3 perlfaq4 - Data Manipulation ($Revision: 1.8 $, $Date: 2001/11/09 08:06:04 $)
7 The section of the FAQ answers questions related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximated on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to an internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers. (See L<perlvar/"$#"> 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 If precision is important, such as when dealing with money, it's good
43 to work with integers and then divide at the last possible moment.
44 For example, work in pennies (1995) instead of dollars and cents
45 (19.95) and divide by 100 at the end.
47 To get rid of the superfluous digits, just use a format (eg,
48 C<printf("%.2f", 19.95)>) to get the required precision.
49 See L<perlop/"Floating-point Arithmetic">.
51 =head2 Why isn't my octal data interpreted correctly?
53 Perl only understands octal and hex numbers as such when they occur
54 as literals in your program. Octal literals in perl must start with
55 a leading "0" and hexadecimal literals must start with a leading "0x".
56 If they are read in from somewhere and assigned, no automatic
57 conversion takes place. You must explicitly use oct() or hex() if you
58 want the values converted to decimal. oct() interprets
59 both hex ("0x350") numbers and octal ones ("0350" or even without the
60 leading "0", like "377"), while hex() only converts hexadecimal ones,
61 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
62 The inverse mapping from decimal to octal can be done with either the
63 "%o" or "%O" sprintf() formats. To get from decimal to hex try either
64 the "%x" or the "%X" formats to sprintf().
66 This problem shows up most often when people try using chmod(), mkdir(),
67 umask(), or sysopen(), which by widespread tradition typically take
70 chmod(644, $file); # WRONG
71 chmod(0644, $file); # right
73 Note the mistake in the first line was specifying the decimal literal
74 644, rather than the intended octal literal 0644. The problem can
77 printf("%#o",644); # prints 01204
79 Surely you had not intended C<chmod(01204, $file);> - did you? If you
80 want to use numeric literals as arguments to chmod() et al. then please
81 try to express them as octal constants, that is with a leading zero and
82 with the following digits restricted to the set 0..7.
84 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
86 Remember that int() merely truncates toward 0. For rounding to a
87 certain number of digits, sprintf() or printf() is usually the easiest
90 printf("%.3f", 3.1415926535); # prints 3.142
92 The POSIX module (part of the standard Perl distribution) implements
93 ceil(), floor(), and a number of other mathematical and trigonometric
97 $ceil = ceil(3.5); # 4
98 $floor = floor(3.5); # 3
100 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
101 module. With 5.004, the Math::Trig module (part of the standard Perl
102 distribution) implements the trigonometric functions. Internally it
103 uses the Math::Complex module and some functions can break out from
104 the real axis into the complex plane, for example the inverse sine of
107 Rounding in financial applications can have serious implications, and
108 the rounding method used should be specified precisely. In these
109 cases, it probably pays not to trust whichever system rounding is
110 being used by Perl, but to instead implement the rounding function you
113 To see why, notice how you'll still have an issue on half-way-point
116 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
118 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
119 0.8 0.8 0.9 0.9 1.0 1.0
121 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
122 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
123 machines) will work pretty much like mathematical integers. Other numbers
126 =head2 How do I convert between numeric representations:
128 As always with Perl there is more than one way to do it. Below
129 are a few examples of approaches to making common conversions
130 between number representations. This is intended to be representational
131 rather than exhaustive.
133 Some of the examples below use the Bit::Vector module from CPAN.
134 The reason you might choose Bit::Vector over the perl built in
135 functions is that it works with numbers of ANY size, that it is
136 optimized for speed on some operations, and for at least some
137 programmers the notation might be familiar.
141 =item B<How do I convert Hexadecimal into decimal:>
143 Using perl's built in conversion of 0x notation:
146 $dec = sprintf("%d", $int);
148 Using the hex function:
150 $int = hex("DEADBEEF");
151 $dec = sprintf("%d", $int);
155 $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
156 $dec = sprintf("%d", $int);
158 Using the CPAN module Bit::Vector:
161 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
162 $dec = $vec->to_Dec();
164 =item B<How do I convert from decimal to hexadecimal:>
168 $hex = sprintf("%X", 3735928559);
172 $hex = unpack("H*", pack("N", 3735928559));
177 $vec = Bit::Vector->new_Dec(32, -559038737);
178 $hex = $vec->to_Hex();
180 And Bit::Vector supports odd bit counts:
183 $vec = Bit::Vector->new_Dec(33, 3735928559);
184 $vec->Resize(32); # suppress leading 0 if unwanted
185 $hex = $vec->to_Hex();
187 =item B<How do I convert from octal to decimal:>
189 Using Perl's built in conversion of numbers with leading zeros:
191 $int = 033653337357; # note the leading 0!
192 $dec = sprintf("%d", $int);
194 Using the oct function:
196 $int = oct("33653337357");
197 $dec = sprintf("%d", $int);
202 $vec = Bit::Vector->new(32);
203 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
204 $dec = $vec->to_Dec();
206 =item B<How do I convert from decimal to octal:>
210 $oct = sprintf("%o", 3735928559);
215 $vec = Bit::Vector->new_Dec(32, -559038737);
216 $oct = reverse join('', $vec->Chunk_List_Read(3));
218 =item B<How do I convert from binary to decimal:>
222 $decimal = ord(pack('B8', '10110110'));
224 Using pack and unpack for larger strings
226 $int = unpack("N", pack("B32",
227 substr("0" x 32 . "11110101011011011111011101111", -32)));
228 $dec = sprintf("%d", $int);
230 # substr() is used to left pad a 32 character string with zeros.
234 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
235 $dec = $vec->to_Dec();
237 =item B<How do I convert from decimal to binary:>
241 $bin = unpack("B*", pack("N", 3735928559));
246 $vec = Bit::Vector->new_Dec(32, -559038737);
247 $bin = $vec->to_Bin();
249 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
250 are left as an exercise to the inclined reader.
254 =head2 Why doesn't & work the way I want it to?
256 The behavior of binary arithmetic operators depends on whether they're
257 used on numbers or strings. The operators treat a string as a series
258 of bits and work with that (the string C<"3"> is the bit pattern
259 C<00110011>). The operators work with the binary form of a number
260 (the number C<3> is treated as the bit pattern C<00000011>).
262 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
263 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
266 Most problems with C<&> and C<|> arise because the programmer thinks
267 they have a number but really it's a string. The rest arise because
270 if ("\020\020" & "\101\101") {
274 but a string consisting of two null bytes (the result of C<"\020\020"
275 & "\101\101">) is not a false value in Perl. You need:
277 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
281 =head2 How do I multiply matrices?
283 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
284 or the PDL extension (also available from CPAN).
286 =head2 How do I perform an operation on a series of integers?
288 To call a function on each element in an array, and collect the
291 @results = map { my_func($_) } @array;
295 @triple = map { 3 * $_ } @single;
297 To call a function on each element of an array, but ignore the
300 foreach $iterator (@array) {
301 some_func($iterator);
304 To call a function on each integer in a (small) range, you B<can> use:
306 @results = map { some_func($_) } (5 .. 25);
308 but you should be aware that the C<..> operator creates an array of
309 all integers in the range. This can take a lot of memory for large
313 for ($i=5; $i < 500_005; $i++) {
314 push(@results, some_func($i));
317 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
318 loop will iterate over the range, without creating the entire range.
320 for my $i (5 .. 500_005) {
321 push(@results, some_func($i));
324 will not create a list of 500,000 integers.
326 =head2 How can I output Roman numerals?
328 Get the http://www.cpan.org/modules/by-module/Roman module.
330 =head2 Why aren't my random numbers random?
332 If you're using a version of Perl before 5.004, you must call C<srand>
333 once at the start of your program to seed the random number generator.
334 5.004 and later automatically call C<srand> at the beginning. Don't
335 call C<srand> more than once--you make your numbers less random, rather
338 Computers are good at being predictable and bad at being random
339 (despite appearances caused by bugs in your programs :-).
340 http://www.cpan.org/doc/FMTEYEWTK/random , courtesy of Tom
341 Phoenix, talks more about this. John von Neumann said, ``Anyone who
342 attempts to generate random numbers by deterministic means is, of
343 course, living in a state of sin.''
345 If you want numbers that are more random than C<rand> with C<srand>
346 provides, you should also check out the Math::TrulyRandom module from
347 CPAN. It uses the imperfections in your system's timer to generate
348 random numbers, but this takes quite a while. If you want a better
349 pseudorandom generator than comes with your operating system, look at
350 ``Numerical Recipes in C'' at http://www.nr.com/ .
354 =head2 How do I find the week-of-the-year/day-of-the-year?
356 The day of the year is in the array returned by localtime() (see
357 L<perlfunc/"localtime">):
359 $day_of_year = (localtime(time()))[7];
361 =head2 How do I find the current century or millennium?
363 Use the following simple functions:
366 return int((((localtime(shift || time))[5] + 1999))/100);
369 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
372 On some systems, you'll find that the POSIX module's strftime() function
373 has been extended in a non-standard way to use a C<%C> format, which they
374 sometimes claim is the "century". It isn't, because on most such systems,
375 this is only the first two digits of the four-digit year, and thus cannot
376 be used to reliably determine the current century or millennium.
378 =head2 How can I compare two dates and find the difference?
380 If you're storing your dates as epoch seconds then simply subtract one
381 from the other. If you've got a structured date (distinct year, day,
382 month, hour, minute, seconds values), then for reasons of accessibility,
383 simplicity, and efficiency, merely use either timelocal or timegm (from
384 the Time::Local module in the standard distribution) to reduce structured
385 dates to epoch seconds. However, if you don't know the precise format of
386 your dates, then you should probably use either of the Date::Manip and
387 Date::Calc modules from CPAN before you go hacking up your own parsing
388 routine to handle arbitrary date formats.
390 =head2 How can I take a string and turn it into epoch seconds?
392 If it's a regular enough string that it always has the same format,
393 you can split it up and pass the parts to C<timelocal> in the standard
394 Time::Local module. Otherwise, you should look into the Date::Calc
395 and Date::Manip modules from CPAN.
397 =head2 How can I find the Julian Day?
399 Use the Time::JulianDay module (part of the Time-modules bundle
400 available from CPAN.)
402 Before you immerse yourself too deeply in this, be sure to verify that
403 it is the I<Julian> Day you really want. Are you interested in a way
404 of getting serial days so that you just can tell how many days they
405 are apart or so that you can do also other date arithmetic? If you
406 are interested in performing date arithmetic, this can be done using
407 modules Date::Manip or Date::Calc.
409 There is too many details and much confusion on this issue to cover in
410 this FAQ, but the term is applied (correctly) to a calendar now
411 supplanted by the Gregorian Calendar, with the Julian Calendar failing
412 to adjust properly for leap years on centennial years (among other
413 annoyances). The term is also used (incorrectly) to mean: [1] days in
414 the Gregorian Calendar; and [2] days since a particular starting time
415 or `epoch', usually 1970 in the Unix world and 1980 in the
416 MS-DOS/Windows world. If you find that it is not the first meaning
417 that you really want, then check out the Date::Manip and Date::Calc
418 modules. (Thanks to David Cassell for most of this text.)
420 =head2 How do I find yesterday's date?
422 The C<time()> function returns the current time in seconds since the
423 epoch. Take twenty-four hours off that:
425 $yesterday = time() - ( 24 * 60 * 60 );
427 Then you can pass this to C<localtime()> and get the individual year,
428 month, day, hour, minute, seconds values.
430 Note very carefully that the code above assumes that your days are
431 twenty-four hours each. For most people, there are two days a year
432 when they aren't: the switch to and from summer time throws this off.
433 A solution to this issue is offered by Russ Allbery.
436 my $now = defined $_[0] ? $_[0] : time;
437 my $then = $now - 60 * 60 * 24;
438 my $ndst = (localtime $now)[8] > 0;
439 my $tdst = (localtime $then)[8] > 0;
440 $then - ($tdst - $ndst) * 60 * 60;
442 # Should give you "this time yesterday" in seconds since epoch relative to
443 # the first argument or the current time if no argument is given and
444 # suitable for passing to localtime or whatever else you need to do with
445 # it. $ndst is whether we're currently in daylight savings time; $tdst is
446 # whether the point 24 hours ago was in daylight savings time. If $tdst
447 # and $ndst are the same, a boundary wasn't crossed, and the correction
448 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
449 # from yesterday's time since we gained an extra hour while going off
450 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
451 # negative hour (add an hour) to yesterday's time since we lost an hour.
453 # All of this is because during those days when one switches off or onto
454 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
456 # The explicit settings of $ndst and $tdst are necessary because localtime
457 # only says it returns the system tm struct, and the system tm struct at
458 # least on Solaris doesn't guarantee any particular positive value (like,
459 # say, 1) for isdst, just a positive value. And that value can
460 # potentially be negative, if DST information isn't available (this sub
461 # just treats those cases like no DST).
463 # Note that between 2am and 3am on the day after the time zone switches
464 # off daylight savings time, the exact hour of "yesterday" corresponding
465 # to the current hour is not clearly defined. Note also that if used
466 # between 2am and 3am the day after the change to daylight savings time,
467 # the result will be between 3am and 4am of the previous day; it's
468 # arguable whether this is correct.
470 # This sub does not attempt to deal with leap seconds (most things don't).
472 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
473 # This code is in the public domain
475 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
477 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
478 Y2K compliant (whatever that means). The programmers you've hired to
479 use it, however, probably are not.
481 Long answer: The question belies a true understanding of the issue.
482 Perl is just as Y2K compliant as your pencil--no more, and no less.
483 Can you use your pencil to write a non-Y2K-compliant memo? Of course
484 you can. Is that the pencil's fault? Of course it isn't.
486 The date and time functions supplied with Perl (gmtime and localtime)
487 supply adequate information to determine the year well beyond 2000
488 (2038 is when trouble strikes for 32-bit machines). The year returned
489 by these functions when used in a list context is the year minus 1900.
490 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
491 number. To avoid the year 2000 problem simply do not treat the year as
492 a 2-digit number. It isn't.
494 When gmtime() and localtime() are used in scalar context they return
495 a timestamp string that contains a fully-expanded year. For example,
496 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
497 2001". There's no year 2000 problem here.
499 That doesn't mean that Perl can't be used to create non-Y2K compliant
500 programs. It can. But so can your pencil. It's the fault of the user,
501 not the language. At the risk of inflaming the NRA: ``Perl doesn't
502 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
507 =head2 How do I validate input?
509 The answer to this question is usually a regular expression, perhaps
510 with auxiliary logic. See the more specific questions (numbers, mail
511 addresses, etc.) for details.
513 =head2 How do I unescape a string?
515 It depends just what you mean by ``escape''. URL escapes are dealt
516 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
517 character are removed with
521 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
523 =head2 How do I remove consecutive pairs of characters?
525 To turn C<"abbcccd"> into C<"abccd">:
527 s/(.)\1/$1/g; # add /s to include newlines
529 Here's a solution that turns "abbcccd" to "abcd":
531 y///cs; # y == tr, but shorter :-)
533 =head2 How do I expand function calls in a string?
535 This is documented in L<perlref>. In general, this is fraught with
536 quoting and readability problems, but it is possible. To interpolate
537 a subroutine call (in list context) into a string:
539 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
541 If you prefer scalar context, similar chicanery is also useful for
542 arbitrary expressions:
544 print "That yields ${\($n + 5)} widgets\n";
546 Version 5.004 of Perl had a bug that gave list context to the
547 expression in C<${...}>, but this is fixed in version 5.005.
549 See also ``How can I expand variables in text strings?'' in this
552 =head2 How do I find matching/nesting anything?
554 This isn't something that can be done in one regular expression, no
555 matter how complicated. To find something between two single
556 characters, a pattern like C</x([^x]*)x/> will get the intervening
557 bits in $1. For multiple ones, then something more like
558 C</alpha(.*?)omega/> would be needed. But none of these deals with
559 nested patterns, nor can they. For that you'll have to write a
562 If you are serious about writing a parser, there are a number of
563 modules or oddities that will make your life a lot easier. There are
564 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
565 and the byacc program. Starting from perl 5.8 the Text::Balanced
566 is part of the standard distribution.
568 One simple destructive, inside-out approach that you might try is to
569 pull out the smallest nesting parts one at a time:
571 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
572 # do something with $1
575 A more complicated and sneaky approach is to make Perl's regular
576 expression engine do it for you. This is courtesy Dean Inada, and
577 rather has the nature of an Obfuscated Perl Contest entry, but it
580 # $_ contains the string to parse
581 # BEGIN and END are the opening and closing markers for the
586 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
587 @$ = (eval{/$re/},$@!~/unmatched/);
588 print join("\n",@$[0..$#$]) if( $$[-1] );
590 =head2 How do I reverse a string?
592 Use reverse() in scalar context, as documented in
595 $reversed = reverse $string;
597 =head2 How do I expand tabs in a string?
599 You can do it yourself:
601 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
603 Or you can just use the Text::Tabs module (part of the standard Perl
607 @expanded_lines = expand(@lines_with_tabs);
609 =head2 How do I reformat a paragraph?
611 Use Text::Wrap (part of the standard Perl distribution):
614 print wrap("\t", ' ', @paragraphs);
616 The paragraphs you give to Text::Wrap should not contain embedded
617 newlines. Text::Wrap doesn't justify the lines (flush-right).
619 Or use the CPAN module Text::Autoformat. Formatting files can be easily
620 done by making a shell alias, like so:
622 alias fmt="perl -i -MText::Autoformat -n0777 \
623 -e 'print autoformat $_, {all=>1}' $*"
625 See the documentation for Text::Autoformat to appreciate its many
628 =head2 How can I access/change the first N letters of a string?
630 There are many ways. If you just want to grab a copy, use
633 $first_byte = substr($a, 0, 1);
635 If you want to modify part of a string, the simplest way is often to
636 use substr() as an lvalue:
638 substr($a, 0, 3) = "Tom";
640 Although those with a pattern matching kind of thought process will
645 =head2 How do I change the Nth occurrence of something?
647 You have to keep track of N yourself. For example, let's say you want
648 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
649 C<"whosoever"> or C<"whomsoever">, case insensitively. These
650 all assume that $_ contains the string to be altered.
654 ++$count == 5 # is it the 5th?
655 ? "${2}soever" # yes, swap
656 : $1 # renege and leave it there
659 In the more general case, you can use the C</g> modifier in a C<while>
660 loop, keeping count of matches.
664 $_ = "One fish two fish red fish blue fish";
665 while (/(\w+)\s+fish\b/gi) {
666 if (++$count == $WANT) {
667 print "The third fish is a $1 one.\n";
671 That prints out: C<"The third fish is a red one."> You can also use a
672 repetition count and repeated pattern like this:
674 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
676 =head2 How can I count the number of occurrences of a substring within a string?
678 There are a number of ways, with varying efficiency. If you want a
679 count of a certain single character (X) within a string, you can use the
680 C<tr///> function like so:
682 $string = "ThisXlineXhasXsomeXx'sXinXit";
683 $count = ($string =~ tr/X//);
684 print "There are $count X characters in the string";
686 This is fine if you are just looking for a single character. However,
687 if you are trying to count multiple character substrings within a
688 larger string, C<tr///> won't work. What you can do is wrap a while()
689 loop around a global pattern match. For example, let's count negative
692 $string = "-9 55 48 -2 23 -76 4 14 -44";
693 while ($string =~ /-\d+/g) { $count++ }
694 print "There are $count negative numbers in the string";
696 =head2 How do I capitalize all the words on one line?
698 To make the first letter of each word upper case:
700 $line =~ s/\b(\w)/\U$1/g;
702 This has the strange effect of turning "C<don't do it>" into "C<Don'T
703 Do It>". Sometimes you might want this. Other times you might need a
704 more thorough solution (Suggested by brian d foy):
707 (^\w) #at the beginning of the line
709 (\s\w) #preceded by whitespace
712 $string =~ /([\w']+)/\u\L$1/g;
714 To make the whole line upper case:
718 To force each word to be lower case, with the first letter upper case:
720 $line =~ s/(\w+)/\u\L$1/g;
722 You can (and probably should) enable locale awareness of those
723 characters by placing a C<use locale> pragma in your program.
724 See L<perllocale> for endless details on locales.
726 This is sometimes referred to as putting something into "title
727 case", but that's not quite accurate. Consider the proper
728 capitalization of the movie I<Dr. Strangelove or: How I Learned to
729 Stop Worrying and Love the Bomb>, for example.
731 =head2 How can I split a [character] delimited string except when inside
732 [character]? (Comma-separated files)
734 Take the example case of trying to split a string that is comma-separated
735 into its different fields. (We'll pretend you said comma-separated, not
736 comma-delimited, which is different and almost never what you mean.) You
737 can't use C<split(/,/)> because you shouldn't split if the comma is inside
738 quotes. For example, take a data line like this:
740 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
742 Due to the restriction of the quotes, this is a fairly complex
743 problem. Thankfully, we have Jeffrey Friedl, author of a highly
744 recommended book on regular expressions, to handle these for us. He
745 suggests (assuming your string is contained in $text):
748 push(@new, $+) while $text =~ m{
749 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
753 push(@new, undef) if substr($text,-1,1) eq ',';
755 If you want to represent quotation marks inside a
756 quotation-mark-delimited field, escape them with backslashes (eg,
757 C<"like \"this\"">. Unescaping them is a task addressed earlier in
760 Alternatively, the Text::ParseWords module (part of the standard Perl
761 distribution) lets you say:
763 use Text::ParseWords;
764 @new = quotewords(",", 0, $text);
766 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
768 =head2 How do I strip blank space from the beginning/end of a string?
770 Although the simplest approach would seem to be
772 $string =~ s/^\s*(.*?)\s*$/$1/;
774 not only is this unnecessarily slow and destructive, it also fails with
775 embedded newlines. It is much faster to do this operation in two steps:
780 Or more nicely written as:
787 This idiom takes advantage of the C<foreach> loop's aliasing
788 behavior to factor out common code. You can do this
789 on several strings at once, or arrays, or even the
790 values of a hash if you use a slice:
792 # trim whitespace in the scalar, the array,
793 # and all the values in the hash
794 foreach ($scalar, @array, @hash{keys %hash}) {
799 =head2 How do I pad a string with blanks or pad a number with zeroes?
801 (This answer contributed by Uri Guttman, with kibitzing from
804 In the following examples, C<$pad_len> is the length to which you wish
805 to pad the string, C<$text> or C<$num> contains the string to be padded,
806 and C<$pad_char> contains the padding character. You can use a single
807 character string constant instead of the C<$pad_char> variable if you
808 know what it is in advance. And in the same way you can use an integer in
809 place of C<$pad_len> if you know the pad length in advance.
811 The simplest method uses the C<sprintf> function. It can pad on the left
812 or right with blanks and on the left with zeroes and it will not
813 truncate the result. The C<pack> function can only pad strings on the
814 right with blanks and it will truncate the result to a maximum length of
817 # Left padding a string with blanks (no truncation):
818 $padded = sprintf("%${pad_len}s", $text);
820 # Right padding a string with blanks (no truncation):
821 $padded = sprintf("%-${pad_len}s", $text);
823 # Left padding a number with 0 (no truncation):
824 $padded = sprintf("%0${pad_len}d", $num);
826 # Right padding a string with blanks using pack (will truncate):
827 $padded = pack("A$pad_len",$text);
829 If you need to pad with a character other than blank or zero you can use
830 one of the following methods. They all generate a pad string with the
831 C<x> operator and combine that with C<$text>. These methods do
832 not truncate C<$text>.
834 Left and right padding with any character, creating a new string:
836 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
837 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
839 Left and right padding with any character, modifying C<$text> directly:
841 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
842 $text .= $pad_char x ( $pad_len - length( $text ) );
844 =head2 How do I extract selected columns from a string?
846 Use substr() or unpack(), both documented in L<perlfunc>.
847 If you prefer thinking in terms of columns instead of widths,
848 you can use this kind of thing:
850 # determine the unpack format needed to split Linux ps output
851 # arguments are cut columns
852 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
858 for my $place (@positions) {
859 $template .= "A" . ($place - $lastpos) . " ";
866 =head2 How do I find the soundex value of a string?
868 Use the standard Text::Soundex module distributed with Perl.
869 Before you do so, you may want to determine whether `soundex' is in
870 fact what you think it is. Knuth's soundex algorithm compresses words
871 into a small space, and so it does not necessarily distinguish between
872 two words which you might want to appear separately. For example, the
873 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
874 If Text::Soundex does not do what you are looking for, you might want
875 to consider the String::Approx module available at CPAN.
877 =head2 How can I expand variables in text strings?
879 Let's assume that you have a string like:
881 $text = 'this has a $foo in it and a $bar';
883 If those were both global variables, then this would
886 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
888 But since they are probably lexicals, or at least, they could
889 be, you'd have to do this:
891 $text =~ s/(\$\w+)/$1/eeg;
892 die if $@; # needed /ee, not /e
894 It's probably better in the general case to treat those
895 variables as entries in some special hash. For example:
901 $text =~ s/\$(\w+)/$user_defs{$1}/g;
903 See also ``How do I expand function calls in a string?'' in this section
906 =head2 What's wrong with always quoting "$vars"?
908 The problem is that those double-quotes force stringification--
909 coercing numbers and references into strings--even when you
910 don't want them to be strings. Think of it this way: double-quote
911 expansion is used to produce new strings. If you already
912 have a string, why do you need more?
914 If you get used to writing odd things like these:
918 somefunc("$var"); # BAD
920 You'll be in trouble. Those should (in 99.8% of the cases) be
921 the simpler and more direct:
927 Otherwise, besides slowing you down, you're going to break code when
928 the thing in the scalar is actually neither a string nor a number, but
934 my $oref = "$aref"; # WRONG
937 You can also get into subtle problems on those few operations in Perl
938 that actually do care about the difference between a string and a
939 number, such as the magical C<++> autoincrement operator or the
942 Stringification also destroys arrays.
945 print "@lines"; # WRONG - extra blanks
946 print @lines; # right
948 =head2 Why don't my <<HERE documents work?
950 Check for these three things:
954 =item 1. There must be no space after the << part.
956 =item 2. There (probably) should be a semicolon at the end.
958 =item 3. You can't (easily) have any space in front of the tag.
962 If you want to indent the text in the here document, you
966 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
971 But the HERE_TARGET must still be flush against the margin.
972 If you want that indented also, you'll have to quote
975 ($quote = <<' FINIS') =~ s/^\s+//gm;
976 ...we will have peace, when you and all your works have
977 perished--and the works of your dark master to whom you
978 would deliver us. You are a liar, Saruman, and a corrupter
979 of men's hearts. --Theoden in /usr/src/perl/taint.c
981 $quote =~ s/\s*--/\n--/;
983 A nice general-purpose fixer-upper function for indented here documents
984 follows. It expects to be called with a here document as its argument.
985 It looks to see whether each line begins with a common substring, and
986 if so, strips that substring off. Otherwise, it takes the amount of leading
987 whitespace found on the first line and removes that much off each
992 my ($white, $leader); # common whitespace and common leading string
993 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
994 ($white, $leader) = ($2, quotemeta($1));
996 ($white, $leader) = (/^(\s+)/, '');
998 s/^\s*?$leader(?:$white)?//gm;
1002 This works with leading special strings, dynamically determined:
1004 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1007 @@@ SAVEI32(runlevel);
1009 @@@ while ( op = (*op->op_ppaddr)() );
1013 MAIN_INTERPRETER_LOOP
1015 Or with a fixed amount of leading whitespace, with remaining
1016 indentation correctly preserved:
1018 $poem = fix<<EVER_ON_AND_ON;
1019 Now far ahead the Road has gone,
1020 And I must follow, if I can,
1021 Pursuing it with eager feet,
1022 Until it joins some larger way
1023 Where many paths and errands meet.
1024 And whither then? I cannot say.
1025 --Bilbo in /usr/src/perl/pp_ctl.c
1030 =head2 What is the difference between a list and an array?
1032 An array has a changeable length. A list does not. An array is something
1033 you can push or pop, while a list is a set of values. Some people make
1034 the distinction that a list is a value while an array is a variable.
1035 Subroutines are passed and return lists, you put things into list
1036 context, you initialize arrays with lists, and you foreach() across
1037 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1038 in scalar context behave like the number of elements in them, subroutines
1039 access their arguments through the array C<@_>, and push/pop/shift only work
1042 As a side note, there's no such thing as a list in scalar context.
1045 $scalar = (2, 5, 7, 9);
1047 you're using the comma operator in scalar context, so it uses the scalar
1048 comma operator. There never was a list there at all! This causes the
1049 last value to be returned: 9.
1051 =head2 What is the difference between $array[1] and @array[1]?
1053 The former is a scalar value; the latter an array slice, making
1054 it a list with one (scalar) value. You should use $ when you want a
1055 scalar value (most of the time) and @ when you want a list with one
1056 scalar value in it (very, very rarely; nearly never, in fact).
1058 Sometimes it doesn't make a difference, but sometimes it does.
1059 For example, compare:
1061 $good[0] = `some program that outputs several lines`;
1065 @bad[0] = `same program that outputs several lines`;
1067 The C<use warnings> pragma and the B<-w> flag will warn you about these
1070 =head2 How can I remove duplicate elements from a list or array?
1072 There are several possible ways, depending on whether the array is
1073 ordered and whether you wish to preserve the ordering.
1079 If @in is sorted, and you want @out to be sorted:
1080 (this assumes all true values in the array)
1082 $prev = "not equal to $in[0]";
1083 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1085 This is nice in that it doesn't use much extra memory, simulating
1086 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1087 guarantees that the expression is true (so that grep picks it up)
1088 even if the $_ is 0, "", or undef.
1092 If you don't know whether @in is sorted:
1095 @out = grep(!$saw{$_}++, @in);
1099 Like (b), but @in contains only small integers:
1101 @out = grep(!$saw[$_]++, @in);
1105 A way to do (b) without any loops or greps:
1109 @out = sort keys %saw; # remove sort if undesired
1113 Like (d), but @in contains only small positive integers:
1117 @out = grep {defined} @ary;
1121 But perhaps you should have been using a hash all along, eh?
1123 =head2 How can I tell whether a certain element is contained in a list or array?
1125 Hearing the word "in" is an I<in>dication that you probably should have
1126 used a hash, not a list or array, to store your data. Hashes are
1127 designed to answer this question quickly and efficiently. Arrays aren't.
1129 That being said, there are several ways to approach this. If you
1130 are going to make this query many times over arbitrary string values,
1131 the fastest way is probably to invert the original array and keep an
1132 associative array lying about whose keys are the first array's values.
1134 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1136 for (@blues) { $is_blue{$_} = 1 }
1138 Now you can check whether $is_blue{$some_color}. It might have been a
1139 good idea to keep the blues all in a hash in the first place.
1141 If the values are all small integers, you could use a simple indexed
1142 array. This kind of an array will take up less space:
1144 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1145 undef @is_tiny_prime;
1146 for (@primes) { $is_tiny_prime[$_] = 1 }
1147 # or simply @istiny_prime[@primes] = (1) x @primes;
1149 Now you check whether $is_tiny_prime[$some_number].
1151 If the values in question are integers instead of strings, you can save
1152 quite a lot of space by using bit strings instead:
1154 @articles = ( 1..10, 150..2000, 2017 );
1156 for (@articles) { vec($read,$_,1) = 1 }
1158 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1162 ($is_there) = grep $_ eq $whatever, @array;
1166 ($is_there) = grep /$whatever/, @array;
1168 These are slow (checks every element even if the first matches),
1169 inefficient (same reason), and potentially buggy (what if there are
1170 regex characters in $whatever?). If you're only testing once, then
1174 foreach $elt (@array) {
1175 if ($elt eq $elt_to_find) {
1180 if ($is_there) { ... }
1182 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1184 Use a hash. Here's code to do both and more. It assumes that
1185 each element is unique in a given array:
1187 @union = @intersection = @difference = ();
1189 foreach $element (@array1, @array2) { $count{$element}++ }
1190 foreach $element (keys %count) {
1191 push @union, $element;
1192 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1195 Note that this is the I<symmetric difference>, that is, all elements in
1196 either A or in B but not in both. Think of it as an xor operation.
1198 =head2 How do I test whether two arrays or hashes are equal?
1200 The following code works for single-level arrays. It uses a stringwise
1201 comparison, and does not distinguish defined versus undefined empty
1202 strings. Modify if you have other needs.
1204 $are_equal = compare_arrays(\@frogs, \@toads);
1206 sub compare_arrays {
1207 my ($first, $second) = @_;
1208 no warnings; # silence spurious -w undef complaints
1209 return 0 unless @$first == @$second;
1210 for (my $i = 0; $i < @$first; $i++) {
1211 return 0 if $first->[$i] ne $second->[$i];
1216 For multilevel structures, you may wish to use an approach more
1217 like this one. It uses the CPAN module FreezeThaw:
1219 use FreezeThaw qw(cmpStr);
1220 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1222 printf "a and b contain %s arrays\n",
1223 cmpStr(\@a, \@b) == 0
1227 This approach also works for comparing hashes. Here
1228 we'll demonstrate two different answers:
1230 use FreezeThaw qw(cmpStr cmpStrHard);
1232 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1236 printf "a and b contain %s hashes\n",
1237 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1239 printf "a and b contain %s hashes\n",
1240 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1243 The first reports that both those the hashes contain the same data,
1244 while the second reports that they do not. Which you prefer is left as
1245 an exercise to the reader.
1247 =head2 How do I find the first array element for which a condition is true?
1249 You can use this if you care about the index:
1251 for ($i= 0; $i < @array; $i++) {
1252 if ($array[$i] eq "Waldo") {
1258 Now C<$found_index> has what you want.
1260 =head2 How do I handle linked lists?
1262 In general, you usually don't need a linked list in Perl, since with
1263 regular arrays, you can push and pop or shift and unshift at either end,
1264 or you can use splice to add and/or remove arbitrary number of elements at
1265 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1266 dynamic arrays. In the absence of shifts and pops, push in general
1267 needs to reallocate on the order every log(N) times, and unshift will
1268 need to copy pointers each time.
1270 If you really, really wanted, you could use structures as described in
1271 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1272 to do. For example, imagine a list node like this:
1279 You could walk the list this way:
1282 for ($node = $head; $node; $node = $node->{LINK}) {
1283 print $node->{VALUE}, " ";
1287 You could add to the list this way:
1290 $tail = append($head, 1); # grow a new head
1291 for $value ( 2 .. 10 ) {
1292 $tail = append($tail, $value);
1296 my($list, $value) = @_;
1297 my $node = { VALUE => $value };
1299 $node->{LINK} = $list->{LINK};
1300 $list->{LINK} = $node;
1302 $_[0] = $node; # replace caller's version
1307 But again, Perl's built-in are virtually always good enough.
1309 =head2 How do I handle circular lists?
1311 Circular lists could be handled in the traditional fashion with linked
1312 lists, or you could just do something like this with an array:
1314 unshift(@array, pop(@array)); # the last shall be first
1315 push(@array, shift(@array)); # and vice versa
1317 =head2 How do I shuffle an array randomly?
1319 If you either have Perl 5.8.0 or later installed, or if you have
1320 Scalar-List-Utils 1.03 or later installed, you can say:
1322 use List::Util 'shuffle';
1324 @shuffled = shuffle(@list);
1326 If not, you can use this:
1328 # fisher_yates_shuffle
1329 # generate a random permutation of an array in place
1330 # As in shuffling a deck of cards
1332 sub fisher_yates_shuffle {
1333 my $deck = shift; # $deck is a reference to an array
1336 my $j = int rand ($i+1);
1337 @$deck[$i,$j] = @$deck[$j,$i];
1341 And here is an example of using it:
1344 # shuffle my mpeg collection
1346 my @mpeg = <audio/*/*.mp3>;
1347 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1350 Note that the above implementation shuffles an array in place,
1351 unlike the List::Util::shuffle() which takes a list and returns
1352 a new shuffled list.
1354 You've probably seen shuffling algorithms that work using splice,
1355 randomly picking another element to swap the current element with
1359 @old = 1 .. 10; # just a demo
1361 push(@new, splice(@old, rand @old, 1));
1364 This is bad because splice is already O(N), and since you do it N times,
1365 you just invented a quadratic algorithm; that is, O(N**2). This does
1366 not scale, although Perl is so efficient that you probably won't notice
1367 this until you have rather largish arrays.
1369 =head2 How do I process/modify each element of an array?
1371 Use C<for>/C<foreach>:
1374 s/foo/bar/; # change that word
1375 y/XZ/ZX/; # swap those letters
1378 Here's another; let's compute spherical volumes:
1380 for (@volumes = @radii) { # @volumes has changed parts
1382 $_ *= (4/3) * 3.14159; # this will be constant folded
1385 If you want to do the same thing to modify the values of the hash,
1386 you may not use the C<values> function, oddly enough. You need a slice:
1388 for $orbit ( @orbits{keys %orbits} ) {
1389 ($orbit **= 3) *= (4/3) * 3.14159;
1392 =head2 How do I select a random element from an array?
1394 Use the rand() function (see L<perlfunc/rand>):
1396 # at the top of the program:
1397 srand; # not needed for 5.004 and later
1400 $index = rand @array;
1401 $element = $array[$index];
1403 Make sure you I<only call srand once per program, if then>.
1404 If you are calling it more than once (such as before each
1405 call to rand), you're almost certainly doing something wrong.
1407 =head2 How do I permute N elements of a list?
1409 Here's a little program that generates all permutations
1410 of all the words on each line of input. The algorithm embodied
1411 in the permute() function should work on any list:
1414 # tsc-permute: permute each word of input
1415 permute([split], []);
1417 my @items = @{ $_[0] };
1418 my @perms = @{ $_[1] };
1422 my(@newitems,@newperms,$i);
1423 foreach $i (0 .. $#items) {
1426 unshift(@newperms, splice(@newitems, $i, 1));
1427 permute([@newitems], [@newperms]);
1432 Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
1433 module from CPAN runs at least an order of magnitude faster. If you don't
1434 have a C compiler (or a binary distribution of Algorithm::Permute), then
1435 you can use List::Permutor which is written in pure Perl, and is still
1436 several times faster than the algorithm above.
1438 =head2 How do I sort an array by (anything)?
1440 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1442 @list = sort { $a <=> $b } @list;
1444 The default sort function is cmp, string comparison, which would
1445 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1446 the numerical comparison operator.
1448 If you have a complicated function needed to pull out the part you
1449 want to sort on, then don't do it inside the sort function. Pull it
1450 out first, because the sort BLOCK can be called many times for the
1451 same element. Here's an example of how to pull out the first word
1452 after the first number on each item, and then sort those words
1457 ($item) = /\d+\s*(\S+)/;
1458 push @idx, uc($item);
1460 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1462 which could also be written this way, using a trick
1463 that's come to be known as the Schwartzian Transform:
1465 @sorted = map { $_->[0] }
1466 sort { $a->[1] cmp $b->[1] }
1467 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1469 If you need to sort on several fields, the following paradigm is useful.
1471 @sorted = sort { field1($a) <=> field1($b) ||
1472 field2($a) cmp field2($b) ||
1473 field3($a) cmp field3($b)
1476 This can be conveniently combined with precalculation of keys as given
1479 See http://www.cpan.org/doc/FMTEYEWTK/sort.html for more about
1482 See also the question below on sorting hashes.
1484 =head2 How do I manipulate arrays of bits?
1486 Use pack() and unpack(), or else vec() and the bitwise operations.
1488 For example, this sets $vec to have bit N set if $ints[N] was set:
1491 foreach(@ints) { vec($vec,$_,1) = 1 }
1493 Here's how, given a vector in $vec, you can
1494 get those bits into your @ints array:
1496 sub bitvec_to_list {
1499 # Find null-byte density then select best algorithm
1500 if ($vec =~ tr/\0// / length $vec > 0.95) {
1503 # This method is faster with mostly null-bytes
1504 while($vec =~ /[^\0]/g ) {
1505 $i = -9 + 8 * pos $vec;
1506 push @ints, $i if vec($vec, ++$i, 1);
1507 push @ints, $i if vec($vec, ++$i, 1);
1508 push @ints, $i if vec($vec, ++$i, 1);
1509 push @ints, $i if vec($vec, ++$i, 1);
1510 push @ints, $i if vec($vec, ++$i, 1);
1511 push @ints, $i if vec($vec, ++$i, 1);
1512 push @ints, $i if vec($vec, ++$i, 1);
1513 push @ints, $i if vec($vec, ++$i, 1);
1516 # This method is a fast general algorithm
1518 my $bits = unpack "b*", $vec;
1519 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1520 push @ints, pos $bits while($bits =~ /1/g);
1525 This method gets faster the more sparse the bit vector is.
1526 (Courtesy of Tim Bunce and Winfried Koenig.)
1528 Or use the CPAN module Bit::Vector:
1530 $vector = Bit::Vector->new($num_of_bits);
1531 $vector->Index_List_Store(@ints);
1532 @ints = $vector->Index_List_Read();
1534 Bit::Vector provides efficient methods for bit vector, sets of small integers
1537 Here's a more extensive illustration using vec():
1540 $vector = "\xff\x0f\xef\xfe";
1541 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1542 unpack("N", $vector), "\n";
1543 $is_set = vec($vector, 23, 1);
1544 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1562 my ($offset, $width, $value) = @_;
1564 vec($vector, $offset, $width) = $value;
1565 print "offset=$offset width=$width value=$value\n";
1571 my $bits = unpack("b*", $vector);
1575 print "vector length in bytes: ", length($vector), "\n";
1576 @bytes = unpack("A8" x length($vector), $bits);
1577 print "bits are: @bytes\n\n";
1580 =head2 Why does defined() return true on empty arrays and hashes?
1582 The short story is that you should probably only use defined on scalars or
1583 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1584 in the 5.004 release or later of Perl for more detail.
1586 =head1 Data: Hashes (Associative Arrays)
1588 =head2 How do I process an entire hash?
1590 Use the each() function (see L<perlfunc/each>) if you don't care
1591 whether it's sorted:
1593 while ( ($key, $value) = each %hash) {
1594 print "$key = $value\n";
1597 If you want it sorted, you'll have to use foreach() on the result of
1598 sorting the keys as shown in an earlier question.
1600 =head2 What happens if I add or remove keys from a hash while iterating over it?
1604 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1605 iterating over it. In Perl 5 you can delete from it, but you still
1606 can't add to it, because that might cause a doubling of the hash table,
1607 in which half the entries get copied up to the new top half of the
1608 table, at which point you've totally bamboozled the iterator code.
1609 Even if the table doesn't double, there's no telling whether your new
1610 entry will be inserted before or after the current iterator position.
1612 Either treasure up your changes and make them after the iterator finishes
1613 or use keys to fetch all the old keys at once, and iterate over the list
1616 =head2 How do I look up a hash element by value?
1618 Create a reverse hash:
1620 %by_value = reverse %by_key;
1621 $key = $by_value{$value};
1623 That's not particularly efficient. It would be more space-efficient
1626 while (($key, $value) = each %by_key) {
1627 $by_value{$value} = $key;
1630 If your hash could have repeated values, the methods above will only find
1631 one of the associated keys. This may or may not worry you. If it does
1632 worry you, you can always reverse the hash into a hash of arrays instead:
1634 while (($key, $value) = each %by_key) {
1635 push @{$key_list_by_value{$value}}, $key;
1638 =head2 How can I know how many entries are in a hash?
1640 If you mean how many keys, then all you have to do is
1641 take the scalar sense of the keys() function:
1643 $num_keys = scalar keys %hash;
1645 The keys() function also resets the iterator, which in void context is
1646 faster for tied hashes than would be iterating through the whole
1647 hash, one key-value pair at a time.
1649 =head2 How do I sort a hash (optionally by value instead of key)?
1651 Internally, hashes are stored in a way that prevents you from imposing
1652 an order on key-value pairs. Instead, you have to sort a list of the
1655 @keys = sort keys %hash; # sorted by key
1657 $hash{$a} cmp $hash{$b}
1658 } keys %hash; # and by value
1660 Here we'll do a reverse numeric sort by value, and if two keys are
1661 identical, sort by length of key, or if that fails, by straight ASCII
1662 comparison of the keys (well, possibly modified by your locale--see
1666 $hash{$b} <=> $hash{$a}
1668 length($b) <=> length($a)
1673 =head2 How can I always keep my hash sorted?
1675 You can look into using the DB_File module and tie() using the
1676 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1677 The Tie::IxHash module from CPAN might also be instructive.
1679 =head2 What's the difference between "delete" and "undef" with hashes?
1681 Hashes are pairs of scalars: the first is the key, the second is the
1682 value. The key will be coerced to a string, although the value can be
1683 any kind of scalar: string, number, or reference. If a key C<$key> is
1684 present in the array, C<exists($key)> will return true. The value for
1685 a given key can be C<undef>, in which case C<$array{$key}> will be
1686 C<undef> while C<$exists{$key}> will return true. This corresponds to
1687 (C<$key>, C<undef>) being in the hash.
1689 Pictures help... here's the C<%ary> table:
1699 And these conditions hold
1703 defined $ary{'d'} is true
1704 defined $ary{'a'} is true
1705 exists $ary{'a'} is true (Perl5 only)
1706 grep ($_ eq 'a', keys %ary) is true
1712 your table now reads:
1723 and these conditions now hold; changes in caps:
1727 defined $ary{'d'} is true
1728 defined $ary{'a'} is FALSE
1729 exists $ary{'a'} is true (Perl5 only)
1730 grep ($_ eq 'a', keys %ary) is true
1732 Notice the last two: you have an undef value, but a defined key!
1738 your table now reads:
1747 and these conditions now hold; changes in caps:
1751 defined $ary{'d'} is true
1752 defined $ary{'a'} is false
1753 exists $ary{'a'} is FALSE (Perl5 only)
1754 grep ($_ eq 'a', keys %ary) is FALSE
1756 See, the whole entry is gone!
1758 =head2 Why don't my tied hashes make the defined/exists distinction?
1760 They may or may not implement the EXISTS() and DEFINED() methods
1761 differently. For example, there isn't the concept of undef with hashes
1762 that are tied to DBM* files. This means the true/false tables above
1763 will give different results when used on such a hash. It also means
1764 that exists and defined do the same thing with a DBM* file, and what
1765 they end up doing is not what they do with ordinary hashes.
1767 =head2 How do I reset an each() operation part-way through?
1769 Using C<keys %hash> in scalar context returns the number of keys in
1770 the hash I<and> resets the iterator associated with the hash. You may
1771 need to do this if you use C<last> to exit a loop early so that when you
1772 re-enter it, the hash iterator has been reset.
1774 =head2 How can I get the unique keys from two hashes?
1776 First you extract the keys from the hashes into lists, then solve
1777 the "removing duplicates" problem described above. For example:
1780 for $element (keys(%foo), keys(%bar)) {
1787 @uniq = keys %{{%foo,%bar}};
1789 Or if you really want to save space:
1792 while (defined ($key = each %foo)) {
1795 while (defined ($key = each %bar)) {
1800 =head2 How can I store a multidimensional array in a DBM file?
1802 Either stringify the structure yourself (no fun), or else
1803 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1804 it on top of either DB_File or GDBM_File.
1806 =head2 How can I make my hash remember the order I put elements into it?
1808 Use the Tie::IxHash from CPAN.
1811 tie(%myhash, Tie::IxHash);
1812 for ($i=0; $i<20; $i++) {
1815 @keys = keys %myhash;
1816 # @keys = (0,1,2,3,...)
1818 =head2 Why does passing a subroutine an undefined element in a hash create it?
1820 If you say something like:
1822 somefunc($hash{"nonesuch key here"});
1824 Then that element "autovivifies"; that is, it springs into existence
1825 whether you store something there or not. That's because functions
1826 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1827 it has to be ready to write it back into the caller's version.
1829 This has been fixed as of Perl5.004.
1831 Normally, merely accessing a key's value for a nonexistent key does
1832 I<not> cause that key to be forever there. This is different than
1835 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1837 Usually a hash ref, perhaps like this:
1842 TITLE => "deputy peon",
1845 PALS => [ "Norbert", "Rhys", "Phineas"],
1848 References are documented in L<perlref> and the upcoming L<perlreftut>.
1849 Examples of complex data structures are given in L<perldsc> and
1850 L<perllol>. Examples of structures and object-oriented classes are
1853 =head2 How can I use a reference as a hash key?
1855 You can't do this directly, but you could use the standard Tie::Refhash
1856 module distributed with Perl.
1860 =head2 How do I handle binary data correctly?
1862 Perl is binary clean, so this shouldn't be a problem. For example,
1863 this works fine (assuming the files are found):
1865 if (`cat /vmunix` =~ /gzip/) {
1866 print "Your kernel is GNU-zip enabled!\n";
1869 On less elegant (read: Byzantine) systems, however, you have
1870 to play tedious games with "text" versus "binary" files. See
1871 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1872 systems are curses out of Microsoft, who seem to be committed to putting
1873 the backward into backward compatibility.
1875 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1877 If you want to deal with multibyte characters, however, there are
1878 some gotchas. See the section on Regular Expressions.
1880 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1882 Assuming that you don't care about IEEE notations like "NaN" or
1883 "Infinity", you probably just want to use a regular expression.
1885 if (/\D/) { print "has nondigits\n" }
1886 if (/^\d+$/) { print "is a whole number\n" }
1887 if (/^-?\d+$/) { print "is an integer\n" }
1888 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1889 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1890 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1891 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1892 { print "a C float" }
1894 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1895 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1896 wrapper function for more convenient access. This function takes
1897 a string and returns the number it found, or C<undef> for input that
1898 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1899 if you just want to say, ``Is this a float?''
1902 use POSIX qw(strtod);
1907 my($num, $unparsed) = strtod($str);
1908 if (($str eq '') || ($unparsed != 0) || $!) {
1915 sub is_numeric { defined getnum($_[0]) }
1917 Or you could check out the String::Scanf module on CPAN instead. The
1918 POSIX module (part of the standard Perl distribution) provides the
1919 C<strtod> and C<strtol> for converting strings to double and longs,
1922 =head2 How do I keep persistent data across program calls?
1924 For some specific applications, you can use one of the DBM modules.
1925 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1926 Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
1927 Storable is part of the standard distribution. Here's one example using
1928 Storable's C<store> and C<retrieve> functions:
1931 store(\%hash, "filename");
1934 $href = retrieve("filename"); # by ref
1935 %hash = %{ retrieve("filename") }; # direct to hash
1937 =head2 How do I print out or copy a recursive data structure?
1939 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1940 for printing out data structures. The Storable module, found on CPAN,
1941 provides a function called C<dclone> that recursively copies its argument.
1943 use Storable qw(dclone);
1946 Where $r1 can be a reference to any kind of data structure you'd like.
1947 It will be deeply copied. Because C<dclone> takes and returns references,
1948 you'd have to add extra punctuation if you had a hash of arrays that
1951 %newhash = %{ dclone(\%oldhash) };
1953 =head2 How do I define methods for every class/object?
1955 Use the UNIVERSAL class (see L<UNIVERSAL>).
1957 =head2 How do I verify a credit card checksum?
1959 Get the Business::CreditCard module from CPAN.
1961 =head2 How do I pack arrays of doubles or floats for XS code?
1963 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1964 If you're doing a lot of float or double processing, consider using
1965 the PDL module from CPAN instead--it makes number-crunching easy.
1967 =head1 AUTHOR AND COPYRIGHT
1969 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1970 All rights reserved.
1972 This documentation is free; you can redistribute it and/or modify it
1973 under the same terms as Perl itself.
1975 Irrespective of its distribution, all code examples in this file
1976 are hereby placed into the public domain. You are permitted and
1977 encouraged to use this code in your own programs for fun
1978 or for profit as you see fit. A simple comment in the code giving
1979 credit would be courteous but is not required.