3 perlfaq4 - Data Manipulation ($Revision: 1.43 $, $Date: 2003/02/23 20:25:09 $)
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 isn't my octal data interpreted correctly?
33 Perl only understands octal and hex numbers as such when they occur as
34 literals in your program. Octal literals in perl must start with a
35 leading "0" and hexadecimal literals must start with a leading "0x".
36 If they are read in from somewhere and assigned, no automatic
37 conversion takes place. You must explicitly use oct() or hex() if you
38 want the values converted to decimal. oct() interprets hex ("0x350"),
39 octal ("0350" or even without the leading "0", like "377") and binary
40 ("0b1010") numbers, while hex() only converts hexadecimal ones, with
41 or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
42 The inverse mapping from decimal to octal can be done with either the
43 "%o" or "%O" sprintf() formats.
45 This problem shows up most often when people try using chmod(), mkdir(),
46 umask(), or sysopen(), which by widespread tradition typically take
49 chmod(644, $file); # WRONG
50 chmod(0644, $file); # right
52 Note the mistake in the first line was specifying the decimal literal
53 644, rather than the intended octal literal 0644. The problem can
56 printf("%#o",644); # prints 01204
58 Surely you had not intended C<chmod(01204, $file);> - did you? If you
59 want to use numeric literals as arguments to chmod() et al. then please
60 try to express them as octal constants, that is with a leading zero and
61 with the following digits restricted to the set 0..7.
63 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
65 Remember that int() merely truncates toward 0. For rounding to a
66 certain number of digits, sprintf() or printf() is usually the easiest
69 printf("%.3f", 3.1415926535); # prints 3.142
71 The POSIX module (part of the standard Perl distribution) implements
72 ceil(), floor(), and a number of other mathematical and trigonometric
76 $ceil = ceil(3.5); # 4
77 $floor = floor(3.5); # 3
79 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
80 module. With 5.004, the Math::Trig module (part of the standard Perl
81 distribution) implements the trigonometric functions. Internally it
82 uses the Math::Complex module and some functions can break out from
83 the real axis into the complex plane, for example the inverse sine of
86 Rounding in financial applications can have serious implications, and
87 the rounding method used should be specified precisely. In these
88 cases, it probably pays not to trust whichever system rounding is
89 being used by Perl, but to instead implement the rounding function you
92 To see why, notice how you'll still have an issue on half-way-point
95 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
97 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
98 0.8 0.8 0.9 0.9 1.0 1.0
100 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
101 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
102 machines) will work pretty much like mathematical integers. Other numbers
105 =head2 How do I convert between numeric representations?
107 As always with Perl there is more than one way to do it. Below
108 are a few examples of approaches to making common conversions
109 between number representations. This is intended to be representational
110 rather than exhaustive.
112 Some of the examples below use the Bit::Vector module from CPAN.
113 The reason you might choose Bit::Vector over the perl built in
114 functions is that it works with numbers of ANY size, that it is
115 optimized for speed on some operations, and for at least some
116 programmers the notation might be familiar.
120 =item How do I convert hexadecimal into decimal
122 Using perl's built in conversion of 0x notation:
125 $dec = sprintf("%d", $int);
127 Using the hex function:
129 $int = hex("DEADBEEF");
130 $dec = sprintf("%d", $int);
134 $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
135 $dec = sprintf("%d", $int);
137 Using the CPAN module Bit::Vector:
140 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
141 $dec = $vec->to_Dec();
143 =item How do I convert from decimal to hexadecimal
147 $hex = sprintf("%X", 3735928559);
151 $hex = unpack("H*", pack("N", 3735928559));
156 $vec = Bit::Vector->new_Dec(32, -559038737);
157 $hex = $vec->to_Hex();
159 And Bit::Vector supports odd bit counts:
162 $vec = Bit::Vector->new_Dec(33, 3735928559);
163 $vec->Resize(32); # suppress leading 0 if unwanted
164 $hex = $vec->to_Hex();
166 =item How do I convert from octal to decimal
168 Using Perl's built in conversion of numbers with leading zeros:
170 $int = 033653337357; # note the leading 0!
171 $dec = sprintf("%d", $int);
173 Using the oct function:
175 $int = oct("33653337357");
176 $dec = sprintf("%d", $int);
181 $vec = Bit::Vector->new(32);
182 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
183 $dec = $vec->to_Dec();
185 =item How do I convert from decimal to octal
189 $oct = sprintf("%o", 3735928559);
194 $vec = Bit::Vector->new_Dec(32, -559038737);
195 $oct = reverse join('', $vec->Chunk_List_Read(3));
197 =item How do I convert from binary to decimal
199 Perl 5.6 lets you write binary numbers directly with
202 $number = 0b10110110;
206 $decimal = ord(pack('B8', '10110110'));
208 Using pack and unpack for larger strings
210 $int = unpack("N", pack("B32",
211 substr("0" x 32 . "11110101011011011111011101111", -32)));
212 $dec = sprintf("%d", $int);
214 # substr() is used to left pad a 32 character string with zeros.
218 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
219 $dec = $vec->to_Dec();
221 =item How do I convert from decimal to binary
225 $bin = unpack("B*", pack("N", 3735928559));
230 $vec = Bit::Vector->new_Dec(32, -559038737);
231 $bin = $vec->to_Bin();
233 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
234 are left as an exercise to the inclined reader.
238 =head2 Why doesn't & work the way I want it to?
240 The behavior of binary arithmetic operators depends on whether they're
241 used on numbers or strings. The operators treat a string as a series
242 of bits and work with that (the string C<"3"> is the bit pattern
243 C<00110011>). The operators work with the binary form of a number
244 (the number C<3> is treated as the bit pattern C<00000011>).
246 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
247 C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
250 Most problems with C<&> and C<|> arise because the programmer thinks
251 they have a number but really it's a string. The rest arise because
254 if ("\020\020" & "\101\101") {
258 but a string consisting of two null bytes (the result of C<"\020\020"
259 & "\101\101">) is not a false value in Perl. You need:
261 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
265 =head2 How do I multiply matrices?
267 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
268 or the PDL extension (also available from CPAN).
270 =head2 How do I perform an operation on a series of integers?
272 To call a function on each element in an array, and collect the
275 @results = map { my_func($_) } @array;
279 @triple = map { 3 * $_ } @single;
281 To call a function on each element of an array, but ignore the
284 foreach $iterator (@array) {
285 some_func($iterator);
288 To call a function on each integer in a (small) range, you B<can> use:
290 @results = map { some_func($_) } (5 .. 25);
292 but you should be aware that the C<..> operator creates an array of
293 all integers in the range. This can take a lot of memory for large
297 for ($i=5; $i < 500_005; $i++) {
298 push(@results, some_func($i));
301 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
302 loop will iterate over the range, without creating the entire range.
304 for my $i (5 .. 500_005) {
305 push(@results, some_func($i));
308 will not create a list of 500,000 integers.
310 =head2 How can I output Roman numerals?
312 Get the http://www.cpan.org/modules/by-module/Roman module.
314 =head2 Why aren't my random numbers random?
316 If you're using a version of Perl before 5.004, you must call C<srand>
317 once at the start of your program to seed the random number generator.
319 BEGIN { srand() if $] < 5.004 }
321 5.004 and later automatically call C<srand> at the beginning. Don't
322 call C<srand> more than once---you make your numbers less random, rather
325 Computers are good at being predictable and bad at being random
326 (despite appearances caused by bugs in your programs :-). see the
327 F<random> article in the "Far More Than You Ever Wanted To Know"
328 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
329 Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
330 who attempts to generate random numbers by deterministic means is, of
331 course, living in a state of sin.''
333 If you want numbers that are more random than C<rand> with C<srand>
334 provides, you should also check out the Math::TrulyRandom module from
335 CPAN. It uses the imperfections in your system's timer to generate
336 random numbers, but this takes quite a while. If you want a better
337 pseudorandom generator than comes with your operating system, look at
338 ``Numerical Recipes in C'' at http://www.nr.com/ .
340 =head2 How do I get a random number between X and Y?
342 Use the following simple function. It selects a random integer between
343 (and possibly including!) the two given integers, e.g.,
344 C<random_int_in(50,120)>
346 sub random_int_in ($$) {
348 # Assumes that the two arguments are integers themselves!
349 return $min if $min == $max;
350 ($min, $max) = ($max, $min) if $min > $max;
351 return $min + int rand(1 + $max - $min);
356 =head2 How do I find the day or week of the year?
358 The localtime function returns the day of the week. Without an
359 argument localtime uses the current time.
361 $day_of_year = (localtime)[7];
363 The POSIX module can also format a date as the day of the year or
366 use POSIX qw/strftime/;
367 my $day_of_year = strftime "%j", localtime;
368 my $week_of_year = strftime "%W", localtime;
370 To get the day of year for any date, use the Time::Local module to get
371 a time in epoch seconds for the argument to localtime.
373 use POSIX qw/strftime/;
375 my $week_of_year = strftime "%W",
376 localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
378 The Date::Calc module provides two functions for to calculate these.
381 my $day_of_year = Day_of_Year( 1987, 12, 18 );
382 my $week_of_year = Week_of_Year( 1987, 12, 18 );
384 =head2 How do I find the current century or millennium?
386 Use the following simple functions:
389 return int((((localtime(shift || time))[5] + 1999))/100);
392 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
395 You can also use the POSIX strftime() function which may be a bit
396 slower but is easier to read and maintain.
398 use POSIX qw/strftime/;
400 my $week_of_the_year = strftime "%W", localtime;
401 my $day_of_the_year = strftime "%j", localtime;
403 On some systems, the POSIX module's strftime() function has
404 been extended in a non-standard way to use a C<%C> format,
405 which they sometimes claim is the "century". It isn't,
406 because on most such systems, this is only the first two
407 digits of the four-digit year, and thus cannot be used to
408 reliably determine the current century or millennium.
410 =head2 How can I compare two dates and find the difference?
412 If you're storing your dates as epoch seconds then simply subtract one
413 from the other. If you've got a structured date (distinct year, day,
414 month, hour, minute, seconds values), then for reasons of accessibility,
415 simplicity, and efficiency, merely use either timelocal or timegm (from
416 the Time::Local module in the standard distribution) to reduce structured
417 dates to epoch seconds. However, if you don't know the precise format of
418 your dates, then you should probably use either of the Date::Manip and
419 Date::Calc modules from CPAN before you go hacking up your own parsing
420 routine to handle arbitrary date formats.
422 =head2 How can I take a string and turn it into epoch seconds?
424 If it's a regular enough string that it always has the same format,
425 you can split it up and pass the parts to C<timelocal> in the standard
426 Time::Local module. Otherwise, you should look into the Date::Calc
427 and Date::Manip modules from CPAN.
429 =head2 How can I find the Julian Day?
431 Use the Time::JulianDay module (part of the Time-modules bundle
432 available from CPAN.)
434 Before you immerse yourself too deeply in this, be sure to verify that
435 it is the I<Julian> Day you really want. Are you interested in a way
436 of getting serial days so that you just can tell how many days they
437 are apart or so that you can do also other date arithmetic? If you
438 are interested in performing date arithmetic, this can be done using
439 modules Date::Manip or Date::Calc.
441 There is too many details and much confusion on this issue to cover in
442 this FAQ, but the term is applied (correctly) to a calendar now
443 supplanted by the Gregorian Calendar, with the Julian Calendar failing
444 to adjust properly for leap years on centennial years (among other
445 annoyances). The term is also used (incorrectly) to mean: [1] days in
446 the Gregorian Calendar; and [2] days since a particular starting time
447 or `epoch', usually 1970 in the Unix world and 1980 in the
448 MS-DOS/Windows world. If you find that it is not the first meaning
449 that you really want, then check out the Date::Manip and Date::Calc
450 modules. (Thanks to David Cassell for most of this text.)
452 =head2 How do I find yesterday's date?
454 If you only need to find the date (and not the same time), you
455 can use the Date::Calc module.
457 use Date::Calc qw(Today Add_Delta_Days);
459 my @date = Add_Delta_Days( Today(), -1 );
463 Most people try to use the time rather than the calendar to
464 figure out dates, but that assumes that your days are
465 twenty-four hours each. For most people, there are two days
466 a year when they aren't: the switch to and from summer time
467 throws this off. Russ Allbery offers this solution.
470 my $now = defined $_[0] ? $_[0] : time;
471 my $then = $now - 60 * 60 * 24;
472 my $ndst = (localtime $now)[8] > 0;
473 my $tdst = (localtime $then)[8] > 0;
474 $then - ($tdst - $ndst) * 60 * 60;
477 Should give you "this time yesterday" in seconds since epoch relative to
478 the first argument or the current time if no argument is given and
479 suitable for passing to localtime or whatever else you need to do with
480 it. $ndst is whether we're currently in daylight savings time; $tdst is
481 whether the point 24 hours ago was in daylight savings time. If $tdst
482 and $ndst are the same, a boundary wasn't crossed, and the correction
483 will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
484 from yesterday's time since we gained an extra hour while going off
485 daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
486 negative hour (add an hour) to yesterday's time since we lost an hour.
488 All of this is because during those days when one switches off or onto
489 DST, a "day" isn't 24 hours long; it's either 23 or 25.
491 The explicit settings of $ndst and $tdst are necessary because localtime
492 only says it returns the system tm struct, and the system tm struct at
493 least on Solaris doesn't guarantee any particular positive value (like,
494 say, 1) for isdst, just a positive value. And that value can
495 potentially be negative, if DST information isn't available (this sub
496 just treats those cases like no DST).
498 Note that between 2am and 3am on the day after the time zone switches
499 off daylight savings time, the exact hour of "yesterday" corresponding
500 to the current hour is not clearly defined. Note also that if used
501 between 2am and 3am the day after the change to daylight savings time,
502 the result will be between 3am and 4am of the previous day; it's
503 arguable whether this is correct.
505 This sub does not attempt to deal with leap seconds (most things don't).
509 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
511 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
512 Y2K compliant (whatever that means). The programmers you've hired to
513 use it, however, probably are not.
515 Long answer: The question belies a true understanding of the issue.
516 Perl is just as Y2K compliant as your pencil--no more, and no less.
517 Can you use your pencil to write a non-Y2K-compliant memo? Of course
518 you can. Is that the pencil's fault? Of course it isn't.
520 The date and time functions supplied with Perl (gmtime and localtime)
521 supply adequate information to determine the year well beyond 2000
522 (2038 is when trouble strikes for 32-bit machines). The year returned
523 by these functions when used in a list context is the year minus 1900.
524 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
525 number. To avoid the year 2000 problem simply do not treat the year as
526 a 2-digit number. It isn't.
528 When gmtime() and localtime() are used in scalar context they return
529 a timestamp string that contains a fully-expanded year. For example,
530 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
531 2001". There's no year 2000 problem here.
533 That doesn't mean that Perl can't be used to create non-Y2K compliant
534 programs. It can. But so can your pencil. It's the fault of the user,
535 not the language. At the risk of inflaming the NRA: ``Perl doesn't
536 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
541 =head2 How do I validate input?
543 The answer to this question is usually a regular expression, perhaps
544 with auxiliary logic. See the more specific questions (numbers, mail
545 addresses, etc.) for details.
547 =head2 How do I unescape a string?
549 It depends just what you mean by ``escape''. URL escapes are dealt
550 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
551 character are removed with
555 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
557 =head2 How do I remove consecutive pairs of characters?
559 To turn C<"abbcccd"> into C<"abccd">:
561 s/(.)\1/$1/g; # add /s to include newlines
563 Here's a solution that turns "abbcccd" to "abcd":
565 y///cs; # y == tr, but shorter :-)
567 =head2 How do I expand function calls in a string?
569 This is documented in L<perlref>. In general, this is fraught with
570 quoting and readability problems, but it is possible. To interpolate
571 a subroutine call (in list context) into a string:
573 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
575 See also ``How can I expand variables in text strings?'' in this
578 =head2 How do I find matching/nesting anything?
580 This isn't something that can be done in one regular expression, no
581 matter how complicated. To find something between two single
582 characters, a pattern like C</x([^x]*)x/> will get the intervening
583 bits in $1. For multiple ones, then something more like
584 C</alpha(.*?)omega/> would be needed. But none of these deals with
585 nested patterns. For balanced expressions using C<(>, C<{>, C<[>
586 or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
587 L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
589 If you are serious about writing a parser, there are a number of
590 modules or oddities that will make your life a lot easier. There are
591 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
592 and the byacc program. Starting from perl 5.8 the Text::Balanced
593 is part of the standard distribution.
595 One simple destructive, inside-out approach that you might try is to
596 pull out the smallest nesting parts one at a time:
598 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
599 # do something with $1
602 A more complicated and sneaky approach is to make Perl's regular
603 expression engine do it for you. This is courtesy Dean Inada, and
604 rather has the nature of an Obfuscated Perl Contest entry, but it
607 # $_ contains the string to parse
608 # BEGIN and END are the opening and closing markers for the
613 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
614 @$ = (eval{/$re/},$@!~/unmatched/i);
615 print join("\n",@$[0..$#$]) if( $$[-1] );
617 =head2 How do I reverse a string?
619 Use reverse() in scalar context, as documented in
622 $reversed = reverse $string;
624 =head2 How do I expand tabs in a string?
626 You can do it yourself:
628 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
630 Or you can just use the Text::Tabs module (part of the standard Perl
634 @expanded_lines = expand(@lines_with_tabs);
636 =head2 How do I reformat a paragraph?
638 Use Text::Wrap (part of the standard Perl distribution):
641 print wrap("\t", ' ', @paragraphs);
643 The paragraphs you give to Text::Wrap should not contain embedded
644 newlines. Text::Wrap doesn't justify the lines (flush-right).
646 Or use the CPAN module Text::Autoformat. Formatting files can be easily
647 done by making a shell alias, like so:
649 alias fmt="perl -i -MText::Autoformat -n0777 \
650 -e 'print autoformat $_, {all=>1}' $*"
652 See the documentation for Text::Autoformat to appreciate its many
655 =head2 How can I access or change N characters of a string?
657 You can access the first characters of a string with substr().
658 To get the first character, for example, start at position 0
659 and grab the string of length 1.
662 $string = "Just another Perl Hacker";
663 $first_char = substr( $string, 0, 1 ); # 'J'
665 To change part of a string, you can use the optional fourth
666 argument which is the replacement string.
668 substr( $string, 13, 4, "Perl 5.8.0" );
670 You can also use substr() as an lvalue.
672 substr( $string, 13, 4 ) = "Perl 5.8.0";
674 =head2 How do I change the Nth occurrence of something?
676 You have to keep track of N yourself. For example, let's say you want
677 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
678 C<"whosoever"> or C<"whomsoever">, case insensitively. These
679 all assume that $_ contains the string to be altered.
683 ++$count == 5 # is it the 5th?
684 ? "${2}soever" # yes, swap
685 : $1 # renege and leave it there
688 In the more general case, you can use the C</g> modifier in a C<while>
689 loop, keeping count of matches.
693 $_ = "One fish two fish red fish blue fish";
694 while (/(\w+)\s+fish\b/gi) {
695 if (++$count == $WANT) {
696 print "The third fish is a $1 one.\n";
700 That prints out: C<"The third fish is a red one."> You can also use a
701 repetition count and repeated pattern like this:
703 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
705 =head2 How can I count the number of occurrences of a substring within a string?
707 There are a number of ways, with varying efficiency. If you want a
708 count of a certain single character (X) within a string, you can use the
709 C<tr///> function like so:
711 $string = "ThisXlineXhasXsomeXx'sXinXit";
712 $count = ($string =~ tr/X//);
713 print "There are $count X characters in the string";
715 This is fine if you are just looking for a single character. However,
716 if you are trying to count multiple character substrings within a
717 larger string, C<tr///> won't work. What you can do is wrap a while()
718 loop around a global pattern match. For example, let's count negative
721 $string = "-9 55 48 -2 23 -76 4 14 -44";
722 while ($string =~ /-\d+/g) { $count++ }
723 print "There are $count negative numbers in the string";
725 Another version uses a global match in list context, then assigns the
726 result to a scalar, producing a count of the number of matches.
728 $count = () = $string =~ /-\d+/g;
730 =head2 How do I capitalize all the words on one line?
732 To make the first letter of each word upper case:
734 $line =~ s/\b(\w)/\U$1/g;
736 This has the strange effect of turning "C<don't do it>" into "C<Don'T
737 Do It>". Sometimes you might want this. Other times you might need a
738 more thorough solution (Suggested by brian d foy):
741 (^\w) #at the beginning of the line
743 (\s\w) #preceded by whitespace
746 $string =~ /([\w']+)/\u\L$1/g;
748 To make the whole line upper case:
752 To force each word to be lower case, with the first letter upper case:
754 $line =~ s/(\w+)/\u\L$1/g;
756 You can (and probably should) enable locale awareness of those
757 characters by placing a C<use locale> pragma in your program.
758 See L<perllocale> for endless details on locales.
760 This is sometimes referred to as putting something into "title
761 case", but that's not quite accurate. Consider the proper
762 capitalization of the movie I<Dr. Strangelove or: How I Learned to
763 Stop Worrying and Love the Bomb>, for example.
765 =head2 How can I split a [character] delimited string except when inside [character]?
767 Several modules can handle this sort of pasing---Text::Balanced,
768 Text::CVS, Text::CVS_XS, and Text::ParseWords, among others.
770 Take the example case of trying to split a string that is
771 comma-separated into its different fields. You can't use C<split(/,/)>
772 because you shouldn't split if the comma is inside quotes. For
773 example, take a data line like this:
775 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
777 Due to the restriction of the quotes, this is a fairly complex
778 problem. Thankfully, we have Jeffrey Friedl, author of
779 I<Mastering Regular Expressions>, to handle these for us. He
780 suggests (assuming your string is contained in $text):
783 push(@new, $+) while $text =~ m{
784 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
788 push(@new, undef) if substr($text,-1,1) eq ',';
790 If you want to represent quotation marks inside a
791 quotation-mark-delimited field, escape them with backslashes (eg,
794 Alternatively, the Text::ParseWords module (part of the standard Perl
795 distribution) lets you say:
797 use Text::ParseWords;
798 @new = quotewords(",", 0, $text);
800 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
802 =head2 How do I strip blank space from the beginning/end of a string?
804 Although the simplest approach would seem to be
806 $string =~ s/^\s*(.*?)\s*$/$1/;
808 not only is this unnecessarily slow and destructive, it also fails with
809 embedded newlines. It is much faster to do this operation in two steps:
814 Or more nicely written as:
821 This idiom takes advantage of the C<foreach> loop's aliasing
822 behavior to factor out common code. You can do this
823 on several strings at once, or arrays, or even the
824 values of a hash if you use a slice:
826 # trim whitespace in the scalar, the array,
827 # and all the values in the hash
828 foreach ($scalar, @array, @hash{keys %hash}) {
833 =head2 How do I pad a string with blanks or pad a number with zeroes?
835 In the following examples, C<$pad_len> is the length to which you wish
836 to pad the string, C<$text> or C<$num> contains the string to be padded,
837 and C<$pad_char> contains the padding character. You can use a single
838 character string constant instead of the C<$pad_char> variable if you
839 know what it is in advance. And in the same way you can use an integer in
840 place of C<$pad_len> if you know the pad length in advance.
842 The simplest method uses the C<sprintf> function. It can pad on the left
843 or right with blanks and on the left with zeroes and it will not
844 truncate the result. The C<pack> function can only pad strings on the
845 right with blanks and it will truncate the result to a maximum length of
848 # Left padding a string with blanks (no truncation):
849 $padded = sprintf("%${pad_len}s", $text);
850 $padded = sprintf("%*s", $pad_len, $text); # same thing
852 # Right padding a string with blanks (no truncation):
853 $padded = sprintf("%-${pad_len}s", $text);
854 $padded = sprintf("%-*s", $pad_len, $text); # same thing
856 # Left padding a number with 0 (no truncation):
857 $padded = sprintf("%0${pad_len}d", $num);
858 $padded = sprintf("%0*d", $pad_len, $num); # same thing
860 # Right padding a string with blanks using pack (will truncate):
861 $padded = pack("A$pad_len",$text);
863 If you need to pad with a character other than blank or zero you can use
864 one of the following methods. They all generate a pad string with the
865 C<x> operator and combine that with C<$text>. These methods do
866 not truncate C<$text>.
868 Left and right padding with any character, creating a new string:
870 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
871 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
873 Left and right padding with any character, modifying C<$text> directly:
875 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
876 $text .= $pad_char x ( $pad_len - length( $text ) );
878 =head2 How do I extract selected columns from a string?
880 Use substr() or unpack(), both documented in L<perlfunc>.
881 If you prefer thinking in terms of columns instead of widths,
882 you can use this kind of thing:
884 # determine the unpack format needed to split Linux ps output
885 # arguments are cut columns
886 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
892 for my $place (@positions) {
893 $template .= "A" . ($place - $lastpos) . " ";
900 =head2 How do I find the soundex value of a string?
902 Use the standard Text::Soundex module distributed with Perl.
903 Before you do so, you may want to determine whether `soundex' is in
904 fact what you think it is. Knuth's soundex algorithm compresses words
905 into a small space, and so it does not necessarily distinguish between
906 two words which you might want to appear separately. For example, the
907 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
908 If Text::Soundex does not do what you are looking for, you might want
909 to consider the String::Approx module available at CPAN.
911 =head2 How can I expand variables in text strings?
913 Let's assume that you have a string like:
915 $text = 'this has a $foo in it and a $bar';
917 If those were both global variables, then this would
920 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
922 But since they are probably lexicals, or at least, they could
923 be, you'd have to do this:
925 $text =~ s/(\$\w+)/$1/eeg;
926 die if $@; # needed /ee, not /e
928 It's probably better in the general case to treat those
929 variables as entries in some special hash. For example:
935 $text =~ s/\$(\w+)/$user_defs{$1}/g;
937 See also ``How do I expand function calls in a string?'' in this section
940 =head2 What's wrong with always quoting "$vars"?
942 The problem is that those double-quotes force stringification--
943 coercing numbers and references into strings--even when you
944 don't want them to be strings. Think of it this way: double-quote
945 expansion is used to produce new strings. If you already
946 have a string, why do you need more?
948 If you get used to writing odd things like these:
952 somefunc("$var"); # BAD
954 You'll be in trouble. Those should (in 99.8% of the cases) be
955 the simpler and more direct:
961 Otherwise, besides slowing you down, you're going to break code when
962 the thing in the scalar is actually neither a string nor a number, but
968 my $oref = "$aref"; # WRONG
971 You can also get into subtle problems on those few operations in Perl
972 that actually do care about the difference between a string and a
973 number, such as the magical C<++> autoincrement operator or the
976 Stringification also destroys arrays.
979 print "@lines"; # WRONG - extra blanks
980 print @lines; # right
982 =head2 Why don't my E<lt>E<lt>HERE documents work?
984 Check for these three things:
988 =item There must be no space after the E<lt>E<lt> part.
990 =item There (probably) should be a semicolon at the end.
992 =item You can't (easily) have any space in front of the tag.
996 If you want to indent the text in the here document, you
1000 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1005 But the HERE_TARGET must still be flush against the margin.
1006 If you want that indented also, you'll have to quote
1009 ($quote = <<' FINIS') =~ s/^\s+//gm;
1010 ...we will have peace, when you and all your works have
1011 perished--and the works of your dark master to whom you
1012 would deliver us. You are a liar, Saruman, and a corrupter
1013 of men's hearts. --Theoden in /usr/src/perl/taint.c
1015 $quote =~ s/\s+--/\n--/;
1017 A nice general-purpose fixer-upper function for indented here documents
1018 follows. It expects to be called with a here document as its argument.
1019 It looks to see whether each line begins with a common substring, and
1020 if so, strips that substring off. Otherwise, it takes the amount of leading
1021 whitespace found on the first line and removes that much off each
1026 my ($white, $leader); # common whitespace and common leading string
1027 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1028 ($white, $leader) = ($2, quotemeta($1));
1030 ($white, $leader) = (/^(\s+)/, '');
1032 s/^\s*?$leader(?:$white)?//gm;
1036 This works with leading special strings, dynamically determined:
1038 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1041 @@@ SAVEI32(runlevel);
1043 @@@ while ( op = (*op->op_ppaddr)() );
1047 MAIN_INTERPRETER_LOOP
1049 Or with a fixed amount of leading whitespace, with remaining
1050 indentation correctly preserved:
1052 $poem = fix<<EVER_ON_AND_ON;
1053 Now far ahead the Road has gone,
1054 And I must follow, if I can,
1055 Pursuing it with eager feet,
1056 Until it joins some larger way
1057 Where many paths and errands meet.
1058 And whither then? I cannot say.
1059 --Bilbo in /usr/src/perl/pp_ctl.c
1064 =head2 What is the difference between a list and an array?
1066 An array has a changeable length. A list does not. An array is something
1067 you can push or pop, while a list is a set of values. Some people make
1068 the distinction that a list is a value while an array is a variable.
1069 Subroutines are passed and return lists, you put things into list
1070 context, you initialize arrays with lists, and you foreach() across
1071 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1072 in scalar context behave like the number of elements in them, subroutines
1073 access their arguments through the array C<@_>, and push/pop/shift only work
1076 As a side note, there's no such thing as a list in scalar context.
1079 $scalar = (2, 5, 7, 9);
1081 you're using the comma operator in scalar context, so it uses the scalar
1082 comma operator. There never was a list there at all! This causes the
1083 last value to be returned: 9.
1085 =head2 What is the difference between $array[1] and @array[1]?
1087 The former is a scalar value; the latter an array slice, making
1088 it a list with one (scalar) value. You should use $ when you want a
1089 scalar value (most of the time) and @ when you want a list with one
1090 scalar value in it (very, very rarely; nearly never, in fact).
1092 Sometimes it doesn't make a difference, but sometimes it does.
1093 For example, compare:
1095 $good[0] = `some program that outputs several lines`;
1099 @bad[0] = `same program that outputs several lines`;
1101 The C<use warnings> pragma and the B<-w> flag will warn you about these
1104 =head2 How can I remove duplicate elements from a list or array?
1106 There are several possible ways, depending on whether the array is
1107 ordered and whether you wish to preserve the ordering.
1113 If @in is sorted, and you want @out to be sorted:
1114 (this assumes all true values in the array)
1116 $prev = "not equal to $in[0]";
1117 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1119 This is nice in that it doesn't use much extra memory, simulating
1120 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1121 guarantees that the expression is true (so that grep picks it up)
1122 even if the $_ is 0, "", or undef.
1126 If you don't know whether @in is sorted:
1129 @out = grep(!$saw{$_}++, @in);
1133 Like (b), but @in contains only small integers:
1135 @out = grep(!$saw[$_]++, @in);
1139 A way to do (b) without any loops or greps:
1143 @out = sort keys %saw; # remove sort if undesired
1147 Like (d), but @in contains only small positive integers:
1151 @out = grep {defined} @ary;
1155 But perhaps you should have been using a hash all along, eh?
1157 =head2 How can I tell whether a certain element is contained in a list or array?
1159 Hearing the word "in" is an I<in>dication that you probably should have
1160 used a hash, not a list or array, to store your data. Hashes are
1161 designed to answer this question quickly and efficiently. Arrays aren't.
1163 That being said, there are several ways to approach this. If you
1164 are going to make this query many times over arbitrary string values,
1165 the fastest way is probably to invert the original array and maintain a
1166 hash whose keys are the first array's values.
1168 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1170 for (@blues) { $is_blue{$_} = 1 }
1172 Now you can check whether $is_blue{$some_color}. It might have been a
1173 good idea to keep the blues all in a hash in the first place.
1175 If the values are all small integers, you could use a simple indexed
1176 array. This kind of an array will take up less space:
1178 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1179 @is_tiny_prime = ();
1180 for (@primes) { $is_tiny_prime[$_] = 1 }
1181 # or simply @istiny_prime[@primes] = (1) x @primes;
1183 Now you check whether $is_tiny_prime[$some_number].
1185 If the values in question are integers instead of strings, you can save
1186 quite a lot of space by using bit strings instead:
1188 @articles = ( 1..10, 150..2000, 2017 );
1190 for (@articles) { vec($read,$_,1) = 1 }
1192 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1196 ($is_there) = grep $_ eq $whatever, @array;
1200 ($is_there) = grep /$whatever/, @array;
1202 These are slow (checks every element even if the first matches),
1203 inefficient (same reason), and potentially buggy (what if there are
1204 regex characters in $whatever?). If you're only testing once, then
1208 foreach $elt (@array) {
1209 if ($elt eq $elt_to_find) {
1214 if ($is_there) { ... }
1216 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1218 Use a hash. Here's code to do both and more. It assumes that
1219 each element is unique in a given array:
1221 @union = @intersection = @difference = ();
1223 foreach $element (@array1, @array2) { $count{$element}++ }
1224 foreach $element (keys %count) {
1225 push @union, $element;
1226 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1229 Note that this is the I<symmetric difference>, that is, all elements in
1230 either A or in B but not in both. Think of it as an xor operation.
1232 =head2 How do I test whether two arrays or hashes are equal?
1234 The following code works for single-level arrays. It uses a stringwise
1235 comparison, and does not distinguish defined versus undefined empty
1236 strings. Modify if you have other needs.
1238 $are_equal = compare_arrays(\@frogs, \@toads);
1240 sub compare_arrays {
1241 my ($first, $second) = @_;
1242 no warnings; # silence spurious -w undef complaints
1243 return 0 unless @$first == @$second;
1244 for (my $i = 0; $i < @$first; $i++) {
1245 return 0 if $first->[$i] ne $second->[$i];
1250 For multilevel structures, you may wish to use an approach more
1251 like this one. It uses the CPAN module FreezeThaw:
1253 use FreezeThaw qw(cmpStr);
1254 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1256 printf "a and b contain %s arrays\n",
1257 cmpStr(\@a, \@b) == 0
1261 This approach also works for comparing hashes. Here
1262 we'll demonstrate two different answers:
1264 use FreezeThaw qw(cmpStr cmpStrHard);
1266 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1270 printf "a and b contain %s hashes\n",
1271 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1273 printf "a and b contain %s hashes\n",
1274 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1277 The first reports that both those the hashes contain the same data,
1278 while the second reports that they do not. Which you prefer is left as
1279 an exercise to the reader.
1281 =head2 How do I find the first array element for which a condition is true?
1283 To find the first array element which satisfies a condition, you can
1284 use the first() function in the List::Util module, which comes with
1285 Perl 5.8. This example finds the first element that contains "Perl".
1287 use List::Util qw(first);
1289 my $element = first { /Perl/ } @array;
1291 If you cannot use List::Util, you can make your own loop to do the
1292 same thing. Once you find the element, you stop the loop with last.
1295 foreach my $element ( @array )
1297 if( /Perl/ ) { $found = $element; last }
1300 If you want the array index, you can iterate through the indices
1301 and check the array element at each index until you find one
1302 that satisfies the condition.
1304 my( $found, $index ) = ( undef, -1 );
1305 for( $i = 0; $i < @array; $i++ )
1307 if( $array[$i] =~ /Perl/ )
1309 $found = $array[$i];
1315 =head2 How do I handle linked lists?
1317 In general, you usually don't need a linked list in Perl, since with
1318 regular arrays, you can push and pop or shift and unshift at either end,
1319 or you can use splice to add and/or remove arbitrary number of elements at
1320 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1321 dynamic arrays. In the absence of shifts and pops, push in general
1322 needs to reallocate on the order every log(N) times, and unshift will
1323 need to copy pointers each time.
1325 If you really, really wanted, you could use structures as described in
1326 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1327 to do. For example, imagine a list node like this:
1334 You could walk the list this way:
1337 for ($node = $head; $node; $node = $node->{LINK}) {
1338 print $node->{VALUE}, " ";
1342 You could add to the list this way:
1345 $tail = append($head, 1); # grow a new head
1346 for $value ( 2 .. 10 ) {
1347 $tail = append($tail, $value);
1351 my($list, $value) = @_;
1352 my $node = { VALUE => $value };
1354 $node->{LINK} = $list->{LINK};
1355 $list->{LINK} = $node;
1357 $_[0] = $node; # replace caller's version
1362 But again, Perl's built-in are virtually always good enough.
1364 =head2 How do I handle circular lists?
1366 Circular lists could be handled in the traditional fashion with linked
1367 lists, or you could just do something like this with an array:
1369 unshift(@array, pop(@array)); # the last shall be first
1370 push(@array, shift(@array)); # and vice versa
1372 =head2 How do I shuffle an array randomly?
1374 If you either have Perl 5.8.0 or later installed, or if you have
1375 Scalar-List-Utils 1.03 or later installed, you can say:
1377 use List::Util 'shuffle';
1379 @shuffled = shuffle(@list);
1381 If not, you can use a Fisher-Yates shuffle.
1383 sub fisher_yates_shuffle {
1384 my $deck = shift; # $deck is a reference to an array
1387 my $j = int rand ($i+1);
1388 @$deck[$i,$j] = @$deck[$j,$i];
1392 # shuffle my mpeg collection
1394 my @mpeg = <audio/*/*.mp3>;
1395 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1398 Note that the above implementation shuffles an array in place,
1399 unlike the List::Util::shuffle() which takes a list and returns
1400 a new shuffled list.
1402 You've probably seen shuffling algorithms that work using splice,
1403 randomly picking another element to swap the current element with
1407 @old = 1 .. 10; # just a demo
1409 push(@new, splice(@old, rand @old, 1));
1412 This is bad because splice is already O(N), and since you do it N times,
1413 you just invented a quadratic algorithm; that is, O(N**2). This does
1414 not scale, although Perl is so efficient that you probably won't notice
1415 this until you have rather largish arrays.
1417 =head2 How do I process/modify each element of an array?
1419 Use C<for>/C<foreach>:
1422 s/foo/bar/; # change that word
1423 y/XZ/ZX/; # swap those letters
1426 Here's another; let's compute spherical volumes:
1428 for (@volumes = @radii) { # @volumes has changed parts
1430 $_ *= (4/3) * 3.14159; # this will be constant folded
1433 which can also be done with map() which is made to transform
1434 one list into another:
1436 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1438 If you want to do the same thing to modify the values of the
1439 hash, you can use the C<values> function. As of Perl 5.6
1440 the values are not copied, so if you modify $orbit (in this
1441 case), you modify the value.
1443 for $orbit ( values %orbits ) {
1444 ($orbit **= 3) *= (4/3) * 3.14159;
1447 Prior to perl 5.6 C<values> returned copies of the values,
1448 so older perl code often contains constructions such as
1449 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1450 the hash is to be modified.
1452 =head2 How do I select a random element from an array?
1454 Use the rand() function (see L<perlfunc/rand>):
1456 # at the top of the program:
1457 srand; # not needed for 5.004 and later
1460 $index = rand @array;
1461 $element = $array[$index];
1463 Make sure you I<only call srand once per program, if then>.
1464 If you are calling it more than once (such as before each
1465 call to rand), you're almost certainly doing something wrong.
1467 =head2 How do I permute N elements of a list?
1469 Use the List::Permutor module on CPAN. If the list is
1470 actually an array, try the Algorithm::Permute module (also
1471 on CPAN). It's written in XS code and is very efficient.
1473 use Algorithm::Permute;
1474 my @array = 'a'..'d';
1475 my $p_iterator = Algorithm::Permute->new ( \@array );
1476 while (my @perm = $p_iterator->next) {
1477 print "next permutation: (@perm)\n";
1480 For even faster execution, you could do:
1482 use Algorithm::Permute;
1483 my @array = 'a'..'d';
1484 Algorithm::Permute::permute {
1485 print "next permutation: (@array)\n";
1488 Here's a little program that generates all permutations of
1489 all the words on each line of input. The algorithm embodied
1490 in the permute() function is discussed in Volume 4 (still
1491 unpublished) of Knuth's I<The Art of Computer Programming>
1492 and will work on any list:
1495 # Fischer-Kause ordered permutation generator
1500 while ( $code->(@_[@idx]) ) {
1502 --$p while $idx[$p-1] > $idx[$p];
1503 my $q = $p or return;
1504 push @idx, reverse splice @idx, $p;
1505 ++$q while $idx[$p-1] > $idx[$q];
1506 @idx[$p-1,$q]=@idx[$q,$p-1];
1510 permute {print"@_\n"} split;
1512 =head2 How do I sort an array by (anything)?
1514 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1516 @list = sort { $a <=> $b } @list;
1518 The default sort function is cmp, string comparison, which would
1519 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1520 the numerical comparison operator.
1522 If you have a complicated function needed to pull out the part you
1523 want to sort on, then don't do it inside the sort function. Pull it
1524 out first, because the sort BLOCK can be called many times for the
1525 same element. Here's an example of how to pull out the first word
1526 after the first number on each item, and then sort those words
1531 ($item) = /\d+\s*(\S+)/;
1532 push @idx, uc($item);
1534 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1536 which could also be written this way, using a trick
1537 that's come to be known as the Schwartzian Transform:
1539 @sorted = map { $_->[0] }
1540 sort { $a->[1] cmp $b->[1] }
1541 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1543 If you need to sort on several fields, the following paradigm is useful.
1545 @sorted = sort { field1($a) <=> field1($b) ||
1546 field2($a) cmp field2($b) ||
1547 field3($a) cmp field3($b)
1550 This can be conveniently combined with precalculation of keys as given
1553 See the F<sort> artitcle article in the "Far More Than You Ever Wanted
1554 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1555 more about this approach.
1557 See also the question below on sorting hashes.
1559 =head2 How do I manipulate arrays of bits?
1561 Use pack() and unpack(), or else vec() and the bitwise operations.
1563 For example, this sets $vec to have bit N set if $ints[N] was set:
1566 foreach(@ints) { vec($vec,$_,1) = 1 }
1568 Here's how, given a vector in $vec, you can
1569 get those bits into your @ints array:
1571 sub bitvec_to_list {
1574 # Find null-byte density then select best algorithm
1575 if ($vec =~ tr/\0// / length $vec > 0.95) {
1578 # This method is faster with mostly null-bytes
1579 while($vec =~ /[^\0]/g ) {
1580 $i = -9 + 8 * pos $vec;
1581 push @ints, $i if vec($vec, ++$i, 1);
1582 push @ints, $i if vec($vec, ++$i, 1);
1583 push @ints, $i if vec($vec, ++$i, 1);
1584 push @ints, $i if vec($vec, ++$i, 1);
1585 push @ints, $i if vec($vec, ++$i, 1);
1586 push @ints, $i if vec($vec, ++$i, 1);
1587 push @ints, $i if vec($vec, ++$i, 1);
1588 push @ints, $i if vec($vec, ++$i, 1);
1591 # This method is a fast general algorithm
1593 my $bits = unpack "b*", $vec;
1594 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1595 push @ints, pos $bits while($bits =~ /1/g);
1600 This method gets faster the more sparse the bit vector is.
1601 (Courtesy of Tim Bunce and Winfried Koenig.)
1603 You can make the while loop a lot shorter with this suggestion
1604 from Benjamin Goldberg:
1606 while($vec =~ /[^\0]+/g ) {
1607 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1610 Or use the CPAN module Bit::Vector:
1612 $vector = Bit::Vector->new($num_of_bits);
1613 $vector->Index_List_Store(@ints);
1614 @ints = $vector->Index_List_Read();
1616 Bit::Vector provides efficient methods for bit vector, sets of small integers
1619 Here's a more extensive illustration using vec():
1622 $vector = "\xff\x0f\xef\xfe";
1623 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1624 unpack("N", $vector), "\n";
1625 $is_set = vec($vector, 23, 1);
1626 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1644 my ($offset, $width, $value) = @_;
1646 vec($vector, $offset, $width) = $value;
1647 print "offset=$offset width=$width value=$value\n";
1653 my $bits = unpack("b*", $vector);
1657 print "vector length in bytes: ", length($vector), "\n";
1658 @bytes = unpack("A8" x length($vector), $bits);
1659 print "bits are: @bytes\n\n";
1662 =head2 Why does defined() return true on empty arrays and hashes?
1664 The short story is that you should probably only use defined on scalars or
1665 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1666 in the 5.004 release or later of Perl for more detail.
1668 =head1 Data: Hashes (Associative Arrays)
1670 =head2 How do I process an entire hash?
1672 Use the each() function (see L<perlfunc/each>) if you don't care
1673 whether it's sorted:
1675 while ( ($key, $value) = each %hash) {
1676 print "$key = $value\n";
1679 If you want it sorted, you'll have to use foreach() on the result of
1680 sorting the keys as shown in an earlier question.
1682 =head2 What happens if I add or remove keys from a hash while iterating over it?
1686 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1687 iterating over it. In Perl 5 you can delete from it, but you still
1688 can't add to it, because that might cause a doubling of the hash table,
1689 in which half the entries get copied up to the new top half of the
1690 table, at which point you've totally bamboozled the iterator code.
1691 Even if the table doesn't double, there's no telling whether your new
1692 entry will be inserted before or after the current iterator position.
1694 Either treasure up your changes and make them after the iterator finishes
1695 or use keys to fetch all the old keys at once, and iterate over the list
1698 =head2 How do I look up a hash element by value?
1700 Create a reverse hash:
1702 %by_value = reverse %by_key;
1703 $key = $by_value{$value};
1705 That's not particularly efficient. It would be more space-efficient
1708 while (($key, $value) = each %by_key) {
1709 $by_value{$value} = $key;
1712 If your hash could have repeated values, the methods above will only find
1713 one of the associated keys. This may or may not worry you. If it does
1714 worry you, you can always reverse the hash into a hash of arrays instead:
1716 while (($key, $value) = each %by_key) {
1717 push @{$key_list_by_value{$value}}, $key;
1720 =head2 How can I know how many entries are in a hash?
1722 If you mean how many keys, then all you have to do is
1723 use the keys() function in a scalar context:
1725 $num_keys = keys %hash;
1727 The keys() function also resets the iterator, which means that you may
1728 see strange results if you use this between uses of other hash operators
1731 =head2 How do I sort a hash (optionally by value instead of key)?
1733 Internally, hashes are stored in a way that prevents you from imposing
1734 an order on key-value pairs. Instead, you have to sort a list of the
1737 @keys = sort keys %hash; # sorted by key
1739 $hash{$a} cmp $hash{$b}
1740 } keys %hash; # and by value
1742 Here we'll do a reverse numeric sort by value, and if two keys are
1743 identical, sort by length of key, or if that fails, by straight ASCII
1744 comparison of the keys (well, possibly modified by your locale--see
1748 $hash{$b} <=> $hash{$a}
1750 length($b) <=> length($a)
1755 =head2 How can I always keep my hash sorted?
1757 You can look into using the DB_File module and tie() using the
1758 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1759 The Tie::IxHash module from CPAN might also be instructive.
1761 =head2 What's the difference between "delete" and "undef" with hashes?
1763 Hashes contain pairs of scalars: the first is the key, the
1764 second is the value. The key will be coerced to a string,
1765 although the value can be any kind of scalar: string,
1766 number, or reference. If a key $key is present in
1767 %hash, C<exists($hash{$key})> will return true. The value
1768 for a given key can be C<undef>, in which case
1769 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1770 will return true. This corresponds to (C<$key>, C<undef>)
1773 Pictures help... here's the %hash table:
1783 And these conditions hold
1787 defined $hash{'d'} is true
1788 defined $hash{'a'} is true
1789 exists $hash{'a'} is true (Perl5 only)
1790 grep ($_ eq 'a', keys %hash) is true
1796 your table now reads:
1807 and these conditions now hold; changes in caps:
1811 defined $hash{'d'} is true
1812 defined $hash{'a'} is FALSE
1813 exists $hash{'a'} is true (Perl5 only)
1814 grep ($_ eq 'a', keys %hash) is true
1816 Notice the last two: you have an undef value, but a defined key!
1822 your table now reads:
1831 and these conditions now hold; changes in caps:
1835 defined $hash{'d'} is true
1836 defined $hash{'a'} is false
1837 exists $hash{'a'} is FALSE (Perl5 only)
1838 grep ($_ eq 'a', keys %hash) is FALSE
1840 See, the whole entry is gone!
1842 =head2 Why don't my tied hashes make the defined/exists distinction?
1844 This depends on the tied hash's implementation of EXISTS().
1845 For example, there isn't the concept of undef with hashes
1846 that are tied to DBM* files. It also means that exists() and
1847 defined() do the same thing with a DBM* file, and what they
1848 end up doing is not what they do with ordinary hashes.
1850 =head2 How do I reset an each() operation part-way through?
1852 Using C<keys %hash> in scalar context returns the number of keys in
1853 the hash I<and> resets the iterator associated with the hash. You may
1854 need to do this if you use C<last> to exit a loop early so that when you
1855 re-enter it, the hash iterator has been reset.
1857 =head2 How can I get the unique keys from two hashes?
1859 First you extract the keys from the hashes into lists, then solve
1860 the "removing duplicates" problem described above. For example:
1863 for $element (keys(%foo), keys(%bar)) {
1870 @uniq = keys %{{%foo,%bar}};
1872 Or if you really want to save space:
1875 while (defined ($key = each %foo)) {
1878 while (defined ($key = each %bar)) {
1883 =head2 How can I store a multidimensional array in a DBM file?
1885 Either stringify the structure yourself (no fun), or else
1886 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1887 it on top of either DB_File or GDBM_File.
1889 =head2 How can I make my hash remember the order I put elements into it?
1891 Use the Tie::IxHash from CPAN.
1894 tie my %myhash, 'Tie::IxHash';
1895 for (my $i=0; $i<20; $i++) {
1898 my @keys = keys %myhash;
1899 # @keys = (0,1,2,3,...)
1901 =head2 Why does passing a subroutine an undefined element in a hash create it?
1903 If you say something like:
1905 somefunc($hash{"nonesuch key here"});
1907 Then that element "autovivifies"; that is, it springs into existence
1908 whether you store something there or not. That's because functions
1909 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1910 it has to be ready to write it back into the caller's version.
1912 This has been fixed as of Perl5.004.
1914 Normally, merely accessing a key's value for a nonexistent key does
1915 I<not> cause that key to be forever there. This is different than
1918 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1920 Usually a hash ref, perhaps like this:
1925 TITLE => "deputy peon",
1928 PALS => [ "Norbert", "Rhys", "Phineas"],
1931 References are documented in L<perlref> and the upcoming L<perlreftut>.
1932 Examples of complex data structures are given in L<perldsc> and
1933 L<perllol>. Examples of structures and object-oriented classes are
1936 =head2 How can I use a reference as a hash key?
1938 You can't do this directly, but you could use the standard Tie::RefHash
1939 module distributed with Perl.
1943 =head2 How do I handle binary data correctly?
1945 Perl is binary clean, so this shouldn't be a problem. For example,
1946 this works fine (assuming the files are found):
1948 if (`cat /vmunix` =~ /gzip/) {
1949 print "Your kernel is GNU-zip enabled!\n";
1952 On less elegant (read: Byzantine) systems, however, you have
1953 to play tedious games with "text" versus "binary" files. See
1954 L<perlfunc/"binmode"> or L<perlopentut>.
1956 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1958 If you want to deal with multibyte characters, however, there are
1959 some gotchas. See the section on Regular Expressions.
1961 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1963 Assuming that you don't care about IEEE notations like "NaN" or
1964 "Infinity", you probably just want to use a regular expression.
1966 if (/\D/) { print "has nondigits\n" }
1967 if (/^\d+$/) { print "is a whole number\n" }
1968 if (/^-?\d+$/) { print "is an integer\n" }
1969 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1970 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1971 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
1972 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1973 { print "a C float\n" }
1975 You can also use the L<Data::Types|Data::Types> module on
1976 the CPAN, which exports functions that validate data types
1977 using these and other regular expressions, or you can use
1978 the C<Regexp::Common> module from CPAN which has regular
1979 expressions to match various types of numbers.
1981 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1982 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1983 wrapper function for more convenient access. This function takes
1984 a string and returns the number it found, or C<undef> for input that
1985 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1986 if you just want to say, ``Is this a float?''
1989 use POSIX qw(strtod);
1994 my($num, $unparsed) = strtod($str);
1995 if (($str eq '') || ($unparsed != 0) || $!) {
2002 sub is_numeric { defined getnum($_[0]) }
2004 Or you could check out the L<String::Scanf|String::Scanf> module on the CPAN
2005 instead. The POSIX module (part of the standard Perl distribution) provides
2006 the C<strtod> and C<strtol> for converting strings to double and longs,
2009 =head2 How do I keep persistent data across program calls?
2011 For some specific applications, you can use one of the DBM modules.
2012 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2013 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2014 of the standard distribution. Here's one example using Storable's C<store>
2015 and C<retrieve> functions:
2018 store(\%hash, "filename");
2021 $href = retrieve("filename"); # by ref
2022 %hash = %{ retrieve("filename") }; # direct to hash
2024 =head2 How do I print out or copy a recursive data structure?
2026 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2027 for printing out data structures. The Storable module, found on CPAN,
2028 provides a function called C<dclone> that recursively copies its argument.
2030 use Storable qw(dclone);
2033 Where $r1 can be a reference to any kind of data structure you'd like.
2034 It will be deeply copied. Because C<dclone> takes and returns references,
2035 you'd have to add extra punctuation if you had a hash of arrays that
2038 %newhash = %{ dclone(\%oldhash) };
2040 =head2 How do I define methods for every class/object?
2042 Use the UNIVERSAL class (see L<UNIVERSAL>).
2044 =head2 How do I verify a credit card checksum?
2046 Get the Business::CreditCard module from CPAN.
2048 =head2 How do I pack arrays of doubles or floats for XS code?
2050 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2051 If you're doing a lot of float or double processing, consider using
2052 the PDL module from CPAN instead--it makes number-crunching easy.
2054 =head1 AUTHOR AND COPYRIGHT
2056 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
2057 All rights reserved.
2059 This documentation is free; you can redistribute it and/or modify it
2060 under the same terms as Perl itself.
2062 Irrespective of its distribution, all code examples in this file
2063 are hereby placed into the public domain. You are permitted and
2064 encouraged to use this code in your own programs for fun
2065 or for profit as you see fit. A simple comment in the code giving
2066 credit would be courteous but is not required.