3 perlfaq4 - Data Manipulation ($Revision: 1.22 $, $Date: 2002/05/16 12:44:24 $)
7 This section of the FAQ answers questions related to manipulating
8 numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
12 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
14 The infinite set that a mathematician thinks of as the real numbers can
15 only be approximated on a computer, since the computer only has a finite
16 number of bits to store an infinite number of, um, numbers.
18 Internally, your computer represents floating-point numbers in binary.
19 Floating-point numbers read in from a file or appearing as literals
20 in your program are converted from their decimal floating-point
21 representation (eg, 19.95) to an internal binary representation.
23 However, 19.95 can't be precisely represented as a binary
24 floating-point number, just like 1/3 can't be exactly represented as a
25 decimal floating-point number. The computer's binary representation
26 of 19.95, therefore, isn't exactly 19.95.
28 When a floating-point number gets printed, the binary floating-point
29 representation is converted back to decimal. These decimal numbers
30 are displayed in either the format you specify with printf(), or the
31 current output format for numbers. (See L<perlvar/"$#"> if you use
32 print. C<$#> has a different default value in Perl5 than it did in
33 Perl4. Changing C<$#> yourself is deprecated.)
35 This affects B<all> computer languages that represent decimal
36 floating-point numbers in binary, not just Perl. Perl provides
37 arbitrary-precision decimal numbers with the Math::BigFloat module
38 (part of the standard Perl distribution), but mathematical operations
39 are consequently slower.
41 If precision is important, such as when dealing with money, it's good
42 to work with integers and then divide at the last possible moment.
43 For example, work in pennies (1995) instead of dollars and cents
44 (19.95) and divide by 100 at the end.
46 To get rid of the superfluous digits, just use a format (eg,
47 C<printf("%.2f", 19.95)>) to get the required precision.
48 See L<perlop/"Floating-point Arithmetic">.
50 =head2 Why isn't my octal data interpreted correctly?
52 Perl only understands octal and hex numbers as such when they occur
53 as literals in your program. Octal literals in perl must start with
54 a leading "0" and hexadecimal literals must start with a leading "0x".
55 If they are read in from somewhere and assigned, no automatic
56 conversion takes place. You must explicitly use oct() or hex() if you
57 want the values converted to decimal. oct() interprets
58 both hex ("0x350") numbers and octal ones ("0350" or even without the
59 leading "0", like "377"), while hex() only converts hexadecimal ones,
60 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
61 The inverse mapping from decimal to octal can be done with either the
62 "%o" or "%O" sprintf() formats. To get from decimal to hex try either
63 the "%x" or the "%X" formats to sprintf().
65 This problem shows up most often when people try using chmod(), mkdir(),
66 umask(), or sysopen(), which by widespread tradition typically take
69 chmod(644, $file); # WRONG
70 chmod(0644, $file); # right
72 Note the mistake in the first line was specifying the decimal literal
73 644, rather than the intended octal literal 0644. The problem can
76 printf("%#o",644); # prints 01204
78 Surely you had not intended C<chmod(01204, $file);> - did you? If you
79 want to use numeric literals as arguments to chmod() et al. then please
80 try to express them as octal constants, that is with a leading zero and
81 with the following digits restricted to the set 0..7.
83 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
85 Remember that int() merely truncates toward 0. For rounding to a
86 certain number of digits, sprintf() or printf() is usually the easiest
89 printf("%.3f", 3.1415926535); # prints 3.142
91 The POSIX module (part of the standard Perl distribution) implements
92 ceil(), floor(), and a number of other mathematical and trigonometric
96 $ceil = ceil(3.5); # 4
97 $floor = floor(3.5); # 3
99 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
100 module. With 5.004, the Math::Trig module (part of the standard Perl
101 distribution) implements the trigonometric functions. Internally it
102 uses the Math::Complex module and some functions can break out from
103 the real axis into the complex plane, for example the inverse sine of
106 Rounding in financial applications can have serious implications, and
107 the rounding method used should be specified precisely. In these
108 cases, it probably pays not to trust whichever system rounding is
109 being used by Perl, but to instead implement the rounding function you
112 To see why, notice how you'll still have an issue on half-way-point
115 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
117 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
118 0.8 0.8 0.9 0.9 1.0 1.0
120 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
121 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
122 machines) will work pretty much like mathematical integers. Other numbers
125 =head2 How do I convert between numeric representations?
127 As always with Perl there is more than one way to do it. Below
128 are a few examples of approaches to making common conversions
129 between number representations. This is intended to be representational
130 rather than exhaustive.
132 Some of the examples below use the Bit::Vector module from CPAN.
133 The reason you might choose Bit::Vector over the perl built in
134 functions is that it works with numbers of ANY size, that it is
135 optimized for speed on some operations, and for at least some
136 programmers the notation might be familiar.
138 =item B<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 B<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 B<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 B<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 B<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 B<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 = $vec->to_Bin();
251 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
252 are left as an exercise to the inclined reader.
255 =head2 Why doesn't & work the way I want it to?
257 The behavior of binary arithmetic operators depends on whether they're
258 used on numbers or strings. The operators treat a string as a series
259 of bits and work with that (the string C<"3"> is the bit pattern
260 C<00110011>). The operators work with the binary form of a number
261 (the number C<3> is treated as the bit pattern C<00000011>).
263 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
264 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
267 Most problems with C<&> and C<|> arise because the programmer thinks
268 they have a number but really it's a string. The rest arise because
271 if ("\020\020" & "\101\101") {
275 but a string consisting of two null bytes (the result of C<"\020\020"
276 & "\101\101">) is not a false value in Perl. You need:
278 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
282 =head2 How do I multiply matrices?
284 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
285 or the PDL extension (also available from CPAN).
287 =head2 How do I perform an operation on a series of integers?
289 To call a function on each element in an array, and collect the
292 @results = map { my_func($_) } @array;
296 @triple = map { 3 * $_ } @single;
298 To call a function on each element of an array, but ignore the
301 foreach $iterator (@array) {
302 some_func($iterator);
305 To call a function on each integer in a (small) range, you B<can> use:
307 @results = map { some_func($_) } (5 .. 25);
309 but you should be aware that the C<..> operator creates an array of
310 all integers in the range. This can take a lot of memory for large
314 for ($i=5; $i < 500_005; $i++) {
315 push(@results, some_func($i));
318 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
319 loop will iterate over the range, without creating the entire range.
321 for my $i (5 .. 500_005) {
322 push(@results, some_func($i));
325 will not create a list of 500,000 integers.
327 =head2 How can I output Roman numerals?
329 Get the http://www.cpan.org/modules/by-module/Roman module.
331 =head2 Why aren't my random numbers random?
333 If you're using a version of Perl before 5.004, you must call C<srand>
334 once at the start of your program to seed the random number generator.
335 5.004 and later automatically call C<srand> at the beginning. Don't
336 call C<srand> more than once--you make your numbers less random, rather
339 Computers are good at being predictable and bad at being random
340 (despite appearances caused by bugs in your programs :-). see the
341 F<random> artitcle in the "Far More Than You Ever Wanted To Know"
342 collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz , courtesy of
343 Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
344 who attempts to generate random numbers by deterministic means is, of
345 course, living in a state of sin.''
347 If you want numbers that are more random than C<rand> with C<srand>
348 provides, you should also check out the Math::TrulyRandom module from
349 CPAN. It uses the imperfections in your system's timer to generate
350 random numbers, but this takes quite a while. If you want a better
351 pseudorandom generator than comes with your operating system, look at
352 ``Numerical Recipes in C'' at http://www.nr.com/ .
354 =head2 How do I get a random number between X and Y?
356 Use the following simple function. It selects a random integer between
357 (and possibly including!) the two given integers, e.g.,
358 C<random_int_in(50,120)>
360 sub random_int_in ($$) {
362 # Assumes that the two arguments are integers themselves!
363 return $min if $min == $max;
364 ($min, $max) = ($max, $min) if $min > $max;
365 return $min + int rand(1 + $max - $min);
370 =head2 How do I find the week-of-the-year/day-of-the-year?
372 The day of the year is in the array returned by localtime() (see
373 L<perlfunc/"localtime">):
375 $day_of_year = (localtime(time()))[7];
377 =head2 How do I find the current century or millennium?
379 Use the following simple functions:
382 return int((((localtime(shift || time))[5] + 1999))/100);
385 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
388 On some systems, you'll find that the POSIX module's strftime() function
389 has been extended in a non-standard way to use a C<%C> format, which they
390 sometimes claim is the "century". It isn't, because on most such systems,
391 this is only the first two digits of the four-digit year, and thus cannot
392 be used to reliably determine the current century or millennium.
394 =head2 How can I compare two dates and find the difference?
396 If you're storing your dates as epoch seconds then simply subtract one
397 from the other. If you've got a structured date (distinct year, day,
398 month, hour, minute, seconds values), then for reasons of accessibility,
399 simplicity, and efficiency, merely use either timelocal or timegm (from
400 the Time::Local module in the standard distribution) to reduce structured
401 dates to epoch seconds. However, if you don't know the precise format of
402 your dates, then you should probably use either of the Date::Manip and
403 Date::Calc modules from CPAN before you go hacking up your own parsing
404 routine to handle arbitrary date formats.
406 =head2 How can I take a string and turn it into epoch seconds?
408 If it's a regular enough string that it always has the same format,
409 you can split it up and pass the parts to C<timelocal> in the standard
410 Time::Local module. Otherwise, you should look into the Date::Calc
411 and Date::Manip modules from CPAN.
413 =head2 How can I find the Julian Day?
415 Use the Time::JulianDay module (part of the Time-modules bundle
416 available from CPAN.)
418 Before you immerse yourself too deeply in this, be sure to verify that
419 it is the I<Julian> Day you really want. Are you interested in a way
420 of getting serial days so that you just can tell how many days they
421 are apart or so that you can do also other date arithmetic? If you
422 are interested in performing date arithmetic, this can be done using
423 modules Date::Manip or Date::Calc.
425 There is too many details and much confusion on this issue to cover in
426 this FAQ, but the term is applied (correctly) to a calendar now
427 supplanted by the Gregorian Calendar, with the Julian Calendar failing
428 to adjust properly for leap years on centennial years (among other
429 annoyances). The term is also used (incorrectly) to mean: [1] days in
430 the Gregorian Calendar; and [2] days since a particular starting time
431 or `epoch', usually 1970 in the Unix world and 1980 in the
432 MS-DOS/Windows world. If you find that it is not the first meaning
433 that you really want, then check out the Date::Manip and Date::Calc
434 modules. (Thanks to David Cassell for most of this text.)
436 =head2 How do I find yesterday's date?
438 The C<time()> function returns the current time in seconds since the
439 epoch. Take twenty-four hours off that:
441 $yesterday = time() - ( 24 * 60 * 60 );
443 Then you can pass this to C<localtime()> and get the individual year,
444 month, day, hour, minute, seconds values.
446 Note very carefully that the code above assumes that your days are
447 twenty-four hours each. For most people, there are two days a year
448 when they aren't: the switch to and from summer time throws this off.
449 A solution to this issue is offered by Russ Allbery.
452 my $now = defined $_[0] ? $_[0] : time;
453 my $then = $now - 60 * 60 * 24;
454 my $ndst = (localtime $now)[8] > 0;
455 my $tdst = (localtime $then)[8] > 0;
456 $then - ($tdst - $ndst) * 60 * 60;
458 # Should give you "this time yesterday" in seconds since epoch relative to
459 # the first argument or the current time if no argument is given and
460 # suitable for passing to localtime or whatever else you need to do with
461 # it. $ndst is whether we're currently in daylight savings time; $tdst is
462 # whether the point 24 hours ago was in daylight savings time. If $tdst
463 # and $ndst are the same, a boundary wasn't crossed, and the correction
464 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
465 # from yesterday's time since we gained an extra hour while going off
466 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
467 # negative hour (add an hour) to yesterday's time since we lost an hour.
469 # All of this is because during those days when one switches off or onto
470 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
472 # The explicit settings of $ndst and $tdst are necessary because localtime
473 # only says it returns the system tm struct, and the system tm struct at
474 # least on Solaris doesn't guarantee any particular positive value (like,
475 # say, 1) for isdst, just a positive value. And that value can
476 # potentially be negative, if DST information isn't available (this sub
477 # just treats those cases like no DST).
479 # Note that between 2am and 3am on the day after the time zone switches
480 # off daylight savings time, the exact hour of "yesterday" corresponding
481 # to the current hour is not clearly defined. Note also that if used
482 # between 2am and 3am the day after the change to daylight savings time,
483 # the result will be between 3am and 4am of the previous day; it's
484 # arguable whether this is correct.
486 # This sub does not attempt to deal with leap seconds (most things don't).
488 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
489 # This code is in the public domain
491 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
493 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
494 Y2K compliant (whatever that means). The programmers you've hired to
495 use it, however, probably are not.
497 Long answer: The question belies a true understanding of the issue.
498 Perl is just as Y2K compliant as your pencil--no more, and no less.
499 Can you use your pencil to write a non-Y2K-compliant memo? Of course
500 you can. Is that the pencil's fault? Of course it isn't.
502 The date and time functions supplied with Perl (gmtime and localtime)
503 supply adequate information to determine the year well beyond 2000
504 (2038 is when trouble strikes for 32-bit machines). The year returned
505 by these functions when used in a list context is the year minus 1900.
506 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
507 number. To avoid the year 2000 problem simply do not treat the year as
508 a 2-digit number. It isn't.
510 When gmtime() and localtime() are used in scalar context they return
511 a timestamp string that contains a fully-expanded year. For example,
512 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
513 2001". There's no year 2000 problem here.
515 That doesn't mean that Perl can't be used to create non-Y2K compliant
516 programs. It can. But so can your pencil. It's the fault of the user,
517 not the language. At the risk of inflaming the NRA: ``Perl doesn't
518 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
523 =head2 How do I validate input?
525 The answer to this question is usually a regular expression, perhaps
526 with auxiliary logic. See the more specific questions (numbers, mail
527 addresses, etc.) for details.
529 =head2 How do I unescape a string?
531 It depends just what you mean by ``escape''. URL escapes are dealt
532 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
533 character are removed with
537 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
539 =head2 How do I remove consecutive pairs of characters?
541 To turn C<"abbcccd"> into C<"abccd">:
543 s/(.)\1/$1/g; # add /s to include newlines
545 Here's a solution that turns "abbcccd" to "abcd":
547 y///cs; # y == tr, but shorter :-)
549 =head2 How do I expand function calls in a string?
551 This is documented in L<perlref>. In general, this is fraught with
552 quoting and readability problems, but it is possible. To interpolate
553 a subroutine call (in list context) into a string:
555 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
557 If you prefer scalar context, similar chicanery is also useful for
558 arbitrary expressions:
560 print "That yields ${\($n + 5)} widgets\n";
562 Version 5.004 of Perl had a bug that gave list context to the
563 expression in C<${...}>, but this is fixed in version 5.005.
565 See also ``How can I expand variables in text strings?'' in this
568 =head2 How do I find matching/nesting anything?
570 This isn't something that can be done in one regular expression, no
571 matter how complicated. To find something between two single
572 characters, a pattern like C</x([^x]*)x/> will get the intervening
573 bits in $1. For multiple ones, then something more like
574 C</alpha(.*?)omega/> would be needed. But none of these deals with
575 nested patterns, nor can they. For that you'll have to write a
578 If you are serious about writing a parser, there are a number of
579 modules or oddities that will make your life a lot easier. There are
580 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
581 and the byacc program. Starting from perl 5.8 the Text::Balanced
582 is part of the standard distribution.
584 One simple destructive, inside-out approach that you might try is to
585 pull out the smallest nesting parts one at a time:
587 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
588 # do something with $1
591 A more complicated and sneaky approach is to make Perl's regular
592 expression engine do it for you. This is courtesy Dean Inada, and
593 rather has the nature of an Obfuscated Perl Contest entry, but it
596 # $_ contains the string to parse
597 # BEGIN and END are the opening and closing markers for the
602 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
603 @$ = (eval{/$re/},$@!~/unmatched/i);
604 print join("\n",@$[0..$#$]) if( $$[-1] );
606 =head2 How do I reverse a string?
608 Use reverse() in scalar context, as documented in
611 $reversed = reverse $string;
613 =head2 How do I expand tabs in a string?
615 You can do it yourself:
617 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
619 Or you can just use the Text::Tabs module (part of the standard Perl
623 @expanded_lines = expand(@lines_with_tabs);
625 =head2 How do I reformat a paragraph?
627 Use Text::Wrap (part of the standard Perl distribution):
630 print wrap("\t", ' ', @paragraphs);
632 The paragraphs you give to Text::Wrap should not contain embedded
633 newlines. Text::Wrap doesn't justify the lines (flush-right).
635 Or use the CPAN module Text::Autoformat. Formatting files can be easily
636 done by making a shell alias, like so:
638 alias fmt="perl -i -MText::Autoformat -n0777 \
639 -e 'print autoformat $_, {all=>1}' $*"
641 See the documentation for Text::Autoformat to appreciate its many
644 =head2 How can I access/change the first N letters of a string?
646 There are many ways. If you just want to grab a copy, use
649 $first_byte = substr($a, 0, 1);
651 If you want to modify part of a string, the simplest way is often to
652 use substr() as an lvalue:
654 substr($a, 0, 3) = "Tom";
656 Although those with a pattern matching kind of thought process will
661 =head2 How do I change the Nth occurrence of something?
663 You have to keep track of N yourself. For example, let's say you want
664 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
665 C<"whosoever"> or C<"whomsoever">, case insensitively. These
666 all assume that $_ contains the string to be altered.
670 ++$count == 5 # is it the 5th?
671 ? "${2}soever" # yes, swap
672 : $1 # renege and leave it there
675 In the more general case, you can use the C</g> modifier in a C<while>
676 loop, keeping count of matches.
680 $_ = "One fish two fish red fish blue fish";
681 while (/(\w+)\s+fish\b/gi) {
682 if (++$count == $WANT) {
683 print "The third fish is a $1 one.\n";
687 That prints out: C<"The third fish is a red one."> You can also use a
688 repetition count and repeated pattern like this:
690 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
692 =head2 How can I count the number of occurrences of a substring within a string?
694 There are a number of ways, with varying efficiency. If you want a
695 count of a certain single character (X) within a string, you can use the
696 C<tr///> function like so:
698 $string = "ThisXlineXhasXsomeXx'sXinXit";
699 $count = ($string =~ tr/X//);
700 print "There are $count X characters in the string";
702 This is fine if you are just looking for a single character. However,
703 if you are trying to count multiple character substrings within a
704 larger string, C<tr///> won't work. What you can do is wrap a while()
705 loop around a global pattern match. For example, let's count negative
708 $string = "-9 55 48 -2 23 -76 4 14 -44";
709 while ($string =~ /-\d+/g) { $count++ }
710 print "There are $count negative numbers in the string";
712 Another version uses a global match in list context, then assigns the
713 result to a scalar, producing a count of the number of matches.
715 $count = () = $string =~ /-\d+/g;
717 =head2 How do I capitalize all the words on one line?
719 To make the first letter of each word upper case:
721 $line =~ s/\b(\w)/\U$1/g;
723 This has the strange effect of turning "C<don't do it>" into "C<Don'T
724 Do It>". Sometimes you might want this. Other times you might need a
725 more thorough solution (Suggested by brian d foy):
728 (^\w) #at the beginning of the line
730 (\s\w) #preceded by whitespace
733 $string =~ /([\w']+)/\u\L$1/g;
735 To make the whole line upper case:
739 To force each word to be lower case, with the first letter upper case:
741 $line =~ s/(\w+)/\u\L$1/g;
743 You can (and probably should) enable locale awareness of those
744 characters by placing a C<use locale> pragma in your program.
745 See L<perllocale> for endless details on locales.
747 This is sometimes referred to as putting something into "title
748 case", but that's not quite accurate. Consider the proper
749 capitalization of the movie I<Dr. Strangelove or: How I Learned to
750 Stop Worrying and Love the Bomb>, for example.
752 =head2 How can I split a [character] delimited string except when inside
753 [character]? (Comma-separated files)
755 Take the example case of trying to split a string that is comma-separated
756 into its different fields. (We'll pretend you said comma-separated, not
757 comma-delimited, which is different and almost never what you mean.) You
758 can't use C<split(/,/)> because you shouldn't split if the comma is inside
759 quotes. For example, take a data line like this:
761 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
763 Due to the restriction of the quotes, this is a fairly complex
764 problem. Thankfully, we have Jeffrey Friedl, author of a highly
765 recommended book on regular expressions, to handle these for us. He
766 suggests (assuming your string is contained in $text):
769 push(@new, $+) while $text =~ m{
770 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
774 push(@new, undef) if substr($text,-1,1) eq ',';
776 If you want to represent quotation marks inside a
777 quotation-mark-delimited field, escape them with backslashes (eg,
778 C<"like \"this\"">. Unescaping them is a task addressed earlier in
781 Alternatively, the Text::ParseWords module (part of the standard Perl
782 distribution) lets you say:
784 use Text::ParseWords;
785 @new = quotewords(",", 0, $text);
787 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
789 =head2 How do I strip blank space from the beginning/end of a string?
791 Although the simplest approach would seem to be
793 $string =~ s/^\s*(.*?)\s*$/$1/;
795 not only is this unnecessarily slow and destructive, it also fails with
796 embedded newlines. It is much faster to do this operation in two steps:
801 Or more nicely written as:
808 This idiom takes advantage of the C<foreach> loop's aliasing
809 behavior to factor out common code. You can do this
810 on several strings at once, or arrays, or even the
811 values of a hash if you use a slice:
813 # trim whitespace in the scalar, the array,
814 # and all the values in the hash
815 foreach ($scalar, @array, @hash{keys %hash}) {
820 =head2 How do I pad a string with blanks or pad a number with zeroes?
822 (This answer contributed by Uri Guttman, with kibitzing from
825 In the following examples, C<$pad_len> is the length to which you wish
826 to pad the string, C<$text> or C<$num> contains the string to be padded,
827 and C<$pad_char> contains the padding character. You can use a single
828 character string constant instead of the C<$pad_char> variable if you
829 know what it is in advance. And in the same way you can use an integer in
830 place of C<$pad_len> if you know the pad length in advance.
832 The simplest method uses the C<sprintf> function. It can pad on the left
833 or right with blanks and on the left with zeroes and it will not
834 truncate the result. The C<pack> function can only pad strings on the
835 right with blanks and it will truncate the result to a maximum length of
838 # Left padding a string with blanks (no truncation):
839 $padded = sprintf("%${pad_len}s", $text);
841 # Right padding a string with blanks (no truncation):
842 $padded = sprintf("%-${pad_len}s", $text);
844 # Left padding a number with 0 (no truncation):
845 $padded = sprintf("%0${pad_len}d", $num);
847 # Right padding a string with blanks using pack (will truncate):
848 $padded = pack("A$pad_len",$text);
850 If you need to pad with a character other than blank or zero you can use
851 one of the following methods. They all generate a pad string with the
852 C<x> operator and combine that with C<$text>. These methods do
853 not truncate C<$text>.
855 Left and right padding with any character, creating a new string:
857 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
858 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
860 Left and right padding with any character, modifying C<$text> directly:
862 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
863 $text .= $pad_char x ( $pad_len - length( $text ) );
865 =head2 How do I extract selected columns from a string?
867 Use substr() or unpack(), both documented in L<perlfunc>.
868 If you prefer thinking in terms of columns instead of widths,
869 you can use this kind of thing:
871 # determine the unpack format needed to split Linux ps output
872 # arguments are cut columns
873 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
879 for my $place (@positions) {
880 $template .= "A" . ($place - $lastpos) . " ";
887 =head2 How do I find the soundex value of a string?
889 Use the standard Text::Soundex module distributed with Perl.
890 Before you do so, you may want to determine whether `soundex' is in
891 fact what you think it is. Knuth's soundex algorithm compresses words
892 into a small space, and so it does not necessarily distinguish between
893 two words which you might want to appear separately. For example, the
894 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
895 If Text::Soundex does not do what you are looking for, you might want
896 to consider the String::Approx module available at CPAN.
898 =head2 How can I expand variables in text strings?
900 Let's assume that you have a string like:
902 $text = 'this has a $foo in it and a $bar';
904 If those were both global variables, then this would
907 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
909 But since they are probably lexicals, or at least, they could
910 be, you'd have to do this:
912 $text =~ s/(\$\w+)/$1/eeg;
913 die if $@; # needed /ee, not /e
915 It's probably better in the general case to treat those
916 variables as entries in some special hash. For example:
922 $text =~ s/\$(\w+)/$user_defs{$1}/g;
924 See also ``How do I expand function calls in a string?'' in this section
927 =head2 What's wrong with always quoting "$vars"?
929 The problem is that those double-quotes force stringification--
930 coercing numbers and references into strings--even when you
931 don't want them to be strings. Think of it this way: double-quote
932 expansion is used to produce new strings. If you already
933 have a string, why do you need more?
935 If you get used to writing odd things like these:
939 somefunc("$var"); # BAD
941 You'll be in trouble. Those should (in 99.8% of the cases) be
942 the simpler and more direct:
948 Otherwise, besides slowing you down, you're going to break code when
949 the thing in the scalar is actually neither a string nor a number, but
955 my $oref = "$aref"; # WRONG
958 You can also get into subtle problems on those few operations in Perl
959 that actually do care about the difference between a string and a
960 number, such as the magical C<++> autoincrement operator or the
963 Stringification also destroys arrays.
966 print "@lines"; # WRONG - extra blanks
967 print @lines; # right
969 =head2 Why don't my <<HERE documents work?
971 Check for these three things:
975 =item 1. There must be no space after the << part.
977 =item 2. There (probably) should be a semicolon at the end.
979 =item 3. You can't (easily) have any space in front of the tag.
983 If you want to indent the text in the here document, you
987 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
992 But the HERE_TARGET must still be flush against the margin.
993 If you want that indented also, you'll have to quote
996 ($quote = <<' FINIS') =~ s/^\s+//gm;
997 ...we will have peace, when you and all your works have
998 perished--and the works of your dark master to whom you
999 would deliver us. You are a liar, Saruman, and a corrupter
1000 of men's hearts. --Theoden in /usr/src/perl/taint.c
1002 $quote =~ s/\s+--/\n--/;
1004 A nice general-purpose fixer-upper function for indented here documents
1005 follows. It expects to be called with a here document as its argument.
1006 It looks to see whether each line begins with a common substring, and
1007 if so, strips that substring off. Otherwise, it takes the amount of leading
1008 whitespace found on the first line and removes that much off each
1013 my ($white, $leader); # common whitespace and common leading string
1014 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1015 ($white, $leader) = ($2, quotemeta($1));
1017 ($white, $leader) = (/^(\s+)/, '');
1019 s/^\s*?$leader(?:$white)?//gm;
1023 This works with leading special strings, dynamically determined:
1025 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1028 @@@ SAVEI32(runlevel);
1030 @@@ while ( op = (*op->op_ppaddr)() );
1034 MAIN_INTERPRETER_LOOP
1036 Or with a fixed amount of leading whitespace, with remaining
1037 indentation correctly preserved:
1039 $poem = fix<<EVER_ON_AND_ON;
1040 Now far ahead the Road has gone,
1041 And I must follow, if I can,
1042 Pursuing it with eager feet,
1043 Until it joins some larger way
1044 Where many paths and errands meet.
1045 And whither then? I cannot say.
1046 --Bilbo in /usr/src/perl/pp_ctl.c
1051 =head2 What is the difference between a list and an array?
1053 An array has a changeable length. A list does not. An array is something
1054 you can push or pop, while a list is a set of values. Some people make
1055 the distinction that a list is a value while an array is a variable.
1056 Subroutines are passed and return lists, you put things into list
1057 context, you initialize arrays with lists, and you foreach() across
1058 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1059 in scalar context behave like the number of elements in them, subroutines
1060 access their arguments through the array C<@_>, and push/pop/shift only work
1063 As a side note, there's no such thing as a list in scalar context.
1066 $scalar = (2, 5, 7, 9);
1068 you're using the comma operator in scalar context, so it uses the scalar
1069 comma operator. There never was a list there at all! This causes the
1070 last value to be returned: 9.
1072 =head2 What is the difference between $array[1] and @array[1]?
1074 The former is a scalar value; the latter an array slice, making
1075 it a list with one (scalar) value. You should use $ when you want a
1076 scalar value (most of the time) and @ when you want a list with one
1077 scalar value in it (very, very rarely; nearly never, in fact).
1079 Sometimes it doesn't make a difference, but sometimes it does.
1080 For example, compare:
1082 $good[0] = `some program that outputs several lines`;
1086 @bad[0] = `same program that outputs several lines`;
1088 The C<use warnings> pragma and the B<-w> flag will warn you about these
1091 =head2 How can I remove duplicate elements from a list or array?
1093 There are several possible ways, depending on whether the array is
1094 ordered and whether you wish to preserve the ordering.
1100 If @in is sorted, and you want @out to be sorted:
1101 (this assumes all true values in the array)
1103 $prev = "not equal to $in[0]";
1104 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1106 This is nice in that it doesn't use much extra memory, simulating
1107 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1108 guarantees that the expression is true (so that grep picks it up)
1109 even if the $_ is 0, "", or undef.
1113 If you don't know whether @in is sorted:
1116 @out = grep(!$saw{$_}++, @in);
1120 Like (b), but @in contains only small integers:
1122 @out = grep(!$saw[$_]++, @in);
1126 A way to do (b) without any loops or greps:
1130 @out = sort keys %saw; # remove sort if undesired
1134 Like (d), but @in contains only small positive integers:
1138 @out = grep {defined} @ary;
1142 But perhaps you should have been using a hash all along, eh?
1144 =head2 How can I tell whether a certain element is contained in a list or array?
1146 Hearing the word "in" is an I<in>dication that you probably should have
1147 used a hash, not a list or array, to store your data. Hashes are
1148 designed to answer this question quickly and efficiently. Arrays aren't.
1150 That being said, there are several ways to approach this. If you
1151 are going to make this query many times over arbitrary string values,
1152 the fastest way is probably to invert the original array and maintain a
1153 hash whose keys are the first array's values.
1155 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1157 for (@blues) { $is_blue{$_} = 1 }
1159 Now you can check whether $is_blue{$some_color}. It might have been a
1160 good idea to keep the blues all in a hash in the first place.
1162 If the values are all small integers, you could use a simple indexed
1163 array. This kind of an array will take up less space:
1165 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1166 @is_tiny_prime = ();
1167 for (@primes) { $is_tiny_prime[$_] = 1 }
1168 # or simply @istiny_prime[@primes] = (1) x @primes;
1170 Now you check whether $is_tiny_prime[$some_number].
1172 If the values in question are integers instead of strings, you can save
1173 quite a lot of space by using bit strings instead:
1175 @articles = ( 1..10, 150..2000, 2017 );
1177 for (@articles) { vec($read,$_,1) = 1 }
1179 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1183 ($is_there) = grep $_ eq $whatever, @array;
1187 ($is_there) = grep /$whatever/, @array;
1189 These are slow (checks every element even if the first matches),
1190 inefficient (same reason), and potentially buggy (what if there are
1191 regex characters in $whatever?). If you're only testing once, then
1195 foreach $elt (@array) {
1196 if ($elt eq $elt_to_find) {
1201 if ($is_there) { ... }
1203 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1205 Use a hash. Here's code to do both and more. It assumes that
1206 each element is unique in a given array:
1208 @union = @intersection = @difference = ();
1210 foreach $element (@array1, @array2) { $count{$element}++ }
1211 foreach $element (keys %count) {
1212 push @union, $element;
1213 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1216 Note that this is the I<symmetric difference>, that is, all elements in
1217 either A or in B but not in both. Think of it as an xor operation.
1219 =head2 How do I test whether two arrays or hashes are equal?
1221 The following code works for single-level arrays. It uses a stringwise
1222 comparison, and does not distinguish defined versus undefined empty
1223 strings. Modify if you have other needs.
1225 $are_equal = compare_arrays(\@frogs, \@toads);
1227 sub compare_arrays {
1228 my ($first, $second) = @_;
1229 no warnings; # silence spurious -w undef complaints
1230 return 0 unless @$first == @$second;
1231 for (my $i = 0; $i < @$first; $i++) {
1232 return 0 if $first->[$i] ne $second->[$i];
1237 For multilevel structures, you may wish to use an approach more
1238 like this one. It uses the CPAN module FreezeThaw:
1240 use FreezeThaw qw(cmpStr);
1241 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1243 printf "a and b contain %s arrays\n",
1244 cmpStr(\@a, \@b) == 0
1248 This approach also works for comparing hashes. Here
1249 we'll demonstrate two different answers:
1251 use FreezeThaw qw(cmpStr cmpStrHard);
1253 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1257 printf "a and b contain %s hashes\n",
1258 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1260 printf "a and b contain %s hashes\n",
1261 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1264 The first reports that both those the hashes contain the same data,
1265 while the second reports that they do not. Which you prefer is left as
1266 an exercise to the reader.
1268 =head2 How do I find the first array element for which a condition is true?
1270 You can use this if you care about the index:
1272 for ($i= 0; $i < @array; $i++) {
1273 if ($array[$i] eq "Waldo") {
1279 Now C<$found_index> has what you want.
1281 =head2 How do I handle linked lists?
1283 In general, you usually don't need a linked list in Perl, since with
1284 regular arrays, you can push and pop or shift and unshift at either end,
1285 or you can use splice to add and/or remove arbitrary number of elements at
1286 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1287 dynamic arrays. In the absence of shifts and pops, push in general
1288 needs to reallocate on the order every log(N) times, and unshift will
1289 need to copy pointers each time.
1291 If you really, really wanted, you could use structures as described in
1292 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1293 to do. For example, imagine a list node like this:
1300 You could walk the list this way:
1303 for ($node = $head; $node; $node = $node->{LINK}) {
1304 print $node->{VALUE}, " ";
1308 You could add to the list this way:
1311 $tail = append($head, 1); # grow a new head
1312 for $value ( 2 .. 10 ) {
1313 $tail = append($tail, $value);
1317 my($list, $value) = @_;
1318 my $node = { VALUE => $value };
1320 $node->{LINK} = $list->{LINK};
1321 $list->{LINK} = $node;
1323 $_[0] = $node; # replace caller's version
1328 But again, Perl's built-in are virtually always good enough.
1330 =head2 How do I handle circular lists?
1332 Circular lists could be handled in the traditional fashion with linked
1333 lists, or you could just do something like this with an array:
1335 unshift(@array, pop(@array)); # the last shall be first
1336 push(@array, shift(@array)); # and vice versa
1338 =head2 How do I shuffle an array randomly?
1340 If you either have Perl 5.8.0 or later installed, or if you have
1341 Scalar-List-Utils 1.03 or later installed, you can say:
1343 use List::Util 'shuffle';
1345 @shuffled = shuffle(@list);
1347 If not, you can use a Fisher-Yates shuffle.
1349 sub fisher_yates_shuffle {
1350 my $deck = shift; # $deck is a reference to an array
1353 my $j = int rand ($i+1);
1354 @$deck[$i,$j] = @$deck[$j,$i];
1358 # shuffle my mpeg collection
1360 my @mpeg = <audio/*/*.mp3>;
1361 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1364 Note that the above implementation shuffles an array in place,
1365 unlike the List::Util::shuffle() which takes a list and returns
1366 a new shuffled list.
1368 You've probably seen shuffling algorithms that work using splice,
1369 randomly picking another element to swap the current element with
1373 @old = 1 .. 10; # just a demo
1375 push(@new, splice(@old, rand @old, 1));
1378 This is bad because splice is already O(N), and since you do it N times,
1379 you just invented a quadratic algorithm; that is, O(N**2). This does
1380 not scale, although Perl is so efficient that you probably won't notice
1381 this until you have rather largish arrays.
1383 =head2 How do I process/modify each element of an array?
1385 Use C<for>/C<foreach>:
1388 s/foo/bar/; # change that word
1389 y/XZ/ZX/; # swap those letters
1392 Here's another; let's compute spherical volumes:
1394 for (@volumes = @radii) { # @volumes has changed parts
1396 $_ *= (4/3) * 3.14159; # this will be constant folded
1399 If you want to do the same thing to modify the values of the
1400 hash, you can use the C<values> function. As of Perl 5.6
1401 the values are not copied, so if you modify $orbit (in this
1402 case), you modify the value.
1404 for $orbit ( values %orbits ) {
1405 ($orbit **= 3) *= (4/3) * 3.14159;
1408 Prior to perl 5.6 C<values> returned copies of the values,
1409 so older perl code often contains constructions such as
1410 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1411 the hash is to be modified.
1413 =head2 How do I select a random element from an array?
1415 Use the rand() function (see L<perlfunc/rand>):
1417 # at the top of the program:
1418 srand; # not needed for 5.004 and later
1421 $index = rand @array;
1422 $element = $array[$index];
1424 Make sure you I<only call srand once per program, if then>.
1425 If you are calling it more than once (such as before each
1426 call to rand), you're almost certainly doing something wrong.
1428 =head2 How do I permute N elements of a list?
1430 Here's a little program that generates all permutations
1431 of all the words on each line of input. The algorithm embodied
1432 in the permute() function should work on any list:
1435 # tsc-permute: permute each word of input
1436 permute([split], []);
1438 my @items = @{ $_[0] };
1439 my @perms = @{ $_[1] };
1443 my(@newitems,@newperms,$i);
1444 foreach $i (0 .. $#items) {
1447 unshift(@newperms, splice(@newitems, $i, 1));
1448 permute([@newitems], [@newperms]);
1453 Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
1454 module from CPAN runs at least an order of magnitude faster. If you don't
1455 have a C compiler (or a binary distribution of Algorithm::Permute), then
1456 you can use List::Permutor which is written in pure Perl, and is still
1457 several times faster than the algorithm above.
1459 =head2 How do I sort an array by (anything)?
1461 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1463 @list = sort { $a <=> $b } @list;
1465 The default sort function is cmp, string comparison, which would
1466 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1467 the numerical comparison operator.
1469 If you have a complicated function needed to pull out the part you
1470 want to sort on, then don't do it inside the sort function. Pull it
1471 out first, because the sort BLOCK can be called many times for the
1472 same element. Here's an example of how to pull out the first word
1473 after the first number on each item, and then sort those words
1478 ($item) = /\d+\s*(\S+)/;
1479 push @idx, uc($item);
1481 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1483 which could also be written this way, using a trick
1484 that's come to be known as the Schwartzian Transform:
1486 @sorted = map { $_->[0] }
1487 sort { $a->[1] cmp $b->[1] }
1488 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1490 If you need to sort on several fields, the following paradigm is useful.
1492 @sorted = sort { field1($a) <=> field1($b) ||
1493 field2($a) cmp field2($b) ||
1494 field3($a) cmp field3($b)
1497 This can be conveniently combined with precalculation of keys as given
1500 See the F<sort> artitcle article in the "Far More Than You Ever Wanted
1501 To Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz for
1502 more about this approach.
1504 See also the question below on sorting hashes.
1506 =head2 How do I manipulate arrays of bits?
1508 Use pack() and unpack(), or else vec() and the bitwise operations.
1510 For example, this sets $vec to have bit N set if $ints[N] was set:
1513 foreach(@ints) { vec($vec,$_,1) = 1 }
1515 Here's how, given a vector in $vec, you can
1516 get those bits into your @ints array:
1518 sub bitvec_to_list {
1521 # Find null-byte density then select best algorithm
1522 if ($vec =~ tr/\0// / length $vec > 0.95) {
1525 # This method is faster with mostly null-bytes
1526 while($vec =~ /[^\0]/g ) {
1527 $i = -9 + 8 * pos $vec;
1528 push @ints, $i if vec($vec, ++$i, 1);
1529 push @ints, $i if vec($vec, ++$i, 1);
1530 push @ints, $i if vec($vec, ++$i, 1);
1531 push @ints, $i if vec($vec, ++$i, 1);
1532 push @ints, $i if vec($vec, ++$i, 1);
1533 push @ints, $i if vec($vec, ++$i, 1);
1534 push @ints, $i if vec($vec, ++$i, 1);
1535 push @ints, $i if vec($vec, ++$i, 1);
1538 # This method is a fast general algorithm
1540 my $bits = unpack "b*", $vec;
1541 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1542 push @ints, pos $bits while($bits =~ /1/g);
1547 This method gets faster the more sparse the bit vector is.
1548 (Courtesy of Tim Bunce and Winfried Koenig.)
1550 You can make the while loop a lot shorter with this suggestion
1551 from Benjamin Goldberg:
1553 while($vec =~ /[^\0]+/g ) {
1554 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1557 Or use the CPAN module Bit::Vector:
1559 $vector = Bit::Vector->new($num_of_bits);
1560 $vector->Index_List_Store(@ints);
1561 @ints = $vector->Index_List_Read();
1563 Bit::Vector provides efficient methods for bit vector, sets of small integers
1566 Here's a more extensive illustration using vec():
1569 $vector = "\xff\x0f\xef\xfe";
1570 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1571 unpack("N", $vector), "\n";
1572 $is_set = vec($vector, 23, 1);
1573 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1591 my ($offset, $width, $value) = @_;
1593 vec($vector, $offset, $width) = $value;
1594 print "offset=$offset width=$width value=$value\n";
1600 my $bits = unpack("b*", $vector);
1604 print "vector length in bytes: ", length($vector), "\n";
1605 @bytes = unpack("A8" x length($vector), $bits);
1606 print "bits are: @bytes\n\n";
1609 =head2 Why does defined() return true on empty arrays and hashes?
1611 The short story is that you should probably only use defined on scalars or
1612 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1613 in the 5.004 release or later of Perl for more detail.
1615 =head1 Data: Hashes (Associative Arrays)
1617 =head2 How do I process an entire hash?
1619 Use the each() function (see L<perlfunc/each>) if you don't care
1620 whether it's sorted:
1622 while ( ($key, $value) = each %hash) {
1623 print "$key = $value\n";
1626 If you want it sorted, you'll have to use foreach() on the result of
1627 sorting the keys as shown in an earlier question.
1629 =head2 What happens if I add or remove keys from a hash while iterating over it?
1633 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1634 iterating over it. In Perl 5 you can delete from it, but you still
1635 can't add to it, because that might cause a doubling of the hash table,
1636 in which half the entries get copied up to the new top half of the
1637 table, at which point you've totally bamboozled the iterator code.
1638 Even if the table doesn't double, there's no telling whether your new
1639 entry will be inserted before or after the current iterator position.
1641 Either treasure up your changes and make them after the iterator finishes
1642 or use keys to fetch all the old keys at once, and iterate over the list
1645 =head2 How do I look up a hash element by value?
1647 Create a reverse hash:
1649 %by_value = reverse %by_key;
1650 $key = $by_value{$value};
1652 That's not particularly efficient. It would be more space-efficient
1655 while (($key, $value) = each %by_key) {
1656 $by_value{$value} = $key;
1659 If your hash could have repeated values, the methods above will only find
1660 one of the associated keys. This may or may not worry you. If it does
1661 worry you, you can always reverse the hash into a hash of arrays instead:
1663 while (($key, $value) = each %by_key) {
1664 push @{$key_list_by_value{$value}}, $key;
1667 =head2 How can I know how many entries are in a hash?
1669 If you mean how many keys, then all you have to do is
1670 use the keys() function in a scalar context:
1672 $num_keys = keys %hash;
1674 The keys() function also resets the iterator, which means that you may
1675 see strange results if you use this between uses of other hash operators
1678 =head2 How do I sort a hash (optionally by value instead of key)?
1680 Internally, hashes are stored in a way that prevents you from imposing
1681 an order on key-value pairs. Instead, you have to sort a list of the
1684 @keys = sort keys %hash; # sorted by key
1686 $hash{$a} cmp $hash{$b}
1687 } keys %hash; # and by value
1689 Here we'll do a reverse numeric sort by value, and if two keys are
1690 identical, sort by length of key, or if that fails, by straight ASCII
1691 comparison of the keys (well, possibly modified by your locale--see
1695 $hash{$b} <=> $hash{$a}
1697 length($b) <=> length($a)
1702 =head2 How can I always keep my hash sorted?
1704 You can look into using the DB_File module and tie() using the
1705 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1706 The Tie::IxHash module from CPAN might also be instructive.
1708 =head2 What's the difference between "delete" and "undef" with hashes?
1710 Hashes are pairs of scalars: the first is the key, the second is the
1711 value. The key will be coerced to a string, although the value can be
1712 any kind of scalar: string, number, or reference. If a key C<$key> is
1713 present in the array, C<exists($key)> will return true. The value for
1714 a given key can be C<undef>, in which case C<$array{$key}> will be
1715 C<undef> while C<$exists{$key}> will return true. This corresponds to
1716 (C<$key>, C<undef>) being in the hash.
1718 Pictures help... here's the C<%ary> table:
1728 And these conditions hold
1732 defined $ary{'d'} is true
1733 defined $ary{'a'} is true
1734 exists $ary{'a'} is true (Perl5 only)
1735 grep ($_ eq 'a', keys %ary) is true
1741 your table now reads:
1752 and these conditions now hold; changes in caps:
1756 defined $ary{'d'} is true
1757 defined $ary{'a'} is FALSE
1758 exists $ary{'a'} is true (Perl5 only)
1759 grep ($_ eq 'a', keys %ary) is true
1761 Notice the last two: you have an undef value, but a defined key!
1767 your table now reads:
1776 and these conditions now hold; changes in caps:
1780 defined $ary{'d'} is true
1781 defined $ary{'a'} is false
1782 exists $ary{'a'} is FALSE (Perl5 only)
1783 grep ($_ eq 'a', keys %ary) is FALSE
1785 See, the whole entry is gone!
1787 =head2 Why don't my tied hashes make the defined/exists distinction?
1789 They may or may not implement the EXISTS() and DEFINED() methods
1790 differently. For example, there isn't the concept of undef with hashes
1791 that are tied to DBM* files. This means the true/false tables above
1792 will give different results when used on such a hash. It also means
1793 that exists and defined do the same thing with a DBM* file, and what
1794 they end up doing is not what they do with ordinary hashes.
1796 =head2 How do I reset an each() operation part-way through?
1798 Using C<keys %hash> in scalar context returns the number of keys in
1799 the hash I<and> resets the iterator associated with the hash. You may
1800 need to do this if you use C<last> to exit a loop early so that when you
1801 re-enter it, the hash iterator has been reset.
1803 =head2 How can I get the unique keys from two hashes?
1805 First you extract the keys from the hashes into lists, then solve
1806 the "removing duplicates" problem described above. For example:
1809 for $element (keys(%foo), keys(%bar)) {
1816 @uniq = keys %{{%foo,%bar}};
1818 Or if you really want to save space:
1821 while (defined ($key = each %foo)) {
1824 while (defined ($key = each %bar)) {
1829 =head2 How can I store a multidimensional array in a DBM file?
1831 Either stringify the structure yourself (no fun), or else
1832 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1833 it on top of either DB_File or GDBM_File.
1835 =head2 How can I make my hash remember the order I put elements into it?
1837 Use the Tie::IxHash from CPAN.
1840 tie(%myhash, Tie::IxHash);
1841 for ($i=0; $i<20; $i++) {
1844 @keys = keys %myhash;
1845 # @keys = (0,1,2,3,...)
1847 =head2 Why does passing a subroutine an undefined element in a hash create it?
1849 If you say something like:
1851 somefunc($hash{"nonesuch key here"});
1853 Then that element "autovivifies"; that is, it springs into existence
1854 whether you store something there or not. That's because functions
1855 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1856 it has to be ready to write it back into the caller's version.
1858 This has been fixed as of Perl5.004.
1860 Normally, merely accessing a key's value for a nonexistent key does
1861 I<not> cause that key to be forever there. This is different than
1864 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1866 Usually a hash ref, perhaps like this:
1871 TITLE => "deputy peon",
1874 PALS => [ "Norbert", "Rhys", "Phineas"],
1877 References are documented in L<perlref> and the upcoming L<perlreftut>.
1878 Examples of complex data structures are given in L<perldsc> and
1879 L<perllol>. Examples of structures and object-oriented classes are
1882 =head2 How can I use a reference as a hash key?
1884 You can't do this directly, but you could use the standard Tie::RefHash
1885 module distributed with Perl.
1889 =head2 How do I handle binary data correctly?
1891 Perl is binary clean, so this shouldn't be a problem. For example,
1892 this works fine (assuming the files are found):
1894 if (`cat /vmunix` =~ /gzip/) {
1895 print "Your kernel is GNU-zip enabled!\n";
1898 On less elegant (read: Byzantine) systems, however, you have
1899 to play tedious games with "text" versus "binary" files. See
1900 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1901 systems are curses out of Microsoft, who seem to be committed to putting
1902 the backward into backward compatibility.
1904 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1906 If you want to deal with multibyte characters, however, there are
1907 some gotchas. See the section on Regular Expressions.
1909 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1911 Assuming that you don't care about IEEE notations like "NaN" or
1912 "Infinity", you probably just want to use a regular expression.
1914 if (/\D/) { print "has nondigits\n" }
1915 if (/^\d+$/) { print "is a whole number\n" }
1916 if (/^-?\d+$/) { print "is an integer\n" }
1917 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1918 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1919 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
1920 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1921 { print "a C float\n" }
1923 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1924 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1925 wrapper function for more convenient access. This function takes
1926 a string and returns the number it found, or C<undef> for input that
1927 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1928 if you just want to say, ``Is this a float?''
1931 use POSIX qw(strtod);
1936 my($num, $unparsed) = strtod($str);
1937 if (($str eq '') || ($unparsed != 0) || $!) {
1944 sub is_numeric { defined getnum($_[0]) }
1946 Or you could check out the String::Scanf module on CPAN instead. The
1947 POSIX module (part of the standard Perl distribution) provides the
1948 C<strtod> and C<strtol> for converting strings to double and longs,
1951 =head2 How do I keep persistent data across program calls?
1953 For some specific applications, you can use one of the DBM modules.
1954 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
1955 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
1956 of the standard distribution. Here's one example using Storable's C<store>
1957 and C<retrieve> functions:
1960 store(\%hash, "filename");
1963 $href = retrieve("filename"); # by ref
1964 %hash = %{ retrieve("filename") }; # direct to hash
1966 =head2 How do I print out or copy a recursive data structure?
1968 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1969 for printing out data structures. The Storable module, found on CPAN,
1970 provides a function called C<dclone> that recursively copies its argument.
1972 use Storable qw(dclone);
1975 Where $r1 can be a reference to any kind of data structure you'd like.
1976 It will be deeply copied. Because C<dclone> takes and returns references,
1977 you'd have to add extra punctuation if you had a hash of arrays that
1980 %newhash = %{ dclone(\%oldhash) };
1982 =head2 How do I define methods for every class/object?
1984 Use the UNIVERSAL class (see L<UNIVERSAL>).
1986 =head2 How do I verify a credit card checksum?
1988 Get the Business::CreditCard module from CPAN.
1990 =head2 How do I pack arrays of doubles or floats for XS code?
1992 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1993 If you're doing a lot of float or double processing, consider using
1994 the PDL module from CPAN instead--it makes number-crunching easy.
1996 =head1 AUTHOR AND COPYRIGHT
1998 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
1999 All rights reserved.
2001 This documentation is free; you can redistribute it and/or modify it
2002 under the same terms as Perl itself.
2004 Irrespective of its distribution, all code examples in this file
2005 are hereby placed into the public domain. You are permitted and
2006 encouraged to use this code in your own programs for fun
2007 or for profit as you see fit. A simple comment in the code giving
2008 credit would be courteous but is not required.