3 perlfaq4 - Data Manipulation ($Revision: 1.67 $, $Date: 2005/08/10 15:55:49 $)
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 $] < 5.004 }
344 5.004 and later automatically call C<srand> at the beginning. Don't
345 call C<srand> more than once---you make your numbers less random, rather
348 Computers are good at being predictable and bad at being random
349 (despite appearances caused by bugs in your programs :-). see the
350 F<random> article in the "Far More Than You Ever Wanted To Know"
351 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
352 Tom Phoenix, talks more about this. John von Neumann said, "Anyone
353 who attempts to generate random numbers by deterministic means is, of
354 course, living in a state of sin."
356 If you want numbers that are more random than C<rand> with C<srand>
357 provides, you should also check out the Math::TrulyRandom module from
358 CPAN. It uses the imperfections in your system's timer to generate
359 random numbers, but this takes quite a while. If you want a better
360 pseudorandom generator than comes with your operating system, look at
361 "Numerical Recipes in C" at http://www.nr.com/ .
363 =head2 How do I get a random number between X and Y?
365 C<rand($x)> returns a number such that
366 C<< 0 <= rand($x) < $x >>. Thus what you want to have perl
367 figure out is a random number in the range from 0 to the
368 difference between your I<X> and I<Y>.
370 That is, to get a number between 10 and 15, inclusive, you
371 want a random number between 0 and 5 that you can then add
374 my $number = 10 + int rand( 15-10+1 );
376 Hence you derive the following simple function to abstract
377 that. It selects a random integer between the two given
378 integers (inclusive), For example: C<random_int_in(50,120)>.
380 sub random_int_in ($$) {
382 # Assumes that the two arguments are integers themselves!
383 return $min if $min == $max;
384 ($min, $max) = ($max, $min) if $min > $max;
385 return $min + int rand(1 + $max - $min);
390 =head2 How do I find the day or week of the year?
392 The localtime function returns the day of the year. Without an
393 argument localtime uses the current time.
395 $day_of_year = (localtime)[7];
397 The POSIX module can also format a date as the day of the year or
400 use POSIX qw/strftime/;
401 my $day_of_year = strftime "%j", localtime;
402 my $week_of_year = strftime "%W", localtime;
404 To get the day of year for any date, use the Time::Local module to get
405 a time in epoch seconds for the argument to localtime.
407 use POSIX qw/strftime/;
409 my $week_of_year = strftime "%W",
410 localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
412 The Date::Calc module provides two functions to calculate these.
415 my $day_of_year = Day_of_Year( 1987, 12, 18 );
416 my $week_of_year = Week_of_Year( 1987, 12, 18 );
418 =head2 How do I find the current century or millennium?
420 Use the following simple functions:
423 return int((((localtime(shift || time))[5] + 1999))/100);
427 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
430 On some systems, the POSIX module's strftime() function has
431 been extended in a non-standard way to use a C<%C> format,
432 which they sometimes claim is the "century". It isn't,
433 because on most such systems, this is only the first two
434 digits of the four-digit year, and thus cannot be used to
435 reliably determine the current century or millennium.
437 =head2 How can I compare two dates and find the difference?
439 (contributed by brian d foy)
441 You could just store all your dates as a number and then subtract. Life
442 isn't always that simple though. If you want to work with formatted
443 dates, the Date::Manip, Date::Calc, or DateTime modules can help you.
446 =head2 How can I take a string and turn it into epoch seconds?
448 If it's a regular enough string that it always has the same format,
449 you can split it up and pass the parts to C<timelocal> in the standard
450 Time::Local module. Otherwise, you should look into the Date::Calc
451 and Date::Manip modules from CPAN.
453 =head2 How can I find the Julian Day?
455 (contributed by brian d foy and Dave Cross)
457 You can use the Time::JulianDay module available on CPAN. Ensure that
458 you really want to find a Julian day, though, as many people have
459 different ideas about Julian days. See
460 http://www.hermetic.ch/cal_stud/jdn.htm for instance.
462 You can also try the DateTime module, which can convert a date/time
465 $ perl -MDateTime -le'print DateTime->today->jd'
468 Or the modified Julian Day
470 $ perl -MDateTime -le'print DateTime->today->mjd'
473 Or even the day of the year (which is what some people think of as a
476 $ perl -MDateTime -le'print DateTime->today->doy'
479 =head2 How do I find yesterday's date?
481 (contributed by brian d foy)
483 Use one of the Date modules. The C<DateTime> module makes it simple, and
484 give you the same time of day, only the day before.
488 my $yesterday = DateTime->now->subtract( days => 1 );
490 print "Yesterday was $yesterday\n";
492 You can also use the C<Date::Calc> module using its Today_and_Now
495 use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
497 my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
501 Most people try to use the time rather than the calendar to figure out
502 dates, but that assumes that days are twenty-four hours each. For
503 most people, there are two days a year when they aren't: the switch to
504 and from summer time throws this off. Let the modules do the work.
506 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
508 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
509 Y2K compliant (whatever that means). The programmers you've hired to
510 use it, however, probably are not.
512 Long answer: The question belies a true understanding of the issue.
513 Perl is just as Y2K compliant as your pencil--no more, and no less.
514 Can you use your pencil to write a non-Y2K-compliant memo? Of course
515 you can. Is that the pencil's fault? Of course it isn't.
517 The date and time functions supplied with Perl (gmtime and localtime)
518 supply adequate information to determine the year well beyond 2000
519 (2038 is when trouble strikes for 32-bit machines). The year returned
520 by these functions when used in a list context is the year minus 1900.
521 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
522 number. To avoid the year 2000 problem simply do not treat the year as
523 a 2-digit number. It isn't.
525 When gmtime() and localtime() are used in scalar context they return
526 a timestamp string that contains a fully-expanded year. For example,
527 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
528 2001". There's no year 2000 problem here.
530 That doesn't mean that Perl can't be used to create non-Y2K compliant
531 programs. It can. But so can your pencil. It's the fault of the user,
532 not the language. At the risk of inflaming the NRA: "Perl doesn't
533 break Y2K, people do." See http://www.perl.org/about/y2k.html for
538 =head2 How do I validate input?
540 (contributed by brian d foy)
542 There are many ways to ensure that values are what you expect or
543 want to accept. Besides the specific examples that we cover in the
544 perlfaq, you can also look at the modules with "Assert" and "Validate"
545 in their names, along with other modules such as C<Regexp::Common>.
547 Some modules have validation for particular types of input, such
548 as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>,
549 and C<Data::Validate::IP>.
551 =head2 How do I unescape a string?
553 It depends just what you mean by "escape". URL escapes are dealt
554 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
555 character are removed with
559 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
561 =head2 How do I remove consecutive pairs of characters?
563 (contributed by brian d foy)
565 You can use the substitution operator to find pairs of characters (or
566 runs of characters) and replace them with a single instance. In this
567 substitution, we find a character in C<(.)>. The memory parentheses
568 store the matched character in the back-reference C<\1> and we use
569 that to require that the same thing immediately follow it. We replace
570 that part of the string with the character in C<$1>.
574 We can also use the transliteration operator, C<tr///>. In this
575 example, the search list side of our C<tr///> contains nothing, but
576 the C<c> option complements that so it contains everything. The
577 replacement list also contains nothing, so the transliteration is
578 almost a no-op since it won't do any replacements (or more exactly,
579 replace the character with itself). However, the C<s> option squashes
580 duplicated and consecutive characters in the string so a character
581 does not show up next to itself
583 my $str = 'Haarlem'; # in the Netherlands
584 $str =~ tr///cs; # Now Harlem, like in New York
586 =head2 How do I expand function calls in a string?
588 (contributed by brian d foy)
590 This is documented in L<perlref>, and although it's not the easiest
591 thing to read, it does work. In each of these examples, we call the
592 function inside the braces of used to dereference a reference. If we
593 have a more than one return value, we can construct and dereference an
594 anonymous array. In this case, we call the function in list context.
596 print "The time values are @{ [localtime] }.\n";
598 If we want to call the function in scalar context, we have to do a bit
599 more work. We can really have any code we like inside the braces, so
600 we simply have to end with the scalar reference, although how you do
601 that is up to you, and you can use code inside the braces.
603 print "The time is ${\(scalar localtime)}.\n"
605 print "The time is ${ my $x = localtime; \$x }.\n";
607 If your function already returns a reference, you don't need to create
608 the reference yourself.
610 sub timestamp { my $t = localtime; \$t }
612 print "The time is ${ timestamp() }.\n";
614 In most cases, it is probably easier to simply use string
615 concatenation, which also forces scalar context.
617 print "The time is " . localtime . ".\n";
619 =head2 How do I find matching/nesting anything?
621 This isn't something that can be done in one regular expression, no
622 matter how complicated. To find something between two single
623 characters, a pattern like C</x([^x]*)x/> will get the intervening
624 bits in $1. For multiple ones, then something more like
625 C</alpha(.*?)omega/> would be needed. But none of these deals with
626 nested patterns. For balanced expressions using C<(>, C<{>, C<[> or
627 C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
628 L<perlre/(??{ code })>. For other cases, you'll have to write a
631 If you are serious about writing a parser, there are a number of
632 modules or oddities that will make your life a lot easier. There are
633 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
634 and the byacc program. Starting from perl 5.8 the Text::Balanced is
635 part of the standard distribution.
637 One simple destructive, inside-out approach that you might try is to
638 pull out the smallest nesting parts one at a time:
640 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
641 # do something with $1
644 A more complicated and sneaky approach is to make Perl's regular
645 expression engine do it for you. This is courtesy Dean Inada, and
646 rather has the nature of an Obfuscated Perl Contest entry, but it
649 # $_ contains the string to parse
650 # BEGIN and END are the opening and closing markers for the
655 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
656 @$ = (eval{/$re/},$@!~/unmatched/i);
657 print join("\n",@$[0..$#$]) if( $$[-1] );
659 =head2 How do I reverse a string?
661 Use reverse() in scalar context, as documented in
664 $reversed = reverse $string;
666 =head2 How do I expand tabs in a string?
668 You can do it yourself:
670 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
672 Or you can just use the Text::Tabs module (part of the standard Perl
676 @expanded_lines = expand(@lines_with_tabs);
678 =head2 How do I reformat a paragraph?
680 Use Text::Wrap (part of the standard Perl distribution):
683 print wrap("\t", ' ', @paragraphs);
685 The paragraphs you give to Text::Wrap should not contain embedded
686 newlines. Text::Wrap doesn't justify the lines (flush-right).
688 Or use the CPAN module Text::Autoformat. Formatting files can be easily
689 done by making a shell alias, like so:
691 alias fmt="perl -i -MText::Autoformat -n0777 \
692 -e 'print autoformat $_, {all=>1}' $*"
694 See the documentation for Text::Autoformat to appreciate its many
697 =head2 How can I access or change N characters of a string?
699 You can access the first characters of a string with substr().
700 To get the first character, for example, start at position 0
701 and grab the string of length 1.
704 $string = "Just another Perl Hacker";
705 $first_char = substr( $string, 0, 1 ); # 'J'
707 To change part of a string, you can use the optional fourth
708 argument which is the replacement string.
710 substr( $string, 13, 4, "Perl 5.8.0" );
712 You can also use substr() as an lvalue.
714 substr( $string, 13, 4 ) = "Perl 5.8.0";
716 =head2 How do I change the Nth occurrence of something?
718 You have to keep track of N yourself. For example, let's say you want
719 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
720 C<"whosoever"> or C<"whomsoever">, case insensitively. These
721 all assume that $_ contains the string to be altered.
725 ++$count == 5 # is it the 5th?
726 ? "${2}soever" # yes, swap
727 : $1 # renege and leave it there
730 In the more general case, you can use the C</g> modifier in a C<while>
731 loop, keeping count of matches.
735 $_ = "One fish two fish red fish blue fish";
736 while (/(\w+)\s+fish\b/gi) {
737 if (++$count == $WANT) {
738 print "The third fish is a $1 one.\n";
742 That prints out: C<"The third fish is a red one."> You can also use a
743 repetition count and repeated pattern like this:
745 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
747 =head2 How can I count the number of occurrences of a substring within a string?
749 There are a number of ways, with varying efficiency. If you want a
750 count of a certain single character (X) within a string, you can use the
751 C<tr///> function like so:
753 $string = "ThisXlineXhasXsomeXx'sXinXit";
754 $count = ($string =~ tr/X//);
755 print "There are $count X characters in the string";
757 This is fine if you are just looking for a single character. However,
758 if you are trying to count multiple character substrings within a
759 larger string, C<tr///> won't work. What you can do is wrap a while()
760 loop around a global pattern match. For example, let's count negative
763 $string = "-9 55 48 -2 23 -76 4 14 -44";
764 while ($string =~ /-\d+/g) { $count++ }
765 print "There are $count negative numbers in the string";
767 Another version uses a global match in list context, then assigns the
768 result to a scalar, producing a count of the number of matches.
770 $count = () = $string =~ /-\d+/g;
772 =head2 How do I capitalize all the words on one line?
774 To make the first letter of each word upper case:
776 $line =~ s/\b(\w)/\U$1/g;
778 This has the strange effect of turning "C<don't do it>" into "C<Don'T
779 Do It>". Sometimes you might want this. Other times you might need a
780 more thorough solution (Suggested by brian d foy):
783 (^\w) #at the beginning of the line
785 (\s\w) #preceded by whitespace
788 $string =~ /([\w']+)/\u\L$1/g;
790 To make the whole line upper case:
794 To force each word to be lower case, with the first letter upper case:
796 $line =~ s/(\w+)/\u\L$1/g;
798 You can (and probably should) enable locale awareness of those
799 characters by placing a C<use locale> pragma in your program.
800 See L<perllocale> for endless details on locales.
802 This is sometimes referred to as putting something into "title
803 case", but that's not quite accurate. Consider the proper
804 capitalization of the movie I<Dr. Strangelove or: How I Learned to
805 Stop Worrying and Love the Bomb>, for example.
807 Damian Conway's L<Text::Autoformat> module provides some smart
808 case transformations:
810 use Text::Autoformat;
811 my $x = "Dr. Strangelove or: How I Learned to Stop ".
812 "Worrying and Love the Bomb";
815 for my $style (qw( sentence title highlight ))
817 print autoformat($x, { case => $style }), "\n";
820 =head2 How can I split a [character] delimited string except when inside [character]?
822 Several modules can handle this sort of pasing---Text::Balanced,
823 Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.
825 Take the example case of trying to split a string that is
826 comma-separated into its different fields. You can't use C<split(/,/)>
827 because you shouldn't split if the comma is inside quotes. For
828 example, take a data line like this:
830 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
832 Due to the restriction of the quotes, this is a fairly complex
833 problem. Thankfully, we have Jeffrey Friedl, author of
834 I<Mastering Regular Expressions>, to handle these for us. He
835 suggests (assuming your string is contained in $text):
838 push(@new, $+) while $text =~ m{
839 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
843 push(@new, undef) if substr($text,-1,1) eq ',';
845 If you want to represent quotation marks inside a
846 quotation-mark-delimited field, escape them with backslashes (eg,
849 Alternatively, the Text::ParseWords module (part of the standard Perl
850 distribution) lets you say:
852 use Text::ParseWords;
853 @new = quotewords(",", 0, $text);
855 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
857 =head2 How do I strip blank space from the beginning/end of a string?
859 (contributed by brian d foy)
861 A substitution can do this for you. For a single line, you want to
862 replace all the leading or trailing whitespace with nothing. You
863 can do that with a pair of substitutions.
868 You can also write that as a single substitution, although it turns
869 out the combined statement is slower than the separate ones. That
870 might not matter to you, though.
874 In this regular expression, the alternation matches either at the
875 beginning or the end of the string since the anchors have a lower
876 precedence than the alternation. With the C</g> flag, the substitution
877 makes all possible matches, so it gets both. Remember, the trailing
878 newline matches the C<\s+>, and the C<$> anchor can match to the
879 physical end of the string, so the newline disappears too. Just add
880 the newline to the output, which has the added benefit of preserving
881 "blank" (consisting entirely of whitespace) lines which the C<^\s+>
882 would remove all by itself.
890 For a multi-line string, you can apply the regular expression
891 to each logical line in the string by adding the C</m> flag (for
892 "multi-line"). With the C</m> flag, the C<$> matches I<before> an
893 embedded newline, so it doesn't remove it. It still removes the
894 newline at the end of the string.
896 $string =~ s/^\s+|\s+$//gm;
898 Remember that lines consisting entirely of whitespace will disappear,
899 since the first part of the alternation can match the entire string
900 and replace it with nothing. If need to keep embedded blank lines,
901 you have to do a little more work. Instead of matching any whitespace
902 (since that includes a newline), just match the other whitespace.
904 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
906 =head2 How do I pad a string with blanks or pad a number with zeroes?
908 In the following examples, C<$pad_len> is the length to which you wish
909 to pad the string, C<$text> or C<$num> contains the string to be padded,
910 and C<$pad_char> contains the padding character. You can use a single
911 character string constant instead of the C<$pad_char> variable if you
912 know what it is in advance. And in the same way you can use an integer in
913 place of C<$pad_len> if you know the pad length in advance.
915 The simplest method uses the C<sprintf> function. It can pad on the left
916 or right with blanks and on the left with zeroes and it will not
917 truncate the result. The C<pack> function can only pad strings on the
918 right with blanks and it will truncate the result to a maximum length of
921 # Left padding a string with blanks (no truncation):
922 $padded = sprintf("%${pad_len}s", $text);
923 $padded = sprintf("%*s", $pad_len, $text); # same thing
925 # Right padding a string with blanks (no truncation):
926 $padded = sprintf("%-${pad_len}s", $text);
927 $padded = sprintf("%-*s", $pad_len, $text); # same thing
929 # Left padding a number with 0 (no truncation):
930 $padded = sprintf("%0${pad_len}d", $num);
931 $padded = sprintf("%0*d", $pad_len, $num); # same thing
933 # Right padding a string with blanks using pack (will truncate):
934 $padded = pack("A$pad_len",$text);
936 If you need to pad with a character other than blank or zero you can use
937 one of the following methods. They all generate a pad string with the
938 C<x> operator and combine that with C<$text>. These methods do
939 not truncate C<$text>.
941 Left and right padding with any character, creating a new string:
943 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
944 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
946 Left and right padding with any character, modifying C<$text> directly:
948 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
949 $text .= $pad_char x ( $pad_len - length( $text ) );
951 =head2 How do I extract selected columns from a string?
953 Use substr() or unpack(), both documented in L<perlfunc>.
954 If you prefer thinking in terms of columns instead of widths,
955 you can use this kind of thing:
957 # determine the unpack format needed to split Linux ps output
958 # arguments are cut columns
959 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
965 for my $place (@positions) {
966 $template .= "A" . ($place - $lastpos) . " ";
973 =head2 How do I find the soundex value of a string?
975 (contributed by brian d foy)
977 You can use the Text::Soundex module. If you want to do fuzzy or close
978 matching, you might also try the String::Approx, and Text::Metaphone,
979 and Text::DoubleMetaphone modules.
981 =head2 How can I expand variables in text strings?
983 Let's assume that you have a string that contains placeholder
986 $text = 'this has a $foo in it and a $bar';
988 You can use a substitution with a double evaluation. The
989 first /e turns C<$1> into C<$foo>, and the second /e turns
990 C<$foo> into its value. You may want to wrap this in an
991 C<eval>: if you try to get the value of an undeclared variable
992 while running under C<use strict>, you get a fatal error.
994 eval { $text =~ s/(\$\w+)/$1/eeg };
997 It's probably better in the general case to treat those
998 variables as entries in some special hash. For example:
1004 $text =~ s/\$(\w+)/$user_defs{$1}/g;
1006 =head2 What's wrong with always quoting "$vars"?
1008 The problem is that those double-quotes force stringification--
1009 coercing numbers and references into strings--even when you
1010 don't want them to be strings. Think of it this way: double-quote
1011 expansion is used to produce new strings. If you already
1012 have a string, why do you need more?
1014 If you get used to writing odd things like these:
1017 $new = "$old"; # BAD
1018 somefunc("$var"); # BAD
1020 You'll be in trouble. Those should (in 99.8% of the cases) be
1021 the simpler and more direct:
1027 Otherwise, besides slowing you down, you're going to break code when
1028 the thing in the scalar is actually neither a string nor a number, but
1034 my $oref = "$aref"; # WRONG
1037 You can also get into subtle problems on those few operations in Perl
1038 that actually do care about the difference between a string and a
1039 number, such as the magical C<++> autoincrement operator or the
1042 Stringification also destroys arrays.
1045 print "@lines"; # WRONG - extra blanks
1046 print @lines; # right
1048 =head2 Why don't my E<lt>E<lt>HERE documents work?
1050 Check for these three things:
1054 =item There must be no space after the E<lt>E<lt> part.
1056 =item There (probably) should be a semicolon at the end.
1058 =item You can't (easily) have any space in front of the tag.
1062 If you want to indent the text in the here document, you
1066 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1071 But the HERE_TARGET must still be flush against the margin.
1072 If you want that indented also, you'll have to quote
1075 ($quote = <<' FINIS') =~ s/^\s+//gm;
1076 ...we will have peace, when you and all your works have
1077 perished--and the works of your dark master to whom you
1078 would deliver us. You are a liar, Saruman, and a corrupter
1079 of men's hearts. --Theoden in /usr/src/perl/taint.c
1081 $quote =~ s/\s+--/\n--/;
1083 A nice general-purpose fixer-upper function for indented here documents
1084 follows. It expects to be called with a here document as its argument.
1085 It looks to see whether each line begins with a common substring, and
1086 if so, strips that substring off. Otherwise, it takes the amount of leading
1087 whitespace found on the first line and removes that much off each
1092 my ($white, $leader); # common whitespace and common leading string
1093 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1094 ($white, $leader) = ($2, quotemeta($1));
1096 ($white, $leader) = (/^(\s+)/, '');
1098 s/^\s*?$leader(?:$white)?//gm;
1102 This works with leading special strings, dynamically determined:
1104 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1107 @@@ SAVEI32(runlevel);
1109 @@@ while ( op = (*op->op_ppaddr)() );
1113 MAIN_INTERPRETER_LOOP
1115 Or with a fixed amount of leading whitespace, with remaining
1116 indentation correctly preserved:
1118 $poem = fix<<EVER_ON_AND_ON;
1119 Now far ahead the Road has gone,
1120 And I must follow, if I can,
1121 Pursuing it with eager feet,
1122 Until it joins some larger way
1123 Where many paths and errands meet.
1124 And whither then? I cannot say.
1125 --Bilbo in /usr/src/perl/pp_ctl.c
1130 =head2 What is the difference between a list and an array?
1132 An array has a changeable length. A list does not. An array is something
1133 you can push or pop, while a list is a set of values. Some people make
1134 the distinction that a list is a value while an array is a variable.
1135 Subroutines are passed and return lists, you put things into list
1136 context, you initialize arrays with lists, and you foreach() across
1137 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1138 in scalar context behave like the number of elements in them, subroutines
1139 access their arguments through the array C<@_>, and push/pop/shift only work
1142 As a side note, there's no such thing as a list in scalar context.
1145 $scalar = (2, 5, 7, 9);
1147 you're using the comma operator in scalar context, so it uses the scalar
1148 comma operator. There never was a list there at all! This causes the
1149 last value to be returned: 9.
1151 =head2 What is the difference between $array[1] and @array[1]?
1153 The former is a scalar value; the latter an array slice, making
1154 it a list with one (scalar) value. You should use $ when you want a
1155 scalar value (most of the time) and @ when you want a list with one
1156 scalar value in it (very, very rarely; nearly never, in fact).
1158 Sometimes it doesn't make a difference, but sometimes it does.
1159 For example, compare:
1161 $good[0] = `some program that outputs several lines`;
1165 @bad[0] = `same program that outputs several lines`;
1167 The C<use warnings> pragma and the B<-w> flag will warn you about these
1170 =head2 How can I remove duplicate elements from a list or array?
1172 (contributed by brian d foy)
1174 Use a hash. When you think the words "unique" or "duplicated", think
1177 If you don't care about the order of the elements, you could just
1178 create the hash then extract the keys. It's not important how you
1179 create that hash: just that you use C<keys> to get the unique
1182 my %hash = map { $_, 1 } @array;
1183 # or a hash slice: @hash{ @array } = ();
1184 # or a foreach: $hash{$_} = 1 foreach ( @array );
1186 my @unique = keys %hash;
1188 You can also go through each element and skip the ones you've seen
1189 before. Use a hash to keep track. The first time the loop sees an
1190 element, that element has no key in C<%Seen>. The C<next> statement
1191 creates the key and immediately uses its value, which is C<undef>, so
1192 the loop continues to the C<push> and increments the value for that
1193 key. The next time the loop sees that same element, its key exists in
1194 the hash I<and> the value for that key is true (since it's not 0 or
1195 undef), so the next skips that iteration and the loop goes to the next
1201 foreach my $elem ( @array )
1203 next if $seen{ $elem }++;
1204 push @unique, $elem;
1207 You can write this more briefly using a grep, which does the
1211 my @unique = grep { ! $seen{ $_ }++ } @array;
1213 =head2 How can I tell whether a certain element is contained in a list or array?
1215 Hearing the word "in" is an I<in>dication that you probably should have
1216 used a hash, not a list or array, to store your data. Hashes are
1217 designed to answer this question quickly and efficiently. Arrays aren't.
1219 That being said, there are several ways to approach this. If you
1220 are going to make this query many times over arbitrary string values,
1221 the fastest way is probably to invert the original array and maintain a
1222 hash whose keys are the first array's values.
1224 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1226 for (@blues) { $is_blue{$_} = 1 }
1228 Now you can check whether $is_blue{$some_color}. It might have been a
1229 good idea to keep the blues all in a hash in the first place.
1231 If the values are all small integers, you could use a simple indexed
1232 array. This kind of an array will take up less space:
1234 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1235 @is_tiny_prime = ();
1236 for (@primes) { $is_tiny_prime[$_] = 1 }
1237 # or simply @istiny_prime[@primes] = (1) x @primes;
1239 Now you check whether $is_tiny_prime[$some_number].
1241 If the values in question are integers instead of strings, you can save
1242 quite a lot of space by using bit strings instead:
1244 @articles = ( 1..10, 150..2000, 2017 );
1246 for (@articles) { vec($read,$_,1) = 1 }
1248 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1252 ($is_there) = grep $_ eq $whatever, @array;
1256 ($is_there) = grep /$whatever/, @array;
1258 These are slow (checks every element even if the first matches),
1259 inefficient (same reason), and potentially buggy (what if there are
1260 regex characters in $whatever?). If you're only testing once, then
1264 foreach $elt (@array) {
1265 if ($elt eq $elt_to_find) {
1270 if ($is_there) { ... }
1272 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1274 Use a hash. Here's code to do both and more. It assumes that
1275 each element is unique in a given array:
1277 @union = @intersection = @difference = ();
1279 foreach $element (@array1, @array2) { $count{$element}++ }
1280 foreach $element (keys %count) {
1281 push @union, $element;
1282 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1285 Note that this is the I<symmetric difference>, that is, all elements in
1286 either A or in B but not in both. Think of it as an xor operation.
1288 =head2 How do I test whether two arrays or hashes are equal?
1290 The following code works for single-level arrays. It uses a stringwise
1291 comparison, and does not distinguish defined versus undefined empty
1292 strings. Modify if you have other needs.
1294 $are_equal = compare_arrays(\@frogs, \@toads);
1296 sub compare_arrays {
1297 my ($first, $second) = @_;
1298 no warnings; # silence spurious -w undef complaints
1299 return 0 unless @$first == @$second;
1300 for (my $i = 0; $i < @$first; $i++) {
1301 return 0 if $first->[$i] ne $second->[$i];
1306 For multilevel structures, you may wish to use an approach more
1307 like this one. It uses the CPAN module FreezeThaw:
1309 use FreezeThaw qw(cmpStr);
1310 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1312 printf "a and b contain %s arrays\n",
1313 cmpStr(\@a, \@b) == 0
1317 This approach also works for comparing hashes. Here
1318 we'll demonstrate two different answers:
1320 use FreezeThaw qw(cmpStr cmpStrHard);
1322 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1326 printf "a and b contain %s hashes\n",
1327 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1329 printf "a and b contain %s hashes\n",
1330 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1333 The first reports that both those the hashes contain the same data,
1334 while the second reports that they do not. Which you prefer is left as
1335 an exercise to the reader.
1337 =head2 How do I find the first array element for which a condition is true?
1339 To find the first array element which satisfies a condition, you can
1340 use the first() function in the List::Util module, which comes with
1341 Perl 5.8. This example finds the first element that contains "Perl".
1343 use List::Util qw(first);
1345 my $element = first { /Perl/ } @array;
1347 If you cannot use List::Util, you can make your own loop to do the
1348 same thing. Once you find the element, you stop the loop with last.
1353 if( /Perl/ ) { $found = $_; last }
1356 If you want the array index, you can iterate through the indices
1357 and check the array element at each index until you find one
1358 that satisfies the condition.
1360 my( $found, $index ) = ( undef, -1 );
1361 for( $i = 0; $i < @array; $i++ )
1363 if( $array[$i] =~ /Perl/ )
1365 $found = $array[$i];
1371 =head2 How do I handle linked lists?
1373 In general, you usually don't need a linked list in Perl, since with
1374 regular arrays, you can push and pop or shift and unshift at either end,
1375 or you can use splice to add and/or remove arbitrary number of elements at
1376 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1377 dynamic arrays. In the absence of shifts and pops, push in general
1378 needs to reallocate on the order every log(N) times, and unshift will
1379 need to copy pointers each time.
1381 If you really, really wanted, you could use structures as described in
1382 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1383 to do. For example, imagine a list node like this:
1390 You could walk the list this way:
1393 for ($node = $head; $node; $node = $node->{LINK}) {
1394 print $node->{VALUE}, " ";
1398 You could add to the list this way:
1401 $tail = append($head, 1); # grow a new head
1402 for $value ( 2 .. 10 ) {
1403 $tail = append($tail, $value);
1407 my($list, $value) = @_;
1408 my $node = { VALUE => $value };
1410 $node->{LINK} = $list->{LINK};
1411 $list->{LINK} = $node;
1413 $_[0] = $node; # replace caller's version
1418 But again, Perl's built-in are virtually always good enough.
1420 =head2 How do I handle circular lists?
1422 Circular lists could be handled in the traditional fashion with linked
1423 lists, or you could just do something like this with an array:
1425 unshift(@array, pop(@array)); # the last shall be first
1426 push(@array, shift(@array)); # and vice versa
1428 =head2 How do I shuffle an array randomly?
1430 If you either have Perl 5.8.0 or later installed, or if you have
1431 Scalar-List-Utils 1.03 or later installed, you can say:
1433 use List::Util 'shuffle';
1435 @shuffled = shuffle(@list);
1437 If not, you can use a Fisher-Yates shuffle.
1439 sub fisher_yates_shuffle {
1440 my $deck = shift; # $deck is a reference to an array
1443 my $j = int rand ($i+1);
1444 @$deck[$i,$j] = @$deck[$j,$i];
1448 # shuffle my mpeg collection
1450 my @mpeg = <audio/*/*.mp3>;
1451 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1454 Note that the above implementation shuffles an array in place,
1455 unlike the List::Util::shuffle() which takes a list and returns
1456 a new shuffled list.
1458 You've probably seen shuffling algorithms that work using splice,
1459 randomly picking another element to swap the current element with
1463 @old = 1 .. 10; # just a demo
1465 push(@new, splice(@old, rand @old, 1));
1468 This is bad because splice is already O(N), and since you do it N times,
1469 you just invented a quadratic algorithm; that is, O(N**2). This does
1470 not scale, although Perl is so efficient that you probably won't notice
1471 this until you have rather largish arrays.
1473 =head2 How do I process/modify each element of an array?
1475 Use C<for>/C<foreach>:
1478 s/foo/bar/; # change that word
1479 tr/XZ/ZX/; # swap those letters
1482 Here's another; let's compute spherical volumes:
1484 for (@volumes = @radii) { # @volumes has changed parts
1486 $_ *= (4/3) * 3.14159; # this will be constant folded
1489 which can also be done with map() which is made to transform
1490 one list into another:
1492 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1494 If you want to do the same thing to modify the values of the
1495 hash, you can use the C<values> function. As of Perl 5.6
1496 the values are not copied, so if you modify $orbit (in this
1497 case), you modify the value.
1499 for $orbit ( values %orbits ) {
1500 ($orbit **= 3) *= (4/3) * 3.14159;
1503 Prior to perl 5.6 C<values> returned copies of the values,
1504 so older perl code often contains constructions such as
1505 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1506 the hash is to be modified.
1508 =head2 How do I select a random element from an array?
1510 Use the rand() function (see L<perlfunc/rand>):
1512 $index = rand @array;
1513 $element = $array[$index];
1516 my $element = $array[ rand @array ];
1518 =head2 How do I permute N elements of a list?
1520 Use the List::Permutor module on CPAN. If the list is
1521 actually an array, try the Algorithm::Permute module (also
1522 on CPAN). It's written in XS code and is very efficient.
1524 use Algorithm::Permute;
1525 my @array = 'a'..'d';
1526 my $p_iterator = Algorithm::Permute->new ( \@array );
1527 while (my @perm = $p_iterator->next) {
1528 print "next permutation: (@perm)\n";
1531 For even faster execution, you could do:
1533 use Algorithm::Permute;
1534 my @array = 'a'..'d';
1535 Algorithm::Permute::permute {
1536 print "next permutation: (@array)\n";
1539 Here's a little program that generates all permutations of
1540 all the words on each line of input. The algorithm embodied
1541 in the permute() function is discussed in Volume 4 (still
1542 unpublished) of Knuth's I<The Art of Computer Programming>
1543 and will work on any list:
1546 # Fischer-Kause ordered permutation generator
1551 while ( $code->(@_[@idx]) ) {
1553 --$p while $idx[$p-1] > $idx[$p];
1554 my $q = $p or return;
1555 push @idx, reverse splice @idx, $p;
1556 ++$q while $idx[$p-1] > $idx[$q];
1557 @idx[$p-1,$q]=@idx[$q,$p-1];
1561 permute {print"@_\n"} split;
1563 =head2 How do I sort an array by (anything)?
1565 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1567 @list = sort { $a <=> $b } @list;
1569 The default sort function is cmp, string comparison, which would
1570 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1571 the numerical comparison operator.
1573 If you have a complicated function needed to pull out the part you
1574 want to sort on, then don't do it inside the sort function. Pull it
1575 out first, because the sort BLOCK can be called many times for the
1576 same element. Here's an example of how to pull out the first word
1577 after the first number on each item, and then sort those words
1582 ($item) = /\d+\s*(\S+)/;
1583 push @idx, uc($item);
1585 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1587 which could also be written this way, using a trick
1588 that's come to be known as the Schwartzian Transform:
1590 @sorted = map { $_->[0] }
1591 sort { $a->[1] cmp $b->[1] }
1592 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1594 If you need to sort on several fields, the following paradigm is useful.
1596 @sorted = sort { field1($a) <=> field1($b) ||
1597 field2($a) cmp field2($b) ||
1598 field3($a) cmp field3($b)
1601 This can be conveniently combined with precalculation of keys as given
1604 See the F<sort> article in the "Far More Than You Ever Wanted
1605 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1606 more about this approach.
1608 See also the question below on sorting hashes.
1610 =head2 How do I manipulate arrays of bits?
1612 Use pack() and unpack(), or else vec() and the bitwise operations.
1614 For example, this sets $vec to have bit N set if $ints[N] was set:
1617 foreach(@ints) { vec($vec,$_,1) = 1 }
1619 Here's how, given a vector in $vec, you can
1620 get those bits into your @ints array:
1622 sub bitvec_to_list {
1625 # Find null-byte density then select best algorithm
1626 if ($vec =~ tr/\0// / length $vec > 0.95) {
1629 # This method is faster with mostly null-bytes
1630 while($vec =~ /[^\0]/g ) {
1631 $i = -9 + 8 * pos $vec;
1632 push @ints, $i if vec($vec, ++$i, 1);
1633 push @ints, $i if vec($vec, ++$i, 1);
1634 push @ints, $i if vec($vec, ++$i, 1);
1635 push @ints, $i if vec($vec, ++$i, 1);
1636 push @ints, $i if vec($vec, ++$i, 1);
1637 push @ints, $i if vec($vec, ++$i, 1);
1638 push @ints, $i if vec($vec, ++$i, 1);
1639 push @ints, $i if vec($vec, ++$i, 1);
1642 # This method is a fast general algorithm
1644 my $bits = unpack "b*", $vec;
1645 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1646 push @ints, pos $bits while($bits =~ /1/g);
1651 This method gets faster the more sparse the bit vector is.
1652 (Courtesy of Tim Bunce and Winfried Koenig.)
1654 You can make the while loop a lot shorter with this suggestion
1655 from Benjamin Goldberg:
1657 while($vec =~ /[^\0]+/g ) {
1658 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1661 Or use the CPAN module Bit::Vector:
1663 $vector = Bit::Vector->new($num_of_bits);
1664 $vector->Index_List_Store(@ints);
1665 @ints = $vector->Index_List_Read();
1667 Bit::Vector provides efficient methods for bit vector, sets of small integers
1670 Here's a more extensive illustration using vec():
1673 $vector = "\xff\x0f\xef\xfe";
1674 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1675 unpack("N", $vector), "\n";
1676 $is_set = vec($vector, 23, 1);
1677 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1695 my ($offset, $width, $value) = @_;
1697 vec($vector, $offset, $width) = $value;
1698 print "offset=$offset width=$width value=$value\n";
1704 my $bits = unpack("b*", $vector);
1708 print "vector length in bytes: ", length($vector), "\n";
1709 @bytes = unpack("A8" x length($vector), $bits);
1710 print "bits are: @bytes\n\n";
1713 =head2 Why does defined() return true on empty arrays and hashes?
1715 The short story is that you should probably only use defined on scalars or
1716 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1717 in the 5.004 release or later of Perl for more detail.
1719 =head1 Data: Hashes (Associative Arrays)
1721 =head2 How do I process an entire hash?
1723 Use the each() function (see L<perlfunc/each>) if you don't care
1724 whether it's sorted:
1726 while ( ($key, $value) = each %hash) {
1727 print "$key = $value\n";
1730 If you want it sorted, you'll have to use foreach() on the result of
1731 sorting the keys as shown in an earlier question.
1733 =head2 What happens if I add or remove keys from a hash while iterating over it?
1735 (contributed by brian d foy)
1737 The easy answer is "Don't do that!"
1739 If you iterate through the hash with each(), you can delete the key
1740 most recently returned without worrying about it. If you delete or add
1741 other keys, the iterator may skip or double up on them since perl
1742 may rearrange the hash table. See the
1743 entry for C<each()> in L<perlfunc>.
1745 =head2 How do I look up a hash element by value?
1747 Create a reverse hash:
1749 %by_value = reverse %by_key;
1750 $key = $by_value{$value};
1752 That's not particularly efficient. It would be more space-efficient
1755 while (($key, $value) = each %by_key) {
1756 $by_value{$value} = $key;
1759 If your hash could have repeated values, the methods above will only find
1760 one of the associated keys. This may or may not worry you. If it does
1761 worry you, you can always reverse the hash into a hash of arrays instead:
1763 while (($key, $value) = each %by_key) {
1764 push @{$key_list_by_value{$value}}, $key;
1767 =head2 How can I know how many entries are in a hash?
1769 If you mean how many keys, then all you have to do is
1770 use the keys() function in a scalar context:
1772 $num_keys = keys %hash;
1774 The keys() function also resets the iterator, which means that you may
1775 see strange results if you use this between uses of other hash operators
1778 =head2 How do I sort a hash (optionally by value instead of key)?
1780 Internally, hashes are stored in a way that prevents you from imposing
1781 an order on key-value pairs. Instead, you have to sort a list of the
1784 @keys = sort keys %hash; # sorted by key
1786 $hash{$a} cmp $hash{$b}
1787 } keys %hash; # and by value
1789 Here we'll do a reverse numeric sort by value, and if two keys are
1790 identical, sort by length of key, or if that fails, by straight ASCII
1791 comparison of the keys (well, possibly modified by your locale--see
1795 $hash{$b} <=> $hash{$a}
1797 length($b) <=> length($a)
1802 =head2 How can I always keep my hash sorted?
1804 You can look into using the DB_File module and tie() using the
1805 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1806 The Tie::IxHash module from CPAN might also be instructive.
1808 =head2 What's the difference between "delete" and "undef" with hashes?
1810 Hashes contain pairs of scalars: the first is the key, the
1811 second is the value. The key will be coerced to a string,
1812 although the value can be any kind of scalar: string,
1813 number, or reference. If a key $key is present in
1814 %hash, C<exists($hash{$key})> will return true. The value
1815 for a given key can be C<undef>, in which case
1816 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1817 will return true. This corresponds to (C<$key>, C<undef>)
1820 Pictures help... here's the %hash table:
1830 And these conditions hold
1834 defined $hash{'d'} is true
1835 defined $hash{'a'} is true
1836 exists $hash{'a'} is true (Perl5 only)
1837 grep ($_ eq 'a', keys %hash) is true
1843 your table now reads:
1854 and these conditions now hold; changes in caps:
1858 defined $hash{'d'} is true
1859 defined $hash{'a'} is FALSE
1860 exists $hash{'a'} is true (Perl5 only)
1861 grep ($_ eq 'a', keys %hash) is true
1863 Notice the last two: you have an undef value, but a defined key!
1869 your table now reads:
1878 and these conditions now hold; changes in caps:
1882 defined $hash{'d'} is true
1883 defined $hash{'a'} is false
1884 exists $hash{'a'} is FALSE (Perl5 only)
1885 grep ($_ eq 'a', keys %hash) is FALSE
1887 See, the whole entry is gone!
1889 =head2 Why don't my tied hashes make the defined/exists distinction?
1891 This depends on the tied hash's implementation of EXISTS().
1892 For example, there isn't the concept of undef with hashes
1893 that are tied to DBM* files. It also means that exists() and
1894 defined() do the same thing with a DBM* file, and what they
1895 end up doing is not what they do with ordinary hashes.
1897 =head2 How do I reset an each() operation part-way through?
1899 Using C<keys %hash> in scalar context returns the number of keys in
1900 the hash I<and> resets the iterator associated with the hash. You may
1901 need to do this if you use C<last> to exit a loop early so that when you
1902 re-enter it, the hash iterator has been reset.
1904 =head2 How can I get the unique keys from two hashes?
1906 First you extract the keys from the hashes into lists, then solve
1907 the "removing duplicates" problem described above. For example:
1910 for $element (keys(%foo), keys(%bar)) {
1917 @uniq = keys %{{%foo,%bar}};
1919 Or if you really want to save space:
1922 while (defined ($key = each %foo)) {
1925 while (defined ($key = each %bar)) {
1930 =head2 How can I store a multidimensional array in a DBM file?
1932 Either stringify the structure yourself (no fun), or else
1933 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1934 it on top of either DB_File or GDBM_File.
1936 =head2 How can I make my hash remember the order I put elements into it?
1938 Use the Tie::IxHash from CPAN.
1941 tie my %myhash, 'Tie::IxHash';
1942 for (my $i=0; $i<20; $i++) {
1945 my @keys = keys %myhash;
1946 # @keys = (0,1,2,3,...)
1948 =head2 Why does passing a subroutine an undefined element in a hash create it?
1950 If you say something like:
1952 somefunc($hash{"nonesuch key here"});
1954 Then that element "autovivifies"; that is, it springs into existence
1955 whether you store something there or not. That's because functions
1956 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1957 it has to be ready to write it back into the caller's version.
1959 This has been fixed as of Perl5.004.
1961 Normally, merely accessing a key's value for a nonexistent key does
1962 I<not> cause that key to be forever there. This is different than
1965 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1967 Usually a hash ref, perhaps like this:
1972 TITLE => "deputy peon",
1975 PALS => [ "Norbert", "Rhys", "Phineas"],
1978 References are documented in L<perlref> and the upcoming L<perlreftut>.
1979 Examples of complex data structures are given in L<perldsc> and
1980 L<perllol>. Examples of structures and object-oriented classes are
1983 =head2 How can I use a reference as a hash key?
1985 You can't do this directly, but you could use the standard Tie::RefHash
1986 module distributed with Perl.
1990 =head2 How do I handle binary data correctly?
1992 Perl is binary clean, so this shouldn't be a problem. For example,
1993 this works fine (assuming the files are found):
1995 if (`cat /vmunix` =~ /gzip/) {
1996 print "Your kernel is GNU-zip enabled!\n";
1999 On less elegant (read: Byzantine) systems, however, you have
2000 to play tedious games with "text" versus "binary" files. See
2001 L<perlfunc/"binmode"> or L<perlopentut>.
2003 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
2005 If you want to deal with multibyte characters, however, there are
2006 some gotchas. See the section on Regular Expressions.
2008 =head2 How do I determine whether a scalar is a number/whole/integer/float?
2010 Assuming that you don't care about IEEE notations like "NaN" or
2011 "Infinity", you probably just want to use a regular expression.
2013 if (/\D/) { print "has nondigits\n" }
2014 if (/^\d+$/) { print "is a whole number\n" }
2015 if (/^-?\d+$/) { print "is an integer\n" }
2016 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2017 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2018 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2019 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2020 { print "a C float\n" }
2022 There are also some commonly used modules for the task.
2023 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2024 internal function C<looks_like_number> for determining
2025 whether a variable looks like a number. L<Data::Types>
2026 exports functions that validate data types using both the
2027 above and other regular expressions. Thirdly, there is
2028 C<Regexp::Common> which has regular expressions to match
2029 various types of numbers. Those three modules are available
2032 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2033 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
2034 wrapper function for more convenient access. This function takes
2035 a string and returns the number it found, or C<undef> for input that
2036 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
2037 if you just want to say, "Is this a float?"
2040 use POSIX qw(strtod);
2045 my($num, $unparsed) = strtod($str);
2046 if (($str eq '') || ($unparsed != 0) || $!) {
2053 sub is_numeric { defined getnum($_[0]) }
2055 Or you could check out the L<String::Scanf> module on the CPAN
2056 instead. The POSIX module (part of the standard Perl distribution) provides
2057 the C<strtod> and C<strtol> for converting strings to double and longs,
2060 =head2 How do I keep persistent data across program calls?
2062 For some specific applications, you can use one of the DBM modules.
2063 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2064 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2065 of the standard distribution. Here's one example using Storable's C<store>
2066 and C<retrieve> functions:
2069 store(\%hash, "filename");
2072 $href = retrieve("filename"); # by ref
2073 %hash = %{ retrieve("filename") }; # direct to hash
2075 =head2 How do I print out or copy a recursive data structure?
2077 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2078 for printing out data structures. The Storable module on CPAN (or the
2079 5.8 release of Perl), provides a function called C<dclone> that recursively
2080 copies its argument.
2082 use Storable qw(dclone);
2085 Where $r1 can be a reference to any kind of data structure you'd like.
2086 It will be deeply copied. Because C<dclone> takes and returns references,
2087 you'd have to add extra punctuation if you had a hash of arrays that
2090 %newhash = %{ dclone(\%oldhash) };
2092 =head2 How do I define methods for every class/object?
2094 Use the UNIVERSAL class (see L<UNIVERSAL>).
2096 =head2 How do I verify a credit card checksum?
2098 Get the Business::CreditCard module from CPAN.
2100 =head2 How do I pack arrays of doubles or floats for XS code?
2102 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2103 If you're doing a lot of float or double processing, consider using
2104 the PDL module from CPAN instead--it makes number-crunching easy.
2106 =head1 AUTHOR AND COPYRIGHT
2108 Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
2109 other authors as noted. All rights reserved.
2111 This documentation is free; you can redistribute it and/or modify it
2112 under the same terms as Perl itself.
2114 Irrespective of its distribution, all code examples in this file
2115 are hereby placed into the public domain. You are permitted and
2116 encouraged to use this code in your own programs for fun
2117 or for profit as you see fit. A simple comment in the code giving
2118 credit would be courteous but is not required.