3 perlfaq4 - Data Manipulation ($Revision: 1.14 $, $Date: 2002/02/08 22:30:23 $)
7 This section of the FAQ answers questions related to manipulating
8 numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
12 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
14 The infinite set that a mathematician thinks of as the real numbers can
15 only be approximated on a computer, since the computer only has a finite
16 number of bits to store an infinite number of, um, numbers.
18 Internally, your computer represents floating-point numbers in binary.
19 Floating-point numbers read in from a file or appearing as literals
20 in your program are converted from their decimal floating-point
21 representation (eg, 19.95) to an internal binary representation.
23 However, 19.95 can't be precisely represented as a binary
24 floating-point number, just like 1/3 can't be exactly represented as a
25 decimal floating-point number. The computer's binary representation
26 of 19.95, therefore, isn't exactly 19.95.
28 When a floating-point number gets printed, the binary floating-point
29 representation is converted back to decimal. These decimal numbers
30 are displayed in either the format you specify with printf(), or the
31 current output format for numbers. (See L<perlvar/"$#"> if you use
32 print. C<$#> has a different default value in Perl5 than it did in
33 Perl4. Changing C<$#> yourself is deprecated.)
35 This affects B<all> computer languages that represent decimal
36 floating-point numbers in binary, not just Perl. Perl provides
37 arbitrary-precision decimal numbers with the Math::BigFloat module
38 (part of the standard Perl distribution), but mathematical operations
39 are consequently slower.
41 If precision is important, such as when dealing with money, it's good
42 to work with integers and then divide at the last possible moment.
43 For example, work in pennies (1995) instead of dollars and cents
44 (19.95) and divide by 100 at the end.
46 To get rid of the superfluous digits, just use a format (eg,
47 C<printf("%.2f", 19.95)>) to get the required precision.
48 See L<perlop/"Floating-point Arithmetic">.
50 =head2 Why isn't my octal data interpreted correctly?
52 Perl only understands octal and hex numbers as such when they occur
53 as literals in your program. Octal literals in perl must start with
54 a leading "0" and hexadecimal literals must start with a leading "0x".
55 If they are read in from somewhere and assigned, no automatic
56 conversion takes place. You must explicitly use oct() or hex() if you
57 want the values converted to decimal. oct() interprets
58 both hex ("0x350") numbers and octal ones ("0350" or even without the
59 leading "0", like "377"), while hex() only converts hexadecimal ones,
60 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
61 The inverse mapping from decimal to octal can be done with either the
62 "%o" or "%O" sprintf() formats. To get from decimal to hex try either
63 the "%x" or the "%X" formats to sprintf().
65 This problem shows up most often when people try using chmod(), mkdir(),
66 umask(), or sysopen(), which by widespread tradition typically take
69 chmod(644, $file); # WRONG
70 chmod(0644, $file); # right
72 Note the mistake in the first line was specifying the decimal literal
73 644, rather than the intended octal literal 0644. The problem can
76 printf("%#o",644); # prints 01204
78 Surely you had not intended C<chmod(01204, $file);> - did you? If you
79 want to use numeric literals as arguments to chmod() et al. then please
80 try to express them as octal constants, that is with a leading zero and
81 with the following digits restricted to the set 0..7.
83 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
85 Remember that int() merely truncates toward 0. For rounding to a
86 certain number of digits, sprintf() or printf() is usually the easiest
89 printf("%.3f", 3.1415926535); # prints 3.142
91 The POSIX module (part of the standard Perl distribution) implements
92 ceil(), floor(), and a number of other mathematical and trigonometric
96 $ceil = ceil(3.5); # 4
97 $floor = floor(3.5); # 3
99 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
100 module. With 5.004, the Math::Trig module (part of the standard Perl
101 distribution) implements the trigonometric functions. Internally it
102 uses the Math::Complex module and some functions can break out from
103 the real axis into the complex plane, for example the inverse sine of
106 Rounding in financial applications can have serious implications, and
107 the rounding method used should be specified precisely. In these
108 cases, it probably pays not to trust whichever system rounding is
109 being used by Perl, but to instead implement the rounding function you
112 To see why, notice how you'll still have an issue on half-way-point
115 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
117 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
118 0.8 0.8 0.9 0.9 1.0 1.0
120 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
121 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
122 machines) will work pretty much like mathematical integers. Other numbers
125 =head2 How do I convert between numeric representations?
127 As always with Perl there is more than one way to do it. Below
128 are a few examples of approaches to making common conversions
129 between number representations. This is intended to be representational
130 rather than exhaustive.
132 Some of the examples below use the Bit::Vector module from CPAN.
133 The reason you might choose Bit::Vector over the perl built in
134 functions is that it works with numbers of ANY size, that it is
135 optimized for speed on some operations, and for at least some
136 programmers the notation might be familiar.
138 =item B<How do I convert Hexadecimal into decimal:>
140 Using perl's built in conversion of 0x notation:
143 $dec = sprintf("%d", $int);
145 Using the hex function:
147 $int = hex("DEADBEEF");
148 $dec = sprintf("%d", $int);
152 $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
153 $dec = sprintf("%d", $int);
155 Using the CPAN module Bit::Vector:
158 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
159 $dec = $vec->to_Dec();
161 =item B<How do I convert from decimal to hexadecimal:>
165 $hex = sprintf("%X", 3735928559);
169 $hex = unpack("H*", pack("N", 3735928559));
174 $vec = Bit::Vector->new_Dec(32, -559038737);
175 $hex = $vec->to_Hex();
177 And Bit::Vector supports odd bit counts:
180 $vec = Bit::Vector->new_Dec(33, 3735928559);
181 $vec->Resize(32); # suppress leading 0 if unwanted
182 $hex = $vec->to_Hex();
184 =item B<How do I convert from octal to decimal:>
186 Using Perl's built in conversion of numbers with leading zeros:
188 $int = 033653337357; # note the leading 0!
189 $dec = sprintf("%d", $int);
191 Using the oct function:
193 $int = oct("33653337357");
194 $dec = sprintf("%d", $int);
199 $vec = Bit::Vector->new(32);
200 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
201 $dec = $vec->to_Dec();
203 =item B<How do I convert from decimal to octal:>
207 $oct = sprintf("%o", 3735928559);
212 $vec = Bit::Vector->new_Dec(32, -559038737);
213 $oct = reverse join('', $vec->Chunk_List_Read(3));
215 =item B<How do I convert from binary to decimal:>
219 $decimal = ord(pack('B8', '10110110'));
221 Using pack and unpack for larger strings
223 $int = unpack("N", pack("B32",
224 substr("0" x 32 . "11110101011011011111011101111", -32)));
225 $dec = sprintf("%d", $int);
227 # substr() is used to left pad a 32 character string with zeros.
231 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
232 $dec = $vec->to_Dec();
234 =item B<How do I convert from decimal to binary:>
238 $bin = unpack("B*", pack("N", 3735928559));
243 $vec = Bit::Vector->new_Dec(32, -559038737);
244 $bin = $vec->to_Bin();
246 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
247 are left as an exercise to the inclined reader.
250 =head2 Why doesn't & work the way I want it to?
252 The behavior of binary arithmetic operators depends on whether they're
253 used on numbers or strings. The operators treat a string as a series
254 of bits and work with that (the string C<"3"> is the bit pattern
255 C<00110011>). The operators work with the binary form of a number
256 (the number C<3> is treated as the bit pattern C<00000011>).
258 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
259 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
262 Most problems with C<&> and C<|> arise because the programmer thinks
263 they have a number but really it's a string. The rest arise because
266 if ("\020\020" & "\101\101") {
270 but a string consisting of two null bytes (the result of C<"\020\020"
271 & "\101\101">) is not a false value in Perl. You need:
273 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
277 =head2 How do I multiply matrices?
279 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
280 or the PDL extension (also available from CPAN).
282 =head2 How do I perform an operation on a series of integers?
284 To call a function on each element in an array, and collect the
287 @results = map { my_func($_) } @array;
291 @triple = map { 3 * $_ } @single;
293 To call a function on each element of an array, but ignore the
296 foreach $iterator (@array) {
297 some_func($iterator);
300 To call a function on each integer in a (small) range, you B<can> use:
302 @results = map { some_func($_) } (5 .. 25);
304 but you should be aware that the C<..> operator creates an array of
305 all integers in the range. This can take a lot of memory for large
309 for ($i=5; $i < 500_005; $i++) {
310 push(@results, some_func($i));
313 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
314 loop will iterate over the range, without creating the entire range.
316 for my $i (5 .. 500_005) {
317 push(@results, some_func($i));
320 will not create a list of 500,000 integers.
322 =head2 How can I output Roman numerals?
324 Get the http://www.cpan.org/modules/by-module/Roman module.
326 =head2 Why aren't my random numbers random?
328 If you're using a version of Perl before 5.004, you must call C<srand>
329 once at the start of your program to seed the random number generator.
330 5.004 and later automatically call C<srand> at the beginning. Don't
331 call C<srand> more than once--you make your numbers less random, rather
334 Computers are good at being predictable and bad at being random
335 (despite appearances caused by bugs in your programs :-). see the
336 F<random> artitcle in the "Far More Than You Ever Wanted To Know"
337 collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz , courtesy of
338 Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
339 who 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/i);
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 the F<sort> artitcle article in the "Far More Than You Ever Wanted
1477 To Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz for
1478 more about this approach.
1480 See also the question below on sorting hashes.
1482 =head2 How do I manipulate arrays of bits?
1484 Use pack() and unpack(), or else vec() and the bitwise operations.
1486 For example, this sets $vec to have bit N set if $ints[N] was set:
1489 foreach(@ints) { vec($vec,$_,1) = 1 }
1491 Here's how, given a vector in $vec, you can
1492 get those bits into your @ints array:
1494 sub bitvec_to_list {
1497 # Find null-byte density then select best algorithm
1498 if ($vec =~ tr/\0// / length $vec > 0.95) {
1501 # This method is faster with mostly null-bytes
1502 while($vec =~ /[^\0]/g ) {
1503 $i = -9 + 8 * pos $vec;
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);
1511 push @ints, $i if vec($vec, ++$i, 1);
1514 # This method is a fast general algorithm
1516 my $bits = unpack "b*", $vec;
1517 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1518 push @ints, pos $bits while($bits =~ /1/g);
1523 This method gets faster the more sparse the bit vector is.
1524 (Courtesy of Tim Bunce and Winfried Koenig.)
1526 Or use the CPAN module Bit::Vector:
1528 $vector = Bit::Vector->new($num_of_bits);
1529 $vector->Index_List_Store(@ints);
1530 @ints = $vector->Index_List_Read();
1532 Bit::Vector provides efficient methods for bit vector, sets of small integers
1535 Here's a more extensive illustration using vec():
1538 $vector = "\xff\x0f\xef\xfe";
1539 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1540 unpack("N", $vector), "\n";
1541 $is_set = vec($vector, 23, 1);
1542 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1560 my ($offset, $width, $value) = @_;
1562 vec($vector, $offset, $width) = $value;
1563 print "offset=$offset width=$width value=$value\n";
1569 my $bits = unpack("b*", $vector);
1573 print "vector length in bytes: ", length($vector), "\n";
1574 @bytes = unpack("A8" x length($vector), $bits);
1575 print "bits are: @bytes\n\n";
1578 =head2 Why does defined() return true on empty arrays and hashes?
1580 The short story is that you should probably only use defined on scalars or
1581 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1582 in the 5.004 release or later of Perl for more detail.
1584 =head1 Data: Hashes (Associative Arrays)
1586 =head2 How do I process an entire hash?
1588 Use the each() function (see L<perlfunc/each>) if you don't care
1589 whether it's sorted:
1591 while ( ($key, $value) = each %hash) {
1592 print "$key = $value\n";
1595 If you want it sorted, you'll have to use foreach() on the result of
1596 sorting the keys as shown in an earlier question.
1598 =head2 What happens if I add or remove keys from a hash while iterating over it?
1602 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1603 iterating over it. In Perl 5 you can delete from it, but you still
1604 can't add to it, because that might cause a doubling of the hash table,
1605 in which half the entries get copied up to the new top half of the
1606 table, at which point you've totally bamboozled the iterator code.
1607 Even if the table doesn't double, there's no telling whether your new
1608 entry will be inserted before or after the current iterator position.
1610 Either treasure up your changes and make them after the iterator finishes
1611 or use keys to fetch all the old keys at once, and iterate over the list
1614 =head2 How do I look up a hash element by value?
1616 Create a reverse hash:
1618 %by_value = reverse %by_key;
1619 $key = $by_value{$value};
1621 That's not particularly efficient. It would be more space-efficient
1624 while (($key, $value) = each %by_key) {
1625 $by_value{$value} = $key;
1628 If your hash could have repeated values, the methods above will only find
1629 one of the associated keys. This may or may not worry you. If it does
1630 worry you, you can always reverse the hash into a hash of arrays instead:
1632 while (($key, $value) = each %by_key) {
1633 push @{$key_list_by_value{$value}}, $key;
1636 =head2 How can I know how many entries are in a hash?
1638 If you mean how many keys, then all you have to do is
1639 use the keys() function in a scalar context:
1641 $num_keys = keys %hash;
1643 The keys() function also resets the iterator, which means that you may
1644 see strange results if you use this between uses of other hash operators
1647 =head2 How do I sort a hash (optionally by value instead of key)?
1649 Internally, hashes are stored in a way that prevents you from imposing
1650 an order on key-value pairs. Instead, you have to sort a list of the
1653 @keys = sort keys %hash; # sorted by key
1655 $hash{$a} cmp $hash{$b}
1656 } keys %hash; # and by value
1658 Here we'll do a reverse numeric sort by value, and if two keys are
1659 identical, sort by length of key, or if that fails, by straight ASCII
1660 comparison of the keys (well, possibly modified by your locale--see
1664 $hash{$b} <=> $hash{$a}
1666 length($b) <=> length($a)
1671 =head2 How can I always keep my hash sorted?
1673 You can look into using the DB_File module and tie() using the
1674 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1675 The Tie::IxHash module from CPAN might also be instructive.
1677 =head2 What's the difference between "delete" and "undef" with hashes?
1679 Hashes are pairs of scalars: the first is the key, the second is the
1680 value. The key will be coerced to a string, although the value can be
1681 any kind of scalar: string, number, or reference. If a key C<$key> is
1682 present in the array, C<exists($key)> will return true. The value for
1683 a given key can be C<undef>, in which case C<$array{$key}> will be
1684 C<undef> while C<$exists{$key}> will return true. This corresponds to
1685 (C<$key>, C<undef>) being in the hash.
1687 Pictures help... here's the C<%ary> table:
1697 And these conditions hold
1701 defined $ary{'d'} is true
1702 defined $ary{'a'} is true
1703 exists $ary{'a'} is true (Perl5 only)
1704 grep ($_ eq 'a', keys %ary) is true
1710 your table now reads:
1721 and these conditions now hold; changes in caps:
1725 defined $ary{'d'} is true
1726 defined $ary{'a'} is FALSE
1727 exists $ary{'a'} is true (Perl5 only)
1728 grep ($_ eq 'a', keys %ary) is true
1730 Notice the last two: you have an undef value, but a defined key!
1736 your table now reads:
1745 and these conditions now hold; changes in caps:
1749 defined $ary{'d'} is true
1750 defined $ary{'a'} is false
1751 exists $ary{'a'} is FALSE (Perl5 only)
1752 grep ($_ eq 'a', keys %ary) is FALSE
1754 See, the whole entry is gone!
1756 =head2 Why don't my tied hashes make the defined/exists distinction?
1758 They may or may not implement the EXISTS() and DEFINED() methods
1759 differently. For example, there isn't the concept of undef with hashes
1760 that are tied to DBM* files. This means the true/false tables above
1761 will give different results when used on such a hash. It also means
1762 that exists and defined do the same thing with a DBM* file, and what
1763 they end up doing is not what they do with ordinary hashes.
1765 =head2 How do I reset an each() operation part-way through?
1767 Using C<keys %hash> in scalar context returns the number of keys in
1768 the hash I<and> resets the iterator associated with the hash. You may
1769 need to do this if you use C<last> to exit a loop early so that when you
1770 re-enter it, the hash iterator has been reset.
1772 =head2 How can I get the unique keys from two hashes?
1774 First you extract the keys from the hashes into lists, then solve
1775 the "removing duplicates" problem described above. For example:
1778 for $element (keys(%foo), keys(%bar)) {
1785 @uniq = keys %{{%foo,%bar}};
1787 Or if you really want to save space:
1790 while (defined ($key = each %foo)) {
1793 while (defined ($key = each %bar)) {
1798 =head2 How can I store a multidimensional array in a DBM file?
1800 Either stringify the structure yourself (no fun), or else
1801 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1802 it on top of either DB_File or GDBM_File.
1804 =head2 How can I make my hash remember the order I put elements into it?
1806 Use the Tie::IxHash from CPAN.
1809 tie(%myhash, Tie::IxHash);
1810 for ($i=0; $i<20; $i++) {
1813 @keys = keys %myhash;
1814 # @keys = (0,1,2,3,...)
1816 =head2 Why does passing a subroutine an undefined element in a hash create it?
1818 If you say something like:
1820 somefunc($hash{"nonesuch key here"});
1822 Then that element "autovivifies"; that is, it springs into existence
1823 whether you store something there or not. That's because functions
1824 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1825 it has to be ready to write it back into the caller's version.
1827 This has been fixed as of Perl5.004.
1829 Normally, merely accessing a key's value for a nonexistent key does
1830 I<not> cause that key to be forever there. This is different than
1833 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1835 Usually a hash ref, perhaps like this:
1840 TITLE => "deputy peon",
1843 PALS => [ "Norbert", "Rhys", "Phineas"],
1846 References are documented in L<perlref> and the upcoming L<perlreftut>.
1847 Examples of complex data structures are given in L<perldsc> and
1848 L<perllol>. Examples of structures and object-oriented classes are
1851 =head2 How can I use a reference as a hash key?
1853 You can't do this directly, but you could use the standard Tie::RefHash
1854 module distributed with Perl.
1858 =head2 How do I handle binary data correctly?
1860 Perl is binary clean, so this shouldn't be a problem. For example,
1861 this works fine (assuming the files are found):
1863 if (`cat /vmunix` =~ /gzip/) {
1864 print "Your kernel is GNU-zip enabled!\n";
1867 On less elegant (read: Byzantine) systems, however, you have
1868 to play tedious games with "text" versus "binary" files. See
1869 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1870 systems are curses out of Microsoft, who seem to be committed to putting
1871 the backward into backward compatibility.
1873 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1875 If you want to deal with multibyte characters, however, there are
1876 some gotchas. See the section on Regular Expressions.
1878 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1880 Assuming that you don't care about IEEE notations like "NaN" or
1881 "Infinity", you probably just want to use a regular expression.
1883 if (/\D/) { print "has nondigits\n" }
1884 if (/^\d+$/) { print "is a whole number\n" }
1885 if (/^-?\d+$/) { print "is an integer\n" }
1886 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1887 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1888 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1889 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1890 { print "a C float" }
1892 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1893 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1894 wrapper function for more convenient access. This function takes
1895 a string and returns the number it found, or C<undef> for input that
1896 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1897 if you just want to say, ``Is this a float?''
1900 use POSIX qw(strtod);
1905 my($num, $unparsed) = strtod($str);
1906 if (($str eq '') || ($unparsed != 0) || $!) {
1913 sub is_numeric { defined getnum($_[0]) }
1915 Or you could check out the String::Scanf module on CPAN instead. The
1916 POSIX module (part of the standard Perl distribution) provides the
1917 C<strtod> and C<strtol> for converting strings to double and longs,
1920 =head2 How do I keep persistent data across program calls?
1922 For some specific applications, you can use one of the DBM modules.
1923 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
1924 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
1925 of the standard distribution. Here's one example using Storable's C<store>
1926 and C<retrieve> functions:
1929 store(\%hash, "filename");
1932 $href = retrieve("filename"); # by ref
1933 %hash = %{ retrieve("filename") }; # direct to hash
1935 =head2 How do I print out or copy a recursive data structure?
1937 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1938 for printing out data structures. The Storable module, found on CPAN,
1939 provides a function called C<dclone> that recursively copies its argument.
1941 use Storable qw(dclone);
1944 Where $r1 can be a reference to any kind of data structure you'd like.
1945 It will be deeply copied. Because C<dclone> takes and returns references,
1946 you'd have to add extra punctuation if you had a hash of arrays that
1949 %newhash = %{ dclone(\%oldhash) };
1951 =head2 How do I define methods for every class/object?
1953 Use the UNIVERSAL class (see L<UNIVERSAL>).
1955 =head2 How do I verify a credit card checksum?
1957 Get the Business::CreditCard module from CPAN.
1959 =head2 How do I pack arrays of doubles or floats for XS code?
1961 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1962 If you're doing a lot of float or double processing, consider using
1963 the PDL module from CPAN instead--it makes number-crunching easy.
1965 =head1 AUTHOR AND COPYRIGHT
1967 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
1968 All rights reserved.
1970 This documentation is free; you can redistribute it and/or modify it
1971 under the same terms as Perl itself.
1973 Irrespective of its distribution, all code examples in this file
1974 are hereby placed into the public domain. You are permitted and
1975 encouraged to use this code in your own programs for fun
1976 or for profit as you see fit. A simple comment in the code giving
1977 credit would be courteous but is not required.