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
244 $bin = unpack("B*", pack("N", 3735928559));
249 $vec = Bit::Vector->new_Dec(32, -559038737);
250 $bin = $vec->to_Bin();
252 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
253 are left as an exercise to the inclined reader.
257 =head2 Why doesn't & work the way I want it to?
259 The behavior of binary arithmetic operators depends on whether they're
260 used on numbers or strings. The operators treat a string as a series
261 of bits and work with that (the string C<"3"> is the bit pattern
262 C<00110011>). The operators work with the binary form of a number
263 (the number C<3> is treated as the bit pattern C<00000011>).
265 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
266 C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
269 Most problems with C<&> and C<|> arise because the programmer thinks
270 they have a number but really it's a string. The rest arise because
273 if ("\020\020" & "\101\101") {
277 but a string consisting of two null bytes (the result of C<"\020\020"
278 & "\101\101">) is not a false value in Perl. You need:
280 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
284 =head2 How do I multiply matrices?
286 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
287 or the PDL extension (also available from CPAN).
289 =head2 How do I perform an operation on a series of integers?
291 To call a function on each element in an array, and collect the
294 @results = map { my_func($_) } @array;
298 @triple = map { 3 * $_ } @single;
300 To call a function on each element of an array, but ignore the
303 foreach $iterator (@array) {
304 some_func($iterator);
307 To call a function on each integer in a (small) range, you B<can> use:
309 @results = map { some_func($_) } (5 .. 25);
311 but you should be aware that the C<..> operator creates an array of
312 all integers in the range. This can take a lot of memory for large
316 for ($i=5; $i < 500_005; $i++) {
317 push(@results, some_func($i));
320 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
321 loop will iterate over the range, without creating the entire range.
323 for my $i (5 .. 500_005) {
324 push(@results, some_func($i));
327 will not create a list of 500,000 integers.
329 =head2 How can I output Roman numerals?
331 Get the http://www.cpan.org/modules/by-module/Roman module.
333 =head2 Why aren't my random numbers random?
335 If you're using a version of Perl before 5.004, you must call C<srand>
336 once at the start of your program to seed the random number generator.
338 BEGIN { srand() if $] < 5.004 }
340 5.004 and later automatically call C<srand> at the beginning. Don't
341 call C<srand> more than once---you make your numbers less random, rather
344 Computers are good at being predictable and bad at being random
345 (despite appearances caused by bugs in your programs :-). see the
346 F<random> article in the "Far More Than You Ever Wanted To Know"
347 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
348 Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
349 who attempts to generate random numbers by deterministic means is, of
350 course, living in a state of sin.''
352 If you want numbers that are more random than C<rand> with C<srand>
353 provides, you should also check out the Math::TrulyRandom module from
354 CPAN. It uses the imperfections in your system's timer to generate
355 random numbers, but this takes quite a while. If you want a better
356 pseudorandom generator than comes with your operating system, look at
357 ``Numerical Recipes in C'' at http://www.nr.com/ .
359 =head2 How do I get a random number between X and Y?
361 Use the following simple function. It selects a random integer between
362 (and possibly including!) the two given integers, e.g.,
363 C<random_int_in(50,120)>
365 sub random_int_in ($$) {
367 # Assumes that the two arguments are integers themselves!
368 return $min if $min == $max;
369 ($min, $max) = ($max, $min) if $min > $max;
370 return $min + int rand(1 + $max - $min);
375 =head2 How do I find the day or week of the year?
377 The localtime function returns the day of the week. Without an
378 argument localtime uses the current time.
380 $day_of_year = (localtime)[7];
382 The POSIX module can also format a date as the day of the year or
385 use POSIX qw/strftime/;
386 my $day_of_year = strftime "%j", localtime;
387 my $week_of_year = strftime "%W", localtime;
389 To get the day of year for any date, use the Time::Local module to get
390 a time in epoch seconds for the argument to localtime.
392 use POSIX qw/strftime/;
394 my $week_of_year = strftime "%W",
395 localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
397 The Date::Calc module provides two functions for to calculate these.
400 my $day_of_year = Day_of_Year( 1987, 12, 18 );
401 my $week_of_year = Week_of_Year( 1987, 12, 18 );
403 =head2 How do I find the current century or millennium?
405 Use the following simple functions:
408 return int((((localtime(shift || time))[5] + 1999))/100);
411 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
414 You can also use the POSIX strftime() function which may be a bit
415 slower but is easier to read and maintain.
417 use POSIX qw/strftime/;
419 my $week_of_the_year = strftime "%W", localtime;
420 my $day_of_the_year = strftime "%j", localtime;
422 On some systems, the POSIX module's strftime() function has
423 been extended in a non-standard way to use a C<%C> format,
424 which they sometimes claim is the "century". It isn't,
425 because on most such systems, this is only the first two
426 digits of the four-digit year, and thus cannot be used to
427 reliably determine the current century or millennium.
429 =head2 How can I compare two dates and find the difference?
431 If you're storing your dates as epoch seconds then simply subtract one
432 from the other. If you've got a structured date (distinct year, day,
433 month, hour, minute, seconds values), then for reasons of accessibility,
434 simplicity, and efficiency, merely use either timelocal or timegm (from
435 the Time::Local module in the standard distribution) to reduce structured
436 dates to epoch seconds. However, if you don't know the precise format of
437 your dates, then you should probably use either of the Date::Manip and
438 Date::Calc modules from CPAN before you go hacking up your own parsing
439 routine to handle arbitrary date formats.
441 =head2 How can I take a string and turn it into epoch seconds?
443 If it's a regular enough string that it always has the same format,
444 you can split it up and pass the parts to C<timelocal> in the standard
445 Time::Local module. Otherwise, you should look into the Date::Calc
446 and Date::Manip modules from CPAN.
448 =head2 How can I find the Julian Day?
450 Use the Time::JulianDay module (part of the Time-modules bundle
451 available from CPAN.)
453 Before you immerse yourself too deeply in this, be sure to verify that
454 it is the I<Julian> Day you really want. Are you interested in a way
455 of getting serial days so that you just can tell how many days they
456 are apart or so that you can do also other date arithmetic? If you
457 are interested in performing date arithmetic, this can be done using
458 modules Date::Manip or Date::Calc.
460 There is too many details and much confusion on this issue to cover in
461 this FAQ, but the term is applied (correctly) to a calendar now
462 supplanted by the Gregorian Calendar, with the Julian Calendar failing
463 to adjust properly for leap years on centennial years (among other
464 annoyances). The term is also used (incorrectly) to mean: [1] days in
465 the Gregorian Calendar; and [2] days since a particular starting time
466 or `epoch', usually 1970 in the Unix world and 1980 in the
467 MS-DOS/Windows world. If you find that it is not the first meaning
468 that you really want, then check out the Date::Manip and Date::Calc
469 modules. (Thanks to David Cassell for most of this text.)
471 =head2 How do I find yesterday's date?
473 If you only need to find the date (and not the same time), you
474 can use the Date::Calc module.
476 use Date::Calc qw(Today Add_Delta_Days);
478 my @date = Add_Delta_Days( Today(), -1 );
482 Most people try to use the time rather than the calendar to
483 figure out dates, but that assumes that your days are
484 twenty-four hours each. For most people, there are two days
485 a year when they aren't: the switch to and from summer time
486 throws this off. Russ Allbery offers this solution.
489 my $now = defined $_[0] ? $_[0] : time;
490 my $then = $now - 60 * 60 * 24;
491 my $ndst = (localtime $now)[8] > 0;
492 my $tdst = (localtime $then)[8] > 0;
493 $then - ($tdst - $ndst) * 60 * 60;
496 Should give you "this time yesterday" in seconds since epoch relative to
497 the first argument or the current time if no argument is given and
498 suitable for passing to localtime or whatever else you need to do with
499 it. $ndst is whether we're currently in daylight savings time; $tdst is
500 whether the point 24 hours ago was in daylight savings time. If $tdst
501 and $ndst are the same, a boundary wasn't crossed, and the correction
502 will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
503 from yesterday's time since we gained an extra hour while going off
504 daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
505 negative hour (add an hour) to yesterday's time since we lost an hour.
507 All of this is because during those days when one switches off or onto
508 DST, a "day" isn't 24 hours long; it's either 23 or 25.
510 The explicit settings of $ndst and $tdst are necessary because localtime
511 only says it returns the system tm struct, and the system tm struct at
512 least on Solaris doesn't guarantee any particular positive value (like,
513 say, 1) for isdst, just a positive value. And that value can
514 potentially be negative, if DST information isn't available (this sub
515 just treats those cases like no DST).
517 Note that between 2am and 3am on the day after the time zone switches
518 off daylight savings time, the exact hour of "yesterday" corresponding
519 to the current hour is not clearly defined. Note also that if used
520 between 2am and 3am the day after the change to daylight savings time,
521 the result will be between 3am and 4am of the previous day; it's
522 arguable whether this is correct.
524 This sub does not attempt to deal with leap seconds (most things don't).
528 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
530 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
531 Y2K compliant (whatever that means). The programmers you've hired to
532 use it, however, probably are not.
534 Long answer: The question belies a true understanding of the issue.
535 Perl is just as Y2K compliant as your pencil--no more, and no less.
536 Can you use your pencil to write a non-Y2K-compliant memo? Of course
537 you can. Is that the pencil's fault? Of course it isn't.
539 The date and time functions supplied with Perl (gmtime and localtime)
540 supply adequate information to determine the year well beyond 2000
541 (2038 is when trouble strikes for 32-bit machines). The year returned
542 by these functions when used in a list context is the year minus 1900.
543 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
544 number. To avoid the year 2000 problem simply do not treat the year as
545 a 2-digit number. It isn't.
547 When gmtime() and localtime() are used in scalar context they return
548 a timestamp string that contains a fully-expanded year. For example,
549 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
550 2001". There's no year 2000 problem here.
552 That doesn't mean that Perl can't be used to create non-Y2K compliant
553 programs. It can. But so can your pencil. It's the fault of the user,
554 not the language. At the risk of inflaming the NRA: ``Perl doesn't
555 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
560 =head2 How do I validate input?
562 The answer to this question is usually a regular expression, perhaps
563 with auxiliary logic. See the more specific questions (numbers, mail
564 addresses, etc.) for details.
566 =head2 How do I unescape a string?
568 It depends just what you mean by ``escape''. URL escapes are dealt
569 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
570 character are removed with
574 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
576 =head2 How do I remove consecutive pairs of characters?
578 To turn C<"abbcccd"> into C<"abccd">:
580 s/(.)\1/$1/g; # add /s to include newlines
582 Here's a solution that turns "abbcccd" to "abcd":
584 y///cs; # y == tr, but shorter :-)
586 =head2 How do I expand function calls in a string?
588 This is documented in L<perlref>. In general, this is fraught with
589 quoting and readability problems, but it is possible. To interpolate
590 a subroutine call (in list context) into a string:
592 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
594 See also ``How can I expand variables in text strings?'' in this
597 =head2 How do I find matching/nesting anything?
599 This isn't something that can be done in one regular expression, no
600 matter how complicated. To find something between two single
601 characters, a pattern like C</x([^x]*)x/> will get the intervening
602 bits in $1. For multiple ones, then something more like
603 C</alpha(.*?)omega/> would be needed. But none of these deals with
604 nested patterns. For balanced expressions using C<(>, C<{>, C<[>
605 or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
606 L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
608 If you are serious about writing a parser, there are a number of
609 modules or oddities that will make your life a lot easier. There are
610 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
611 and the byacc program. Starting from perl 5.8 the Text::Balanced
612 is part of the standard distribution.
614 One simple destructive, inside-out approach that you might try is to
615 pull out the smallest nesting parts one at a time:
617 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
618 # do something with $1
621 A more complicated and sneaky approach is to make Perl's regular
622 expression engine do it for you. This is courtesy Dean Inada, and
623 rather has the nature of an Obfuscated Perl Contest entry, but it
626 # $_ contains the string to parse
627 # BEGIN and END are the opening and closing markers for the
632 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
633 @$ = (eval{/$re/},$@!~/unmatched/i);
634 print join("\n",@$[0..$#$]) if( $$[-1] );
636 =head2 How do I reverse a string?
638 Use reverse() in scalar context, as documented in
641 $reversed = reverse $string;
643 =head2 How do I expand tabs in a string?
645 You can do it yourself:
647 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
649 Or you can just use the Text::Tabs module (part of the standard Perl
653 @expanded_lines = expand(@lines_with_tabs);
655 =head2 How do I reformat a paragraph?
657 Use Text::Wrap (part of the standard Perl distribution):
660 print wrap("\t", ' ', @paragraphs);
662 The paragraphs you give to Text::Wrap should not contain embedded
663 newlines. Text::Wrap doesn't justify the lines (flush-right).
665 Or use the CPAN module Text::Autoformat. Formatting files can be easily
666 done by making a shell alias, like so:
668 alias fmt="perl -i -MText::Autoformat -n0777 \
669 -e 'print autoformat $_, {all=>1}' $*"
671 See the documentation for Text::Autoformat to appreciate its many
674 =head2 How can I access or change N characters of a string?
676 You can access the first characters of a string with substr().
677 To get the first character, for example, start at position 0
678 and grab the string of length 1.
681 $string = "Just another Perl Hacker";
682 $first_char = substr( $string, 0, 1 ); # 'J'
684 To change part of a string, you can use the optional fourth
685 argument which is the replacement string.
687 substr( $string, 13, 4, "Perl 5.8.0" );
689 You can also use substr() as an lvalue.
691 substr( $string, 13, 4 ) = "Perl 5.8.0";
693 =head2 How do I change the Nth occurrence of something?
695 You have to keep track of N yourself. For example, let's say you want
696 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
697 C<"whosoever"> or C<"whomsoever">, case insensitively. These
698 all assume that $_ contains the string to be altered.
702 ++$count == 5 # is it the 5th?
703 ? "${2}soever" # yes, swap
704 : $1 # renege and leave it there
707 In the more general case, you can use the C</g> modifier in a C<while>
708 loop, keeping count of matches.
712 $_ = "One fish two fish red fish blue fish";
713 while (/(\w+)\s+fish\b/gi) {
714 if (++$count == $WANT) {
715 print "The third fish is a $1 one.\n";
719 That prints out: C<"The third fish is a red one."> You can also use a
720 repetition count and repeated pattern like this:
722 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
724 =head2 How can I count the number of occurrences of a substring within a string?
726 There are a number of ways, with varying efficiency. If you want a
727 count of a certain single character (X) within a string, you can use the
728 C<tr///> function like so:
730 $string = "ThisXlineXhasXsomeXx'sXinXit";
731 $count = ($string =~ tr/X//);
732 print "There are $count X characters in the string";
734 This is fine if you are just looking for a single character. However,
735 if you are trying to count multiple character substrings within a
736 larger string, C<tr///> won't work. What you can do is wrap a while()
737 loop around a global pattern match. For example, let's count negative
740 $string = "-9 55 48 -2 23 -76 4 14 -44";
741 while ($string =~ /-\d+/g) { $count++ }
742 print "There are $count negative numbers in the string";
744 Another version uses a global match in list context, then assigns the
745 result to a scalar, producing a count of the number of matches.
747 $count = () = $string =~ /-\d+/g;
749 =head2 How do I capitalize all the words on one line?
751 To make the first letter of each word upper case:
753 $line =~ s/\b(\w)/\U$1/g;
755 This has the strange effect of turning "C<don't do it>" into "C<Don'T
756 Do It>". Sometimes you might want this. Other times you might need a
757 more thorough solution (Suggested by brian d foy):
760 (^\w) #at the beginning of the line
762 (\s\w) #preceded by whitespace
765 $string =~ /([\w']+)/\u\L$1/g;
767 To make the whole line upper case:
771 To force each word to be lower case, with the first letter upper case:
773 $line =~ s/(\w+)/\u\L$1/g;
775 You can (and probably should) enable locale awareness of those
776 characters by placing a C<use locale> pragma in your program.
777 See L<perllocale> for endless details on locales.
779 This is sometimes referred to as putting something into "title
780 case", but that's not quite accurate. Consider the proper
781 capitalization of the movie I<Dr. Strangelove or: How I Learned to
782 Stop Worrying and Love the Bomb>, for example.
784 Damian Conway's L<Text::Autoformat> module provides some smart
785 case transformations:
787 use Text::Autoformat;
788 my $x = "Dr. Strangelove or: How I Learned to Stop ".
789 "Worrying and Love the Bomb";
792 for my $style (qw( sentence title highlight ))
794 print autoformat($x, { case => $style }), "\n";
797 =head2 How can I split a [character] delimited string except when inside [character]?
799 Several modules can handle this sort of pasing---Text::Balanced,
800 Text::CVS, Text::CVS_XS, and Text::ParseWords, among others.
802 Take the example case of trying to split a string that is
803 comma-separated into its different fields. You can't use C<split(/,/)>
804 because you shouldn't split if the comma is inside quotes. For
805 example, take a data line like this:
807 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
809 Due to the restriction of the quotes, this is a fairly complex
810 problem. Thankfully, we have Jeffrey Friedl, author of
811 I<Mastering Regular Expressions>, to handle these for us. He
812 suggests (assuming your string is contained in $text):
815 push(@new, $+) while $text =~ m{
816 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
820 push(@new, undef) if substr($text,-1,1) eq ',';
822 If you want to represent quotation marks inside a
823 quotation-mark-delimited field, escape them with backslashes (eg,
826 Alternatively, the Text::ParseWords module (part of the standard Perl
827 distribution) lets you say:
829 use Text::ParseWords;
830 @new = quotewords(",", 0, $text);
832 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
834 =head2 How do I strip blank space from the beginning/end of a string?
836 Although the simplest approach would seem to be
838 $string =~ s/^\s*(.*?)\s*$/$1/;
840 not only is this unnecessarily slow and destructive, it also fails with
841 embedded newlines. It is much faster to do this operation in two steps:
846 Or more nicely written as:
853 This idiom takes advantage of the C<foreach> loop's aliasing
854 behavior to factor out common code. You can do this
855 on several strings at once, or arrays, or even the
856 values of a hash if you use a slice:
858 # trim whitespace in the scalar, the array,
859 # and all the values in the hash
860 foreach ($scalar, @array, @hash{keys %hash}) {
865 =head2 How do I pad a string with blanks or pad a number with zeroes?
867 In the following examples, C<$pad_len> is the length to which you wish
868 to pad the string, C<$text> or C<$num> contains the string to be padded,
869 and C<$pad_char> contains the padding character. You can use a single
870 character string constant instead of the C<$pad_char> variable if you
871 know what it is in advance. And in the same way you can use an integer in
872 place of C<$pad_len> if you know the pad length in advance.
874 The simplest method uses the C<sprintf> function. It can pad on the left
875 or right with blanks and on the left with zeroes and it will not
876 truncate the result. The C<pack> function can only pad strings on the
877 right with blanks and it will truncate the result to a maximum length of
880 # Left padding a string with blanks (no truncation):
881 $padded = sprintf("%${pad_len}s", $text);
882 $padded = sprintf("%*s", $pad_len, $text); # same thing
884 # Right padding a string with blanks (no truncation):
885 $padded = sprintf("%-${pad_len}s", $text);
886 $padded = sprintf("%-*s", $pad_len, $text); # same thing
888 # Left padding a number with 0 (no truncation):
889 $padded = sprintf("%0${pad_len}d", $num);
890 $padded = sprintf("%0*d", $pad_len, $num); # same thing
892 # Right padding a string with blanks using pack (will truncate):
893 $padded = pack("A$pad_len",$text);
895 If you need to pad with a character other than blank or zero you can use
896 one of the following methods. They all generate a pad string with the
897 C<x> operator and combine that with C<$text>. These methods do
898 not truncate C<$text>.
900 Left and right padding with any character, creating a new string:
902 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
903 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
905 Left and right padding with any character, modifying C<$text> directly:
907 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
908 $text .= $pad_char x ( $pad_len - length( $text ) );
910 =head2 How do I extract selected columns from a string?
912 Use substr() or unpack(), both documented in L<perlfunc>.
913 If you prefer thinking in terms of columns instead of widths,
914 you can use this kind of thing:
916 # determine the unpack format needed to split Linux ps output
917 # arguments are cut columns
918 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
924 for my $place (@positions) {
925 $template .= "A" . ($place - $lastpos) . " ";
932 =head2 How do I find the soundex value of a string?
934 Use the standard Text::Soundex module distributed with Perl.
935 Before you do so, you may want to determine whether `soundex' is in
936 fact what you think it is. Knuth's soundex algorithm compresses words
937 into a small space, and so it does not necessarily distinguish between
938 two words which you might want to appear separately. For example, the
939 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
940 If Text::Soundex does not do what you are looking for, you might want
941 to consider the String::Approx module available at CPAN.
943 =head2 How can I expand variables in text strings?
945 Let's assume that you have a string like:
947 $text = 'this has a $foo in it and a $bar';
949 If those were both global variables, then this would
952 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
954 But since they are probably lexicals, or at least, they could
955 be, you'd have to do this:
957 $text =~ s/(\$\w+)/$1/eeg;
958 die if $@; # needed /ee, not /e
960 It's probably better in the general case to treat those
961 variables as entries in some special hash. For example:
967 $text =~ s/\$(\w+)/$user_defs{$1}/g;
969 See also ``How do I expand function calls in a string?'' in this section
972 =head2 What's wrong with always quoting "$vars"?
974 The problem is that those double-quotes force stringification--
975 coercing numbers and references into strings--even when you
976 don't want them to be strings. Think of it this way: double-quote
977 expansion is used to produce new strings. If you already
978 have a string, why do you need more?
980 If you get used to writing odd things like these:
984 somefunc("$var"); # BAD
986 You'll be in trouble. Those should (in 99.8% of the cases) be
987 the simpler and more direct:
993 Otherwise, besides slowing you down, you're going to break code when
994 the thing in the scalar is actually neither a string nor a number, but
1000 my $oref = "$aref"; # WRONG
1003 You can also get into subtle problems on those few operations in Perl
1004 that actually do care about the difference between a string and a
1005 number, such as the magical C<++> autoincrement operator or the
1008 Stringification also destroys arrays.
1011 print "@lines"; # WRONG - extra blanks
1012 print @lines; # right
1014 =head2 Why don't my E<lt>E<lt>HERE documents work?
1016 Check for these three things:
1020 =item There must be no space after the E<lt>E<lt> part.
1022 =item There (probably) should be a semicolon at the end.
1024 =item You can't (easily) have any space in front of the tag.
1028 If you want to indent the text in the here document, you
1032 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1037 But the HERE_TARGET must still be flush against the margin.
1038 If you want that indented also, you'll have to quote
1041 ($quote = <<' FINIS') =~ s/^\s+//gm;
1042 ...we will have peace, when you and all your works have
1043 perished--and the works of your dark master to whom you
1044 would deliver us. You are a liar, Saruman, and a corrupter
1045 of men's hearts. --Theoden in /usr/src/perl/taint.c
1047 $quote =~ s/\s+--/\n--/;
1049 A nice general-purpose fixer-upper function for indented here documents
1050 follows. It expects to be called with a here document as its argument.
1051 It looks to see whether each line begins with a common substring, and
1052 if so, strips that substring off. Otherwise, it takes the amount of leading
1053 whitespace found on the first line and removes that much off each
1058 my ($white, $leader); # common whitespace and common leading string
1059 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1060 ($white, $leader) = ($2, quotemeta($1));
1062 ($white, $leader) = (/^(\s+)/, '');
1064 s/^\s*?$leader(?:$white)?//gm;
1068 This works with leading special strings, dynamically determined:
1070 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1073 @@@ SAVEI32(runlevel);
1075 @@@ while ( op = (*op->op_ppaddr)() );
1079 MAIN_INTERPRETER_LOOP
1081 Or with a fixed amount of leading whitespace, with remaining
1082 indentation correctly preserved:
1084 $poem = fix<<EVER_ON_AND_ON;
1085 Now far ahead the Road has gone,
1086 And I must follow, if I can,
1087 Pursuing it with eager feet,
1088 Until it joins some larger way
1089 Where many paths and errands meet.
1090 And whither then? I cannot say.
1091 --Bilbo in /usr/src/perl/pp_ctl.c
1096 =head2 What is the difference between a list and an array?
1098 An array has a changeable length. A list does not. An array is something
1099 you can push or pop, while a list is a set of values. Some people make
1100 the distinction that a list is a value while an array is a variable.
1101 Subroutines are passed and return lists, you put things into list
1102 context, you initialize arrays with lists, and you foreach() across
1103 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1104 in scalar context behave like the number of elements in them, subroutines
1105 access their arguments through the array C<@_>, and push/pop/shift only work
1108 As a side note, there's no such thing as a list in scalar context.
1111 $scalar = (2, 5, 7, 9);
1113 you're using the comma operator in scalar context, so it uses the scalar
1114 comma operator. There never was a list there at all! This causes the
1115 last value to be returned: 9.
1117 =head2 What is the difference between $array[1] and @array[1]?
1119 The former is a scalar value; the latter an array slice, making
1120 it a list with one (scalar) value. You should use $ when you want a
1121 scalar value (most of the time) and @ when you want a list with one
1122 scalar value in it (very, very rarely; nearly never, in fact).
1124 Sometimes it doesn't make a difference, but sometimes it does.
1125 For example, compare:
1127 $good[0] = `some program that outputs several lines`;
1131 @bad[0] = `same program that outputs several lines`;
1133 The C<use warnings> pragma and the B<-w> flag will warn you about these
1136 =head2 How can I remove duplicate elements from a list or array?
1138 There are several possible ways, depending on whether the array is
1139 ordered and whether you wish to preserve the ordering.
1145 If @in is sorted, and you want @out to be sorted:
1146 (this assumes all true values in the array)
1148 $prev = "not equal to $in[0]";
1149 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1151 This is nice in that it doesn't use much extra memory, simulating
1152 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1153 guarantees that the expression is true (so that grep picks it up)
1154 even if the $_ is 0, "", or undef.
1158 If you don't know whether @in is sorted:
1161 @out = grep(!$saw{$_}++, @in);
1165 Like (b), but @in contains only small integers:
1167 @out = grep(!$saw[$_]++, @in);
1171 A way to do (b) without any loops or greps:
1175 @out = sort keys %saw; # remove sort if undesired
1179 Like (d), but @in contains only small positive integers:
1183 @out = grep {defined} @ary;
1187 But perhaps you should have been using a hash all along, eh?
1189 =head2 How can I tell whether a certain element is contained in a list or array?
1191 Hearing the word "in" is an I<in>dication that you probably should have
1192 used a hash, not a list or array, to store your data. Hashes are
1193 designed to answer this question quickly and efficiently. Arrays aren't.
1195 That being said, there are several ways to approach this. If you
1196 are going to make this query many times over arbitrary string values,
1197 the fastest way is probably to invert the original array and maintain a
1198 hash whose keys are the first array's values.
1200 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1202 for (@blues) { $is_blue{$_} = 1 }
1204 Now you can check whether $is_blue{$some_color}. It might have been a
1205 good idea to keep the blues all in a hash in the first place.
1207 If the values are all small integers, you could use a simple indexed
1208 array. This kind of an array will take up less space:
1210 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1211 @is_tiny_prime = ();
1212 for (@primes) { $is_tiny_prime[$_] = 1 }
1213 # or simply @istiny_prime[@primes] = (1) x @primes;
1215 Now you check whether $is_tiny_prime[$some_number].
1217 If the values in question are integers instead of strings, you can save
1218 quite a lot of space by using bit strings instead:
1220 @articles = ( 1..10, 150..2000, 2017 );
1222 for (@articles) { vec($read,$_,1) = 1 }
1224 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1228 ($is_there) = grep $_ eq $whatever, @array;
1232 ($is_there) = grep /$whatever/, @array;
1234 These are slow (checks every element even if the first matches),
1235 inefficient (same reason), and potentially buggy (what if there are
1236 regex characters in $whatever?). If you're only testing once, then
1240 foreach $elt (@array) {
1241 if ($elt eq $elt_to_find) {
1246 if ($is_there) { ... }
1248 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1250 Use a hash. Here's code to do both and more. It assumes that
1251 each element is unique in a given array:
1253 @union = @intersection = @difference = ();
1255 foreach $element (@array1, @array2) { $count{$element}++ }
1256 foreach $element (keys %count) {
1257 push @union, $element;
1258 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1261 Note that this is the I<symmetric difference>, that is, all elements in
1262 either A or in B but not in both. Think of it as an xor operation.
1264 =head2 How do I test whether two arrays or hashes are equal?
1266 The following code works for single-level arrays. It uses a stringwise
1267 comparison, and does not distinguish defined versus undefined empty
1268 strings. Modify if you have other needs.
1270 $are_equal = compare_arrays(\@frogs, \@toads);
1272 sub compare_arrays {
1273 my ($first, $second) = @_;
1274 no warnings; # silence spurious -w undef complaints
1275 return 0 unless @$first == @$second;
1276 for (my $i = 0; $i < @$first; $i++) {
1277 return 0 if $first->[$i] ne $second->[$i];
1282 For multilevel structures, you may wish to use an approach more
1283 like this one. It uses the CPAN module FreezeThaw:
1285 use FreezeThaw qw(cmpStr);
1286 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1288 printf "a and b contain %s arrays\n",
1289 cmpStr(\@a, \@b) == 0
1293 This approach also works for comparing hashes. Here
1294 we'll demonstrate two different answers:
1296 use FreezeThaw qw(cmpStr cmpStrHard);
1298 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1302 printf "a and b contain %s hashes\n",
1303 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1305 printf "a and b contain %s hashes\n",
1306 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1309 The first reports that both those the hashes contain the same data,
1310 while the second reports that they do not. Which you prefer is left as
1311 an exercise to the reader.
1313 =head2 How do I find the first array element for which a condition is true?
1315 To find the first array element which satisfies a condition, you can
1316 use the first() function in the List::Util module, which comes with
1317 Perl 5.8. This example finds the first element that contains "Perl".
1319 use List::Util qw(first);
1321 my $element = first { /Perl/ } @array;
1323 If you cannot use List::Util, you can make your own loop to do the
1324 same thing. Once you find the element, you stop the loop with last.
1327 foreach my $element ( @array )
1329 if( /Perl/ ) { $found = $element; last }
1332 If you want the array index, you can iterate through the indices
1333 and check the array element at each index until you find one
1334 that satisfies the condition.
1336 my( $found, $index ) = ( undef, -1 );
1337 for( $i = 0; $i < @array; $i++ )
1339 if( $array[$i] =~ /Perl/ )
1341 $found = $array[$i];
1347 =head2 How do I handle linked lists?
1349 In general, you usually don't need a linked list in Perl, since with
1350 regular arrays, you can push and pop or shift and unshift at either end,
1351 or you can use splice to add and/or remove arbitrary number of elements at
1352 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1353 dynamic arrays. In the absence of shifts and pops, push in general
1354 needs to reallocate on the order every log(N) times, and unshift will
1355 need to copy pointers each time.
1357 If you really, really wanted, you could use structures as described in
1358 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1359 to do. For example, imagine a list node like this:
1366 You could walk the list this way:
1369 for ($node = $head; $node; $node = $node->{LINK}) {
1370 print $node->{VALUE}, " ";
1374 You could add to the list this way:
1377 $tail = append($head, 1); # grow a new head
1378 for $value ( 2 .. 10 ) {
1379 $tail = append($tail, $value);
1383 my($list, $value) = @_;
1384 my $node = { VALUE => $value };
1386 $node->{LINK} = $list->{LINK};
1387 $list->{LINK} = $node;
1389 $_[0] = $node; # replace caller's version
1394 But again, Perl's built-in are virtually always good enough.
1396 =head2 How do I handle circular lists?
1398 Circular lists could be handled in the traditional fashion with linked
1399 lists, or you could just do something like this with an array:
1401 unshift(@array, pop(@array)); # the last shall be first
1402 push(@array, shift(@array)); # and vice versa
1404 =head2 How do I shuffle an array randomly?
1406 If you either have Perl 5.8.0 or later installed, or if you have
1407 Scalar-List-Utils 1.03 or later installed, you can say:
1409 use List::Util 'shuffle';
1411 @shuffled = shuffle(@list);
1413 If not, you can use a Fisher-Yates shuffle.
1415 sub fisher_yates_shuffle {
1416 my $deck = shift; # $deck is a reference to an array
1419 my $j = int rand ($i+1);
1420 @$deck[$i,$j] = @$deck[$j,$i];
1424 # shuffle my mpeg collection
1426 my @mpeg = <audio/*/*.mp3>;
1427 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1430 Note that the above implementation shuffles an array in place,
1431 unlike the List::Util::shuffle() which takes a list and returns
1432 a new shuffled list.
1434 You've probably seen shuffling algorithms that work using splice,
1435 randomly picking another element to swap the current element with
1439 @old = 1 .. 10; # just a demo
1441 push(@new, splice(@old, rand @old, 1));
1444 This is bad because splice is already O(N), and since you do it N times,
1445 you just invented a quadratic algorithm; that is, O(N**2). This does
1446 not scale, although Perl is so efficient that you probably won't notice
1447 this until you have rather largish arrays.
1449 =head2 How do I process/modify each element of an array?
1451 Use C<for>/C<foreach>:
1454 s/foo/bar/; # change that word
1455 y/XZ/ZX/; # swap those letters
1458 Here's another; let's compute spherical volumes:
1460 for (@volumes = @radii) { # @volumes has changed parts
1462 $_ *= (4/3) * 3.14159; # this will be constant folded
1465 which can also be done with map() which is made to transform
1466 one list into another:
1468 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1470 If you want to do the same thing to modify the values of the
1471 hash, you can use the C<values> function. As of Perl 5.6
1472 the values are not copied, so if you modify $orbit (in this
1473 case), you modify the value.
1475 for $orbit ( values %orbits ) {
1476 ($orbit **= 3) *= (4/3) * 3.14159;
1479 Prior to perl 5.6 C<values> returned copies of the values,
1480 so older perl code often contains constructions such as
1481 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1482 the hash is to be modified.
1484 =head2 How do I select a random element from an array?
1486 Use the rand() function (see L<perlfunc/rand>):
1488 # at the top of the program:
1489 srand; # not needed for 5.004 and later
1492 $index = rand @array;
1493 $element = $array[$index];
1495 Make sure you I<only call srand once per program, if then>.
1496 If you are calling it more than once (such as before each
1497 call to rand), you're almost certainly doing something wrong.
1499 =head2 How do I permute N elements of a list?
1501 Use the List::Permutor module on CPAN. If the list is
1502 actually an array, try the Algorithm::Permute module (also
1503 on CPAN). It's written in XS code and is very efficient.
1505 use Algorithm::Permute;
1506 my @array = 'a'..'d';
1507 my $p_iterator = Algorithm::Permute->new ( \@array );
1508 while (my @perm = $p_iterator->next) {
1509 print "next permutation: (@perm)\n";
1512 For even faster execution, you could do:
1514 use Algorithm::Permute;
1515 my @array = 'a'..'d';
1516 Algorithm::Permute::permute {
1517 print "next permutation: (@array)\n";
1520 Here's a little program that generates all permutations of
1521 all the words on each line of input. The algorithm embodied
1522 in the permute() function is discussed in Volume 4 (still
1523 unpublished) of Knuth's I<The Art of Computer Programming>
1524 and will work on any list:
1527 # Fischer-Kause ordered permutation generator
1532 while ( $code->(@_[@idx]) ) {
1534 --$p while $idx[$p-1] > $idx[$p];
1535 my $q = $p or return;
1536 push @idx, reverse splice @idx, $p;
1537 ++$q while $idx[$p-1] > $idx[$q];
1538 @idx[$p-1,$q]=@idx[$q,$p-1];
1542 permute {print"@_\n"} split;
1544 =head2 How do I sort an array by (anything)?
1546 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1548 @list = sort { $a <=> $b } @list;
1550 The default sort function is cmp, string comparison, which would
1551 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1552 the numerical comparison operator.
1554 If you have a complicated function needed to pull out the part you
1555 want to sort on, then don't do it inside the sort function. Pull it
1556 out first, because the sort BLOCK can be called many times for the
1557 same element. Here's an example of how to pull out the first word
1558 after the first number on each item, and then sort those words
1563 ($item) = /\d+\s*(\S+)/;
1564 push @idx, uc($item);
1566 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1568 which could also be written this way, using a trick
1569 that's come to be known as the Schwartzian Transform:
1571 @sorted = map { $_->[0] }
1572 sort { $a->[1] cmp $b->[1] }
1573 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1575 If you need to sort on several fields, the following paradigm is useful.
1577 @sorted = sort { field1($a) <=> field1($b) ||
1578 field2($a) cmp field2($b) ||
1579 field3($a) cmp field3($b)
1582 This can be conveniently combined with precalculation of keys as given
1585 See the F<sort> artitcle article in the "Far More Than You Ever Wanted
1586 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1587 more about this approach.
1589 See also the question below on sorting hashes.
1591 =head2 How do I manipulate arrays of bits?
1593 Use pack() and unpack(), or else vec() and the bitwise operations.
1595 For example, this sets $vec to have bit N set if $ints[N] was set:
1598 foreach(@ints) { vec($vec,$_,1) = 1 }
1600 Here's how, given a vector in $vec, you can
1601 get those bits into your @ints array:
1603 sub bitvec_to_list {
1606 # Find null-byte density then select best algorithm
1607 if ($vec =~ tr/\0// / length $vec > 0.95) {
1610 # This method is faster with mostly null-bytes
1611 while($vec =~ /[^\0]/g ) {
1612 $i = -9 + 8 * pos $vec;
1613 push @ints, $i if vec($vec, ++$i, 1);
1614 push @ints, $i if vec($vec, ++$i, 1);
1615 push @ints, $i if vec($vec, ++$i, 1);
1616 push @ints, $i if vec($vec, ++$i, 1);
1617 push @ints, $i if vec($vec, ++$i, 1);
1618 push @ints, $i if vec($vec, ++$i, 1);
1619 push @ints, $i if vec($vec, ++$i, 1);
1620 push @ints, $i if vec($vec, ++$i, 1);
1623 # This method is a fast general algorithm
1625 my $bits = unpack "b*", $vec;
1626 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1627 push @ints, pos $bits while($bits =~ /1/g);
1632 This method gets faster the more sparse the bit vector is.
1633 (Courtesy of Tim Bunce and Winfried Koenig.)
1635 You can make the while loop a lot shorter with this suggestion
1636 from Benjamin Goldberg:
1638 while($vec =~ /[^\0]+/g ) {
1639 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1642 Or use the CPAN module Bit::Vector:
1644 $vector = Bit::Vector->new($num_of_bits);
1645 $vector->Index_List_Store(@ints);
1646 @ints = $vector->Index_List_Read();
1648 Bit::Vector provides efficient methods for bit vector, sets of small integers
1651 Here's a more extensive illustration using vec():
1654 $vector = "\xff\x0f\xef\xfe";
1655 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1656 unpack("N", $vector), "\n";
1657 $is_set = vec($vector, 23, 1);
1658 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1676 my ($offset, $width, $value) = @_;
1678 vec($vector, $offset, $width) = $value;
1679 print "offset=$offset width=$width value=$value\n";
1685 my $bits = unpack("b*", $vector);
1689 print "vector length in bytes: ", length($vector), "\n";
1690 @bytes = unpack("A8" x length($vector), $bits);
1691 print "bits are: @bytes\n\n";
1694 =head2 Why does defined() return true on empty arrays and hashes?
1696 The short story is that you should probably only use defined on scalars or
1697 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1698 in the 5.004 release or later of Perl for more detail.
1700 =head1 Data: Hashes (Associative Arrays)
1702 =head2 How do I process an entire hash?
1704 Use the each() function (see L<perlfunc/each>) if you don't care
1705 whether it's sorted:
1707 while ( ($key, $value) = each %hash) {
1708 print "$key = $value\n";
1711 If you want it sorted, you'll have to use foreach() on the result of
1712 sorting the keys as shown in an earlier question.
1714 =head2 What happens if I add or remove keys from a hash while iterating over it?
1718 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1719 iterating over it. In Perl 5 you can delete from it, but you still
1720 can't add to it, because that might cause a doubling of the hash table,
1721 in which half the entries get copied up to the new top half of the
1722 table, at which point you've totally bamboozled the iterator code.
1723 Even if the table doesn't double, there's no telling whether your new
1724 entry will be inserted before or after the current iterator position.
1726 Either treasure up your changes and make them after the iterator finishes
1727 or use keys to fetch all the old keys at once, and iterate over the list
1730 =head2 How do I look up a hash element by value?
1732 Create a reverse hash:
1734 %by_value = reverse %by_key;
1735 $key = $by_value{$value};
1737 That's not particularly efficient. It would be more space-efficient
1740 while (($key, $value) = each %by_key) {
1741 $by_value{$value} = $key;
1744 If your hash could have repeated values, the methods above will only find
1745 one of the associated keys. This may or may not worry you. If it does
1746 worry you, you can always reverse the hash into a hash of arrays instead:
1748 while (($key, $value) = each %by_key) {
1749 push @{$key_list_by_value{$value}}, $key;
1752 =head2 How can I know how many entries are in a hash?
1754 If you mean how many keys, then all you have to do is
1755 use the keys() function in a scalar context:
1757 $num_keys = keys %hash;
1759 The keys() function also resets the iterator, which means that you may
1760 see strange results if you use this between uses of other hash operators
1763 =head2 How do I sort a hash (optionally by value instead of key)?
1765 Internally, hashes are stored in a way that prevents you from imposing
1766 an order on key-value pairs. Instead, you have to sort a list of the
1769 @keys = sort keys %hash; # sorted by key
1771 $hash{$a} cmp $hash{$b}
1772 } keys %hash; # and by value
1774 Here we'll do a reverse numeric sort by value, and if two keys are
1775 identical, sort by length of key, or if that fails, by straight ASCII
1776 comparison of the keys (well, possibly modified by your locale--see
1780 $hash{$b} <=> $hash{$a}
1782 length($b) <=> length($a)
1787 =head2 How can I always keep my hash sorted?
1789 You can look into using the DB_File module and tie() using the
1790 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1791 The Tie::IxHash module from CPAN might also be instructive.
1793 =head2 What's the difference between "delete" and "undef" with hashes?
1795 Hashes contain pairs of scalars: the first is the key, the
1796 second is the value. The key will be coerced to a string,
1797 although the value can be any kind of scalar: string,
1798 number, or reference. If a key $key is present in
1799 %hash, C<exists($hash{$key})> will return true. The value
1800 for a given key can be C<undef>, in which case
1801 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1802 will return true. This corresponds to (C<$key>, C<undef>)
1805 Pictures help... here's the %hash table:
1815 And these conditions hold
1819 defined $hash{'d'} is true
1820 defined $hash{'a'} is true
1821 exists $hash{'a'} is true (Perl5 only)
1822 grep ($_ eq 'a', keys %hash) is true
1828 your table now reads:
1839 and these conditions now hold; changes in caps:
1843 defined $hash{'d'} is true
1844 defined $hash{'a'} is FALSE
1845 exists $hash{'a'} is true (Perl5 only)
1846 grep ($_ eq 'a', keys %hash) is true
1848 Notice the last two: you have an undef value, but a defined key!
1854 your table now reads:
1863 and these conditions now hold; changes in caps:
1867 defined $hash{'d'} is true
1868 defined $hash{'a'} is false
1869 exists $hash{'a'} is FALSE (Perl5 only)
1870 grep ($_ eq 'a', keys %hash) is FALSE
1872 See, the whole entry is gone!
1874 =head2 Why don't my tied hashes make the defined/exists distinction?
1876 This depends on the tied hash's implementation of EXISTS().
1877 For example, there isn't the concept of undef with hashes
1878 that are tied to DBM* files. It also means that exists() and
1879 defined() do the same thing with a DBM* file, and what they
1880 end up doing is not what they do with ordinary hashes.
1882 =head2 How do I reset an each() operation part-way through?
1884 Using C<keys %hash> in scalar context returns the number of keys in
1885 the hash I<and> resets the iterator associated with the hash. You may
1886 need to do this if you use C<last> to exit a loop early so that when you
1887 re-enter it, the hash iterator has been reset.
1889 =head2 How can I get the unique keys from two hashes?
1891 First you extract the keys from the hashes into lists, then solve
1892 the "removing duplicates" problem described above. For example:
1895 for $element (keys(%foo), keys(%bar)) {
1902 @uniq = keys %{{%foo,%bar}};
1904 Or if you really want to save space:
1907 while (defined ($key = each %foo)) {
1910 while (defined ($key = each %bar)) {
1915 =head2 How can I store a multidimensional array in a DBM file?
1917 Either stringify the structure yourself (no fun), or else
1918 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1919 it on top of either DB_File or GDBM_File.
1921 =head2 How can I make my hash remember the order I put elements into it?
1923 Use the Tie::IxHash from CPAN.
1926 tie my %myhash, 'Tie::IxHash';
1927 for (my $i=0; $i<20; $i++) {
1930 my @keys = keys %myhash;
1931 # @keys = (0,1,2,3,...)
1933 =head2 Why does passing a subroutine an undefined element in a hash create it?
1935 If you say something like:
1937 somefunc($hash{"nonesuch key here"});
1939 Then that element "autovivifies"; that is, it springs into existence
1940 whether you store something there or not. That's because functions
1941 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1942 it has to be ready to write it back into the caller's version.
1944 This has been fixed as of Perl5.004.
1946 Normally, merely accessing a key's value for a nonexistent key does
1947 I<not> cause that key to be forever there. This is different than
1950 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1952 Usually a hash ref, perhaps like this:
1957 TITLE => "deputy peon",
1960 PALS => [ "Norbert", "Rhys", "Phineas"],
1963 References are documented in L<perlref> and the upcoming L<perlreftut>.
1964 Examples of complex data structures are given in L<perldsc> and
1965 L<perllol>. Examples of structures and object-oriented classes are
1968 =head2 How can I use a reference as a hash key?
1970 You can't do this directly, but you could use the standard Tie::RefHash
1971 module distributed with Perl.
1975 =head2 How do I handle binary data correctly?
1977 Perl is binary clean, so this shouldn't be a problem. For example,
1978 this works fine (assuming the files are found):
1980 if (`cat /vmunix` =~ /gzip/) {
1981 print "Your kernel is GNU-zip enabled!\n";
1984 On less elegant (read: Byzantine) systems, however, you have
1985 to play tedious games with "text" versus "binary" files. See
1986 L<perlfunc/"binmode"> or L<perlopentut>.
1988 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1990 If you want to deal with multibyte characters, however, there are
1991 some gotchas. See the section on Regular Expressions.
1993 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1995 Assuming that you don't care about IEEE notations like "NaN" or
1996 "Infinity", you probably just want to use a regular expression.
1998 if (/\D/) { print "has nondigits\n" }
1999 if (/^\d+$/) { print "is a whole number\n" }
2000 if (/^-?\d+$/) { print "is an integer\n" }
2001 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2002 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2003 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2004 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2005 { print "a C float\n" }
2007 There are also some commonly used modules for the task.
2008 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2009 internal function C<looks_like_number> for determining
2010 whether a variable looks like a number. L<Data::Types>
2011 exports functions that validate data types using both the
2012 above and other regular expressions. Thirdly, there is
2013 C<Regexp::Common> which has regular expressions to match
2014 various types of numbers. Those three modules are available
2017 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2018 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
2019 wrapper function for more convenient access. This function takes
2020 a string and returns the number it found, or C<undef> for input that
2021 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
2022 if you just want to say, ``Is this a float?''
2025 use POSIX qw(strtod);
2030 my($num, $unparsed) = strtod($str);
2031 if (($str eq '') || ($unparsed != 0) || $!) {
2038 sub is_numeric { defined getnum($_[0]) }
2040 Or you could check out the L<String::Scanf> module on the CPAN
2041 instead. The POSIX module (part of the standard Perl distribution) provides
2042 the C<strtod> and C<strtol> for converting strings to double and longs,
2045 =head2 How do I keep persistent data across program calls?
2047 For some specific applications, you can use one of the DBM modules.
2048 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2049 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2050 of the standard distribution. Here's one example using Storable's C<store>
2051 and C<retrieve> functions:
2054 store(\%hash, "filename");
2057 $href = retrieve("filename"); # by ref
2058 %hash = %{ retrieve("filename") }; # direct to hash
2060 =head2 How do I print out or copy a recursive data structure?
2062 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2063 for printing out data structures. The Storable module on CPAN (or the
2064 5.8 release of Perl), provides a function called C<dclone> that recursively
2065 copies its argument.
2067 use Storable qw(dclone);
2070 Where $r1 can be a reference to any kind of data structure you'd like.
2071 It will be deeply copied. Because C<dclone> takes and returns references,
2072 you'd have to add extra punctuation if you had a hash of arrays that
2075 %newhash = %{ dclone(\%oldhash) };
2077 =head2 How do I define methods for every class/object?
2079 Use the UNIVERSAL class (see L<UNIVERSAL>).
2081 =head2 How do I verify a credit card checksum?
2083 Get the Business::CreditCard module from CPAN.
2085 =head2 How do I pack arrays of doubles or floats for XS code?
2087 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2088 If you're doing a lot of float or double processing, consider using
2089 the PDL module from CPAN instead--it makes number-crunching easy.
2091 =head1 AUTHOR AND COPYRIGHT
2093 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
2094 All rights reserved.
2096 This documentation is free; you can redistribute it and/or modify it
2097 under the same terms as Perl itself.
2099 Irrespective of its distribution, all code examples in this file
2100 are hereby placed into the public domain. You are permitted and
2101 encouraged to use this code in your own programs for fun
2102 or for profit as you see fit. A simple comment in the code giving
2103 credit would be courteous but is not required.