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68dc0745 | 1 | =head1 NAME |
2 | ||
213329dd | 3 | perlfaq4 - Data Manipulation ($Revision: 1.8 $, $Date: 2001/11/09 08:06:04 $) |
68dc0745 | 4 | |
5 | =head1 DESCRIPTION | |
6 | ||
a6dd486b | 7 | The section of the FAQ answers questions related to the manipulation |
68dc0745 | 8 | of data as numbers, dates, strings, arrays, hashes, and miscellaneous |
9 | data issues. | |
10 | ||
11 | =head1 Data: Numbers | |
12 | ||
46fc3d4c | 13 | =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? |
14 | ||
5a964f20 | 15 | The infinite set that a mathematician thinks of as the real numbers can |
a6dd486b | 16 | only be approximated on a computer, since the computer only has a finite |
5a964f20 TC |
17 | number of bits to store an infinite number of, um, numbers. |
18 | ||
46fc3d4c | 19 | Internally, your computer represents floating-point numbers in binary. |
92c2ed05 GS |
20 | Floating-point numbers read in from a file or appearing as literals |
21 | in your program are converted from their decimal floating-point | |
a6dd486b | 22 | representation (eg, 19.95) to an internal binary representation. |
46fc3d4c | 23 | |
24 | However, 19.95 can't be precisely represented as a binary | |
25 | floating-point number, just like 1/3 can't be exactly represented as a | |
26 | decimal floating-point number. The computer's binary representation | |
27 | of 19.95, therefore, isn't exactly 19.95. | |
28 | ||
29 | When a floating-point number gets printed, the binary floating-point | |
30 | representation is converted back to decimal. These decimal numbers | |
31 | are displayed in either the format you specify with printf(), or the | |
a6dd486b | 32 | current output format for numbers. (See L<perlvar/"$#"> if you use |
46fc3d4c | 33 | print. C<$#> has a different default value in Perl5 than it did in |
87275199 | 34 | Perl4. Changing C<$#> yourself is deprecated.) |
46fc3d4c | 35 | |
36 | This affects B<all> computer languages that represent decimal | |
37 | floating-point numbers in binary, not just Perl. Perl provides | |
38 | arbitrary-precision decimal numbers with the Math::BigFloat module | |
39 | (part of the standard Perl distribution), but mathematical operations | |
40 | are consequently slower. | |
41 | ||
80ba158a | 42 | If precision is important, such as when dealing with money, it's good |
1affb2ee MS |
43 | to work with integers and then divide at the last possible moment. |
44 | For example, work in pennies (1995) instead of dollars and cents | |
6b927632 | 45 | (19.95) and divide by 100 at the end. |
1affb2ee | 46 | |
46fc3d4c | 47 | To get rid of the superfluous digits, just use a format (eg, |
48 | C<printf("%.2f", 19.95)>) to get the required precision. | |
65acb1b1 | 49 | See L<perlop/"Floating-point Arithmetic">. |
46fc3d4c | 50 | |
68dc0745 | 51 | =head2 Why isn't my octal data interpreted correctly? |
52 | ||
53 | Perl only understands octal and hex numbers as such when they occur | |
33ce146f PP |
54 | as literals in your program. Octal literals in perl must start with |
55 | a leading "0" and hexadecimal literals must start with a leading "0x". | |
56 | If they are read in from somewhere and assigned, no automatic | |
57 | conversion takes place. You must explicitly use oct() or hex() if you | |
58 | want the values converted to decimal. oct() interprets | |
68dc0745 | 59 | both hex ("0x350") numbers and octal ones ("0350" or even without the |
60 | leading "0", like "377"), while hex() only converts hexadecimal ones, | |
61 | with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef". | |
33ce146f PP |
62 | The inverse mapping from decimal to octal can be done with either the |
63 | "%o" or "%O" sprintf() formats. To get from decimal to hex try either | |
64 | the "%x" or the "%X" formats to sprintf(). | |
68dc0745 | 65 | |
66 | This problem shows up most often when people try using chmod(), mkdir(), | |
33ce146f PP |
67 | umask(), or sysopen(), which by widespread tradition typically take |
68 | permissions in octal. | |
68dc0745 | 69 | |
33ce146f | 70 | chmod(644, $file); # WRONG |
68dc0745 | 71 | chmod(0644, $file); # right |
72 | ||
33ce146f PP |
73 | Note the mistake in the first line was specifying the decimal literal |
74 | 644, rather than the intended octal literal 0644. The problem can | |
75 | be seen with: | |
76 | ||
434f7166 | 77 | printf("%#o",644); # prints 01204 |
33ce146f PP |
78 | |
79 | Surely you had not intended C<chmod(01204, $file);> - did you? If you | |
80 | want to use numeric literals as arguments to chmod() et al. then please | |
81 | try to express them as octal constants, that is with a leading zero and | |
82 | with the following digits restricted to the set 0..7. | |
83 | ||
65acb1b1 | 84 | =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? |
68dc0745 | 85 | |
92c2ed05 GS |
86 | Remember that int() merely truncates toward 0. For rounding to a |
87 | certain number of digits, sprintf() or printf() is usually the easiest | |
88 | route. | |
89 | ||
90 | printf("%.3f", 3.1415926535); # prints 3.142 | |
68dc0745 | 91 | |
87275199 | 92 | The POSIX module (part of the standard Perl distribution) implements |
68dc0745 | 93 | ceil(), floor(), and a number of other mathematical and trigonometric |
94 | functions. | |
95 | ||
92c2ed05 GS |
96 | use POSIX; |
97 | $ceil = ceil(3.5); # 4 | |
98 | $floor = floor(3.5); # 3 | |
99 | ||
a6dd486b | 100 | In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex |
87275199 | 101 | module. With 5.004, the Math::Trig module (part of the standard Perl |
46fc3d4c | 102 | distribution) implements the trigonometric functions. Internally it |
103 | uses the Math::Complex module and some functions can break out from | |
104 | the real axis into the complex plane, for example the inverse sine of | |
105 | 2. | |
68dc0745 | 106 | |
107 | Rounding in financial applications can have serious implications, and | |
108 | the rounding method used should be specified precisely. In these | |
109 | cases, it probably pays not to trust whichever system rounding is | |
110 | being used by Perl, but to instead implement the rounding function you | |
111 | need yourself. | |
112 | ||
65acb1b1 TC |
113 | To see why, notice how you'll still have an issue on half-way-point |
114 | alternation: | |
115 | ||
116 | for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} | |
117 | ||
118 | 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 | |
119 | 0.8 0.8 0.9 0.9 1.0 1.0 | |
120 | ||
121 | Don't blame Perl. It's the same as in C. IEEE says we have to do this. | |
122 | Perl numbers whose absolute values are integers under 2**31 (on 32 bit | |
123 | machines) will work pretty much like mathematical integers. Other numbers | |
124 | are not guaranteed. | |
125 | ||
6761e064 | 126 | =head2 How do I convert between numeric representations: |
68dc0745 | 127 | |
6761e064 JH |
128 | As always with Perl there is more than one way to do it. Below |
129 | are a few examples of approaches to making common conversions | |
130 | between number representations. This is intended to be representational | |
131 | rather than exhaustive. | |
68dc0745 | 132 | |
6761e064 JH |
133 | Some of the examples below use the Bit::Vector module from CPAN. |
134 | The reason you might choose Bit::Vector over the perl built in | |
135 | functions is that it works with numbers of ANY size, that it is | |
136 | optimized for speed on some operations, and for at least some | |
137 | programmers the notation might be familiar. | |
d92eb7b0 | 138 | |
6761e064 | 139 | =item B<How do I convert Hexadecimal into decimal:> |
d92eb7b0 | 140 | |
6761e064 JH |
141 | Using perl's built in conversion of 0x notation: |
142 | ||
143 | $int = 0xDEADBEEF; | |
144 | $dec = sprintf("%d", $int); | |
f7686833 | 145 | |
6761e064 JH |
146 | Using the hex function: |
147 | ||
148 | $int = hex("DEADBEEF"); | |
149 | $dec = sprintf("%d", $int); | |
150 | ||
151 | Using pack: | |
152 | ||
153 | $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); | |
154 | $dec = sprintf("%d", $int); | |
155 | ||
156 | Using the CPAN module Bit::Vector: | |
157 | ||
158 | use Bit::Vector; | |
159 | $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); | |
160 | $dec = $vec->to_Dec(); | |
161 | ||
162 | =item B<How do I convert from decimal to hexadecimal:> | |
163 | ||
164 | Using sprint: | |
165 | ||
166 | $hex = sprintf("%X", 3735928559); | |
167 | ||
168 | Using unpack | |
169 | ||
170 | $hex = unpack("H*", pack("N", 3735928559)); | |
171 | ||
172 | Using Bit::Vector | |
173 | ||
174 | use Bit::Vector; | |
175 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
176 | $hex = $vec->to_Hex(); | |
177 | ||
178 | And Bit::Vector supports odd bit counts: | |
179 | ||
180 | use Bit::Vector; | |
181 | $vec = Bit::Vector->new_Dec(33, 3735928559); | |
182 | $vec->Resize(32); # suppress leading 0 if unwanted | |
183 | $hex = $vec->to_Hex(); | |
184 | ||
185 | =item B<How do I convert from octal to decimal:> | |
186 | ||
187 | Using Perl's built in conversion of numbers with leading zeros: | |
188 | ||
189 | $int = 033653337357; # note the leading 0! | |
190 | $dec = sprintf("%d", $int); | |
191 | ||
192 | Using the oct function: | |
193 | ||
194 | $int = oct("33653337357"); | |
195 | $dec = sprintf("%d", $int); | |
196 | ||
197 | Using Bit::Vector: | |
198 | ||
199 | use Bit::Vector; | |
200 | $vec = Bit::Vector->new(32); | |
201 | $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); | |
202 | $dec = $vec->to_Dec(); | |
203 | ||
204 | =item B<How do I convert from decimal to octal:> | |
205 | ||
206 | Using sprintf: | |
207 | ||
208 | $oct = sprintf("%o", 3735928559); | |
209 | ||
210 | Using Bit::Vector | |
211 | ||
212 | use Bit::Vector; | |
213 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
214 | $oct = reverse join('', $vec->Chunk_List_Read(3)); | |
215 | ||
216 | =item B<How do I convert from binary to decimal:> | |
217 | ||
218 | Using pack and ord | |
d92eb7b0 GS |
219 | |
220 | $decimal = ord(pack('B8', '10110110')); | |
68dc0745 | 221 | |
6761e064 JH |
222 | Using pack and unpack for larger strings |
223 | ||
224 | $int = unpack("N", pack("B32", | |
225 | substr("0" x 32 . "11110101011011011111011101111", -32))); | |
226 | $dec = sprintf("%d", $int); | |
227 | ||
5efd7060 | 228 | # substr() is used to left pad a 32 character string with zeros. |
6761e064 JH |
229 | |
230 | Using Bit::Vector: | |
231 | ||
232 | $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); | |
233 | $dec = $vec->to_Dec(); | |
234 | ||
235 | =item B<How do I convert from decimal to binary:> | |
236 | ||
237 | Using unpack; | |
238 | ||
239 | $bin = unpack("B*", pack("N", 3735928559)); | |
240 | ||
241 | Using Bit::Vector: | |
242 | ||
243 | use Bit::Vector; | |
244 | $vec = Bit::Vector->new_Dec(32, -559038737); | |
245 | $bin = $vec->to_Bin(); | |
246 | ||
247 | The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) | |
248 | are left as an exercise to the inclined reader. | |
68dc0745 | 249 | |
68dc0745 | 250 | |
65acb1b1 TC |
251 | =head2 Why doesn't & work the way I want it to? |
252 | ||
253 | The behavior of binary arithmetic operators depends on whether they're | |
254 | used on numbers or strings. The operators treat a string as a series | |
255 | of bits and work with that (the string C<"3"> is the bit pattern | |
256 | C<00110011>). The operators work with the binary form of a number | |
257 | (the number C<3> is treated as the bit pattern C<00000011>). | |
258 | ||
259 | So, saying C<11 & 3> performs the "and" operation on numbers (yielding | |
260 | C<1>). Saying C<"11" & "3"> performs the "and" operation on strings | |
261 | (yielding C<"1">). | |
262 | ||
263 | Most problems with C<&> and C<|> arise because the programmer thinks | |
264 | they have a number but really it's a string. The rest arise because | |
265 | the programmer says: | |
266 | ||
267 | if ("\020\020" & "\101\101") { | |
268 | # ... | |
269 | } | |
270 | ||
271 | but a string consisting of two null bytes (the result of C<"\020\020" | |
272 | & "\101\101">) is not a false value in Perl. You need: | |
273 | ||
274 | if ( ("\020\020" & "\101\101") !~ /[^\000]/) { | |
275 | # ... | |
276 | } | |
277 | ||
68dc0745 | 278 | =head2 How do I multiply matrices? |
279 | ||
280 | Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) | |
281 | or the PDL extension (also available from CPAN). | |
282 | ||
283 | =head2 How do I perform an operation on a series of integers? | |
284 | ||
285 | To call a function on each element in an array, and collect the | |
286 | results, use: | |
287 | ||
288 | @results = map { my_func($_) } @array; | |
289 | ||
290 | For example: | |
291 | ||
292 | @triple = map { 3 * $_ } @single; | |
293 | ||
294 | To call a function on each element of an array, but ignore the | |
295 | results: | |
296 | ||
297 | foreach $iterator (@array) { | |
65acb1b1 | 298 | some_func($iterator); |
68dc0745 | 299 | } |
300 | ||
301 | To call a function on each integer in a (small) range, you B<can> use: | |
302 | ||
65acb1b1 | 303 | @results = map { some_func($_) } (5 .. 25); |
68dc0745 | 304 | |
305 | but you should be aware that the C<..> operator creates an array of | |
306 | all integers in the range. This can take a lot of memory for large | |
307 | ranges. Instead use: | |
308 | ||
309 | @results = (); | |
310 | for ($i=5; $i < 500_005; $i++) { | |
65acb1b1 | 311 | push(@results, some_func($i)); |
68dc0745 | 312 | } |
313 | ||
87275199 GS |
314 | This situation has been fixed in Perl5.005. Use of C<..> in a C<for> |
315 | loop will iterate over the range, without creating the entire range. | |
316 | ||
317 | for my $i (5 .. 500_005) { | |
318 | push(@results, some_func($i)); | |
319 | } | |
320 | ||
321 | will not create a list of 500,000 integers. | |
322 | ||
68dc0745 | 323 | =head2 How can I output Roman numerals? |
324 | ||
a93751fa | 325 | Get the http://www.cpan.org/modules/by-module/Roman module. |
68dc0745 | 326 | |
327 | =head2 Why aren't my random numbers random? | |
328 | ||
65acb1b1 TC |
329 | If you're using a version of Perl before 5.004, you must call C<srand> |
330 | once at the start of your program to seed the random number generator. | |
331 | 5.004 and later automatically call C<srand> at the beginning. Don't | |
332 | call C<srand> more than once--you make your numbers less random, rather | |
333 | than more. | |
92c2ed05 | 334 | |
65acb1b1 TC |
335 | Computers are good at being predictable and bad at being random |
336 | (despite appearances caused by bugs in your programs :-). | |
a93751fa | 337 | http://www.cpan.org/doc/FMTEYEWTK/random , courtesy of Tom |
a6dd486b | 338 | Phoenix, talks more about this. John von Neumann said, ``Anyone who |
65acb1b1 TC |
339 | attempts to generate random numbers by deterministic means is, of |
340 | course, living in a state of sin.'' | |
341 | ||
342 | If you want numbers that are more random than C<rand> with C<srand> | |
343 | provides, you should also check out the Math::TrulyRandom module from | |
344 | CPAN. It uses the imperfections in your system's timer to generate | |
345 | random numbers, but this takes quite a while. If you want a better | |
92c2ed05 | 346 | pseudorandom generator than comes with your operating system, look at |
65acb1b1 | 347 | ``Numerical Recipes in C'' at http://www.nr.com/ . |
68dc0745 | 348 | |
349 | =head1 Data: Dates | |
350 | ||
351 | =head2 How do I find the week-of-the-year/day-of-the-year? | |
352 | ||
353 | The day of the year is in the array returned by localtime() (see | |
354 | L<perlfunc/"localtime">): | |
355 | ||
356 | $day_of_year = (localtime(time()))[7]; | |
357 | ||
d92eb7b0 GS |
358 | =head2 How do I find the current century or millennium? |
359 | ||
360 | Use the following simple functions: | |
361 | ||
362 | sub get_century { | |
363 | return int((((localtime(shift || time))[5] + 1999))/100); | |
364 | } | |
365 | sub get_millennium { | |
366 | return 1+int((((localtime(shift || time))[5] + 1899))/1000); | |
367 | } | |
368 | ||
369 | On some systems, you'll find that the POSIX module's strftime() function | |
370 | has been extended in a non-standard way to use a C<%C> format, which they | |
371 | sometimes claim is the "century". It isn't, because on most such systems, | |
372 | this is only the first two digits of the four-digit year, and thus cannot | |
373 | be used to reliably determine the current century or millennium. | |
374 | ||
92c2ed05 | 375 | =head2 How can I compare two dates and find the difference? |
68dc0745 | 376 | |
92c2ed05 GS |
377 | If you're storing your dates as epoch seconds then simply subtract one |
378 | from the other. If you've got a structured date (distinct year, day, | |
d92eb7b0 GS |
379 | month, hour, minute, seconds values), then for reasons of accessibility, |
380 | simplicity, and efficiency, merely use either timelocal or timegm (from | |
381 | the Time::Local module in the standard distribution) to reduce structured | |
382 | dates to epoch seconds. However, if you don't know the precise format of | |
383 | your dates, then you should probably use either of the Date::Manip and | |
384 | Date::Calc modules from CPAN before you go hacking up your own parsing | |
385 | routine to handle arbitrary date formats. | |
68dc0745 | 386 | |
387 | =head2 How can I take a string and turn it into epoch seconds? | |
388 | ||
389 | If it's a regular enough string that it always has the same format, | |
92c2ed05 GS |
390 | you can split it up and pass the parts to C<timelocal> in the standard |
391 | Time::Local module. Otherwise, you should look into the Date::Calc | |
392 | and Date::Manip modules from CPAN. | |
68dc0745 | 393 | |
394 | =head2 How can I find the Julian Day? | |
395 | ||
2a2bf5f4 JH |
396 | Use the Time::JulianDay module (part of the Time-modules bundle |
397 | available from CPAN.) | |
d92eb7b0 | 398 | |
89435c96 MS |
399 | Before you immerse yourself too deeply in this, be sure to verify that |
400 | it is the I<Julian> Day you really want. Are you interested in a way | |
401 | of getting serial days so that you just can tell how many days they | |
402 | are apart or so that you can do also other date arithmetic? If you | |
d92eb7b0 | 403 | are interested in performing date arithmetic, this can be done using |
2a2bf5f4 | 404 | modules Date::Manip or Date::Calc. |
89435c96 MS |
405 | |
406 | There is too many details and much confusion on this issue to cover in | |
407 | this FAQ, but the term is applied (correctly) to a calendar now | |
408 | supplanted by the Gregorian Calendar, with the Julian Calendar failing | |
409 | to adjust properly for leap years on centennial years (among other | |
410 | annoyances). The term is also used (incorrectly) to mean: [1] days in | |
411 | the Gregorian Calendar; and [2] days since a particular starting time | |
412 | or `epoch', usually 1970 in the Unix world and 1980 in the | |
413 | MS-DOS/Windows world. If you find that it is not the first meaning | |
414 | that you really want, then check out the Date::Manip and Date::Calc | |
415 | modules. (Thanks to David Cassell for most of this text.) | |
be94a901 | 416 | |
65acb1b1 TC |
417 | =head2 How do I find yesterday's date? |
418 | ||
419 | The C<time()> function returns the current time in seconds since the | |
d92eb7b0 | 420 | epoch. Take twenty-four hours off that: |
65acb1b1 TC |
421 | |
422 | $yesterday = time() - ( 24 * 60 * 60 ); | |
423 | ||
424 | Then you can pass this to C<localtime()> and get the individual year, | |
425 | month, day, hour, minute, seconds values. | |
426 | ||
d92eb7b0 GS |
427 | Note very carefully that the code above assumes that your days are |
428 | twenty-four hours each. For most people, there are two days a year | |
429 | when they aren't: the switch to and from summer time throws this off. | |
430 | A solution to this issue is offered by Russ Allbery. | |
431 | ||
432 | sub yesterday { | |
433 | my $now = defined $_[0] ? $_[0] : time; | |
434 | my $then = $now - 60 * 60 * 24; | |
435 | my $ndst = (localtime $now)[8] > 0; | |
436 | my $tdst = (localtime $then)[8] > 0; | |
437 | $then - ($tdst - $ndst) * 60 * 60; | |
438 | } | |
439 | # Should give you "this time yesterday" in seconds since epoch relative to | |
440 | # the first argument or the current time if no argument is given and | |
441 | # suitable for passing to localtime or whatever else you need to do with | |
442 | # it. $ndst is whether we're currently in daylight savings time; $tdst is | |
443 | # whether the point 24 hours ago was in daylight savings time. If $tdst | |
444 | # and $ndst are the same, a boundary wasn't crossed, and the correction | |
445 | # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more | |
446 | # from yesterday's time since we gained an extra hour while going off | |
447 | # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a | |
448 | # negative hour (add an hour) to yesterday's time since we lost an hour. | |
449 | # | |
450 | # All of this is because during those days when one switches off or onto | |
451 | # DST, a "day" isn't 24 hours long; it's either 23 or 25. | |
452 | # | |
453 | # The explicit settings of $ndst and $tdst are necessary because localtime | |
454 | # only says it returns the system tm struct, and the system tm struct at | |
87275199 | 455 | # least on Solaris doesn't guarantee any particular positive value (like, |
d92eb7b0 GS |
456 | # say, 1) for isdst, just a positive value. And that value can |
457 | # potentially be negative, if DST information isn't available (this sub | |
458 | # just treats those cases like no DST). | |
459 | # | |
460 | # Note that between 2am and 3am on the day after the time zone switches | |
461 | # off daylight savings time, the exact hour of "yesterday" corresponding | |
462 | # to the current hour is not clearly defined. Note also that if used | |
463 | # between 2am and 3am the day after the change to daylight savings time, | |
464 | # the result will be between 3am and 4am of the previous day; it's | |
465 | # arguable whether this is correct. | |
466 | # | |
467 | # This sub does not attempt to deal with leap seconds (most things don't). | |
468 | # | |
469 | # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu> | |
470 | # This code is in the public domain | |
471 | ||
87275199 | 472 | =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? |
68dc0745 | 473 | |
65acb1b1 TC |
474 | Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is |
475 | Y2K compliant (whatever that means). The programmers you've hired to | |
476 | use it, however, probably are not. | |
477 | ||
478 | Long answer: The question belies a true understanding of the issue. | |
479 | Perl is just as Y2K compliant as your pencil--no more, and no less. | |
480 | Can you use your pencil to write a non-Y2K-compliant memo? Of course | |
481 | you can. Is that the pencil's fault? Of course it isn't. | |
92c2ed05 | 482 | |
87275199 | 483 | The date and time functions supplied with Perl (gmtime and localtime) |
65acb1b1 TC |
484 | supply adequate information to determine the year well beyond 2000 |
485 | (2038 is when trouble strikes for 32-bit machines). The year returned | |
90fdbbb7 | 486 | by these functions when used in a list context is the year minus 1900. |
65acb1b1 TC |
487 | For years between 1910 and 1999 this I<happens> to be a 2-digit decimal |
488 | number. To avoid the year 2000 problem simply do not treat the year as | |
489 | a 2-digit number. It isn't. | |
68dc0745 | 490 | |
5a964f20 | 491 | When gmtime() and localtime() are used in scalar context they return |
68dc0745 | 492 | a timestamp string that contains a fully-expanded year. For example, |
493 | C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 | |
494 | 2001". There's no year 2000 problem here. | |
495 | ||
5a964f20 TC |
496 | That doesn't mean that Perl can't be used to create non-Y2K compliant |
497 | programs. It can. But so can your pencil. It's the fault of the user, | |
498 | not the language. At the risk of inflaming the NRA: ``Perl doesn't | |
499 | break Y2K, people do.'' See http://language.perl.com/news/y2k.html for | |
500 | a longer exposition. | |
501 | ||
68dc0745 | 502 | =head1 Data: Strings |
503 | ||
504 | =head2 How do I validate input? | |
505 | ||
506 | The answer to this question is usually a regular expression, perhaps | |
5a964f20 | 507 | with auxiliary logic. See the more specific questions (numbers, mail |
68dc0745 | 508 | addresses, etc.) for details. |
509 | ||
510 | =head2 How do I unescape a string? | |
511 | ||
92c2ed05 GS |
512 | It depends just what you mean by ``escape''. URL escapes are dealt |
513 | with in L<perlfaq9>. Shell escapes with the backslash (C<\>) | |
a6dd486b | 514 | character are removed with |
68dc0745 | 515 | |
516 | s/\\(.)/$1/g; | |
517 | ||
92c2ed05 | 518 | This won't expand C<"\n"> or C<"\t"> or any other special escapes. |
68dc0745 | 519 | |
520 | =head2 How do I remove consecutive pairs of characters? | |
521 | ||
92c2ed05 | 522 | To turn C<"abbcccd"> into C<"abccd">: |
68dc0745 | 523 | |
d92eb7b0 GS |
524 | s/(.)\1/$1/g; # add /s to include newlines |
525 | ||
526 | Here's a solution that turns "abbcccd" to "abcd": | |
527 | ||
528 | y///cs; # y == tr, but shorter :-) | |
68dc0745 | 529 | |
530 | =head2 How do I expand function calls in a string? | |
531 | ||
532 | This is documented in L<perlref>. In general, this is fraught with | |
533 | quoting and readability problems, but it is possible. To interpolate | |
5a964f20 | 534 | a subroutine call (in list context) into a string: |
68dc0745 | 535 | |
536 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; | |
537 | ||
538 | If you prefer scalar context, similar chicanery is also useful for | |
539 | arbitrary expressions: | |
540 | ||
541 | print "That yields ${\($n + 5)} widgets\n"; | |
542 | ||
92c2ed05 GS |
543 | Version 5.004 of Perl had a bug that gave list context to the |
544 | expression in C<${...}>, but this is fixed in version 5.005. | |
545 | ||
546 | See also ``How can I expand variables in text strings?'' in this | |
547 | section of the FAQ. | |
46fc3d4c | 548 | |
68dc0745 | 549 | =head2 How do I find matching/nesting anything? |
550 | ||
92c2ed05 GS |
551 | This isn't something that can be done in one regular expression, no |
552 | matter how complicated. To find something between two single | |
553 | characters, a pattern like C</x([^x]*)x/> will get the intervening | |
554 | bits in $1. For multiple ones, then something more like | |
555 | C</alpha(.*?)omega/> would be needed. But none of these deals with | |
556 | nested patterns, nor can they. For that you'll have to write a | |
557 | parser. | |
558 | ||
559 | If you are serious about writing a parser, there are a number of | |
6a2af475 GS |
560 | modules or oddities that will make your life a lot easier. There are |
561 | the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; | |
83df6a1d JH |
562 | and the byacc program. Starting from perl 5.8 the Text::Balanced |
563 | is part of the standard distribution. | |
68dc0745 | 564 | |
92c2ed05 GS |
565 | One simple destructive, inside-out approach that you might try is to |
566 | pull out the smallest nesting parts one at a time: | |
5a964f20 | 567 | |
d92eb7b0 | 568 | while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { |
5a964f20 TC |
569 | # do something with $1 |
570 | } | |
571 | ||
65acb1b1 TC |
572 | A more complicated and sneaky approach is to make Perl's regular |
573 | expression engine do it for you. This is courtesy Dean Inada, and | |
574 | rather has the nature of an Obfuscated Perl Contest entry, but it | |
575 | really does work: | |
576 | ||
577 | # $_ contains the string to parse | |
578 | # BEGIN and END are the opening and closing markers for the | |
579 | # nested text. | |
c47ff5f1 | 580 | |
65acb1b1 TC |
581 | @( = ('(',''); |
582 | @) = (')',''); | |
583 | ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; | |
584 | @$ = (eval{/$re/},$@!~/unmatched/); | |
585 | print join("\n",@$[0..$#$]) if( $$[-1] ); | |
586 | ||
68dc0745 | 587 | =head2 How do I reverse a string? |
588 | ||
5a964f20 | 589 | Use reverse() in scalar context, as documented in |
68dc0745 | 590 | L<perlfunc/reverse>. |
591 | ||
592 | $reversed = reverse $string; | |
593 | ||
594 | =head2 How do I expand tabs in a string? | |
595 | ||
5a964f20 | 596 | You can do it yourself: |
68dc0745 | 597 | |
598 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; | |
599 | ||
87275199 | 600 | Or you can just use the Text::Tabs module (part of the standard Perl |
68dc0745 | 601 | distribution). |
602 | ||
603 | use Text::Tabs; | |
604 | @expanded_lines = expand(@lines_with_tabs); | |
605 | ||
606 | =head2 How do I reformat a paragraph? | |
607 | ||
87275199 | 608 | Use Text::Wrap (part of the standard Perl distribution): |
68dc0745 | 609 | |
610 | use Text::Wrap; | |
611 | print wrap("\t", ' ', @paragraphs); | |
612 | ||
92c2ed05 | 613 | The paragraphs you give to Text::Wrap should not contain embedded |
46fc3d4c | 614 | newlines. Text::Wrap doesn't justify the lines (flush-right). |
615 | ||
bc06af74 JH |
616 | Or use the CPAN module Text::Autoformat. Formatting files can be easily |
617 | done by making a shell alias, like so: | |
618 | ||
619 | alias fmt="perl -i -MText::Autoformat -n0777 \ | |
620 | -e 'print autoformat $_, {all=>1}' $*" | |
621 | ||
622 | See the documentation for Text::Autoformat to appreciate its many | |
623 | capabilities. | |
624 | ||
68dc0745 | 625 | =head2 How can I access/change the first N letters of a string? |
626 | ||
627 | There are many ways. If you just want to grab a copy, use | |
92c2ed05 | 628 | substr(): |
68dc0745 | 629 | |
630 | $first_byte = substr($a, 0, 1); | |
631 | ||
632 | If you want to modify part of a string, the simplest way is often to | |
633 | use substr() as an lvalue: | |
634 | ||
635 | substr($a, 0, 3) = "Tom"; | |
636 | ||
92c2ed05 | 637 | Although those with a pattern matching kind of thought process will |
a6dd486b | 638 | likely prefer |
68dc0745 | 639 | |
640 | $a =~ s/^.../Tom/; | |
641 | ||
642 | =head2 How do I change the Nth occurrence of something? | |
643 | ||
92c2ed05 GS |
644 | You have to keep track of N yourself. For example, let's say you want |
645 | to change the fifth occurrence of C<"whoever"> or C<"whomever"> into | |
d92eb7b0 GS |
646 | C<"whosoever"> or C<"whomsoever">, case insensitively. These |
647 | all assume that $_ contains the string to be altered. | |
68dc0745 | 648 | |
649 | $count = 0; | |
650 | s{((whom?)ever)}{ | |
651 | ++$count == 5 # is it the 5th? | |
652 | ? "${2}soever" # yes, swap | |
653 | : $1 # renege and leave it there | |
d92eb7b0 | 654 | }ige; |
68dc0745 | 655 | |
5a964f20 TC |
656 | In the more general case, you can use the C</g> modifier in a C<while> |
657 | loop, keeping count of matches. | |
658 | ||
659 | $WANT = 3; | |
660 | $count = 0; | |
d92eb7b0 | 661 | $_ = "One fish two fish red fish blue fish"; |
5a964f20 TC |
662 | while (/(\w+)\s+fish\b/gi) { |
663 | if (++$count == $WANT) { | |
664 | print "The third fish is a $1 one.\n"; | |
5a964f20 TC |
665 | } |
666 | } | |
667 | ||
92c2ed05 | 668 | That prints out: C<"The third fish is a red one."> You can also use a |
5a964f20 TC |
669 | repetition count and repeated pattern like this: |
670 | ||
671 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; | |
672 | ||
68dc0745 | 673 | =head2 How can I count the number of occurrences of a substring within a string? |
674 | ||
a6dd486b | 675 | There are a number of ways, with varying efficiency. If you want a |
68dc0745 | 676 | count of a certain single character (X) within a string, you can use the |
677 | C<tr///> function like so: | |
678 | ||
368c9434 | 679 | $string = "ThisXlineXhasXsomeXx'sXinXit"; |
68dc0745 | 680 | $count = ($string =~ tr/X//); |
d92eb7b0 | 681 | print "There are $count X characters in the string"; |
68dc0745 | 682 | |
683 | This is fine if you are just looking for a single character. However, | |
684 | if you are trying to count multiple character substrings within a | |
685 | larger string, C<tr///> won't work. What you can do is wrap a while() | |
686 | loop around a global pattern match. For example, let's count negative | |
687 | integers: | |
688 | ||
689 | $string = "-9 55 48 -2 23 -76 4 14 -44"; | |
690 | while ($string =~ /-\d+/g) { $count++ } | |
691 | print "There are $count negative numbers in the string"; | |
692 | ||
693 | =head2 How do I capitalize all the words on one line? | |
694 | ||
695 | To make the first letter of each word upper case: | |
3fe9a6f1 | 696 | |
68dc0745 | 697 | $line =~ s/\b(\w)/\U$1/g; |
698 | ||
46fc3d4c | 699 | This has the strange effect of turning "C<don't do it>" into "C<Don'T |
a6dd486b | 700 | Do It>". Sometimes you might want this. Other times you might need a |
24f1ba9b | 701 | more thorough solution (Suggested by brian d foy): |
46fc3d4c | 702 | |
703 | $string =~ s/ ( | |
704 | (^\w) #at the beginning of the line | |
705 | | # or | |
706 | (\s\w) #preceded by whitespace | |
707 | ) | |
708 | /\U$1/xg; | |
709 | $string =~ /([\w']+)/\u\L$1/g; | |
710 | ||
68dc0745 | 711 | To make the whole line upper case: |
3fe9a6f1 | 712 | |
68dc0745 | 713 | $line = uc($line); |
714 | ||
715 | To force each word to be lower case, with the first letter upper case: | |
3fe9a6f1 | 716 | |
68dc0745 | 717 | $line =~ s/(\w+)/\u\L$1/g; |
718 | ||
5a964f20 TC |
719 | You can (and probably should) enable locale awareness of those |
720 | characters by placing a C<use locale> pragma in your program. | |
92c2ed05 | 721 | See L<perllocale> for endless details on locales. |
5a964f20 | 722 | |
65acb1b1 | 723 | This is sometimes referred to as putting something into "title |
d92eb7b0 | 724 | case", but that's not quite accurate. Consider the proper |
65acb1b1 TC |
725 | capitalization of the movie I<Dr. Strangelove or: How I Learned to |
726 | Stop Worrying and Love the Bomb>, for example. | |
727 | ||
68dc0745 | 728 | =head2 How can I split a [character] delimited string except when inside |
729 | [character]? (Comma-separated files) | |
730 | ||
731 | Take the example case of trying to split a string that is comma-separated | |
732 | into its different fields. (We'll pretend you said comma-separated, not | |
733 | comma-delimited, which is different and almost never what you mean.) You | |
734 | can't use C<split(/,/)> because you shouldn't split if the comma is inside | |
735 | quotes. For example, take a data line like this: | |
736 | ||
737 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" | |
738 | ||
739 | Due to the restriction of the quotes, this is a fairly complex | |
740 | problem. Thankfully, we have Jeffrey Friedl, author of a highly | |
741 | recommended book on regular expressions, to handle these for us. He | |
742 | suggests (assuming your string is contained in $text): | |
743 | ||
744 | @new = (); | |
745 | push(@new, $+) while $text =~ m{ | |
746 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes | |
747 | | ([^,]+),? | |
748 | | , | |
749 | }gx; | |
750 | push(@new, undef) if substr($text,-1,1) eq ','; | |
751 | ||
46fc3d4c | 752 | If you want to represent quotation marks inside a |
753 | quotation-mark-delimited field, escape them with backslashes (eg, | |
2ceaccd7 | 754 | C<"like \"this\"">. Unescaping them is a task addressed earlier in |
46fc3d4c | 755 | this section. |
756 | ||
87275199 | 757 | Alternatively, the Text::ParseWords module (part of the standard Perl |
68dc0745 | 758 | distribution) lets you say: |
759 | ||
760 | use Text::ParseWords; | |
761 | @new = quotewords(",", 0, $text); | |
762 | ||
a6dd486b | 763 | There's also a Text::CSV (Comma-Separated Values) module on CPAN. |
65acb1b1 | 764 | |
68dc0745 | 765 | =head2 How do I strip blank space from the beginning/end of a string? |
766 | ||
a6dd486b | 767 | Although the simplest approach would seem to be |
68dc0745 | 768 | |
769 | $string =~ s/^\s*(.*?)\s*$/$1/; | |
770 | ||
a6dd486b | 771 | not only is this unnecessarily slow and destructive, it also fails with |
d92eb7b0 | 772 | embedded newlines. It is much faster to do this operation in two steps: |
68dc0745 | 773 | |
774 | $string =~ s/^\s+//; | |
775 | $string =~ s/\s+$//; | |
776 | ||
777 | Or more nicely written as: | |
778 | ||
779 | for ($string) { | |
780 | s/^\s+//; | |
781 | s/\s+$//; | |
782 | } | |
783 | ||
5e3006a4 | 784 | This idiom takes advantage of the C<foreach> loop's aliasing |
5a964f20 TC |
785 | behavior to factor out common code. You can do this |
786 | on several strings at once, or arrays, or even the | |
d92eb7b0 | 787 | values of a hash if you use a slice: |
5a964f20 TC |
788 | |
789 | # trim whitespace in the scalar, the array, | |
790 | # and all the values in the hash | |
791 | foreach ($scalar, @array, @hash{keys %hash}) { | |
792 | s/^\s+//; | |
793 | s/\s+$//; | |
794 | } | |
795 | ||
65acb1b1 TC |
796 | =head2 How do I pad a string with blanks or pad a number with zeroes? |
797 | ||
d92eb7b0 GS |
798 | (This answer contributed by Uri Guttman, with kibitzing from |
799 | Bart Lateur.) | |
65acb1b1 TC |
800 | |
801 | In the following examples, C<$pad_len> is the length to which you wish | |
d92eb7b0 GS |
802 | to pad the string, C<$text> or C<$num> contains the string to be padded, |
803 | and C<$pad_char> contains the padding character. You can use a single | |
804 | character string constant instead of the C<$pad_char> variable if you | |
805 | know what it is in advance. And in the same way you can use an integer in | |
806 | place of C<$pad_len> if you know the pad length in advance. | |
65acb1b1 | 807 | |
d92eb7b0 GS |
808 | The simplest method uses the C<sprintf> function. It can pad on the left |
809 | or right with blanks and on the left with zeroes and it will not | |
810 | truncate the result. The C<pack> function can only pad strings on the | |
811 | right with blanks and it will truncate the result to a maximum length of | |
812 | C<$pad_len>. | |
65acb1b1 | 813 | |
d92eb7b0 GS |
814 | # Left padding a string with blanks (no truncation): |
815 | $padded = sprintf("%${pad_len}s", $text); | |
65acb1b1 | 816 | |
d92eb7b0 GS |
817 | # Right padding a string with blanks (no truncation): |
818 | $padded = sprintf("%-${pad_len}s", $text); | |
65acb1b1 | 819 | |
d92eb7b0 GS |
820 | # Left padding a number with 0 (no truncation): |
821 | $padded = sprintf("%0${pad_len}d", $num); | |
65acb1b1 | 822 | |
d92eb7b0 GS |
823 | # Right padding a string with blanks using pack (will truncate): |
824 | $padded = pack("A$pad_len",$text); | |
65acb1b1 | 825 | |
d92eb7b0 GS |
826 | If you need to pad with a character other than blank or zero you can use |
827 | one of the following methods. They all generate a pad string with the | |
828 | C<x> operator and combine that with C<$text>. These methods do | |
829 | not truncate C<$text>. | |
65acb1b1 | 830 | |
d92eb7b0 | 831 | Left and right padding with any character, creating a new string: |
65acb1b1 | 832 | |
d92eb7b0 GS |
833 | $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; |
834 | $padded = $text . $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 835 | |
d92eb7b0 | 836 | Left and right padding with any character, modifying C<$text> directly: |
65acb1b1 | 837 | |
d92eb7b0 GS |
838 | substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); |
839 | $text .= $pad_char x ( $pad_len - length( $text ) ); | |
65acb1b1 | 840 | |
68dc0745 | 841 | =head2 How do I extract selected columns from a string? |
842 | ||
843 | Use substr() or unpack(), both documented in L<perlfunc>. | |
5a964f20 TC |
844 | If you prefer thinking in terms of columns instead of widths, |
845 | you can use this kind of thing: | |
846 | ||
847 | # determine the unpack format needed to split Linux ps output | |
848 | # arguments are cut columns | |
849 | my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72); | |
850 | ||
851 | sub cut2fmt { | |
852 | my(@positions) = @_; | |
853 | my $template = ''; | |
854 | my $lastpos = 1; | |
855 | for my $place (@positions) { | |
856 | $template .= "A" . ($place - $lastpos) . " "; | |
857 | $lastpos = $place; | |
858 | } | |
859 | $template .= "A*"; | |
860 | return $template; | |
861 | } | |
68dc0745 | 862 | |
863 | =head2 How do I find the soundex value of a string? | |
864 | ||
87275199 | 865 | Use the standard Text::Soundex module distributed with Perl. |
a6dd486b | 866 | Before you do so, you may want to determine whether `soundex' is in |
d92eb7b0 GS |
867 | fact what you think it is. Knuth's soundex algorithm compresses words |
868 | into a small space, and so it does not necessarily distinguish between | |
869 | two words which you might want to appear separately. For example, the | |
870 | last names `Knuth' and `Kant' are both mapped to the soundex code K530. | |
871 | If Text::Soundex does not do what you are looking for, you might want | |
872 | to consider the String::Approx module available at CPAN. | |
68dc0745 | 873 | |
874 | =head2 How can I expand variables in text strings? | |
875 | ||
876 | Let's assume that you have a string like: | |
877 | ||
878 | $text = 'this has a $foo in it and a $bar'; | |
5a964f20 TC |
879 | |
880 | If those were both global variables, then this would | |
881 | suffice: | |
882 | ||
65acb1b1 | 883 | $text =~ s/\$(\w+)/${$1}/g; # no /e needed |
68dc0745 | 884 | |
5a964f20 TC |
885 | But since they are probably lexicals, or at least, they could |
886 | be, you'd have to do this: | |
68dc0745 | 887 | |
888 | $text =~ s/(\$\w+)/$1/eeg; | |
65acb1b1 | 889 | die if $@; # needed /ee, not /e |
68dc0745 | 890 | |
5a964f20 TC |
891 | It's probably better in the general case to treat those |
892 | variables as entries in some special hash. For example: | |
893 | ||
894 | %user_defs = ( | |
895 | foo => 23, | |
896 | bar => 19, | |
897 | ); | |
898 | $text =~ s/\$(\w+)/$user_defs{$1}/g; | |
68dc0745 | 899 | |
92c2ed05 | 900 | See also ``How do I expand function calls in a string?'' in this section |
46fc3d4c | 901 | of the FAQ. |
902 | ||
68dc0745 | 903 | =head2 What's wrong with always quoting "$vars"? |
904 | ||
a6dd486b JB |
905 | The problem is that those double-quotes force stringification-- |
906 | coercing numbers and references into strings--even when you | |
907 | don't want them to be strings. Think of it this way: double-quote | |
65acb1b1 TC |
908 | expansion is used to produce new strings. If you already |
909 | have a string, why do you need more? | |
68dc0745 | 910 | |
911 | If you get used to writing odd things like these: | |
912 | ||
913 | print "$var"; # BAD | |
914 | $new = "$old"; # BAD | |
915 | somefunc("$var"); # BAD | |
916 | ||
917 | You'll be in trouble. Those should (in 99.8% of the cases) be | |
918 | the simpler and more direct: | |
919 | ||
920 | print $var; | |
921 | $new = $old; | |
922 | somefunc($var); | |
923 | ||
924 | Otherwise, besides slowing you down, you're going to break code when | |
925 | the thing in the scalar is actually neither a string nor a number, but | |
926 | a reference: | |
927 | ||
928 | func(\@array); | |
929 | sub func { | |
930 | my $aref = shift; | |
931 | my $oref = "$aref"; # WRONG | |
932 | } | |
933 | ||
934 | You can also get into subtle problems on those few operations in Perl | |
935 | that actually do care about the difference between a string and a | |
936 | number, such as the magical C<++> autoincrement operator or the | |
937 | syscall() function. | |
938 | ||
5a964f20 TC |
939 | Stringification also destroys arrays. |
940 | ||
941 | @lines = `command`; | |
942 | print "@lines"; # WRONG - extra blanks | |
943 | print @lines; # right | |
944 | ||
c47ff5f1 | 945 | =head2 Why don't my <<HERE documents work? |
68dc0745 | 946 | |
947 | Check for these three things: | |
948 | ||
949 | =over 4 | |
950 | ||
951 | =item 1. There must be no space after the << part. | |
952 | ||
953 | =item 2. There (probably) should be a semicolon at the end. | |
954 | ||
955 | =item 3. You can't (easily) have any space in front of the tag. | |
956 | ||
957 | =back | |
958 | ||
5a964f20 TC |
959 | If you want to indent the text in the here document, you |
960 | can do this: | |
961 | ||
962 | # all in one | |
963 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; | |
964 | your text | |
965 | goes here | |
966 | HERE_TARGET | |
967 | ||
968 | But the HERE_TARGET must still be flush against the margin. | |
969 | If you want that indented also, you'll have to quote | |
970 | in the indentation. | |
971 | ||
972 | ($quote = <<' FINIS') =~ s/^\s+//gm; | |
973 | ...we will have peace, when you and all your works have | |
974 | perished--and the works of your dark master to whom you | |
975 | would deliver us. You are a liar, Saruman, and a corrupter | |
976 | of men's hearts. --Theoden in /usr/src/perl/taint.c | |
977 | FINIS | |
978 | $quote =~ s/\s*--/\n--/; | |
979 | ||
980 | A nice general-purpose fixer-upper function for indented here documents | |
981 | follows. It expects to be called with a here document as its argument. | |
982 | It looks to see whether each line begins with a common substring, and | |
a6dd486b JB |
983 | if so, strips that substring off. Otherwise, it takes the amount of leading |
984 | whitespace found on the first line and removes that much off each | |
5a964f20 TC |
985 | subsequent line. |
986 | ||
987 | sub fix { | |
988 | local $_ = shift; | |
a6dd486b | 989 | my ($white, $leader); # common whitespace and common leading string |
5a964f20 TC |
990 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { |
991 | ($white, $leader) = ($2, quotemeta($1)); | |
992 | } else { | |
993 | ($white, $leader) = (/^(\s+)/, ''); | |
994 | } | |
995 | s/^\s*?$leader(?:$white)?//gm; | |
996 | return $_; | |
997 | } | |
998 | ||
c8db1d39 | 999 | This works with leading special strings, dynamically determined: |
5a964f20 TC |
1000 | |
1001 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; | |
1002 | @@@ int | |
1003 | @@@ runops() { | |
1004 | @@@ SAVEI32(runlevel); | |
1005 | @@@ runlevel++; | |
d92eb7b0 | 1006 | @@@ while ( op = (*op->op_ppaddr)() ); |
5a964f20 TC |
1007 | @@@ TAINT_NOT; |
1008 | @@@ return 0; | |
1009 | @@@ } | |
1010 | MAIN_INTERPRETER_LOOP | |
1011 | ||
a6dd486b | 1012 | Or with a fixed amount of leading whitespace, with remaining |
5a964f20 TC |
1013 | indentation correctly preserved: |
1014 | ||
1015 | $poem = fix<<EVER_ON_AND_ON; | |
1016 | Now far ahead the Road has gone, | |
1017 | And I must follow, if I can, | |
1018 | Pursuing it with eager feet, | |
1019 | Until it joins some larger way | |
1020 | Where many paths and errands meet. | |
1021 | And whither then? I cannot say. | |
1022 | --Bilbo in /usr/src/perl/pp_ctl.c | |
1023 | EVER_ON_AND_ON | |
1024 | ||
68dc0745 | 1025 | =head1 Data: Arrays |
1026 | ||
65acb1b1 TC |
1027 | =head2 What is the difference between a list and an array? |
1028 | ||
1029 | An array has a changeable length. A list does not. An array is something | |
1030 | you can push or pop, while a list is a set of values. Some people make | |
1031 | the distinction that a list is a value while an array is a variable. | |
1032 | Subroutines are passed and return lists, you put things into list | |
1033 | context, you initialize arrays with lists, and you foreach() across | |
1034 | a list. C<@> variables are arrays, anonymous arrays are arrays, arrays | |
1035 | in scalar context behave like the number of elements in them, subroutines | |
a6dd486b | 1036 | access their arguments through the array C<@_>, and push/pop/shift only work |
65acb1b1 TC |
1037 | on arrays. |
1038 | ||
1039 | As a side note, there's no such thing as a list in scalar context. | |
1040 | When you say | |
1041 | ||
1042 | $scalar = (2, 5, 7, 9); | |
1043 | ||
d92eb7b0 GS |
1044 | you're using the comma operator in scalar context, so it uses the scalar |
1045 | comma operator. There never was a list there at all! This causes the | |
1046 | last value to be returned: 9. | |
65acb1b1 | 1047 | |
68dc0745 | 1048 | =head2 What is the difference between $array[1] and @array[1]? |
1049 | ||
a6dd486b | 1050 | The former is a scalar value; the latter an array slice, making |
68dc0745 | 1051 | it a list with one (scalar) value. You should use $ when you want a |
1052 | scalar value (most of the time) and @ when you want a list with one | |
1053 | scalar value in it (very, very rarely; nearly never, in fact). | |
1054 | ||
1055 | Sometimes it doesn't make a difference, but sometimes it does. | |
1056 | For example, compare: | |
1057 | ||
1058 | $good[0] = `some program that outputs several lines`; | |
1059 | ||
1060 | with | |
1061 | ||
1062 | @bad[0] = `same program that outputs several lines`; | |
1063 | ||
9f1b1f2d GS |
1064 | The C<use warnings> pragma and the B<-w> flag will warn you about these |
1065 | matters. | |
68dc0745 | 1066 | |
d92eb7b0 | 1067 | =head2 How can I remove duplicate elements from a list or array? |
68dc0745 | 1068 | |
1069 | There are several possible ways, depending on whether the array is | |
1070 | ordered and whether you wish to preserve the ordering. | |
1071 | ||
1072 | =over 4 | |
1073 | ||
551e1d92 RB |
1074 | =item a) |
1075 | ||
1076 | If @in is sorted, and you want @out to be sorted: | |
5a964f20 | 1077 | (this assumes all true values in the array) |
68dc0745 | 1078 | |
a4341a65 | 1079 | $prev = "not equal to $in[0]"; |
3bc5ef3e | 1080 | @out = grep($_ ne $prev && ($prev = $_, 1), @in); |
68dc0745 | 1081 | |
c8db1d39 | 1082 | This is nice in that it doesn't use much extra memory, simulating |
3bc5ef3e HG |
1083 | uniq(1)'s behavior of removing only adjacent duplicates. The ", 1" |
1084 | guarantees that the expression is true (so that grep picks it up) | |
1085 | even if the $_ is 0, "", or undef. | |
68dc0745 | 1086 | |
551e1d92 RB |
1087 | =item b) |
1088 | ||
1089 | If you don't know whether @in is sorted: | |
68dc0745 | 1090 | |
1091 | undef %saw; | |
1092 | @out = grep(!$saw{$_}++, @in); | |
1093 | ||
551e1d92 RB |
1094 | =item c) |
1095 | ||
1096 | Like (b), but @in contains only small integers: | |
68dc0745 | 1097 | |
1098 | @out = grep(!$saw[$_]++, @in); | |
1099 | ||
551e1d92 RB |
1100 | =item d) |
1101 | ||
1102 | A way to do (b) without any loops or greps: | |
68dc0745 | 1103 | |
1104 | undef %saw; | |
1105 | @saw{@in} = (); | |
1106 | @out = sort keys %saw; # remove sort if undesired | |
1107 | ||
551e1d92 RB |
1108 | =item e) |
1109 | ||
1110 | Like (d), but @in contains only small positive integers: | |
68dc0745 | 1111 | |
1112 | undef @ary; | |
1113 | @ary[@in] = @in; | |
87275199 | 1114 | @out = grep {defined} @ary; |
68dc0745 | 1115 | |
1116 | =back | |
1117 | ||
65acb1b1 TC |
1118 | But perhaps you should have been using a hash all along, eh? |
1119 | ||
ddbc1f16 | 1120 | =head2 How can I tell whether a certain element is contained in a list or array? |
5a964f20 TC |
1121 | |
1122 | Hearing the word "in" is an I<in>dication that you probably should have | |
1123 | used a hash, not a list or array, to store your data. Hashes are | |
1124 | designed to answer this question quickly and efficiently. Arrays aren't. | |
68dc0745 | 1125 | |
5a964f20 TC |
1126 | That being said, there are several ways to approach this. If you |
1127 | are going to make this query many times over arbitrary string values, | |
1128 | the fastest way is probably to invert the original array and keep an | |
68dc0745 | 1129 | associative array lying about whose keys are the first array's values. |
1130 | ||
1131 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; | |
1132 | undef %is_blue; | |
1133 | for (@blues) { $is_blue{$_} = 1 } | |
1134 | ||
1135 | Now you can check whether $is_blue{$some_color}. It might have been a | |
1136 | good idea to keep the blues all in a hash in the first place. | |
1137 | ||
1138 | If the values are all small integers, you could use a simple indexed | |
1139 | array. This kind of an array will take up less space: | |
1140 | ||
1141 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); | |
1142 | undef @is_tiny_prime; | |
d92eb7b0 GS |
1143 | for (@primes) { $is_tiny_prime[$_] = 1 } |
1144 | # or simply @istiny_prime[@primes] = (1) x @primes; | |
68dc0745 | 1145 | |
1146 | Now you check whether $is_tiny_prime[$some_number]. | |
1147 | ||
1148 | If the values in question are integers instead of strings, you can save | |
1149 | quite a lot of space by using bit strings instead: | |
1150 | ||
1151 | @articles = ( 1..10, 150..2000, 2017 ); | |
1152 | undef $read; | |
7b8d334a | 1153 | for (@articles) { vec($read,$_,1) = 1 } |
68dc0745 | 1154 | |
1155 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. | |
1156 | ||
1157 | Please do not use | |
1158 | ||
a6dd486b | 1159 | ($is_there) = grep $_ eq $whatever, @array; |
68dc0745 | 1160 | |
1161 | or worse yet | |
1162 | ||
a6dd486b | 1163 | ($is_there) = grep /$whatever/, @array; |
68dc0745 | 1164 | |
1165 | These are slow (checks every element even if the first matches), | |
1166 | inefficient (same reason), and potentially buggy (what if there are | |
d92eb7b0 | 1167 | regex characters in $whatever?). If you're only testing once, then |
65acb1b1 TC |
1168 | use: |
1169 | ||
1170 | $is_there = 0; | |
1171 | foreach $elt (@array) { | |
1172 | if ($elt eq $elt_to_find) { | |
1173 | $is_there = 1; | |
1174 | last; | |
1175 | } | |
1176 | } | |
1177 | if ($is_there) { ... } | |
68dc0745 | 1178 | |
1179 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? | |
1180 | ||
1181 | Use a hash. Here's code to do both and more. It assumes that | |
1182 | each element is unique in a given array: | |
1183 | ||
1184 | @union = @intersection = @difference = (); | |
1185 | %count = (); | |
1186 | foreach $element (@array1, @array2) { $count{$element}++ } | |
1187 | foreach $element (keys %count) { | |
1188 | push @union, $element; | |
1189 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; | |
1190 | } | |
1191 | ||
d92eb7b0 | 1192 | Note that this is the I<symmetric difference>, that is, all elements in |
a6dd486b | 1193 | either A or in B but not in both. Think of it as an xor operation. |
d92eb7b0 | 1194 | |
65acb1b1 TC |
1195 | =head2 How do I test whether two arrays or hashes are equal? |
1196 | ||
1197 | The following code works for single-level arrays. It uses a stringwise | |
1198 | comparison, and does not distinguish defined versus undefined empty | |
1199 | strings. Modify if you have other needs. | |
1200 | ||
1201 | $are_equal = compare_arrays(\@frogs, \@toads); | |
1202 | ||
1203 | sub compare_arrays { | |
1204 | my ($first, $second) = @_; | |
9f1b1f2d | 1205 | no warnings; # silence spurious -w undef complaints |
65acb1b1 TC |
1206 | return 0 unless @$first == @$second; |
1207 | for (my $i = 0; $i < @$first; $i++) { | |
1208 | return 0 if $first->[$i] ne $second->[$i]; | |
1209 | } | |
1210 | return 1; | |
1211 | } | |
1212 | ||
1213 | For multilevel structures, you may wish to use an approach more | |
1214 | like this one. It uses the CPAN module FreezeThaw: | |
1215 | ||
1216 | use FreezeThaw qw(cmpStr); | |
1217 | @a = @b = ( "this", "that", [ "more", "stuff" ] ); | |
1218 | ||
1219 | printf "a and b contain %s arrays\n", | |
1220 | cmpStr(\@a, \@b) == 0 | |
1221 | ? "the same" | |
1222 | : "different"; | |
1223 | ||
1224 | This approach also works for comparing hashes. Here | |
1225 | we'll demonstrate two different answers: | |
1226 | ||
1227 | use FreezeThaw qw(cmpStr cmpStrHard); | |
1228 | ||
1229 | %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); | |
1230 | $a{EXTRA} = \%b; | |
1231 | $b{EXTRA} = \%a; | |
1232 | ||
1233 | printf "a and b contain %s hashes\n", | |
1234 | cmpStr(\%a, \%b) == 0 ? "the same" : "different"; | |
1235 | ||
1236 | printf "a and b contain %s hashes\n", | |
1237 | cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; | |
1238 | ||
1239 | ||
1240 | The first reports that both those the hashes contain the same data, | |
1241 | while the second reports that they do not. Which you prefer is left as | |
1242 | an exercise to the reader. | |
1243 | ||
68dc0745 | 1244 | =head2 How do I find the first array element for which a condition is true? |
1245 | ||
1246 | You can use this if you care about the index: | |
1247 | ||
65acb1b1 | 1248 | for ($i= 0; $i < @array; $i++) { |
68dc0745 | 1249 | if ($array[$i] eq "Waldo") { |
1250 | $found_index = $i; | |
1251 | last; | |
1252 | } | |
1253 | } | |
1254 | ||
1255 | Now C<$found_index> has what you want. | |
1256 | ||
1257 | =head2 How do I handle linked lists? | |
1258 | ||
1259 | In general, you usually don't need a linked list in Perl, since with | |
1260 | regular arrays, you can push and pop or shift and unshift at either end, | |
5a964f20 | 1261 | or you can use splice to add and/or remove arbitrary number of elements at |
87275199 | 1262 | arbitrary points. Both pop and shift are both O(1) operations on Perl's |
5a964f20 TC |
1263 | dynamic arrays. In the absence of shifts and pops, push in general |
1264 | needs to reallocate on the order every log(N) times, and unshift will | |
1265 | need to copy pointers each time. | |
68dc0745 | 1266 | |
1267 | If you really, really wanted, you could use structures as described in | |
1268 | L<perldsc> or L<perltoot> and do just what the algorithm book tells you | |
65acb1b1 TC |
1269 | to do. For example, imagine a list node like this: |
1270 | ||
1271 | $node = { | |
1272 | VALUE => 42, | |
1273 | LINK => undef, | |
1274 | }; | |
1275 | ||
1276 | You could walk the list this way: | |
1277 | ||
1278 | print "List: "; | |
1279 | for ($node = $head; $node; $node = $node->{LINK}) { | |
1280 | print $node->{VALUE}, " "; | |
1281 | } | |
1282 | print "\n"; | |
1283 | ||
a6dd486b | 1284 | You could add to the list this way: |
65acb1b1 TC |
1285 | |
1286 | my ($head, $tail); | |
1287 | $tail = append($head, 1); # grow a new head | |
1288 | for $value ( 2 .. 10 ) { | |
1289 | $tail = append($tail, $value); | |
1290 | } | |
1291 | ||
1292 | sub append { | |
1293 | my($list, $value) = @_; | |
1294 | my $node = { VALUE => $value }; | |
1295 | if ($list) { | |
1296 | $node->{LINK} = $list->{LINK}; | |
1297 | $list->{LINK} = $node; | |
1298 | } else { | |
1299 | $_[0] = $node; # replace caller's version | |
1300 | } | |
1301 | return $node; | |
1302 | } | |
1303 | ||
1304 | But again, Perl's built-in are virtually always good enough. | |
68dc0745 | 1305 | |
1306 | =head2 How do I handle circular lists? | |
1307 | ||
1308 | Circular lists could be handled in the traditional fashion with linked | |
1309 | lists, or you could just do something like this with an array: | |
1310 | ||
1311 | unshift(@array, pop(@array)); # the last shall be first | |
1312 | push(@array, shift(@array)); # and vice versa | |
1313 | ||
1314 | =head2 How do I shuffle an array randomly? | |
1315 | ||
45bbf655 JH |
1316 | If you either have Perl 5.8.0 or later installed, or if you have |
1317 | Scalar-List-Utils 1.03 or later installed, you can say: | |
1318 | ||
1319 | use List::Util 'shuffle'; | |
1320 | ||
1321 | @shuffled = shuffle(@list); | |
1322 | ||
1323 | If not, you can use this: | |
5a964f20 | 1324 | |
cc30d1a7 JH |
1325 | # fisher_yates_shuffle |
1326 | # generate a random permutation of an array in place | |
1327 | # As in shuffling a deck of cards | |
1328 | # | |
5a964f20 | 1329 | sub fisher_yates_shuffle { |
cc30d1a7 JH |
1330 | my $deck = shift; # $deck is a reference to an array |
1331 | my $i = @$deck; | |
8caf10e0 | 1332 | while (--$i) { |
5a964f20 | 1333 | my $j = int rand ($i+1); |
cc30d1a7 | 1334 | @$deck[$i,$j] = @$deck[$j,$i]; |
5a964f20 TC |
1335 | } |
1336 | } | |
1337 | ||
cc30d1a7 JH |
1338 | And here is an example of using it: |
1339 | ||
1340 | # | |
1341 | # shuffle my mpeg collection | |
1342 | # | |
1343 | my @mpeg = <audio/*/*.mp3>; | |
1344 | fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place | |
1345 | print @mpeg; | |
5a964f20 | 1346 | |
45bbf655 JH |
1347 | Note that the above implementation shuffles an array in place, |
1348 | unlike the List::Util::shuffle() which takes a list and returns | |
1349 | a new shuffled list. | |
1350 | ||
d92eb7b0 | 1351 | You've probably seen shuffling algorithms that work using splice, |
a6dd486b | 1352 | randomly picking another element to swap the current element with |
68dc0745 | 1353 | |
1354 | srand; | |
1355 | @new = (); | |
1356 | @old = 1 .. 10; # just a demo | |
1357 | while (@old) { | |
1358 | push(@new, splice(@old, rand @old, 1)); | |
1359 | } | |
1360 | ||
5a964f20 TC |
1361 | This is bad because splice is already O(N), and since you do it N times, |
1362 | you just invented a quadratic algorithm; that is, O(N**2). This does | |
1363 | not scale, although Perl is so efficient that you probably won't notice | |
1364 | this until you have rather largish arrays. | |
68dc0745 | 1365 | |
1366 | =head2 How do I process/modify each element of an array? | |
1367 | ||
1368 | Use C<for>/C<foreach>: | |
1369 | ||
1370 | for (@lines) { | |
5a964f20 TC |
1371 | s/foo/bar/; # change that word |
1372 | y/XZ/ZX/; # swap those letters | |
68dc0745 | 1373 | } |
1374 | ||
1375 | Here's another; let's compute spherical volumes: | |
1376 | ||
5a964f20 | 1377 | for (@volumes = @radii) { # @volumes has changed parts |
68dc0745 | 1378 | $_ **= 3; |
1379 | $_ *= (4/3) * 3.14159; # this will be constant folded | |
1380 | } | |
1381 | ||
5a964f20 TC |
1382 | If you want to do the same thing to modify the values of the hash, |
1383 | you may not use the C<values> function, oddly enough. You need a slice: | |
1384 | ||
1385 | for $orbit ( @orbits{keys %orbits} ) { | |
1386 | ($orbit **= 3) *= (4/3) * 3.14159; | |
1387 | } | |
1388 | ||
68dc0745 | 1389 | =head2 How do I select a random element from an array? |
1390 | ||
1391 | Use the rand() function (see L<perlfunc/rand>): | |
1392 | ||
5a964f20 | 1393 | # at the top of the program: |
68dc0745 | 1394 | srand; # not needed for 5.004 and later |
5a964f20 TC |
1395 | |
1396 | # then later on | |
68dc0745 | 1397 | $index = rand @array; |
1398 | $element = $array[$index]; | |
1399 | ||
5a964f20 TC |
1400 | Make sure you I<only call srand once per program, if then>. |
1401 | If you are calling it more than once (such as before each | |
1402 | call to rand), you're almost certainly doing something wrong. | |
1403 | ||
68dc0745 | 1404 | =head2 How do I permute N elements of a list? |
1405 | ||
1406 | Here's a little program that generates all permutations | |
1407 | of all the words on each line of input. The algorithm embodied | |
5a964f20 | 1408 | in the permute() function should work on any list: |
68dc0745 | 1409 | |
1410 | #!/usr/bin/perl -n | |
5a964f20 TC |
1411 | # tsc-permute: permute each word of input |
1412 | permute([split], []); | |
1413 | sub permute { | |
1414 | my @items = @{ $_[0] }; | |
1415 | my @perms = @{ $_[1] }; | |
1416 | unless (@items) { | |
1417 | print "@perms\n"; | |
68dc0745 | 1418 | } else { |
5a964f20 TC |
1419 | my(@newitems,@newperms,$i); |
1420 | foreach $i (0 .. $#items) { | |
1421 | @newitems = @items; | |
1422 | @newperms = @perms; | |
1423 | unshift(@newperms, splice(@newitems, $i, 1)); | |
1424 | permute([@newitems], [@newperms]); | |
68dc0745 | 1425 | } |
1426 | } | |
1427 | } | |
1428 | ||
b8d2732a RH |
1429 | Unfortunately, this algorithm is very inefficient. The Algorithm::Permute |
1430 | module from CPAN runs at least an order of magnitude faster. If you don't | |
1431 | have a C compiler (or a binary distribution of Algorithm::Permute), then | |
1432 | you can use List::Permutor which is written in pure Perl, and is still | |
f8620f40 | 1433 | several times faster than the algorithm above. |
b8d2732a | 1434 | |
68dc0745 | 1435 | =head2 How do I sort an array by (anything)? |
1436 | ||
1437 | Supply a comparison function to sort() (described in L<perlfunc/sort>): | |
1438 | ||
1439 | @list = sort { $a <=> $b } @list; | |
1440 | ||
1441 | The default sort function is cmp, string comparison, which would | |
c47ff5f1 | 1442 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is |
68dc0745 | 1443 | the numerical comparison operator. |
1444 | ||
1445 | If you have a complicated function needed to pull out the part you | |
1446 | want to sort on, then don't do it inside the sort function. Pull it | |
1447 | out first, because the sort BLOCK can be called many times for the | |
1448 | same element. Here's an example of how to pull out the first word | |
1449 | after the first number on each item, and then sort those words | |
1450 | case-insensitively. | |
1451 | ||
1452 | @idx = (); | |
1453 | for (@data) { | |
1454 | ($item) = /\d+\s*(\S+)/; | |
1455 | push @idx, uc($item); | |
1456 | } | |
1457 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; | |
1458 | ||
a6dd486b | 1459 | which could also be written this way, using a trick |
68dc0745 | 1460 | that's come to be known as the Schwartzian Transform: |
1461 | ||
1462 | @sorted = map { $_->[0] } | |
1463 | sort { $a->[1] cmp $b->[1] } | |
d92eb7b0 | 1464 | map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; |
68dc0745 | 1465 | |
1466 | If you need to sort on several fields, the following paradigm is useful. | |
1467 | ||
1468 | @sorted = sort { field1($a) <=> field1($b) || | |
1469 | field2($a) cmp field2($b) || | |
1470 | field3($a) cmp field3($b) | |
1471 | } @data; | |
1472 | ||
1473 | This can be conveniently combined with precalculation of keys as given | |
1474 | above. | |
1475 | ||
a93751fa | 1476 | See http://www.cpan.org/doc/FMTEYEWTK/sort.html for more about |
68dc0745 | 1477 | this approach. |
1478 | ||
1479 | See also the question below on sorting hashes. | |
1480 | ||
1481 | =head2 How do I manipulate arrays of bits? | |
1482 | ||
1483 | Use pack() and unpack(), or else vec() and the bitwise operations. | |
1484 | ||
1485 | For example, this sets $vec to have bit N set if $ints[N] was set: | |
1486 | ||
1487 | $vec = ''; | |
1488 | foreach(@ints) { vec($vec,$_,1) = 1 } | |
1489 | ||
cc30d1a7 | 1490 | Here's how, given a vector in $vec, you can |
68dc0745 | 1491 | get those bits into your @ints array: |
1492 | ||
1493 | sub bitvec_to_list { | |
1494 | my $vec = shift; | |
1495 | my @ints; | |
1496 | # Find null-byte density then select best algorithm | |
1497 | if ($vec =~ tr/\0// / length $vec > 0.95) { | |
1498 | use integer; | |
1499 | my $i; | |
1500 | # This method is faster with mostly null-bytes | |
1501 | while($vec =~ /[^\0]/g ) { | |
1502 | $i = -9 + 8 * pos $vec; | |
1503 | push @ints, $i if vec($vec, ++$i, 1); | |
1504 | push @ints, $i if vec($vec, ++$i, 1); | |
1505 | push @ints, $i if vec($vec, ++$i, 1); | |
1506 | push @ints, $i if vec($vec, ++$i, 1); | |
1507 | push @ints, $i if vec($vec, ++$i, 1); | |
1508 | push @ints, $i if vec($vec, ++$i, 1); | |
1509 | push @ints, $i if vec($vec, ++$i, 1); | |
1510 | push @ints, $i if vec($vec, ++$i, 1); | |
1511 | } | |
1512 | } else { | |
1513 | # This method is a fast general algorithm | |
1514 | use integer; | |
1515 | my $bits = unpack "b*", $vec; | |
1516 | push @ints, 0 if $bits =~ s/^(\d)// && $1; | |
1517 | push @ints, pos $bits while($bits =~ /1/g); | |
1518 | } | |
1519 | return \@ints; | |
1520 | } | |
1521 | ||
1522 | This method gets faster the more sparse the bit vector is. | |
1523 | (Courtesy of Tim Bunce and Winfried Koenig.) | |
1524 | ||
cc30d1a7 JH |
1525 | Or use the CPAN module Bit::Vector: |
1526 | ||
1527 | $vector = Bit::Vector->new($num_of_bits); | |
1528 | $vector->Index_List_Store(@ints); | |
1529 | @ints = $vector->Index_List_Read(); | |
1530 | ||
1531 | Bit::Vector provides efficient methods for bit vector, sets of small integers | |
1532 | and "big int" math. | |
1533 | ||
1534 | Here's a more extensive illustration using vec(): | |
65acb1b1 TC |
1535 | |
1536 | # vec demo | |
1537 | $vector = "\xff\x0f\xef\xfe"; | |
1538 | print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", | |
1539 | unpack("N", $vector), "\n"; | |
1540 | $is_set = vec($vector, 23, 1); | |
1541 | print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; | |
1542 | pvec($vector); | |
1543 | ||
1544 | set_vec(1,1,1); | |
1545 | set_vec(3,1,1); | |
1546 | set_vec(23,1,1); | |
1547 | ||
1548 | set_vec(3,1,3); | |
1549 | set_vec(3,2,3); | |
1550 | set_vec(3,4,3); | |
1551 | set_vec(3,4,7); | |
1552 | set_vec(3,8,3); | |
1553 | set_vec(3,8,7); | |
1554 | ||
1555 | set_vec(0,32,17); | |
1556 | set_vec(1,32,17); | |
1557 | ||
1558 | sub set_vec { | |
1559 | my ($offset, $width, $value) = @_; | |
1560 | my $vector = ''; | |
1561 | vec($vector, $offset, $width) = $value; | |
1562 | print "offset=$offset width=$width value=$value\n"; | |
1563 | pvec($vector); | |
1564 | } | |
1565 | ||
1566 | sub pvec { | |
1567 | my $vector = shift; | |
1568 | my $bits = unpack("b*", $vector); | |
1569 | my $i = 0; | |
1570 | my $BASE = 8; | |
1571 | ||
1572 | print "vector length in bytes: ", length($vector), "\n"; | |
1573 | @bytes = unpack("A8" x length($vector), $bits); | |
1574 | print "bits are: @bytes\n\n"; | |
1575 | } | |
1576 | ||
68dc0745 | 1577 | =head2 Why does defined() return true on empty arrays and hashes? |
1578 | ||
65acb1b1 TC |
1579 | The short story is that you should probably only use defined on scalars or |
1580 | functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> | |
1581 | in the 5.004 release or later of Perl for more detail. | |
68dc0745 | 1582 | |
1583 | =head1 Data: Hashes (Associative Arrays) | |
1584 | ||
1585 | =head2 How do I process an entire hash? | |
1586 | ||
1587 | Use the each() function (see L<perlfunc/each>) if you don't care | |
1588 | whether it's sorted: | |
1589 | ||
5a964f20 | 1590 | while ( ($key, $value) = each %hash) { |
68dc0745 | 1591 | print "$key = $value\n"; |
1592 | } | |
1593 | ||
1594 | If you want it sorted, you'll have to use foreach() on the result of | |
1595 | sorting the keys as shown in an earlier question. | |
1596 | ||
1597 | =head2 What happens if I add or remove keys from a hash while iterating over it? | |
1598 | ||
d92eb7b0 GS |
1599 | Don't do that. :-) |
1600 | ||
1601 | [lwall] In Perl 4, you were not allowed to modify a hash at all while | |
87275199 | 1602 | iterating over it. In Perl 5 you can delete from it, but you still |
d92eb7b0 GS |
1603 | can't add to it, because that might cause a doubling of the hash table, |
1604 | in which half the entries get copied up to the new top half of the | |
87275199 | 1605 | table, at which point you've totally bamboozled the iterator code. |
d92eb7b0 GS |
1606 | Even if the table doesn't double, there's no telling whether your new |
1607 | entry will be inserted before or after the current iterator position. | |
1608 | ||
a6dd486b | 1609 | Either treasure up your changes and make them after the iterator finishes |
d92eb7b0 GS |
1610 | or use keys to fetch all the old keys at once, and iterate over the list |
1611 | of keys. | |
68dc0745 | 1612 | |
1613 | =head2 How do I look up a hash element by value? | |
1614 | ||
1615 | Create a reverse hash: | |
1616 | ||
1617 | %by_value = reverse %by_key; | |
1618 | $key = $by_value{$value}; | |
1619 | ||
1620 | That's not particularly efficient. It would be more space-efficient | |
1621 | to use: | |
1622 | ||
1623 | while (($key, $value) = each %by_key) { | |
1624 | $by_value{$value} = $key; | |
1625 | } | |
1626 | ||
d92eb7b0 GS |
1627 | If your hash could have repeated values, the methods above will only find |
1628 | one of the associated keys. This may or may not worry you. If it does | |
1629 | worry you, you can always reverse the hash into a hash of arrays instead: | |
1630 | ||
1631 | while (($key, $value) = each %by_key) { | |
1632 | push @{$key_list_by_value{$value}}, $key; | |
1633 | } | |
68dc0745 | 1634 | |
1635 | =head2 How can I know how many entries are in a hash? | |
1636 | ||
1637 | If you mean how many keys, then all you have to do is | |
1638 | take the scalar sense of the keys() function: | |
1639 | ||
3fe9a6f1 | 1640 | $num_keys = scalar keys %hash; |
68dc0745 | 1641 | |
a6dd486b | 1642 | The keys() function also resets the iterator, which in void context is |
d92eb7b0 GS |
1643 | faster for tied hashes than would be iterating through the whole |
1644 | hash, one key-value pair at a time. | |
68dc0745 | 1645 | |
1646 | =head2 How do I sort a hash (optionally by value instead of key)? | |
1647 | ||
1648 | Internally, hashes are stored in a way that prevents you from imposing | |
1649 | an order on key-value pairs. Instead, you have to sort a list of the | |
1650 | keys or values: | |
1651 | ||
1652 | @keys = sort keys %hash; # sorted by key | |
1653 | @keys = sort { | |
1654 | $hash{$a} cmp $hash{$b} | |
1655 | } keys %hash; # and by value | |
1656 | ||
1657 | Here we'll do a reverse numeric sort by value, and if two keys are | |
a6dd486b JB |
1658 | identical, sort by length of key, or if that fails, by straight ASCII |
1659 | comparison of the keys (well, possibly modified by your locale--see | |
68dc0745 | 1660 | L<perllocale>). |
1661 | ||
1662 | @keys = sort { | |
1663 | $hash{$b} <=> $hash{$a} | |
1664 | || | |
1665 | length($b) <=> length($a) | |
1666 | || | |
1667 | $a cmp $b | |
1668 | } keys %hash; | |
1669 | ||
1670 | =head2 How can I always keep my hash sorted? | |
1671 | ||
1672 | You can look into using the DB_File module and tie() using the | |
1673 | $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">. | |
5a964f20 | 1674 | The Tie::IxHash module from CPAN might also be instructive. |
68dc0745 | 1675 | |
1676 | =head2 What's the difference between "delete" and "undef" with hashes? | |
1677 | ||
1678 | Hashes are pairs of scalars: the first is the key, the second is the | |
1679 | value. The key will be coerced to a string, although the value can be | |
1680 | any kind of scalar: string, number, or reference. If a key C<$key> is | |
1681 | present in the array, C<exists($key)> will return true. The value for | |
1682 | a given key can be C<undef>, in which case C<$array{$key}> will be | |
1683 | C<undef> while C<$exists{$key}> will return true. This corresponds to | |
1684 | (C<$key>, C<undef>) being in the hash. | |
1685 | ||
1686 | Pictures help... here's the C<%ary> table: | |
1687 | ||
1688 | keys values | |
1689 | +------+------+ | |
1690 | | a | 3 | | |
1691 | | x | 7 | | |
1692 | | d | 0 | | |
1693 | | e | 2 | | |
1694 | +------+------+ | |
1695 | ||
1696 | And these conditions hold | |
1697 | ||
1698 | $ary{'a'} is true | |
1699 | $ary{'d'} is false | |
1700 | defined $ary{'d'} is true | |
1701 | defined $ary{'a'} is true | |
87275199 | 1702 | exists $ary{'a'} is true (Perl5 only) |
68dc0745 | 1703 | grep ($_ eq 'a', keys %ary) is true |
1704 | ||
1705 | If you now say | |
1706 | ||
1707 | undef $ary{'a'} | |
1708 | ||
1709 | your table now reads: | |
1710 | ||
1711 | ||
1712 | keys values | |
1713 | +------+------+ | |
1714 | | a | undef| | |
1715 | | x | 7 | | |
1716 | | d | 0 | | |
1717 | | e | 2 | | |
1718 | +------+------+ | |
1719 | ||
1720 | and these conditions now hold; changes in caps: | |
1721 | ||
1722 | $ary{'a'} is FALSE | |
1723 | $ary{'d'} is false | |
1724 | defined $ary{'d'} is true | |
1725 | defined $ary{'a'} is FALSE | |
87275199 | 1726 | exists $ary{'a'} is true (Perl5 only) |
68dc0745 | 1727 | grep ($_ eq 'a', keys %ary) is true |
1728 | ||
1729 | Notice the last two: you have an undef value, but a defined key! | |
1730 | ||
1731 | Now, consider this: | |
1732 | ||
1733 | delete $ary{'a'} | |
1734 | ||
1735 | your table now reads: | |
1736 | ||
1737 | keys values | |
1738 | +------+------+ | |
1739 | | x | 7 | | |
1740 | | d | 0 | | |
1741 | | e | 2 | | |
1742 | +------+------+ | |
1743 | ||
1744 | and these conditions now hold; changes in caps: | |
1745 | ||
1746 | $ary{'a'} is false | |
1747 | $ary{'d'} is false | |
1748 | defined $ary{'d'} is true | |
1749 | defined $ary{'a'} is false | |
87275199 | 1750 | exists $ary{'a'} is FALSE (Perl5 only) |
68dc0745 | 1751 | grep ($_ eq 'a', keys %ary) is FALSE |
1752 | ||
1753 | See, the whole entry is gone! | |
1754 | ||
1755 | =head2 Why don't my tied hashes make the defined/exists distinction? | |
1756 | ||
1757 | They may or may not implement the EXISTS() and DEFINED() methods | |
1758 | differently. For example, there isn't the concept of undef with hashes | |
1759 | that are tied to DBM* files. This means the true/false tables above | |
1760 | will give different results when used on such a hash. It also means | |
1761 | that exists and defined do the same thing with a DBM* file, and what | |
1762 | they end up doing is not what they do with ordinary hashes. | |
1763 | ||
1764 | =head2 How do I reset an each() operation part-way through? | |
1765 | ||
5a964f20 | 1766 | Using C<keys %hash> in scalar context returns the number of keys in |
68dc0745 | 1767 | the hash I<and> resets the iterator associated with the hash. You may |
1768 | need to do this if you use C<last> to exit a loop early so that when you | |
46fc3d4c | 1769 | re-enter it, the hash iterator has been reset. |
68dc0745 | 1770 | |
1771 | =head2 How can I get the unique keys from two hashes? | |
1772 | ||
d92eb7b0 GS |
1773 | First you extract the keys from the hashes into lists, then solve |
1774 | the "removing duplicates" problem described above. For example: | |
68dc0745 | 1775 | |
1776 | %seen = (); | |
1777 | for $element (keys(%foo), keys(%bar)) { | |
1778 | $seen{$element}++; | |
1779 | } | |
1780 | @uniq = keys %seen; | |
1781 | ||
1782 | Or more succinctly: | |
1783 | ||
1784 | @uniq = keys %{{%foo,%bar}}; | |
1785 | ||
1786 | Or if you really want to save space: | |
1787 | ||
1788 | %seen = (); | |
1789 | while (defined ($key = each %foo)) { | |
1790 | $seen{$key}++; | |
1791 | } | |
1792 | while (defined ($key = each %bar)) { | |
1793 | $seen{$key}++; | |
1794 | } | |
1795 | @uniq = keys %seen; | |
1796 | ||
1797 | =head2 How can I store a multidimensional array in a DBM file? | |
1798 | ||
1799 | Either stringify the structure yourself (no fun), or else | |
1800 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer | |
1801 | it on top of either DB_File or GDBM_File. | |
1802 | ||
1803 | =head2 How can I make my hash remember the order I put elements into it? | |
1804 | ||
1805 | Use the Tie::IxHash from CPAN. | |
1806 | ||
46fc3d4c | 1807 | use Tie::IxHash; |
1808 | tie(%myhash, Tie::IxHash); | |
1809 | for ($i=0; $i<20; $i++) { | |
1810 | $myhash{$i} = 2*$i; | |
1811 | } | |
1812 | @keys = keys %myhash; | |
1813 | # @keys = (0,1,2,3,...) | |
1814 | ||
68dc0745 | 1815 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
1816 | ||
1817 | If you say something like: | |
1818 | ||
1819 | somefunc($hash{"nonesuch key here"}); | |
1820 | ||
1821 | Then that element "autovivifies"; that is, it springs into existence | |
1822 | whether you store something there or not. That's because functions | |
1823 | get scalars passed in by reference. If somefunc() modifies C<$_[0]>, | |
1824 | it has to be ready to write it back into the caller's version. | |
1825 | ||
87275199 | 1826 | This has been fixed as of Perl5.004. |
68dc0745 | 1827 | |
1828 | Normally, merely accessing a key's value for a nonexistent key does | |
1829 | I<not> cause that key to be forever there. This is different than | |
1830 | awk's behavior. | |
1831 | ||
fc36a67e | 1832 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
68dc0745 | 1833 | |
65acb1b1 TC |
1834 | Usually a hash ref, perhaps like this: |
1835 | ||
1836 | $record = { | |
1837 | NAME => "Jason", | |
1838 | EMPNO => 132, | |
1839 | TITLE => "deputy peon", | |
1840 | AGE => 23, | |
1841 | SALARY => 37_000, | |
1842 | PALS => [ "Norbert", "Rhys", "Phineas"], | |
1843 | }; | |
1844 | ||
1845 | References are documented in L<perlref> and the upcoming L<perlreftut>. | |
1846 | Examples of complex data structures are given in L<perldsc> and | |
1847 | L<perllol>. Examples of structures and object-oriented classes are | |
1848 | in L<perltoot>. | |
68dc0745 | 1849 | |
1850 | =head2 How can I use a reference as a hash key? | |
1851 | ||
f7686833 | 1852 | You can't do this directly, but you could use the standard Tie::Refhash |
87275199 | 1853 | module distributed with Perl. |
68dc0745 | 1854 | |
1855 | =head1 Data: Misc | |
1856 | ||
1857 | =head2 How do I handle binary data correctly? | |
1858 | ||
1859 | Perl is binary clean, so this shouldn't be a problem. For example, | |
1860 | this works fine (assuming the files are found): | |
1861 | ||
1862 | if (`cat /vmunix` =~ /gzip/) { | |
1863 | print "Your kernel is GNU-zip enabled!\n"; | |
1864 | } | |
1865 | ||
d92eb7b0 GS |
1866 | On less elegant (read: Byzantine) systems, however, you have |
1867 | to play tedious games with "text" versus "binary" files. See | |
1868 | L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking | |
1869 | systems are curses out of Microsoft, who seem to be committed to putting | |
1870 | the backward into backward compatibility. | |
68dc0745 | 1871 | |
1872 | If you're concerned about 8-bit ASCII data, then see L<perllocale>. | |
1873 | ||
54310121 | 1874 | If you want to deal with multibyte characters, however, there are |
68dc0745 | 1875 | some gotchas. See the section on Regular Expressions. |
1876 | ||
1877 | =head2 How do I determine whether a scalar is a number/whole/integer/float? | |
1878 | ||
1879 | Assuming that you don't care about IEEE notations like "NaN" or | |
1880 | "Infinity", you probably just want to use a regular expression. | |
1881 | ||
65acb1b1 TC |
1882 | if (/\D/) { print "has nondigits\n" } |
1883 | if (/^\d+$/) { print "is a whole number\n" } | |
1884 | if (/^-?\d+$/) { print "is an integer\n" } | |
1885 | if (/^[+-]?\d+$/) { print "is a +/- integer\n" } | |
1886 | if (/^-?\d+\.?\d*$/) { print "is a real number\n" } | |
1887 | if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" } | |
1888 | if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) | |
1889 | { print "a C float" } | |
68dc0745 | 1890 | |
5a964f20 TC |
1891 | If you're on a POSIX system, Perl's supports the C<POSIX::strtod> |
1892 | function. Its semantics are somewhat cumbersome, so here's a C<getnum> | |
1893 | wrapper function for more convenient access. This function takes | |
1894 | a string and returns the number it found, or C<undef> for input that | |
1895 | isn't a C float. The C<is_numeric> function is a front end to C<getnum> | |
1896 | if you just want to say, ``Is this a float?'' | |
1897 | ||
1898 | sub getnum { | |
1899 | use POSIX qw(strtod); | |
1900 | my $str = shift; | |
1901 | $str =~ s/^\s+//; | |
1902 | $str =~ s/\s+$//; | |
1903 | $! = 0; | |
1904 | my($num, $unparsed) = strtod($str); | |
1905 | if (($str eq '') || ($unparsed != 0) || $!) { | |
1906 | return undef; | |
1907 | } else { | |
1908 | return $num; | |
1909 | } | |
1910 | } | |
1911 | ||
072dc14b | 1912 | sub is_numeric { defined getnum($_[0]) } |
5a964f20 | 1913 | |
6cecdcac GS |
1914 | Or you could check out the String::Scanf module on CPAN instead. The |
1915 | POSIX module (part of the standard Perl distribution) provides the | |
bf4acbe4 | 1916 | C<strtod> and C<strtol> for converting strings to double and longs, |
6cecdcac | 1917 | respectively. |
68dc0745 | 1918 | |
1919 | =head2 How do I keep persistent data across program calls? | |
1920 | ||
1921 | For some specific applications, you can use one of the DBM modules. | |
65acb1b1 | 1922 | See L<AnyDBM_File>. More generically, you should consult the FreezeThaw, |
83df6a1d JH |
1923 | Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8 |
1924 | Storable is part of the standard distribution. Here's one example using | |
65acb1b1 TC |
1925 | Storable's C<store> and C<retrieve> functions: |
1926 | ||
1927 | use Storable; | |
1928 | store(\%hash, "filename"); | |
1929 | ||
1930 | # later on... | |
1931 | $href = retrieve("filename"); # by ref | |
1932 | %hash = %{ retrieve("filename") }; # direct to hash | |
68dc0745 | 1933 | |
1934 | =head2 How do I print out or copy a recursive data structure? | |
1935 | ||
65acb1b1 TC |
1936 | The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great |
1937 | for printing out data structures. The Storable module, found on CPAN, | |
1938 | provides a function called C<dclone> that recursively copies its argument. | |
1939 | ||
1940 | use Storable qw(dclone); | |
1941 | $r2 = dclone($r1); | |
68dc0745 | 1942 | |
65acb1b1 TC |
1943 | Where $r1 can be a reference to any kind of data structure you'd like. |
1944 | It will be deeply copied. Because C<dclone> takes and returns references, | |
1945 | you'd have to add extra punctuation if you had a hash of arrays that | |
1946 | you wanted to copy. | |
68dc0745 | 1947 | |
65acb1b1 | 1948 | %newhash = %{ dclone(\%oldhash) }; |
68dc0745 | 1949 | |
1950 | =head2 How do I define methods for every class/object? | |
1951 | ||
1952 | Use the UNIVERSAL class (see L<UNIVERSAL>). | |
1953 | ||
1954 | =head2 How do I verify a credit card checksum? | |
1955 | ||
1956 | Get the Business::CreditCard module from CPAN. | |
1957 | ||
65acb1b1 TC |
1958 | =head2 How do I pack arrays of doubles or floats for XS code? |
1959 | ||
1960 | The kgbpack.c code in the PGPLOT module on CPAN does just this. | |
1961 | If you're doing a lot of float or double processing, consider using | |
1962 | the PDL module from CPAN instead--it makes number-crunching easy. | |
1963 | ||
68dc0745 | 1964 | =head1 AUTHOR AND COPYRIGHT |
1965 | ||
65acb1b1 | 1966 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 TC |
1967 | All rights reserved. |
1968 | ||
5a7beb56 JH |
1969 | This documentation is free; you can redistribute it and/or modify it |
1970 | under the same terms as Perl itself. | |
5a964f20 TC |
1971 | |
1972 | Irrespective of its distribution, all code examples in this file | |
1973 | are hereby placed into the public domain. You are permitted and | |
1974 | encouraged to use this code in your own programs for fun | |
1975 | or for profit as you see fit. A simple comment in the code giving | |
1976 | credit would be courteous but is not required. |