3 perlfaq4 - Data Manipulation ($Revision: 1.24 $, $Date: 1998/07/05 15:07:20 $)
7 The section of the FAQ answers question related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximate on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file, or appearing as literals
21 in your program, are converted from their decimal floating-point
22 representation (eg, 19.95) to the internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers (see L<perlvar/"$#"> if you use
33 print. C<$#> has a different default value in Perl5 than it did in
34 Perl4. Changing C<$#> yourself is deprecated.
36 This affects B<all> computer languages that represent decimal
37 floating-point numbers in binary, not just Perl. Perl provides
38 arbitrary-precision decimal numbers with the Math::BigFloat module
39 (part of the standard Perl distribution), but mathematical operations
40 are consequently slower.
42 To get rid of the superfluous digits, just use a format (eg,
43 C<printf("%.2f", 19.95)>) to get the required precision.
44 See L<perlop/"Floating-point Arithmetic">.
46 =head2 Why isn't my octal data interpreted correctly?
48 Perl only understands octal and hex numbers as such when they occur
49 as literals in your program. If they are read in from somewhere and
50 assigned, no automatic conversion takes place. You must explicitly
51 use oct() or hex() if you want the values converted. oct() interprets
52 both hex ("0x350") numbers and octal ones ("0350" or even without the
53 leading "0", like "377"), while hex() only converts hexadecimal ones,
54 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
56 This problem shows up most often when people try using chmod(), mkdir(),
57 umask(), or sysopen(), which all want permissions in octal.
59 chmod(644, $file); # WRONG -- perl -w catches this
60 chmod(0644, $file); # right
62 =head2 Does perl have a round function? What about ceil() and floor()? Trig functions?
64 Remember that int() merely truncates toward 0. For rounding to a certain
65 number of digits, sprintf() or printf() is usually the easiest route.
67 The POSIX module (part of the standard perl distribution) implements
68 ceil(), floor(), and a number of other mathematical and trigonometric
71 In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex
72 module. With 5.004, the Math::Trig module (part of the standard perl
73 distribution) implements the trigonometric functions. Internally it
74 uses the Math::Complex module and some functions can break out from
75 the real axis into the complex plane, for example the inverse sine of
78 Rounding in financial applications can have serious implications, and
79 the rounding method used should be specified precisely. In these
80 cases, it probably pays not to trust whichever system rounding is
81 being used by Perl, but to instead implement the rounding function you
84 =head2 How do I convert bits into ints?
86 To turn a string of 1s and 0s like '10110110' into a scalar containing
87 its binary value, use the pack() function (documented in
90 $decimal = pack('B8', '10110110');
92 Here's an example of going the other way:
94 $binary_string = join('', unpack('B*', "\x29"));
96 =head2 How do I multiply matrices?
98 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
99 or the PDL extension (also available from CPAN).
101 =head2 How do I perform an operation on a series of integers?
103 To call a function on each element in an array, and collect the
106 @results = map { my_func($_) } @array;
110 @triple = map { 3 * $_ } @single;
112 To call a function on each element of an array, but ignore the
115 foreach $iterator (@array) {
119 To call a function on each integer in a (small) range, you B<can> use:
121 @results = map { &my_func($_) } (5 .. 25);
123 but you should be aware that the C<..> operator creates an array of
124 all integers in the range. This can take a lot of memory for large
128 for ($i=5; $i < 500_005; $i++) {
129 push(@results, &my_func($i));
132 =head2 How can I output Roman numerals?
134 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
136 =head2 Why aren't my random numbers random?
138 John von Neumann said, ``Anyone who attempts to generate random numbers by
139 deterministic means is, of course, living in a state of sin.''
141 The short explanation is that you're getting pseudorandom numbers, not
142 random ones, because that's how these things work. A longer explanation
143 is available on http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy
146 You should also check out the Math::TrulyRandom module from CPAN.
150 =head2 How do I find the week-of-the-year/day-of-the-year?
152 The day of the year is in the array returned by localtime() (see
153 L<perlfunc/"localtime">):
155 $day_of_year = (localtime(time()))[7];
157 or more legibly (in 5.004 or higher):
160 $day_of_year = localtime(time())->yday;
162 You can find the week of the year by dividing this by 7:
164 $week_of_year = int($day_of_year / 7);
166 Of course, this believes that weeks start at zero.
168 =head2 How can I compare two date strings?
170 Use the Date::Manip or Date::DateCalc modules from CPAN.
172 =head2 How can I take a string and turn it into epoch seconds?
174 If it's a regular enough string that it always has the same format,
175 you can split it up and pass the parts to timelocal in the standard
176 Time::Local module. Otherwise, you should look into one of the
177 Date modules from CPAN.
179 =head2 How can I find the Julian Day?
181 Neither Date::Manip nor Date::DateCalc deal with Julian days.
182 Instead, there is an example of Julian date calculation in
183 http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz,
186 =head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
188 Perl is just as Y2K compliant as your pencil--no more, and no less.
189 The date and time functions supplied with perl (gmtime and localtime)
190 supply adequate information to determine the year well beyond 2000
191 (2038 is when trouble strikes for 32-bit machines). The year returned
192 by these functions when used in an array context is the year minus 1900.
193 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
194 number. To avoid the year 2000 problem simply do not treat the year as
195 a 2-digit number. It isn't.
197 When gmtime() and localtime() are used in scalar context they return
198 a timestamp string that contains a fully-expanded year. For example,
199 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
200 2001". There's no year 2000 problem here.
202 That doesn't mean that Perl can't be used to create non-Y2K compliant
203 programs. It can. But so can your pencil. It's the fault of the user,
204 not the language. At the risk of inflaming the NRA: ``Perl doesn't
205 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
210 =head2 How do I validate input?
212 The answer to this question is usually a regular expression, perhaps
213 with auxiliary logic. See the more specific questions (numbers, mail
214 addresses, etc.) for details.
216 =head2 How do I unescape a string?
218 It depends just what you mean by "escape". URL escapes are dealt with
219 in L<perlfaq9>. Shell escapes with the backslash (\)
220 character are removed with:
224 Note that this won't expand \n or \t or any other special escapes.
226 =head2 How do I remove consecutive pairs of characters?
228 To turn "abbcccd" into "abccd":
232 =head2 How do I expand function calls in a string?
234 This is documented in L<perlref>. In general, this is fraught with
235 quoting and readability problems, but it is possible. To interpolate
236 a subroutine call (in list context) into a string:
238 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
240 If you prefer scalar context, similar chicanery is also useful for
241 arbitrary expressions:
243 print "That yields ${\($n + 5)} widgets\n";
245 See also "How can I expand variables in text strings?" in this section
248 =head2 How do I find matching/nesting anything?
250 This isn't something that can be tackled in one regular expression, no
251 matter how complicated. To find something between two single characters,
252 a pattern like C</x([^x]*)x/> will get the intervening bits in $1. For
253 multiple ones, then something more like C</alpha(.*?)omega/> would
254 be needed. But none of these deals with nested patterns, nor can they.
255 For that you'll have to write a parser.
257 One destructive, inside-out approach that you might try is to pull
258 out the smallest nesting parts one at a time:
260 while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) {
261 # do something with $1
264 =head2 How do I reverse a string?
266 Use reverse() in scalar context, as documented in
269 $reversed = reverse $string;
271 =head2 How do I expand tabs in a string?
273 You can do it yourself:
275 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
277 Or you can just use the Text::Tabs module (part of the standard perl
281 @expanded_lines = expand(@lines_with_tabs);
283 =head2 How do I reformat a paragraph?
285 Use Text::Wrap (part of the standard perl distribution):
288 print wrap("\t", ' ', @paragraphs);
290 The paragraphs you give to Text::Wrap may not contain embedded
291 newlines. Text::Wrap doesn't justify the lines (flush-right).
293 =head2 How can I access/change the first N letters of a string?
295 There are many ways. If you just want to grab a copy, use
298 $first_byte = substr($a, 0, 1);
300 If you want to modify part of a string, the simplest way is often to
301 use substr() as an lvalue:
303 substr($a, 0, 3) = "Tom";
305 Although those with a regexp kind of thought process will likely prefer
309 =head2 How do I change the Nth occurrence of something?
311 You have to keep track. For example, let's say you want
312 to change the fifth occurrence of "whoever" or "whomever"
313 into "whosoever" or "whomsoever", case insensitively.
317 ++$count == 5 # is it the 5th?
318 ? "${2}soever" # yes, swap
319 : $1 # renege and leave it there
322 In the more general case, you can use the C</g> modifier in a C<while>
323 loop, keeping count of matches.
327 while (/(\w+)\s+fish\b/gi) {
328 if (++$count == $WANT) {
329 print "The third fish is a $1 one.\n";
330 # Warning: don't `last' out of this loop
334 That prints out: "The third fish is a red one." You can also use a
335 repetition count and repeated pattern like this:
337 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
339 =head2 How can I count the number of occurrences of a substring within a string?
341 There are a number of ways, with varying efficiency: If you want a
342 count of a certain single character (X) within a string, you can use the
343 C<tr///> function like so:
345 $string = "ThisXlineXhasXsomeXx'sXinXit":
346 $count = ($string =~ tr/X//);
347 print "There are $count X charcters in the string";
349 This is fine if you are just looking for a single character. However,
350 if you are trying to count multiple character substrings within a
351 larger string, C<tr///> won't work. What you can do is wrap a while()
352 loop around a global pattern match. For example, let's count negative
355 $string = "-9 55 48 -2 23 -76 4 14 -44";
356 while ($string =~ /-\d+/g) { $count++ }
357 print "There are $count negative numbers in the string";
359 =head2 How do I capitalize all the words on one line?
361 To make the first letter of each word upper case:
363 $line =~ s/\b(\w)/\U$1/g;
365 This has the strange effect of turning "C<don't do it>" into "C<Don'T
366 Do It>". Sometimes you might want this, instead (Suggested by Brian
367 Foy E<lt>comdog@computerdog.comE<gt>):
370 (^\w) #at the beginning of the line
372 (\s\w) #preceded by whitespace
375 $string =~ /([\w']+)/\u\L$1/g;
377 To make the whole line upper case:
381 To force each word to be lower case, with the first letter upper case:
383 $line =~ s/(\w+)/\u\L$1/g;
385 You can (and probably should) enable locale awareness of those
386 characters by placing a C<use locale> pragma in your program.
387 See L<perllocale> for endless details.
389 =head2 How can I split a [character] delimited string except when inside
390 [character]? (Comma-separated files)
392 Take the example case of trying to split a string that is comma-separated
393 into its different fields. (We'll pretend you said comma-separated, not
394 comma-delimited, which is different and almost never what you mean.) You
395 can't use C<split(/,/)> because you shouldn't split if the comma is inside
396 quotes. For example, take a data line like this:
398 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
400 Due to the restriction of the quotes, this is a fairly complex
401 problem. Thankfully, we have Jeffrey Friedl, author of a highly
402 recommended book on regular expressions, to handle these for us. He
403 suggests (assuming your string is contained in $text):
406 push(@new, $+) while $text =~ m{
407 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
411 push(@new, undef) if substr($text,-1,1) eq ',';
413 If you want to represent quotation marks inside a
414 quotation-mark-delimited field, escape them with backslashes (eg,
415 C<"like \"this\"">. Unescaping them is a task addressed earlier in
418 Alternatively, the Text::ParseWords module (part of the standard perl
419 distribution) lets you say:
421 use Text::ParseWords;
422 @new = quotewords(",", 0, $text);
424 =head2 How do I strip blank space from the beginning/end of a string?
426 Although the simplest approach would seem to be:
428 $string =~ s/^\s*(.*?)\s*$/$1/;
430 This is unneccesarily slow, destructive, and fails with embedded newlines.
431 It is much better faster to do this in two steps:
436 Or more nicely written as:
443 This idiom takes advantage of the for(each) loop's aliasing
444 behavior to factor out common code. You can do this
445 on several strings at once, or arrays, or even the
446 values of a hash if you use a slide:
448 # trim whitespace in the scalar, the array,
449 # and all the values in the hash
450 foreach ($scalar, @array, @hash{keys %hash}) {
455 =head2 How do I extract selected columns from a string?
457 Use substr() or unpack(), both documented in L<perlfunc>.
458 If you prefer thinking in terms of columns instead of widths,
459 you can use this kind of thing:
461 # determine the unpack format needed to split Linux ps output
462 # arguments are cut columns
463 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
469 for my $place (@positions) {
470 $template .= "A" . ($place - $lastpos) . " ";
477 =head2 How do I find the soundex value of a string?
479 Use the standard Text::Soundex module distributed with perl.
481 =head2 How can I expand variables in text strings?
483 Let's assume that you have a string like:
485 $text = 'this has a $foo in it and a $bar';
487 If those were both global variables, then this would
490 $text =~ s/\$(\w+)/${$1}/g;
492 But since they are probably lexicals, or at least, they could
493 be, you'd have to do this:
495 $text =~ s/(\$\w+)/$1/eeg;
496 die if $@; # needed on /ee, not /e
498 It's probably better in the general case to treat those
499 variables as entries in some special hash. For example:
505 $text =~ s/\$(\w+)/$user_defs{$1}/g;
507 See also "How do I expand function calls in a string?" in this section
510 =head2 What's wrong with always quoting "$vars"?
512 The problem is that those double-quotes force stringification,
513 coercing numbers and references into strings, even when you
514 don't want them to be.
516 If you get used to writing odd things like these:
520 somefunc("$var"); # BAD
522 You'll be in trouble. Those should (in 99.8% of the cases) be
523 the simpler and more direct:
529 Otherwise, besides slowing you down, you're going to break code when
530 the thing in the scalar is actually neither a string nor a number, but
536 my $oref = "$aref"; # WRONG
539 You can also get into subtle problems on those few operations in Perl
540 that actually do care about the difference between a string and a
541 number, such as the magical C<++> autoincrement operator or the
544 Stringification also destroys arrays.
547 print "@lines"; # WRONG - extra blanks
548 print @lines; # right
550 =head2 Why don't my <<HERE documents work?
552 Check for these three things:
556 =item 1. There must be no space after the << part.
558 =item 2. There (probably) should be a semicolon at the end.
560 =item 3. You can't (easily) have any space in front of the tag.
564 If you want to indent the text in the here document, you
568 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
573 But the HERE_TARGET must still be flush against the margin.
574 If you want that indented also, you'll have to quote
577 ($quote = <<' FINIS') =~ s/^\s+//gm;
578 ...we will have peace, when you and all your works have
579 perished--and the works of your dark master to whom you
580 would deliver us. You are a liar, Saruman, and a corrupter
581 of men's hearts. --Theoden in /usr/src/perl/taint.c
583 $quote =~ s/\s*--/\n--/;
585 A nice general-purpose fixer-upper function for indented here documents
586 follows. It expects to be called with a here document as its argument.
587 It looks to see whether each line begins with a common substring, and
588 if so, strips that off. Otherwise, it takes the amount of leading
589 white space found on the first line and removes that much off each
594 my ($white, $leader); # common white space and common leading string
595 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
596 ($white, $leader) = ($2, quotemeta($1));
598 ($white, $leader) = (/^(\s+)/, '');
600 s/^\s*?$leader(?:$white)?//gm;
604 This works with leading special strings, dynamically determined:
606 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
609 @@@ SAVEI32(runlevel);
611 @@@ while ( op = (*op->op_ppaddr)() ) ;
615 MAIN_INTERPRETER_LOOP
617 Or with a fixed amount of leading white space, with remaining
618 indentation correctly preserved:
620 $poem = fix<<EVER_ON_AND_ON;
621 Now far ahead the Road has gone,
622 And I must follow, if I can,
623 Pursuing it with eager feet,
624 Until it joins some larger way
625 Where many paths and errands meet.
626 And whither then? I cannot say.
627 --Bilbo in /usr/src/perl/pp_ctl.c
632 =head2 What is the difference between $array[1] and @array[1]?
634 The former is a scalar value, the latter an array slice, which makes
635 it a list with one (scalar) value. You should use $ when you want a
636 scalar value (most of the time) and @ when you want a list with one
637 scalar value in it (very, very rarely; nearly never, in fact).
639 Sometimes it doesn't make a difference, but sometimes it does.
640 For example, compare:
642 $good[0] = `some program that outputs several lines`;
646 @bad[0] = `same program that outputs several lines`;
648 The B<-w> flag will warn you about these matters.
650 =head2 How can I extract just the unique elements of an array?
652 There are several possible ways, depending on whether the array is
653 ordered and whether you wish to preserve the ordering.
657 =item a) If @in is sorted, and you want @out to be sorted:
658 (this assumes all true values in the array)
661 @out = grep($_ ne $prev && ($prev = $_), @in);
663 This is nice in that it doesn't use much extra memory, simulating
664 uniq(1)'s behavior of removing only adjacent duplicates. It's less
665 nice in that it won't work with false values like undef, 0, or "";
666 "0 but true" is ok, though.
668 =item b) If you don't know whether @in is sorted:
671 @out = grep(!$saw{$_}++, @in);
673 =item c) Like (b), but @in contains only small integers:
675 @out = grep(!$saw[$_]++, @in);
677 =item d) A way to do (b) without any loops or greps:
681 @out = sort keys %saw; # remove sort if undesired
683 =item e) Like (d), but @in contains only small positive integers:
691 =head2 How can I tell whether a list or array contains a certain element?
693 Hearing the word "in" is an I<in>dication that you probably should have
694 used a hash, not a list or array, to store your data. Hashes are
695 designed to answer this question quickly and efficiently. Arrays aren't.
697 That being said, there are several ways to approach this. If you
698 are going to make this query many times over arbitrary string values,
699 the fastest way is probably to invert the original array and keep an
700 associative array lying about whose keys are the first array's values.
702 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
704 for (@blues) { $is_blue{$_} = 1 }
706 Now you can check whether $is_blue{$some_color}. It might have been a
707 good idea to keep the blues all in a hash in the first place.
709 If the values are all small integers, you could use a simple indexed
710 array. This kind of an array will take up less space:
712 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
713 undef @is_tiny_prime;
714 for (@primes) { $is_tiny_prime[$_] = 1; }
716 Now you check whether $is_tiny_prime[$some_number].
718 If the values in question are integers instead of strings, you can save
719 quite a lot of space by using bit strings instead:
721 @articles = ( 1..10, 150..2000, 2017 );
723 for (@articles) { vec($read,$_,1) = 1 }
725 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
729 $is_there = grep $_ eq $whatever, @array;
733 $is_there = grep /$whatever/, @array;
735 These are slow (checks every element even if the first matches),
736 inefficient (same reason), and potentially buggy (what if there are
737 regexp characters in $whatever?).
739 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
741 Use a hash. Here's code to do both and more. It assumes that
742 each element is unique in a given array:
744 @union = @intersection = @difference = ();
746 foreach $element (@array1, @array2) { $count{$element}++ }
747 foreach $element (keys %count) {
748 push @union, $element;
749 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
752 =head2 How do I find the first array element for which a condition is true?
754 You can use this if you care about the index:
756 for ($i=0; $i < @array; $i++) {
757 if ($array[$i] eq "Waldo") {
763 Now C<$found_index> has what you want.
765 =head2 How do I handle linked lists?
767 In general, you usually don't need a linked list in Perl, since with
768 regular arrays, you can push and pop or shift and unshift at either end,
769 or you can use splice to add and/or remove arbitrary number of elements at
770 arbitrary points. Both pop and shift are both O(1) operations on perl's
771 dynamic arrays. In the absence of shifts and pops, push in general
772 needs to reallocate on the order every log(N) times, and unshift will
773 need to copy pointers each time.
775 If you really, really wanted, you could use structures as described in
776 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
779 =head2 How do I handle circular lists?
781 Circular lists could be handled in the traditional fashion with linked
782 lists, or you could just do something like this with an array:
784 unshift(@array, pop(@array)); # the last shall be first
785 push(@array, shift(@array)); # and vice versa
787 =head2 How do I shuffle an array randomly?
791 # fisher_yates_shuffle( \@array ) :
792 # generate a random permutation of @array in place
793 sub fisher_yates_shuffle {
796 for ($i = @$array; --$i; ) {
797 my $j = int rand ($i+1);
799 @$array[$i,$j] = @$array[$j,$i];
803 fisher_yates_shuffle( \@array ); # permutes @array in place
805 You've probably seen shuffling algorithms that works using splice,
806 randomly picking another element to swap the current element with:
810 @old = 1 .. 10; # just a demo
812 push(@new, splice(@old, rand @old, 1));
815 This is bad because splice is already O(N), and since you do it N times,
816 you just invented a quadratic algorithm; that is, O(N**2). This does
817 not scale, although Perl is so efficient that you probably won't notice
818 this until you have rather largish arrays.
820 =head2 How do I process/modify each element of an array?
822 Use C<for>/C<foreach>:
825 s/foo/bar/; # change that word
826 y/XZ/ZX/; # swap those letters
829 Here's another; let's compute spherical volumes:
831 for (@volumes = @radii) { # @volumes has changed parts
833 $_ *= (4/3) * 3.14159; # this will be constant folded
836 If you want to do the same thing to modify the values of the hash,
837 you may not use the C<values> function, oddly enough. You need a slice:
839 for $orbit ( @orbits{keys %orbits} ) {
840 ($orbit **= 3) *= (4/3) * 3.14159;
843 =head2 How do I select a random element from an array?
845 Use the rand() function (see L<perlfunc/rand>):
847 # at the top of the program:
848 srand; # not needed for 5.004 and later
851 $index = rand @array;
852 $element = $array[$index];
854 Make sure you I<only call srand once per program, if then>.
855 If you are calling it more than once (such as before each
856 call to rand), you're almost certainly doing something wrong.
858 =head2 How do I permute N elements of a list?
860 Here's a little program that generates all permutations
861 of all the words on each line of input. The algorithm embodied
862 in the permute() function should work on any list:
865 # tsc-permute: permute each word of input
866 permute([split], []);
868 my @items = @{ $_[0] };
869 my @perms = @{ $_[1] };
873 my(@newitems,@newperms,$i);
874 foreach $i (0 .. $#items) {
877 unshift(@newperms, splice(@newitems, $i, 1));
878 permute([@newitems], [@newperms]);
883 =head2 How do I sort an array by (anything)?
885 Supply a comparison function to sort() (described in L<perlfunc/sort>):
887 @list = sort { $a <=> $b } @list;
889 The default sort function is cmp, string comparison, which would
890 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
891 the numerical comparison operator.
893 If you have a complicated function needed to pull out the part you
894 want to sort on, then don't do it inside the sort function. Pull it
895 out first, because the sort BLOCK can be called many times for the
896 same element. Here's an example of how to pull out the first word
897 after the first number on each item, and then sort those words
902 ($item) = /\d+\s*(\S+)/;
903 push @idx, uc($item);
905 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
907 Which could also be written this way, using a trick
908 that's come to be known as the Schwartzian Transform:
910 @sorted = map { $_->[0] }
911 sort { $a->[1] cmp $b->[1] }
912 map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
914 If you need to sort on several fields, the following paradigm is useful.
916 @sorted = sort { field1($a) <=> field1($b) ||
917 field2($a) cmp field2($b) ||
918 field3($a) cmp field3($b)
921 This can be conveniently combined with precalculation of keys as given
924 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
927 See also the question below on sorting hashes.
929 =head2 How do I manipulate arrays of bits?
931 Use pack() and unpack(), or else vec() and the bitwise operations.
933 For example, this sets $vec to have bit N set if $ints[N] was set:
936 foreach(@ints) { vec($vec,$_,1) = 1 }
938 And here's how, given a vector in $vec, you can
939 get those bits into your @ints array:
944 # Find null-byte density then select best algorithm
945 if ($vec =~ tr/\0// / length $vec > 0.95) {
948 # This method is faster with mostly null-bytes
949 while($vec =~ /[^\0]/g ) {
950 $i = -9 + 8 * pos $vec;
951 push @ints, $i if vec($vec, ++$i, 1);
952 push @ints, $i if vec($vec, ++$i, 1);
953 push @ints, $i if vec($vec, ++$i, 1);
954 push @ints, $i if vec($vec, ++$i, 1);
955 push @ints, $i if vec($vec, ++$i, 1);
956 push @ints, $i if vec($vec, ++$i, 1);
957 push @ints, $i if vec($vec, ++$i, 1);
958 push @ints, $i if vec($vec, ++$i, 1);
961 # This method is a fast general algorithm
963 my $bits = unpack "b*", $vec;
964 push @ints, 0 if $bits =~ s/^(\d)// && $1;
965 push @ints, pos $bits while($bits =~ /1/g);
970 This method gets faster the more sparse the bit vector is.
971 (Courtesy of Tim Bunce and Winfried Koenig.)
973 =head2 Why does defined() return true on empty arrays and hashes?
975 See L<perlfunc/defined> in the 5.004 release or later of Perl.
977 =head1 Data: Hashes (Associative Arrays)
979 =head2 How do I process an entire hash?
981 Use the each() function (see L<perlfunc/each>) if you don't care
984 while ( ($key, $value) = each %hash) {
985 print "$key = $value\n";
988 If you want it sorted, you'll have to use foreach() on the result of
989 sorting the keys as shown in an earlier question.
991 =head2 What happens if I add or remove keys from a hash while iterating over it?
995 =head2 How do I look up a hash element by value?
997 Create a reverse hash:
999 %by_value = reverse %by_key;
1000 $key = $by_value{$value};
1002 That's not particularly efficient. It would be more space-efficient
1005 while (($key, $value) = each %by_key) {
1006 $by_value{$value} = $key;
1009 If your hash could have repeated values, the methods above will only
1010 find one of the associated keys. This may or may not worry you.
1012 =head2 How can I know how many entries are in a hash?
1014 If you mean how many keys, then all you have to do is
1015 take the scalar sense of the keys() function:
1017 $num_keys = scalar keys %hash;
1019 In void context it just resets the iterator, which is faster
1022 =head2 How do I sort a hash (optionally by value instead of key)?
1024 Internally, hashes are stored in a way that prevents you from imposing
1025 an order on key-value pairs. Instead, you have to sort a list of the
1028 @keys = sort keys %hash; # sorted by key
1030 $hash{$a} cmp $hash{$b}
1031 } keys %hash; # and by value
1033 Here we'll do a reverse numeric sort by value, and if two keys are
1034 identical, sort by length of key, and if that fails, by straight ASCII
1035 comparison of the keys (well, possibly modified by your locale -- see
1039 $hash{$b} <=> $hash{$a}
1041 length($b) <=> length($a)
1046 =head2 How can I always keep my hash sorted?
1048 You can look into using the DB_File module and tie() using the
1049 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1050 The Tie::IxHash module from CPAN might also be instructive.
1052 =head2 What's the difference between "delete" and "undef" with hashes?
1054 Hashes are pairs of scalars: the first is the key, the second is the
1055 value. The key will be coerced to a string, although the value can be
1056 any kind of scalar: string, number, or reference. If a key C<$key> is
1057 present in the array, C<exists($key)> will return true. The value for
1058 a given key can be C<undef>, in which case C<$array{$key}> will be
1059 C<undef> while C<$exists{$key}> will return true. This corresponds to
1060 (C<$key>, C<undef>) being in the hash.
1062 Pictures help... here's the C<%ary> table:
1072 And these conditions hold
1076 defined $ary{'d'} is true
1077 defined $ary{'a'} is true
1078 exists $ary{'a'} is true (perl5 only)
1079 grep ($_ eq 'a', keys %ary) is true
1085 your table now reads:
1096 and these conditions now hold; changes in caps:
1100 defined $ary{'d'} is true
1101 defined $ary{'a'} is FALSE
1102 exists $ary{'a'} is true (perl5 only)
1103 grep ($_ eq 'a', keys %ary) is true
1105 Notice the last two: you have an undef value, but a defined key!
1111 your table now reads:
1120 and these conditions now hold; changes in caps:
1124 defined $ary{'d'} is true
1125 defined $ary{'a'} is false
1126 exists $ary{'a'} is FALSE (perl5 only)
1127 grep ($_ eq 'a', keys %ary) is FALSE
1129 See, the whole entry is gone!
1131 =head2 Why don't my tied hashes make the defined/exists distinction?
1133 They may or may not implement the EXISTS() and DEFINED() methods
1134 differently. For example, there isn't the concept of undef with hashes
1135 that are tied to DBM* files. This means the true/false tables above
1136 will give different results when used on such a hash. It also means
1137 that exists and defined do the same thing with a DBM* file, and what
1138 they end up doing is not what they do with ordinary hashes.
1140 =head2 How do I reset an each() operation part-way through?
1142 Using C<keys %hash> in scalar context returns the number of keys in
1143 the hash I<and> resets the iterator associated with the hash. You may
1144 need to do this if you use C<last> to exit a loop early so that when you
1145 re-enter it, the hash iterator has been reset.
1147 =head2 How can I get the unique keys from two hashes?
1149 First you extract the keys from the hashes into arrays, and then solve
1150 the uniquifying the array problem described above. For example:
1153 for $element (keys(%foo), keys(%bar)) {
1160 @uniq = keys %{{%foo,%bar}};
1162 Or if you really want to save space:
1165 while (defined ($key = each %foo)) {
1168 while (defined ($key = each %bar)) {
1173 =head2 How can I store a multidimensional array in a DBM file?
1175 Either stringify the structure yourself (no fun), or else
1176 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1177 it on top of either DB_File or GDBM_File.
1179 =head2 How can I make my hash remember the order I put elements into it?
1181 Use the Tie::IxHash from CPAN.
1184 tie(%myhash, Tie::IxHash);
1185 for ($i=0; $i<20; $i++) {
1188 @keys = keys %myhash;
1189 # @keys = (0,1,2,3,...)
1191 =head2 Why does passing a subroutine an undefined element in a hash create it?
1193 If you say something like:
1195 somefunc($hash{"nonesuch key here"});
1197 Then that element "autovivifies"; that is, it springs into existence
1198 whether you store something there or not. That's because functions
1199 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1200 it has to be ready to write it back into the caller's version.
1202 This has been fixed as of perl5.004.
1204 Normally, merely accessing a key's value for a nonexistent key does
1205 I<not> cause that key to be forever there. This is different than
1208 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1210 Use references (documented in L<perlref>). Examples of complex data
1211 structures are given in L<perldsc> and L<perllol>. Examples of
1212 structures and object-oriented classes are in L<perltoot>.
1214 =head2 How can I use a reference as a hash key?
1216 You can't do this directly, but you could use the standard Tie::Refhash
1217 module distributed with perl.
1221 =head2 How do I handle binary data correctly?
1223 Perl is binary clean, so this shouldn't be a problem. For example,
1224 this works fine (assuming the files are found):
1226 if (`cat /vmunix` =~ /gzip/) {
1227 print "Your kernel is GNU-zip enabled!\n";
1230 On some systems, however, you have to play tedious games with "text"
1231 versus "binary" files. See L<perlfunc/"binmode">.
1233 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1235 If you want to deal with multibyte characters, however, there are
1236 some gotchas. See the section on Regular Expressions.
1238 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1240 Assuming that you don't care about IEEE notations like "NaN" or
1241 "Infinity", you probably just want to use a regular expression.
1243 warn "has nondigits" if /\D/;
1244 warn "not a natural number" unless /^\d+$/; # rejects -3
1245 warn "not an integer" unless /^-?\d+$/; # rejects +3
1246 warn "not an integer" unless /^[+-]?\d+$/;
1247 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
1248 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
1249 warn "not a C float"
1250 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
1252 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1253 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1254 wrapper function for more convenient access. This function takes
1255 a string and returns the number it found, or C<undef> for input that
1256 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1257 if you just want to say, ``Is this a float?''
1260 use POSIX qw(strtod);
1265 my($num, $unparsed) = strtod($str);
1266 if (($str eq '') || ($unparsed != 0) || $!) {
1273 sub is_numeric { defined &getnum }
1275 Or you could check out
1276 http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
1277 instead. The POSIX module (part of the standard Perl distribution)
1278 provides the C<strtol> and C<strtod> for converting strings to double
1279 and longs, respectively.
1281 =head2 How do I keep persistent data across program calls?
1283 For some specific applications, you can use one of the DBM modules.
1284 See L<AnyDBM_File>. More generically, you should consult the
1285 FreezeThaw, Storable, or Class::Eroot modules from CPAN.
1287 =head2 How do I print out or copy a recursive data structure?
1289 The Data::Dumper module on CPAN is nice for printing out
1290 data structures, and FreezeThaw for copying them. For example:
1292 use FreezeThaw qw(freeze thaw);
1293 $new = thaw freeze $old;
1295 Where $old can be (a reference to) any kind of data structure you'd like.
1296 It will be deeply copied.
1298 =head2 How do I define methods for every class/object?
1300 Use the UNIVERSAL class (see L<UNIVERSAL>).
1302 =head2 How do I verify a credit card checksum?
1304 Get the Business::CreditCard module from CPAN.
1306 =head1 AUTHOR AND COPYRIGHT
1308 Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
1309 All rights reserved.
1311 When included as part of the Standard Version of Perl, or as part of
1312 its complete documentation whether printed or otherwise, this work
1313 may be distributed only under the terms of Perl's Artistic License.
1314 Any distribution of this file or derivatives thereof I<outside>
1315 of that package require that special arrangements be made with
1318 Irrespective of its distribution, all code examples in this file
1319 are hereby placed into the public domain. You are permitted and
1320 encouraged to use this code in your own programs for fun
1321 or for profit as you see fit. A simple comment in the code giving
1322 credit would be courteous but is not required.