3 perlfaq4 - Data Manipulation ($Revision: 7954 $)
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> show 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 Perl only understands octal and hex numbers as such when they occur as
52 literals in your program. Octal literals in perl must start with a
53 leading C<0> and hexadecimal literals must start with a leading C<0x>.
54 If they are read in from somewhere and assigned, no automatic
55 conversion takes place. You must explicitly use C<oct()> or C<hex()> if you
56 want the values converted to decimal. C<oct()> interprets hexadecimal (C<0x350>),
57 octal (C<0350> or even without the leading C<0>, like C<377>) and binary
58 (C<0b1010>) numbers, while C<hex()> only converts hexadecimal ones, with
59 or without a leading C<0x>, such as C<0x255>, C<3A>, C<ff>, or C<deadbeef>.
60 The inverse mapping from decimal to octal can be done with either the
61 <%o> or C<%O> C<sprintf()> formats.
63 This problem shows up most often when people try using C<chmod()>,
64 C<mkdir()>, C<umask()>, or C<sysopen()>, which by widespread tradition
65 typically take permissions in octal.
67 chmod(644, $file); # WRONG
68 chmod(0644, $file); # right
70 Note the mistake in the first line was specifying the decimal literal
71 C<644>, rather than the intended octal literal C<0644>. The problem can
74 printf("%#o",644); # prints 01204
76 Surely you had not intended C<chmod(01204, $file);> - did you? If you
77 want to use numeric literals as arguments to chmod() et al. then please
78 try to express them as octal constants, that is with a leading zero and
79 with the following digits restricted to the set C<0..7>.
81 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
83 Remember that C<int()> merely truncates toward 0. For rounding to a
84 certain number of digits, C<sprintf()> or C<printf()> is usually the
87 printf("%.3f", 3.1415926535); # prints 3.142
89 The C<POSIX> module (part of the standard Perl distribution)
90 implements C<ceil()>, C<floor()>, and a number of other mathematical
91 and trigonometric functions.
94 $ceil = ceil(3.5); # 4
95 $floor = floor(3.5); # 3
97 In 5.000 to 5.003 perls, trigonometry was done in the C<Math::Complex>
98 module. With 5.004, the C<Math::Trig> module (part of the standard Perl
99 distribution) implements the trigonometric functions. Internally it
100 uses the C<Math::Complex> module and some functions can break out from
101 the real axis into the complex plane, for example the inverse sine of
104 Rounding in financial applications can have serious implications, and
105 the rounding method used should be specified precisely. In these
106 cases, it probably pays not to trust whichever system rounding is
107 being used by Perl, but to instead implement the rounding function you
110 To see why, notice how you'll still have an issue on half-way-point
113 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
115 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
116 0.8 0.8 0.9 0.9 1.0 1.0
118 Don't blame Perl. It's the same as in C. IEEE says we have to do
119 this. Perl numbers whose absolute values are integers under 2**31 (on
120 32 bit machines) will work pretty much like mathematical integers.
121 Other numbers are not guaranteed.
123 =head2 How do I convert between numeric representations/bases/radixes?
125 As always with Perl there is more than one way to do it. Below are a
126 few examples of approaches to making common conversions between number
127 representations. This is intended to be representational rather than
130 Some of the examples later in L<perlfaq4> use the C<Bit::Vector>
131 module from CPAN. The reason you might choose C<Bit::Vector> over the
132 perl built in functions is that it works with numbers of ANY size,
133 that it is optimized for speed on some operations, and for at least
134 some programmers the notation might be familiar.
138 =item How do I convert hexadecimal into decimal
140 Using perl's built in conversion of C<0x> notation:
144 Using the C<hex> function:
146 $dec = hex("DEADBEEF");
150 $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
152 Using the CPAN module C<Bit::Vector>:
155 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
156 $dec = $vec->to_Dec();
158 =item How do I convert from decimal to hexadecimal
162 $hex = sprintf("%X", 3735928559); # upper case A-F
163 $hex = sprintf("%x", 3735928559); # lower case a-f
167 $hex = unpack("H*", pack("N", 3735928559));
169 Using C<Bit::Vector>:
172 $vec = Bit::Vector->new_Dec(32, -559038737);
173 $hex = $vec->to_Hex();
175 And C<Bit::Vector> supports odd bit counts:
178 $vec = Bit::Vector->new_Dec(33, 3735928559);
179 $vec->Resize(32); # suppress leading 0 if unwanted
180 $hex = $vec->to_Hex();
182 =item How do I convert from octal to decimal
184 Using Perl's built in conversion of numbers with leading zeros:
186 $dec = 033653337357; # note the leading 0!
188 Using the C<oct> function:
190 $dec = oct("33653337357");
192 Using C<Bit::Vector>:
195 $vec = Bit::Vector->new(32);
196 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
197 $dec = $vec->to_Dec();
199 =item How do I convert from decimal to octal
203 $oct = sprintf("%o", 3735928559);
205 Using C<Bit::Vector>:
208 $vec = Bit::Vector->new_Dec(32, -559038737);
209 $oct = reverse join('', $vec->Chunk_List_Read(3));
211 =item How do I convert from binary to decimal
213 Perl 5.6 lets you write binary numbers directly with
216 $number = 0b10110110;
220 my $input = "10110110";
221 $decimal = oct( "0b$input" );
223 Using C<pack> and C<ord>:
225 $decimal = ord(pack('B8', '10110110'));
227 Using C<pack> and C<unpack> for larger strings:
229 $int = unpack("N", pack("B32",
230 substr("0" x 32 . "11110101011011011111011101111", -32)));
231 $dec = sprintf("%d", $int);
233 # substr() is used to left pad a 32 character string with zeros.
235 Using C<Bit::Vector>:
237 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
238 $dec = $vec->to_Dec();
240 =item How do I convert from decimal to binary
242 Using C<sprintf> (perl 5.6+):
244 $bin = sprintf("%b", 3735928559);
248 $bin = unpack("B*", pack("N", 3735928559));
250 Using C<Bit::Vector>:
253 $vec = Bit::Vector->new_Dec(32, -559038737);
254 $bin = $vec->to_Bin();
256 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
257 are left as an exercise to the inclined reader.
261 =head2 Why doesn't & work the way I want it to?
263 The behavior of binary arithmetic operators depends on whether they're
264 used on numbers or strings. The operators treat a string as a series
265 of bits and work with that (the string C<"3"> is the bit pattern
266 C<00110011>). The operators work with the binary form of a number
267 (the number C<3> is treated as the bit pattern C<00000011>).
269 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
270 C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
273 Most problems with C<&> and C<|> arise because the programmer thinks
274 they have a number but really it's a string. The rest arise because
277 if ("\020\020" & "\101\101") {
281 but a string consisting of two null bytes (the result of C<"\020\020"
282 & "\101\101">) is not a false value in Perl. You need:
284 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
288 =head2 How do I multiply matrices?
290 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
291 or the PDL extension (also available from CPAN).
293 =head2 How do I perform an operation on a series of integers?
295 To call a function on each element in an array, and collect the
298 @results = map { my_func($_) } @array;
302 @triple = map { 3 * $_ } @single;
304 To call a function on each element of an array, but ignore the
307 foreach $iterator (@array) {
308 some_func($iterator);
311 To call a function on each integer in a (small) range, you B<can> use:
313 @results = map { some_func($_) } (5 .. 25);
315 but you should be aware that the C<..> operator creates an array of
316 all integers in the range. This can take a lot of memory for large
320 for ($i=5; $i < 500_005; $i++) {
321 push(@results, some_func($i));
324 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
325 loop will iterate over the range, without creating the entire range.
327 for my $i (5 .. 500_005) {
328 push(@results, some_func($i));
331 will not create a list of 500,000 integers.
333 =head2 How can I output Roman numerals?
335 Get the http://www.cpan.org/modules/by-module/Roman module.
337 =head2 Why aren't my random numbers random?
339 If you're using a version of Perl before 5.004, you must call C<srand>
340 once at the start of your program to seed the random number generator.
342 BEGIN { srand() if $] < 5.004 }
344 5.004 and later automatically call C<srand> at the beginning. Don't
345 call C<srand> more than once--you make your numbers less random,
348 Computers are good at being predictable and bad at being random
349 (despite appearances caused by bugs in your programs :-). see the
350 F<random> article in the "Far More Than You Ever Wanted To Know"
351 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy
352 of Tom Phoenix, talks more about this. John von Neumann said, "Anyone
353 who attempts to generate random numbers by deterministic means is, of
354 course, living in a state of sin."
356 If you want numbers that are more random than C<rand> with C<srand>
357 provides, you should also check out the C<Math::TrulyRandom> module from
358 CPAN. It uses the imperfections in your system's timer to generate
359 random numbers, but this takes quite a while. If you want a better
360 pseudorandom generator than comes with your operating system, look at
361 "Numerical Recipes in C" at http://www.nr.com/ .
363 =head2 How do I get a random number between X and Y?
365 To get a random number between two values, you can use the
366 C<rand()> builtin to get a random number between 0 and
368 C<rand($x)> returns a number such that
369 C<< 0 <= rand($x) < $x >>. Thus what you want to have perl
370 figure out is a random number in the range from 0 to the
371 difference between your I<X> and I<Y>.
373 That is, to get a number between 10 and 15, inclusive, you
374 want a random number between 0 and 5 that you can then add
377 my $number = 10 + int rand( 15-10+1 );
379 Hence you derive the following simple function to abstract
380 that. It selects a random integer between the two given
381 integers (inclusive), For example: C<random_int_between(50,120)>.
383 sub random_int_between {
385 # Assumes that the two arguments are integers themselves!
386 return $min if $min == $max;
387 ($min, $max) = ($max, $min) if $min > $max;
388 return $min + int rand(1 + $max - $min);
393 =head2 How do I find the day or week of the year?
395 The localtime function returns the day of the year. Without an
396 argument localtime uses the current time.
398 $day_of_year = (localtime)[7];
400 The C<POSIX> module can also format a date as the day of the year or
403 use POSIX qw/strftime/;
404 my $day_of_year = strftime "%j", localtime;
405 my $week_of_year = strftime "%W", localtime;
407 To get the day of year for any date, use C<POSIX>'s C<mktime> to get
408 a time in epoch seconds for the argument to localtime.
410 use POSIX qw/mktime strftime/;
411 my $week_of_year = strftime "%W",
412 localtime( mktime( 0, 0, 0, 18, 11, 87 ) );
414 The C<Date::Calc> module provides two functions to calculate these.
417 my $day_of_year = Day_of_Year( 1987, 12, 18 );
418 my $week_of_year = Week_of_Year( 1987, 12, 18 );
420 =head2 How do I find the current century or millennium?
422 Use the following simple functions:
425 return int((((localtime(shift || time))[5] + 1999))/100);
429 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
432 On some systems, the C<POSIX> module's C<strftime()> function has been
433 extended in a non-standard way to use a C<%C> format, which they
434 sometimes claim is the "century". It isn't, because on most such
435 systems, this is only the first two digits of the four-digit year, and
436 thus cannot be used to reliably determine the current century or
439 =head2 How can I compare two dates and find the difference?
441 (contributed by brian d foy)
443 You could just store all your dates as a number and then subtract.
444 Life isn't always that simple though. If you want to work with
445 formatted dates, the C<Date::Manip>, C<Date::Calc>, or C<DateTime>
446 modules can help you.
448 =head2 How can I take a string and turn it into epoch seconds?
450 If it's a regular enough string that it always has the same format,
451 you can split it up and pass the parts to C<timelocal> in the standard
452 C<Time::Local> module. Otherwise, you should look into the C<Date::Calc>
453 and C<Date::Manip> modules from CPAN.
455 =head2 How can I find the Julian Day?
457 (contributed by brian d foy and Dave Cross)
459 You can use the C<Time::JulianDay> module available on CPAN. Ensure
460 that you really want to find a Julian day, though, as many people have
461 different ideas about Julian days. See
462 http://www.hermetic.ch/cal_stud/jdn.htm for instance.
464 You can also try the C<DateTime> module, which can convert a date/time
467 $ perl -MDateTime -le'print DateTime->today->jd'
470 Or the modified Julian Day
472 $ perl -MDateTime -le'print DateTime->today->mjd'
475 Or even the day of the year (which is what some people think of as a
478 $ perl -MDateTime -le'print DateTime->today->doy'
481 =head2 How do I find yesterday's date?
483 (contributed by brian d foy)
485 Use one of the Date modules. The C<DateTime> module makes it simple, and
486 give you the same time of day, only the day before.
490 my $yesterday = DateTime->now->subtract( days => 1 );
492 print "Yesterday was $yesterday\n";
494 You can also use the C<Date::Calc> module using its Today_and_Now
497 use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
499 my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
503 Most people try to use the time rather than the calendar to figure out
504 dates, but that assumes that days are twenty-four hours each. For
505 most people, there are two days a year when they aren't: the switch to
506 and from summer time throws this off. Let the modules do the work.
508 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
510 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
511 Y2K compliant (whatever that means). The programmers you've hired to
512 use it, however, probably are not.
514 Long answer: The question belies a true understanding of the issue.
515 Perl is just as Y2K compliant as your pencil--no more, and no less.
516 Can you use your pencil to write a non-Y2K-compliant memo? Of course
517 you can. Is that the pencil's fault? Of course it isn't.
519 The date and time functions supplied with Perl (gmtime and localtime)
520 supply adequate information to determine the year well beyond 2000
521 (2038 is when trouble strikes for 32-bit machines). The year returned
522 by these functions when used in a list context is the year minus 1900.
523 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
524 number. To avoid the year 2000 problem simply do not treat the year as
525 a 2-digit number. It isn't.
527 When gmtime() and localtime() are used in scalar context they return
528 a timestamp string that contains a fully-expanded year. For example,
529 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
530 2001". There's no year 2000 problem here.
532 That doesn't mean that Perl can't be used to create non-Y2K compliant
533 programs. It can. But so can your pencil. It's the fault of the user,
534 not the language. At the risk of inflaming the NRA: "Perl doesn't
535 break Y2K, people do." See http://www.perl.org/about/y2k.html for
540 =head2 How do I validate input?
542 (contributed by brian d foy)
544 There are many ways to ensure that values are what you expect or
545 want to accept. Besides the specific examples that we cover in the
546 perlfaq, you can also look at the modules with "Assert" and "Validate"
547 in their names, along with other modules such as C<Regexp::Common>.
549 Some modules have validation for particular types of input, such
550 as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>,
551 and C<Data::Validate::IP>.
553 =head2 How do I unescape a string?
555 It depends just what you mean by "escape". URL escapes are dealt
556 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
557 character are removed with
561 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
563 =head2 How do I remove consecutive pairs of characters?
565 (contributed by brian d foy)
567 You can use the substitution operator to find pairs of characters (or
568 runs of characters) and replace them with a single instance. In this
569 substitution, we find a character in C<(.)>. The memory parentheses
570 store the matched character in the back-reference C<\1> and we use
571 that to require that the same thing immediately follow it. We replace
572 that part of the string with the character in C<$1>.
576 We can also use the transliteration operator, C<tr///>. In this
577 example, the search list side of our C<tr///> contains nothing, but
578 the C<c> option complements that so it contains everything. The
579 replacement list also contains nothing, so the transliteration is
580 almost a no-op since it won't do any replacements (or more exactly,
581 replace the character with itself). However, the C<s> option squashes
582 duplicated and consecutive characters in the string so a character
583 does not show up next to itself
585 my $str = 'Haarlem'; # in the Netherlands
586 $str =~ tr///cs; # Now Harlem, like in New York
588 =head2 How do I expand function calls in a string?
590 (contributed by brian d foy)
592 This is documented in L<perlref>, and although it's not the easiest
593 thing to read, it does work. In each of these examples, we call the
594 function inside the braces used to dereference a reference. If we
595 have more than one return value, we can construct and dereference an
596 anonymous array. In this case, we call the function in list context.
598 print "The time values are @{ [localtime] }.\n";
600 If we want to call the function in scalar context, we have to do a bit
601 more work. We can really have any code we like inside the braces, so
602 we simply have to end with the scalar reference, although how you do
603 that is up to you, and you can use code inside the braces. Note that
604 the use of parens creates a list context, so we need C<scalar> to
605 force the scalar context on the function:
607 print "The time is ${\(scalar localtime)}.\n"
609 print "The time is ${ my $x = localtime; \$x }.\n";
611 If your function already returns a reference, you don't need to create
612 the reference yourself.
614 sub timestamp { my $t = localtime; \$t }
616 print "The time is ${ timestamp() }.\n";
618 The C<Interpolation> module can also do a lot of magic for you. You can
619 specify a variable name, in this case C<E>, to set up a tied hash that
620 does the interpolation for you. It has several other methods to do this
623 use Interpolation E => 'eval';
624 print "The time values are $E{localtime()}.\n";
626 In most cases, it is probably easier to simply use string concatenation,
627 which also forces scalar context.
629 print "The time is " . localtime() . ".\n";
631 =head2 How do I find matching/nesting anything?
633 This isn't something that can be done in one regular expression, no
634 matter how complicated. To find something between two single
635 characters, a pattern like C</x([^x]*)x/> will get the intervening
636 bits in $1. For multiple ones, then something more like
637 C</alpha(.*?)omega/> would be needed. But none of these deals with
638 nested patterns. For balanced expressions using C<(>, C<{>, C<[> or
639 C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
640 L<perlre/(??{ code })>. For other cases, you'll have to write a
643 If you are serious about writing a parser, there are a number of
644 modules or oddities that will make your life a lot easier. There are
645 the CPAN modules C<Parse::RecDescent>, C<Parse::Yapp>, and
646 C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8
647 the C<Text::Balanced> is part of the standard distribution.
649 One simple destructive, inside-out approach that you might try is to
650 pull out the smallest nesting parts one at a time:
652 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
653 # do something with $1
656 A more complicated and sneaky approach is to make Perl's regular
657 expression engine do it for you. This is courtesy Dean Inada, and
658 rather has the nature of an Obfuscated Perl Contest entry, but it
661 # $_ contains the string to parse
662 # BEGIN and END are the opening and closing markers for the
667 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
668 @$ = (eval{/$re/},$@!~/unmatched/i);
669 print join("\n",@$[0..$#$]) if( $$[-1] );
671 =head2 How do I reverse a string?
673 Use C<reverse()> in scalar context, as documented in
676 $reversed = reverse $string;
678 =head2 How do I expand tabs in a string?
680 You can do it yourself:
682 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
684 Or you can just use the C<Text::Tabs> module (part of the standard Perl
688 @expanded_lines = expand(@lines_with_tabs);
690 =head2 How do I reformat a paragraph?
692 Use C<Text::Wrap> (part of the standard Perl distribution):
695 print wrap("\t", ' ', @paragraphs);
697 The paragraphs you give to C<Text::Wrap> should not contain embedded
698 newlines. C<Text::Wrap> doesn't justify the lines (flush-right).
700 Or use the CPAN module C<Text::Autoformat>. Formatting files can be
701 easily done by making a shell alias, like so:
703 alias fmt="perl -i -MText::Autoformat -n0777 \
704 -e 'print autoformat $_, {all=>1}' $*"
706 See the documentation for C<Text::Autoformat> to appreciate its many
709 =head2 How can I access or change N characters of a string?
711 You can access the first characters of a string with substr().
712 To get the first character, for example, start at position 0
713 and grab the string of length 1.
716 $string = "Just another Perl Hacker";
717 $first_char = substr( $string, 0, 1 ); # 'J'
719 To change part of a string, you can use the optional fourth
720 argument which is the replacement string.
722 substr( $string, 13, 4, "Perl 5.8.0" );
724 You can also use substr() as an lvalue.
726 substr( $string, 13, 4 ) = "Perl 5.8.0";
728 =head2 How do I change the Nth occurrence of something?
730 You have to keep track of N yourself. For example, let's say you want
731 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
732 C<"whosoever"> or C<"whomsoever">, case insensitively. These
733 all assume that $_ contains the string to be altered.
737 ++$count == 5 # is it the 5th?
738 ? "${2}soever" # yes, swap
739 : $1 # renege and leave it there
742 In the more general case, you can use the C</g> modifier in a C<while>
743 loop, keeping count of matches.
747 $_ = "One fish two fish red fish blue fish";
748 while (/(\w+)\s+fish\b/gi) {
749 if (++$count == $WANT) {
750 print "The third fish is a $1 one.\n";
754 That prints out: C<"The third fish is a red one."> You can also use a
755 repetition count and repeated pattern like this:
757 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
759 =head2 How can I count the number of occurrences of a substring within a string?
761 There are a number of ways, with varying efficiency. If you want a
762 count of a certain single character (X) within a string, you can use the
763 C<tr///> function like so:
765 $string = "ThisXlineXhasXsomeXx'sXinXit";
766 $count = ($string =~ tr/X//);
767 print "There are $count X characters in the string";
769 This is fine if you are just looking for a single character. However,
770 if you are trying to count multiple character substrings within a
771 larger string, C<tr///> won't work. What you can do is wrap a while()
772 loop around a global pattern match. For example, let's count negative
775 $string = "-9 55 48 -2 23 -76 4 14 -44";
776 while ($string =~ /-\d+/g) { $count++ }
777 print "There are $count negative numbers in the string";
779 Another version uses a global match in list context, then assigns the
780 result to a scalar, producing a count of the number of matches.
782 $count = () = $string =~ /-\d+/g;
784 =head2 How do I capitalize all the words on one line?
786 To make the first letter of each word upper case:
788 $line =~ s/\b(\w)/\U$1/g;
790 This has the strange effect of turning "C<don't do it>" into "C<Don'T
791 Do It>". Sometimes you might want this. Other times you might need a
792 more thorough solution (Suggested by brian d foy):
795 (^\w) #at the beginning of the line
797 (\s\w) #preceded by whitespace
801 $string =~ s/([\w']+)/\u\L$1/g;
803 To make the whole line upper case:
807 To force each word to be lower case, with the first letter upper case:
809 $line =~ s/(\w+)/\u\L$1/g;
811 You can (and probably should) enable locale awareness of those
812 characters by placing a C<use locale> pragma in your program.
813 See L<perllocale> for endless details on locales.
815 This is sometimes referred to as putting something into "title
816 case", but that's not quite accurate. Consider the proper
817 capitalization of the movie I<Dr. Strangelove or: How I Learned to
818 Stop Worrying and Love the Bomb>, for example.
820 Damian Conway's L<Text::Autoformat> module provides some smart
821 case transformations:
823 use Text::Autoformat;
824 my $x = "Dr. Strangelove or: How I Learned to Stop ".
825 "Worrying and Love the Bomb";
828 for my $style (qw( sentence title highlight )) {
829 print autoformat($x, { case => $style }), "\n";
832 =head2 How can I split a [character] delimited string except when inside [character]?
834 Several modules can handle this sort of parsing--C<Text::Balanced>,
835 C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others.
837 Take the example case of trying to split a string that is
838 comma-separated into its different fields. You can't use C<split(/,/)>
839 because you shouldn't split if the comma is inside quotes. For
840 example, take a data line like this:
842 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
844 Due to the restriction of the quotes, this is a fairly complex
845 problem. Thankfully, we have Jeffrey Friedl, author of
846 I<Mastering Regular Expressions>, to handle these for us. He
847 suggests (assuming your string is contained in C<$text>):
850 push(@new, $+) while $text =~ m{
851 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
855 push(@new, undef) if substr($text,-1,1) eq ',';
857 If you want to represent quotation marks inside a
858 quotation-mark-delimited field, escape them with backslashes (eg,
861 Alternatively, the C<Text::ParseWords> module (part of the standard
862 Perl distribution) lets you say:
864 use Text::ParseWords;
865 @new = quotewords(",", 0, $text);
867 =head2 How do I strip blank space from the beginning/end of a string?
869 (contributed by brian d foy)
871 A substitution can do this for you. For a single line, you want to
872 replace all the leading or trailing whitespace with nothing. You
873 can do that with a pair of substitutions.
878 You can also write that as a single substitution, although it turns
879 out the combined statement is slower than the separate ones. That
880 might not matter to you, though.
884 In this regular expression, the alternation matches either at the
885 beginning or the end of the string since the anchors have a lower
886 precedence than the alternation. With the C</g> flag, the substitution
887 makes all possible matches, so it gets both. Remember, the trailing
888 newline matches the C<\s+>, and the C<$> anchor can match to the
889 physical end of the string, so the newline disappears too. Just add
890 the newline to the output, which has the added benefit of preserving
891 "blank" (consisting entirely of whitespace) lines which the C<^\s+>
892 would remove all by itself.
900 For a multi-line string, you can apply the regular expression
901 to each logical line in the string by adding the C</m> flag (for
902 "multi-line"). With the C</m> flag, the C<$> matches I<before> an
903 embedded newline, so it doesn't remove it. It still removes the
904 newline at the end of the string.
906 $string =~ s/^\s+|\s+$//gm;
908 Remember that lines consisting entirely of whitespace will disappear,
909 since the first part of the alternation can match the entire string
910 and replace it with nothing. If need to keep embedded blank lines,
911 you have to do a little more work. Instead of matching any whitespace
912 (since that includes a newline), just match the other whitespace.
914 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
916 =head2 How do I pad a string with blanks or pad a number with zeroes?
918 In the following examples, C<$pad_len> is the length to which you wish
919 to pad the string, C<$text> or C<$num> contains the string to be padded,
920 and C<$pad_char> contains the padding character. You can use a single
921 character string constant instead of the C<$pad_char> variable if you
922 know what it is in advance. And in the same way you can use an integer in
923 place of C<$pad_len> if you know the pad length in advance.
925 The simplest method uses the C<sprintf> function. It can pad on the left
926 or right with blanks and on the left with zeroes and it will not
927 truncate the result. The C<pack> function can only pad strings on the
928 right with blanks and it will truncate the result to a maximum length of
931 # Left padding a string with blanks (no truncation):
932 $padded = sprintf("%${pad_len}s", $text);
933 $padded = sprintf("%*s", $pad_len, $text); # same thing
935 # Right padding a string with blanks (no truncation):
936 $padded = sprintf("%-${pad_len}s", $text);
937 $padded = sprintf("%-*s", $pad_len, $text); # same thing
939 # Left padding a number with 0 (no truncation):
940 $padded = sprintf("%0${pad_len}d", $num);
941 $padded = sprintf("%0*d", $pad_len, $num); # same thing
943 # Right padding a string with blanks using pack (will truncate):
944 $padded = pack("A$pad_len",$text);
946 If you need to pad with a character other than blank or zero you can use
947 one of the following methods. They all generate a pad string with the
948 C<x> operator and combine that with C<$text>. These methods do
949 not truncate C<$text>.
951 Left and right padding with any character, creating a new string:
953 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
954 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
956 Left and right padding with any character, modifying C<$text> directly:
958 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
959 $text .= $pad_char x ( $pad_len - length( $text ) );
961 =head2 How do I extract selected columns from a string?
963 (contributed by brian d foy)
965 If you know where the columns that contain the data, you can
966 use C<substr> to extract a single column.
968 my $column = substr( $line, $start_column, $length );
970 You can use C<split> if the columns are separated by whitespace or
971 some other delimiter, as long as whitespace or the delimiter cannot
972 appear as part of the data.
974 my $line = ' fred barney betty ';
975 my @columns = split /\s+/, $line;
976 # ( '', 'fred', 'barney', 'betty' );
978 my $line = 'fred||barney||betty';
979 my @columns = split /\|/, $line;
980 # ( 'fred', '', 'barney', '', 'betty' );
982 If you want to work with comma-separated values, don't do this since
983 that format is a bit more complicated. Use one of the modules that
984 handle that fornat, such as C<Text::CSV>, C<Text::CSV_XS>, or
987 If you want to break apart an entire line of fixed columns, you can use
988 C<unpack> with the A (ASCII) format. by using a number after the format
989 specifier, you can denote the column width. See the C<pack> and C<unpack>
990 entries in L<perlfunc> for more details.
992 my @fields = unpack( $line, "A8 A8 A8 A16 A4" );
994 Note that spaces in the format argument to C<unpack> do not denote literal
995 spaces. If you have space separated data, you may want C<split> instead.
997 =head2 How do I find the soundex value of a string?
999 (contributed by brian d foy)
1001 You can use the Text::Soundex module. If you want to do fuzzy or close
1002 matching, you might also try the C<String::Approx>, and
1003 C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules.
1005 =head2 How can I expand variables in text strings?
1007 (contributed by brian d foy)
1009 For example, I'll use a string that has two Perl scalar variables
1010 in it. In this example, I want to expand C<$foo> and C<$bar> to
1011 their variable's values.
1015 $string = 'Say hello to $foo and $bar';
1017 One way I can do this involves the substitution operator and a double
1018 C</e> flag. The first C</e> evaluates C<$1> on the replacement side and
1019 turns it into C<$foo>. The second /e starts with C<$foo> and replaces
1020 it with its value. C<$foo>, then, turns into 'Fred', and that's finally
1021 what's left in the string.
1023 $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'
1025 The C</e> will also silently ignore violations of strict, replacing
1026 undefined variable names with the empty string.
1028 I could also pull the values from a hash instead of evaluating
1029 variable names. Using a single C</e>, I can check the hash to ensure
1030 the value exists, and if it doesn't, I can replace the missing value
1031 with a marker, in this case C<???> to signal that I missed something:
1033 my $string = 'This has $foo and $bar';
1035 my %Replacements = (
1039 # $string =~ s/\$(\w+)/$Replacements{$1}/g;
1040 $string =~ s/\$(\w+)/
1041 exists $Replacements{$1} ? $Replacements{$1} : '???'
1046 =head2 What's wrong with always quoting "$vars"?
1048 The problem is that those double-quotes force
1049 stringification--coercing numbers and references into strings--even
1050 when you don't want them to be strings. Think of it this way:
1051 double-quote expansion is used to produce new strings. If you already
1052 have a string, why do you need more?
1054 If you get used to writing odd things like these:
1057 $new = "$old"; # BAD
1058 somefunc("$var"); # BAD
1060 You'll be in trouble. Those should (in 99.8% of the cases) be
1061 the simpler and more direct:
1067 Otherwise, besides slowing you down, you're going to break code when
1068 the thing in the scalar is actually neither a string nor a number, but
1074 my $oref = "$aref"; # WRONG
1077 You can also get into subtle problems on those few operations in Perl
1078 that actually do care about the difference between a string and a
1079 number, such as the magical C<++> autoincrement operator or the
1082 Stringification also destroys arrays.
1085 print "@lines"; # WRONG - extra blanks
1086 print @lines; # right
1088 =head2 Why don't my E<lt>E<lt>HERE documents work?
1090 Check for these three things:
1094 =item There must be no space after the E<lt>E<lt> part.
1096 =item There (probably) should be a semicolon at the end.
1098 =item You can't (easily) have any space in front of the tag.
1102 If you want to indent the text in the here document, you
1106 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1111 But the HERE_TARGET must still be flush against the margin.
1112 If you want that indented also, you'll have to quote
1115 ($quote = <<' FINIS') =~ s/^\s+//gm;
1116 ...we will have peace, when you and all your works have
1117 perished--and the works of your dark master to whom you
1118 would deliver us. You are a liar, Saruman, and a corrupter
1119 of men's hearts. --Theoden in /usr/src/perl/taint.c
1121 $quote =~ s/\s+--/\n--/;
1123 A nice general-purpose fixer-upper function for indented here documents
1124 follows. It expects to be called with a here document as its argument.
1125 It looks to see whether each line begins with a common substring, and
1126 if so, strips that substring off. Otherwise, it takes the amount of leading
1127 whitespace found on the first line and removes that much off each
1132 my ($white, $leader); # common whitespace and common leading string
1133 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1134 ($white, $leader) = ($2, quotemeta($1));
1136 ($white, $leader) = (/^(\s+)/, '');
1138 s/^\s*?$leader(?:$white)?//gm;
1142 This works with leading special strings, dynamically determined:
1144 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1147 @@@ SAVEI32(runlevel);
1149 @@@ while ( op = (*op->op_ppaddr)() );
1153 MAIN_INTERPRETER_LOOP
1155 Or with a fixed amount of leading whitespace, with remaining
1156 indentation correctly preserved:
1158 $poem = fix<<EVER_ON_AND_ON;
1159 Now far ahead the Road has gone,
1160 And I must follow, if I can,
1161 Pursuing it with eager feet,
1162 Until it joins some larger way
1163 Where many paths and errands meet.
1164 And whither then? I cannot say.
1165 --Bilbo in /usr/src/perl/pp_ctl.c
1170 =head2 What is the difference between a list and an array?
1172 An array has a changeable length. A list does not. An array is
1173 something you can push or pop, while a list is a set of values. Some
1174 people make the distinction that a list is a value while an array is a
1175 variable. Subroutines are passed and return lists, you put things into
1176 list context, you initialize arrays with lists, and you C<foreach()>
1177 across a list. C<@> variables are arrays, anonymous arrays are
1178 arrays, arrays in scalar context behave like the number of elements in
1179 them, subroutines access their arguments through the array C<@_>, and
1180 C<push>/C<pop>/C<shift> only work on arrays.
1182 As a side note, there's no such thing as a list in scalar context.
1185 $scalar = (2, 5, 7, 9);
1187 you're using the comma operator in scalar context, so it uses the scalar
1188 comma operator. There never was a list there at all! This causes the
1189 last value to be returned: 9.
1191 =head2 What is the difference between $array[1] and @array[1]?
1193 The former is a scalar value; the latter an array slice, making
1194 it a list with one (scalar) value. You should use $ when you want a
1195 scalar value (most of the time) and @ when you want a list with one
1196 scalar value in it (very, very rarely; nearly never, in fact).
1198 Sometimes it doesn't make a difference, but sometimes it does.
1199 For example, compare:
1201 $good[0] = `some program that outputs several lines`;
1205 @bad[0] = `same program that outputs several lines`;
1207 The C<use warnings> pragma and the B<-w> flag will warn you about these
1210 =head2 How can I remove duplicate elements from a list or array?
1212 (contributed by brian d foy)
1214 Use a hash. When you think the words "unique" or "duplicated", think
1217 If you don't care about the order of the elements, you could just
1218 create the hash then extract the keys. It's not important how you
1219 create that hash: just that you use C<keys> to get the unique
1222 my %hash = map { $_, 1 } @array;
1223 # or a hash slice: @hash{ @array } = ();
1224 # or a foreach: $hash{$_} = 1 foreach ( @array );
1226 my @unique = keys %hash;
1228 If you want to use a module, try the C<uniq> function from
1229 C<List::MoreUtils>. In list context it returns the unique elements,
1230 preserving their order in the list. In scalar context, it returns the
1231 number of unique elements.
1233 use List::MoreUtils qw(uniq);
1235 my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
1236 my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7
1238 You can also go through each element and skip the ones you've seen
1239 before. Use a hash to keep track. The first time the loop sees an
1240 element, that element has no key in C<%Seen>. The C<next> statement
1241 creates the key and immediately uses its value, which is C<undef>, so
1242 the loop continues to the C<push> and increments the value for that
1243 key. The next time the loop sees that same element, its key exists in
1244 the hash I<and> the value for that key is true (since it's not 0 or
1245 C<undef>), so the next skips that iteration and the loop goes to the
1251 foreach my $elem ( @array )
1253 next if $seen{ $elem }++;
1254 push @unique, $elem;
1257 You can write this more briefly using a grep, which does the
1261 my @unique = grep { ! $seen{ $_ }++ } @array;
1263 =head2 How can I tell whether a certain element is contained in a list or array?
1265 (portions of this answer contributed by Anno Siegel)
1267 Hearing the word "in" is an I<in>dication that you probably should have
1268 used a hash, not a list or array, to store your data. Hashes are
1269 designed to answer this question quickly and efficiently. Arrays aren't.
1271 That being said, there are several ways to approach this. If you
1272 are going to make this query many times over arbitrary string values,
1273 the fastest way is probably to invert the original array and maintain a
1274 hash whose keys are the first array's values.
1276 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1278 for (@blues) { $is_blue{$_} = 1 }
1280 Now you can check whether C<$is_blue{$some_color}>. It might have
1281 been a good idea to keep the blues all in a hash in the first place.
1283 If the values are all small integers, you could use a simple indexed
1284 array. This kind of an array will take up less space:
1286 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1287 @is_tiny_prime = ();
1288 for (@primes) { $is_tiny_prime[$_] = 1 }
1289 # or simply @istiny_prime[@primes] = (1) x @primes;
1291 Now you check whether $is_tiny_prime[$some_number].
1293 If the values in question are integers instead of strings, you can save
1294 quite a lot of space by using bit strings instead:
1296 @articles = ( 1..10, 150..2000, 2017 );
1298 for (@articles) { vec($read,$_,1) = 1 }
1300 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1302 These methods guarantee fast individual tests but require a re-organization
1303 of the original list or array. They only pay off if you have to test
1304 multiple values against the same array.
1306 If you are testing only once, the standard module C<List::Util> exports
1307 the function C<first> for this purpose. It works by stopping once it
1308 finds the element. It's written in C for speed, and its Perl equivalant
1309 looks like this subroutine:
1314 return $_ if &{$code}();
1319 If speed is of little concern, the common idiom uses grep in scalar context
1320 (which returns the number of items that passed its condition) to traverse the
1321 entire list. This does have the benefit of telling you how many matches it
1324 my $is_there = grep $_ eq $whatever, @array;
1326 If you want to actually extract the matching elements, simply use grep in
1329 my @matches = grep $_ eq $whatever, @array;
1331 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1333 Use a hash. Here's code to do both and more. It assumes that each
1334 element is unique in a given array:
1336 @union = @intersection = @difference = ();
1338 foreach $element (@array1, @array2) { $count{$element}++ }
1339 foreach $element (keys %count) {
1340 push @union, $element;
1341 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1344 Note that this is the I<symmetric difference>, that is, all elements
1345 in either A or in B but not in both. Think of it as an xor operation.
1347 =head2 How do I test whether two arrays or hashes are equal?
1349 The following code works for single-level arrays. It uses a
1350 stringwise comparison, and does not distinguish defined versus
1351 undefined empty strings. Modify if you have other needs.
1353 $are_equal = compare_arrays(\@frogs, \@toads);
1355 sub compare_arrays {
1356 my ($first, $second) = @_;
1357 no warnings; # silence spurious -w undef complaints
1358 return 0 unless @$first == @$second;
1359 for (my $i = 0; $i < @$first; $i++) {
1360 return 0 if $first->[$i] ne $second->[$i];
1365 For multilevel structures, you may wish to use an approach more
1366 like this one. It uses the CPAN module C<FreezeThaw>:
1368 use FreezeThaw qw(cmpStr);
1369 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1371 printf "a and b contain %s arrays\n",
1372 cmpStr(\@a, \@b) == 0
1376 This approach also works for comparing hashes. Here we'll demonstrate
1377 two different answers:
1379 use FreezeThaw qw(cmpStr cmpStrHard);
1381 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1385 printf "a and b contain %s hashes\n",
1386 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1388 printf "a and b contain %s hashes\n",
1389 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1392 The first reports that both those the hashes contain the same data,
1393 while the second reports that they do not. Which you prefer is left as
1394 an exercise to the reader.
1396 =head2 How do I find the first array element for which a condition is true?
1398 To find the first array element which satisfies a condition, you can
1399 use the C<first()> function in the C<List::Util> module, which comes
1400 with Perl 5.8. This example finds the first element that contains
1403 use List::Util qw(first);
1405 my $element = first { /Perl/ } @array;
1407 If you cannot use C<List::Util>, you can make your own loop to do the
1408 same thing. Once you find the element, you stop the loop with last.
1411 foreach ( @array ) {
1412 if( /Perl/ ) { $found = $_; last }
1415 If you want the array index, you can iterate through the indices
1416 and check the array element at each index until you find one
1417 that satisfies the condition.
1419 my( $found, $index ) = ( undef, -1 );
1420 for( $i = 0; $i < @array; $i++ ) {
1421 if( $array[$i] =~ /Perl/ ) {
1422 $found = $array[$i];
1428 =head2 How do I handle linked lists?
1430 In general, you usually don't need a linked list in Perl, since with
1431 regular arrays, you can push and pop or shift and unshift at either
1432 end, or you can use splice to add and/or remove arbitrary number of
1433 elements at arbitrary points. Both pop and shift are both O(1)
1434 operations on Perl's dynamic arrays. In the absence of shifts and
1435 pops, push in general needs to reallocate on the order every log(N)
1436 times, and unshift will need to copy pointers each time.
1438 If you really, really wanted, you could use structures as described in
1439 L<perldsc> or L<perltoot> and do just what the algorithm book tells
1440 you to do. For example, imagine a list node like this:
1447 You could walk the list this way:
1450 for ($node = $head; $node; $node = $node->{LINK}) {
1451 print $node->{VALUE}, " ";
1455 You could add to the list this way:
1458 $tail = append($head, 1); # grow a new head
1459 for $value ( 2 .. 10 ) {
1460 $tail = append($tail, $value);
1464 my($list, $value) = @_;
1465 my $node = { VALUE => $value };
1467 $node->{LINK} = $list->{LINK};
1468 $list->{LINK} = $node;
1471 $_[0] = $node; # replace caller's version
1476 But again, Perl's built-in are virtually always good enough.
1478 =head2 How do I handle circular lists?
1480 Circular lists could be handled in the traditional fashion with linked
1481 lists, or you could just do something like this with an array:
1483 unshift(@array, pop(@array)); # the last shall be first
1484 push(@array, shift(@array)); # and vice versa
1486 You can also use C<Tie::Cycle>:
1490 tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
1492 print $cycle; # FFFFFF
1493 print $cycle; # 000000
1494 print $cycle; # FFFF00
1496 =head2 How do I shuffle an array randomly?
1498 If you either have Perl 5.8.0 or later installed, or if you have
1499 Scalar-List-Utils 1.03 or later installed, you can say:
1501 use List::Util 'shuffle';
1503 @shuffled = shuffle(@list);
1505 If not, you can use a Fisher-Yates shuffle.
1507 sub fisher_yates_shuffle {
1508 my $deck = shift; # $deck is a reference to an array
1511 my $j = int rand ($i+1);
1512 @$deck[$i,$j] = @$deck[$j,$i];
1516 # shuffle my mpeg collection
1518 my @mpeg = <audio/*/*.mp3>;
1519 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1522 Note that the above implementation shuffles an array in place,
1523 unlike the C<List::Util::shuffle()> which takes a list and returns
1524 a new shuffled list.
1526 You've probably seen shuffling algorithms that work using splice,
1527 randomly picking another element to swap the current element with
1531 @old = 1 .. 10; # just a demo
1533 push(@new, splice(@old, rand @old, 1));
1536 This is bad because splice is already O(N), and since you do it N
1537 times, you just invented a quadratic algorithm; that is, O(N**2).
1538 This does not scale, although Perl is so efficient that you probably
1539 won't notice this until you have rather largish arrays.
1541 =head2 How do I process/modify each element of an array?
1543 Use C<for>/C<foreach>:
1546 s/foo/bar/; # change that word
1547 tr/XZ/ZX/; # swap those letters
1550 Here's another; let's compute spherical volumes:
1552 for (@volumes = @radii) { # @volumes has changed parts
1554 $_ *= (4/3) * 3.14159; # this will be constant folded
1557 which can also be done with C<map()> which is made to transform
1558 one list into another:
1560 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1562 If you want to do the same thing to modify the values of the
1563 hash, you can use the C<values> function. As of Perl 5.6
1564 the values are not copied, so if you modify $orbit (in this
1565 case), you modify the value.
1567 for $orbit ( values %orbits ) {
1568 ($orbit **= 3) *= (4/3) * 3.14159;
1571 Prior to perl 5.6 C<values> returned copies of the values,
1572 so older perl code often contains constructions such as
1573 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1574 the hash is to be modified.
1576 =head2 How do I select a random element from an array?
1578 Use the C<rand()> function (see L<perlfunc/rand>):
1580 $index = rand @array;
1581 $element = $array[$index];
1585 my $element = $array[ rand @array ];
1587 =head2 How do I permute N elements of a list?
1589 Use the C<List::Permutor> module on CPAN. If the list is actually an
1590 array, try the C<Algorithm::Permute> module (also on CPAN). It's
1591 written in XS code and is very efficient.
1593 use Algorithm::Permute;
1594 my @array = 'a'..'d';
1595 my $p_iterator = Algorithm::Permute->new ( \@array );
1596 while (my @perm = $p_iterator->next) {
1597 print "next permutation: (@perm)\n";
1600 For even faster execution, you could do:
1602 use Algorithm::Permute;
1603 my @array = 'a'..'d';
1604 Algorithm::Permute::permute {
1605 print "next permutation: (@array)\n";
1608 Here's a little program that generates all permutations of
1609 all the words on each line of input. The algorithm embodied
1610 in the C<permute()> function is discussed in Volume 4 (still
1611 unpublished) of Knuth's I<The Art of Computer Programming>
1612 and will work on any list:
1615 # Fischer-Kause ordered permutation generator
1620 while ( $code->(@_[@idx]) ) {
1622 --$p while $idx[$p-1] > $idx[$p];
1623 my $q = $p or return;
1624 push @idx, reverse splice @idx, $p;
1625 ++$q while $idx[$p-1] > $idx[$q];
1626 @idx[$p-1,$q]=@idx[$q,$p-1];
1630 permute {print"@_\n"} split;
1632 =head2 How do I sort an array by (anything)?
1634 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1636 @list = sort { $a <=> $b } @list;
1638 The default sort function is cmp, string comparison, which would
1639 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1640 the numerical comparison operator.
1642 If you have a complicated function needed to pull out the part you
1643 want to sort on, then don't do it inside the sort function. Pull it
1644 out first, because the sort BLOCK can be called many times for the
1645 same element. Here's an example of how to pull out the first word
1646 after the first number on each item, and then sort those words
1651 ($item) = /\d+\s*(\S+)/;
1652 push @idx, uc($item);
1654 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1656 which could also be written this way, using a trick
1657 that's come to be known as the Schwartzian Transform:
1659 @sorted = map { $_->[0] }
1660 sort { $a->[1] cmp $b->[1] }
1661 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1663 If you need to sort on several fields, the following paradigm is useful.
1666 field1($a) <=> field1($b) ||
1667 field2($a) cmp field2($b) ||
1668 field3($a) cmp field3($b)
1671 This can be conveniently combined with precalculation of keys as given
1674 See the F<sort> article in the "Far More Than You Ever Wanted
1675 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1676 more about this approach.
1678 See also the question later in L<perlfaq4> on sorting hashes.
1680 =head2 How do I manipulate arrays of bits?
1682 Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise
1685 For example, this sets C<$vec> to have bit N set if C<$ints[N]> was
1689 foreach(@ints) { vec($vec,$_,1) = 1 }
1691 Here's how, given a vector in C<$vec>, you can get those bits into your
1694 sub bitvec_to_list {
1697 # Find null-byte density then select best algorithm
1698 if ($vec =~ tr/\0// / length $vec > 0.95) {
1702 # This method is faster with mostly null-bytes
1703 while($vec =~ /[^\0]/g ) {
1704 $i = -9 + 8 * pos $vec;
1705 push @ints, $i if vec($vec, ++$i, 1);
1706 push @ints, $i if vec($vec, ++$i, 1);
1707 push @ints, $i if vec($vec, ++$i, 1);
1708 push @ints, $i if vec($vec, ++$i, 1);
1709 push @ints, $i if vec($vec, ++$i, 1);
1710 push @ints, $i if vec($vec, ++$i, 1);
1711 push @ints, $i if vec($vec, ++$i, 1);
1712 push @ints, $i if vec($vec, ++$i, 1);
1716 # This method is a fast general algorithm
1718 my $bits = unpack "b*", $vec;
1719 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1720 push @ints, pos $bits while($bits =~ /1/g);
1726 This method gets faster the more sparse the bit vector is.
1727 (Courtesy of Tim Bunce and Winfried Koenig.)
1729 You can make the while loop a lot shorter with this suggestion
1730 from Benjamin Goldberg:
1732 while($vec =~ /[^\0]+/g ) {
1733 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1736 Or use the CPAN module C<Bit::Vector>:
1738 $vector = Bit::Vector->new($num_of_bits);
1739 $vector->Index_List_Store(@ints);
1740 @ints = $vector->Index_List_Read();
1742 C<Bit::Vector> provides efficient methods for bit vector, sets of
1743 small integers and "big int" math.
1745 Here's a more extensive illustration using vec():
1748 $vector = "\xff\x0f\xef\xfe";
1749 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1750 unpack("N", $vector), "\n";
1751 $is_set = vec($vector, 23, 1);
1752 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1770 my ($offset, $width, $value) = @_;
1772 vec($vector, $offset, $width) = $value;
1773 print "offset=$offset width=$width value=$value\n";
1779 my $bits = unpack("b*", $vector);
1783 print "vector length in bytes: ", length($vector), "\n";
1784 @bytes = unpack("A8" x length($vector), $bits);
1785 print "bits are: @bytes\n\n";
1788 =head2 Why does defined() return true on empty arrays and hashes?
1790 The short story is that you should probably only use defined on scalars or
1791 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1792 in the 5.004 release or later of Perl for more detail.
1794 =head1 Data: Hashes (Associative Arrays)
1796 =head2 How do I process an entire hash?
1798 Use the each() function (see L<perlfunc/each>) if you don't care
1799 whether it's sorted:
1801 while ( ($key, $value) = each %hash) {
1802 print "$key = $value\n";
1805 If you want it sorted, you'll have to use foreach() on the result of
1806 sorting the keys as shown in an earlier question.
1808 =head2 What happens if I add or remove keys from a hash while iterating over it?
1810 (contributed by brian d foy)
1812 The easy answer is "Don't do that!"
1814 If you iterate through the hash with each(), you can delete the key
1815 most recently returned without worrying about it. If you delete or add
1816 other keys, the iterator may skip or double up on them since perl
1817 may rearrange the hash table. See the
1818 entry for C<each()> in L<perlfunc>.
1820 =head2 How do I look up a hash element by value?
1822 Create a reverse hash:
1824 %by_value = reverse %by_key;
1825 $key = $by_value{$value};
1827 That's not particularly efficient. It would be more space-efficient
1830 while (($key, $value) = each %by_key) {
1831 $by_value{$value} = $key;
1834 If your hash could have repeated values, the methods above will only find
1835 one of the associated keys. This may or may not worry you. If it does
1836 worry you, you can always reverse the hash into a hash of arrays instead:
1838 while (($key, $value) = each %by_key) {
1839 push @{$key_list_by_value{$value}}, $key;
1842 =head2 How can I know how many entries are in a hash?
1844 If you mean how many keys, then all you have to do is
1845 use the keys() function in a scalar context:
1847 $num_keys = keys %hash;
1849 The keys() function also resets the iterator, which means that you may
1850 see strange results if you use this between uses of other hash operators
1853 =head2 How do I sort a hash (optionally by value instead of key)?
1855 (contributed by brian d foy)
1857 To sort a hash, start with the keys. In this example, we give the list of
1858 keys to the sort function which then compares them ASCIIbetically (which
1859 might be affected by your locale settings). The output list has the keys
1860 in ASCIIbetical order. Once we have the keys, we can go through them to
1861 create a report which lists the keys in ASCIIbetical order.
1863 my @keys = sort { $a cmp $b } keys %hash;
1865 foreach my $key ( @keys )
1867 printf "%-20s %6d\n", $key, $hash{$value};
1870 We could get more fancy in the C<sort()> block though. Instead of
1871 comparing the keys, we can compute a value with them and use that
1872 value as the comparison.
1874 For instance, to make our report order case-insensitive, we use
1875 the C<\L> sequence in a double-quoted string to make everything
1876 lowercase. The C<sort()> block then compares the lowercased
1877 values to determine in which order to put the keys.
1879 my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
1881 Note: if the computation is expensive or the hash has many elements,
1882 you may want to look at the Schwartzian Transform to cache the
1883 computation results.
1885 If we want to sort by the hash value instead, we use the hash key
1886 to look it up. We still get out a list of keys, but this time they
1887 are ordered by their value.
1889 my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
1891 From there we can get more complex. If the hash values are the same,
1892 we can provide a secondary sort on the hash key.
1895 $hash{$a} <=> $hash{$b}
1900 =head2 How can I always keep my hash sorted?
1901 X<hash tie sort DB_File Tie::IxHash>
1903 You can look into using the C<DB_File> module and C<tie()> using the
1904 C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory
1905 Databases">. The C<Tie::IxHash> module from CPAN might also be
1906 instructive. Although this does keep your hash sorted, you might not
1907 like the slow down you suffer from the tie interface. Are you sure you
1910 =head2 What's the difference between "delete" and "undef" with hashes?
1912 Hashes contain pairs of scalars: the first is the key, the
1913 second is the value. The key will be coerced to a string,
1914 although the value can be any kind of scalar: string,
1915 number, or reference. If a key C<$key> is present in
1916 %hash, C<exists($hash{$key})> will return true. The value
1917 for a given key can be C<undef>, in which case
1918 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1919 will return true. This corresponds to (C<$key>, C<undef>)
1922 Pictures help... here's the C<%hash> table:
1932 And these conditions hold
1936 defined $hash{'d'} is true
1937 defined $hash{'a'} is true
1938 exists $hash{'a'} is true (Perl5 only)
1939 grep ($_ eq 'a', keys %hash) is true
1945 your table now reads:
1956 and these conditions now hold; changes in caps:
1960 defined $hash{'d'} is true
1961 defined $hash{'a'} is FALSE
1962 exists $hash{'a'} is true (Perl5 only)
1963 grep ($_ eq 'a', keys %hash) is true
1965 Notice the last two: you have an undef value, but a defined key!
1971 your table now reads:
1980 and these conditions now hold; changes in caps:
1984 defined $hash{'d'} is true
1985 defined $hash{'a'} is false
1986 exists $hash{'a'} is FALSE (Perl5 only)
1987 grep ($_ eq 'a', keys %hash) is FALSE
1989 See, the whole entry is gone!
1991 =head2 Why don't my tied hashes make the defined/exists distinction?
1993 This depends on the tied hash's implementation of EXISTS().
1994 For example, there isn't the concept of undef with hashes
1995 that are tied to DBM* files. It also means that exists() and
1996 defined() do the same thing with a DBM* file, and what they
1997 end up doing is not what they do with ordinary hashes.
1999 =head2 How do I reset an each() operation part-way through?
2001 Using C<keys %hash> in scalar context returns the number of keys in
2002 the hash I<and> resets the iterator associated with the hash. You may
2003 need to do this if you use C<last> to exit a loop early so that when
2004 you re-enter it, the hash iterator has been reset.
2006 =head2 How can I get the unique keys from two hashes?
2008 First you extract the keys from the hashes into lists, then solve
2009 the "removing duplicates" problem described above. For example:
2012 for $element (keys(%foo), keys(%bar)) {
2019 @uniq = keys %{{%foo,%bar}};
2021 Or if you really want to save space:
2024 while (defined ($key = each %foo)) {
2027 while (defined ($key = each %bar)) {
2032 =head2 How can I store a multidimensional array in a DBM file?
2034 Either stringify the structure yourself (no fun), or else
2035 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
2036 it on top of either DB_File or GDBM_File.
2038 =head2 How can I make my hash remember the order I put elements into it?
2040 Use the C<Tie::IxHash> from CPAN.
2044 tie my %myhash, 'Tie::IxHash';
2046 for (my $i=0; $i<20; $i++) {
2050 my @keys = keys %myhash;
2051 # @keys = (0,1,2,3,...)
2053 =head2 Why does passing a subroutine an undefined element in a hash create it?
2055 If you say something like:
2057 somefunc($hash{"nonesuch key here"});
2059 Then that element "autovivifies"; that is, it springs into existence
2060 whether you store something there or not. That's because functions
2061 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
2062 it has to be ready to write it back into the caller's version.
2064 This has been fixed as of Perl5.004.
2066 Normally, merely accessing a key's value for a nonexistent key does
2067 I<not> cause that key to be forever there. This is different than
2070 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
2072 Usually a hash ref, perhaps like this:
2077 TITLE => "deputy peon",
2080 PALS => [ "Norbert", "Rhys", "Phineas"],
2083 References are documented in L<perlref> and the upcoming L<perlreftut>.
2084 Examples of complex data structures are given in L<perldsc> and
2085 L<perllol>. Examples of structures and object-oriented classes are
2088 =head2 How can I use a reference as a hash key?
2090 (contributed by brian d foy)
2092 Hash keys are strings, so you can't really use a reference as the key.
2093 When you try to do that, perl turns the reference into its stringified
2094 form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get
2095 back the reference from the stringified form, at least without doing
2096 some extra work on your own. Also remember that hash keys must be
2097 unique, but two different variables can store the same reference (and
2098 those variables can change later).
2100 The C<Tie::RefHash> module, which is distributed with perl, might be
2101 what you want. It handles that extra work.
2105 =head2 How do I handle binary data correctly?
2107 Perl is binary clean, so it can handle binary data just fine.
2108 On Windows or DOS, however, you have to use C<binmode> for binary
2109 files to avoid conversions for line endings. In general, you should
2110 use C<binmode> any time you want to work with binary data.
2112 Also see L<perlfunc/"binmode"> or L<perlopentut>.
2114 If you're concerned about 8-bit textual data then see L<perllocale>.
2115 If you want to deal with multibyte characters, however, there are
2116 some gotchas. See the section on Regular Expressions.
2118 =head2 How do I determine whether a scalar is a number/whole/integer/float?
2120 Assuming that you don't care about IEEE notations like "NaN" or
2121 "Infinity", you probably just want to use a regular expression.
2123 if (/\D/) { print "has nondigits\n" }
2124 if (/^\d+$/) { print "is a whole number\n" }
2125 if (/^-?\d+$/) { print "is an integer\n" }
2126 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2127 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2128 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2129 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2130 { print "a C float\n" }
2132 There are also some commonly used modules for the task.
2133 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2134 internal function C<looks_like_number> for determining whether a
2135 variable looks like a number. L<Data::Types> exports functions that
2136 validate data types using both the above and other regular
2137 expressions. Thirdly, there is C<Regexp::Common> which has regular
2138 expressions to match various types of numbers. Those three modules are
2139 available from the CPAN.
2141 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2142 function. Its semantics are somewhat cumbersome, so here's a
2143 C<getnum> wrapper function for more convenient access. This function
2144 takes a string and returns the number it found, or C<undef> for input
2145 that isn't a C float. The C<is_numeric> function is a front end to
2146 C<getnum> if you just want to say, "Is this a float?"
2149 use POSIX qw(strtod);
2154 my($num, $unparsed) = strtod($str);
2155 if (($str eq '') || ($unparsed != 0) || $!) {
2163 sub is_numeric { defined getnum($_[0]) }
2165 Or you could check out the L<String::Scanf> module on the CPAN
2166 instead. The C<POSIX> module (part of the standard Perl distribution)
2167 provides the C<strtod> and C<strtol> for converting strings to double
2168 and longs, respectively.
2170 =head2 How do I keep persistent data across program calls?
2172 For some specific applications, you can use one of the DBM modules.
2173 See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw>
2174 or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part
2175 of the standard distribution. Here's one example using C<Storable>'s C<store>
2176 and C<retrieve> functions:
2179 store(\%hash, "filename");
2182 $href = retrieve("filename"); # by ref
2183 %hash = %{ retrieve("filename") }; # direct to hash
2185 =head2 How do I print out or copy a recursive data structure?
2187 The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great
2188 for printing out data structures. The C<Storable> module on CPAN (or the
2189 5.8 release of Perl), provides a function called C<dclone> that recursively
2190 copies its argument.
2192 use Storable qw(dclone);
2195 Where C<$r1> can be a reference to any kind of data structure you'd like.
2196 It will be deeply copied. Because C<dclone> takes and returns references,
2197 you'd have to add extra punctuation if you had a hash of arrays that
2200 %newhash = %{ dclone(\%oldhash) };
2202 =head2 How do I define methods for every class/object?
2204 Use the C<UNIVERSAL> class (see L<UNIVERSAL>).
2206 =head2 How do I verify a credit card checksum?
2208 Get the C<Business::CreditCard> module from CPAN.
2210 =head2 How do I pack arrays of doubles or floats for XS code?
2212 The kgbpack.c code in the C<PGPLOT> module on CPAN does just this.
2213 If you're doing a lot of float or double processing, consider using
2214 the C<PDL> module from CPAN instead--it makes number-crunching easy.
2218 Revision: $Revision: 7954 $
2220 Date: $Date: 2006-10-16 22:07:58 +0200 (lun, 16 oct 2006) $
2222 See L<perlfaq> for source control details and availability.
2224 =head1 AUTHOR AND COPYRIGHT
2226 Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
2227 other authors as noted. All rights reserved.
2229 This documentation is free; you can redistribute it and/or modify it
2230 under the same terms as Perl itself.
2232 Irrespective of its distribution, all code examples in this file
2233 are hereby placed into the public domain. You are permitted and
2234 encouraged to use this code in your own programs for fun
2235 or for profit as you see fit. A simple comment in the code giving
2236 credit would be courteous but is not required.