3 perlfaq4 - Data Manipulation ($Revision: 1.73 $, $Date: 2005/12/31 00:54:37 $)
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 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 The C<Interpolation> module can also do a lot of magic for you. You can
615 specify a variable name, in this case C<E>, to set up a tied hash that
616 does the interpolation for you. It has several other methods to do this
619 use Interpolation E => 'eval';
620 print "The time values are $E{localtime()}.\n";
622 In most cases, it is probably easier to simply use string concatenation,
623 which also forces scalar context.
625 print "The time is " . localtime . ".\n";
627 =head2 How do I find matching/nesting anything?
629 This isn't something that can be done in one regular expression, no
630 matter how complicated. To find something between two single
631 characters, a pattern like C</x([^x]*)x/> will get the intervening
632 bits in $1. For multiple ones, then something more like
633 C</alpha(.*?)omega/> would be needed. But none of these deals with
634 nested patterns. For balanced expressions using C<(>, C<{>, C<[> or
635 C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
636 L<perlre/(??{ code })>. For other cases, you'll have to write a
639 If you are serious about writing a parser, there are a number of
640 modules or oddities that will make your life a lot easier. There are
641 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
642 and the byacc program. Starting from perl 5.8 the Text::Balanced is
643 part of the standard distribution.
645 One simple destructive, inside-out approach that you might try is to
646 pull out the smallest nesting parts one at a time:
648 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
649 # do something with $1
652 A more complicated and sneaky approach is to make Perl's regular
653 expression engine do it for you. This is courtesy Dean Inada, and
654 rather has the nature of an Obfuscated Perl Contest entry, but it
657 # $_ contains the string to parse
658 # BEGIN and END are the opening and closing markers for the
663 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
664 @$ = (eval{/$re/},$@!~/unmatched/i);
665 print join("\n",@$[0..$#$]) if( $$[-1] );
667 =head2 How do I reverse a string?
669 Use reverse() in scalar context, as documented in
672 $reversed = reverse $string;
674 =head2 How do I expand tabs in a string?
676 You can do it yourself:
678 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
680 Or you can just use the Text::Tabs module (part of the standard Perl
684 @expanded_lines = expand(@lines_with_tabs);
686 =head2 How do I reformat a paragraph?
688 Use Text::Wrap (part of the standard Perl distribution):
691 print wrap("\t", ' ', @paragraphs);
693 The paragraphs you give to Text::Wrap should not contain embedded
694 newlines. Text::Wrap doesn't justify the lines (flush-right).
696 Or use the CPAN module Text::Autoformat. Formatting files can be easily
697 done by making a shell alias, like so:
699 alias fmt="perl -i -MText::Autoformat -n0777 \
700 -e 'print autoformat $_, {all=>1}' $*"
702 See the documentation for Text::Autoformat to appreciate its many
705 =head2 How can I access or change N characters of a string?
707 You can access the first characters of a string with substr().
708 To get the first character, for example, start at position 0
709 and grab the string of length 1.
712 $string = "Just another Perl Hacker";
713 $first_char = substr( $string, 0, 1 ); # 'J'
715 To change part of a string, you can use the optional fourth
716 argument which is the replacement string.
718 substr( $string, 13, 4, "Perl 5.8.0" );
720 You can also use substr() as an lvalue.
722 substr( $string, 13, 4 ) = "Perl 5.8.0";
724 =head2 How do I change the Nth occurrence of something?
726 You have to keep track of N yourself. For example, let's say you want
727 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
728 C<"whosoever"> or C<"whomsoever">, case insensitively. These
729 all assume that $_ contains the string to be altered.
733 ++$count == 5 # is it the 5th?
734 ? "${2}soever" # yes, swap
735 : $1 # renege and leave it there
738 In the more general case, you can use the C</g> modifier in a C<while>
739 loop, keeping count of matches.
743 $_ = "One fish two fish red fish blue fish";
744 while (/(\w+)\s+fish\b/gi) {
745 if (++$count == $WANT) {
746 print "The third fish is a $1 one.\n";
750 That prints out: C<"The third fish is a red one."> You can also use a
751 repetition count and repeated pattern like this:
753 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
755 =head2 How can I count the number of occurrences of a substring within a string?
757 There are a number of ways, with varying efficiency. If you want a
758 count of a certain single character (X) within a string, you can use the
759 C<tr///> function like so:
761 $string = "ThisXlineXhasXsomeXx'sXinXit";
762 $count = ($string =~ tr/X//);
763 print "There are $count X characters in the string";
765 This is fine if you are just looking for a single character. However,
766 if you are trying to count multiple character substrings within a
767 larger string, C<tr///> won't work. What you can do is wrap a while()
768 loop around a global pattern match. For example, let's count negative
771 $string = "-9 55 48 -2 23 -76 4 14 -44";
772 while ($string =~ /-\d+/g) { $count++ }
773 print "There are $count negative numbers in the string";
775 Another version uses a global match in list context, then assigns the
776 result to a scalar, producing a count of the number of matches.
778 $count = () = $string =~ /-\d+/g;
780 =head2 How do I capitalize all the words on one line?
782 To make the first letter of each word upper case:
784 $line =~ s/\b(\w)/\U$1/g;
786 This has the strange effect of turning "C<don't do it>" into "C<Don'T
787 Do It>". Sometimes you might want this. Other times you might need a
788 more thorough solution (Suggested by brian d foy):
791 (^\w) #at the beginning of the line
793 (\s\w) #preceded by whitespace
796 $string =~ /([\w']+)/\u\L$1/g;
798 To make the whole line upper case:
802 To force each word to be lower case, with the first letter upper case:
804 $line =~ s/(\w+)/\u\L$1/g;
806 You can (and probably should) enable locale awareness of those
807 characters by placing a C<use locale> pragma in your program.
808 See L<perllocale> for endless details on locales.
810 This is sometimes referred to as putting something into "title
811 case", but that's not quite accurate. Consider the proper
812 capitalization of the movie I<Dr. Strangelove or: How I Learned to
813 Stop Worrying and Love the Bomb>, for example.
815 Damian Conway's L<Text::Autoformat> module provides some smart
816 case transformations:
818 use Text::Autoformat;
819 my $x = "Dr. Strangelove or: How I Learned to Stop ".
820 "Worrying and Love the Bomb";
823 for my $style (qw( sentence title highlight ))
825 print autoformat($x, { case => $style }), "\n";
828 =head2 How can I split a [character] delimited string except when inside [character]?
830 Several modules can handle this sort of pasing---Text::Balanced,
831 Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.
833 Take the example case of trying to split a string that is
834 comma-separated into its different fields. You can't use C<split(/,/)>
835 because you shouldn't split if the comma is inside quotes. For
836 example, take a data line like this:
838 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
840 Due to the restriction of the quotes, this is a fairly complex
841 problem. Thankfully, we have Jeffrey Friedl, author of
842 I<Mastering Regular Expressions>, to handle these for us. He
843 suggests (assuming your string is contained in $text):
846 push(@new, $+) while $text =~ m{
847 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
851 push(@new, undef) if substr($text,-1,1) eq ',';
853 If you want to represent quotation marks inside a
854 quotation-mark-delimited field, escape them with backslashes (eg,
857 Alternatively, the Text::ParseWords module (part of the standard Perl
858 distribution) lets you say:
860 use Text::ParseWords;
861 @new = quotewords(",", 0, $text);
863 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
865 =head2 How do I strip blank space from the beginning/end of a string?
867 (contributed by brian d foy)
869 A substitution can do this for you. For a single line, you want to
870 replace all the leading or trailing whitespace with nothing. You
871 can do that with a pair of substitutions.
876 You can also write that as a single substitution, although it turns
877 out the combined statement is slower than the separate ones. That
878 might not matter to you, though.
882 In this regular expression, the alternation matches either at the
883 beginning or the end of the string since the anchors have a lower
884 precedence than the alternation. With the C</g> flag, the substitution
885 makes all possible matches, so it gets both. Remember, the trailing
886 newline matches the C<\s+>, and the C<$> anchor can match to the
887 physical end of the string, so the newline disappears too. Just add
888 the newline to the output, which has the added benefit of preserving
889 "blank" (consisting entirely of whitespace) lines which the C<^\s+>
890 would remove all by itself.
898 For a multi-line string, you can apply the regular expression
899 to each logical line in the string by adding the C</m> flag (for
900 "multi-line"). With the C</m> flag, the C<$> matches I<before> an
901 embedded newline, so it doesn't remove it. It still removes the
902 newline at the end of the string.
904 $string =~ s/^\s+|\s+$//gm;
906 Remember that lines consisting entirely of whitespace will disappear,
907 since the first part of the alternation can match the entire string
908 and replace it with nothing. If need to keep embedded blank lines,
909 you have to do a little more work. Instead of matching any whitespace
910 (since that includes a newline), just match the other whitespace.
912 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
914 =head2 How do I pad a string with blanks or pad a number with zeroes?
916 In the following examples, C<$pad_len> is the length to which you wish
917 to pad the string, C<$text> or C<$num> contains the string to be padded,
918 and C<$pad_char> contains the padding character. You can use a single
919 character string constant instead of the C<$pad_char> variable if you
920 know what it is in advance. And in the same way you can use an integer in
921 place of C<$pad_len> if you know the pad length in advance.
923 The simplest method uses the C<sprintf> function. It can pad on the left
924 or right with blanks and on the left with zeroes and it will not
925 truncate the result. The C<pack> function can only pad strings on the
926 right with blanks and it will truncate the result to a maximum length of
929 # Left padding a string with blanks (no truncation):
930 $padded = sprintf("%${pad_len}s", $text);
931 $padded = sprintf("%*s", $pad_len, $text); # same thing
933 # Right padding a string with blanks (no truncation):
934 $padded = sprintf("%-${pad_len}s", $text);
935 $padded = sprintf("%-*s", $pad_len, $text); # same thing
937 # Left padding a number with 0 (no truncation):
938 $padded = sprintf("%0${pad_len}d", $num);
939 $padded = sprintf("%0*d", $pad_len, $num); # same thing
941 # Right padding a string with blanks using pack (will truncate):
942 $padded = pack("A$pad_len",$text);
944 If you need to pad with a character other than blank or zero you can use
945 one of the following methods. They all generate a pad string with the
946 C<x> operator and combine that with C<$text>. These methods do
947 not truncate C<$text>.
949 Left and right padding with any character, creating a new string:
951 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
952 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
954 Left and right padding with any character, modifying C<$text> directly:
956 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
957 $text .= $pad_char x ( $pad_len - length( $text ) );
959 =head2 How do I extract selected columns from a string?
961 Use substr() or unpack(), both documented in L<perlfunc>.
962 If you prefer thinking in terms of columns instead of widths,
963 you can use this kind of thing:
965 # determine the unpack format needed to split Linux ps output
966 # arguments are cut columns
967 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
973 for my $place (@positions) {
974 $template .= "A" . ($place - $lastpos) . " ";
981 =head2 How do I find the soundex value of a string?
983 (contributed by brian d foy)
985 You can use the Text::Soundex module. If you want to do fuzzy or close
986 matching, you might also try the String::Approx, and Text::Metaphone,
987 and Text::DoubleMetaphone modules.
989 =head2 How can I expand variables in text strings?
991 Let's assume that you have a string that contains placeholder
994 $text = 'this has a $foo in it and a $bar';
996 You can use a substitution with a double evaluation. The
997 first /e turns C<$1> into C<$foo>, and the second /e turns
998 C<$foo> into its value. You may want to wrap this in an
999 C<eval>: if you try to get the value of an undeclared variable
1000 while running under C<use strict>, you get a fatal error.
1002 eval { $text =~ s/(\$\w+)/$1/eeg };
1005 It's probably better in the general case to treat those
1006 variables as entries in some special hash. For example:
1012 $text =~ s/\$(\w+)/$user_defs{$1}/g;
1014 =head2 What's wrong with always quoting "$vars"?
1016 The problem is that those double-quotes force stringification--
1017 coercing numbers and references into strings--even when you
1018 don't want them to be strings. Think of it this way: double-quote
1019 expansion is used to produce new strings. If you already
1020 have a string, why do you need more?
1022 If you get used to writing odd things like these:
1025 $new = "$old"; # BAD
1026 somefunc("$var"); # BAD
1028 You'll be in trouble. Those should (in 99.8% of the cases) be
1029 the simpler and more direct:
1035 Otherwise, besides slowing you down, you're going to break code when
1036 the thing in the scalar is actually neither a string nor a number, but
1042 my $oref = "$aref"; # WRONG
1045 You can also get into subtle problems on those few operations in Perl
1046 that actually do care about the difference between a string and a
1047 number, such as the magical C<++> autoincrement operator or the
1050 Stringification also destroys arrays.
1053 print "@lines"; # WRONG - extra blanks
1054 print @lines; # right
1056 =head2 Why don't my E<lt>E<lt>HERE documents work?
1058 Check for these three things:
1062 =item There must be no space after the E<lt>E<lt> part.
1064 =item There (probably) should be a semicolon at the end.
1066 =item You can't (easily) have any space in front of the tag.
1070 If you want to indent the text in the here document, you
1074 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1079 But the HERE_TARGET must still be flush against the margin.
1080 If you want that indented also, you'll have to quote
1083 ($quote = <<' FINIS') =~ s/^\s+//gm;
1084 ...we will have peace, when you and all your works have
1085 perished--and the works of your dark master to whom you
1086 would deliver us. You are a liar, Saruman, and a corrupter
1087 of men's hearts. --Theoden in /usr/src/perl/taint.c
1089 $quote =~ s/\s+--/\n--/;
1091 A nice general-purpose fixer-upper function for indented here documents
1092 follows. It expects to be called with a here document as its argument.
1093 It looks to see whether each line begins with a common substring, and
1094 if so, strips that substring off. Otherwise, it takes the amount of leading
1095 whitespace found on the first line and removes that much off each
1100 my ($white, $leader); # common whitespace and common leading string
1101 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1102 ($white, $leader) = ($2, quotemeta($1));
1104 ($white, $leader) = (/^(\s+)/, '');
1106 s/^\s*?$leader(?:$white)?//gm;
1110 This works with leading special strings, dynamically determined:
1112 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1115 @@@ SAVEI32(runlevel);
1117 @@@ while ( op = (*op->op_ppaddr)() );
1121 MAIN_INTERPRETER_LOOP
1123 Or with a fixed amount of leading whitespace, with remaining
1124 indentation correctly preserved:
1126 $poem = fix<<EVER_ON_AND_ON;
1127 Now far ahead the Road has gone,
1128 And I must follow, if I can,
1129 Pursuing it with eager feet,
1130 Until it joins some larger way
1131 Where many paths and errands meet.
1132 And whither then? I cannot say.
1133 --Bilbo in /usr/src/perl/pp_ctl.c
1138 =head2 What is the difference between a list and an array?
1140 An array has a changeable length. A list does not. An array is something
1141 you can push or pop, while a list is a set of values. Some people make
1142 the distinction that a list is a value while an array is a variable.
1143 Subroutines are passed and return lists, you put things into list
1144 context, you initialize arrays with lists, and you foreach() across
1145 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1146 in scalar context behave like the number of elements in them, subroutines
1147 access their arguments through the array C<@_>, and push/pop/shift only work
1150 As a side note, there's no such thing as a list in scalar context.
1153 $scalar = (2, 5, 7, 9);
1155 you're using the comma operator in scalar context, so it uses the scalar
1156 comma operator. There never was a list there at all! This causes the
1157 last value to be returned: 9.
1159 =head2 What is the difference between $array[1] and @array[1]?
1161 The former is a scalar value; the latter an array slice, making
1162 it a list with one (scalar) value. You should use $ when you want a
1163 scalar value (most of the time) and @ when you want a list with one
1164 scalar value in it (very, very rarely; nearly never, in fact).
1166 Sometimes it doesn't make a difference, but sometimes it does.
1167 For example, compare:
1169 $good[0] = `some program that outputs several lines`;
1173 @bad[0] = `same program that outputs several lines`;
1175 The C<use warnings> pragma and the B<-w> flag will warn you about these
1178 =head2 How can I remove duplicate elements from a list or array?
1180 (contributed by brian d foy)
1182 Use a hash. When you think the words "unique" or "duplicated", think
1185 If you don't care about the order of the elements, you could just
1186 create the hash then extract the keys. It's not important how you
1187 create that hash: just that you use C<keys> to get the unique
1190 my %hash = map { $_, 1 } @array;
1191 # or a hash slice: @hash{ @array } = ();
1192 # or a foreach: $hash{$_} = 1 foreach ( @array );
1194 my @unique = keys %hash;
1196 You can also go through each element and skip the ones you've seen
1197 before. Use a hash to keep track. The first time the loop sees an
1198 element, that element has no key in C<%Seen>. The C<next> statement
1199 creates the key and immediately uses its value, which is C<undef>, so
1200 the loop continues to the C<push> and increments the value for that
1201 key. The next time the loop sees that same element, its key exists in
1202 the hash I<and> the value for that key is true (since it's not 0 or
1203 undef), so the next skips that iteration and the loop goes to the next
1209 foreach my $elem ( @array )
1211 next if $seen{ $elem }++;
1212 push @unique, $elem;
1215 You can write this more briefly using a grep, which does the
1219 my @unique = grep { ! $seen{ $_ }++ } @array;
1221 =head2 How can I tell whether a certain element is contained in a list or array?
1223 (portions of this answer contributed by Anno Siegel)
1225 Hearing the word "in" is an I<in>dication that you probably should have
1226 used a hash, not a list or array, to store your data. Hashes are
1227 designed to answer this question quickly and efficiently. Arrays aren't.
1229 That being said, there are several ways to approach this. If you
1230 are going to make this query many times over arbitrary string values,
1231 the fastest way is probably to invert the original array and maintain a
1232 hash whose keys are the first array's values.
1234 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1236 for (@blues) { $is_blue{$_} = 1 }
1238 Now you can check whether $is_blue{$some_color}. It might have been a
1239 good idea to keep the blues all in a hash in the first place.
1241 If the values are all small integers, you could use a simple indexed
1242 array. This kind of an array will take up less space:
1244 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1245 @is_tiny_prime = ();
1246 for (@primes) { $is_tiny_prime[$_] = 1 }
1247 # or simply @istiny_prime[@primes] = (1) x @primes;
1249 Now you check whether $is_tiny_prime[$some_number].
1251 If the values in question are integers instead of strings, you can save
1252 quite a lot of space by using bit strings instead:
1254 @articles = ( 1..10, 150..2000, 2017 );
1256 for (@articles) { vec($read,$_,1) = 1 }
1258 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1260 These methods guarantee fast individual tests but require a re-organization
1261 of the original list or array. They only pay off if you have to test
1262 multiple values against the same array.
1264 If you are testing only once, the standard module List::Util exports
1265 the function C<first> for this purpose. It works by stopping once it
1266 finds the element. It's written in C for speed, and its Perl equivalant
1267 looks like this subroutine:
1272 return $_ if &{$code}();
1277 If speed is of little concern, the common idiom uses grep in scalar context
1278 (which returns the number of items that passed its condition) to traverse the
1279 entire list. This does have the benefit of telling you how many matches it
1282 my $is_there = grep $_ eq $whatever, @array;
1284 If you want to actually extract the matching elements, simply use grep in
1287 my @matches = grep $_ eq $whatever, @array;
1289 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1291 Use a hash. Here's code to do both and more. It assumes that
1292 each element is unique in a given array:
1294 @union = @intersection = @difference = ();
1296 foreach $element (@array1, @array2) { $count{$element}++ }
1297 foreach $element (keys %count) {
1298 push @union, $element;
1299 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1302 Note that this is the I<symmetric difference>, that is, all elements in
1303 either A or in B but not in both. Think of it as an xor operation.
1305 =head2 How do I test whether two arrays or hashes are equal?
1307 The following code works for single-level arrays. It uses a stringwise
1308 comparison, and does not distinguish defined versus undefined empty
1309 strings. Modify if you have other needs.
1311 $are_equal = compare_arrays(\@frogs, \@toads);
1313 sub compare_arrays {
1314 my ($first, $second) = @_;
1315 no warnings; # silence spurious -w undef complaints
1316 return 0 unless @$first == @$second;
1317 for (my $i = 0; $i < @$first; $i++) {
1318 return 0 if $first->[$i] ne $second->[$i];
1323 For multilevel structures, you may wish to use an approach more
1324 like this one. It uses the CPAN module FreezeThaw:
1326 use FreezeThaw qw(cmpStr);
1327 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1329 printf "a and b contain %s arrays\n",
1330 cmpStr(\@a, \@b) == 0
1334 This approach also works for comparing hashes. Here
1335 we'll demonstrate two different answers:
1337 use FreezeThaw qw(cmpStr cmpStrHard);
1339 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1343 printf "a and b contain %s hashes\n",
1344 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1346 printf "a and b contain %s hashes\n",
1347 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1350 The first reports that both those the hashes contain the same data,
1351 while the second reports that they do not. Which you prefer is left as
1352 an exercise to the reader.
1354 =head2 How do I find the first array element for which a condition is true?
1356 To find the first array element which satisfies a condition, you can
1357 use the first() function in the List::Util module, which comes with
1358 Perl 5.8. This example finds the first element that contains "Perl".
1360 use List::Util qw(first);
1362 my $element = first { /Perl/ } @array;
1364 If you cannot use List::Util, you can make your own loop to do the
1365 same thing. Once you find the element, you stop the loop with last.
1370 if( /Perl/ ) { $found = $_; last }
1373 If you want the array index, you can iterate through the indices
1374 and check the array element at each index until you find one
1375 that satisfies the condition.
1377 my( $found, $index ) = ( undef, -1 );
1378 for( $i = 0; $i < @array; $i++ )
1380 if( $array[$i] =~ /Perl/ )
1382 $found = $array[$i];
1388 =head2 How do I handle linked lists?
1390 In general, you usually don't need a linked list in Perl, since with
1391 regular arrays, you can push and pop or shift and unshift at either end,
1392 or you can use splice to add and/or remove arbitrary number of elements at
1393 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1394 dynamic arrays. In the absence of shifts and pops, push in general
1395 needs to reallocate on the order every log(N) times, and unshift will
1396 need to copy pointers each time.
1398 If you really, really wanted, you could use structures as described in
1399 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1400 to do. For example, imagine a list node like this:
1407 You could walk the list this way:
1410 for ($node = $head; $node; $node = $node->{LINK}) {
1411 print $node->{VALUE}, " ";
1415 You could add to the list this way:
1418 $tail = append($head, 1); # grow a new head
1419 for $value ( 2 .. 10 ) {
1420 $tail = append($tail, $value);
1424 my($list, $value) = @_;
1425 my $node = { VALUE => $value };
1427 $node->{LINK} = $list->{LINK};
1428 $list->{LINK} = $node;
1430 $_[0] = $node; # replace caller's version
1435 But again, Perl's built-in are virtually always good enough.
1437 =head2 How do I handle circular lists?
1439 Circular lists could be handled in the traditional fashion with linked
1440 lists, or you could just do something like this with an array:
1442 unshift(@array, pop(@array)); # the last shall be first
1443 push(@array, shift(@array)); # and vice versa
1445 =head2 How do I shuffle an array randomly?
1447 If you either have Perl 5.8.0 or later installed, or if you have
1448 Scalar-List-Utils 1.03 or later installed, you can say:
1450 use List::Util 'shuffle';
1452 @shuffled = shuffle(@list);
1454 If not, you can use a Fisher-Yates shuffle.
1456 sub fisher_yates_shuffle {
1457 my $deck = shift; # $deck is a reference to an array
1460 my $j = int rand ($i+1);
1461 @$deck[$i,$j] = @$deck[$j,$i];
1465 # shuffle my mpeg collection
1467 my @mpeg = <audio/*/*.mp3>;
1468 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1471 Note that the above implementation shuffles an array in place,
1472 unlike the List::Util::shuffle() which takes a list and returns
1473 a new shuffled list.
1475 You've probably seen shuffling algorithms that work using splice,
1476 randomly picking another element to swap the current element with
1480 @old = 1 .. 10; # just a demo
1482 push(@new, splice(@old, rand @old, 1));
1485 This is bad because splice is already O(N), and since you do it N times,
1486 you just invented a quadratic algorithm; that is, O(N**2). This does
1487 not scale, although Perl is so efficient that you probably won't notice
1488 this until you have rather largish arrays.
1490 =head2 How do I process/modify each element of an array?
1492 Use C<for>/C<foreach>:
1495 s/foo/bar/; # change that word
1496 tr/XZ/ZX/; # swap those letters
1499 Here's another; let's compute spherical volumes:
1501 for (@volumes = @radii) { # @volumes has changed parts
1503 $_ *= (4/3) * 3.14159; # this will be constant folded
1506 which can also be done with map() which is made to transform
1507 one list into another:
1509 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1511 If you want to do the same thing to modify the values of the
1512 hash, you can use the C<values> function. As of Perl 5.6
1513 the values are not copied, so if you modify $orbit (in this
1514 case), you modify the value.
1516 for $orbit ( values %orbits ) {
1517 ($orbit **= 3) *= (4/3) * 3.14159;
1520 Prior to perl 5.6 C<values> returned copies of the values,
1521 so older perl code often contains constructions such as
1522 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1523 the hash is to be modified.
1525 =head2 How do I select a random element from an array?
1527 Use the rand() function (see L<perlfunc/rand>):
1529 $index = rand @array;
1530 $element = $array[$index];
1533 my $element = $array[ rand @array ];
1535 =head2 How do I permute N elements of a list?
1537 Use the List::Permutor module on CPAN. If the list is
1538 actually an array, try the Algorithm::Permute module (also
1539 on CPAN). It's written in XS code and is very efficient.
1541 use Algorithm::Permute;
1542 my @array = 'a'..'d';
1543 my $p_iterator = Algorithm::Permute->new ( \@array );
1544 while (my @perm = $p_iterator->next) {
1545 print "next permutation: (@perm)\n";
1548 For even faster execution, you could do:
1550 use Algorithm::Permute;
1551 my @array = 'a'..'d';
1552 Algorithm::Permute::permute {
1553 print "next permutation: (@array)\n";
1556 Here's a little program that generates all permutations of
1557 all the words on each line of input. The algorithm embodied
1558 in the permute() function is discussed in Volume 4 (still
1559 unpublished) of Knuth's I<The Art of Computer Programming>
1560 and will work on any list:
1563 # Fischer-Kause ordered permutation generator
1568 while ( $code->(@_[@idx]) ) {
1570 --$p while $idx[$p-1] > $idx[$p];
1571 my $q = $p or return;
1572 push @idx, reverse splice @idx, $p;
1573 ++$q while $idx[$p-1] > $idx[$q];
1574 @idx[$p-1,$q]=@idx[$q,$p-1];
1578 permute {print"@_\n"} split;
1580 =head2 How do I sort an array by (anything)?
1582 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1584 @list = sort { $a <=> $b } @list;
1586 The default sort function is cmp, string comparison, which would
1587 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1588 the numerical comparison operator.
1590 If you have a complicated function needed to pull out the part you
1591 want to sort on, then don't do it inside the sort function. Pull it
1592 out first, because the sort BLOCK can be called many times for the
1593 same element. Here's an example of how to pull out the first word
1594 after the first number on each item, and then sort those words
1599 ($item) = /\d+\s*(\S+)/;
1600 push @idx, uc($item);
1602 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1604 which could also be written this way, using a trick
1605 that's come to be known as the Schwartzian Transform:
1607 @sorted = map { $_->[0] }
1608 sort { $a->[1] cmp $b->[1] }
1609 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1611 If you need to sort on several fields, the following paradigm is useful.
1613 @sorted = sort { field1($a) <=> field1($b) ||
1614 field2($a) cmp field2($b) ||
1615 field3($a) cmp field3($b)
1618 This can be conveniently combined with precalculation of keys as given
1621 See the F<sort> article in the "Far More Than You Ever Wanted
1622 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1623 more about this approach.
1625 See also the question below on sorting hashes.
1627 =head2 How do I manipulate arrays of bits?
1629 Use pack() and unpack(), or else vec() and the bitwise operations.
1631 For example, this sets $vec to have bit N set if $ints[N] was set:
1634 foreach(@ints) { vec($vec,$_,1) = 1 }
1636 Here's how, given a vector in $vec, you can
1637 get those bits into your @ints array:
1639 sub bitvec_to_list {
1642 # Find null-byte density then select best algorithm
1643 if ($vec =~ tr/\0// / length $vec > 0.95) {
1646 # This method is faster with mostly null-bytes
1647 while($vec =~ /[^\0]/g ) {
1648 $i = -9 + 8 * pos $vec;
1649 push @ints, $i if vec($vec, ++$i, 1);
1650 push @ints, $i if vec($vec, ++$i, 1);
1651 push @ints, $i if vec($vec, ++$i, 1);
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);
1659 # This method is a fast general algorithm
1661 my $bits = unpack "b*", $vec;
1662 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1663 push @ints, pos $bits while($bits =~ /1/g);
1668 This method gets faster the more sparse the bit vector is.
1669 (Courtesy of Tim Bunce and Winfried Koenig.)
1671 You can make the while loop a lot shorter with this suggestion
1672 from Benjamin Goldberg:
1674 while($vec =~ /[^\0]+/g ) {
1675 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1678 Or use the CPAN module Bit::Vector:
1680 $vector = Bit::Vector->new($num_of_bits);
1681 $vector->Index_List_Store(@ints);
1682 @ints = $vector->Index_List_Read();
1684 Bit::Vector provides efficient methods for bit vector, sets of small integers
1687 Here's a more extensive illustration using vec():
1690 $vector = "\xff\x0f\xef\xfe";
1691 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1692 unpack("N", $vector), "\n";
1693 $is_set = vec($vector, 23, 1);
1694 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1712 my ($offset, $width, $value) = @_;
1714 vec($vector, $offset, $width) = $value;
1715 print "offset=$offset width=$width value=$value\n";
1721 my $bits = unpack("b*", $vector);
1725 print "vector length in bytes: ", length($vector), "\n";
1726 @bytes = unpack("A8" x length($vector), $bits);
1727 print "bits are: @bytes\n\n";
1730 =head2 Why does defined() return true on empty arrays and hashes?
1732 The short story is that you should probably only use defined on scalars or
1733 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1734 in the 5.004 release or later of Perl for more detail.
1736 =head1 Data: Hashes (Associative Arrays)
1738 =head2 How do I process an entire hash?
1740 Use the each() function (see L<perlfunc/each>) if you don't care
1741 whether it's sorted:
1743 while ( ($key, $value) = each %hash) {
1744 print "$key = $value\n";
1747 If you want it sorted, you'll have to use foreach() on the result of
1748 sorting the keys as shown in an earlier question.
1750 =head2 What happens if I add or remove keys from a hash while iterating over it?
1752 (contributed by brian d foy)
1754 The easy answer is "Don't do that!"
1756 If you iterate through the hash with each(), you can delete the key
1757 most recently returned without worrying about it. If you delete or add
1758 other keys, the iterator may skip or double up on them since perl
1759 may rearrange the hash table. See the
1760 entry for C<each()> in L<perlfunc>.
1762 =head2 How do I look up a hash element by value?
1764 Create a reverse hash:
1766 %by_value = reverse %by_key;
1767 $key = $by_value{$value};
1769 That's not particularly efficient. It would be more space-efficient
1772 while (($key, $value) = each %by_key) {
1773 $by_value{$value} = $key;
1776 If your hash could have repeated values, the methods above will only find
1777 one of the associated keys. This may or may not worry you. If it does
1778 worry you, you can always reverse the hash into a hash of arrays instead:
1780 while (($key, $value) = each %by_key) {
1781 push @{$key_list_by_value{$value}}, $key;
1784 =head2 How can I know how many entries are in a hash?
1786 If you mean how many keys, then all you have to do is
1787 use the keys() function in a scalar context:
1789 $num_keys = keys %hash;
1791 The keys() function also resets the iterator, which means that you may
1792 see strange results if you use this between uses of other hash operators
1795 =head2 How do I sort a hash (optionally by value instead of key)?
1797 (contributed by brian d foy)
1799 To sort a hash, start with the keys. In this example, we give the list of
1800 keys to the sort function which then compares them ASCIIbetically (which
1801 might be affected by your locale settings). The output list has the keys
1802 in ASCIIbetical order. Once we have the keys, we can go through them to
1803 create a report which lists the keys in ASCIIbetical order.
1805 my @keys = sort { $a cmp $b } keys %hash;
1807 foreach my $key ( @keys )
1809 printf "%-20s %6d\n", $key, $hash{$value};
1812 We could get more fancy in the C<sort()> block though. Instead of
1813 comparing the keys, we can compute a value with them and use that
1814 value as the comparison.
1816 For instance, to make our report order case-insensitive, we use
1817 the C<\L> sequence in a double-quoted string to make everything
1818 lowercase. The C<sort()> block then compares the lowercased
1819 values to determine in which order to put the keys.
1821 my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
1823 Note: if the computation is expensive or the hash has many elements,
1824 you may want to look at the Schwartzian Transform to cache the
1825 computation results.
1827 If we want to sort by the hash value instead, we use the hash key
1828 to look it up. We still get out a list of keys, but this time they
1829 are ordered by their value.
1831 my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
1833 From there we can get more complex. If the hash values are the same,
1834 we can provide a secondary sort on the hash key.
1837 $hash{$a} <=> $hash{$b}
1842 =head2 How can I always keep my hash sorted?
1844 You can look into using the DB_File module and tie() using the
1845 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1846 The Tie::IxHash module from CPAN might also be instructive.
1848 =head2 What's the difference between "delete" and "undef" with hashes?
1850 Hashes contain pairs of scalars: the first is the key, the
1851 second is the value. The key will be coerced to a string,
1852 although the value can be any kind of scalar: string,
1853 number, or reference. If a key $key is present in
1854 %hash, C<exists($hash{$key})> will return true. The value
1855 for a given key can be C<undef>, in which case
1856 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1857 will return true. This corresponds to (C<$key>, C<undef>)
1860 Pictures help... here's the %hash table:
1870 And these conditions hold
1874 defined $hash{'d'} is true
1875 defined $hash{'a'} is true
1876 exists $hash{'a'} is true (Perl5 only)
1877 grep ($_ eq 'a', keys %hash) is true
1883 your table now reads:
1894 and these conditions now hold; changes in caps:
1898 defined $hash{'d'} is true
1899 defined $hash{'a'} is FALSE
1900 exists $hash{'a'} is true (Perl5 only)
1901 grep ($_ eq 'a', keys %hash) is true
1903 Notice the last two: you have an undef value, but a defined key!
1909 your table now reads:
1918 and these conditions now hold; changes in caps:
1922 defined $hash{'d'} is true
1923 defined $hash{'a'} is false
1924 exists $hash{'a'} is FALSE (Perl5 only)
1925 grep ($_ eq 'a', keys %hash) is FALSE
1927 See, the whole entry is gone!
1929 =head2 Why don't my tied hashes make the defined/exists distinction?
1931 This depends on the tied hash's implementation of EXISTS().
1932 For example, there isn't the concept of undef with hashes
1933 that are tied to DBM* files. It also means that exists() and
1934 defined() do the same thing with a DBM* file, and what they
1935 end up doing is not what they do with ordinary hashes.
1937 =head2 How do I reset an each() operation part-way through?
1939 Using C<keys %hash> in scalar context returns the number of keys in
1940 the hash I<and> resets the iterator associated with the hash. You may
1941 need to do this if you use C<last> to exit a loop early so that when you
1942 re-enter it, the hash iterator has been reset.
1944 =head2 How can I get the unique keys from two hashes?
1946 First you extract the keys from the hashes into lists, then solve
1947 the "removing duplicates" problem described above. For example:
1950 for $element (keys(%foo), keys(%bar)) {
1957 @uniq = keys %{{%foo,%bar}};
1959 Or if you really want to save space:
1962 while (defined ($key = each %foo)) {
1965 while (defined ($key = each %bar)) {
1970 =head2 How can I store a multidimensional array in a DBM file?
1972 Either stringify the structure yourself (no fun), or else
1973 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1974 it on top of either DB_File or GDBM_File.
1976 =head2 How can I make my hash remember the order I put elements into it?
1978 Use the Tie::IxHash from CPAN.
1981 tie my %myhash, 'Tie::IxHash';
1982 for (my $i=0; $i<20; $i++) {
1985 my @keys = keys %myhash;
1986 # @keys = (0,1,2,3,...)
1988 =head2 Why does passing a subroutine an undefined element in a hash create it?
1990 If you say something like:
1992 somefunc($hash{"nonesuch key here"});
1994 Then that element "autovivifies"; that is, it springs into existence
1995 whether you store something there or not. That's because functions
1996 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1997 it has to be ready to write it back into the caller's version.
1999 This has been fixed as of Perl5.004.
2001 Normally, merely accessing a key's value for a nonexistent key does
2002 I<not> cause that key to be forever there. This is different than
2005 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
2007 Usually a hash ref, perhaps like this:
2012 TITLE => "deputy peon",
2015 PALS => [ "Norbert", "Rhys", "Phineas"],
2018 References are documented in L<perlref> and the upcoming L<perlreftut>.
2019 Examples of complex data structures are given in L<perldsc> and
2020 L<perllol>. Examples of structures and object-oriented classes are
2023 =head2 How can I use a reference as a hash key?
2025 (contributed by brian d foy)
2027 Hash keys are strings, so you can't really use a reference as the key.
2028 When you try to do that, perl turns the reference into its stringified
2029 form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get back
2030 the reference from the stringified form, at least without doing some
2031 extra work on your own. Also remember that hash keys must be unique, but
2032 two different variables can store the same reference (and those variables
2035 The Tie::RefHash module, which is distributed with perl, might be what
2036 you want. It handles that extra work.
2040 =head2 How do I handle binary data correctly?
2042 Perl is binary clean, so this shouldn't be a problem. For example,
2043 this works fine (assuming the files are found):
2045 if (`cat /vmunix` =~ /gzip/) {
2046 print "Your kernel is GNU-zip enabled!\n";
2049 On less elegant (read: Byzantine) systems, however, you have
2050 to play tedious games with "text" versus "binary" files. See
2051 L<perlfunc/"binmode"> or L<perlopentut>.
2053 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
2055 If you want to deal with multibyte characters, however, there are
2056 some gotchas. See the section on Regular Expressions.
2058 =head2 How do I determine whether a scalar is a number/whole/integer/float?
2060 Assuming that you don't care about IEEE notations like "NaN" or
2061 "Infinity", you probably just want to use a regular expression.
2063 if (/\D/) { print "has nondigits\n" }
2064 if (/^\d+$/) { print "is a whole number\n" }
2065 if (/^-?\d+$/) { print "is an integer\n" }
2066 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2067 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2068 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2069 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2070 { print "a C float\n" }
2072 There are also some commonly used modules for the task.
2073 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2074 internal function C<looks_like_number> for determining
2075 whether a variable looks like a number. L<Data::Types>
2076 exports functions that validate data types using both the
2077 above and other regular expressions. Thirdly, there is
2078 C<Regexp::Common> which has regular expressions to match
2079 various types of numbers. Those three modules are available
2082 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2083 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
2084 wrapper function for more convenient access. This function takes
2085 a string and returns the number it found, or C<undef> for input that
2086 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
2087 if you just want to say, "Is this a float?"
2090 use POSIX qw(strtod);
2095 my($num, $unparsed) = strtod($str);
2096 if (($str eq '') || ($unparsed != 0) || $!) {
2103 sub is_numeric { defined getnum($_[0]) }
2105 Or you could check out the L<String::Scanf> module on the CPAN
2106 instead. The POSIX module (part of the standard Perl distribution) provides
2107 the C<strtod> and C<strtol> for converting strings to double and longs,
2110 =head2 How do I keep persistent data across program calls?
2112 For some specific applications, you can use one of the DBM modules.
2113 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2114 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2115 of the standard distribution. Here's one example using Storable's C<store>
2116 and C<retrieve> functions:
2119 store(\%hash, "filename");
2122 $href = retrieve("filename"); # by ref
2123 %hash = %{ retrieve("filename") }; # direct to hash
2125 =head2 How do I print out or copy a recursive data structure?
2127 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2128 for printing out data structures. The Storable module on CPAN (or the
2129 5.8 release of Perl), provides a function called C<dclone> that recursively
2130 copies its argument.
2132 use Storable qw(dclone);
2135 Where $r1 can be a reference to any kind of data structure you'd like.
2136 It will be deeply copied. Because C<dclone> takes and returns references,
2137 you'd have to add extra punctuation if you had a hash of arrays that
2140 %newhash = %{ dclone(\%oldhash) };
2142 =head2 How do I define methods for every class/object?
2144 Use the UNIVERSAL class (see L<UNIVERSAL>).
2146 =head2 How do I verify a credit card checksum?
2148 Get the Business::CreditCard module from CPAN.
2150 =head2 How do I pack arrays of doubles or floats for XS code?
2152 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2153 If you're doing a lot of float or double processing, consider using
2154 the PDL module from CPAN instead--it makes number-crunching easy.
2156 =head1 AUTHOR AND COPYRIGHT
2158 Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
2159 other authors as noted. All rights reserved.
2161 This documentation is free; you can redistribute it and/or modify it
2162 under the same terms as Perl itself.
2164 Irrespective of its distribution, all code examples in this file
2165 are hereby placed into the public domain. You are permitted and
2166 encouraged to use this code in your own programs for fun
2167 or for profit as you see fit. A simple comment in the code giving
2168 credit would be courteous but is not required.