3 perlfaq4 - Data Manipulation
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 in binary.
15 Digital (as in powers of two) computers cannot store all numbers
16 exactly. Some real numbers lose precision in the process. This is a
17 problem with how computers store numbers and affects all computer
18 languages, not just Perl.
20 L<perlnumber> shows the gory details of number representations and
23 To limit the number of decimal places in your numbers, you can use the
24 printf or sprintf function. See the L<"Floating Point
25 Arithmetic"|perlop> for more details.
29 my $number = sprintf "%.2f", 10/3;
31 =head2 Why is int() broken?
33 Your C<int()> is most probably working just fine. It's the numbers that
34 aren't quite what you think.
36 First, see the answer to "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 (contributed by brian d foy)
53 You're probably trying to convert a string to a number, which Perl only
54 converts as a decimal number. When Perl converts a string to a number, it
55 ignores leading spaces and zeroes, then assumes the rest of the digits
60 print $string + 0; # prints 644
62 print $string + 44; # prints 688, certainly not octal!
64 This problem usually involves one of the Perl built-ins that has the
65 same name a unix command that uses octal numbers as arguments on the
66 command line. In this example, C<chmod> on the command line knows that
67 its first argument is octal because that's what it does:
69 %prompt> chmod 644 file
71 If you want to use the same literal digits (644) in Perl, you have to tell
72 Perl to treat them as octal numbers either by prefixing the digits with
73 a C<0> or using C<oct>:
75 chmod( 0644, $file); # right, has leading zero
76 chmod( oct(644), $file ); # also correct
78 The problem comes in when you take your numbers from something that Perl
79 thinks is a string, such as a command line argument in C<@ARGV>:
81 chmod( $ARGV[0], $file); # wrong, even if "0644"
83 chmod( oct($ARGV[0]), $file ); # correct, treat string as octal
85 You can always check the value you're using by printing it in octal
86 notation to ensure it matches what you think it should be. Print it
87 in octal and decimal format:
89 printf "0%o %d", $number, $number;
91 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
93 Remember that C<int()> merely truncates toward 0. For rounding to a
94 certain number of digits, C<sprintf()> or C<printf()> is usually the
97 printf("%.3f", 3.1415926535); # prints 3.142
99 The C<POSIX> module (part of the standard Perl distribution)
100 implements C<ceil()>, C<floor()>, and a number of other mathematical
101 and trigonometric functions.
104 $ceil = ceil(3.5); # 4
105 $floor = floor(3.5); # 3
107 In 5.000 to 5.003 perls, trigonometry was done in the C<Math::Complex>
108 module. With 5.004, the C<Math::Trig> module (part of the standard Perl
109 distribution) implements the trigonometric functions. Internally it
110 uses the C<Math::Complex> module and some functions can break out from
111 the real axis into the complex plane, for example the inverse sine of
114 Rounding in financial applications can have serious implications, and
115 the rounding method used should be specified precisely. In these
116 cases, it probably pays not to trust whichever system rounding is
117 being used by Perl, but to instead implement the rounding function you
120 To see why, notice how you'll still have an issue on half-way-point
123 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
125 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
126 0.8 0.8 0.9 0.9 1.0 1.0
128 Don't blame Perl. It's the same as in C. IEEE says we have to do
129 this. Perl numbers whose absolute values are integers under 2**31 (on
130 32 bit machines) will work pretty much like mathematical integers.
131 Other numbers are not guaranteed.
133 =head2 How do I convert between numeric representations/bases/radixes?
135 As always with Perl there is more than one way to do it. Below are a
136 few examples of approaches to making common conversions between number
137 representations. This is intended to be representational rather than
140 Some of the examples later in L<perlfaq4> use the C<Bit::Vector>
141 module from CPAN. The reason you might choose C<Bit::Vector> over the
142 perl built in functions is that it works with numbers of ANY size,
143 that it is optimized for speed on some operations, and for at least
144 some programmers the notation might be familiar.
148 =item How do I convert hexadecimal into decimal
150 Using perl's built in conversion of C<0x> notation:
154 Using the C<hex> function:
156 $dec = hex("DEADBEEF");
160 $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
162 Using the CPAN module C<Bit::Vector>:
165 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
166 $dec = $vec->to_Dec();
168 =item How do I convert from decimal to hexadecimal
172 $hex = sprintf("%X", 3735928559); # upper case A-F
173 $hex = sprintf("%x", 3735928559); # lower case a-f
177 $hex = unpack("H*", pack("N", 3735928559));
179 Using C<Bit::Vector>:
182 $vec = Bit::Vector->new_Dec(32, -559038737);
183 $hex = $vec->to_Hex();
185 And C<Bit::Vector> supports odd bit counts:
188 $vec = Bit::Vector->new_Dec(33, 3735928559);
189 $vec->Resize(32); # suppress leading 0 if unwanted
190 $hex = $vec->to_Hex();
192 =item How do I convert from octal to decimal
194 Using Perl's built in conversion of numbers with leading zeros:
196 $dec = 033653337357; # note the leading 0!
198 Using the C<oct> function:
200 $dec = oct("33653337357");
202 Using C<Bit::Vector>:
205 $vec = Bit::Vector->new(32);
206 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
207 $dec = $vec->to_Dec();
209 =item How do I convert from decimal to octal
213 $oct = sprintf("%o", 3735928559);
215 Using C<Bit::Vector>:
218 $vec = Bit::Vector->new_Dec(32, -559038737);
219 $oct = reverse join('', $vec->Chunk_List_Read(3));
221 =item How do I convert from binary to decimal
223 Perl 5.6 lets you write binary numbers directly with
226 $number = 0b10110110;
230 my $input = "10110110";
231 $decimal = oct( "0b$input" );
233 Using C<pack> and C<ord>:
235 $decimal = ord(pack('B8', '10110110'));
237 Using C<pack> and C<unpack> for larger strings:
239 $int = unpack("N", pack("B32",
240 substr("0" x 32 . "11110101011011011111011101111", -32)));
241 $dec = sprintf("%d", $int);
243 # substr() is used to left pad a 32 character string with zeros.
245 Using C<Bit::Vector>:
247 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
248 $dec = $vec->to_Dec();
250 =item How do I convert from decimal to binary
252 Using C<sprintf> (perl 5.6+):
254 $bin = sprintf("%b", 3735928559);
258 $bin = unpack("B*", pack("N", 3735928559));
260 Using C<Bit::Vector>:
263 $vec = Bit::Vector->new_Dec(32, -559038737);
264 $bin = $vec->to_Bin();
266 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
267 are left as an exercise to the inclined reader.
271 =head2 Why doesn't & work the way I want it to?
273 The behavior of binary arithmetic operators depends on whether they're
274 used on numbers or strings. The operators treat a string as a series
275 of bits and work with that (the string C<"3"> is the bit pattern
276 C<00110011>). The operators work with the binary form of a number
277 (the number C<3> is treated as the bit pattern C<00000011>).
279 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
280 C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
283 Most problems with C<&> and C<|> arise because the programmer thinks
284 they have a number but really it's a string. The rest arise because
287 if ("\020\020" & "\101\101") {
291 but a string consisting of two null bytes (the result of C<"\020\020"
292 & "\101\101">) is not a false value in Perl. You need:
294 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
298 =head2 How do I multiply matrices?
300 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
301 or the PDL extension (also available from CPAN).
303 =head2 How do I perform an operation on a series of integers?
305 To call a function on each element in an array, and collect the
308 @results = map { my_func($_) } @array;
312 @triple = map { 3 * $_ } @single;
314 To call a function on each element of an array, but ignore the
317 foreach $iterator (@array) {
318 some_func($iterator);
321 To call a function on each integer in a (small) range, you B<can> use:
323 @results = map { some_func($_) } (5 .. 25);
325 but you should be aware that the C<..> operator creates an array of
326 all integers in the range. This can take a lot of memory for large
330 for ($i=5; $i < 500_005; $i++) {
331 push(@results, some_func($i));
334 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
335 loop will iterate over the range, without creating the entire range.
337 for my $i (5 .. 500_005) {
338 push(@results, some_func($i));
341 will not create a list of 500,000 integers.
343 =head2 How can I output Roman numerals?
345 Get the http://www.cpan.org/modules/by-module/Roman module.
347 =head2 Why aren't my random numbers random?
349 If you're using a version of Perl before 5.004, you must call C<srand>
350 once at the start of your program to seed the random number generator.
352 BEGIN { srand() if $] < 5.004 }
354 5.004 and later automatically call C<srand> at the beginning. Don't
355 call C<srand> more than once--you make your numbers less random,
358 Computers are good at being predictable and bad at being random
359 (despite appearances caused by bugs in your programs :-). see the
360 F<random> article in the "Far More Than You Ever Wanted To Know"
361 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy
362 of Tom Phoenix, talks more about this. John von Neumann said, "Anyone
363 who attempts to generate random numbers by deterministic means is, of
364 course, living in a state of sin."
366 If you want numbers that are more random than C<rand> with C<srand>
367 provides, you should also check out the C<Math::TrulyRandom> module from
368 CPAN. It uses the imperfections in your system's timer to generate
369 random numbers, but this takes quite a while. If you want a better
370 pseudorandom generator than comes with your operating system, look at
371 "Numerical Recipes in C" at http://www.nr.com/ .
373 =head2 How do I get a random number between X and Y?
375 To get a random number between two values, you can use the C<rand()>
376 built-in to get a random number between 0 and 1. From there, you shift
377 that into the range that you want.
379 C<rand($x)> returns a number such that C<< 0 <= rand($x) < $x >>. Thus
380 what you want to have perl figure out is a random number in the range
381 from 0 to the difference between your I<X> and I<Y>.
383 That is, to get a number between 10 and 15, inclusive, you want a
384 random number between 0 and 5 that you can then add to 10.
386 my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )
388 Hence you derive the following simple function to abstract
389 that. It selects a random integer between the two given
390 integers (inclusive), For example: C<random_int_between(50,120)>.
392 sub random_int_between {
394 # Assumes that the two arguments are integers themselves!
395 return $min if $min == $max;
396 ($min, $max) = ($max, $min) if $min > $max;
397 return $min + int rand(1 + $max - $min);
402 =head2 How do I find the day or week of the year?
404 The localtime function returns the day of the year. Without an
405 argument localtime uses the current time.
407 $day_of_year = (localtime)[7];
409 The C<POSIX> module can also format a date as the day of the year or
412 use POSIX qw/strftime/;
413 my $day_of_year = strftime "%j", localtime;
414 my $week_of_year = strftime "%W", localtime;
416 To get the day of year for any date, use C<POSIX>'s C<mktime> to get
417 a time in epoch seconds for the argument to localtime.
419 use POSIX qw/mktime strftime/;
420 my $week_of_year = strftime "%W",
421 localtime( mktime( 0, 0, 0, 18, 11, 87 ) );
423 The C<Date::Calc> module provides two functions to calculate these.
426 my $day_of_year = Day_of_Year( 1987, 12, 18 );
427 my $week_of_year = Week_of_Year( 1987, 12, 18 );
429 =head2 How do I find the current century or millennium?
431 Use the following simple functions:
434 return int((((localtime(shift || time))[5] + 1999))/100);
438 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
441 On some systems, the C<POSIX> module's C<strftime()> function has been
442 extended in a non-standard way to use a C<%C> format, which they
443 sometimes claim is the "century". It isn't, because on most such
444 systems, this is only the first two digits of the four-digit year, and
445 thus cannot be used to reliably determine the current century or
448 =head2 How can I compare two dates and find the difference?
450 (contributed by brian d foy)
452 You could just store all your dates as a number and then subtract.
453 Life isn't always that simple though. If you want to work with
454 formatted dates, the C<Date::Manip>, C<Date::Calc>, or C<DateTime>
455 modules can help you.
457 =head2 How can I take a string and turn it into epoch seconds?
459 If it's a regular enough string that it always has the same format,
460 you can split it up and pass the parts to C<timelocal> in the standard
461 C<Time::Local> module. Otherwise, you should look into the C<Date::Calc>
462 and C<Date::Manip> modules from CPAN.
464 =head2 How can I find the Julian Day?
466 (contributed by brian d foy and Dave Cross)
468 You can use the C<Time::JulianDay> module available on CPAN. Ensure
469 that you really want to find a Julian day, though, as many people have
470 different ideas about Julian days. See
471 http://www.hermetic.ch/cal_stud/jdn.htm for instance.
473 You can also try the C<DateTime> module, which can convert a date/time
476 $ perl -MDateTime -le'print DateTime->today->jd'
479 Or the modified Julian Day
481 $ perl -MDateTime -le'print DateTime->today->mjd'
484 Or even the day of the year (which is what some people think of as a
487 $ perl -MDateTime -le'print DateTime->today->doy'
490 =head2 How do I find yesterday's date?
491 X<date> X<yesterday> X<DateTime> X<Date::Calc> X<Time::Local>
492 X<daylight saving time> X<day> X<Today_and_Now> X<localtime>
495 (contributed by brian d foy)
497 Use one of the Date modules. The C<DateTime> module makes it simple, and
498 give you the same time of day, only the day before.
502 my $yesterday = DateTime->now->subtract( days => 1 );
504 print "Yesterday was $yesterday\n";
506 You can also use the C<Date::Calc> module using its C<Today_and_Now>
509 use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
511 my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
513 print "@date_time\n";
515 Most people try to use the time rather than the calendar to figure out
516 dates, but that assumes that days are twenty-four hours each. For
517 most people, there are two days a year when they aren't: the switch to
518 and from summer time throws this off. Let the modules do the work.
520 If you absolutely must do it yourself (or can't use one of the
521 modules), here's a solution using C<Time::Local>, which comes with
524 # contributed by Gunnar Hjalmarsson
526 my $today = timelocal 0, 0, 12, ( localtime )[3..5];
527 my ($d, $m, $y) = ( localtime $today-86400 )[3..5];
528 printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;
530 In this case, you measure the day starting at noon, and subtract 24
531 hours. Even if the length of the calendar day is 23 or 25 hours,
532 you'll still end up on the previous calendar day, although not at
533 noon. Since you don't care about the time, the one hour difference
534 doesn't matter and you end up with the previous date.
536 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
538 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
539 Y2K compliant (whatever that means). The programmers you've hired to
540 use it, however, probably are not.
542 Long answer: The question belies a true understanding of the issue.
543 Perl is just as Y2K compliant as your pencil--no more, and no less.
544 Can you use your pencil to write a non-Y2K-compliant memo? Of course
545 you can. Is that the pencil's fault? Of course it isn't.
547 The date and time functions supplied with Perl (gmtime and localtime)
548 supply adequate information to determine the year well beyond 2000
549 (2038 is when trouble strikes for 32-bit machines). The year returned
550 by these functions when used in a list context is the year minus 1900.
551 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
552 number. To avoid the year 2000 problem simply do not treat the year as
553 a 2-digit number. It isn't.
555 When gmtime() and localtime() are used in scalar context they return
556 a timestamp string that contains a fully-expanded year. For example,
557 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
558 2001". There's no year 2000 problem here.
560 That doesn't mean that Perl can't be used to create non-Y2K compliant
561 programs. It can. But so can your pencil. It's the fault of the user,
562 not the language. At the risk of inflaming the NRA: "Perl doesn't
563 break Y2K, people do." See http://www.perl.org/about/y2k.html for
568 =head2 How do I validate input?
570 (contributed by brian d foy)
572 There are many ways to ensure that values are what you expect or
573 want to accept. Besides the specific examples that we cover in the
574 perlfaq, you can also look at the modules with "Assert" and "Validate"
575 in their names, along with other modules such as C<Regexp::Common>.
577 Some modules have validation for particular types of input, such
578 as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>,
579 and C<Data::Validate::IP>.
581 =head2 How do I unescape a string?
583 It depends just what you mean by "escape". URL escapes are dealt
584 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
585 character are removed with
589 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
591 =head2 How do I remove consecutive pairs of characters?
593 (contributed by brian d foy)
595 You can use the substitution operator to find pairs of characters (or
596 runs of characters) and replace them with a single instance. In this
597 substitution, we find a character in C<(.)>. The memory parentheses
598 store the matched character in the back-reference C<\1> and we use
599 that to require that the same thing immediately follow it. We replace
600 that part of the string with the character in C<$1>.
604 We can also use the transliteration operator, C<tr///>. In this
605 example, the search list side of our C<tr///> contains nothing, but
606 the C<c> option complements that so it contains everything. The
607 replacement list also contains nothing, so the transliteration is
608 almost a no-op since it won't do any replacements (or more exactly,
609 replace the character with itself). However, the C<s> option squashes
610 duplicated and consecutive characters in the string so a character
611 does not show up next to itself
613 my $str = 'Haarlem'; # in the Netherlands
614 $str =~ tr///cs; # Now Harlem, like in New York
616 =head2 How do I expand function calls in a string?
618 (contributed by brian d foy)
620 This is documented in L<perlref>, and although it's not the easiest
621 thing to read, it does work. In each of these examples, we call the
622 function inside the braces used to dereference a reference. If we
623 have more than one return value, we can construct and dereference an
624 anonymous array. In this case, we call the function in list context.
626 print "The time values are @{ [localtime] }.\n";
628 If we want to call the function in scalar context, we have to do a bit
629 more work. We can really have any code we like inside the braces, so
630 we simply have to end with the scalar reference, although how you do
631 that is up to you, and you can use code inside the braces. Note that
632 the use of parens creates a list context, so we need C<scalar> to
633 force the scalar context on the function:
635 print "The time is ${\(scalar localtime)}.\n"
637 print "The time is ${ my $x = localtime; \$x }.\n";
639 If your function already returns a reference, you don't need to create
640 the reference yourself.
642 sub timestamp { my $t = localtime; \$t }
644 print "The time is ${ timestamp() }.\n";
646 The C<Interpolation> module can also do a lot of magic for you. You can
647 specify a variable name, in this case C<E>, to set up a tied hash that
648 does the interpolation for you. It has several other methods to do this
651 use Interpolation E => 'eval';
652 print "The time values are $E{localtime()}.\n";
654 In most cases, it is probably easier to simply use string concatenation,
655 which also forces scalar context.
657 print "The time is " . localtime() . ".\n";
659 =head2 How do I find matching/nesting anything?
661 This isn't something that can be done in one regular expression, no
662 matter how complicated. To find something between two single
663 characters, a pattern like C</x([^x]*)x/> will get the intervening
664 bits in $1. For multiple ones, then something more like
665 C</alpha(.*?)omega/> would be needed. But none of these deals with
666 nested patterns. For balanced expressions using C<(>, C<{>, C<[> or
667 C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
668 L<perlre/(??{ code })>. For other cases, you'll have to write a
671 If you are serious about writing a parser, there are a number of
672 modules or oddities that will make your life a lot easier. There are
673 the CPAN modules C<Parse::RecDescent>, C<Parse::Yapp>, and
674 C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8
675 the C<Text::Balanced> is part of the standard distribution.
677 One simple destructive, inside-out approach that you might try is to
678 pull out the smallest nesting parts one at a time:
680 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
681 # do something with $1
684 A more complicated and sneaky approach is to make Perl's regular
685 expression engine do it for you. This is courtesy Dean Inada, and
686 rather has the nature of an Obfuscated Perl Contest entry, but it
689 # $_ contains the string to parse
690 # BEGIN and END are the opening and closing markers for the
695 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
696 @$ = (eval{/$re/},$@!~/unmatched/i);
697 print join("\n",@$[0..$#$]) if( $$[-1] );
699 =head2 How do I reverse a string?
701 Use C<reverse()> in scalar context, as documented in
704 $reversed = reverse $string;
706 =head2 How do I expand tabs in a string?
708 You can do it yourself:
710 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
712 Or you can just use the C<Text::Tabs> module (part of the standard Perl
716 @expanded_lines = expand(@lines_with_tabs);
718 =head2 How do I reformat a paragraph?
720 Use C<Text::Wrap> (part of the standard Perl distribution):
723 print wrap("\t", ' ', @paragraphs);
725 The paragraphs you give to C<Text::Wrap> should not contain embedded
726 newlines. C<Text::Wrap> doesn't justify the lines (flush-right).
728 Or use the CPAN module C<Text::Autoformat>. Formatting files can be
729 easily done by making a shell alias, like so:
731 alias fmt="perl -i -MText::Autoformat -n0777 \
732 -e 'print autoformat $_, {all=>1}' $*"
734 See the documentation for C<Text::Autoformat> to appreciate its many
737 =head2 How can I access or change N characters of a string?
739 You can access the first characters of a string with substr().
740 To get the first character, for example, start at position 0
741 and grab the string of length 1.
744 $string = "Just another Perl Hacker";
745 $first_char = substr( $string, 0, 1 ); # 'J'
747 To change part of a string, you can use the optional fourth
748 argument which is the replacement string.
750 substr( $string, 13, 4, "Perl 5.8.0" );
752 You can also use substr() as an lvalue.
754 substr( $string, 13, 4 ) = "Perl 5.8.0";
756 =head2 How do I change the Nth occurrence of something?
758 You have to keep track of N yourself. For example, let's say you want
759 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
760 C<"whosoever"> or C<"whomsoever">, case insensitively. These
761 all assume that $_ contains the string to be altered.
765 ++$count == 5 # is it the 5th?
766 ? "${2}soever" # yes, swap
767 : $1 # renege and leave it there
770 In the more general case, you can use the C</g> modifier in a C<while>
771 loop, keeping count of matches.
775 $_ = "One fish two fish red fish blue fish";
776 while (/(\w+)\s+fish\b/gi) {
777 if (++$count == $WANT) {
778 print "The third fish is a $1 one.\n";
782 That prints out: C<"The third fish is a red one."> You can also use a
783 repetition count and repeated pattern like this:
785 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
787 =head2 How can I count the number of occurrences of a substring within a string?
789 There are a number of ways, with varying efficiency. If you want a
790 count of a certain single character (X) within a string, you can use the
791 C<tr///> function like so:
793 $string = "ThisXlineXhasXsomeXx'sXinXit";
794 $count = ($string =~ tr/X//);
795 print "There are $count X characters in the string";
797 This is fine if you are just looking for a single character. However,
798 if you are trying to count multiple character substrings within a
799 larger string, C<tr///> won't work. What you can do is wrap a while()
800 loop around a global pattern match. For example, let's count negative
803 $string = "-9 55 48 -2 23 -76 4 14 -44";
804 while ($string =~ /-\d+/g) { $count++ }
805 print "There are $count negative numbers in the string";
807 Another version uses a global match in list context, then assigns the
808 result to a scalar, producing a count of the number of matches.
810 $count = () = $string =~ /-\d+/g;
812 =head2 How do I capitalize all the words on one line?
813 X<Text::Autoformat> X<capitalize> X<case, title> X<case, sentence>
815 (contributed by brian d foy)
817 Damian Conway's L<Text::Autoformat> handles all of the thinking
820 use Text::Autoformat;
821 my $x = "Dr. Strangelove or: How I Learned to Stop ".
822 "Worrying and Love the Bomb";
825 for my $style (qw( sentence title highlight )) {
826 print autoformat($x, { case => $style }), "\n";
829 How do you want to capitalize those words?
831 FRED AND BARNEY'S LODGE # all uppercase
832 Fred And Barney's Lodge # title case
833 Fred and Barney's Lodge # highlight case
835 It's not as easy a problem as it looks. How many words do you think
836 are in there? Wait for it... wait for it.... If you answered 5
837 you're right. Perl words are groups of C<\w+>, but that's not what
838 you want to capitalize. How is Perl supposed to know not to capitalize
839 that C<s> after the apostrophe? You could try a regular expression:
842 (^\w) #at the beginning of the line
844 (\s\w) #preceded by whitespace
848 $string =~ s/([\w']+)/\u\L$1/g;
850 Now, what if you don't want to capitalize that "and"? Just use
851 L<Text::Autoformat> and get on with the next problem. :)
853 =head2 How can I split a [character] delimited string except when inside [character]?
855 Several modules can handle this sort of parsing--C<Text::Balanced>,
856 C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others.
858 Take the example case of trying to split a string that is
859 comma-separated into its different fields. You can't use C<split(/,/)>
860 because you shouldn't split if the comma is inside quotes. For
861 example, take a data line like this:
863 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
865 Due to the restriction of the quotes, this is a fairly complex
866 problem. Thankfully, we have Jeffrey Friedl, author of
867 I<Mastering Regular Expressions>, to handle these for us. He
868 suggests (assuming your string is contained in C<$text>):
871 push(@new, $+) while $text =~ m{
872 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
876 push(@new, undef) if substr($text,-1,1) eq ',';
878 If you want to represent quotation marks inside a
879 quotation-mark-delimited field, escape them with backslashes (eg,
882 Alternatively, the C<Text::ParseWords> module (part of the standard
883 Perl distribution) lets you say:
885 use Text::ParseWords;
886 @new = quotewords(",", 0, $text);
888 =head2 How do I strip blank space from the beginning/end of a string?
890 (contributed by brian d foy)
892 A substitution can do this for you. For a single line, you want to
893 replace all the leading or trailing whitespace with nothing. You
894 can do that with a pair of substitutions.
899 You can also write that as a single substitution, although it turns
900 out the combined statement is slower than the separate ones. That
901 might not matter to you, though.
905 In this regular expression, the alternation matches either at the
906 beginning or the end of the string since the anchors have a lower
907 precedence than the alternation. With the C</g> flag, the substitution
908 makes all possible matches, so it gets both. Remember, the trailing
909 newline matches the C<\s+>, and the C<$> anchor can match to the
910 physical end of the string, so the newline disappears too. Just add
911 the newline to the output, which has the added benefit of preserving
912 "blank" (consisting entirely of whitespace) lines which the C<^\s+>
913 would remove all by itself.
921 For a multi-line string, you can apply the regular expression
922 to each logical line in the string by adding the C</m> flag (for
923 "multi-line"). With the C</m> flag, the C<$> matches I<before> an
924 embedded newline, so it doesn't remove it. It still removes the
925 newline at the end of the string.
927 $string =~ s/^\s+|\s+$//gm;
929 Remember that lines consisting entirely of whitespace will disappear,
930 since the first part of the alternation can match the entire string
931 and replace it with nothing. If need to keep embedded blank lines,
932 you have to do a little more work. Instead of matching any whitespace
933 (since that includes a newline), just match the other whitespace.
935 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
937 =head2 How do I pad a string with blanks or pad a number with zeroes?
939 In the following examples, C<$pad_len> is the length to which you wish
940 to pad the string, C<$text> or C<$num> contains the string to be padded,
941 and C<$pad_char> contains the padding character. You can use a single
942 character string constant instead of the C<$pad_char> variable if you
943 know what it is in advance. And in the same way you can use an integer in
944 place of C<$pad_len> if you know the pad length in advance.
946 The simplest method uses the C<sprintf> function. It can pad on the left
947 or right with blanks and on the left with zeroes and it will not
948 truncate the result. The C<pack> function can only pad strings on the
949 right with blanks and it will truncate the result to a maximum length of
952 # Left padding a string with blanks (no truncation):
953 $padded = sprintf("%${pad_len}s", $text);
954 $padded = sprintf("%*s", $pad_len, $text); # same thing
956 # Right padding a string with blanks (no truncation):
957 $padded = sprintf("%-${pad_len}s", $text);
958 $padded = sprintf("%-*s", $pad_len, $text); # same thing
960 # Left padding a number with 0 (no truncation):
961 $padded = sprintf("%0${pad_len}d", $num);
962 $padded = sprintf("%0*d", $pad_len, $num); # same thing
964 # Right padding a string with blanks using pack (will truncate):
965 $padded = pack("A$pad_len",$text);
967 If you need to pad with a character other than blank or zero you can use
968 one of the following methods. They all generate a pad string with the
969 C<x> operator and combine that with C<$text>. These methods do
970 not truncate C<$text>.
972 Left and right padding with any character, creating a new string:
974 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
975 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
977 Left and right padding with any character, modifying C<$text> directly:
979 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
980 $text .= $pad_char x ( $pad_len - length( $text ) );
982 =head2 How do I extract selected columns from a string?
984 (contributed by brian d foy)
986 If you know where the columns that contain the data, you can
987 use C<substr> to extract a single column.
989 my $column = substr( $line, $start_column, $length );
991 You can use C<split> if the columns are separated by whitespace or
992 some other delimiter, as long as whitespace or the delimiter cannot
993 appear as part of the data.
995 my $line = ' fred barney betty ';
996 my @columns = split /\s+/, $line;
997 # ( '', 'fred', 'barney', 'betty' );
999 my $line = 'fred||barney||betty';
1000 my @columns = split /\|/, $line;
1001 # ( 'fred', '', 'barney', '', 'betty' );
1003 If you want to work with comma-separated values, don't do this since
1004 that format is a bit more complicated. Use one of the modules that
1005 handle that format, such as C<Text::CSV>, C<Text::CSV_XS>, or
1008 If you want to break apart an entire line of fixed columns, you can use
1009 C<unpack> with the A (ASCII) format. By using a number after the format
1010 specifier, you can denote the column width. See the C<pack> and C<unpack>
1011 entries in L<perlfunc> for more details.
1013 my @fields = unpack( $line, "A8 A8 A8 A16 A4" );
1015 Note that spaces in the format argument to C<unpack> do not denote literal
1016 spaces. If you have space separated data, you may want C<split> instead.
1018 =head2 How do I find the soundex value of a string?
1020 (contributed by brian d foy)
1022 You can use the Text::Soundex module. If you want to do fuzzy or close
1023 matching, you might also try the C<String::Approx>, and
1024 C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules.
1026 =head2 How can I expand variables in text strings?
1028 (contributed by brian d foy)
1030 If you can avoid it, don't, or if you can use a templating system,
1031 such as C<Text::Template> or C<Template> Toolkit, do that instead. You
1032 might even be able to get the job done with C<sprintf> or C<printf>:
1034 my $string = sprintf 'Say hello to %s and %s', $foo, $bar;
1036 However, for the one-off simple case where I don't want to pull out a
1037 full templating system, I'll use a string that has two Perl scalar
1038 variables in it. In this example, I want to expand C<$foo> and C<$bar>
1039 to their variable's values:
1043 $string = 'Say hello to $foo and $bar';
1045 One way I can do this involves the substitution operator and a double
1046 C</e> flag. The first C</e> evaluates C<$1> on the replacement side and
1047 turns it into C<$foo>. The second /e starts with C<$foo> and replaces
1048 it with its value. C<$foo>, then, turns into 'Fred', and that's finally
1049 what's left in the string:
1051 $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'
1053 The C</e> will also silently ignore violations of strict, replacing
1054 undefined variable names with the empty string. Since I'm using the
1055 C</e> flag (twice even!), I have all of the same security problems I
1056 have with C<eval> in its string form. If there's something odd in
1057 C<$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then
1058 I could get myself in trouble.
1060 To get around the security problem, I could also pull the values from
1061 a hash instead of evaluating variable names. Using a single C</e>, I
1062 can check the hash to ensure the value exists, and if it doesn't, I
1063 can replace the missing value with a marker, in this case C<???> to
1064 signal that I missed something:
1066 my $string = 'This has $foo and $bar';
1068 my %Replacements = (
1072 # $string =~ s/\$(\w+)/$Replacements{$1}/g;
1073 $string =~ s/\$(\w+)/
1074 exists $Replacements{$1} ? $Replacements{$1} : '???'
1079 =head2 What's wrong with always quoting "$vars"?
1081 The problem is that those double-quotes force
1082 stringification--coercing numbers and references into strings--even
1083 when you don't want them to be strings. Think of it this way:
1084 double-quote expansion is used to produce new strings. If you already
1085 have a string, why do you need more?
1087 If you get used to writing odd things like these:
1090 $new = "$old"; # BAD
1091 somefunc("$var"); # BAD
1093 You'll be in trouble. Those should (in 99.8% of the cases) be
1094 the simpler and more direct:
1100 Otherwise, besides slowing you down, you're going to break code when
1101 the thing in the scalar is actually neither a string nor a number, but
1107 my $oref = "$aref"; # WRONG
1110 You can also get into subtle problems on those few operations in Perl
1111 that actually do care about the difference between a string and a
1112 number, such as the magical C<++> autoincrement operator or the
1115 Stringification also destroys arrays.
1118 print "@lines"; # WRONG - extra blanks
1119 print @lines; # right
1121 =head2 Why don't my E<lt>E<lt>HERE documents work?
1123 Check for these three things:
1127 =item There must be no space after the E<lt>E<lt> part.
1129 =item There (probably) should be a semicolon at the end.
1131 =item You can't (easily) have any space in front of the tag.
1135 If you want to indent the text in the here document, you
1139 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1144 But the HERE_TARGET must still be flush against the margin.
1145 If you want that indented also, you'll have to quote
1148 ($quote = <<' FINIS') =~ s/^\s+//gm;
1149 ...we will have peace, when you and all your works have
1150 perished--and the works of your dark master to whom you
1151 would deliver us. You are a liar, Saruman, and a corrupter
1152 of men's hearts. --Theoden in /usr/src/perl/taint.c
1154 $quote =~ s/\s+--/\n--/;
1156 A nice general-purpose fixer-upper function for indented here documents
1157 follows. It expects to be called with a here document as its argument.
1158 It looks to see whether each line begins with a common substring, and
1159 if so, strips that substring off. Otherwise, it takes the amount of leading
1160 whitespace found on the first line and removes that much off each
1165 my ($white, $leader); # common whitespace and common leading string
1166 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1167 ($white, $leader) = ($2, quotemeta($1));
1169 ($white, $leader) = (/^(\s+)/, '');
1171 s/^\s*?$leader(?:$white)?//gm;
1175 This works with leading special strings, dynamically determined:
1177 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1180 @@@ SAVEI32(runlevel);
1182 @@@ while ( op = (*op->op_ppaddr)() );
1186 MAIN_INTERPRETER_LOOP
1188 Or with a fixed amount of leading whitespace, with remaining
1189 indentation correctly preserved:
1191 $poem = fix<<EVER_ON_AND_ON;
1192 Now far ahead the Road has gone,
1193 And I must follow, if I can,
1194 Pursuing it with eager feet,
1195 Until it joins some larger way
1196 Where many paths and errands meet.
1197 And whither then? I cannot say.
1198 --Bilbo in /usr/src/perl/pp_ctl.c
1203 =head2 What is the difference between a list and an array?
1205 An array has a changeable length. A list does not. An array is
1206 something you can push or pop, while a list is a set of values. Some
1207 people make the distinction that a list is a value while an array is a
1208 variable. Subroutines are passed and return lists, you put things into
1209 list context, you initialize arrays with lists, and you C<foreach()>
1210 across a list. C<@> variables are arrays, anonymous arrays are
1211 arrays, arrays in scalar context behave like the number of elements in
1212 them, subroutines access their arguments through the array C<@_>, and
1213 C<push>/C<pop>/C<shift> only work on arrays.
1215 As a side note, there's no such thing as a list in scalar context.
1218 $scalar = (2, 5, 7, 9);
1220 you're using the comma operator in scalar context, so it uses the scalar
1221 comma operator. There never was a list there at all! This causes the
1222 last value to be returned: 9.
1224 =head2 What is the difference between $array[1] and @array[1]?
1226 The former is a scalar value; the latter an array slice, making
1227 it a list with one (scalar) value. You should use $ when you want a
1228 scalar value (most of the time) and @ when you want a list with one
1229 scalar value in it (very, very rarely; nearly never, in fact).
1231 Sometimes it doesn't make a difference, but sometimes it does.
1232 For example, compare:
1234 $good[0] = `some program that outputs several lines`;
1238 @bad[0] = `same program that outputs several lines`;
1240 The C<use warnings> pragma and the B<-w> flag will warn you about these
1243 =head2 How can I remove duplicate elements from a list or array?
1245 (contributed by brian d foy)
1247 Use a hash. When you think the words "unique" or "duplicated", think
1250 If you don't care about the order of the elements, you could just
1251 create the hash then extract the keys. It's not important how you
1252 create that hash: just that you use C<keys> to get the unique
1255 my %hash = map { $_, 1 } @array;
1256 # or a hash slice: @hash{ @array } = ();
1257 # or a foreach: $hash{$_} = 1 foreach ( @array );
1259 my @unique = keys %hash;
1261 If you want to use a module, try the C<uniq> function from
1262 C<List::MoreUtils>. In list context it returns the unique elements,
1263 preserving their order in the list. In scalar context, it returns the
1264 number of unique elements.
1266 use List::MoreUtils qw(uniq);
1268 my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
1269 my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7
1271 You can also go through each element and skip the ones you've seen
1272 before. Use a hash to keep track. The first time the loop sees an
1273 element, that element has no key in C<%Seen>. The C<next> statement
1274 creates the key and immediately uses its value, which is C<undef>, so
1275 the loop continues to the C<push> and increments the value for that
1276 key. The next time the loop sees that same element, its key exists in
1277 the hash I<and> the value for that key is true (since it's not 0 or
1278 C<undef>), so the next skips that iteration and the loop goes to the
1284 foreach my $elem ( @array )
1286 next if $seen{ $elem }++;
1287 push @unique, $elem;
1290 You can write this more briefly using a grep, which does the
1294 my @unique = grep { ! $seen{ $_ }++ } @array;
1296 =head2 How can I tell whether a certain element is contained in a list or array?
1298 (portions of this answer contributed by Anno Siegel and brian d foy)
1300 Hearing the word "in" is an I<in>dication that you probably should have
1301 used a hash, not a list or array, to store your data. Hashes are
1302 designed to answer this question quickly and efficiently. Arrays aren't.
1304 That being said, there are several ways to approach this. In Perl 5.10
1305 and later, you can use the smart match operator to check that an item is
1306 contained in an array or a hash:
1310 if( $item ~~ @array )
1312 say "The array contains $item"
1315 if( $item ~~ %hash )
1317 say "The hash contains $item"
1320 With earlier versions of Perl, you have to do a bit more work. If you
1321 are going to make this query many times over arbitrary string values,
1322 the fastest way is probably to invert the original array and maintain a
1323 hash whose keys are the first array's values:
1325 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1327 for (@blues) { $is_blue{$_} = 1 }
1329 Now you can check whether C<$is_blue{$some_color}>. It might have
1330 been a good idea to keep the blues all in a hash in the first place.
1332 If the values are all small integers, you could use a simple indexed
1333 array. This kind of an array will take up less space:
1335 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1336 @is_tiny_prime = ();
1337 for (@primes) { $is_tiny_prime[$_] = 1 }
1338 # or simply @istiny_prime[@primes] = (1) x @primes;
1340 Now you check whether $is_tiny_prime[$some_number].
1342 If the values in question are integers instead of strings, you can save
1343 quite a lot of space by using bit strings instead:
1345 @articles = ( 1..10, 150..2000, 2017 );
1347 for (@articles) { vec($read,$_,1) = 1 }
1349 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1351 These methods guarantee fast individual tests but require a re-organization
1352 of the original list or array. They only pay off if you have to test
1353 multiple values against the same array.
1355 If you are testing only once, the standard module C<List::Util> exports
1356 the function C<first> for this purpose. It works by stopping once it
1357 finds the element. It's written in C for speed, and its Perl equivalent
1358 looks like this subroutine:
1363 return $_ if &{$code}();
1368 If speed is of little concern, the common idiom uses grep in scalar context
1369 (which returns the number of items that passed its condition) to traverse the
1370 entire list. This does have the benefit of telling you how many matches it
1373 my $is_there = grep $_ eq $whatever, @array;
1375 If you want to actually extract the matching elements, simply use grep in
1378 my @matches = grep $_ eq $whatever, @array;
1380 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1382 Use a hash. Here's code to do both and more. It assumes that each
1383 element is unique in a given array:
1385 @union = @intersection = @difference = ();
1387 foreach $element (@array1, @array2) { $count{$element}++ }
1388 foreach $element (keys %count) {
1389 push @union, $element;
1390 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1393 Note that this is the I<symmetric difference>, that is, all elements
1394 in either A or in B but not in both. Think of it as an xor operation.
1396 =head2 How do I test whether two arrays or hashes are equal?
1398 With Perl 5.10 and later, the smart match operator can give you the answer
1399 with the least amount of work:
1403 if( @array1 ~~ @array2 )
1405 say "The arrays are the same";
1408 if( %hash1 ~~ %hash2 ) # doesn't check values!
1410 say "The hash keys are the same";
1413 The following code works for single-level arrays. It uses a
1414 stringwise comparison, and does not distinguish defined versus
1415 undefined empty strings. Modify if you have other needs.
1417 $are_equal = compare_arrays(\@frogs, \@toads);
1419 sub compare_arrays {
1420 my ($first, $second) = @_;
1421 no warnings; # silence spurious -w undef complaints
1422 return 0 unless @$first == @$second;
1423 for (my $i = 0; $i < @$first; $i++) {
1424 return 0 if $first->[$i] ne $second->[$i];
1429 For multilevel structures, you may wish to use an approach more
1430 like this one. It uses the CPAN module C<FreezeThaw>:
1432 use FreezeThaw qw(cmpStr);
1433 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1435 printf "a and b contain %s arrays\n",
1436 cmpStr(\@a, \@b) == 0
1440 This approach also works for comparing hashes. Here we'll demonstrate
1441 two different answers:
1443 use FreezeThaw qw(cmpStr cmpStrHard);
1445 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1449 printf "a and b contain %s hashes\n",
1450 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1452 printf "a and b contain %s hashes\n",
1453 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1456 The first reports that both those the hashes contain the same data,
1457 while the second reports that they do not. Which you prefer is left as
1458 an exercise to the reader.
1460 =head2 How do I find the first array element for which a condition is true?
1462 To find the first array element which satisfies a condition, you can
1463 use the C<first()> function in the C<List::Util> module, which comes
1464 with Perl 5.8. This example finds the first element that contains
1467 use List::Util qw(first);
1469 my $element = first { /Perl/ } @array;
1471 If you cannot use C<List::Util>, you can make your own loop to do the
1472 same thing. Once you find the element, you stop the loop with last.
1475 foreach ( @array ) {
1476 if( /Perl/ ) { $found = $_; last }
1479 If you want the array index, you can iterate through the indices
1480 and check the array element at each index until you find one
1481 that satisfies the condition.
1483 my( $found, $index ) = ( undef, -1 );
1484 for( $i = 0; $i < @array; $i++ ) {
1485 if( $array[$i] =~ /Perl/ ) {
1486 $found = $array[$i];
1492 =head2 How do I handle linked lists?
1494 In general, you usually don't need a linked list in Perl, since with
1495 regular arrays, you can push and pop or shift and unshift at either
1496 end, or you can use splice to add and/or remove arbitrary number of
1497 elements at arbitrary points. Both pop and shift are O(1)
1498 operations on Perl's dynamic arrays. In the absence of shifts and
1499 pops, push in general needs to reallocate on the order every log(N)
1500 times, and unshift will need to copy pointers each time.
1502 If you really, really wanted, you could use structures as described in
1503 L<perldsc> or L<perltoot> and do just what the algorithm book tells
1504 you to do. For example, imagine a list node like this:
1511 You could walk the list this way:
1514 for ($node = $head; $node; $node = $node->{LINK}) {
1515 print $node->{VALUE}, " ";
1519 You could add to the list this way:
1522 $tail = append($head, 1); # grow a new head
1523 for $value ( 2 .. 10 ) {
1524 $tail = append($tail, $value);
1528 my($list, $value) = @_;
1529 my $node = { VALUE => $value };
1531 $node->{LINK} = $list->{LINK};
1532 $list->{LINK} = $node;
1535 $_[0] = $node; # replace caller's version
1540 But again, Perl's built-in are virtually always good enough.
1542 =head2 How do I handle circular lists?
1543 X<circular> X<array> X<Tie::Cycle> X<Array::Iterator::Circular>
1546 (contributed by brian d foy)
1548 If you want to cycle through an array endlessly, you can increment the
1549 index modulo the number of elements in the array:
1551 my @array = qw( a b c );
1555 print $array[ $i++ % @array ], "\n";
1559 You can also use C<Tie::Cycle> to use a scalar that always has the
1560 next element of the circular array:
1564 tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
1566 print $cycle; # FFFFFF
1567 print $cycle; # 000000
1568 print $cycle; # FFFF00
1570 The C<Array::Iterator::Circular> creates an iterator object for
1573 use Array::Iterator::Circular;
1575 my $color_iterator = Array::Iterator::Circular->new(
1576 qw(red green blue orange)
1579 foreach ( 1 .. 20 ) {
1580 print $color_iterator->next, "\n";
1583 =head2 How do I shuffle an array randomly?
1585 If you either have Perl 5.8.0 or later installed, or if you have
1586 Scalar-List-Utils 1.03 or later installed, you can say:
1588 use List::Util 'shuffle';
1590 @shuffled = shuffle(@list);
1592 If not, you can use a Fisher-Yates shuffle.
1594 sub fisher_yates_shuffle {
1595 my $deck = shift; # $deck is a reference to an array
1596 return unless @$deck; # must not be empty!
1600 my $j = int rand ($i+1);
1601 @$deck[$i,$j] = @$deck[$j,$i];
1605 # shuffle my mpeg collection
1607 my @mpeg = <audio/*/*.mp3>;
1608 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1611 Note that the above implementation shuffles an array in place,
1612 unlike the C<List::Util::shuffle()> which takes a list and returns
1613 a new shuffled list.
1615 You've probably seen shuffling algorithms that work using splice,
1616 randomly picking another element to swap the current element with
1620 @old = 1 .. 10; # just a demo
1622 push(@new, splice(@old, rand @old, 1));
1625 This is bad because splice is already O(N), and since you do it N
1626 times, you just invented a quadratic algorithm; that is, O(N**2).
1627 This does not scale, although Perl is so efficient that you probably
1628 won't notice this until you have rather largish arrays.
1630 =head2 How do I process/modify each element of an array?
1632 Use C<for>/C<foreach>:
1635 s/foo/bar/; # change that word
1636 tr/XZ/ZX/; # swap those letters
1639 Here's another; let's compute spherical volumes:
1641 for (@volumes = @radii) { # @volumes has changed parts
1643 $_ *= (4/3) * 3.14159; # this will be constant folded
1646 which can also be done with C<map()> which is made to transform
1647 one list into another:
1649 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1651 If you want to do the same thing to modify the values of the
1652 hash, you can use the C<values> function. As of Perl 5.6
1653 the values are not copied, so if you modify $orbit (in this
1654 case), you modify the value.
1656 for $orbit ( values %orbits ) {
1657 ($orbit **= 3) *= (4/3) * 3.14159;
1660 Prior to perl 5.6 C<values> returned copies of the values,
1661 so older perl code often contains constructions such as
1662 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1663 the hash is to be modified.
1665 =head2 How do I select a random element from an array?
1667 Use the C<rand()> function (see L<perlfunc/rand>):
1669 $index = rand @array;
1670 $element = $array[$index];
1674 my $element = $array[ rand @array ];
1676 =head2 How do I permute N elements of a list?
1677 X<List::Permuter> X<permute> X<Algorithm::Loops> X<Knuth>
1678 X<The Art of Computer Programming> X<Fischer-Krause>
1680 Use the C<List::Permutor> module on CPAN. If the list is actually an
1681 array, try the C<Algorithm::Permute> module (also on CPAN). It's
1682 written in XS code and is very efficient:
1684 use Algorithm::Permute;
1686 my @array = 'a'..'d';
1687 my $p_iterator = Algorithm::Permute->new ( \@array );
1689 while (my @perm = $p_iterator->next) {
1690 print "next permutation: (@perm)\n";
1693 For even faster execution, you could do:
1695 use Algorithm::Permute;
1697 my @array = 'a'..'d';
1699 Algorithm::Permute::permute {
1700 print "next permutation: (@array)\n";
1703 Here's a little program that generates all permutations of all the
1704 words on each line of input. The algorithm embodied in the
1705 C<permute()> function is discussed in Volume 4 (still unpublished) of
1706 Knuth's I<The Art of Computer Programming> and will work on any list:
1709 # Fischer-Krause ordered permutation generator
1714 while ( $code->(@_[@idx]) ) {
1716 --$p while $idx[$p-1] > $idx[$p];
1717 my $q = $p or return;
1718 push @idx, reverse splice @idx, $p;
1719 ++$q while $idx[$p-1] > $idx[$q];
1720 @idx[$p-1,$q]=@idx[$q,$p-1];
1724 permute { print "@_\n" } split;
1726 The C<Algorithm::Loops> module also provides the C<NextPermute> and
1727 C<NextPermuteNum> functions which efficiently find all unique permutations
1728 of an array, even if it contains duplicate values, modifying it in-place:
1729 if its elements are in reverse-sorted order then the array is reversed,
1730 making it sorted, and it returns false; otherwise the next
1731 permutation is returned.
1733 C<NextPermute> uses string order and C<NextPermuteNum> numeric order, so
1734 you can enumerate all the permutations of C<0..9> like this:
1736 use Algorithm::Loops qw(NextPermuteNum);
1739 do { print "@list\n" } while NextPermuteNum @list;
1741 =head2 How do I sort an array by (anything)?
1743 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1745 @list = sort { $a <=> $b } @list;
1747 The default sort function is cmp, string comparison, which would
1748 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1749 the numerical comparison operator.
1751 If you have a complicated function needed to pull out the part you
1752 want to sort on, then don't do it inside the sort function. Pull it
1753 out first, because the sort BLOCK can be called many times for the
1754 same element. Here's an example of how to pull out the first word
1755 after the first number on each item, and then sort those words
1760 ($item) = /\d+\s*(\S+)/;
1761 push @idx, uc($item);
1763 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1765 which could also be written this way, using a trick
1766 that's come to be known as the Schwartzian Transform:
1768 @sorted = map { $_->[0] }
1769 sort { $a->[1] cmp $b->[1] }
1770 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1772 If you need to sort on several fields, the following paradigm is useful.
1775 field1($a) <=> field1($b) ||
1776 field2($a) cmp field2($b) ||
1777 field3($a) cmp field3($b)
1780 This can be conveniently combined with precalculation of keys as given
1783 See the F<sort> article in the "Far More Than You Ever Wanted
1784 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1785 more about this approach.
1787 See also the question later in L<perlfaq4> on sorting hashes.
1789 =head2 How do I manipulate arrays of bits?
1791 Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise
1794 For example, you don't have to store individual bits in an array
1795 (which would mean that you're wasting a lot of space). To convert an
1796 array of bits to a string, use C<vec()> to set the right bits. This
1797 sets C<$vec> to have bit N set only if C<$ints[N]> was set:
1799 @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
1801 foreach( 0 .. $#ints ) {
1802 vec($vec,$_,1) = 1 if $ints[$_];
1805 The string C<$vec> only takes up as many bits as it needs. For
1806 instance, if you had 16 entries in C<@ints>, C<$vec> only needs two
1807 bytes to store them (not counting the scalar variable overhead).
1809 Here's how, given a vector in C<$vec>, you can get those bits into
1810 your C<@ints> array:
1812 sub bitvec_to_list {
1815 # Find null-byte density then select best algorithm
1816 if ($vec =~ tr/\0// / length $vec > 0.95) {
1820 # This method is faster with mostly null-bytes
1821 while($vec =~ /[^\0]/g ) {
1822 $i = -9 + 8 * pos $vec;
1823 push @ints, $i if vec($vec, ++$i, 1);
1824 push @ints, $i if vec($vec, ++$i, 1);
1825 push @ints, $i if vec($vec, ++$i, 1);
1826 push @ints, $i if vec($vec, ++$i, 1);
1827 push @ints, $i if vec($vec, ++$i, 1);
1828 push @ints, $i if vec($vec, ++$i, 1);
1829 push @ints, $i if vec($vec, ++$i, 1);
1830 push @ints, $i if vec($vec, ++$i, 1);
1834 # This method is a fast general algorithm
1836 my $bits = unpack "b*", $vec;
1837 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1838 push @ints, pos $bits while($bits =~ /1/g);
1844 This method gets faster the more sparse the bit vector is.
1845 (Courtesy of Tim Bunce and Winfried Koenig.)
1847 You can make the while loop a lot shorter with this suggestion
1848 from Benjamin Goldberg:
1850 while($vec =~ /[^\0]+/g ) {
1851 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1854 Or use the CPAN module C<Bit::Vector>:
1856 $vector = Bit::Vector->new($num_of_bits);
1857 $vector->Index_List_Store(@ints);
1858 @ints = $vector->Index_List_Read();
1860 C<Bit::Vector> provides efficient methods for bit vector, sets of
1861 small integers and "big int" math.
1863 Here's a more extensive illustration using vec():
1866 $vector = "\xff\x0f\xef\xfe";
1867 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1868 unpack("N", $vector), "\n";
1869 $is_set = vec($vector, 23, 1);
1870 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1888 my ($offset, $width, $value) = @_;
1890 vec($vector, $offset, $width) = $value;
1891 print "offset=$offset width=$width value=$value\n";
1897 my $bits = unpack("b*", $vector);
1901 print "vector length in bytes: ", length($vector), "\n";
1902 @bytes = unpack("A8" x length($vector), $bits);
1903 print "bits are: @bytes\n\n";
1906 =head2 Why does defined() return true on empty arrays and hashes?
1908 The short story is that you should probably only use defined on scalars or
1909 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1910 in the 5.004 release or later of Perl for more detail.
1912 =head1 Data: Hashes (Associative Arrays)
1914 =head2 How do I process an entire hash?
1916 (contributed by brian d foy)
1918 There are a couple of ways that you can process an entire hash. You
1919 can get a list of keys, then go through each key, or grab a one
1920 key-value pair at a time.
1922 To go through all of the keys, use the C<keys> function. This extracts
1923 all of the keys of the hash and gives them back to you as a list. You
1924 can then get the value through the particular key you're processing:
1926 foreach my $key ( keys %hash ) {
1927 my $value = $hash{$key}
1931 Once you have the list of keys, you can process that list before you
1932 process the hash elements. For instance, you can sort the keys so you
1933 can process them in lexical order:
1935 foreach my $key ( sort keys %hash ) {
1936 my $value = $hash{$key}
1940 Or, you might want to only process some of the items. If you only want
1941 to deal with the keys that start with C<text:>, you can select just
1942 those using C<grep>:
1944 foreach my $key ( grep /^text:/, keys %hash ) {
1945 my $value = $hash{$key}
1949 If the hash is very large, you might not want to create a long list of
1950 keys. To save some memory, you can grab one key-value pair at a time using
1951 C<each()>, which returns a pair you haven't seen yet:
1953 while( my( $key, $value ) = each( %hash ) ) {
1957 The C<each> operator returns the pairs in apparently random order, so if
1958 ordering matters to you, you'll have to stick with the C<keys> method.
1960 The C<each()> operator can be a bit tricky though. You can't add or
1961 delete keys of the hash while you're using it without possibly
1962 skipping or re-processing some pairs after Perl internally rehashes
1963 all of the elements. Additionally, a hash has only one iterator, so if
1964 you use C<keys>, C<values>, or C<each> on the same hash, you can reset
1965 the iterator and mess up your processing. See the C<each> entry in
1966 L<perlfunc> for more details.
1968 =head2 How do I merge two hashes?
1969 X<hash> X<merge> X<slice, hash>
1971 (contributed by brian d foy)
1973 Before you decide to merge two hashes, you have to decide what to do
1974 if both hashes contain keys that are the same and if you want to leave
1975 the original hashes as they were.
1977 If you want to preserve the original hashes, copy one hash (C<%hash1>)
1978 to a new hash (C<%new_hash>), then add the keys from the other hash
1979 (C<%hash2> to the new hash. Checking that the key already exists in
1980 C<%new_hash> gives you a chance to decide what to do with the
1983 my %new_hash = %hash1; # make a copy; leave %hash1 alone
1985 foreach my $key2 ( keys %hash2 )
1987 if( exists $new_hash{$key2} )
1989 warn "Key [$key2] is in both hashes!";
1990 # handle the duplicate (perhaps only warning)
1996 $new_hash{$key2} = $hash2{$key2};
2000 If you don't want to create a new hash, you can still use this looping
2001 technique; just change the C<%new_hash> to C<%hash1>.
2003 foreach my $key2 ( keys %hash2 )
2005 if( exists $hash1{$key2} )
2007 warn "Key [$key2] is in both hashes!";
2008 # handle the duplicate (perhaps only warning)
2014 $hash1{$key2} = $hash2{$key2};
2018 If you don't care that one hash overwrites keys and values from the other, you
2019 could just use a hash slice to add one hash to another. In this case, values
2020 from C<%hash2> replace values from C<%hash1> when they have keys in common:
2022 @hash1{ keys %hash2 } = values %hash2;
2024 =head2 What happens if I add or remove keys from a hash while iterating over it?
2026 (contributed by brian d foy)
2028 The easy answer is "Don't do that!"
2030 If you iterate through the hash with each(), you can delete the key
2031 most recently returned without worrying about it. If you delete or add
2032 other keys, the iterator may skip or double up on them since perl
2033 may rearrange the hash table. See the
2034 entry for C<each()> in L<perlfunc>.
2036 =head2 How do I look up a hash element by value?
2038 Create a reverse hash:
2040 %by_value = reverse %by_key;
2041 $key = $by_value{$value};
2043 That's not particularly efficient. It would be more space-efficient
2046 while (($key, $value) = each %by_key) {
2047 $by_value{$value} = $key;
2050 If your hash could have repeated values, the methods above will only find
2051 one of the associated keys. This may or may not worry you. If it does
2052 worry you, you can always reverse the hash into a hash of arrays instead:
2054 while (($key, $value) = each %by_key) {
2055 push @{$key_list_by_value{$value}}, $key;
2058 =head2 How can I know how many entries are in a hash?
2060 (contributed by brian d foy)
2062 This is very similar to "How do I process an entire hash?", also in
2063 L<perlfaq4>, but a bit simpler in the common cases.
2065 You can use the C<keys()> built-in function in scalar context to find out
2066 have many entries you have in a hash:
2068 my $key_count = keys %hash; # must be scalar context!
2070 If you want to find out how many entries have a defined value, that's
2071 a bit different. You have to check each value. A C<grep> is handy:
2073 my $defined_value_count = grep { defined } values %hash;
2075 You can use that same structure to count the entries any way that
2076 you like. If you want the count of the keys with vowels in them,
2077 you just test for that instead:
2079 my $vowel_count = grep { /[aeiou]/ } keys %hash;
2081 The C<grep> in scalar context returns the count. If you want the list
2082 of matching items, just use it in list context instead:
2084 my @defined_values = grep { defined } values %hash;
2086 The C<keys()> function also resets the iterator, which means that you may
2087 see strange results if you use this between uses of other hash operators
2090 =head2 How do I sort a hash (optionally by value instead of key)?
2092 (contributed by brian d foy)
2094 To sort a hash, start with the keys. In this example, we give the list of
2095 keys to the sort function which then compares them ASCIIbetically (which
2096 might be affected by your locale settings). The output list has the keys
2097 in ASCIIbetical order. Once we have the keys, we can go through them to
2098 create a report which lists the keys in ASCIIbetical order.
2100 my @keys = sort { $a cmp $b } keys %hash;
2102 foreach my $key ( @keys )
2104 printf "%-20s %6d\n", $key, $hash{$key};
2107 We could get more fancy in the C<sort()> block though. Instead of
2108 comparing the keys, we can compute a value with them and use that
2109 value as the comparison.
2111 For instance, to make our report order case-insensitive, we use
2112 the C<\L> sequence in a double-quoted string to make everything
2113 lowercase. The C<sort()> block then compares the lowercased
2114 values to determine in which order to put the keys.
2116 my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
2118 Note: if the computation is expensive or the hash has many elements,
2119 you may want to look at the Schwartzian Transform to cache the
2120 computation results.
2122 If we want to sort by the hash value instead, we use the hash key
2123 to look it up. We still get out a list of keys, but this time they
2124 are ordered by their value.
2126 my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
2128 From there we can get more complex. If the hash values are the same,
2129 we can provide a secondary sort on the hash key.
2132 $hash{$a} <=> $hash{$b}
2137 =head2 How can I always keep my hash sorted?
2138 X<hash tie sort DB_File Tie::IxHash>
2140 You can look into using the C<DB_File> module and C<tie()> using the
2141 C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory
2142 Databases">. The C<Tie::IxHash> module from CPAN might also be
2143 instructive. Although this does keep your hash sorted, you might not
2144 like the slow down you suffer from the tie interface. Are you sure you
2147 =head2 What's the difference between "delete" and "undef" with hashes?
2149 Hashes contain pairs of scalars: the first is the key, the
2150 second is the value. The key will be coerced to a string,
2151 although the value can be any kind of scalar: string,
2152 number, or reference. If a key C<$key> is present in
2153 %hash, C<exists($hash{$key})> will return true. The value
2154 for a given key can be C<undef>, in which case
2155 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
2156 will return true. This corresponds to (C<$key>, C<undef>)
2159 Pictures help... Here's the C<%hash> table:
2169 And these conditions hold
2173 defined $hash{'d'} is true
2174 defined $hash{'a'} is true
2175 exists $hash{'a'} is true (Perl 5 only)
2176 grep ($_ eq 'a', keys %hash) is true
2182 your table now reads:
2193 and these conditions now hold; changes in caps:
2197 defined $hash{'d'} is true
2198 defined $hash{'a'} is FALSE
2199 exists $hash{'a'} is true (Perl 5 only)
2200 grep ($_ eq 'a', keys %hash) is true
2202 Notice the last two: you have an undef value, but a defined key!
2208 your table now reads:
2217 and these conditions now hold; changes in caps:
2221 defined $hash{'d'} is true
2222 defined $hash{'a'} is false
2223 exists $hash{'a'} is FALSE (Perl 5 only)
2224 grep ($_ eq 'a', keys %hash) is FALSE
2226 See, the whole entry is gone!
2228 =head2 Why don't my tied hashes make the defined/exists distinction?
2230 This depends on the tied hash's implementation of EXISTS().
2231 For example, there isn't the concept of undef with hashes
2232 that are tied to DBM* files. It also means that exists() and
2233 defined() do the same thing with a DBM* file, and what they
2234 end up doing is not what they do with ordinary hashes.
2236 =head2 How do I reset an each() operation part-way through?
2238 (contributed by brian d foy)
2240 You can use the C<keys> or C<values> functions to reset C<each>. To
2241 simply reset the iterator used by C<each> without doing anything else,
2242 use one of them in void context:
2244 keys %hash; # resets iterator, nothing else.
2245 values %hash; # resets iterator, nothing else.
2247 See the documentation for C<each> in L<perlfunc>.
2249 =head2 How can I get the unique keys from two hashes?
2251 First you extract the keys from the hashes into lists, then solve
2252 the "removing duplicates" problem described above. For example:
2255 for $element (keys(%foo), keys(%bar)) {
2262 @uniq = keys %{{%foo,%bar}};
2264 Or if you really want to save space:
2267 while (defined ($key = each %foo)) {
2270 while (defined ($key = each %bar)) {
2275 =head2 How can I store a multidimensional array in a DBM file?
2277 Either stringify the structure yourself (no fun), or else
2278 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
2279 it on top of either DB_File or GDBM_File.
2281 =head2 How can I make my hash remember the order I put elements into it?
2283 Use the C<Tie::IxHash> from CPAN.
2287 tie my %myhash, 'Tie::IxHash';
2289 for (my $i=0; $i<20; $i++) {
2293 my @keys = keys %myhash;
2294 # @keys = (0,1,2,3,...)
2296 =head2 Why does passing a subroutine an undefined element in a hash create it?
2298 (contributed by brian d foy)
2300 Are you using a really old version of Perl?
2302 Normally, accessing a hash key's value for a nonexistent key will
2303 I<not> create the key.
2306 my $value = $hash{ 'foo' };
2307 print "This won't print\n" if exists $hash{ 'foo' };
2309 Passing C<$hash{ 'foo' }> to a subroutine used to be a special case, though.
2310 Since you could assign directly to C<$_[0]>, Perl had to be ready to
2311 make that assignment so it created the hash key ahead of time:
2313 my_sub( $hash{ 'foo' } );
2314 print "This will print before 5.004\n" if exists $hash{ 'foo' };
2317 # $_[0] = 'bar'; # create hash key in case you do this
2321 Since Perl 5.004, however, this situation is a special case and Perl
2322 creates the hash key only when you make the assignment:
2324 my_sub( $hash{ 'foo' } );
2325 print "This will print, even after 5.004\n" if exists $hash{ 'foo' };
2331 However, if you want the old behavior (and think carefully about that
2332 because it's a weird side effect), you can pass a hash slice instead.
2333 Perl 5.004 didn't make this a special case:
2335 my_sub( @hash{ qw/foo/ } );
2337 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
2339 Usually a hash ref, perhaps like this:
2344 TITLE => "deputy peon",
2347 PALS => [ "Norbert", "Rhys", "Phineas"],
2350 References are documented in L<perlref> and the upcoming L<perlreftut>.
2351 Examples of complex data structures are given in L<perldsc> and
2352 L<perllol>. Examples of structures and object-oriented classes are
2355 =head2 How can I use a reference as a hash key?
2357 (contributed by brian d foy and Ben Morrow)
2359 Hash keys are strings, so you can't really use a reference as the key.
2360 When you try to do that, perl turns the reference into its stringified
2361 form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get
2362 back the reference from the stringified form, at least without doing
2363 some extra work on your own.
2365 Remember that the entry in the hash will still be there even if
2366 the referenced variable goes out of scope, and that it is entirely
2367 possible for Perl to subsequently allocate a different variable at
2368 the same address. This will mean a new variable might accidentally
2369 be associated with the value for an old.
2371 If you have Perl 5.10 or later, and you just want to store a value
2372 against the reference for lookup later, you can use the core
2373 Hash::Util::Fieldhash module. This will also handle renaming the
2374 keys if you use multiple threads (which causes all variables to be
2375 reallocated at new addresses, changing their stringification), and
2376 garbage-collecting the entries when the referenced variable goes out
2379 If you actually need to be able to get a real reference back from
2380 each hash entry, you can use the Tie::RefHash module, which does the
2381 required work for you.
2385 =head2 How do I handle binary data correctly?
2387 Perl is binary clean, so it can handle binary data just fine.
2388 On Windows or DOS, however, you have to use C<binmode> for binary
2389 files to avoid conversions for line endings. In general, you should
2390 use C<binmode> any time you want to work with binary data.
2392 Also see L<perlfunc/"binmode"> or L<perlopentut>.
2394 If you're concerned about 8-bit textual data then see L<perllocale>.
2395 If you want to deal with multibyte characters, however, there are
2396 some gotchas. See the section on Regular Expressions.
2398 =head2 How do I determine whether a scalar is a number/whole/integer/float?
2400 Assuming that you don't care about IEEE notations like "NaN" or
2401 "Infinity", you probably just want to use a regular expression.
2403 if (/\D/) { print "has nondigits\n" }
2404 if (/^\d+$/) { print "is a whole number\n" }
2405 if (/^-?\d+$/) { print "is an integer\n" }
2406 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2407 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2408 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2409 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2410 { print "a C float\n" }
2412 There are also some commonly used modules for the task.
2413 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2414 internal function C<looks_like_number> for determining whether a
2415 variable looks like a number. L<Data::Types> exports functions that
2416 validate data types using both the above and other regular
2417 expressions. Thirdly, there is C<Regexp::Common> which has regular
2418 expressions to match various types of numbers. Those three modules are
2419 available from the CPAN.
2421 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2422 function. Its semantics are somewhat cumbersome, so here's a
2423 C<getnum> wrapper function for more convenient access. This function
2424 takes a string and returns the number it found, or C<undef> for input
2425 that isn't a C float. The C<is_numeric> function is a front end to
2426 C<getnum> if you just want to say, "Is this a float?"
2429 use POSIX qw(strtod);
2434 my($num, $unparsed) = strtod($str);
2435 if (($str eq '') || ($unparsed != 0) || $!) {
2443 sub is_numeric { defined getnum($_[0]) }
2445 Or you could check out the L<String::Scanf> module on the CPAN
2446 instead. The C<POSIX> module (part of the standard Perl distribution)
2447 provides the C<strtod> and C<strtol> for converting strings to double
2448 and longs, respectively.
2450 =head2 How do I keep persistent data across program calls?
2452 For some specific applications, you can use one of the DBM modules.
2453 See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw>
2454 or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part
2455 of the standard distribution. Here's one example using C<Storable>'s C<store>
2456 and C<retrieve> functions:
2459 store(\%hash, "filename");
2462 $href = retrieve("filename"); # by ref
2463 %hash = %{ retrieve("filename") }; # direct to hash
2465 =head2 How do I print out or copy a recursive data structure?
2467 The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great
2468 for printing out data structures. The C<Storable> module on CPAN (or the
2469 5.8 release of Perl), provides a function called C<dclone> that recursively
2470 copies its argument.
2472 use Storable qw(dclone);
2475 Where C<$r1> can be a reference to any kind of data structure you'd like.
2476 It will be deeply copied. Because C<dclone> takes and returns references,
2477 you'd have to add extra punctuation if you had a hash of arrays that
2480 %newhash = %{ dclone(\%oldhash) };
2482 =head2 How do I define methods for every class/object?
2484 (contributed by Ben Morrow)
2486 You can use the C<UNIVERSAL> class (see L<UNIVERSAL>). However, please
2487 be very careful to consider the consequences of doing this: adding
2488 methods to every object is very likely to have unintended
2489 consequences. If possible, it would be better to have all your object
2490 inherit from some common base class, or to use an object system like
2491 Moose that supports roles.
2493 =head2 How do I verify a credit card checksum?
2495 Get the C<Business::CreditCard> module from CPAN.
2497 =head2 How do I pack arrays of doubles or floats for XS code?
2499 The arrays.h/arrays.c code in the C<PGPLOT> module on CPAN does just this.
2500 If you're doing a lot of float or double processing, consider using
2501 the C<PDL> module from CPAN instead--it makes number-crunching easy.
2503 See L<http://search.cpan.org/dist/PGPLOT> for the code.
2507 Revision: $Revision$
2511 See L<perlfaq> for source control details and availability.
2513 =head1 AUTHOR AND COPYRIGHT
2515 Copyright (c) 1997-2009 Tom Christiansen, Nathan Torkington, and
2516 other authors as noted. All rights reserved.
2518 This documentation is free; you can redistribute it and/or modify it
2519 under the same terms as Perl itself.
2521 Irrespective of its distribution, all code examples in this file
2522 are hereby placed into the public domain. You are permitted and
2523 encouraged to use this code in your own programs for fun
2524 or for profit as you see fit. A simple comment in the code giving
2525 credit would be courteous but is not required.