perl distribution): |
68dc0745 | 632 | |
633 | use Text::Wrap; | |
634 | print wrap("\t", ' ', @paragraphs); | |
635 | ||
92c2ed05 | 636 | The paragraphs you give to Text::Wrap should not contain embedded |
46fc3d4c | 637 | newlines. Text::Wrap doesn't justify the lines (flush-right). |
638 | ||
bc06af74 JH |
639 | Or use the CPAN module Text::Autoformat. Formatting files can be easily |
640 | done by making a shell alias, like so: | |
641 | ||
642 | alias fmt="perl -i -MText::Autoformat -n0777 \ | |
643 | -e 'print autoformat $_, {all=>1}' $*" | |
644 | ||
645 | See the documentation for Text::Autoformat to appreciate its many | |
646 | capabilities. | |
647 | ||
68dc0745 | 648 | =head2 How can I access/change the first N letters of a string? |
649 | ||
650 | There are many ways. If you just want to grab a copy, use | |
92c2ed05 | 651 | substr(): |
68dc0745 | 652 | |
653 | $first_byte = substr($a, 0, 1); | |
654 | ||
655 | If you want to modify part of a string, the simplest way is often to | |
656 | use substr() as an lvalue: | |
657 | ||
658 | substr($a, 0, 3) = "Tom"; | |
659 | ||
92c2ed05 | 660 | Although those with a pattern matching kind of thought process will |
a6dd486b | 661 | likely prefer |
68dc0745 | 662 | |
663 | $a =~ s/^.../Tom/; | |
664 | ||
665 | =head2 How do I change the Nth occurrence of something? | |
666 | ||
92c2ed05 GS |
667 | You have to keep track of N yourself. For example, let's say you want |
668 | to change the fifth occurrence of C<"whoever"> or C<"whomever"> into | |
d92eb7b0 GS |
669 | C<"whosoever"> or C<"whomsoever">, case insensitively. These |
670 | all assume that $_ contains the string to be altered. | |
68dc0745 | 671 | |
672 | $count = 0; | |
673 | s{((whom?)ever)}{ | |
674 | ++$count == 5 # is it the 5th? | |
675 | ? "${2}soever" # yes, swap | |
676 | : $1 # renege and leave it there | |
d92eb7b0 | 677 | }ige; |
68dc0745 | 678 | |
5a964f20 TC |
679 | In the more general case, you can use the C</g> modifier in a C<while> |
680 | loop, keeping count of matches. | |
681 | ||
682 | $WANT = 3; | |
683 | $count = 0; | |
d92eb7b0 | 684 | $_ = "One fish two fish red fish blue fish"; |
5a964f20 TC |
685 | while (/(\w+)\s+fish\b/gi) { |
686 | if (++$count == $WANT) { | |
687 | print "The third fish is a $1 one.\n"; | |
5a964f20 TC |
688 | } |
689 | } | |
690 | ||
92c2ed05 | 691 | That prints out: C<"The third fish is a red one."> You can also use a |
5a964f20 TC |
692 | repetition count and repeated pattern like this: |
693 | ||
694 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; | |
695 | ||
68dc0745 | 696 | =head2 How can I count the number of occurrences of a substring within a string? |
697 | ||
a6dd486b | 698 | There are a number of ways, with varying efficiency. If you want a |
68dc0745 | 699 | count of a certain single character (X) within a string, you can use the |
700 | C<tr///> function like so: | |
701 | ||
368c9434 | 702 | $string = "ThisXlineXhasXsomeXx'sXinXit"; |
68dc0745 | 703 | $count = ($string =~ tr/X//); |
d92eb7b0 | 704 | print "There are $count X characters in the string"; |
68dc0745 | 705 | |
706 | This is fine if you are just looking for a single character. However, | |
707 | if you are trying to count multiple character substrings within a | |
708 | larger string, C<tr///> won't work. What you can do is wrap a while() | |
709 | loop around a global pattern match. For example, let's count negative | |
710 | integers: | |
711 | ||
712 | $string = "-9 55 48 -2 23 -76 4 14 -44"; | |
713 | while ($string =~ /-\d+/g) { $count++ } | |
714 | print "There are $count negative numbers in the string"; | |
715 | ||
881bdbd4 JH |
716 | Another version uses a global match in list context, then assigns the |
717 | result to a scalar, producing a count of the number of matches. | |
718 | ||
719 | $count = () = $string =~ /-\d+/g; | |
720 | ||
68dc0745 | 721 | =head2 How do I capitalize all the words on one line? |
722 | ||
723 | To make the first letter of each word upper case: | |
3fe9a6f1 | 724 | |
68dc0745 | 725 | $line =~ s/\b(\w)/\U$1/g; |
726 | ||
46fc3d4c | 727 | This has the strange effect of turning "C<don't do it>" into "C<Don'T |
a6dd486b | 728 | Do It>". Sometimes you might want this. Other times you might need a |
24f1ba9b | 729 | more thorough solution (Suggested by brian d foy): |
46fc3d4c | 730 | |
731 | $string =~ s/ ( | |
732 | (^\w) #at the beginning of the line | |
733 | | # or | |
734 | (\s\w) #preceded by whitespace | |
735 | ) | |
736 | /\U$1/xg; | |
737 | $string =~ /([\w']+)/\u\L$1/g; | |
738 | ||
68dc0745 | 739 | To make the whole line upper case: |
3fe9a6f1 | 740 | |
68dc0745 | 741 | $line = uc($line); |
742 | ||
743 | To force each word to be lower case, with the first letter upper case: | |
3fe9a6f1 | 744 | |
68dc0745 | 745 | $line =~ s/(\w+)/\u\L$1/g; |
746 | ||
5a964f20 TC |
747 | You can (and probably should) enable locale awareness of those |
748 | characters by placing a C<use locale> pragma in your program. | |
92c2ed05 | 749 | See L<perllocale> for endless details on locales. |
5a964f20 | 750 | |
65acb1b1 | 751 | This is sometimes referred to as putting something into "title |
d92eb7b0 | 752 | case", but that's not quite accurate. Consider the proper |
65acb1b1 TC |
753 | capitalization of the movie I<Dr. Strangelove or: How I Learned to |
754 | Stop Worrying and Love the Bomb>, for example. | |
755 | ||
68dc0745 | 756 | =head2 How can I split a [character] delimited string except when inside |
757 | [character]? (Comma-separated files) | |
758 | ||
759 | Take the example case of trying to split a string that is comma-separated | |
760 | into its different fields. (We'll pretend you said comma-separated, not | |
761 | comma-delimited, which is different and almost never what you mean.) You | |
762 | can't use C<split(/,/)> because you shouldn't split if the comma is inside | |
763 | quotes. For example, take a data line like this: | |
764 | ||
765 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" | |
766 | ||
767 | Due to the restriction of the quotes, this is a fairly complex | |
768 | problem. Thankfully, we have Jeffrey Friedl, author of a highly | |
769 | recommended book on regular expressions, to handle these for us. He | |
770 | suggests (assuming your string is contained in $text): | |
771 | ||
772 | @new = (); | |
773 | push(@new, $+) while $text =~ m{ | |
774 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes | |
775 | | ([^,]+),? | |
776 | | , | |
777 | }gx; | |
778 | push(@new, undef) if substr($text,-1,1) eq ','; | |
779 | ||
46fc3d4c | 780 | If you want to represent quotation marks inside a |
781 | quotation-mark-delimited field, escape them with backslashes (eg, | |
2ceaccd7 | 782 | C<"like \"this\"">. Unescaping them is a task addressed earlier in |
46fc3d4c | 783 | this section. |
784 | ||
87275199 | 785 | Alternatively, the Text::ParseWords module (part of the standard Perl |
68dc0745 | 786 | distribution) lets you say: |
787 | ||
788 | use Text::ParseWords; | |
789 | @new = quotewords(",", 0, $text); | |
790 | ||
a6dd486b | 791 | There's also a Text::CSV (Comma-Separated Values) module on CPAN. |
65acb1b1 | 792 | |
68dc0745 | 793 | =head2 How do I strip blank space from the beginning/end of a string? |
794 | ||
a6dd486b | 795 | Although the simplest approach would seem to be |
68dc0745 | 796 | |
797 | $string =~ s/^\s*(.*?)\s*$/$1/; | |
798 | ||
a6dd486b | 799 | not only is this unnecessarily slow and destructive, it also fails with |
d92eb7b0 | 800 | embedded newlines. It is much faster to do this operation in two steps: |
68dc0745 | 801 | |
802 | $string =~ s/^\s+//; | |
803 | $string =~ s/\s+$//; | |
804 | ||
805 | Or more nicely written as: | |
806 | ||
807 | for ($string) { | |
808 | s/^\s+//; | |
809 | s/\s+$//; | |
810 | } | |
811 | ||
5e3006a4 | 812 | This idiom takes advantage of the C<foreach> loop's aliasing |
5a964f20 TC |
813 | behavior to factor out common code. You can do this |
814 | on several strings at once, or arrays, or even the | |
d92eb7b0 | 815 | values of a hash if you use a slice: |
5a964f20 TC |
816 | |
817 | # trim whitespace in the scalar, the array, | |
818 | # and all the values in the hash | |
819 | foreach ($scalar, @array, @hash{keys %hash}) { | |
820 | s/^\s+//; | |
821 | s/\s+$//; | |
822 | } | |
823 | ||
65acb1b1 TC |
824 | =head2 How do I pad a string with blanks or pad a number with zeroes? |
825 | ||
d92eb7b0 GS |
826 | (This answer contributed by Uri Guttman, with kibitzing from |
827 | Bart Lateur.) | |
65acb1b1 TC |
828 | |
829 | In the following examples, C<$pad_len> is the length to which you wish | |
d92eb7b0 GS |
830 | to pad the string, C<$text> or C<$num> contains the string to be padded, |
831 | and C<$pad_char> contains the padding character. You can use a single | |
832 | character string constant instead of the C<$pad_char> variable if you | |
833 | know what it is in advance. And in the same way you can use an integer in | |
834 | place of C<$pad_len> if you know the pad length in advance. | |
65acb1b1 | 835 | |
d92eb7b0 GS |
836 | The simplest method uses the C<sprintf> function. It can pad on the left |
837 | or right with blanks and on the left with zeroes and it will not | |
838 | truncate the result. The C<pack> function can only pad strings on the | |
839 | right with blanks and it will truncate the result to a maximum length of | |
840 | C<$pad_len>. | |
65acb1b1 | 841 | |
d92eb7b0 GS |
842 | # Left padding a string with blanks (no truncation): |
843 | $padded = sprintf("%${pad_len}s", $text); | |
65acb1b1 | 844 | |
d92eb7b0 GS |
845 | # Right padding a string with blanks (no truncation): |
846 | $padded = sprintf("%-${pad_len}s", $text); | |
65acb1b1 | 847 | |
d92eb7b0 GS |
848 | # Left padding a number with 0 (no truncation): |
849 | $padded = sprintf("%0${pad_len}d", $num); | |
65acb1b1 | 850 | |
d92eb7b0 GS |
851 | # Right padding a string with blanks using pack (will truncate): |
852 | $padded = pack("A$pad_len",$text); | |
65acb1b1 | 853 | |
d92eb7b0 GS |
854 | If you need to pad with a character other than blank or zero you can use |
855 | one of the following methods. They all generate a pad string with the | |
856 | C<x> operator and combine that with C<$text>. These methods do | |
857 | not truncate C<$text>. | |
65acb1b1 | 858 | |
d92eb7b0 | 859 | Left and right padding with any character, creating a new string: |
65acb1b1 | 860 | |
d92eb7b0 GS |
861 | $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; |
862 | $padded = $text . $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 863 | |
d92eb7b0 | 864 | Left and right padding with any character, modifying C<$text> directly: |
65acb1b1 | 865 | |
d92eb7b0 GS |
866 | substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); |
867 | $text .= $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 868 | |
68dc0745 | 869 | =head2 How do I extract selected columns from a string? |
870 | ||
871 | Use substr() or unpack(), both documented in L<perlfunc>. | |
5a964f20 TC |
872 | If you prefer thinking in terms of columns instead of widths, |
873 | you can use this kind of thing: | |
874 | ||
875 | # determine the unpack format needed to split Linux ps output | |
876 | # arguments are cut columns | |
877 | my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72); | |
878 | ||
879 | sub cut2fmt { | |
880 | my(@positions) = @_; | |
881 | my $template = ''; | |
882 | my $lastpos = 1; | |
883 | for my $place (@positions) { | |
884 | $template .= "A" . ($place - $lastpos) . " "; | |
885 | $lastpos = $place; | |
886 | } | |
887 | $template .= "A*"; | |
888 | return $template; | |
889 | } | |
68dc0745 | 890 | |
891 | =head2 How do I find the soundex value of a string? | |
892 | ||
87275199 | 893 | Use the standard Text::Soundex module distributed with Perl. |
a6dd486b | 894 | Before you do so, you may want to determine whether `soundex' is in |
d92eb7b0 GS |
895 | fact what you think it is. Knuth's soundex algorithm compresses words |
896 | into a small space, and so it does not necessarily distinguish between | |
897 | two words which you might want to appear separately. For example, the | |
898 | last names `Knuth' and `Kant' are both mapped to the soundex code K530. | |
899 | If Text::Soundex does not do what you are looking for, you might want | |
900 | to consider the String::Approx module available at CPAN. | |
68dc0745 | 901 | |
902 | =head2 How can I expand variables in text strings? | |
903 | ||
904 | Let's assume that you have a string like: | |
905 | ||
906 | $text = 'this has a $foo in it and a $bar'; | |
5a964f20 TC |
907 | |
908 | If those were both global variables, then this would | |
909 | suffice: | |
910 | ||
65acb1b1 | 911 | $text =~ s/\$(\w+)/${$1}/g; # no /e needed |
68dc0745 | 912 | |
5a964f20 TC |
913 | But since they are probably lexicals, or at least, they could |
914 | be, you'd have to do this: | |
68dc0745 | 915 | |
916 | $text =~ s/(\$\w+)/$1/eeg; | |
65acb1b1 | 917 | die if $@; # needed /ee, not /e |
68dc0745 | 918 | |
5a964f20 TC |
919 | It's probably better in the general case to treat those |
920 | variables as entries in some special hash. For example: | |
921 | ||
922 | %user_defs = ( | |
923 | foo => 23, | |
924 | bar => 19, | |
925 | ); | |
926 | $text =~ s/\$(\w+)/$user_defs{$1}/g; | |
68dc0745 | 927 | |
92c2ed05 | 928 | See also ``How do I expand function calls in a string?'' in this section |
46fc3d4c | 929 | of the FAQ. |
930 | ||
68dc0745 | 931 | =head2 What's wrong with always quoting "$vars"? |
932 | ||
a6dd486b JB |
933 | The problem is that those double-quotes force stringification-- |
934 | coercing numbers and references into strings--even when you | |
935 | don't want them to be strings. Think of it this way: double-quote | |
65acb1b1 TC |
936 | expansion is used to produce new strings. If you already |
937 | have a string, why do you need more? | |
68dc0745 | 938 | |
939 | If you get used to writing odd things like these: | |
940 | ||
941 | print "$var"; # BAD | |
942 | $new = "$old"; # BAD | |
943 | somefunc("$var"); # BAD | |
944 | ||
945 | You'll be in trouble. Those should (in 99.8% of the cases) be | |
946 | the simpler and more direct: | |
947 | ||
948 | print $var; | |
949 | $new = $old; | |
950 | somefunc($var); | |
951 | ||
952 | Otherwise, besides slowing you down, you're going to break code when | |
953 | the thing in the scalar is actually neither a string nor a number, but | |
954 | a reference: | |
955 | ||
956 | func(\@array); | |
957 | sub func { | |
958 | my $aref = shift; | |
959 | my $oref = "$aref"; # WRONG | |
960 | } | |
961 | ||
962 | You can also get into subtle problems on those few operations in Perl | |
963 | that actually do care about the difference between a string and a | |
964 | number, such as the magical C<++> autoincrement operator or the | |
965 | syscall() function. | |
966 | ||
5a964f20 TC |
967 | Stringification also destroys arrays. |
968 | ||
969 | @lines = `command`; | |
970 | print "@lines"; # WRONG - extra blanks | |
971 | print @lines; # right | |
972 | ||
c47ff5f1 | 973 | =head2 Why don't my <<HERE documents work? |
68dc0745 | 974 | |
975 | Check for these three things: | |
976 | ||
977 | =over 4 | |
978 | ||
979 | =item 1. There must be no space after the << part. | |
980 | ||
981 | =item 2. There (probably) should be a semicolon at the end. | |
982 | ||
983 | =item 3. You can't (easily) have any space in front of the tag. | |
984 | ||
985 | =back | |
986 | ||
5a964f20 TC |
987 | If you want to indent the text in the here document, you |
988 | can do this: | |
989 | ||
990 | # all in one | |
991 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; | |
992 | your text | |
993 | goes here | |
994 | HERE_TARGET | |
995 | ||
996 | But the HERE_TARGET must still be flush against the margin. | |
997 | If you want that indented also, you'll have to quote | |
998 | in the indentation. | |
999 | ||
1000 | ($quote = <<' FINIS') =~ s/^\s+//gm; | |
1001 | ...we will have peace, when you and all your works have | |
1002 | perished--and the works of your dark master to whom you | |
1003 | would deliver us. You are a liar, Saruman, and a corrupter | |
1004 | of men's hearts. --Theoden in /usr/src/perl/taint.c | |
1005 | FINIS | |
83ded9ee | 1006 | $quote =~ s/\s+--/\n--/; |
5a964f20 TC |
1007 | |
1008 | A nice general-purpose fixer-upper function for indented here documents | |
1009 | follows. It expects to be called with a here document as its argument. | |
1010 | It looks to see whether each line begins with a common substring, and | |
a6dd486b JB |
1011 | if so, strips that substring off. Otherwise, it takes the amount of leading |
1012 | whitespace found on the first line and removes that much off each | |
5a964f20 TC |
1013 | subsequent line. |
1014 | ||
1015 | sub fix { | |
1016 | local $_ = shift; | |
a6dd486b | 1017 | my ($white, $leader); # common whitespace and common leading string |
5a964f20 TC |
1018 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { |
1019 | ($white, $leader) = ($2, quotemeta($1)); | |
1020 | } else { | |
1021 | ($white, $leader) = (/^(\s+)/, ''); | |
1022 | } | |
1023 | s/^\s*?$leader(?:$white)?//gm; | |
1024 | return $_; | |
1025 | } | |
1026 | ||
c8db1d39 | 1027 | This works with leading special strings, dynamically determined: |
5a964f20 TC |
1028 | |
1029 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; | |
1030 | @@@ int | |
1031 | @@@ runops() { | |
1032 | @@@ SAVEI32(runlevel); | |
1033 | @@@ runlevel++; | |
d92eb7b0 | 1034 | @@@ while ( op = (*op->op_ppaddr)() ); |
5a964f20 TC |
1035 | @@@ TAINT_NOT; |
1036 | @@@ return 0; | |
1037 | @@@ } | |
1038 | MAIN_INTERPRETER_LOOP | |
1039 | ||
a6dd486b | 1040 | Or with a fixed amount of leading whitespace, with remaining |
5a964f20 TC |
1041 | indentation correctly preserved: |
1042 | ||
1043 | $poem = fix<<EVER_ON_AND_ON; | |
1044 | Now far ahead the Road has gone, | |
1045 | And I must follow, if I can, | |
1046 | Pursuing it with eager feet, | |
1047 | Until it joins some larger way | |
1048 | Where many paths and errands meet. | |
1049 | And whither then? I cannot say. | |
1050 | --Bilbo in /usr/src/perl/pp_ctl.c | |
1051 | EVER_ON_AND_ON | |
1052 | ||
68dc0745 | 1053 | =head1 Data: Arrays |
1054 | ||
65acb1b1 TC |
1055 | =head2 What is the difference between a list and an array? |
1056 | ||
1057 | An array has a changeable length. A list does not. An array is something | |
1058 | you can push or pop, while a list is a set of values. Some people make | |
1059 | the distinction that a list is a value while an array is a variable. | |
1060 | Subroutines are passed and return lists, you put things into list | |
1061 | context, you initialize arrays with lists, and you foreach() across | |
1062 | a list. C<@> variables are arrays, anonymous arrays are arrays, arrays | |
1063 | in scalar context behave like the number of elements in them, subroutines | |
a6dd486b | 1064 | access their arguments through the array C<@_>, and push/pop/shift only work |
65acb1b1 TC |
1065 | on arrays. |
1066 | ||
1067 | As a side note, there's no such thing as a list in scalar context. | |
1068 | When you say | |
1069 | ||
1070 | $scalar = (2, 5, 7, 9); | |
1071 | ||
d92eb7b0 GS |
1072 | you're using the comma operator in scalar context, so it uses the scalar |
1073 | comma operator. There never was a list there at all! This causes the | |
1074 | last value to be returned: 9. | |
65acb1b1 | 1075 | |
68dc0745 | 1076 | =head2 What is the difference between $array[1] and @array[1]? |
1077 | ||
a6dd486b | 1078 | The former is a scalar value; the latter an array slice, making |
68dc0745 | 1079 | it a list with one (scalar) value. You should use $ when you want a |
1080 | scalar value (most of the time) and @ when you want a list with one | |
1081 | scalar value in it (very, very rarely; nearly never, in fact). | |
1082 | ||
1083 | Sometimes it doesn't make a difference, but sometimes it does. | |
1084 | For example, compare: | |
1085 | ||
1086 | $good[0] = `some program that outputs several lines`; | |
1087 | ||
1088 | with | |
1089 | ||
1090 | @bad[0] = `same program that outputs several lines`; | |
1091 | ||
9f1b1f2d GS |
1092 | The C<use warnings> pragma and the B<-w> flag will warn you about these |
1093 | matters. | |
68dc0745 | 1094 | |
d92eb7b0 | 1095 | =head2 How can I remove duplicate elements from a list or array? |
68dc0745 | 1096 | |
1097 | There are several possible ways, depending on whether the array is | |
1098 | ordered and whether you wish to preserve the ordering. | |
1099 | ||
1100 | =over 4 | |
1101 | ||
551e1d92 RB |
1102 | =item a) |
1103 | ||
1104 | If @in is sorted, and you want @out to be sorted: | |
5a964f20 | 1105 | (this assumes all true values in the array) |
68dc0745 | 1106 | |
a4341a65 | 1107 | $prev = "not equal to $in[0]"; |
3bc5ef3e | 1108 | @out = grep($_ ne $prev && ($prev = $_, 1), @in); |
68dc0745 | 1109 | |
c8db1d39 | 1110 | This is nice in that it doesn't use much extra memory, simulating |
3bc5ef3e HG |
1111 | uniq(1)'s behavior of removing only adjacent duplicates. The ", 1" |
1112 | guarantees that the expression is true (so that grep picks it up) | |
1113 | even if the $_ is 0, "", or undef. | |
68dc0745 | 1114 | |
551e1d92 RB |
1115 | =item b) |
1116 | ||
1117 | If you don't know whether @in is sorted: | |
68dc0745 | 1118 | |
1119 | undef %saw; | |
1120 | @out = grep(!$saw{$_}++, @in); | |
1121 | ||
551e1d92 RB |
1122 | =item c) |
1123 | ||
1124 | Like (b), but @in contains only small integers: | |
68dc0745 | 1125 | |
1126 | @out = grep(!$saw[$_]++, @in); | |
1127 | ||
551e1d92 RB |
1128 | =item d) |
1129 | ||
1130 | A way to do (b) without any loops or greps: | |
68dc0745 | 1131 | |
1132 | undef %saw; | |
1133 | @saw{@in} = (); | |
1134 | @out = sort keys %saw; # remove sort if undesired | |
1135 | ||
551e1d92 RB |
1136 | =item e) |
1137 | ||
1138 | Like (d), but @in contains only small positive integers: | |
68dc0745 | 1139 | |
1140 | undef @ary; | |
1141 | @ary[@in] = @in; | |
87275199 | 1142 | @out = grep {defined} @ary; |
68dc0745 | 1143 | |
1144 | =back | |
1145 | ||
65acb1b1 TC |
1146 | But perhaps you should have been using a hash all along, eh? |
1147 | ||
ddbc1f16 | 1148 | =head2 How can I tell whether a certain element is contained in a list or array? |
5a964f20 TC |
1149 | |
1150 | Hearing the word "in" is an I<in>dication that you probably should have | |
1151 | used a hash, not a list or array, to store your data. Hashes are | |
1152 | designed to answer this question quickly and efficiently. Arrays aren't. | |
68dc0745 | 1153 | |
5a964f20 TC |
1154 | That being said, there are several ways to approach this. If you |
1155 | are going to make this query many times over arbitrary string values, | |
881bdbd4 JH |
1156 | the fastest way is probably to invert the original array and maintain a |
1157 | hash whose keys are the first array's values. | |
68dc0745 | 1158 | |
1159 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; | |
881bdbd4 | 1160 | %is_blue = (); |
68dc0745 | 1161 | for (@blues) { $is_blue{$_} = 1 } |
1162 | ||
1163 | Now you can check whether $is_blue{$some_color}. It might have been a | |
1164 | good idea to keep the blues all in a hash in the first place. | |
1165 | ||
1166 | If the values are all small integers, you could use a simple indexed | |
1167 | array. This kind of an array will take up less space: | |
1168 | ||
1169 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); | |
881bdbd4 | 1170 | @is_tiny_prime = (); |
d92eb7b0 GS |
1171 | for (@primes) { $is_tiny_prime[$_] = 1 } |
1172 | # or simply @istiny_prime[@primes] = (1) x @primes; | |
68dc0745 | 1173 | |
1174 | Now you check whether $is_tiny_prime[$some_number]. | |
1175 | ||
1176 | If the values in question are integers instead of strings, you can save | |
1177 | quite a lot of space by using bit strings instead: | |
1178 | ||
1179 | @articles = ( 1..10, 150..2000, 2017 ); | |
1180 | undef $read; | |
7b8d334a | 1181 | for (@articles) { vec($read,$_,1) = 1 } |
68dc0745 | 1182 | |
1183 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. | |
1184 | ||
1185 | Please do not use | |
1186 | ||
a6dd486b | 1187 | ($is_there) = grep $_ eq $whatever, @array; |
68dc0745 | 1188 | |
1189 | or worse yet | |
1190 | ||
a6dd486b | 1191 | ($is_there) = grep /$whatever/, @array; |
68dc0745 | 1192 | |
1193 | These are slow (checks every element even if the first matches), | |
1194 | inefficient (same reason), and potentially buggy (what if there are | |
d92eb7b0 | 1195 | regex characters in $whatever?). If you're only testing once, then |
65acb1b1 TC |
1196 | use: |
1197 | ||
1198 | $is_there = 0; | |
1199 | foreach $elt (@array) { | |
1200 | if ($elt eq $elt_to_find) { | |
1201 | $is_there = 1; | |
1202 | last; | |
1203 | } | |
1204 | } | |
1205 | if ($is_there) { ... } | |
68dc0745 | 1206 | |
1207 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? | |
1208 | ||
1209 | Use a hash. Here's code to do both and more. It assumes that | |
1210 | each element is unique in a given array: | |
1211 | ||
1212 | @union = @intersection = @difference = (); | |
1213 | %count = (); | |
1214 | foreach $element (@array1, @array2) { $count{$element}++ } | |
1215 | foreach $element (keys %count) { | |
1216 | push @union, $element; | |
1217 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; | |
1218 | } | |
1219 | ||
d92eb7b0 | 1220 | Note that this is the I<symmetric difference>, that is, all elements in |
a6dd486b | 1221 | either A or in B but not in both. Think of it as an xor operation. |
d92eb7b0 | 1222 | |
65acb1b1 TC |
1223 | =head2 How do I test whether two arrays or hashes are equal? |
1224 | ||
1225 | The following code works for single-level arrays. It uses a stringwise | |
1226 | comparison, and does not distinguish defined versus undefined empty | |
1227 | strings. Modify if you have other needs. | |
1228 | ||
1229 | $are_equal = compare_arrays(\@frogs, \@toads); | |
1230 | ||
1231 | sub compare_arrays { | |
1232 | my ($first, $second) = @_; | |
9f1b1f2d | 1233 | no warnings; # silence spurious -w undef complaints |
65acb1b1 TC |
1234 | return 0 unless @$first == @$second; |
1235 | for (my $i = 0; $i < @$first; $i++) { | |
1236 | return 0 if $first->[$i] ne $second->[$i]; | |
1237 | } | |
1238 | return 1; | |
1239 | } | |
1240 | ||
1241 | For multilevel structures, you may wish to use an approach more | |
1242 | like this one. It uses the CPAN module FreezeThaw: | |
1243 | ||
1244 | use FreezeThaw qw(cmpStr); | |
1245 | @a = @b = ( "this", "that", [ "more", "stuff" ] ); | |
1246 | ||
1247 | printf "a and b contain %s arrays\n", | |
1248 | cmpStr(\@a, \@b) == 0 | |
1249 | ? "the same" | |
1250 | : "different"; | |
1251 | ||
1252 | This approach also works for comparing hashes. Here | |
1253 | we'll demonstrate two different answers: | |
1254 | ||
1255 | use FreezeThaw qw(cmpStr cmpStrHard); | |
1256 | ||
1257 | %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); | |
1258 | $a{EXTRA} = \%b; | |
1259 | $b{EXTRA} = \%a; | |
1260 | ||
1261 | printf "a and b contain %s hashes\n", | |
1262 | cmpStr(\%a, \%b) == 0 ? "the same" : "different"; | |
1263 | ||
1264 | printf "a and b contain %s hashes\n", | |
1265 | cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; | |
1266 | ||
1267 | ||
1268 | The first reports that both those the hashes contain the same data, | |
1269 | while the second reports that they do not. Which you prefer is left as | |
1270 | an exercise to the reader. | |
1271 | ||
68dc0745 | 1272 | =head2 How do I find the first array element for which a condition is true? |
1273 | ||
1274 | You can use this if you care about the index: | |
1275 | ||
65acb1b1 | 1276 | for ($i= 0; $i < @array; $i++) { |
68dc0745 | 1277 | if ($array[$i] eq "Waldo") { |
1278 | $found_index = $i; | |
1279 | last; | |
1280 | } | |
1281 | } | |
1282 | ||
1283 | Now C<$found_index> has what you want. | |
1284 | ||
1285 | =head2 How do I handle linked lists? | |
1286 | ||
1287 | In general, you usually don't need a linked list in Perl, since with | |
1288 | regular arrays, you can push and pop or shift and unshift at either end, | |
5a964f20 | 1289 | or you can use splice to add and/or remove arbitrary number of elements at |
87275199 | 1290 | arbitrary points. Both pop and shift are both O(1) operations on Perl's |
5a964f20 TC |
1291 | dynamic arrays. In the absence of shifts and pops, push in general |
1292 | needs to reallocate on the order every log(N) times, and unshift will | |
1293 | need to copy pointers each time. | |
68dc0745 | 1294 | |
1295 | If you really, really wanted, you could use structures as described in | |
1296 | L<perldsc> or L<perltoot> and do just what the algorithm book tells you | |
65acb1b1 TC |
1297 | to do. For example, imagine a list node like this: |
1298 | ||
1299 | $node = { | |
1300 | VALUE => 42, | |
1301 | LINK => undef, | |
1302 | }; | |
1303 | ||
1304 | You could walk the list this way: | |
1305 | ||
1306 | print "List: "; | |
1307 | for ($node = $head; $node; $node = $node->{LINK}) { | |
1308 | print $node->{VALUE}, " "; | |
1309 | } | |
1310 | print "\n"; | |
1311 | ||
a6dd486b | 1312 | You could add to the list this way: |
65acb1b1 TC |
1313 | |
1314 | my ($head, $tail); | |
1315 | $tail = append($head, 1); # grow a new head | |
1316 | for $value ( 2 .. 10 ) { | |
1317 | $tail = append($tail, $value); | |
1318 | } | |
1319 | ||
1320 | sub append { | |
1321 | my($list, $value) = @_; | |
1322 | my $node = { VALUE => $value }; | |
1323 | if ($list) { | |
1324 | $node->{LINK} = $list->{LINK}; | |
1325 | $list->{LINK} = $node; | |
1326 | } else { | |
1327 | $_[0] = $node; # replace caller's version | |
1328 | } | |
1329 | return $node; | |
1330 | } | |
1331 | ||
1332 | But again, Perl's built-in are virtually always good enough. | |
68dc0745 | 1333 | |
1334 | =head2 How do I handle circular lists? | |
1335 | ||
1336 | Circular lists could be handled in the traditional fashion with linked | |
1337 | lists, or you could just do something like this with an array: | |
1338 | ||
1339 | unshift(@array, pop(@array)); # the last shall be first | |
1340 | push(@array, shift(@array)); # and vice versa | |
1341 | ||
1342 | =head2 How do I shuffle an array randomly? | |
1343 | ||
45bbf655 JH |
1344 | If you either have Perl 5.8.0 or later installed, or if you have |
1345 | Scalar-List-Utils 1.03 or later installed, you can say: | |
1346 | ||
f05bbc40 | 1347 | use List::Util 'shuffle'; |
45bbf655 JH |
1348 | |
1349 | @shuffled = shuffle(@list); | |
1350 | ||
f05bbc40 | 1351 | If not, you can use a Fisher-Yates shuffle. |
5a964f20 | 1352 | |
5a964f20 | 1353 | sub fisher_yates_shuffle { |
cc30d1a7 JH |
1354 | my $deck = shift; # $deck is a reference to an array |
1355 | my $i = @$deck; | |
f05bbc40 | 1356 | while ($i--) { |
5a964f20 | 1357 | my $j = int rand ($i+1); |
cc30d1a7 | 1358 | @$deck[$i,$j] = @$deck[$j,$i]; |
5a964f20 TC |
1359 | } |
1360 | } | |
1361 | ||
cc30d1a7 JH |
1362 | # shuffle my mpeg collection |
1363 | # | |
1364 | my @mpeg = <audio/*/*.mp3>; | |
1365 | fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place | |
1366 | print @mpeg; | |
5a964f20 | 1367 | |
45bbf655 JH |
1368 | Note that the above implementation shuffles an array in place, |
1369 | unlike the List::Util::shuffle() which takes a list and returns | |
1370 | a new shuffled list. | |
1371 | ||
d92eb7b0 | 1372 | You've probably seen shuffling algorithms that work using splice, |
a6dd486b | 1373 | randomly picking another element to swap the current element with |
68dc0745 | 1374 | |
1375 | srand; | |
1376 | @new = (); | |
1377 | @old = 1 .. 10; # just a demo | |
1378 | while (@old) { | |
1379 | push(@new, splice(@old, rand @old, 1)); | |
1380 | } | |
1381 | ||
5a964f20 TC |
1382 | This is bad because splice is already O(N), and since you do it N times, |
1383 | you just invented a quadratic algorithm; that is, O(N**2). This does | |
1384 | not scale, although Perl is so efficient that you probably won't notice | |
1385 | this until you have rather largish arrays. | |
68dc0745 | 1386 | |
1387 | =head2 How do I process/modify each element of an array? | |
1388 | ||
1389 | Use C<for>/C<foreach>: | |
1390 | ||
1391 | for (@lines) { | |
5a964f20 TC |
1392 | s/foo/bar/; # change that word |
1393 | y/XZ/ZX/; # swap those letters | |
68dc0745 | 1394 | } |
1395 | ||
1396 | Here's another; let's compute spherical volumes: | |
1397 | ||
5a964f20 | 1398 | for (@volumes = @radii) { # @volumes has changed parts |
68dc0745 | 1399 | $_ **= 3; |
1400 | $_ *= (4/3) * 3.14159; # this will be constant folded | |
1401 | } | |
1402 | ||
76817d6d JH |
1403 | If you want to do the same thing to modify the values of the |
1404 | hash, you can use the C<values> function. As of Perl 5.6 | |
1405 | the values are not copied, so if you modify $orbit (in this | |
1406 | case), you modify the value. | |
5a964f20 | 1407 | |
76817d6d | 1408 | for $orbit ( values %orbits ) { |
5a964f20 TC |
1409 | ($orbit **= 3) *= (4/3) * 3.14159; |
1410 | } | |
818c4caa | 1411 | |
76817d6d JH |
1412 | Prior to perl 5.6 C<values> returned copies of the values, |
1413 | so older perl code often contains constructions such as | |
1414 | C<@orbits{keys %orbits}> instead of C<values %orbits> where | |
1415 | the hash is to be modified. | |
818c4caa | 1416 | |
68dc0745 | 1417 | =head2 How do I select a random element from an array? |
1418 | ||
1419 | Use the rand() function (see L<perlfunc/rand>): | |
1420 | ||
5a964f20 | 1421 | # at the top of the program: |
68dc0745 | 1422 | srand; # not needed for 5.004 and later |
5a964f20 TC |
1423 | |
1424 | # then later on | |
68dc0745 | 1425 | $index = rand @array; |
1426 | $element = $array[$index]; | |
1427 | ||
5a964f20 TC |
1428 | Make sure you I<only call srand once per program, if then>. |
1429 | If you are calling it more than once (such as before each | |
1430 | call to rand), you're almost certainly doing something wrong. | |
1431 | ||
68dc0745 | 1432 | =head2 How do I permute N elements of a list? |
1433 | ||
1434 | Here's a little program that generates all permutations | |
1435 | of all the words on each line of input. The algorithm embodied | |
5a964f20 | 1436 | in the permute() function should work on any list: |
68dc0745 | 1437 | |
1438 | #!/usr/bin/perl -n | |
5a964f20 TC |
1439 | # tsc-permute: permute each word of input |
1440 | permute([split], []); | |
1441 | sub permute { | |
1442 | my @items = @{ $_[0] }; | |
1443 | my @perms = @{ $_[1] }; | |
1444 | unless (@items) { | |
1445 | print "@perms\n"; | |
68dc0745 | 1446 | } else { |
5a964f20 TC |
1447 | my(@newitems,@newperms,$i); |
1448 | foreach $i (0 .. $#items) { | |
1449 | @newitems = @items; | |
1450 | @newperms = @perms; | |
1451 | unshift(@newperms, splice(@newitems, $i, 1)); | |
1452 | permute([@newitems], [@newperms]); | |
68dc0745 | 1453 | } |
1454 | } | |
1455 | } | |
1456 | ||
b8d2732a RH |
1457 | Unfortunately, this algorithm is very inefficient. The Algorithm::Permute |
1458 | module from CPAN runs at least an order of magnitude faster. If you don't | |
1459 | have a C compiler (or a binary distribution of Algorithm::Permute), then | |
1460 | you can use List::Permutor which is written in pure Perl, and is still | |
f8620f40 | 1461 | several times faster than the algorithm above. |
b8d2732a | 1462 | |
68dc0745 | 1463 | =head2 How do I sort an array by (anything)? |
1464 | ||
1465 | Supply a comparison function to sort() (described in L<perlfunc/sort>): | |
1466 | ||
1467 | @list = sort { $a <=> $b } @list; | |
1468 | ||
1469 | The default sort function is cmp, string comparison, which would | |
c47ff5f1 | 1470 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is |
68dc0745 | 1471 | the numerical comparison operator. |
1472 | ||
1473 | If you have a complicated function needed to pull out the part you | |
1474 | want to sort on, then don't do it inside the sort function. Pull it | |
1475 | out first, because the sort BLOCK can be called many times for the | |
1476 | same element. Here's an example of how to pull out the first word | |
1477 | after the first number on each item, and then sort those words | |
1478 | case-insensitively. | |
1479 | ||
1480 | @idx = (); | |
1481 | for (@data) { | |
1482 | ($item) = /\d+\s*(\S+)/; | |
1483 | push @idx, uc($item); | |
1484 | } | |
1485 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; | |
1486 | ||
a6dd486b | 1487 | which could also be written this way, using a trick |
68dc0745 | 1488 | that's come to be known as the Schwartzian Transform: |
1489 | ||
1490 | @sorted = map { $_->[0] } | |
1491 | sort { $a->[1] cmp $b->[1] } | |
d92eb7b0 | 1492 | map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; |
68dc0745 | 1493 | |
1494 | If you need to sort on several fields, the following paradigm is useful. | |
1495 | ||
1496 | @sorted = sort { field1($a) <=> field1($b) || | |
1497 | field2($a) cmp field2($b) || | |
1498 | field3($a) cmp field3($b) | |
1499 | } @data; | |
1500 | ||
1501 | This can be conveniently combined with precalculation of keys as given | |
1502 | above. | |
1503 | ||
06a5f41f JH |
1504 | See the F<sort> artitcle article in the "Far More Than You Ever Wanted |
1505 | To Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz for | |
1506 | more about this approach. | |
68dc0745 | 1507 | |
1508 | See also the question below on sorting hashes. | |
1509 | ||
1510 | =head2 How do I manipulate arrays of bits? | |
1511 | ||
1512 | Use pack() and unpack(), or else vec() and the bitwise operations. | |
1513 | ||
1514 | For example, this sets $vec to have bit N set if $ints[N] was set: | |
1515 | ||
1516 | $vec = ''; | |
1517 | foreach(@ints) { vec($vec,$_,1) = 1 } | |
1518 | ||
cc30d1a7 | 1519 | Here's how, given a vector in $vec, you can |
68dc0745 | 1520 | get those bits into your @ints array: |
1521 | ||
1522 | sub bitvec_to_list { | |
1523 | my $vec = shift; | |
1524 | my @ints; | |
1525 | # Find null-byte density then select best algorithm | |
1526 | if ($vec =~ tr/\0// / length $vec > 0.95) { | |
1527 | use integer; | |
1528 | my $i; | |
1529 | # This method is faster with mostly null-bytes | |
1530 | while($vec =~ /[^\0]/g ) { | |
1531 | $i = -9 + 8 * pos $vec; | |
1532 | push @ints, $i if vec($vec, ++$i, 1); | |
1533 | push @ints, $i if vec($vec, ++$i, 1); | |
1534 | push @ints, $i if vec($vec, ++$i, 1); | |
1535 | push @ints, $i if vec($vec, ++$i, 1); | |
1536 | push @ints, $i if vec($vec, ++$i, 1); | |
1537 | push @ints, $i if vec($vec, ++$i, 1); | |
1538 | push @ints, $i if vec($vec, ++$i, 1); | |
1539 | push @ints, $i if vec($vec, ++$i, 1); | |
1540 | } | |
1541 | } else { | |
1542 | # This method is a fast general algorithm | |
1543 | use integer; | |
1544 | my $bits = unpack "b*", $vec; | |
1545 | push @ints, 0 if $bits =~ s/^(\d)// && $1; | |
1546 | push @ints, pos $bits while($bits =~ /1/g); | |
1547 | } | |
1548 | return \@ints; | |
1549 | } | |
1550 | ||
1551 | This method gets faster the more sparse the bit vector is. | |
1552 | (Courtesy of Tim Bunce and Winfried Koenig.) | |
1553 | ||
76817d6d JH |
1554 | You can make the while loop a lot shorter with this suggestion |
1555 | from Benjamin Goldberg: | |
1556 | ||
1557 | while($vec =~ /[^\0]+/g ) { | |
1558 | push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8; | |
1559 | } | |
1560 | ||
cc30d1a7 JH |
1561 | Or use the CPAN module Bit::Vector: |
1562 | ||
1563 | $vector = Bit::Vector->new($num_of_bits); | |
1564 | $vector->Index_List_Store(@ints); | |
1565 | @ints = $vector->Index_List_Read(); | |
1566 | ||
1567 | Bit::Vector provides efficient methods for bit vector, sets of small integers | |
1568 | and "big int" math. | |
1569 | ||
1570 | Here's a more extensive illustration using vec(): | |
65acb1b1 TC |
1571 | |
1572 | # vec demo | |
1573 | $vector = "\xff\x0f\xef\xfe"; | |
1574 | print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", | |
1575 | unpack("N", $vector), "\n"; | |
1576 | $is_set = vec($vector, 23, 1); | |
1577 | print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; | |
1578 | pvec($vector); | |
1579 | ||
1580 | set_vec(1,1,1); | |
1581 | set_vec(3,1,1); | |
1582 | set_vec(23,1,1); | |
1583 | ||
1584 | set_vec(3,1,3); | |
1585 | set_vec(3,2,3); | |
1586 | set_vec(3,4,3); | |
1587 | set_vec(3,4,7); | |
1588 | set_vec(3,8,3); | |
1589 | set_vec(3,8,7); | |
1590 | ||
1591 | set_vec(0,32,17); | |
1592 | set_vec(1,32,17); | |
1593 | ||
1594 | sub set_vec { | |
1595 | my ($offset, $width, $value) = @_; | |
1596 | my $vector = ''; | |
1597 | vec($vector, $offset, $width) = $value; | |
1598 | print "offset=$offset width=$width value=$value\n"; | |
1599 | pvec($vector); | |
1600 | } | |
1601 | ||
1602 | sub pvec { | |
1603 | my $vector = shift; | |
1604 | my $bits = unpack("b*", $vector); | |
1605 | my $i = 0; | |
1606 | my $BASE = 8; | |
1607 | ||
1608 | print "vector length in bytes: ", length($vector), "\n"; | |
1609 | @bytes = unpack("A8" x length($vector), $bits); | |
1610 | print "bits are: @bytes\n\n"; | |
1611 | } | |
1612 | ||
68dc0745 | 1613 | =head2 Why does defined() return true on empty arrays and hashes? |
1614 | ||
65acb1b1 TC |
1615 | The short story is that you should probably only use defined on scalars or |
1616 | functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> | |
1617 | in the 5.004 release or later of Perl for more detail. | |
68dc0745 | 1618 | |
1619 | =head1 Data: Hashes (Associative Arrays) | |
1620 | ||
1621 | =head2 How do I process an entire hash? | |
1622 | ||
1623 | Use the each() function (see L<perlfunc/each>) if you don't care | |
1624 | whether it's sorted: | |
1625 | ||
5a964f20 | 1626 | while ( ($key, $value) = each %hash) { |
68dc0745 | 1627 | print "$key = $value\n"; |
1628 | } | |
1629 | ||
1630 | If you want it sorted, you'll have to use foreach() on the result of | |
1631 | sorting the keys as shown in an earlier question. | |
1632 | ||
1633 | =head2 What happens if I add or remove keys from a hash while iterating over it? | |
1634 | ||
d92eb7b0 GS |
1635 | Don't do that. :-) |
1636 | ||
1637 | [lwall] In Perl 4, you were not allowed to modify a hash at all while | |
87275199 | 1638 | iterating over it. In Perl 5 you can delete from it, but you still |
d92eb7b0 GS |
1639 | can't add to it, because that might cause a doubling of the hash table, |
1640 | in which half the entries get copied up to the new top half of the | |
87275199 | 1641 | table, at which point you've totally bamboozled the iterator code. |
d92eb7b0 GS |
1642 | Even if the table doesn't double, there's no telling whether your new |
1643 | entry will be inserted before or after the current iterator position. | |
1644 | ||
a6dd486b | 1645 | Either treasure up your changes and make them after the iterator finishes |
d92eb7b0 GS |
1646 | or use keys to fetch all the old keys at once, and iterate over the list |
1647 | of keys. | |
68dc0745 | 1648 | |
1649 | =head2 How do I look up a hash element by value? | |
1650 | ||
1651 | Create a reverse hash: | |
1652 | ||
1653 | %by_value = reverse %by_key; | |
1654 | $key = $by_value{$value}; | |
1655 | ||
1656 | That's not particularly efficient. It would be more space-efficient | |
1657 | to use: | |
1658 | ||
1659 | while (($key, $value) = each %by_key) { | |
1660 | $by_value{$value} = $key; | |
1661 | } | |
1662 | ||
d92eb7b0 GS |
1663 | If your hash could have repeated values, the methods above will only find |
1664 | one of the associated keys. This may or may not worry you. If it does | |
1665 | worry you, you can always reverse the hash into a hash of arrays instead: | |
1666 | ||
1667 | while (($key, $value) = each %by_key) { | |
1668 | push @{$key_list_by_value{$value}}, $key; | |
1669 | } | |
68dc0745 | 1670 | |
1671 | =head2 How can I know how many entries are in a hash? | |
1672 | ||
1673 | If you mean how many keys, then all you have to do is | |
875e5c2f | 1674 | use the keys() function in a scalar context: |
68dc0745 | 1675 | |
875e5c2f | 1676 | $num_keys = keys %hash; |
68dc0745 | 1677 | |
875e5c2f JH |
1678 | The keys() function also resets the iterator, which means that you may |
1679 | see strange results if you use this between uses of other hash operators | |
1680 | such as each(). | |
68dc0745 | 1681 | |
1682 | =head2 How do I sort a hash (optionally by value instead of key)? | |
1683 | ||
1684 | Internally, hashes are stored in a way that prevents you from imposing | |
1685 | an order on key-value pairs. Instead, you have to sort a list of the | |
1686 | keys or values: | |
1687 | ||
1688 | @keys = sort keys %hash; # sorted by key | |
1689 | @keys = sort { | |
1690 | $hash{$a} cmp $hash{$b} | |
1691 | } keys %hash; # and by value | |
1692 | ||
1693 | Here we'll do a reverse numeric sort by value, and if two keys are | |
a6dd486b JB |
1694 | identical, sort by length of key, or if that fails, by straight ASCII |
1695 | comparison of the keys (well, possibly modified by your locale--see | |
68dc0745 | 1696 | L<perllocale>). |
1697 | ||
1698 | @keys = sort { | |
1699 | $hash{$b} <=> $hash{$a} | |
1700 | || | |
1701 | length($b) <=> length($a) | |
1702 | || | |
1703 | $a cmp $b | |
1704 | } keys %hash; | |
1705 | ||
1706 | =head2 How can I always keep my hash sorted? | |
1707 | ||
1708 | You can look into using the DB_File module and tie() using the | |
1709 | $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">. | |
5a964f20 | 1710 | The Tie::IxHash module from CPAN might also be instructive. |
68dc0745 | 1711 | |
1712 | =head2 What's the difference between "delete" and "undef" with hashes? | |
1713 | ||
92993692 JH |
1714 | Hashes contain pairs of scalars: the first is the key, the |
1715 | second is the value. The key will be coerced to a string, | |
1716 | although the value can be any kind of scalar: string, | |
1717 | number, or reference. If a key $key is present in | |
1718 | %hash, C<exists($hash{$key})> will return true. The value | |
1719 | for a given key can be C<undef>, in which case | |
1720 | C<$hash{$key}> will be C<undef> while C<exists $hash{$key}> | |
1721 | will return true. This corresponds to (C<$key>, C<undef>) | |
1722 | being in the hash. | |
68dc0745 | 1723 | |
92993692 | 1724 | Pictures help... here's the %hash table: |
68dc0745 | 1725 | |
1726 | keys values | |
1727 | +------+------+ | |
1728 | | a | 3 | | |
1729 | | x | 7 | | |
1730 | | d | 0 | | |
1731 | | e | 2 | | |
1732 | +------+------+ | |
1733 | ||
1734 | And these conditions hold | |
1735 | ||
92993692 JH |
1736 | $hash{'a'} is true |
1737 | $hash{'d'} is false | |
1738 | defined $hash{'d'} is true | |
1739 | defined $hash{'a'} is true | |
1740 | exists $hash{'a'} is true (Perl5 only) | |
1741 | grep ($_ eq 'a', keys %hash) is true | |
68dc0745 | 1742 | |
1743 | If you now say | |
1744 | ||
92993692 | 1745 | undef $hash{'a'} |
68dc0745 | 1746 | |
1747 | your table now reads: | |
1748 | ||
1749 | ||
1750 | keys values | |
1751 | +------+------+ | |
1752 | | a | undef| | |
1753 | | x | 7 | | |
1754 | | d | 0 | | |
1755 | | e | 2 | | |
1756 | +------+------+ | |
1757 | ||
1758 | and these conditions now hold; changes in caps: | |
1759 | ||
92993692 JH |
1760 | $hash{'a'} is FALSE |
1761 | $hash{'d'} is false | |
1762 | defined $hash{'d'} is true | |
1763 | defined $hash{'a'} is FALSE | |
1764 | exists $hash{'a'} is true (Perl5 only) | |
1765 | grep ($_ eq 'a', keys %hash) is true | |
68dc0745 | 1766 | |
1767 | Notice the last two: you have an undef value, but a defined key! | |
1768 | ||
1769 | Now, consider this: | |
1770 | ||
92993692 | 1771 | delete $hash{'a'} |
68dc0745 | 1772 | |
1773 | your table now reads: | |
1774 | ||
1775 | keys values | |
1776 | +------+------+ | |
1777 | | x | 7 | | |
1778 | | d | 0 | | |
1779 | | e | 2 | | |
1780 | +------+------+ | |
1781 | ||
1782 | and these conditions now hold; changes in caps: | |
1783 | ||
92993692 JH |
1784 | $hash{'a'} is false |
1785 | $hash{'d'} is false | |
1786 | defined $hash{'d'} is true | |
1787 | defined $hash{'a'} is false | |
1788 | exists $hash{'a'} is FALSE (Perl5 only) | |
1789 | grep ($_ eq 'a', keys %hash) is FALSE | |
68dc0745 | 1790 | |
1791 | See, the whole entry is gone! | |
1792 | ||
1793 | =head2 Why don't my tied hashes make the defined/exists distinction? | |
1794 | ||
92993692 JH |
1795 | This depends on the tied hash's implementation of EXISTS(). |
1796 | For example, there isn't the concept of undef with hashes | |
1797 | that are tied to DBM* files. It also means that exists() and | |
1798 | defined() do the same thing with a DBM* file, and what they | |
1799 | end up doing is not what they do with ordinary hashes. | |
68dc0745 | 1800 | |
1801 | =head2 How do I reset an each() operation part-way through? | |
1802 | ||
5a964f20 | 1803 | Using C<keys %hash> in scalar context returns the number of keys in |
68dc0745 | 1804 | the hash I<and> resets the iterator associated with the hash. You may |
1805 | need to do this if you use C<last> to exit a loop early so that when you | |
46fc3d4c | 1806 | re-enter it, the hash iterator has been reset. |
68dc0745 | 1807 | |
1808 | =head2 How can I get the unique keys from two hashes? | |
1809 | ||
d92eb7b0 GS |
1810 | First you extract the keys from the hashes into lists, then solve |
1811 | the "removing duplicates" problem described above. For example: | |
68dc0745 | 1812 | |
1813 | %seen = (); | |
1814 | for $element (keys(%foo), keys(%bar)) { | |
1815 | $seen{$element}++; | |
1816 | } | |
1817 | @uniq = keys %seen; | |
1818 | ||
1819 | Or more succinctly: | |
1820 | ||
1821 | @uniq = keys %{{%foo,%bar}}; | |
1822 | ||
1823 | Or if you really want to save space: | |
1824 | ||
1825 | %seen = (); | |
1826 | while (defined ($key = each %foo)) { | |
1827 | $seen{$key}++; | |
1828 | } | |
1829 | while (defined ($key = each %bar)) { | |
1830 | $seen{$key}++; | |
1831 | } | |
1832 | @uniq = keys %seen; | |
1833 | ||
1834 | =head2 How can I store a multidimensional array in a DBM file? | |
1835 | ||
1836 | Either stringify the structure yourself (no fun), or else | |
1837 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer | |
1838 | it on top of either DB_File or GDBM_File. | |
1839 | ||
1840 | =head2 How can I make my hash remember the order I put elements into it? | |
1841 | ||
1842 | Use the Tie::IxHash from CPAN. | |
1843 | ||
46fc3d4c | 1844 | use Tie::IxHash; |
1845 | tie(%myhash, Tie::IxHash); | |
1846 | for ($i=0; $i<20; $i++) { | |
1847 | $myhash{$i} = 2*$i; | |
1848 | } | |
1849 | @keys = keys %myhash; | |
1850 | # @keys = (0,1,2,3,...) | |
1851 | ||
68dc0745 | 1852 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
1853 | ||
1854 | If you say something like: | |
1855 | ||
1856 | somefunc($hash{"nonesuch key here"}); | |
1857 | ||
1858 | Then that element "autovivifies"; that is, it springs into existence | |
1859 | whether you store something there or not. That's because functions | |
1860 | get scalars passed in by reference. If somefunc() modifies C<$_[0]>, | |
1861 | it has to be ready to write it back into the caller's version. | |
1862 | ||
87275199 | 1863 | This has been fixed as of Perl5.004. |
68dc0745 | 1864 | |
1865 | Normally, merely accessing a key's value for a nonexistent key does | |
1866 | I<not> cause that key to be forever there. This is different than | |
1867 | awk's behavior. | |
1868 | ||
fc36a67e | 1869 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
68dc0745 | 1870 | |
65acb1b1 TC |
1871 | Usually a hash ref, perhaps like this: |
1872 | ||
1873 | $record = { | |
1874 | NAME => "Jason", | |
1875 | EMPNO => 132, | |
1876 | TITLE => "deputy peon", | |
1877 | AGE => 23, | |
1878 | SALARY => 37_000, | |
1879 | PALS => [ "Norbert", "Rhys", "Phineas"], | |
1880 | }; | |
1881 | ||
1882 | References are documented in L<perlref> and the upcoming L<perlreftut>. | |
1883 | Examples of complex data structures are given in L<perldsc> and | |
1884 | L<perllol>. Examples of structures and object-oriented classes are | |
1885 | in L<perltoot>. | |
68dc0745 | 1886 | |
1887 | =head2 How can I use a reference as a hash key? | |
1888 | ||
fe854a6f | 1889 | You can't do this directly, but you could use the standard Tie::RefHash |
87275199 | 1890 | module distributed with Perl. |
68dc0745 | 1891 | |
1892 | =head1 Data: Misc | |
1893 | ||
1894 | =head2 How do I handle binary data correctly? | |
1895 | ||
1896 | Perl is binary clean, so this shouldn't be a problem. For example, | |
1897 | this works fine (assuming the files are found): | |
1898 | ||
1899 | if (`cat /vmunix` =~ /gzip/) { | |
1900 | print "Your kernel is GNU-zip enabled!\n"; | |
1901 | } | |
1902 | ||
d92eb7b0 GS |
1903 | On less elegant (read: Byzantine) systems, however, you have |
1904 | to play tedious games with "text" versus "binary" files. See | |
1905 | L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking | |
1906 | systems are curses out of Microsoft, who seem to be committed to putting | |
1907 | the backward into backward compatibility. | |
68dc0745 | 1908 | |
1909 | If you're concerned about 8-bit ASCII data, then see L<perllocale>. | |
1910 | ||
54310121 | 1911 | If you want to deal with multibyte characters, however, there are |
68dc0745 | 1912 | some gotchas. See the section on Regular Expressions. |
1913 | ||
1914 | =head2 How do I determine whether a scalar is a number/whole/integer/float? | |
1915 | ||
1916 | Assuming that you don't care about IEEE notations like "NaN" or | |
1917 | "Infinity", you probably just want to use a regular expression. | |
1918 | ||
65acb1b1 TC |
1919 | if (/\D/) { print "has nondigits\n" } |
1920 | if (/^\d+$/) { print "is a whole number\n" } | |
1921 | if (/^-?\d+$/) { print "is an integer\n" } | |
1922 | if (/^[+-]?\d+$/) { print "is a +/- integer\n" } | |
1923 | if (/^-?\d+\.?\d*$/) { print "is a real number\n" } | |
881bdbd4 | 1924 | if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" } |
65acb1b1 | 1925 | if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) |
881bdbd4 | 1926 | { print "a C float\n" } |
68dc0745 | 1927 | |
92993692 JH |
1928 | You can also use the L<Data::Types|Data::Types> module on |
1929 | the CPAN, which exports functions that validate data types | |
f0f835c2 A |
1930 | using these and other regular expressions, or you can use |
1931 | the C<Regexp::Common> module from CPAN which has regular | |
1932 | expressions to match various types of numbers. | |
b5b6f210 | 1933 | |
5a964f20 TC |
1934 | If you're on a POSIX system, Perl's supports the C<POSIX::strtod> |
1935 | function. Its semantics are somewhat cumbersome, so here's a C<getnum> | |
1936 | wrapper function for more convenient access. This function takes | |
1937 | a string and returns the number it found, or C<undef> for input that | |
1938 | isn't a C float. The C<is_numeric> function is a front end to C<getnum> | |
1939 | if you just want to say, ``Is this a float?'' | |
1940 | ||
1941 | sub getnum { | |
1942 | use POSIX qw(strtod); | |
1943 | my $str = shift; | |
1944 | $str =~ s/^\s+//; | |
1945 | $str =~ s/\s+$//; | |
1946 | $! = 0; | |
1947 | my($num, $unparsed) = strtod($str); | |
1948 | if (($str eq '') || ($unparsed != 0) || $!) { | |
1949 | return undef; | |
1950 | } else { | |
1951 | return $num; | |
1952 | } | |
1953 | } | |
1954 | ||
072dc14b | 1955 | sub is_numeric { defined getnum($_[0]) } |
5a964f20 | 1956 | |
b5b6f210 JH |
1957 | Or you could check out the L<String::Scanf|String::Scanf> module on the CPAN |
1958 | instead. The POSIX module (part of the standard Perl distribution) provides | |
1959 | the C<strtod> and C<strtol> for converting strings to double and longs, | |
6cecdcac | 1960 | respectively. |
68dc0745 | 1961 | |
1962 | =head2 How do I keep persistent data across program calls? | |
1963 | ||
1964 | For some specific applications, you can use one of the DBM modules. | |
fe854a6f AT |
1965 | See L<AnyDBM_File>. More generically, you should consult the FreezeThaw |
1966 | or Storable modules from CPAN. Starting from Perl 5.8 Storable is part | |
1967 | of the standard distribution. Here's one example using Storable's C<store> | |
1968 | and C<retrieve> functions: | |
65acb1b1 TC |
1969 | |
1970 | use Storable; | |
1971 | store(\%hash, "filename"); | |
1972 | ||
1973 | # later on... | |
1974 | $href = retrieve("filename"); # by ref | |
1975 | %hash = %{ retrieve("filename") }; # direct to hash | |
68dc0745 | 1976 | |
1977 | =head2 How do I print out or copy a recursive data structure? | |
1978 | ||
65acb1b1 TC |
1979 | The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great |
1980 | for printing out data structures. The Storable module, found on CPAN, | |
1981 | provides a function called C<dclone> that recursively copies its argument. | |
1982 | ||
1983 | use Storable qw(dclone); | |
1984 | $r2 = dclone($r1); | |
68dc0745 | 1985 | |
65acb1b1 TC |
1986 | Where $r1 can be a reference to any kind of data structure you'd like. |
1987 | It will be deeply copied. Because C<dclone> takes and returns references, | |
1988 | you'd have to add extra punctuation if you had a hash of arrays that | |
1989 | you wanted to copy. | |
68dc0745 | 1990 | |
65acb1b1 | 1991 | %newhash = %{ dclone(\%oldhash) }; |
68dc0745 | 1992 | |
1993 | =head2 How do I define methods for every class/object? | |
1994 | ||
1995 | Use the UNIVERSAL class (see L<UNIVERSAL>). | |
1996 | ||
1997 | =head2 How do I verify a credit card checksum? | |
1998 | ||
1999 | Get the Business::CreditCard module from CPAN. | |
2000 | ||
65acb1b1 TC |
2001 | =head2 How do I pack arrays of doubles or floats for XS code? |
2002 | ||
2003 | The kgbpack.c code in the PGPLOT module on CPAN does just this. | |
2004 | If you're doing a lot of float or double processing, consider using | |
2005 | the PDL module from CPAN instead--it makes number-crunching easy. | |
2006 | ||
68dc0745 | 2007 | =head1 AUTHOR AND COPYRIGHT |
2008 | ||
0bc0ad85 | 2009 | Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington. |
5a964f20 TC |
2010 | All rights reserved. |
2011 | ||
5a7beb56 JH |
2012 | This documentation is free; you can redistribute it and/or modify it |
2013 | under the same terms as Perl itself. | |
5a964f20 TC |
2014 | |
2015 | Irrespective of its distribution, all code examples in this file | |
2016 | are hereby placed into the public domain. You are permitted and | |
2017 | encouraged to use this code in your own programs for fun | |
2018 | or for profit as you see fit. A simple comment in the code giving | |
2019 | credit would be courteous but is not required. |