3 perlfaq4 - Data Manipulation ($Revision: 1.54 $, $Date: 2003/11/30 00:50:08 $)
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 week. 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 for 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);
426 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
429 On some systems, the POSIX module's strftime() function has
430 been extended in a non-standard way to use a C<%C> format,
431 which they sometimes claim is the "century". It isn't,
432 because on most such systems, this is only the first two
433 digits of the four-digit year, and thus cannot be used to
434 reliably determine the current century or millennium.
436 =head2 How can I compare two dates and find the difference?
438 If you're storing your dates as epoch seconds then simply subtract one
439 from the other. If you've got a structured date (distinct year, day,
440 month, hour, minute, seconds values), then for reasons of accessibility,
441 simplicity, and efficiency, merely use either timelocal or timegm (from
442 the Time::Local module in the standard distribution) to reduce structured
443 dates to epoch seconds. However, if you don't know the precise format of
444 your dates, then you should probably use either of the Date::Manip and
445 Date::Calc modules from CPAN before you go hacking up your own parsing
446 routine to handle arbitrary date formats.
448 =head2 How can I take a string and turn it into epoch seconds?
450 If it's a regular enough string that it always has the same format,
451 you can split it up and pass the parts to C<timelocal> in the standard
452 Time::Local module. Otherwise, you should look into the Date::Calc
453 and Date::Manip modules from CPAN.
455 =head2 How can I find the Julian Day?
457 Use the Time::JulianDay module (part of the Time-modules bundle
458 available from CPAN.)
460 Before you immerse yourself too deeply in this, be sure to verify that
461 it is the I<Julian> Day you really want. Are you interested in a way
462 of getting serial days so that you just can tell how many days they
463 are apart or so that you can do also other date arithmetic? If you
464 are interested in performing date arithmetic, this can be done using
465 modules Date::Manip or Date::Calc.
467 There is too many details and much confusion on this issue to cover in
468 this FAQ, but the term is applied (correctly) to a calendar now
469 supplanted by the Gregorian Calendar, with the Julian Calendar failing
470 to adjust properly for leap years on centennial years (among other
471 annoyances). The term is also used (incorrectly) to mean: [1] days in
472 the Gregorian Calendar; and [2] days since a particular starting time
473 or `epoch', usually 1970 in the Unix world and 1980 in the
474 MS-DOS/Windows world. If you find that it is not the first meaning
475 that you really want, then check out the Date::Manip and Date::Calc
476 modules. (Thanks to David Cassell for most of this text.)
478 =head2 How do I find yesterday's date?
480 If you only need to find the date (and not the same time), you
481 can use the Date::Calc module.
483 use Date::Calc qw(Today Add_Delta_Days);
485 my @date = Add_Delta_Days( Today(), -1 );
489 Most people try to use the time rather than the calendar to
490 figure out dates, but that assumes that your days are
491 twenty-four hours each. For most people, there are two days
492 a year when they aren't: the switch to and from summer time
493 throws this off. Russ Allbery offers this solution.
496 my $now = defined $_[0] ? $_[0] : time;
497 my $then = $now - 60 * 60 * 24;
498 my $ndst = (localtime $now)[8] > 0;
499 my $tdst = (localtime $then)[8] > 0;
500 $then - ($tdst - $ndst) * 60 * 60;
503 Should give you "this time yesterday" in seconds since epoch relative to
504 the first argument or the current time if no argument is given and
505 suitable for passing to localtime or whatever else you need to do with
506 it. $ndst is whether we're currently in daylight savings time; $tdst is
507 whether the point 24 hours ago was in daylight savings time. If $tdst
508 and $ndst are the same, a boundary wasn't crossed, and the correction
509 will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
510 from yesterday's time since we gained an extra hour while going off
511 daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
512 negative hour (add an hour) to yesterday's time since we lost an hour.
514 All of this is because during those days when one switches off or onto
515 DST, a "day" isn't 24 hours long; it's either 23 or 25.
517 The explicit settings of $ndst and $tdst are necessary because localtime
518 only says it returns the system tm struct, and the system tm struct at
519 least on Solaris doesn't guarantee any particular positive value (like,
520 say, 1) for isdst, just a positive value. And that value can
521 potentially be negative, if DST information isn't available (this sub
522 just treats those cases like no DST).
524 Note that between 2am and 3am on the day after the time zone switches
525 off daylight savings time, the exact hour of "yesterday" corresponding
526 to the current hour is not clearly defined. Note also that if used
527 between 2am and 3am the day after the change to daylight savings time,
528 the result will be between 3am and 4am of the previous day; it's
529 arguable whether this is correct.
531 This sub does not attempt to deal with leap seconds (most things don't).
535 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
537 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
538 Y2K compliant (whatever that means). The programmers you've hired to
539 use it, however, probably are not.
541 Long answer: The question belies a true understanding of the issue.
542 Perl is just as Y2K compliant as your pencil--no more, and no less.
543 Can you use your pencil to write a non-Y2K-compliant memo? Of course
544 you can. Is that the pencil's fault? Of course it isn't.
546 The date and time functions supplied with Perl (gmtime and localtime)
547 supply adequate information to determine the year well beyond 2000
548 (2038 is when trouble strikes for 32-bit machines). The year returned
549 by these functions when used in a list context is the year minus 1900.
550 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
551 number. To avoid the year 2000 problem simply do not treat the year as
552 a 2-digit number. It isn't.
554 When gmtime() and localtime() are used in scalar context they return
555 a timestamp string that contains a fully-expanded year. For example,
556 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
557 2001". There's no year 2000 problem here.
559 That doesn't mean that Perl can't be used to create non-Y2K compliant
560 programs. It can. But so can your pencil. It's the fault of the user,
561 not the language. At the risk of inflaming the NRA: ``Perl doesn't
562 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
567 =head2 How do I validate input?
569 The answer to this question is usually a regular expression, perhaps
570 with auxiliary logic. See the more specific questions (numbers, mail
571 addresses, etc.) for details.
573 =head2 How do I unescape a string?
575 It depends just what you mean by ``escape''. URL escapes are dealt
576 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
577 character are removed with
581 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
583 =head2 How do I remove consecutive pairs of characters?
585 To turn C<"abbcccd"> into C<"abccd">:
587 s/(.)\1/$1/g; # add /s to include newlines
589 Here's a solution that turns "abbcccd" to "abcd":
591 y///cs; # y == tr, but shorter :-)
593 =head2 How do I expand function calls in a string?
595 This is documented in L<perlref>. In general, this is fraught with
596 quoting and readability problems, but it is possible. To interpolate
597 a subroutine call (in list context) into a string:
599 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
601 See also ``How can I expand variables in text strings?'' in this
604 =head2 How do I find matching/nesting anything?
606 This isn't something that can be done in one regular expression, no
607 matter how complicated. To find something between two single
608 characters, a pattern like C</x([^x]*)x/> will get the intervening
609 bits in $1. For multiple ones, then something more like
610 C</alpha(.*?)omega/> would be needed. But none of these deals with
611 nested patterns. For balanced expressions using C<(>, C<{>, C<[>
612 or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
613 L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
615 If you are serious about writing a parser, there are a number of
616 modules or oddities that will make your life a lot easier. There are
617 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
618 and the byacc program. Starting from perl 5.8 the Text::Balanced
619 is part of the standard distribution.
621 One simple destructive, inside-out approach that you might try is to
622 pull out the smallest nesting parts one at a time:
624 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
625 # do something with $1
628 A more complicated and sneaky approach is to make Perl's regular
629 expression engine do it for you. This is courtesy Dean Inada, and
630 rather has the nature of an Obfuscated Perl Contest entry, but it
633 # $_ contains the string to parse
634 # BEGIN and END are the opening and closing markers for the
639 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
640 @$ = (eval{/$re/},$@!~/unmatched/i);
641 print join("\n",@$[0..$#$]) if( $$[-1] );
643 =head2 How do I reverse a string?
645 Use reverse() in scalar context, as documented in
648 $reversed = reverse $string;
650 =head2 How do I expand tabs in a string?
652 You can do it yourself:
654 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
656 Or you can just use the Text::Tabs module (part of the standard Perl
660 @expanded_lines = expand(@lines_with_tabs);
662 =head2 How do I reformat a paragraph?
664 Use Text::Wrap (part of the standard Perl distribution):
667 print wrap("\t", ' ', @paragraphs);
669 The paragraphs you give to Text::Wrap should not contain embedded
670 newlines. Text::Wrap doesn't justify the lines (flush-right).
672 Or use the CPAN module Text::Autoformat. Formatting files can be easily
673 done by making a shell alias, like so:
675 alias fmt="perl -i -MText::Autoformat -n0777 \
676 -e 'print autoformat $_, {all=>1}' $*"
678 See the documentation for Text::Autoformat to appreciate its many
681 =head2 How can I access or change N characters of a string?
683 You can access the first characters of a string with substr().
684 To get the first character, for example, start at position 0
685 and grab the string of length 1.
688 $string = "Just another Perl Hacker";
689 $first_char = substr( $string, 0, 1 ); # 'J'
691 To change part of a string, you can use the optional fourth
692 argument which is the replacement string.
694 substr( $string, 13, 4, "Perl 5.8.0" );
696 You can also use substr() as an lvalue.
698 substr( $string, 13, 4 ) = "Perl 5.8.0";
700 =head2 How do I change the Nth occurrence of something?
702 You have to keep track of N yourself. For example, let's say you want
703 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
704 C<"whosoever"> or C<"whomsoever">, case insensitively. These
705 all assume that $_ contains the string to be altered.
709 ++$count == 5 # is it the 5th?
710 ? "${2}soever" # yes, swap
711 : $1 # renege and leave it there
714 In the more general case, you can use the C</g> modifier in a C<while>
715 loop, keeping count of matches.
719 $_ = "One fish two fish red fish blue fish";
720 while (/(\w+)\s+fish\b/gi) {
721 if (++$count == $WANT) {
722 print "The third fish is a $1 one.\n";
726 That prints out: C<"The third fish is a red one."> You can also use a
727 repetition count and repeated pattern like this:
729 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
731 =head2 How can I count the number of occurrences of a substring within a string?
733 There are a number of ways, with varying efficiency. If you want a
734 count of a certain single character (X) within a string, you can use the
735 C<tr///> function like so:
737 $string = "ThisXlineXhasXsomeXx'sXinXit";
738 $count = ($string =~ tr/X//);
739 print "There are $count X characters in the string";
741 This is fine if you are just looking for a single character. However,
742 if you are trying to count multiple character substrings within a
743 larger string, C<tr///> won't work. What you can do is wrap a while()
744 loop around a global pattern match. For example, let's count negative
747 $string = "-9 55 48 -2 23 -76 4 14 -44";
748 while ($string =~ /-\d+/g) { $count++ }
749 print "There are $count negative numbers in the string";
751 Another version uses a global match in list context, then assigns the
752 result to a scalar, producing a count of the number of matches.
754 $count = () = $string =~ /-\d+/g;
756 =head2 How do I capitalize all the words on one line?
758 To make the first letter of each word upper case:
760 $line =~ s/\b(\w)/\U$1/g;
762 This has the strange effect of turning "C<don't do it>" into "C<Don'T
763 Do It>". Sometimes you might want this. Other times you might need a
764 more thorough solution (Suggested by brian d foy):
767 (^\w) #at the beginning of the line
769 (\s\w) #preceded by whitespace
772 $string =~ /([\w']+)/\u\L$1/g;
774 To make the whole line upper case:
778 To force each word to be lower case, with the first letter upper case:
780 $line =~ s/(\w+)/\u\L$1/g;
782 You can (and probably should) enable locale awareness of those
783 characters by placing a C<use locale> pragma in your program.
784 See L<perllocale> for endless details on locales.
786 This is sometimes referred to as putting something into "title
787 case", but that's not quite accurate. Consider the proper
788 capitalization of the movie I<Dr. Strangelove or: How I Learned to
789 Stop Worrying and Love the Bomb>, for example.
791 Damian Conway's L<Text::Autoformat> module provides some smart
792 case transformations:
794 use Text::Autoformat;
795 my $x = "Dr. Strangelove or: How I Learned to Stop ".
796 "Worrying and Love the Bomb";
799 for my $style (qw( sentence title highlight ))
801 print autoformat($x, { case => $style }), "\n";
804 =head2 How can I split a [character] delimited string except when inside [character]?
806 Several modules can handle this sort of pasing---Text::Balanced,
807 Text::CVS, Text::CVS_XS, and Text::ParseWords, among others.
809 Take the example case of trying to split a string that is
810 comma-separated into its different fields. You can't use C<split(/,/)>
811 because you shouldn't split if the comma is inside quotes. For
812 example, take a data line like this:
814 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
816 Due to the restriction of the quotes, this is a fairly complex
817 problem. Thankfully, we have Jeffrey Friedl, author of
818 I<Mastering Regular Expressions>, to handle these for us. He
819 suggests (assuming your string is contained in $text):
822 push(@new, $+) while $text =~ m{
823 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
827 push(@new, undef) if substr($text,-1,1) eq ',';
829 If you want to represent quotation marks inside a
830 quotation-mark-delimited field, escape them with backslashes (eg,
833 Alternatively, the Text::ParseWords module (part of the standard Perl
834 distribution) lets you say:
836 use Text::ParseWords;
837 @new = quotewords(",", 0, $text);
839 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
841 =head2 How do I strip blank space from the beginning/end of a string?
843 Although the simplest approach would seem to be
845 $string =~ s/^\s*(.*?)\s*$/$1/;
847 not only is this unnecessarily slow and destructive, it also fails with
848 embedded newlines. It is much faster to do this operation in two steps:
853 Or more nicely written as:
860 This idiom takes advantage of the C<foreach> loop's aliasing
861 behavior to factor out common code. You can do this
862 on several strings at once, or arrays, or even the
863 values of a hash if you use a slice:
865 # trim whitespace in the scalar, the array,
866 # and all the values in the hash
867 foreach ($scalar, @array, @hash{keys %hash}) {
872 =head2 How do I pad a string with blanks or pad a number with zeroes?
874 In the following examples, C<$pad_len> is the length to which you wish
875 to pad the string, C<$text> or C<$num> contains the string to be padded,
876 and C<$pad_char> contains the padding character. You can use a single
877 character string constant instead of the C<$pad_char> variable if you
878 know what it is in advance. And in the same way you can use an integer in
879 place of C<$pad_len> if you know the pad length in advance.
881 The simplest method uses the C<sprintf> function. It can pad on the left
882 or right with blanks and on the left with zeroes and it will not
883 truncate the result. The C<pack> function can only pad strings on the
884 right with blanks and it will truncate the result to a maximum length of
887 # Left padding a string with blanks (no truncation):
888 $padded = sprintf("%${pad_len}s", $text);
889 $padded = sprintf("%*s", $pad_len, $text); # same thing
891 # Right padding a string with blanks (no truncation):
892 $padded = sprintf("%-${pad_len}s", $text);
893 $padded = sprintf("%-*s", $pad_len, $text); # same thing
895 # Left padding a number with 0 (no truncation):
896 $padded = sprintf("%0${pad_len}d", $num);
897 $padded = sprintf("%0*d", $pad_len, $num); # same thing
899 # Right padding a string with blanks using pack (will truncate):
900 $padded = pack("A$pad_len",$text);
902 If you need to pad with a character other than blank or zero you can use
903 one of the following methods. They all generate a pad string with the
904 C<x> operator and combine that with C<$text>. These methods do
905 not truncate C<$text>.
907 Left and right padding with any character, creating a new string:
909 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
910 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
912 Left and right padding with any character, modifying C<$text> directly:
914 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
915 $text .= $pad_char x ( $pad_len - length( $text ) );
917 =head2 How do I extract selected columns from a string?
919 Use substr() or unpack(), both documented in L<perlfunc>.
920 If you prefer thinking in terms of columns instead of widths,
921 you can use this kind of thing:
923 # determine the unpack format needed to split Linux ps output
924 # arguments are cut columns
925 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
931 for my $place (@positions) {
932 $template .= "A" . ($place - $lastpos) . " ";
939 =head2 How do I find the soundex value of a string?
941 Use the standard Text::Soundex module distributed with Perl.
942 Before you do so, you may want to determine whether `soundex' is in
943 fact what you think it is. Knuth's soundex algorithm compresses words
944 into a small space, and so it does not necessarily distinguish between
945 two words which you might want to appear separately. For example, the
946 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
947 If Text::Soundex does not do what you are looking for, you might want
948 to consider the String::Approx module available at CPAN.
950 =head2 How can I expand variables in text strings?
952 Let's assume that you have a string like:
954 $text = 'this has a $foo in it and a $bar';
956 If those were both global variables, then this would
959 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
961 But since they are probably lexicals, or at least, they could
962 be, you'd have to do this:
964 $text =~ s/(\$\w+)/$1/eeg;
965 die if $@; # needed /ee, not /e
967 It's probably better in the general case to treat those
968 variables as entries in some special hash. For example:
974 $text =~ s/\$(\w+)/$user_defs{$1}/g;
976 See also ``How do I expand function calls in a string?'' in this section
979 =head2 What's wrong with always quoting "$vars"?
981 The problem is that those double-quotes force stringification--
982 coercing numbers and references into strings--even when you
983 don't want them to be strings. Think of it this way: double-quote
984 expansion is used to produce new strings. If you already
985 have a string, why do you need more?
987 If you get used to writing odd things like these:
991 somefunc("$var"); # BAD
993 You'll be in trouble. Those should (in 99.8% of the cases) be
994 the simpler and more direct:
1000 Otherwise, besides slowing you down, you're going to break code when
1001 the thing in the scalar is actually neither a string nor a number, but
1007 my $oref = "$aref"; # WRONG
1010 You can also get into subtle problems on those few operations in Perl
1011 that actually do care about the difference between a string and a
1012 number, such as the magical C<++> autoincrement operator or the
1015 Stringification also destroys arrays.
1018 print "@lines"; # WRONG - extra blanks
1019 print @lines; # right
1021 =head2 Why don't my E<lt>E<lt>HERE documents work?
1023 Check for these three things:
1027 =item There must be no space after the E<lt>E<lt> part.
1029 =item There (probably) should be a semicolon at the end.
1031 =item You can't (easily) have any space in front of the tag.
1035 If you want to indent the text in the here document, you
1039 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1044 But the HERE_TARGET must still be flush against the margin.
1045 If you want that indented also, you'll have to quote
1048 ($quote = <<' FINIS') =~ s/^\s+//gm;
1049 ...we will have peace, when you and all your works have
1050 perished--and the works of your dark master to whom you
1051 would deliver us. You are a liar, Saruman, and a corrupter
1052 of men's hearts. --Theoden in /usr/src/perl/taint.c
1054 $quote =~ s/\s+--/\n--/;
1056 A nice general-purpose fixer-upper function for indented here documents
1057 follows. It expects to be called with a here document as its argument.
1058 It looks to see whether each line begins with a common substring, and
1059 if so, strips that substring off. Otherwise, it takes the amount of leading
1060 whitespace found on the first line and removes that much off each
1065 my ($white, $leader); # common whitespace and common leading string
1066 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1067 ($white, $leader) = ($2, quotemeta($1));
1069 ($white, $leader) = (/^(\s+)/, '');
1071 s/^\s*?$leader(?:$white)?//gm;
1075 This works with leading special strings, dynamically determined:
1077 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1080 @@@ SAVEI32(runlevel);
1082 @@@ while ( op = (*op->op_ppaddr)() );
1086 MAIN_INTERPRETER_LOOP
1088 Or with a fixed amount of leading whitespace, with remaining
1089 indentation correctly preserved:
1091 $poem = fix<<EVER_ON_AND_ON;
1092 Now far ahead the Road has gone,
1093 And I must follow, if I can,
1094 Pursuing it with eager feet,
1095 Until it joins some larger way
1096 Where many paths and errands meet.
1097 And whither then? I cannot say.
1098 --Bilbo in /usr/src/perl/pp_ctl.c
1103 =head2 What is the difference between a list and an array?
1105 An array has a changeable length. A list does not. An array is something
1106 you can push or pop, while a list is a set of values. Some people make
1107 the distinction that a list is a value while an array is a variable.
1108 Subroutines are passed and return lists, you put things into list
1109 context, you initialize arrays with lists, and you foreach() across
1110 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1111 in scalar context behave like the number of elements in them, subroutines
1112 access their arguments through the array C<@_>, and push/pop/shift only work
1115 As a side note, there's no such thing as a list in scalar context.
1118 $scalar = (2, 5, 7, 9);
1120 you're using the comma operator in scalar context, so it uses the scalar
1121 comma operator. There never was a list there at all! This causes the
1122 last value to be returned: 9.
1124 =head2 What is the difference between $array[1] and @array[1]?
1126 The former is a scalar value; the latter an array slice, making
1127 it a list with one (scalar) value. You should use $ when you want a
1128 scalar value (most of the time) and @ when you want a list with one
1129 scalar value in it (very, very rarely; nearly never, in fact).
1131 Sometimes it doesn't make a difference, but sometimes it does.
1132 For example, compare:
1134 $good[0] = `some program that outputs several lines`;
1138 @bad[0] = `same program that outputs several lines`;
1140 The C<use warnings> pragma and the B<-w> flag will warn you about these
1143 =head2 How can I remove duplicate elements from a list or array?
1145 There are several possible ways, depending on whether the array is
1146 ordered and whether you wish to preserve the ordering.
1152 If @in is sorted, and you want @out to be sorted:
1153 (this assumes all true values in the array)
1155 $prev = "not equal to $in[0]";
1156 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1158 This is nice in that it doesn't use much extra memory, simulating
1159 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1160 guarantees that the expression is true (so that grep picks it up)
1161 even if the $_ is 0, "", or undef.
1165 If you don't know whether @in is sorted:
1168 @out = grep(!$saw{$_}++, @in);
1172 Like (b), but @in contains only small integers:
1174 @out = grep(!$saw[$_]++, @in);
1178 A way to do (b) without any loops or greps:
1182 @out = sort keys %saw; # remove sort if undesired
1186 Like (d), but @in contains only small positive integers:
1190 @out = grep {defined} @ary;
1194 But perhaps you should have been using a hash all along, eh?
1196 =head2 How can I tell whether a certain element is contained in a list or array?
1198 Hearing the word "in" is an I<in>dication that you probably should have
1199 used a hash, not a list or array, to store your data. Hashes are
1200 designed to answer this question quickly and efficiently. Arrays aren't.
1202 That being said, there are several ways to approach this. If you
1203 are going to make this query many times over arbitrary string values,
1204 the fastest way is probably to invert the original array and maintain a
1205 hash whose keys are the first array's values.
1207 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1209 for (@blues) { $is_blue{$_} = 1 }
1211 Now you can check whether $is_blue{$some_color}. It might have been a
1212 good idea to keep the blues all in a hash in the first place.
1214 If the values are all small integers, you could use a simple indexed
1215 array. This kind of an array will take up less space:
1217 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1218 @is_tiny_prime = ();
1219 for (@primes) { $is_tiny_prime[$_] = 1 }
1220 # or simply @istiny_prime[@primes] = (1) x @primes;
1222 Now you check whether $is_tiny_prime[$some_number].
1224 If the values in question are integers instead of strings, you can save
1225 quite a lot of space by using bit strings instead:
1227 @articles = ( 1..10, 150..2000, 2017 );
1229 for (@articles) { vec($read,$_,1) = 1 }
1231 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1235 ($is_there) = grep $_ eq $whatever, @array;
1239 ($is_there) = grep /$whatever/, @array;
1241 These are slow (checks every element even if the first matches),
1242 inefficient (same reason), and potentially buggy (what if there are
1243 regex characters in $whatever?). If you're only testing once, then
1247 foreach $elt (@array) {
1248 if ($elt eq $elt_to_find) {
1253 if ($is_there) { ... }
1255 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1257 Use a hash. Here's code to do both and more. It assumes that
1258 each element is unique in a given array:
1260 @union = @intersection = @difference = ();
1262 foreach $element (@array1, @array2) { $count{$element}++ }
1263 foreach $element (keys %count) {
1264 push @union, $element;
1265 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1268 Note that this is the I<symmetric difference>, that is, all elements in
1269 either A or in B but not in both. Think of it as an xor operation.
1271 =head2 How do I test whether two arrays or hashes are equal?
1273 The following code works for single-level arrays. It uses a stringwise
1274 comparison, and does not distinguish defined versus undefined empty
1275 strings. Modify if you have other needs.
1277 $are_equal = compare_arrays(\@frogs, \@toads);
1279 sub compare_arrays {
1280 my ($first, $second) = @_;
1281 no warnings; # silence spurious -w undef complaints
1282 return 0 unless @$first == @$second;
1283 for (my $i = 0; $i < @$first; $i++) {
1284 return 0 if $first->[$i] ne $second->[$i];
1289 For multilevel structures, you may wish to use an approach more
1290 like this one. It uses the CPAN module FreezeThaw:
1292 use FreezeThaw qw(cmpStr);
1293 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1295 printf "a and b contain %s arrays\n",
1296 cmpStr(\@a, \@b) == 0
1300 This approach also works for comparing hashes. Here
1301 we'll demonstrate two different answers:
1303 use FreezeThaw qw(cmpStr cmpStrHard);
1305 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1309 printf "a and b contain %s hashes\n",
1310 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1312 printf "a and b contain %s hashes\n",
1313 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1316 The first reports that both those the hashes contain the same data,
1317 while the second reports that they do not. Which you prefer is left as
1318 an exercise to the reader.
1320 =head2 How do I find the first array element for which a condition is true?
1322 To find the first array element which satisfies a condition, you can
1323 use the first() function in the List::Util module, which comes with
1324 Perl 5.8. This example finds the first element that contains "Perl".
1326 use List::Util qw(first);
1328 my $element = first { /Perl/ } @array;
1330 If you cannot use List::Util, you can make your own loop to do the
1331 same thing. Once you find the element, you stop the loop with last.
1334 foreach my $element ( @array )
1336 if( /Perl/ ) { $found = $element; last }
1339 If you want the array index, you can iterate through the indices
1340 and check the array element at each index until you find one
1341 that satisfies the condition.
1343 my( $found, $index ) = ( undef, -1 );
1344 for( $i = 0; $i < @array; $i++ )
1346 if( $array[$i] =~ /Perl/ )
1348 $found = $array[$i];
1354 =head2 How do I handle linked lists?
1356 In general, you usually don't need a linked list in Perl, since with
1357 regular arrays, you can push and pop or shift and unshift at either end,
1358 or you can use splice to add and/or remove arbitrary number of elements at
1359 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1360 dynamic arrays. In the absence of shifts and pops, push in general
1361 needs to reallocate on the order every log(N) times, and unshift will
1362 need to copy pointers each time.
1364 If you really, really wanted, you could use structures as described in
1365 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1366 to do. For example, imagine a list node like this:
1373 You could walk the list this way:
1376 for ($node = $head; $node; $node = $node->{LINK}) {
1377 print $node->{VALUE}, " ";
1381 You could add to the list this way:
1384 $tail = append($head, 1); # grow a new head
1385 for $value ( 2 .. 10 ) {
1386 $tail = append($tail, $value);
1390 my($list, $value) = @_;
1391 my $node = { VALUE => $value };
1393 $node->{LINK} = $list->{LINK};
1394 $list->{LINK} = $node;
1396 $_[0] = $node; # replace caller's version
1401 But again, Perl's built-in are virtually always good enough.
1403 =head2 How do I handle circular lists?
1405 Circular lists could be handled in the traditional fashion with linked
1406 lists, or you could just do something like this with an array:
1408 unshift(@array, pop(@array)); # the last shall be first
1409 push(@array, shift(@array)); # and vice versa
1411 =head2 How do I shuffle an array randomly?
1413 If you either have Perl 5.8.0 or later installed, or if you have
1414 Scalar-List-Utils 1.03 or later installed, you can say:
1416 use List::Util 'shuffle';
1418 @shuffled = shuffle(@list);
1420 If not, you can use a Fisher-Yates shuffle.
1422 sub fisher_yates_shuffle {
1423 my $deck = shift; # $deck is a reference to an array
1426 my $j = int rand ($i+1);
1427 @$deck[$i,$j] = @$deck[$j,$i];
1431 # shuffle my mpeg collection
1433 my @mpeg = <audio/*/*.mp3>;
1434 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1437 Note that the above implementation shuffles an array in place,
1438 unlike the List::Util::shuffle() which takes a list and returns
1439 a new shuffled list.
1441 You've probably seen shuffling algorithms that work using splice,
1442 randomly picking another element to swap the current element with
1446 @old = 1 .. 10; # just a demo
1448 push(@new, splice(@old, rand @old, 1));
1451 This is bad because splice is already O(N), and since you do it N times,
1452 you just invented a quadratic algorithm; that is, O(N**2). This does
1453 not scale, although Perl is so efficient that you probably won't notice
1454 this until you have rather largish arrays.
1456 =head2 How do I process/modify each element of an array?
1458 Use C<for>/C<foreach>:
1461 s/foo/bar/; # change that word
1462 y/XZ/ZX/; # swap those letters
1465 Here's another; let's compute spherical volumes:
1467 for (@volumes = @radii) { # @volumes has changed parts
1469 $_ *= (4/3) * 3.14159; # this will be constant folded
1472 which can also be done with map() which is made to transform
1473 one list into another:
1475 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1477 If you want to do the same thing to modify the values of the
1478 hash, you can use the C<values> function. As of Perl 5.6
1479 the values are not copied, so if you modify $orbit (in this
1480 case), you modify the value.
1482 for $orbit ( values %orbits ) {
1483 ($orbit **= 3) *= (4/3) * 3.14159;
1486 Prior to perl 5.6 C<values> returned copies of the values,
1487 so older perl code often contains constructions such as
1488 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1489 the hash is to be modified.
1491 =head2 How do I select a random element from an array?
1493 Use the rand() function (see L<perlfunc/rand>):
1495 $index = rand @array;
1496 $element = $array[$index];
1499 my $element = $array[ rand @array ];
1501 =head2 How do I permute N elements of a list?
1503 Use the List::Permutor module on CPAN. If the list is
1504 actually an array, try the Algorithm::Permute module (also
1505 on CPAN). It's written in XS code and is very efficient.
1507 use Algorithm::Permute;
1508 my @array = 'a'..'d';
1509 my $p_iterator = Algorithm::Permute->new ( \@array );
1510 while (my @perm = $p_iterator->next) {
1511 print "next permutation: (@perm)\n";
1514 For even faster execution, you could do:
1516 use Algorithm::Permute;
1517 my @array = 'a'..'d';
1518 Algorithm::Permute::permute {
1519 print "next permutation: (@array)\n";
1522 Here's a little program that generates all permutations of
1523 all the words on each line of input. The algorithm embodied
1524 in the permute() function is discussed in Volume 4 (still
1525 unpublished) of Knuth's I<The Art of Computer Programming>
1526 and will work on any list:
1529 # Fischer-Kause ordered permutation generator
1534 while ( $code->(@_[@idx]) ) {
1536 --$p while $idx[$p-1] > $idx[$p];
1537 my $q = $p or return;
1538 push @idx, reverse splice @idx, $p;
1539 ++$q while $idx[$p-1] > $idx[$q];
1540 @idx[$p-1,$q]=@idx[$q,$p-1];
1544 permute {print"@_\n"} split;
1546 =head2 How do I sort an array by (anything)?
1548 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1550 @list = sort { $a <=> $b } @list;
1552 The default sort function is cmp, string comparison, which would
1553 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1554 the numerical comparison operator.
1556 If you have a complicated function needed to pull out the part you
1557 want to sort on, then don't do it inside the sort function. Pull it
1558 out first, because the sort BLOCK can be called many times for the
1559 same element. Here's an example of how to pull out the first word
1560 after the first number on each item, and then sort those words
1565 ($item) = /\d+\s*(\S+)/;
1566 push @idx, uc($item);
1568 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1570 which could also be written this way, using a trick
1571 that's come to be known as the Schwartzian Transform:
1573 @sorted = map { $_->[0] }
1574 sort { $a->[1] cmp $b->[1] }
1575 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1577 If you need to sort on several fields, the following paradigm is useful.
1579 @sorted = sort { field1($a) <=> field1($b) ||
1580 field2($a) cmp field2($b) ||
1581 field3($a) cmp field3($b)
1584 This can be conveniently combined with precalculation of keys as given
1587 See the F<sort> article in the "Far More Than You Ever Wanted
1588 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1589 more about this approach.
1591 See also the question below on sorting hashes.
1593 =head2 How do I manipulate arrays of bits?
1595 Use pack() and unpack(), or else vec() and the bitwise operations.
1597 For example, this sets $vec to have bit N set if $ints[N] was set:
1600 foreach(@ints) { vec($vec,$_,1) = 1 }
1602 Here's how, given a vector in $vec, you can
1603 get those bits into your @ints array:
1605 sub bitvec_to_list {
1608 # Find null-byte density then select best algorithm
1609 if ($vec =~ tr/\0// / length $vec > 0.95) {
1612 # This method is faster with mostly null-bytes
1613 while($vec =~ /[^\0]/g ) {
1614 $i = -9 + 8 * pos $vec;
1615 push @ints, $i if vec($vec, ++$i, 1);
1616 push @ints, $i if vec($vec, ++$i, 1);
1617 push @ints, $i if vec($vec, ++$i, 1);
1618 push @ints, $i if vec($vec, ++$i, 1);
1619 push @ints, $i if vec($vec, ++$i, 1);
1620 push @ints, $i if vec($vec, ++$i, 1);
1621 push @ints, $i if vec($vec, ++$i, 1);
1622 push @ints, $i if vec($vec, ++$i, 1);
1625 # This method is a fast general algorithm
1627 my $bits = unpack "b*", $vec;
1628 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1629 push @ints, pos $bits while($bits =~ /1/g);
1634 This method gets faster the more sparse the bit vector is.
1635 (Courtesy of Tim Bunce and Winfried Koenig.)
1637 You can make the while loop a lot shorter with this suggestion
1638 from Benjamin Goldberg:
1640 while($vec =~ /[^\0]+/g ) {
1641 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1644 Or use the CPAN module Bit::Vector:
1646 $vector = Bit::Vector->new($num_of_bits);
1647 $vector->Index_List_Store(@ints);
1648 @ints = $vector->Index_List_Read();
1650 Bit::Vector provides efficient methods for bit vector, sets of small integers
1653 Here's a more extensive illustration using vec():
1656 $vector = "\xff\x0f\xef\xfe";
1657 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1658 unpack("N", $vector), "\n";
1659 $is_set = vec($vector, 23, 1);
1660 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1678 my ($offset, $width, $value) = @_;
1680 vec($vector, $offset, $width) = $value;
1681 print "offset=$offset width=$width value=$value\n";
1687 my $bits = unpack("b*", $vector);
1691 print "vector length in bytes: ", length($vector), "\n";
1692 @bytes = unpack("A8" x length($vector), $bits);
1693 print "bits are: @bytes\n\n";
1696 =head2 Why does defined() return true on empty arrays and hashes?
1698 The short story is that you should probably only use defined on scalars or
1699 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1700 in the 5.004 release or later of Perl for more detail.
1702 =head1 Data: Hashes (Associative Arrays)
1704 =head2 How do I process an entire hash?
1706 Use the each() function (see L<perlfunc/each>) if you don't care
1707 whether it's sorted:
1709 while ( ($key, $value) = each %hash) {
1710 print "$key = $value\n";
1713 If you want it sorted, you'll have to use foreach() on the result of
1714 sorting the keys as shown in an earlier question.
1716 =head2 What happens if I add or remove keys from a hash while iterating over it?
1720 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1721 iterating over it. In Perl 5 you can delete from it, but you still
1722 can't add to it, because that might cause a doubling of the hash table,
1723 in which half the entries get copied up to the new top half of the
1724 table, at which point you've totally bamboozled the iterator code.
1725 Even if the table doesn't double, there's no telling whether your new
1726 entry will be inserted before or after the current iterator position.
1728 Either treasure up your changes and make them after the iterator finishes
1729 or use keys to fetch all the old keys at once, and iterate over the list
1732 =head2 How do I look up a hash element by value?
1734 Create a reverse hash:
1736 %by_value = reverse %by_key;
1737 $key = $by_value{$value};
1739 That's not particularly efficient. It would be more space-efficient
1742 while (($key, $value) = each %by_key) {
1743 $by_value{$value} = $key;
1746 If your hash could have repeated values, the methods above will only find
1747 one of the associated keys. This may or may not worry you. If it does
1748 worry you, you can always reverse the hash into a hash of arrays instead:
1750 while (($key, $value) = each %by_key) {
1751 push @{$key_list_by_value{$value}}, $key;
1754 =head2 How can I know how many entries are in a hash?
1756 If you mean how many keys, then all you have to do is
1757 use the keys() function in a scalar context:
1759 $num_keys = keys %hash;
1761 The keys() function also resets the iterator, which means that you may
1762 see strange results if you use this between uses of other hash operators
1765 =head2 How do I sort a hash (optionally by value instead of key)?
1767 Internally, hashes are stored in a way that prevents you from imposing
1768 an order on key-value pairs. Instead, you have to sort a list of the
1771 @keys = sort keys %hash; # sorted by key
1773 $hash{$a} cmp $hash{$b}
1774 } keys %hash; # and by value
1776 Here we'll do a reverse numeric sort by value, and if two keys are
1777 identical, sort by length of key, or if that fails, by straight ASCII
1778 comparison of the keys (well, possibly modified by your locale--see
1782 $hash{$b} <=> $hash{$a}
1784 length($b) <=> length($a)
1789 =head2 How can I always keep my hash sorted?
1791 You can look into using the DB_File module and tie() using the
1792 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1793 The Tie::IxHash module from CPAN might also be instructive.
1795 =head2 What's the difference between "delete" and "undef" with hashes?
1797 Hashes contain pairs of scalars: the first is the key, the
1798 second is the value. The key will be coerced to a string,
1799 although the value can be any kind of scalar: string,
1800 number, or reference. If a key $key is present in
1801 %hash, C<exists($hash{$key})> will return true. The value
1802 for a given key can be C<undef>, in which case
1803 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1804 will return true. This corresponds to (C<$key>, C<undef>)
1807 Pictures help... here's the %hash table:
1817 And these conditions hold
1821 defined $hash{'d'} is true
1822 defined $hash{'a'} is true
1823 exists $hash{'a'} is true (Perl5 only)
1824 grep ($_ eq 'a', keys %hash) is true
1830 your table now reads:
1841 and these conditions now hold; changes in caps:
1845 defined $hash{'d'} is true
1846 defined $hash{'a'} is FALSE
1847 exists $hash{'a'} is true (Perl5 only)
1848 grep ($_ eq 'a', keys %hash) is true
1850 Notice the last two: you have an undef value, but a defined key!
1856 your table now reads:
1865 and these conditions now hold; changes in caps:
1869 defined $hash{'d'} is true
1870 defined $hash{'a'} is false
1871 exists $hash{'a'} is FALSE (Perl5 only)
1872 grep ($_ eq 'a', keys %hash) is FALSE
1874 See, the whole entry is gone!
1876 =head2 Why don't my tied hashes make the defined/exists distinction?
1878 This depends on the tied hash's implementation of EXISTS().
1879 For example, there isn't the concept of undef with hashes
1880 that are tied to DBM* files. It also means that exists() and
1881 defined() do the same thing with a DBM* file, and what they
1882 end up doing is not what they do with ordinary hashes.
1884 =head2 How do I reset an each() operation part-way through?
1886 Using C<keys %hash> in scalar context returns the number of keys in
1887 the hash I<and> resets the iterator associated with the hash. You may
1888 need to do this if you use C<last> to exit a loop early so that when you
1889 re-enter it, the hash iterator has been reset.
1891 =head2 How can I get the unique keys from two hashes?
1893 First you extract the keys from the hashes into lists, then solve
1894 the "removing duplicates" problem described above. For example:
1897 for $element (keys(%foo), keys(%bar)) {
1904 @uniq = keys %{{%foo,%bar}};
1906 Or if you really want to save space:
1909 while (defined ($key = each %foo)) {
1912 while (defined ($key = each %bar)) {
1917 =head2 How can I store a multidimensional array in a DBM file?
1919 Either stringify the structure yourself (no fun), or else
1920 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1921 it on top of either DB_File or GDBM_File.
1923 =head2 How can I make my hash remember the order I put elements into it?
1925 Use the Tie::IxHash from CPAN.
1928 tie my %myhash, 'Tie::IxHash';
1929 for (my $i=0; $i<20; $i++) {
1932 my @keys = keys %myhash;
1933 # @keys = (0,1,2,3,...)
1935 =head2 Why does passing a subroutine an undefined element in a hash create it?
1937 If you say something like:
1939 somefunc($hash{"nonesuch key here"});
1941 Then that element "autovivifies"; that is, it springs into existence
1942 whether you store something there or not. That's because functions
1943 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1944 it has to be ready to write it back into the caller's version.
1946 This has been fixed as of Perl5.004.
1948 Normally, merely accessing a key's value for a nonexistent key does
1949 I<not> cause that key to be forever there. This is different than
1952 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1954 Usually a hash ref, perhaps like this:
1959 TITLE => "deputy peon",
1962 PALS => [ "Norbert", "Rhys", "Phineas"],
1965 References are documented in L<perlref> and the upcoming L<perlreftut>.
1966 Examples of complex data structures are given in L<perldsc> and
1967 L<perllol>. Examples of structures and object-oriented classes are
1970 =head2 How can I use a reference as a hash key?
1972 You can't do this directly, but you could use the standard Tie::RefHash
1973 module distributed with Perl.
1977 =head2 How do I handle binary data correctly?
1979 Perl is binary clean, so this shouldn't be a problem. For example,
1980 this works fine (assuming the files are found):
1982 if (`cat /vmunix` =~ /gzip/) {
1983 print "Your kernel is GNU-zip enabled!\n";
1986 On less elegant (read: Byzantine) systems, however, you have
1987 to play tedious games with "text" versus "binary" files. See
1988 L<perlfunc/"binmode"> or L<perlopentut>.
1990 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1992 If you want to deal with multibyte characters, however, there are
1993 some gotchas. See the section on Regular Expressions.
1995 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1997 Assuming that you don't care about IEEE notations like "NaN" or
1998 "Infinity", you probably just want to use a regular expression.
2000 if (/\D/) { print "has nondigits\n" }
2001 if (/^\d+$/) { print "is a whole number\n" }
2002 if (/^-?\d+$/) { print "is an integer\n" }
2003 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2004 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2005 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2006 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2007 { print "a C float\n" }
2009 There are also some commonly used modules for the task.
2010 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2011 internal function C<looks_like_number> for determining
2012 whether a variable looks like a number. L<Data::Types>
2013 exports functions that validate data types using both the
2014 above and other regular expressions. Thirdly, there is
2015 C<Regexp::Common> which has regular expressions to match
2016 various types of numbers. Those three modules are available
2019 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2020 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
2021 wrapper function for more convenient access. This function takes
2022 a string and returns the number it found, or C<undef> for input that
2023 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
2024 if you just want to say, ``Is this a float?''
2027 use POSIX qw(strtod);
2032 my($num, $unparsed) = strtod($str);
2033 if (($str eq '') || ($unparsed != 0) || $!) {
2040 sub is_numeric { defined getnum($_[0]) }
2042 Or you could check out the L<String::Scanf> module on the CPAN
2043 instead. The POSIX module (part of the standard Perl distribution) provides
2044 the C<strtod> and C<strtol> for converting strings to double and longs,
2047 =head2 How do I keep persistent data across program calls?
2049 For some specific applications, you can use one of the DBM modules.
2050 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2051 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2052 of the standard distribution. Here's one example using Storable's C<store>
2053 and C<retrieve> functions:
2056 store(\%hash, "filename");
2059 $href = retrieve("filename"); # by ref
2060 %hash = %{ retrieve("filename") }; # direct to hash
2062 =head2 How do I print out or copy a recursive data structure?
2064 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2065 for printing out data structures. The Storable module on CPAN (or the
2066 5.8 release of Perl), provides a function called C<dclone> that recursively
2067 copies its argument.
2069 use Storable qw(dclone);
2072 Where $r1 can be a reference to any kind of data structure you'd like.
2073 It will be deeply copied. Because C<dclone> takes and returns references,
2074 you'd have to add extra punctuation if you had a hash of arrays that
2077 %newhash = %{ dclone(\%oldhash) };
2079 =head2 How do I define methods for every class/object?
2081 Use the UNIVERSAL class (see L<UNIVERSAL>).
2083 =head2 How do I verify a credit card checksum?
2085 Get the Business::CreditCard module from CPAN.
2087 =head2 How do I pack arrays of doubles or floats for XS code?
2089 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2090 If you're doing a lot of float or double processing, consider using
2091 the PDL module from CPAN instead--it makes number-crunching easy.
2093 =head1 AUTHOR AND COPYRIGHT
2095 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
2096 All rights reserved.
2098 This documentation is free; you can redistribute it and/or modify it
2099 under the same terms as Perl itself.
2101 Irrespective of its distribution, all code examples in this file
2102 are hereby placed into the public domain. You are permitted and
2103 encouraged to use this code in your own programs for fun
2104 or for profit as you see fit. A simple comment in the code giving
2105 credit would be courteous but is not required.