3 perlfaq4 - Data Manipulation ($Revision: 1.24 $, $Date: 2002/05/20 16:50:08 $)
7 This section of the FAQ answers questions related to manipulating
8 numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
12 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
14 The infinite set that a mathematician thinks of as the real numbers can
15 only be approximated on a computer, since the computer only has a finite
16 number of bits to store an infinite number of, um, numbers.
18 Internally, your computer represents floating-point numbers in binary.
19 Floating-point numbers read in from a file or appearing as literals
20 in your program are converted from their decimal floating-point
21 representation (eg, 19.95) to an internal binary representation.
23 However, 19.95 can't be precisely represented as a binary
24 floating-point number, just like 1/3 can't be exactly represented as a
25 decimal floating-point number. The computer's binary representation
26 of 19.95, therefore, isn't exactly 19.95.
28 When a floating-point number gets printed, the binary floating-point
29 representation is converted back to decimal. These decimal numbers
30 are displayed in either the format you specify with printf(), or the
31 current output format for numbers. (See L<perlvar/"$#"> 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.
140 =item How do I convert hexadecimal into decimal
142 Using perl's built in conversion of 0x notation:
145 $dec = sprintf("%d", $int);
147 Using the hex function:
149 $int = hex("DEADBEEF");
150 $dec = sprintf("%d", $int);
154 $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
155 $dec = sprintf("%d", $int);
157 Using the CPAN module Bit::Vector:
160 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
161 $dec = $vec->to_Dec();
163 =item How do I convert from decimal to hexadecimal
167 $hex = sprintf("%X", 3735928559);
171 $hex = unpack("H*", pack("N", 3735928559));
176 $vec = Bit::Vector->new_Dec(32, -559038737);
177 $hex = $vec->to_Hex();
179 And Bit::Vector supports odd bit counts:
182 $vec = Bit::Vector->new_Dec(33, 3735928559);
183 $vec->Resize(32); # suppress leading 0 if unwanted
184 $hex = $vec->to_Hex();
186 =item How do I convert from octal to decimal
188 Using Perl's built in conversion of numbers with leading zeros:
190 $int = 033653337357; # note the leading 0!
191 $dec = sprintf("%d", $int);
193 Using the oct function:
195 $int = oct("33653337357");
196 $dec = sprintf("%d", $int);
201 $vec = Bit::Vector->new(32);
202 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
203 $dec = $vec->to_Dec();
205 =item How do I convert from decimal to octal
209 $oct = sprintf("%o", 3735928559);
214 $vec = Bit::Vector->new_Dec(32, -559038737);
215 $oct = reverse join('', $vec->Chunk_List_Read(3));
217 =item How do I convert from binary to decimal
219 Perl 5.6 lets you write binary numbers directly with
222 $number = 0b10110110;
226 $decimal = ord(pack('B8', '10110110'));
228 Using pack and unpack for larger strings
230 $int = unpack("N", pack("B32",
231 substr("0" x 32 . "11110101011011011111011101111", -32)));
232 $dec = sprintf("%d", $int);
234 # substr() is used to left pad a 32 character string with zeros.
238 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
239 $dec = $vec->to_Dec();
241 =item How do I convert from decimal to binary
245 $bin = unpack("B*", pack("N", 3735928559));
250 $vec = Bit::Vector->new_Dec(32, -559038737);
251 $bin = $vec->to_Bin();
253 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
254 are left as an exercise to the inclined reader.
258 =head2 Why doesn't & work the way I want it to?
260 The behavior of binary arithmetic operators depends on whether they're
261 used on numbers or strings. The operators treat a string as a series
262 of bits and work with that (the string C<"3"> is the bit pattern
263 C<00110011>). The operators work with the binary form of a number
264 (the number C<3> is treated as the bit pattern C<00000011>).
266 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
267 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
270 Most problems with C<&> and C<|> arise because the programmer thinks
271 they have a number but really it's a string. The rest arise because
274 if ("\020\020" & "\101\101") {
278 but a string consisting of two null bytes (the result of C<"\020\020"
279 & "\101\101">) is not a false value in Perl. You need:
281 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
285 =head2 How do I multiply matrices?
287 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
288 or the PDL extension (also available from CPAN).
290 =head2 How do I perform an operation on a series of integers?
292 To call a function on each element in an array, and collect the
295 @results = map { my_func($_) } @array;
299 @triple = map { 3 * $_ } @single;
301 To call a function on each element of an array, but ignore the
304 foreach $iterator (@array) {
305 some_func($iterator);
308 To call a function on each integer in a (small) range, you B<can> use:
310 @results = map { some_func($_) } (5 .. 25);
312 but you should be aware that the C<..> operator creates an array of
313 all integers in the range. This can take a lot of memory for large
317 for ($i=5; $i < 500_005; $i++) {
318 push(@results, some_func($i));
321 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
322 loop will iterate over the range, without creating the entire range.
324 for my $i (5 .. 500_005) {
325 push(@results, some_func($i));
328 will not create a list of 500,000 integers.
330 =head2 How can I output Roman numerals?
332 Get the http://www.cpan.org/modules/by-module/Roman module.
334 =head2 Why aren't my random numbers random?
336 If you're using a version of Perl before 5.004, you must call C<srand>
337 once at the start of your program to seed the random number generator.
338 5.004 and later automatically call C<srand> at the beginning. Don't
339 call C<srand> more than once--you make your numbers less random, rather
342 Computers are good at being predictable and bad at being random
343 (despite appearances caused by bugs in your programs :-). see the
344 F<random> artitcle in the "Far More Than You Ever Wanted To Know"
345 collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz , courtesy of
346 Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
347 who attempts to generate random numbers by deterministic means is, of
348 course, living in a state of sin.''
350 If you want numbers that are more random than C<rand> with C<srand>
351 provides, you should also check out the Math::TrulyRandom module from
352 CPAN. It uses the imperfections in your system's timer to generate
353 random numbers, but this takes quite a while. If you want a better
354 pseudorandom generator than comes with your operating system, look at
355 ``Numerical Recipes in C'' at http://www.nr.com/ .
357 =head2 How do I get a random number between X and Y?
359 Use the following simple function. It selects a random integer between
360 (and possibly including!) the two given integers, e.g.,
361 C<random_int_in(50,120)>
363 sub random_int_in ($$) {
365 # Assumes that the two arguments are integers themselves!
366 return $min if $min == $max;
367 ($min, $max) = ($max, $min) if $min > $max;
368 return $min + int rand(1 + $max - $min);
373 =head2 How do I find the week-of-the-year/day-of-the-year?
375 The day of the year is in the array returned by localtime() (see
376 L<perlfunc/"localtime">):
378 $day_of_year = (localtime(time()))[7];
380 =head2 How do I find the current century or millennium?
382 Use the following simple functions:
385 return int((((localtime(shift || time))[5] + 1999))/100);
388 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
391 On some systems, you'll find that the POSIX module's strftime() function
392 has been extended in a non-standard way to use a C<%C> format, which they
393 sometimes claim is the "century". It isn't, because on most such systems,
394 this is only the first two digits of the four-digit year, and thus cannot
395 be used to reliably determine the current century or millennium.
397 =head2 How can I compare two dates and find the difference?
399 If you're storing your dates as epoch seconds then simply subtract one
400 from the other. If you've got a structured date (distinct year, day,
401 month, hour, minute, seconds values), then for reasons of accessibility,
402 simplicity, and efficiency, merely use either timelocal or timegm (from
403 the Time::Local module in the standard distribution) to reduce structured
404 dates to epoch seconds. However, if you don't know the precise format of
405 your dates, then you should probably use either of the Date::Manip and
406 Date::Calc modules from CPAN before you go hacking up your own parsing
407 routine to handle arbitrary date formats.
409 =head2 How can I take a string and turn it into epoch seconds?
411 If it's a regular enough string that it always has the same format,
412 you can split it up and pass the parts to C<timelocal> in the standard
413 Time::Local module. Otherwise, you should look into the Date::Calc
414 and Date::Manip modules from CPAN.
416 =head2 How can I find the Julian Day?
418 Use the Time::JulianDay module (part of the Time-modules bundle
419 available from CPAN.)
421 Before you immerse yourself too deeply in this, be sure to verify that
422 it is the I<Julian> Day you really want. Are you interested in a way
423 of getting serial days so that you just can tell how many days they
424 are apart or so that you can do also other date arithmetic? If you
425 are interested in performing date arithmetic, this can be done using
426 modules Date::Manip or Date::Calc.
428 There is too many details and much confusion on this issue to cover in
429 this FAQ, but the term is applied (correctly) to a calendar now
430 supplanted by the Gregorian Calendar, with the Julian Calendar failing
431 to adjust properly for leap years on centennial years (among other
432 annoyances). The term is also used (incorrectly) to mean: [1] days in
433 the Gregorian Calendar; and [2] days since a particular starting time
434 or `epoch', usually 1970 in the Unix world and 1980 in the
435 MS-DOS/Windows world. If you find that it is not the first meaning
436 that you really want, then check out the Date::Manip and Date::Calc
437 modules. (Thanks to David Cassell for most of this text.)
439 =head2 How do I find yesterday's date?
441 The C<time()> function returns the current time in seconds since the
442 epoch. Take twenty-four hours off that:
444 $yesterday = time() - ( 24 * 60 * 60 );
446 Then you can pass this to C<localtime()> and get the individual year,
447 month, day, hour, minute, seconds values.
449 Note very carefully that the code above assumes that your days are
450 twenty-four hours each. For most people, there are two days a year
451 when they aren't: the switch to and from summer time throws this off.
452 A solution to this issue is offered by Russ Allbery.
455 my $now = defined $_[0] ? $_[0] : time;
456 my $then = $now - 60 * 60 * 24;
457 my $ndst = (localtime $now)[8] > 0;
458 my $tdst = (localtime $then)[8] > 0;
459 $then - ($tdst - $ndst) * 60 * 60;
461 # Should give you "this time yesterday" in seconds since epoch relative to
462 # the first argument or the current time if no argument is given and
463 # suitable for passing to localtime or whatever else you need to do with
464 # it. $ndst is whether we're currently in daylight savings time; $tdst is
465 # whether the point 24 hours ago was in daylight savings time. If $tdst
466 # and $ndst are the same, a boundary wasn't crossed, and the correction
467 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
468 # from yesterday's time since we gained an extra hour while going off
469 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
470 # negative hour (add an hour) to yesterday's time since we lost an hour.
472 # All of this is because during those days when one switches off or onto
473 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
475 # The explicit settings of $ndst and $tdst are necessary because localtime
476 # only says it returns the system tm struct, and the system tm struct at
477 # least on Solaris doesn't guarantee any particular positive value (like,
478 # say, 1) for isdst, just a positive value. And that value can
479 # potentially be negative, if DST information isn't available (this sub
480 # just treats those cases like no DST).
482 # Note that between 2am and 3am on the day after the time zone switches
483 # off daylight savings time, the exact hour of "yesterday" corresponding
484 # to the current hour is not clearly defined. Note also that if used
485 # between 2am and 3am the day after the change to daylight savings time,
486 # the result will be between 3am and 4am of the previous day; it's
487 # arguable whether this is correct.
489 # This sub does not attempt to deal with leap seconds (most things don't).
491 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
492 # This code is in the public domain
494 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
496 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
497 Y2K compliant (whatever that means). The programmers you've hired to
498 use it, however, probably are not.
500 Long answer: The question belies a true understanding of the issue.
501 Perl is just as Y2K compliant as your pencil--no more, and no less.
502 Can you use your pencil to write a non-Y2K-compliant memo? Of course
503 you can. Is that the pencil's fault? Of course it isn't.
505 The date and time functions supplied with Perl (gmtime and localtime)
506 supply adequate information to determine the year well beyond 2000
507 (2038 is when trouble strikes for 32-bit machines). The year returned
508 by these functions when used in a list context is the year minus 1900.
509 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
510 number. To avoid the year 2000 problem simply do not treat the year as
511 a 2-digit number. It isn't.
513 When gmtime() and localtime() are used in scalar context they return
514 a timestamp string that contains a fully-expanded year. For example,
515 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
516 2001". There's no year 2000 problem here.
518 That doesn't mean that Perl can't be used to create non-Y2K compliant
519 programs. It can. But so can your pencil. It's the fault of the user,
520 not the language. At the risk of inflaming the NRA: ``Perl doesn't
521 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
526 =head2 How do I validate input?
528 The answer to this question is usually a regular expression, perhaps
529 with auxiliary logic. See the more specific questions (numbers, mail
530 addresses, etc.) for details.
532 =head2 How do I unescape a string?
534 It depends just what you mean by ``escape''. URL escapes are dealt
535 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
536 character are removed with
540 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
542 =head2 How do I remove consecutive pairs of characters?
544 To turn C<"abbcccd"> into C<"abccd">:
546 s/(.)\1/$1/g; # add /s to include newlines
548 Here's a solution that turns "abbcccd" to "abcd":
550 y///cs; # y == tr, but shorter :-)
552 =head2 How do I expand function calls in a string?
554 This is documented in L<perlref>. In general, this is fraught with
555 quoting and readability problems, but it is possible. To interpolate
556 a subroutine call (in list context) into a string:
558 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
560 If you prefer scalar context, similar chicanery is also useful for
561 arbitrary expressions:
563 print "That yields ${\($n + 5)} widgets\n";
565 Version 5.004 of Perl had a bug that gave list context to the
566 expression in C<${...}>, but this is fixed in version 5.005.
568 See also ``How can I expand variables in text strings?'' in this
571 =head2 How do I find matching/nesting anything?
573 This isn't something that can be done in one regular expression, no
574 matter how complicated. To find something between two single
575 characters, a pattern like C</x([^x]*)x/> will get the intervening
576 bits in $1. For multiple ones, then something more like
577 C</alpha(.*?)omega/> would be needed. But none of these deals with
578 nested patterns, nor can they. For that you'll have to write a
581 If you are serious about writing a parser, there are a number of
582 modules or oddities that will make your life a lot easier. There are
583 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
584 and the byacc program. Starting from perl 5.8 the Text::Balanced
585 is part of the standard distribution.
587 One simple destructive, inside-out approach that you might try is to
588 pull out the smallest nesting parts one at a time:
590 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
591 # do something with $1
594 A more complicated and sneaky approach is to make Perl's regular
595 expression engine do it for you. This is courtesy Dean Inada, and
596 rather has the nature of an Obfuscated Perl Contest entry, but it
599 # $_ contains the string to parse
600 # BEGIN and END are the opening and closing markers for the
605 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
606 @$ = (eval{/$re/},$@!~/unmatched/i);
607 print join("\n",@$[0..$#$]) if( $$[-1] );
609 =head2 How do I reverse a string?
611 Use reverse() in scalar context, as documented in
614 $reversed = reverse $string;
616 =head2 How do I expand tabs in a string?
618 You can do it yourself:
620 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
622 Or you can just use the Text::Tabs module (part of the standard Perl
626 @expanded_lines = expand(@lines_with_tabs);
628 =head2 How do I reformat a paragraph?
630 Use Text::Wrap (part of the standard Perl distribution):
633 print wrap("\t", ' ', @paragraphs);
635 The paragraphs you give to Text::Wrap should not contain embedded
636 newlines. Text::Wrap doesn't justify the lines (flush-right).
638 Or use the CPAN module Text::Autoformat. Formatting files can be easily
639 done by making a shell alias, like so:
641 alias fmt="perl -i -MText::Autoformat -n0777 \
642 -e 'print autoformat $_, {all=>1}' $*"
644 See the documentation for Text::Autoformat to appreciate its many
647 =head2 How can I access/change the first N letters of a string?
649 There are many ways. If you just want to grab a copy, use
652 $first_byte = substr($a, 0, 1);
654 If you want to modify part of a string, the simplest way is often to
655 use substr() as an lvalue:
657 substr($a, 0, 3) = "Tom";
659 Although those with a pattern matching kind of thought process will
664 =head2 How do I change the Nth occurrence of something?
666 You have to keep track of N yourself. For example, let's say you want
667 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
668 C<"whosoever"> or C<"whomsoever">, case insensitively. These
669 all assume that $_ contains the string to be altered.
673 ++$count == 5 # is it the 5th?
674 ? "${2}soever" # yes, swap
675 : $1 # renege and leave it there
678 In the more general case, you can use the C</g> modifier in a C<while>
679 loop, keeping count of matches.
683 $_ = "One fish two fish red fish blue fish";
684 while (/(\w+)\s+fish\b/gi) {
685 if (++$count == $WANT) {
686 print "The third fish is a $1 one.\n";
690 That prints out: C<"The third fish is a red one."> You can also use a
691 repetition count and repeated pattern like this:
693 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
695 =head2 How can I count the number of occurrences of a substring within a string?
697 There are a number of ways, with varying efficiency. If you want a
698 count of a certain single character (X) within a string, you can use the
699 C<tr///> function like so:
701 $string = "ThisXlineXhasXsomeXx'sXinXit";
702 $count = ($string =~ tr/X//);
703 print "There are $count X characters in the string";
705 This is fine if you are just looking for a single character. However,
706 if you are trying to count multiple character substrings within a
707 larger string, C<tr///> won't work. What you can do is wrap a while()
708 loop around a global pattern match. For example, let's count negative
711 $string = "-9 55 48 -2 23 -76 4 14 -44";
712 while ($string =~ /-\d+/g) { $count++ }
713 print "There are $count negative numbers in the string";
715 Another version uses a global match in list context, then assigns the
716 result to a scalar, producing a count of the number of matches.
718 $count = () = $string =~ /-\d+/g;
720 =head2 How do I capitalize all the words on one line?
722 To make the first letter of each word upper case:
724 $line =~ s/\b(\w)/\U$1/g;
726 This has the strange effect of turning "C<don't do it>" into "C<Don'T
727 Do It>". Sometimes you might want this. Other times you might need a
728 more thorough solution (Suggested by brian d foy):
731 (^\w) #at the beginning of the line
733 (\s\w) #preceded by whitespace
736 $string =~ /([\w']+)/\u\L$1/g;
738 To make the whole line upper case:
742 To force each word to be lower case, with the first letter upper case:
744 $line =~ s/(\w+)/\u\L$1/g;
746 You can (and probably should) enable locale awareness of those
747 characters by placing a C<use locale> pragma in your program.
748 See L<perllocale> for endless details on locales.
750 This is sometimes referred to as putting something into "title
751 case", but that's not quite accurate. Consider the proper
752 capitalization of the movie I<Dr. Strangelove or: How I Learned to
753 Stop Worrying and Love the Bomb>, for example.
755 =head2 How can I split a [character] delimited string except when inside
756 [character]? (Comma-separated files)
758 Take the example case of trying to split a string that is comma-separated
759 into its different fields. (We'll pretend you said comma-separated, not
760 comma-delimited, which is different and almost never what you mean.) You
761 can't use C<split(/,/)> because you shouldn't split if the comma is inside
762 quotes. For example, take a data line like this:
764 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
766 Due to the restriction of the quotes, this is a fairly complex
767 problem. Thankfully, we have Jeffrey Friedl, author of a highly
768 recommended book on regular expressions, to handle these for us. He
769 suggests (assuming your string is contained in $text):
772 push(@new, $+) while $text =~ m{
773 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
777 push(@new, undef) if substr($text,-1,1) eq ',';
779 If you want to represent quotation marks inside a
780 quotation-mark-delimited field, escape them with backslashes (eg,
781 C<"like \"this\"">. Unescaping them is a task addressed earlier in
784 Alternatively, the Text::ParseWords module (part of the standard Perl
785 distribution) lets you say:
787 use Text::ParseWords;
788 @new = quotewords(",", 0, $text);
790 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
792 =head2 How do I strip blank space from the beginning/end of a string?
794 Although the simplest approach would seem to be
796 $string =~ s/^\s*(.*?)\s*$/$1/;
798 not only is this unnecessarily slow and destructive, it also fails with
799 embedded newlines. It is much faster to do this operation in two steps:
804 Or more nicely written as:
811 This idiom takes advantage of the C<foreach> loop's aliasing
812 behavior to factor out common code. You can do this
813 on several strings at once, or arrays, or even the
814 values of a hash if you use a slice:
816 # trim whitespace in the scalar, the array,
817 # and all the values in the hash
818 foreach ($scalar, @array, @hash{keys %hash}) {
823 =head2 How do I pad a string with blanks or pad a number with zeroes?
825 (This answer contributed by Uri Guttman, with kibitzing from
828 In the following examples, C<$pad_len> is the length to which you wish
829 to pad the string, C<$text> or C<$num> contains the string to be padded,
830 and C<$pad_char> contains the padding character. You can use a single
831 character string constant instead of the C<$pad_char> variable if you
832 know what it is in advance. And in the same way you can use an integer in
833 place of C<$pad_len> if you know the pad length in advance.
835 The simplest method uses the C<sprintf> function. It can pad on the left
836 or right with blanks and on the left with zeroes and it will not
837 truncate the result. The C<pack> function can only pad strings on the
838 right with blanks and it will truncate the result to a maximum length of
841 # Left padding a string with blanks (no truncation):
842 $padded = sprintf("%${pad_len}s", $text);
844 # Right padding a string with blanks (no truncation):
845 $padded = sprintf("%-${pad_len}s", $text);
847 # Left padding a number with 0 (no truncation):
848 $padded = sprintf("%0${pad_len}d", $num);
850 # Right padding a string with blanks using pack (will truncate):
851 $padded = pack("A$pad_len",$text);
853 If you need to pad with a character other than blank or zero you can use
854 one of the following methods. They all generate a pad string with the
855 C<x> operator and combine that with C<$text>. These methods do
856 not truncate C<$text>.
858 Left and right padding with any character, creating a new string:
860 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
861 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
863 Left and right padding with any character, modifying C<$text> directly:
865 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
866 $text .= $pad_char x ( $pad_len - length( $text ) );
868 =head2 How do I extract selected columns from a string?
870 Use substr() or unpack(), both documented in L<perlfunc>.
871 If you prefer thinking in terms of columns instead of widths,
872 you can use this kind of thing:
874 # determine the unpack format needed to split Linux ps output
875 # arguments are cut columns
876 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
882 for my $place (@positions) {
883 $template .= "A" . ($place - $lastpos) . " ";
890 =head2 How do I find the soundex value of a string?
892 Use the standard Text::Soundex module distributed with Perl.
893 Before you do so, you may want to determine whether `soundex' is in
894 fact what you think it is. Knuth's soundex algorithm compresses words
895 into a small space, and so it does not necessarily distinguish between
896 two words which you might want to appear separately. For example, the
897 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
898 If Text::Soundex does not do what you are looking for, you might want
899 to consider the String::Approx module available at CPAN.
901 =head2 How can I expand variables in text strings?
903 Let's assume that you have a string like:
905 $text = 'this has a $foo in it and a $bar';
907 If those were both global variables, then this would
910 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
912 But since they are probably lexicals, or at least, they could
913 be, you'd have to do this:
915 $text =~ s/(\$\w+)/$1/eeg;
916 die if $@; # needed /ee, not /e
918 It's probably better in the general case to treat those
919 variables as entries in some special hash. For example:
925 $text =~ s/\$(\w+)/$user_defs{$1}/g;
927 See also ``How do I expand function calls in a string?'' in this section
930 =head2 What's wrong with always quoting "$vars"?
932 The problem is that those double-quotes force stringification--
933 coercing numbers and references into strings--even when you
934 don't want them to be strings. Think of it this way: double-quote
935 expansion is used to produce new strings. If you already
936 have a string, why do you need more?
938 If you get used to writing odd things like these:
942 somefunc("$var"); # BAD
944 You'll be in trouble. Those should (in 99.8% of the cases) be
945 the simpler and more direct:
951 Otherwise, besides slowing you down, you're going to break code when
952 the thing in the scalar is actually neither a string nor a number, but
958 my $oref = "$aref"; # WRONG
961 You can also get into subtle problems on those few operations in Perl
962 that actually do care about the difference between a string and a
963 number, such as the magical C<++> autoincrement operator or the
966 Stringification also destroys arrays.
969 print "@lines"; # WRONG - extra blanks
970 print @lines; # right
972 =head2 Why don't my <<HERE documents work?
974 Check for these three things:
978 =item 1. There must be no space after the << part.
980 =item 2. There (probably) should be a semicolon at the end.
982 =item 3. You can't (easily) have any space in front of the tag.
986 If you want to indent the text in the here document, you
990 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
995 But the HERE_TARGET must still be flush against the margin.
996 If you want that indented also, you'll have to quote
999 ($quote = <<' FINIS') =~ s/^\s+//gm;
1000 ...we will have peace, when you and all your works have
1001 perished--and the works of your dark master to whom you
1002 would deliver us. You are a liar, Saruman, and a corrupter
1003 of men's hearts. --Theoden in /usr/src/perl/taint.c
1005 $quote =~ s/\s+--/\n--/;
1007 A nice general-purpose fixer-upper function for indented here documents
1008 follows. It expects to be called with a here document as its argument.
1009 It looks to see whether each line begins with a common substring, and
1010 if so, strips that substring off. Otherwise, it takes the amount of leading
1011 whitespace found on the first line and removes that much off each
1016 my ($white, $leader); # common whitespace and common leading string
1017 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1018 ($white, $leader) = ($2, quotemeta($1));
1020 ($white, $leader) = (/^(\s+)/, '');
1022 s/^\s*?$leader(?:$white)?//gm;
1026 This works with leading special strings, dynamically determined:
1028 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1031 @@@ SAVEI32(runlevel);
1033 @@@ while ( op = (*op->op_ppaddr)() );
1037 MAIN_INTERPRETER_LOOP
1039 Or with a fixed amount of leading whitespace, with remaining
1040 indentation correctly preserved:
1042 $poem = fix<<EVER_ON_AND_ON;
1043 Now far ahead the Road has gone,
1044 And I must follow, if I can,
1045 Pursuing it with eager feet,
1046 Until it joins some larger way
1047 Where many paths and errands meet.
1048 And whither then? I cannot say.
1049 --Bilbo in /usr/src/perl/pp_ctl.c
1054 =head2 What is the difference between a list and an array?
1056 An array has a changeable length. A list does not. An array is something
1057 you can push or pop, while a list is a set of values. Some people make
1058 the distinction that a list is a value while an array is a variable.
1059 Subroutines are passed and return lists, you put things into list
1060 context, you initialize arrays with lists, and you foreach() across
1061 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1062 in scalar context behave like the number of elements in them, subroutines
1063 access their arguments through the array C<@_>, and push/pop/shift only work
1066 As a side note, there's no such thing as a list in scalar context.
1069 $scalar = (2, 5, 7, 9);
1071 you're using the comma operator in scalar context, so it uses the scalar
1072 comma operator. There never was a list there at all! This causes the
1073 last value to be returned: 9.
1075 =head2 What is the difference between $array[1] and @array[1]?
1077 The former is a scalar value; the latter an array slice, making
1078 it a list with one (scalar) value. You should use $ when you want a
1079 scalar value (most of the time) and @ when you want a list with one
1080 scalar value in it (very, very rarely; nearly never, in fact).
1082 Sometimes it doesn't make a difference, but sometimes it does.
1083 For example, compare:
1085 $good[0] = `some program that outputs several lines`;
1089 @bad[0] = `same program that outputs several lines`;
1091 The C<use warnings> pragma and the B<-w> flag will warn you about these
1094 =head2 How can I remove duplicate elements from a list or array?
1096 There are several possible ways, depending on whether the array is
1097 ordered and whether you wish to preserve the ordering.
1103 If @in is sorted, and you want @out to be sorted:
1104 (this assumes all true values in the array)
1106 $prev = "not equal to $in[0]";
1107 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1109 This is nice in that it doesn't use much extra memory, simulating
1110 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1111 guarantees that the expression is true (so that grep picks it up)
1112 even if the $_ is 0, "", or undef.
1116 If you don't know whether @in is sorted:
1119 @out = grep(!$saw{$_}++, @in);
1123 Like (b), but @in contains only small integers:
1125 @out = grep(!$saw[$_]++, @in);
1129 A way to do (b) without any loops or greps:
1133 @out = sort keys %saw; # remove sort if undesired
1137 Like (d), but @in contains only small positive integers:
1141 @out = grep {defined} @ary;
1145 But perhaps you should have been using a hash all along, eh?
1147 =head2 How can I tell whether a certain element is contained in a list or array?
1149 Hearing the word "in" is an I<in>dication that you probably should have
1150 used a hash, not a list or array, to store your data. Hashes are
1151 designed to answer this question quickly and efficiently. Arrays aren't.
1153 That being said, there are several ways to approach this. If you
1154 are going to make this query many times over arbitrary string values,
1155 the fastest way is probably to invert the original array and maintain a
1156 hash whose keys are the first array's values.
1158 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1160 for (@blues) { $is_blue{$_} = 1 }
1162 Now you can check whether $is_blue{$some_color}. It might have been a
1163 good idea to keep the blues all in a hash in the first place.
1165 If the values are all small integers, you could use a simple indexed
1166 array. This kind of an array will take up less space:
1168 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1169 @is_tiny_prime = ();
1170 for (@primes) { $is_tiny_prime[$_] = 1 }
1171 # or simply @istiny_prime[@primes] = (1) x @primes;
1173 Now you check whether $is_tiny_prime[$some_number].
1175 If the values in question are integers instead of strings, you can save
1176 quite a lot of space by using bit strings instead:
1178 @articles = ( 1..10, 150..2000, 2017 );
1180 for (@articles) { vec($read,$_,1) = 1 }
1182 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1186 ($is_there) = grep $_ eq $whatever, @array;
1190 ($is_there) = grep /$whatever/, @array;
1192 These are slow (checks every element even if the first matches),
1193 inefficient (same reason), and potentially buggy (what if there are
1194 regex characters in $whatever?). If you're only testing once, then
1198 foreach $elt (@array) {
1199 if ($elt eq $elt_to_find) {
1204 if ($is_there) { ... }
1206 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1208 Use a hash. Here's code to do both and more. It assumes that
1209 each element is unique in a given array:
1211 @union = @intersection = @difference = ();
1213 foreach $element (@array1, @array2) { $count{$element}++ }
1214 foreach $element (keys %count) {
1215 push @union, $element;
1216 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1219 Note that this is the I<symmetric difference>, that is, all elements in
1220 either A or in B but not in both. Think of it as an xor operation.
1222 =head2 How do I test whether two arrays or hashes are equal?
1224 The following code works for single-level arrays. It uses a stringwise
1225 comparison, and does not distinguish defined versus undefined empty
1226 strings. Modify if you have other needs.
1228 $are_equal = compare_arrays(\@frogs, \@toads);
1230 sub compare_arrays {
1231 my ($first, $second) = @_;
1232 no warnings; # silence spurious -w undef complaints
1233 return 0 unless @$first == @$second;
1234 for (my $i = 0; $i < @$first; $i++) {
1235 return 0 if $first->[$i] ne $second->[$i];
1240 For multilevel structures, you may wish to use an approach more
1241 like this one. It uses the CPAN module FreezeThaw:
1243 use FreezeThaw qw(cmpStr);
1244 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1246 printf "a and b contain %s arrays\n",
1247 cmpStr(\@a, \@b) == 0
1251 This approach also works for comparing hashes. Here
1252 we'll demonstrate two different answers:
1254 use FreezeThaw qw(cmpStr cmpStrHard);
1256 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1260 printf "a and b contain %s hashes\n",
1261 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1263 printf "a and b contain %s hashes\n",
1264 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1267 The first reports that both those the hashes contain the same data,
1268 while the second reports that they do not. Which you prefer is left as
1269 an exercise to the reader.
1271 =head2 How do I find the first array element for which a condition is true?
1273 You can use this if you care about the index:
1275 for ($i= 0; $i < @array; $i++) {
1276 if ($array[$i] eq "Waldo") {
1282 Now C<$found_index> has what you want.
1284 =head2 How do I handle linked lists?
1286 In general, you usually don't need a linked list in Perl, since with
1287 regular arrays, you can push and pop or shift and unshift at either end,
1288 or you can use splice to add and/or remove arbitrary number of elements at
1289 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1290 dynamic arrays. In the absence of shifts and pops, push in general
1291 needs to reallocate on the order every log(N) times, and unshift will
1292 need to copy pointers each time.
1294 If you really, really wanted, you could use structures as described in
1295 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1296 to do. For example, imagine a list node like this:
1303 You could walk the list this way:
1306 for ($node = $head; $node; $node = $node->{LINK}) {
1307 print $node->{VALUE}, " ";
1311 You could add to the list this way:
1314 $tail = append($head, 1); # grow a new head
1315 for $value ( 2 .. 10 ) {
1316 $tail = append($tail, $value);
1320 my($list, $value) = @_;
1321 my $node = { VALUE => $value };
1323 $node->{LINK} = $list->{LINK};
1324 $list->{LINK} = $node;
1326 $_[0] = $node; # replace caller's version
1331 But again, Perl's built-in are virtually always good enough.
1333 =head2 How do I handle circular lists?
1335 Circular lists could be handled in the traditional fashion with linked
1336 lists, or you could just do something like this with an array:
1338 unshift(@array, pop(@array)); # the last shall be first
1339 push(@array, shift(@array)); # and vice versa
1341 =head2 How do I shuffle an array randomly?
1343 If you either have Perl 5.8.0 or later installed, or if you have
1344 Scalar-List-Utils 1.03 or later installed, you can say:
1346 use List::Util 'shuffle';
1348 @shuffled = shuffle(@list);
1350 If not, you can use a Fisher-Yates shuffle.
1352 sub fisher_yates_shuffle {
1353 my $deck = shift; # $deck is a reference to an array
1356 my $j = int rand ($i+1);
1357 @$deck[$i,$j] = @$deck[$j,$i];
1361 # shuffle my mpeg collection
1363 my @mpeg = <audio/*/*.mp3>;
1364 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1367 Note that the above implementation shuffles an array in place,
1368 unlike the List::Util::shuffle() which takes a list and returns
1369 a new shuffled list.
1371 You've probably seen shuffling algorithms that work using splice,
1372 randomly picking another element to swap the current element with
1376 @old = 1 .. 10; # just a demo
1378 push(@new, splice(@old, rand @old, 1));
1381 This is bad because splice is already O(N), and since you do it N times,
1382 you just invented a quadratic algorithm; that is, O(N**2). This does
1383 not scale, although Perl is so efficient that you probably won't notice
1384 this until you have rather largish arrays.
1386 =head2 How do I process/modify each element of an array?
1388 Use C<for>/C<foreach>:
1391 s/foo/bar/; # change that word
1392 y/XZ/ZX/; # swap those letters
1395 Here's another; let's compute spherical volumes:
1397 for (@volumes = @radii) { # @volumes has changed parts
1399 $_ *= (4/3) * 3.14159; # this will be constant folded
1402 If you want to do the same thing to modify the values of the
1403 hash, you can use the C<values> function. As of Perl 5.6
1404 the values are not copied, so if you modify $orbit (in this
1405 case), you modify the value.
1407 for $orbit ( values %orbits ) {
1408 ($orbit **= 3) *= (4/3) * 3.14159;
1411 Prior to perl 5.6 C<values> returned copies of the values,
1412 so older perl code often contains constructions such as
1413 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1414 the hash is to be modified.
1416 =head2 How do I select a random element from an array?
1418 Use the rand() function (see L<perlfunc/rand>):
1420 # at the top of the program:
1421 srand; # not needed for 5.004 and later
1424 $index = rand @array;
1425 $element = $array[$index];
1427 Make sure you I<only call srand once per program, if then>.
1428 If you are calling it more than once (such as before each
1429 call to rand), you're almost certainly doing something wrong.
1431 =head2 How do I permute N elements of a list?
1433 Here's a little program that generates all permutations
1434 of all the words on each line of input. The algorithm embodied
1435 in the permute() function should work on any list:
1438 # tsc-permute: permute each word of input
1439 permute([split], []);
1441 my @items = @{ $_[0] };
1442 my @perms = @{ $_[1] };
1446 my(@newitems,@newperms,$i);
1447 foreach $i (0 .. $#items) {
1450 unshift(@newperms, splice(@newitems, $i, 1));
1451 permute([@newitems], [@newperms]);
1456 Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
1457 module from CPAN runs at least an order of magnitude faster. If you don't
1458 have a C compiler (or a binary distribution of Algorithm::Permute), then
1459 you can use List::Permutor which is written in pure Perl, and is still
1460 several times faster than the algorithm above.
1462 =head2 How do I sort an array by (anything)?
1464 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1466 @list = sort { $a <=> $b } @list;
1468 The default sort function is cmp, string comparison, which would
1469 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1470 the numerical comparison operator.
1472 If you have a complicated function needed to pull out the part you
1473 want to sort on, then don't do it inside the sort function. Pull it
1474 out first, because the sort BLOCK can be called many times for the
1475 same element. Here's an example of how to pull out the first word
1476 after the first number on each item, and then sort those words
1481 ($item) = /\d+\s*(\S+)/;
1482 push @idx, uc($item);
1484 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1486 which could also be written this way, using a trick
1487 that's come to be known as the Schwartzian Transform:
1489 @sorted = map { $_->[0] }
1490 sort { $a->[1] cmp $b->[1] }
1491 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1493 If you need to sort on several fields, the following paradigm is useful.
1495 @sorted = sort { field1($a) <=> field1($b) ||
1496 field2($a) cmp field2($b) ||
1497 field3($a) cmp field3($b)
1500 This can be conveniently combined with precalculation of keys as given
1503 See the F<sort> artitcle article in the "Far More Than You Ever Wanted
1504 To Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz for
1505 more about this approach.
1507 See also the question below on sorting hashes.
1509 =head2 How do I manipulate arrays of bits?
1511 Use pack() and unpack(), or else vec() and the bitwise operations.
1513 For example, this sets $vec to have bit N set if $ints[N] was set:
1516 foreach(@ints) { vec($vec,$_,1) = 1 }
1518 Here's how, given a vector in $vec, you can
1519 get those bits into your @ints array:
1521 sub bitvec_to_list {
1524 # Find null-byte density then select best algorithm
1525 if ($vec =~ tr/\0// / length $vec > 0.95) {
1528 # This method is faster with mostly null-bytes
1529 while($vec =~ /[^\0]/g ) {
1530 $i = -9 + 8 * pos $vec;
1531 push @ints, $i if vec($vec, ++$i, 1);
1532 push @ints, $i if vec($vec, ++$i, 1);
1533 push @ints, $i if vec($vec, ++$i, 1);
1534 push @ints, $i if vec($vec, ++$i, 1);
1535 push @ints, $i if vec($vec, ++$i, 1);
1536 push @ints, $i if vec($vec, ++$i, 1);
1537 push @ints, $i if vec($vec, ++$i, 1);
1538 push @ints, $i if vec($vec, ++$i, 1);
1541 # This method is a fast general algorithm
1543 my $bits = unpack "b*", $vec;
1544 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1545 push @ints, pos $bits while($bits =~ /1/g);
1550 This method gets faster the more sparse the bit vector is.
1551 (Courtesy of Tim Bunce and Winfried Koenig.)
1553 You can make the while loop a lot shorter with this suggestion
1554 from Benjamin Goldberg:
1556 while($vec =~ /[^\0]+/g ) {
1557 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1560 Or use the CPAN module Bit::Vector:
1562 $vector = Bit::Vector->new($num_of_bits);
1563 $vector->Index_List_Store(@ints);
1564 @ints = $vector->Index_List_Read();
1566 Bit::Vector provides efficient methods for bit vector, sets of small integers
1569 Here's a more extensive illustration using vec():
1572 $vector = "\xff\x0f\xef\xfe";
1573 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1574 unpack("N", $vector), "\n";
1575 $is_set = vec($vector, 23, 1);
1576 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1594 my ($offset, $width, $value) = @_;
1596 vec($vector, $offset, $width) = $value;
1597 print "offset=$offset width=$width value=$value\n";
1603 my $bits = unpack("b*", $vector);
1607 print "vector length in bytes: ", length($vector), "\n";
1608 @bytes = unpack("A8" x length($vector), $bits);
1609 print "bits are: @bytes\n\n";
1612 =head2 Why does defined() return true on empty arrays and hashes?
1614 The short story is that you should probably only use defined on scalars or
1615 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1616 in the 5.004 release or later of Perl for more detail.
1618 =head1 Data: Hashes (Associative Arrays)
1620 =head2 How do I process an entire hash?
1622 Use the each() function (see L<perlfunc/each>) if you don't care
1623 whether it's sorted:
1625 while ( ($key, $value) = each %hash) {
1626 print "$key = $value\n";
1629 If you want it sorted, you'll have to use foreach() on the result of
1630 sorting the keys as shown in an earlier question.
1632 =head2 What happens if I add or remove keys from a hash while iterating over it?
1636 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1637 iterating over it. In Perl 5 you can delete from it, but you still
1638 can't add to it, because that might cause a doubling of the hash table,
1639 in which half the entries get copied up to the new top half of the
1640 table, at which point you've totally bamboozled the iterator code.
1641 Even if the table doesn't double, there's no telling whether your new
1642 entry will be inserted before or after the current iterator position.
1644 Either treasure up your changes and make them after the iterator finishes
1645 or use keys to fetch all the old keys at once, and iterate over the list
1648 =head2 How do I look up a hash element by value?
1650 Create a reverse hash:
1652 %by_value = reverse %by_key;
1653 $key = $by_value{$value};
1655 That's not particularly efficient. It would be more space-efficient
1658 while (($key, $value) = each %by_key) {
1659 $by_value{$value} = $key;
1662 If your hash could have repeated values, the methods above will only find
1663 one of the associated keys. This may or may not worry you. If it does
1664 worry you, you can always reverse the hash into a hash of arrays instead:
1666 while (($key, $value) = each %by_key) {
1667 push @{$key_list_by_value{$value}}, $key;
1670 =head2 How can I know how many entries are in a hash?
1672 If you mean how many keys, then all you have to do is
1673 use the keys() function in a scalar context:
1675 $num_keys = keys %hash;
1677 The keys() function also resets the iterator, which means that you may
1678 see strange results if you use this between uses of other hash operators
1681 =head2 How do I sort a hash (optionally by value instead of key)?
1683 Internally, hashes are stored in a way that prevents you from imposing
1684 an order on key-value pairs. Instead, you have to sort a list of the
1687 @keys = sort keys %hash; # sorted by key
1689 $hash{$a} cmp $hash{$b}
1690 } keys %hash; # and by value
1692 Here we'll do a reverse numeric sort by value, and if two keys are
1693 identical, sort by length of key, or if that fails, by straight ASCII
1694 comparison of the keys (well, possibly modified by your locale--see
1698 $hash{$b} <=> $hash{$a}
1700 length($b) <=> length($a)
1705 =head2 How can I always keep my hash sorted?
1707 You can look into using the DB_File module and tie() using the
1708 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1709 The Tie::IxHash module from CPAN might also be instructive.
1711 =head2 What's the difference between "delete" and "undef" with hashes?
1713 Hashes contain pairs of scalars: the first is the key, the
1714 second is the value. The key will be coerced to a string,
1715 although the value can be any kind of scalar: string,
1716 number, or reference. If a key $key is present in
1717 %hash, C<exists($hash{$key})> will return true. The value
1718 for a given key can be C<undef>, in which case
1719 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1720 will return true. This corresponds to (C<$key>, C<undef>)
1723 Pictures help... here's the %hash table:
1733 And these conditions hold
1737 defined $hash{'d'} is true
1738 defined $hash{'a'} is true
1739 exists $hash{'a'} is true (Perl5 only)
1740 grep ($_ eq 'a', keys %hash) is true
1746 your table now reads:
1757 and these conditions now hold; changes in caps:
1761 defined $hash{'d'} is true
1762 defined $hash{'a'} is FALSE
1763 exists $hash{'a'} is true (Perl5 only)
1764 grep ($_ eq 'a', keys %hash) is true
1766 Notice the last two: you have an undef value, but a defined key!
1772 your table now reads:
1781 and these conditions now hold; changes in caps:
1785 defined $hash{'d'} is true
1786 defined $hash{'a'} is false
1787 exists $hash{'a'} is FALSE (Perl5 only)
1788 grep ($_ eq 'a', keys %hash) is FALSE
1790 See, the whole entry is gone!
1792 =head2 Why don't my tied hashes make the defined/exists distinction?
1794 This depends on the tied hash's implementation of EXISTS().
1795 For example, there isn't the concept of undef with hashes
1796 that are tied to DBM* files. It also means that exists() and
1797 defined() do the same thing with a DBM* file, and what they
1798 end up doing is not what they do with ordinary hashes.
1800 =head2 How do I reset an each() operation part-way through?
1802 Using C<keys %hash> in scalar context returns the number of keys in
1803 the hash I<and> resets the iterator associated with the hash. You may
1804 need to do this if you use C<last> to exit a loop early so that when you
1805 re-enter it, the hash iterator has been reset.
1807 =head2 How can I get the unique keys from two hashes?
1809 First you extract the keys from the hashes into lists, then solve
1810 the "removing duplicates" problem described above. For example:
1813 for $element (keys(%foo), keys(%bar)) {
1820 @uniq = keys %{{%foo,%bar}};
1822 Or if you really want to save space:
1825 while (defined ($key = each %foo)) {
1828 while (defined ($key = each %bar)) {
1833 =head2 How can I store a multidimensional array in a DBM file?
1835 Either stringify the structure yourself (no fun), or else
1836 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1837 it on top of either DB_File or GDBM_File.
1839 =head2 How can I make my hash remember the order I put elements into it?
1841 Use the Tie::IxHash from CPAN.
1844 tie(%myhash, Tie::IxHash);
1845 for ($i=0; $i<20; $i++) {
1848 @keys = keys %myhash;
1849 # @keys = (0,1,2,3,...)
1851 =head2 Why does passing a subroutine an undefined element in a hash create it?
1853 If you say something like:
1855 somefunc($hash{"nonesuch key here"});
1857 Then that element "autovivifies"; that is, it springs into existence
1858 whether you store something there or not. That's because functions
1859 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1860 it has to be ready to write it back into the caller's version.
1862 This has been fixed as of Perl5.004.
1864 Normally, merely accessing a key's value for a nonexistent key does
1865 I<not> cause that key to be forever there. This is different than
1868 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1870 Usually a hash ref, perhaps like this:
1875 TITLE => "deputy peon",
1878 PALS => [ "Norbert", "Rhys", "Phineas"],
1881 References are documented in L<perlref> and the upcoming L<perlreftut>.
1882 Examples of complex data structures are given in L<perldsc> and
1883 L<perllol>. Examples of structures and object-oriented classes are
1886 =head2 How can I use a reference as a hash key?
1888 You can't do this directly, but you could use the standard Tie::RefHash
1889 module distributed with Perl.
1893 =head2 How do I handle binary data correctly?
1895 Perl is binary clean, so this shouldn't be a problem. For example,
1896 this works fine (assuming the files are found):
1898 if (`cat /vmunix` =~ /gzip/) {
1899 print "Your kernel is GNU-zip enabled!\n";
1902 On less elegant (read: Byzantine) systems, however, you have
1903 to play tedious games with "text" versus "binary" files. See
1904 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1905 systems are curses out of Microsoft, who seem to be committed to putting
1906 the backward into backward compatibility.
1908 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1910 If you want to deal with multibyte characters, however, there are
1911 some gotchas. See the section on Regular Expressions.
1913 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1915 Assuming that you don't care about IEEE notations like "NaN" or
1916 "Infinity", you probably just want to use a regular expression.
1918 if (/\D/) { print "has nondigits\n" }
1919 if (/^\d+$/) { print "is a whole number\n" }
1920 if (/^-?\d+$/) { print "is an integer\n" }
1921 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1922 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1923 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
1924 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1925 { print "a C float\n" }
1927 You can also use the L<Data::Types|Data::Types> module on
1928 the CPAN, which exports functions that validate data types
1929 using these and other regular expressions.
1931 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1932 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1933 wrapper function for more convenient access. This function takes
1934 a string and returns the number it found, or C<undef> for input that
1935 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1936 if you just want to say, ``Is this a float?''
1939 use POSIX qw(strtod);
1944 my($num, $unparsed) = strtod($str);
1945 if (($str eq '') || ($unparsed != 0) || $!) {
1952 sub is_numeric { defined getnum($_[0]) }
1954 Or you could check out the L<String::Scanf|String::Scanf> module on the CPAN
1955 instead. The POSIX module (part of the standard Perl distribution) provides
1956 the C<strtod> and C<strtol> for converting strings to double and longs,
1959 =head2 How do I keep persistent data across program calls?
1961 For some specific applications, you can use one of the DBM modules.
1962 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
1963 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
1964 of the standard distribution. Here's one example using Storable's C<store>
1965 and C<retrieve> functions:
1968 store(\%hash, "filename");
1971 $href = retrieve("filename"); # by ref
1972 %hash = %{ retrieve("filename") }; # direct to hash
1974 =head2 How do I print out or copy a recursive data structure?
1976 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1977 for printing out data structures. The Storable module, found on CPAN,
1978 provides a function called C<dclone> that recursively copies its argument.
1980 use Storable qw(dclone);
1983 Where $r1 can be a reference to any kind of data structure you'd like.
1984 It will be deeply copied. Because C<dclone> takes and returns references,
1985 you'd have to add extra punctuation if you had a hash of arrays that
1988 %newhash = %{ dclone(\%oldhash) };
1990 =head2 How do I define methods for every class/object?
1992 Use the UNIVERSAL class (see L<UNIVERSAL>).
1994 =head2 How do I verify a credit card checksum?
1996 Get the Business::CreditCard module from CPAN.
1998 =head2 How do I pack arrays of doubles or floats for XS code?
2000 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2001 If you're doing a lot of float or double processing, consider using
2002 the PDL module from CPAN instead--it makes number-crunching easy.
2004 =head1 AUTHOR AND COPYRIGHT
2006 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
2007 All rights reserved.
2009 This documentation is free; you can redistribute it and/or modify it
2010 under the same terms as Perl itself.
2012 Irrespective of its distribution, all code examples in this file
2013 are hereby placed into the public domain. You are permitted and
2014 encouraged to use this code in your own programs for fun
2015 or for profit as you see fit. A simple comment in the code giving
2016 credit would be courteous but is not required.