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 http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz
252 =head2 How do I find yesterday's date?
254 The C<time()> function returns the current time in seconds since the
255 epoch. Take one day off that:
257 $yesterday = time() - ( 24 * 60 * 60 );
259 Then you can pass this to C<localtime()> and get the individual year,
260 month, day, hour, minute, seconds values.
262 =head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
264 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
265 Y2K compliant (whatever that means). The programmers you've hired to
266 use it, however, probably are not.
268 Long answer: The question belies a true understanding of the issue.
269 Perl is just as Y2K compliant as your pencil--no more, and no less.
270 Can you use your pencil to write a non-Y2K-compliant memo? Of course
271 you can. Is that the pencil's fault? Of course it isn't.
273 The date and time functions supplied with perl (gmtime and localtime)
274 supply adequate information to determine the year well beyond 2000
275 (2038 is when trouble strikes for 32-bit machines). The year returned
276 by these functions when used in an array context is the year minus 1900.
277 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
278 number. To avoid the year 2000 problem simply do not treat the year as
279 a 2-digit number. It isn't.
281 When gmtime() and localtime() are used in scalar context they return
282 a timestamp string that contains a fully-expanded year. For example,
283 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
284 2001". There's no year 2000 problem here.
286 That doesn't mean that Perl can't be used to create non-Y2K compliant
287 programs. It can. But so can your pencil. It's the fault of the user,
288 not the language. At the risk of inflaming the NRA: ``Perl doesn't
289 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
294 =head2 How do I validate input?
296 The answer to this question is usually a regular expression, perhaps
297 with auxiliary logic. See the more specific questions (numbers, mail
298 addresses, etc.) for details.
300 =head2 How do I unescape a string?
302 It depends just what you mean by ``escape''. URL escapes are dealt
303 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
304 character are removed with:
308 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
310 =head2 How do I remove consecutive pairs of characters?
312 To turn C<"abbcccd"> into C<"abccd">:
316 =head2 How do I expand function calls in a string?
318 This is documented in L<perlref>. In general, this is fraught with
319 quoting and readability problems, but it is possible. To interpolate
320 a subroutine call (in list context) into a string:
322 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
324 If you prefer scalar context, similar chicanery is also useful for
325 arbitrary expressions:
327 print "That yields ${\($n + 5)} widgets\n";
329 Version 5.004 of Perl had a bug that gave list context to the
330 expression in C<${...}>, but this is fixed in version 5.005.
332 See also ``How can I expand variables in text strings?'' in this
335 =head2 How do I find matching/nesting anything?
337 This isn't something that can be done in one regular expression, no
338 matter how complicated. To find something between two single
339 characters, a pattern like C</x([^x]*)x/> will get the intervening
340 bits in $1. For multiple ones, then something more like
341 C</alpha(.*?)omega/> would be needed. But none of these deals with
342 nested patterns, nor can they. For that you'll have to write a
345 If you are serious about writing a parser, there are a number of
346 modules or oddities that will make your life a lot easier. There is
347 the CPAN module Parse::RecDescent, the standard module Text::Balanced,
348 the byacc program, the CPAN module Parse::Yapp, and Mark-Jason
349 Dominus's excellent I<py> tool at http://www.plover.com/~mjd/perl/py/
352 One simple destructive, inside-out approach that you might try is to
353 pull out the smallest nesting parts one at a time:
355 while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) {
356 # do something with $1
359 A more complicated and sneaky approach is to make Perl's regular
360 expression engine do it for you. This is courtesy Dean Inada, and
361 rather has the nature of an Obfuscated Perl Contest entry, but it
364 # $_ contains the string to parse
365 # BEGIN and END are the opening and closing markers for the
370 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
371 @$ = (eval{/$re/},$@!~/unmatched/);
372 print join("\n",@$[0..$#$]) if( $$[-1] );
374 =head2 How do I reverse a string?
376 Use reverse() in scalar context, as documented in
379 $reversed = reverse $string;
381 =head2 How do I expand tabs in a string?
383 You can do it yourself:
385 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
387 Or you can just use the Text::Tabs module (part of the standard perl
391 @expanded_lines = expand(@lines_with_tabs);
393 =head2 How do I reformat a paragraph?
395 Use Text::Wrap (part of the standard perl distribution):
398 print wrap("\t", ' ', @paragraphs);
400 The paragraphs you give to Text::Wrap should not contain embedded
401 newlines. Text::Wrap doesn't justify the lines (flush-right).
403 =head2 How can I access/change the first N letters of a string?
405 There are many ways. If you just want to grab a copy, use
408 $first_byte = substr($a, 0, 1);
410 If you want to modify part of a string, the simplest way is often to
411 use substr() as an lvalue:
413 substr($a, 0, 3) = "Tom";
415 Although those with a pattern matching kind of thought process will
420 =head2 How do I change the Nth occurrence of something?
422 You have to keep track of N yourself. For example, let's say you want
423 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
424 C<"whosoever"> or C<"whomsoever">, case insensitively.
428 ++$count == 5 # is it the 5th?
429 ? "${2}soever" # yes, swap
430 : $1 # renege and leave it there
433 In the more general case, you can use the C</g> modifier in a C<while>
434 loop, keeping count of matches.
438 while (/(\w+)\s+fish\b/gi) {
439 if (++$count == $WANT) {
440 print "The third fish is a $1 one.\n";
441 # Warning: don't `last' out of this loop
445 That prints out: C<"The third fish is a red one."> You can also use a
446 repetition count and repeated pattern like this:
448 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
450 =head2 How can I count the number of occurrences of a substring within a string?
452 There are a number of ways, with varying efficiency: If you want a
453 count of a certain single character (X) within a string, you can use the
454 C<tr///> function like so:
456 $string = "ThisXlineXhasXsomeXx'sXinXit";
457 $count = ($string =~ tr/X//);
458 print "There are $count X charcters in the string";
460 This is fine if you are just looking for a single character. However,
461 if you are trying to count multiple character substrings within a
462 larger string, C<tr///> won't work. What you can do is wrap a while()
463 loop around a global pattern match. For example, let's count negative
466 $string = "-9 55 48 -2 23 -76 4 14 -44";
467 while ($string =~ /-\d+/g) { $count++ }
468 print "There are $count negative numbers in the string";
470 =head2 How do I capitalize all the words on one line?
472 To make the first letter of each word upper case:
474 $line =~ s/\b(\w)/\U$1/g;
476 This has the strange effect of turning "C<don't do it>" into "C<Don'T
477 Do It>". Sometimes you might want this, instead (Suggested by Brian
481 (^\w) #at the beginning of the line
483 (\s\w) #preceded by whitespace
486 $string =~ /([\w']+)/\u\L$1/g;
488 To make the whole line upper case:
492 To force each word to be lower case, with the first letter upper case:
494 $line =~ s/(\w+)/\u\L$1/g;
496 You can (and probably should) enable locale awareness of those
497 characters by placing a C<use locale> pragma in your program.
498 See L<perllocale> for endless details on locales.
500 This is sometimes referred to as putting something into "title
501 case", but that's not quite accurate. Consdier the proper
502 capitalization of the movie I<Dr. Strangelove or: How I Learned to
503 Stop Worrying and Love the Bomb>, for example.
505 =head2 How can I split a [character] delimited string except when inside
506 [character]? (Comma-separated files)
508 Take the example case of trying to split a string that is comma-separated
509 into its different fields. (We'll pretend you said comma-separated, not
510 comma-delimited, which is different and almost never what you mean.) You
511 can't use C<split(/,/)> because you shouldn't split if the comma is inside
512 quotes. For example, take a data line like this:
514 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
516 Due to the restriction of the quotes, this is a fairly complex
517 problem. Thankfully, we have Jeffrey Friedl, author of a highly
518 recommended book on regular expressions, to handle these for us. He
519 suggests (assuming your string is contained in $text):
522 push(@new, $+) while $text =~ m{
523 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
527 push(@new, undef) if substr($text,-1,1) eq ',';
529 If you want to represent quotation marks inside a
530 quotation-mark-delimited field, escape them with backslashes (eg,
531 C<"like \"this\"">. Unescaping them is a task addressed earlier in
534 Alternatively, the Text::ParseWords module (part of the standard perl
535 distribution) lets you say:
537 use Text::ParseWords;
538 @new = quotewords(",", 0, $text);
540 There's also a Text::CSV module on CPAN.
542 =head2 How do I strip blank space from the beginning/end of a string?
544 Although the simplest approach would seem to be:
546 $string =~ s/^\s*(.*?)\s*$/$1/;
548 This is unnecessarily slow, destructive, and fails with embedded newlines.
549 It is much better faster to do this in two steps:
554 Or more nicely written as:
561 This idiom takes advantage of the C<foreach> loop's aliasing
562 behavior to factor out common code. You can do this
563 on several strings at once, or arrays, or even the
564 values of a hash if you use a slide:
566 # trim whitespace in the scalar, the array,
567 # and all the values in the hash
568 foreach ($scalar, @array, @hash{keys %hash}) {
573 =head2 How do I pad a string with blanks or pad a number with zeroes?
575 (This answer contributed by Uri Guttman)
577 In the following examples, C<$pad_len> is the length to which you wish
578 to pad the string, C<$text> or C<$num> contains the string to be
579 padded, and C<$pad_char> contains the padding character. You can use a
580 single character string constant instead of the C<$pad_char> variable
581 if you know what it is in advance.
583 The simplest method use the C<sprintf> function. It can pad on the
584 left or right with blanks and on the left with zeroes.
586 # Left padding with blank:
587 $padded = sprintf( "%${pad_len}s", $text ) ;
589 # Right padding with blank:
590 $padded = sprintf( "%${pad_len}s", $text ) ;
592 # Left padding with 0:
593 $padded = sprintf( "%0${pad_len}d", $num ) ;
595 If you need to pad with a character other than blank or zero you can use
596 one of the following methods.
598 These methods generate a pad string with the C<x> operator and
599 concatenate that with the original text.
601 Left and right padding with any character:
603 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text ;
604 $padded = $text . $pad_char x ( $pad_len - length( $text ) ) ;
606 Or you can left or right pad $text directly:
608 $text .= $pad_char x ( $pad_len - length( $text ) ) ;
609 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ) ;
611 =head2 How do I extract selected columns from a string?
613 Use substr() or unpack(), both documented in L<perlfunc>.
614 If you prefer thinking in terms of columns instead of widths,
615 you can use this kind of thing:
617 # determine the unpack format needed to split Linux ps output
618 # arguments are cut columns
619 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
625 for my $place (@positions) {
626 $template .= "A" . ($place - $lastpos) . " ";
633 =head2 How do I find the soundex value of a string?
635 Use the standard Text::Soundex module distributed with perl.
637 =head2 How can I expand variables in text strings?
639 Let's assume that you have a string like:
641 $text = 'this has a $foo in it and a $bar';
643 If those were both global variables, then this would
646 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
648 But since they are probably lexicals, or at least, they could
649 be, you'd have to do this:
651 $text =~ s/(\$\w+)/$1/eeg;
652 die if $@; # needed /ee, not /e
654 It's probably better in the general case to treat those
655 variables as entries in some special hash. For example:
661 $text =~ s/\$(\w+)/$user_defs{$1}/g;
663 See also ``How do I expand function calls in a string?'' in this section
666 =head2 What's wrong with always quoting "$vars"?
668 The problem is that those double-quotes force stringification,
669 coercing numbers and references into strings, even when you
670 don't want them to be. Think of it this way: double-quote
671 expansion is used to produce new strings. If you already
672 have a string, why do you need more?
674 If you get used to writing odd things like these:
678 somefunc("$var"); # BAD
680 You'll be in trouble. Those should (in 99.8% of the cases) be
681 the simpler and more direct:
687 Otherwise, besides slowing you down, you're going to break code when
688 the thing in the scalar is actually neither a string nor a number, but
694 my $oref = "$aref"; # WRONG
697 You can also get into subtle problems on those few operations in Perl
698 that actually do care about the difference between a string and a
699 number, such as the magical C<++> autoincrement operator or the
702 Stringification also destroys arrays.
705 print "@lines"; # WRONG - extra blanks
706 print @lines; # right
708 =head2 Why don't my E<lt>E<lt>HERE documents work?
710 Check for these three things:
714 =item 1. There must be no space after the << part.
716 =item 2. There (probably) should be a semicolon at the end.
718 =item 3. You can't (easily) have any space in front of the tag.
722 If you want to indent the text in the here document, you
726 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
731 But the HERE_TARGET must still be flush against the margin.
732 If you want that indented also, you'll have to quote
735 ($quote = <<' FINIS') =~ s/^\s+//gm;
736 ...we will have peace, when you and all your works have
737 perished--and the works of your dark master to whom you
738 would deliver us. You are a liar, Saruman, and a corrupter
739 of men's hearts. --Theoden in /usr/src/perl/taint.c
741 $quote =~ s/\s*--/\n--/;
743 A nice general-purpose fixer-upper function for indented here documents
744 follows. It expects to be called with a here document as its argument.
745 It looks to see whether each line begins with a common substring, and
746 if so, strips that off. Otherwise, it takes the amount of leading
747 white space found on the first line and removes that much off each
752 my ($white, $leader); # common white space and common leading string
753 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
754 ($white, $leader) = ($2, quotemeta($1));
756 ($white, $leader) = (/^(\s+)/, '');
758 s/^\s*?$leader(?:$white)?//gm;
762 This works with leading special strings, dynamically determined:
764 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
767 @@@ SAVEI32(runlevel);
769 @@@ while ( op = (*op->op_ppaddr)() ) ;
773 MAIN_INTERPRETER_LOOP
775 Or with a fixed amount of leading white space, with remaining
776 indentation correctly preserved:
778 $poem = fix<<EVER_ON_AND_ON;
779 Now far ahead the Road has gone,
780 And I must follow, if I can,
781 Pursuing it with eager feet,
782 Until it joins some larger way
783 Where many paths and errands meet.
784 And whither then? I cannot say.
785 --Bilbo in /usr/src/perl/pp_ctl.c
790 =head2 What is the difference between a list and an array?
792 An array has a changeable length. A list does not. An array is something
793 you can push or pop, while a list is a set of values. Some people make
794 the distinction that a list is a value while an array is a variable.
795 Subroutines are passed and return lists, you put things into list
796 context, you initialize arrays with lists, and you foreach() across
797 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
798 in scalar context behave like the number of elements in them, subroutines
799 access their arguments through the array C<@_>, push/pop/shift only work
802 As a side note, there's no such thing as a list in scalar context.
805 $scalar = (2, 5, 7, 9);
807 you're using the comma operator in scalar context, so it evaluates the
808 left hand side, then evaluates and returns the left hand side. This
809 causes the last value to be returned: 9.
811 =head2 What is the difference between $array[1] and @array[1]?
813 The former is a scalar value, the latter an array slice, which makes
814 it a list with one (scalar) value. You should use $ when you want a
815 scalar value (most of the time) and @ when you want a list with one
816 scalar value in it (very, very rarely; nearly never, in fact).
818 Sometimes it doesn't make a difference, but sometimes it does.
819 For example, compare:
821 $good[0] = `some program that outputs several lines`;
825 @bad[0] = `same program that outputs several lines`;
827 The B<-w> flag will warn you about these matters.
829 =head2 How can I extract just the unique elements of an array?
831 There are several possible ways, depending on whether the array is
832 ordered and whether you wish to preserve the ordering.
836 =item a) If @in is sorted, and you want @out to be sorted:
837 (this assumes all true values in the array)
840 @out = grep($_ ne $prev && ($prev = $_), @in);
842 This is nice in that it doesn't use much extra memory, simulating
843 uniq(1)'s behavior of removing only adjacent duplicates. It's less
844 nice in that it won't work with false values like undef, 0, or "";
845 "0 but true" is ok, though.
847 =item b) If you don't know whether @in is sorted:
850 @out = grep(!$saw{$_}++, @in);
852 =item c) Like (b), but @in contains only small integers:
854 @out = grep(!$saw[$_]++, @in);
856 =item d) A way to do (b) without any loops or greps:
860 @out = sort keys %saw; # remove sort if undesired
862 =item e) Like (d), but @in contains only small positive integers:
870 But perhaps you should have been using a hash all along, eh?
872 =head2 How can I tell whether a list or array contains a certain element?
874 Hearing the word "in" is an I<in>dication that you probably should have
875 used a hash, not a list or array, to store your data. Hashes are
876 designed to answer this question quickly and efficiently. Arrays aren't.
878 That being said, there are several ways to approach this. If you
879 are going to make this query many times over arbitrary string values,
880 the fastest way is probably to invert the original array and keep an
881 associative array lying about whose keys are the first array's values.
883 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
885 for (@blues) { $is_blue{$_} = 1 }
887 Now you can check whether $is_blue{$some_color}. It might have been a
888 good idea to keep the blues all in a hash in the first place.
890 If the values are all small integers, you could use a simple indexed
891 array. This kind of an array will take up less space:
893 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
894 undef @is_tiny_prime;
895 for (@primes) { $is_tiny_prime[$_] = 1; }
897 Now you check whether $is_tiny_prime[$some_number].
899 If the values in question are integers instead of strings, you can save
900 quite a lot of space by using bit strings instead:
902 @articles = ( 1..10, 150..2000, 2017 );
904 for (@articles) { vec($read,$_,1) = 1 }
906 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
910 $is_there = grep $_ eq $whatever, @array;
914 $is_there = grep /$whatever/, @array;
916 These are slow (checks every element even if the first matches),
917 inefficient (same reason), and potentially buggy (what if there are
918 regexp characters in $whatever?). If you're only testing once, then
922 foreach $elt (@array) {
923 if ($elt eq $elt_to_find) {
928 if ($is_there) { ... }
930 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
932 Use a hash. Here's code to do both and more. It assumes that
933 each element is unique in a given array:
935 @union = @intersection = @difference = ();
937 foreach $element (@array1, @array2) { $count{$element}++ }
938 foreach $element (keys %count) {
939 push @union, $element;
940 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
943 =head2 How do I test whether two arrays or hashes are equal?
945 The following code works for single-level arrays. It uses a stringwise
946 comparison, and does not distinguish defined versus undefined empty
947 strings. Modify if you have other needs.
949 $are_equal = compare_arrays(\@frogs, \@toads);
952 my ($first, $second) = @_;
953 local $^W = 0; # silence spurious -w undef complaints
954 return 0 unless @$first == @$second;
955 for (my $i = 0; $i < @$first; $i++) {
956 return 0 if $first->[$i] ne $second->[$i];
961 For multilevel structures, you may wish to use an approach more
962 like this one. It uses the CPAN module FreezeThaw:
964 use FreezeThaw qw(cmpStr);
965 @a = @b = ( "this", "that", [ "more", "stuff" ] );
967 printf "a and b contain %s arrays\n",
968 cmpStr(\@a, \@b) == 0
972 This approach also works for comparing hashes. Here
973 we'll demonstrate two different answers:
975 use FreezeThaw qw(cmpStr cmpStrHard);
977 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
981 printf "a and b contain %s hashes\n",
982 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
984 printf "a and b contain %s hashes\n",
985 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
988 The first reports that both those the hashes contain the same data,
989 while the second reports that they do not. Which you prefer is left as
990 an exercise to the reader.
992 =head2 How do I find the first array element for which a condition is true?
994 You can use this if you care about the index:
996 for ($i= 0; $i < @array; $i++) {
997 if ($array[$i] eq "Waldo") {
1003 Now C<$found_index> has what you want.
1005 =head2 How do I handle linked lists?
1007 In general, you usually don't need a linked list in Perl, since with
1008 regular arrays, you can push and pop or shift and unshift at either end,
1009 or you can use splice to add and/or remove arbitrary number of elements at
1010 arbitrary points. Both pop and shift are both O(1) operations on perl's
1011 dynamic arrays. In the absence of shifts and pops, push in general
1012 needs to reallocate on the order every log(N) times, and unshift will
1013 need to copy pointers each time.
1015 If you really, really wanted, you could use structures as described in
1016 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1017 to do. For example, imagine a list node like this:
1024 You could walk the list this way:
1027 for ($node = $head; $node; $node = $node->{LINK}) {
1028 print $node->{VALUE}, " ";
1032 You could grow the list this way:
1035 $tail = append($head, 1); # grow a new head
1036 for $value ( 2 .. 10 ) {
1037 $tail = append($tail, $value);
1041 my($list, $value) = @_;
1042 my $node = { VALUE => $value };
1044 $node->{LINK} = $list->{LINK};
1045 $list->{LINK} = $node;
1047 $_[0] = $node; # replace caller's version
1052 But again, Perl's built-in are virtually always good enough.
1054 =head2 How do I handle circular lists?
1056 Circular lists could be handled in the traditional fashion with linked
1057 lists, or you could just do something like this with an array:
1059 unshift(@array, pop(@array)); # the last shall be first
1060 push(@array, shift(@array)); # and vice versa
1062 =head2 How do I shuffle an array randomly?
1066 # fisher_yates_shuffle( \@array ) :
1067 # generate a random permutation of @array in place
1068 sub fisher_yates_shuffle {
1071 for ($i = @$array; --$i; ) {
1072 my $j = int rand ($i+1);
1074 @$array[$i,$j] = @$array[$j,$i];
1078 fisher_yates_shuffle( \@array ); # permutes @array in place
1080 You've probably seen shuffling algorithms that works using splice,
1081 randomly picking another element to swap the current element with:
1085 @old = 1 .. 10; # just a demo
1087 push(@new, splice(@old, rand @old, 1));
1090 This is bad because splice is already O(N), and since you do it N times,
1091 you just invented a quadratic algorithm; that is, O(N**2). This does
1092 not scale, although Perl is so efficient that you probably won't notice
1093 this until you have rather largish arrays.
1095 =head2 How do I process/modify each element of an array?
1097 Use C<for>/C<foreach>:
1100 s/foo/bar/; # change that word
1101 y/XZ/ZX/; # swap those letters
1104 Here's another; let's compute spherical volumes:
1106 for (@volumes = @radii) { # @volumes has changed parts
1108 $_ *= (4/3) * 3.14159; # this will be constant folded
1111 If you want to do the same thing to modify the values of the hash,
1112 you may not use the C<values> function, oddly enough. You need a slice:
1114 for $orbit ( @orbits{keys %orbits} ) {
1115 ($orbit **= 3) *= (4/3) * 3.14159;
1118 =head2 How do I select a random element from an array?
1120 Use the rand() function (see L<perlfunc/rand>):
1122 # at the top of the program:
1123 srand; # not needed for 5.004 and later
1126 $index = rand @array;
1127 $element = $array[$index];
1129 Make sure you I<only call srand once per program, if then>.
1130 If you are calling it more than once (such as before each
1131 call to rand), you're almost certainly doing something wrong.
1133 =head2 How do I permute N elements of a list?
1135 Here's a little program that generates all permutations
1136 of all the words on each line of input. The algorithm embodied
1137 in the permute() function should work on any list:
1140 # tsc-permute: permute each word of input
1141 permute([split], []);
1143 my @items = @{ $_[0] };
1144 my @perms = @{ $_[1] };
1148 my(@newitems,@newperms,$i);
1149 foreach $i (0 .. $#items) {
1152 unshift(@newperms, splice(@newitems, $i, 1));
1153 permute([@newitems], [@newperms]);
1158 =head2 How do I sort an array by (anything)?
1160 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1162 @list = sort { $a <=> $b } @list;
1164 The default sort function is cmp, string comparison, which would
1165 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
1166 the numerical comparison operator.
1168 If you have a complicated function needed to pull out the part you
1169 want to sort on, then don't do it inside the sort function. Pull it
1170 out first, because the sort BLOCK can be called many times for the
1171 same element. Here's an example of how to pull out the first word
1172 after the first number on each item, and then sort those words
1177 ($item) = /\d+\s*(\S+)/;
1178 push @idx, uc($item);
1180 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1182 Which could also be written this way, using a trick
1183 that's come to be known as the Schwartzian Transform:
1185 @sorted = map { $_->[0] }
1186 sort { $a->[1] cmp $b->[1] }
1187 map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
1189 If you need to sort on several fields, the following paradigm is useful.
1191 @sorted = sort { field1($a) <=> field1($b) ||
1192 field2($a) cmp field2($b) ||
1193 field3($a) cmp field3($b)
1196 This can be conveniently combined with precalculation of keys as given
1199 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1202 See also the question below on sorting hashes.
1204 =head2 How do I manipulate arrays of bits?
1206 Use pack() and unpack(), or else vec() and the bitwise operations.
1208 For example, this sets $vec to have bit N set if $ints[N] was set:
1211 foreach(@ints) { vec($vec,$_,1) = 1 }
1213 And here's how, given a vector in $vec, you can
1214 get those bits into your @ints array:
1216 sub bitvec_to_list {
1219 # Find null-byte density then select best algorithm
1220 if ($vec =~ tr/\0// / length $vec > 0.95) {
1223 # This method is faster with mostly null-bytes
1224 while($vec =~ /[^\0]/g ) {
1225 $i = -9 + 8 * pos $vec;
1226 push @ints, $i if vec($vec, ++$i, 1);
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);
1236 # This method is a fast general algorithm
1238 my $bits = unpack "b*", $vec;
1239 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1240 push @ints, pos $bits while($bits =~ /1/g);
1245 This method gets faster the more sparse the bit vector is.
1246 (Courtesy of Tim Bunce and Winfried Koenig.)
1248 Here's a demo on how to use vec():
1251 $vector = "\xff\x0f\xef\xfe";
1252 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1253 unpack("N", $vector), "\n";
1254 $is_set = vec($vector, 23, 1);
1255 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1273 my ($offset, $width, $value) = @_;
1275 vec($vector, $offset, $width) = $value;
1276 print "offset=$offset width=$width value=$value\n";
1282 my $bits = unpack("b*", $vector);
1286 print "vector length in bytes: ", length($vector), "\n";
1287 @bytes = unpack("A8" x length($vector), $bits);
1288 print "bits are: @bytes\n\n";
1291 =head2 Why does defined() return true on empty arrays and hashes?
1293 The short story is that you should probably only use defined on scalars or
1294 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1295 in the 5.004 release or later of Perl for more detail.
1297 =head1 Data: Hashes (Associative Arrays)
1299 =head2 How do I process an entire hash?
1301 Use the each() function (see L<perlfunc/each>) if you don't care
1302 whether it's sorted:
1304 while ( ($key, $value) = each %hash) {
1305 print "$key = $value\n";
1308 If you want it sorted, you'll have to use foreach() on the result of
1309 sorting the keys as shown in an earlier question.
1311 =head2 What happens if I add or remove keys from a hash while iterating over it?
1315 =head2 How do I look up a hash element by value?
1317 Create a reverse hash:
1319 %by_value = reverse %by_key;
1320 $key = $by_value{$value};
1322 That's not particularly efficient. It would be more space-efficient
1325 while (($key, $value) = each %by_key) {
1326 $by_value{$value} = $key;
1329 If your hash could have repeated values, the methods above will only
1330 find one of the associated keys. This may or may not worry you.
1332 =head2 How can I know how many entries are in a hash?
1334 If you mean how many keys, then all you have to do is
1335 take the scalar sense of the keys() function:
1337 $num_keys = scalar keys %hash;
1339 In void context it just resets the iterator, which is faster
1342 =head2 How do I sort a hash (optionally by value instead of key)?
1344 Internally, hashes are stored in a way that prevents you from imposing
1345 an order on key-value pairs. Instead, you have to sort a list of the
1348 @keys = sort keys %hash; # sorted by key
1350 $hash{$a} cmp $hash{$b}
1351 } keys %hash; # and by value
1353 Here we'll do a reverse numeric sort by value, and if two keys are
1354 identical, sort by length of key, and if that fails, by straight ASCII
1355 comparison of the keys (well, possibly modified by your locale -- see
1359 $hash{$b} <=> $hash{$a}
1361 length($b) <=> length($a)
1366 =head2 How can I always keep my hash sorted?
1368 You can look into using the DB_File module and tie() using the
1369 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1370 The Tie::IxHash module from CPAN might also be instructive.
1372 =head2 What's the difference between "delete" and "undef" with hashes?
1374 Hashes are pairs of scalars: the first is the key, the second is the
1375 value. The key will be coerced to a string, although the value can be
1376 any kind of scalar: string, number, or reference. If a key C<$key> is
1377 present in the array, C<exists($key)> will return true. The value for
1378 a given key can be C<undef>, in which case C<$array{$key}> will be
1379 C<undef> while C<$exists{$key}> will return true. This corresponds to
1380 (C<$key>, C<undef>) being in the hash.
1382 Pictures help... here's the C<%ary> table:
1392 And these conditions hold
1396 defined $ary{'d'} is true
1397 defined $ary{'a'} is true
1398 exists $ary{'a'} is true (perl5 only)
1399 grep ($_ eq 'a', keys %ary) is true
1405 your table now reads:
1416 and these conditions now hold; changes in caps:
1420 defined $ary{'d'} is true
1421 defined $ary{'a'} is FALSE
1422 exists $ary{'a'} is true (perl5 only)
1423 grep ($_ eq 'a', keys %ary) is true
1425 Notice the last two: you have an undef value, but a defined key!
1431 your table now reads:
1440 and these conditions now hold; changes in caps:
1444 defined $ary{'d'} is true
1445 defined $ary{'a'} is false
1446 exists $ary{'a'} is FALSE (perl5 only)
1447 grep ($_ eq 'a', keys %ary) is FALSE
1449 See, the whole entry is gone!
1451 =head2 Why don't my tied hashes make the defined/exists distinction?
1453 They may or may not implement the EXISTS() and DEFINED() methods
1454 differently. For example, there isn't the concept of undef with hashes
1455 that are tied to DBM* files. This means the true/false tables above
1456 will give different results when used on such a hash. It also means
1457 that exists and defined do the same thing with a DBM* file, and what
1458 they end up doing is not what they do with ordinary hashes.
1460 =head2 How do I reset an each() operation part-way through?
1462 Using C<keys %hash> in scalar context returns the number of keys in
1463 the hash I<and> resets the iterator associated with the hash. You may
1464 need to do this if you use C<last> to exit a loop early so that when you
1465 re-enter it, the hash iterator has been reset.
1467 =head2 How can I get the unique keys from two hashes?
1469 First you extract the keys from the hashes into arrays, and then solve
1470 the uniquifying the array problem described above. For example:
1473 for $element (keys(%foo), keys(%bar)) {
1480 @uniq = keys %{{%foo,%bar}};
1482 Or if you really want to save space:
1485 while (defined ($key = each %foo)) {
1488 while (defined ($key = each %bar)) {
1493 =head2 How can I store a multidimensional array in a DBM file?
1495 Either stringify the structure yourself (no fun), or else
1496 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1497 it on top of either DB_File or GDBM_File.
1499 =head2 How can I make my hash remember the order I put elements into it?
1501 Use the Tie::IxHash from CPAN.
1504 tie(%myhash, Tie::IxHash);
1505 for ($i=0; $i<20; $i++) {
1508 @keys = keys %myhash;
1509 # @keys = (0,1,2,3,...)
1511 =head2 Why does passing a subroutine an undefined element in a hash create it?
1513 If you say something like:
1515 somefunc($hash{"nonesuch key here"});
1517 Then that element "autovivifies"; that is, it springs into existence
1518 whether you store something there or not. That's because functions
1519 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1520 it has to be ready to write it back into the caller's version.
1522 This has been fixed as of perl5.004.
1524 Normally, merely accessing a key's value for a nonexistent key does
1525 I<not> cause that key to be forever there. This is different than
1528 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1530 Usually a hash ref, perhaps like this:
1535 TITLE => "deputy peon",
1538 PALS => [ "Norbert", "Rhys", "Phineas"],
1541 References are documented in L<perlref> and the upcoming L<perlreftut>.
1542 Examples of complex data structures are given in L<perldsc> and
1543 L<perllol>. Examples of structures and object-oriented classes are
1546 =head2 How can I use a reference as a hash key?
1548 You can't do this directly, but you could use the standard Tie::Refhash
1549 module distributed with perl.
1553 =head2 How do I handle binary data correctly?
1555 Perl is binary clean, so this shouldn't be a problem. For example,
1556 this works fine (assuming the files are found):
1558 if (`cat /vmunix` =~ /gzip/) {
1559 print "Your kernel is GNU-zip enabled!\n";
1562 On some legacy systems, however, you have to play tedious games with
1563 "text" versus "binary" files. See L<perlfunc/"binmode">, or the upcoming
1564 L<perlopentut> manpage.
1566 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1568 If you want to deal with multibyte characters, however, there are
1569 some gotchas. See the section on Regular Expressions.
1571 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1573 Assuming that you don't care about IEEE notations like "NaN" or
1574 "Infinity", you probably just want to use a regular expression.
1576 if (/\D/) { print "has nondigits\n" }
1577 if (/^\d+$/) { print "is a whole number\n" }
1578 if (/^-?\d+$/) { print "is an integer\n" }
1579 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1580 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1581 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1582 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1583 { print "a C float" }
1585 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1586 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1587 wrapper function for more convenient access. This function takes
1588 a string and returns the number it found, or C<undef> for input that
1589 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1590 if you just want to say, ``Is this a float?''
1593 use POSIX qw(strtod);
1598 my($num, $unparsed) = strtod($str);
1599 if (($str eq '') || ($unparsed != 0) || $!) {
1606 sub is_numeric { defined &getnum }
1608 Or you could check out
1609 http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
1610 instead. The POSIX module (part of the standard Perl distribution)
1611 provides the C<strtol> and C<strtod> for converting strings to double
1612 and longs, respectively.
1614 =head2 How do I keep persistent data across program calls?
1616 For some specific applications, you can use one of the DBM modules.
1617 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1618 Storable, or Class::Eroot modules from CPAN. Here's one example using
1619 Storable's C<store> and C<retrieve> functions:
1622 store(\%hash, "filename");
1625 $href = retrieve("filename"); # by ref
1626 %hash = %{ retrieve("filename") }; # direct to hash
1628 =head2 How do I print out or copy a recursive data structure?
1630 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1631 for printing out data structures. The Storable module, found on CPAN,
1632 provides a function called C<dclone> that recursively copies its argument.
1634 use Storable qw(dclone);
1637 Where $r1 can be a reference to any kind of data structure you'd like.
1638 It will be deeply copied. Because C<dclone> takes and returns references,
1639 you'd have to add extra punctuation if you had a hash of arrays that
1642 %newhash = %{ dclone(\%oldhash) };
1644 =head2 How do I define methods for every class/object?
1646 Use the UNIVERSAL class (see L<UNIVERSAL>).
1648 =head2 How do I verify a credit card checksum?
1650 Get the Business::CreditCard module from CPAN.
1652 =head2 How do I pack arrays of doubles or floats for XS code?
1654 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1655 If you're doing a lot of float or double processing, consider using
1656 the PDL module from CPAN instead--it makes number-crunching easy.
1658 =head1 AUTHOR AND COPYRIGHT
1660 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1661 All rights reserved.
1663 When included as part of the Standard Version of Perl, or as part of
1664 its complete documentation whether printed or otherwise, this work
1665 may be distributed only under the terms of Perl's Artistic License.
1666 Any distribution of this file or derivatives thereof I<outside>
1667 of that package require that special arrangements be made with
1670 Irrespective of its distribution, all code examples in this file
1671 are hereby placed into the public domain. You are permitted and
1672 encouraged to use this code in your own programs for fun
1673 or for profit as you see fit. A simple comment in the code giving
1674 credit would be courteous but is not required.