3 perlfaq4 - Data Manipulation ($Revision: 1.10 $, $Date: 2002/01/01 22:26:45 $)
7 The section of the FAQ answers questions related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximated on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to an internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers. (See L<perlvar/"$#"> 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.cpan.org/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 :-). see the
337 F<random> artitcle in the "Far More Than You Ever Wanted To Know"
338 collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz , courtesy of
339 Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
340 who attempts to generate random numbers by deterministic means is, of
341 course, living in a state of sin.''
343 If you want numbers that are more random than C<rand> with C<srand>
344 provides, you should also check out the Math::TrulyRandom module from
345 CPAN. It uses the imperfections in your system's timer to generate
346 random numbers, but this takes quite a while. If you want a better
347 pseudorandom generator than comes with your operating system, look at
348 ``Numerical Recipes in C'' at http://www.nr.com/ .
352 =head2 How do I find the week-of-the-year/day-of-the-year?
354 The day of the year is in the array returned by localtime() (see
355 L<perlfunc/"localtime">):
357 $day_of_year = (localtime(time()))[7];
359 =head2 How do I find the current century or millennium?
361 Use the following simple functions:
364 return int((((localtime(shift || time))[5] + 1999))/100);
367 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
370 On some systems, you'll find that the POSIX module's strftime() function
371 has been extended in a non-standard way to use a C<%C> format, which they
372 sometimes claim is the "century". It isn't, because on most such systems,
373 this is only the first two digits of the four-digit year, and thus cannot
374 be used to reliably determine the current century or millennium.
376 =head2 How can I compare two dates and find the difference?
378 If you're storing your dates as epoch seconds then simply subtract one
379 from the other. If you've got a structured date (distinct year, day,
380 month, hour, minute, seconds values), then for reasons of accessibility,
381 simplicity, and efficiency, merely use either timelocal or timegm (from
382 the Time::Local module in the standard distribution) to reduce structured
383 dates to epoch seconds. However, if you don't know the precise format of
384 your dates, then you should probably use either of the Date::Manip and
385 Date::Calc modules from CPAN before you go hacking up your own parsing
386 routine to handle arbitrary date formats.
388 =head2 How can I take a string and turn it into epoch seconds?
390 If it's a regular enough string that it always has the same format,
391 you can split it up and pass the parts to C<timelocal> in the standard
392 Time::Local module. Otherwise, you should look into the Date::Calc
393 and Date::Manip modules from CPAN.
395 =head2 How can I find the Julian Day?
397 Use the Time::JulianDay module (part of the Time-modules bundle
398 available from CPAN.)
400 Before you immerse yourself too deeply in this, be sure to verify that
401 it is the I<Julian> Day you really want. Are you interested in a way
402 of getting serial days so that you just can tell how many days they
403 are apart or so that you can do also other date arithmetic? If you
404 are interested in performing date arithmetic, this can be done using
405 modules Date::Manip or Date::Calc.
407 There is too many details and much confusion on this issue to cover in
408 this FAQ, but the term is applied (correctly) to a calendar now
409 supplanted by the Gregorian Calendar, with the Julian Calendar failing
410 to adjust properly for leap years on centennial years (among other
411 annoyances). The term is also used (incorrectly) to mean: [1] days in
412 the Gregorian Calendar; and [2] days since a particular starting time
413 or `epoch', usually 1970 in the Unix world and 1980 in the
414 MS-DOS/Windows world. If you find that it is not the first meaning
415 that you really want, then check out the Date::Manip and Date::Calc
416 modules. (Thanks to David Cassell for most of this text.)
418 =head2 How do I find yesterday's date?
420 The C<time()> function returns the current time in seconds since the
421 epoch. Take twenty-four hours off that:
423 $yesterday = time() - ( 24 * 60 * 60 );
425 Then you can pass this to C<localtime()> and get the individual year,
426 month, day, hour, minute, seconds values.
428 Note very carefully that the code above assumes that your days are
429 twenty-four hours each. For most people, there are two days a year
430 when they aren't: the switch to and from summer time throws this off.
431 A solution to this issue is offered by Russ Allbery.
434 my $now = defined $_[0] ? $_[0] : time;
435 my $then = $now - 60 * 60 * 24;
436 my $ndst = (localtime $now)[8] > 0;
437 my $tdst = (localtime $then)[8] > 0;
438 $then - ($tdst - $ndst) * 60 * 60;
440 # Should give you "this time yesterday" in seconds since epoch relative to
441 # the first argument or the current time if no argument is given and
442 # suitable for passing to localtime or whatever else you need to do with
443 # it. $ndst is whether we're currently in daylight savings time; $tdst is
444 # whether the point 24 hours ago was in daylight savings time. If $tdst
445 # and $ndst are the same, a boundary wasn't crossed, and the correction
446 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
447 # from yesterday's time since we gained an extra hour while going off
448 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
449 # negative hour (add an hour) to yesterday's time since we lost an hour.
451 # All of this is because during those days when one switches off or onto
452 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
454 # The explicit settings of $ndst and $tdst are necessary because localtime
455 # only says it returns the system tm struct, and the system tm struct at
456 # least on Solaris doesn't guarantee any particular positive value (like,
457 # say, 1) for isdst, just a positive value. And that value can
458 # potentially be negative, if DST information isn't available (this sub
459 # just treats those cases like no DST).
461 # Note that between 2am and 3am on the day after the time zone switches
462 # off daylight savings time, the exact hour of "yesterday" corresponding
463 # to the current hour is not clearly defined. Note also that if used
464 # between 2am and 3am the day after the change to daylight savings time,
465 # the result will be between 3am and 4am of the previous day; it's
466 # arguable whether this is correct.
468 # This sub does not attempt to deal with leap seconds (most things don't).
470 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
471 # This code is in the public domain
473 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
475 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
476 Y2K compliant (whatever that means). The programmers you've hired to
477 use it, however, probably are not.
479 Long answer: The question belies a true understanding of the issue.
480 Perl is just as Y2K compliant as your pencil--no more, and no less.
481 Can you use your pencil to write a non-Y2K-compliant memo? Of course
482 you can. Is that the pencil's fault? Of course it isn't.
484 The date and time functions supplied with Perl (gmtime and localtime)
485 supply adequate information to determine the year well beyond 2000
486 (2038 is when trouble strikes for 32-bit machines). The year returned
487 by these functions when used in a list context is the year minus 1900.
488 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
489 number. To avoid the year 2000 problem simply do not treat the year as
490 a 2-digit number. It isn't.
492 When gmtime() and localtime() are used in scalar context they return
493 a timestamp string that contains a fully-expanded year. For example,
494 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
495 2001". There's no year 2000 problem here.
497 That doesn't mean that Perl can't be used to create non-Y2K compliant
498 programs. It can. But so can your pencil. It's the fault of the user,
499 not the language. At the risk of inflaming the NRA: ``Perl doesn't
500 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
505 =head2 How do I validate input?
507 The answer to this question is usually a regular expression, perhaps
508 with auxiliary logic. See the more specific questions (numbers, mail
509 addresses, etc.) for details.
511 =head2 How do I unescape a string?
513 It depends just what you mean by ``escape''. URL escapes are dealt
514 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
515 character are removed with
519 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
521 =head2 How do I remove consecutive pairs of characters?
523 To turn C<"abbcccd"> into C<"abccd">:
525 s/(.)\1/$1/g; # add /s to include newlines
527 Here's a solution that turns "abbcccd" to "abcd":
529 y///cs; # y == tr, but shorter :-)
531 =head2 How do I expand function calls in a string?
533 This is documented in L<perlref>. In general, this is fraught with
534 quoting and readability problems, but it is possible. To interpolate
535 a subroutine call (in list context) into a string:
537 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
539 If you prefer scalar context, similar chicanery is also useful for
540 arbitrary expressions:
542 print "That yields ${\($n + 5)} widgets\n";
544 Version 5.004 of Perl had a bug that gave list context to the
545 expression in C<${...}>, but this is fixed in version 5.005.
547 See also ``How can I expand variables in text strings?'' in this
550 =head2 How do I find matching/nesting anything?
552 This isn't something that can be done in one regular expression, no
553 matter how complicated. To find something between two single
554 characters, a pattern like C</x([^x]*)x/> will get the intervening
555 bits in $1. For multiple ones, then something more like
556 C</alpha(.*?)omega/> would be needed. But none of these deals with
557 nested patterns, nor can they. For that you'll have to write a
560 If you are serious about writing a parser, there are a number of
561 modules or oddities that will make your life a lot easier. There are
562 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
563 and the byacc program. Starting from perl 5.8 the Text::Balanced
564 is part of the standard distribution.
566 One simple destructive, inside-out approach that you might try is to
567 pull out the smallest nesting parts one at a time:
569 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
570 # do something with $1
573 A more complicated and sneaky approach is to make Perl's regular
574 expression engine do it for you. This is courtesy Dean Inada, and
575 rather has the nature of an Obfuscated Perl Contest entry, but it
578 # $_ contains the string to parse
579 # BEGIN and END are the opening and closing markers for the
584 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
585 @$ = (eval{/$re/},$@!~/unmatched/);
586 print join("\n",@$[0..$#$]) if( $$[-1] );
588 =head2 How do I reverse a string?
590 Use reverse() in scalar context, as documented in
593 $reversed = reverse $string;
595 =head2 How do I expand tabs in a string?
597 You can do it yourself:
599 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
601 Or you can just use the Text::Tabs module (part of the standard Perl
605 @expanded_lines = expand(@lines_with_tabs);
607 =head2 How do I reformat a paragraph?
609 Use Text::Wrap (part of the standard Perl distribution):
612 print wrap("\t", ' ', @paragraphs);
614 The paragraphs you give to Text::Wrap should not contain embedded
615 newlines. Text::Wrap doesn't justify the lines (flush-right).
617 Or use the CPAN module Text::Autoformat. Formatting files can be easily
618 done by making a shell alias, like so:
620 alias fmt="perl -i -MText::Autoformat -n0777 \
621 -e 'print autoformat $_, {all=>1}' $*"
623 See the documentation for Text::Autoformat to appreciate its many
626 =head2 How can I access/change the first N letters of a string?
628 There are many ways. If you just want to grab a copy, use
631 $first_byte = substr($a, 0, 1);
633 If you want to modify part of a string, the simplest way is often to
634 use substr() as an lvalue:
636 substr($a, 0, 3) = "Tom";
638 Although those with a pattern matching kind of thought process will
643 =head2 How do I change the Nth occurrence of something?
645 You have to keep track of N yourself. For example, let's say you want
646 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
647 C<"whosoever"> or C<"whomsoever">, case insensitively. These
648 all assume that $_ contains the string to be altered.
652 ++$count == 5 # is it the 5th?
653 ? "${2}soever" # yes, swap
654 : $1 # renege and leave it there
657 In the more general case, you can use the C</g> modifier in a C<while>
658 loop, keeping count of matches.
662 $_ = "One fish two fish red fish blue fish";
663 while (/(\w+)\s+fish\b/gi) {
664 if (++$count == $WANT) {
665 print "The third fish is a $1 one.\n";
669 That prints out: C<"The third fish is a red one."> You can also use a
670 repetition count and repeated pattern like this:
672 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
674 =head2 How can I count the number of occurrences of a substring within a string?
676 There are a number of ways, with varying efficiency. If you want a
677 count of a certain single character (X) within a string, you can use the
678 C<tr///> function like so:
680 $string = "ThisXlineXhasXsomeXx'sXinXit";
681 $count = ($string =~ tr/X//);
682 print "There are $count X characters in the string";
684 This is fine if you are just looking for a single character. However,
685 if you are trying to count multiple character substrings within a
686 larger string, C<tr///> won't work. What you can do is wrap a while()
687 loop around a global pattern match. For example, let's count negative
690 $string = "-9 55 48 -2 23 -76 4 14 -44";
691 while ($string =~ /-\d+/g) { $count++ }
692 print "There are $count negative numbers in the string";
694 =head2 How do I capitalize all the words on one line?
696 To make the first letter of each word upper case:
698 $line =~ s/\b(\w)/\U$1/g;
700 This has the strange effect of turning "C<don't do it>" into "C<Don'T
701 Do It>". Sometimes you might want this. Other times you might need a
702 more thorough solution (Suggested by brian d foy):
705 (^\w) #at the beginning of the line
707 (\s\w) #preceded by whitespace
710 $string =~ /([\w']+)/\u\L$1/g;
712 To make the whole line upper case:
716 To force each word to be lower case, with the first letter upper case:
718 $line =~ s/(\w+)/\u\L$1/g;
720 You can (and probably should) enable locale awareness of those
721 characters by placing a C<use locale> pragma in your program.
722 See L<perllocale> for endless details on locales.
724 This is sometimes referred to as putting something into "title
725 case", but that's not quite accurate. Consider the proper
726 capitalization of the movie I<Dr. Strangelove or: How I Learned to
727 Stop Worrying and Love the Bomb>, for example.
729 =head2 How can I split a [character] delimited string except when inside
730 [character]? (Comma-separated files)
732 Take the example case of trying to split a string that is comma-separated
733 into its different fields. (We'll pretend you said comma-separated, not
734 comma-delimited, which is different and almost never what you mean.) You
735 can't use C<split(/,/)> because you shouldn't split if the comma is inside
736 quotes. For example, take a data line like this:
738 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
740 Due to the restriction of the quotes, this is a fairly complex
741 problem. Thankfully, we have Jeffrey Friedl, author of a highly
742 recommended book on regular expressions, to handle these for us. He
743 suggests (assuming your string is contained in $text):
746 push(@new, $+) while $text =~ m{
747 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
751 push(@new, undef) if substr($text,-1,1) eq ',';
753 If you want to represent quotation marks inside a
754 quotation-mark-delimited field, escape them with backslashes (eg,
755 C<"like \"this\"">. Unescaping them is a task addressed earlier in
758 Alternatively, the Text::ParseWords module (part of the standard Perl
759 distribution) lets you say:
761 use Text::ParseWords;
762 @new = quotewords(",", 0, $text);
764 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
766 =head2 How do I strip blank space from the beginning/end of a string?
768 Although the simplest approach would seem to be
770 $string =~ s/^\s*(.*?)\s*$/$1/;
772 not only is this unnecessarily slow and destructive, it also fails with
773 embedded newlines. It is much faster to do this operation in two steps:
778 Or more nicely written as:
785 This idiom takes advantage of the C<foreach> loop's aliasing
786 behavior to factor out common code. You can do this
787 on several strings at once, or arrays, or even the
788 values of a hash if you use a slice:
790 # trim whitespace in the scalar, the array,
791 # and all the values in the hash
792 foreach ($scalar, @array, @hash{keys %hash}) {
797 =head2 How do I pad a string with blanks or pad a number with zeroes?
799 (This answer contributed by Uri Guttman, with kibitzing from
802 In the following examples, C<$pad_len> is the length to which you wish
803 to pad the string, C<$text> or C<$num> contains the string to be padded,
804 and C<$pad_char> contains the padding character. You can use a single
805 character string constant instead of the C<$pad_char> variable if you
806 know what it is in advance. And in the same way you can use an integer in
807 place of C<$pad_len> if you know the pad length in advance.
809 The simplest method uses the C<sprintf> function. It can pad on the left
810 or right with blanks and on the left with zeroes and it will not
811 truncate the result. The C<pack> function can only pad strings on the
812 right with blanks and it will truncate the result to a maximum length of
815 # Left padding a string with blanks (no truncation):
816 $padded = sprintf("%${pad_len}s", $text);
818 # Right padding a string with blanks (no truncation):
819 $padded = sprintf("%-${pad_len}s", $text);
821 # Left padding a number with 0 (no truncation):
822 $padded = sprintf("%0${pad_len}d", $num);
824 # Right padding a string with blanks using pack (will truncate):
825 $padded = pack("A$pad_len",$text);
827 If you need to pad with a character other than blank or zero you can use
828 one of the following methods. They all generate a pad string with the
829 C<x> operator and combine that with C<$text>. These methods do
830 not truncate C<$text>.
832 Left and right padding with any character, creating a new string:
834 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
835 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
837 Left and right padding with any character, modifying C<$text> directly:
839 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
840 $text .= $pad_char x ( $pad_len - length( $text ) );
842 =head2 How do I extract selected columns from a string?
844 Use substr() or unpack(), both documented in L<perlfunc>.
845 If you prefer thinking in terms of columns instead of widths,
846 you can use this kind of thing:
848 # determine the unpack format needed to split Linux ps output
849 # arguments are cut columns
850 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
856 for my $place (@positions) {
857 $template .= "A" . ($place - $lastpos) . " ";
864 =head2 How do I find the soundex value of a string?
866 Use the standard Text::Soundex module distributed with Perl.
867 Before you do so, you may want to determine whether `soundex' is in
868 fact what you think it is. Knuth's soundex algorithm compresses words
869 into a small space, and so it does not necessarily distinguish between
870 two words which you might want to appear separately. For example, the
871 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
872 If Text::Soundex does not do what you are looking for, you might want
873 to consider the String::Approx module available at CPAN.
875 =head2 How can I expand variables in text strings?
877 Let's assume that you have a string like:
879 $text = 'this has a $foo in it and a $bar';
881 If those were both global variables, then this would
884 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
886 But since they are probably lexicals, or at least, they could
887 be, you'd have to do this:
889 $text =~ s/(\$\w+)/$1/eeg;
890 die if $@; # needed /ee, not /e
892 It's probably better in the general case to treat those
893 variables as entries in some special hash. For example:
899 $text =~ s/\$(\w+)/$user_defs{$1}/g;
901 See also ``How do I expand function calls in a string?'' in this section
904 =head2 What's wrong with always quoting "$vars"?
906 The problem is that those double-quotes force stringification--
907 coercing numbers and references into strings--even when you
908 don't want them to be strings. Think of it this way: double-quote
909 expansion is used to produce new strings. If you already
910 have a string, why do you need more?
912 If you get used to writing odd things like these:
916 somefunc("$var"); # BAD
918 You'll be in trouble. Those should (in 99.8% of the cases) be
919 the simpler and more direct:
925 Otherwise, besides slowing you down, you're going to break code when
926 the thing in the scalar is actually neither a string nor a number, but
932 my $oref = "$aref"; # WRONG
935 You can also get into subtle problems on those few operations in Perl
936 that actually do care about the difference between a string and a
937 number, such as the magical C<++> autoincrement operator or the
940 Stringification also destroys arrays.
943 print "@lines"; # WRONG - extra blanks
944 print @lines; # right
946 =head2 Why don't my <<HERE documents work?
948 Check for these three things:
952 =item 1. There must be no space after the << part.
954 =item 2. There (probably) should be a semicolon at the end.
956 =item 3. You can't (easily) have any space in front of the tag.
960 If you want to indent the text in the here document, you
964 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
969 But the HERE_TARGET must still be flush against the margin.
970 If you want that indented also, you'll have to quote
973 ($quote = <<' FINIS') =~ s/^\s+//gm;
974 ...we will have peace, when you and all your works have
975 perished--and the works of your dark master to whom you
976 would deliver us. You are a liar, Saruman, and a corrupter
977 of men's hearts. --Theoden in /usr/src/perl/taint.c
979 $quote =~ s/\s*--/\n--/;
981 A nice general-purpose fixer-upper function for indented here documents
982 follows. It expects to be called with a here document as its argument.
983 It looks to see whether each line begins with a common substring, and
984 if so, strips that substring off. Otherwise, it takes the amount of leading
985 whitespace found on the first line and removes that much off each
990 my ($white, $leader); # common whitespace and common leading string
991 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
992 ($white, $leader) = ($2, quotemeta($1));
994 ($white, $leader) = (/^(\s+)/, '');
996 s/^\s*?$leader(?:$white)?//gm;
1000 This works with leading special strings, dynamically determined:
1002 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1005 @@@ SAVEI32(runlevel);
1007 @@@ while ( op = (*op->op_ppaddr)() );
1011 MAIN_INTERPRETER_LOOP
1013 Or with a fixed amount of leading whitespace, with remaining
1014 indentation correctly preserved:
1016 $poem = fix<<EVER_ON_AND_ON;
1017 Now far ahead the Road has gone,
1018 And I must follow, if I can,
1019 Pursuing it with eager feet,
1020 Until it joins some larger way
1021 Where many paths and errands meet.
1022 And whither then? I cannot say.
1023 --Bilbo in /usr/src/perl/pp_ctl.c
1028 =head2 What is the difference between a list and an array?
1030 An array has a changeable length. A list does not. An array is something
1031 you can push or pop, while a list is a set of values. Some people make
1032 the distinction that a list is a value while an array is a variable.
1033 Subroutines are passed and return lists, you put things into list
1034 context, you initialize arrays with lists, and you foreach() across
1035 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1036 in scalar context behave like the number of elements in them, subroutines
1037 access their arguments through the array C<@_>, and push/pop/shift only work
1040 As a side note, there's no such thing as a list in scalar context.
1043 $scalar = (2, 5, 7, 9);
1045 you're using the comma operator in scalar context, so it uses the scalar
1046 comma operator. There never was a list there at all! This causes the
1047 last value to be returned: 9.
1049 =head2 What is the difference between $array[1] and @array[1]?
1051 The former is a scalar value; the latter an array slice, making
1052 it a list with one (scalar) value. You should use $ when you want a
1053 scalar value (most of the time) and @ when you want a list with one
1054 scalar value in it (very, very rarely; nearly never, in fact).
1056 Sometimes it doesn't make a difference, but sometimes it does.
1057 For example, compare:
1059 $good[0] = `some program that outputs several lines`;
1063 @bad[0] = `same program that outputs several lines`;
1065 The C<use warnings> pragma and the B<-w> flag will warn you about these
1068 =head2 How can I remove duplicate elements from a list or array?
1070 There are several possible ways, depending on whether the array is
1071 ordered and whether you wish to preserve the ordering.
1077 If @in is sorted, and you want @out to be sorted:
1078 (this assumes all true values in the array)
1080 $prev = "not equal to $in[0]";
1081 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1083 This is nice in that it doesn't use much extra memory, simulating
1084 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1085 guarantees that the expression is true (so that grep picks it up)
1086 even if the $_ is 0, "", or undef.
1090 If you don't know whether @in is sorted:
1093 @out = grep(!$saw{$_}++, @in);
1097 Like (b), but @in contains only small integers:
1099 @out = grep(!$saw[$_]++, @in);
1103 A way to do (b) without any loops or greps:
1107 @out = sort keys %saw; # remove sort if undesired
1111 Like (d), but @in contains only small positive integers:
1115 @out = grep {defined} @ary;
1119 But perhaps you should have been using a hash all along, eh?
1121 =head2 How can I tell whether a certain element is contained in a list or array?
1123 Hearing the word "in" is an I<in>dication that you probably should have
1124 used a hash, not a list or array, to store your data. Hashes are
1125 designed to answer this question quickly and efficiently. Arrays aren't.
1127 That being said, there are several ways to approach this. If you
1128 are going to make this query many times over arbitrary string values,
1129 the fastest way is probably to invert the original array and keep an
1130 associative array lying about whose keys are the first array's values.
1132 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1134 for (@blues) { $is_blue{$_} = 1 }
1136 Now you can check whether $is_blue{$some_color}. It might have been a
1137 good idea to keep the blues all in a hash in the first place.
1139 If the values are all small integers, you could use a simple indexed
1140 array. This kind of an array will take up less space:
1142 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1143 undef @is_tiny_prime;
1144 for (@primes) { $is_tiny_prime[$_] = 1 }
1145 # or simply @istiny_prime[@primes] = (1) x @primes;
1147 Now you check whether $is_tiny_prime[$some_number].
1149 If the values in question are integers instead of strings, you can save
1150 quite a lot of space by using bit strings instead:
1152 @articles = ( 1..10, 150..2000, 2017 );
1154 for (@articles) { vec($read,$_,1) = 1 }
1156 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1160 ($is_there) = grep $_ eq $whatever, @array;
1164 ($is_there) = grep /$whatever/, @array;
1166 These are slow (checks every element even if the first matches),
1167 inefficient (same reason), and potentially buggy (what if there are
1168 regex characters in $whatever?). If you're only testing once, then
1172 foreach $elt (@array) {
1173 if ($elt eq $elt_to_find) {
1178 if ($is_there) { ... }
1180 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1182 Use a hash. Here's code to do both and more. It assumes that
1183 each element is unique in a given array:
1185 @union = @intersection = @difference = ();
1187 foreach $element (@array1, @array2) { $count{$element}++ }
1188 foreach $element (keys %count) {
1189 push @union, $element;
1190 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1193 Note that this is the I<symmetric difference>, that is, all elements in
1194 either A or in B but not in both. Think of it as an xor operation.
1196 =head2 How do I test whether two arrays or hashes are equal?
1198 The following code works for single-level arrays. It uses a stringwise
1199 comparison, and does not distinguish defined versus undefined empty
1200 strings. Modify if you have other needs.
1202 $are_equal = compare_arrays(\@frogs, \@toads);
1204 sub compare_arrays {
1205 my ($first, $second) = @_;
1206 no warnings; # silence spurious -w undef complaints
1207 return 0 unless @$first == @$second;
1208 for (my $i = 0; $i < @$first; $i++) {
1209 return 0 if $first->[$i] ne $second->[$i];
1214 For multilevel structures, you may wish to use an approach more
1215 like this one. It uses the CPAN module FreezeThaw:
1217 use FreezeThaw qw(cmpStr);
1218 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1220 printf "a and b contain %s arrays\n",
1221 cmpStr(\@a, \@b) == 0
1225 This approach also works for comparing hashes. Here
1226 we'll demonstrate two different answers:
1228 use FreezeThaw qw(cmpStr cmpStrHard);
1230 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1234 printf "a and b contain %s hashes\n",
1235 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1237 printf "a and b contain %s hashes\n",
1238 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1241 The first reports that both those the hashes contain the same data,
1242 while the second reports that they do not. Which you prefer is left as
1243 an exercise to the reader.
1245 =head2 How do I find the first array element for which a condition is true?
1247 You can use this if you care about the index:
1249 for ($i= 0; $i < @array; $i++) {
1250 if ($array[$i] eq "Waldo") {
1256 Now C<$found_index> has what you want.
1258 =head2 How do I handle linked lists?
1260 In general, you usually don't need a linked list in Perl, since with
1261 regular arrays, you can push and pop or shift and unshift at either end,
1262 or you can use splice to add and/or remove arbitrary number of elements at
1263 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1264 dynamic arrays. In the absence of shifts and pops, push in general
1265 needs to reallocate on the order every log(N) times, and unshift will
1266 need to copy pointers each time.
1268 If you really, really wanted, you could use structures as described in
1269 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1270 to do. For example, imagine a list node like this:
1277 You could walk the list this way:
1280 for ($node = $head; $node; $node = $node->{LINK}) {
1281 print $node->{VALUE}, " ";
1285 You could add to the list this way:
1288 $tail = append($head, 1); # grow a new head
1289 for $value ( 2 .. 10 ) {
1290 $tail = append($tail, $value);
1294 my($list, $value) = @_;
1295 my $node = { VALUE => $value };
1297 $node->{LINK} = $list->{LINK};
1298 $list->{LINK} = $node;
1300 $_[0] = $node; # replace caller's version
1305 But again, Perl's built-in are virtually always good enough.
1307 =head2 How do I handle circular lists?
1309 Circular lists could be handled in the traditional fashion with linked
1310 lists, or you could just do something like this with an array:
1312 unshift(@array, pop(@array)); # the last shall be first
1313 push(@array, shift(@array)); # and vice versa
1315 =head2 How do I shuffle an array randomly?
1317 If you either have Perl 5.8.0 or later installed, or if you have
1318 Scalar-List-Utils 1.03 or later installed, you can say:
1320 use List::Util 'shuffle';
1322 @shuffled = shuffle(@list);
1324 If not, you can use this:
1326 # fisher_yates_shuffle
1327 # generate a random permutation of an array in place
1328 # As in shuffling a deck of cards
1330 sub fisher_yates_shuffle {
1331 my $deck = shift; # $deck is a reference to an array
1334 my $j = int rand ($i+1);
1335 @$deck[$i,$j] = @$deck[$j,$i];
1339 And here is an example of using it:
1342 # shuffle my mpeg collection
1344 my @mpeg = <audio/*/*.mp3>;
1345 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1348 Note that the above implementation shuffles an array in place,
1349 unlike the List::Util::shuffle() which takes a list and returns
1350 a new shuffled list.
1352 You've probably seen shuffling algorithms that work using splice,
1353 randomly picking another element to swap the current element with
1357 @old = 1 .. 10; # just a demo
1359 push(@new, splice(@old, rand @old, 1));
1362 This is bad because splice is already O(N), and since you do it N times,
1363 you just invented a quadratic algorithm; that is, O(N**2). This does
1364 not scale, although Perl is so efficient that you probably won't notice
1365 this until you have rather largish arrays.
1367 =head2 How do I process/modify each element of an array?
1369 Use C<for>/C<foreach>:
1372 s/foo/bar/; # change that word
1373 y/XZ/ZX/; # swap those letters
1376 Here's another; let's compute spherical volumes:
1378 for (@volumes = @radii) { # @volumes has changed parts
1380 $_ *= (4/3) * 3.14159; # this will be constant folded
1383 If you want to do the same thing to modify the values of the hash,
1384 you may not use the C<values> function, oddly enough. You need a slice:
1386 for $orbit ( @orbits{keys %orbits} ) {
1387 ($orbit **= 3) *= (4/3) * 3.14159;
1390 =head2 How do I select a random element from an array?
1392 Use the rand() function (see L<perlfunc/rand>):
1394 # at the top of the program:
1395 srand; # not needed for 5.004 and later
1398 $index = rand @array;
1399 $element = $array[$index];
1401 Make sure you I<only call srand once per program, if then>.
1402 If you are calling it more than once (such as before each
1403 call to rand), you're almost certainly doing something wrong.
1405 =head2 How do I permute N elements of a list?
1407 Here's a little program that generates all permutations
1408 of all the words on each line of input. The algorithm embodied
1409 in the permute() function should work on any list:
1412 # tsc-permute: permute each word of input
1413 permute([split], []);
1415 my @items = @{ $_[0] };
1416 my @perms = @{ $_[1] };
1420 my(@newitems,@newperms,$i);
1421 foreach $i (0 .. $#items) {
1424 unshift(@newperms, splice(@newitems, $i, 1));
1425 permute([@newitems], [@newperms]);
1430 Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
1431 module from CPAN runs at least an order of magnitude faster. If you don't
1432 have a C compiler (or a binary distribution of Algorithm::Permute), then
1433 you can use List::Permutor which is written in pure Perl, and is still
1434 several times faster than the algorithm above.
1436 =head2 How do I sort an array by (anything)?
1438 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1440 @list = sort { $a <=> $b } @list;
1442 The default sort function is cmp, string comparison, which would
1443 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1444 the numerical comparison operator.
1446 If you have a complicated function needed to pull out the part you
1447 want to sort on, then don't do it inside the sort function. Pull it
1448 out first, because the sort BLOCK can be called many times for the
1449 same element. Here's an example of how to pull out the first word
1450 after the first number on each item, and then sort those words
1455 ($item) = /\d+\s*(\S+)/;
1456 push @idx, uc($item);
1458 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1460 which could also be written this way, using a trick
1461 that's come to be known as the Schwartzian Transform:
1463 @sorted = map { $_->[0] }
1464 sort { $a->[1] cmp $b->[1] }
1465 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1467 If you need to sort on several fields, the following paradigm is useful.
1469 @sorted = sort { field1($a) <=> field1($b) ||
1470 field2($a) cmp field2($b) ||
1471 field3($a) cmp field3($b)
1474 This can be conveniently combined with precalculation of keys as given
1477 See the F<sort> artitcle article in the "Far More Than You Ever Wanted
1478 To Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz for
1479 more about this approach.
1481 See also the question below on sorting hashes.
1483 =head2 How do I manipulate arrays of bits?
1485 Use pack() and unpack(), or else vec() and the bitwise operations.
1487 For example, this sets $vec to have bit N set if $ints[N] was set:
1490 foreach(@ints) { vec($vec,$_,1) = 1 }
1492 Here's how, given a vector in $vec, you can
1493 get those bits into your @ints array:
1495 sub bitvec_to_list {
1498 # Find null-byte density then select best algorithm
1499 if ($vec =~ tr/\0// / length $vec > 0.95) {
1502 # This method is faster with mostly null-bytes
1503 while($vec =~ /[^\0]/g ) {
1504 $i = -9 + 8 * pos $vec;
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);
1511 push @ints, $i if vec($vec, ++$i, 1);
1512 push @ints, $i if vec($vec, ++$i, 1);
1515 # This method is a fast general algorithm
1517 my $bits = unpack "b*", $vec;
1518 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1519 push @ints, pos $bits while($bits =~ /1/g);
1524 This method gets faster the more sparse the bit vector is.
1525 (Courtesy of Tim Bunce and Winfried Koenig.)
1527 Or use the CPAN module Bit::Vector:
1529 $vector = Bit::Vector->new($num_of_bits);
1530 $vector->Index_List_Store(@ints);
1531 @ints = $vector->Index_List_Read();
1533 Bit::Vector provides efficient methods for bit vector, sets of small integers
1536 Here's a more extensive illustration using vec():
1539 $vector = "\xff\x0f\xef\xfe";
1540 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1541 unpack("N", $vector), "\n";
1542 $is_set = vec($vector, 23, 1);
1543 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1561 my ($offset, $width, $value) = @_;
1563 vec($vector, $offset, $width) = $value;
1564 print "offset=$offset width=$width value=$value\n";
1570 my $bits = unpack("b*", $vector);
1574 print "vector length in bytes: ", length($vector), "\n";
1575 @bytes = unpack("A8" x length($vector), $bits);
1576 print "bits are: @bytes\n\n";
1579 =head2 Why does defined() return true on empty arrays and hashes?
1581 The short story is that you should probably only use defined on scalars or
1582 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1583 in the 5.004 release or later of Perl for more detail.
1585 =head1 Data: Hashes (Associative Arrays)
1587 =head2 How do I process an entire hash?
1589 Use the each() function (see L<perlfunc/each>) if you don't care
1590 whether it's sorted:
1592 while ( ($key, $value) = each %hash) {
1593 print "$key = $value\n";
1596 If you want it sorted, you'll have to use foreach() on the result of
1597 sorting the keys as shown in an earlier question.
1599 =head2 What happens if I add or remove keys from a hash while iterating over it?
1603 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1604 iterating over it. In Perl 5 you can delete from it, but you still
1605 can't add to it, because that might cause a doubling of the hash table,
1606 in which half the entries get copied up to the new top half of the
1607 table, at which point you've totally bamboozled the iterator code.
1608 Even if the table doesn't double, there's no telling whether your new
1609 entry will be inserted before or after the current iterator position.
1611 Either treasure up your changes and make them after the iterator finishes
1612 or use keys to fetch all the old keys at once, and iterate over the list
1615 =head2 How do I look up a hash element by value?
1617 Create a reverse hash:
1619 %by_value = reverse %by_key;
1620 $key = $by_value{$value};
1622 That's not particularly efficient. It would be more space-efficient
1625 while (($key, $value) = each %by_key) {
1626 $by_value{$value} = $key;
1629 If your hash could have repeated values, the methods above will only find
1630 one of the associated keys. This may or may not worry you. If it does
1631 worry you, you can always reverse the hash into a hash of arrays instead:
1633 while (($key, $value) = each %by_key) {
1634 push @{$key_list_by_value{$value}}, $key;
1637 =head2 How can I know how many entries are in a hash?
1639 If you mean how many keys, then all you have to do is
1640 use the keys() function in a scalar context:
1642 $num_keys = keys %hash;
1644 The keys() function also resets the iterator, which means that you may
1645 see strange results if you use this between uses of other hash operators
1648 =head2 How do I sort a hash (optionally by value instead of key)?
1650 Internally, hashes are stored in a way that prevents you from imposing
1651 an order on key-value pairs. Instead, you have to sort a list of the
1654 @keys = sort keys %hash; # sorted by key
1656 $hash{$a} cmp $hash{$b}
1657 } keys %hash; # and by value
1659 Here we'll do a reverse numeric sort by value, and if two keys are
1660 identical, sort by length of key, or if that fails, by straight ASCII
1661 comparison of the keys (well, possibly modified by your locale--see
1665 $hash{$b} <=> $hash{$a}
1667 length($b) <=> length($a)
1672 =head2 How can I always keep my hash sorted?
1674 You can look into using the DB_File module and tie() using the
1675 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1676 The Tie::IxHash module from CPAN might also be instructive.
1678 =head2 What's the difference between "delete" and "undef" with hashes?
1680 Hashes are pairs of scalars: the first is the key, the second is the
1681 value. The key will be coerced to a string, although the value can be
1682 any kind of scalar: string, number, or reference. If a key C<$key> is
1683 present in the array, C<exists($key)> will return true. The value for
1684 a given key can be C<undef>, in which case C<$array{$key}> will be
1685 C<undef> while C<$exists{$key}> will return true. This corresponds to
1686 (C<$key>, C<undef>) being in the hash.
1688 Pictures help... here's the C<%ary> table:
1698 And these conditions hold
1702 defined $ary{'d'} is true
1703 defined $ary{'a'} is true
1704 exists $ary{'a'} is true (Perl5 only)
1705 grep ($_ eq 'a', keys %ary) is true
1711 your table now reads:
1722 and these conditions now hold; changes in caps:
1726 defined $ary{'d'} is true
1727 defined $ary{'a'} is FALSE
1728 exists $ary{'a'} is true (Perl5 only)
1729 grep ($_ eq 'a', keys %ary) is true
1731 Notice the last two: you have an undef value, but a defined key!
1737 your table now reads:
1746 and these conditions now hold; changes in caps:
1750 defined $ary{'d'} is true
1751 defined $ary{'a'} is false
1752 exists $ary{'a'} is FALSE (Perl5 only)
1753 grep ($_ eq 'a', keys %ary) is FALSE
1755 See, the whole entry is gone!
1757 =head2 Why don't my tied hashes make the defined/exists distinction?
1759 They may or may not implement the EXISTS() and DEFINED() methods
1760 differently. For example, there isn't the concept of undef with hashes
1761 that are tied to DBM* files. This means the true/false tables above
1762 will give different results when used on such a hash. It also means
1763 that exists and defined do the same thing with a DBM* file, and what
1764 they end up doing is not what they do with ordinary hashes.
1766 =head2 How do I reset an each() operation part-way through?
1768 Using C<keys %hash> in scalar context returns the number of keys in
1769 the hash I<and> resets the iterator associated with the hash. You may
1770 need to do this if you use C<last> to exit a loop early so that when you
1771 re-enter it, the hash iterator has been reset.
1773 =head2 How can I get the unique keys from two hashes?
1775 First you extract the keys from the hashes into lists, then solve
1776 the "removing duplicates" problem described above. For example:
1779 for $element (keys(%foo), keys(%bar)) {
1786 @uniq = keys %{{%foo,%bar}};
1788 Or if you really want to save space:
1791 while (defined ($key = each %foo)) {
1794 while (defined ($key = each %bar)) {
1799 =head2 How can I store a multidimensional array in a DBM file?
1801 Either stringify the structure yourself (no fun), or else
1802 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1803 it on top of either DB_File or GDBM_File.
1805 =head2 How can I make my hash remember the order I put elements into it?
1807 Use the Tie::IxHash from CPAN.
1810 tie(%myhash, Tie::IxHash);
1811 for ($i=0; $i<20; $i++) {
1814 @keys = keys %myhash;
1815 # @keys = (0,1,2,3,...)
1817 =head2 Why does passing a subroutine an undefined element in a hash create it?
1819 If you say something like:
1821 somefunc($hash{"nonesuch key here"});
1823 Then that element "autovivifies"; that is, it springs into existence
1824 whether you store something there or not. That's because functions
1825 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1826 it has to be ready to write it back into the caller's version.
1828 This has been fixed as of Perl5.004.
1830 Normally, merely accessing a key's value for a nonexistent key does
1831 I<not> cause that key to be forever there. This is different than
1834 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1836 Usually a hash ref, perhaps like this:
1841 TITLE => "deputy peon",
1844 PALS => [ "Norbert", "Rhys", "Phineas"],
1847 References are documented in L<perlref> and the upcoming L<perlreftut>.
1848 Examples of complex data structures are given in L<perldsc> and
1849 L<perllol>. Examples of structures and object-oriented classes are
1852 =head2 How can I use a reference as a hash key?
1854 You can't do this directly, but you could use the standard Tie::RefHash
1855 module distributed with Perl.
1859 =head2 How do I handle binary data correctly?
1861 Perl is binary clean, so this shouldn't be a problem. For example,
1862 this works fine (assuming the files are found):
1864 if (`cat /vmunix` =~ /gzip/) {
1865 print "Your kernel is GNU-zip enabled!\n";
1868 On less elegant (read: Byzantine) systems, however, you have
1869 to play tedious games with "text" versus "binary" files. See
1870 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1871 systems are curses out of Microsoft, who seem to be committed to putting
1872 the backward into backward compatibility.
1874 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1876 If you want to deal with multibyte characters, however, there are
1877 some gotchas. See the section on Regular Expressions.
1879 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1881 Assuming that you don't care about IEEE notations like "NaN" or
1882 "Infinity", you probably just want to use a regular expression.
1884 if (/\D/) { print "has nondigits\n" }
1885 if (/^\d+$/) { print "is a whole number\n" }
1886 if (/^-?\d+$/) { print "is an integer\n" }
1887 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1888 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1889 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1890 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1891 { print "a C float" }
1893 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1894 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1895 wrapper function for more convenient access. This function takes
1896 a string and returns the number it found, or C<undef> for input that
1897 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1898 if you just want to say, ``Is this a float?''
1901 use POSIX qw(strtod);
1906 my($num, $unparsed) = strtod($str);
1907 if (($str eq '') || ($unparsed != 0) || $!) {
1914 sub is_numeric { defined getnum($_[0]) }
1916 Or you could check out the String::Scanf module on CPAN instead. The
1917 POSIX module (part of the standard Perl distribution) provides the
1918 C<strtod> and C<strtol> for converting strings to double and longs,
1921 =head2 How do I keep persistent data across program calls?
1923 For some specific applications, you can use one of the DBM modules.
1924 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
1925 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
1926 of the standard distribution. Here's one example using Storable's C<store>
1927 and C<retrieve> functions:
1930 store(\%hash, "filename");
1933 $href = retrieve("filename"); # by ref
1934 %hash = %{ retrieve("filename") }; # direct to hash
1936 =head2 How do I print out or copy a recursive data structure?
1938 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1939 for printing out data structures. The Storable module, found on CPAN,
1940 provides a function called C<dclone> that recursively copies its argument.
1942 use Storable qw(dclone);
1945 Where $r1 can be a reference to any kind of data structure you'd like.
1946 It will be deeply copied. Because C<dclone> takes and returns references,
1947 you'd have to add extra punctuation if you had a hash of arrays that
1950 %newhash = %{ dclone(\%oldhash) };
1952 =head2 How do I define methods for every class/object?
1954 Use the UNIVERSAL class (see L<UNIVERSAL>).
1956 =head2 How do I verify a credit card checksum?
1958 Get the Business::CreditCard module from CPAN.
1960 =head2 How do I pack arrays of doubles or floats for XS code?
1962 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1963 If you're doing a lot of float or double processing, consider using
1964 the PDL module from CPAN instead--it makes number-crunching easy.
1966 =head1 AUTHOR AND COPYRIGHT
1968 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1969 All rights reserved.
1971 This documentation is free; you can redistribute it and/or modify it
1972 under the same terms as Perl itself.
1974 Irrespective of its distribution, all code examples in this file
1975 are hereby placed into the public domain. You are permitted and
1976 encouraged to use this code in your own programs for fun
1977 or for profit as you see fit. A simple comment in the code giving
1978 credit would be courteous but is not required.