3 perlfaq4 - Data Manipulation ($Revision: 1.7 $, $Date: 2001/10/26 19:46:03 $)
7 The section of the FAQ answers questions related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximated on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to an internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers. (See L<perlvar/"$#"> 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.
139 =item B<How do I convert Hexadecimal into decimal:>
141 Using perl's built in conversion of 0x notation:
144 $dec = sprintf("%d", $int);
146 Using the hex function:
148 $int = hex("DEADBEEF");
149 $dec = sprintf("%d", $int);
153 $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
154 $dec = sprintf("%d", $int);
156 Using the CPAN module Bit::Vector:
159 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
160 $dec = $vec->to_Dec();
162 =item B<How do I convert from decimal to hexadecimal:>
166 $hex = sprintf("%X", 3735928559);
170 $hex = unpack("H*", pack("N", 3735928559));
175 $vec = Bit::Vector->new_Dec(32, -559038737);
176 $hex = $vec->to_Hex();
178 And Bit::Vector supports odd bit counts:
181 $vec = Bit::Vector->new_Dec(33, 3735928559);
182 $vec->Resize(32); # suppress leading 0 if unwanted
183 $hex = $vec->to_Hex();
185 =item B<How do I convert from octal to decimal:>
187 Using Perl's built in conversion of numbers with leading zeros:
189 $int = 033653337357; # note the leading 0!
190 $dec = sprintf("%d", $int);
192 Using the oct function:
194 $int = oct("33653337357");
195 $dec = sprintf("%d", $int);
200 $vec = Bit::Vector->new(32);
201 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
202 $dec = $vec->to_Dec();
204 =item B<How do I convert from decimal to octal:>
208 $oct = sprintf("%o", 3735928559);
213 $vec = Bit::Vector->new_Dec(32, -559038737);
214 $oct = reverse join('', $vec->Chunk_List_Read(3));
216 =item B<How do I convert from binary to decimal:>
220 $decimal = ord(pack('B8', '10110110'));
222 Using pack and unpack for larger strings
224 $int = unpack("N", pack("B32",
225 substr("0" x 32 . "11110101011011011111011101111", -32)));
226 $dec = sprintf("%d", $int);
228 # substr() is used to left pad a 32 character string with zeros.
232 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
233 $dec = $vec->to_Dec();
235 =item B<How do I convert from decimal to binary:>
239 $bin = unpack("B*", pack("N", 3735928559));
244 $vec = Bit::Vector->new_Dec(32, -559038737);
245 $bin = $vec->to_Bin();
247 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
248 are left as an exercise to the inclined reader.
251 =head2 Why doesn't & work the way I want it to?
253 The behavior of binary arithmetic operators depends on whether they're
254 used on numbers or strings. The operators treat a string as a series
255 of bits and work with that (the string C<"3"> is the bit pattern
256 C<00110011>). The operators work with the binary form of a number
257 (the number C<3> is treated as the bit pattern C<00000011>).
259 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
260 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
263 Most problems with C<&> and C<|> arise because the programmer thinks
264 they have a number but really it's a string. The rest arise because
267 if ("\020\020" & "\101\101") {
271 but a string consisting of two null bytes (the result of C<"\020\020"
272 & "\101\101">) is not a false value in Perl. You need:
274 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
278 =head2 How do I multiply matrices?
280 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
281 or the PDL extension (also available from CPAN).
283 =head2 How do I perform an operation on a series of integers?
285 To call a function on each element in an array, and collect the
288 @results = map { my_func($_) } @array;
292 @triple = map { 3 * $_ } @single;
294 To call a function on each element of an array, but ignore the
297 foreach $iterator (@array) {
298 some_func($iterator);
301 To call a function on each integer in a (small) range, you B<can> use:
303 @results = map { some_func($_) } (5 .. 25);
305 but you should be aware that the C<..> operator creates an array of
306 all integers in the range. This can take a lot of memory for large
310 for ($i=5; $i < 500_005; $i++) {
311 push(@results, some_func($i));
314 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
315 loop will iterate over the range, without creating the entire range.
317 for my $i (5 .. 500_005) {
318 push(@results, some_func($i));
321 will not create a list of 500,000 integers.
323 =head2 How can I output Roman numerals?
325 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
327 =head2 Why aren't my random numbers random?
329 If you're using a version of Perl before 5.004, you must call C<srand>
330 once at the start of your program to seed the random number generator.
331 5.004 and later automatically call C<srand> at the beginning. Don't
332 call C<srand> more than once--you make your numbers less random, rather
335 Computers are good at being predictable and bad at being random
336 (despite appearances caused by bugs in your programs :-).
337 http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
338 Phoenix, talks more about this. John von Neumann said, ``Anyone who
339 attempts to generate random numbers by deterministic means is, of
340 course, living in a state of sin.''
342 If you want numbers that are more random than C<rand> with C<srand>
343 provides, you should also check out the Math::TrulyRandom module from
344 CPAN. It uses the imperfections in your system's timer to generate
345 random numbers, but this takes quite a while. If you want a better
346 pseudorandom generator than comes with your operating system, look at
347 ``Numerical Recipes in C'' at http://www.nr.com/ .
351 =head2 How do I find the week-of-the-year/day-of-the-year?
353 The day of the year is in the array returned by localtime() (see
354 L<perlfunc/"localtime">):
356 $day_of_year = (localtime(time()))[7];
358 =head2 How do I find the current century or millennium?
360 Use the following simple functions:
363 return int((((localtime(shift || time))[5] + 1999))/100);
366 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
369 On some systems, you'll find that the POSIX module's strftime() function
370 has been extended in a non-standard way to use a C<%C> format, which they
371 sometimes claim is the "century". It isn't, because on most such systems,
372 this is only the first two digits of the four-digit year, and thus cannot
373 be used to reliably determine the current century or millennium.
375 =head2 How can I compare two dates and find the difference?
377 If you're storing your dates as epoch seconds then simply subtract one
378 from the other. If you've got a structured date (distinct year, day,
379 month, hour, minute, seconds values), then for reasons of accessibility,
380 simplicity, and efficiency, merely use either timelocal or timegm (from
381 the Time::Local module in the standard distribution) to reduce structured
382 dates to epoch seconds. However, if you don't know the precise format of
383 your dates, then you should probably use either of the Date::Manip and
384 Date::Calc modules from CPAN before you go hacking up your own parsing
385 routine to handle arbitrary date formats.
387 =head2 How can I take a string and turn it into epoch seconds?
389 If it's a regular enough string that it always has the same format,
390 you can split it up and pass the parts to C<timelocal> in the standard
391 Time::Local module. Otherwise, you should look into the Date::Calc
392 and Date::Manip modules from CPAN.
394 =head2 How can I find the Julian Day?
396 Use the Time::JulianDay module (part of the Time-modules bundle
397 available from CPAN.)
399 Before you immerse yourself too deeply in this, be sure to verify that
400 it is the I<Julian> Day you really want. Are you interested in a way
401 of getting serial days so that you just can tell how many days they
402 are apart or so that you can do also other date arithmetic? If you
403 are interested in performing date arithmetic, this can be done using
404 modules Date::Manip or Date::Calc.
406 There is too many details and much confusion on this issue to cover in
407 this FAQ, but the term is applied (correctly) to a calendar now
408 supplanted by the Gregorian Calendar, with the Julian Calendar failing
409 to adjust properly for leap years on centennial years (among other
410 annoyances). The term is also used (incorrectly) to mean: [1] days in
411 the Gregorian Calendar; and [2] days since a particular starting time
412 or `epoch', usually 1970 in the Unix world and 1980 in the
413 MS-DOS/Windows world. If you find that it is not the first meaning
414 that you really want, then check out the Date::Manip and Date::Calc
415 modules. (Thanks to David Cassell for most of this text.)
417 =head2 How do I find yesterday's date?
419 The C<time()> function returns the current time in seconds since the
420 epoch. Take twenty-four hours off that:
422 $yesterday = time() - ( 24 * 60 * 60 );
424 Then you can pass this to C<localtime()> and get the individual year,
425 month, day, hour, minute, seconds values.
427 Note very carefully that the code above assumes that your days are
428 twenty-four hours each. For most people, there are two days a year
429 when they aren't: the switch to and from summer time throws this off.
430 A solution to this issue is offered by Russ Allbery.
433 my $now = defined $_[0] ? $_[0] : time;
434 my $then = $now - 60 * 60 * 24;
435 my $ndst = (localtime $now)[8] > 0;
436 my $tdst = (localtime $then)[8] > 0;
437 $then - ($tdst - $ndst) * 60 * 60;
439 # Should give you "this time yesterday" in seconds since epoch relative to
440 # the first argument or the current time if no argument is given and
441 # suitable for passing to localtime or whatever else you need to do with
442 # it. $ndst is whether we're currently in daylight savings time; $tdst is
443 # whether the point 24 hours ago was in daylight savings time. If $tdst
444 # and $ndst are the same, a boundary wasn't crossed, and the correction
445 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
446 # from yesterday's time since we gained an extra hour while going off
447 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
448 # negative hour (add an hour) to yesterday's time since we lost an hour.
450 # All of this is because during those days when one switches off or onto
451 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
453 # The explicit settings of $ndst and $tdst are necessary because localtime
454 # only says it returns the system tm struct, and the system tm struct at
455 # least on Solaris doesn't guarantee any particular positive value (like,
456 # say, 1) for isdst, just a positive value. And that value can
457 # potentially be negative, if DST information isn't available (this sub
458 # just treats those cases like no DST).
460 # Note that between 2am and 3am on the day after the time zone switches
461 # off daylight savings time, the exact hour of "yesterday" corresponding
462 # to the current hour is not clearly defined. Note also that if used
463 # between 2am and 3am the day after the change to daylight savings time,
464 # the result will be between 3am and 4am of the previous day; it's
465 # arguable whether this is correct.
467 # This sub does not attempt to deal with leap seconds (most things don't).
469 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
470 # This code is in the public domain
472 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
474 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
475 Y2K compliant (whatever that means). The programmers you've hired to
476 use it, however, probably are not.
478 Long answer: The question belies a true understanding of the issue.
479 Perl is just as Y2K compliant as your pencil--no more, and no less.
480 Can you use your pencil to write a non-Y2K-compliant memo? Of course
481 you can. Is that the pencil's fault? Of course it isn't.
483 The date and time functions supplied with Perl (gmtime and localtime)
484 supply adequate information to determine the year well beyond 2000
485 (2038 is when trouble strikes for 32-bit machines). The year returned
486 by these functions when used in a list context is the year minus 1900.
487 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
488 number. To avoid the year 2000 problem simply do not treat the year as
489 a 2-digit number. It isn't.
491 When gmtime() and localtime() are used in scalar context they return
492 a timestamp string that contains a fully-expanded year. For example,
493 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
494 2001". There's no year 2000 problem here.
496 That doesn't mean that Perl can't be used to create non-Y2K compliant
497 programs. It can. But so can your pencil. It's the fault of the user,
498 not the language. At the risk of inflaming the NRA: ``Perl doesn't
499 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
504 =head2 How do I validate input?
506 The answer to this question is usually a regular expression, perhaps
507 with auxiliary logic. See the more specific questions (numbers, mail
508 addresses, etc.) for details.
510 =head2 How do I unescape a string?
512 It depends just what you mean by ``escape''. URL escapes are dealt
513 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
514 character are removed with
518 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
520 =head2 How do I remove consecutive pairs of characters?
522 To turn C<"abbcccd"> into C<"abccd">:
524 s/(.)\1/$1/g; # add /s to include newlines
526 Here's a solution that turns "abbcccd" to "abcd":
528 y///cs; # y == tr, but shorter :-)
530 =head2 How do I expand function calls in a string?
532 This is documented in L<perlref>. In general, this is fraught with
533 quoting and readability problems, but it is possible. To interpolate
534 a subroutine call (in list context) into a string:
536 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
538 If you prefer scalar context, similar chicanery is also useful for
539 arbitrary expressions:
541 print "That yields ${\($n + 5)} widgets\n";
543 Version 5.004 of Perl had a bug that gave list context to the
544 expression in C<${...}>, but this is fixed in version 5.005.
546 See also ``How can I expand variables in text strings?'' in this
549 =head2 How do I find matching/nesting anything?
551 This isn't something that can be done in one regular expression, no
552 matter how complicated. To find something between two single
553 characters, a pattern like C</x([^x]*)x/> will get the intervening
554 bits in $1. For multiple ones, then something more like
555 C</alpha(.*?)omega/> would be needed. But none of these deals with
556 nested patterns, nor can they. For that you'll have to write a
559 If you are serious about writing a parser, there are a number of
560 modules or oddities that will make your life a lot easier. There are
561 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
562 and the byacc program. Starting from perl 5.8 the Text::Balanced
563 is part of the standard distribution.
565 One simple destructive, inside-out approach that you might try is to
566 pull out the smallest nesting parts one at a time:
568 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
569 # do something with $1
572 A more complicated and sneaky approach is to make Perl's regular
573 expression engine do it for you. This is courtesy Dean Inada, and
574 rather has the nature of an Obfuscated Perl Contest entry, but it
577 # $_ contains the string to parse
578 # BEGIN and END are the opening and closing markers for the
583 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
584 @$ = (eval{/$re/},$@!~/unmatched/);
585 print join("\n",@$[0..$#$]) if( $$[-1] );
587 =head2 How do I reverse a string?
589 Use reverse() in scalar context, as documented in
592 $reversed = reverse $string;
594 =head2 How do I expand tabs in a string?
596 You can do it yourself:
598 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
600 Or you can just use the Text::Tabs module (part of the standard Perl
604 @expanded_lines = expand(@lines_with_tabs);
606 =head2 How do I reformat a paragraph?
608 Use Text::Wrap (part of the standard Perl distribution):
611 print wrap("\t", ' ', @paragraphs);
613 The paragraphs you give to Text::Wrap should not contain embedded
614 newlines. Text::Wrap doesn't justify the lines (flush-right).
616 Or use the CPAN module Text::Autoformat. Formatting files can be easily
617 done by making a shell alias, like so:
619 alias fmt="perl -i -MText::Autoformat -n0777 \
620 -e 'print autoformat $_, {all=>1}' $*"
622 See the documentation for Text::Autoformat to appreciate its many
625 =head2 How can I access/change the first N letters of a string?
627 There are many ways. If you just want to grab a copy, use
630 $first_byte = substr($a, 0, 1);
632 If you want to modify part of a string, the simplest way is often to
633 use substr() as an lvalue:
635 substr($a, 0, 3) = "Tom";
637 Although those with a pattern matching kind of thought process will
642 =head2 How do I change the Nth occurrence of something?
644 You have to keep track of N yourself. For example, let's say you want
645 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
646 C<"whosoever"> or C<"whomsoever">, case insensitively. These
647 all assume that $_ contains the string to be altered.
651 ++$count == 5 # is it the 5th?
652 ? "${2}soever" # yes, swap
653 : $1 # renege and leave it there
656 In the more general case, you can use the C</g> modifier in a C<while>
657 loop, keeping count of matches.
661 $_ = "One fish two fish red fish blue fish";
662 while (/(\w+)\s+fish\b/gi) {
663 if (++$count == $WANT) {
664 print "The third fish is a $1 one.\n";
668 That prints out: C<"The third fish is a red one."> You can also use a
669 repetition count and repeated pattern like this:
671 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
673 =head2 How can I count the number of occurrences of a substring within a string?
675 There are a number of ways, with varying efficiency. If you want a
676 count of a certain single character (X) within a string, you can use the
677 C<tr///> function like so:
679 $string = "ThisXlineXhasXsomeXx'sXinXit";
680 $count = ($string =~ tr/X//);
681 print "There are $count X characters in the string";
683 This is fine if you are just looking for a single character. However,
684 if you are trying to count multiple character substrings within a
685 larger string, C<tr///> won't work. What you can do is wrap a while()
686 loop around a global pattern match. For example, let's count negative
689 $string = "-9 55 48 -2 23 -76 4 14 -44";
690 while ($string =~ /-\d+/g) { $count++ }
691 print "There are $count negative numbers in the string";
693 =head2 How do I capitalize all the words on one line?
695 To make the first letter of each word upper case:
697 $line =~ s/\b(\w)/\U$1/g;
699 This has the strange effect of turning "C<don't do it>" into "C<Don'T
700 Do It>". Sometimes you might want this. Other times you might need a
701 more thorough solution (Suggested by brian d foy):
704 (^\w) #at the beginning of the line
706 (\s\w) #preceded by whitespace
709 $string =~ /([\w']+)/\u\L$1/g;
711 To make the whole line upper case:
715 To force each word to be lower case, with the first letter upper case:
717 $line =~ s/(\w+)/\u\L$1/g;
719 You can (and probably should) enable locale awareness of those
720 characters by placing a C<use locale> pragma in your program.
721 See L<perllocale> for endless details on locales.
723 This is sometimes referred to as putting something into "title
724 case", but that's not quite accurate. Consider the proper
725 capitalization of the movie I<Dr. Strangelove or: How I Learned to
726 Stop Worrying and Love the Bomb>, for example.
728 =head2 How can I split a [character] delimited string except when inside
729 [character]? (Comma-separated files)
731 Take the example case of trying to split a string that is comma-separated
732 into its different fields. (We'll pretend you said comma-separated, not
733 comma-delimited, which is different and almost never what you mean.) You
734 can't use C<split(/,/)> because you shouldn't split if the comma is inside
735 quotes. For example, take a data line like this:
737 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
739 Due to the restriction of the quotes, this is a fairly complex
740 problem. Thankfully, we have Jeffrey Friedl, author of a highly
741 recommended book on regular expressions, to handle these for us. He
742 suggests (assuming your string is contained in $text):
745 push(@new, $+) while $text =~ m{
746 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
750 push(@new, undef) if substr($text,-1,1) eq ',';
752 If you want to represent quotation marks inside a
753 quotation-mark-delimited field, escape them with backslashes (eg,
754 C<"like \"this\"">. Unescaping them is a task addressed earlier in
757 Alternatively, the Text::ParseWords module (part of the standard Perl
758 distribution) lets you say:
760 use Text::ParseWords;
761 @new = quotewords(",", 0, $text);
763 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
765 =head2 How do I strip blank space from the beginning/end of a string?
767 Although the simplest approach would seem to be
769 $string =~ s/^\s*(.*?)\s*$/$1/;
771 not only is this unnecessarily slow and destructive, it also fails with
772 embedded newlines. It is much faster to do this operation in two steps:
777 Or more nicely written as:
784 This idiom takes advantage of the C<foreach> loop's aliasing
785 behavior to factor out common code. You can do this
786 on several strings at once, or arrays, or even the
787 values of a hash if you use a slice:
789 # trim whitespace in the scalar, the array,
790 # and all the values in the hash
791 foreach ($scalar, @array, @hash{keys %hash}) {
796 =head2 How do I pad a string with blanks or pad a number with zeroes?
798 (This answer contributed by Uri Guttman, with kibitzing from
801 In the following examples, C<$pad_len> is the length to which you wish
802 to pad the string, C<$text> or C<$num> contains the string to be padded,
803 and C<$pad_char> contains the padding character. You can use a single
804 character string constant instead of the C<$pad_char> variable if you
805 know what it is in advance. And in the same way you can use an integer in
806 place of C<$pad_len> if you know the pad length in advance.
808 The simplest method uses the C<sprintf> function. It can pad on the left
809 or right with blanks and on the left with zeroes and it will not
810 truncate the result. The C<pack> function can only pad strings on the
811 right with blanks and it will truncate the result to a maximum length of
814 # Left padding a string with blanks (no truncation):
815 $padded = sprintf("%${pad_len}s", $text);
817 # Right padding a string with blanks (no truncation):
818 $padded = sprintf("%-${pad_len}s", $text);
820 # Left padding a number with 0 (no truncation):
821 $padded = sprintf("%0${pad_len}d", $num);
823 # Right padding a string with blanks using pack (will truncate):
824 $padded = pack("A$pad_len",$text);
826 If you need to pad with a character other than blank or zero you can use
827 one of the following methods. They all generate a pad string with the
828 C<x> operator and combine that with C<$text>. These methods do
829 not truncate C<$text>.
831 Left and right padding with any character, creating a new string:
833 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
834 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
836 Left and right padding with any character, modifying C<$text> directly:
838 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
839 $text .= $pad_char x ( $pad_len - length( $text ) );
841 =head2 How do I extract selected columns from a string?
843 Use substr() or unpack(), both documented in L<perlfunc>.
844 If you prefer thinking in terms of columns instead of widths,
845 you can use this kind of thing:
847 # determine the unpack format needed to split Linux ps output
848 # arguments are cut columns
849 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
855 for my $place (@positions) {
856 $template .= "A" . ($place - $lastpos) . " ";
863 =head2 How do I find the soundex value of a string?
865 Use the standard Text::Soundex module distributed with Perl.
866 Before you do so, you may want to determine whether `soundex' is in
867 fact what you think it is. Knuth's soundex algorithm compresses words
868 into a small space, and so it does not necessarily distinguish between
869 two words which you might want to appear separately. For example, the
870 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
871 If Text::Soundex does not do what you are looking for, you might want
872 to consider the String::Approx module available at CPAN.
874 =head2 How can I expand variables in text strings?
876 Let's assume that you have a string like:
878 $text = 'this has a $foo in it and a $bar';
880 If those were both global variables, then this would
883 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
885 But since they are probably lexicals, or at least, they could
886 be, you'd have to do this:
888 $text =~ s/(\$\w+)/$1/eeg;
889 die if $@; # needed /ee, not /e
891 It's probably better in the general case to treat those
892 variables as entries in some special hash. For example:
898 $text =~ s/\$(\w+)/$user_defs{$1}/g;
900 See also ``How do I expand function calls in a string?'' in this section
903 =head2 What's wrong with always quoting "$vars"?
905 The problem is that those double-quotes force stringification--
906 coercing numbers and references into strings--even when you
907 don't want them to be strings. Think of it this way: double-quote
908 expansion is used to produce new strings. If you already
909 have a string, why do you need more?
911 If you get used to writing odd things like these:
915 somefunc("$var"); # BAD
917 You'll be in trouble. Those should (in 99.8% of the cases) be
918 the simpler and more direct:
924 Otherwise, besides slowing you down, you're going to break code when
925 the thing in the scalar is actually neither a string nor a number, but
931 my $oref = "$aref"; # WRONG
934 You can also get into subtle problems on those few operations in Perl
935 that actually do care about the difference between a string and a
936 number, such as the magical C<++> autoincrement operator or the
939 Stringification also destroys arrays.
942 print "@lines"; # WRONG - extra blanks
943 print @lines; # right
945 =head2 Why don't my <<HERE documents work?
947 Check for these three things:
951 =item 1. There must be no space after the << part.
953 =item 2. There (probably) should be a semicolon at the end.
955 =item 3. You can't (easily) have any space in front of the tag.
959 If you want to indent the text in the here document, you
963 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
968 But the HERE_TARGET must still be flush against the margin.
969 If you want that indented also, you'll have to quote
972 ($quote = <<' FINIS') =~ s/^\s+//gm;
973 ...we will have peace, when you and all your works have
974 perished--and the works of your dark master to whom you
975 would deliver us. You are a liar, Saruman, and a corrupter
976 of men's hearts. --Theoden in /usr/src/perl/taint.c
978 $quote =~ s/\s*--/\n--/;
980 A nice general-purpose fixer-upper function for indented here documents
981 follows. It expects to be called with a here document as its argument.
982 It looks to see whether each line begins with a common substring, and
983 if so, strips that substring off. Otherwise, it takes the amount of leading
984 whitespace found on the first line and removes that much off each
989 my ($white, $leader); # common whitespace and common leading string
990 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
991 ($white, $leader) = ($2, quotemeta($1));
993 ($white, $leader) = (/^(\s+)/, '');
995 s/^\s*?$leader(?:$white)?//gm;
999 This works with leading special strings, dynamically determined:
1001 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1004 @@@ SAVEI32(runlevel);
1006 @@@ while ( op = (*op->op_ppaddr)() );
1010 MAIN_INTERPRETER_LOOP
1012 Or with a fixed amount of leading whitespace, with remaining
1013 indentation correctly preserved:
1015 $poem = fix<<EVER_ON_AND_ON;
1016 Now far ahead the Road has gone,
1017 And I must follow, if I can,
1018 Pursuing it with eager feet,
1019 Until it joins some larger way
1020 Where many paths and errands meet.
1021 And whither then? I cannot say.
1022 --Bilbo in /usr/src/perl/pp_ctl.c
1027 =head2 What is the difference between a list and an array?
1029 An array has a changeable length. A list does not. An array is something
1030 you can push or pop, while a list is a set of values. Some people make
1031 the distinction that a list is a value while an array is a variable.
1032 Subroutines are passed and return lists, you put things into list
1033 context, you initialize arrays with lists, and you foreach() across
1034 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1035 in scalar context behave like the number of elements in them, subroutines
1036 access their arguments through the array C<@_>, and push/pop/shift only work
1039 As a side note, there's no such thing as a list in scalar context.
1042 $scalar = (2, 5, 7, 9);
1044 you're using the comma operator in scalar context, so it uses the scalar
1045 comma operator. There never was a list there at all! This causes the
1046 last value to be returned: 9.
1048 =head2 What is the difference between $array[1] and @array[1]?
1050 The former is a scalar value; the latter an array slice, making
1051 it a list with one (scalar) value. You should use $ when you want a
1052 scalar value (most of the time) and @ when you want a list with one
1053 scalar value in it (very, very rarely; nearly never, in fact).
1055 Sometimes it doesn't make a difference, but sometimes it does.
1056 For example, compare:
1058 $good[0] = `some program that outputs several lines`;
1062 @bad[0] = `same program that outputs several lines`;
1064 The C<use warnings> pragma and the B<-w> flag will warn you about these
1067 =head2 How can I remove duplicate elements from a list or array?
1069 There are several possible ways, depending on whether the array is
1070 ordered and whether you wish to preserve the ordering.
1076 If @in is sorted, and you want @out to be sorted:
1077 (this assumes all true values in the array)
1079 $prev = "not equal to $in[0]";
1080 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1082 This is nice in that it doesn't use much extra memory, simulating
1083 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1084 guarantees that the expression is true (so that grep picks it up)
1085 even if the $_ is 0, "", or undef.
1089 If you don't know whether @in is sorted:
1092 @out = grep(!$saw{$_}++, @in);
1096 Like (b), but @in contains only small integers:
1098 @out = grep(!$saw[$_]++, @in);
1102 A way to do (b) without any loops or greps:
1106 @out = sort keys %saw; # remove sort if undesired
1110 Like (d), but @in contains only small positive integers:
1114 @out = grep {defined} @ary;
1118 But perhaps you should have been using a hash all along, eh?
1120 =head2 How can I tell whether a certain element is contained in a list or array?
1122 Hearing the word "in" is an I<in>dication that you probably should have
1123 used a hash, not a list or array, to store your data. Hashes are
1124 designed to answer this question quickly and efficiently. Arrays aren't.
1126 That being said, there are several ways to approach this. If you
1127 are going to make this query many times over arbitrary string values,
1128 the fastest way is probably to invert the original array and keep an
1129 associative array lying about whose keys are the first array's values.
1131 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1133 for (@blues) { $is_blue{$_} = 1 }
1135 Now you can check whether $is_blue{$some_color}. It might have been a
1136 good idea to keep the blues all in a hash in the first place.
1138 If the values are all small integers, you could use a simple indexed
1139 array. This kind of an array will take up less space:
1141 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1142 undef @is_tiny_prime;
1143 for (@primes) { $is_tiny_prime[$_] = 1 }
1144 # or simply @istiny_prime[@primes] = (1) x @primes;
1146 Now you check whether $is_tiny_prime[$some_number].
1148 If the values in question are integers instead of strings, you can save
1149 quite a lot of space by using bit strings instead:
1151 @articles = ( 1..10, 150..2000, 2017 );
1153 for (@articles) { vec($read,$_,1) = 1 }
1155 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1159 ($is_there) = grep $_ eq $whatever, @array;
1163 ($is_there) = grep /$whatever/, @array;
1165 These are slow (checks every element even if the first matches),
1166 inefficient (same reason), and potentially buggy (what if there are
1167 regex characters in $whatever?). If you're only testing once, then
1171 foreach $elt (@array) {
1172 if ($elt eq $elt_to_find) {
1177 if ($is_there) { ... }
1179 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1181 Use a hash. Here's code to do both and more. It assumes that
1182 each element is unique in a given array:
1184 @union = @intersection = @difference = ();
1186 foreach $element (@array1, @array2) { $count{$element}++ }
1187 foreach $element (keys %count) {
1188 push @union, $element;
1189 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1192 Note that this is the I<symmetric difference>, that is, all elements in
1193 either A or in B but not in both. Think of it as an xor operation.
1195 =head2 How do I test whether two arrays or hashes are equal?
1197 The following code works for single-level arrays. It uses a stringwise
1198 comparison, and does not distinguish defined versus undefined empty
1199 strings. Modify if you have other needs.
1201 $are_equal = compare_arrays(\@frogs, \@toads);
1203 sub compare_arrays {
1204 my ($first, $second) = @_;
1205 no warnings; # silence spurious -w undef complaints
1206 return 0 unless @$first == @$second;
1207 for (my $i = 0; $i < @$first; $i++) {
1208 return 0 if $first->[$i] ne $second->[$i];
1213 For multilevel structures, you may wish to use an approach more
1214 like this one. It uses the CPAN module FreezeThaw:
1216 use FreezeThaw qw(cmpStr);
1217 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1219 printf "a and b contain %s arrays\n",
1220 cmpStr(\@a, \@b) == 0
1224 This approach also works for comparing hashes. Here
1225 we'll demonstrate two different answers:
1227 use FreezeThaw qw(cmpStr cmpStrHard);
1229 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1233 printf "a and b contain %s hashes\n",
1234 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1236 printf "a and b contain %s hashes\n",
1237 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1240 The first reports that both those the hashes contain the same data,
1241 while the second reports that they do not. Which you prefer is left as
1242 an exercise to the reader.
1244 =head2 How do I find the first array element for which a condition is true?
1246 You can use this if you care about the index:
1248 for ($i= 0; $i < @array; $i++) {
1249 if ($array[$i] eq "Waldo") {
1255 Now C<$found_index> has what you want.
1257 =head2 How do I handle linked lists?
1259 In general, you usually don't need a linked list in Perl, since with
1260 regular arrays, you can push and pop or shift and unshift at either end,
1261 or you can use splice to add and/or remove arbitrary number of elements at
1262 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1263 dynamic arrays. In the absence of shifts and pops, push in general
1264 needs to reallocate on the order every log(N) times, and unshift will
1265 need to copy pointers each time.
1267 If you really, really wanted, you could use structures as described in
1268 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1269 to do. For example, imagine a list node like this:
1276 You could walk the list this way:
1279 for ($node = $head; $node; $node = $node->{LINK}) {
1280 print $node->{VALUE}, " ";
1284 You could add to the list this way:
1287 $tail = append($head, 1); # grow a new head
1288 for $value ( 2 .. 10 ) {
1289 $tail = append($tail, $value);
1293 my($list, $value) = @_;
1294 my $node = { VALUE => $value };
1296 $node->{LINK} = $list->{LINK};
1297 $list->{LINK} = $node;
1299 $_[0] = $node; # replace caller's version
1304 But again, Perl's built-in are virtually always good enough.
1306 =head2 How do I handle circular lists?
1308 Circular lists could be handled in the traditional fashion with linked
1309 lists, or you could just do something like this with an array:
1311 unshift(@array, pop(@array)); # the last shall be first
1312 push(@array, shift(@array)); # and vice versa
1314 =head2 How do I shuffle an array randomly?
1316 If you either have Perl 5.8.0 or later installed, or if you have
1317 Scalar-List-Utils 1.03 or later installed, you can say:
1319 use List::Util 'shuffle';
1321 @shuffled = shuffle(@list);
1323 If not, you can use this:
1325 # fisher_yates_shuffle
1326 # generate a random permutation of an array in place
1327 # As in shuffling a deck of cards
1329 sub fisher_yates_shuffle {
1330 my $deck = shift; # $deck is a reference to an array
1333 my $j = int rand ($i+1);
1334 @$deck[$i,$j] = @$deck[$j,$i];
1338 And here is an example of using it:
1341 # shuffle my mpeg collection
1343 my @mpeg = <audio/*/*.mp3>;
1344 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1347 Note that the above implementation shuffles an array in place,
1348 unlike the List::Util::shuffle() which takes a list and returns
1349 a new shuffled list.
1351 You've probably seen shuffling algorithms that work using splice,
1352 randomly picking another element to swap the current element with
1356 @old = 1 .. 10; # just a demo
1358 push(@new, splice(@old, rand @old, 1));
1361 This is bad because splice is already O(N), and since you do it N times,
1362 you just invented a quadratic algorithm; that is, O(N**2). This does
1363 not scale, although Perl is so efficient that you probably won't notice
1364 this until you have rather largish arrays.
1366 =head2 How do I process/modify each element of an array?
1368 Use C<for>/C<foreach>:
1371 s/foo/bar/; # change that word
1372 y/XZ/ZX/; # swap those letters
1375 Here's another; let's compute spherical volumes:
1377 for (@volumes = @radii) { # @volumes has changed parts
1379 $_ *= (4/3) * 3.14159; # this will be constant folded
1382 If you want to do the same thing to modify the values of the hash,
1383 you may not use the C<values> function, oddly enough. You need a slice:
1385 for $orbit ( @orbits{keys %orbits} ) {
1386 ($orbit **= 3) *= (4/3) * 3.14159;
1389 =head2 How do I select a random element from an array?
1391 Use the rand() function (see L<perlfunc/rand>):
1393 # at the top of the program:
1394 srand; # not needed for 5.004 and later
1397 $index = rand @array;
1398 $element = $array[$index];
1400 Make sure you I<only call srand once per program, if then>.
1401 If you are calling it more than once (such as before each
1402 call to rand), you're almost certainly doing something wrong.
1404 =head2 How do I permute N elements of a list?
1406 Here's a little program that generates all permutations
1407 of all the words on each line of input. The algorithm embodied
1408 in the permute() function should work on any list:
1411 # tsc-permute: permute each word of input
1412 permute([split], []);
1414 my @items = @{ $_[0] };
1415 my @perms = @{ $_[1] };
1419 my(@newitems,@newperms,$i);
1420 foreach $i (0 .. $#items) {
1423 unshift(@newperms, splice(@newitems, $i, 1));
1424 permute([@newitems], [@newperms]);
1429 Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
1430 module from CPAN runs at least an order of magnitude faster. If you don't
1431 have a C compiler (or a binary distribution of Algorithm::Permute), then
1432 you can use List::Permutor which is written in pure Perl, and is still
1433 several times faster than the algorithm above.
1435 =head2 How do I sort an array by (anything)?
1437 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1439 @list = sort { $a <=> $b } @list;
1441 The default sort function is cmp, string comparison, which would
1442 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1443 the numerical comparison operator.
1445 If you have a complicated function needed to pull out the part you
1446 want to sort on, then don't do it inside the sort function. Pull it
1447 out first, because the sort BLOCK can be called many times for the
1448 same element. Here's an example of how to pull out the first word
1449 after the first number on each item, and then sort those words
1454 ($item) = /\d+\s*(\S+)/;
1455 push @idx, uc($item);
1457 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1459 which could also be written this way, using a trick
1460 that's come to be known as the Schwartzian Transform:
1462 @sorted = map { $_->[0] }
1463 sort { $a->[1] cmp $b->[1] }
1464 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1466 If you need to sort on several fields, the following paradigm is useful.
1468 @sorted = sort { field1($a) <=> field1($b) ||
1469 field2($a) cmp field2($b) ||
1470 field3($a) cmp field3($b)
1473 This can be conveniently combined with precalculation of keys as given
1476 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1479 See also the question below on sorting hashes.
1481 =head2 How do I manipulate arrays of bits?
1483 Use pack() and unpack(), or else vec() and the bitwise operations.
1485 For example, this sets $vec to have bit N set if $ints[N] was set:
1488 foreach(@ints) { vec($vec,$_,1) = 1 }
1490 Here's how, given a vector in $vec, you can
1491 get those bits into your @ints array:
1493 sub bitvec_to_list {
1496 # Find null-byte density then select best algorithm
1497 if ($vec =~ tr/\0// / length $vec > 0.95) {
1500 # This method is faster with mostly null-bytes
1501 while($vec =~ /[^\0]/g ) {
1502 $i = -9 + 8 * pos $vec;
1503 push @ints, $i if vec($vec, ++$i, 1);
1504 push @ints, $i if vec($vec, ++$i, 1);
1505 push @ints, $i if vec($vec, ++$i, 1);
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);
1513 # This method is a fast general algorithm
1515 my $bits = unpack "b*", $vec;
1516 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1517 push @ints, pos $bits while($bits =~ /1/g);
1522 This method gets faster the more sparse the bit vector is.
1523 (Courtesy of Tim Bunce and Winfried Koenig.)
1525 Or use the CPAN module Bit::Vector:
1527 $vector = Bit::Vector->new($num_of_bits);
1528 $vector->Index_List_Store(@ints);
1529 @ints = $vector->Index_List_Read();
1531 Bit::Vector provides efficient methods for bit vector, sets of small integers
1534 Here's a more extensive illustration using vec():
1537 $vector = "\xff\x0f\xef\xfe";
1538 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1539 unpack("N", $vector), "\n";
1540 $is_set = vec($vector, 23, 1);
1541 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1559 my ($offset, $width, $value) = @_;
1561 vec($vector, $offset, $width) = $value;
1562 print "offset=$offset width=$width value=$value\n";
1568 my $bits = unpack("b*", $vector);
1572 print "vector length in bytes: ", length($vector), "\n";
1573 @bytes = unpack("A8" x length($vector), $bits);
1574 print "bits are: @bytes\n\n";
1577 =head2 Why does defined() return true on empty arrays and hashes?
1579 The short story is that you should probably only use defined on scalars or
1580 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1581 in the 5.004 release or later of Perl for more detail.
1583 =head1 Data: Hashes (Associative Arrays)
1585 =head2 How do I process an entire hash?
1587 Use the each() function (see L<perlfunc/each>) if you don't care
1588 whether it's sorted:
1590 while ( ($key, $value) = each %hash) {
1591 print "$key = $value\n";
1594 If you want it sorted, you'll have to use foreach() on the result of
1595 sorting the keys as shown in an earlier question.
1597 =head2 What happens if I add or remove keys from a hash while iterating over it?
1601 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1602 iterating over it. In Perl 5 you can delete from it, but you still
1603 can't add to it, because that might cause a doubling of the hash table,
1604 in which half the entries get copied up to the new top half of the
1605 table, at which point you've totally bamboozled the iterator code.
1606 Even if the table doesn't double, there's no telling whether your new
1607 entry will be inserted before or after the current iterator position.
1609 Either treasure up your changes and make them after the iterator finishes
1610 or use keys to fetch all the old keys at once, and iterate over the list
1613 =head2 How do I look up a hash element by value?
1615 Create a reverse hash:
1617 %by_value = reverse %by_key;
1618 $key = $by_value{$value};
1620 That's not particularly efficient. It would be more space-efficient
1623 while (($key, $value) = each %by_key) {
1624 $by_value{$value} = $key;
1627 If your hash could have repeated values, the methods above will only find
1628 one of the associated keys. This may or may not worry you. If it does
1629 worry you, you can always reverse the hash into a hash of arrays instead:
1631 while (($key, $value) = each %by_key) {
1632 push @{$key_list_by_value{$value}}, $key;
1635 =head2 How can I know how many entries are in a hash?
1637 If you mean how many keys, then all you have to do is
1638 take the scalar sense of the keys() function:
1640 $num_keys = scalar keys %hash;
1642 The keys() function also resets the iterator, which in void context is
1643 faster for tied hashes than would be iterating through the whole
1644 hash, one key-value pair at a time.
1646 =head2 How do I sort a hash (optionally by value instead of key)?
1648 Internally, hashes are stored in a way that prevents you from imposing
1649 an order on key-value pairs. Instead, you have to sort a list of the
1652 @keys = sort keys %hash; # sorted by key
1654 $hash{$a} cmp $hash{$b}
1655 } keys %hash; # and by value
1657 Here we'll do a reverse numeric sort by value, and if two keys are
1658 identical, sort by length of key, or if that fails, by straight ASCII
1659 comparison of the keys (well, possibly modified by your locale--see
1663 $hash{$b} <=> $hash{$a}
1665 length($b) <=> length($a)
1670 =head2 How can I always keep my hash sorted?
1672 You can look into using the DB_File module and tie() using the
1673 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1674 The Tie::IxHash module from CPAN might also be instructive.
1676 =head2 What's the difference between "delete" and "undef" with hashes?
1678 Hashes are pairs of scalars: the first is the key, the second is the
1679 value. The key will be coerced to a string, although the value can be
1680 any kind of scalar: string, number, or reference. If a key C<$key> is
1681 present in the array, C<exists($key)> will return true. The value for
1682 a given key can be C<undef>, in which case C<$array{$key}> will be
1683 C<undef> while C<$exists{$key}> will return true. This corresponds to
1684 (C<$key>, C<undef>) being in the hash.
1686 Pictures help... here's the C<%ary> table:
1696 And these conditions hold
1700 defined $ary{'d'} is true
1701 defined $ary{'a'} is true
1702 exists $ary{'a'} is true (Perl5 only)
1703 grep ($_ eq 'a', keys %ary) is true
1709 your table now reads:
1720 and these conditions now hold; changes in caps:
1724 defined $ary{'d'} is true
1725 defined $ary{'a'} is FALSE
1726 exists $ary{'a'} is true (Perl5 only)
1727 grep ($_ eq 'a', keys %ary) is true
1729 Notice the last two: you have an undef value, but a defined key!
1735 your table now reads:
1744 and these conditions now hold; changes in caps:
1748 defined $ary{'d'} is true
1749 defined $ary{'a'} is false
1750 exists $ary{'a'} is FALSE (Perl5 only)
1751 grep ($_ eq 'a', keys %ary) is FALSE
1753 See, the whole entry is gone!
1755 =head2 Why don't my tied hashes make the defined/exists distinction?
1757 They may or may not implement the EXISTS() and DEFINED() methods
1758 differently. For example, there isn't the concept of undef with hashes
1759 that are tied to DBM* files. This means the true/false tables above
1760 will give different results when used on such a hash. It also means
1761 that exists and defined do the same thing with a DBM* file, and what
1762 they end up doing is not what they do with ordinary hashes.
1764 =head2 How do I reset an each() operation part-way through?
1766 Using C<keys %hash> in scalar context returns the number of keys in
1767 the hash I<and> resets the iterator associated with the hash. You may
1768 need to do this if you use C<last> to exit a loop early so that when you
1769 re-enter it, the hash iterator has been reset.
1771 =head2 How can I get the unique keys from two hashes?
1773 First you extract the keys from the hashes into lists, then solve
1774 the "removing duplicates" problem described above. For example:
1777 for $element (keys(%foo), keys(%bar)) {
1784 @uniq = keys %{{%foo,%bar}};
1786 Or if you really want to save space:
1789 while (defined ($key = each %foo)) {
1792 while (defined ($key = each %bar)) {
1797 =head2 How can I store a multidimensional array in a DBM file?
1799 Either stringify the structure yourself (no fun), or else
1800 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1801 it on top of either DB_File or GDBM_File.
1803 =head2 How can I make my hash remember the order I put elements into it?
1805 Use the Tie::IxHash from CPAN.
1808 tie(%myhash, Tie::IxHash);
1809 for ($i=0; $i<20; $i++) {
1812 @keys = keys %myhash;
1813 # @keys = (0,1,2,3,...)
1815 =head2 Why does passing a subroutine an undefined element in a hash create it?
1817 If you say something like:
1819 somefunc($hash{"nonesuch key here"});
1821 Then that element "autovivifies"; that is, it springs into existence
1822 whether you store something there or not. That's because functions
1823 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1824 it has to be ready to write it back into the caller's version.
1826 This has been fixed as of Perl5.004.
1828 Normally, merely accessing a key's value for a nonexistent key does
1829 I<not> cause that key to be forever there. This is different than
1832 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1834 Usually a hash ref, perhaps like this:
1839 TITLE => "deputy peon",
1842 PALS => [ "Norbert", "Rhys", "Phineas"],
1845 References are documented in L<perlref> and the upcoming L<perlreftut>.
1846 Examples of complex data structures are given in L<perldsc> and
1847 L<perllol>. Examples of structures and object-oriented classes are
1850 =head2 How can I use a reference as a hash key?
1852 You can't do this directly, but you could use the standard Tie::Refhash
1853 module distributed with Perl.
1857 =head2 How do I handle binary data correctly?
1859 Perl is binary clean, so this shouldn't be a problem. For example,
1860 this works fine (assuming the files are found):
1862 if (`cat /vmunix` =~ /gzip/) {
1863 print "Your kernel is GNU-zip enabled!\n";
1866 On less elegant (read: Byzantine) systems, however, you have
1867 to play tedious games with "text" versus "binary" files. See
1868 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1869 systems are curses out of Microsoft, who seem to be committed to putting
1870 the backward into backward compatibility.
1872 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1874 If you want to deal with multibyte characters, however, there are
1875 some gotchas. See the section on Regular Expressions.
1877 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1879 Assuming that you don't care about IEEE notations like "NaN" or
1880 "Infinity", you probably just want to use a regular expression.
1882 if (/\D/) { print "has nondigits\n" }
1883 if (/^\d+$/) { print "is a whole number\n" }
1884 if (/^-?\d+$/) { print "is an integer\n" }
1885 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1886 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1887 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1888 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1889 { print "a C float" }
1891 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1892 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1893 wrapper function for more convenient access. This function takes
1894 a string and returns the number it found, or C<undef> for input that
1895 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1896 if you just want to say, ``Is this a float?''
1899 use POSIX qw(strtod);
1904 my($num, $unparsed) = strtod($str);
1905 if (($str eq '') || ($unparsed != 0) || $!) {
1912 sub is_numeric { defined getnum($_[0]) }
1914 Or you could check out the String::Scanf module on CPAN instead. The
1915 POSIX module (part of the standard Perl distribution) provides the
1916 C<strtod> and C<strtol> for converting strings to double and longs,
1919 =head2 How do I keep persistent data across program calls?
1921 For some specific applications, you can use one of the DBM modules.
1922 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1923 Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
1924 Storable is part of the standard distribution. Here's one example using
1925 Storable's C<store> and C<retrieve> functions:
1928 store(\%hash, "filename");
1931 $href = retrieve("filename"); # by ref
1932 %hash = %{ retrieve("filename") }; # direct to hash
1934 =head2 How do I print out or copy a recursive data structure?
1936 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1937 for printing out data structures. The Storable module, found on CPAN,
1938 provides a function called C<dclone> that recursively copies its argument.
1940 use Storable qw(dclone);
1943 Where $r1 can be a reference to any kind of data structure you'd like.
1944 It will be deeply copied. Because C<dclone> takes and returns references,
1945 you'd have to add extra punctuation if you had a hash of arrays that
1948 %newhash = %{ dclone(\%oldhash) };
1950 =head2 How do I define methods for every class/object?
1952 Use the UNIVERSAL class (see L<UNIVERSAL>).
1954 =head2 How do I verify a credit card checksum?
1956 Get the Business::CreditCard module from CPAN.
1958 =head2 How do I pack arrays of doubles or floats for XS code?
1960 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1961 If you're doing a lot of float or double processing, consider using
1962 the PDL module from CPAN instead--it makes number-crunching easy.
1964 =head1 AUTHOR AND COPYRIGHT
1966 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1967 All rights reserved.
1969 This documentation is free; you can redistribute it and/or modify it
1970 under the same terms as Perl itself.
1972 Irrespective of its distribution, all code examples in this file
1973 are hereby placed into the public domain. You are permitted and
1974 encouraged to use this code in your own programs for fun
1975 or for profit as you see fit. A simple comment in the code giving
1976 credit would be courteous but is not required.