3 perlfaq4 - Data Manipulation ($Revision: 1.40 $, $Date: 1999/01/08 04:26:39 $)
7 The section of the FAQ answers question related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximate on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to the internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers (see L<perlvar/"$#"> 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 $floor = floor(3.5); # 3
78 In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex
79 module. With 5.004, the Math::Trig module (part of the standard perl
80 distribution) implements the trigonometric functions. Internally it
81 uses the Math::Complex module and some functions can break out from
82 the real axis into the complex plane, for example the inverse sine of
85 Rounding in financial applications can have serious implications, and
86 the rounding method used should be specified precisely. In these
87 cases, it probably pays not to trust whichever system rounding is
88 being used by Perl, but to instead implement the rounding function you
91 To see why, notice how you'll still have an issue on half-way-point
94 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
96 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
97 0.8 0.8 0.9 0.9 1.0 1.0
99 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
100 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
101 machines) will work pretty much like mathematical integers. Other numbers
104 =head2 How do I convert bits into ints?
106 To turn a string of 1s and 0s like C<10110110> into a scalar containing
107 its binary value, use the pack() function (documented in
110 $decimal = pack('B8', '10110110');
112 Here's an example of going the other way:
114 $binary_string = join('', unpack('B*', "\x29"));
116 =head2 Why doesn't & work the way I want it to?
118 The behavior of binary arithmetic operators depends on whether they're
119 used on numbers or strings. The operators treat a string as a series
120 of bits and work with that (the string C<"3"> is the bit pattern
121 C<00110011>). The operators work with the binary form of a number
122 (the number C<3> is treated as the bit pattern C<00000011>).
124 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
125 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
128 Most problems with C<&> and C<|> arise because the programmer thinks
129 they have a number but really it's a string. The rest arise because
132 if ("\020\020" & "\101\101") {
136 but a string consisting of two null bytes (the result of C<"\020\020"
137 & "\101\101">) is not a false value in Perl. You need:
139 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
143 =head2 How do I multiply matrices?
145 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
146 or the PDL extension (also available from CPAN).
148 =head2 How do I perform an operation on a series of integers?
150 To call a function on each element in an array, and collect the
153 @results = map { my_func($_) } @array;
157 @triple = map { 3 * $_ } @single;
159 To call a function on each element of an array, but ignore the
162 foreach $iterator (@array) {
163 some_func($iterator);
166 To call a function on each integer in a (small) range, you B<can> use:
168 @results = map { some_func($_) } (5 .. 25);
170 but you should be aware that the C<..> operator creates an array of
171 all integers in the range. This can take a lot of memory for large
175 for ($i=5; $i < 500_005; $i++) {
176 push(@results, some_func($i));
179 =head2 How can I output Roman numerals?
181 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
183 =head2 Why aren't my random numbers random?
185 If you're using a version of Perl before 5.004, you must call C<srand>
186 once at the start of your program to seed the random number generator.
187 5.004 and later automatically call C<srand> at the beginning. Don't
188 call C<srand> more than once--you make your numbers less random, rather
191 Computers are good at being predictable and bad at being random
192 (despite appearances caused by bugs in your programs :-).
193 http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
194 Phoenix, talks more about this.. John von Neumann said, ``Anyone who
195 attempts to generate random numbers by deterministic means is, of
196 course, living in a state of sin.''
198 If you want numbers that are more random than C<rand> with C<srand>
199 provides, you should also check out the Math::TrulyRandom module from
200 CPAN. It uses the imperfections in your system's timer to generate
201 random numbers, but this takes quite a while. If you want a better
202 pseudorandom generator than comes with your operating system, look at
203 ``Numerical Recipes in C'' at http://www.nr.com/ .
207 =head2 How do I find the week-of-the-year/day-of-the-year?
209 The day of the year is in the array returned by localtime() (see
210 L<perlfunc/"localtime">):
212 $day_of_year = (localtime(time()))[7];
214 or more legibly (in 5.004 or higher):
217 $day_of_year = localtime(time())->yday;
219 You can find the week of the year by dividing this by 7:
221 $week_of_year = int($day_of_year / 7);
223 Of course, this believes that weeks start at zero. The Date::Calc
224 module from CPAN has a lot of date calculation functions, including
225 day of the year, week of the year, and so on. Note that not
226 all businesses consider ``week 1'' to be the same; for example,
227 American businesses often consider the first week with a Monday
228 in it to be Work Week #1, despite ISO 8601, which considers
229 WW1 to be the first week with a Thursday in it.
231 =head2 How can I compare two dates and find the difference?
233 If you're storing your dates as epoch seconds then simply subtract one
234 from the other. If you've got a structured date (distinct year, day,
235 month, hour, minute, seconds values) then use one of the Date::Manip
236 and Date::Calc modules from CPAN.
238 =head2 How can I take a string and turn it into epoch seconds?
240 If it's a regular enough string that it always has the same format,
241 you can split it up and pass the parts to C<timelocal> in the standard
242 Time::Local module. Otherwise, you should look into the Date::Calc
243 and Date::Manip modules from CPAN.
245 =head2 How can I find the Julian Day?
247 Neither Date::Manip nor Date::Calc deal with Julian days. Instead,
248 there is an example of Julian date calculation that should help you in
249 Time::JulianDay (part of the Time-modules bundle) which can be found at
250 http://www.perl.com/CPAN/modules/by-module/Time/.
253 =head2 How do I find yesterday's date?
255 The C<time()> function returns the current time in seconds since the
256 epoch. Take one day off that:
258 $yesterday = time() - ( 24 * 60 * 60 );
260 Then you can pass this to C<localtime()> and get the individual year,
261 month, day, hour, minute, seconds values.
263 =head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
265 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
266 Y2K compliant (whatever that means). The programmers you've hired to
267 use it, however, probably are not.
269 Long answer: The question belies a true understanding of the issue.
270 Perl is just as Y2K compliant as your pencil--no more, and no less.
271 Can you use your pencil to write a non-Y2K-compliant memo? Of course
272 you can. Is that the pencil's fault? Of course it isn't.
274 The date and time functions supplied with perl (gmtime and localtime)
275 supply adequate information to determine the year well beyond 2000
276 (2038 is when trouble strikes for 32-bit machines). The year returned
277 by these functions when used in an array context is the year minus 1900.
278 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
279 number. To avoid the year 2000 problem simply do not treat the year as
280 a 2-digit number. It isn't.
282 When gmtime() and localtime() are used in scalar context they return
283 a timestamp string that contains a fully-expanded year. For example,
284 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
285 2001". There's no year 2000 problem here.
287 That doesn't mean that Perl can't be used to create non-Y2K compliant
288 programs. It can. But so can your pencil. It's the fault of the user,
289 not the language. At the risk of inflaming the NRA: ``Perl doesn't
290 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
295 =head2 How do I validate input?
297 The answer to this question is usually a regular expression, perhaps
298 with auxiliary logic. See the more specific questions (numbers, mail
299 addresses, etc.) for details.
301 =head2 How do I unescape a string?
303 It depends just what you mean by ``escape''. URL escapes are dealt
304 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
305 character are removed with:
309 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
311 =head2 How do I remove consecutive pairs of characters?
313 To turn C<"abbcccd"> into C<"abccd">:
317 =head2 How do I expand function calls in a string?
319 This is documented in L<perlref>. In general, this is fraught with
320 quoting and readability problems, but it is possible. To interpolate
321 a subroutine call (in list context) into a string:
323 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
325 If you prefer scalar context, similar chicanery is also useful for
326 arbitrary expressions:
328 print "That yields ${\($n + 5)} widgets\n";
330 Version 5.004 of Perl had a bug that gave list context to the
331 expression in C<${...}>, but this is fixed in version 5.005.
333 See also ``How can I expand variables in text strings?'' in this
336 =head2 How do I find matching/nesting anything?
338 This isn't something that can be done in one regular expression, no
339 matter how complicated. To find something between two single
340 characters, a pattern like C</x([^x]*)x/> will get the intervening
341 bits in $1. For multiple ones, then something more like
342 C</alpha(.*?)omega/> would be needed. But none of these deals with
343 nested patterns, nor can they. For that you'll have to write a
346 If you are serious about writing a parser, there are a number of
347 modules or oddities that will make your life a lot easier. There is
348 the CPAN module Parse::RecDescent, the standard module Text::Balanced,
349 the byacc program, the CPAN module Parse::Yapp, and Mark-Jason
350 Dominus's excellent I<py> tool at http://www.plover.com/~mjd/perl/py/
353 One simple destructive, inside-out approach that you might try is to
354 pull out the smallest nesting parts one at a time:
356 while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) {
357 # do something with $1
360 A more complicated and sneaky approach is to make Perl's regular
361 expression engine do it for you. This is courtesy Dean Inada, and
362 rather has the nature of an Obfuscated Perl Contest entry, but it
365 # $_ contains the string to parse
366 # BEGIN and END are the opening and closing markers for the
371 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
372 @$ = (eval{/$re/},$@!~/unmatched/);
373 print join("\n",@$[0..$#$]) if( $$[-1] );
375 =head2 How do I reverse a string?
377 Use reverse() in scalar context, as documented in
380 $reversed = reverse $string;
382 =head2 How do I expand tabs in a string?
384 You can do it yourself:
386 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
388 Or you can just use the Text::Tabs module (part of the standard perl
392 @expanded_lines = expand(@lines_with_tabs);
394 =head2 How do I reformat a paragraph?
396 Use Text::Wrap (part of the standard perl distribution):
399 print wrap("\t", ' ', @paragraphs);
401 The paragraphs you give to Text::Wrap should not contain embedded
402 newlines. Text::Wrap doesn't justify the lines (flush-right).
404 =head2 How can I access/change the first N letters of a string?
406 There are many ways. If you just want to grab a copy, use
409 $first_byte = substr($a, 0, 1);
411 If you want to modify part of a string, the simplest way is often to
412 use substr() as an lvalue:
414 substr($a, 0, 3) = "Tom";
416 Although those with a pattern matching kind of thought process will
421 =head2 How do I change the Nth occurrence of something?
423 You have to keep track of N yourself. For example, let's say you want
424 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
425 C<"whosoever"> or C<"whomsoever">, case insensitively.
429 ++$count == 5 # is it the 5th?
430 ? "${2}soever" # yes, swap
431 : $1 # renege and leave it there
434 In the more general case, you can use the C</g> modifier in a C<while>
435 loop, keeping count of matches.
439 while (/(\w+)\s+fish\b/gi) {
440 if (++$count == $WANT) {
441 print "The third fish is a $1 one.\n";
442 # Warning: don't `last' out of this loop
446 That prints out: C<"The third fish is a red one."> You can also use a
447 repetition count and repeated pattern like this:
449 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
451 =head2 How can I count the number of occurrences of a substring within a string?
453 There are a number of ways, with varying efficiency: If you want a
454 count of a certain single character (X) within a string, you can use the
455 C<tr///> function like so:
457 $string = "ThisXlineXhasXsomeXx'sXinXit";
458 $count = ($string =~ tr/X//);
459 print "There are $count X charcters in the string";
461 This is fine if you are just looking for a single character. However,
462 if you are trying to count multiple character substrings within a
463 larger string, C<tr///> won't work. What you can do is wrap a while()
464 loop around a global pattern match. For example, let's count negative
467 $string = "-9 55 48 -2 23 -76 4 14 -44";
468 while ($string =~ /-\d+/g) { $count++ }
469 print "There are $count negative numbers in the string";
471 =head2 How do I capitalize all the words on one line?
473 To make the first letter of each word upper case:
475 $line =~ s/\b(\w)/\U$1/g;
477 This has the strange effect of turning "C<don't do it>" into "C<Don'T
478 Do It>". Sometimes you might want this, instead (Suggested by Brian
482 (^\w) #at the beginning of the line
484 (\s\w) #preceded by whitespace
487 $string =~ /([\w']+)/\u\L$1/g;
489 To make the whole line upper case:
493 To force each word to be lower case, with the first letter upper case:
495 $line =~ s/(\w+)/\u\L$1/g;
497 You can (and probably should) enable locale awareness of those
498 characters by placing a C<use locale> pragma in your program.
499 See L<perllocale> for endless details on locales.
501 This is sometimes referred to as putting something into "title
502 case", but that's not quite accurate. Consdier the proper
503 capitalization of the movie I<Dr. Strangelove or: How I Learned to
504 Stop Worrying and Love the Bomb>, for example.
506 =head2 How can I split a [character] delimited string except when inside
507 [character]? (Comma-separated files)
509 Take the example case of trying to split a string that is comma-separated
510 into its different fields. (We'll pretend you said comma-separated, not
511 comma-delimited, which is different and almost never what you mean.) You
512 can't use C<split(/,/)> because you shouldn't split if the comma is inside
513 quotes. For example, take a data line like this:
515 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
517 Due to the restriction of the quotes, this is a fairly complex
518 problem. Thankfully, we have Jeffrey Friedl, author of a highly
519 recommended book on regular expressions, to handle these for us. He
520 suggests (assuming your string is contained in $text):
523 push(@new, $+) while $text =~ m{
524 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
528 push(@new, undef) if substr($text,-1,1) eq ',';
530 If you want to represent quotation marks inside a
531 quotation-mark-delimited field, escape them with backslashes (eg,
532 C<"like \"this\"">. Unescaping them is a task addressed earlier in
535 Alternatively, the Text::ParseWords module (part of the standard perl
536 distribution) lets you say:
538 use Text::ParseWords;
539 @new = quotewords(",", 0, $text);
541 There's also a Text::CSV module on CPAN.
543 =head2 How do I strip blank space from the beginning/end of a string?
545 Although the simplest approach would seem to be:
547 $string =~ s/^\s*(.*?)\s*$/$1/;
549 This is unnecessarily slow, destructive, and fails with embedded newlines.
550 It is much better faster to do this in two steps:
555 Or more nicely written as:
562 This idiom takes advantage of the C<foreach> loop's aliasing
563 behavior to factor out common code. You can do this
564 on several strings at once, or arrays, or even the
565 values of a hash if you use a slide:
567 # trim whitespace in the scalar, the array,
568 # and all the values in the hash
569 foreach ($scalar, @array, @hash{keys %hash}) {
574 =head2 How do I pad a string with blanks or pad a number with zeroes?
576 (This answer contributed by Uri Guttman)
578 In the following examples, C<$pad_len> is the length to which you wish
579 to pad the string, C<$text> or C<$num> contains the string to be
580 padded, and C<$pad_char> contains the padding character. You can use a
581 single character string constant instead of the C<$pad_char> variable
582 if you know what it is in advance.
584 The simplest method use the C<sprintf> function. It can pad on the
585 left or right with blanks and on the left with zeroes.
587 # Left padding with blank:
588 $padded = sprintf( "%${pad_len}s", $text ) ;
590 # Right padding with blank:
591 $padded = sprintf( "%${pad_len}s", $text ) ;
593 # Left padding with 0:
594 $padded = sprintf( "%0${pad_len}d", $num ) ;
596 If you need to pad with a character other than blank or zero you can use
597 one of the following methods.
599 These methods generate a pad string with the C<x> operator and
600 concatenate that with the original text.
602 Left and right padding with any character:
604 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text ;
605 $padded = $text . $pad_char x ( $pad_len - length( $text ) ) ;
607 Or you can left or right pad $text directly:
609 $text .= $pad_char x ( $pad_len - length( $text ) ) ;
610 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ) ;
612 =head2 How do I extract selected columns from a string?
614 Use substr() or unpack(), both documented in L<perlfunc>.
615 If you prefer thinking in terms of columns instead of widths,
616 you can use this kind of thing:
618 # determine the unpack format needed to split Linux ps output
619 # arguments are cut columns
620 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
626 for my $place (@positions) {
627 $template .= "A" . ($place - $lastpos) . " ";
634 =head2 How do I find the soundex value of a string?
636 Use the standard Text::Soundex module distributed with perl.
638 =head2 How can I expand variables in text strings?
640 Let's assume that you have a string like:
642 $text = 'this has a $foo in it and a $bar';
644 If those were both global variables, then this would
647 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
649 But since they are probably lexicals, or at least, they could
650 be, you'd have to do this:
652 $text =~ s/(\$\w+)/$1/eeg;
653 die if $@; # needed /ee, not /e
655 It's probably better in the general case to treat those
656 variables as entries in some special hash. For example:
662 $text =~ s/\$(\w+)/$user_defs{$1}/g;
664 See also ``How do I expand function calls in a string?'' in this section
667 =head2 What's wrong with always quoting "$vars"?
669 The problem is that those double-quotes force stringification,
670 coercing numbers and references into strings, even when you
671 don't want them to be. Think of it this way: double-quote
672 expansion is used to produce new strings. If you already
673 have a string, why do you need more?
675 If you get used to writing odd things like these:
679 somefunc("$var"); # BAD
681 You'll be in trouble. Those should (in 99.8% of the cases) be
682 the simpler and more direct:
688 Otherwise, besides slowing you down, you're going to break code when
689 the thing in the scalar is actually neither a string nor a number, but
695 my $oref = "$aref"; # WRONG
698 You can also get into subtle problems on those few operations in Perl
699 that actually do care about the difference between a string and a
700 number, such as the magical C<++> autoincrement operator or the
703 Stringification also destroys arrays.
706 print "@lines"; # WRONG - extra blanks
707 print @lines; # right
709 =head2 Why don't my E<lt>E<lt>HERE documents work?
711 Check for these three things:
715 =item 1. There must be no space after the << part.
717 =item 2. There (probably) should be a semicolon at the end.
719 =item 3. You can't (easily) have any space in front of the tag.
723 If you want to indent the text in the here document, you
727 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
732 But the HERE_TARGET must still be flush against the margin.
733 If you want that indented also, you'll have to quote
736 ($quote = <<' FINIS') =~ s/^\s+//gm;
737 ...we will have peace, when you and all your works have
738 perished--and the works of your dark master to whom you
739 would deliver us. You are a liar, Saruman, and a corrupter
740 of men's hearts. --Theoden in /usr/src/perl/taint.c
742 $quote =~ s/\s*--/\n--/;
744 A nice general-purpose fixer-upper function for indented here documents
745 follows. It expects to be called with a here document as its argument.
746 It looks to see whether each line begins with a common substring, and
747 if so, strips that off. Otherwise, it takes the amount of leading
748 white space found on the first line and removes that much off each
753 my ($white, $leader); # common white space and common leading string
754 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
755 ($white, $leader) = ($2, quotemeta($1));
757 ($white, $leader) = (/^(\s+)/, '');
759 s/^\s*?$leader(?:$white)?//gm;
763 This works with leading special strings, dynamically determined:
765 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
768 @@@ SAVEI32(runlevel);
770 @@@ while ( op = (*op->op_ppaddr)() ) ;
774 MAIN_INTERPRETER_LOOP
776 Or with a fixed amount of leading white space, with remaining
777 indentation correctly preserved:
779 $poem = fix<<EVER_ON_AND_ON;
780 Now far ahead the Road has gone,
781 And I must follow, if I can,
782 Pursuing it with eager feet,
783 Until it joins some larger way
784 Where many paths and errands meet.
785 And whither then? I cannot say.
786 --Bilbo in /usr/src/perl/pp_ctl.c
791 =head2 What is the difference between a list and an array?
793 An array has a changeable length. A list does not. An array is something
794 you can push or pop, while a list is a set of values. Some people make
795 the distinction that a list is a value while an array is a variable.
796 Subroutines are passed and return lists, you put things into list
797 context, you initialize arrays with lists, and you foreach() across
798 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
799 in scalar context behave like the number of elements in them, subroutines
800 access their arguments through the array C<@_>, push/pop/shift only work
803 As a side note, there's no such thing as a list in scalar context.
806 $scalar = (2, 5, 7, 9);
808 you're using the comma operator in scalar context, so it evaluates the
809 left hand side, then evaluates and returns the left hand side. This
810 causes the last value to be returned: 9.
812 =head2 What is the difference between $array[1] and @array[1]?
814 The former is a scalar value, the latter an array slice, which makes
815 it a list with one (scalar) value. You should use $ when you want a
816 scalar value (most of the time) and @ when you want a list with one
817 scalar value in it (very, very rarely; nearly never, in fact).
819 Sometimes it doesn't make a difference, but sometimes it does.
820 For example, compare:
822 $good[0] = `some program that outputs several lines`;
826 @bad[0] = `same program that outputs several lines`;
828 The B<-w> flag will warn you about these matters.
830 =head2 How can I extract just the unique elements of an array?
832 There are several possible ways, depending on whether the array is
833 ordered and whether you wish to preserve the ordering.
837 =item a) If @in is sorted, and you want @out to be sorted:
838 (this assumes all true values in the array)
841 @out = grep($_ ne $prev && ($prev = $_), @in);
843 This is nice in that it doesn't use much extra memory, simulating
844 uniq(1)'s behavior of removing only adjacent duplicates. It's less
845 nice in that it won't work with false values like undef, 0, or "";
846 "0 but true" is ok, though.
848 =item b) If you don't know whether @in is sorted:
851 @out = grep(!$saw{$_}++, @in);
853 =item c) Like (b), but @in contains only small integers:
855 @out = grep(!$saw[$_]++, @in);
857 =item d) A way to do (b) without any loops or greps:
861 @out = sort keys %saw; # remove sort if undesired
863 =item e) Like (d), but @in contains only small positive integers:
871 But perhaps you should have been using a hash all along, eh?
873 =head2 How can I tell whether a list or array contains a certain element?
875 Hearing the word "in" is an I<in>dication that you probably should have
876 used a hash, not a list or array, to store your data. Hashes are
877 designed to answer this question quickly and efficiently. Arrays aren't.
879 That being said, there are several ways to approach this. If you
880 are going to make this query many times over arbitrary string values,
881 the fastest way is probably to invert the original array and keep an
882 associative array lying about whose keys are the first array's values.
884 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
886 for (@blues) { $is_blue{$_} = 1 }
888 Now you can check whether $is_blue{$some_color}. It might have been a
889 good idea to keep the blues all in a hash in the first place.
891 If the values are all small integers, you could use a simple indexed
892 array. This kind of an array will take up less space:
894 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
895 undef @is_tiny_prime;
896 for (@primes) { $is_tiny_prime[$_] = 1; }
898 Now you check whether $is_tiny_prime[$some_number].
900 If the values in question are integers instead of strings, you can save
901 quite a lot of space by using bit strings instead:
903 @articles = ( 1..10, 150..2000, 2017 );
905 for (@articles) { vec($read,$_,1) = 1 }
907 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
911 $is_there = grep $_ eq $whatever, @array;
915 $is_there = grep /$whatever/, @array;
917 These are slow (checks every element even if the first matches),
918 inefficient (same reason), and potentially buggy (what if there are
919 regexp characters in $whatever?). If you're only testing once, then
923 foreach $elt (@array) {
924 if ($elt eq $elt_to_find) {
929 if ($is_there) { ... }
931 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
933 Use a hash. Here's code to do both and more. It assumes that
934 each element is unique in a given array:
936 @union = @intersection = @difference = ();
938 foreach $element (@array1, @array2) { $count{$element}++ }
939 foreach $element (keys %count) {
940 push @union, $element;
941 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
944 =head2 How do I test whether two arrays or hashes are equal?
946 The following code works for single-level arrays. It uses a stringwise
947 comparison, and does not distinguish defined versus undefined empty
948 strings. Modify if you have other needs.
950 $are_equal = compare_arrays(\@frogs, \@toads);
953 my ($first, $second) = @_;
954 local $^W = 0; # silence spurious -w undef complaints
955 return 0 unless @$first == @$second;
956 for (my $i = 0; $i < @$first; $i++) {
957 return 0 if $first->[$i] ne $second->[$i];
962 For multilevel structures, you may wish to use an approach more
963 like this one. It uses the CPAN module FreezeThaw:
965 use FreezeThaw qw(cmpStr);
966 @a = @b = ( "this", "that", [ "more", "stuff" ] );
968 printf "a and b contain %s arrays\n",
969 cmpStr(\@a, \@b) == 0
973 This approach also works for comparing hashes. Here
974 we'll demonstrate two different answers:
976 use FreezeThaw qw(cmpStr cmpStrHard);
978 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
982 printf "a and b contain %s hashes\n",
983 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
985 printf "a and b contain %s hashes\n",
986 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
989 The first reports that both those the hashes contain the same data,
990 while the second reports that they do not. Which you prefer is left as
991 an exercise to the reader.
993 =head2 How do I find the first array element for which a condition is true?
995 You can use this if you care about the index:
997 for ($i= 0; $i < @array; $i++) {
998 if ($array[$i] eq "Waldo") {
1004 Now C<$found_index> has what you want.
1006 =head2 How do I handle linked lists?
1008 In general, you usually don't need a linked list in Perl, since with
1009 regular arrays, you can push and pop or shift and unshift at either end,
1010 or you can use splice to add and/or remove arbitrary number of elements at
1011 arbitrary points. Both pop and shift are both O(1) operations on perl's
1012 dynamic arrays. In the absence of shifts and pops, push in general
1013 needs to reallocate on the order every log(N) times, and unshift will
1014 need to copy pointers each time.
1016 If you really, really wanted, you could use structures as described in
1017 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1018 to do. For example, imagine a list node like this:
1025 You could walk the list this way:
1028 for ($node = $head; $node; $node = $node->{LINK}) {
1029 print $node->{VALUE}, " ";
1033 You could grow the list this way:
1036 $tail = append($head, 1); # grow a new head
1037 for $value ( 2 .. 10 ) {
1038 $tail = append($tail, $value);
1042 my($list, $value) = @_;
1043 my $node = { VALUE => $value };
1045 $node->{LINK} = $list->{LINK};
1046 $list->{LINK} = $node;
1048 $_[0] = $node; # replace caller's version
1053 But again, Perl's built-in are virtually always good enough.
1055 =head2 How do I handle circular lists?
1057 Circular lists could be handled in the traditional fashion with linked
1058 lists, or you could just do something like this with an array:
1060 unshift(@array, pop(@array)); # the last shall be first
1061 push(@array, shift(@array)); # and vice versa
1063 =head2 How do I shuffle an array randomly?
1067 # fisher_yates_shuffle( \@array ) :
1068 # generate a random permutation of @array in place
1069 sub fisher_yates_shuffle {
1072 for ($i = @$array; --$i; ) {
1073 my $j = int rand ($i+1);
1075 @$array[$i,$j] = @$array[$j,$i];
1079 fisher_yates_shuffle( \@array ); # permutes @array in place
1081 You've probably seen shuffling algorithms that works using splice,
1082 randomly picking another element to swap the current element with:
1086 @old = 1 .. 10; # just a demo
1088 push(@new, splice(@old, rand @old, 1));
1091 This is bad because splice is already O(N), and since you do it N times,
1092 you just invented a quadratic algorithm; that is, O(N**2). This does
1093 not scale, although Perl is so efficient that you probably won't notice
1094 this until you have rather largish arrays.
1096 =head2 How do I process/modify each element of an array?
1098 Use C<for>/C<foreach>:
1101 s/foo/bar/; # change that word
1102 y/XZ/ZX/; # swap those letters
1105 Here's another; let's compute spherical volumes:
1107 for (@volumes = @radii) { # @volumes has changed parts
1109 $_ *= (4/3) * 3.14159; # this will be constant folded
1112 If you want to do the same thing to modify the values of the hash,
1113 you may not use the C<values> function, oddly enough. You need a slice:
1115 for $orbit ( @orbits{keys %orbits} ) {
1116 ($orbit **= 3) *= (4/3) * 3.14159;
1119 =head2 How do I select a random element from an array?
1121 Use the rand() function (see L<perlfunc/rand>):
1123 # at the top of the program:
1124 srand; # not needed for 5.004 and later
1127 $index = rand @array;
1128 $element = $array[$index];
1130 Make sure you I<only call srand once per program, if then>.
1131 If you are calling it more than once (such as before each
1132 call to rand), you're almost certainly doing something wrong.
1134 =head2 How do I permute N elements of a list?
1136 Here's a little program that generates all permutations
1137 of all the words on each line of input. The algorithm embodied
1138 in the permute() function should work on any list:
1141 # tsc-permute: permute each word of input
1142 permute([split], []);
1144 my @items = @{ $_[0] };
1145 my @perms = @{ $_[1] };
1149 my(@newitems,@newperms,$i);
1150 foreach $i (0 .. $#items) {
1153 unshift(@newperms, splice(@newitems, $i, 1));
1154 permute([@newitems], [@newperms]);
1159 =head2 How do I sort an array by (anything)?
1161 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1163 @list = sort { $a <=> $b } @list;
1165 The default sort function is cmp, string comparison, which would
1166 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
1167 the numerical comparison operator.
1169 If you have a complicated function needed to pull out the part you
1170 want to sort on, then don't do it inside the sort function. Pull it
1171 out first, because the sort BLOCK can be called many times for the
1172 same element. Here's an example of how to pull out the first word
1173 after the first number on each item, and then sort those words
1178 ($item) = /\d+\s*(\S+)/;
1179 push @idx, uc($item);
1181 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1183 Which could also be written this way, using a trick
1184 that's come to be known as the Schwartzian Transform:
1186 @sorted = map { $_->[0] }
1187 sort { $a->[1] cmp $b->[1] }
1188 map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
1190 If you need to sort on several fields, the following paradigm is useful.
1192 @sorted = sort { field1($a) <=> field1($b) ||
1193 field2($a) cmp field2($b) ||
1194 field3($a) cmp field3($b)
1197 This can be conveniently combined with precalculation of keys as given
1200 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1203 See also the question below on sorting hashes.
1205 =head2 How do I manipulate arrays of bits?
1207 Use pack() and unpack(), or else vec() and the bitwise operations.
1209 For example, this sets $vec to have bit N set if $ints[N] was set:
1212 foreach(@ints) { vec($vec,$_,1) = 1 }
1214 And here's how, given a vector in $vec, you can
1215 get those bits into your @ints array:
1217 sub bitvec_to_list {
1220 # Find null-byte density then select best algorithm
1221 if ($vec =~ tr/\0// / length $vec > 0.95) {
1224 # This method is faster with mostly null-bytes
1225 while($vec =~ /[^\0]/g ) {
1226 $i = -9 + 8 * pos $vec;
1227 push @ints, $i if vec($vec, ++$i, 1);
1228 push @ints, $i if vec($vec, ++$i, 1);
1229 push @ints, $i if vec($vec, ++$i, 1);
1230 push @ints, $i if vec($vec, ++$i, 1);
1231 push @ints, $i if vec($vec, ++$i, 1);
1232 push @ints, $i if vec($vec, ++$i, 1);
1233 push @ints, $i if vec($vec, ++$i, 1);
1234 push @ints, $i if vec($vec, ++$i, 1);
1237 # This method is a fast general algorithm
1239 my $bits = unpack "b*", $vec;
1240 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1241 push @ints, pos $bits while($bits =~ /1/g);
1246 This method gets faster the more sparse the bit vector is.
1247 (Courtesy of Tim Bunce and Winfried Koenig.)
1249 Here's a demo on how to use vec():
1252 $vector = "\xff\x0f\xef\xfe";
1253 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1254 unpack("N", $vector), "\n";
1255 $is_set = vec($vector, 23, 1);
1256 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1274 my ($offset, $width, $value) = @_;
1276 vec($vector, $offset, $width) = $value;
1277 print "offset=$offset width=$width value=$value\n";
1283 my $bits = unpack("b*", $vector);
1287 print "vector length in bytes: ", length($vector), "\n";
1288 @bytes = unpack("A8" x length($vector), $bits);
1289 print "bits are: @bytes\n\n";
1292 =head2 Why does defined() return true on empty arrays and hashes?
1294 The short story is that you should probably only use defined on scalars or
1295 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1296 in the 5.004 release or later of Perl for more detail.
1298 =head1 Data: Hashes (Associative Arrays)
1300 =head2 How do I process an entire hash?
1302 Use the each() function (see L<perlfunc/each>) if you don't care
1303 whether it's sorted:
1305 while ( ($key, $value) = each %hash) {
1306 print "$key = $value\n";
1309 If you want it sorted, you'll have to use foreach() on the result of
1310 sorting the keys as shown in an earlier question.
1312 =head2 What happens if I add or remove keys from a hash while iterating over it?
1316 =head2 How do I look up a hash element by value?
1318 Create a reverse hash:
1320 %by_value = reverse %by_key;
1321 $key = $by_value{$value};
1323 That's not particularly efficient. It would be more space-efficient
1326 while (($key, $value) = each %by_key) {
1327 $by_value{$value} = $key;
1330 If your hash could have repeated values, the methods above will only
1331 find one of the associated keys. This may or may not worry you.
1333 =head2 How can I know how many entries are in a hash?
1335 If you mean how many keys, then all you have to do is
1336 take the scalar sense of the keys() function:
1338 $num_keys = scalar keys %hash;
1340 In void context it just resets the iterator, which is faster
1343 =head2 How do I sort a hash (optionally by value instead of key)?
1345 Internally, hashes are stored in a way that prevents you from imposing
1346 an order on key-value pairs. Instead, you have to sort a list of the
1349 @keys = sort keys %hash; # sorted by key
1351 $hash{$a} cmp $hash{$b}
1352 } keys %hash; # and by value
1354 Here we'll do a reverse numeric sort by value, and if two keys are
1355 identical, sort by length of key, and if that fails, by straight ASCII
1356 comparison of the keys (well, possibly modified by your locale -- see
1360 $hash{$b} <=> $hash{$a}
1362 length($b) <=> length($a)
1367 =head2 How can I always keep my hash sorted?
1369 You can look into using the DB_File module and tie() using the
1370 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1371 The Tie::IxHash module from CPAN might also be instructive.
1373 =head2 What's the difference between "delete" and "undef" with hashes?
1375 Hashes are pairs of scalars: the first is the key, the second is the
1376 value. The key will be coerced to a string, although the value can be
1377 any kind of scalar: string, number, or reference. If a key C<$key> is
1378 present in the array, C<exists($key)> will return true. The value for
1379 a given key can be C<undef>, in which case C<$array{$key}> will be
1380 C<undef> while C<$exists{$key}> will return true. This corresponds to
1381 (C<$key>, C<undef>) being in the hash.
1383 Pictures help... here's the C<%ary> table:
1393 And these conditions hold
1397 defined $ary{'d'} is true
1398 defined $ary{'a'} is true
1399 exists $ary{'a'} is true (perl5 only)
1400 grep ($_ eq 'a', keys %ary) is true
1406 your table now reads:
1417 and these conditions now hold; changes in caps:
1421 defined $ary{'d'} is true
1422 defined $ary{'a'} is FALSE
1423 exists $ary{'a'} is true (perl5 only)
1424 grep ($_ eq 'a', keys %ary) is true
1426 Notice the last two: you have an undef value, but a defined key!
1432 your table now reads:
1441 and these conditions now hold; changes in caps:
1445 defined $ary{'d'} is true
1446 defined $ary{'a'} is false
1447 exists $ary{'a'} is FALSE (perl5 only)
1448 grep ($_ eq 'a', keys %ary) is FALSE
1450 See, the whole entry is gone!
1452 =head2 Why don't my tied hashes make the defined/exists distinction?
1454 They may or may not implement the EXISTS() and DEFINED() methods
1455 differently. For example, there isn't the concept of undef with hashes
1456 that are tied to DBM* files. This means the true/false tables above
1457 will give different results when used on such a hash. It also means
1458 that exists and defined do the same thing with a DBM* file, and what
1459 they end up doing is not what they do with ordinary hashes.
1461 =head2 How do I reset an each() operation part-way through?
1463 Using C<keys %hash> in scalar context returns the number of keys in
1464 the hash I<and> resets the iterator associated with the hash. You may
1465 need to do this if you use C<last> to exit a loop early so that when you
1466 re-enter it, the hash iterator has been reset.
1468 =head2 How can I get the unique keys from two hashes?
1470 First you extract the keys from the hashes into arrays, and then solve
1471 the uniquifying the array problem described above. For example:
1474 for $element (keys(%foo), keys(%bar)) {
1481 @uniq = keys %{{%foo,%bar}};
1483 Or if you really want to save space:
1486 while (defined ($key = each %foo)) {
1489 while (defined ($key = each %bar)) {
1494 =head2 How can I store a multidimensional array in a DBM file?
1496 Either stringify the structure yourself (no fun), or else
1497 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1498 it on top of either DB_File or GDBM_File.
1500 =head2 How can I make my hash remember the order I put elements into it?
1502 Use the Tie::IxHash from CPAN.
1505 tie(%myhash, Tie::IxHash);
1506 for ($i=0; $i<20; $i++) {
1509 @keys = keys %myhash;
1510 # @keys = (0,1,2,3,...)
1512 =head2 Why does passing a subroutine an undefined element in a hash create it?
1514 If you say something like:
1516 somefunc($hash{"nonesuch key here"});
1518 Then that element "autovivifies"; that is, it springs into existence
1519 whether you store something there or not. That's because functions
1520 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1521 it has to be ready to write it back into the caller's version.
1523 This has been fixed as of perl5.004.
1525 Normally, merely accessing a key's value for a nonexistent key does
1526 I<not> cause that key to be forever there. This is different than
1529 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1531 Usually a hash ref, perhaps like this:
1536 TITLE => "deputy peon",
1539 PALS => [ "Norbert", "Rhys", "Phineas"],
1542 References are documented in L<perlref> and the upcoming L<perlreftut>.
1543 Examples of complex data structures are given in L<perldsc> and
1544 L<perllol>. Examples of structures and object-oriented classes are
1547 =head2 How can I use a reference as a hash key?
1549 You can't do this directly, but you could use the standard Tie::Refhash
1550 module distributed with perl.
1554 =head2 How do I handle binary data correctly?
1556 Perl is binary clean, so this shouldn't be a problem. For example,
1557 this works fine (assuming the files are found):
1559 if (`cat /vmunix` =~ /gzip/) {
1560 print "Your kernel is GNU-zip enabled!\n";
1563 On some legacy systems, however, you have to play tedious games with
1564 "text" versus "binary" files. See L<perlfunc/"binmode">, or the upcoming
1565 L<perlopentut> manpage.
1567 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1569 If you want to deal with multibyte characters, however, there are
1570 some gotchas. See the section on Regular Expressions.
1572 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1574 Assuming that you don't care about IEEE notations like "NaN" or
1575 "Infinity", you probably just want to use a regular expression.
1577 if (/\D/) { print "has nondigits\n" }
1578 if (/^\d+$/) { print "is a whole number\n" }
1579 if (/^-?\d+$/) { print "is an integer\n" }
1580 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1581 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1582 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1583 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1584 { print "a C float" }
1586 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1587 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1588 wrapper function for more convenient access. This function takes
1589 a string and returns the number it found, or C<undef> for input that
1590 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1591 if you just want to say, ``Is this a float?''
1594 use POSIX qw(strtod);
1599 my($num, $unparsed) = strtod($str);
1600 if (($str eq '') || ($unparsed != 0) || $!) {
1607 sub is_numeric { defined &getnum }
1609 Or you could check out String::Scanf which can be found at
1610 http://www.perl.com/CPAN/modules/by-module/String/.
1611 The POSIX module (part of the standard Perl distribution) provides
1612 the C<strtol> and C<strtod> for converting strings to double
1613 and longs, respectively.
1615 =head2 How do I keep persistent data across program calls?
1617 For some specific applications, you can use one of the DBM modules.
1618 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1619 Storable, or Class::Eroot modules from CPAN. Here's one example using
1620 Storable's C<store> and C<retrieve> functions:
1623 store(\%hash, "filename");
1626 $href = retrieve("filename"); # by ref
1627 %hash = %{ retrieve("filename") }; # direct to hash
1629 =head2 How do I print out or copy a recursive data structure?
1631 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1632 for printing out data structures. The Storable module, found on CPAN,
1633 provides a function called C<dclone> that recursively copies its argument.
1635 use Storable qw(dclone);
1638 Where $r1 can be a reference to any kind of data structure you'd like.
1639 It will be deeply copied. Because C<dclone> takes and returns references,
1640 you'd have to add extra punctuation if you had a hash of arrays that
1643 %newhash = %{ dclone(\%oldhash) };
1645 =head2 How do I define methods for every class/object?
1647 Use the UNIVERSAL class (see L<UNIVERSAL>).
1649 =head2 How do I verify a credit card checksum?
1651 Get the Business::CreditCard module from CPAN.
1653 =head2 How do I pack arrays of doubles or floats for XS code?
1655 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1656 If you're doing a lot of float or double processing, consider using
1657 the PDL module from CPAN instead--it makes number-crunching easy.
1659 =head1 AUTHOR AND COPYRIGHT
1661 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1662 All rights reserved.
1664 When included as part of the Standard Version of Perl, or as part of
1665 its complete documentation whether printed or otherwise, this work
1666 may be distributed only under the terms of Perl's Artistic Licence.
1667 Any distribution of this file or derivatives thereof I<outside>
1668 of that package require that special arrangements be made with
1671 Irrespective of its distribution, all code examples in this file
1672 are hereby placed into the public domain. You are permitted and
1673 encouraged to use this code in your own programs for fun
1674 or for profit as you see fit. A simple comment in the code giving
1675 credit would be courteous but is not required.