be defensive about setting {host,group,pass}cat (from Andy Dougherty)
[p5sagit/p5-mst-13.2.git] / pod / perlfaq4.pod
CommitLineData
68dc0745 1=head1 NAME
2
d92eb7b0 3perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
68dc0745 4
5=head1 DESCRIPTION
6
7The section of the FAQ answers question related to the manipulation
8of data as numbers, dates, strings, arrays, hashes, and miscellaneous
9data issues.
10
11=head1 Data: Numbers
12
46fc3d4c 13=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
14
5a964f20 15The infinite set that a mathematician thinks of as the real numbers can
16only be approximate on a computer, since the computer only has a finite
17number of bits to store an infinite number of, um, numbers.
18
46fc3d4c 19Internally, your computer represents floating-point numbers in binary.
92c2ed05 20Floating-point numbers read in from a file or appearing as literals
21in your program are converted from their decimal floating-point
46fc3d4c 22representation (eg, 19.95) to the internal binary representation.
23
24However, 19.95 can't be precisely represented as a binary
25floating-point number, just like 1/3 can't be exactly represented as a
26decimal floating-point number. The computer's binary representation
27of 19.95, therefore, isn't exactly 19.95.
28
29When a floating-point number gets printed, the binary floating-point
30representation is converted back to decimal. These decimal numbers
31are displayed in either the format you specify with printf(), or the
32current output format for numbers (see L<perlvar/"$#"> if you use
33print. C<$#> has a different default value in Perl5 than it did in
87275199 34Perl4. Changing C<$#> yourself is deprecated.)
46fc3d4c 35
36This affects B<all> computer languages that represent decimal
37floating-point numbers in binary, not just Perl. Perl provides
38arbitrary-precision decimal numbers with the Math::BigFloat module
39(part of the standard Perl distribution), but mathematical operations
40are consequently slower.
41
42To get rid of the superfluous digits, just use a format (eg,
43C<printf("%.2f", 19.95)>) to get the required precision.
65acb1b1 44See L<perlop/"Floating-point Arithmetic">.
46fc3d4c 45
68dc0745 46=head2 Why isn't my octal data interpreted correctly?
47
48Perl only understands octal and hex numbers as such when they occur
49as literals in your program. If they are read in from somewhere and
50assigned, no automatic conversion takes place. You must explicitly
51use oct() or hex() if you want the values converted. oct() interprets
52both hex ("0x350") numbers and octal ones ("0350" or even without the
53leading "0", like "377"), while hex() only converts hexadecimal ones,
54with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
55
56This problem shows up most often when people try using chmod(), mkdir(),
57umask(), or sysopen(), which all want permissions in octal.
58
59 chmod(644, $file); # WRONG -- perl -w catches this
60 chmod(0644, $file); # right
61
65acb1b1 62=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
68dc0745 63
92c2ed05 64Remember that int() merely truncates toward 0. For rounding to a
65certain number of digits, sprintf() or printf() is usually the easiest
66route.
67
68 printf("%.3f", 3.1415926535); # prints 3.142
68dc0745 69
87275199 70The POSIX module (part of the standard Perl distribution) implements
68dc0745 71ceil(), floor(), and a number of other mathematical and trigonometric
72functions.
73
92c2ed05 74 use POSIX;
75 $ceil = ceil(3.5); # 4
76 $floor = floor(3.5); # 3
77
46fc3d4c 78In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex
87275199 79module. With 5.004, the Math::Trig module (part of the standard Perl
46fc3d4c 80distribution) implements the trigonometric functions. Internally it
81uses the Math::Complex module and some functions can break out from
82the real axis into the complex plane, for example the inverse sine of
832.
68dc0745 84
85Rounding in financial applications can have serious implications, and
86the rounding method used should be specified precisely. In these
87cases, it probably pays not to trust whichever system rounding is
88being used by Perl, but to instead implement the rounding function you
89need yourself.
90
65acb1b1 91To see why, notice how you'll still have an issue on half-way-point
92alternation:
93
94 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
95
96 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
97 0.8 0.8 0.9 0.9 1.0 1.0
98
99Don't blame Perl. It's the same as in C. IEEE says we have to do this.
100Perl numbers whose absolute values are integers under 2**31 (on 32 bit
101machines) will work pretty much like mathematical integers. Other numbers
102are not guaranteed.
103
68dc0745 104=head2 How do I convert bits into ints?
105
92c2ed05 106To turn a string of 1s and 0s like C<10110110> into a scalar containing
d92eb7b0 107its binary value, use the pack() and unpack() functions (documented in
87275199 108L<perlfunc/"pack"> and L<perlfunc/"unpack">):
68dc0745 109
d92eb7b0 110 $decimal = unpack('c', pack('B8', '10110110'));
111
112This packs the string C<10110110> into an eight bit binary structure.
87275199 113This is then unpacked as a character, which returns its ordinal value.
d92eb7b0 114
115This does the same thing:
116
117 $decimal = ord(pack('B8', '10110110'));
68dc0745 118
119Here's an example of going the other way:
120
d92eb7b0 121 $binary_string = unpack('B*', "\x29");
68dc0745 122
65acb1b1 123=head2 Why doesn't & work the way I want it to?
124
125The behavior of binary arithmetic operators depends on whether they're
126used on numbers or strings. The operators treat a string as a series
127of bits and work with that (the string C<"3"> is the bit pattern
128C<00110011>). The operators work with the binary form of a number
129(the number C<3> is treated as the bit pattern C<00000011>).
130
131So, saying C<11 & 3> performs the "and" operation on numbers (yielding
132C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
133(yielding C<"1">).
134
135Most problems with C<&> and C<|> arise because the programmer thinks
136they have a number but really it's a string. The rest arise because
137the programmer says:
138
139 if ("\020\020" & "\101\101") {
140 # ...
141 }
142
143but a string consisting of two null bytes (the result of C<"\020\020"
144& "\101\101">) is not a false value in Perl. You need:
145
146 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
147 # ...
148 }
149
68dc0745 150=head2 How do I multiply matrices?
151
152Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
153or the PDL extension (also available from CPAN).
154
155=head2 How do I perform an operation on a series of integers?
156
157To call a function on each element in an array, and collect the
158results, use:
159
160 @results = map { my_func($_) } @array;
161
162For example:
163
164 @triple = map { 3 * $_ } @single;
165
166To call a function on each element of an array, but ignore the
167results:
168
169 foreach $iterator (@array) {
65acb1b1 170 some_func($iterator);
68dc0745 171 }
172
173To call a function on each integer in a (small) range, you B<can> use:
174
65acb1b1 175 @results = map { some_func($_) } (5 .. 25);
68dc0745 176
177but you should be aware that the C<..> operator creates an array of
178all integers in the range. This can take a lot of memory for large
179ranges. Instead use:
180
181 @results = ();
182 for ($i=5; $i < 500_005; $i++) {
65acb1b1 183 push(@results, some_func($i));
68dc0745 184 }
185
87275199 186This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
187loop will iterate over the range, without creating the entire range.
188
189 for my $i (5 .. 500_005) {
190 push(@results, some_func($i));
191 }
192
193will not create a list of 500,000 integers.
194
68dc0745 195=head2 How can I output Roman numerals?
196
197Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
198
199=head2 Why aren't my random numbers random?
200
65acb1b1 201If you're using a version of Perl before 5.004, you must call C<srand>
202once at the start of your program to seed the random number generator.
2035.004 and later automatically call C<srand> at the beginning. Don't
204call C<srand> more than once--you make your numbers less random, rather
205than more.
92c2ed05 206
65acb1b1 207Computers are good at being predictable and bad at being random
208(despite appearances caused by bugs in your programs :-).
209http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
210Phoenix, talks more about this.. John von Neumann said, ``Anyone who
211attempts to generate random numbers by deterministic means is, of
212course, living in a state of sin.''
213
214If you want numbers that are more random than C<rand> with C<srand>
215provides, you should also check out the Math::TrulyRandom module from
216CPAN. It uses the imperfections in your system's timer to generate
217random numbers, but this takes quite a while. If you want a better
92c2ed05 218pseudorandom generator than comes with your operating system, look at
65acb1b1 219``Numerical Recipes in C'' at http://www.nr.com/ .
68dc0745 220
221=head1 Data: Dates
222
223=head2 How do I find the week-of-the-year/day-of-the-year?
224
225The day of the year is in the array returned by localtime() (see
226L<perlfunc/"localtime">):
227
228 $day_of_year = (localtime(time()))[7];
229
230or more legibly (in 5.004 or higher):
231
232 use Time::localtime;
233 $day_of_year = localtime(time())->yday;
234
235You can find the week of the year by dividing this by 7:
236
237 $week_of_year = int($day_of_year / 7);
238
92c2ed05 239Of course, this believes that weeks start at zero. The Date::Calc
240module from CPAN has a lot of date calculation functions, including
5e3006a4 241day of the year, week of the year, and so on. Note that not
65acb1b1 242all businesses consider ``week 1'' to be the same; for example,
243American businesses often consider the first week with a Monday
244in it to be Work Week #1, despite ISO 8601, which considers
245WW1 to be the first week with a Thursday in it.
68dc0745 246
d92eb7b0 247=head2 How do I find the current century or millennium?
248
249Use the following simple functions:
250
251 sub get_century {
252 return int((((localtime(shift || time))[5] + 1999))/100);
253 }
254 sub get_millennium {
255 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
256 }
257
258On some systems, you'll find that the POSIX module's strftime() function
259has been extended in a non-standard way to use a C<%C> format, which they
260sometimes claim is the "century". It isn't, because on most such systems,
261this is only the first two digits of the four-digit year, and thus cannot
262be used to reliably determine the current century or millennium.
263
92c2ed05 264=head2 How can I compare two dates and find the difference?
68dc0745 265
92c2ed05 266If you're storing your dates as epoch seconds then simply subtract one
267from the other. If you've got a structured date (distinct year, day,
d92eb7b0 268month, hour, minute, seconds values), then for reasons of accessibility,
269simplicity, and efficiency, merely use either timelocal or timegm (from
270the Time::Local module in the standard distribution) to reduce structured
271dates to epoch seconds. However, if you don't know the precise format of
272your dates, then you should probably use either of the Date::Manip and
273Date::Calc modules from CPAN before you go hacking up your own parsing
274routine to handle arbitrary date formats.
68dc0745 275
276=head2 How can I take a string and turn it into epoch seconds?
277
278If it's a regular enough string that it always has the same format,
92c2ed05 279you can split it up and pass the parts to C<timelocal> in the standard
280Time::Local module. Otherwise, you should look into the Date::Calc
281and Date::Manip modules from CPAN.
68dc0745 282
283=head2 How can I find the Julian Day?
284
d92eb7b0 285You could use Date::Calc's Delta_Days function and calculate the number
286of days from there. Assuming that's what you really want, that is.
287
288Before you immerse yourself too deeply in this, be sure to verify that it
289is the I<Julian> Day you really want. Are they really just interested in
290a way of getting serial days so that they can do date arithmetic? If you
291are interested in performing date arithmetic, this can be done using
292either Date::Manip or Date::Calc, without converting to Julian Day first.
293
294There is too much confusion on this issue to cover in this FAQ, but the
295term is applied (correctly) to a calendar now supplanted by the Gregorian
296Calendar, with the Julian Calendar failing to adjust properly for leap
297years on centennial years (among other annoyances). The term is also used
298(incorrectly) to mean: [1] days in the Gregorian Calendar; and [2] days
299since a particular starting time or `epoch', usually 1970 in the Unix
300world and 1980 in the MS-DOS/Windows world. If you find that it is not
301the first meaning that you really want, then check out the Date::Manip
302and Date::Calc modules. (Thanks to David Cassell for most of this text.)
be94a901 303
d92eb7b0 304There is also an example of Julian date calculation that should help you in
305http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz
68dc0745 306
65acb1b1 307=head2 How do I find yesterday's date?
308
309The C<time()> function returns the current time in seconds since the
d92eb7b0 310epoch. Take twenty-four hours off that:
65acb1b1 311
312 $yesterday = time() - ( 24 * 60 * 60 );
313
314Then you can pass this to C<localtime()> and get the individual year,
315month, day, hour, minute, seconds values.
316
d92eb7b0 317Note very carefully that the code above assumes that your days are
318twenty-four hours each. For most people, there are two days a year
319when they aren't: the switch to and from summer time throws this off.
320A solution to this issue is offered by Russ Allbery.
321
322 sub yesterday {
323 my $now = defined $_[0] ? $_[0] : time;
324 my $then = $now - 60 * 60 * 24;
325 my $ndst = (localtime $now)[8] > 0;
326 my $tdst = (localtime $then)[8] > 0;
327 $then - ($tdst - $ndst) * 60 * 60;
328 }
329 # Should give you "this time yesterday" in seconds since epoch relative to
330 # the first argument or the current time if no argument is given and
331 # suitable for passing to localtime or whatever else you need to do with
332 # it. $ndst is whether we're currently in daylight savings time; $tdst is
333 # whether the point 24 hours ago was in daylight savings time. If $tdst
334 # and $ndst are the same, a boundary wasn't crossed, and the correction
335 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
336 # from yesterday's time since we gained an extra hour while going off
337 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
338 # negative hour (add an hour) to yesterday's time since we lost an hour.
339 #
340 # All of this is because during those days when one switches off or onto
341 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
342 #
343 # The explicit settings of $ndst and $tdst are necessary because localtime
344 # only says it returns the system tm struct, and the system tm struct at
87275199 345 # least on Solaris doesn't guarantee any particular positive value (like,
d92eb7b0 346 # say, 1) for isdst, just a positive value. And that value can
347 # potentially be negative, if DST information isn't available (this sub
348 # just treats those cases like no DST).
349 #
350 # Note that between 2am and 3am on the day after the time zone switches
351 # off daylight savings time, the exact hour of "yesterday" corresponding
352 # to the current hour is not clearly defined. Note also that if used
353 # between 2am and 3am the day after the change to daylight savings time,
354 # the result will be between 3am and 4am of the previous day; it's
355 # arguable whether this is correct.
356 #
357 # This sub does not attempt to deal with leap seconds (most things don't).
358 #
359 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
360 # This code is in the public domain
361
87275199 362=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
68dc0745 363
65acb1b1 364Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
365Y2K compliant (whatever that means). The programmers you've hired to
366use it, however, probably are not.
367
368Long answer: The question belies a true understanding of the issue.
369Perl is just as Y2K compliant as your pencil--no more, and no less.
370Can you use your pencil to write a non-Y2K-compliant memo? Of course
371you can. Is that the pencil's fault? Of course it isn't.
92c2ed05 372
87275199 373The date and time functions supplied with Perl (gmtime and localtime)
65acb1b1 374supply adequate information to determine the year well beyond 2000
375(2038 is when trouble strikes for 32-bit machines). The year returned
376by these functions when used in an array context is the year minus 1900.
377For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
378number. To avoid the year 2000 problem simply do not treat the year as
379a 2-digit number. It isn't.
68dc0745 380
5a964f20 381When gmtime() and localtime() are used in scalar context they return
68dc0745 382a timestamp string that contains a fully-expanded year. For example,
383C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
3842001". There's no year 2000 problem here.
385
5a964f20 386That doesn't mean that Perl can't be used to create non-Y2K compliant
387programs. It can. But so can your pencil. It's the fault of the user,
388not the language. At the risk of inflaming the NRA: ``Perl doesn't
389break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
390a longer exposition.
391
68dc0745 392=head1 Data: Strings
393
394=head2 How do I validate input?
395
396The answer to this question is usually a regular expression, perhaps
5a964f20 397with auxiliary logic. See the more specific questions (numbers, mail
68dc0745 398addresses, etc.) for details.
399
400=head2 How do I unescape a string?
401
92c2ed05 402It depends just what you mean by ``escape''. URL escapes are dealt
403with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
68dc0745 404character are removed with:
405
406 s/\\(.)/$1/g;
407
92c2ed05 408This won't expand C<"\n"> or C<"\t"> or any other special escapes.
68dc0745 409
410=head2 How do I remove consecutive pairs of characters?
411
92c2ed05 412To turn C<"abbcccd"> into C<"abccd">:
68dc0745 413
d92eb7b0 414 s/(.)\1/$1/g; # add /s to include newlines
415
416Here's a solution that turns "abbcccd" to "abcd":
417
418 y///cs; # y == tr, but shorter :-)
68dc0745 419
420=head2 How do I expand function calls in a string?
421
422This is documented in L<perlref>. In general, this is fraught with
423quoting and readability problems, but it is possible. To interpolate
5a964f20 424a subroutine call (in list context) into a string:
68dc0745 425
426 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
427
428If you prefer scalar context, similar chicanery is also useful for
429arbitrary expressions:
430
431 print "That yields ${\($n + 5)} widgets\n";
432
92c2ed05 433Version 5.004 of Perl had a bug that gave list context to the
434expression in C<${...}>, but this is fixed in version 5.005.
435
436See also ``How can I expand variables in text strings?'' in this
437section of the FAQ.
46fc3d4c 438
68dc0745 439=head2 How do I find matching/nesting anything?
440
92c2ed05 441This isn't something that can be done in one regular expression, no
442matter how complicated. To find something between two single
443characters, a pattern like C</x([^x]*)x/> will get the intervening
444bits in $1. For multiple ones, then something more like
445C</alpha(.*?)omega/> would be needed. But none of these deals with
446nested patterns, nor can they. For that you'll have to write a
447parser.
448
449If you are serious about writing a parser, there are a number of
450modules or oddities that will make your life a lot easier. There is
451the CPAN module Parse::RecDescent, the standard module Text::Balanced,
65acb1b1 452the byacc program, the CPAN module Parse::Yapp, and Mark-Jason
87275199 453Dominus's excellent I<py> tool at http://www.plover.com/%7Emjd/perl/py/
65acb1b1 454.
68dc0745 455
92c2ed05 456One simple destructive, inside-out approach that you might try is to
457pull out the smallest nesting parts one at a time:
5a964f20 458
d92eb7b0 459 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
5a964f20 460 # do something with $1
461 }
462
65acb1b1 463A more complicated and sneaky approach is to make Perl's regular
464expression engine do it for you. This is courtesy Dean Inada, and
465rather has the nature of an Obfuscated Perl Contest entry, but it
466really does work:
467
468 # $_ contains the string to parse
469 # BEGIN and END are the opening and closing markers for the
470 # nested text.
471
472 @( = ('(','');
473 @) = (')','');
474 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
475 @$ = (eval{/$re/},$@!~/unmatched/);
476 print join("\n",@$[0..$#$]) if( $$[-1] );
477
68dc0745 478=head2 How do I reverse a string?
479
5a964f20 480Use reverse() in scalar context, as documented in
68dc0745 481L<perlfunc/reverse>.
482
483 $reversed = reverse $string;
484
485=head2 How do I expand tabs in a string?
486
5a964f20 487You can do it yourself:
68dc0745 488
489 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
490
87275199 491Or you can just use the Text::Tabs module (part of the standard Perl
68dc0745 492distribution).
493
494 use Text::Tabs;
495 @expanded_lines = expand(@lines_with_tabs);
496
497=head2 How do I reformat a paragraph?
498
87275199 499Use Text::Wrap (part of the standard Perl distribution):
68dc0745 500
501 use Text::Wrap;
502 print wrap("\t", ' ', @paragraphs);
503
92c2ed05 504The paragraphs you give to Text::Wrap should not contain embedded
46fc3d4c 505newlines. Text::Wrap doesn't justify the lines (flush-right).
506
68dc0745 507=head2 How can I access/change the first N letters of a string?
508
509There are many ways. If you just want to grab a copy, use
92c2ed05 510substr():
68dc0745 511
512 $first_byte = substr($a, 0, 1);
513
514If you want to modify part of a string, the simplest way is often to
515use substr() as an lvalue:
516
517 substr($a, 0, 3) = "Tom";
518
92c2ed05 519Although those with a pattern matching kind of thought process will
520likely prefer:
68dc0745 521
522 $a =~ s/^.../Tom/;
523
524=head2 How do I change the Nth occurrence of something?
525
92c2ed05 526You have to keep track of N yourself. For example, let's say you want
527to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
d92eb7b0 528C<"whosoever"> or C<"whomsoever">, case insensitively. These
529all assume that $_ contains the string to be altered.
68dc0745 530
531 $count = 0;
532 s{((whom?)ever)}{
533 ++$count == 5 # is it the 5th?
534 ? "${2}soever" # yes, swap
535 : $1 # renege and leave it there
d92eb7b0 536 }ige;
68dc0745 537
5a964f20 538In the more general case, you can use the C</g> modifier in a C<while>
539loop, keeping count of matches.
540
541 $WANT = 3;
542 $count = 0;
d92eb7b0 543 $_ = "One fish two fish red fish blue fish";
5a964f20 544 while (/(\w+)\s+fish\b/gi) {
545 if (++$count == $WANT) {
546 print "The third fish is a $1 one.\n";
5a964f20 547 }
548 }
549
92c2ed05 550That prints out: C<"The third fish is a red one."> You can also use a
5a964f20 551repetition count and repeated pattern like this:
552
553 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
554
68dc0745 555=head2 How can I count the number of occurrences of a substring within a string?
556
557There are a number of ways, with varying efficiency: If you want a
558count of a certain single character (X) within a string, you can use the
559C<tr///> function like so:
560
368c9434 561 $string = "ThisXlineXhasXsomeXx'sXinXit";
68dc0745 562 $count = ($string =~ tr/X//);
d92eb7b0 563 print "There are $count X characters in the string";
68dc0745 564
565This is fine if you are just looking for a single character. However,
566if you are trying to count multiple character substrings within a
567larger string, C<tr///> won't work. What you can do is wrap a while()
568loop around a global pattern match. For example, let's count negative
569integers:
570
571 $string = "-9 55 48 -2 23 -76 4 14 -44";
572 while ($string =~ /-\d+/g) { $count++ }
573 print "There are $count negative numbers in the string";
574
575=head2 How do I capitalize all the words on one line?
576
577To make the first letter of each word upper case:
3fe9a6f1 578
68dc0745 579 $line =~ s/\b(\w)/\U$1/g;
580
46fc3d4c 581This has the strange effect of turning "C<don't do it>" into "C<Don'T
87275199 582Do It>". Sometimes you might want this, instead (Suggested by brian d.
583foy):
46fc3d4c 584
585 $string =~ s/ (
586 (^\w) #at the beginning of the line
587 | # or
588 (\s\w) #preceded by whitespace
589 )
590 /\U$1/xg;
591 $string =~ /([\w']+)/\u\L$1/g;
592
68dc0745 593To make the whole line upper case:
3fe9a6f1 594
68dc0745 595 $line = uc($line);
596
597To force each word to be lower case, with the first letter upper case:
3fe9a6f1 598
68dc0745 599 $line =~ s/(\w+)/\u\L$1/g;
600
5a964f20 601You can (and probably should) enable locale awareness of those
602characters by placing a C<use locale> pragma in your program.
92c2ed05 603See L<perllocale> for endless details on locales.
5a964f20 604
65acb1b1 605This is sometimes referred to as putting something into "title
d92eb7b0 606case", but that's not quite accurate. Consider the proper
65acb1b1 607capitalization of the movie I<Dr. Strangelove or: How I Learned to
608Stop Worrying and Love the Bomb>, for example.
609
68dc0745 610=head2 How can I split a [character] delimited string except when inside
611[character]? (Comma-separated files)
612
613Take the example case of trying to split a string that is comma-separated
614into its different fields. (We'll pretend you said comma-separated, not
615comma-delimited, which is different and almost never what you mean.) You
616can't use C<split(/,/)> because you shouldn't split if the comma is inside
617quotes. For example, take a data line like this:
618
619 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
620
621Due to the restriction of the quotes, this is a fairly complex
622problem. Thankfully, we have Jeffrey Friedl, author of a highly
623recommended book on regular expressions, to handle these for us. He
624suggests (assuming your string is contained in $text):
625
626 @new = ();
627 push(@new, $+) while $text =~ m{
628 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
629 | ([^,]+),?
630 | ,
631 }gx;
632 push(@new, undef) if substr($text,-1,1) eq ',';
633
46fc3d4c 634If you want to represent quotation marks inside a
635quotation-mark-delimited field, escape them with backslashes (eg,
2ceaccd7 636C<"like \"this\"">. Unescaping them is a task addressed earlier in
46fc3d4c 637this section.
638
87275199 639Alternatively, the Text::ParseWords module (part of the standard Perl
68dc0745 640distribution) lets you say:
641
642 use Text::ParseWords;
643 @new = quotewords(",", 0, $text);
644
65acb1b1 645There's also a Text::CSV module on CPAN.
646
68dc0745 647=head2 How do I strip blank space from the beginning/end of a string?
648
5a964f20 649Although the simplest approach would seem to be:
68dc0745 650
651 $string =~ s/^\s*(.*?)\s*$/$1/;
652
d92eb7b0 653Not only is this unnecessarily slow and destructive, it also fails with
654embedded newlines. It is much faster to do this operation in two steps:
68dc0745 655
656 $string =~ s/^\s+//;
657 $string =~ s/\s+$//;
658
659Or more nicely written as:
660
661 for ($string) {
662 s/^\s+//;
663 s/\s+$//;
664 }
665
5e3006a4 666This idiom takes advantage of the C<foreach> loop's aliasing
5a964f20 667behavior to factor out common code. You can do this
668on several strings at once, or arrays, or even the
d92eb7b0 669values of a hash if you use a slice:
5a964f20 670
671 # trim whitespace in the scalar, the array,
672 # and all the values in the hash
673 foreach ($scalar, @array, @hash{keys %hash}) {
674 s/^\s+//;
675 s/\s+$//;
676 }
677
65acb1b1 678=head2 How do I pad a string with blanks or pad a number with zeroes?
679
d92eb7b0 680(This answer contributed by Uri Guttman, with kibitzing from
681Bart Lateur.)
65acb1b1 682
683In the following examples, C<$pad_len> is the length to which you wish
d92eb7b0 684to pad the string, C<$text> or C<$num> contains the string to be padded,
685and C<$pad_char> contains the padding character. You can use a single
686character string constant instead of the C<$pad_char> variable if you
687know what it is in advance. And in the same way you can use an integer in
688place of C<$pad_len> if you know the pad length in advance.
65acb1b1 689
d92eb7b0 690The simplest method uses the C<sprintf> function. It can pad on the left
691or right with blanks and on the left with zeroes and it will not
692truncate the result. The C<pack> function can only pad strings on the
693right with blanks and it will truncate the result to a maximum length of
694C<$pad_len>.
65acb1b1 695
d92eb7b0 696 # Left padding a string with blanks (no truncation):
697 $padded = sprintf("%${pad_len}s", $text);
65acb1b1 698
d92eb7b0 699 # Right padding a string with blanks (no truncation):
700 $padded = sprintf("%-${pad_len}s", $text);
65acb1b1 701
d92eb7b0 702 # Left padding a number with 0 (no truncation):
703 $padded = sprintf("%0${pad_len}d", $num);
65acb1b1 704
d92eb7b0 705 # Right padding a string with blanks using pack (will truncate):
706 $padded = pack("A$pad_len",$text);
65acb1b1 707
d92eb7b0 708If you need to pad with a character other than blank or zero you can use
709one of the following methods. They all generate a pad string with the
710C<x> operator and combine that with C<$text>. These methods do
711not truncate C<$text>.
65acb1b1 712
d92eb7b0 713Left and right padding with any character, creating a new string:
65acb1b1 714
d92eb7b0 715 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
716 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
65acb1b1 717
d92eb7b0 718Left and right padding with any character, modifying C<$text> directly:
65acb1b1 719
d92eb7b0 720 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
721 $text .= $pad_char x ( $pad_len - length( $text ) );
65acb1b1 722
68dc0745 723=head2 How do I extract selected columns from a string?
724
725Use substr() or unpack(), both documented in L<perlfunc>.
5a964f20 726If you prefer thinking in terms of columns instead of widths,
727you can use this kind of thing:
728
729 # determine the unpack format needed to split Linux ps output
730 # arguments are cut columns
731 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
732
733 sub cut2fmt {
734 my(@positions) = @_;
735 my $template = '';
736 my $lastpos = 1;
737 for my $place (@positions) {
738 $template .= "A" . ($place - $lastpos) . " ";
739 $lastpos = $place;
740 }
741 $template .= "A*";
742 return $template;
743 }
68dc0745 744
745=head2 How do I find the soundex value of a string?
746
87275199 747Use the standard Text::Soundex module distributed with Perl.
d92eb7b0 748But before you do so, you may want to determine whether `soundex' is in
749fact what you think it is. Knuth's soundex algorithm compresses words
750into a small space, and so it does not necessarily distinguish between
751two words which you might want to appear separately. For example, the
752last names `Knuth' and `Kant' are both mapped to the soundex code K530.
753If Text::Soundex does not do what you are looking for, you might want
754to consider the String::Approx module available at CPAN.
68dc0745 755
756=head2 How can I expand variables in text strings?
757
758Let's assume that you have a string like:
759
760 $text = 'this has a $foo in it and a $bar';
5a964f20 761
762If those were both global variables, then this would
763suffice:
764
65acb1b1 765 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
68dc0745 766
5a964f20 767But since they are probably lexicals, or at least, they could
768be, you'd have to do this:
68dc0745 769
770 $text =~ s/(\$\w+)/$1/eeg;
65acb1b1 771 die if $@; # needed /ee, not /e
68dc0745 772
5a964f20 773It's probably better in the general case to treat those
774variables as entries in some special hash. For example:
775
776 %user_defs = (
777 foo => 23,
778 bar => 19,
779 );
780 $text =~ s/\$(\w+)/$user_defs{$1}/g;
68dc0745 781
92c2ed05 782See also ``How do I expand function calls in a string?'' in this section
46fc3d4c 783of the FAQ.
784
68dc0745 785=head2 What's wrong with always quoting "$vars"?
786
787The problem is that those double-quotes force stringification,
788coercing numbers and references into strings, even when you
65acb1b1 789don't want them to be. Think of it this way: double-quote
790expansion is used to produce new strings. If you already
791have a string, why do you need more?
68dc0745 792
793If you get used to writing odd things like these:
794
795 print "$var"; # BAD
796 $new = "$old"; # BAD
797 somefunc("$var"); # BAD
798
799You'll be in trouble. Those should (in 99.8% of the cases) be
800the simpler and more direct:
801
802 print $var;
803 $new = $old;
804 somefunc($var);
805
806Otherwise, besides slowing you down, you're going to break code when
807the thing in the scalar is actually neither a string nor a number, but
808a reference:
809
810 func(\@array);
811 sub func {
812 my $aref = shift;
813 my $oref = "$aref"; # WRONG
814 }
815
816You can also get into subtle problems on those few operations in Perl
817that actually do care about the difference between a string and a
818number, such as the magical C<++> autoincrement operator or the
819syscall() function.
820
5a964f20 821Stringification also destroys arrays.
822
823 @lines = `command`;
824 print "@lines"; # WRONG - extra blanks
825 print @lines; # right
826
65acb1b1 827=head2 Why don't my E<lt>E<lt>HERE documents work?
68dc0745 828
829Check for these three things:
830
831=over 4
832
833=item 1. There must be no space after the << part.
834
835=item 2. There (probably) should be a semicolon at the end.
836
837=item 3. You can't (easily) have any space in front of the tag.
838
839=back
840
5a964f20 841If you want to indent the text in the here document, you
842can do this:
843
844 # all in one
845 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
846 your text
847 goes here
848 HERE_TARGET
849
850But the HERE_TARGET must still be flush against the margin.
851If you want that indented also, you'll have to quote
852in the indentation.
853
854 ($quote = <<' FINIS') =~ s/^\s+//gm;
855 ...we will have peace, when you and all your works have
856 perished--and the works of your dark master to whom you
857 would deliver us. You are a liar, Saruman, and a corrupter
858 of men's hearts. --Theoden in /usr/src/perl/taint.c
859 FINIS
860 $quote =~ s/\s*--/\n--/;
861
862A nice general-purpose fixer-upper function for indented here documents
863follows. It expects to be called with a here document as its argument.
864It looks to see whether each line begins with a common substring, and
865if so, strips that off. Otherwise, it takes the amount of leading
866white space found on the first line and removes that much off each
867subsequent line.
868
869 sub fix {
870 local $_ = shift;
871 my ($white, $leader); # common white space and common leading string
872 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
873 ($white, $leader) = ($2, quotemeta($1));
874 } else {
875 ($white, $leader) = (/^(\s+)/, '');
876 }
877 s/^\s*?$leader(?:$white)?//gm;
878 return $_;
879 }
880
c8db1d39 881This works with leading special strings, dynamically determined:
5a964f20 882
883 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
884 @@@ int
885 @@@ runops() {
886 @@@ SAVEI32(runlevel);
887 @@@ runlevel++;
d92eb7b0 888 @@@ while ( op = (*op->op_ppaddr)() );
5a964f20 889 @@@ TAINT_NOT;
890 @@@ return 0;
891 @@@ }
892 MAIN_INTERPRETER_LOOP
893
894Or with a fixed amount of leading white space, with remaining
895indentation correctly preserved:
896
897 $poem = fix<<EVER_ON_AND_ON;
898 Now far ahead the Road has gone,
899 And I must follow, if I can,
900 Pursuing it with eager feet,
901 Until it joins some larger way
902 Where many paths and errands meet.
903 And whither then? I cannot say.
904 --Bilbo in /usr/src/perl/pp_ctl.c
905 EVER_ON_AND_ON
906
68dc0745 907=head1 Data: Arrays
908
65acb1b1 909=head2 What is the difference between a list and an array?
910
911An array has a changeable length. A list does not. An array is something
912you can push or pop, while a list is a set of values. Some people make
913the distinction that a list is a value while an array is a variable.
914Subroutines are passed and return lists, you put things into list
915context, you initialize arrays with lists, and you foreach() across
916a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
917in scalar context behave like the number of elements in them, subroutines
918access their arguments through the array C<@_>, push/pop/shift only work
919on arrays.
920
921As a side note, there's no such thing as a list in scalar context.
922When you say
923
924 $scalar = (2, 5, 7, 9);
925
d92eb7b0 926you're using the comma operator in scalar context, so it uses the scalar
927comma operator. There never was a list there at all! This causes the
928last value to be returned: 9.
65acb1b1 929
68dc0745 930=head2 What is the difference between $array[1] and @array[1]?
931
932The former is a scalar value, the latter an array slice, which makes
933it a list with one (scalar) value. You should use $ when you want a
934scalar value (most of the time) and @ when you want a list with one
935scalar value in it (very, very rarely; nearly never, in fact).
936
937Sometimes it doesn't make a difference, but sometimes it does.
938For example, compare:
939
940 $good[0] = `some program that outputs several lines`;
941
942with
943
944 @bad[0] = `same program that outputs several lines`;
945
946The B<-w> flag will warn you about these matters.
947
d92eb7b0 948=head2 How can I remove duplicate elements from a list or array?
68dc0745 949
950There are several possible ways, depending on whether the array is
951ordered and whether you wish to preserve the ordering.
952
953=over 4
954
955=item a) If @in is sorted, and you want @out to be sorted:
5a964f20 956(this assumes all true values in the array)
68dc0745 957
958 $prev = 'nonesuch';
959 @out = grep($_ ne $prev && ($prev = $_), @in);
960
c8db1d39 961This is nice in that it doesn't use much extra memory, simulating
962uniq(1)'s behavior of removing only adjacent duplicates. It's less
963nice in that it won't work with false values like undef, 0, or "";
87275199 964"0 but true" is OK, though.
68dc0745 965
966=item b) If you don't know whether @in is sorted:
967
968 undef %saw;
969 @out = grep(!$saw{$_}++, @in);
970
971=item c) Like (b), but @in contains only small integers:
972
973 @out = grep(!$saw[$_]++, @in);
974
975=item d) A way to do (b) without any loops or greps:
976
977 undef %saw;
978 @saw{@in} = ();
979 @out = sort keys %saw; # remove sort if undesired
980
981=item e) Like (d), but @in contains only small positive integers:
982
983 undef @ary;
984 @ary[@in] = @in;
87275199 985 @out = grep {defined} @ary;
68dc0745 986
987=back
988
65acb1b1 989But perhaps you should have been using a hash all along, eh?
990
5a964f20 991=head2 How can I tell whether a list or array contains a certain element?
992
993Hearing the word "in" is an I<in>dication that you probably should have
994used a hash, not a list or array, to store your data. Hashes are
995designed to answer this question quickly and efficiently. Arrays aren't.
68dc0745 996
5a964f20 997That being said, there are several ways to approach this. If you
998are going to make this query many times over arbitrary string values,
999the fastest way is probably to invert the original array and keep an
68dc0745 1000associative array lying about whose keys are the first array's values.
1001
1002 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1003 undef %is_blue;
1004 for (@blues) { $is_blue{$_} = 1 }
1005
1006Now you can check whether $is_blue{$some_color}. It might have been a
1007good idea to keep the blues all in a hash in the first place.
1008
1009If the values are all small integers, you could use a simple indexed
1010array. This kind of an array will take up less space:
1011
1012 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1013 undef @is_tiny_prime;
d92eb7b0 1014 for (@primes) { $is_tiny_prime[$_] = 1 }
1015 # or simply @istiny_prime[@primes] = (1) x @primes;
68dc0745 1016
1017Now you check whether $is_tiny_prime[$some_number].
1018
1019If the values in question are integers instead of strings, you can save
1020quite a lot of space by using bit strings instead:
1021
1022 @articles = ( 1..10, 150..2000, 2017 );
1023 undef $read;
7b8d334a 1024 for (@articles) { vec($read,$_,1) = 1 }
68dc0745 1025
1026Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1027
1028Please do not use
1029
1030 $is_there = grep $_ eq $whatever, @array;
1031
1032or worse yet
1033
1034 $is_there = grep /$whatever/, @array;
1035
1036These are slow (checks every element even if the first matches),
1037inefficient (same reason), and potentially buggy (what if there are
d92eb7b0 1038regex characters in $whatever?). If you're only testing once, then
65acb1b1 1039use:
1040
1041 $is_there = 0;
1042 foreach $elt (@array) {
1043 if ($elt eq $elt_to_find) {
1044 $is_there = 1;
1045 last;
1046 }
1047 }
1048 if ($is_there) { ... }
68dc0745 1049
1050=head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1051
1052Use a hash. Here's code to do both and more. It assumes that
1053each element is unique in a given array:
1054
1055 @union = @intersection = @difference = ();
1056 %count = ();
1057 foreach $element (@array1, @array2) { $count{$element}++ }
1058 foreach $element (keys %count) {
1059 push @union, $element;
1060 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1061 }
1062
d92eb7b0 1063Note that this is the I<symmetric difference>, that is, all elements in
1064either A or in B, but not in both. Think of it as an xor operation.
1065
65acb1b1 1066=head2 How do I test whether two arrays or hashes are equal?
1067
1068The following code works for single-level arrays. It uses a stringwise
1069comparison, and does not distinguish defined versus undefined empty
1070strings. Modify if you have other needs.
1071
1072 $are_equal = compare_arrays(\@frogs, \@toads);
1073
1074 sub compare_arrays {
1075 my ($first, $second) = @_;
1076 local $^W = 0; # silence spurious -w undef complaints
1077 return 0 unless @$first == @$second;
1078 for (my $i = 0; $i < @$first; $i++) {
1079 return 0 if $first->[$i] ne $second->[$i];
1080 }
1081 return 1;
1082 }
1083
1084For multilevel structures, you may wish to use an approach more
1085like this one. It uses the CPAN module FreezeThaw:
1086
1087 use FreezeThaw qw(cmpStr);
1088 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1089
1090 printf "a and b contain %s arrays\n",
1091 cmpStr(\@a, \@b) == 0
1092 ? "the same"
1093 : "different";
1094
1095This approach also works for comparing hashes. Here
1096we'll demonstrate two different answers:
1097
1098 use FreezeThaw qw(cmpStr cmpStrHard);
1099
1100 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1101 $a{EXTRA} = \%b;
1102 $b{EXTRA} = \%a;
1103
1104 printf "a and b contain %s hashes\n",
1105 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1106
1107 printf "a and b contain %s hashes\n",
1108 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1109
1110
1111The first reports that both those the hashes contain the same data,
1112while the second reports that they do not. Which you prefer is left as
1113an exercise to the reader.
1114
68dc0745 1115=head2 How do I find the first array element for which a condition is true?
1116
1117You can use this if you care about the index:
1118
65acb1b1 1119 for ($i= 0; $i < @array; $i++) {
68dc0745 1120 if ($array[$i] eq "Waldo") {
1121 $found_index = $i;
1122 last;
1123 }
1124 }
1125
1126Now C<$found_index> has what you want.
1127
1128=head2 How do I handle linked lists?
1129
1130In general, you usually don't need a linked list in Perl, since with
1131regular arrays, you can push and pop or shift and unshift at either end,
5a964f20 1132or you can use splice to add and/or remove arbitrary number of elements at
87275199 1133arbitrary points. Both pop and shift are both O(1) operations on Perl's
5a964f20 1134dynamic arrays. In the absence of shifts and pops, push in general
1135needs to reallocate on the order every log(N) times, and unshift will
1136need to copy pointers each time.
68dc0745 1137
1138If you really, really wanted, you could use structures as described in
1139L<perldsc> or L<perltoot> and do just what the algorithm book tells you
65acb1b1 1140to do. For example, imagine a list node like this:
1141
1142 $node = {
1143 VALUE => 42,
1144 LINK => undef,
1145 };
1146
1147You could walk the list this way:
1148
1149 print "List: ";
1150 for ($node = $head; $node; $node = $node->{LINK}) {
1151 print $node->{VALUE}, " ";
1152 }
1153 print "\n";
1154
1155You could grow the list this way:
1156
1157 my ($head, $tail);
1158 $tail = append($head, 1); # grow a new head
1159 for $value ( 2 .. 10 ) {
1160 $tail = append($tail, $value);
1161 }
1162
1163 sub append {
1164 my($list, $value) = @_;
1165 my $node = { VALUE => $value };
1166 if ($list) {
1167 $node->{LINK} = $list->{LINK};
1168 $list->{LINK} = $node;
1169 } else {
1170 $_[0] = $node; # replace caller's version
1171 }
1172 return $node;
1173 }
1174
1175But again, Perl's built-in are virtually always good enough.
68dc0745 1176
1177=head2 How do I handle circular lists?
1178
1179Circular lists could be handled in the traditional fashion with linked
1180lists, or you could just do something like this with an array:
1181
1182 unshift(@array, pop(@array)); # the last shall be first
1183 push(@array, shift(@array)); # and vice versa
1184
1185=head2 How do I shuffle an array randomly?
1186
5a964f20 1187Use this:
1188
1189 # fisher_yates_shuffle( \@array ) :
1190 # generate a random permutation of @array in place
1191 sub fisher_yates_shuffle {
1192 my $array = shift;
1193 my $i;
1194 for ($i = @$array; --$i; ) {
1195 my $j = int rand ($i+1);
1196 next if $i == $j;
1197 @$array[$i,$j] = @$array[$j,$i];
1198 }
1199 }
1200
1201 fisher_yates_shuffle( \@array ); # permutes @array in place
1202
d92eb7b0 1203You've probably seen shuffling algorithms that work using splice,
68dc0745 1204randomly picking another element to swap the current element with:
1205
1206 srand;
1207 @new = ();
1208 @old = 1 .. 10; # just a demo
1209 while (@old) {
1210 push(@new, splice(@old, rand @old, 1));
1211 }
1212
5a964f20 1213This is bad because splice is already O(N), and since you do it N times,
1214you just invented a quadratic algorithm; that is, O(N**2). This does
1215not scale, although Perl is so efficient that you probably won't notice
1216this until you have rather largish arrays.
68dc0745 1217
1218=head2 How do I process/modify each element of an array?
1219
1220Use C<for>/C<foreach>:
1221
1222 for (@lines) {
5a964f20 1223 s/foo/bar/; # change that word
1224 y/XZ/ZX/; # swap those letters
68dc0745 1225 }
1226
1227Here's another; let's compute spherical volumes:
1228
5a964f20 1229 for (@volumes = @radii) { # @volumes has changed parts
68dc0745 1230 $_ **= 3;
1231 $_ *= (4/3) * 3.14159; # this will be constant folded
1232 }
1233
5a964f20 1234If you want to do the same thing to modify the values of the hash,
1235you may not use the C<values> function, oddly enough. You need a slice:
1236
1237 for $orbit ( @orbits{keys %orbits} ) {
1238 ($orbit **= 3) *= (4/3) * 3.14159;
1239 }
1240
68dc0745 1241=head2 How do I select a random element from an array?
1242
1243Use the rand() function (see L<perlfunc/rand>):
1244
5a964f20 1245 # at the top of the program:
68dc0745 1246 srand; # not needed for 5.004 and later
5a964f20 1247
1248 # then later on
68dc0745 1249 $index = rand @array;
1250 $element = $array[$index];
1251
5a964f20 1252Make sure you I<only call srand once per program, if then>.
1253If you are calling it more than once (such as before each
1254call to rand), you're almost certainly doing something wrong.
1255
68dc0745 1256=head2 How do I permute N elements of a list?
1257
1258Here's a little program that generates all permutations
1259of all the words on each line of input. The algorithm embodied
5a964f20 1260in the permute() function should work on any list:
68dc0745 1261
1262 #!/usr/bin/perl -n
5a964f20 1263 # tsc-permute: permute each word of input
1264 permute([split], []);
1265 sub permute {
1266 my @items = @{ $_[0] };
1267 my @perms = @{ $_[1] };
1268 unless (@items) {
1269 print "@perms\n";
68dc0745 1270 } else {
5a964f20 1271 my(@newitems,@newperms,$i);
1272 foreach $i (0 .. $#items) {
1273 @newitems = @items;
1274 @newperms = @perms;
1275 unshift(@newperms, splice(@newitems, $i, 1));
1276 permute([@newitems], [@newperms]);
68dc0745 1277 }
1278 }
1279 }
1280
1281=head2 How do I sort an array by (anything)?
1282
1283Supply a comparison function to sort() (described in L<perlfunc/sort>):
1284
1285 @list = sort { $a <=> $b } @list;
1286
1287The default sort function is cmp, string comparison, which would
1288sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
1289the numerical comparison operator.
1290
1291If you have a complicated function needed to pull out the part you
1292want to sort on, then don't do it inside the sort function. Pull it
1293out first, because the sort BLOCK can be called many times for the
1294same element. Here's an example of how to pull out the first word
1295after the first number on each item, and then sort those words
1296case-insensitively.
1297
1298 @idx = ();
1299 for (@data) {
1300 ($item) = /\d+\s*(\S+)/;
1301 push @idx, uc($item);
1302 }
1303 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1304
1305Which could also be written this way, using a trick
1306that's come to be known as the Schwartzian Transform:
1307
1308 @sorted = map { $_->[0] }
1309 sort { $a->[1] cmp $b->[1] }
d92eb7b0 1310 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
68dc0745 1311
1312If you need to sort on several fields, the following paradigm is useful.
1313
1314 @sorted = sort { field1($a) <=> field1($b) ||
1315 field2($a) cmp field2($b) ||
1316 field3($a) cmp field3($b)
1317 } @data;
1318
1319This can be conveniently combined with precalculation of keys as given
1320above.
1321
1322See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1323this approach.
1324
1325See also the question below on sorting hashes.
1326
1327=head2 How do I manipulate arrays of bits?
1328
1329Use pack() and unpack(), or else vec() and the bitwise operations.
1330
1331For example, this sets $vec to have bit N set if $ints[N] was set:
1332
1333 $vec = '';
1334 foreach(@ints) { vec($vec,$_,1) = 1 }
1335
1336And here's how, given a vector in $vec, you can
1337get those bits into your @ints array:
1338
1339 sub bitvec_to_list {
1340 my $vec = shift;
1341 my @ints;
1342 # Find null-byte density then select best algorithm
1343 if ($vec =~ tr/\0// / length $vec > 0.95) {
1344 use integer;
1345 my $i;
1346 # This method is faster with mostly null-bytes
1347 while($vec =~ /[^\0]/g ) {
1348 $i = -9 + 8 * pos $vec;
1349 push @ints, $i if vec($vec, ++$i, 1);
1350 push @ints, $i if vec($vec, ++$i, 1);
1351 push @ints, $i if vec($vec, ++$i, 1);
1352 push @ints, $i if vec($vec, ++$i, 1);
1353 push @ints, $i if vec($vec, ++$i, 1);
1354 push @ints, $i if vec($vec, ++$i, 1);
1355 push @ints, $i if vec($vec, ++$i, 1);
1356 push @ints, $i if vec($vec, ++$i, 1);
1357 }
1358 } else {
1359 # This method is a fast general algorithm
1360 use integer;
1361 my $bits = unpack "b*", $vec;
1362 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1363 push @ints, pos $bits while($bits =~ /1/g);
1364 }
1365 return \@ints;
1366 }
1367
1368This method gets faster the more sparse the bit vector is.
1369(Courtesy of Tim Bunce and Winfried Koenig.)
1370
65acb1b1 1371Here's a demo on how to use vec():
1372
1373 # vec demo
1374 $vector = "\xff\x0f\xef\xfe";
1375 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1376 unpack("N", $vector), "\n";
1377 $is_set = vec($vector, 23, 1);
1378 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1379 pvec($vector);
1380
1381 set_vec(1,1,1);
1382 set_vec(3,1,1);
1383 set_vec(23,1,1);
1384
1385 set_vec(3,1,3);
1386 set_vec(3,2,3);
1387 set_vec(3,4,3);
1388 set_vec(3,4,7);
1389 set_vec(3,8,3);
1390 set_vec(3,8,7);
1391
1392 set_vec(0,32,17);
1393 set_vec(1,32,17);
1394
1395 sub set_vec {
1396 my ($offset, $width, $value) = @_;
1397 my $vector = '';
1398 vec($vector, $offset, $width) = $value;
1399 print "offset=$offset width=$width value=$value\n";
1400 pvec($vector);
1401 }
1402
1403 sub pvec {
1404 my $vector = shift;
1405 my $bits = unpack("b*", $vector);
1406 my $i = 0;
1407 my $BASE = 8;
1408
1409 print "vector length in bytes: ", length($vector), "\n";
1410 @bytes = unpack("A8" x length($vector), $bits);
1411 print "bits are: @bytes\n\n";
1412 }
1413
68dc0745 1414=head2 Why does defined() return true on empty arrays and hashes?
1415
65acb1b1 1416The short story is that you should probably only use defined on scalars or
1417functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1418in the 5.004 release or later of Perl for more detail.
68dc0745 1419
1420=head1 Data: Hashes (Associative Arrays)
1421
1422=head2 How do I process an entire hash?
1423
1424Use the each() function (see L<perlfunc/each>) if you don't care
1425whether it's sorted:
1426
5a964f20 1427 while ( ($key, $value) = each %hash) {
68dc0745 1428 print "$key = $value\n";
1429 }
1430
1431If you want it sorted, you'll have to use foreach() on the result of
1432sorting the keys as shown in an earlier question.
1433
1434=head2 What happens if I add or remove keys from a hash while iterating over it?
1435
d92eb7b0 1436Don't do that. :-)
1437
1438[lwall] In Perl 4, you were not allowed to modify a hash at all while
87275199 1439iterating over it. In Perl 5 you can delete from it, but you still
d92eb7b0 1440can't add to it, because that might cause a doubling of the hash table,
1441in which half the entries get copied up to the new top half of the
87275199 1442table, at which point you've totally bamboozled the iterator code.
d92eb7b0 1443Even if the table doesn't double, there's no telling whether your new
1444entry will be inserted before or after the current iterator position.
1445
1446Either treasure up your changes and make them after the iterator finishes,
1447or use keys to fetch all the old keys at once, and iterate over the list
1448of keys.
68dc0745 1449
1450=head2 How do I look up a hash element by value?
1451
1452Create a reverse hash:
1453
1454 %by_value = reverse %by_key;
1455 $key = $by_value{$value};
1456
1457That's not particularly efficient. It would be more space-efficient
1458to use:
1459
1460 while (($key, $value) = each %by_key) {
1461 $by_value{$value} = $key;
1462 }
1463
d92eb7b0 1464If your hash could have repeated values, the methods above will only find
1465one of the associated keys. This may or may not worry you. If it does
1466worry you, you can always reverse the hash into a hash of arrays instead:
1467
1468 while (($key, $value) = each %by_key) {
1469 push @{$key_list_by_value{$value}}, $key;
1470 }
68dc0745 1471
1472=head2 How can I know how many entries are in a hash?
1473
1474If you mean how many keys, then all you have to do is
1475take the scalar sense of the keys() function:
1476
3fe9a6f1 1477 $num_keys = scalar keys %hash;
68dc0745 1478
d92eb7b0 1479In void context, the keys() function just resets the iterator, which is
1480faster for tied hashes than would be iterating through the whole
1481hash, one key-value pair at a time.
68dc0745 1482
1483=head2 How do I sort a hash (optionally by value instead of key)?
1484
1485Internally, hashes are stored in a way that prevents you from imposing
1486an order on key-value pairs. Instead, you have to sort a list of the
1487keys or values:
1488
1489 @keys = sort keys %hash; # sorted by key
1490 @keys = sort {
1491 $hash{$a} cmp $hash{$b}
1492 } keys %hash; # and by value
1493
1494Here we'll do a reverse numeric sort by value, and if two keys are
1495identical, sort by length of key, and if that fails, by straight ASCII
1496comparison of the keys (well, possibly modified by your locale -- see
1497L<perllocale>).
1498
1499 @keys = sort {
1500 $hash{$b} <=> $hash{$a}
1501 ||
1502 length($b) <=> length($a)
1503 ||
1504 $a cmp $b
1505 } keys %hash;
1506
1507=head2 How can I always keep my hash sorted?
1508
1509You can look into using the DB_File module and tie() using the
1510$DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
5a964f20 1511The Tie::IxHash module from CPAN might also be instructive.
68dc0745 1512
1513=head2 What's the difference between "delete" and "undef" with hashes?
1514
1515Hashes are pairs of scalars: the first is the key, the second is the
1516value. The key will be coerced to a string, although the value can be
1517any kind of scalar: string, number, or reference. If a key C<$key> is
1518present in the array, C<exists($key)> will return true. The value for
1519a given key can be C<undef>, in which case C<$array{$key}> will be
1520C<undef> while C<$exists{$key}> will return true. This corresponds to
1521(C<$key>, C<undef>) being in the hash.
1522
1523Pictures help... here's the C<%ary> table:
1524
1525 keys values
1526 +------+------+
1527 | a | 3 |
1528 | x | 7 |
1529 | d | 0 |
1530 | e | 2 |
1531 +------+------+
1532
1533And these conditions hold
1534
1535 $ary{'a'} is true
1536 $ary{'d'} is false
1537 defined $ary{'d'} is true
1538 defined $ary{'a'} is true
87275199 1539 exists $ary{'a'} is true (Perl5 only)
68dc0745 1540 grep ($_ eq 'a', keys %ary) is true
1541
1542If you now say
1543
1544 undef $ary{'a'}
1545
1546your table now reads:
1547
1548
1549 keys values
1550 +------+------+
1551 | a | undef|
1552 | x | 7 |
1553 | d | 0 |
1554 | e | 2 |
1555 +------+------+
1556
1557and these conditions now hold; changes in caps:
1558
1559 $ary{'a'} is FALSE
1560 $ary{'d'} is false
1561 defined $ary{'d'} is true
1562 defined $ary{'a'} is FALSE
87275199 1563 exists $ary{'a'} is true (Perl5 only)
68dc0745 1564 grep ($_ eq 'a', keys %ary) is true
1565
1566Notice the last two: you have an undef value, but a defined key!
1567
1568Now, consider this:
1569
1570 delete $ary{'a'}
1571
1572your table now reads:
1573
1574 keys values
1575 +------+------+
1576 | x | 7 |
1577 | d | 0 |
1578 | e | 2 |
1579 +------+------+
1580
1581and these conditions now hold; changes in caps:
1582
1583 $ary{'a'} is false
1584 $ary{'d'} is false
1585 defined $ary{'d'} is true
1586 defined $ary{'a'} is false
87275199 1587 exists $ary{'a'} is FALSE (Perl5 only)
68dc0745 1588 grep ($_ eq 'a', keys %ary) is FALSE
1589
1590See, the whole entry is gone!
1591
1592=head2 Why don't my tied hashes make the defined/exists distinction?
1593
1594They may or may not implement the EXISTS() and DEFINED() methods
1595differently. For example, there isn't the concept of undef with hashes
1596that are tied to DBM* files. This means the true/false tables above
1597will give different results when used on such a hash. It also means
1598that exists and defined do the same thing with a DBM* file, and what
1599they end up doing is not what they do with ordinary hashes.
1600
1601=head2 How do I reset an each() operation part-way through?
1602
5a964f20 1603Using C<keys %hash> in scalar context returns the number of keys in
68dc0745 1604the hash I<and> resets the iterator associated with the hash. You may
1605need to do this if you use C<last> to exit a loop early so that when you
46fc3d4c 1606re-enter it, the hash iterator has been reset.
68dc0745 1607
1608=head2 How can I get the unique keys from two hashes?
1609
d92eb7b0 1610First you extract the keys from the hashes into lists, then solve
1611the "removing duplicates" problem described above. For example:
68dc0745 1612
1613 %seen = ();
1614 for $element (keys(%foo), keys(%bar)) {
1615 $seen{$element}++;
1616 }
1617 @uniq = keys %seen;
1618
1619Or more succinctly:
1620
1621 @uniq = keys %{{%foo,%bar}};
1622
1623Or if you really want to save space:
1624
1625 %seen = ();
1626 while (defined ($key = each %foo)) {
1627 $seen{$key}++;
1628 }
1629 while (defined ($key = each %bar)) {
1630 $seen{$key}++;
1631 }
1632 @uniq = keys %seen;
1633
1634=head2 How can I store a multidimensional array in a DBM file?
1635
1636Either stringify the structure yourself (no fun), or else
1637get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1638it on top of either DB_File or GDBM_File.
1639
1640=head2 How can I make my hash remember the order I put elements into it?
1641
1642Use the Tie::IxHash from CPAN.
1643
46fc3d4c 1644 use Tie::IxHash;
1645 tie(%myhash, Tie::IxHash);
1646 for ($i=0; $i<20; $i++) {
1647 $myhash{$i} = 2*$i;
1648 }
1649 @keys = keys %myhash;
1650 # @keys = (0,1,2,3,...)
1651
68dc0745 1652=head2 Why does passing a subroutine an undefined element in a hash create it?
1653
1654If you say something like:
1655
1656 somefunc($hash{"nonesuch key here"});
1657
1658Then that element "autovivifies"; that is, it springs into existence
1659whether you store something there or not. That's because functions
1660get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1661it has to be ready to write it back into the caller's version.
1662
87275199 1663This has been fixed as of Perl5.004.
68dc0745 1664
1665Normally, merely accessing a key's value for a nonexistent key does
1666I<not> cause that key to be forever there. This is different than
1667awk's behavior.
1668
fc36a67e 1669=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
68dc0745 1670
65acb1b1 1671Usually a hash ref, perhaps like this:
1672
1673 $record = {
1674 NAME => "Jason",
1675 EMPNO => 132,
1676 TITLE => "deputy peon",
1677 AGE => 23,
1678 SALARY => 37_000,
1679 PALS => [ "Norbert", "Rhys", "Phineas"],
1680 };
1681
1682References are documented in L<perlref> and the upcoming L<perlreftut>.
1683Examples of complex data structures are given in L<perldsc> and
1684L<perllol>. Examples of structures and object-oriented classes are
1685in L<perltoot>.
68dc0745 1686
1687=head2 How can I use a reference as a hash key?
1688
1689You can't do this directly, but you could use the standard Tie::Refhash
87275199 1690module distributed with Perl.
68dc0745 1691
1692=head1 Data: Misc
1693
1694=head2 How do I handle binary data correctly?
1695
1696Perl is binary clean, so this shouldn't be a problem. For example,
1697this works fine (assuming the files are found):
1698
1699 if (`cat /vmunix` =~ /gzip/) {
1700 print "Your kernel is GNU-zip enabled!\n";
1701 }
1702
d92eb7b0 1703On less elegant (read: Byzantine) systems, however, you have
1704to play tedious games with "text" versus "binary" files. See
1705L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1706systems are curses out of Microsoft, who seem to be committed to putting
1707the backward into backward compatibility.
68dc0745 1708
1709If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1710
54310121 1711If you want to deal with multibyte characters, however, there are
68dc0745 1712some gotchas. See the section on Regular Expressions.
1713
1714=head2 How do I determine whether a scalar is a number/whole/integer/float?
1715
1716Assuming that you don't care about IEEE notations like "NaN" or
1717"Infinity", you probably just want to use a regular expression.
1718
65acb1b1 1719 if (/\D/) { print "has nondigits\n" }
1720 if (/^\d+$/) { print "is a whole number\n" }
1721 if (/^-?\d+$/) { print "is an integer\n" }
1722 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1723 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1724 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1725 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1726 { print "a C float" }
68dc0745 1727
5a964f20 1728If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1729function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1730wrapper function for more convenient access. This function takes
1731a string and returns the number it found, or C<undef> for input that
1732isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1733if you just want to say, ``Is this a float?''
1734
1735 sub getnum {
1736 use POSIX qw(strtod);
1737 my $str = shift;
1738 $str =~ s/^\s+//;
1739 $str =~ s/\s+$//;
1740 $! = 0;
1741 my($num, $unparsed) = strtod($str);
1742 if (($str eq '') || ($unparsed != 0) || $!) {
1743 return undef;
1744 } else {
1745 return $num;
1746 }
1747 }
1748
072dc14b 1749 sub is_numeric { defined getnum($_[0]) }
5a964f20 1750
d92eb7b0 1751Or you could check out
1752http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
1753instead. The POSIX module (part of the standard Perl distribution)
1754provides the C<strtol> and C<strtod> for converting strings to double
68dc0745 1755and longs, respectively.
1756
1757=head2 How do I keep persistent data across program calls?
1758
1759For some specific applications, you can use one of the DBM modules.
65acb1b1 1760See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1761Storable, or Class::Eroot modules from CPAN. Here's one example using
1762Storable's C<store> and C<retrieve> functions:
1763
1764 use Storable;
1765 store(\%hash, "filename");
1766
1767 # later on...
1768 $href = retrieve("filename"); # by ref
1769 %hash = %{ retrieve("filename") }; # direct to hash
68dc0745 1770
1771=head2 How do I print out or copy a recursive data structure?
1772
65acb1b1 1773The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1774for printing out data structures. The Storable module, found on CPAN,
1775provides a function called C<dclone> that recursively copies its argument.
1776
1777 use Storable qw(dclone);
1778 $r2 = dclone($r1);
68dc0745 1779
65acb1b1 1780Where $r1 can be a reference to any kind of data structure you'd like.
1781It will be deeply copied. Because C<dclone> takes and returns references,
1782you'd have to add extra punctuation if you had a hash of arrays that
1783you wanted to copy.
68dc0745 1784
65acb1b1 1785 %newhash = %{ dclone(\%oldhash) };
68dc0745 1786
1787=head2 How do I define methods for every class/object?
1788
1789Use the UNIVERSAL class (see L<UNIVERSAL>).
1790
1791=head2 How do I verify a credit card checksum?
1792
1793Get the Business::CreditCard module from CPAN.
1794
65acb1b1 1795=head2 How do I pack arrays of doubles or floats for XS code?
1796
1797The kgbpack.c code in the PGPLOT module on CPAN does just this.
1798If you're doing a lot of float or double processing, consider using
1799the PDL module from CPAN instead--it makes number-crunching easy.
1800
68dc0745 1801=head1 AUTHOR AND COPYRIGHT
1802
65acb1b1 1803Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
5a964f20 1804All rights reserved.
1805
1806When included as part of the Standard Version of Perl, or as part of
1807its complete documentation whether printed or otherwise, this work
d92eb7b0 1808may be distributed only under the terms of Perl's Artistic License.
5a964f20 1809Any distribution of this file or derivatives thereof I<outside>
1810of that package require that special arrangements be made with
1811copyright holder.
1812
1813Irrespective of its distribution, all code examples in this file
1814are hereby placed into the public domain. You are permitted and
1815encouraged to use this code in your own programs for fun
1816or for profit as you see fit. A simple comment in the code giving
1817credit would be courteous but is not required.