3 perlfaq4 - Data Manipulation ($Revision: 1.25 $, $Date: 1998/07/16 22:49:55 $)
7 The section of the FAQ answers question related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximate on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to the internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers (see L<perlvar/"$#"> 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.
182 =head2 How can I compare two dates and find the difference?
184 If you're storing your dates as epoch seconds then simply subtract one
185 from the other. If you've got a structured date (distinct year, day,
186 month, hour, minute, seconds values) then use one of the Date::Manip
187 and Date::Calc modules from CPAN.
189 =head2 How can I take a string and turn it into epoch seconds?
191 If it's a regular enough string that it always has the same format,
192 you can split it up and pass the parts to C<timelocal> in the standard
193 Time::Local module. Otherwise, you should look into the Date::Calc
194 and Date::Manip modules from CPAN.
196 =head2 How can I find the Julian Day?
198 Neither Date::Manip nor Date::Calc deal with Julian days. Instead,
199 there is an example of Julian date calculation that should help you in
200 http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz
203 =head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
205 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl
208 Long answer: Perl is just as Y2K compliant as your pencil--no more,
209 and no less. The date and time functions supplied with perl (gmtime
210 and localtime) supply adequate information to determine the year well
211 beyond 2000 (2038 is when trouble strikes for 32-bit machines). The
212 year returned by these functions when used in an array context is the
213 year minus 1900. For years between 1910 and 1999 this I<happens> to
214 be a 2-digit decimal number. To avoid the year 2000 problem simply do
215 not treat the year as a 2-digit number. It isn't.
217 When gmtime() and localtime() are used in scalar context they return
218 a timestamp string that contains a fully-expanded year. For example,
219 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
220 2001". There's no year 2000 problem here.
222 That doesn't mean that Perl can't be used to create non-Y2K compliant
223 programs. It can. But so can your pencil. It's the fault of the user,
224 not the language. At the risk of inflaming the NRA: ``Perl doesn't
225 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
230 =head2 How do I validate input?
232 The answer to this question is usually a regular expression, perhaps
233 with auxiliary logic. See the more specific questions (numbers, mail
234 addresses, etc.) for details.
236 =head2 How do I unescape a string?
238 It depends just what you mean by ``escape''. URL escapes are dealt
239 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
240 character are removed with:
244 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
246 =head2 How do I remove consecutive pairs of characters?
248 To turn C<"abbcccd"> into C<"abccd">:
252 =head2 How do I expand function calls in a string?
254 This is documented in L<perlref>. In general, this is fraught with
255 quoting and readability problems, but it is possible. To interpolate
256 a subroutine call (in list context) into a string:
258 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
260 If you prefer scalar context, similar chicanery is also useful for
261 arbitrary expressions:
263 print "That yields ${\($n + 5)} widgets\n";
265 Version 5.004 of Perl had a bug that gave list context to the
266 expression in C<${...}>, but this is fixed in version 5.005.
268 See also ``How can I expand variables in text strings?'' in this
271 =head2 How do I find matching/nesting anything?
273 This isn't something that can be done in one regular expression, no
274 matter how complicated. To find something between two single
275 characters, a pattern like C</x([^x]*)x/> will get the intervening
276 bits in $1. For multiple ones, then something more like
277 C</alpha(.*?)omega/> would be needed. But none of these deals with
278 nested patterns, nor can they. For that you'll have to write a
281 If you are serious about writing a parser, there are a number of
282 modules or oddities that will make your life a lot easier. There is
283 the CPAN module Parse::RecDescent, the standard module Text::Balanced,
284 the byacc program, and Mark-Jason Dominus's excellent I<py> tool at
285 http://www.plover.com/~mjd/perl/py/ .
287 One simple destructive, inside-out approach that you might try is to
288 pull out the smallest nesting parts one at a time:
290 while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) {
291 # do something with $1
294 =head2 How do I reverse a string?
296 Use reverse() in scalar context, as documented in
299 $reversed = reverse $string;
301 =head2 How do I expand tabs in a string?
303 You can do it yourself:
305 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
307 Or you can just use the Text::Tabs module (part of the standard perl
311 @expanded_lines = expand(@lines_with_tabs);
313 =head2 How do I reformat a paragraph?
315 Use Text::Wrap (part of the standard perl distribution):
318 print wrap("\t", ' ', @paragraphs);
320 The paragraphs you give to Text::Wrap should not contain embedded
321 newlines. Text::Wrap doesn't justify the lines (flush-right).
323 =head2 How can I access/change the first N letters of a string?
325 There are many ways. If you just want to grab a copy, use
328 $first_byte = substr($a, 0, 1);
330 If you want to modify part of a string, the simplest way is often to
331 use substr() as an lvalue:
333 substr($a, 0, 3) = "Tom";
335 Although those with a pattern matching kind of thought process will
340 =head2 How do I change the Nth occurrence of something?
342 You have to keep track of N yourself. For example, let's say you want
343 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
344 C<"whosoever"> or C<"whomsoever">, case insensitively.
348 ++$count == 5 # is it the 5th?
349 ? "${2}soever" # yes, swap
350 : $1 # renege and leave it there
353 In the more general case, you can use the C</g> modifier in a C<while>
354 loop, keeping count of matches.
358 while (/(\w+)\s+fish\b/gi) {
359 if (++$count == $WANT) {
360 print "The third fish is a $1 one.\n";
361 # Warning: don't `last' out of this loop
365 That prints out: C<"The third fish is a red one."> You can also use a
366 repetition count and repeated pattern like this:
368 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
370 =head2 How can I count the number of occurrences of a substring within a string?
372 There are a number of ways, with varying efficiency: If you want a
373 count of a certain single character (X) within a string, you can use the
374 C<tr///> function like so:
376 $string = "ThisXlineXhasXsomeXx'sXinXit":
377 $count = ($string =~ tr/X//);
378 print "There are $count X charcters in the string";
380 This is fine if you are just looking for a single character. However,
381 if you are trying to count multiple character substrings within a
382 larger string, C<tr///> won't work. What you can do is wrap a while()
383 loop around a global pattern match. For example, let's count negative
386 $string = "-9 55 48 -2 23 -76 4 14 -44";
387 while ($string =~ /-\d+/g) { $count++ }
388 print "There are $count negative numbers in the string";
390 =head2 How do I capitalize all the words on one line?
392 To make the first letter of each word upper case:
394 $line =~ s/\b(\w)/\U$1/g;
396 This has the strange effect of turning "C<don't do it>" into "C<Don'T
397 Do It>". Sometimes you might want this, instead (Suggested by Brian
401 (^\w) #at the beginning of the line
403 (\s\w) #preceded by whitespace
406 $string =~ /([\w']+)/\u\L$1/g;
408 To make the whole line upper case:
412 To force each word to be lower case, with the first letter upper case:
414 $line =~ s/(\w+)/\u\L$1/g;
416 You can (and probably should) enable locale awareness of those
417 characters by placing a C<use locale> pragma in your program.
418 See L<perllocale> for endless details on locales.
420 =head2 How can I split a [character] delimited string except when inside
421 [character]? (Comma-separated files)
423 Take the example case of trying to split a string that is comma-separated
424 into its different fields. (We'll pretend you said comma-separated, not
425 comma-delimited, which is different and almost never what you mean.) You
426 can't use C<split(/,/)> because you shouldn't split if the comma is inside
427 quotes. For example, take a data line like this:
429 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
431 Due to the restriction of the quotes, this is a fairly complex
432 problem. Thankfully, we have Jeffrey Friedl, author of a highly
433 recommended book on regular expressions, to handle these for us. He
434 suggests (assuming your string is contained in $text):
437 push(@new, $+) while $text =~ m{
438 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
442 push(@new, undef) if substr($text,-1,1) eq ',';
444 If you want to represent quotation marks inside a
445 quotation-mark-delimited field, escape them with backslashes (eg,
446 C<"like \"this\"">. Unescaping them is a task addressed earlier in
449 Alternatively, the Text::ParseWords module (part of the standard perl
450 distribution) lets you say:
452 use Text::ParseWords;
453 @new = quotewords(",", 0, $text);
455 =head2 How do I strip blank space from the beginning/end of a string?
457 Although the simplest approach would seem to be:
459 $string =~ s/^\s*(.*?)\s*$/$1/;
461 This is unneccesarily slow, destructive, and fails with embedded newlines.
462 It is much better faster to do this in two steps:
467 Or more nicely written as:
474 This idiom takes advantage of the C<for(each)> loop's aliasing
475 behavior to factor out common code. You can do this
476 on several strings at once, or arrays, or even the
477 values of a hash if you use a slide:
479 # trim whitespace in the scalar, the array,
480 # and all the values in the hash
481 foreach ($scalar, @array, @hash{keys %hash}) {
486 =head2 How do I extract selected columns from a string?
488 Use substr() or unpack(), both documented in L<perlfunc>.
489 If you prefer thinking in terms of columns instead of widths,
490 you can use this kind of thing:
492 # determine the unpack format needed to split Linux ps output
493 # arguments are cut columns
494 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
500 for my $place (@positions) {
501 $template .= "A" . ($place - $lastpos) . " ";
508 =head2 How do I find the soundex value of a string?
510 Use the standard Text::Soundex module distributed with perl.
512 =head2 How can I expand variables in text strings?
514 Let's assume that you have a string like:
516 $text = 'this has a $foo in it and a $bar';
518 If those were both global variables, then this would
521 $text =~ s/\$(\w+)/${$1}/g;
523 But since they are probably lexicals, or at least, they could
524 be, you'd have to do this:
526 $text =~ s/(\$\w+)/$1/eeg;
527 die if $@; # needed on /ee, not /e
529 It's probably better in the general case to treat those
530 variables as entries in some special hash. For example:
536 $text =~ s/\$(\w+)/$user_defs{$1}/g;
538 See also ``How do I expand function calls in a string?'' in this section
541 =head2 What's wrong with always quoting "$vars"?
543 The problem is that those double-quotes force stringification,
544 coercing numbers and references into strings, even when you
545 don't want them to be.
547 If you get used to writing odd things like these:
551 somefunc("$var"); # BAD
553 You'll be in trouble. Those should (in 99.8% of the cases) be
554 the simpler and more direct:
560 Otherwise, besides slowing you down, you're going to break code when
561 the thing in the scalar is actually neither a string nor a number, but
567 my $oref = "$aref"; # WRONG
570 You can also get into subtle problems on those few operations in Perl
571 that actually do care about the difference between a string and a
572 number, such as the magical C<++> autoincrement operator or the
575 Stringification also destroys arrays.
578 print "@lines"; # WRONG - extra blanks
579 print @lines; # right
581 =head2 Why don't my <<HERE documents work?
583 Check for these three things:
587 =item 1. There must be no space after the << part.
589 =item 2. There (probably) should be a semicolon at the end.
591 =item 3. You can't (easily) have any space in front of the tag.
595 If you want to indent the text in the here document, you
599 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
604 But the HERE_TARGET must still be flush against the margin.
605 If you want that indented also, you'll have to quote
608 ($quote = <<' FINIS') =~ s/^\s+//gm;
609 ...we will have peace, when you and all your works have
610 perished--and the works of your dark master to whom you
611 would deliver us. You are a liar, Saruman, and a corrupter
612 of men's hearts. --Theoden in /usr/src/perl/taint.c
614 $quote =~ s/\s*--/\n--/;
616 A nice general-purpose fixer-upper function for indented here documents
617 follows. It expects to be called with a here document as its argument.
618 It looks to see whether each line begins with a common substring, and
619 if so, strips that off. Otherwise, it takes the amount of leading
620 white space found on the first line and removes that much off each
625 my ($white, $leader); # common white space and common leading string
626 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
627 ($white, $leader) = ($2, quotemeta($1));
629 ($white, $leader) = (/^(\s+)/, '');
631 s/^\s*?$leader(?:$white)?//gm;
635 This works with leading special strings, dynamically determined:
637 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
640 @@@ SAVEI32(runlevel);
642 @@@ while ( op = (*op->op_ppaddr)() ) ;
646 MAIN_INTERPRETER_LOOP
648 Or with a fixed amount of leading white space, with remaining
649 indentation correctly preserved:
651 $poem = fix<<EVER_ON_AND_ON;
652 Now far ahead the Road has gone,
653 And I must follow, if I can,
654 Pursuing it with eager feet,
655 Until it joins some larger way
656 Where many paths and errands meet.
657 And whither then? I cannot say.
658 --Bilbo in /usr/src/perl/pp_ctl.c
663 =head2 What is the difference between $array[1] and @array[1]?
665 The former is a scalar value, the latter an array slice, which makes
666 it a list with one (scalar) value. You should use $ when you want a
667 scalar value (most of the time) and @ when you want a list with one
668 scalar value in it (very, very rarely; nearly never, in fact).
670 Sometimes it doesn't make a difference, but sometimes it does.
671 For example, compare:
673 $good[0] = `some program that outputs several lines`;
677 @bad[0] = `same program that outputs several lines`;
679 The B<-w> flag will warn you about these matters.
681 =head2 How can I extract just the unique elements of an array?
683 There are several possible ways, depending on whether the array is
684 ordered and whether you wish to preserve the ordering.
688 =item a) If @in is sorted, and you want @out to be sorted:
689 (this assumes all true values in the array)
692 @out = grep($_ ne $prev && ($prev = $_), @in);
694 This is nice in that it doesn't use much extra memory, simulating
695 uniq(1)'s behavior of removing only adjacent duplicates. It's less
696 nice in that it won't work with false values like undef, 0, or "";
697 "0 but true" is ok, though.
699 =item b) If you don't know whether @in is sorted:
702 @out = grep(!$saw{$_}++, @in);
704 =item c) Like (b), but @in contains only small integers:
706 @out = grep(!$saw[$_]++, @in);
708 =item d) A way to do (b) without any loops or greps:
712 @out = sort keys %saw; # remove sort if undesired
714 =item e) Like (d), but @in contains only small positive integers:
722 =head2 How can I tell whether a list or array contains a certain element?
724 Hearing the word "in" is an I<in>dication that you probably should have
725 used a hash, not a list or array, to store your data. Hashes are
726 designed to answer this question quickly and efficiently. Arrays aren't.
728 That being said, there are several ways to approach this. If you
729 are going to make this query many times over arbitrary string values,
730 the fastest way is probably to invert the original array and keep an
731 associative array lying about whose keys are the first array's values.
733 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
735 for (@blues) { $is_blue{$_} = 1 }
737 Now you can check whether $is_blue{$some_color}. It might have been a
738 good idea to keep the blues all in a hash in the first place.
740 If the values are all small integers, you could use a simple indexed
741 array. This kind of an array will take up less space:
743 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
744 undef @is_tiny_prime;
745 for (@primes) { $is_tiny_prime[$_] = 1; }
747 Now you check whether $is_tiny_prime[$some_number].
749 If the values in question are integers instead of strings, you can save
750 quite a lot of space by using bit strings instead:
752 @articles = ( 1..10, 150..2000, 2017 );
754 for (@articles) { vec($read,$_,1) = 1 }
756 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
760 $is_there = grep $_ eq $whatever, @array;
764 $is_there = grep /$whatever/, @array;
766 These are slow (checks every element even if the first matches),
767 inefficient (same reason), and potentially buggy (what if there are
768 regexp characters in $whatever?).
770 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
772 Use a hash. Here's code to do both and more. It assumes that
773 each element is unique in a given array:
775 @union = @intersection = @difference = ();
777 foreach $element (@array1, @array2) { $count{$element}++ }
778 foreach $element (keys %count) {
779 push @union, $element;
780 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
783 =head2 How do I find the first array element for which a condition is true?
785 You can use this if you care about the index:
787 for ($i=0; $i < @array; $i++) {
788 if ($array[$i] eq "Waldo") {
794 Now C<$found_index> has what you want.
796 =head2 How do I handle linked lists?
798 In general, you usually don't need a linked list in Perl, since with
799 regular arrays, you can push and pop or shift and unshift at either end,
800 or you can use splice to add and/or remove arbitrary number of elements at
801 arbitrary points. Both pop and shift are both O(1) operations on perl's
802 dynamic arrays. In the absence of shifts and pops, push in general
803 needs to reallocate on the order every log(N) times, and unshift will
804 need to copy pointers each time.
806 If you really, really wanted, you could use structures as described in
807 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
810 =head2 How do I handle circular lists?
812 Circular lists could be handled in the traditional fashion with linked
813 lists, or you could just do something like this with an array:
815 unshift(@array, pop(@array)); # the last shall be first
816 push(@array, shift(@array)); # and vice versa
818 =head2 How do I shuffle an array randomly?
822 # fisher_yates_shuffle( \@array ) :
823 # generate a random permutation of @array in place
824 sub fisher_yates_shuffle {
827 for ($i = @$array; --$i; ) {
828 my $j = int rand ($i+1);
830 @$array[$i,$j] = @$array[$j,$i];
834 fisher_yates_shuffle( \@array ); # permutes @array in place
836 You've probably seen shuffling algorithms that works using splice,
837 randomly picking another element to swap the current element with:
841 @old = 1 .. 10; # just a demo
843 push(@new, splice(@old, rand @old, 1));
846 This is bad because splice is already O(N), and since you do it N times,
847 you just invented a quadratic algorithm; that is, O(N**2). This does
848 not scale, although Perl is so efficient that you probably won't notice
849 this until you have rather largish arrays.
851 =head2 How do I process/modify each element of an array?
853 Use C<for>/C<foreach>:
856 s/foo/bar/; # change that word
857 y/XZ/ZX/; # swap those letters
860 Here's another; let's compute spherical volumes:
862 for (@volumes = @radii) { # @volumes has changed parts
864 $_ *= (4/3) * 3.14159; # this will be constant folded
867 If you want to do the same thing to modify the values of the hash,
868 you may not use the C<values> function, oddly enough. You need a slice:
870 for $orbit ( @orbits{keys %orbits} ) {
871 ($orbit **= 3) *= (4/3) * 3.14159;
874 =head2 How do I select a random element from an array?
876 Use the rand() function (see L<perlfunc/rand>):
878 # at the top of the program:
879 srand; # not needed for 5.004 and later
882 $index = rand @array;
883 $element = $array[$index];
885 Make sure you I<only call srand once per program, if then>.
886 If you are calling it more than once (such as before each
887 call to rand), you're almost certainly doing something wrong.
889 =head2 How do I permute N elements of a list?
891 Here's a little program that generates all permutations
892 of all the words on each line of input. The algorithm embodied
893 in the permute() function should work on any list:
896 # tsc-permute: permute each word of input
897 permute([split], []);
899 my @items = @{ $_[0] };
900 my @perms = @{ $_[1] };
904 my(@newitems,@newperms,$i);
905 foreach $i (0 .. $#items) {
908 unshift(@newperms, splice(@newitems, $i, 1));
909 permute([@newitems], [@newperms]);
914 =head2 How do I sort an array by (anything)?
916 Supply a comparison function to sort() (described in L<perlfunc/sort>):
918 @list = sort { $a <=> $b } @list;
920 The default sort function is cmp, string comparison, which would
921 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
922 the numerical comparison operator.
924 If you have a complicated function needed to pull out the part you
925 want to sort on, then don't do it inside the sort function. Pull it
926 out first, because the sort BLOCK can be called many times for the
927 same element. Here's an example of how to pull out the first word
928 after the first number on each item, and then sort those words
933 ($item) = /\d+\s*(\S+)/;
934 push @idx, uc($item);
936 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
938 Which could also be written this way, using a trick
939 that's come to be known as the Schwartzian Transform:
941 @sorted = map { $_->[0] }
942 sort { $a->[1] cmp $b->[1] }
943 map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
945 If you need to sort on several fields, the following paradigm is useful.
947 @sorted = sort { field1($a) <=> field1($b) ||
948 field2($a) cmp field2($b) ||
949 field3($a) cmp field3($b)
952 This can be conveniently combined with precalculation of keys as given
955 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
958 See also the question below on sorting hashes.
960 =head2 How do I manipulate arrays of bits?
962 Use pack() and unpack(), or else vec() and the bitwise operations.
964 For example, this sets $vec to have bit N set if $ints[N] was set:
967 foreach(@ints) { vec($vec,$_,1) = 1 }
969 And here's how, given a vector in $vec, you can
970 get those bits into your @ints array:
975 # Find null-byte density then select best algorithm
976 if ($vec =~ tr/\0// / length $vec > 0.95) {
979 # This method is faster with mostly null-bytes
980 while($vec =~ /[^\0]/g ) {
981 $i = -9 + 8 * pos $vec;
982 push @ints, $i if vec($vec, ++$i, 1);
983 push @ints, $i if vec($vec, ++$i, 1);
984 push @ints, $i if vec($vec, ++$i, 1);
985 push @ints, $i if vec($vec, ++$i, 1);
986 push @ints, $i if vec($vec, ++$i, 1);
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);
992 # This method is a fast general algorithm
994 my $bits = unpack "b*", $vec;
995 push @ints, 0 if $bits =~ s/^(\d)// && $1;
996 push @ints, pos $bits while($bits =~ /1/g);
1001 This method gets faster the more sparse the bit vector is.
1002 (Courtesy of Tim Bunce and Winfried Koenig.)
1004 =head2 Why does defined() return true on empty arrays and hashes?
1006 See L<perlfunc/defined> in the 5.004 release or later of Perl.
1008 =head1 Data: Hashes (Associative Arrays)
1010 =head2 How do I process an entire hash?
1012 Use the each() function (see L<perlfunc/each>) if you don't care
1013 whether it's sorted:
1015 while ( ($key, $value) = each %hash) {
1016 print "$key = $value\n";
1019 If you want it sorted, you'll have to use foreach() on the result of
1020 sorting the keys as shown in an earlier question.
1022 =head2 What happens if I add or remove keys from a hash while iterating over it?
1026 =head2 How do I look up a hash element by value?
1028 Create a reverse hash:
1030 %by_value = reverse %by_key;
1031 $key = $by_value{$value};
1033 That's not particularly efficient. It would be more space-efficient
1036 while (($key, $value) = each %by_key) {
1037 $by_value{$value} = $key;
1040 If your hash could have repeated values, the methods above will only
1041 find one of the associated keys. This may or may not worry you.
1043 =head2 How can I know how many entries are in a hash?
1045 If you mean how many keys, then all you have to do is
1046 take the scalar sense of the keys() function:
1048 $num_keys = scalar keys %hash;
1050 In void context it just resets the iterator, which is faster
1053 =head2 How do I sort a hash (optionally by value instead of key)?
1055 Internally, hashes are stored in a way that prevents you from imposing
1056 an order on key-value pairs. Instead, you have to sort a list of the
1059 @keys = sort keys %hash; # sorted by key
1061 $hash{$a} cmp $hash{$b}
1062 } keys %hash; # and by value
1064 Here we'll do a reverse numeric sort by value, and if two keys are
1065 identical, sort by length of key, and if that fails, by straight ASCII
1066 comparison of the keys (well, possibly modified by your locale -- see
1070 $hash{$b} <=> $hash{$a}
1072 length($b) <=> length($a)
1077 =head2 How can I always keep my hash sorted?
1079 You can look into using the DB_File module and tie() using the
1080 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1081 The Tie::IxHash module from CPAN might also be instructive.
1083 =head2 What's the difference between "delete" and "undef" with hashes?
1085 Hashes are pairs of scalars: the first is the key, the second is the
1086 value. The key will be coerced to a string, although the value can be
1087 any kind of scalar: string, number, or reference. If a key C<$key> is
1088 present in the array, C<exists($key)> will return true. The value for
1089 a given key can be C<undef>, in which case C<$array{$key}> will be
1090 C<undef> while C<$exists{$key}> will return true. This corresponds to
1091 (C<$key>, C<undef>) being in the hash.
1093 Pictures help... here's the C<%ary> table:
1103 And these conditions hold
1107 defined $ary{'d'} is true
1108 defined $ary{'a'} is true
1109 exists $ary{'a'} is true (perl5 only)
1110 grep ($_ eq 'a', keys %ary) is true
1116 your table now reads:
1127 and these conditions now hold; changes in caps:
1131 defined $ary{'d'} is true
1132 defined $ary{'a'} is FALSE
1133 exists $ary{'a'} is true (perl5 only)
1134 grep ($_ eq 'a', keys %ary) is true
1136 Notice the last two: you have an undef value, but a defined key!
1142 your table now reads:
1151 and these conditions now hold; changes in caps:
1155 defined $ary{'d'} is true
1156 defined $ary{'a'} is false
1157 exists $ary{'a'} is FALSE (perl5 only)
1158 grep ($_ eq 'a', keys %ary) is FALSE
1160 See, the whole entry is gone!
1162 =head2 Why don't my tied hashes make the defined/exists distinction?
1164 They may or may not implement the EXISTS() and DEFINED() methods
1165 differently. For example, there isn't the concept of undef with hashes
1166 that are tied to DBM* files. This means the true/false tables above
1167 will give different results when used on such a hash. It also means
1168 that exists and defined do the same thing with a DBM* file, and what
1169 they end up doing is not what they do with ordinary hashes.
1171 =head2 How do I reset an each() operation part-way through?
1173 Using C<keys %hash> in scalar context returns the number of keys in
1174 the hash I<and> resets the iterator associated with the hash. You may
1175 need to do this if you use C<last> to exit a loop early so that when you
1176 re-enter it, the hash iterator has been reset.
1178 =head2 How can I get the unique keys from two hashes?
1180 First you extract the keys from the hashes into arrays, and then solve
1181 the uniquifying the array problem described above. For example:
1184 for $element (keys(%foo), keys(%bar)) {
1191 @uniq = keys %{{%foo,%bar}};
1193 Or if you really want to save space:
1196 while (defined ($key = each %foo)) {
1199 while (defined ($key = each %bar)) {
1204 =head2 How can I store a multidimensional array in a DBM file?
1206 Either stringify the structure yourself (no fun), or else
1207 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1208 it on top of either DB_File or GDBM_File.
1210 =head2 How can I make my hash remember the order I put elements into it?
1212 Use the Tie::IxHash from CPAN.
1215 tie(%myhash, Tie::IxHash);
1216 for ($i=0; $i<20; $i++) {
1219 @keys = keys %myhash;
1220 # @keys = (0,1,2,3,...)
1222 =head2 Why does passing a subroutine an undefined element in a hash create it?
1224 If you say something like:
1226 somefunc($hash{"nonesuch key here"});
1228 Then that element "autovivifies"; that is, it springs into existence
1229 whether you store something there or not. That's because functions
1230 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1231 it has to be ready to write it back into the caller's version.
1233 This has been fixed as of perl5.004.
1235 Normally, merely accessing a key's value for a nonexistent key does
1236 I<not> cause that key to be forever there. This is different than
1239 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1241 Use references (documented in L<perlref>). Examples of complex data
1242 structures are given in L<perldsc> and L<perllol>. Examples of
1243 structures and object-oriented classes are in L<perltoot>.
1245 =head2 How can I use a reference as a hash key?
1247 You can't do this directly, but you could use the standard Tie::Refhash
1248 module distributed with perl.
1252 =head2 How do I handle binary data correctly?
1254 Perl is binary clean, so this shouldn't be a problem. For example,
1255 this works fine (assuming the files are found):
1257 if (`cat /vmunix` =~ /gzip/) {
1258 print "Your kernel is GNU-zip enabled!\n";
1261 On some systems, however, you have to play tedious games with "text"
1262 versus "binary" files. See L<perlfunc/"binmode">.
1264 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1266 If you want to deal with multibyte characters, however, there are
1267 some gotchas. See the section on Regular Expressions.
1269 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1271 Assuming that you don't care about IEEE notations like "NaN" or
1272 "Infinity", you probably just want to use a regular expression.
1274 warn "has nondigits" if /\D/;
1275 warn "not a natural number" unless /^\d+$/; # rejects -3
1276 warn "not an integer" unless /^-?\d+$/; # rejects +3
1277 warn "not an integer" unless /^[+-]?\d+$/;
1278 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
1279 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
1280 warn "not a C float"
1281 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
1283 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1284 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1285 wrapper function for more convenient access. This function takes
1286 a string and returns the number it found, or C<undef> for input that
1287 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1288 if you just want to say, ``Is this a float?''
1291 use POSIX qw(strtod);
1296 my($num, $unparsed) = strtod($str);
1297 if (($str eq '') || ($unparsed != 0) || $!) {
1304 sub is_numeric { defined &getnum }
1306 Or you could check out
1307 http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
1308 instead. The POSIX module (part of the standard Perl distribution)
1309 provides the C<strtol> and C<strtod> for converting strings to double
1310 and longs, respectively.
1312 =head2 How do I keep persistent data across program calls?
1314 For some specific applications, you can use one of the DBM modules.
1315 See L<AnyDBM_File>. More generically, you should consult the
1316 FreezeThaw, Storable, or Class::Eroot modules from CPAN.
1318 =head2 How do I print out or copy a recursive data structure?
1320 The Data::Dumper module on CPAN is nice for printing out
1321 data structures, and FreezeThaw for copying them. For example:
1323 use FreezeThaw qw(freeze thaw);
1324 $new = thaw freeze $old;
1326 Where $old can be (a reference to) any kind of data structure you'd like.
1327 It will be deeply copied.
1329 =head2 How do I define methods for every class/object?
1331 Use the UNIVERSAL class (see L<UNIVERSAL>).
1333 =head2 How do I verify a credit card checksum?
1335 Get the Business::CreditCard module from CPAN.
1337 =head1 AUTHOR AND COPYRIGHT
1339 Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
1340 All rights reserved.
1342 When included as part of the Standard Version of Perl, or as part of
1343 its complete documentation whether printed or otherwise, this work
1344 may be distributed only under the terms of Perl's Artistic License.
1345 Any distribution of this file or derivatives thereof I<outside>
1346 of that package require that special arrangements be made with
1349 Irrespective of its distribution, all code examples in this file
1350 are hereby placed into the public domain. You are permitted and
1351 encouraged to use this code in your own programs for fun
1352 or for profit as you see fit. A simple comment in the code giving
1353 credit would be courteous but is not required.