3 perlfaq4 - Data Manipulation ($Revision: 1.61 $, $Date: 2005/03/11 16:27:53 $)
7 This section of the FAQ answers questions related to manipulating
8 numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
12 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
14 Internally, your computer represents floating-point numbers
15 in binary. Digital (as in powers of two) computers cannot
16 store all numbers exactly. Some real numbers lose precision
17 in the process. This is a problem with how computers store
18 numbers and affects all computer languages, not just Perl.
20 L<perlnumber> show the gory details of number
21 representations and conversions.
23 To limit the number of decimal places in your numbers, you
24 can use the printf or sprintf function. See the
25 L<"Floating Point Arithmetic"|perlop> for more details.
29 my $number = sprintf "%.2f", 10/3;
31 =head2 Why is int() broken?
33 Your int() is most probably working just fine. It's the numbers that
34 aren't quite what you think.
36 First, see the above item "Why am I getting long decimals
37 (eg, 19.9499999999999) instead of the numbers I should be getting
42 print int(0.6/0.2-2), "\n";
44 will in most computers print 0, not 1, because even such simple
45 numbers as 0.6 and 0.2 cannot be presented exactly by floating-point
46 numbers. What you think in the above as 'three' is really more like
47 2.9999999999999995559.
49 =head2 Why isn't my octal data interpreted correctly?
51 Perl only understands octal and hex numbers as such when they occur as
52 literals in your program. Octal literals in perl must start with a
53 leading "0" and hexadecimal literals must start with a leading "0x".
54 If they are read in from somewhere and assigned, no automatic
55 conversion takes place. You must explicitly use oct() or hex() if you
56 want the values converted to decimal. oct() interprets hex ("0x350"),
57 octal ("0350" or even without the leading "0", like "377") and binary
58 ("0b1010") numbers, while hex() only converts hexadecimal ones, with
59 or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
60 The inverse mapping from decimal to octal can be done with either the
61 "%o" or "%O" sprintf() formats.
63 This problem shows up most often when people try using chmod(), mkdir(),
64 umask(), or sysopen(), which by widespread tradition typically take
67 chmod(644, $file); # WRONG
68 chmod(0644, $file); # right
70 Note the mistake in the first line was specifying the decimal literal
71 644, rather than the intended octal literal 0644. The problem can
74 printf("%#o",644); # prints 01204
76 Surely you had not intended C<chmod(01204, $file);> - did you? If you
77 want to use numeric literals as arguments to chmod() et al. then please
78 try to express them as octal constants, that is with a leading zero and
79 with the following digits restricted to the set 0..7.
81 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
83 Remember that int() merely truncates toward 0. For rounding to a
84 certain number of digits, sprintf() or printf() is usually the easiest
87 printf("%.3f", 3.1415926535); # prints 3.142
89 The POSIX module (part of the standard Perl distribution) implements
90 ceil(), floor(), and a number of other mathematical and trigonometric
94 $ceil = ceil(3.5); # 4
95 $floor = floor(3.5); # 3
97 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
98 module. With 5.004, the Math::Trig module (part of the standard Perl
99 distribution) implements the trigonometric functions. Internally it
100 uses the Math::Complex module and some functions can break out from
101 the real axis into the complex plane, for example the inverse sine of
104 Rounding in financial applications can have serious implications, and
105 the rounding method used should be specified precisely. In these
106 cases, it probably pays not to trust whichever system rounding is
107 being used by Perl, but to instead implement the rounding function you
110 To see why, notice how you'll still have an issue on half-way-point
113 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
115 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
116 0.8 0.8 0.9 0.9 1.0 1.0
118 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
119 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
120 machines) will work pretty much like mathematical integers. Other numbers
123 =head2 How do I convert between numeric representations/bases/radixes?
125 As always with Perl there is more than one way to do it. Below
126 are a few examples of approaches to making common conversions
127 between number representations. This is intended to be representational
128 rather than exhaustive.
130 Some of the examples below use the Bit::Vector module from CPAN.
131 The reason you might choose Bit::Vector over the perl built in
132 functions is that it works with numbers of ANY size, that it is
133 optimized for speed on some operations, and for at least some
134 programmers the notation might be familiar.
138 =item How do I convert hexadecimal into decimal
140 Using perl's built in conversion of 0x notation:
144 Using the hex function:
146 $dec = hex("DEADBEEF");
150 $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
152 Using the CPAN module Bit::Vector:
155 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
156 $dec = $vec->to_Dec();
158 =item How do I convert from decimal to hexadecimal
162 $hex = sprintf("%X", 3735928559); # upper case A-F
163 $hex = sprintf("%x", 3735928559); # lower case a-f
167 $hex = unpack("H*", pack("N", 3735928559));
172 $vec = Bit::Vector->new_Dec(32, -559038737);
173 $hex = $vec->to_Hex();
175 And Bit::Vector supports odd bit counts:
178 $vec = Bit::Vector->new_Dec(33, 3735928559);
179 $vec->Resize(32); # suppress leading 0 if unwanted
180 $hex = $vec->to_Hex();
182 =item How do I convert from octal to decimal
184 Using Perl's built in conversion of numbers with leading zeros:
186 $dec = 033653337357; # note the leading 0!
188 Using the oct function:
190 $dec = oct("33653337357");
195 $vec = Bit::Vector->new(32);
196 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
197 $dec = $vec->to_Dec();
199 =item How do I convert from decimal to octal
203 $oct = sprintf("%o", 3735928559);
208 $vec = Bit::Vector->new_Dec(32, -559038737);
209 $oct = reverse join('', $vec->Chunk_List_Read(3));
211 =item How do I convert from binary to decimal
213 Perl 5.6 lets you write binary numbers directly with
216 $number = 0b10110110;
220 my $input = "10110110";
221 $decimal = oct( "0b$input" );
225 $decimal = ord(pack('B8', '10110110'));
227 Using pack and unpack for larger strings:
229 $int = unpack("N", pack("B32",
230 substr("0" x 32 . "11110101011011011111011101111", -32)));
231 $dec = sprintf("%d", $int);
233 # substr() is used to left pad a 32 character string with zeros.
237 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
238 $dec = $vec->to_Dec();
240 =item How do I convert from decimal to binary
242 Using sprintf (perl 5.6+):
244 $bin = sprintf("%b", 3735928559);
248 $bin = unpack("B*", pack("N", 3735928559));
253 $vec = Bit::Vector->new_Dec(32, -559038737);
254 $bin = $vec->to_Bin();
256 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
257 are left as an exercise to the inclined reader.
261 =head2 Why doesn't & work the way I want it to?
263 The behavior of binary arithmetic operators depends on whether they're
264 used on numbers or strings. The operators treat a string as a series
265 of bits and work with that (the string C<"3"> is the bit pattern
266 C<00110011>). The operators work with the binary form of a number
267 (the number C<3> is treated as the bit pattern C<00000011>).
269 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
270 C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
273 Most problems with C<&> and C<|> arise because the programmer thinks
274 they have a number but really it's a string. The rest arise because
277 if ("\020\020" & "\101\101") {
281 but a string consisting of two null bytes (the result of C<"\020\020"
282 & "\101\101">) is not a false value in Perl. You need:
284 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
288 =head2 How do I multiply matrices?
290 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
291 or the PDL extension (also available from CPAN).
293 =head2 How do I perform an operation on a series of integers?
295 To call a function on each element in an array, and collect the
298 @results = map { my_func($_) } @array;
302 @triple = map { 3 * $_ } @single;
304 To call a function on each element of an array, but ignore the
307 foreach $iterator (@array) {
308 some_func($iterator);
311 To call a function on each integer in a (small) range, you B<can> use:
313 @results = map { some_func($_) } (5 .. 25);
315 but you should be aware that the C<..> operator creates an array of
316 all integers in the range. This can take a lot of memory for large
320 for ($i=5; $i < 500_005; $i++) {
321 push(@results, some_func($i));
324 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
325 loop will iterate over the range, without creating the entire range.
327 for my $i (5 .. 500_005) {
328 push(@results, some_func($i));
331 will not create a list of 500,000 integers.
333 =head2 How can I output Roman numerals?
335 Get the http://www.cpan.org/modules/by-module/Roman module.
337 =head2 Why aren't my random numbers random?
339 If you're using a version of Perl before 5.004, you must call C<srand>
340 once at the start of your program to seed the random number generator.
342 BEGIN { srand() if $] < 5.004 }
344 5.004 and later automatically call C<srand> at the beginning. Don't
345 call C<srand> more than once---you make your numbers less random, rather
348 Computers are good at being predictable and bad at being random
349 (despite appearances caused by bugs in your programs :-). see the
350 F<random> article in the "Far More Than You Ever Wanted To Know"
351 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
352 Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
353 who attempts to generate random numbers by deterministic means is, of
354 course, living in a state of sin.''
356 If you want numbers that are more random than C<rand> with C<srand>
357 provides, you should also check out the Math::TrulyRandom module from
358 CPAN. It uses the imperfections in your system's timer to generate
359 random numbers, but this takes quite a while. If you want a better
360 pseudorandom generator than comes with your operating system, look at
361 ``Numerical Recipes in C'' at http://www.nr.com/ .
363 =head2 How do I get a random number between X and Y?
365 C<rand($x)> returns a number such that
366 C<< 0 <= rand($x) < $x >>. Thus what you want to have perl
367 figure out is a random number in the range from 0 to the
368 difference between your I<X> and I<Y>.
370 That is, to get a number between 10 and 15, inclusive, you
371 want a random number between 0 and 5 that you can then add
374 my $number = 10 + int rand( 15-10+1 );
376 Hence you derive the following simple function to abstract
377 that. It selects a random integer between the two given
378 integers (inclusive), For example: C<random_int_in(50,120)>.
380 sub random_int_in ($$) {
382 # Assumes that the two arguments are integers themselves!
383 return $min if $min == $max;
384 ($min, $max) = ($max, $min) if $min > $max;
385 return $min + int rand(1 + $max - $min);
390 =head2 How do I find the day or week of the year?
392 The localtime function returns the day of the year. Without an
393 argument localtime uses the current time.
395 $day_of_year = (localtime)[7];
397 The POSIX module can also format a date as the day of the year or
400 use POSIX qw/strftime/;
401 my $day_of_year = strftime "%j", localtime;
402 my $week_of_year = strftime "%W", localtime;
404 To get the day of year for any date, use the Time::Local module to get
405 a time in epoch seconds for the argument to localtime.
407 use POSIX qw/strftime/;
409 my $week_of_year = strftime "%W",
410 localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
412 The Date::Calc module provides two functions for to calculate these.
415 my $day_of_year = Day_of_Year( 1987, 12, 18 );
416 my $week_of_year = Week_of_Year( 1987, 12, 18 );
418 =head2 How do I find the current century or millennium?
420 Use the following simple functions:
423 return int((((localtime(shift || time))[5] + 1999))/100);
427 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
430 On some systems, the POSIX module's strftime() function has
431 been extended in a non-standard way to use a C<%C> format,
432 which they sometimes claim is the "century". It isn't,
433 because on most such systems, this is only the first two
434 digits of the four-digit year, and thus cannot be used to
435 reliably determine the current century or millennium.
437 =head2 How can I compare two dates and find the difference?
439 If you're storing your dates as epoch seconds then simply subtract one
440 from the other. If you've got a structured date (distinct year, day,
441 month, hour, minute, seconds values), then for reasons of accessibility,
442 simplicity, and efficiency, merely use either timelocal or timegm (from
443 the Time::Local module in the standard distribution) to reduce structured
444 dates to epoch seconds. However, if you don't know the precise format of
445 your dates, then you should probably use either of the Date::Manip and
446 Date::Calc modules from CPAN before you go hacking up your own parsing
447 routine to handle arbitrary date formats.
449 =head2 How can I take a string and turn it into epoch seconds?
451 If it's a regular enough string that it always has the same format,
452 you can split it up and pass the parts to C<timelocal> in the standard
453 Time::Local module. Otherwise, you should look into the Date::Calc
454 and Date::Manip modules from CPAN.
456 =head2 How can I find the Julian Day?
458 (contributed by brian d foy and Dave Cross)
460 You can use the Time::JulianDay module available on CPAN. Ensure that
461 you really want to find a Julian day, though, as many people have
462 different ideas about Julian days. See
463 http://www.hermetic.ch/cal_stud/jdn.htm for instance.
465 You can also try the DateTime module, which can convert a date/time
468 $ perl -MDateTime -le'print DateTime->today->jd'
471 Or the modified Julian Day
473 $ perl -MDateTime -le'print DateTime->today->mjd'
476 Or even the day of the year (which is what some people think of as a
479 $ perl -MDateTime -le'print DateTime->today->doy'
482 =head2 How do I find yesterday's date?
484 If you only need to find the date (and not the same time), you
485 can use the Date::Calc module.
487 use Date::Calc qw(Today Add_Delta_Days);
489 my @date = Add_Delta_Days( Today(), -1 );
493 Most people try to use the time rather than the calendar to
494 figure out dates, but that assumes that your days are
495 twenty-four hours each. For most people, there are two days
496 a year when they aren't: the switch to and from summer time
497 throws this off. Russ Allbery offers this solution.
500 my $now = defined $_[0] ? $_[0] : time;
501 my $then = $now - 60 * 60 * 24;
502 my $ndst = (localtime $now)[8] > 0;
503 my $tdst = (localtime $then)[8] > 0;
504 $then - ($tdst - $ndst) * 60 * 60;
507 Should give you "this time yesterday" in seconds since epoch relative to
508 the first argument or the current time if no argument is given and
509 suitable for passing to localtime or whatever else you need to do with
510 it. $ndst is whether we're currently in daylight savings time; $tdst is
511 whether the point 24 hours ago was in daylight savings time. If $tdst
512 and $ndst are the same, a boundary wasn't crossed, and the correction
513 will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
514 from yesterday's time since we gained an extra hour while going off
515 daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
516 negative hour (add an hour) to yesterday's time since we lost an hour.
518 All of this is because during those days when one switches off or onto
519 DST, a "day" isn't 24 hours long; it's either 23 or 25.
521 The explicit settings of $ndst and $tdst are necessary because localtime
522 only says it returns the system tm struct, and the system tm struct at
523 least on Solaris doesn't guarantee any particular positive value (like,
524 say, 1) for isdst, just a positive value. And that value can
525 potentially be negative, if DST information isn't available (this sub
526 just treats those cases like no DST).
528 Note that between 2am and 3am on the day after the time zone switches
529 off daylight savings time, the exact hour of "yesterday" corresponding
530 to the current hour is not clearly defined. Note also that if used
531 between 2am and 3am the day after the change to daylight savings time,
532 the result will be between 3am and 4am of the previous day; it's
533 arguable whether this is correct.
535 This sub does not attempt to deal with leap seconds (most things don't).
539 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
541 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
542 Y2K compliant (whatever that means). The programmers you've hired to
543 use it, however, probably are not.
545 Long answer: The question belies a true understanding of the issue.
546 Perl is just as Y2K compliant as your pencil--no more, and no less.
547 Can you use your pencil to write a non-Y2K-compliant memo? Of course
548 you can. Is that the pencil's fault? Of course it isn't.
550 The date and time functions supplied with Perl (gmtime and localtime)
551 supply adequate information to determine the year well beyond 2000
552 (2038 is when trouble strikes for 32-bit machines). The year returned
553 by these functions when used in a list context is the year minus 1900.
554 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
555 number. To avoid the year 2000 problem simply do not treat the year as
556 a 2-digit number. It isn't.
558 When gmtime() and localtime() are used in scalar context they return
559 a timestamp string that contains a fully-expanded year. For example,
560 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
561 2001". There's no year 2000 problem here.
563 That doesn't mean that Perl can't be used to create non-Y2K compliant
564 programs. It can. But so can your pencil. It's the fault of the user,
565 not the language. At the risk of inflaming the NRA: ``Perl doesn't
566 break Y2K, people do.'' See http://www.perl.org/about/y2k.html for
571 =head2 How do I validate input?
573 The answer to this question is usually a regular expression, perhaps
574 with auxiliary logic. See the more specific questions (numbers, mail
575 addresses, etc.) for details.
577 =head2 How do I unescape a string?
579 It depends just what you mean by ``escape''. URL escapes are dealt
580 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
581 character are removed with
585 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
587 =head2 How do I remove consecutive pairs of characters?
589 To turn C<"abbcccd"> into C<"abccd">:
591 s/(.)\1/$1/g; # add /s to include newlines
593 Here's a solution that turns "abbcccd" to "abcd":
595 y///cs; # y == tr, but shorter :-)
597 =head2 How do I expand function calls in a string?
599 This is documented in L<perlref>. In general, this is fraught with
600 quoting and readability problems, but it is possible. To interpolate
601 a subroutine call (in list context) into a string:
603 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
605 =head2 How do I find matching/nesting anything?
607 This isn't something that can be done in one regular expression, no
608 matter how complicated. To find something between two single
609 characters, a pattern like C</x([^x]*)x/> will get the intervening
610 bits in $1. For multiple ones, then something more like
611 C</alpha(.*?)omega/> would be needed. But none of these deals with
612 nested patterns. For balanced expressions using C<(>, C<{>, C<[>
613 or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
614 L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
616 If you are serious about writing a parser, there are a number of
617 modules or oddities that will make your life a lot easier. There are
618 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
619 and the byacc program. Starting from perl 5.8 the Text::Balanced
620 is part of the standard distribution.
622 One simple destructive, inside-out approach that you might try is to
623 pull out the smallest nesting parts one at a time:
625 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
626 # do something with $1
629 A more complicated and sneaky approach is to make Perl's regular
630 expression engine do it for you. This is courtesy Dean Inada, and
631 rather has the nature of an Obfuscated Perl Contest entry, but it
634 # $_ contains the string to parse
635 # BEGIN and END are the opening and closing markers for the
640 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
641 @$ = (eval{/$re/},$@!~/unmatched/i);
642 print join("\n",@$[0..$#$]) if( $$[-1] );
644 =head2 How do I reverse a string?
646 Use reverse() in scalar context, as documented in
649 $reversed = reverse $string;
651 =head2 How do I expand tabs in a string?
653 You can do it yourself:
655 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
657 Or you can just use the Text::Tabs module (part of the standard Perl
661 @expanded_lines = expand(@lines_with_tabs);
663 =head2 How do I reformat a paragraph?
665 Use Text::Wrap (part of the standard Perl distribution):
668 print wrap("\t", ' ', @paragraphs);
670 The paragraphs you give to Text::Wrap should not contain embedded
671 newlines. Text::Wrap doesn't justify the lines (flush-right).
673 Or use the CPAN module Text::Autoformat. Formatting files can be easily
674 done by making a shell alias, like so:
676 alias fmt="perl -i -MText::Autoformat -n0777 \
677 -e 'print autoformat $_, {all=>1}' $*"
679 See the documentation for Text::Autoformat to appreciate its many
682 =head2 How can I access or change N characters of a string?
684 You can access the first characters of a string with substr().
685 To get the first character, for example, start at position 0
686 and grab the string of length 1.
689 $string = "Just another Perl Hacker";
690 $first_char = substr( $string, 0, 1 ); # 'J'
692 To change part of a string, you can use the optional fourth
693 argument which is the replacement string.
695 substr( $string, 13, 4, "Perl 5.8.0" );
697 You can also use substr() as an lvalue.
699 substr( $string, 13, 4 ) = "Perl 5.8.0";
701 =head2 How do I change the Nth occurrence of something?
703 You have to keep track of N yourself. For example, let's say you want
704 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
705 C<"whosoever"> or C<"whomsoever">, case insensitively. These
706 all assume that $_ contains the string to be altered.
710 ++$count == 5 # is it the 5th?
711 ? "${2}soever" # yes, swap
712 : $1 # renege and leave it there
715 In the more general case, you can use the C</g> modifier in a C<while>
716 loop, keeping count of matches.
720 $_ = "One fish two fish red fish blue fish";
721 while (/(\w+)\s+fish\b/gi) {
722 if (++$count == $WANT) {
723 print "The third fish is a $1 one.\n";
727 That prints out: C<"The third fish is a red one."> You can also use a
728 repetition count and repeated pattern like this:
730 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
732 =head2 How can I count the number of occurrences of a substring within a string?
734 There are a number of ways, with varying efficiency. If you want a
735 count of a certain single character (X) within a string, you can use the
736 C<tr///> function like so:
738 $string = "ThisXlineXhasXsomeXx'sXinXit";
739 $count = ($string =~ tr/X//);
740 print "There are $count X characters in the string";
742 This is fine if you are just looking for a single character. However,
743 if you are trying to count multiple character substrings within a
744 larger string, C<tr///> won't work. What you can do is wrap a while()
745 loop around a global pattern match. For example, let's count negative
748 $string = "-9 55 48 -2 23 -76 4 14 -44";
749 while ($string =~ /-\d+/g) { $count++ }
750 print "There are $count negative numbers in the string";
752 Another version uses a global match in list context, then assigns the
753 result to a scalar, producing a count of the number of matches.
755 $count = () = $string =~ /-\d+/g;
757 =head2 How do I capitalize all the words on one line?
759 To make the first letter of each word upper case:
761 $line =~ s/\b(\w)/\U$1/g;
763 This has the strange effect of turning "C<don't do it>" into "C<Don'T
764 Do It>". Sometimes you might want this. Other times you might need a
765 more thorough solution (Suggested by brian d foy):
768 (^\w) #at the beginning of the line
770 (\s\w) #preceded by whitespace
773 $string =~ /([\w']+)/\u\L$1/g;
775 To make the whole line upper case:
779 To force each word to be lower case, with the first letter upper case:
781 $line =~ s/(\w+)/\u\L$1/g;
783 You can (and probably should) enable locale awareness of those
784 characters by placing a C<use locale> pragma in your program.
785 See L<perllocale> for endless details on locales.
787 This is sometimes referred to as putting something into "title
788 case", but that's not quite accurate. Consider the proper
789 capitalization of the movie I<Dr. Strangelove or: How I Learned to
790 Stop Worrying and Love the Bomb>, for example.
792 Damian Conway's L<Text::Autoformat> module provides some smart
793 case transformations:
795 use Text::Autoformat;
796 my $x = "Dr. Strangelove or: How I Learned to Stop ".
797 "Worrying and Love the Bomb";
800 for my $style (qw( sentence title highlight ))
802 print autoformat($x, { case => $style }), "\n";
805 =head2 How can I split a [character] delimited string except when inside [character]?
807 Several modules can handle this sort of pasing---Text::Balanced,
808 Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.
810 Take the example case of trying to split a string that is
811 comma-separated into its different fields. You can't use C<split(/,/)>
812 because you shouldn't split if the comma is inside quotes. For
813 example, take a data line like this:
815 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
817 Due to the restriction of the quotes, this is a fairly complex
818 problem. Thankfully, we have Jeffrey Friedl, author of
819 I<Mastering Regular Expressions>, to handle these for us. He
820 suggests (assuming your string is contained in $text):
823 push(@new, $+) while $text =~ m{
824 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
828 push(@new, undef) if substr($text,-1,1) eq ',';
830 If you want to represent quotation marks inside a
831 quotation-mark-delimited field, escape them with backslashes (eg,
834 Alternatively, the Text::ParseWords module (part of the standard Perl
835 distribution) lets you say:
837 use Text::ParseWords;
838 @new = quotewords(",", 0, $text);
840 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
842 =head2 How do I strip blank space from the beginning/end of a string?
844 Although the simplest approach would seem to be
846 $string =~ s/^\s*(.*?)\s*$/$1/;
848 not only is this unnecessarily slow and destructive, it also fails with
849 embedded newlines. It is much faster to do this operation in two steps:
854 Or more nicely written as:
861 This idiom takes advantage of the C<foreach> loop's aliasing
862 behavior to factor out common code. You can do this
863 on several strings at once, or arrays, or even the
864 values of a hash if you use a slice:
866 # trim whitespace in the scalar, the array,
867 # and all the values in the hash
868 foreach ($scalar, @array, @hash{keys %hash}) {
873 =head2 How do I pad a string with blanks or pad a number with zeroes?
875 In the following examples, C<$pad_len> is the length to which you wish
876 to pad the string, C<$text> or C<$num> contains the string to be padded,
877 and C<$pad_char> contains the padding character. You can use a single
878 character string constant instead of the C<$pad_char> variable if you
879 know what it is in advance. And in the same way you can use an integer in
880 place of C<$pad_len> if you know the pad length in advance.
882 The simplest method uses the C<sprintf> function. It can pad on the left
883 or right with blanks and on the left with zeroes and it will not
884 truncate the result. The C<pack> function can only pad strings on the
885 right with blanks and it will truncate the result to a maximum length of
888 # Left padding a string with blanks (no truncation):
889 $padded = sprintf("%${pad_len}s", $text);
890 $padded = sprintf("%*s", $pad_len, $text); # same thing
892 # Right padding a string with blanks (no truncation):
893 $padded = sprintf("%-${pad_len}s", $text);
894 $padded = sprintf("%-*s", $pad_len, $text); # same thing
896 # Left padding a number with 0 (no truncation):
897 $padded = sprintf("%0${pad_len}d", $num);
898 $padded = sprintf("%0*d", $pad_len, $num); # same thing
900 # Right padding a string with blanks using pack (will truncate):
901 $padded = pack("A$pad_len",$text);
903 If you need to pad with a character other than blank or zero you can use
904 one of the following methods. They all generate a pad string with the
905 C<x> operator and combine that with C<$text>. These methods do
906 not truncate C<$text>.
908 Left and right padding with any character, creating a new string:
910 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
911 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
913 Left and right padding with any character, modifying C<$text> directly:
915 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
916 $text .= $pad_char x ( $pad_len - length( $text ) );
918 =head2 How do I extract selected columns from a string?
920 Use substr() or unpack(), both documented in L<perlfunc>.
921 If you prefer thinking in terms of columns instead of widths,
922 you can use this kind of thing:
924 # determine the unpack format needed to split Linux ps output
925 # arguments are cut columns
926 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
932 for my $place (@positions) {
933 $template .= "A" . ($place - $lastpos) . " ";
940 =head2 How do I find the soundex value of a string?
942 (contributed by brian d foy)
944 You can use the Text::Soundex module. If you want to do fuzzy or close
945 matching, you might also try the String::Approx, and Text::Metaphone,
946 and Text::DoubleMetaphone modules.
948 =head2 How can I expand variables in text strings?
950 Let's assume that you have a string that contains placeholder
953 $text = 'this has a $foo in it and a $bar';
955 You can use a substitution with a double evaluation. The
956 first /e turns C<$1> into C<$foo>, and the second /e turns
957 C<$foo> into its value. You may want to wrap this in an
958 C<eval>: if you try to get the value of an undeclared variable
959 while running under C<use strict>, you get a fatal error.
961 eval { $text =~ s/(\$\w+)/$1/eeg };
964 It's probably better in the general case to treat those
965 variables as entries in some special hash. For example:
971 $text =~ s/\$(\w+)/$user_defs{$1}/g;
973 =head2 What's wrong with always quoting "$vars"?
975 The problem is that those double-quotes force stringification--
976 coercing numbers and references into strings--even when you
977 don't want them to be strings. Think of it this way: double-quote
978 expansion is used to produce new strings. If you already
979 have a string, why do you need more?
981 If you get used to writing odd things like these:
985 somefunc("$var"); # BAD
987 You'll be in trouble. Those should (in 99.8% of the cases) be
988 the simpler and more direct:
994 Otherwise, besides slowing you down, you're going to break code when
995 the thing in the scalar is actually neither a string nor a number, but
1001 my $oref = "$aref"; # WRONG
1004 You can also get into subtle problems on those few operations in Perl
1005 that actually do care about the difference between a string and a
1006 number, such as the magical C<++> autoincrement operator or the
1009 Stringification also destroys arrays.
1012 print "@lines"; # WRONG - extra blanks
1013 print @lines; # right
1015 =head2 Why don't my E<lt>E<lt>HERE documents work?
1017 Check for these three things:
1021 =item There must be no space after the E<lt>E<lt> part.
1023 =item There (probably) should be a semicolon at the end.
1025 =item You can't (easily) have any space in front of the tag.
1029 If you want to indent the text in the here document, you
1033 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1038 But the HERE_TARGET must still be flush against the margin.
1039 If you want that indented also, you'll have to quote
1042 ($quote = <<' FINIS') =~ s/^\s+//gm;
1043 ...we will have peace, when you and all your works have
1044 perished--and the works of your dark master to whom you
1045 would deliver us. You are a liar, Saruman, and a corrupter
1046 of men's hearts. --Theoden in /usr/src/perl/taint.c
1048 $quote =~ s/\s+--/\n--/;
1050 A nice general-purpose fixer-upper function for indented here documents
1051 follows. It expects to be called with a here document as its argument.
1052 It looks to see whether each line begins with a common substring, and
1053 if so, strips that substring off. Otherwise, it takes the amount of leading
1054 whitespace found on the first line and removes that much off each
1059 my ($white, $leader); # common whitespace and common leading string
1060 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1061 ($white, $leader) = ($2, quotemeta($1));
1063 ($white, $leader) = (/^(\s+)/, '');
1065 s/^\s*?$leader(?:$white)?//gm;
1069 This works with leading special strings, dynamically determined:
1071 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1074 @@@ SAVEI32(runlevel);
1076 @@@ while ( op = (*op->op_ppaddr)() );
1080 MAIN_INTERPRETER_LOOP
1082 Or with a fixed amount of leading whitespace, with remaining
1083 indentation correctly preserved:
1085 $poem = fix<<EVER_ON_AND_ON;
1086 Now far ahead the Road has gone,
1087 And I must follow, if I can,
1088 Pursuing it with eager feet,
1089 Until it joins some larger way
1090 Where many paths and errands meet.
1091 And whither then? I cannot say.
1092 --Bilbo in /usr/src/perl/pp_ctl.c
1097 =head2 What is the difference between a list and an array?
1099 An array has a changeable length. A list does not. An array is something
1100 you can push or pop, while a list is a set of values. Some people make
1101 the distinction that a list is a value while an array is a variable.
1102 Subroutines are passed and return lists, you put things into list
1103 context, you initialize arrays with lists, and you foreach() across
1104 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1105 in scalar context behave like the number of elements in them, subroutines
1106 access their arguments through the array C<@_>, and push/pop/shift only work
1109 As a side note, there's no such thing as a list in scalar context.
1112 $scalar = (2, 5, 7, 9);
1114 you're using the comma operator in scalar context, so it uses the scalar
1115 comma operator. There never was a list there at all! This causes the
1116 last value to be returned: 9.
1118 =head2 What is the difference between $array[1] and @array[1]?
1120 The former is a scalar value; the latter an array slice, making
1121 it a list with one (scalar) value. You should use $ when you want a
1122 scalar value (most of the time) and @ when you want a list with one
1123 scalar value in it (very, very rarely; nearly never, in fact).
1125 Sometimes it doesn't make a difference, but sometimes it does.
1126 For example, compare:
1128 $good[0] = `some program that outputs several lines`;
1132 @bad[0] = `same program that outputs several lines`;
1134 The C<use warnings> pragma and the B<-w> flag will warn you about these
1137 =head2 How can I remove duplicate elements from a list or array?
1139 There are several possible ways, depending on whether the array is
1140 ordered and whether you wish to preserve the ordering.
1146 If @in is sorted, and you want @out to be sorted:
1147 (this assumes all true values in the array)
1149 $prev = "not equal to $in[0]";
1150 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1152 This is nice in that it doesn't use much extra memory, simulating
1153 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1154 guarantees that the expression is true (so that grep picks it up)
1155 even if the $_ is 0, "", or undef.
1159 If you don't know whether @in is sorted:
1162 @out = grep(!$saw{$_}++, @in);
1166 Like (b), but @in contains only small integers:
1168 @out = grep(!$saw[$_]++, @in);
1172 A way to do (b) without any loops or greps:
1176 @out = sort keys %saw; # remove sort if undesired
1180 Like (d), but @in contains only small positive integers:
1184 @out = grep {defined} @ary;
1188 But perhaps you should have been using a hash all along, eh?
1190 =head2 How can I tell whether a certain element is contained in a list or array?
1192 Hearing the word "in" is an I<in>dication that you probably should have
1193 used a hash, not a list or array, to store your data. Hashes are
1194 designed to answer this question quickly and efficiently. Arrays aren't.
1196 That being said, there are several ways to approach this. If you
1197 are going to make this query many times over arbitrary string values,
1198 the fastest way is probably to invert the original array and maintain a
1199 hash whose keys are the first array's values.
1201 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1203 for (@blues) { $is_blue{$_} = 1 }
1205 Now you can check whether $is_blue{$some_color}. It might have been a
1206 good idea to keep the blues all in a hash in the first place.
1208 If the values are all small integers, you could use a simple indexed
1209 array. This kind of an array will take up less space:
1211 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1212 @is_tiny_prime = ();
1213 for (@primes) { $is_tiny_prime[$_] = 1 }
1214 # or simply @istiny_prime[@primes] = (1) x @primes;
1216 Now you check whether $is_tiny_prime[$some_number].
1218 If the values in question are integers instead of strings, you can save
1219 quite a lot of space by using bit strings instead:
1221 @articles = ( 1..10, 150..2000, 2017 );
1223 for (@articles) { vec($read,$_,1) = 1 }
1225 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1229 ($is_there) = grep $_ eq $whatever, @array;
1233 ($is_there) = grep /$whatever/, @array;
1235 These are slow (checks every element even if the first matches),
1236 inefficient (same reason), and potentially buggy (what if there are
1237 regex characters in $whatever?). If you're only testing once, then
1241 foreach $elt (@array) {
1242 if ($elt eq $elt_to_find) {
1247 if ($is_there) { ... }
1249 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1251 Use a hash. Here's code to do both and more. It assumes that
1252 each element is unique in a given array:
1254 @union = @intersection = @difference = ();
1256 foreach $element (@array1, @array2) { $count{$element}++ }
1257 foreach $element (keys %count) {
1258 push @union, $element;
1259 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1262 Note that this is the I<symmetric difference>, that is, all elements in
1263 either A or in B but not in both. Think of it as an xor operation.
1265 =head2 How do I test whether two arrays or hashes are equal?
1267 The following code works for single-level arrays. It uses a stringwise
1268 comparison, and does not distinguish defined versus undefined empty
1269 strings. Modify if you have other needs.
1271 $are_equal = compare_arrays(\@frogs, \@toads);
1273 sub compare_arrays {
1274 my ($first, $second) = @_;
1275 no warnings; # silence spurious -w undef complaints
1276 return 0 unless @$first == @$second;
1277 for (my $i = 0; $i < @$first; $i++) {
1278 return 0 if $first->[$i] ne $second->[$i];
1283 For multilevel structures, you may wish to use an approach more
1284 like this one. It uses the CPAN module FreezeThaw:
1286 use FreezeThaw qw(cmpStr);
1287 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1289 printf "a and b contain %s arrays\n",
1290 cmpStr(\@a, \@b) == 0
1294 This approach also works for comparing hashes. Here
1295 we'll demonstrate two different answers:
1297 use FreezeThaw qw(cmpStr cmpStrHard);
1299 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1303 printf "a and b contain %s hashes\n",
1304 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1306 printf "a and b contain %s hashes\n",
1307 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1310 The first reports that both those the hashes contain the same data,
1311 while the second reports that they do not. Which you prefer is left as
1312 an exercise to the reader.
1314 =head2 How do I find the first array element for which a condition is true?
1316 To find the first array element which satisfies a condition, you can
1317 use the first() function in the List::Util module, which comes with
1318 Perl 5.8. This example finds the first element that contains "Perl".
1320 use List::Util qw(first);
1322 my $element = first { /Perl/ } @array;
1324 If you cannot use List::Util, you can make your own loop to do the
1325 same thing. Once you find the element, you stop the loop with last.
1328 foreach my $element ( @array )
1330 if( /Perl/ ) { $found = $element; last }
1333 If you want the array index, you can iterate through the indices
1334 and check the array element at each index until you find one
1335 that satisfies the condition.
1337 my( $found, $index ) = ( undef, -1 );
1338 for( $i = 0; $i < @array; $i++ )
1340 if( $array[$i] =~ /Perl/ )
1342 $found = $array[$i];
1348 =head2 How do I handle linked lists?
1350 In general, you usually don't need a linked list in Perl, since with
1351 regular arrays, you can push and pop or shift and unshift at either end,
1352 or you can use splice to add and/or remove arbitrary number of elements at
1353 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1354 dynamic arrays. In the absence of shifts and pops, push in general
1355 needs to reallocate on the order every log(N) times, and unshift will
1356 need to copy pointers each time.
1358 If you really, really wanted, you could use structures as described in
1359 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1360 to do. For example, imagine a list node like this:
1367 You could walk the list this way:
1370 for ($node = $head; $node; $node = $node->{LINK}) {
1371 print $node->{VALUE}, " ";
1375 You could add to the list this way:
1378 $tail = append($head, 1); # grow a new head
1379 for $value ( 2 .. 10 ) {
1380 $tail = append($tail, $value);
1384 my($list, $value) = @_;
1385 my $node = { VALUE => $value };
1387 $node->{LINK} = $list->{LINK};
1388 $list->{LINK} = $node;
1390 $_[0] = $node; # replace caller's version
1395 But again, Perl's built-in are virtually always good enough.
1397 =head2 How do I handle circular lists?
1399 Circular lists could be handled in the traditional fashion with linked
1400 lists, or you could just do something like this with an array:
1402 unshift(@array, pop(@array)); # the last shall be first
1403 push(@array, shift(@array)); # and vice versa
1405 =head2 How do I shuffle an array randomly?
1407 If you either have Perl 5.8.0 or later installed, or if you have
1408 Scalar-List-Utils 1.03 or later installed, you can say:
1410 use List::Util 'shuffle';
1412 @shuffled = shuffle(@list);
1414 If not, you can use a Fisher-Yates shuffle.
1416 sub fisher_yates_shuffle {
1417 my $deck = shift; # $deck is a reference to an array
1420 my $j = int rand ($i+1);
1421 @$deck[$i,$j] = @$deck[$j,$i];
1425 # shuffle my mpeg collection
1427 my @mpeg = <audio/*/*.mp3>;
1428 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1431 Note that the above implementation shuffles an array in place,
1432 unlike the List::Util::shuffle() which takes a list and returns
1433 a new shuffled list.
1435 You've probably seen shuffling algorithms that work using splice,
1436 randomly picking another element to swap the current element with
1440 @old = 1 .. 10; # just a demo
1442 push(@new, splice(@old, rand @old, 1));
1445 This is bad because splice is already O(N), and since you do it N times,
1446 you just invented a quadratic algorithm; that is, O(N**2). This does
1447 not scale, although Perl is so efficient that you probably won't notice
1448 this until you have rather largish arrays.
1450 =head2 How do I process/modify each element of an array?
1452 Use C<for>/C<foreach>:
1455 s/foo/bar/; # change that word
1456 y/XZ/ZX/; # swap those letters
1459 Here's another; let's compute spherical volumes:
1461 for (@volumes = @radii) { # @volumes has changed parts
1463 $_ *= (4/3) * 3.14159; # this will be constant folded
1466 which can also be done with map() which is made to transform
1467 one list into another:
1469 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1471 If you want to do the same thing to modify the values of the
1472 hash, you can use the C<values> function. As of Perl 5.6
1473 the values are not copied, so if you modify $orbit (in this
1474 case), you modify the value.
1476 for $orbit ( values %orbits ) {
1477 ($orbit **= 3) *= (4/3) * 3.14159;
1480 Prior to perl 5.6 C<values> returned copies of the values,
1481 so older perl code often contains constructions such as
1482 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1483 the hash is to be modified.
1485 =head2 How do I select a random element from an array?
1487 Use the rand() function (see L<perlfunc/rand>):
1489 $index = rand @array;
1490 $element = $array[$index];
1493 my $element = $array[ rand @array ];
1495 =head2 How do I permute N elements of a list?
1497 Use the List::Permutor module on CPAN. If the list is
1498 actually an array, try the Algorithm::Permute module (also
1499 on CPAN). It's written in XS code and is very efficient.
1501 use Algorithm::Permute;
1502 my @array = 'a'..'d';
1503 my $p_iterator = Algorithm::Permute->new ( \@array );
1504 while (my @perm = $p_iterator->next) {
1505 print "next permutation: (@perm)\n";
1508 For even faster execution, you could do:
1510 use Algorithm::Permute;
1511 my @array = 'a'..'d';
1512 Algorithm::Permute::permute {
1513 print "next permutation: (@array)\n";
1516 Here's a little program that generates all permutations of
1517 all the words on each line of input. The algorithm embodied
1518 in the permute() function is discussed in Volume 4 (still
1519 unpublished) of Knuth's I<The Art of Computer Programming>
1520 and will work on any list:
1523 # Fischer-Kause ordered permutation generator
1528 while ( $code->(@_[@idx]) ) {
1530 --$p while $idx[$p-1] > $idx[$p];
1531 my $q = $p or return;
1532 push @idx, reverse splice @idx, $p;
1533 ++$q while $idx[$p-1] > $idx[$q];
1534 @idx[$p-1,$q]=@idx[$q,$p-1];
1538 permute {print"@_\n"} split;
1540 =head2 How do I sort an array by (anything)?
1542 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1544 @list = sort { $a <=> $b } @list;
1546 The default sort function is cmp, string comparison, which would
1547 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1548 the numerical comparison operator.
1550 If you have a complicated function needed to pull out the part you
1551 want to sort on, then don't do it inside the sort function. Pull it
1552 out first, because the sort BLOCK can be called many times for the
1553 same element. Here's an example of how to pull out the first word
1554 after the first number on each item, and then sort those words
1559 ($item) = /\d+\s*(\S+)/;
1560 push @idx, uc($item);
1562 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1564 which could also be written this way, using a trick
1565 that's come to be known as the Schwartzian Transform:
1567 @sorted = map { $_->[0] }
1568 sort { $a->[1] cmp $b->[1] }
1569 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1571 If you need to sort on several fields, the following paradigm is useful.
1573 @sorted = sort { field1($a) <=> field1($b) ||
1574 field2($a) cmp field2($b) ||
1575 field3($a) cmp field3($b)
1578 This can be conveniently combined with precalculation of keys as given
1581 See the F<sort> article in the "Far More Than You Ever Wanted
1582 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1583 more about this approach.
1585 See also the question below on sorting hashes.
1587 =head2 How do I manipulate arrays of bits?
1589 Use pack() and unpack(), or else vec() and the bitwise operations.
1591 For example, this sets $vec to have bit N set if $ints[N] was set:
1594 foreach(@ints) { vec($vec,$_,1) = 1 }
1596 Here's how, given a vector in $vec, you can
1597 get those bits into your @ints array:
1599 sub bitvec_to_list {
1602 # Find null-byte density then select best algorithm
1603 if ($vec =~ tr/\0// / length $vec > 0.95) {
1606 # This method is faster with mostly null-bytes
1607 while($vec =~ /[^\0]/g ) {
1608 $i = -9 + 8 * pos $vec;
1609 push @ints, $i if vec($vec, ++$i, 1);
1610 push @ints, $i if vec($vec, ++$i, 1);
1611 push @ints, $i if vec($vec, ++$i, 1);
1612 push @ints, $i if vec($vec, ++$i, 1);
1613 push @ints, $i if vec($vec, ++$i, 1);
1614 push @ints, $i if vec($vec, ++$i, 1);
1615 push @ints, $i if vec($vec, ++$i, 1);
1616 push @ints, $i if vec($vec, ++$i, 1);
1619 # This method is a fast general algorithm
1621 my $bits = unpack "b*", $vec;
1622 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1623 push @ints, pos $bits while($bits =~ /1/g);
1628 This method gets faster the more sparse the bit vector is.
1629 (Courtesy of Tim Bunce and Winfried Koenig.)
1631 You can make the while loop a lot shorter with this suggestion
1632 from Benjamin Goldberg:
1634 while($vec =~ /[^\0]+/g ) {
1635 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1638 Or use the CPAN module Bit::Vector:
1640 $vector = Bit::Vector->new($num_of_bits);
1641 $vector->Index_List_Store(@ints);
1642 @ints = $vector->Index_List_Read();
1644 Bit::Vector provides efficient methods for bit vector, sets of small integers
1647 Here's a more extensive illustration using vec():
1650 $vector = "\xff\x0f\xef\xfe";
1651 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1652 unpack("N", $vector), "\n";
1653 $is_set = vec($vector, 23, 1);
1654 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1672 my ($offset, $width, $value) = @_;
1674 vec($vector, $offset, $width) = $value;
1675 print "offset=$offset width=$width value=$value\n";
1681 my $bits = unpack("b*", $vector);
1685 print "vector length in bytes: ", length($vector), "\n";
1686 @bytes = unpack("A8" x length($vector), $bits);
1687 print "bits are: @bytes\n\n";
1690 =head2 Why does defined() return true on empty arrays and hashes?
1692 The short story is that you should probably only use defined on scalars or
1693 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1694 in the 5.004 release or later of Perl for more detail.
1696 =head1 Data: Hashes (Associative Arrays)
1698 =head2 How do I process an entire hash?
1700 Use the each() function (see L<perlfunc/each>) if you don't care
1701 whether it's sorted:
1703 while ( ($key, $value) = each %hash) {
1704 print "$key = $value\n";
1707 If you want it sorted, you'll have to use foreach() on the result of
1708 sorting the keys as shown in an earlier question.
1710 =head2 What happens if I add or remove keys from a hash while iterating over it?
1712 (contributed by brian d foy)
1714 The easy answer is "Don't do that!"
1716 If you iterate through the hash with each(), you can delete the key
1717 most recently returned without worrying about it. If you delete or add
1718 other keys, the iterator may skip or double up on them since perl
1719 may rearrange the hash table. See the
1720 entry for C<each()> in L<perlfunc>.
1722 =head2 How do I look up a hash element by value?
1724 Create a reverse hash:
1726 %by_value = reverse %by_key;
1727 $key = $by_value{$value};
1729 That's not particularly efficient. It would be more space-efficient
1732 while (($key, $value) = each %by_key) {
1733 $by_value{$value} = $key;
1736 If your hash could have repeated values, the methods above will only find
1737 one of the associated keys. This may or may not worry you. If it does
1738 worry you, you can always reverse the hash into a hash of arrays instead:
1740 while (($key, $value) = each %by_key) {
1741 push @{$key_list_by_value{$value}}, $key;
1744 =head2 How can I know how many entries are in a hash?
1746 If you mean how many keys, then all you have to do is
1747 use the keys() function in a scalar context:
1749 $num_keys = keys %hash;
1751 The keys() function also resets the iterator, which means that you may
1752 see strange results if you use this between uses of other hash operators
1755 =head2 How do I sort a hash (optionally by value instead of key)?
1757 Internally, hashes are stored in a way that prevents you from imposing
1758 an order on key-value pairs. Instead, you have to sort a list of the
1761 @keys = sort keys %hash; # sorted by key
1763 $hash{$a} cmp $hash{$b}
1764 } keys %hash; # and by value
1766 Here we'll do a reverse numeric sort by value, and if two keys are
1767 identical, sort by length of key, or if that fails, by straight ASCII
1768 comparison of the keys (well, possibly modified by your locale--see
1772 $hash{$b} <=> $hash{$a}
1774 length($b) <=> length($a)
1779 =head2 How can I always keep my hash sorted?
1781 You can look into using the DB_File module and tie() using the
1782 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1783 The Tie::IxHash module from CPAN might also be instructive.
1785 =head2 What's the difference between "delete" and "undef" with hashes?
1787 Hashes contain pairs of scalars: the first is the key, the
1788 second is the value. The key will be coerced to a string,
1789 although the value can be any kind of scalar: string,
1790 number, or reference. If a key $key is present in
1791 %hash, C<exists($hash{$key})> will return true. The value
1792 for a given key can be C<undef>, in which case
1793 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1794 will return true. This corresponds to (C<$key>, C<undef>)
1797 Pictures help... here's the %hash table:
1807 And these conditions hold
1811 defined $hash{'d'} is true
1812 defined $hash{'a'} is true
1813 exists $hash{'a'} is true (Perl5 only)
1814 grep ($_ eq 'a', keys %hash) is true
1820 your table now reads:
1831 and these conditions now hold; changes in caps:
1835 defined $hash{'d'} is true
1836 defined $hash{'a'} is FALSE
1837 exists $hash{'a'} is true (Perl5 only)
1838 grep ($_ eq 'a', keys %hash) is true
1840 Notice the last two: you have an undef value, but a defined key!
1846 your table now reads:
1855 and these conditions now hold; changes in caps:
1859 defined $hash{'d'} is true
1860 defined $hash{'a'} is false
1861 exists $hash{'a'} is FALSE (Perl5 only)
1862 grep ($_ eq 'a', keys %hash) is FALSE
1864 See, the whole entry is gone!
1866 =head2 Why don't my tied hashes make the defined/exists distinction?
1868 This depends on the tied hash's implementation of EXISTS().
1869 For example, there isn't the concept of undef with hashes
1870 that are tied to DBM* files. It also means that exists() and
1871 defined() do the same thing with a DBM* file, and what they
1872 end up doing is not what they do with ordinary hashes.
1874 =head2 How do I reset an each() operation part-way through?
1876 Using C<keys %hash> in scalar context returns the number of keys in
1877 the hash I<and> resets the iterator associated with the hash. You may
1878 need to do this if you use C<last> to exit a loop early so that when you
1879 re-enter it, the hash iterator has been reset.
1881 =head2 How can I get the unique keys from two hashes?
1883 First you extract the keys from the hashes into lists, then solve
1884 the "removing duplicates" problem described above. For example:
1887 for $element (keys(%foo), keys(%bar)) {
1894 @uniq = keys %{{%foo,%bar}};
1896 Or if you really want to save space:
1899 while (defined ($key = each %foo)) {
1902 while (defined ($key = each %bar)) {
1907 =head2 How can I store a multidimensional array in a DBM file?
1909 Either stringify the structure yourself (no fun), or else
1910 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1911 it on top of either DB_File or GDBM_File.
1913 =head2 How can I make my hash remember the order I put elements into it?
1915 Use the Tie::IxHash from CPAN.
1918 tie my %myhash, 'Tie::IxHash';
1919 for (my $i=0; $i<20; $i++) {
1922 my @keys = keys %myhash;
1923 # @keys = (0,1,2,3,...)
1925 =head2 Why does passing a subroutine an undefined element in a hash create it?
1927 If you say something like:
1929 somefunc($hash{"nonesuch key here"});
1931 Then that element "autovivifies"; that is, it springs into existence
1932 whether you store something there or not. That's because functions
1933 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1934 it has to be ready to write it back into the caller's version.
1936 This has been fixed as of Perl5.004.
1938 Normally, merely accessing a key's value for a nonexistent key does
1939 I<not> cause that key to be forever there. This is different than
1942 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1944 Usually a hash ref, perhaps like this:
1949 TITLE => "deputy peon",
1952 PALS => [ "Norbert", "Rhys", "Phineas"],
1955 References are documented in L<perlref> and the upcoming L<perlreftut>.
1956 Examples of complex data structures are given in L<perldsc> and
1957 L<perllol>. Examples of structures and object-oriented classes are
1960 =head2 How can I use a reference as a hash key?
1962 You can't do this directly, but you could use the standard Tie::RefHash
1963 module distributed with Perl.
1967 =head2 How do I handle binary data correctly?
1969 Perl is binary clean, so this shouldn't be a problem. For example,
1970 this works fine (assuming the files are found):
1972 if (`cat /vmunix` =~ /gzip/) {
1973 print "Your kernel is GNU-zip enabled!\n";
1976 On less elegant (read: Byzantine) systems, however, you have
1977 to play tedious games with "text" versus "binary" files. See
1978 L<perlfunc/"binmode"> or L<perlopentut>.
1980 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1982 If you want to deal with multibyte characters, however, there are
1983 some gotchas. See the section on Regular Expressions.
1985 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1987 Assuming that you don't care about IEEE notations like "NaN" or
1988 "Infinity", you probably just want to use a regular expression.
1990 if (/\D/) { print "has nondigits\n" }
1991 if (/^\d+$/) { print "is a whole number\n" }
1992 if (/^-?\d+$/) { print "is an integer\n" }
1993 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1994 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1995 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
1996 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1997 { print "a C float\n" }
1999 There are also some commonly used modules for the task.
2000 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2001 internal function C<looks_like_number> for determining
2002 whether a variable looks like a number. L<Data::Types>
2003 exports functions that validate data types using both the
2004 above and other regular expressions. Thirdly, there is
2005 C<Regexp::Common> which has regular expressions to match
2006 various types of numbers. Those three modules are available
2009 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2010 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
2011 wrapper function for more convenient access. This function takes
2012 a string and returns the number it found, or C<undef> for input that
2013 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
2014 if you just want to say, ``Is this a float?''
2017 use POSIX qw(strtod);
2022 my($num, $unparsed) = strtod($str);
2023 if (($str eq '') || ($unparsed != 0) || $!) {
2030 sub is_numeric { defined getnum($_[0]) }
2032 Or you could check out the L<String::Scanf> module on the CPAN
2033 instead. The POSIX module (part of the standard Perl distribution) provides
2034 the C<strtod> and C<strtol> for converting strings to double and longs,
2037 =head2 How do I keep persistent data across program calls?
2039 For some specific applications, you can use one of the DBM modules.
2040 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2041 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2042 of the standard distribution. Here's one example using Storable's C<store>
2043 and C<retrieve> functions:
2046 store(\%hash, "filename");
2049 $href = retrieve("filename"); # by ref
2050 %hash = %{ retrieve("filename") }; # direct to hash
2052 =head2 How do I print out or copy a recursive data structure?
2054 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2055 for printing out data structures. The Storable module on CPAN (or the
2056 5.8 release of Perl), provides a function called C<dclone> that recursively
2057 copies its argument.
2059 use Storable qw(dclone);
2062 Where $r1 can be a reference to any kind of data structure you'd like.
2063 It will be deeply copied. Because C<dclone> takes and returns references,
2064 you'd have to add extra punctuation if you had a hash of arrays that
2067 %newhash = %{ dclone(\%oldhash) };
2069 =head2 How do I define methods for every class/object?
2071 Use the UNIVERSAL class (see L<UNIVERSAL>).
2073 =head2 How do I verify a credit card checksum?
2075 Get the Business::CreditCard module from CPAN.
2077 =head2 How do I pack arrays of doubles or floats for XS code?
2079 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2080 If you're doing a lot of float or double processing, consider using
2081 the PDL module from CPAN instead--it makes number-crunching easy.
2083 =head1 AUTHOR AND COPYRIGHT
2085 Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
2086 other authors as noted. All rights reserved.
2088 This documentation is free; you can redistribute it and/or modify it
2089 under the same terms as Perl itself.
2091 Irrespective of its distribution, all code examples in this file
2092 are hereby placed into the public domain. You are permitted and
2093 encouraged to use this code in your own programs for fun
2094 or for profit as you see fit. A simple comment in the code giving
2095 credit would be courteous but is not required.