3 perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 2003/09/20 06:37:43 $)
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 Use the following simple function. It selects a random integer between
366 (and possibly including!) the two given integers, e.g.,
367 C<random_int_in(50,120)>
369 sub random_int_in ($$) {
371 # Assumes that the two arguments are integers themselves!
372 return $min if $min == $max;
373 ($min, $max) = ($max, $min) if $min > $max;
374 return $min + int rand(1 + $max - $min);
379 =head2 How do I find the day or week of the year?
381 The localtime function returns the day of the week. Without an
382 argument localtime uses the current time.
384 $day_of_year = (localtime)[7];
386 The POSIX module can also format a date as the day of the year or
389 use POSIX qw/strftime/;
390 my $day_of_year = strftime "%j", localtime;
391 my $week_of_year = strftime "%W", localtime;
393 To get the day of year for any date, use the Time::Local module to get
394 a time in epoch seconds for the argument to localtime.
396 use POSIX qw/strftime/;
398 my $week_of_year = strftime "%W",
399 localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
401 The Date::Calc module provides two functions for to calculate these.
404 my $day_of_year = Day_of_Year( 1987, 12, 18 );
405 my $week_of_year = Week_of_Year( 1987, 12, 18 );
407 =head2 How do I find the current century or millennium?
409 Use the following simple functions:
412 return int((((localtime(shift || time))[5] + 1999))/100);
415 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
418 You can also use the POSIX strftime() function which may be a bit
419 slower but is easier to read and maintain.
421 use POSIX qw/strftime/;
423 my $week_of_the_year = strftime "%W", localtime;
424 my $day_of_the_year = strftime "%j", localtime;
426 On some systems, the POSIX module's strftime() function has
427 been extended in a non-standard way to use a C<%C> format,
428 which they sometimes claim is the "century". It isn't,
429 because on most such systems, this is only the first two
430 digits of the four-digit year, and thus cannot be used to
431 reliably determine the current century or millennium.
433 =head2 How can I compare two dates and find the difference?
435 If you're storing your dates as epoch seconds then simply subtract one
436 from the other. If you've got a structured date (distinct year, day,
437 month, hour, minute, seconds values), then for reasons of accessibility,
438 simplicity, and efficiency, merely use either timelocal or timegm (from
439 the Time::Local module in the standard distribution) to reduce structured
440 dates to epoch seconds. However, if you don't know the precise format of
441 your dates, then you should probably use either of the Date::Manip and
442 Date::Calc modules from CPAN before you go hacking up your own parsing
443 routine to handle arbitrary date formats.
445 =head2 How can I take a string and turn it into epoch seconds?
447 If it's a regular enough string that it always has the same format,
448 you can split it up and pass the parts to C<timelocal> in the standard
449 Time::Local module. Otherwise, you should look into the Date::Calc
450 and Date::Manip modules from CPAN.
452 =head2 How can I find the Julian Day?
454 Use the Time::JulianDay module (part of the Time-modules bundle
455 available from CPAN.)
457 Before you immerse yourself too deeply in this, be sure to verify that
458 it is the I<Julian> Day you really want. Are you interested in a way
459 of getting serial days so that you just can tell how many days they
460 are apart or so that you can do also other date arithmetic? If you
461 are interested in performing date arithmetic, this can be done using
462 modules Date::Manip or Date::Calc.
464 There is too many details and much confusion on this issue to cover in
465 this FAQ, but the term is applied (correctly) to a calendar now
466 supplanted by the Gregorian Calendar, with the Julian Calendar failing
467 to adjust properly for leap years on centennial years (among other
468 annoyances). The term is also used (incorrectly) to mean: [1] days in
469 the Gregorian Calendar; and [2] days since a particular starting time
470 or `epoch', usually 1970 in the Unix world and 1980 in the
471 MS-DOS/Windows world. If you find that it is not the first meaning
472 that you really want, then check out the Date::Manip and Date::Calc
473 modules. (Thanks to David Cassell for most of this text.)
475 =head2 How do I find yesterday's date?
477 If you only need to find the date (and not the same time), you
478 can use the Date::Calc module.
480 use Date::Calc qw(Today Add_Delta_Days);
482 my @date = Add_Delta_Days( Today(), -1 );
486 Most people try to use the time rather than the calendar to
487 figure out dates, but that assumes that your days are
488 twenty-four hours each. For most people, there are two days
489 a year when they aren't: the switch to and from summer time
490 throws this off. Russ Allbery offers this solution.
493 my $now = defined $_[0] ? $_[0] : time;
494 my $then = $now - 60 * 60 * 24;
495 my $ndst = (localtime $now)[8] > 0;
496 my $tdst = (localtime $then)[8] > 0;
497 $then - ($tdst - $ndst) * 60 * 60;
500 Should give you "this time yesterday" in seconds since epoch relative to
501 the first argument or the current time if no argument is given and
502 suitable for passing to localtime or whatever else you need to do with
503 it. $ndst is whether we're currently in daylight savings time; $tdst is
504 whether the point 24 hours ago was in daylight savings time. If $tdst
505 and $ndst are the same, a boundary wasn't crossed, and the correction
506 will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
507 from yesterday's time since we gained an extra hour while going off
508 daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
509 negative hour (add an hour) to yesterday's time since we lost an hour.
511 All of this is because during those days when one switches off or onto
512 DST, a "day" isn't 24 hours long; it's either 23 or 25.
514 The explicit settings of $ndst and $tdst are necessary because localtime
515 only says it returns the system tm struct, and the system tm struct at
516 least on Solaris doesn't guarantee any particular positive value (like,
517 say, 1) for isdst, just a positive value. And that value can
518 potentially be negative, if DST information isn't available (this sub
519 just treats those cases like no DST).
521 Note that between 2am and 3am on the day after the time zone switches
522 off daylight savings time, the exact hour of "yesterday" corresponding
523 to the current hour is not clearly defined. Note also that if used
524 between 2am and 3am the day after the change to daylight savings time,
525 the result will be between 3am and 4am of the previous day; it's
526 arguable whether this is correct.
528 This sub does not attempt to deal with leap seconds (most things don't).
532 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
534 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
535 Y2K compliant (whatever that means). The programmers you've hired to
536 use it, however, probably are not.
538 Long answer: The question belies a true understanding of the issue.
539 Perl is just as Y2K compliant as your pencil--no more, and no less.
540 Can you use your pencil to write a non-Y2K-compliant memo? Of course
541 you can. Is that the pencil's fault? Of course it isn't.
543 The date and time functions supplied with Perl (gmtime and localtime)
544 supply adequate information to determine the year well beyond 2000
545 (2038 is when trouble strikes for 32-bit machines). The year returned
546 by these functions when used in a list context is the year minus 1900.
547 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
548 number. To avoid the year 2000 problem simply do not treat the year as
549 a 2-digit number. It isn't.
551 When gmtime() and localtime() are used in scalar context they return
552 a timestamp string that contains a fully-expanded year. For example,
553 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
554 2001". There's no year 2000 problem here.
556 That doesn't mean that Perl can't be used to create non-Y2K compliant
557 programs. It can. But so can your pencil. It's the fault of the user,
558 not the language. At the risk of inflaming the NRA: ``Perl doesn't
559 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
564 =head2 How do I validate input?
566 The answer to this question is usually a regular expression, perhaps
567 with auxiliary logic. See the more specific questions (numbers, mail
568 addresses, etc.) for details.
570 =head2 How do I unescape a string?
572 It depends just what you mean by ``escape''. URL escapes are dealt
573 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
574 character are removed with
578 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
580 =head2 How do I remove consecutive pairs of characters?
582 To turn C<"abbcccd"> into C<"abccd">:
584 s/(.)\1/$1/g; # add /s to include newlines
586 Here's a solution that turns "abbcccd" to "abcd":
588 y///cs; # y == tr, but shorter :-)
590 =head2 How do I expand function calls in a string?
592 This is documented in L<perlref>. In general, this is fraught with
593 quoting and readability problems, but it is possible. To interpolate
594 a subroutine call (in list context) into a string:
596 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
598 See also ``How can I expand variables in text strings?'' in this
601 =head2 How do I find matching/nesting anything?
603 This isn't something that can be done in one regular expression, no
604 matter how complicated. To find something between two single
605 characters, a pattern like C</x([^x]*)x/> will get the intervening
606 bits in $1. For multiple ones, then something more like
607 C</alpha(.*?)omega/> would be needed. But none of these deals with
608 nested patterns. For balanced expressions using C<(>, C<{>, C<[>
609 or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
610 L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
612 If you are serious about writing a parser, there are a number of
613 modules or oddities that will make your life a lot easier. There are
614 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
615 and the byacc program. Starting from perl 5.8 the Text::Balanced
616 is part of the standard distribution.
618 One simple destructive, inside-out approach that you might try is to
619 pull out the smallest nesting parts one at a time:
621 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
622 # do something with $1
625 A more complicated and sneaky approach is to make Perl's regular
626 expression engine do it for you. This is courtesy Dean Inada, and
627 rather has the nature of an Obfuscated Perl Contest entry, but it
630 # $_ contains the string to parse
631 # BEGIN and END are the opening and closing markers for the
636 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
637 @$ = (eval{/$re/},$@!~/unmatched/i);
638 print join("\n",@$[0..$#$]) if( $$[-1] );
640 =head2 How do I reverse a string?
642 Use reverse() in scalar context, as documented in
645 $reversed = reverse $string;
647 =head2 How do I expand tabs in a string?
649 You can do it yourself:
651 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
653 Or you can just use the Text::Tabs module (part of the standard Perl
657 @expanded_lines = expand(@lines_with_tabs);
659 =head2 How do I reformat a paragraph?
661 Use Text::Wrap (part of the standard Perl distribution):
664 print wrap("\t", ' ', @paragraphs);
666 The paragraphs you give to Text::Wrap should not contain embedded
667 newlines. Text::Wrap doesn't justify the lines (flush-right).
669 Or use the CPAN module Text::Autoformat. Formatting files can be easily
670 done by making a shell alias, like so:
672 alias fmt="perl -i -MText::Autoformat -n0777 \
673 -e 'print autoformat $_, {all=>1}' $*"
675 See the documentation for Text::Autoformat to appreciate its many
678 =head2 How can I access or change N characters of a string?
680 You can access the first characters of a string with substr().
681 To get the first character, for example, start at position 0
682 and grab the string of length 1.
685 $string = "Just another Perl Hacker";
686 $first_char = substr( $string, 0, 1 ); # 'J'
688 To change part of a string, you can use the optional fourth
689 argument which is the replacement string.
691 substr( $string, 13, 4, "Perl 5.8.0" );
693 You can also use substr() as an lvalue.
695 substr( $string, 13, 4 ) = "Perl 5.8.0";
697 =head2 How do I change the Nth occurrence of something?
699 You have to keep track of N yourself. For example, let's say you want
700 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
701 C<"whosoever"> or C<"whomsoever">, case insensitively. These
702 all assume that $_ contains the string to be altered.
706 ++$count == 5 # is it the 5th?
707 ? "${2}soever" # yes, swap
708 : $1 # renege and leave it there
711 In the more general case, you can use the C</g> modifier in a C<while>
712 loop, keeping count of matches.
716 $_ = "One fish two fish red fish blue fish";
717 while (/(\w+)\s+fish\b/gi) {
718 if (++$count == $WANT) {
719 print "The third fish is a $1 one.\n";
723 That prints out: C<"The third fish is a red one."> You can also use a
724 repetition count and repeated pattern like this:
726 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
728 =head2 How can I count the number of occurrences of a substring within a string?
730 There are a number of ways, with varying efficiency. If you want a
731 count of a certain single character (X) within a string, you can use the
732 C<tr///> function like so:
734 $string = "ThisXlineXhasXsomeXx'sXinXit";
735 $count = ($string =~ tr/X//);
736 print "There are $count X characters in the string";
738 This is fine if you are just looking for a single character. However,
739 if you are trying to count multiple character substrings within a
740 larger string, C<tr///> won't work. What you can do is wrap a while()
741 loop around a global pattern match. For example, let's count negative
744 $string = "-9 55 48 -2 23 -76 4 14 -44";
745 while ($string =~ /-\d+/g) { $count++ }
746 print "There are $count negative numbers in the string";
748 Another version uses a global match in list context, then assigns the
749 result to a scalar, producing a count of the number of matches.
751 $count = () = $string =~ /-\d+/g;
753 =head2 How do I capitalize all the words on one line?
755 To make the first letter of each word upper case:
757 $line =~ s/\b(\w)/\U$1/g;
759 This has the strange effect of turning "C<don't do it>" into "C<Don'T
760 Do It>". Sometimes you might want this. Other times you might need a
761 more thorough solution (Suggested by brian d foy):
764 (^\w) #at the beginning of the line
766 (\s\w) #preceded by whitespace
769 $string =~ /([\w']+)/\u\L$1/g;
771 To make the whole line upper case:
775 To force each word to be lower case, with the first letter upper case:
777 $line =~ s/(\w+)/\u\L$1/g;
779 You can (and probably should) enable locale awareness of those
780 characters by placing a C<use locale> pragma in your program.
781 See L<perllocale> for endless details on locales.
783 This is sometimes referred to as putting something into "title
784 case", but that's not quite accurate. Consider the proper
785 capitalization of the movie I<Dr. Strangelove or: How I Learned to
786 Stop Worrying and Love the Bomb>, for example.
788 Damian Conway's L<Text::Autoformat> module provides some smart
789 case transformations:
791 use Text::Autoformat;
792 my $x = "Dr. Strangelove or: How I Learned to Stop ".
793 "Worrying and Love the Bomb";
796 for my $style (qw( sentence title highlight ))
798 print autoformat($x, { case => $style }), "\n";
801 =head2 How can I split a [character] delimited string except when inside [character]?
803 Several modules can handle this sort of pasing---Text::Balanced,
804 Text::CVS, Text::CVS_XS, and Text::ParseWords, among others.
806 Take the example case of trying to split a string that is
807 comma-separated into its different fields. You can't use C<split(/,/)>
808 because you shouldn't split if the comma is inside quotes. For
809 example, take a data line like this:
811 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
813 Due to the restriction of the quotes, this is a fairly complex
814 problem. Thankfully, we have Jeffrey Friedl, author of
815 I<Mastering Regular Expressions>, to handle these for us. He
816 suggests (assuming your string is contained in $text):
819 push(@new, $+) while $text =~ m{
820 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
824 push(@new, undef) if substr($text,-1,1) eq ',';
826 If you want to represent quotation marks inside a
827 quotation-mark-delimited field, escape them with backslashes (eg,
830 Alternatively, the Text::ParseWords module (part of the standard Perl
831 distribution) lets you say:
833 use Text::ParseWords;
834 @new = quotewords(",", 0, $text);
836 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
838 =head2 How do I strip blank space from the beginning/end of a string?
840 Although the simplest approach would seem to be
842 $string =~ s/^\s*(.*?)\s*$/$1/;
844 not only is this unnecessarily slow and destructive, it also fails with
845 embedded newlines. It is much faster to do this operation in two steps:
850 Or more nicely written as:
857 This idiom takes advantage of the C<foreach> loop's aliasing
858 behavior to factor out common code. You can do this
859 on several strings at once, or arrays, or even the
860 values of a hash if you use a slice:
862 # trim whitespace in the scalar, the array,
863 # and all the values in the hash
864 foreach ($scalar, @array, @hash{keys %hash}) {
869 =head2 How do I pad a string with blanks or pad a number with zeroes?
871 In the following examples, C<$pad_len> is the length to which you wish
872 to pad the string, C<$text> or C<$num> contains the string to be padded,
873 and C<$pad_char> contains the padding character. You can use a single
874 character string constant instead of the C<$pad_char> variable if you
875 know what it is in advance. And in the same way you can use an integer in
876 place of C<$pad_len> if you know the pad length in advance.
878 The simplest method uses the C<sprintf> function. It can pad on the left
879 or right with blanks and on the left with zeroes and it will not
880 truncate the result. The C<pack> function can only pad strings on the
881 right with blanks and it will truncate the result to a maximum length of
884 # Left padding a string with blanks (no truncation):
885 $padded = sprintf("%${pad_len}s", $text);
886 $padded = sprintf("%*s", $pad_len, $text); # same thing
888 # Right padding a string with blanks (no truncation):
889 $padded = sprintf("%-${pad_len}s", $text);
890 $padded = sprintf("%-*s", $pad_len, $text); # same thing
892 # Left padding a number with 0 (no truncation):
893 $padded = sprintf("%0${pad_len}d", $num);
894 $padded = sprintf("%0*d", $pad_len, $num); # same thing
896 # Right padding a string with blanks using pack (will truncate):
897 $padded = pack("A$pad_len",$text);
899 If you need to pad with a character other than blank or zero you can use
900 one of the following methods. They all generate a pad string with the
901 C<x> operator and combine that with C<$text>. These methods do
902 not truncate C<$text>.
904 Left and right padding with any character, creating a new string:
906 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
907 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
909 Left and right padding with any character, modifying C<$text> directly:
911 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
912 $text .= $pad_char x ( $pad_len - length( $text ) );
914 =head2 How do I extract selected columns from a string?
916 Use substr() or unpack(), both documented in L<perlfunc>.
917 If you prefer thinking in terms of columns instead of widths,
918 you can use this kind of thing:
920 # determine the unpack format needed to split Linux ps output
921 # arguments are cut columns
922 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
928 for my $place (@positions) {
929 $template .= "A" . ($place - $lastpos) . " ";
936 =head2 How do I find the soundex value of a string?
938 Use the standard Text::Soundex module distributed with Perl.
939 Before you do so, you may want to determine whether `soundex' is in
940 fact what you think it is. Knuth's soundex algorithm compresses words
941 into a small space, and so it does not necessarily distinguish between
942 two words which you might want to appear separately. For example, the
943 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
944 If Text::Soundex does not do what you are looking for, you might want
945 to consider the String::Approx module available at CPAN.
947 =head2 How can I expand variables in text strings?
949 Let's assume that you have a string like:
951 $text = 'this has a $foo in it and a $bar';
953 If those were both global variables, then this would
956 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
958 But since they are probably lexicals, or at least, they could
959 be, you'd have to do this:
961 $text =~ s/(\$\w+)/$1/eeg;
962 die if $@; # needed /ee, not /e
964 It's probably better in the general case to treat those
965 variables as entries in some special hash. For example:
971 $text =~ s/\$(\w+)/$user_defs{$1}/g;
973 See also ``How do I expand function calls in a string?'' in this section
976 =head2 What's wrong with always quoting "$vars"?
978 The problem is that those double-quotes force stringification--
979 coercing numbers and references into strings--even when you
980 don't want them to be strings. Think of it this way: double-quote
981 expansion is used to produce new strings. If you already
982 have a string, why do you need more?
984 If you get used to writing odd things like these:
988 somefunc("$var"); # BAD
990 You'll be in trouble. Those should (in 99.8% of the cases) be
991 the simpler and more direct:
997 Otherwise, besides slowing you down, you're going to break code when
998 the thing in the scalar is actually neither a string nor a number, but
1004 my $oref = "$aref"; # WRONG
1007 You can also get into subtle problems on those few operations in Perl
1008 that actually do care about the difference between a string and a
1009 number, such as the magical C<++> autoincrement operator or the
1012 Stringification also destroys arrays.
1015 print "@lines"; # WRONG - extra blanks
1016 print @lines; # right
1018 =head2 Why don't my E<lt>E<lt>HERE documents work?
1020 Check for these three things:
1024 =item There must be no space after the E<lt>E<lt> part.
1026 =item There (probably) should be a semicolon at the end.
1028 =item You can't (easily) have any space in front of the tag.
1032 If you want to indent the text in the here document, you
1036 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1041 But the HERE_TARGET must still be flush against the margin.
1042 If you want that indented also, you'll have to quote
1045 ($quote = <<' FINIS') =~ s/^\s+//gm;
1046 ...we will have peace, when you and all your works have
1047 perished--and the works of your dark master to whom you
1048 would deliver us. You are a liar, Saruman, and a corrupter
1049 of men's hearts. --Theoden in /usr/src/perl/taint.c
1051 $quote =~ s/\s+--/\n--/;
1053 A nice general-purpose fixer-upper function for indented here documents
1054 follows. It expects to be called with a here document as its argument.
1055 It looks to see whether each line begins with a common substring, and
1056 if so, strips that substring off. Otherwise, it takes the amount of leading
1057 whitespace found on the first line and removes that much off each
1062 my ($white, $leader); # common whitespace and common leading string
1063 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1064 ($white, $leader) = ($2, quotemeta($1));
1066 ($white, $leader) = (/^(\s+)/, '');
1068 s/^\s*?$leader(?:$white)?//gm;
1072 This works with leading special strings, dynamically determined:
1074 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1077 @@@ SAVEI32(runlevel);
1079 @@@ while ( op = (*op->op_ppaddr)() );
1083 MAIN_INTERPRETER_LOOP
1085 Or with a fixed amount of leading whitespace, with remaining
1086 indentation correctly preserved:
1088 $poem = fix<<EVER_ON_AND_ON;
1089 Now far ahead the Road has gone,
1090 And I must follow, if I can,
1091 Pursuing it with eager feet,
1092 Until it joins some larger way
1093 Where many paths and errands meet.
1094 And whither then? I cannot say.
1095 --Bilbo in /usr/src/perl/pp_ctl.c
1100 =head2 What is the difference between a list and an array?
1102 An array has a changeable length. A list does not. An array is something
1103 you can push or pop, while a list is a set of values. Some people make
1104 the distinction that a list is a value while an array is a variable.
1105 Subroutines are passed and return lists, you put things into list
1106 context, you initialize arrays with lists, and you foreach() across
1107 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1108 in scalar context behave like the number of elements in them, subroutines
1109 access their arguments through the array C<@_>, and push/pop/shift only work
1112 As a side note, there's no such thing as a list in scalar context.
1115 $scalar = (2, 5, 7, 9);
1117 you're using the comma operator in scalar context, so it uses the scalar
1118 comma operator. There never was a list there at all! This causes the
1119 last value to be returned: 9.
1121 =head2 What is the difference between $array[1] and @array[1]?
1123 The former is a scalar value; the latter an array slice, making
1124 it a list with one (scalar) value. You should use $ when you want a
1125 scalar value (most of the time) and @ when you want a list with one
1126 scalar value in it (very, very rarely; nearly never, in fact).
1128 Sometimes it doesn't make a difference, but sometimes it does.
1129 For example, compare:
1131 $good[0] = `some program that outputs several lines`;
1135 @bad[0] = `same program that outputs several lines`;
1137 The C<use warnings> pragma and the B<-w> flag will warn you about these
1140 =head2 How can I remove duplicate elements from a list or array?
1142 There are several possible ways, depending on whether the array is
1143 ordered and whether you wish to preserve the ordering.
1149 If @in is sorted, and you want @out to be sorted:
1150 (this assumes all true values in the array)
1152 $prev = "not equal to $in[0]";
1153 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1155 This is nice in that it doesn't use much extra memory, simulating
1156 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1157 guarantees that the expression is true (so that grep picks it up)
1158 even if the $_ is 0, "", or undef.
1162 If you don't know whether @in is sorted:
1165 @out = grep(!$saw{$_}++, @in);
1169 Like (b), but @in contains only small integers:
1171 @out = grep(!$saw[$_]++, @in);
1175 A way to do (b) without any loops or greps:
1179 @out = sort keys %saw; # remove sort if undesired
1183 Like (d), but @in contains only small positive integers:
1187 @out = grep {defined} @ary;
1191 But perhaps you should have been using a hash all along, eh?
1193 =head2 How can I tell whether a certain element is contained in a list or array?
1195 Hearing the word "in" is an I<in>dication that you probably should have
1196 used a hash, not a list or array, to store your data. Hashes are
1197 designed to answer this question quickly and efficiently. Arrays aren't.
1199 That being said, there are several ways to approach this. If you
1200 are going to make this query many times over arbitrary string values,
1201 the fastest way is probably to invert the original array and maintain a
1202 hash whose keys are the first array's values.
1204 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1206 for (@blues) { $is_blue{$_} = 1 }
1208 Now you can check whether $is_blue{$some_color}. It might have been a
1209 good idea to keep the blues all in a hash in the first place.
1211 If the values are all small integers, you could use a simple indexed
1212 array. This kind of an array will take up less space:
1214 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1215 @is_tiny_prime = ();
1216 for (@primes) { $is_tiny_prime[$_] = 1 }
1217 # or simply @istiny_prime[@primes] = (1) x @primes;
1219 Now you check whether $is_tiny_prime[$some_number].
1221 If the values in question are integers instead of strings, you can save
1222 quite a lot of space by using bit strings instead:
1224 @articles = ( 1..10, 150..2000, 2017 );
1226 for (@articles) { vec($read,$_,1) = 1 }
1228 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1232 ($is_there) = grep $_ eq $whatever, @array;
1236 ($is_there) = grep /$whatever/, @array;
1238 These are slow (checks every element even if the first matches),
1239 inefficient (same reason), and potentially buggy (what if there are
1240 regex characters in $whatever?). If you're only testing once, then
1244 foreach $elt (@array) {
1245 if ($elt eq $elt_to_find) {
1250 if ($is_there) { ... }
1252 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1254 Use a hash. Here's code to do both and more. It assumes that
1255 each element is unique in a given array:
1257 @union = @intersection = @difference = ();
1259 foreach $element (@array1, @array2) { $count{$element}++ }
1260 foreach $element (keys %count) {
1261 push @union, $element;
1262 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1265 Note that this is the I<symmetric difference>, that is, all elements in
1266 either A or in B but not in both. Think of it as an xor operation.
1268 =head2 How do I test whether two arrays or hashes are equal?
1270 The following code works for single-level arrays. It uses a stringwise
1271 comparison, and does not distinguish defined versus undefined empty
1272 strings. Modify if you have other needs.
1274 $are_equal = compare_arrays(\@frogs, \@toads);
1276 sub compare_arrays {
1277 my ($first, $second) = @_;
1278 no warnings; # silence spurious -w undef complaints
1279 return 0 unless @$first == @$second;
1280 for (my $i = 0; $i < @$first; $i++) {
1281 return 0 if $first->[$i] ne $second->[$i];
1286 For multilevel structures, you may wish to use an approach more
1287 like this one. It uses the CPAN module FreezeThaw:
1289 use FreezeThaw qw(cmpStr);
1290 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1292 printf "a and b contain %s arrays\n",
1293 cmpStr(\@a, \@b) == 0
1297 This approach also works for comparing hashes. Here
1298 we'll demonstrate two different answers:
1300 use FreezeThaw qw(cmpStr cmpStrHard);
1302 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1306 printf "a and b contain %s hashes\n",
1307 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1309 printf "a and b contain %s hashes\n",
1310 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1313 The first reports that both those the hashes contain the same data,
1314 while the second reports that they do not. Which you prefer is left as
1315 an exercise to the reader.
1317 =head2 How do I find the first array element for which a condition is true?
1319 To find the first array element which satisfies a condition, you can
1320 use the first() function in the List::Util module, which comes with
1321 Perl 5.8. This example finds the first element that contains "Perl".
1323 use List::Util qw(first);
1325 my $element = first { /Perl/ } @array;
1327 If you cannot use List::Util, you can make your own loop to do the
1328 same thing. Once you find the element, you stop the loop with last.
1331 foreach my $element ( @array )
1333 if( /Perl/ ) { $found = $element; last }
1336 If you want the array index, you can iterate through the indices
1337 and check the array element at each index until you find one
1338 that satisfies the condition.
1340 my( $found, $index ) = ( undef, -1 );
1341 for( $i = 0; $i < @array; $i++ )
1343 if( $array[$i] =~ /Perl/ )
1345 $found = $array[$i];
1351 =head2 How do I handle linked lists?
1353 In general, you usually don't need a linked list in Perl, since with
1354 regular arrays, you can push and pop or shift and unshift at either end,
1355 or you can use splice to add and/or remove arbitrary number of elements at
1356 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1357 dynamic arrays. In the absence of shifts and pops, push in general
1358 needs to reallocate on the order every log(N) times, and unshift will
1359 need to copy pointers each time.
1361 If you really, really wanted, you could use structures as described in
1362 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1363 to do. For example, imagine a list node like this:
1370 You could walk the list this way:
1373 for ($node = $head; $node; $node = $node->{LINK}) {
1374 print $node->{VALUE}, " ";
1378 You could add to the list this way:
1381 $tail = append($head, 1); # grow a new head
1382 for $value ( 2 .. 10 ) {
1383 $tail = append($tail, $value);
1387 my($list, $value) = @_;
1388 my $node = { VALUE => $value };
1390 $node->{LINK} = $list->{LINK};
1391 $list->{LINK} = $node;
1393 $_[0] = $node; # replace caller's version
1398 But again, Perl's built-in are virtually always good enough.
1400 =head2 How do I handle circular lists?
1402 Circular lists could be handled in the traditional fashion with linked
1403 lists, or you could just do something like this with an array:
1405 unshift(@array, pop(@array)); # the last shall be first
1406 push(@array, shift(@array)); # and vice versa
1408 =head2 How do I shuffle an array randomly?
1410 If you either have Perl 5.8.0 or later installed, or if you have
1411 Scalar-List-Utils 1.03 or later installed, you can say:
1413 use List::Util 'shuffle';
1415 @shuffled = shuffle(@list);
1417 If not, you can use a Fisher-Yates shuffle.
1419 sub fisher_yates_shuffle {
1420 my $deck = shift; # $deck is a reference to an array
1423 my $j = int rand ($i+1);
1424 @$deck[$i,$j] = @$deck[$j,$i];
1428 # shuffle my mpeg collection
1430 my @mpeg = <audio/*/*.mp3>;
1431 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1434 Note that the above implementation shuffles an array in place,
1435 unlike the List::Util::shuffle() which takes a list and returns
1436 a new shuffled list.
1438 You've probably seen shuffling algorithms that work using splice,
1439 randomly picking another element to swap the current element with
1443 @old = 1 .. 10; # just a demo
1445 push(@new, splice(@old, rand @old, 1));
1448 This is bad because splice is already O(N), and since you do it N times,
1449 you just invented a quadratic algorithm; that is, O(N**2). This does
1450 not scale, although Perl is so efficient that you probably won't notice
1451 this until you have rather largish arrays.
1453 =head2 How do I process/modify each element of an array?
1455 Use C<for>/C<foreach>:
1458 s/foo/bar/; # change that word
1459 y/XZ/ZX/; # swap those letters
1462 Here's another; let's compute spherical volumes:
1464 for (@volumes = @radii) { # @volumes has changed parts
1466 $_ *= (4/3) * 3.14159; # this will be constant folded
1469 which can also be done with map() which is made to transform
1470 one list into another:
1472 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1474 If you want to do the same thing to modify the values of the
1475 hash, you can use the C<values> function. As of Perl 5.6
1476 the values are not copied, so if you modify $orbit (in this
1477 case), you modify the value.
1479 for $orbit ( values %orbits ) {
1480 ($orbit **= 3) *= (4/3) * 3.14159;
1483 Prior to perl 5.6 C<values> returned copies of the values,
1484 so older perl code often contains constructions such as
1485 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1486 the hash is to be modified.
1488 =head2 How do I select a random element from an array?
1490 Use the rand() function (see L<perlfunc/rand>):
1492 # at the top of the program:
1493 srand; # not needed for 5.004 and later
1496 $index = rand @array;
1497 $element = $array[$index];
1499 Make sure you I<only call srand once per program, if then>.
1500 If you are calling it more than once (such as before each
1501 call to rand), you're almost certainly doing something wrong.
1503 =head2 How do I permute N elements of a list?
1505 Use the List::Permutor module on CPAN. If the list is
1506 actually an array, try the Algorithm::Permute module (also
1507 on CPAN). It's written in XS code and is very efficient.
1509 use Algorithm::Permute;
1510 my @array = 'a'..'d';
1511 my $p_iterator = Algorithm::Permute->new ( \@array );
1512 while (my @perm = $p_iterator->next) {
1513 print "next permutation: (@perm)\n";
1516 For even faster execution, you could do:
1518 use Algorithm::Permute;
1519 my @array = 'a'..'d';
1520 Algorithm::Permute::permute {
1521 print "next permutation: (@array)\n";
1524 Here's a little program that generates all permutations of
1525 all the words on each line of input. The algorithm embodied
1526 in the permute() function is discussed in Volume 4 (still
1527 unpublished) of Knuth's I<The Art of Computer Programming>
1528 and will work on any list:
1531 # Fischer-Kause ordered permutation generator
1536 while ( $code->(@_[@idx]) ) {
1538 --$p while $idx[$p-1] > $idx[$p];
1539 my $q = $p or return;
1540 push @idx, reverse splice @idx, $p;
1541 ++$q while $idx[$p-1] > $idx[$q];
1542 @idx[$p-1,$q]=@idx[$q,$p-1];
1546 permute {print"@_\n"} split;
1548 =head2 How do I sort an array by (anything)?
1550 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1552 @list = sort { $a <=> $b } @list;
1554 The default sort function is cmp, string comparison, which would
1555 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1556 the numerical comparison operator.
1558 If you have a complicated function needed to pull out the part you
1559 want to sort on, then don't do it inside the sort function. Pull it
1560 out first, because the sort BLOCK can be called many times for the
1561 same element. Here's an example of how to pull out the first word
1562 after the first number on each item, and then sort those words
1567 ($item) = /\d+\s*(\S+)/;
1568 push @idx, uc($item);
1570 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1572 which could also be written this way, using a trick
1573 that's come to be known as the Schwartzian Transform:
1575 @sorted = map { $_->[0] }
1576 sort { $a->[1] cmp $b->[1] }
1577 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1579 If you need to sort on several fields, the following paradigm is useful.
1581 @sorted = sort { field1($a) <=> field1($b) ||
1582 field2($a) cmp field2($b) ||
1583 field3($a) cmp field3($b)
1586 This can be conveniently combined with precalculation of keys as given
1589 See the F<sort> artitcle article in the "Far More Than You Ever Wanted
1590 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1591 more about this approach.
1593 See also the question below on sorting hashes.
1595 =head2 How do I manipulate arrays of bits?
1597 Use pack() and unpack(), or else vec() and the bitwise operations.
1599 For example, this sets $vec to have bit N set if $ints[N] was set:
1602 foreach(@ints) { vec($vec,$_,1) = 1 }
1604 Here's how, given a vector in $vec, you can
1605 get those bits into your @ints array:
1607 sub bitvec_to_list {
1610 # Find null-byte density then select best algorithm
1611 if ($vec =~ tr/\0// / length $vec > 0.95) {
1614 # This method is faster with mostly null-bytes
1615 while($vec =~ /[^\0]/g ) {
1616 $i = -9 + 8 * pos $vec;
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);
1623 push @ints, $i if vec($vec, ++$i, 1);
1624 push @ints, $i if vec($vec, ++$i, 1);
1627 # This method is a fast general algorithm
1629 my $bits = unpack "b*", $vec;
1630 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1631 push @ints, pos $bits while($bits =~ /1/g);
1636 This method gets faster the more sparse the bit vector is.
1637 (Courtesy of Tim Bunce and Winfried Koenig.)
1639 You can make the while loop a lot shorter with this suggestion
1640 from Benjamin Goldberg:
1642 while($vec =~ /[^\0]+/g ) {
1643 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1646 Or use the CPAN module Bit::Vector:
1648 $vector = Bit::Vector->new($num_of_bits);
1649 $vector->Index_List_Store(@ints);
1650 @ints = $vector->Index_List_Read();
1652 Bit::Vector provides efficient methods for bit vector, sets of small integers
1655 Here's a more extensive illustration using vec():
1658 $vector = "\xff\x0f\xef\xfe";
1659 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1660 unpack("N", $vector), "\n";
1661 $is_set = vec($vector, 23, 1);
1662 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1680 my ($offset, $width, $value) = @_;
1682 vec($vector, $offset, $width) = $value;
1683 print "offset=$offset width=$width value=$value\n";
1689 my $bits = unpack("b*", $vector);
1693 print "vector length in bytes: ", length($vector), "\n";
1694 @bytes = unpack("A8" x length($vector), $bits);
1695 print "bits are: @bytes\n\n";
1698 =head2 Why does defined() return true on empty arrays and hashes?
1700 The short story is that you should probably only use defined on scalars or
1701 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1702 in the 5.004 release or later of Perl for more detail.
1704 =head1 Data: Hashes (Associative Arrays)
1706 =head2 How do I process an entire hash?
1708 Use the each() function (see L<perlfunc/each>) if you don't care
1709 whether it's sorted:
1711 while ( ($key, $value) = each %hash) {
1712 print "$key = $value\n";
1715 If you want it sorted, you'll have to use foreach() on the result of
1716 sorting the keys as shown in an earlier question.
1718 =head2 What happens if I add or remove keys from a hash while iterating over it?
1722 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1723 iterating over it. In Perl 5 you can delete from it, but you still
1724 can't add to it, because that might cause a doubling of the hash table,
1725 in which half the entries get copied up to the new top half of the
1726 table, at which point you've totally bamboozled the iterator code.
1727 Even if the table doesn't double, there's no telling whether your new
1728 entry will be inserted before or after the current iterator position.
1730 Either treasure up your changes and make them after the iterator finishes
1731 or use keys to fetch all the old keys at once, and iterate over the list
1734 =head2 How do I look up a hash element by value?
1736 Create a reverse hash:
1738 %by_value = reverse %by_key;
1739 $key = $by_value{$value};
1741 That's not particularly efficient. It would be more space-efficient
1744 while (($key, $value) = each %by_key) {
1745 $by_value{$value} = $key;
1748 If your hash could have repeated values, the methods above will only find
1749 one of the associated keys. This may or may not worry you. If it does
1750 worry you, you can always reverse the hash into a hash of arrays instead:
1752 while (($key, $value) = each %by_key) {
1753 push @{$key_list_by_value{$value}}, $key;
1756 =head2 How can I know how many entries are in a hash?
1758 If you mean how many keys, then all you have to do is
1759 use the keys() function in a scalar context:
1761 $num_keys = keys %hash;
1763 The keys() function also resets the iterator, which means that you may
1764 see strange results if you use this between uses of other hash operators
1767 =head2 How do I sort a hash (optionally by value instead of key)?
1769 Internally, hashes are stored in a way that prevents you from imposing
1770 an order on key-value pairs. Instead, you have to sort a list of the
1773 @keys = sort keys %hash; # sorted by key
1775 $hash{$a} cmp $hash{$b}
1776 } keys %hash; # and by value
1778 Here we'll do a reverse numeric sort by value, and if two keys are
1779 identical, sort by length of key, or if that fails, by straight ASCII
1780 comparison of the keys (well, possibly modified by your locale--see
1784 $hash{$b} <=> $hash{$a}
1786 length($b) <=> length($a)
1791 =head2 How can I always keep my hash sorted?
1793 You can look into using the DB_File module and tie() using the
1794 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1795 The Tie::IxHash module from CPAN might also be instructive.
1797 =head2 What's the difference between "delete" and "undef" with hashes?
1799 Hashes contain pairs of scalars: the first is the key, the
1800 second is the value. The key will be coerced to a string,
1801 although the value can be any kind of scalar: string,
1802 number, or reference. If a key $key is present in
1803 %hash, C<exists($hash{$key})> will return true. The value
1804 for a given key can be C<undef>, in which case
1805 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1806 will return true. This corresponds to (C<$key>, C<undef>)
1809 Pictures help... here's the %hash table:
1819 And these conditions hold
1823 defined $hash{'d'} is true
1824 defined $hash{'a'} is true
1825 exists $hash{'a'} is true (Perl5 only)
1826 grep ($_ eq 'a', keys %hash) is true
1832 your table now reads:
1843 and these conditions now hold; changes in caps:
1847 defined $hash{'d'} is true
1848 defined $hash{'a'} is FALSE
1849 exists $hash{'a'} is true (Perl5 only)
1850 grep ($_ eq 'a', keys %hash) is true
1852 Notice the last two: you have an undef value, but a defined key!
1858 your table now reads:
1867 and these conditions now hold; changes in caps:
1871 defined $hash{'d'} is true
1872 defined $hash{'a'} is false
1873 exists $hash{'a'} is FALSE (Perl5 only)
1874 grep ($_ eq 'a', keys %hash) is FALSE
1876 See, the whole entry is gone!
1878 =head2 Why don't my tied hashes make the defined/exists distinction?
1880 This depends on the tied hash's implementation of EXISTS().
1881 For example, there isn't the concept of undef with hashes
1882 that are tied to DBM* files. It also means that exists() and
1883 defined() do the same thing with a DBM* file, and what they
1884 end up doing is not what they do with ordinary hashes.
1886 =head2 How do I reset an each() operation part-way through?
1888 Using C<keys %hash> in scalar context returns the number of keys in
1889 the hash I<and> resets the iterator associated with the hash. You may
1890 need to do this if you use C<last> to exit a loop early so that when you
1891 re-enter it, the hash iterator has been reset.
1893 =head2 How can I get the unique keys from two hashes?
1895 First you extract the keys from the hashes into lists, then solve
1896 the "removing duplicates" problem described above. For example:
1899 for $element (keys(%foo), keys(%bar)) {
1906 @uniq = keys %{{%foo,%bar}};
1908 Or if you really want to save space:
1911 while (defined ($key = each %foo)) {
1914 while (defined ($key = each %bar)) {
1919 =head2 How can I store a multidimensional array in a DBM file?
1921 Either stringify the structure yourself (no fun), or else
1922 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1923 it on top of either DB_File or GDBM_File.
1925 =head2 How can I make my hash remember the order I put elements into it?
1927 Use the Tie::IxHash from CPAN.
1930 tie my %myhash, 'Tie::IxHash';
1931 for (my $i=0; $i<20; $i++) {
1934 my @keys = keys %myhash;
1935 # @keys = (0,1,2,3,...)
1937 =head2 Why does passing a subroutine an undefined element in a hash create it?
1939 If you say something like:
1941 somefunc($hash{"nonesuch key here"});
1943 Then that element "autovivifies"; that is, it springs into existence
1944 whether you store something there or not. That's because functions
1945 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1946 it has to be ready to write it back into the caller's version.
1948 This has been fixed as of Perl5.004.
1950 Normally, merely accessing a key's value for a nonexistent key does
1951 I<not> cause that key to be forever there. This is different than
1954 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1956 Usually a hash ref, perhaps like this:
1961 TITLE => "deputy peon",
1964 PALS => [ "Norbert", "Rhys", "Phineas"],
1967 References are documented in L<perlref> and the upcoming L<perlreftut>.
1968 Examples of complex data structures are given in L<perldsc> and
1969 L<perllol>. Examples of structures and object-oriented classes are
1972 =head2 How can I use a reference as a hash key?
1974 You can't do this directly, but you could use the standard Tie::RefHash
1975 module distributed with Perl.
1979 =head2 How do I handle binary data correctly?
1981 Perl is binary clean, so this shouldn't be a problem. For example,
1982 this works fine (assuming the files are found):
1984 if (`cat /vmunix` =~ /gzip/) {
1985 print "Your kernel is GNU-zip enabled!\n";
1988 On less elegant (read: Byzantine) systems, however, you have
1989 to play tedious games with "text" versus "binary" files. See
1990 L<perlfunc/"binmode"> or L<perlopentut>.
1992 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1994 If you want to deal with multibyte characters, however, there are
1995 some gotchas. See the section on Regular Expressions.
1997 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1999 Assuming that you don't care about IEEE notations like "NaN" or
2000 "Infinity", you probably just want to use a regular expression.
2002 if (/\D/) { print "has nondigits\n" }
2003 if (/^\d+$/) { print "is a whole number\n" }
2004 if (/^-?\d+$/) { print "is an integer\n" }
2005 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2006 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2007 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2008 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2009 { print "a C float\n" }
2011 There are also some commonly used modules for the task.
2012 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2013 internal function C<looks_like_number> for determining
2014 whether a variable looks like a number. L<Data::Types>
2015 exports functions that validate data types using both the
2016 above and other regular expressions. Thirdly, there is
2017 C<Regexp::Common> which has regular expressions to match
2018 various types of numbers. Those three modules are available
2021 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2022 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
2023 wrapper function for more convenient access. This function takes
2024 a string and returns the number it found, or C<undef> for input that
2025 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
2026 if you just want to say, ``Is this a float?''
2029 use POSIX qw(strtod);
2034 my($num, $unparsed) = strtod($str);
2035 if (($str eq '') || ($unparsed != 0) || $!) {
2042 sub is_numeric { defined getnum($_[0]) }
2044 Or you could check out the L<String::Scanf> module on the CPAN
2045 instead. The POSIX module (part of the standard Perl distribution) provides
2046 the C<strtod> and C<strtol> for converting strings to double and longs,
2049 =head2 How do I keep persistent data across program calls?
2051 For some specific applications, you can use one of the DBM modules.
2052 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2053 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2054 of the standard distribution. Here's one example using Storable's C<store>
2055 and C<retrieve> functions:
2058 store(\%hash, "filename");
2061 $href = retrieve("filename"); # by ref
2062 %hash = %{ retrieve("filename") }; # direct to hash
2064 =head2 How do I print out or copy a recursive data structure?
2066 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2067 for printing out data structures. The Storable module on CPAN (or the
2068 5.8 release of Perl), provides a function called C<dclone> that recursively
2069 copies its argument.
2071 use Storable qw(dclone);
2074 Where $r1 can be a reference to any kind of data structure you'd like.
2075 It will be deeply copied. Because C<dclone> takes and returns references,
2076 you'd have to add extra punctuation if you had a hash of arrays that
2079 %newhash = %{ dclone(\%oldhash) };
2081 =head2 How do I define methods for every class/object?
2083 Use the UNIVERSAL class (see L<UNIVERSAL>).
2085 =head2 How do I verify a credit card checksum?
2087 Get the Business::CreditCard module from CPAN.
2089 =head2 How do I pack arrays of doubles or floats for XS code?
2091 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2092 If you're doing a lot of float or double processing, consider using
2093 the PDL module from CPAN instead--it makes number-crunching easy.
2095 =head1 AUTHOR AND COPYRIGHT
2097 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
2098 All rights reserved.
2100 This documentation is free; you can redistribute it and/or modify it
2101 under the same terms as Perl itself.
2103 Irrespective of its distribution, all code examples in this file
2104 are hereby placed into the public domain. You are permitted and
2105 encouraged to use this code in your own programs for fun
2106 or for profit as you see fit. A simple comment in the code giving
2107 credit would be courteous but is not required.