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, $d;
530 In this case, you measure the day starting at noon, and subtract 24
531 hours. Even if the length of the calendar day is 23 or 25 hours,
532 you'll still end up on the previous calendar day, although not at
533 noon. Since you don't care about the time, the one hour difference
534 doesn't matter and you end up with the previous date.
536 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
538 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
539 Y2K compliant (whatever that means). The programmers you've hired to
540 use it, however, probably are not.
542 Long answer: The question belies a true understanding of the issue.
543 Perl is just as Y2K compliant as your pencil--no more, and no less.
544 Can you use your pencil to write a non-Y2K-compliant memo? Of course
545 you can. Is that the pencil's fault? Of course it isn't.
547 The date and time functions supplied with Perl (gmtime and localtime)
548 supply adequate information to determine the year well beyond 2000 and
549 2038. The year returned by these functions when used in a list
550 context is the year minus 1900. For years between 1910 and 1999 this
551 I<happens> to be a 2-digit decimal number. To avoid the year 2000
552 problem simply do not treat the year as a 2-digit number. It isn't.
554 When gmtime() and localtime() are used in scalar context they return
555 a timestamp string that contains a fully-expanded year. For example,
556 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
557 2001". There's no year 2000 problem here.
559 That doesn't mean that Perl can't be used to create non-Y2K compliant
560 programs. It can. But so can your pencil. It's the fault of the user,
561 not the language. At the risk of inflaming the NRA: "Perl doesn't
562 break Y2K, people do." See http://www.perl.org/about/y2k.html for
567 =head2 How do I validate input?
569 (contributed by brian d foy)
571 There are many ways to ensure that values are what you expect or
572 want to accept. Besides the specific examples that we cover in the
573 perlfaq, you can also look at the modules with "Assert" and "Validate"
574 in their names, along with other modules such as C<Regexp::Common>.
576 Some modules have validation for particular types of input, such
577 as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>,
578 and C<Data::Validate::IP>.
580 =head2 How do I unescape a string?
582 It depends just what you mean by "escape". URL escapes are dealt
583 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
584 character are removed with
588 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
590 =head2 How do I remove consecutive pairs of characters?
592 (contributed by brian d foy)
594 You can use the substitution operator to find pairs of characters (or
595 runs of characters) and replace them with a single instance. In this
596 substitution, we find a character in C<(.)>. The memory parentheses
597 store the matched character in the back-reference C<\1> and we use
598 that to require that the same thing immediately follow it. We replace
599 that part of the string with the character in C<$1>.
603 We can also use the transliteration operator, C<tr///>. In this
604 example, the search list side of our C<tr///> contains nothing, but
605 the C<c> option complements that so it contains everything. The
606 replacement list also contains nothing, so the transliteration is
607 almost a no-op since it won't do any replacements (or more exactly,
608 replace the character with itself). However, the C<s> option squashes
609 duplicated and consecutive characters in the string so a character
610 does not show up next to itself
612 my $str = 'Haarlem'; # in the Netherlands
613 $str =~ tr///cs; # Now Harlem, like in New York
615 =head2 How do I expand function calls in a string?
617 (contributed by brian d foy)
619 This is documented in L<perlref>, and although it's not the easiest
620 thing to read, it does work. In each of these examples, we call the
621 function inside the braces used to dereference a reference. If we
622 have more than one return value, we can construct and dereference an
623 anonymous array. In this case, we call the function in list context.
625 print "The time values are @{ [localtime] }.\n";
627 If we want to call the function in scalar context, we have to do a bit
628 more work. We can really have any code we like inside the braces, so
629 we simply have to end with the scalar reference, although how you do
630 that is up to you, and you can use code inside the braces. Note that
631 the use of parens creates a list context, so we need C<scalar> to
632 force the scalar context on the function:
634 print "The time is ${\(scalar localtime)}.\n"
636 print "The time is ${ my $x = localtime; \$x }.\n";
638 If your function already returns a reference, you don't need to create
639 the reference yourself.
641 sub timestamp { my $t = localtime; \$t }
643 print "The time is ${ timestamp() }.\n";
645 The C<Interpolation> module can also do a lot of magic for you. You can
646 specify a variable name, in this case C<E>, to set up a tied hash that
647 does the interpolation for you. It has several other methods to do this
650 use Interpolation E => 'eval';
651 print "The time values are $E{localtime()}.\n";
653 In most cases, it is probably easier to simply use string concatenation,
654 which also forces scalar context.
656 print "The time is " . localtime() . ".\n";
658 =head2 How do I find matching/nesting anything?
660 This isn't something that can be done in one regular expression, no
661 matter how complicated. To find something between two single
662 characters, a pattern like C</x([^x]*)x/> will get the intervening
663 bits in $1. For multiple ones, then something more like
664 C</alpha(.*?)omega/> would be needed. But none of these deals with
665 nested patterns. For balanced expressions using C<(>, C<{>, C<[> or
666 C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
667 L<perlre/(??{ code })>. For other cases, you'll have to write a
670 If you are serious about writing a parser, there are a number of
671 modules or oddities that will make your life a lot easier. There are
672 the CPAN modules C<Parse::RecDescent>, C<Parse::Yapp>, and
673 C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8
674 the C<Text::Balanced> is part of the standard distribution.
676 One simple destructive, inside-out approach that you might try is to
677 pull out the smallest nesting parts one at a time:
679 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
680 # do something with $1
683 A more complicated and sneaky approach is to make Perl's regular
684 expression engine do it for you. This is courtesy Dean Inada, and
685 rather has the nature of an Obfuscated Perl Contest entry, but it
688 # $_ contains the string to parse
689 # BEGIN and END are the opening and closing markers for the
694 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
695 @$ = (eval{/$re/},$@!~/unmatched/i);
696 print join("\n",@$[0..$#$]) if( $$[-1] );
698 =head2 How do I reverse a string?
700 Use C<reverse()> in scalar context, as documented in
703 $reversed = reverse $string;
705 =head2 How do I expand tabs in a string?
707 You can do it yourself:
709 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
711 Or you can just use the C<Text::Tabs> module (part of the standard Perl
715 @expanded_lines = expand(@lines_with_tabs);
717 =head2 How do I reformat a paragraph?
719 Use C<Text::Wrap> (part of the standard Perl distribution):
722 print wrap("\t", ' ', @paragraphs);
724 The paragraphs you give to C<Text::Wrap> should not contain embedded
725 newlines. C<Text::Wrap> doesn't justify the lines (flush-right).
727 Or use the CPAN module C<Text::Autoformat>. Formatting files can be
728 easily done by making a shell alias, like so:
730 alias fmt="perl -i -MText::Autoformat -n0777 \
731 -e 'print autoformat $_, {all=>1}' $*"
733 See the documentation for C<Text::Autoformat> to appreciate its many
736 =head2 How can I access or change N characters of a string?
738 You can access the first characters of a string with substr().
739 To get the first character, for example, start at position 0
740 and grab the string of length 1.
743 $string = "Just another Perl Hacker";
744 $first_char = substr( $string, 0, 1 ); # 'J'
746 To change part of a string, you can use the optional fourth
747 argument which is the replacement string.
749 substr( $string, 13, 4, "Perl 5.8.0" );
751 You can also use substr() as an lvalue.
753 substr( $string, 13, 4 ) = "Perl 5.8.0";
755 =head2 How do I change the Nth occurrence of something?
757 You have to keep track of N yourself. For example, let's say you want
758 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
759 C<"whosoever"> or C<"whomsoever">, case insensitively. These
760 all assume that $_ contains the string to be altered.
764 ++$count == 5 # is it the 5th?
765 ? "${2}soever" # yes, swap
766 : $1 # renege and leave it there
769 In the more general case, you can use the C</g> modifier in a C<while>
770 loop, keeping count of matches.
774 $_ = "One fish two fish red fish blue fish";
775 while (/(\w+)\s+fish\b/gi) {
776 if (++$count == $WANT) {
777 print "The third fish is a $1 one.\n";
781 That prints out: C<"The third fish is a red one."> You can also use a
782 repetition count and repeated pattern like this:
784 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
786 =head2 How can I count the number of occurrences of a substring within a string?
788 There are a number of ways, with varying efficiency. If you want a
789 count of a certain single character (X) within a string, you can use the
790 C<tr///> function like so:
792 $string = "ThisXlineXhasXsomeXx'sXinXit";
793 $count = ($string =~ tr/X//);
794 print "There are $count X characters in the string";
796 This is fine if you are just looking for a single character. However,
797 if you are trying to count multiple character substrings within a
798 larger string, C<tr///> won't work. What you can do is wrap a while()
799 loop around a global pattern match. For example, let's count negative
802 $string = "-9 55 48 -2 23 -76 4 14 -44";
803 while ($string =~ /-\d+/g) { $count++ }
804 print "There are $count negative numbers in the string";
806 Another version uses a global match in list context, then assigns the
807 result to a scalar, producing a count of the number of matches.
809 $count = () = $string =~ /-\d+/g;
811 =head2 Does Perl have a Year 2038 problem?
813 No, all of Perl's built in date and time functions and modules will
814 work to about 2 billion years before and after 1970.
816 Many systems cannot count time past the year 2038. Older versions of
817 Perl were dependent on the system to do date calculation and thus
818 shared their 2038 bug.
820 =head2 How do I capitalize all the words on one line?
821 X<Text::Autoformat> X<capitalize> X<case, title> X<case, sentence>
823 (contributed by brian d foy)
825 Damian Conway's L<Text::Autoformat> handles all of the thinking
828 use Text::Autoformat;
829 my $x = "Dr. Strangelove or: How I Learned to Stop ".
830 "Worrying and Love the Bomb";
833 for my $style (qw( sentence title highlight )) {
834 print autoformat($x, { case => $style }), "\n";
837 How do you want to capitalize those words?
839 FRED AND BARNEY'S LODGE # all uppercase
840 Fred And Barney's Lodge # title case
841 Fred and Barney's Lodge # highlight case
843 It's not as easy a problem as it looks. How many words do you think
844 are in there? Wait for it... wait for it.... If you answered 5
845 you're right. Perl words are groups of C<\w+>, but that's not what
846 you want to capitalize. How is Perl supposed to know not to capitalize
847 that C<s> after the apostrophe? You could try a regular expression:
850 (^\w) #at the beginning of the line
852 (\s\w) #preceded by whitespace
856 $string =~ s/([\w']+)/\u\L$1/g;
858 Now, what if you don't want to capitalize that "and"? Just use
859 L<Text::Autoformat> and get on with the next problem. :)
861 =head2 How can I split a [character] delimited string except when inside [character]?
863 Several modules can handle this sort of parsing--C<Text::Balanced>,
864 C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others.
866 Take the example case of trying to split a string that is
867 comma-separated into its different fields. You can't use C<split(/,/)>
868 because you shouldn't split if the comma is inside quotes. For
869 example, take a data line like this:
871 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
873 Due to the restriction of the quotes, this is a fairly complex
874 problem. Thankfully, we have Jeffrey Friedl, author of
875 I<Mastering Regular Expressions>, to handle these for us. He
876 suggests (assuming your string is contained in C<$text>):
879 push(@new, $+) while $text =~ m{
880 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
884 push(@new, undef) if substr($text,-1,1) eq ',';
886 If you want to represent quotation marks inside a
887 quotation-mark-delimited field, escape them with backslashes (eg,
890 Alternatively, the C<Text::ParseWords> module (part of the standard
891 Perl distribution) lets you say:
893 use Text::ParseWords;
894 @new = quotewords(",", 0, $text);
896 =head2 How do I strip blank space from the beginning/end of a string?
898 (contributed by brian d foy)
900 A substitution can do this for you. For a single line, you want to
901 replace all the leading or trailing whitespace with nothing. You
902 can do that with a pair of substitutions.
907 You can also write that as a single substitution, although it turns
908 out the combined statement is slower than the separate ones. That
909 might not matter to you, though.
913 In this regular expression, the alternation matches either at the
914 beginning or the end of the string since the anchors have a lower
915 precedence than the alternation. With the C</g> flag, the substitution
916 makes all possible matches, so it gets both. Remember, the trailing
917 newline matches the C<\s+>, and the C<$> anchor can match to the
918 physical end of the string, so the newline disappears too. Just add
919 the newline to the output, which has the added benefit of preserving
920 "blank" (consisting entirely of whitespace) lines which the C<^\s+>
921 would remove all by itself.
929 For a multi-line string, you can apply the regular expression
930 to each logical line in the string by adding the C</m> flag (for
931 "multi-line"). With the C</m> flag, the C<$> matches I<before> an
932 embedded newline, so it doesn't remove it. It still removes the
933 newline at the end of the string.
935 $string =~ s/^\s+|\s+$//gm;
937 Remember that lines consisting entirely of whitespace will disappear,
938 since the first part of the alternation can match the entire string
939 and replace it with nothing. If need to keep embedded blank lines,
940 you have to do a little more work. Instead of matching any whitespace
941 (since that includes a newline), just match the other whitespace.
943 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
945 =head2 How do I pad a string with blanks or pad a number with zeroes?
947 In the following examples, C<$pad_len> is the length to which you wish
948 to pad the string, C<$text> or C<$num> contains the string to be padded,
949 and C<$pad_char> contains the padding character. You can use a single
950 character string constant instead of the C<$pad_char> variable if you
951 know what it is in advance. And in the same way you can use an integer in
952 place of C<$pad_len> if you know the pad length in advance.
954 The simplest method uses the C<sprintf> function. It can pad on the left
955 or right with blanks and on the left with zeroes and it will not
956 truncate the result. The C<pack> function can only pad strings on the
957 right with blanks and it will truncate the result to a maximum length of
960 # Left padding a string with blanks (no truncation):
961 $padded = sprintf("%${pad_len}s", $text);
962 $padded = sprintf("%*s", $pad_len, $text); # same thing
964 # Right padding a string with blanks (no truncation):
965 $padded = sprintf("%-${pad_len}s", $text);
966 $padded = sprintf("%-*s", $pad_len, $text); # same thing
968 # Left padding a number with 0 (no truncation):
969 $padded = sprintf("%0${pad_len}d", $num);
970 $padded = sprintf("%0*d", $pad_len, $num); # same thing
972 # Right padding a string with blanks using pack (will truncate):
973 $padded = pack("A$pad_len",$text);
975 If you need to pad with a character other than blank or zero you can use
976 one of the following methods. They all generate a pad string with the
977 C<x> operator and combine that with C<$text>. These methods do
978 not truncate C<$text>.
980 Left and right padding with any character, creating a new string:
982 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
983 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
985 Left and right padding with any character, modifying C<$text> directly:
987 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
988 $text .= $pad_char x ( $pad_len - length( $text ) );
990 =head2 How do I extract selected columns from a string?
992 (contributed by brian d foy)
994 If you know where the columns that contain the data, you can
995 use C<substr> to extract a single column.
997 my $column = substr( $line, $start_column, $length );
999 You can use C<split> if the columns are separated by whitespace or
1000 some other delimiter, as long as whitespace or the delimiter cannot
1001 appear as part of the data.
1003 my $line = ' fred barney betty ';
1004 my @columns = split /\s+/, $line;
1005 # ( '', 'fred', 'barney', 'betty' );
1007 my $line = 'fred||barney||betty';
1008 my @columns = split /\|/, $line;
1009 # ( 'fred', '', 'barney', '', 'betty' );
1011 If you want to work with comma-separated values, don't do this since
1012 that format is a bit more complicated. Use one of the modules that
1013 handle that format, such as C<Text::CSV>, C<Text::CSV_XS>, or
1016 If you want to break apart an entire line of fixed columns, you can use
1017 C<unpack> with the A (ASCII) format. by using a number after the format
1018 specifier, you can denote the column width. See the C<pack> and C<unpack>
1019 entries in L<perlfunc> for more details.
1021 my @fields = unpack( $line, "A8 A8 A8 A16 A4" );
1023 Note that spaces in the format argument to C<unpack> do not denote literal
1024 spaces. If you have space separated data, you may want C<split> instead.
1026 =head2 How do I find the soundex value of a string?
1028 (contributed by brian d foy)
1030 You can use the Text::Soundex module. If you want to do fuzzy or close
1031 matching, you might also try the C<String::Approx>, and
1032 C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules.
1034 =head2 How can I expand variables in text strings?
1036 (contributed by brian d foy)
1038 If you can avoid it, don't, or if you can use a templating system,
1039 such as C<Text::Template> or C<Template> Toolkit, do that instead. You
1040 might even be able to get the job done with C<sprintf> or C<printf>:
1042 my $string = sprintf 'Say hello to %s and %s', $foo, $bar;
1044 However, for the one-off simple case where I don't want to pull out a
1045 full templating system, I'll use a string that has two Perl scalar
1046 variables in it. In this example, I want to expand C<$foo> and C<$bar>
1047 to their variable's values:
1051 $string = 'Say hello to $foo and $bar';
1053 One way I can do this involves the substitution operator and a double
1054 C</e> flag. The first C</e> evaluates C<$1> on the replacement side and
1055 turns it into C<$foo>. The second /e starts with C<$foo> and replaces
1056 it with its value. C<$foo>, then, turns into 'Fred', and that's finally
1057 what's left in the string:
1059 $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'
1061 The C</e> will also silently ignore violations of strict, replacing
1062 undefined variable names with the empty string. Since I'm using the
1063 C</e> flag (twice even!), I have all of the same security problems I
1064 have with C<eval> in its string form. If there's something odd in
1065 C<$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then
1066 I could get myself in trouble.
1068 To get around the security problem, I could also pull the values from
1069 a hash instead of evaluating variable names. Using a single C</e>, I
1070 can check the hash to ensure the value exists, and if it doesn't, I
1071 can replace the missing value with a marker, in this case C<???> to
1072 signal that I missed something:
1074 my $string = 'This has $foo and $bar';
1076 my %Replacements = (
1080 # $string =~ s/\$(\w+)/$Replacements{$1}/g;
1081 $string =~ s/\$(\w+)/
1082 exists $Replacements{$1} ? $Replacements{$1} : '???'
1087 =head2 What's wrong with always quoting "$vars"?
1089 The problem is that those double-quotes force
1090 stringification--coercing numbers and references into strings--even
1091 when you don't want them to be strings. Think of it this way:
1092 double-quote expansion is used to produce new strings. If you already
1093 have a string, why do you need more?
1095 If you get used to writing odd things like these:
1098 $new = "$old"; # BAD
1099 somefunc("$var"); # BAD
1101 You'll be in trouble. Those should (in 99.8% of the cases) be
1102 the simpler and more direct:
1108 Otherwise, besides slowing you down, you're going to break code when
1109 the thing in the scalar is actually neither a string nor a number, but
1115 my $oref = "$aref"; # WRONG
1118 You can also get into subtle problems on those few operations in Perl
1119 that actually do care about the difference between a string and a
1120 number, such as the magical C<++> autoincrement operator or the
1123 Stringification also destroys arrays.
1126 print "@lines"; # WRONG - extra blanks
1127 print @lines; # right
1129 =head2 Why don't my E<lt>E<lt>HERE documents work?
1131 Check for these three things:
1135 =item There must be no space after the E<lt>E<lt> part.
1137 =item There (probably) should be a semicolon at the end.
1139 =item You can't (easily) have any space in front of the tag.
1143 If you want to indent the text in the here document, you
1147 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1152 But the HERE_TARGET must still be flush against the margin.
1153 If you want that indented also, you'll have to quote
1156 ($quote = <<' FINIS') =~ s/^\s+//gm;
1157 ...we will have peace, when you and all your works have
1158 perished--and the works of your dark master to whom you
1159 would deliver us. You are a liar, Saruman, and a corrupter
1160 of men's hearts. --Theoden in /usr/src/perl/taint.c
1162 $quote =~ s/\s+--/\n--/;
1164 A nice general-purpose fixer-upper function for indented here documents
1165 follows. It expects to be called with a here document as its argument.
1166 It looks to see whether each line begins with a common substring, and
1167 if so, strips that substring off. Otherwise, it takes the amount of leading
1168 whitespace found on the first line and removes that much off each
1173 my ($white, $leader); # common whitespace and common leading string
1174 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1175 ($white, $leader) = ($2, quotemeta($1));
1177 ($white, $leader) = (/^(\s+)/, '');
1179 s/^\s*?$leader(?:$white)?//gm;
1183 This works with leading special strings, dynamically determined:
1185 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1188 @@@ SAVEI32(runlevel);
1190 @@@ while ( op = (*op->op_ppaddr)() );
1194 MAIN_INTERPRETER_LOOP
1196 Or with a fixed amount of leading whitespace, with remaining
1197 indentation correctly preserved:
1199 $poem = fix<<EVER_ON_AND_ON;
1200 Now far ahead the Road has gone,
1201 And I must follow, if I can,
1202 Pursuing it with eager feet,
1203 Until it joins some larger way
1204 Where many paths and errands meet.
1205 And whither then? I cannot say.
1206 --Bilbo in /usr/src/perl/pp_ctl.c
1211 =head2 What is the difference between a list and an array?
1213 An array has a changeable length. A list does not. An array is
1214 something you can push or pop, while a list is a set of values. Some
1215 people make the distinction that a list is a value while an array is a
1216 variable. Subroutines are passed and return lists, you put things into
1217 list context, you initialize arrays with lists, and you C<foreach()>
1218 across a list. C<@> variables are arrays, anonymous arrays are
1219 arrays, arrays in scalar context behave like the number of elements in
1220 them, subroutines access their arguments through the array C<@_>, and
1221 C<push>/C<pop>/C<shift> only work on arrays.
1223 As a side note, there's no such thing as a list in scalar context.
1226 $scalar = (2, 5, 7, 9);
1228 you're using the comma operator in scalar context, so it uses the scalar
1229 comma operator. There never was a list there at all! This causes the
1230 last value to be returned: 9.
1232 =head2 What is the difference between $array[1] and @array[1]?
1234 The former is a scalar value; the latter an array slice, making
1235 it a list with one (scalar) value. You should use $ when you want a
1236 scalar value (most of the time) and @ when you want a list with one
1237 scalar value in it (very, very rarely; nearly never, in fact).
1239 Sometimes it doesn't make a difference, but sometimes it does.
1240 For example, compare:
1242 $good[0] = `some program that outputs several lines`;
1246 @bad[0] = `same program that outputs several lines`;
1248 The C<use warnings> pragma and the B<-w> flag will warn you about these
1251 =head2 How can I remove duplicate elements from a list or array?
1253 (contributed by brian d foy)
1255 Use a hash. When you think the words "unique" or "duplicated", think
1258 If you don't care about the order of the elements, you could just
1259 create the hash then extract the keys. It's not important how you
1260 create that hash: just that you use C<keys> to get the unique
1263 my %hash = map { $_, 1 } @array;
1264 # or a hash slice: @hash{ @array } = ();
1265 # or a foreach: $hash{$_} = 1 foreach ( @array );
1267 my @unique = keys %hash;
1269 If you want to use a module, try the C<uniq> function from
1270 C<List::MoreUtils>. In list context it returns the unique elements,
1271 preserving their order in the list. In scalar context, it returns the
1272 number of unique elements.
1274 use List::MoreUtils qw(uniq);
1276 my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
1277 my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7
1279 You can also go through each element and skip the ones you've seen
1280 before. Use a hash to keep track. The first time the loop sees an
1281 element, that element has no key in C<%Seen>. The C<next> statement
1282 creates the key and immediately uses its value, which is C<undef>, so
1283 the loop continues to the C<push> and increments the value for that
1284 key. The next time the loop sees that same element, its key exists in
1285 the hash I<and> the value for that key is true (since it's not 0 or
1286 C<undef>), so the next skips that iteration and the loop goes to the
1292 foreach my $elem ( @array )
1294 next if $seen{ $elem }++;
1295 push @unique, $elem;
1298 You can write this more briefly using a grep, which does the
1302 my @unique = grep { ! $seen{ $_ }++ } @array;
1304 =head2 How can I tell whether a certain element is contained in a list or array?
1306 (portions of this answer contributed by Anno Siegel and brian d foy)
1308 Hearing the word "in" is an I<in>dication that you probably should have
1309 used a hash, not a list or array, to store your data. Hashes are
1310 designed to answer this question quickly and efficiently. Arrays aren't.
1312 That being said, there are several ways to approach this. In Perl 5.10
1313 and later, you can use the smart match operator to check that an item is
1314 contained in an array or a hash:
1318 if( $item ~~ @array )
1320 say "The array contains $item"
1323 if( $item ~~ %hash )
1325 say "The hash contains $item"
1328 With earlier versions of Perl, you have to do a bit more work. If you
1329 are going to make this query many times over arbitrary string values,
1330 the fastest way is probably to invert the original array and maintain a
1331 hash whose keys are the first array's values:
1333 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1335 for (@blues) { $is_blue{$_} = 1 }
1337 Now you can check whether C<$is_blue{$some_color}>. It might have
1338 been a good idea to keep the blues all in a hash in the first place.
1340 If the values are all small integers, you could use a simple indexed
1341 array. This kind of an array will take up less space:
1343 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1344 @is_tiny_prime = ();
1345 for (@primes) { $is_tiny_prime[$_] = 1 }
1346 # or simply @istiny_prime[@primes] = (1) x @primes;
1348 Now you check whether $is_tiny_prime[$some_number].
1350 If the values in question are integers instead of strings, you can save
1351 quite a lot of space by using bit strings instead:
1353 @articles = ( 1..10, 150..2000, 2017 );
1355 for (@articles) { vec($read,$_,1) = 1 }
1357 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1359 These methods guarantee fast individual tests but require a re-organization
1360 of the original list or array. They only pay off if you have to test
1361 multiple values against the same array.
1363 If you are testing only once, the standard module C<List::Util> exports
1364 the function C<first> for this purpose. It works by stopping once it
1365 finds the element. It's written in C for speed, and its Perl equivalent
1366 looks like this subroutine:
1371 return $_ if &{$code}();
1376 If speed is of little concern, the common idiom uses grep in scalar context
1377 (which returns the number of items that passed its condition) to traverse the
1378 entire list. This does have the benefit of telling you how many matches it
1381 my $is_there = grep $_ eq $whatever, @array;
1383 If you want to actually extract the matching elements, simply use grep in
1386 my @matches = grep $_ eq $whatever, @array;
1388 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1390 Use a hash. Here's code to do both and more. It assumes that each
1391 element is unique in a given array:
1393 @union = @intersection = @difference = ();
1395 foreach $element (@array1, @array2) { $count{$element}++ }
1396 foreach $element (keys %count) {
1397 push @union, $element;
1398 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1401 Note that this is the I<symmetric difference>, that is, all elements
1402 in either A or in B but not in both. Think of it as an xor operation.
1404 =head2 How do I test whether two arrays or hashes are equal?
1406 With Perl 5.10 and later, the smart match operator can give you the answer
1407 with the least amount of work:
1411 if( @array1 ~~ @array2 )
1413 say "The arrays are the same";
1416 if( %hash1 ~~ %hash2 ) # doesn't check values!
1418 say "The hash keys are the same";
1421 The following code works for single-level arrays. It uses a
1422 stringwise comparison, and does not distinguish defined versus
1423 undefined empty strings. Modify if you have other needs.
1425 $are_equal = compare_arrays(\@frogs, \@toads);
1427 sub compare_arrays {
1428 my ($first, $second) = @_;
1429 no warnings; # silence spurious -w undef complaints
1430 return 0 unless @$first == @$second;
1431 for (my $i = 0; $i < @$first; $i++) {
1432 return 0 if $first->[$i] ne $second->[$i];
1437 For multilevel structures, you may wish to use an approach more
1438 like this one. It uses the CPAN module C<FreezeThaw>:
1440 use FreezeThaw qw(cmpStr);
1441 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1443 printf "a and b contain %s arrays\n",
1444 cmpStr(\@a, \@b) == 0
1448 This approach also works for comparing hashes. Here we'll demonstrate
1449 two different answers:
1451 use FreezeThaw qw(cmpStr cmpStrHard);
1453 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1457 printf "a and b contain %s hashes\n",
1458 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1460 printf "a and b contain %s hashes\n",
1461 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1464 The first reports that both those the hashes contain the same data,
1465 while the second reports that they do not. Which you prefer is left as
1466 an exercise to the reader.
1468 =head2 How do I find the first array element for which a condition is true?
1470 To find the first array element which satisfies a condition, you can
1471 use the C<first()> function in the C<List::Util> module, which comes
1472 with Perl 5.8. This example finds the first element that contains
1475 use List::Util qw(first);
1477 my $element = first { /Perl/ } @array;
1479 If you cannot use C<List::Util>, you can make your own loop to do the
1480 same thing. Once you find the element, you stop the loop with last.
1483 foreach ( @array ) {
1484 if( /Perl/ ) { $found = $_; last }
1487 If you want the array index, you can iterate through the indices
1488 and check the array element at each index until you find one
1489 that satisfies the condition.
1491 my( $found, $index ) = ( undef, -1 );
1492 for( $i = 0; $i < @array; $i++ ) {
1493 if( $array[$i] =~ /Perl/ ) {
1494 $found = $array[$i];
1500 =head2 How do I handle linked lists?
1502 In general, you usually don't need a linked list in Perl, since with
1503 regular arrays, you can push and pop or shift and unshift at either
1504 end, or you can use splice to add and/or remove arbitrary number of
1505 elements at arbitrary points. Both pop and shift are O(1)
1506 operations on Perl's dynamic arrays. In the absence of shifts and
1507 pops, push in general needs to reallocate on the order every log(N)
1508 times, and unshift will need to copy pointers each time.
1510 If you really, really wanted, you could use structures as described in
1511 L<perldsc> or L<perltoot> and do just what the algorithm book tells
1512 you to do. For example, imagine a list node like this:
1519 You could walk the list this way:
1522 for ($node = $head; $node; $node = $node->{LINK}) {
1523 print $node->{VALUE}, " ";
1527 You could add to the list this way:
1530 $tail = append($head, 1); # grow a new head
1531 for $value ( 2 .. 10 ) {
1532 $tail = append($tail, $value);
1536 my($list, $value) = @_;
1537 my $node = { VALUE => $value };
1539 $node->{LINK} = $list->{LINK};
1540 $list->{LINK} = $node;
1543 $_[0] = $node; # replace caller's version
1548 But again, Perl's built-in are virtually always good enough.
1550 =head2 How do I handle circular lists?
1551 X<circular> X<array> X<Tie::Cycle> X<Array::Iterator::Circular>
1554 (contributed by brian d foy)
1556 If you want to cycle through an array endlessy, you can increment the
1557 index modulo the number of elements in the array:
1559 my @array = qw( a b c );
1563 print $array[ $i++ % @array ], "\n";
1567 You can also use C<Tie::Cycle> to use a scalar that always has the
1568 next element of the circular array:
1572 tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
1574 print $cycle; # FFFFFF
1575 print $cycle; # 000000
1576 print $cycle; # FFFF00
1578 The C<Array::Iterator::Circular> creates an iterator object for
1581 use Array::Iterator::Circular;
1583 my $color_iterator = Array::Iterator::Circular->new(
1584 qw(red green blue orange)
1587 foreach ( 1 .. 20 ) {
1588 print $color_iterator->next, "\n";
1591 =head2 How do I shuffle an array randomly?
1593 If you either have Perl 5.8.0 or later installed, or if you have
1594 Scalar-List-Utils 1.03 or later installed, you can say:
1596 use List::Util 'shuffle';
1598 @shuffled = shuffle(@list);
1600 If not, you can use a Fisher-Yates shuffle.
1602 sub fisher_yates_shuffle {
1603 my $deck = shift; # $deck is a reference to an array
1604 return unless @$deck; # must not be empty!
1608 my $j = int rand ($i+1);
1609 @$deck[$i,$j] = @$deck[$j,$i];
1613 # shuffle my mpeg collection
1615 my @mpeg = <audio/*/*.mp3>;
1616 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1619 Note that the above implementation shuffles an array in place,
1620 unlike the C<List::Util::shuffle()> which takes a list and returns
1621 a new shuffled list.
1623 You've probably seen shuffling algorithms that work using splice,
1624 randomly picking another element to swap the current element with
1628 @old = 1 .. 10; # just a demo
1630 push(@new, splice(@old, rand @old, 1));
1633 This is bad because splice is already O(N), and since you do it N
1634 times, you just invented a quadratic algorithm; that is, O(N**2).
1635 This does not scale, although Perl is so efficient that you probably
1636 won't notice this until you have rather largish arrays.
1638 =head2 How do I process/modify each element of an array?
1640 Use C<for>/C<foreach>:
1643 s/foo/bar/; # change that word
1644 tr/XZ/ZX/; # swap those letters
1647 Here's another; let's compute spherical volumes:
1649 for (@volumes = @radii) { # @volumes has changed parts
1651 $_ *= (4/3) * 3.14159; # this will be constant folded
1654 which can also be done with C<map()> which is made to transform
1655 one list into another:
1657 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1659 If you want to do the same thing to modify the values of the
1660 hash, you can use the C<values> function. As of Perl 5.6
1661 the values are not copied, so if you modify $orbit (in this
1662 case), you modify the value.
1664 for $orbit ( values %orbits ) {
1665 ($orbit **= 3) *= (4/3) * 3.14159;
1668 Prior to perl 5.6 C<values> returned copies of the values,
1669 so older perl code often contains constructions such as
1670 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1671 the hash is to be modified.
1673 =head2 How do I select a random element from an array?
1675 Use the C<rand()> function (see L<perlfunc/rand>):
1677 $index = rand @array;
1678 $element = $array[$index];
1682 my $element = $array[ rand @array ];
1684 =head2 How do I permute N elements of a list?
1685 X<List::Permuter> X<permute> X<Algorithm::Loops> X<Knuth>
1686 X<The Art of Computer Programming> X<Fischer-Krause>
1688 Use the C<List::Permutor> module on CPAN. If the list is actually an
1689 array, try the C<Algorithm::Permute> module (also on CPAN). It's
1690 written in XS code and is very efficient:
1692 use Algorithm::Permute;
1694 my @array = 'a'..'d';
1695 my $p_iterator = Algorithm::Permute->new ( \@array );
1697 while (my @perm = $p_iterator->next) {
1698 print "next permutation: (@perm)\n";
1701 For even faster execution, you could do:
1703 use Algorithm::Permute;
1705 my @array = 'a'..'d';
1707 Algorithm::Permute::permute {
1708 print "next permutation: (@array)\n";
1711 Here's a little program that generates all permutations of all the
1712 words on each line of input. The algorithm embodied in the
1713 C<permute()> function is discussed in Volume 4 (still unpublished) of
1714 Knuth's I<The Art of Computer Programming> and will work on any list:
1717 # Fischer-Krause ordered permutation generator
1722 while ( $code->(@_[@idx]) ) {
1724 --$p while $idx[$p-1] > $idx[$p];
1725 my $q = $p or return;
1726 push @idx, reverse splice @idx, $p;
1727 ++$q while $idx[$p-1] > $idx[$q];
1728 @idx[$p-1,$q]=@idx[$q,$p-1];
1732 permute { print "@_\n" } split;
1734 The C<Algorithm::Loops> module also provides the C<NextPermute> and
1735 C<NextPermuteNum> functions which efficiently find all unique permutations
1736 of an array, even if it contains duplicate values, modifying it in-place:
1737 if its elements are in reverse-sorted order then the array is reversed,
1738 making it sorted, and it returns false; otherwise the next
1739 permutation is returned.
1741 C<NextPermute> uses string order and C<NextPermuteNum> numeric order, so
1742 you can enumerate all the permutations of C<0..9> like this:
1744 use Algorithm::Loops qw(NextPermuteNum);
1747 do { print "@list\n" } while NextPermuteNum @list;
1749 =head2 How do I sort an array by (anything)?
1751 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1753 @list = sort { $a <=> $b } @list;
1755 The default sort function is cmp, string comparison, which would
1756 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1757 the numerical comparison operator.
1759 If you have a complicated function needed to pull out the part you
1760 want to sort on, then don't do it inside the sort function. Pull it
1761 out first, because the sort BLOCK can be called many times for the
1762 same element. Here's an example of how to pull out the first word
1763 after the first number on each item, and then sort those words
1768 ($item) = /\d+\s*(\S+)/;
1769 push @idx, uc($item);
1771 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1773 which could also be written this way, using a trick
1774 that's come to be known as the Schwartzian Transform:
1776 @sorted = map { $_->[0] }
1777 sort { $a->[1] cmp $b->[1] }
1778 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1780 If you need to sort on several fields, the following paradigm is useful.
1783 field1($a) <=> field1($b) ||
1784 field2($a) cmp field2($b) ||
1785 field3($a) cmp field3($b)
1788 This can be conveniently combined with precalculation of keys as given
1791 See the F<sort> article in the "Far More Than You Ever Wanted
1792 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1793 more about this approach.
1795 See also the question later in L<perlfaq4> on sorting hashes.
1797 =head2 How do I manipulate arrays of bits?
1799 Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise
1802 For example, you don't have to store individual bits in an array
1803 (which would mean that you're wasting a lot of space). To convert an
1804 array of bits to a string, use C<vec()> to set the right bits. This
1805 sets C<$vec> to have bit N set only if C<$ints[N]> was set:
1807 @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
1809 foreach( 0 .. $#ints ) {
1810 vec($vec,$_,1) = 1 if $ints[$_];
1813 The string C<$vec> only takes up as many bits as it needs. For
1814 instance, if you had 16 entries in C<@ints>, C<$vec> only needs two
1815 bytes to store them (not counting the scalar variable overhead).
1817 Here's how, given a vector in C<$vec>, you can get those bits into
1818 your C<@ints> array:
1820 sub bitvec_to_list {
1823 # Find null-byte density then select best algorithm
1824 if ($vec =~ tr/\0// / length $vec > 0.95) {
1828 # This method is faster with mostly null-bytes
1829 while($vec =~ /[^\0]/g ) {
1830 $i = -9 + 8 * pos $vec;
1831 push @ints, $i if vec($vec, ++$i, 1);
1832 push @ints, $i if vec($vec, ++$i, 1);
1833 push @ints, $i if vec($vec, ++$i, 1);
1834 push @ints, $i if vec($vec, ++$i, 1);
1835 push @ints, $i if vec($vec, ++$i, 1);
1836 push @ints, $i if vec($vec, ++$i, 1);
1837 push @ints, $i if vec($vec, ++$i, 1);
1838 push @ints, $i if vec($vec, ++$i, 1);
1842 # This method is a fast general algorithm
1844 my $bits = unpack "b*", $vec;
1845 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1846 push @ints, pos $bits while($bits =~ /1/g);
1852 This method gets faster the more sparse the bit vector is.
1853 (Courtesy of Tim Bunce and Winfried Koenig.)
1855 You can make the while loop a lot shorter with this suggestion
1856 from Benjamin Goldberg:
1858 while($vec =~ /[^\0]+/g ) {
1859 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1862 Or use the CPAN module C<Bit::Vector>:
1864 $vector = Bit::Vector->new($num_of_bits);
1865 $vector->Index_List_Store(@ints);
1866 @ints = $vector->Index_List_Read();
1868 C<Bit::Vector> provides efficient methods for bit vector, sets of
1869 small integers and "big int" math.
1871 Here's a more extensive illustration using vec():
1874 $vector = "\xff\x0f\xef\xfe";
1875 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1876 unpack("N", $vector), "\n";
1877 $is_set = vec($vector, 23, 1);
1878 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1896 my ($offset, $width, $value) = @_;
1898 vec($vector, $offset, $width) = $value;
1899 print "offset=$offset width=$width value=$value\n";
1905 my $bits = unpack("b*", $vector);
1909 print "vector length in bytes: ", length($vector), "\n";
1910 @bytes = unpack("A8" x length($vector), $bits);
1911 print "bits are: @bytes\n\n";
1914 =head2 Why does defined() return true on empty arrays and hashes?
1916 The short story is that you should probably only use defined on scalars or
1917 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1918 in the 5.004 release or later of Perl for more detail.
1920 =head1 Data: Hashes (Associative Arrays)
1922 =head2 How do I process an entire hash?
1924 (contributed by brian d foy)
1926 There are a couple of ways that you can process an entire hash. You
1927 can get a list of keys, then go through each key, or grab a one
1928 key-value pair at a time.
1930 To go through all of the keys, use the C<keys> function. This extracts
1931 all of the keys of the hash and gives them back to you as a list. You
1932 can then get the value through the particular key you're processing:
1934 foreach my $key ( keys %hash ) {
1935 my $value = $hash{$key}
1939 Once you have the list of keys, you can process that list before you
1940 process the hash elements. For instance, you can sort the keys so you
1941 can process them in lexical order:
1943 foreach my $key ( sort keys %hash ) {
1944 my $value = $hash{$key}
1948 Or, you might want to only process some of the items. If you only want
1949 to deal with the keys that start with C<text:>, you can select just
1950 those using C<grep>:
1952 foreach my $key ( grep /^text:/, keys %hash ) {
1953 my $value = $hash{$key}
1957 If the hash is very large, you might not want to create a long list of
1958 keys. To save some memory, you can grab one key-value pair at a time using
1959 C<each()>, which returns a pair you haven't seen yet:
1961 while( my( $key, $value ) = each( %hash ) ) {
1965 The C<each> operator returns the pairs in apparently random order, so if
1966 ordering matters to you, you'll have to stick with the C<keys> method.
1968 The C<each()> operator can be a bit tricky though. You can't add or
1969 delete keys of the hash while you're using it without possibly
1970 skipping or re-processing some pairs after Perl internally rehashes
1971 all of the elements. Additionally, a hash has only one iterator, so if
1972 you use C<keys>, C<values>, or C<each> on the same hash, you can reset
1973 the iterator and mess up your processing. See the C<each> entry in
1974 L<perlfunc> for more details.
1976 =head2 How do I merge two hashes?
1977 X<hash> X<merge> X<slice, hash>
1979 (contributed by brian d foy)
1981 Before you decide to merge two hashes, you have to decide what to do
1982 if both hashes contain keys that are the same and if you want to leave
1983 the original hashes as they were.
1985 If you want to preserve the original hashes, copy one hash (C<%hash1>)
1986 to a new hash (C<%new_hash>), then add the keys from the other hash
1987 (C<%hash2> to the new hash. Checking that the key already exists in
1988 C<%new_hash> gives you a chance to decide what to do with the
1991 my %new_hash = %hash1; # make a copy; leave %hash1 alone
1993 foreach my $key2 ( keys %hash2 )
1995 if( exists $new_hash{$key2} )
1997 warn "Key [$key2] is in both hashes!";
1998 # handle the duplicate (perhaps only warning)
2004 $new_hash{$key2} = $hash2{$key2};
2008 If you don't want to create a new hash, you can still use this looping
2009 technique; just change the C<%new_hash> to C<%hash1>.
2011 foreach my $key2 ( keys %hash2 )
2013 if( exists $hash1{$key2} )
2015 warn "Key [$key2] is in both hashes!";
2016 # handle the duplicate (perhaps only warning)
2022 $hash1{$key2} = $hash2{$key2};
2026 If you don't care that one hash overwrites keys and values from the other, you
2027 could just use a hash slice to add one hash to another. In this case, values
2028 from C<%hash2> replace values from C<%hash1> when they have keys in common:
2030 @hash1{ keys %hash2 } = values %hash2;
2032 =head2 What happens if I add or remove keys from a hash while iterating over it?
2034 (contributed by brian d foy)
2036 The easy answer is "Don't do that!"
2038 If you iterate through the hash with each(), you can delete the key
2039 most recently returned without worrying about it. If you delete or add
2040 other keys, the iterator may skip or double up on them since perl
2041 may rearrange the hash table. See the
2042 entry for C<each()> in L<perlfunc>.
2044 =head2 How do I look up a hash element by value?
2046 Create a reverse hash:
2048 %by_value = reverse %by_key;
2049 $key = $by_value{$value};
2051 That's not particularly efficient. It would be more space-efficient
2054 while (($key, $value) = each %by_key) {
2055 $by_value{$value} = $key;
2058 If your hash could have repeated values, the methods above will only find
2059 one of the associated keys. This may or may not worry you. If it does
2060 worry you, you can always reverse the hash into a hash of arrays instead:
2062 while (($key, $value) = each %by_key) {
2063 push @{$key_list_by_value{$value}}, $key;
2066 =head2 How can I know how many entries are in a hash?
2068 (contributed by brian d foy)
2070 This is very similar to "How do I process an entire hash?", also in
2071 L<perlfaq4>, but a bit simpler in the common cases.
2073 You can use the C<keys()> built-in function in scalar context to find out
2074 have many entries you have in a hash:
2076 my $key_count = keys %hash; # must be scalar context!
2078 If you want to find out how many entries have a defined value, that's
2079 a bit different. You have to check each value. A C<grep> is handy:
2081 my $defined_value_count = grep { defined } values %hash;
2083 You can use that same structure to count the entries any way that
2084 you like. If you want the count of the keys with vowels in them,
2085 you just test for that instead:
2087 my $vowel_count = grep { /[aeiou]/ } keys %hash;
2089 The C<grep> in scalar context returns the count. If you want the list
2090 of matching items, just use it in list context instead:
2092 my @defined_values = grep { defined } values %hash;
2094 The C<keys()> function also resets the iterator, which means that you may
2095 see strange results if you use this between uses of other hash operators
2098 =head2 How do I sort a hash (optionally by value instead of key)?
2100 (contributed by brian d foy)
2102 To sort a hash, start with the keys. In this example, we give the list of
2103 keys to the sort function which then compares them ASCIIbetically (which
2104 might be affected by your locale settings). The output list has the keys
2105 in ASCIIbetical order. Once we have the keys, we can go through them to
2106 create a report which lists the keys in ASCIIbetical order.
2108 my @keys = sort { $a cmp $b } keys %hash;
2110 foreach my $key ( @keys )
2112 printf "%-20s %6d\n", $key, $hash{$key};
2115 We could get more fancy in the C<sort()> block though. Instead of
2116 comparing the keys, we can compute a value with them and use that
2117 value as the comparison.
2119 For instance, to make our report order case-insensitive, we use
2120 the C<\L> sequence in a double-quoted string to make everything
2121 lowercase. The C<sort()> block then compares the lowercased
2122 values to determine in which order to put the keys.
2124 my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
2126 Note: if the computation is expensive or the hash has many elements,
2127 you may want to look at the Schwartzian Transform to cache the
2128 computation results.
2130 If we want to sort by the hash value instead, we use the hash key
2131 to look it up. We still get out a list of keys, but this time they
2132 are ordered by their value.
2134 my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
2136 From there we can get more complex. If the hash values are the same,
2137 we can provide a secondary sort on the hash key.
2140 $hash{$a} <=> $hash{$b}
2145 =head2 How can I always keep my hash sorted?
2146 X<hash tie sort DB_File Tie::IxHash>
2148 You can look into using the C<DB_File> module and C<tie()> using the
2149 C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory
2150 Databases">. The C<Tie::IxHash> module from CPAN might also be
2151 instructive. Although this does keep your hash sorted, you might not
2152 like the slow down you suffer from the tie interface. Are you sure you
2155 =head2 What's the difference between "delete" and "undef" with hashes?
2157 Hashes contain pairs of scalars: the first is the key, the
2158 second is the value. The key will be coerced to a string,
2159 although the value can be any kind of scalar: string,
2160 number, or reference. If a key C<$key> is present in
2161 %hash, C<exists($hash{$key})> will return true. The value
2162 for a given key can be C<undef>, in which case
2163 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
2164 will return true. This corresponds to (C<$key>, C<undef>)
2167 Pictures help... here's the C<%hash> table:
2177 And these conditions hold
2181 defined $hash{'d'} is true
2182 defined $hash{'a'} is true
2183 exists $hash{'a'} is true (Perl 5 only)
2184 grep ($_ eq 'a', keys %hash) is true
2190 your table now reads:
2201 and these conditions now hold; changes in caps:
2205 defined $hash{'d'} is true
2206 defined $hash{'a'} is FALSE
2207 exists $hash{'a'} is true (Perl 5 only)
2208 grep ($_ eq 'a', keys %hash) is true
2210 Notice the last two: you have an undef value, but a defined key!
2216 your table now reads:
2225 and these conditions now hold; changes in caps:
2229 defined $hash{'d'} is true
2230 defined $hash{'a'} is false
2231 exists $hash{'a'} is FALSE (Perl 5 only)
2232 grep ($_ eq 'a', keys %hash) is FALSE
2234 See, the whole entry is gone!
2236 =head2 Why don't my tied hashes make the defined/exists distinction?
2238 This depends on the tied hash's implementation of EXISTS().
2239 For example, there isn't the concept of undef with hashes
2240 that are tied to DBM* files. It also means that exists() and
2241 defined() do the same thing with a DBM* file, and what they
2242 end up doing is not what they do with ordinary hashes.
2244 =head2 How do I reset an each() operation part-way through?
2246 (contributed by brian d foy)
2248 You can use the C<keys> or C<values> functions to reset C<each>. To
2249 simply reset the iterator used by C<each> without doing anything else,
2250 use one of them in void context:
2252 keys %hash; # resets iterator, nothing else.
2253 values %hash; # resets iterator, nothing else.
2255 See the documentation for C<each> in L<perlfunc>.
2257 =head2 How can I get the unique keys from two hashes?
2259 First you extract the keys from the hashes into lists, then solve
2260 the "removing duplicates" problem described above. For example:
2263 for $element (keys(%foo), keys(%bar)) {
2270 @uniq = keys %{{%foo,%bar}};
2272 Or if you really want to save space:
2275 while (defined ($key = each %foo)) {
2278 while (defined ($key = each %bar)) {
2283 =head2 How can I store a multidimensional array in a DBM file?
2285 Either stringify the structure yourself (no fun), or else
2286 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
2287 it on top of either DB_File or GDBM_File.
2289 =head2 How can I make my hash remember the order I put elements into it?
2291 Use the C<Tie::IxHash> from CPAN.
2295 tie my %myhash, 'Tie::IxHash';
2297 for (my $i=0; $i<20; $i++) {
2301 my @keys = keys %myhash;
2302 # @keys = (0,1,2,3,...)
2304 =head2 Why does passing a subroutine an undefined element in a hash create it?
2306 (contributed by brian d foy)
2308 Are you using a really old version of Perl?
2310 Normally, accessing a hash key's value for a nonexistent key will
2311 I<not> create the key.
2314 my $value = $hash{ 'foo' };
2315 print "This won't print\n" if exists $hash{ 'foo' };
2317 Passing C<$hash{ 'foo' }> to a subroutine used to be a special case, though.
2318 Since you could assign directly to C<$_[0]>, Perl had to be ready to
2319 make that assignment so it created the hash key ahead of time:
2321 my_sub( $hash{ 'foo' } );
2322 print "This will print before 5.004\n" if exists $hash{ 'foo' };
2325 # $_[0] = 'bar'; # create hash key in case you do this
2329 Since Perl 5.004, however, this situation is a special case and Perl
2330 creates the hash key only when you make the assignment:
2332 my_sub( $hash{ 'foo' } );
2333 print "This will print, even after 5.004\n" if exists $hash{ 'foo' };
2339 However, if you want the old behavior (and think carefully about that
2340 because it's a weird side effect), you can pass a hash slice instead.
2341 Perl 5.004 didn't make this a special case:
2343 my_sub( @hash{ qw/foo/ } );
2345 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
2347 Usually a hash ref, perhaps like this:
2352 TITLE => "deputy peon",
2355 PALS => [ "Norbert", "Rhys", "Phineas"],
2358 References are documented in L<perlref> and the upcoming L<perlreftut>.
2359 Examples of complex data structures are given in L<perldsc> and
2360 L<perllol>. Examples of structures and object-oriented classes are
2363 =head2 How can I use a reference as a hash key?
2365 (contributed by brian d foy and Ben Morrow)
2367 Hash keys are strings, so you can't really use a reference as the key.
2368 When you try to do that, perl turns the reference into its stringified
2369 form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get
2370 back the reference from the stringified form, at least without doing
2371 some extra work on your own.
2373 Remember that the entry in the hash will still be there even if
2374 the referenced variable goes out of scope, and that it is entirely
2375 possible for Perl to subsequently allocate a different variable at
2376 the same address. This will mean a new variable might accidentally
2377 be associated with the value for an old.
2379 If you have Perl 5.10 or later, and you just want to store a value
2380 against the reference for lookup later, you can use the core
2381 Hash::Util::Fieldhash module. This will also handle renaming the
2382 keys if you use multiple threads (which causes all variables to be
2383 reallocated at new addresses, changing their stringification), and
2384 garbage-collecting the entries when the referenced variable goes out
2387 If you actually need to be able to get a real reference back from
2388 each hash entry, you can use the Tie::RefHash module, which does the
2389 required work for you.
2393 =head2 How do I handle binary data correctly?
2395 Perl is binary clean, so it can handle binary data just fine.
2396 On Windows or DOS, however, you have to use C<binmode> for binary
2397 files to avoid conversions for line endings. In general, you should
2398 use C<binmode> any time you want to work with binary data.
2400 Also see L<perlfunc/"binmode"> or L<perlopentut>.
2402 If you're concerned about 8-bit textual data then see L<perllocale>.
2403 If you want to deal with multibyte characters, however, there are
2404 some gotchas. See the section on Regular Expressions.
2406 =head2 How do I determine whether a scalar is a number/whole/integer/float?
2408 Assuming that you don't care about IEEE notations like "NaN" or
2409 "Infinity", you probably just want to use a regular expression.
2411 if (/\D/) { print "has nondigits\n" }
2412 if (/^\d+$/) { print "is a whole number\n" }
2413 if (/^-?\d+$/) { print "is an integer\n" }
2414 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2415 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2416 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2417 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2418 { print "a C float\n" }
2420 There are also some commonly used modules for the task.
2421 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2422 internal function C<looks_like_number> for determining whether a
2423 variable looks like a number. L<Data::Types> exports functions that
2424 validate data types using both the above and other regular
2425 expressions. Thirdly, there is C<Regexp::Common> which has regular
2426 expressions to match various types of numbers. Those three modules are
2427 available from the CPAN.
2429 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2430 function. Its semantics are somewhat cumbersome, so here's a
2431 C<getnum> wrapper function for more convenient access. This function
2432 takes a string and returns the number it found, or C<undef> for input
2433 that isn't a C float. The C<is_numeric> function is a front end to
2434 C<getnum> if you just want to say, "Is this a float?"
2437 use POSIX qw(strtod);
2442 my($num, $unparsed) = strtod($str);
2443 if (($str eq '') || ($unparsed != 0) || $!) {
2451 sub is_numeric { defined getnum($_[0]) }
2453 Or you could check out the L<String::Scanf> module on the CPAN
2454 instead. The C<POSIX> module (part of the standard Perl distribution)
2455 provides the C<strtod> and C<strtol> for converting strings to double
2456 and longs, respectively.
2458 =head2 How do I keep persistent data across program calls?
2460 For some specific applications, you can use one of the DBM modules.
2461 See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw>
2462 or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part
2463 of the standard distribution. Here's one example using C<Storable>'s C<store>
2464 and C<retrieve> functions:
2467 store(\%hash, "filename");
2470 $href = retrieve("filename"); # by ref
2471 %hash = %{ retrieve("filename") }; # direct to hash
2473 =head2 How do I print out or copy a recursive data structure?
2475 The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great
2476 for printing out data structures. The C<Storable> module on CPAN (or the
2477 5.8 release of Perl), provides a function called C<dclone> that recursively
2478 copies its argument.
2480 use Storable qw(dclone);
2483 Where C<$r1> can be a reference to any kind of data structure you'd like.
2484 It will be deeply copied. Because C<dclone> takes and returns references,
2485 you'd have to add extra punctuation if you had a hash of arrays that
2488 %newhash = %{ dclone(\%oldhash) };
2490 =head2 How do I define methods for every class/object?
2492 (contributed by Ben Morrow)
2494 You can use the C<UNIVERSAL> class (see L<UNIVERSAL>). However, please
2495 be very careful to consider the consequences of doing this: adding
2496 methods to every object is very likely to have unintended
2497 consequences. If possible, it would be better to have all your object
2498 inherit from some common base class, or to use an object system like
2499 Moose that supports roles.
2501 =head2 How do I verify a credit card checksum?
2503 Get the C<Business::CreditCard> module from CPAN.
2505 =head2 How do I pack arrays of doubles or floats for XS code?
2507 The arrays.h/arrays.c code in the C<PGPLOT> module on CPAN does just this.
2508 If you're doing a lot of float or double processing, consider using
2509 the C<PDL> module from CPAN instead--it makes number-crunching easy.
2511 See L<http://search.cpan.org/dist/PGPLOT> for the code.
2515 Revision: $Revision$
2519 See L<perlfaq> for source control details and availability.
2521 =head1 AUTHOR AND COPYRIGHT
2523 Copyright (c) 1997-2009 Tom Christiansen, Nathan Torkington, and
2524 other authors as noted. All rights reserved.
2526 This documentation is free; you can redistribute it and/or modify it
2527 under the same terms as Perl itself.
2529 Irrespective of its distribution, all code examples in this file
2530 are hereby placed into the public domain. You are permitted and
2531 encouraged to use this code in your own programs for fun
2532 or for profit as you see fit. A simple comment in the code giving
2533 credit would be courteous but is not required.