3 perlfaq4 - Data Manipulation ($Revision: 1.44 $, $Date: 2003/07/28 17:35:21 $)
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?
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:
143 $dec = sprintf("%d", $int);
145 Using the hex function:
147 $int = hex("DEADBEEF");
148 $dec = sprintf("%d", $int);
152 $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
153 $dec = sprintf("%d", $int);
155 Using the CPAN module Bit::Vector:
158 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
159 $dec = $vec->to_Dec();
161 =item How do I convert from decimal to hexadecimal
165 $hex = sprintf("%X", 3735928559);
169 $hex = unpack("H*", pack("N", 3735928559));
174 $vec = Bit::Vector->new_Dec(32, -559038737);
175 $hex = $vec->to_Hex();
177 And Bit::Vector supports odd bit counts:
180 $vec = Bit::Vector->new_Dec(33, 3735928559);
181 $vec->Resize(32); # suppress leading 0 if unwanted
182 $hex = $vec->to_Hex();
184 =item How do I convert from octal to decimal
186 Using Perl's built in conversion of numbers with leading zeros:
188 $int = 033653337357; # note the leading 0!
189 $dec = sprintf("%d", $int);
191 Using the oct function:
193 $int = oct("33653337357");
194 $dec = sprintf("%d", $int);
199 $vec = Bit::Vector->new(32);
200 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
201 $dec = $vec->to_Dec();
203 =item How do I convert from decimal to octal
207 $oct = sprintf("%o", 3735928559);
212 $vec = Bit::Vector->new_Dec(32, -559038737);
213 $oct = reverse join('', $vec->Chunk_List_Read(3));
215 =item How do I convert from binary to decimal
217 Perl 5.6 lets you write binary numbers directly with
220 $number = 0b10110110;
224 $decimal = ord(pack('B8', '10110110'));
226 Using pack and unpack for larger strings
228 $int = unpack("N", pack("B32",
229 substr("0" x 32 . "11110101011011011111011101111", -32)));
230 $dec = sprintf("%d", $int);
232 # substr() is used to left pad a 32 character string with zeros.
236 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
237 $dec = $vec->to_Dec();
239 =item How do I convert from decimal to binary
243 $bin = unpack("B*", pack("N", 3735928559));
248 $vec = Bit::Vector->new_Dec(32, -559038737);
249 $bin = $vec->to_Bin();
251 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
252 are left as an exercise to the inclined reader.
256 =head2 Why doesn't & work the way I want it to?
258 The behavior of binary arithmetic operators depends on whether they're
259 used on numbers or strings. The operators treat a string as a series
260 of bits and work with that (the string C<"3"> is the bit pattern
261 C<00110011>). The operators work with the binary form of a number
262 (the number C<3> is treated as the bit pattern C<00000011>).
264 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
265 C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
268 Most problems with C<&> and C<|> arise because the programmer thinks
269 they have a number but really it's a string. The rest arise because
272 if ("\020\020" & "\101\101") {
276 but a string consisting of two null bytes (the result of C<"\020\020"
277 & "\101\101">) is not a false value in Perl. You need:
279 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
283 =head2 How do I multiply matrices?
285 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
286 or the PDL extension (also available from CPAN).
288 =head2 How do I perform an operation on a series of integers?
290 To call a function on each element in an array, and collect the
293 @results = map { my_func($_) } @array;
297 @triple = map { 3 * $_ } @single;
299 To call a function on each element of an array, but ignore the
302 foreach $iterator (@array) {
303 some_func($iterator);
306 To call a function on each integer in a (small) range, you B<can> use:
308 @results = map { some_func($_) } (5 .. 25);
310 but you should be aware that the C<..> operator creates an array of
311 all integers in the range. This can take a lot of memory for large
315 for ($i=5; $i < 500_005; $i++) {
316 push(@results, some_func($i));
319 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
320 loop will iterate over the range, without creating the entire range.
322 for my $i (5 .. 500_005) {
323 push(@results, some_func($i));
326 will not create a list of 500,000 integers.
328 =head2 How can I output Roman numerals?
330 Get the http://www.cpan.org/modules/by-module/Roman module.
332 =head2 Why aren't my random numbers random?
334 If you're using a version of Perl before 5.004, you must call C<srand>
335 once at the start of your program to seed the random number generator.
337 BEGIN { srand() if $] < 5.004 }
339 5.004 and later automatically call C<srand> at the beginning. Don't
340 call C<srand> more than once---you make your numbers less random, rather
343 Computers are good at being predictable and bad at being random
344 (despite appearances caused by bugs in your programs :-). see the
345 F<random> article in the "Far More Than You Ever Wanted To Know"
346 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
347 Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
348 who attempts to generate random numbers by deterministic means is, of
349 course, living in a state of sin.''
351 If you want numbers that are more random than C<rand> with C<srand>
352 provides, you should also check out the Math::TrulyRandom module from
353 CPAN. It uses the imperfections in your system's timer to generate
354 random numbers, but this takes quite a while. If you want a better
355 pseudorandom generator than comes with your operating system, look at
356 ``Numerical Recipes in C'' at http://www.nr.com/ .
358 =head2 How do I get a random number between X and Y?
360 Use the following simple function. It selects a random integer between
361 (and possibly including!) the two given integers, e.g.,
362 C<random_int_in(50,120)>
364 sub random_int_in ($$) {
366 # Assumes that the two arguments are integers themselves!
367 return $min if $min == $max;
368 ($min, $max) = ($max, $min) if $min > $max;
369 return $min + int rand(1 + $max - $min);
374 =head2 How do I find the day or week of the year?
376 The localtime function returns the day of the week. Without an
377 argument localtime uses the current time.
379 $day_of_year = (localtime)[7];
381 The POSIX module can also format a date as the day of the year or
384 use POSIX qw/strftime/;
385 my $day_of_year = strftime "%j", localtime;
386 my $week_of_year = strftime "%W", localtime;
388 To get the day of year for any date, use the Time::Local module to get
389 a time in epoch seconds for the argument to localtime.
391 use POSIX qw/strftime/;
393 my $week_of_year = strftime "%W",
394 localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
396 The Date::Calc module provides two functions for to calculate these.
399 my $day_of_year = Day_of_Year( 1987, 12, 18 );
400 my $week_of_year = Week_of_Year( 1987, 12, 18 );
402 =head2 How do I find the current century or millennium?
404 Use the following simple functions:
407 return int((((localtime(shift || time))[5] + 1999))/100);
410 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
413 You can also use the POSIX strftime() function which may be a bit
414 slower but is easier to read and maintain.
416 use POSIX qw/strftime/;
418 my $week_of_the_year = strftime "%W", localtime;
419 my $day_of_the_year = strftime "%j", localtime;
421 On some systems, the POSIX module's strftime() function has
422 been extended in a non-standard way to use a C<%C> format,
423 which they sometimes claim is the "century". It isn't,
424 because on most such systems, this is only the first two
425 digits of the four-digit year, and thus cannot be used to
426 reliably determine the current century or millennium.
428 =head2 How can I compare two dates and find the difference?
430 If you're storing your dates as epoch seconds then simply subtract one
431 from the other. If you've got a structured date (distinct year, day,
432 month, hour, minute, seconds values), then for reasons of accessibility,
433 simplicity, and efficiency, merely use either timelocal or timegm (from
434 the Time::Local module in the standard distribution) to reduce structured
435 dates to epoch seconds. However, if you don't know the precise format of
436 your dates, then you should probably use either of the Date::Manip and
437 Date::Calc modules from CPAN before you go hacking up your own parsing
438 routine to handle arbitrary date formats.
440 =head2 How can I take a string and turn it into epoch seconds?
442 If it's a regular enough string that it always has the same format,
443 you can split it up and pass the parts to C<timelocal> in the standard
444 Time::Local module. Otherwise, you should look into the Date::Calc
445 and Date::Manip modules from CPAN.
447 =head2 How can I find the Julian Day?
449 Use the Time::JulianDay module (part of the Time-modules bundle
450 available from CPAN.)
452 Before you immerse yourself too deeply in this, be sure to verify that
453 it is the I<Julian> Day you really want. Are you interested in a way
454 of getting serial days so that you just can tell how many days they
455 are apart or so that you can do also other date arithmetic? If you
456 are interested in performing date arithmetic, this can be done using
457 modules Date::Manip or Date::Calc.
459 There is too many details and much confusion on this issue to cover in
460 this FAQ, but the term is applied (correctly) to a calendar now
461 supplanted by the Gregorian Calendar, with the Julian Calendar failing
462 to adjust properly for leap years on centennial years (among other
463 annoyances). The term is also used (incorrectly) to mean: [1] days in
464 the Gregorian Calendar; and [2] days since a particular starting time
465 or `epoch', usually 1970 in the Unix world and 1980 in the
466 MS-DOS/Windows world. If you find that it is not the first meaning
467 that you really want, then check out the Date::Manip and Date::Calc
468 modules. (Thanks to David Cassell for most of this text.)
470 =head2 How do I find yesterday's date?
472 If you only need to find the date (and not the same time), you
473 can use the Date::Calc module.
475 use Date::Calc qw(Today Add_Delta_Days);
477 my @date = Add_Delta_Days( Today(), -1 );
481 Most people try to use the time rather than the calendar to
482 figure out dates, but that assumes that your days are
483 twenty-four hours each. For most people, there are two days
484 a year when they aren't: the switch to and from summer time
485 throws this off. Russ Allbery offers this solution.
488 my $now = defined $_[0] ? $_[0] : time;
489 my $then = $now - 60 * 60 * 24;
490 my $ndst = (localtime $now)[8] > 0;
491 my $tdst = (localtime $then)[8] > 0;
492 $then - ($tdst - $ndst) * 60 * 60;
495 Should give you "this time yesterday" in seconds since epoch relative to
496 the first argument or the current time if no argument is given and
497 suitable for passing to localtime or whatever else you need to do with
498 it. $ndst is whether we're currently in daylight savings time; $tdst is
499 whether the point 24 hours ago was in daylight savings time. If $tdst
500 and $ndst are the same, a boundary wasn't crossed, and the correction
501 will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
502 from yesterday's time since we gained an extra hour while going off
503 daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
504 negative hour (add an hour) to yesterday's time since we lost an hour.
506 All of this is because during those days when one switches off or onto
507 DST, a "day" isn't 24 hours long; it's either 23 or 25.
509 The explicit settings of $ndst and $tdst are necessary because localtime
510 only says it returns the system tm struct, and the system tm struct at
511 least on Solaris doesn't guarantee any particular positive value (like,
512 say, 1) for isdst, just a positive value. And that value can
513 potentially be negative, if DST information isn't available (this sub
514 just treats those cases like no DST).
516 Note that between 2am and 3am on the day after the time zone switches
517 off daylight savings time, the exact hour of "yesterday" corresponding
518 to the current hour is not clearly defined. Note also that if used
519 between 2am and 3am the day after the change to daylight savings time,
520 the result will be between 3am and 4am of the previous day; it's
521 arguable whether this is correct.
523 This sub does not attempt to deal with leap seconds (most things don't).
527 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
529 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
530 Y2K compliant (whatever that means). The programmers you've hired to
531 use it, however, probably are not.
533 Long answer: The question belies a true understanding of the issue.
534 Perl is just as Y2K compliant as your pencil--no more, and no less.
535 Can you use your pencil to write a non-Y2K-compliant memo? Of course
536 you can. Is that the pencil's fault? Of course it isn't.
538 The date and time functions supplied with Perl (gmtime and localtime)
539 supply adequate information to determine the year well beyond 2000
540 (2038 is when trouble strikes for 32-bit machines). The year returned
541 by these functions when used in a list context is the year minus 1900.
542 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
543 number. To avoid the year 2000 problem simply do not treat the year as
544 a 2-digit number. It isn't.
546 When gmtime() and localtime() are used in scalar context they return
547 a timestamp string that contains a fully-expanded year. For example,
548 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
549 2001". There's no year 2000 problem here.
551 That doesn't mean that Perl can't be used to create non-Y2K compliant
552 programs. It can. But so can your pencil. It's the fault of the user,
553 not the language. At the risk of inflaming the NRA: ``Perl doesn't
554 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
559 =head2 How do I validate input?
561 The answer to this question is usually a regular expression, perhaps
562 with auxiliary logic. See the more specific questions (numbers, mail
563 addresses, etc.) for details.
565 =head2 How do I unescape a string?
567 It depends just what you mean by ``escape''. URL escapes are dealt
568 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
569 character are removed with
573 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
575 =head2 How do I remove consecutive pairs of characters?
577 To turn C<"abbcccd"> into C<"abccd">:
579 s/(.)\1/$1/g; # add /s to include newlines
581 Here's a solution that turns "abbcccd" to "abcd":
583 y///cs; # y == tr, but shorter :-)
585 =head2 How do I expand function calls in a string?
587 This is documented in L<perlref>. In general, this is fraught with
588 quoting and readability problems, but it is possible. To interpolate
589 a subroutine call (in list context) into a string:
591 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
593 See also ``How can I expand variables in text strings?'' in this
596 =head2 How do I find matching/nesting anything?
598 This isn't something that can be done in one regular expression, no
599 matter how complicated. To find something between two single
600 characters, a pattern like C</x([^x]*)x/> will get the intervening
601 bits in $1. For multiple ones, then something more like
602 C</alpha(.*?)omega/> would be needed. But none of these deals with
603 nested patterns. For balanced expressions using C<(>, C<{>, C<[>
604 or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
605 L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
607 If you are serious about writing a parser, there are a number of
608 modules or oddities that will make your life a lot easier. There are
609 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
610 and the byacc program. Starting from perl 5.8 the Text::Balanced
611 is part of the standard distribution.
613 One simple destructive, inside-out approach that you might try is to
614 pull out the smallest nesting parts one at a time:
616 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
617 # do something with $1
620 A more complicated and sneaky approach is to make Perl's regular
621 expression engine do it for you. This is courtesy Dean Inada, and
622 rather has the nature of an Obfuscated Perl Contest entry, but it
625 # $_ contains the string to parse
626 # BEGIN and END are the opening and closing markers for the
631 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
632 @$ = (eval{/$re/},$@!~/unmatched/i);
633 print join("\n",@$[0..$#$]) if( $$[-1] );
635 =head2 How do I reverse a string?
637 Use reverse() in scalar context, as documented in
640 $reversed = reverse $string;
642 =head2 How do I expand tabs in a string?
644 You can do it yourself:
646 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
648 Or you can just use the Text::Tabs module (part of the standard Perl
652 @expanded_lines = expand(@lines_with_tabs);
654 =head2 How do I reformat a paragraph?
656 Use Text::Wrap (part of the standard Perl distribution):
659 print wrap("\t", ' ', @paragraphs);
661 The paragraphs you give to Text::Wrap should not contain embedded
662 newlines. Text::Wrap doesn't justify the lines (flush-right).
664 Or use the CPAN module Text::Autoformat. Formatting files can be easily
665 done by making a shell alias, like so:
667 alias fmt="perl -i -MText::Autoformat -n0777 \
668 -e 'print autoformat $_, {all=>1}' $*"
670 See the documentation for Text::Autoformat to appreciate its many
673 =head2 How can I access or change N characters of a string?
675 You can access the first characters of a string with substr().
676 To get the first character, for example, start at position 0
677 and grab the string of length 1.
680 $string = "Just another Perl Hacker";
681 $first_char = substr( $string, 0, 1 ); # 'J'
683 To change part of a string, you can use the optional fourth
684 argument which is the replacement string.
686 substr( $string, 13, 4, "Perl 5.8.0" );
688 You can also use substr() as an lvalue.
690 substr( $string, 13, 4 ) = "Perl 5.8.0";
692 =head2 How do I change the Nth occurrence of something?
694 You have to keep track of N yourself. For example, let's say you want
695 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
696 C<"whosoever"> or C<"whomsoever">, case insensitively. These
697 all assume that $_ contains the string to be altered.
701 ++$count == 5 # is it the 5th?
702 ? "${2}soever" # yes, swap
703 : $1 # renege and leave it there
706 In the more general case, you can use the C</g> modifier in a C<while>
707 loop, keeping count of matches.
711 $_ = "One fish two fish red fish blue fish";
712 while (/(\w+)\s+fish\b/gi) {
713 if (++$count == $WANT) {
714 print "The third fish is a $1 one.\n";
718 That prints out: C<"The third fish is a red one."> You can also use a
719 repetition count and repeated pattern like this:
721 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
723 =head2 How can I count the number of occurrences of a substring within a string?
725 There are a number of ways, with varying efficiency. If you want a
726 count of a certain single character (X) within a string, you can use the
727 C<tr///> function like so:
729 $string = "ThisXlineXhasXsomeXx'sXinXit";
730 $count = ($string =~ tr/X//);
731 print "There are $count X characters in the string";
733 This is fine if you are just looking for a single character. However,
734 if you are trying to count multiple character substrings within a
735 larger string, C<tr///> won't work. What you can do is wrap a while()
736 loop around a global pattern match. For example, let's count negative
739 $string = "-9 55 48 -2 23 -76 4 14 -44";
740 while ($string =~ /-\d+/g) { $count++ }
741 print "There are $count negative numbers in the string";
743 Another version uses a global match in list context, then assigns the
744 result to a scalar, producing a count of the number of matches.
746 $count = () = $string =~ /-\d+/g;
748 =head2 How do I capitalize all the words on one line?
750 To make the first letter of each word upper case:
752 $line =~ s/\b(\w)/\U$1/g;
754 This has the strange effect of turning "C<don't do it>" into "C<Don'T
755 Do It>". Sometimes you might want this. Other times you might need a
756 more thorough solution (Suggested by brian d foy):
759 (^\w) #at the beginning of the line
761 (\s\w) #preceded by whitespace
764 $string =~ /([\w']+)/\u\L$1/g;
766 To make the whole line upper case:
770 To force each word to be lower case, with the first letter upper case:
772 $line =~ s/(\w+)/\u\L$1/g;
774 You can (and probably should) enable locale awareness of those
775 characters by placing a C<use locale> pragma in your program.
776 See L<perllocale> for endless details on locales.
778 This is sometimes referred to as putting something into "title
779 case", but that's not quite accurate. Consider the proper
780 capitalization of the movie I<Dr. Strangelove or: How I Learned to
781 Stop Worrying and Love the Bomb>, for example.
783 Damian Conway's L<Text::Autoformat> module provides some smart
784 case transformations:
786 use Text::Autoformat;
787 my $x = "Dr. Strangelove or: How I Learned to Stop ".
788 "Worrying and Love the Bomb";
791 for my $style (qw( sentence title highlight ))
793 print autoformat($x, { case => $style }), "\n";
796 =head2 How can I split a [character] delimited string except when inside [character]?
798 Several modules can handle this sort of pasing---Text::Balanced,
799 Text::CVS, Text::CVS_XS, and Text::ParseWords, among others.
801 Take the example case of trying to split a string that is
802 comma-separated into its different fields. You can't use C<split(/,/)>
803 because you shouldn't split if the comma is inside quotes. For
804 example, take a data line like this:
806 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
808 Due to the restriction of the quotes, this is a fairly complex
809 problem. Thankfully, we have Jeffrey Friedl, author of
810 I<Mastering Regular Expressions>, to handle these for us. He
811 suggests (assuming your string is contained in $text):
814 push(@new, $+) while $text =~ m{
815 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
819 push(@new, undef) if substr($text,-1,1) eq ',';
821 If you want to represent quotation marks inside a
822 quotation-mark-delimited field, escape them with backslashes (eg,
825 Alternatively, the Text::ParseWords module (part of the standard Perl
826 distribution) lets you say:
828 use Text::ParseWords;
829 @new = quotewords(",", 0, $text);
831 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
833 =head2 How do I strip blank space from the beginning/end of a string?
835 Although the simplest approach would seem to be
837 $string =~ s/^\s*(.*?)\s*$/$1/;
839 not only is this unnecessarily slow and destructive, it also fails with
840 embedded newlines. It is much faster to do this operation in two steps:
845 Or more nicely written as:
852 This idiom takes advantage of the C<foreach> loop's aliasing
853 behavior to factor out common code. You can do this
854 on several strings at once, or arrays, or even the
855 values of a hash if you use a slice:
857 # trim whitespace in the scalar, the array,
858 # and all the values in the hash
859 foreach ($scalar, @array, @hash{keys %hash}) {
864 =head2 How do I pad a string with blanks or pad a number with zeroes?
866 In the following examples, C<$pad_len> is the length to which you wish
867 to pad the string, C<$text> or C<$num> contains the string to be padded,
868 and C<$pad_char> contains the padding character. You can use a single
869 character string constant instead of the C<$pad_char> variable if you
870 know what it is in advance. And in the same way you can use an integer in
871 place of C<$pad_len> if you know the pad length in advance.
873 The simplest method uses the C<sprintf> function. It can pad on the left
874 or right with blanks and on the left with zeroes and it will not
875 truncate the result. The C<pack> function can only pad strings on the
876 right with blanks and it will truncate the result to a maximum length of
879 # Left padding a string with blanks (no truncation):
880 $padded = sprintf("%${pad_len}s", $text);
881 $padded = sprintf("%*s", $pad_len, $text); # same thing
883 # Right padding a string with blanks (no truncation):
884 $padded = sprintf("%-${pad_len}s", $text);
885 $padded = sprintf("%-*s", $pad_len, $text); # same thing
887 # Left padding a number with 0 (no truncation):
888 $padded = sprintf("%0${pad_len}d", $num);
889 $padded = sprintf("%0*d", $pad_len, $num); # same thing
891 # Right padding a string with blanks using pack (will truncate):
892 $padded = pack("A$pad_len",$text);
894 If you need to pad with a character other than blank or zero you can use
895 one of the following methods. They all generate a pad string with the
896 C<x> operator and combine that with C<$text>. These methods do
897 not truncate C<$text>.
899 Left and right padding with any character, creating a new string:
901 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
902 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
904 Left and right padding with any character, modifying C<$text> directly:
906 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
907 $text .= $pad_char x ( $pad_len - length( $text ) );
909 =head2 How do I extract selected columns from a string?
911 Use substr() or unpack(), both documented in L<perlfunc>.
912 If you prefer thinking in terms of columns instead of widths,
913 you can use this kind of thing:
915 # determine the unpack format needed to split Linux ps output
916 # arguments are cut columns
917 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
923 for my $place (@positions) {
924 $template .= "A" . ($place - $lastpos) . " ";
931 =head2 How do I find the soundex value of a string?
933 Use the standard Text::Soundex module distributed with Perl.
934 Before you do so, you may want to determine whether `soundex' is in
935 fact what you think it is. Knuth's soundex algorithm compresses words
936 into a small space, and so it does not necessarily distinguish between
937 two words which you might want to appear separately. For example, the
938 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
939 If Text::Soundex does not do what you are looking for, you might want
940 to consider the String::Approx module available at CPAN.
942 =head2 How can I expand variables in text strings?
944 Let's assume that you have a string like:
946 $text = 'this has a $foo in it and a $bar';
948 If those were both global variables, then this would
951 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
953 But since they are probably lexicals, or at least, they could
954 be, you'd have to do this:
956 $text =~ s/(\$\w+)/$1/eeg;
957 die if $@; # needed /ee, not /e
959 It's probably better in the general case to treat those
960 variables as entries in some special hash. For example:
966 $text =~ s/\$(\w+)/$user_defs{$1}/g;
968 See also ``How do I expand function calls in a string?'' in this section
971 =head2 What's wrong with always quoting "$vars"?
973 The problem is that those double-quotes force stringification--
974 coercing numbers and references into strings--even when you
975 don't want them to be strings. Think of it this way: double-quote
976 expansion is used to produce new strings. If you already
977 have a string, why do you need more?
979 If you get used to writing odd things like these:
983 somefunc("$var"); # BAD
985 You'll be in trouble. Those should (in 99.8% of the cases) be
986 the simpler and more direct:
992 Otherwise, besides slowing you down, you're going to break code when
993 the thing in the scalar is actually neither a string nor a number, but
999 my $oref = "$aref"; # WRONG
1002 You can also get into subtle problems on those few operations in Perl
1003 that actually do care about the difference between a string and a
1004 number, such as the magical C<++> autoincrement operator or the
1007 Stringification also destroys arrays.
1010 print "@lines"; # WRONG - extra blanks
1011 print @lines; # right
1013 =head2 Why don't my E<lt>E<lt>HERE documents work?
1015 Check for these three things:
1019 =item There must be no space after the E<lt>E<lt> part.
1021 =item There (probably) should be a semicolon at the end.
1023 =item You can't (easily) have any space in front of the tag.
1027 If you want to indent the text in the here document, you
1031 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1036 But the HERE_TARGET must still be flush against the margin.
1037 If you want that indented also, you'll have to quote
1040 ($quote = <<' FINIS') =~ s/^\s+//gm;
1041 ...we will have peace, when you and all your works have
1042 perished--and the works of your dark master to whom you
1043 would deliver us. You are a liar, Saruman, and a corrupter
1044 of men's hearts. --Theoden in /usr/src/perl/taint.c
1046 $quote =~ s/\s+--/\n--/;
1048 A nice general-purpose fixer-upper function for indented here documents
1049 follows. It expects to be called with a here document as its argument.
1050 It looks to see whether each line begins with a common substring, and
1051 if so, strips that substring off. Otherwise, it takes the amount of leading
1052 whitespace found on the first line and removes that much off each
1057 my ($white, $leader); # common whitespace and common leading string
1058 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1059 ($white, $leader) = ($2, quotemeta($1));
1061 ($white, $leader) = (/^(\s+)/, '');
1063 s/^\s*?$leader(?:$white)?//gm;
1067 This works with leading special strings, dynamically determined:
1069 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1072 @@@ SAVEI32(runlevel);
1074 @@@ while ( op = (*op->op_ppaddr)() );
1078 MAIN_INTERPRETER_LOOP
1080 Or with a fixed amount of leading whitespace, with remaining
1081 indentation correctly preserved:
1083 $poem = fix<<EVER_ON_AND_ON;
1084 Now far ahead the Road has gone,
1085 And I must follow, if I can,
1086 Pursuing it with eager feet,
1087 Until it joins some larger way
1088 Where many paths and errands meet.
1089 And whither then? I cannot say.
1090 --Bilbo in /usr/src/perl/pp_ctl.c
1095 =head2 What is the difference between a list and an array?
1097 An array has a changeable length. A list does not. An array is something
1098 you can push or pop, while a list is a set of values. Some people make
1099 the distinction that a list is a value while an array is a variable.
1100 Subroutines are passed and return lists, you put things into list
1101 context, you initialize arrays with lists, and you foreach() across
1102 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1103 in scalar context behave like the number of elements in them, subroutines
1104 access their arguments through the array C<@_>, and push/pop/shift only work
1107 As a side note, there's no such thing as a list in scalar context.
1110 $scalar = (2, 5, 7, 9);
1112 you're using the comma operator in scalar context, so it uses the scalar
1113 comma operator. There never was a list there at all! This causes the
1114 last value to be returned: 9.
1116 =head2 What is the difference between $array[1] and @array[1]?
1118 The former is a scalar value; the latter an array slice, making
1119 it a list with one (scalar) value. You should use $ when you want a
1120 scalar value (most of the time) and @ when you want a list with one
1121 scalar value in it (very, very rarely; nearly never, in fact).
1123 Sometimes it doesn't make a difference, but sometimes it does.
1124 For example, compare:
1126 $good[0] = `some program that outputs several lines`;
1130 @bad[0] = `same program that outputs several lines`;
1132 The C<use warnings> pragma and the B<-w> flag will warn you about these
1135 =head2 How can I remove duplicate elements from a list or array?
1137 There are several possible ways, depending on whether the array is
1138 ordered and whether you wish to preserve the ordering.
1144 If @in is sorted, and you want @out to be sorted:
1145 (this assumes all true values in the array)
1147 $prev = "not equal to $in[0]";
1148 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1150 This is nice in that it doesn't use much extra memory, simulating
1151 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1152 guarantees that the expression is true (so that grep picks it up)
1153 even if the $_ is 0, "", or undef.
1157 If you don't know whether @in is sorted:
1160 @out = grep(!$saw{$_}++, @in);
1164 Like (b), but @in contains only small integers:
1166 @out = grep(!$saw[$_]++, @in);
1170 A way to do (b) without any loops or greps:
1174 @out = sort keys %saw; # remove sort if undesired
1178 Like (d), but @in contains only small positive integers:
1182 @out = grep {defined} @ary;
1186 But perhaps you should have been using a hash all along, eh?
1188 =head2 How can I tell whether a certain element is contained in a list or array?
1190 Hearing the word "in" is an I<in>dication that you probably should have
1191 used a hash, not a list or array, to store your data. Hashes are
1192 designed to answer this question quickly and efficiently. Arrays aren't.
1194 That being said, there are several ways to approach this. If you
1195 are going to make this query many times over arbitrary string values,
1196 the fastest way is probably to invert the original array and maintain a
1197 hash whose keys are the first array's values.
1199 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1201 for (@blues) { $is_blue{$_} = 1 }
1203 Now you can check whether $is_blue{$some_color}. It might have been a
1204 good idea to keep the blues all in a hash in the first place.
1206 If the values are all small integers, you could use a simple indexed
1207 array. This kind of an array will take up less space:
1209 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1210 @is_tiny_prime = ();
1211 for (@primes) { $is_tiny_prime[$_] = 1 }
1212 # or simply @istiny_prime[@primes] = (1) x @primes;
1214 Now you check whether $is_tiny_prime[$some_number].
1216 If the values in question are integers instead of strings, you can save
1217 quite a lot of space by using bit strings instead:
1219 @articles = ( 1..10, 150..2000, 2017 );
1221 for (@articles) { vec($read,$_,1) = 1 }
1223 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1227 ($is_there) = grep $_ eq $whatever, @array;
1231 ($is_there) = grep /$whatever/, @array;
1233 These are slow (checks every element even if the first matches),
1234 inefficient (same reason), and potentially buggy (what if there are
1235 regex characters in $whatever?). If you're only testing once, then
1239 foreach $elt (@array) {
1240 if ($elt eq $elt_to_find) {
1245 if ($is_there) { ... }
1247 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1249 Use a hash. Here's code to do both and more. It assumes that
1250 each element is unique in a given array:
1252 @union = @intersection = @difference = ();
1254 foreach $element (@array1, @array2) { $count{$element}++ }
1255 foreach $element (keys %count) {
1256 push @union, $element;
1257 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1260 Note that this is the I<symmetric difference>, that is, all elements in
1261 either A or in B but not in both. Think of it as an xor operation.
1263 =head2 How do I test whether two arrays or hashes are equal?
1265 The following code works for single-level arrays. It uses a stringwise
1266 comparison, and does not distinguish defined versus undefined empty
1267 strings. Modify if you have other needs.
1269 $are_equal = compare_arrays(\@frogs, \@toads);
1271 sub compare_arrays {
1272 my ($first, $second) = @_;
1273 no warnings; # silence spurious -w undef complaints
1274 return 0 unless @$first == @$second;
1275 for (my $i = 0; $i < @$first; $i++) {
1276 return 0 if $first->[$i] ne $second->[$i];
1281 For multilevel structures, you may wish to use an approach more
1282 like this one. It uses the CPAN module FreezeThaw:
1284 use FreezeThaw qw(cmpStr);
1285 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1287 printf "a and b contain %s arrays\n",
1288 cmpStr(\@a, \@b) == 0
1292 This approach also works for comparing hashes. Here
1293 we'll demonstrate two different answers:
1295 use FreezeThaw qw(cmpStr cmpStrHard);
1297 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1301 printf "a and b contain %s hashes\n",
1302 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1304 printf "a and b contain %s hashes\n",
1305 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1308 The first reports that both those the hashes contain the same data,
1309 while the second reports that they do not. Which you prefer is left as
1310 an exercise to the reader.
1312 =head2 How do I find the first array element for which a condition is true?
1314 To find the first array element which satisfies a condition, you can
1315 use the first() function in the List::Util module, which comes with
1316 Perl 5.8. This example finds the first element that contains "Perl".
1318 use List::Util qw(first);
1320 my $element = first { /Perl/ } @array;
1322 If you cannot use List::Util, you can make your own loop to do the
1323 same thing. Once you find the element, you stop the loop with last.
1326 foreach my $element ( @array )
1328 if( /Perl/ ) { $found = $element; last }
1331 If you want the array index, you can iterate through the indices
1332 and check the array element at each index until you find one
1333 that satisfies the condition.
1335 my( $found, $index ) = ( undef, -1 );
1336 for( $i = 0; $i < @array; $i++ )
1338 if( $array[$i] =~ /Perl/ )
1340 $found = $array[$i];
1346 =head2 How do I handle linked lists?
1348 In general, you usually don't need a linked list in Perl, since with
1349 regular arrays, you can push and pop or shift and unshift at either end,
1350 or you can use splice to add and/or remove arbitrary number of elements at
1351 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1352 dynamic arrays. In the absence of shifts and pops, push in general
1353 needs to reallocate on the order every log(N) times, and unshift will
1354 need to copy pointers each time.
1356 If you really, really wanted, you could use structures as described in
1357 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1358 to do. For example, imagine a list node like this:
1365 You could walk the list this way:
1368 for ($node = $head; $node; $node = $node->{LINK}) {
1369 print $node->{VALUE}, " ";
1373 You could add to the list this way:
1376 $tail = append($head, 1); # grow a new head
1377 for $value ( 2 .. 10 ) {
1378 $tail = append($tail, $value);
1382 my($list, $value) = @_;
1383 my $node = { VALUE => $value };
1385 $node->{LINK} = $list->{LINK};
1386 $list->{LINK} = $node;
1388 $_[0] = $node; # replace caller's version
1393 But again, Perl's built-in are virtually always good enough.
1395 =head2 How do I handle circular lists?
1397 Circular lists could be handled in the traditional fashion with linked
1398 lists, or you could just do something like this with an array:
1400 unshift(@array, pop(@array)); # the last shall be first
1401 push(@array, shift(@array)); # and vice versa
1403 =head2 How do I shuffle an array randomly?
1405 If you either have Perl 5.8.0 or later installed, or if you have
1406 Scalar-List-Utils 1.03 or later installed, you can say:
1408 use List::Util 'shuffle';
1410 @shuffled = shuffle(@list);
1412 If not, you can use a Fisher-Yates shuffle.
1414 sub fisher_yates_shuffle {
1415 my $deck = shift; # $deck is a reference to an array
1418 my $j = int rand ($i+1);
1419 @$deck[$i,$j] = @$deck[$j,$i];
1423 # shuffle my mpeg collection
1425 my @mpeg = <audio/*/*.mp3>;
1426 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1429 Note that the above implementation shuffles an array in place,
1430 unlike the List::Util::shuffle() which takes a list and returns
1431 a new shuffled list.
1433 You've probably seen shuffling algorithms that work using splice,
1434 randomly picking another element to swap the current element with
1438 @old = 1 .. 10; # just a demo
1440 push(@new, splice(@old, rand @old, 1));
1443 This is bad because splice is already O(N), and since you do it N times,
1444 you just invented a quadratic algorithm; that is, O(N**2). This does
1445 not scale, although Perl is so efficient that you probably won't notice
1446 this until you have rather largish arrays.
1448 =head2 How do I process/modify each element of an array?
1450 Use C<for>/C<foreach>:
1453 s/foo/bar/; # change that word
1454 y/XZ/ZX/; # swap those letters
1457 Here's another; let's compute spherical volumes:
1459 for (@volumes = @radii) { # @volumes has changed parts
1461 $_ *= (4/3) * 3.14159; # this will be constant folded
1464 which can also be done with map() which is made to transform
1465 one list into another:
1467 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1469 If you want to do the same thing to modify the values of the
1470 hash, you can use the C<values> function. As of Perl 5.6
1471 the values are not copied, so if you modify $orbit (in this
1472 case), you modify the value.
1474 for $orbit ( values %orbits ) {
1475 ($orbit **= 3) *= (4/3) * 3.14159;
1478 Prior to perl 5.6 C<values> returned copies of the values,
1479 so older perl code often contains constructions such as
1480 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1481 the hash is to be modified.
1483 =head2 How do I select a random element from an array?
1485 Use the rand() function (see L<perlfunc/rand>):
1487 # at the top of the program:
1488 srand; # not needed for 5.004 and later
1491 $index = rand @array;
1492 $element = $array[$index];
1494 Make sure you I<only call srand once per program, if then>.
1495 If you are calling it more than once (such as before each
1496 call to rand), you're almost certainly doing something wrong.
1498 =head2 How do I permute N elements of a list?
1500 Use the List::Permutor module on CPAN. If the list is
1501 actually an array, try the Algorithm::Permute module (also
1502 on CPAN). It's written in XS code and is very efficient.
1504 use Algorithm::Permute;
1505 my @array = 'a'..'d';
1506 my $p_iterator = Algorithm::Permute->new ( \@array );
1507 while (my @perm = $p_iterator->next) {
1508 print "next permutation: (@perm)\n";
1511 For even faster execution, you could do:
1513 use Algorithm::Permute;
1514 my @array = 'a'..'d';
1515 Algorithm::Permute::permute {
1516 print "next permutation: (@array)\n";
1519 Here's a little program that generates all permutations of
1520 all the words on each line of input. The algorithm embodied
1521 in the permute() function is discussed in Volume 4 (still
1522 unpublished) of Knuth's I<The Art of Computer Programming>
1523 and will work on any list:
1526 # Fischer-Kause ordered permutation generator
1531 while ( $code->(@_[@idx]) ) {
1533 --$p while $idx[$p-1] > $idx[$p];
1534 my $q = $p or return;
1535 push @idx, reverse splice @idx, $p;
1536 ++$q while $idx[$p-1] > $idx[$q];
1537 @idx[$p-1,$q]=@idx[$q,$p-1];
1541 permute {print"@_\n"} split;
1543 =head2 How do I sort an array by (anything)?
1545 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1547 @list = sort { $a <=> $b } @list;
1549 The default sort function is cmp, string comparison, which would
1550 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1551 the numerical comparison operator.
1553 If you have a complicated function needed to pull out the part you
1554 want to sort on, then don't do it inside the sort function. Pull it
1555 out first, because the sort BLOCK can be called many times for the
1556 same element. Here's an example of how to pull out the first word
1557 after the first number on each item, and then sort those words
1562 ($item) = /\d+\s*(\S+)/;
1563 push @idx, uc($item);
1565 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1567 which could also be written this way, using a trick
1568 that's come to be known as the Schwartzian Transform:
1570 @sorted = map { $_->[0] }
1571 sort { $a->[1] cmp $b->[1] }
1572 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1574 If you need to sort on several fields, the following paradigm is useful.
1576 @sorted = sort { field1($a) <=> field1($b) ||
1577 field2($a) cmp field2($b) ||
1578 field3($a) cmp field3($b)
1581 This can be conveniently combined with precalculation of keys as given
1584 See the F<sort> artitcle article in the "Far More Than You Ever Wanted
1585 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1586 more about this approach.
1588 See also the question below on sorting hashes.
1590 =head2 How do I manipulate arrays of bits?
1592 Use pack() and unpack(), or else vec() and the bitwise operations.
1594 For example, this sets $vec to have bit N set if $ints[N] was set:
1597 foreach(@ints) { vec($vec,$_,1) = 1 }
1599 Here's how, given a vector in $vec, you can
1600 get those bits into your @ints array:
1602 sub bitvec_to_list {
1605 # Find null-byte density then select best algorithm
1606 if ($vec =~ tr/\0// / length $vec > 0.95) {
1609 # This method is faster with mostly null-bytes
1610 while($vec =~ /[^\0]/g ) {
1611 $i = -9 + 8 * pos $vec;
1612 push @ints, $i if vec($vec, ++$i, 1);
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);
1622 # This method is a fast general algorithm
1624 my $bits = unpack "b*", $vec;
1625 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1626 push @ints, pos $bits while($bits =~ /1/g);
1631 This method gets faster the more sparse the bit vector is.
1632 (Courtesy of Tim Bunce and Winfried Koenig.)
1634 You can make the while loop a lot shorter with this suggestion
1635 from Benjamin Goldberg:
1637 while($vec =~ /[^\0]+/g ) {
1638 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1641 Or use the CPAN module Bit::Vector:
1643 $vector = Bit::Vector->new($num_of_bits);
1644 $vector->Index_List_Store(@ints);
1645 @ints = $vector->Index_List_Read();
1647 Bit::Vector provides efficient methods for bit vector, sets of small integers
1650 Here's a more extensive illustration using vec():
1653 $vector = "\xff\x0f\xef\xfe";
1654 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1655 unpack("N", $vector), "\n";
1656 $is_set = vec($vector, 23, 1);
1657 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1675 my ($offset, $width, $value) = @_;
1677 vec($vector, $offset, $width) = $value;
1678 print "offset=$offset width=$width value=$value\n";
1684 my $bits = unpack("b*", $vector);
1688 print "vector length in bytes: ", length($vector), "\n";
1689 @bytes = unpack("A8" x length($vector), $bits);
1690 print "bits are: @bytes\n\n";
1693 =head2 Why does defined() return true on empty arrays and hashes?
1695 The short story is that you should probably only use defined on scalars or
1696 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1697 in the 5.004 release or later of Perl for more detail.
1699 =head1 Data: Hashes (Associative Arrays)
1701 =head2 How do I process an entire hash?
1703 Use the each() function (see L<perlfunc/each>) if you don't care
1704 whether it's sorted:
1706 while ( ($key, $value) = each %hash) {
1707 print "$key = $value\n";
1710 If you want it sorted, you'll have to use foreach() on the result of
1711 sorting the keys as shown in an earlier question.
1713 =head2 What happens if I add or remove keys from a hash while iterating over it?
1717 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1718 iterating over it. In Perl 5 you can delete from it, but you still
1719 can't add to it, because that might cause a doubling of the hash table,
1720 in which half the entries get copied up to the new top half of the
1721 table, at which point you've totally bamboozled the iterator code.
1722 Even if the table doesn't double, there's no telling whether your new
1723 entry will be inserted before or after the current iterator position.
1725 Either treasure up your changes and make them after the iterator finishes
1726 or use keys to fetch all the old keys at once, and iterate over the list
1729 =head2 How do I look up a hash element by value?
1731 Create a reverse hash:
1733 %by_value = reverse %by_key;
1734 $key = $by_value{$value};
1736 That's not particularly efficient. It would be more space-efficient
1739 while (($key, $value) = each %by_key) {
1740 $by_value{$value} = $key;
1743 If your hash could have repeated values, the methods above will only find
1744 one of the associated keys. This may or may not worry you. If it does
1745 worry you, you can always reverse the hash into a hash of arrays instead:
1747 while (($key, $value) = each %by_key) {
1748 push @{$key_list_by_value{$value}}, $key;
1751 =head2 How can I know how many entries are in a hash?
1753 If you mean how many keys, then all you have to do is
1754 use the keys() function in a scalar context:
1756 $num_keys = keys %hash;
1758 The keys() function also resets the iterator, which means that you may
1759 see strange results if you use this between uses of other hash operators
1762 =head2 How do I sort a hash (optionally by value instead of key)?
1764 Internally, hashes are stored in a way that prevents you from imposing
1765 an order on key-value pairs. Instead, you have to sort a list of the
1768 @keys = sort keys %hash; # sorted by key
1770 $hash{$a} cmp $hash{$b}
1771 } keys %hash; # and by value
1773 Here we'll do a reverse numeric sort by value, and if two keys are
1774 identical, sort by length of key, or if that fails, by straight ASCII
1775 comparison of the keys (well, possibly modified by your locale--see
1779 $hash{$b} <=> $hash{$a}
1781 length($b) <=> length($a)
1786 =head2 How can I always keep my hash sorted?
1788 You can look into using the DB_File module and tie() using the
1789 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1790 The Tie::IxHash module from CPAN might also be instructive.
1792 =head2 What's the difference between "delete" and "undef" with hashes?
1794 Hashes contain pairs of scalars: the first is the key, the
1795 second is the value. The key will be coerced to a string,
1796 although the value can be any kind of scalar: string,
1797 number, or reference. If a key $key is present in
1798 %hash, C<exists($hash{$key})> will return true. The value
1799 for a given key can be C<undef>, in which case
1800 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1801 will return true. This corresponds to (C<$key>, C<undef>)
1804 Pictures help... here's the %hash table:
1814 And these conditions hold
1818 defined $hash{'d'} is true
1819 defined $hash{'a'} is true
1820 exists $hash{'a'} is true (Perl5 only)
1821 grep ($_ eq 'a', keys %hash) is true
1827 your table now reads:
1838 and these conditions now hold; changes in caps:
1842 defined $hash{'d'} is true
1843 defined $hash{'a'} is FALSE
1844 exists $hash{'a'} is true (Perl5 only)
1845 grep ($_ eq 'a', keys %hash) is true
1847 Notice the last two: you have an undef value, but a defined key!
1853 your table now reads:
1862 and these conditions now hold; changes in caps:
1866 defined $hash{'d'} is true
1867 defined $hash{'a'} is false
1868 exists $hash{'a'} is FALSE (Perl5 only)
1869 grep ($_ eq 'a', keys %hash) is FALSE
1871 See, the whole entry is gone!
1873 =head2 Why don't my tied hashes make the defined/exists distinction?
1875 This depends on the tied hash's implementation of EXISTS().
1876 For example, there isn't the concept of undef with hashes
1877 that are tied to DBM* files. It also means that exists() and
1878 defined() do the same thing with a DBM* file, and what they
1879 end up doing is not what they do with ordinary hashes.
1881 =head2 How do I reset an each() operation part-way through?
1883 Using C<keys %hash> in scalar context returns the number of keys in
1884 the hash I<and> resets the iterator associated with the hash. You may
1885 need to do this if you use C<last> to exit a loop early so that when you
1886 re-enter it, the hash iterator has been reset.
1888 =head2 How can I get the unique keys from two hashes?
1890 First you extract the keys from the hashes into lists, then solve
1891 the "removing duplicates" problem described above. For example:
1894 for $element (keys(%foo), keys(%bar)) {
1901 @uniq = keys %{{%foo,%bar}};
1903 Or if you really want to save space:
1906 while (defined ($key = each %foo)) {
1909 while (defined ($key = each %bar)) {
1914 =head2 How can I store a multidimensional array in a DBM file?
1916 Either stringify the structure yourself (no fun), or else
1917 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1918 it on top of either DB_File or GDBM_File.
1920 =head2 How can I make my hash remember the order I put elements into it?
1922 Use the Tie::IxHash from CPAN.
1925 tie my %myhash, 'Tie::IxHash';
1926 for (my $i=0; $i<20; $i++) {
1929 my @keys = keys %myhash;
1930 # @keys = (0,1,2,3,...)
1932 =head2 Why does passing a subroutine an undefined element in a hash create it?
1934 If you say something like:
1936 somefunc($hash{"nonesuch key here"});
1938 Then that element "autovivifies"; that is, it springs into existence
1939 whether you store something there or not. That's because functions
1940 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1941 it has to be ready to write it back into the caller's version.
1943 This has been fixed as of Perl5.004.
1945 Normally, merely accessing a key's value for a nonexistent key does
1946 I<not> cause that key to be forever there. This is different than
1949 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1951 Usually a hash ref, perhaps like this:
1956 TITLE => "deputy peon",
1959 PALS => [ "Norbert", "Rhys", "Phineas"],
1962 References are documented in L<perlref> and the upcoming L<perlreftut>.
1963 Examples of complex data structures are given in L<perldsc> and
1964 L<perllol>. Examples of structures and object-oriented classes are
1967 =head2 How can I use a reference as a hash key?
1969 You can't do this directly, but you could use the standard Tie::RefHash
1970 module distributed with Perl.
1974 =head2 How do I handle binary data correctly?
1976 Perl is binary clean, so this shouldn't be a problem. For example,
1977 this works fine (assuming the files are found):
1979 if (`cat /vmunix` =~ /gzip/) {
1980 print "Your kernel is GNU-zip enabled!\n";
1983 On less elegant (read: Byzantine) systems, however, you have
1984 to play tedious games with "text" versus "binary" files. See
1985 L<perlfunc/"binmode"> or L<perlopentut>.
1987 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1989 If you want to deal with multibyte characters, however, there are
1990 some gotchas. See the section on Regular Expressions.
1992 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1994 Assuming that you don't care about IEEE notations like "NaN" or
1995 "Infinity", you probably just want to use a regular expression.
1997 if (/\D/) { print "has nondigits\n" }
1998 if (/^\d+$/) { print "is a whole number\n" }
1999 if (/^-?\d+$/) { print "is an integer\n" }
2000 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2001 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2002 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2003 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
2004 { print "a C float\n" }
2006 There are also some commonly used modules for the task.
2007 L<Scalar::Util> (distributed with 5.8) provides access to perl's
2008 internal function C<looks_like_number> for determining
2009 whether a variable looks like a number. L<Data::Types>
2010 exports functions that validate data types using both the
2011 above and other regular expressions. Thirdly, there is
2012 C<Regexp::Common> which has regular expressions to match
2013 various types of numbers. Those three modules are available
2016 If you're on a POSIX system, Perl supports the C<POSIX::strtod>
2017 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
2018 wrapper function for more convenient access. This function takes
2019 a string and returns the number it found, or C<undef> for input that
2020 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
2021 if you just want to say, ``Is this a float?''
2024 use POSIX qw(strtod);
2029 my($num, $unparsed) = strtod($str);
2030 if (($str eq '') || ($unparsed != 0) || $!) {
2037 sub is_numeric { defined getnum($_[0]) }
2039 Or you could check out the L<String::Scanf> module on the CPAN
2040 instead. The POSIX module (part of the standard Perl distribution) provides
2041 the C<strtod> and C<strtol> for converting strings to double and longs,
2044 =head2 How do I keep persistent data across program calls?
2046 For some specific applications, you can use one of the DBM modules.
2047 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2048 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2049 of the standard distribution. Here's one example using Storable's C<store>
2050 and C<retrieve> functions:
2053 store(\%hash, "filename");
2056 $href = retrieve("filename"); # by ref
2057 %hash = %{ retrieve("filename") }; # direct to hash
2059 =head2 How do I print out or copy a recursive data structure?
2061 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2062 for printing out data structures. The Storable module on CPAN (or the
2063 5.8 release of Perl), provides a function called C<dclone> that recursively
2064 copies its argument.
2066 use Storable qw(dclone);
2069 Where $r1 can be a reference to any kind of data structure you'd like.
2070 It will be deeply copied. Because C<dclone> takes and returns references,
2071 you'd have to add extra punctuation if you had a hash of arrays that
2074 %newhash = %{ dclone(\%oldhash) };
2076 =head2 How do I define methods for every class/object?
2078 Use the UNIVERSAL class (see L<UNIVERSAL>).
2080 =head2 How do I verify a credit card checksum?
2082 Get the Business::CreditCard module from CPAN.
2084 =head2 How do I pack arrays of doubles or floats for XS code?
2086 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2087 If you're doing a lot of float or double processing, consider using
2088 the PDL module from CPAN instead--it makes number-crunching easy.
2090 =head1 AUTHOR AND COPYRIGHT
2092 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
2093 All rights reserved.
2095 This documentation is free; you can redistribute it and/or modify it
2096 under the same terms as Perl itself.
2098 Irrespective of its distribution, all code examples in this file
2099 are hereby placed into the public domain. You are permitted and
2100 encouraged to use this code in your own programs for fun
2101 or for profit as you see fit. A simple comment in the code giving
2102 credit would be courteous but is not required.