3 perlfaq4 - Data Manipulation ($Revision: 1.19 $, $Date: 1997/04/24 22:43:57 $)
7 The section of the FAQ answers question related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximate on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file, or appearing as literals
21 in your program, are converted from their decimal floating-point
22 representation (eg, 19.95) to the internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers (see L<perlvar/"$#"> 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 (2038
191 is when trouble strikes). The year returned by these functions when used
192 in an array context is the year minus 1900. For years between 1910 and
193 1999 this I<happens> to be a 2-digit decimal number. To avoid the year
194 2000 problem simply do not treat the year as a 2-digit number. It isn't.
196 When gmtime() and localtime() are used in scalar context they return
197 a timestamp string that contains a fully-expanded year. For example,
198 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
199 2001". There's no year 2000 problem here.
201 That doesn't mean that Perl can't be used to create non-Y2K compliant
202 programs. It can. But so can your pencil. It's the fault of the user,
203 not the language. At the risk of inflaming the NRA: ``Perl doesn't
204 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
209 =head2 How do I validate input?
211 The answer to this question is usually a regular expression, perhaps
212 with auxiliary logic. See the more specific questions (numbers, mail
213 addresses, etc.) for details.
215 =head2 How do I unescape a string?
217 It depends just what you mean by "escape". URL escapes are dealt with
218 in L<perlfaq9>. Shell escapes with the backslash (\)
219 character are removed with:
223 Note that this won't expand \n or \t or any other special escapes.
225 =head2 How do I remove consecutive pairs of characters?
227 To turn "abbcccd" into "abccd":
231 =head2 How do I expand function calls in a string?
233 This is documented in L<perlref>. In general, this is fraught with
234 quoting and readability problems, but it is possible. To interpolate
235 a subroutine call (in list context) into a string:
237 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
239 If you prefer scalar context, similar chicanery is also useful for
240 arbitrary expressions:
242 print "That yields ${\($n + 5)} widgets\n";
244 See also "How can I expand variables in text strings?" in this section
247 =head2 How do I find matching/nesting anything?
249 This isn't something that can be tackled in one regular expression, no
250 matter how complicated. To find something between two single characters,
251 a pattern like C</x([^x]*)x/> will get the intervening bits in $1. For
252 multiple ones, then something more like C</alpha(.*?)omega/> would
253 be needed. But none of these deals with nested patterns, nor can they.
254 For that you'll have to write a parser.
256 One destructive, inside-out approach that you might try is to pull
257 out the smallest nesting parts one at a time:
259 while (s/BEGIN(.*?)END//gs) {
260 # do something with $1
263 =head2 How do I reverse a string?
265 Use reverse() in scalar context, as documented in
268 $reversed = reverse $string;
270 =head2 How do I expand tabs in a string?
272 You can do it yourself:
274 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
276 Or you can just use the Text::Tabs module (part of the standard perl
280 @expanded_lines = expand(@lines_with_tabs);
282 =head2 How do I reformat a paragraph?
284 Use Text::Wrap (part of the standard perl distribution):
287 print wrap("\t", ' ', @paragraphs);
289 The paragraphs you give to Text::Wrap may not contain embedded
290 newlines. Text::Wrap doesn't justify the lines (flush-right).
292 =head2 How can I access/change the first N letters of a string?
294 There are many ways. If you just want to grab a copy, use
297 $first_byte = substr($a, 0, 1);
299 If you want to modify part of a string, the simplest way is often to
300 use substr() as an lvalue:
302 substr($a, 0, 3) = "Tom";
304 Although those with a regexp kind of thought process will likely prefer
308 =head2 How do I change the Nth occurrence of something?
310 You have to keep track. For example, let's say you want
311 to change the fifth occurrence of "whoever" or "whomever"
312 into "whosoever" or "whomsoever", case insensitively.
316 ++$count == 5 # is it the 5th?
317 ? "${2}soever" # yes, swap
318 : $1 # renege and leave it there
321 In the more general case, you can use the C</g> modifier in a C<while>
322 loop, keeping count of matches.
326 while (/(\w+)\s+fish\b/gi) {
327 if (++$count == $WANT) {
328 print "The third fish is a $1 one.\n";
329 # Warning: don't `last' out of this loop
333 That prints out: "The third fish is a red one." You can also use a
334 repetition count and repeated pattern like this:
336 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
338 =head2 How can I count the number of occurrences of a substring within a string?
340 There are a number of ways, with varying efficiency: If you want a
341 count of a certain single character (X) within a string, you can use the
342 C<tr///> function like so:
344 $string = "ThisXlineXhasXsomeXx'sXinXit":
345 $count = ($string =~ tr/X//);
346 print "There are $count X charcters in the string";
348 This is fine if you are just looking for a single character. However,
349 if you are trying to count multiple character substrings within a
350 larger string, C<tr///> won't work. What you can do is wrap a while()
351 loop around a global pattern match. For example, let's count negative
354 $string = "-9 55 48 -2 23 -76 4 14 -44";
355 while ($string =~ /-\d+/g) { $count++ }
356 print "There are $count negative numbers in the string";
358 =head2 How do I capitalize all the words on one line?
360 To make the first letter of each word upper case:
362 $line =~ s/\b(\w)/\U$1/g;
364 This has the strange effect of turning "C<don't do it>" into "C<Don'T
365 Do It>". Sometimes you might want this, instead (Suggested by Brian
366 Foy E<lt>comdog@computerdog.comE<gt>):
369 (^\w) #at the beginning of the line
371 (\s\w) #preceded by whitespace
374 $string =~ /([\w']+)/\u\L$1/g;
376 To make the whole line upper case:
380 To force each word to be lower case, with the first letter upper case:
382 $line =~ s/(\w+)/\u\L$1/g;
384 You can (and probably should) enable locale awareness of those
385 characters by placing a C<use locale> pragma in your program.
386 See L<perllocale> for endless details.
388 =head2 How can I split a [character] delimited string except when inside
389 [character]? (Comma-separated files)
391 Take the example case of trying to split a string that is comma-separated
392 into its different fields. (We'll pretend you said comma-separated, not
393 comma-delimited, which is different and almost never what you mean.) You
394 can't use C<split(/,/)> because you shouldn't split if the comma is inside
395 quotes. For example, take a data line like this:
397 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
399 Due to the restriction of the quotes, this is a fairly complex
400 problem. Thankfully, we have Jeffrey Friedl, author of a highly
401 recommended book on regular expressions, to handle these for us. He
402 suggests (assuming your string is contained in $text):
405 push(@new, $+) while $text =~ m{
406 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
410 push(@new, undef) if substr($text,-1,1) eq ',';
412 If you want to represent quotation marks inside a
413 quotation-mark-delimited field, escape them with backslashes (eg,
414 C<"like \"this\"">. Unescaping them is a task addressed earlier in
417 Alternatively, the Text::ParseWords module (part of the standard perl
418 distribution) lets you say:
420 use Text::ParseWords;
421 @new = quotewords(",", 0, $text);
423 =head2 How do I strip blank space from the beginning/end of a string?
425 Although the simplest approach would seem to be:
427 $string =~ s/^\s*(.*?)\s*$/$1/;
429 This is unneccesarily slow, destructive, and fails with embedded newlines.
430 It is much better faster to do this in two steps:
435 Or more nicely written as:
442 This idiom takes advantage of the for(each) loop's aliasing
443 behavior to factor out common code. You can do this
444 on several strings at once, or arrays, or even the
445 values of a hash if you use a slide:
447 # trim whitespace in the scalar, the array,
448 # and all the values in the hash
449 foreach ($scalar, @array, @hash{keys %hash}) {
454 =head2 How do I extract selected columns from a string?
456 Use substr() or unpack(), both documented in L<perlfunc>.
457 If you prefer thinking in terms of columns instead of widths,
458 you can use this kind of thing:
460 # determine the unpack format needed to split Linux ps output
461 # arguments are cut columns
462 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
468 for my $place (@positions) {
469 $template .= "A" . ($place - $lastpos) . " ";
476 =head2 How do I find the soundex value of a string?
478 Use the standard Text::Soundex module distributed with perl.
480 =head2 How can I expand variables in text strings?
482 Let's assume that you have a string like:
484 $text = 'this has a $foo in it and a $bar';
486 If those were both global variables, then this would
489 $text =~ s/\$(\w+)/${$1}/g;
491 But since they are probably lexicals, or at least, they could
492 be, you'd have to do this:
494 $text =~ s/(\$\w+)/$1/eeg;
495 die if $@; # needed on /ee, not /e
497 It's probably better in the general case to treat those
498 variables as entries in some special hash. For example:
504 $text =~ s/\$(\w+)/$user_defs{$1}/g;
506 See also "How do I expand function calls in a string?" in this section
509 =head2 What's wrong with always quoting "$vars"?
511 The problem is that those double-quotes force stringification,
512 coercing numbers and references into strings, even when you
513 don't want them to be.
515 If you get used to writing odd things like these:
519 somefunc("$var"); # BAD
521 You'll be in trouble. Those should (in 99.8% of the cases) be
522 the simpler and more direct:
528 Otherwise, besides slowing you down, you're going to break code when
529 the thing in the scalar is actually neither a string nor a number, but
535 my $oref = "$aref"; # WRONG
538 You can also get into subtle problems on those few operations in Perl
539 that actually do care about the difference between a string and a
540 number, such as the magical C<++> autoincrement operator or the
543 Stringification also destroys arrays.
546 print "@lines"; # WRONG - extra blanks
547 print @lines; # right
549 =head2 Why don't my <<HERE documents work?
551 Check for these three things:
555 =item 1. There must be no space after the << part.
557 =item 2. There (probably) should be a semicolon at the end.
559 =item 3. You can't (easily) have any space in front of the tag.
563 If you want to indent the text in the here document, you
567 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
572 But the HERE_TARGET must still be flush against the margin.
573 If you want that indented also, you'll have to quote
576 ($quote = <<' FINIS') =~ s/^\s+//gm;
577 ...we will have peace, when you and all your works have
578 perished--and the works of your dark master to whom you
579 would deliver us. You are a liar, Saruman, and a corrupter
580 of men's hearts. --Theoden in /usr/src/perl/taint.c
582 $quote =~ s/\s*--/\n--/;
584 A nice general-purpose fixer-upper function for indented here documents
585 follows. It expects to be called with a here document as its argument.
586 It looks to see whether each line begins with a common substring, and
587 if so, strips that off. Otherwise, it takes the amount of leading
588 white space found on the first line and removes that much off each
593 my ($white, $leader); # common white space and common leading string
594 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
595 ($white, $leader) = ($2, quotemeta($1));
597 ($white, $leader) = (/^(\s+)/, '');
599 s/^\s*?$leader(?:$white)?//gm;
603 This owrks with leading special strings, dynamically determined:
605 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
608 @@@ SAVEI32(runlevel);
610 @@@ while ( op = (*op->op_ppaddr)() ) ;
614 MAIN_INTERPRETER_LOOP
616 Or with a fixed amount of leading white space, with remaining
617 indentation correctly preserved:
619 $poem = fix<<EVER_ON_AND_ON;
620 Now far ahead the Road has gone,
621 And I must follow, if I can,
622 Pursuing it with eager feet,
623 Until it joins some larger way
624 Where many paths and errands meet.
625 And whither then? I cannot say.
626 --Bilbo in /usr/src/perl/pp_ctl.c
631 =head2 What is the difference between $array[1] and @array[1]?
633 The former is a scalar value, the latter an array slice, which makes
634 it a list with one (scalar) value. You should use $ when you want a
635 scalar value (most of the time) and @ when you want a list with one
636 scalar value in it (very, very rarely; nearly never, in fact).
638 Sometimes it doesn't make a difference, but sometimes it does.
639 For example, compare:
641 $good[0] = `some program that outputs several lines`;
645 @bad[0] = `same program that outputs several lines`;
647 The B<-w> flag will warn you about these matters.
649 =head2 How can I extract just the unique elements of an array?
651 There are several possible ways, depending on whether the array is
652 ordered and whether you wish to preserve the ordering.
656 =item a) If @in is sorted, and you want @out to be sorted:
657 (this assumes all true values in the array)
660 @out = grep($_ ne $prev && ($prev = $_), @in);
662 This is nice in that it doesn't use much extra memory,
663 simulating uniq(1)'s behavior of removing only adjacent
666 =item b) If you don't know whether @in is sorted:
669 @out = grep(!$saw{$_}++, @in);
671 =item c) Like (b), but @in contains only small integers:
673 @out = grep(!$saw[$_]++, @in);
675 =item d) A way to do (b) without any loops or greps:
679 @out = sort keys %saw; # remove sort if undesired
681 =item e) Like (d), but @in contains only small positive integers:
689 =head2 How can I tell whether a list or array contains a certain element?
691 Hearing the word "in" is an I<in>dication that you probably should have
692 used a hash, not a list or array, to store your data. Hashes are
693 designed to answer this question quickly and efficiently. Arrays aren't.
695 That being said, there are several ways to approach this. If you
696 are going to make this query many times over arbitrary string values,
697 the fastest way is probably to invert the original array and keep an
698 associative array lying about whose keys are the first array's values.
700 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
702 for (@blues) { $is_blue{$_} = 1 }
704 Now you can check whether $is_blue{$some_color}. It might have been a
705 good idea to keep the blues all in a hash in the first place.
707 If the values are all small integers, you could use a simple indexed
708 array. This kind of an array will take up less space:
710 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
711 undef @is_tiny_prime;
712 for (@primes) { $is_tiny_prime[$_] = 1; }
714 Now you check whether $is_tiny_prime[$some_number].
716 If the values in question are integers instead of strings, you can save
717 quite a lot of space by using bit strings instead:
719 @articles = ( 1..10, 150..2000, 2017 );
721 for (@articles) { vec($read,$_,1) = 1 }
723 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
727 $is_there = grep $_ eq $whatever, @array;
731 $is_there = grep /$whatever/, @array;
733 These are slow (checks every element even if the first matches),
734 inefficient (same reason), and potentially buggy (what if there are
735 regexp characters in $whatever?).
737 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
739 Use a hash. Here's code to do both and more. It assumes that
740 each element is unique in a given array:
742 @union = @intersection = @difference = ();
744 foreach $element (@array1, @array2) { $count{$element}++ }
745 foreach $element (keys %count) {
746 push @union, $element;
747 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
750 =head2 How do I find the first array element for which a condition is true?
752 You can use this if you care about the index:
754 for ($i=0; $i < @array; $i++) {
755 if ($array[$i] eq "Waldo") {
761 Now C<$found_index> has what you want.
763 =head2 How do I handle linked lists?
765 In general, you usually don't need a linked list in Perl, since with
766 regular arrays, you can push and pop or shift and unshift at either end,
767 or you can use splice to add and/or remove arbitrary number of elements at
768 arbitrary points. Both pop and shift are both O(1) operations on perl's
769 dynamic arrays. In the absence of shifts and pops, push in general
770 needs to reallocate on the order every log(N) times, and unshift will
771 need to copy pointers each time.
773 If you really, really wanted, you could use structures as described in
774 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
777 =head2 How do I handle circular lists?
779 Circular lists could be handled in the traditional fashion with linked
780 lists, or you could just do something like this with an array:
782 unshift(@array, pop(@array)); # the last shall be first
783 push(@array, shift(@array)); # and vice versa
785 =head2 How do I shuffle an array randomly?
789 # fisher_yates_shuffle( \@array ) :
790 # generate a random permutation of @array in place
791 sub fisher_yates_shuffle {
794 for ($i = @$array; --$i; ) {
795 my $j = int rand ($i+1);
797 @$array[$i,$j] = @$array[$j,$i];
801 fisher_yates_shuffle( \@array ); # permutes @array in place
803 You've probably seen shuffling algorithms that works using splice,
804 randomly picking another element to swap the current element with:
808 @old = 1 .. 10; # just a demo
810 push(@new, splice(@old, rand @old, 1));
813 This is bad because splice is already O(N), and since you do it N times,
814 you just invented a quadratic algorithm; that is, O(N**2). This does
815 not scale, although Perl is so efficient that you probably won't notice
816 this until you have rather largish arrays.
818 =head2 How do I process/modify each element of an array?
820 Use C<for>/C<foreach>:
823 s/foo/bar/; # change that word
824 y/XZ/ZX/; # swap those letters
827 Here's another; let's compute spherical volumes:
829 for (@volumes = @radii) { # @volumes has changed parts
831 $_ *= (4/3) * 3.14159; # this will be constant folded
834 If you want to do the same thing to modify the values of the hash,
835 you may not use the C<values> function, oddly enough. You need a slice:
837 for $orbit ( @orbits{keys %orbits} ) {
838 ($orbit **= 3) *= (4/3) * 3.14159;
841 =head2 How do I select a random element from an array?
843 Use the rand() function (see L<perlfunc/rand>):
845 # at the top of the program:
846 srand; # not needed for 5.004 and later
849 $index = rand @array;
850 $element = $array[$index];
852 Make sure you I<only call srand once per program, if then>.
853 If you are calling it more than once (such as before each
854 call to rand), you're almost certainly doing something wrong.
856 =head2 How do I permute N elements of a list?
858 Here's a little program that generates all permutations
859 of all the words on each line of input. The algorithm embodied
860 in the permute() function should work on any list:
863 # tsc-permute: permute each word of input
864 permute([split], []);
866 my @items = @{ $_[0] };
867 my @perms = @{ $_[1] };
871 my(@newitems,@newperms,$i);
872 foreach $i (0 .. $#items) {
875 unshift(@newperms, splice(@newitems, $i, 1));
876 permute([@newitems], [@newperms]);
881 =head2 How do I sort an array by (anything)?
883 Supply a comparison function to sort() (described in L<perlfunc/sort>):
885 @list = sort { $a <=> $b } @list;
887 The default sort function is cmp, string comparison, which would
888 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
889 the numerical comparison operator.
891 If you have a complicated function needed to pull out the part you
892 want to sort on, then don't do it inside the sort function. Pull it
893 out first, because the sort BLOCK can be called many times for the
894 same element. Here's an example of how to pull out the first word
895 after the first number on each item, and then sort those words
900 ($item) = /\d+\s*(\S+)/;
901 push @idx, uc($item);
903 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
905 Which could also be written this way, using a trick
906 that's come to be known as the Schwartzian Transform:
908 @sorted = map { $_->[0] }
909 sort { $a->[1] cmp $b->[1] }
910 map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
912 If you need to sort on several fields, the following paradigm is useful.
914 @sorted = sort { field1($a) <=> field1($b) ||
915 field2($a) cmp field2($b) ||
916 field3($a) cmp field3($b)
919 This can be conveniently combined with precalculation of keys as given
922 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
925 See also the question below on sorting hashes.
927 =head2 How do I manipulate arrays of bits?
929 Use pack() and unpack(), or else vec() and the bitwise operations.
931 For example, this sets $vec to have bit N set if $ints[N] was set:
934 foreach(@ints) { vec($vec,$_,1) = 1 }
936 And here's how, given a vector in $vec, you can
937 get those bits into your @ints array:
942 # Find null-byte density then select best algorithm
943 if ($vec =~ tr/\0// / length $vec > 0.95) {
946 # This method is faster with mostly null-bytes
947 while($vec =~ /[^\0]/g ) {
948 $i = -9 + 8 * pos $vec;
949 push @ints, $i if vec($vec, ++$i, 1);
950 push @ints, $i if vec($vec, ++$i, 1);
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);
959 # This method is a fast general algorithm
961 my $bits = unpack "b*", $vec;
962 push @ints, 0 if $bits =~ s/^(\d)// && $1;
963 push @ints, pos $bits while($bits =~ /1/g);
968 This method gets faster the more sparse the bit vector is.
969 (Courtesy of Tim Bunce and Winfried Koenig.)
971 =head2 Why does defined() return true on empty arrays and hashes?
973 See L<perlfunc/defined> in the 5.004 release or later of Perl.
975 =head1 Data: Hashes (Associative Arrays)
977 =head2 How do I process an entire hash?
979 Use the each() function (see L<perlfunc/each>) if you don't care
982 while ( ($key, $value) = each %hash) {
983 print "$key = $value\n";
986 If you want it sorted, you'll have to use foreach() on the result of
987 sorting the keys as shown in an earlier question.
989 =head2 What happens if I add or remove keys from a hash while iterating over it?
993 =head2 How do I look up a hash element by value?
995 Create a reverse hash:
997 %by_value = reverse %by_key;
998 $key = $by_value{$value};
1000 That's not particularly efficient. It would be more space-efficient
1003 while (($key, $value) = each %by_key) {
1004 $by_value{$value} = $key;
1007 If your hash could have repeated values, the methods above will only
1008 find one of the associated keys. This may or may not worry you.
1010 =head2 How can I know how many entries are in a hash?
1012 If you mean how many keys, then all you have to do is
1013 take the scalar sense of the keys() function:
1015 $num_keys = scalar keys %hash;
1017 In void context it just resets the iterator, which is faster
1020 =head2 How do I sort a hash (optionally by value instead of key)?
1022 Internally, hashes are stored in a way that prevents you from imposing
1023 an order on key-value pairs. Instead, you have to sort a list of the
1026 @keys = sort keys %hash; # sorted by key
1028 $hash{$a} cmp $hash{$b}
1029 } keys %hash; # and by value
1031 Here we'll do a reverse numeric sort by value, and if two keys are
1032 identical, sort by length of key, and if that fails, by straight ASCII
1033 comparison of the keys (well, possibly modified by your locale -- see
1037 $hash{$b} <=> $hash{$a}
1039 length($b) <=> length($a)
1044 =head2 How can I always keep my hash sorted?
1046 You can look into using the DB_File module and tie() using the
1047 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1048 The Tie::IxHash module from CPAN might also be instructive.
1050 =head2 What's the difference between "delete" and "undef" with hashes?
1052 Hashes are pairs of scalars: the first is the key, the second is the
1053 value. The key will be coerced to a string, although the value can be
1054 any kind of scalar: string, number, or reference. If a key C<$key> is
1055 present in the array, C<exists($key)> will return true. The value for
1056 a given key can be C<undef>, in which case C<$array{$key}> will be
1057 C<undef> while C<$exists{$key}> will return true. This corresponds to
1058 (C<$key>, C<undef>) being in the hash.
1060 Pictures help... here's the C<%ary> table:
1070 And these conditions hold
1074 defined $ary{'d'} is true
1075 defined $ary{'a'} is true
1076 exists $ary{'a'} is true (perl5 only)
1077 grep ($_ eq 'a', keys %ary) is true
1083 your table now reads:
1094 and these conditions now hold; changes in caps:
1098 defined $ary{'d'} is true
1099 defined $ary{'a'} is FALSE
1100 exists $ary{'a'} is true (perl5 only)
1101 grep ($_ eq 'a', keys %ary) is true
1103 Notice the last two: you have an undef value, but a defined key!
1109 your table now reads:
1118 and these conditions now hold; changes in caps:
1122 defined $ary{'d'} is true
1123 defined $ary{'a'} is false
1124 exists $ary{'a'} is FALSE (perl5 only)
1125 grep ($_ eq 'a', keys %ary) is FALSE
1127 See, the whole entry is gone!
1129 =head2 Why don't my tied hashes make the defined/exists distinction?
1131 They may or may not implement the EXISTS() and DEFINED() methods
1132 differently. For example, there isn't the concept of undef with hashes
1133 that are tied to DBM* files. This means the true/false tables above
1134 will give different results when used on such a hash. It also means
1135 that exists and defined do the same thing with a DBM* file, and what
1136 they end up doing is not what they do with ordinary hashes.
1138 =head2 How do I reset an each() operation part-way through?
1140 Using C<keys %hash> in scalar context returns the number of keys in
1141 the hash I<and> resets the iterator associated with the hash. You may
1142 need to do this if you use C<last> to exit a loop early so that when you
1143 re-enter it, the hash iterator has been reset.
1145 =head2 How can I get the unique keys from two hashes?
1147 First you extract the keys from the hashes into arrays, and then solve
1148 the uniquifying the array problem described above. For example:
1151 for $element (keys(%foo), keys(%bar)) {
1158 @uniq = keys %{{%foo,%bar}};
1160 Or if you really want to save space:
1163 while (defined ($key = each %foo)) {
1166 while (defined ($key = each %bar)) {
1171 =head2 How can I store a multidimensional array in a DBM file?
1173 Either stringify the structure yourself (no fun), or else
1174 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1175 it on top of either DB_File or GDBM_File.
1177 =head2 How can I make my hash remember the order I put elements into it?
1179 Use the Tie::IxHash from CPAN.
1182 tie(%myhash, Tie::IxHash);
1183 for ($i=0; $i<20; $i++) {
1186 @keys = keys %myhash;
1187 # @keys = (0,1,2,3,...)
1189 =head2 Why does passing a subroutine an undefined element in a hash create it?
1191 If you say something like:
1193 somefunc($hash{"nonesuch key here"});
1195 Then that element "autovivifies"; that is, it springs into existence
1196 whether you store something there or not. That's because functions
1197 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1198 it has to be ready to write it back into the caller's version.
1200 This has been fixed as of perl5.004.
1202 Normally, merely accessing a key's value for a nonexistent key does
1203 I<not> cause that key to be forever there. This is different than
1206 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1208 Use references (documented in L<perlref>). Examples of complex data
1209 structures are given in L<perldsc> and L<perllol>. Examples of
1210 structures and object-oriented classes are in L<perltoot>.
1212 =head2 How can I use a reference as a hash key?
1214 You can't do this directly, but you could use the standard Tie::Refhash
1215 module distributed with perl.
1219 =head2 How do I handle binary data correctly?
1221 Perl is binary clean, so this shouldn't be a problem. For example,
1222 this works fine (assuming the files are found):
1224 if (`cat /vmunix` =~ /gzip/) {
1225 print "Your kernel is GNU-zip enabled!\n";
1228 On some systems, however, you have to play tedious games with "text"
1229 versus "binary" files. See L<perlfunc/"binmode">.
1231 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1233 If you want to deal with multibyte characters, however, there are
1234 some gotchas. See the section on Regular Expressions.
1236 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1238 Assuming that you don't care about IEEE notations like "NaN" or
1239 "Infinity", you probably just want to use a regular expression.
1241 warn "has nondigits" if /\D/;
1242 warn "not a natural number" unless /^\d+$/; # rejects -3
1243 warn "not an integer" unless /^-?\d+$/; # rejects +3
1244 warn "not an integer" unless /^[+-]?\d+$/;
1245 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
1246 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
1247 warn "not a C float"
1248 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
1250 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1251 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1252 wrapper function for more convenient access. This function takes
1253 a string and returns the number it found, or C<undef> for input that
1254 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1255 if you just want to say, ``Is this a float?''
1258 use POSIX qw(strtod);
1263 my($num, $unparsed) = strtod($str);
1264 if (($str eq '') || ($unparsed != 0) || $!) {
1271 sub is_numeric { defined &getnum }
1273 Or you could check out
1274 http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
1275 instead. The POSIX module (part of the standard Perl distribution)
1276 provides the C<strtol> and C<strtod> for converting strings to double
1277 and longs, respectively.
1279 =head2 How do I keep persistent data across program calls?
1281 For some specific applications, you can use one of the DBM modules.
1282 See L<AnyDBM_File>. More generically, you should consult the
1283 FreezeThaw, Storable, or Class::Eroot modules from CPAN.
1285 =head2 How do I print out or copy a recursive data structure?
1287 The Data::Dumper module on CPAN is nice for printing out
1288 data structures, and FreezeThaw for copying them. For example:
1290 use FreezeThaw qw(freeze thaw);
1291 $new = thaw freeze $old;
1293 Where $old can be (a reference to) any kind of data structure you'd like.
1294 It will be deeply copied.
1296 =head2 How do I define methods for every class/object?
1298 Use the UNIVERSAL class (see L<UNIVERSAL>).
1300 =head2 How do I verify a credit card checksum?
1302 Get the Business::CreditCard module from CPAN.
1304 =head1 AUTHOR AND COPYRIGHT
1306 Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
1307 All rights reserved.
1309 When included as part of the Standard Version of Perl, or as part of
1310 its complete documentation whether printed or otherwise, this work
1311 may be distributed only under the terms of Perl's Artistic License.
1312 Any distribution of this file or derivatives thereof I<outside>
1313 of that package require that special arrangements be made with
1316 Irrespective of its distribution, all code examples in this file
1317 are hereby placed into the public domain. You are permitted and
1318 encouraged to use this code in your own programs for fun
1319 or for profit as you see fit. A simple comment in the code giving
1320 credit would be courteous but is not required.