3 perlfaq4 - Data Manipulation ($Revision: 6816 $)
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.
605 print "The time is ${\(scalar localtime)}.\n"
607 print "The time is ${ my $x = localtime; \$x }.\n";
609 If your function already returns a reference, you don't need to create
610 the reference yourself.
612 sub timestamp { my $t = localtime; \$t }
614 print "The time is ${ timestamp() }.\n";
616 The C<Interpolation> module can also do a lot of magic for you. You can
617 specify a variable name, in this case C<E>, to set up a tied hash that
618 does the interpolation for you. It has several other methods to do this
621 use Interpolation E => 'eval';
622 print "The time values are $E{localtime()}.\n";
624 In most cases, it is probably easier to simply use string concatenation,
625 which also forces scalar context.
627 print "The time is " . localtime() . ".\n";
629 =head2 How do I find matching/nesting anything?
631 This isn't something that can be done in one regular expression, no
632 matter how complicated. To find something between two single
633 characters, a pattern like C</x([^x]*)x/> will get the intervening
634 bits in $1. For multiple ones, then something more like
635 C</alpha(.*?)omega/> would be needed. But none of these deals with
636 nested patterns. For balanced expressions using C<(>, C<{>, C<[> or
637 C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
638 L<perlre/(??{ code })>. For other cases, you'll have to write a
641 If you are serious about writing a parser, there are a number of
642 modules or oddities that will make your life a lot easier. There are
643 the CPAN modules C<Parse::RecDescent>, C<Parse::Yapp>, and
644 C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8
645 the C<Text::Balanced> is part of the standard distribution.
647 One simple destructive, inside-out approach that you might try is to
648 pull out the smallest nesting parts one at a time:
650 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
651 # do something with $1
654 A more complicated and sneaky approach is to make Perl's regular
655 expression engine do it for you. This is courtesy Dean Inada, and
656 rather has the nature of an Obfuscated Perl Contest entry, but it
659 # $_ contains the string to parse
660 # BEGIN and END are the opening and closing markers for the
665 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
666 @$ = (eval{/$re/},$@!~/unmatched/i);
667 print join("\n",@$[0..$#$]) if( $$[-1] );
669 =head2 How do I reverse a string?
671 Use C<reverse()> in scalar context, as documented in
674 $reversed = reverse $string;
676 =head2 How do I expand tabs in a string?
678 You can do it yourself:
680 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
682 Or you can just use the C<Text::Tabs> module (part of the standard Perl
686 @expanded_lines = expand(@lines_with_tabs);
688 =head2 How do I reformat a paragraph?
690 Use C<Text::Wrap> (part of the standard Perl distribution):
693 print wrap("\t", ' ', @paragraphs);
695 The paragraphs you give to C<Text::Wrap> should not contain embedded
696 newlines. C<Text::Wrap> doesn't justify the lines (flush-right).
698 Or use the CPAN module C<Text::Autoformat>. Formatting files can be
699 easily done by making a shell alias, like so:
701 alias fmt="perl -i -MText::Autoformat -n0777 \
702 -e 'print autoformat $_, {all=>1}' $*"
704 See the documentation for C<Text::Autoformat> to appreciate its many
707 =head2 How can I access or change N characters of a string?
709 You can access the first characters of a string with substr().
710 To get the first character, for example, start at position 0
711 and grab the string of length 1.
714 $string = "Just another Perl Hacker";
715 $first_char = substr( $string, 0, 1 ); # 'J'
717 To change part of a string, you can use the optional fourth
718 argument which is the replacement string.
720 substr( $string, 13, 4, "Perl 5.8.0" );
722 You can also use substr() as an lvalue.
724 substr( $string, 13, 4 ) = "Perl 5.8.0";
726 =head2 How do I change the Nth occurrence of something?
728 You have to keep track of N yourself. For example, let's say you want
729 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
730 C<"whosoever"> or C<"whomsoever">, case insensitively. These
731 all assume that $_ contains the string to be altered.
735 ++$count == 5 # is it the 5th?
736 ? "${2}soever" # yes, swap
737 : $1 # renege and leave it there
740 In the more general case, you can use the C</g> modifier in a C<while>
741 loop, keeping count of matches.
745 $_ = "One fish two fish red fish blue fish";
746 while (/(\w+)\s+fish\b/gi) {
747 if (++$count == $WANT) {
748 print "The third fish is a $1 one.\n";
752 That prints out: C<"The third fish is a red one."> You can also use a
753 repetition count and repeated pattern like this:
755 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
757 =head2 How can I count the number of occurrences of a substring within a string?
759 There are a number of ways, with varying efficiency. If you want a
760 count of a certain single character (X) within a string, you can use the
761 C<tr///> function like so:
763 $string = "ThisXlineXhasXsomeXx'sXinXit";
764 $count = ($string =~ tr/X//);
765 print "There are $count X characters in the string";
767 This is fine if you are just looking for a single character. However,
768 if you are trying to count multiple character substrings within a
769 larger string, C<tr///> won't work. What you can do is wrap a while()
770 loop around a global pattern match. For example, let's count negative
773 $string = "-9 55 48 -2 23 -76 4 14 -44";
774 while ($string =~ /-\d+/g) { $count++ }
775 print "There are $count negative numbers in the string";
777 Another version uses a global match in list context, then assigns the
778 result to a scalar, producing a count of the number of matches.
780 $count = () = $string =~ /-\d+/g;
782 =head2 How do I capitalize all the words on one line?
784 To make the first letter of each word upper case:
786 $line =~ s/\b(\w)/\U$1/g;
788 This has the strange effect of turning "C<don't do it>" into "C<Don'T
789 Do It>". Sometimes you might want this. Other times you might need a
790 more thorough solution (Suggested by brian d foy):
793 (^\w) #at the beginning of the line
795 (\s\w) #preceded by whitespace
799 $string =~ s/([\w']+)/\u\L$1/g;
801 To make the whole line upper case:
805 To force each word to be lower case, with the first letter upper case:
807 $line =~ s/(\w+)/\u\L$1/g;
809 You can (and probably should) enable locale awareness of those
810 characters by placing a C<use locale> pragma in your program.
811 See L<perllocale> for endless details on locales.
813 This is sometimes referred to as putting something into "title
814 case", but that's not quite accurate. Consider the proper
815 capitalization of the movie I<Dr. Strangelove or: How I Learned to
816 Stop Worrying and Love the Bomb>, for example.
818 Damian Conway's L<Text::Autoformat> module provides some smart
819 case transformations:
821 use Text::Autoformat;
822 my $x = "Dr. Strangelove or: How I Learned to Stop ".
823 "Worrying and Love the Bomb";
826 for my $style (qw( sentence title highlight )) {
827 print autoformat($x, { case => $style }), "\n";
830 =head2 How can I split a [character] delimited string except when inside [character]?
832 Several modules can handle this sort of parsing--C<Text::Balanced>,
833 C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others.
835 Take the example case of trying to split a string that is
836 comma-separated into its different fields. You can't use C<split(/,/)>
837 because you shouldn't split if the comma is inside quotes. For
838 example, take a data line like this:
840 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
842 Due to the restriction of the quotes, this is a fairly complex
843 problem. Thankfully, we have Jeffrey Friedl, author of
844 I<Mastering Regular Expressions>, to handle these for us. He
845 suggests (assuming your string is contained in C<$text>):
848 push(@new, $+) while $text =~ m{
849 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
853 push(@new, undef) if substr($text,-1,1) eq ',';
855 If you want to represent quotation marks inside a
856 quotation-mark-delimited field, escape them with backslashes (eg,
859 Alternatively, the C<Text::ParseWords> module (part of the standard
860 Perl distribution) lets you say:
862 use Text::ParseWords;
863 @new = quotewords(",", 0, $text);
865 =head2 How do I strip blank space from the beginning/end of a string?
867 (contributed by brian d foy)
869 A substitution can do this for you. For a single line, you want to
870 replace all the leading or trailing whitespace with nothing. You
871 can do that with a pair of substitutions.
876 You can also write that as a single substitution, although it turns
877 out the combined statement is slower than the separate ones. That
878 might not matter to you, though.
882 In this regular expression, the alternation matches either at the
883 beginning or the end of the string since the anchors have a lower
884 precedence than the alternation. With the C</g> flag, the substitution
885 makes all possible matches, so it gets both. Remember, the trailing
886 newline matches the C<\s+>, and the C<$> anchor can match to the
887 physical end of the string, so the newline disappears too. Just add
888 the newline to the output, which has the added benefit of preserving
889 "blank" (consisting entirely of whitespace) lines which the C<^\s+>
890 would remove all by itself.
898 For a multi-line string, you can apply the regular expression
899 to each logical line in the string by adding the C</m> flag (for
900 "multi-line"). With the C</m> flag, the C<$> matches I<before> an
901 embedded newline, so it doesn't remove it. It still removes the
902 newline at the end of the string.
904 $string =~ s/^\s+|\s+$//gm;
906 Remember that lines consisting entirely of whitespace will disappear,
907 since the first part of the alternation can match the entire string
908 and replace it with nothing. If need to keep embedded blank lines,
909 you have to do a little more work. Instead of matching any whitespace
910 (since that includes a newline), just match the other whitespace.
912 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
914 =head2 How do I pad a string with blanks or pad a number with zeroes?
916 In the following examples, C<$pad_len> is the length to which you wish
917 to pad the string, C<$text> or C<$num> contains the string to be padded,
918 and C<$pad_char> contains the padding character. You can use a single
919 character string constant instead of the C<$pad_char> variable if you
920 know what it is in advance. And in the same way you can use an integer in
921 place of C<$pad_len> if you know the pad length in advance.
923 The simplest method uses the C<sprintf> function. It can pad on the left
924 or right with blanks and on the left with zeroes and it will not
925 truncate the result. The C<pack> function can only pad strings on the
926 right with blanks and it will truncate the result to a maximum length of
929 # Left padding a string with blanks (no truncation):
930 $padded = sprintf("%${pad_len}s", $text);
931 $padded = sprintf("%*s", $pad_len, $text); # same thing
933 # Right padding a string with blanks (no truncation):
934 $padded = sprintf("%-${pad_len}s", $text);
935 $padded = sprintf("%-*s", $pad_len, $text); # same thing
937 # Left padding a number with 0 (no truncation):
938 $padded = sprintf("%0${pad_len}d", $num);
939 $padded = sprintf("%0*d", $pad_len, $num); # same thing
941 # Right padding a string with blanks using pack (will truncate):
942 $padded = pack("A$pad_len",$text);
944 If you need to pad with a character other than blank or zero you can use
945 one of the following methods. They all generate a pad string with the
946 C<x> operator and combine that with C<$text>. These methods do
947 not truncate C<$text>.
949 Left and right padding with any character, creating a new string:
951 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
952 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
954 Left and right padding with any character, modifying C<$text> directly:
956 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
957 $text .= $pad_char x ( $pad_len - length( $text ) );
959 =head2 How do I extract selected columns from a string?
961 Use C<substr()> or C<unpack()>, both documented in L<perlfunc>.
962 If you prefer thinking in terms of columns instead of widths,
963 you can use this kind of thing:
965 # determine the unpack format needed to split Linux ps output
966 # arguments are cut columns
967 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
973 for my $place (@positions) {
974 $template .= "A" . ($place - $lastpos) . " ";
981 =head2 How do I find the soundex value of a string?
983 (contributed by brian d foy)
985 You can use the Text::Soundex module. If you want to do fuzzy or close
986 matching, you might also try the C<String::Approx>, and
987 C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules.
989 =head2 How can I expand variables in text strings?
991 Let's assume that you have a string that contains placeholder
994 $text = 'this has a $foo in it and a $bar';
996 You can use a substitution with a double evaluation. The
997 first /e turns C<$1> into C<$foo>, and the second /e turns
998 C<$foo> into its value. You may want to wrap this in an
999 C<eval>: if you try to get the value of an undeclared variable
1000 while running under C<use strict>, you get a fatal error.
1002 eval { $text =~ s/(\$\w+)/$1/eeg };
1005 It's probably better in the general case to treat those
1006 variables as entries in some special hash. For example:
1012 $text =~ s/\$(\w+)/$user_defs{$1}/g;
1014 =head2 What's wrong with always quoting "$vars"?
1016 The problem is that those double-quotes force
1017 stringification--coercing numbers and references into
1018 strings--even when you don't want them to be strings. Think
1019 of it this way: double-quote expansion is used to produce
1020 new strings. If you already have a string, why do you need
1023 If you get used to writing odd things like these:
1026 $new = "$old"; # BAD
1027 somefunc("$var"); # BAD
1029 You'll be in trouble. Those should (in 99.8% of the cases) be
1030 the simpler and more direct:
1036 Otherwise, besides slowing you down, you're going to break code when
1037 the thing in the scalar is actually neither a string nor a number, but
1043 my $oref = "$aref"; # WRONG
1046 You can also get into subtle problems on those few operations in Perl
1047 that actually do care about the difference between a string and a
1048 number, such as the magical C<++> autoincrement operator or the
1051 Stringification also destroys arrays.
1054 print "@lines"; # WRONG - extra blanks
1055 print @lines; # right
1057 =head2 Why don't my E<lt>E<lt>HERE documents work?
1059 Check for these three things:
1063 =item There must be no space after the E<lt>E<lt> part.
1065 =item There (probably) should be a semicolon at the end.
1067 =item You can't (easily) have any space in front of the tag.
1071 If you want to indent the text in the here document, you
1075 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1080 But the HERE_TARGET must still be flush against the margin.
1081 If you want that indented also, you'll have to quote
1084 ($quote = <<' FINIS') =~ s/^\s+//gm;
1085 ...we will have peace, when you and all your works have
1086 perished--and the works of your dark master to whom you
1087 would deliver us. You are a liar, Saruman, and a corrupter
1088 of men's hearts. --Theoden in /usr/src/perl/taint.c
1090 $quote =~ s/\s+--/\n--/;
1092 A nice general-purpose fixer-upper function for indented here documents
1093 follows. It expects to be called with a here document as its argument.
1094 It looks to see whether each line begins with a common substring, and
1095 if so, strips that substring off. Otherwise, it takes the amount of leading
1096 whitespace found on the first line and removes that much off each
1101 my ($white, $leader); # common whitespace and common leading string
1102 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1103 ($white, $leader) = ($2, quotemeta($1));
1105 ($white, $leader) = (/^(\s+)/, '');
1107 s/^\s*?$leader(?:$white)?//gm;
1111 This works with leading special strings, dynamically determined:
1113 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1116 @@@ SAVEI32(runlevel);
1118 @@@ while ( op = (*op->op_ppaddr)() );
1122 MAIN_INTERPRETER_LOOP
1124 Or with a fixed amount of leading whitespace, with remaining
1125 indentation correctly preserved:
1127 $poem = fix<<EVER_ON_AND_ON;
1128 Now far ahead the Road has gone,
1129 And I must follow, if I can,
1130 Pursuing it with eager feet,
1131 Until it joins some larger way
1132 Where many paths and errands meet.
1133 And whither then? I cannot say.
1134 --Bilbo in /usr/src/perl/pp_ctl.c
1139 =head2 What is the difference between a list and an array?
1141 An array has a changeable length. A list does not. An array is
1142 something you can push or pop, while a list is a set of values. Some
1143 people make the distinction that a list is a value while an array is a
1144 variable. Subroutines are passed and return lists, you put things into
1145 list context, you initialize arrays with lists, and you C<foreach()>
1146 across a list. C<@> variables are arrays, anonymous arrays are
1147 arrays, arrays in scalar context behave like the number of elements in
1148 them, subroutines access their arguments through the array C<@_>, and
1149 C<push>/C<pop>/C<shift> only work on arrays.
1151 As a side note, there's no such thing as a list in scalar context.
1154 $scalar = (2, 5, 7, 9);
1156 you're using the comma operator in scalar context, so it uses the scalar
1157 comma operator. There never was a list there at all! This causes the
1158 last value to be returned: 9.
1160 =head2 What is the difference between $array[1] and @array[1]?
1162 The former is a scalar value; the latter an array slice, making
1163 it a list with one (scalar) value. You should use $ when you want a
1164 scalar value (most of the time) and @ when you want a list with one
1165 scalar value in it (very, very rarely; nearly never, in fact).
1167 Sometimes it doesn't make a difference, but sometimes it does.
1168 For example, compare:
1170 $good[0] = `some program that outputs several lines`;
1174 @bad[0] = `same program that outputs several lines`;
1176 The C<use warnings> pragma and the B<-w> flag will warn you about these
1179 =head2 How can I remove duplicate elements from a list or array?
1181 (contributed by brian d foy)
1183 Use a hash. When you think the words "unique" or "duplicated", think
1186 If you don't care about the order of the elements, you could just
1187 create the hash then extract the keys. It's not important how you
1188 create that hash: just that you use C<keys> to get the unique
1191 my %hash = map { $_, 1 } @array;
1192 # or a hash slice: @hash{ @array } = ();
1193 # or a foreach: $hash{$_} = 1 foreach ( @array );
1195 my @unique = keys %hash;
1197 If you want to use a module, try the C<uniq> function from
1198 C<List::MoreUtils>. In list context it returns the unique elements,
1199 preserving their order in the list. In scalar context, it returns the
1200 number of unique elements.
1202 use List::MoreUtils qw(uniq);
1204 my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
1205 my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7
1207 You can also go through each element and skip the ones you've seen
1208 before. Use a hash to keep track. The first time the loop sees an
1209 element, that element has no key in C<%Seen>. The C<next> statement
1210 creates the key and immediately uses its value, which is C<undef>, so
1211 the loop continues to the C<push> and increments the value for that
1212 key. The next time the loop sees that same element, its key exists in
1213 the hash I<and> the value for that key is true (since it's not 0 or
1214 C<undef>), so the next skips that iteration and the loop goes to the
1220 foreach my $elem ( @array )
1222 next if $seen{ $elem }++;
1223 push @unique, $elem;
1226 You can write this more briefly using a grep, which does the
1230 my @unique = grep { ! $seen{ $_ }++ } @array;
1232 =head2 How can I tell whether a certain element is contained in a list or array?
1234 (portions of this answer contributed by Anno Siegel)
1236 Hearing the word "in" is an I<in>dication that you probably should have
1237 used a hash, not a list or array, to store your data. Hashes are
1238 designed to answer this question quickly and efficiently. Arrays aren't.
1240 That being said, there are several ways to approach this. If you
1241 are going to make this query many times over arbitrary string values,
1242 the fastest way is probably to invert the original array and maintain a
1243 hash whose keys are the first array's values.
1245 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1247 for (@blues) { $is_blue{$_} = 1 }
1249 Now you can check whether C<$is_blue{$some_color}>. It might have
1250 been a good idea to keep the blues all in a hash in the first place.
1252 If the values are all small integers, you could use a simple indexed
1253 array. This kind of an array will take up less space:
1255 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1256 @is_tiny_prime = ();
1257 for (@primes) { $is_tiny_prime[$_] = 1 }
1258 # or simply @istiny_prime[@primes] = (1) x @primes;
1260 Now you check whether $is_tiny_prime[$some_number].
1262 If the values in question are integers instead of strings, you can save
1263 quite a lot of space by using bit strings instead:
1265 @articles = ( 1..10, 150..2000, 2017 );
1267 for (@articles) { vec($read,$_,1) = 1 }
1269 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1271 These methods guarantee fast individual tests but require a re-organization
1272 of the original list or array. They only pay off if you have to test
1273 multiple values against the same array.
1275 If you are testing only once, the standard module C<List::Util> exports
1276 the function C<first> for this purpose. It works by stopping once it
1277 finds the element. It's written in C for speed, and its Perl equivalant
1278 looks like this subroutine:
1283 return $_ if &{$code}();
1288 If speed is of little concern, the common idiom uses grep in scalar context
1289 (which returns the number of items that passed its condition) to traverse the
1290 entire list. This does have the benefit of telling you how many matches it
1293 my $is_there = grep $_ eq $whatever, @array;
1295 If you want to actually extract the matching elements, simply use grep in
1298 my @matches = grep $_ eq $whatever, @array;
1300 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1302 Use a hash. Here's code to do both and more. It assumes that each
1303 element is unique in a given array:
1305 @union = @intersection = @difference = ();
1307 foreach $element (@array1, @array2) { $count{$element}++ }
1308 foreach $element (keys %count) {
1309 push @union, $element;
1310 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1313 Note that this is the I<symmetric difference>, that is, all elements
1314 in either A or in B but not in both. Think of it as an xor operation.
1316 =head2 How do I test whether two arrays or hashes are equal?
1318 The following code works for single-level arrays. It uses a
1319 stringwise comparison, and does not distinguish defined versus
1320 undefined empty strings. Modify if you have other needs.
1322 $are_equal = compare_arrays(\@frogs, \@toads);
1324 sub compare_arrays {
1325 my ($first, $second) = @_;
1326 no warnings; # silence spurious -w undef complaints
1327 return 0 unless @$first == @$second;
1328 for (my $i = 0; $i < @$first; $i++) {
1329 return 0 if $first->[$i] ne $second->[$i];
1334 For multilevel structures, you may wish to use an approach more
1335 like this one. It uses the CPAN module C<FreezeThaw>:
1337 use FreezeThaw qw(cmpStr);
1338 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1340 printf "a and b contain %s arrays\n",
1341 cmpStr(\@a, \@b) == 0
1345 This approach also works for comparing hashes. Here we'll demonstrate
1346 two different answers:
1348 use FreezeThaw qw(cmpStr cmpStrHard);
1350 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1354 printf "a and b contain %s hashes\n",
1355 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1357 printf "a and b contain %s hashes\n",
1358 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1361 The first reports that both those the hashes contain the same data,
1362 while the second reports that they do not. Which you prefer is left as
1363 an exercise to the reader.
1365 =head2 How do I find the first array element for which a condition is true?
1367 To find the first array element which satisfies a condition, you can
1368 use the C<first()> function in the C<List::Util> module, which comes
1369 with Perl 5.8. This example finds the first element that contains
1372 use List::Util qw(first);
1374 my $element = first { /Perl/ } @array;
1376 If you cannot use C<List::Util>, you can make your own loop to do the
1377 same thing. Once you find the element, you stop the loop with last.
1380 foreach ( @array ) {
1381 if( /Perl/ ) { $found = $_; last }
1384 If you want the array index, you can iterate through the indices
1385 and check the array element at each index until you find one
1386 that satisfies the condition.
1388 my( $found, $index ) = ( undef, -1 );
1389 for( $i = 0; $i < @array; $i++ ) {
1390 if( $array[$i] =~ /Perl/ ) {
1391 $found = $array[$i];
1397 =head2 How do I handle linked lists?
1399 In general, you usually don't need a linked list in Perl, since with
1400 regular arrays, you can push and pop or shift and unshift at either
1401 end, or you can use splice to add and/or remove arbitrary number of
1402 elements at arbitrary points. Both pop and shift are both O(1)
1403 operations on Perl's dynamic arrays. In the absence of shifts and
1404 pops, push in general needs to reallocate on the order every log(N)
1405 times, and unshift will need to copy pointers each time.
1407 If you really, really wanted, you could use structures as described in
1408 L<perldsc> or L<perltoot> and do just what the algorithm book tells
1409 you to do. For example, imagine a list node like this:
1416 You could walk the list this way:
1419 for ($node = $head; $node; $node = $node->{LINK}) {
1420 print $node->{VALUE}, " ";
1424 You could add to the list this way:
1427 $tail = append($head, 1); # grow a new head
1428 for $value ( 2 .. 10 ) {
1429 $tail = append($tail, $value);
1433 my($list, $value) = @_;
1434 my $node = { VALUE => $value };
1436 $node->{LINK} = $list->{LINK};
1437 $list->{LINK} = $node;
1440 $_[0] = $node; # replace caller's version
1445 But again, Perl's built-in are virtually always good enough.
1447 =head2 How do I handle circular lists?
1449 Circular lists could be handled in the traditional fashion with linked
1450 lists, or you could just do something like this with an array:
1452 unshift(@array, pop(@array)); # the last shall be first
1453 push(@array, shift(@array)); # and vice versa
1455 You can also use C<Tie::Cycle>:
1459 tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
1461 print $cycle; # FFFFFF
1462 print $cycle; # 000000
1463 print $cycle; # FFFF00
1465 =head2 How do I shuffle an array randomly?
1467 If you either have Perl 5.8.0 or later installed, or if you have
1468 Scalar-List-Utils 1.03 or later installed, you can say:
1470 use List::Util 'shuffle';
1472 @shuffled = shuffle(@list);
1474 If not, you can use a Fisher-Yates shuffle.
1476 sub fisher_yates_shuffle {
1477 my $deck = shift; # $deck is a reference to an array
1480 my $j = int rand ($i+1);
1481 @$deck[$i,$j] = @$deck[$j,$i];
1485 # shuffle my mpeg collection
1487 my @mpeg = <audio/*/*.mp3>;
1488 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1491 Note that the above implementation shuffles an array in place,
1492 unlike the C<List::Util::shuffle()> which takes a list and returns
1493 a new shuffled list.
1495 You've probably seen shuffling algorithms that work using splice,
1496 randomly picking another element to swap the current element with
1500 @old = 1 .. 10; # just a demo
1502 push(@new, splice(@old, rand @old, 1));
1505 This is bad because splice is already O(N), and since you do it N
1506 times, you just invented a quadratic algorithm; that is, O(N**2).
1507 This does not scale, although Perl is so efficient that you probably
1508 won't notice this until you have rather largish arrays.
1510 =head2 How do I process/modify each element of an array?
1512 Use C<for>/C<foreach>:
1515 s/foo/bar/; # change that word
1516 tr/XZ/ZX/; # swap those letters
1519 Here's another; let's compute spherical volumes:
1521 for (@volumes = @radii) { # @volumes has changed parts
1523 $_ *= (4/3) * 3.14159; # this will be constant folded
1526 which can also be done with C<map()> which is made to transform
1527 one list into another:
1529 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1531 If you want to do the same thing to modify the values of the
1532 hash, you can use the C<values> function. As of Perl 5.6
1533 the values are not copied, so if you modify $orbit (in this
1534 case), you modify the value.
1536 for $orbit ( values %orbits ) {
1537 ($orbit **= 3) *= (4/3) * 3.14159;
1540 Prior to perl 5.6 C<values> returned copies of the values,
1541 so older perl code often contains constructions such as
1542 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1543 the hash is to be modified.
1545 =head2 How do I select a random element from an array?
1547 Use the C<rand()> function (see L<perlfunc/rand>):
1549 $index = rand @array;
1550 $element = $array[$index];
1554 my $element = $array[ rand @array ];
1556 =head2 How do I permute N elements of a list?
1558 Use the C<List::Permutor> module on CPAN. If the list is actually an
1559 array, try the C<Algorithm::Permute> module (also on CPAN). It's
1560 written in XS code and is very efficient.
1562 use Algorithm::Permute;
1563 my @array = 'a'..'d';
1564 my $p_iterator = Algorithm::Permute->new ( \@array );
1565 while (my @perm = $p_iterator->next) {
1566 print "next permutation: (@perm)\n";
1569 For even faster execution, you could do:
1571 use Algorithm::Permute;
1572 my @array = 'a'..'d';
1573 Algorithm::Permute::permute {
1574 print "next permutation: (@array)\n";
1577 Here's a little program that generates all permutations of
1578 all the words on each line of input. The algorithm embodied
1579 in the C<permute()> function is discussed in Volume 4 (still
1580 unpublished) of Knuth's I<The Art of Computer Programming>
1581 and will work on any list:
1584 # Fischer-Kause ordered permutation generator
1589 while ( $code->(@_[@idx]) ) {
1591 --$p while $idx[$p-1] > $idx[$p];
1592 my $q = $p or return;
1593 push @idx, reverse splice @idx, $p;
1594 ++$q while $idx[$p-1] > $idx[$q];
1595 @idx[$p-1,$q]=@idx[$q,$p-1];
1599 permute {print"@_\n"} split;
1601 =head2 How do I sort an array by (anything)?
1603 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1605 @list = sort { $a <=> $b } @list;
1607 The default sort function is cmp, string comparison, which would
1608 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1609 the numerical comparison operator.
1611 If you have a complicated function needed to pull out the part you
1612 want to sort on, then don't do it inside the sort function. Pull it
1613 out first, because the sort BLOCK can be called many times for the
1614 same element. Here's an example of how to pull out the first word
1615 after the first number on each item, and then sort those words
1620 ($item) = /\d+\s*(\S+)/;
1621 push @idx, uc($item);
1623 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1625 which could also be written this way, using a trick
1626 that's come to be known as the Schwartzian Transform:
1628 @sorted = map { $_->[0] }
1629 sort { $a->[1] cmp $b->[1] }
1630 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1632 If you need to sort on several fields, the following paradigm is useful.
1635 field1($a) <=> field1($b) ||
1636 field2($a) cmp field2($b) ||
1637 field3($a) cmp field3($b)
1640 This can be conveniently combined with precalculation of keys as given
1643 See the F<sort> article in the "Far More Than You Ever Wanted
1644 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1645 more about this approach.
1647 See also the question later in L<perlfaq4> on sorting hashes.
1649 =head2 How do I manipulate arrays of bits?
1651 Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise
1654 For example, this sets C<$vec> to have bit N set if C<$ints[N]> was
1658 foreach(@ints) { vec($vec,$_,1) = 1 }
1660 Here's how, given a vector in C<$vec>, you can get those bits into your
1663 sub bitvec_to_list {
1666 # Find null-byte density then select best algorithm
1667 if ($vec =~ tr/\0// / length $vec > 0.95) {
1671 # This method is faster with mostly null-bytes
1672 while($vec =~ /[^\0]/g ) {
1673 $i = -9 + 8 * pos $vec;
1674 push @ints, $i if vec($vec, ++$i, 1);
1675 push @ints, $i if vec($vec, ++$i, 1);
1676 push @ints, $i if vec($vec, ++$i, 1);
1677 push @ints, $i if vec($vec, ++$i, 1);
1678 push @ints, $i if vec($vec, ++$i, 1);
1679 push @ints, $i if vec($vec, ++$i, 1);
1680 push @ints, $i if vec($vec, ++$i, 1);
1681 push @ints, $i if vec($vec, ++$i, 1);
1685 # This method is a fast general algorithm
1687 my $bits = unpack "b*", $vec;
1688 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1689 push @ints, pos $bits while($bits =~ /1/g);
1695 This method gets faster the more sparse the bit vector is.
1696 (Courtesy of Tim Bunce and Winfried Koenig.)
1698 You can make the while loop a lot shorter with this suggestion
1699 from Benjamin Goldberg:
1701 while($vec =~ /[^\0]+/g ) {
1702 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1705 Or use the CPAN module C<Bit::Vector>:
1707 $vector = Bit::Vector->new($num_of_bits);
1708 $vector->Index_List_Store(@ints);
1709 @ints = $vector->Index_List_Read();
1711 C<Bit::Vector> provides efficient methods for bit vector, sets of
1712 small integers and "big int" math.
1714 Here's a more extensive illustration using vec():
1717 $vector = "\xff\x0f\xef\xfe";
1718 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1719 unpack("N", $vector), "\n";
1720 $is_set = vec($vector, 23, 1);
1721 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1739 my ($offset, $width, $value) = @_;
1741 vec($vector, $offset, $width) = $value;
1742 print "offset=$offset width=$width value=$value\n";
1748 my $bits = unpack("b*", $vector);
1752 print "vector length in bytes: ", length($vector), "\n";
1753 @bytes = unpack("A8" x length($vector), $bits);
1754 print "bits are: @bytes\n\n";
1757 =head2 Why does defined() return true on empty arrays and hashes?
1759 The short story is that you should probably only use defined on scalars or
1760 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1761 in the 5.004 release or later of Perl for more detail.
1763 =head1 Data: Hashes (Associative Arrays)
1765 =head2 How do I process an entire hash?
1767 Use the each() function (see L<perlfunc/each>) if you don't care
1768 whether it's sorted:
1770 while ( ($key, $value) = each %hash) {
1771 print "$key = $value\n";
1774 If you want it sorted, you'll have to use foreach() on the result of
1775 sorting the keys as shown in an earlier question.
1777 =head2 What happens if I add or remove keys from a hash while iterating over it?
1779 (contributed by brian d foy)
1781 The easy answer is "Don't do that!"
1783 If you iterate through the hash with each(), you can delete the key
1784 most recently returned without worrying about it. If you delete or add
1785 other keys, the iterator may skip or double up on them since perl
1786 may rearrange the hash table. See the
1787 entry for C<each()> in L<perlfunc>.
1789 =head2 How do I look up a hash element by value?
1791 Create a reverse hash:
1793 %by_value = reverse %by_key;
1794 $key = $by_value{$value};
1796 That's not particularly efficient. It would be more space-efficient
1799 while (($key, $value) = each %by_key) {
1800 $by_value{$value} = $key;
1803 If your hash could have repeated values, the methods above will only find
1804 one of the associated keys. This may or may not worry you. If it does
1805 worry you, you can always reverse the hash into a hash of arrays instead:
1807 while (($key, $value) = each %by_key) {
1808 push @{$key_list_by_value{$value}}, $key;
1811 =head2 How can I know how many entries are in a hash?
1813 If you mean how many keys, then all you have to do is
1814 use the keys() function in a scalar context:
1816 $num_keys = keys %hash;
1818 The keys() function also resets the iterator, which means that you may
1819 see strange results if you use this between uses of other hash operators
1822 =head2 How do I sort a hash (optionally by value instead of key)?
1824 (contributed by brian d foy)
1826 To sort a hash, start with the keys. In this example, we give the list of
1827 keys to the sort function which then compares them ASCIIbetically (which
1828 might be affected by your locale settings). The output list has the keys
1829 in ASCIIbetical order. Once we have the keys, we can go through them to
1830 create a report which lists the keys in ASCIIbetical order.
1832 my @keys = sort { $a cmp $b } keys %hash;
1834 foreach my $key ( @keys )
1836 printf "%-20s %6d\n", $key, $hash{$value};
1839 We could get more fancy in the C<sort()> block though. Instead of
1840 comparing the keys, we can compute a value with them and use that
1841 value as the comparison.
1843 For instance, to make our report order case-insensitive, we use
1844 the C<\L> sequence in a double-quoted string to make everything
1845 lowercase. The C<sort()> block then compares the lowercased
1846 values to determine in which order to put the keys.
1848 my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
1850 Note: if the computation is expensive or the hash has many elements,
1851 you may want to look at the Schwartzian Transform to cache the
1852 computation results.
1854 If we want to sort by the hash value instead, we use the hash key
1855 to look it up. We still get out a list of keys, but this time they
1856 are ordered by their value.
1858 my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
1860 From there we can get more complex. If the hash values are the same,
1861 we can provide a secondary sort on the hash key.
1864 $hash{$a} <=> $hash{$b}
1869 =head2 How can I always keep my hash sorted?
1870 X<hash tie sort DB_File Tie::IxHash>
1872 You can look into using the C<DB_File> module and C<tie()> using the
1873 C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory
1874 Databases">. The C<Tie::IxHash> module from CPAN might also be
1875 instructive. Although this does keep your hash sorted, you might not
1876 like the slow down you suffer from the tie interface. Are you sure you
1879 =head2 What's the difference between "delete" and "undef" with hashes?
1881 Hashes contain pairs of scalars: the first is the key, the
1882 second is the value. The key will be coerced to a string,
1883 although the value can be any kind of scalar: string,
1884 number, or reference. If a key C<$key> is present in
1885 %hash, C<exists($hash{$key})> will return true. The value
1886 for a given key can be C<undef>, in which case
1887 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1888 will return true. This corresponds to (C<$key>, C<undef>)
1891 Pictures help... here's the C<%hash> table:
1901 And these conditions hold
1905 defined $hash{'d'} is true
1906 defined $hash{'a'} is true
1907 exists $hash{'a'} is true (Perl5 only)
1908 grep ($_ eq 'a', keys %hash) is true
1914 your table now reads:
1925 and these conditions now hold; changes in caps:
1929 defined $hash{'d'} is true
1930 defined $hash{'a'} is FALSE
1931 exists $hash{'a'} is true (Perl5 only)
1932 grep ($_ eq 'a', keys %hash) is true
1934 Notice the last two: you have an undef value, but a defined key!
1940 your table now reads:
1949 and these conditions now hold; changes in caps:
1953 defined $hash{'d'} is true
1954 defined $hash{'a'} is false
1955 exists $hash{'a'} is FALSE (Perl5 only)
1956 grep ($_ eq 'a', keys %hash) is FALSE
1958 See, the whole entry is gone!
1960 =head2 Why don't my tied hashes make the defined/exists distinction?
1962 This depends on the tied hash's implementation of EXISTS().
1963 For example, there isn't the concept of undef with hashes
1964 that are tied to DBM* files. It also means that exists() and
1965 defined() do the same thing with a DBM* file, and what they
1966 end up doing is not what they do with ordinary hashes.
1968 =head2 How do I reset an each() operation part-way through?
1970 Using C<keys %hash> in scalar context returns the number of keys in
1971 the hash I<and> resets the iterator associated with the hash. You may
1972 need to do this if you use C<last> to exit a loop early so that when
1973 you re-enter it, the hash iterator has been reset.
1975 =head2 How can I get the unique keys from two hashes?
1977 First you extract the keys from the hashes into lists, then solve
1978 the "removing duplicates" problem described above. For example:
1981 for $element (keys(%foo), keys(%bar)) {
1988 @uniq = keys %{{%foo,%bar}};
1990 Or if you really want to save space:
1993 while (defined ($key = each %foo)) {
1996 while (defined ($key = each %bar)) {
2001 =head2 How can I store a multidimensional array in a DBM file?
2003 Either stringify the structure yourself (no fun), or else
2004 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
2005 it on top of either DB_File or GDBM_File.
2007 =head2 How can I make my hash remember the order I put elements into it?
2009 Use the C<Tie::IxHash> from CPAN.
2013 tie my %myhash, 'Tie::IxHash';
2015 for (my $i=0; $i<20; $i++) {
2019 my @keys = keys %myhash;
2020 # @keys = (0,1,2,3,...)
2022 =head2 Why does passing a subroutine an undefined element in a hash create it?
2024 If you say something like:
2026 somefunc($hash{"nonesuch key here"});
2028 Then that element "autovivifies"; that is, it springs into existence
2029 whether you store something there or not. That's because functions
2030 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
2031 it has to be ready to write it back into the caller's version.
2033 This has been fixed as of Perl5.004.
2035 Normally, merely accessing a key's value for a nonexistent key does
2036 I<not> cause that key to be forever there. This is different than
2039 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
2041 Usually a hash ref, perhaps like this:
2046 TITLE => "deputy peon",
2049 PALS => [ "Norbert", "Rhys", "Phineas"],
2052 References are documented in L<perlref> and the upcoming L<perlreftut>.
2053 Examples of complex data structures are given in L<perldsc> and
2054 L<perllol>. Examples of structures and object-oriented classes are
2057 =head2 How can I use a reference as a hash key?
2059 (contributed by brian d foy)
2061 Hash keys are strings, so you can't really use a reference as the key.
2062 When you try to do that, perl turns the reference into its stringified
2063 form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get
2064 back the reference from the stringified form, at least without doing
2065 some extra work on your own. Also remember that hash keys must be
2066 unique, but two different variables can store the same reference (and
2067 those variables can change later).
2069 The C<Tie::RefHash> module, which is distributed with perl, might be
2070 what you want. It handles that extra work.
2074 =head2 How do I handle binary data correctly?
2076 Perl is binary clean, so it can handle binary data just fine.
2077 On Windows or DOS, however, you have to use C<binmode> for binary
2078 files to avoid conversions for line endings. In general, you should
2079 use C<binmode> any time you want to work with binary data.
2081 Also see L<perlfunc/"binmode"> or L<perlopentut>.
2083 If you're concerned about 8-bit textual data then see L<perllocale>.
2084 If you want to deal with multibyte characters, however, there are
2085 some gotchas. See the section on Regular Expressions.
2087 =head2 How do I determine whether a scalar is a number/whole/integer/float?
2089 Assuming that you don't care about IEEE notations like "NaN" or
2090 "Infinity", you probably just want to use a regular expression.
2092 if (/\D/) { print "has nondigits\n" }
2093 if (/^\d+$/) { print "is a whole number\n" }
2094 if (/^-?\d+$/) { print "is an integer\n" }
2095 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2096 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2097 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2098 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2099 { print "a C float\n" }
2101 There are also some commonly used modules for the task.
2102 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2103 internal function C<looks_like_number> for determining whether a
2104 variable looks like a number. L<Data::Types> exports functions that
2105 validate data types using both the above and other regular
2106 expressions. Thirdly, there is C<Regexp::Common> which has regular
2107 expressions to match various types of numbers. Those three modules are
2108 available from the CPAN.
2110 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2111 function. Its semantics are somewhat cumbersome, so here's a
2112 C<getnum> wrapper function for more convenient access. This function
2113 takes a string and returns the number it found, or C<undef> for input
2114 that isn't a C float. The C<is_numeric> function is a front end to
2115 C<getnum> if you just want to say, "Is this a float?"
2118 use POSIX qw(strtod);
2123 my($num, $unparsed) = strtod($str);
2124 if (($str eq '') || ($unparsed != 0) || $!) {
2132 sub is_numeric { defined getnum($_[0]) }
2134 Or you could check out the L<String::Scanf> module on the CPAN
2135 instead. The C<POSIX> module (part of the standard Perl distribution)
2136 provides the C<strtod> and C<strtol> for converting strings to double
2137 and longs, respectively.
2139 =head2 How do I keep persistent data across program calls?
2141 For some specific applications, you can use one of the DBM modules.
2142 See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw>
2143 or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part
2144 of the standard distribution. Here's one example using C<Storable>'s C<store>
2145 and C<retrieve> functions:
2148 store(\%hash, "filename");
2151 $href = retrieve("filename"); # by ref
2152 %hash = %{ retrieve("filename") }; # direct to hash
2154 =head2 How do I print out or copy a recursive data structure?
2156 The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great
2157 for printing out data structures. The C<Storable> module on CPAN (or the
2158 5.8 release of Perl), provides a function called C<dclone> that recursively
2159 copies its argument.
2161 use Storable qw(dclone);
2164 Where C<$r1> can be a reference to any kind of data structure you'd like.
2165 It will be deeply copied. Because C<dclone> takes and returns references,
2166 you'd have to add extra punctuation if you had a hash of arrays that
2169 %newhash = %{ dclone(\%oldhash) };
2171 =head2 How do I define methods for every class/object?
2173 Use the C<UNIVERSAL> class (see L<UNIVERSAL>).
2175 =head2 How do I verify a credit card checksum?
2177 Get the C<Business::CreditCard> module from CPAN.
2179 =head2 How do I pack arrays of doubles or floats for XS code?
2181 The kgbpack.c code in the C<PGPLOT> module on CPAN does just this.
2182 If you're doing a lot of float or double processing, consider using
2183 the C<PDL> module from CPAN instead--it makes number-crunching easy.
2187 Revision: $Revision: 6816 $
2189 Date: $Date: 2006-08-20 21:20:03 +0200 (dim, 20 aoĆ» 2006) $
2191 See L<perlfaq> for source control details and availability.
2193 =head1 AUTHOR AND COPYRIGHT
2195 Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
2196 other authors as noted. All rights reserved.
2198 This documentation is free; you can redistribute it and/or modify it
2199 under the same terms as Perl itself.
2201 Irrespective of its distribution, all code examples in this file
2202 are hereby placed into the public domain. You are permitted and
2203 encouraged to use this code in your own programs for fun
2204 or for profit as you see fit. A simple comment in the code giving
2205 credit would be courteous but is not required.