3 perlfaq4 - Data Manipulation ($Revision: 1.26 $, $Date: 1998/08/05 12:04:00 $)
7 The section of the FAQ answers question related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximate on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to the internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers (see L<perlvar/"$#"> 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
65 certain number of digits, sprintf() or printf() is usually the easiest
68 printf("%.3f", 3.1415926535); # prints 3.142
70 The POSIX module (part of the standard perl distribution) implements
71 ceil(), floor(), and a number of other mathematical and trigonometric
75 $ceil = ceil(3.5); # 4
76 $floor = floor(3.5); # 3
78 In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex
79 module. With 5.004, the Math::Trig module (part of the standard perl
80 distribution) implements the trigonometric functions. Internally it
81 uses the Math::Complex module and some functions can break out from
82 the real axis into the complex plane, for example the inverse sine of
85 Rounding in financial applications can have serious implications, and
86 the rounding method used should be specified precisely. In these
87 cases, it probably pays not to trust whichever system rounding is
88 being used by Perl, but to instead implement the rounding function you
91 =head2 How do I convert bits into ints?
93 To turn a string of 1s and 0s like C<10110110> into a scalar containing
94 its binary value, use the pack() function (documented in
97 $decimal = pack('B8', '10110110');
99 Here's an example of going the other way:
101 $binary_string = join('', unpack('B*', "\x29"));
103 =head2 How do I multiply matrices?
105 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
106 or the PDL extension (also available from CPAN).
108 =head2 How do I perform an operation on a series of integers?
110 To call a function on each element in an array, and collect the
113 @results = map { my_func($_) } @array;
117 @triple = map { 3 * $_ } @single;
119 To call a function on each element of an array, but ignore the
122 foreach $iterator (@array) {
126 To call a function on each integer in a (small) range, you B<can> use:
128 @results = map { &my_func($_) } (5 .. 25);
130 but you should be aware that the C<..> operator creates an array of
131 all integers in the range. This can take a lot of memory for large
135 for ($i=5; $i < 500_005; $i++) {
136 push(@results, &my_func($i));
139 =head2 How can I output Roman numerals?
141 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
143 =head2 Why aren't my random numbers random?
145 The short explanation is that you're getting pseudorandom numbers, not
146 random ones, because computers are good at being predictable and bad
147 at being random (despite appearances caused by bugs in your programs
148 :-). A longer explanation is available on
149 http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
150 Phoenix. John von Neumann said, ``Anyone who attempts to generate
151 random numbers by deterministic means is, of course, living in a state
154 You should also check out the Math::TrulyRandom module from CPAN. It
155 uses the imperfections in your system's timer to generate random
156 numbers, but this takes quite a while. If you want a better
157 pseudorandom generator than comes with your operating system, look at
158 ``Numerical Recipes in C'' at http://nr.harvard.edu/nr/bookc.html .
162 =head2 How do I find the week-of-the-year/day-of-the-year?
164 The day of the year is in the array returned by localtime() (see
165 L<perlfunc/"localtime">):
167 $day_of_year = (localtime(time()))[7];
169 or more legibly (in 5.004 or higher):
172 $day_of_year = localtime(time())->yday;
174 You can find the week of the year by dividing this by 7:
176 $week_of_year = int($day_of_year / 7);
178 Of course, this believes that weeks start at zero. The Date::Calc
179 module from CPAN has a lot of date calculation functions, including
180 day of the year, week of the year, and so on. Note that not
181 all business consider ``week 1'' to be the same; for example,
182 American business often consider the first week with a Monday
183 in it to be Work Week #1, despite ISO 8601, which consider
184 WW1 to be the frist week with a Thursday in it.
186 =head2 How can I compare two dates and find the difference?
188 If you're storing your dates as epoch seconds then simply subtract one
189 from the other. If you've got a structured date (distinct year, day,
190 month, hour, minute, seconds values) then use one of the Date::Manip
191 and Date::Calc modules from CPAN.
193 =head2 How can I take a string and turn it into epoch seconds?
195 If it's a regular enough string that it always has the same format,
196 you can split it up and pass the parts to C<timelocal> in the standard
197 Time::Local module. Otherwise, you should look into the Date::Calc
198 and Date::Manip modules from CPAN.
200 =head2 How can I find the Julian Day?
202 Neither Date::Manip nor Date::Calc deal with Julian days. Instead,
203 there is an example of Julian date calculation that should help you in
204 http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz
207 =head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
209 Short answer: No, Perl does not have a Year 2000 problem. Yes,
210 Perl is Y2K compliant. The programmers you've hired to use it,
211 however, probably are not.
213 Long answer: Perl is just as Y2K compliant as your pencil--no more,
214 and no less. The date and time functions supplied with perl (gmtime
215 and localtime) supply adequate information to determine the year well
216 beyond 2000 (2038 is when trouble strikes for 32-bit machines). The
217 year returned by these functions when used in an array context is the
218 year minus 1900. For years between 1910 and 1999 this I<happens> to
219 be a 2-digit decimal number. To avoid the year 2000 problem simply do
220 not treat the year as a 2-digit number. It isn't.
222 When gmtime() and localtime() are used in scalar context they return
223 a timestamp string that contains a fully-expanded year. For example,
224 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
225 2001". There's no year 2000 problem here.
227 That doesn't mean that Perl can't be used to create non-Y2K compliant
228 programs. It can. But so can your pencil. It's the fault of the user,
229 not the language. At the risk of inflaming the NRA: ``Perl doesn't
230 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
235 =head2 How do I validate input?
237 The answer to this question is usually a regular expression, perhaps
238 with auxiliary logic. See the more specific questions (numbers, mail
239 addresses, etc.) for details.
241 =head2 How do I unescape a string?
243 It depends just what you mean by ``escape''. URL escapes are dealt
244 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
245 character are removed with:
249 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
251 =head2 How do I remove consecutive pairs of characters?
253 To turn C<"abbcccd"> into C<"abccd">:
257 =head2 How do I expand function calls in a string?
259 This is documented in L<perlref>. In general, this is fraught with
260 quoting and readability problems, but it is possible. To interpolate
261 a subroutine call (in list context) into a string:
263 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
265 If you prefer scalar context, similar chicanery is also useful for
266 arbitrary expressions:
268 print "That yields ${\($n + 5)} widgets\n";
270 Version 5.004 of Perl had a bug that gave list context to the
271 expression in C<${...}>, but this is fixed in version 5.005.
273 See also ``How can I expand variables in text strings?'' in this
276 =head2 How do I find matching/nesting anything?
278 This isn't something that can be done in one regular expression, no
279 matter how complicated. To find something between two single
280 characters, a pattern like C</x([^x]*)x/> will get the intervening
281 bits in $1. For multiple ones, then something more like
282 C</alpha(.*?)omega/> would be needed. But none of these deals with
283 nested patterns, nor can they. For that you'll have to write a
286 If you are serious about writing a parser, there are a number of
287 modules or oddities that will make your life a lot easier. There is
288 the CPAN module Parse::RecDescent, the standard module Text::Balanced,
289 the byacc program, and Mark-Jason Dominus's excellent I<py> tool at
290 http://www.plover.com/~mjd/perl/py/ .
292 One simple destructive, inside-out approach that you might try is to
293 pull out the smallest nesting parts one at a time:
295 while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) {
296 # do something with $1
299 =head2 How do I reverse a string?
301 Use reverse() in scalar context, as documented in
304 $reversed = reverse $string;
306 =head2 How do I expand tabs in a string?
308 You can do it yourself:
310 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
312 Or you can just use the Text::Tabs module (part of the standard perl
316 @expanded_lines = expand(@lines_with_tabs);
318 =head2 How do I reformat a paragraph?
320 Use Text::Wrap (part of the standard perl distribution):
323 print wrap("\t", ' ', @paragraphs);
325 The paragraphs you give to Text::Wrap should not contain embedded
326 newlines. Text::Wrap doesn't justify the lines (flush-right).
328 =head2 How can I access/change the first N letters of a string?
330 There are many ways. If you just want to grab a copy, use
333 $first_byte = substr($a, 0, 1);
335 If you want to modify part of a string, the simplest way is often to
336 use substr() as an lvalue:
338 substr($a, 0, 3) = "Tom";
340 Although those with a pattern matching kind of thought process will
345 =head2 How do I change the Nth occurrence of something?
347 You have to keep track of N yourself. For example, let's say you want
348 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
349 C<"whosoever"> or C<"whomsoever">, case insensitively.
353 ++$count == 5 # is it the 5th?
354 ? "${2}soever" # yes, swap
355 : $1 # renege and leave it there
358 In the more general case, you can use the C</g> modifier in a C<while>
359 loop, keeping count of matches.
363 while (/(\w+)\s+fish\b/gi) {
364 if (++$count == $WANT) {
365 print "The third fish is a $1 one.\n";
366 # Warning: don't `last' out of this loop
370 That prints out: C<"The third fish is a red one."> You can also use a
371 repetition count and repeated pattern like this:
373 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
375 =head2 How can I count the number of occurrences of a substring within a string?
377 There are a number of ways, with varying efficiency: If you want a
378 count of a certain single character (X) within a string, you can use the
379 C<tr///> function like so:
381 $string = "ThisXlineXhasXsomeXx'sXinXit";
382 $count = ($string =~ tr/X//);
383 print "There are $count X charcters in the string";
385 This is fine if you are just looking for a single character. However,
386 if you are trying to count multiple character substrings within a
387 larger string, C<tr///> won't work. What you can do is wrap a while()
388 loop around a global pattern match. For example, let's count negative
391 $string = "-9 55 48 -2 23 -76 4 14 -44";
392 while ($string =~ /-\d+/g) { $count++ }
393 print "There are $count negative numbers in the string";
395 =head2 How do I capitalize all the words on one line?
397 To make the first letter of each word upper case:
399 $line =~ s/\b(\w)/\U$1/g;
401 This has the strange effect of turning "C<don't do it>" into "C<Don'T
402 Do It>". Sometimes you might want this, instead (Suggested by Brian
406 (^\w) #at the beginning of the line
408 (\s\w) #preceded by whitespace
411 $string =~ /([\w']+)/\u\L$1/g;
413 To make the whole line upper case:
417 To force each word to be lower case, with the first letter upper case:
419 $line =~ s/(\w+)/\u\L$1/g;
421 You can (and probably should) enable locale awareness of those
422 characters by placing a C<use locale> pragma in your program.
423 See L<perllocale> for endless details on locales.
425 =head2 How can I split a [character] delimited string except when inside
426 [character]? (Comma-separated files)
428 Take the example case of trying to split a string that is comma-separated
429 into its different fields. (We'll pretend you said comma-separated, not
430 comma-delimited, which is different and almost never what you mean.) You
431 can't use C<split(/,/)> because you shouldn't split if the comma is inside
432 quotes. For example, take a data line like this:
434 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
436 Due to the restriction of the quotes, this is a fairly complex
437 problem. Thankfully, we have Jeffrey Friedl, author of a highly
438 recommended book on regular expressions, to handle these for us. He
439 suggests (assuming your string is contained in $text):
442 push(@new, $+) while $text =~ m{
443 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
447 push(@new, undef) if substr($text,-1,1) eq ',';
449 If you want to represent quotation marks inside a
450 quotation-mark-delimited field, escape them with backslashes (eg,
451 C<"like \"this\"">. Unescaping them is a task addressed earlier in
454 Alternatively, the Text::ParseWords module (part of the standard perl
455 distribution) lets you say:
457 use Text::ParseWords;
458 @new = quotewords(",", 0, $text);
460 =head2 How do I strip blank space from the beginning/end of a string?
462 Although the simplest approach would seem to be:
464 $string =~ s/^\s*(.*?)\s*$/$1/;
466 This is unneccesarily slow, destructive, and fails with embedded newlines.
467 It is much better faster to do this in two steps:
472 Or more nicely written as:
479 This idiom takes advantage of the C<foreach> loop's aliasing
480 behavior to factor out common code. You can do this
481 on several strings at once, or arrays, or even the
482 values of a hash if you use a slide:
484 # trim whitespace in the scalar, the array,
485 # and all the values in the hash
486 foreach ($scalar, @array, @hash{keys %hash}) {
491 =head2 How do I extract selected columns from a string?
493 Use substr() or unpack(), both documented in L<perlfunc>.
494 If you prefer thinking in terms of columns instead of widths,
495 you can use this kind of thing:
497 # determine the unpack format needed to split Linux ps output
498 # arguments are cut columns
499 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
505 for my $place (@positions) {
506 $template .= "A" . ($place - $lastpos) . " ";
513 =head2 How do I find the soundex value of a string?
515 Use the standard Text::Soundex module distributed with perl.
517 =head2 How can I expand variables in text strings?
519 Let's assume that you have a string like:
521 $text = 'this has a $foo in it and a $bar';
523 If those were both global variables, then this would
526 $text =~ s/\$(\w+)/${$1}/g;
528 But since they are probably lexicals, or at least, they could
529 be, you'd have to do this:
531 $text =~ s/(\$\w+)/$1/eeg;
532 die if $@; # needed on /ee, not /e
534 It's probably better in the general case to treat those
535 variables as entries in some special hash. For example:
541 $text =~ s/\$(\w+)/$user_defs{$1}/g;
543 See also ``How do I expand function calls in a string?'' in this section
546 =head2 What's wrong with always quoting "$vars"?
548 The problem is that those double-quotes force stringification,
549 coercing numbers and references into strings, even when you
550 don't want them to be.
552 If you get used to writing odd things like these:
556 somefunc("$var"); # BAD
558 You'll be in trouble. Those should (in 99.8% of the cases) be
559 the simpler and more direct:
565 Otherwise, besides slowing you down, you're going to break code when
566 the thing in the scalar is actually neither a string nor a number, but
572 my $oref = "$aref"; # WRONG
575 You can also get into subtle problems on those few operations in Perl
576 that actually do care about the difference between a string and a
577 number, such as the magical C<++> autoincrement operator or the
580 Stringification also destroys arrays.
583 print "@lines"; # WRONG - extra blanks
584 print @lines; # right
586 =head2 Why don't my <<HERE documents work?
588 Check for these three things:
592 =item 1. There must be no space after the << part.
594 =item 2. There (probably) should be a semicolon at the end.
596 =item 3. You can't (easily) have any space in front of the tag.
600 If you want to indent the text in the here document, you
604 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
609 But the HERE_TARGET must still be flush against the margin.
610 If you want that indented also, you'll have to quote
613 ($quote = <<' FINIS') =~ s/^\s+//gm;
614 ...we will have peace, when you and all your works have
615 perished--and the works of your dark master to whom you
616 would deliver us. You are a liar, Saruman, and a corrupter
617 of men's hearts. --Theoden in /usr/src/perl/taint.c
619 $quote =~ s/\s*--/\n--/;
621 A nice general-purpose fixer-upper function for indented here documents
622 follows. It expects to be called with a here document as its argument.
623 It looks to see whether each line begins with a common substring, and
624 if so, strips that off. Otherwise, it takes the amount of leading
625 white space found on the first line and removes that much off each
630 my ($white, $leader); # common white space and common leading string
631 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
632 ($white, $leader) = ($2, quotemeta($1));
634 ($white, $leader) = (/^(\s+)/, '');
636 s/^\s*?$leader(?:$white)?//gm;
640 This works with leading special strings, dynamically determined:
642 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
645 @@@ SAVEI32(runlevel);
647 @@@ while ( op = (*op->op_ppaddr)() ) ;
651 MAIN_INTERPRETER_LOOP
653 Or with a fixed amount of leading white space, with remaining
654 indentation correctly preserved:
656 $poem = fix<<EVER_ON_AND_ON;
657 Now far ahead the Road has gone,
658 And I must follow, if I can,
659 Pursuing it with eager feet,
660 Until it joins some larger way
661 Where many paths and errands meet.
662 And whither then? I cannot say.
663 --Bilbo in /usr/src/perl/pp_ctl.c
668 =head2 What is the difference between $array[1] and @array[1]?
670 The former is a scalar value, the latter an array slice, which makes
671 it a list with one (scalar) value. You should use $ when you want a
672 scalar value (most of the time) and @ when you want a list with one
673 scalar value in it (very, very rarely; nearly never, in fact).
675 Sometimes it doesn't make a difference, but sometimes it does.
676 For example, compare:
678 $good[0] = `some program that outputs several lines`;
682 @bad[0] = `same program that outputs several lines`;
684 The B<-w> flag will warn you about these matters.
686 =head2 How can I extract just the unique elements of an array?
688 There are several possible ways, depending on whether the array is
689 ordered and whether you wish to preserve the ordering.
693 =item a) If @in is sorted, and you want @out to be sorted:
694 (this assumes all true values in the array)
697 @out = grep($_ ne $prev && ($prev = $_), @in);
699 This is nice in that it doesn't use much extra memory, simulating
700 uniq(1)'s behavior of removing only adjacent duplicates. It's less
701 nice in that it won't work with false values like undef, 0, or "";
702 "0 but true" is ok, though.
704 =item b) If you don't know whether @in is sorted:
707 @out = grep(!$saw{$_}++, @in);
709 =item c) Like (b), but @in contains only small integers:
711 @out = grep(!$saw[$_]++, @in);
713 =item d) A way to do (b) without any loops or greps:
717 @out = sort keys %saw; # remove sort if undesired
719 =item e) Like (d), but @in contains only small positive integers:
727 =head2 How can I tell whether a list or array contains a certain element?
729 Hearing the word "in" is an I<in>dication that you probably should have
730 used a hash, not a list or array, to store your data. Hashes are
731 designed to answer this question quickly and efficiently. Arrays aren't.
733 That being said, there are several ways to approach this. If you
734 are going to make this query many times over arbitrary string values,
735 the fastest way is probably to invert the original array and keep an
736 associative array lying about whose keys are the first array's values.
738 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
740 for (@blues) { $is_blue{$_} = 1 }
742 Now you can check whether $is_blue{$some_color}. It might have been a
743 good idea to keep the blues all in a hash in the first place.
745 If the values are all small integers, you could use a simple indexed
746 array. This kind of an array will take up less space:
748 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
749 undef @is_tiny_prime;
750 for (@primes) { $is_tiny_prime[$_] = 1; }
752 Now you check whether $is_tiny_prime[$some_number].
754 If the values in question are integers instead of strings, you can save
755 quite a lot of space by using bit strings instead:
757 @articles = ( 1..10, 150..2000, 2017 );
759 for (@articles) { vec($read,$_,1) = 1 }
761 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
765 $is_there = grep $_ eq $whatever, @array;
769 $is_there = grep /$whatever/, @array;
771 These are slow (checks every element even if the first matches),
772 inefficient (same reason), and potentially buggy (what if there are
773 regexp characters in $whatever?).
775 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
777 Use a hash. Here's code to do both and more. It assumes that
778 each element is unique in a given array:
780 @union = @intersection = @difference = ();
782 foreach $element (@array1, @array2) { $count{$element}++ }
783 foreach $element (keys %count) {
784 push @union, $element;
785 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
788 =head2 How do I find the first array element for which a condition is true?
790 You can use this if you care about the index:
792 for ($i=0; $i < @array; $i++) {
793 if ($array[$i] eq "Waldo") {
799 Now C<$found_index> has what you want.
801 =head2 How do I handle linked lists?
803 In general, you usually don't need a linked list in Perl, since with
804 regular arrays, you can push and pop or shift and unshift at either end,
805 or you can use splice to add and/or remove arbitrary number of elements at
806 arbitrary points. Both pop and shift are both O(1) operations on perl's
807 dynamic arrays. In the absence of shifts and pops, push in general
808 needs to reallocate on the order every log(N) times, and unshift will
809 need to copy pointers each time.
811 If you really, really wanted, you could use structures as described in
812 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
815 =head2 How do I handle circular lists?
817 Circular lists could be handled in the traditional fashion with linked
818 lists, or you could just do something like this with an array:
820 unshift(@array, pop(@array)); # the last shall be first
821 push(@array, shift(@array)); # and vice versa
823 =head2 How do I shuffle an array randomly?
827 # fisher_yates_shuffle( \@array ) :
828 # generate a random permutation of @array in place
829 sub fisher_yates_shuffle {
832 for ($i = @$array; --$i; ) {
833 my $j = int rand ($i+1);
835 @$array[$i,$j] = @$array[$j,$i];
839 fisher_yates_shuffle( \@array ); # permutes @array in place
841 You've probably seen shuffling algorithms that works using splice,
842 randomly picking another element to swap the current element with:
846 @old = 1 .. 10; # just a demo
848 push(@new, splice(@old, rand @old, 1));
851 This is bad because splice is already O(N), and since you do it N times,
852 you just invented a quadratic algorithm; that is, O(N**2). This does
853 not scale, although Perl is so efficient that you probably won't notice
854 this until you have rather largish arrays.
856 =head2 How do I process/modify each element of an array?
858 Use C<for>/C<foreach>:
861 s/foo/bar/; # change that word
862 y/XZ/ZX/; # swap those letters
865 Here's another; let's compute spherical volumes:
867 for (@volumes = @radii) { # @volumes has changed parts
869 $_ *= (4/3) * 3.14159; # this will be constant folded
872 If you want to do the same thing to modify the values of the hash,
873 you may not use the C<values> function, oddly enough. You need a slice:
875 for $orbit ( @orbits{keys %orbits} ) {
876 ($orbit **= 3) *= (4/3) * 3.14159;
879 =head2 How do I select a random element from an array?
881 Use the rand() function (see L<perlfunc/rand>):
883 # at the top of the program:
884 srand; # not needed for 5.004 and later
887 $index = rand @array;
888 $element = $array[$index];
890 Make sure you I<only call srand once per program, if then>.
891 If you are calling it more than once (such as before each
892 call to rand), you're almost certainly doing something wrong.
894 =head2 How do I permute N elements of a list?
896 Here's a little program that generates all permutations
897 of all the words on each line of input. The algorithm embodied
898 in the permute() function should work on any list:
901 # tsc-permute: permute each word of input
902 permute([split], []);
904 my @items = @{ $_[0] };
905 my @perms = @{ $_[1] };
909 my(@newitems,@newperms,$i);
910 foreach $i (0 .. $#items) {
913 unshift(@newperms, splice(@newitems, $i, 1));
914 permute([@newitems], [@newperms]);
919 =head2 How do I sort an array by (anything)?
921 Supply a comparison function to sort() (described in L<perlfunc/sort>):
923 @list = sort { $a <=> $b } @list;
925 The default sort function is cmp, string comparison, which would
926 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
927 the numerical comparison operator.
929 If you have a complicated function needed to pull out the part you
930 want to sort on, then don't do it inside the sort function. Pull it
931 out first, because the sort BLOCK can be called many times for the
932 same element. Here's an example of how to pull out the first word
933 after the first number on each item, and then sort those words
938 ($item) = /\d+\s*(\S+)/;
939 push @idx, uc($item);
941 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
943 Which could also be written this way, using a trick
944 that's come to be known as the Schwartzian Transform:
946 @sorted = map { $_->[0] }
947 sort { $a->[1] cmp $b->[1] }
948 map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
950 If you need to sort on several fields, the following paradigm is useful.
952 @sorted = sort { field1($a) <=> field1($b) ||
953 field2($a) cmp field2($b) ||
954 field3($a) cmp field3($b)
957 This can be conveniently combined with precalculation of keys as given
960 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
963 See also the question below on sorting hashes.
965 =head2 How do I manipulate arrays of bits?
967 Use pack() and unpack(), or else vec() and the bitwise operations.
969 For example, this sets $vec to have bit N set if $ints[N] was set:
972 foreach(@ints) { vec($vec,$_,1) = 1 }
974 And here's how, given a vector in $vec, you can
975 get those bits into your @ints array:
980 # Find null-byte density then select best algorithm
981 if ($vec =~ tr/\0// / length $vec > 0.95) {
984 # This method is faster with mostly null-bytes
985 while($vec =~ /[^\0]/g ) {
986 $i = -9 + 8 * pos $vec;
987 push @ints, $i if vec($vec, ++$i, 1);
988 push @ints, $i if vec($vec, ++$i, 1);
989 push @ints, $i if vec($vec, ++$i, 1);
990 push @ints, $i if vec($vec, ++$i, 1);
991 push @ints, $i if vec($vec, ++$i, 1);
992 push @ints, $i if vec($vec, ++$i, 1);
993 push @ints, $i if vec($vec, ++$i, 1);
994 push @ints, $i if vec($vec, ++$i, 1);
997 # This method is a fast general algorithm
999 my $bits = unpack "b*", $vec;
1000 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1001 push @ints, pos $bits while($bits =~ /1/g);
1006 This method gets faster the more sparse the bit vector is.
1007 (Courtesy of Tim Bunce and Winfried Koenig.)
1009 =head2 Why does defined() return true on empty arrays and hashes?
1011 See L<perlfunc/defined> in the 5.004 release or later of Perl.
1013 =head1 Data: Hashes (Associative Arrays)
1015 =head2 How do I process an entire hash?
1017 Use the each() function (see L<perlfunc/each>) if you don't care
1018 whether it's sorted:
1020 while ( ($key, $value) = each %hash) {
1021 print "$key = $value\n";
1024 If you want it sorted, you'll have to use foreach() on the result of
1025 sorting the keys as shown in an earlier question.
1027 =head2 What happens if I add or remove keys from a hash while iterating over it?
1031 =head2 How do I look up a hash element by value?
1033 Create a reverse hash:
1035 %by_value = reverse %by_key;
1036 $key = $by_value{$value};
1038 That's not particularly efficient. It would be more space-efficient
1041 while (($key, $value) = each %by_key) {
1042 $by_value{$value} = $key;
1045 If your hash could have repeated values, the methods above will only
1046 find one of the associated keys. This may or may not worry you.
1048 =head2 How can I know how many entries are in a hash?
1050 If you mean how many keys, then all you have to do is
1051 take the scalar sense of the keys() function:
1053 $num_keys = scalar keys %hash;
1055 In void context it just resets the iterator, which is faster
1058 =head2 How do I sort a hash (optionally by value instead of key)?
1060 Internally, hashes are stored in a way that prevents you from imposing
1061 an order on key-value pairs. Instead, you have to sort a list of the
1064 @keys = sort keys %hash; # sorted by key
1066 $hash{$a} cmp $hash{$b}
1067 } keys %hash; # and by value
1069 Here we'll do a reverse numeric sort by value, and if two keys are
1070 identical, sort by length of key, and if that fails, by straight ASCII
1071 comparison of the keys (well, possibly modified by your locale -- see
1075 $hash{$b} <=> $hash{$a}
1077 length($b) <=> length($a)
1082 =head2 How can I always keep my hash sorted?
1084 You can look into using the DB_File module and tie() using the
1085 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1086 The Tie::IxHash module from CPAN might also be instructive.
1088 =head2 What's the difference between "delete" and "undef" with hashes?
1090 Hashes are pairs of scalars: the first is the key, the second is the
1091 value. The key will be coerced to a string, although the value can be
1092 any kind of scalar: string, number, or reference. If a key C<$key> is
1093 present in the array, C<exists($key)> will return true. The value for
1094 a given key can be C<undef>, in which case C<$array{$key}> will be
1095 C<undef> while C<$exists{$key}> will return true. This corresponds to
1096 (C<$key>, C<undef>) being in the hash.
1098 Pictures help... here's the C<%ary> table:
1108 And these conditions hold
1112 defined $ary{'d'} is true
1113 defined $ary{'a'} is true
1114 exists $ary{'a'} is true (perl5 only)
1115 grep ($_ eq 'a', keys %ary) is true
1121 your table now reads:
1132 and these conditions now hold; changes in caps:
1136 defined $ary{'d'} is true
1137 defined $ary{'a'} is FALSE
1138 exists $ary{'a'} is true (perl5 only)
1139 grep ($_ eq 'a', keys %ary) is true
1141 Notice the last two: you have an undef value, but a defined key!
1147 your table now reads:
1156 and these conditions now hold; changes in caps:
1160 defined $ary{'d'} is true
1161 defined $ary{'a'} is false
1162 exists $ary{'a'} is FALSE (perl5 only)
1163 grep ($_ eq 'a', keys %ary) is FALSE
1165 See, the whole entry is gone!
1167 =head2 Why don't my tied hashes make the defined/exists distinction?
1169 They may or may not implement the EXISTS() and DEFINED() methods
1170 differently. For example, there isn't the concept of undef with hashes
1171 that are tied to DBM* files. This means the true/false tables above
1172 will give different results when used on such a hash. It also means
1173 that exists and defined do the same thing with a DBM* file, and what
1174 they end up doing is not what they do with ordinary hashes.
1176 =head2 How do I reset an each() operation part-way through?
1178 Using C<keys %hash> in scalar context returns the number of keys in
1179 the hash I<and> resets the iterator associated with the hash. You may
1180 need to do this if you use C<last> to exit a loop early so that when you
1181 re-enter it, the hash iterator has been reset.
1183 =head2 How can I get the unique keys from two hashes?
1185 First you extract the keys from the hashes into arrays, and then solve
1186 the uniquifying the array problem described above. For example:
1189 for $element (keys(%foo), keys(%bar)) {
1196 @uniq = keys %{{%foo,%bar}};
1198 Or if you really want to save space:
1201 while (defined ($key = each %foo)) {
1204 while (defined ($key = each %bar)) {
1209 =head2 How can I store a multidimensional array in a DBM file?
1211 Either stringify the structure yourself (no fun), or else
1212 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1213 it on top of either DB_File or GDBM_File.
1215 =head2 How can I make my hash remember the order I put elements into it?
1217 Use the Tie::IxHash from CPAN.
1220 tie(%myhash, Tie::IxHash);
1221 for ($i=0; $i<20; $i++) {
1224 @keys = keys %myhash;
1225 # @keys = (0,1,2,3,...)
1227 =head2 Why does passing a subroutine an undefined element in a hash create it?
1229 If you say something like:
1231 somefunc($hash{"nonesuch key here"});
1233 Then that element "autovivifies"; that is, it springs into existence
1234 whether you store something there or not. That's because functions
1235 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1236 it has to be ready to write it back into the caller's version.
1238 This has been fixed as of perl5.004.
1240 Normally, merely accessing a key's value for a nonexistent key does
1241 I<not> cause that key to be forever there. This is different than
1244 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1246 Use references (documented in L<perlref>). Examples of complex data
1247 structures are given in L<perldsc> and L<perllol>. Examples of
1248 structures and object-oriented classes are in L<perltoot>.
1250 =head2 How can I use a reference as a hash key?
1252 You can't do this directly, but you could use the standard Tie::Refhash
1253 module distributed with perl.
1257 =head2 How do I handle binary data correctly?
1259 Perl is binary clean, so this shouldn't be a problem. For example,
1260 this works fine (assuming the files are found):
1262 if (`cat /vmunix` =~ /gzip/) {
1263 print "Your kernel is GNU-zip enabled!\n";
1266 On some systems, however, you have to play tedious games with "text"
1267 versus "binary" files. See L<perlfunc/"binmode">.
1269 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1271 If you want to deal with multibyte characters, however, there are
1272 some gotchas. See the section on Regular Expressions.
1274 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1276 Assuming that you don't care about IEEE notations like "NaN" or
1277 "Infinity", you probably just want to use a regular expression.
1279 warn "has nondigits" if /\D/;
1280 warn "not a natural number" unless /^\d+$/; # rejects -3
1281 warn "not an integer" unless /^-?\d+$/; # rejects +3
1282 warn "not an integer" unless /^[+-]?\d+$/;
1283 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
1284 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
1285 warn "not a C float"
1286 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
1288 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1289 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1290 wrapper function for more convenient access. This function takes
1291 a string and returns the number it found, or C<undef> for input that
1292 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1293 if you just want to say, ``Is this a float?''
1296 use POSIX qw(strtod);
1301 my($num, $unparsed) = strtod($str);
1302 if (($str eq '') || ($unparsed != 0) || $!) {
1309 sub is_numeric { defined &getnum }
1311 Or you could check out
1312 http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
1313 instead. The POSIX module (part of the standard Perl distribution)
1314 provides the C<strtol> and C<strtod> for converting strings to double
1315 and longs, respectively.
1317 =head2 How do I keep persistent data across program calls?
1319 For some specific applications, you can use one of the DBM modules.
1320 See L<AnyDBM_File>. More generically, you should consult the
1321 FreezeThaw, Storable, or Class::Eroot modules from CPAN.
1323 =head2 How do I print out or copy a recursive data structure?
1325 The Data::Dumper module on CPAN is nice for printing out
1326 data structures, and FreezeThaw for copying them. For example:
1328 use FreezeThaw qw(freeze thaw);
1329 $new = thaw freeze $old;
1331 Where $old can be (a reference to) any kind of data structure you'd like.
1332 It will be deeply copied.
1334 =head2 How do I define methods for every class/object?
1336 Use the UNIVERSAL class (see L<UNIVERSAL>).
1338 =head2 How do I verify a credit card checksum?
1340 Get the Business::CreditCard module from CPAN.
1342 =head1 AUTHOR AND COPYRIGHT
1344 Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
1345 All rights reserved.
1347 When included as part of the Standard Version of Perl, or as part of
1348 its complete documentation whether printed or otherwise, this work
1349 may be distributed only under the terms of Perl's Artistic License.
1350 Any distribution of this file or derivatives thereof I<outside>
1351 of that package require that special arrangements be made with
1354 Irrespective of its distribution, all code examples in this file
1355 are hereby placed into the public domain. You are permitted and
1356 encouraged to use this code in your own programs for fun
1357 or for profit as you see fit. A simple comment in the code giving
1358 credit would be courteous but is not required.