3 perlfaq4 - Data Manipulation ($Revision: 1.39 $, $Date: 2003/01/03 20:06:21 $)
7 This section of the FAQ answers questions related to manipulating
8 numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
12 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
14 Internally, your computer represents floating-point numbers
15 in binary. Digital (as in powers of two) computers cannot
16 store all numbers exactly. Some real numbers lose precision
17 in the process. This is a problem with how computers store
18 numbers and affects all computer languages, not just Perl.
20 L<perlnumber> show the gory details of number
21 representations and conversions.
23 To limit the number of decimal places in your numbers, you
24 can use the printf or sprintf function. See the
25 L<"Floating Point Arithmetic"|perlop> for more details.
29 my $number = sprintf "%.2f", 10/3;
31 =head2 Why isn't my octal data interpreted correctly?
33 Perl only understands octal and hex numbers as such when they occur as
34 literals in your program. Octal literals in perl must start with a
35 leading "0" and hexadecimal literals must start with a leading "0x".
36 If they are read in from somewhere and assigned, no automatic
37 conversion takes place. You must explicitly use oct() or hex() if you
38 want the values converted to decimal. oct() interprets hex ("0x350"),
39 octal ("0350" or even without the leading "0", like "377") and binary
40 ("0b1010") numbers, while hex() only converts hexadecimal ones, with
41 or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
42 The inverse mapping from decimal to octal can be done with either the
43 "%o" or "%O" sprintf() formats.
45 This problem shows up most often when people try using chmod(), mkdir(),
46 umask(), or sysopen(), which by widespread tradition typically take
49 chmod(644, $file); # WRONG
50 chmod(0644, $file); # right
52 Note the mistake in the first line was specifying the decimal literal
53 644, rather than the intended octal literal 0644. The problem can
56 printf("%#o",644); # prints 01204
58 Surely you had not intended C<chmod(01204, $file);> - did you? If you
59 want to use numeric literals as arguments to chmod() et al. then please
60 try to express them as octal constants, that is with a leading zero and
61 with the following digits restricted to the set 0..7.
63 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
65 Remember that int() merely truncates toward 0. For rounding to a
66 certain number of digits, sprintf() or printf() is usually the easiest
69 printf("%.3f", 3.1415926535); # prints 3.142
71 The POSIX module (part of the standard Perl distribution) implements
72 ceil(), floor(), and a number of other mathematical and trigonometric
76 $ceil = ceil(3.5); # 4
77 $floor = floor(3.5); # 3
79 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
80 module. With 5.004, the Math::Trig module (part of the standard Perl
81 distribution) implements the trigonometric functions. Internally it
82 uses the Math::Complex module and some functions can break out from
83 the real axis into the complex plane, for example the inverse sine of
86 Rounding in financial applications can have serious implications, and
87 the rounding method used should be specified precisely. In these
88 cases, it probably pays not to trust whichever system rounding is
89 being used by Perl, but to instead implement the rounding function you
92 To see why, notice how you'll still have an issue on half-way-point
95 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
97 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
98 0.8 0.8 0.9 0.9 1.0 1.0
100 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
101 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
102 machines) will work pretty much like mathematical integers. Other numbers
105 =head2 How do I convert between numeric representations?
107 As always with Perl there is more than one way to do it. Below
108 are a few examples of approaches to making common conversions
109 between number representations. This is intended to be representational
110 rather than exhaustive.
112 Some of the examples below use the Bit::Vector module from CPAN.
113 The reason you might choose Bit::Vector over the perl built in
114 functions is that it works with numbers of ANY size, that it is
115 optimized for speed on some operations, and for at least some
116 programmers the notation might be familiar.
120 =item How do I convert hexadecimal into decimal
122 Using perl's built in conversion of 0x notation:
125 $dec = sprintf("%d", $int);
127 Using the hex function:
129 $int = hex("DEADBEEF");
130 $dec = sprintf("%d", $int);
134 $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
135 $dec = sprintf("%d", $int);
137 Using the CPAN module Bit::Vector:
140 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
141 $dec = $vec->to_Dec();
143 =item How do I convert from decimal to hexadecimal
147 $hex = sprintf("%X", 3735928559);
151 $hex = unpack("H*", pack("N", 3735928559));
156 $vec = Bit::Vector->new_Dec(32, -559038737);
157 $hex = $vec->to_Hex();
159 And Bit::Vector supports odd bit counts:
162 $vec = Bit::Vector->new_Dec(33, 3735928559);
163 $vec->Resize(32); # suppress leading 0 if unwanted
164 $hex = $vec->to_Hex();
166 =item How do I convert from octal to decimal
168 Using Perl's built in conversion of numbers with leading zeros:
170 $int = 033653337357; # note the leading 0!
171 $dec = sprintf("%d", $int);
173 Using the oct function:
175 $int = oct("33653337357");
176 $dec = sprintf("%d", $int);
181 $vec = Bit::Vector->new(32);
182 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
183 $dec = $vec->to_Dec();
185 =item How do I convert from decimal to octal
189 $oct = sprintf("%o", 3735928559);
194 $vec = Bit::Vector->new_Dec(32, -559038737);
195 $oct = reverse join('', $vec->Chunk_List_Read(3));
197 =item How do I convert from binary to decimal
199 Perl 5.6 lets you write binary numbers directly with
202 $number = 0b10110110;
206 $decimal = ord(pack('B8', '10110110'));
208 Using pack and unpack for larger strings
210 $int = unpack("N", pack("B32",
211 substr("0" x 32 . "11110101011011011111011101111", -32)));
212 $dec = sprintf("%d", $int);
214 # substr() is used to left pad a 32 character string with zeros.
218 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
219 $dec = $vec->to_Dec();
221 =item How do I convert from decimal to binary
225 $bin = unpack("B*", pack("N", 3735928559));
230 $vec = Bit::Vector->new_Dec(32, -559038737);
231 $bin = $vec->to_Bin();
233 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
234 are left as an exercise to the inclined reader.
238 =head2 Why doesn't & work the way I want it to?
240 The behavior of binary arithmetic operators depends on whether they're
241 used on numbers or strings. The operators treat a string as a series
242 of bits and work with that (the string C<"3"> is the bit pattern
243 C<00110011>). The operators work with the binary form of a number
244 (the number C<3> is treated as the bit pattern C<00000011>).
246 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
247 C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
250 Most problems with C<&> and C<|> arise because the programmer thinks
251 they have a number but really it's a string. The rest arise because
254 if ("\020\020" & "\101\101") {
258 but a string consisting of two null bytes (the result of C<"\020\020"
259 & "\101\101">) is not a false value in Perl. You need:
261 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
265 =head2 How do I multiply matrices?
267 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
268 or the PDL extension (also available from CPAN).
270 =head2 How do I perform an operation on a series of integers?
272 To call a function on each element in an array, and collect the
275 @results = map { my_func($_) } @array;
279 @triple = map { 3 * $_ } @single;
281 To call a function on each element of an array, but ignore the
284 foreach $iterator (@array) {
285 some_func($iterator);
288 To call a function on each integer in a (small) range, you B<can> use:
290 @results = map { some_func($_) } (5 .. 25);
292 but you should be aware that the C<..> operator creates an array of
293 all integers in the range. This can take a lot of memory for large
297 for ($i=5; $i < 500_005; $i++) {
298 push(@results, some_func($i));
301 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
302 loop will iterate over the range, without creating the entire range.
304 for my $i (5 .. 500_005) {
305 push(@results, some_func($i));
308 will not create a list of 500,000 integers.
310 =head2 How can I output Roman numerals?
312 Get the http://www.cpan.org/modules/by-module/Roman module.
314 =head2 Why aren't my random numbers random?
316 If you're using a version of Perl before 5.004, you must call C<srand>
317 once at the start of your program to seed the random number generator.
319 BEGIN { srand() if $[ < 5.004 }
321 5.004 and later automatically call C<srand> at the beginning. Don't
322 call C<srand> more than once---you make your numbers less random, rather
325 Computers are good at being predictable and bad at being random
326 (despite appearances caused by bugs in your programs :-). see the
327 F<random> article in the "Far More Than You Ever Wanted To Know"
328 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
329 Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
330 who attempts to generate random numbers by deterministic means is, of
331 course, living in a state of sin.''
333 If you want numbers that are more random than C<rand> with C<srand>
334 provides, you should also check out the Math::TrulyRandom module from
335 CPAN. It uses the imperfections in your system's timer to generate
336 random numbers, but this takes quite a while. If you want a better
337 pseudorandom generator than comes with your operating system, look at
338 ``Numerical Recipes in C'' at http://www.nr.com/ .
340 =head2 How do I get a random number between X and Y?
342 Use the following simple function. It selects a random integer between
343 (and possibly including!) the two given integers, e.g.,
344 C<random_int_in(50,120)>
346 sub random_int_in ($$) {
348 # Assumes that the two arguments are integers themselves!
349 return $min if $min == $max;
350 ($min, $max) = ($max, $min) if $min > $max;
351 return $min + int rand(1 + $max - $min);
356 =head2 How do I find the week-of-the-year/day-of-the-year?
358 The day of the year is in the array returned by localtime() (see
359 L<perlfunc/"localtime">):
361 $day_of_year = (localtime(time()))[7];
363 =head2 How do I find the current century or millennium?
365 Use the following simple functions:
368 return int((((localtime(shift || time))[5] + 1999))/100);
371 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
374 You can also use the POSIX strftime() function which may be a bit
375 slower but is easier to read and maintain.
377 use POSIX qw/strftime/;
379 my $week_of_the_year = strftime "%W", localtime;
380 my $day_of_the_year = strftime "%j", localtime;
382 On some systems, the POSIX module's strftime() function has
383 been extended in a non-standard way to use a C<%C> format,
384 which they sometimes claim is the "century". It isn't,
385 because on most such systems, this is only the first two
386 digits of the four-digit year, and thus cannot be used to
387 reliably determine the current century or millennium.
389 =head2 How can I compare two dates and find the difference?
391 If you're storing your dates as epoch seconds then simply subtract one
392 from the other. If you've got a structured date (distinct year, day,
393 month, hour, minute, seconds values), then for reasons of accessibility,
394 simplicity, and efficiency, merely use either timelocal or timegm (from
395 the Time::Local module in the standard distribution) to reduce structured
396 dates to epoch seconds. However, if you don't know the precise format of
397 your dates, then you should probably use either of the Date::Manip and
398 Date::Calc modules from CPAN before you go hacking up your own parsing
399 routine to handle arbitrary date formats.
401 =head2 How can I take a string and turn it into epoch seconds?
403 If it's a regular enough string that it always has the same format,
404 you can split it up and pass the parts to C<timelocal> in the standard
405 Time::Local module. Otherwise, you should look into the Date::Calc
406 and Date::Manip modules from CPAN.
408 =head2 How can I find the Julian Day?
410 Use the Time::JulianDay module (part of the Time-modules bundle
411 available from CPAN.)
413 Before you immerse yourself too deeply in this, be sure to verify that
414 it is the I<Julian> Day you really want. Are you interested in a way
415 of getting serial days so that you just can tell how many days they
416 are apart or so that you can do also other date arithmetic? If you
417 are interested in performing date arithmetic, this can be done using
418 modules Date::Manip or Date::Calc.
420 There is too many details and much confusion on this issue to cover in
421 this FAQ, but the term is applied (correctly) to a calendar now
422 supplanted by the Gregorian Calendar, with the Julian Calendar failing
423 to adjust properly for leap years on centennial years (among other
424 annoyances). The term is also used (incorrectly) to mean: [1] days in
425 the Gregorian Calendar; and [2] days since a particular starting time
426 or `epoch', usually 1970 in the Unix world and 1980 in the
427 MS-DOS/Windows world. If you find that it is not the first meaning
428 that you really want, then check out the Date::Manip and Date::Calc
429 modules. (Thanks to David Cassell for most of this text.)
431 =head2 How do I find yesterday's date?
433 If you only need to find the date (and not the same time), you
434 can use the Date::Calc module.
436 use Date::Calc qw(Today Add_Delta_Days);
438 my @date = Add_Delta_Days( Today(), -1 );
442 Most people try to use the time rather than the calendar to
443 figure out dates, but that assumes that your days are
444 twenty-four hours each. For most people, there are two days
445 a year when they aren't: the switch to and from summer time
446 throws this off. Russ Allbery offers this solution.
449 my $now = defined $_[0] ? $_[0] : time;
450 my $then = $now - 60 * 60 * 24;
451 my $ndst = (localtime $now)[8] > 0;
452 my $tdst = (localtime $then)[8] > 0;
453 $then - ($tdst - $ndst) * 60 * 60;
456 Should give you "this time yesterday" in seconds since epoch relative to
457 the first argument or the current time if no argument is given and
458 suitable for passing to localtime or whatever else you need to do with
459 it. $ndst is whether we're currently in daylight savings time; $tdst is
460 whether the point 24 hours ago was in daylight savings time. If $tdst
461 and $ndst are the same, a boundary wasn't crossed, and the correction
462 will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
463 from yesterday's time since we gained an extra hour while going off
464 daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
465 negative hour (add an hour) to yesterday's time since we lost an hour.
467 All of this is because during those days when one switches off or onto
468 DST, a "day" isn't 24 hours long; it's either 23 or 25.
470 The explicit settings of $ndst and $tdst are necessary because localtime
471 only says it returns the system tm struct, and the system tm struct at
472 least on Solaris doesn't guarantee any particular positive value (like,
473 say, 1) for isdst, just a positive value. And that value can
474 potentially be negative, if DST information isn't available (this sub
475 just treats those cases like no DST).
477 Note that between 2am and 3am on the day after the time zone switches
478 off daylight savings time, the exact hour of "yesterday" corresponding
479 to the current hour is not clearly defined. Note also that if used
480 between 2am and 3am the day after the change to daylight savings time,
481 the result will be between 3am and 4am of the previous day; it's
482 arguable whether this is correct.
484 This sub does not attempt to deal with leap seconds (most things don't).
488 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
490 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
491 Y2K compliant (whatever that means). The programmers you've hired to
492 use it, however, probably are not.
494 Long answer: The question belies a true understanding of the issue.
495 Perl is just as Y2K compliant as your pencil--no more, and no less.
496 Can you use your pencil to write a non-Y2K-compliant memo? Of course
497 you can. Is that the pencil's fault? Of course it isn't.
499 The date and time functions supplied with Perl (gmtime and localtime)
500 supply adequate information to determine the year well beyond 2000
501 (2038 is when trouble strikes for 32-bit machines). The year returned
502 by these functions when used in a list context is the year minus 1900.
503 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
504 number. To avoid the year 2000 problem simply do not treat the year as
505 a 2-digit number. It isn't.
507 When gmtime() and localtime() are used in scalar context they return
508 a timestamp string that contains a fully-expanded year. For example,
509 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
510 2001". There's no year 2000 problem here.
512 That doesn't mean that Perl can't be used to create non-Y2K compliant
513 programs. It can. But so can your pencil. It's the fault of the user,
514 not the language. At the risk of inflaming the NRA: ``Perl doesn't
515 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
520 =head2 How do I validate input?
522 The answer to this question is usually a regular expression, perhaps
523 with auxiliary logic. See the more specific questions (numbers, mail
524 addresses, etc.) for details.
526 =head2 How do I unescape a string?
528 It depends just what you mean by ``escape''. URL escapes are dealt
529 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
530 character are removed with
534 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
536 =head2 How do I remove consecutive pairs of characters?
538 To turn C<"abbcccd"> into C<"abccd">:
540 s/(.)\1/$1/g; # add /s to include newlines
542 Here's a solution that turns "abbcccd" to "abcd":
544 y///cs; # y == tr, but shorter :-)
546 =head2 How do I expand function calls in a string?
548 This is documented in L<perlref>. In general, this is fraught with
549 quoting and readability problems, but it is possible. To interpolate
550 a subroutine call (in list context) into a string:
552 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
554 See also ``How can I expand variables in text strings?'' in this
557 =head2 How do I find matching/nesting anything?
559 This isn't something that can be done in one regular expression, no
560 matter how complicated. To find something between two single
561 characters, a pattern like C</x([^x]*)x/> will get the intervening
562 bits in $1. For multiple ones, then something more like
563 C</alpha(.*?)omega/> would be needed. But none of these deals with
564 nested patterns. For balanced expressions using C<(>, C<{>, C<[>
565 or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
566 L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
568 If you are serious about writing a parser, there are a number of
569 modules or oddities that will make your life a lot easier. There are
570 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
571 and the byacc program. Starting from perl 5.8 the Text::Balanced
572 is part of the standard distribution.
574 One simple destructive, inside-out approach that you might try is to
575 pull out the smallest nesting parts one at a time:
577 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
578 # do something with $1
581 A more complicated and sneaky approach is to make Perl's regular
582 expression engine do it for you. This is courtesy Dean Inada, and
583 rather has the nature of an Obfuscated Perl Contest entry, but it
586 # $_ contains the string to parse
587 # BEGIN and END are the opening and closing markers for the
592 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
593 @$ = (eval{/$re/},$@!~/unmatched/i);
594 print join("\n",@$[0..$#$]) if( $$[-1] );
596 =head2 How do I reverse a string?
598 Use reverse() in scalar context, as documented in
601 $reversed = reverse $string;
603 =head2 How do I expand tabs in a string?
605 You can do it yourself:
607 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
609 Or you can just use the Text::Tabs module (part of the standard Perl
613 @expanded_lines = expand(@lines_with_tabs);
615 =head2 How do I reformat a paragraph?
617 Use Text::Wrap (part of the standard Perl distribution):
620 print wrap("\t", ' ', @paragraphs);
622 The paragraphs you give to Text::Wrap should not contain embedded
623 newlines. Text::Wrap doesn't justify the lines (flush-right).
625 Or use the CPAN module Text::Autoformat. Formatting files can be easily
626 done by making a shell alias, like so:
628 alias fmt="perl -i -MText::Autoformat -n0777 \
629 -e 'print autoformat $_, {all=>1}' $*"
631 See the documentation for Text::Autoformat to appreciate its many
634 =head2 How can I access or change N characters of a string?
636 You can access the first characters of a string with substr().
637 To get the first character, for example, start at position 0
638 and grab the string of length 1.
641 $string = "Just another Perl Hacker";
642 $first_char = substr( $string, 0, 1 ); # 'J'
644 To change part of a string, you can use the optional fourth
645 argument which is the replacement string.
647 substr( $string, 13, 4, "Perl 5.8.0" );
649 You can also use substr() as an lvalue.
651 substr( $string, 13, 4 ) = "Perl 5.8.0";
653 =head2 How do I change the Nth occurrence of something?
655 You have to keep track of N yourself. For example, let's say you want
656 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
657 C<"whosoever"> or C<"whomsoever">, case insensitively. These
658 all assume that $_ contains the string to be altered.
662 ++$count == 5 # is it the 5th?
663 ? "${2}soever" # yes, swap
664 : $1 # renege and leave it there
667 In the more general case, you can use the C</g> modifier in a C<while>
668 loop, keeping count of matches.
672 $_ = "One fish two fish red fish blue fish";
673 while (/(\w+)\s+fish\b/gi) {
674 if (++$count == $WANT) {
675 print "The third fish is a $1 one.\n";
679 That prints out: C<"The third fish is a red one."> You can also use a
680 repetition count and repeated pattern like this:
682 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
684 =head2 How can I count the number of occurrences of a substring within a string?
686 There are a number of ways, with varying efficiency. If you want a
687 count of a certain single character (X) within a string, you can use the
688 C<tr///> function like so:
690 $string = "ThisXlineXhasXsomeXx'sXinXit";
691 $count = ($string =~ tr/X//);
692 print "There are $count X characters in the string";
694 This is fine if you are just looking for a single character. However,
695 if you are trying to count multiple character substrings within a
696 larger string, C<tr///> won't work. What you can do is wrap a while()
697 loop around a global pattern match. For example, let's count negative
700 $string = "-9 55 48 -2 23 -76 4 14 -44";
701 while ($string =~ /-\d+/g) { $count++ }
702 print "There are $count negative numbers in the string";
704 Another version uses a global match in list context, then assigns the
705 result to a scalar, producing a count of the number of matches.
707 $count = () = $string =~ /-\d+/g;
709 =head2 How do I capitalize all the words on one line?
711 To make the first letter of each word upper case:
713 $line =~ s/\b(\w)/\U$1/g;
715 This has the strange effect of turning "C<don't do it>" into "C<Don'T
716 Do It>". Sometimes you might want this. Other times you might need a
717 more thorough solution (Suggested by brian d foy):
720 (^\w) #at the beginning of the line
722 (\s\w) #preceded by whitespace
725 $string =~ /([\w']+)/\u\L$1/g;
727 To make the whole line upper case:
731 To force each word to be lower case, with the first letter upper case:
733 $line =~ s/(\w+)/\u\L$1/g;
735 You can (and probably should) enable locale awareness of those
736 characters by placing a C<use locale> pragma in your program.
737 See L<perllocale> for endless details on locales.
739 This is sometimes referred to as putting something into "title
740 case", but that's not quite accurate. Consider the proper
741 capitalization of the movie I<Dr. Strangelove or: How I Learned to
742 Stop Worrying and Love the Bomb>, for example.
744 =head2 How can I split a [character] delimited string except when inside [character]?
746 Several modules can handle this sort of pasing---Text::Balanced,
747 Text::CVS, Text::CVS_XS, and Text::ParseWords, among others.
749 Take the example case of trying to split a string that is
750 comma-separated into its different fields. You can't use C<split(/,/)>
751 because you shouldn't split if the comma is inside quotes. For
752 example, take a data line like this:
754 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
756 Due to the restriction of the quotes, this is a fairly complex
757 problem. Thankfully, we have Jeffrey Friedl, author of
758 I<Mastering Regular Expressions>, to handle these for us. He
759 suggests (assuming your string is contained in $text):
762 push(@new, $+) while $text =~ m{
763 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
767 push(@new, undef) if substr($text,-1,1) eq ',';
769 If you want to represent quotation marks inside a
770 quotation-mark-delimited field, escape them with backslashes (eg,
773 Alternatively, the Text::ParseWords module (part of the standard Perl
774 distribution) lets you say:
776 use Text::ParseWords;
777 @new = quotewords(",", 0, $text);
779 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
781 =head2 How do I strip blank space from the beginning/end of a string?
783 Although the simplest approach would seem to be
785 $string =~ s/^\s*(.*?)\s*$/$1/;
787 not only is this unnecessarily slow and destructive, it also fails with
788 embedded newlines. It is much faster to do this operation in two steps:
793 Or more nicely written as:
800 This idiom takes advantage of the C<foreach> loop's aliasing
801 behavior to factor out common code. You can do this
802 on several strings at once, or arrays, or even the
803 values of a hash if you use a slice:
805 # trim whitespace in the scalar, the array,
806 # and all the values in the hash
807 foreach ($scalar, @array, @hash{keys %hash}) {
812 =head2 How do I pad a string with blanks or pad a number with zeroes?
814 (This answer contributed by Uri Guttman, with kibitzing from
817 In the following examples, C<$pad_len> is the length to which you wish
818 to pad the string, C<$text> or C<$num> contains the string to be padded,
819 and C<$pad_char> contains the padding character. You can use a single
820 character string constant instead of the C<$pad_char> variable if you
821 know what it is in advance. And in the same way you can use an integer in
822 place of C<$pad_len> if you know the pad length in advance.
824 The simplest method uses the C<sprintf> function. It can pad on the left
825 or right with blanks and on the left with zeroes and it will not
826 truncate the result. The C<pack> function can only pad strings on the
827 right with blanks and it will truncate the result to a maximum length of
830 # Left padding a string with blanks (no truncation):
831 $padded = sprintf("%${pad_len}s", $text);
833 # Right padding a string with blanks (no truncation):
834 $padded = sprintf("%-${pad_len}s", $text);
836 # Left padding a number with 0 (no truncation):
837 $padded = sprintf("%0${pad_len}d", $num);
839 # Right padding a string with blanks using pack (will truncate):
840 $padded = pack("A$pad_len",$text);
842 If you need to pad with a character other than blank or zero you can use
843 one of the following methods. They all generate a pad string with the
844 C<x> operator and combine that with C<$text>. These methods do
845 not truncate C<$text>.
847 Left and right padding with any character, creating a new string:
849 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
850 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
852 Left and right padding with any character, modifying C<$text> directly:
854 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
855 $text .= $pad_char x ( $pad_len - length( $text ) );
857 =head2 How do I extract selected columns from a string?
859 Use substr() or unpack(), both documented in L<perlfunc>.
860 If you prefer thinking in terms of columns instead of widths,
861 you can use this kind of thing:
863 # determine the unpack format needed to split Linux ps output
864 # arguments are cut columns
865 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
871 for my $place (@positions) {
872 $template .= "A" . ($place - $lastpos) . " ";
879 =head2 How do I find the soundex value of a string?
881 Use the standard Text::Soundex module distributed with Perl.
882 Before you do so, you may want to determine whether `soundex' is in
883 fact what you think it is. Knuth's soundex algorithm compresses words
884 into a small space, and so it does not necessarily distinguish between
885 two words which you might want to appear separately. For example, the
886 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
887 If Text::Soundex does not do what you are looking for, you might want
888 to consider the String::Approx module available at CPAN.
890 =head2 How can I expand variables in text strings?
892 Let's assume that you have a string like:
894 $text = 'this has a $foo in it and a $bar';
896 If those were both global variables, then this would
899 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
901 But since they are probably lexicals, or at least, they could
902 be, you'd have to do this:
904 $text =~ s/(\$\w+)/$1/eeg;
905 die if $@; # needed /ee, not /e
907 It's probably better in the general case to treat those
908 variables as entries in some special hash. For example:
914 $text =~ s/\$(\w+)/$user_defs{$1}/g;
916 See also ``How do I expand function calls in a string?'' in this section
919 =head2 What's wrong with always quoting "$vars"?
921 The problem is that those double-quotes force stringification--
922 coercing numbers and references into strings--even when you
923 don't want them to be strings. Think of it this way: double-quote
924 expansion is used to produce new strings. If you already
925 have a string, why do you need more?
927 If you get used to writing odd things like these:
931 somefunc("$var"); # BAD
933 You'll be in trouble. Those should (in 99.8% of the cases) be
934 the simpler and more direct:
940 Otherwise, besides slowing you down, you're going to break code when
941 the thing in the scalar is actually neither a string nor a number, but
947 my $oref = "$aref"; # WRONG
950 You can also get into subtle problems on those few operations in Perl
951 that actually do care about the difference between a string and a
952 number, such as the magical C<++> autoincrement operator or the
955 Stringification also destroys arrays.
958 print "@lines"; # WRONG - extra blanks
959 print @lines; # right
961 =head2 Why don't my <<HERE documents work?
963 Check for these three things:
967 =item There must be no space after the << part.
969 =item There (probably) should be a semicolon at the end.
971 =item You can't (easily) have any space in front of the tag.
975 If you want to indent the text in the here document, you
979 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
984 But the HERE_TARGET must still be flush against the margin.
985 If you want that indented also, you'll have to quote
988 ($quote = <<' FINIS') =~ s/^\s+//gm;
989 ...we will have peace, when you and all your works have
990 perished--and the works of your dark master to whom you
991 would deliver us. You are a liar, Saruman, and a corrupter
992 of men's hearts. --Theoden in /usr/src/perl/taint.c
994 $quote =~ s/\s+--/\n--/;
996 A nice general-purpose fixer-upper function for indented here documents
997 follows. It expects to be called with a here document as its argument.
998 It looks to see whether each line begins with a common substring, and
999 if so, strips that substring off. Otherwise, it takes the amount of leading
1000 whitespace found on the first line and removes that much off each
1005 my ($white, $leader); # common whitespace and common leading string
1006 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1007 ($white, $leader) = ($2, quotemeta($1));
1009 ($white, $leader) = (/^(\s+)/, '');
1011 s/^\s*?$leader(?:$white)?//gm;
1015 This works with leading special strings, dynamically determined:
1017 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1020 @@@ SAVEI32(runlevel);
1022 @@@ while ( op = (*op->op_ppaddr)() );
1026 MAIN_INTERPRETER_LOOP
1028 Or with a fixed amount of leading whitespace, with remaining
1029 indentation correctly preserved:
1031 $poem = fix<<EVER_ON_AND_ON;
1032 Now far ahead the Road has gone,
1033 And I must follow, if I can,
1034 Pursuing it with eager feet,
1035 Until it joins some larger way
1036 Where many paths and errands meet.
1037 And whither then? I cannot say.
1038 --Bilbo in /usr/src/perl/pp_ctl.c
1043 =head2 What is the difference between a list and an array?
1045 An array has a changeable length. A list does not. An array is something
1046 you can push or pop, while a list is a set of values. Some people make
1047 the distinction that a list is a value while an array is a variable.
1048 Subroutines are passed and return lists, you put things into list
1049 context, you initialize arrays with lists, and you foreach() across
1050 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1051 in scalar context behave like the number of elements in them, subroutines
1052 access their arguments through the array C<@_>, and push/pop/shift only work
1055 As a side note, there's no such thing as a list in scalar context.
1058 $scalar = (2, 5, 7, 9);
1060 you're using the comma operator in scalar context, so it uses the scalar
1061 comma operator. There never was a list there at all! This causes the
1062 last value to be returned: 9.
1064 =head2 What is the difference between $array[1] and @array[1]?
1066 The former is a scalar value; the latter an array slice, making
1067 it a list with one (scalar) value. You should use $ when you want a
1068 scalar value (most of the time) and @ when you want a list with one
1069 scalar value in it (very, very rarely; nearly never, in fact).
1071 Sometimes it doesn't make a difference, but sometimes it does.
1072 For example, compare:
1074 $good[0] = `some program that outputs several lines`;
1078 @bad[0] = `same program that outputs several lines`;
1080 The C<use warnings> pragma and the B<-w> flag will warn you about these
1083 =head2 How can I remove duplicate elements from a list or array?
1085 There are several possible ways, depending on whether the array is
1086 ordered and whether you wish to preserve the ordering.
1092 If @in is sorted, and you want @out to be sorted:
1093 (this assumes all true values in the array)
1095 $prev = "not equal to $in[0]";
1096 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1098 This is nice in that it doesn't use much extra memory, simulating
1099 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1100 guarantees that the expression is true (so that grep picks it up)
1101 even if the $_ is 0, "", or undef.
1105 If you don't know whether @in is sorted:
1108 @out = grep(!$saw{$_}++, @in);
1112 Like (b), but @in contains only small integers:
1114 @out = grep(!$saw[$_]++, @in);
1118 A way to do (b) without any loops or greps:
1122 @out = sort keys %saw; # remove sort if undesired
1126 Like (d), but @in contains only small positive integers:
1130 @out = grep {defined} @ary;
1134 But perhaps you should have been using a hash all along, eh?
1136 =head2 How can I tell whether a certain element is contained in a list or array?
1138 Hearing the word "in" is an I<in>dication that you probably should have
1139 used a hash, not a list or array, to store your data. Hashes are
1140 designed to answer this question quickly and efficiently. Arrays aren't.
1142 That being said, there are several ways to approach this. If you
1143 are going to make this query many times over arbitrary string values,
1144 the fastest way is probably to invert the original array and maintain a
1145 hash whose keys are the first array's values.
1147 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1149 for (@blues) { $is_blue{$_} = 1 }
1151 Now you can check whether $is_blue{$some_color}. It might have been a
1152 good idea to keep the blues all in a hash in the first place.
1154 If the values are all small integers, you could use a simple indexed
1155 array. This kind of an array will take up less space:
1157 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1158 @is_tiny_prime = ();
1159 for (@primes) { $is_tiny_prime[$_] = 1 }
1160 # or simply @istiny_prime[@primes] = (1) x @primes;
1162 Now you check whether $is_tiny_prime[$some_number].
1164 If the values in question are integers instead of strings, you can save
1165 quite a lot of space by using bit strings instead:
1167 @articles = ( 1..10, 150..2000, 2017 );
1169 for (@articles) { vec($read,$_,1) = 1 }
1171 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1175 ($is_there) = grep $_ eq $whatever, @array;
1179 ($is_there) = grep /$whatever/, @array;
1181 These are slow (checks every element even if the first matches),
1182 inefficient (same reason), and potentially buggy (what if there are
1183 regex characters in $whatever?). If you're only testing once, then
1187 foreach $elt (@array) {
1188 if ($elt eq $elt_to_find) {
1193 if ($is_there) { ... }
1195 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1197 Use a hash. Here's code to do both and more. It assumes that
1198 each element is unique in a given array:
1200 @union = @intersection = @difference = ();
1202 foreach $element (@array1, @array2) { $count{$element}++ }
1203 foreach $element (keys %count) {
1204 push @union, $element;
1205 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1208 Note that this is the I<symmetric difference>, that is, all elements in
1209 either A or in B but not in both. Think of it as an xor operation.
1211 =head2 How do I test whether two arrays or hashes are equal?
1213 The following code works for single-level arrays. It uses a stringwise
1214 comparison, and does not distinguish defined versus undefined empty
1215 strings. Modify if you have other needs.
1217 $are_equal = compare_arrays(\@frogs, \@toads);
1219 sub compare_arrays {
1220 my ($first, $second) = @_;
1221 no warnings; # silence spurious -w undef complaints
1222 return 0 unless @$first == @$second;
1223 for (my $i = 0; $i < @$first; $i++) {
1224 return 0 if $first->[$i] ne $second->[$i];
1229 For multilevel structures, you may wish to use an approach more
1230 like this one. It uses the CPAN module FreezeThaw:
1232 use FreezeThaw qw(cmpStr);
1233 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1235 printf "a and b contain %s arrays\n",
1236 cmpStr(\@a, \@b) == 0
1240 This approach also works for comparing hashes. Here
1241 we'll demonstrate two different answers:
1243 use FreezeThaw qw(cmpStr cmpStrHard);
1245 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1249 printf "a and b contain %s hashes\n",
1250 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1252 printf "a and b contain %s hashes\n",
1253 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1256 The first reports that both those the hashes contain the same data,
1257 while the second reports that they do not. Which you prefer is left as
1258 an exercise to the reader.
1260 =head2 How do I find the first array element for which a condition is true?
1262 To find the first array element which satisfies a condition, you can
1263 use the first() function in the List::Util module, which comes with
1264 Perl 5.8. This example finds the first element that contains "Perl".
1266 use List::Util qw(first);
1268 my $element = first { /Perl/ } @array;
1270 If you cannot use List::Util, you can make your own loop to do the
1271 same thing. Once you find the element, you stop the loop with last.
1274 foreach my $element ( @array )
1276 if( /Perl/ ) { $found = $element; last }
1279 If you want the array index, you can iterate through the indices
1280 and check the array element at each index until you find one
1281 that satisfies the condition.
1283 my( $found, $index ) = ( undef, -1 );
1284 for( $i = 0; $i < @array; $i++ )
1286 if( $array[$i] =~ /Perl/ )
1288 $found = $array[$i];
1294 =head2 How do I handle linked lists?
1296 In general, you usually don't need a linked list in Perl, since with
1297 regular arrays, you can push and pop or shift and unshift at either end,
1298 or you can use splice to add and/or remove arbitrary number of elements at
1299 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1300 dynamic arrays. In the absence of shifts and pops, push in general
1301 needs to reallocate on the order every log(N) times, and unshift will
1302 need to copy pointers each time.
1304 If you really, really wanted, you could use structures as described in
1305 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1306 to do. For example, imagine a list node like this:
1313 You could walk the list this way:
1316 for ($node = $head; $node; $node = $node->{LINK}) {
1317 print $node->{VALUE}, " ";
1321 You could add to the list this way:
1324 $tail = append($head, 1); # grow a new head
1325 for $value ( 2 .. 10 ) {
1326 $tail = append($tail, $value);
1330 my($list, $value) = @_;
1331 my $node = { VALUE => $value };
1333 $node->{LINK} = $list->{LINK};
1334 $list->{LINK} = $node;
1336 $_[0] = $node; # replace caller's version
1341 But again, Perl's built-in are virtually always good enough.
1343 =head2 How do I handle circular lists?
1345 Circular lists could be handled in the traditional fashion with linked
1346 lists, or you could just do something like this with an array:
1348 unshift(@array, pop(@array)); # the last shall be first
1349 push(@array, shift(@array)); # and vice versa
1351 =head2 How do I shuffle an array randomly?
1353 If you either have Perl 5.8.0 or later installed, or if you have
1354 Scalar-List-Utils 1.03 or later installed, you can say:
1356 use List::Util 'shuffle';
1358 @shuffled = shuffle(@list);
1360 If not, you can use a Fisher-Yates shuffle.
1362 sub fisher_yates_shuffle {
1363 my $deck = shift; # $deck is a reference to an array
1366 my $j = int rand ($i+1);
1367 @$deck[$i,$j] = @$deck[$j,$i];
1371 # shuffle my mpeg collection
1373 my @mpeg = <audio/*/*.mp3>;
1374 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1377 Note that the above implementation shuffles an array in place,
1378 unlike the List::Util::shuffle() which takes a list and returns
1379 a new shuffled list.
1381 You've probably seen shuffling algorithms that work using splice,
1382 randomly picking another element to swap the current element with
1386 @old = 1 .. 10; # just a demo
1388 push(@new, splice(@old, rand @old, 1));
1391 This is bad because splice is already O(N), and since you do it N times,
1392 you just invented a quadratic algorithm; that is, O(N**2). This does
1393 not scale, although Perl is so efficient that you probably won't notice
1394 this until you have rather largish arrays.
1396 =head2 How do I process/modify each element of an array?
1398 Use C<for>/C<foreach>:
1401 s/foo/bar/; # change that word
1402 y/XZ/ZX/; # swap those letters
1405 Here's another; let's compute spherical volumes:
1407 for (@volumes = @radii) { # @volumes has changed parts
1409 $_ *= (4/3) * 3.14159; # this will be constant folded
1412 which can also be done with map() which is made to transform
1413 one list into another:
1415 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
1417 If you want to do the same thing to modify the values of the
1418 hash, you can use the C<values> function. As of Perl 5.6
1419 the values are not copied, so if you modify $orbit (in this
1420 case), you modify the value.
1422 for $orbit ( values %orbits ) {
1423 ($orbit **= 3) *= (4/3) * 3.14159;
1426 Prior to perl 5.6 C<values> returned copies of the values,
1427 so older perl code often contains constructions such as
1428 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1429 the hash is to be modified.
1431 =head2 How do I select a random element from an array?
1433 Use the rand() function (see L<perlfunc/rand>):
1435 # at the top of the program:
1436 srand; # not needed for 5.004 and later
1439 $index = rand @array;
1440 $element = $array[$index];
1442 Make sure you I<only call srand once per program, if then>.
1443 If you are calling it more than once (such as before each
1444 call to rand), you're almost certainly doing something wrong.
1446 =head2 How do I permute N elements of a list?
1448 Use the List::Permutor module on CPAN. If the list is
1449 actually an array, try the Algorithm::Permute module (also
1450 on CPAN). It's written in XS code and is very efficient.
1452 use Algorithm::Permute;
1453 my @array = 'a'..'d';
1454 my $p_iterator = Algorithm::Permute->new ( \@array );
1455 while (my @perm = $p_iterator->next) {
1456 print "next permutation: (@perm)\n";
1459 For even faster execution, you could do:
1461 use Algorithm::Permute;
1462 my @array = 'a'..'d';
1463 Algorithm::Permute::permute {
1464 print "next permutation: (@array)\n";
1467 Here's a little program that generates all permutations of
1468 all the words on each line of input. The algorithm embodied
1469 in the permute() function is discussed in Volume 4 (still
1470 unpublished) of Knuth's I<The Art of Computer Programming>
1471 and will work on any list:
1474 # Fischer-Kause ordered permutation generator
1479 while ( $code->(@_[@idx]) ) {
1481 --$p while $idx[$p-1] > $idx[$p];
1482 my $q = $p or return;
1483 push @idx, reverse splice @idx, $p;
1484 ++$q while $idx[$p-1] > $idx[$q];
1485 @idx[$p-1,$q]=@idx[$q,$p-1];
1489 permute {print"@_\n"} split;
1491 =head2 How do I sort an array by (anything)?
1493 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1495 @list = sort { $a <=> $b } @list;
1497 The default sort function is cmp, string comparison, which would
1498 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1499 the numerical comparison operator.
1501 If you have a complicated function needed to pull out the part you
1502 want to sort on, then don't do it inside the sort function. Pull it
1503 out first, because the sort BLOCK can be called many times for the
1504 same element. Here's an example of how to pull out the first word
1505 after the first number on each item, and then sort those words
1510 ($item) = /\d+\s*(\S+)/;
1511 push @idx, uc($item);
1513 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1515 which could also be written this way, using a trick
1516 that's come to be known as the Schwartzian Transform:
1518 @sorted = map { $_->[0] }
1519 sort { $a->[1] cmp $b->[1] }
1520 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1522 If you need to sort on several fields, the following paradigm is useful.
1524 @sorted = sort { field1($a) <=> field1($b) ||
1525 field2($a) cmp field2($b) ||
1526 field3($a) cmp field3($b)
1529 This can be conveniently combined with precalculation of keys as given
1532 See the F<sort> artitcle article in the "Far More Than You Ever Wanted
1533 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
1534 more about this approach.
1536 See also the question below on sorting hashes.
1538 =head2 How do I manipulate arrays of bits?
1540 Use pack() and unpack(), or else vec() and the bitwise operations.
1542 For example, this sets $vec to have bit N set if $ints[N] was set:
1545 foreach(@ints) { vec($vec,$_,1) = 1 }
1547 Here's how, given a vector in $vec, you can
1548 get those bits into your @ints array:
1550 sub bitvec_to_list {
1553 # Find null-byte density then select best algorithm
1554 if ($vec =~ tr/\0// / length $vec > 0.95) {
1557 # This method is faster with mostly null-bytes
1558 while($vec =~ /[^\0]/g ) {
1559 $i = -9 + 8 * pos $vec;
1560 push @ints, $i if vec($vec, ++$i, 1);
1561 push @ints, $i if vec($vec, ++$i, 1);
1562 push @ints, $i if vec($vec, ++$i, 1);
1563 push @ints, $i if vec($vec, ++$i, 1);
1564 push @ints, $i if vec($vec, ++$i, 1);
1565 push @ints, $i if vec($vec, ++$i, 1);
1566 push @ints, $i if vec($vec, ++$i, 1);
1567 push @ints, $i if vec($vec, ++$i, 1);
1570 # This method is a fast general algorithm
1572 my $bits = unpack "b*", $vec;
1573 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1574 push @ints, pos $bits while($bits =~ /1/g);
1579 This method gets faster the more sparse the bit vector is.
1580 (Courtesy of Tim Bunce and Winfried Koenig.)
1582 You can make the while loop a lot shorter with this suggestion
1583 from Benjamin Goldberg:
1585 while($vec =~ /[^\0]+/g ) {
1586 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1589 Or use the CPAN module Bit::Vector:
1591 $vector = Bit::Vector->new($num_of_bits);
1592 $vector->Index_List_Store(@ints);
1593 @ints = $vector->Index_List_Read();
1595 Bit::Vector provides efficient methods for bit vector, sets of small integers
1598 Here's a more extensive illustration using vec():
1601 $vector = "\xff\x0f\xef\xfe";
1602 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1603 unpack("N", $vector), "\n";
1604 $is_set = vec($vector, 23, 1);
1605 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1623 my ($offset, $width, $value) = @_;
1625 vec($vector, $offset, $width) = $value;
1626 print "offset=$offset width=$width value=$value\n";
1632 my $bits = unpack("b*", $vector);
1636 print "vector length in bytes: ", length($vector), "\n";
1637 @bytes = unpack("A8" x length($vector), $bits);
1638 print "bits are: @bytes\n\n";
1641 =head2 Why does defined() return true on empty arrays and hashes?
1643 The short story is that you should probably only use defined on scalars or
1644 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1645 in the 5.004 release or later of Perl for more detail.
1647 =head1 Data: Hashes (Associative Arrays)
1649 =head2 How do I process an entire hash?
1651 Use the each() function (see L<perlfunc/each>) if you don't care
1652 whether it's sorted:
1654 while ( ($key, $value) = each %hash) {
1655 print "$key = $value\n";
1658 If you want it sorted, you'll have to use foreach() on the result of
1659 sorting the keys as shown in an earlier question.
1661 =head2 What happens if I add or remove keys from a hash while iterating over it?
1665 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1666 iterating over it. In Perl 5 you can delete from it, but you still
1667 can't add to it, because that might cause a doubling of the hash table,
1668 in which half the entries get copied up to the new top half of the
1669 table, at which point you've totally bamboozled the iterator code.
1670 Even if the table doesn't double, there's no telling whether your new
1671 entry will be inserted before or after the current iterator position.
1673 Either treasure up your changes and make them after the iterator finishes
1674 or use keys to fetch all the old keys at once, and iterate over the list
1677 =head2 How do I look up a hash element by value?
1679 Create a reverse hash:
1681 %by_value = reverse %by_key;
1682 $key = $by_value{$value};
1684 That's not particularly efficient. It would be more space-efficient
1687 while (($key, $value) = each %by_key) {
1688 $by_value{$value} = $key;
1691 If your hash could have repeated values, the methods above will only find
1692 one of the associated keys. This may or may not worry you. If it does
1693 worry you, you can always reverse the hash into a hash of arrays instead:
1695 while (($key, $value) = each %by_key) {
1696 push @{$key_list_by_value{$value}}, $key;
1699 =head2 How can I know how many entries are in a hash?
1701 If you mean how many keys, then all you have to do is
1702 use the keys() function in a scalar context:
1704 $num_keys = keys %hash;
1706 The keys() function also resets the iterator, which means that you may
1707 see strange results if you use this between uses of other hash operators
1710 =head2 How do I sort a hash (optionally by value instead of key)?
1712 Internally, hashes are stored in a way that prevents you from imposing
1713 an order on key-value pairs. Instead, you have to sort a list of the
1716 @keys = sort keys %hash; # sorted by key
1718 $hash{$a} cmp $hash{$b}
1719 } keys %hash; # and by value
1721 Here we'll do a reverse numeric sort by value, and if two keys are
1722 identical, sort by length of key, or if that fails, by straight ASCII
1723 comparison of the keys (well, possibly modified by your locale--see
1727 $hash{$b} <=> $hash{$a}
1729 length($b) <=> length($a)
1734 =head2 How can I always keep my hash sorted?
1736 You can look into using the DB_File module and tie() using the
1737 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1738 The Tie::IxHash module from CPAN might also be instructive.
1740 =head2 What's the difference between "delete" and "undef" with hashes?
1742 Hashes contain pairs of scalars: the first is the key, the
1743 second is the value. The key will be coerced to a string,
1744 although the value can be any kind of scalar: string,
1745 number, or reference. If a key $key is present in
1746 %hash, C<exists($hash{$key})> will return true. The value
1747 for a given key can be C<undef>, in which case
1748 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1749 will return true. This corresponds to (C<$key>, C<undef>)
1752 Pictures help... here's the %hash table:
1762 And these conditions hold
1766 defined $hash{'d'} is true
1767 defined $hash{'a'} is true
1768 exists $hash{'a'} is true (Perl5 only)
1769 grep ($_ eq 'a', keys %hash) is true
1775 your table now reads:
1786 and these conditions now hold; changes in caps:
1790 defined $hash{'d'} is true
1791 defined $hash{'a'} is FALSE
1792 exists $hash{'a'} is true (Perl5 only)
1793 grep ($_ eq 'a', keys %hash) is true
1795 Notice the last two: you have an undef value, but a defined key!
1801 your table now reads:
1810 and these conditions now hold; changes in caps:
1814 defined $hash{'d'} is true
1815 defined $hash{'a'} is false
1816 exists $hash{'a'} is FALSE (Perl5 only)
1817 grep ($_ eq 'a', keys %hash) is FALSE
1819 See, the whole entry is gone!
1821 =head2 Why don't my tied hashes make the defined/exists distinction?
1823 This depends on the tied hash's implementation of EXISTS().
1824 For example, there isn't the concept of undef with hashes
1825 that are tied to DBM* files. It also means that exists() and
1826 defined() do the same thing with a DBM* file, and what they
1827 end up doing is not what they do with ordinary hashes.
1829 =head2 How do I reset an each() operation part-way through?
1831 Using C<keys %hash> in scalar context returns the number of keys in
1832 the hash I<and> resets the iterator associated with the hash. You may
1833 need to do this if you use C<last> to exit a loop early so that when you
1834 re-enter it, the hash iterator has been reset.
1836 =head2 How can I get the unique keys from two hashes?
1838 First you extract the keys from the hashes into lists, then solve
1839 the "removing duplicates" problem described above. For example:
1842 for $element (keys(%foo), keys(%bar)) {
1849 @uniq = keys %{{%foo,%bar}};
1851 Or if you really want to save space:
1854 while (defined ($key = each %foo)) {
1857 while (defined ($key = each %bar)) {
1862 =head2 How can I store a multidimensional array in a DBM file?
1864 Either stringify the structure yourself (no fun), or else
1865 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1866 it on top of either DB_File or GDBM_File.
1868 =head2 How can I make my hash remember the order I put elements into it?
1870 Use the Tie::IxHash from CPAN.
1873 tie my %myhash, Tie::IxHash;
1874 for (my $i=0; $i<20; $i++) {
1877 my @keys = keys %myhash;
1878 # @keys = (0,1,2,3,...)
1880 =head2 Why does passing a subroutine an undefined element in a hash create it?
1882 If you say something like:
1884 somefunc($hash{"nonesuch key here"});
1886 Then that element "autovivifies"; that is, it springs into existence
1887 whether you store something there or not. That's because functions
1888 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1889 it has to be ready to write it back into the caller's version.
1891 This has been fixed as of Perl5.004.
1893 Normally, merely accessing a key's value for a nonexistent key does
1894 I<not> cause that key to be forever there. This is different than
1897 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1899 Usually a hash ref, perhaps like this:
1904 TITLE => "deputy peon",
1907 PALS => [ "Norbert", "Rhys", "Phineas"],
1910 References are documented in L<perlref> and the upcoming L<perlreftut>.
1911 Examples of complex data structures are given in L<perldsc> and
1912 L<perllol>. Examples of structures and object-oriented classes are
1915 =head2 How can I use a reference as a hash key?
1917 You can't do this directly, but you could use the standard Tie::RefHash
1918 module distributed with Perl.
1922 =head2 How do I handle binary data correctly?
1924 Perl is binary clean, so this shouldn't be a problem. For example,
1925 this works fine (assuming the files are found):
1927 if (`cat /vmunix` =~ /gzip/) {
1928 print "Your kernel is GNU-zip enabled!\n";
1931 On less elegant (read: Byzantine) systems, however, you have
1932 to play tedious games with "text" versus "binary" files. See
1933 L<perlfunc/"binmode"> or L<perlopentut>.
1935 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1937 If you want to deal with multibyte characters, however, there are
1938 some gotchas. See the section on Regular Expressions.
1940 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1942 Assuming that you don't care about IEEE notations like "NaN" or
1943 "Infinity", you probably just want to use a regular expression.
1945 if (/\D/) { print "has nondigits\n" }
1946 if (/^\d+$/) { print "is a whole number\n" }
1947 if (/^-?\d+$/) { print "is an integer\n" }
1948 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1949 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1950 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
1951 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1952 { print "a C float\n" }
1954 You can also use the L<Data::Types|Data::Types> module on
1955 the CPAN, which exports functions that validate data types
1956 using these and other regular expressions, or you can use
1957 the C<Regexp::Common> module from CPAN which has regular
1958 expressions to match various types of numbers.
1960 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1961 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1962 wrapper function for more convenient access. This function takes
1963 a string and returns the number it found, or C<undef> for input that
1964 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1965 if you just want to say, ``Is this a float?''
1968 use POSIX qw(strtod);
1973 my($num, $unparsed) = strtod($str);
1974 if (($str eq '') || ($unparsed != 0) || $!) {
1981 sub is_numeric { defined getnum($_[0]) }
1983 Or you could check out the L<String::Scanf|String::Scanf> module on the CPAN
1984 instead. The POSIX module (part of the standard Perl distribution) provides
1985 the C<strtod> and C<strtol> for converting strings to double and longs,
1988 =head2 How do I keep persistent data across program calls?
1990 For some specific applications, you can use one of the DBM modules.
1991 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
1992 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
1993 of the standard distribution. Here's one example using Storable's C<store>
1994 and C<retrieve> functions:
1997 store(\%hash, "filename");
2000 $href = retrieve("filename"); # by ref
2001 %hash = %{ retrieve("filename") }; # direct to hash
2003 =head2 How do I print out or copy a recursive data structure?
2005 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
2006 for printing out data structures. The Storable module, found on CPAN,
2007 provides a function called C<dclone> that recursively copies its argument.
2009 use Storable qw(dclone);
2012 Where $r1 can be a reference to any kind of data structure you'd like.
2013 It will be deeply copied. Because C<dclone> takes and returns references,
2014 you'd have to add extra punctuation if you had a hash of arrays that
2017 %newhash = %{ dclone(\%oldhash) };
2019 =head2 How do I define methods for every class/object?
2021 Use the UNIVERSAL class (see L<UNIVERSAL>).
2023 =head2 How do I verify a credit card checksum?
2025 Get the Business::CreditCard module from CPAN.
2027 =head2 How do I pack arrays of doubles or floats for XS code?
2029 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2030 If you're doing a lot of float or double processing, consider using
2031 the PDL module from CPAN instead--it makes number-crunching easy.
2033 =head1 AUTHOR AND COPYRIGHT
2035 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
2036 All rights reserved.
2038 This documentation is free; you can redistribute it and/or modify it
2039 under the same terms as Perl itself.
2041 Irrespective of its distribution, all code examples in this file
2042 are hereby placed into the public domain. You are permitted and
2043 encouraged to use this code in your own programs for fun
2044 or for profit as you see fit. A simple comment in the code giving
2045 credit would be courteous but is not required.