3 perlfaq4 - Data Manipulation ($Revision: 3606 $)
7 This section of the FAQ answers questions related to manipulating
8 numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
12 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
14 Internally, your computer represents floating-point numbers
15 in binary. Digital (as in powers of two) computers cannot
16 store all numbers exactly. Some real numbers lose precision
17 in the process. This is a problem with how computers store
18 numbers and affects all computer languages, not just Perl.
20 L<perlnumber> show the gory details of number
21 representations and conversions.
23 To limit the number of decimal places in your numbers, you
24 can use the printf or sprintf function. See the
25 L<"Floating Point Arithmetic"|perlop> for more details.
29 my $number = sprintf "%.2f", 10/3;
31 =head2 Why is int() broken?
33 Your int() is most probably working just fine. It's the numbers that
34 aren't quite what you think.
36 First, see the above item "Why am I getting long decimals
37 (eg, 19.9499999999999) instead of the numbers I should be getting
42 print int(0.6/0.2-2), "\n";
44 will in most computers print 0, not 1, because even such simple
45 numbers as 0.6 and 0.2 cannot be presented exactly by floating-point
46 numbers. What you think in the above as 'three' is really more like
47 2.9999999999999995559.
49 =head2 Why isn't my octal data interpreted correctly?
51 Perl only understands octal and hex numbers as such when they occur as
52 literals in your program. Octal literals in perl must start with a
53 leading "0" and hexadecimal literals must start with a leading "0x".
54 If they are read in from somewhere and assigned, no automatic
55 conversion takes place. You must explicitly use oct() or hex() if you
56 want the values converted to decimal. oct() interprets hex ("0x350"),
57 octal ("0350" or even without the leading "0", like "377") and binary
58 ("0b1010") numbers, while hex() only converts hexadecimal ones, with
59 or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
60 The inverse mapping from decimal to octal can be done with either the
61 "%o" or "%O" sprintf() formats.
63 This problem shows up most often when people try using chmod(), mkdir(),
64 umask(), or sysopen(), which by widespread tradition typically take
67 chmod(644, $file); # WRONG
68 chmod(0644, $file); # right
70 Note the mistake in the first line was specifying the decimal literal
71 644, rather than the intended octal literal 0644. The problem can
74 printf("%#o",644); # prints 01204
76 Surely you had not intended C<chmod(01204, $file);> - did you? If you
77 want to use numeric literals as arguments to chmod() et al. then please
78 try to express them as octal constants, that is with a leading zero and
79 with the following digits restricted to the set 0..7.
81 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
83 Remember that int() merely truncates toward 0. For rounding to a
84 certain number of digits, sprintf() or printf() is usually the easiest
87 printf("%.3f", 3.1415926535); # prints 3.142
89 The POSIX module (part of the standard Perl distribution) implements
90 ceil(), floor(), and a number of other mathematical and trigonometric
94 $ceil = ceil(3.5); # 4
95 $floor = floor(3.5); # 3
97 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
98 module. With 5.004, the Math::Trig module (part of the standard Perl
99 distribution) implements the trigonometric functions. Internally it
100 uses the Math::Complex module and some functions can break out from
101 the real axis into the complex plane, for example the inverse sine of
104 Rounding in financial applications can have serious implications, and
105 the rounding method used should be specified precisely. In these
106 cases, it probably pays not to trust whichever system rounding is
107 being used by Perl, but to instead implement the rounding function you
110 To see why, notice how you'll still have an issue on half-way-point
113 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
115 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
116 0.8 0.8 0.9 0.9 1.0 1.0
118 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
119 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
120 machines) will work pretty much like mathematical integers. Other numbers
123 =head2 How do I convert between numeric representations/bases/radixes?
125 As always with Perl there is more than one way to do it. Below
126 are a few examples of approaches to making common conversions
127 between number representations. This is intended to be representational
128 rather than exhaustive.
130 Some of the examples below use the Bit::Vector module from CPAN.
131 The reason you might choose Bit::Vector over the perl built in
132 functions is that it works with numbers of ANY size, that it is
133 optimized for speed on some operations, and for at least some
134 programmers the notation might be familiar.
138 =item How do I convert hexadecimal into decimal
140 Using perl's built in conversion of 0x notation:
144 Using the hex function:
146 $dec = hex("DEADBEEF");
150 $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
152 Using the CPAN module Bit::Vector:
155 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
156 $dec = $vec->to_Dec();
158 =item How do I convert from decimal to hexadecimal
162 $hex = sprintf("%X", 3735928559); # upper case A-F
163 $hex = sprintf("%x", 3735928559); # lower case a-f
167 $hex = unpack("H*", pack("N", 3735928559));
172 $vec = Bit::Vector->new_Dec(32, -559038737);
173 $hex = $vec->to_Hex();
175 And Bit::Vector supports odd bit counts:
178 $vec = Bit::Vector->new_Dec(33, 3735928559);
179 $vec->Resize(32); # suppress leading 0 if unwanted
180 $hex = $vec->to_Hex();
182 =item How do I convert from octal to decimal
184 Using Perl's built in conversion of numbers with leading zeros:
186 $dec = 033653337357; # note the leading 0!
188 Using the oct function:
190 $dec = oct("33653337357");
195 $vec = Bit::Vector->new(32);
196 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
197 $dec = $vec->to_Dec();
199 =item How do I convert from decimal to octal
203 $oct = sprintf("%o", 3735928559);
208 $vec = Bit::Vector->new_Dec(32, -559038737);
209 $oct = reverse join('', $vec->Chunk_List_Read(3));
211 =item How do I convert from binary to decimal
213 Perl 5.6 lets you write binary numbers directly with
216 $number = 0b10110110;
220 my $input = "10110110";
221 $decimal = oct( "0b$input" );
225 $decimal = ord(pack('B8', '10110110'));
227 Using pack and unpack for larger strings:
229 $int = unpack("N", pack("B32",
230 substr("0" x 32 . "11110101011011011111011101111", -32)));
231 $dec = sprintf("%d", $int);
233 # substr() is used to left pad a 32 character string with zeros.
237 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
238 $dec = $vec->to_Dec();
240 =item How do I convert from decimal to binary
242 Using sprintf (perl 5.6+):
244 $bin = sprintf("%b", 3735928559);
248 $bin = unpack("B*", pack("N", 3735928559));
253 $vec = Bit::Vector->new_Dec(32, -559038737);
254 $bin = $vec->to_Bin();
256 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
257 are left as an exercise to the inclined reader.
261 =head2 Why doesn't & work the way I want it to?
263 The behavior of binary arithmetic operators depends on whether they're
264 used on numbers or strings. The operators treat a string as a series
265 of bits and work with that (the string C<"3"> is the bit pattern
266 C<00110011>). The operators work with the binary form of a number
267 (the number C<3> is treated as the bit pattern C<00000011>).
269 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
270 C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
273 Most problems with C<&> and C<|> arise because the programmer thinks
274 they have a number but really it's a string. The rest arise because
277 if ("\020\020" & "\101\101") {
281 but a string consisting of two null bytes (the result of C<"\020\020"
282 & "\101\101">) is not a false value in Perl. You need:
284 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
288 =head2 How do I multiply matrices?
290 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
291 or the PDL extension (also available from CPAN).
293 =head2 How do I perform an operation on a series of integers?
295 To call a function on each element in an array, and collect the
298 @results = map { my_func($_) } @array;
302 @triple = map { 3 * $_ } @single;
304 To call a function on each element of an array, but ignore the
307 foreach $iterator (@array) {
308 some_func($iterator);
311 To call a function on each integer in a (small) range, you B<can> use:
313 @results = map { some_func($_) } (5 .. 25);
315 but you should be aware that the C<..> operator creates an array of
316 all integers in the range. This can take a lot of memory for large
320 for ($i=5; $i < 500_005; $i++) {
321 push(@results, some_func($i));
324 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
325 loop will iterate over the range, without creating the entire range.
327 for my $i (5 .. 500_005) {
328 push(@results, some_func($i));
331 will not create a list of 500,000 integers.
333 =head2 How can I output Roman numerals?
335 Get the http://www.cpan.org/modules/by-module/Roman module.
337 =head2 Why aren't my random numbers random?
339 If you're using a version of Perl before 5.004, you must call C<srand>
340 once at the start of your program to seed the random number generator.
342 BEGIN { srand() if $] < 5.004 }
344 5.004 and later automatically call C<srand> at the beginning. Don't
345 call C<srand> more than once---you make your numbers less random, rather
348 Computers are good at being predictable and bad at being random
349 (despite appearances caused by bugs in your programs :-). see the
350 F<random> article in the "Far More Than You Ever Wanted To Know"
351 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
352 Tom Phoenix, talks more about this. John von Neumann said, "Anyone
353 who attempts to generate random numbers by deterministic means is, of
354 course, living in a state of sin."
356 If you want numbers that are more random than C<rand> with C<srand>
357 provides, you should also check out the Math::TrulyRandom module from
358 CPAN. It uses the imperfections in your system's timer to generate
359 random numbers, but this takes quite a while. If you want a better
360 pseudorandom generator than comes with your operating system, look at
361 "Numerical Recipes in C" at http://www.nr.com/ .
363 =head2 How do I get a random number between X and Y?
365 To get a random number between two values, you can use the
366 C<rand()> builtin to get a random number between 0 and
368 C<rand($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 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 the Time::Local module to get
408 a time in epoch seconds for the argument to localtime.
410 use POSIX qw/strftime/;
412 my $week_of_year = strftime "%W",
413 localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
415 The Date::Calc module provides two functions to calculate these.
418 my $day_of_year = Day_of_Year( 1987, 12, 18 );
419 my $week_of_year = Week_of_Year( 1987, 12, 18 );
421 =head2 How do I find the current century or millennium?
423 Use the following simple functions:
426 return int((((localtime(shift || time))[5] + 1999))/100);
430 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
433 On some systems, the POSIX module's strftime() function has
434 been extended in a non-standard way to use a C<%C> format,
435 which they sometimes claim is the "century". It isn't,
436 because on most such systems, this is only the first two
437 digits of the four-digit year, and thus cannot be used to
438 reliably determine the current century or millennium.
440 =head2 How can I compare two dates and find the difference?
442 (contributed by brian d foy)
444 You could just store all your dates as a number and then subtract. Life
445 isn't always that simple though. If you want to work with formatted
446 dates, the Date::Manip, Date::Calc, or DateTime modules can help you.
449 =head2 How can I take a string and turn it into epoch seconds?
451 If it's a regular enough string that it always has the same format,
452 you can split it up and pass the parts to C<timelocal> in the standard
453 Time::Local module. Otherwise, you should look into the Date::Calc
454 and Date::Manip modules from CPAN.
456 =head2 How can I find the Julian Day?
458 (contributed by brian d foy and Dave Cross)
460 You can use the Time::JulianDay module available on CPAN. Ensure that
461 you really want to find a Julian day, though, as many people have
462 different ideas about Julian days. See
463 http://www.hermetic.ch/cal_stud/jdn.htm for instance.
465 You can also try the DateTime module, which can convert a date/time
468 $ perl -MDateTime -le'print DateTime->today->jd'
471 Or the modified Julian Day
473 $ perl -MDateTime -le'print DateTime->today->mjd'
476 Or even the day of the year (which is what some people think of as a
479 $ perl -MDateTime -le'print DateTime->today->doy'
482 =head2 How do I find yesterday's date?
484 (contributed by brian d foy)
486 Use one of the Date modules. The C<DateTime> module makes it simple, and
487 give you the same time of day, only the day before.
491 my $yesterday = DateTime->now->subtract( days => 1 );
493 print "Yesterday was $yesterday\n";
495 You can also use the C<Date::Calc> module using its Today_and_Now
498 use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
500 my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
504 Most people try to use the time rather than the calendar to figure out
505 dates, but that assumes that days are twenty-four hours each. For
506 most people, there are two days a year when they aren't: the switch to
507 and from summer time throws this off. Let the modules do the work.
509 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
511 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
512 Y2K compliant (whatever that means). The programmers you've hired to
513 use it, however, probably are not.
515 Long answer: The question belies a true understanding of the issue.
516 Perl is just as Y2K compliant as your pencil--no more, and no less.
517 Can you use your pencil to write a non-Y2K-compliant memo? Of course
518 you can. Is that the pencil's fault? Of course it isn't.
520 The date and time functions supplied with Perl (gmtime and localtime)
521 supply adequate information to determine the year well beyond 2000
522 (2038 is when trouble strikes for 32-bit machines). The year returned
523 by these functions when used in a list context is the year minus 1900.
524 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
525 number. To avoid the year 2000 problem simply do not treat the year as
526 a 2-digit number. It isn't.
528 When gmtime() and localtime() are used in scalar context they return
529 a timestamp string that contains a fully-expanded year. For example,
530 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
531 2001". There's no year 2000 problem here.
533 That doesn't mean that Perl can't be used to create non-Y2K compliant
534 programs. It can. But so can your pencil. It's the fault of the user,
535 not the language. At the risk of inflaming the NRA: "Perl doesn't
536 break Y2K, people do." See http://www.perl.org/about/y2k.html for
541 =head2 How do I validate input?
543 (contributed by brian d foy)
545 There are many ways to ensure that values are what you expect or
546 want to accept. Besides the specific examples that we cover in the
547 perlfaq, you can also look at the modules with "Assert" and "Validate"
548 in their names, along with other modules such as C<Regexp::Common>.
550 Some modules have validation for particular types of input, such
551 as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>,
552 and C<Data::Validate::IP>.
554 =head2 How do I unescape a string?
556 It depends just what you mean by "escape". URL escapes are dealt
557 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
558 character are removed with
562 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
564 =head2 How do I remove consecutive pairs of characters?
566 (contributed by brian d foy)
568 You can use the substitution operator to find pairs of characters (or
569 runs of characters) and replace them with a single instance. In this
570 substitution, we find a character in C<(.)>. The memory parentheses
571 store the matched character in the back-reference C<\1> and we use
572 that to require that the same thing immediately follow it. We replace
573 that part of the string with the character in C<$1>.
577 We can also use the transliteration operator, C<tr///>. In this
578 example, the search list side of our C<tr///> contains nothing, but
579 the C<c> option complements that so it contains everything. The
580 replacement list also contains nothing, so the transliteration is
581 almost a no-op since it won't do any replacements (or more exactly,
582 replace the character with itself). However, the C<s> option squashes
583 duplicated and consecutive characters in the string so a character
584 does not show up next to itself
586 my $str = 'Haarlem'; # in the Netherlands
587 $str =~ tr///cs; # Now Harlem, like in New York
589 =head2 How do I expand function calls in a string?
591 (contributed by brian d foy)
593 This is documented in L<perlref>, and although it's not the easiest
594 thing to read, it does work. In each of these examples, we call the
595 function inside the braces used to dereference a reference. If we
596 have a more than one return value, we can construct and dereference an
597 anonymous array. In this case, we call the function in list context.
599 print "The time values are @{ [localtime] }.\n";
601 If we want to call the function in scalar context, we have to do a bit
602 more work. We can really have any code we like inside the braces, so
603 we simply have to end with the scalar reference, although how you do
604 that is up to you, and you can use code inside the braces.
606 print "The time is ${\(scalar localtime)}.\n"
608 print "The time is ${ my $x = localtime; \$x }.\n";
610 If your function already returns a reference, you don't need to create
611 the reference yourself.
613 sub timestamp { my $t = localtime; \$t }
615 print "The time is ${ timestamp() }.\n";
617 The C<Interpolation> module can also do a lot of magic for you. You can
618 specify a variable name, in this case C<E>, to set up a tied hash that
619 does the interpolation for you. It has several other methods to do this
622 use Interpolation E => 'eval';
623 print "The time values are $E{localtime()}.\n";
625 In most cases, it is probably easier to simply use string concatenation,
626 which also forces scalar context.
628 print "The time is " . localtime . ".\n";
630 =head2 How do I find matching/nesting anything?
632 This isn't something that can be done in one regular expression, no
633 matter how complicated. To find something between two single
634 characters, a pattern like C</x([^x]*)x/> will get the intervening
635 bits in $1. For multiple ones, then something more like
636 C</alpha(.*?)omega/> would be needed. But none of these deals with
637 nested patterns. For balanced expressions using C<(>, C<{>, C<[> or
638 C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
639 L<perlre/(??{ code })>. For other cases, you'll have to write a
642 If you are serious about writing a parser, there are a number of
643 modules or oddities that will make your life a lot easier. There are
644 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
645 and the byacc program. Starting from perl 5.8 the Text::Balanced is
646 part of the standard distribution.
648 One simple destructive, inside-out approach that you might try is to
649 pull out the smallest nesting parts one at a time:
651 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
652 # do something with $1
655 A more complicated and sneaky approach is to make Perl's regular
656 expression engine do it for you. This is courtesy Dean Inada, and
657 rather has the nature of an Obfuscated Perl Contest entry, but it
660 # $_ contains the string to parse
661 # BEGIN and END are the opening and closing markers for the
666 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
667 @$ = (eval{/$re/},$@!~/unmatched/i);
668 print join("\n",@$[0..$#$]) if( $$[-1] );
670 =head2 How do I reverse a string?
672 Use reverse() in scalar context, as documented in
675 $reversed = reverse $string;
677 =head2 How do I expand tabs in a string?
679 You can do it yourself:
681 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
683 Or you can just use the Text::Tabs module (part of the standard Perl
687 @expanded_lines = expand(@lines_with_tabs);
689 =head2 How do I reformat a paragraph?
691 Use Text::Wrap (part of the standard Perl distribution):
694 print wrap("\t", ' ', @paragraphs);
696 The paragraphs you give to Text::Wrap should not contain embedded
697 newlines. Text::Wrap doesn't justify the lines (flush-right).
699 Or use the CPAN module Text::Autoformat. Formatting files can be easily
700 done by making a shell alias, like so:
702 alias fmt="perl -i -MText::Autoformat -n0777 \
703 -e 'print autoformat $_, {all=>1}' $*"
705 See the documentation for Text::Autoformat to appreciate its many
708 =head2 How can I access or change N characters of a string?
710 You can access the first characters of a string with substr().
711 To get the first character, for example, start at position 0
712 and grab the string of length 1.
715 $string = "Just another Perl Hacker";
716 $first_char = substr( $string, 0, 1 ); # 'J'
718 To change part of a string, you can use the optional fourth
719 argument which is the replacement string.
721 substr( $string, 13, 4, "Perl 5.8.0" );
723 You can also use substr() as an lvalue.
725 substr( $string, 13, 4 ) = "Perl 5.8.0";
727 =head2 How do I change the Nth occurrence of something?
729 You have to keep track of N yourself. For example, let's say you want
730 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
731 C<"whosoever"> or C<"whomsoever">, case insensitively. These
732 all assume that $_ contains the string to be altered.
736 ++$count == 5 # is it the 5th?
737 ? "${2}soever" # yes, swap
738 : $1 # renege and leave it there
741 In the more general case, you can use the C</g> modifier in a C<while>
742 loop, keeping count of matches.
746 $_ = "One fish two fish red fish blue fish";
747 while (/(\w+)\s+fish\b/gi) {
748 if (++$count == $WANT) {
749 print "The third fish is a $1 one.\n";
753 That prints out: C<"The third fish is a red one."> You can also use a
754 repetition count and repeated pattern like this:
756 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
758 =head2 How can I count the number of occurrences of a substring within a string?
760 There are a number of ways, with varying efficiency. If you want a
761 count of a certain single character (X) within a string, you can use the
762 C<tr///> function like so:
764 $string = "ThisXlineXhasXsomeXx'sXinXit";
765 $count = ($string =~ tr/X//);
766 print "There are $count X characters in the string";
768 This is fine if you are just looking for a single character. However,
769 if you are trying to count multiple character substrings within a
770 larger string, C<tr///> won't work. What you can do is wrap a while()
771 loop around a global pattern match. For example, let's count negative
774 $string = "-9 55 48 -2 23 -76 4 14 -44";
775 while ($string =~ /-\d+/g) { $count++ }
776 print "There are $count negative numbers in the string";
778 Another version uses a global match in list context, then assigns the
779 result to a scalar, producing a count of the number of matches.
781 $count = () = $string =~ /-\d+/g;
783 =head2 How do I capitalize all the words on one line?
785 To make the first letter of each word upper case:
787 $line =~ s/\b(\w)/\U$1/g;
789 This has the strange effect of turning "C<don't do it>" into "C<Don'T
790 Do It>". Sometimes you might want this. Other times you might need a
791 more thorough solution (Suggested by brian d foy):
794 (^\w) #at the beginning of the line
796 (\s\w) #preceded by whitespace
799 $string =~ /([\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 $style (qw( sentence title highlight ))
828 print autoformat($x, { case => $style }), "\n";
831 =head2 How can I split a [character] delimited string except when inside [character]?
833 Several modules can handle this sort of pasing---Text::Balanced,
834 Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.
836 Take the example case of trying to split a string that is
837 comma-separated into its different fields. You can't use C<split(/,/)>
838 because you shouldn't split if the comma is inside quotes. For
839 example, take a data line like this:
841 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
843 Due to the restriction of the quotes, this is a fairly complex
844 problem. Thankfully, we have Jeffrey Friedl, author of
845 I<Mastering Regular Expressions>, to handle these for us. He
846 suggests (assuming your string is contained in $text):
849 push(@new, $+) while $text =~ m{
850 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
854 push(@new, undef) if substr($text,-1,1) eq ',';
856 If you want to represent quotation marks inside a
857 quotation-mark-delimited field, escape them with backslashes (eg,
860 Alternatively, the Text::ParseWords module (part of the standard Perl
861 distribution) lets you say:
863 use Text::ParseWords;
864 @new = quotewords(",", 0, $text);
866 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
868 =head2 How do I strip blank space from the beginning/end of a string?
870 (contributed by brian d foy)
872 A substitution can do this for you. For a single line, you want to
873 replace all the leading or trailing whitespace with nothing. You
874 can do that with a pair of substitutions.
879 You can also write that as a single substitution, although it turns
880 out the combined statement is slower than the separate ones. That
881 might not matter to you, though.
885 In this regular expression, the alternation matches either at the
886 beginning or the end of the string since the anchors have a lower
887 precedence than the alternation. With the C</g> flag, the substitution
888 makes all possible matches, so it gets both. Remember, the trailing
889 newline matches the C<\s+>, and the C<$> anchor can match to the
890 physical end of the string, so the newline disappears too. Just add
891 the newline to the output, which has the added benefit of preserving
892 "blank" (consisting entirely of whitespace) lines which the C<^\s+>
893 would remove all by itself.
901 For a multi-line string, you can apply the regular expression
902 to each logical line in the string by adding the C</m> flag (for
903 "multi-line"). With the C</m> flag, the C<$> matches I<before> an
904 embedded newline, so it doesn't remove it. It still removes the
905 newline at the end of the string.
907 $string =~ s/^\s+|\s+$//gm;
909 Remember that lines consisting entirely of whitespace will disappear,
910 since the first part of the alternation can match the entire string
911 and replace it with nothing. If need to keep embedded blank lines,
912 you have to do a little more work. Instead of matching any whitespace
913 (since that includes a newline), just match the other whitespace.
915 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
917 =head2 How do I pad a string with blanks or pad a number with zeroes?
919 In the following examples, C<$pad_len> is the length to which you wish
920 to pad the string, C<$text> or C<$num> contains the string to be padded,
921 and C<$pad_char> contains the padding character. You can use a single
922 character string constant instead of the C<$pad_char> variable if you
923 know what it is in advance. And in the same way you can use an integer in
924 place of C<$pad_len> if you know the pad length in advance.
926 The simplest method uses the C<sprintf> function. It can pad on the left
927 or right with blanks and on the left with zeroes and it will not
928 truncate the result. The C<pack> function can only pad strings on the
929 right with blanks and it will truncate the result to a maximum length of
932 # Left padding a string with blanks (no truncation):
933 $padded = sprintf("%${pad_len}s", $text);
934 $padded = sprintf("%*s", $pad_len, $text); # same thing
936 # Right padding a string with blanks (no truncation):
937 $padded = sprintf("%-${pad_len}s", $text);
938 $padded = sprintf("%-*s", $pad_len, $text); # same thing
940 # Left padding a number with 0 (no truncation):
941 $padded = sprintf("%0${pad_len}d", $num);
942 $padded = sprintf("%0*d", $pad_len, $num); # same thing
944 # Right padding a string with blanks using pack (will truncate):
945 $padded = pack("A$pad_len",$text);
947 If you need to pad with a character other than blank or zero you can use
948 one of the following methods. They all generate a pad string with the
949 C<x> operator and combine that with C<$text>. These methods do
950 not truncate C<$text>.
952 Left and right padding with any character, creating a new string:
954 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
955 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
957 Left and right padding with any character, modifying C<$text> directly:
959 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
960 $text .= $pad_char x ( $pad_len - length( $text ) );
962 =head2 How do I extract selected columns from a string?
964 Use substr() or unpack(), both documented in L<perlfunc>.
965 If you prefer thinking in terms of columns instead of widths,
966 you can use this kind of thing:
968 # determine the unpack format needed to split Linux ps output
969 # arguments are cut columns
970 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
976 for my $place (@positions) {
977 $template .= "A" . ($place - $lastpos) . " ";
984 =head2 How do I find the soundex value of a string?
986 (contributed by brian d foy)
988 You can use the Text::Soundex module. If you want to do fuzzy or close
989 matching, you might also try the String::Approx, and Text::Metaphone,
990 and Text::DoubleMetaphone modules.
992 =head2 How can I expand variables in text strings?
994 Let's assume that you have a string that contains placeholder
997 $text = 'this has a $foo in it and a $bar';
999 You can use a substitution with a double evaluation. The
1000 first /e turns C<$1> into C<$foo>, and the second /e turns
1001 C<$foo> into its value. You may want to wrap this in an
1002 C<eval>: if you try to get the value of an undeclared variable
1003 while running under C<use strict>, you get a fatal error.
1005 eval { $text =~ s/(\$\w+)/$1/eeg };
1008 It's probably better in the general case to treat those
1009 variables as entries in some special hash. For example:
1015 $text =~ s/\$(\w+)/$user_defs{$1}/g;
1017 =head2 What's wrong with always quoting "$vars"?
1019 The problem is that those double-quotes force stringification--
1020 coercing numbers and references into strings--even when you
1021 don't want them to be strings. Think of it this way: double-quote
1022 expansion is used to produce new strings. If you already
1023 have a string, why do you need more?
1025 If you get used to writing odd things like these:
1028 $new = "$old"; # BAD
1029 somefunc("$var"); # BAD
1031 You'll be in trouble. Those should (in 99.8% of the cases) be
1032 the simpler and more direct:
1038 Otherwise, besides slowing you down, you're going to break code when
1039 the thing in the scalar is actually neither a string nor a number, but
1045 my $oref = "$aref"; # WRONG
1048 You can also get into subtle problems on those few operations in Perl
1049 that actually do care about the difference between a string and a
1050 number, such as the magical C<++> autoincrement operator or the
1053 Stringification also destroys arrays.
1056 print "@lines"; # WRONG - extra blanks
1057 print @lines; # right
1059 =head2 Why don't my E<lt>E<lt>HERE documents work?
1061 Check for these three things:
1065 =item There must be no space after the E<lt>E<lt> part.
1067 =item There (probably) should be a semicolon at the end.
1069 =item You can't (easily) have any space in front of the tag.
1073 If you want to indent the text in the here document, you
1077 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1082 But the HERE_TARGET must still be flush against the margin.
1083 If you want that indented also, you'll have to quote
1086 ($quote = <<' FINIS') =~ s/^\s+//gm;
1087 ...we will have peace, when you and all your works have
1088 perished--and the works of your dark master to whom you
1089 would deliver us. You are a liar, Saruman, and a corrupter
1090 of men's hearts. --Theoden in /usr/src/perl/taint.c
1092 $quote =~ s/\s+--/\n--/;
1094 A nice general-purpose fixer-upper function for indented here documents
1095 follows. It expects to be called with a here document as its argument.
1096 It looks to see whether each line begins with a common substring, and
1097 if so, strips that substring off. Otherwise, it takes the amount of leading
1098 whitespace found on the first line and removes that much off each
1103 my ($white, $leader); # common whitespace and common leading string
1104 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1105 ($white, $leader) = ($2, quotemeta($1));
1107 ($white, $leader) = (/^(\s+)/, '');
1109 s/^\s*?$leader(?:$white)?//gm;
1113 This works with leading special strings, dynamically determined:
1115 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1118 @@@ SAVEI32(runlevel);
1120 @@@ while ( op = (*op->op_ppaddr)() );
1124 MAIN_INTERPRETER_LOOP
1126 Or with a fixed amount of leading whitespace, with remaining
1127 indentation correctly preserved:
1129 $poem = fix<<EVER_ON_AND_ON;
1130 Now far ahead the Road has gone,
1131 And I must follow, if I can,
1132 Pursuing it with eager feet,
1133 Until it joins some larger way
1134 Where many paths and errands meet.
1135 And whither then? I cannot say.
1136 --Bilbo in /usr/src/perl/pp_ctl.c
1141 =head2 What is the difference between a list and an array?
1143 An array has a changeable length. A list does not. An array is something
1144 you can push or pop, while a list is a set of values. Some people make
1145 the distinction that a list is a value while an array is a variable.
1146 Subroutines are passed and return lists, you put things into list
1147 context, you initialize arrays with lists, and you foreach() across
1148 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1149 in scalar context behave like the number of elements in them, subroutines
1150 access their arguments through the array C<@_>, and push/pop/shift only work
1153 As a side note, there's no such thing as a list in scalar context.
1156 $scalar = (2, 5, 7, 9);
1158 you're using the comma operator in scalar context, so it uses the scalar
1159 comma operator. There never was a list there at all! This causes the
1160 last value to be returned: 9.
1162 =head2 What is the difference between $array[1] and @array[1]?
1164 The former is a scalar value; the latter an array slice, making
1165 it a list with one (scalar) value. You should use $ when you want a
1166 scalar value (most of the time) and @ when you want a list with one
1167 scalar value in it (very, very rarely; nearly never, in fact).
1169 Sometimes it doesn't make a difference, but sometimes it does.
1170 For example, compare:
1172 $good[0] = `some program that outputs several lines`;
1176 @bad[0] = `same program that outputs several lines`;
1178 The C<use warnings> pragma and the B<-w> flag will warn you about these
1181 =head2 How can I remove duplicate elements from a list or array?
1183 (contributed by brian d foy)
1185 Use a hash. When you think the words "unique" or "duplicated", think
1188 If you don't care about the order of the elements, you could just
1189 create the hash then extract the keys. It's not important how you
1190 create that hash: just that you use C<keys> to get the unique
1193 my %hash = map { $_, 1 } @array;
1194 # or a hash slice: @hash{ @array } = ();
1195 # or a foreach: $hash{$_} = 1 foreach ( @array );
1197 my @unique = keys %hash;
1199 You can also go through each element and skip the ones you've seen
1200 before. Use a hash to keep track. The first time the loop sees an
1201 element, that element has no key in C<%Seen>. The C<next> statement
1202 creates the key and immediately uses its value, which is C<undef>, so
1203 the loop continues to the C<push> and increments the value for that
1204 key. The next time the loop sees that same element, its key exists in
1205 the hash I<and> the value for that key is true (since it's not 0 or
1206 undef), so the next skips that iteration and the loop goes to the next
1212 foreach my $elem ( @array )
1214 next if $seen{ $elem }++;
1215 push @unique, $elem;
1218 You can write this more briefly using a grep, which does the
1222 my @unique = grep { ! $seen{ $_ }++ } @array;
1224 =head2 How can I tell whether a certain element is contained in a list or array?
1226 (portions of this answer contributed by Anno Siegel)
1228 Hearing the word "in" is an I<in>dication that you probably should have
1229 used a hash, not a list or array, to store your data. Hashes are
1230 designed to answer this question quickly and efficiently. Arrays aren't.
1232 That being said, there are several ways to approach this. If you
1233 are going to make this query many times over arbitrary string values,
1234 the fastest way is probably to invert the original array and maintain a
1235 hash whose keys are the first array's values.
1237 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1239 for (@blues) { $is_blue{$_} = 1 }
1241 Now you can check whether $is_blue{$some_color}. It might have been a
1242 good idea to keep the blues all in a hash in the first place.
1244 If the values are all small integers, you could use a simple indexed
1245 array. This kind of an array will take up less space:
1247 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1248 @is_tiny_prime = ();
1249 for (@primes) { $is_tiny_prime[$_] = 1 }
1250 # or simply @istiny_prime[@primes] = (1) x @primes;
1252 Now you check whether $is_tiny_prime[$some_number].
1254 If the values in question are integers instead of strings, you can save
1255 quite a lot of space by using bit strings instead:
1257 @articles = ( 1..10, 150..2000, 2017 );
1259 for (@articles) { vec($read,$_,1) = 1 }
1261 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1263 These methods guarantee fast individual tests but require a re-organization
1264 of the original list or array. They only pay off if you have to test
1265 multiple values against the same array.
1267 If you are testing only once, the standard module List::Util exports
1268 the function C<first> for this purpose. It works by stopping once it
1269 finds the element. It's written in C for speed, and its Perl equivalant
1270 looks like this subroutine:
1275 return $_ if &{$code}();
1280 If speed is of little concern, the common idiom uses grep in scalar context
1281 (which returns the number of items that passed its condition) to traverse the
1282 entire list. This does have the benefit of telling you how many matches it
1285 my $is_there = grep $_ eq $whatever, @array;
1287 If you want to actually extract the matching elements, simply use grep in
1290 my @matches = grep $_ eq $whatever, @array;
1292 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1294 Use a hash. Here's code to do both and more. It assumes that
1295 each element is unique in a given array:
1297 @union = @intersection = @difference = ();
1299 foreach $element (@array1, @array2) { $count{$element}++ }
1300 foreach $element (keys %count) {
1301 push @union, $element;
1302 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1305 Note that this is the I<symmetric difference>, that is, all elements in
1306 either A or in B but not in both. Think of it as an xor operation.
1308 =head2 How do I test whether two arrays or hashes are equal?
1310 The following code works for single-level arrays. It uses a stringwise
1311 comparison, and does not distinguish defined versus undefined empty
1312 strings. Modify if you have other needs.
1314 $are_equal = compare_arrays(\@frogs, \@toads);
1316 sub compare_arrays {
1317 my ($first, $second) = @_;
1318 no warnings; # silence spurious -w undef complaints
1319 return 0 unless @$first == @$second;
1320 for (my $i = 0; $i < @$first; $i++) {
1321 return 0 if $first->[$i] ne $second->[$i];
1326 For multilevel structures, you may wish to use an approach more
1327 like this one. It uses the CPAN module FreezeThaw:
1329 use FreezeThaw qw(cmpStr);
1330 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1332 printf "a and b contain %s arrays\n",
1333 cmpStr(\@a, \@b) == 0
1337 This approach also works for comparing hashes. Here
1338 we'll demonstrate two different answers:
1340 use FreezeThaw qw(cmpStr cmpStrHard);
1342 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1346 printf "a and b contain %s hashes\n",
1347 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1349 printf "a and b contain %s hashes\n",
1350 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1353 The first reports that both those the hashes contain the same data,
1354 while the second reports that they do not. Which you prefer is left as
1355 an exercise to the reader.
1357 =head2 How do I find the first array element for which a condition is true?
1359 To find the first array element which satisfies a condition, you can
1360 use the first() function in the List::Util module, which comes with
1361 Perl 5.8. This example finds the first element that contains "Perl".
1363 use List::Util qw(first);
1365 my $element = first { /Perl/ } @array;
1367 If you cannot use List::Util, you can make your own loop to do the
1368 same thing. Once you find the element, you stop the loop with last.
1373 if( /Perl/ ) { $found = $_; last }
1376 If you want the array index, you can iterate through the indices
1377 and check the array element at each index until you find one
1378 that satisfies the condition.
1380 my( $found, $index ) = ( undef, -1 );
1381 for( $i = 0; $i < @array; $i++ )
1383 if( $array[$i] =~ /Perl/ )
1385 $found = $array[$i];
1391 =head2 How do I handle linked lists?
1393 In general, you usually don't need a linked list in Perl, since with
1394 regular arrays, you can push and pop or shift and unshift at either end,
1395 or you can use splice to add and/or remove arbitrary number of elements at
1396 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1397 dynamic arrays. In the absence of shifts and pops, push in general
1398 needs to reallocate on the order every log(N) times, and unshift will
1399 need to copy pointers each time.
1401 If you really, really wanted, you could use structures as described in
1402 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1403 to do. For example, imagine a list node like this:
1410 You could walk the list this way:
1413 for ($node = $head; $node; $node = $node->{LINK}) {
1414 print $node->{VALUE}, " ";
1418 You could add to the list this way:
1421 $tail = append($head, 1); # grow a new head
1422 for $value ( 2 .. 10 ) {
1423 $tail = append($tail, $value);
1427 my($list, $value) = @_;
1428 my $node = { VALUE => $value };
1430 $node->{LINK} = $list->{LINK};
1431 $list->{LINK} = $node;
1433 $_[0] = $node; # replace caller's version
1438 But again, Perl's built-in are virtually always good enough.
1440 =head2 How do I handle circular lists?
1442 Circular lists could be handled in the traditional fashion with linked
1443 lists, or you could just do something like this with an array:
1445 unshift(@array, pop(@array)); # the last shall be first
1446 push(@array, shift(@array)); # and vice versa
1448 =head2 How do I shuffle an array randomly?
1450 If you either have Perl 5.8.0 or later installed, or if you have
1451 Scalar-List-Utils 1.03 or later installed, you can say:
1453 use List::Util 'shuffle';
1455 @shuffled = shuffle(@list);
1457 If not, you can use a Fisher-Yates shuffle.
1459 sub fisher_yates_shuffle {
1460 my $deck = shift; # $deck is a reference to an array
1463 my $j = int rand ($i+1);
1464 @$deck[$i,$j] = @$deck[$j,$i];
1468 # shuffle my mpeg collection
1470 my @mpeg = <audio/*/*.mp3>;
1471 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1474 Note that the above implementation shuffles an array in place,
1475 unlike the List::Util::shuffle() which takes a list and returns
1476 a new shuffled list.
1478 You've probably seen shuffling algorithms that work using splice,
1479 randomly picking another element to swap the current element with
1483 @old = 1 .. 10; # just a demo
1485 push(@new, splice(@old, rand @old, 1));
1488 This is bad because splice is already O(N), and since you do it N times,
1489 you just invented a quadratic algorithm; that is, O(N**2). This does
1490 not scale, although Perl is so efficient that you probably won't notice
1491 this until you have rather largish arrays.
1493 =head2 How do I process/modify each element of an array?
1495 Use C<for>/C<foreach>:
1498 s/foo/bar/; # change that word
1499 tr/XZ/ZX/; # swap those letters
1502 Here's another; let's compute spherical volumes:
1504 for (@volumes = @radii) { # @volumes has changed parts
1506 $_ *= (4/3) * 3.14159; # this will be constant folded
1509 which can also be done with map() which is made to transform
1510 one list into another:
1512 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1514 If you want to do the same thing to modify the values of the
1515 hash, you can use the C<values> function. As of Perl 5.6
1516 the values are not copied, so if you modify $orbit (in this
1517 case), you modify the value.
1519 for $orbit ( values %orbits ) {
1520 ($orbit **= 3) *= (4/3) * 3.14159;
1523 Prior to perl 5.6 C<values> returned copies of the values,
1524 so older perl code often contains constructions such as
1525 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1526 the hash is to be modified.
1528 =head2 How do I select a random element from an array?
1530 Use the rand() function (see L<perlfunc/rand>):
1532 $index = rand @array;
1533 $element = $array[$index];
1536 my $element = $array[ rand @array ];
1538 =head2 How do I permute N elements of a list?
1540 Use the List::Permutor module on CPAN. If the list is
1541 actually an array, try the Algorithm::Permute module (also
1542 on CPAN). It's written in XS code and is very efficient.
1544 use Algorithm::Permute;
1545 my @array = 'a'..'d';
1546 my $p_iterator = Algorithm::Permute->new ( \@array );
1547 while (my @perm = $p_iterator->next) {
1548 print "next permutation: (@perm)\n";
1551 For even faster execution, you could do:
1553 use Algorithm::Permute;
1554 my @array = 'a'..'d';
1555 Algorithm::Permute::permute {
1556 print "next permutation: (@array)\n";
1559 Here's a little program that generates all permutations of
1560 all the words on each line of input. The algorithm embodied
1561 in the permute() function is discussed in Volume 4 (still
1562 unpublished) of Knuth's I<The Art of Computer Programming>
1563 and will work on any list:
1566 # Fischer-Kause ordered permutation generator
1571 while ( $code->(@_[@idx]) ) {
1573 --$p while $idx[$p-1] > $idx[$p];
1574 my $q = $p or return;
1575 push @idx, reverse splice @idx, $p;
1576 ++$q while $idx[$p-1] > $idx[$q];
1577 @idx[$p-1,$q]=@idx[$q,$p-1];
1581 permute {print"@_\n"} split;
1583 =head2 How do I sort an array by (anything)?
1585 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1587 @list = sort { $a <=> $b } @list;
1589 The default sort function is cmp, string comparison, which would
1590 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1591 the numerical comparison operator.
1593 If you have a complicated function needed to pull out the part you
1594 want to sort on, then don't do it inside the sort function. Pull it
1595 out first, because the sort BLOCK can be called many times for the
1596 same element. Here's an example of how to pull out the first word
1597 after the first number on each item, and then sort those words
1602 ($item) = /\d+\s*(\S+)/;
1603 push @idx, uc($item);
1605 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1607 which could also be written this way, using a trick
1608 that's come to be known as the Schwartzian Transform:
1610 @sorted = map { $_->[0] }
1611 sort { $a->[1] cmp $b->[1] }
1612 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1614 If you need to sort on several fields, the following paradigm is useful.
1616 @sorted = sort { field1($a) <=> field1($b) ||
1617 field2($a) cmp field2($b) ||
1618 field3($a) cmp field3($b)
1621 This can be conveniently combined with precalculation of keys as given
1624 See the F<sort> article in the "Far More Than You Ever Wanted
1625 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1626 more about this approach.
1628 See also the question below on sorting hashes.
1630 =head2 How do I manipulate arrays of bits?
1632 Use pack() and unpack(), or else vec() and the bitwise operations.
1634 For example, this sets $vec to have bit N set if $ints[N] was set:
1637 foreach(@ints) { vec($vec,$_,1) = 1 }
1639 Here's how, given a vector in $vec, you can
1640 get those bits into your @ints array:
1642 sub bitvec_to_list {
1645 # Find null-byte density then select best algorithm
1646 if ($vec =~ tr/\0// / length $vec > 0.95) {
1649 # This method is faster with mostly null-bytes
1650 while($vec =~ /[^\0]/g ) {
1651 $i = -9 + 8 * pos $vec;
1652 push @ints, $i if vec($vec, ++$i, 1);
1653 push @ints, $i if vec($vec, ++$i, 1);
1654 push @ints, $i if vec($vec, ++$i, 1);
1655 push @ints, $i if vec($vec, ++$i, 1);
1656 push @ints, $i if vec($vec, ++$i, 1);
1657 push @ints, $i if vec($vec, ++$i, 1);
1658 push @ints, $i if vec($vec, ++$i, 1);
1659 push @ints, $i if vec($vec, ++$i, 1);
1662 # This method is a fast general algorithm
1664 my $bits = unpack "b*", $vec;
1665 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1666 push @ints, pos $bits while($bits =~ /1/g);
1671 This method gets faster the more sparse the bit vector is.
1672 (Courtesy of Tim Bunce and Winfried Koenig.)
1674 You can make the while loop a lot shorter with this suggestion
1675 from Benjamin Goldberg:
1677 while($vec =~ /[^\0]+/g ) {
1678 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1681 Or use the CPAN module Bit::Vector:
1683 $vector = Bit::Vector->new($num_of_bits);
1684 $vector->Index_List_Store(@ints);
1685 @ints = $vector->Index_List_Read();
1687 Bit::Vector provides efficient methods for bit vector, sets of small integers
1690 Here's a more extensive illustration using vec():
1693 $vector = "\xff\x0f\xef\xfe";
1694 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1695 unpack("N", $vector), "\n";
1696 $is_set = vec($vector, 23, 1);
1697 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1715 my ($offset, $width, $value) = @_;
1717 vec($vector, $offset, $width) = $value;
1718 print "offset=$offset width=$width value=$value\n";
1724 my $bits = unpack("b*", $vector);
1728 print "vector length in bytes: ", length($vector), "\n";
1729 @bytes = unpack("A8" x length($vector), $bits);
1730 print "bits are: @bytes\n\n";
1733 =head2 Why does defined() return true on empty arrays and hashes?
1735 The short story is that you should probably only use defined on scalars or
1736 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1737 in the 5.004 release or later of Perl for more detail.
1739 =head1 Data: Hashes (Associative Arrays)
1741 =head2 How do I process an entire hash?
1743 Use the each() function (see L<perlfunc/each>) if you don't care
1744 whether it's sorted:
1746 while ( ($key, $value) = each %hash) {
1747 print "$key = $value\n";
1750 If you want it sorted, you'll have to use foreach() on the result of
1751 sorting the keys as shown in an earlier question.
1753 =head2 What happens if I add or remove keys from a hash while iterating over it?
1755 (contributed by brian d foy)
1757 The easy answer is "Don't do that!"
1759 If you iterate through the hash with each(), you can delete the key
1760 most recently returned without worrying about it. If you delete or add
1761 other keys, the iterator may skip or double up on them since perl
1762 may rearrange the hash table. See the
1763 entry for C<each()> in L<perlfunc>.
1765 =head2 How do I look up a hash element by value?
1767 Create a reverse hash:
1769 %by_value = reverse %by_key;
1770 $key = $by_value{$value};
1772 That's not particularly efficient. It would be more space-efficient
1775 while (($key, $value) = each %by_key) {
1776 $by_value{$value} = $key;
1779 If your hash could have repeated values, the methods above will only find
1780 one of the associated keys. This may or may not worry you. If it does
1781 worry you, you can always reverse the hash into a hash of arrays instead:
1783 while (($key, $value) = each %by_key) {
1784 push @{$key_list_by_value{$value}}, $key;
1787 =head2 How can I know how many entries are in a hash?
1789 If you mean how many keys, then all you have to do is
1790 use the keys() function in a scalar context:
1792 $num_keys = keys %hash;
1794 The keys() function also resets the iterator, which means that you may
1795 see strange results if you use this between uses of other hash operators
1798 =head2 How do I sort a hash (optionally by value instead of key)?
1800 (contributed by brian d foy)
1802 To sort a hash, start with the keys. In this example, we give the list of
1803 keys to the sort function which then compares them ASCIIbetically (which
1804 might be affected by your locale settings). The output list has the keys
1805 in ASCIIbetical order. Once we have the keys, we can go through them to
1806 create a report which lists the keys in ASCIIbetical order.
1808 my @keys = sort { $a cmp $b } keys %hash;
1810 foreach my $key ( @keys )
1812 printf "%-20s %6d\n", $key, $hash{$value};
1815 We could get more fancy in the C<sort()> block though. Instead of
1816 comparing the keys, we can compute a value with them and use that
1817 value as the comparison.
1819 For instance, to make our report order case-insensitive, we use
1820 the C<\L> sequence in a double-quoted string to make everything
1821 lowercase. The C<sort()> block then compares the lowercased
1822 values to determine in which order to put the keys.
1824 my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
1826 Note: if the computation is expensive or the hash has many elements,
1827 you may want to look at the Schwartzian Transform to cache the
1828 computation results.
1830 If we want to sort by the hash value instead, we use the hash key
1831 to look it up. We still get out a list of keys, but this time they
1832 are ordered by their value.
1834 my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
1836 From there we can get more complex. If the hash values are the same,
1837 we can provide a secondary sort on the hash key.
1840 $hash{$a} <=> $hash{$b}
1845 =head2 How can I always keep my hash sorted?
1847 You can look into using the DB_File module and tie() using the
1848 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1849 The Tie::IxHash module from CPAN might also be instructive.
1851 =head2 What's the difference between "delete" and "undef" with hashes?
1853 Hashes contain pairs of scalars: the first is the key, the
1854 second is the value. The key will be coerced to a string,
1855 although the value can be any kind of scalar: string,
1856 number, or reference. If a key $key is present in
1857 %hash, C<exists($hash{$key})> will return true. The value
1858 for a given key can be C<undef>, in which case
1859 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1860 will return true. This corresponds to (C<$key>, C<undef>)
1863 Pictures help... here's the %hash table:
1873 And these conditions hold
1877 defined $hash{'d'} is true
1878 defined $hash{'a'} is true
1879 exists $hash{'a'} is true (Perl5 only)
1880 grep ($_ eq 'a', keys %hash) is true
1886 your table now reads:
1897 and these conditions now hold; changes in caps:
1901 defined $hash{'d'} is true
1902 defined $hash{'a'} is FALSE
1903 exists $hash{'a'} is true (Perl5 only)
1904 grep ($_ eq 'a', keys %hash) is true
1906 Notice the last two: you have an undef value, but a defined key!
1912 your table now reads:
1921 and these conditions now hold; changes in caps:
1925 defined $hash{'d'} is true
1926 defined $hash{'a'} is false
1927 exists $hash{'a'} is FALSE (Perl5 only)
1928 grep ($_ eq 'a', keys %hash) is FALSE
1930 See, the whole entry is gone!
1932 =head2 Why don't my tied hashes make the defined/exists distinction?
1934 This depends on the tied hash's implementation of EXISTS().
1935 For example, there isn't the concept of undef with hashes
1936 that are tied to DBM* files. It also means that exists() and
1937 defined() do the same thing with a DBM* file, and what they
1938 end up doing is not what they do with ordinary hashes.
1940 =head2 How do I reset an each() operation part-way through?
1942 Using C<keys %hash> in scalar context returns the number of keys in
1943 the hash I<and> resets the iterator associated with the hash. You may
1944 need to do this if you use C<last> to exit a loop early so that when you
1945 re-enter it, the hash iterator has been reset.
1947 =head2 How can I get the unique keys from two hashes?
1949 First you extract the keys from the hashes into lists, then solve
1950 the "removing duplicates" problem described above. For example:
1953 for $element (keys(%foo), keys(%bar)) {
1960 @uniq = keys %{{%foo,%bar}};
1962 Or if you really want to save space:
1965 while (defined ($key = each %foo)) {
1968 while (defined ($key = each %bar)) {
1973 =head2 How can I store a multidimensional array in a DBM file?
1975 Either stringify the structure yourself (no fun), or else
1976 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1977 it on top of either DB_File or GDBM_File.
1979 =head2 How can I make my hash remember the order I put elements into it?
1981 Use the Tie::IxHash from CPAN.
1984 tie my %myhash, 'Tie::IxHash';
1985 for (my $i=0; $i<20; $i++) {
1988 my @keys = keys %myhash;
1989 # @keys = (0,1,2,3,...)
1991 =head2 Why does passing a subroutine an undefined element in a hash create it?
1993 If you say something like:
1995 somefunc($hash{"nonesuch key here"});
1997 Then that element "autovivifies"; that is, it springs into existence
1998 whether you store something there or not. That's because functions
1999 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
2000 it has to be ready to write it back into the caller's version.
2002 This has been fixed as of Perl5.004.
2004 Normally, merely accessing a key's value for a nonexistent key does
2005 I<not> cause that key to be forever there. This is different than
2008 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
2010 Usually a hash ref, perhaps like this:
2015 TITLE => "deputy peon",
2018 PALS => [ "Norbert", "Rhys", "Phineas"],
2021 References are documented in L<perlref> and the upcoming L<perlreftut>.
2022 Examples of complex data structures are given in L<perldsc> and
2023 L<perllol>. Examples of structures and object-oriented classes are
2026 =head2 How can I use a reference as a hash key?
2028 (contributed by brian d foy)
2030 Hash keys are strings, so you can't really use a reference as the key.
2031 When you try to do that, perl turns the reference into its stringified
2032 form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get back
2033 the reference from the stringified form, at least without doing some
2034 extra work on your own. Also remember that hash keys must be unique, but
2035 two different variables can store the same reference (and those variables
2038 The Tie::RefHash module, which is distributed with perl, might be what
2039 you want. It handles that extra work.
2043 =head2 How do I handle binary data correctly?
2045 Perl is binary clean, so this shouldn't be a problem. For example,
2046 this works fine (assuming the files are found):
2048 if (`cat /vmunix` =~ /gzip/) {
2049 print "Your kernel is GNU-zip enabled!\n";
2052 On less elegant (read: Byzantine) systems, however, you have
2053 to play tedious games with "text" versus "binary" files. See
2054 L<perlfunc/"binmode"> or L<perlopentut>.
2056 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
2058 If you want to deal with multibyte characters, however, there are
2059 some gotchas. See the section on Regular Expressions.
2061 =head2 How do I determine whether a scalar is a number/whole/integer/float?
2063 Assuming that you don't care about IEEE notations like "NaN" or
2064 "Infinity", you probably just want to use a regular expression.
2066 if (/\D/) { print "has nondigits\n" }
2067 if (/^\d+$/) { print "is a whole number\n" }
2068 if (/^-?\d+$/) { print "is an integer\n" }
2069 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2070 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2071 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2072 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2073 { print "a C float\n" }
2075 There are also some commonly used modules for the task.
2076 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2077 internal function C<looks_like_number> for determining
2078 whether a variable looks like a number. L<Data::Types>
2079 exports functions that validate data types using both the
2080 above and other regular expressions. Thirdly, there is
2081 C<Regexp::Common> which has regular expressions to match
2082 various types of numbers. Those three modules are available
2085 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2086 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
2087 wrapper function for more convenient access. This function takes
2088 a string and returns the number it found, or C<undef> for input that
2089 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
2090 if you just want to say, "Is this a float?"
2093 use POSIX qw(strtod);
2098 my($num, $unparsed) = strtod($str);
2099 if (($str eq '') || ($unparsed != 0) || $!) {
2106 sub is_numeric { defined getnum($_[0]) }
2108 Or you could check out the L<String::Scanf> module on the CPAN
2109 instead. The POSIX module (part of the standard Perl distribution) provides
2110 the C<strtod> and C<strtol> for converting strings to double and longs,
2113 =head2 How do I keep persistent data across program calls?
2115 For some specific applications, you can use one of the DBM modules.
2116 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2117 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2118 of the standard distribution. Here's one example using Storable's C<store>
2119 and C<retrieve> functions:
2122 store(\%hash, "filename");
2125 $href = retrieve("filename"); # by ref
2126 %hash = %{ retrieve("filename") }; # direct to hash
2128 =head2 How do I print out or copy a recursive data structure?
2130 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2131 for printing out data structures. The Storable module on CPAN (or the
2132 5.8 release of Perl), provides a function called C<dclone> that recursively
2133 copies its argument.
2135 use Storable qw(dclone);
2138 Where $r1 can be a reference to any kind of data structure you'd like.
2139 It will be deeply copied. Because C<dclone> takes and returns references,
2140 you'd have to add extra punctuation if you had a hash of arrays that
2143 %newhash = %{ dclone(\%oldhash) };
2145 =head2 How do I define methods for every class/object?
2147 Use the UNIVERSAL class (see L<UNIVERSAL>).
2149 =head2 How do I verify a credit card checksum?
2151 Get the Business::CreditCard module from CPAN.
2153 =head2 How do I pack arrays of doubles or floats for XS code?
2155 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2156 If you're doing a lot of float or double processing, consider using
2157 the PDL module from CPAN instead--it makes number-crunching easy.
2161 Revision: $Revision: 3606 $
2163 Date: $Date: 2006-03-06 12:05:47 +0100 (lun, 06 mar 2006) $
2165 See L<perlfaq> for source control details and availability.
2167 =head1 AUTHOR AND COPYRIGHT
2169 Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
2170 other authors as noted. All rights reserved.
2172 This documentation is free; you can redistribute it and/or modify it
2173 under the same terms as Perl itself.
2175 Irrespective of its distribution, all code examples in this file
2176 are hereby placed into the public domain. You are permitted and
2177 encouraged to use this code in your own programs for fun
2178 or for profit as you see fit. A simple comment in the code giving
2179 credit would be courteous but is not required.