3 perlfaq4 - Data Manipulation ($Revision: 1.25 $, $Date: 2002/05/30 07:04:25 $)
7 This section of the FAQ answers questions related to manipulating
8 numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
12 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
14 The infinite set that a mathematician thinks of as the real numbers can
15 only be approximated on a computer, since the computer only has a finite
16 number of bits to store an infinite number of, um, numbers.
18 Internally, your computer represents floating-point numbers in binary.
19 Floating-point numbers read in from a file or appearing as literals
20 in your program are converted from their decimal floating-point
21 representation (eg, 19.95) to an internal binary representation.
23 However, 19.95 can't be precisely represented as a binary
24 floating-point number, just like 1/3 can't be exactly represented as a
25 decimal floating-point number. The computer's binary representation
26 of 19.95, therefore, isn't exactly 19.95.
28 When a floating-point number gets printed, the binary floating-point
29 representation is converted back to decimal. These decimal numbers
30 are displayed in either the format you specify with printf(), or the
31 current output format for numbers. (See L<perlvar/"$#"> 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. For balanced expressions using C<(>, C<{>, C<[>
579 or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
580 L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
582 If you are serious about writing a parser, there are a number of
583 modules or oddities that will make your life a lot easier. There are
584 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
585 and the byacc program. Starting from perl 5.8 the Text::Balanced
586 is part of the standard distribution.
588 One simple destructive, inside-out approach that you might try is to
589 pull out the smallest nesting parts one at a time:
591 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
592 # do something with $1
595 A more complicated and sneaky approach is to make Perl's regular
596 expression engine do it for you. This is courtesy Dean Inada, and
597 rather has the nature of an Obfuscated Perl Contest entry, but it
600 # $_ contains the string to parse
601 # BEGIN and END are the opening and closing markers for the
606 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
607 @$ = (eval{/$re/},$@!~/unmatched/i);
608 print join("\n",@$[0..$#$]) if( $$[-1] );
610 =head2 How do I reverse a string?
612 Use reverse() in scalar context, as documented in
615 $reversed = reverse $string;
617 =head2 How do I expand tabs in a string?
619 You can do it yourself:
621 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
623 Or you can just use the Text::Tabs module (part of the standard Perl
627 @expanded_lines = expand(@lines_with_tabs);
629 =head2 How do I reformat a paragraph?
631 Use Text::Wrap (part of the standard Perl distribution):
634 print wrap("\t", ' ', @paragraphs);
636 The paragraphs you give to Text::Wrap should not contain embedded
637 newlines. Text::Wrap doesn't justify the lines (flush-right).
639 Or use the CPAN module Text::Autoformat. Formatting files can be easily
640 done by making a shell alias, like so:
642 alias fmt="perl -i -MText::Autoformat -n0777 \
643 -e 'print autoformat $_, {all=>1}' $*"
645 See the documentation for Text::Autoformat to appreciate its many
648 =head2 How can I access/change the first N letters of a string?
650 There are many ways. If you just want to grab a copy, use
653 $first_byte = substr($a, 0, 1);
655 If you want to modify part of a string, the simplest way is often to
656 use substr() as an lvalue:
658 substr($a, 0, 3) = "Tom";
660 Although those with a pattern matching kind of thought process will
665 =head2 How do I change the Nth occurrence of something?
667 You have to keep track of N yourself. For example, let's say you want
668 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
669 C<"whosoever"> or C<"whomsoever">, case insensitively. These
670 all assume that $_ contains the string to be altered.
674 ++$count == 5 # is it the 5th?
675 ? "${2}soever" # yes, swap
676 : $1 # renege and leave it there
679 In the more general case, you can use the C</g> modifier in a C<while>
680 loop, keeping count of matches.
684 $_ = "One fish two fish red fish blue fish";
685 while (/(\w+)\s+fish\b/gi) {
686 if (++$count == $WANT) {
687 print "The third fish is a $1 one.\n";
691 That prints out: C<"The third fish is a red one."> You can also use a
692 repetition count and repeated pattern like this:
694 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
696 =head2 How can I count the number of occurrences of a substring within a string?
698 There are a number of ways, with varying efficiency. If you want a
699 count of a certain single character (X) within a string, you can use the
700 C<tr///> function like so:
702 $string = "ThisXlineXhasXsomeXx'sXinXit";
703 $count = ($string =~ tr/X//);
704 print "There are $count X characters in the string";
706 This is fine if you are just looking for a single character. However,
707 if you are trying to count multiple character substrings within a
708 larger string, C<tr///> won't work. What you can do is wrap a while()
709 loop around a global pattern match. For example, let's count negative
712 $string = "-9 55 48 -2 23 -76 4 14 -44";
713 while ($string =~ /-\d+/g) { $count++ }
714 print "There are $count negative numbers in the string";
716 Another version uses a global match in list context, then assigns the
717 result to a scalar, producing a count of the number of matches.
719 $count = () = $string =~ /-\d+/g;
721 =head2 How do I capitalize all the words on one line?
723 To make the first letter of each word upper case:
725 $line =~ s/\b(\w)/\U$1/g;
727 This has the strange effect of turning "C<don't do it>" into "C<Don'T
728 Do It>". Sometimes you might want this. Other times you might need a
729 more thorough solution (Suggested by brian d foy):
732 (^\w) #at the beginning of the line
734 (\s\w) #preceded by whitespace
737 $string =~ /([\w']+)/\u\L$1/g;
739 To make the whole line upper case:
743 To force each word to be lower case, with the first letter upper case:
745 $line =~ s/(\w+)/\u\L$1/g;
747 You can (and probably should) enable locale awareness of those
748 characters by placing a C<use locale> pragma in your program.
749 See L<perllocale> for endless details on locales.
751 This is sometimes referred to as putting something into "title
752 case", but that's not quite accurate. Consider the proper
753 capitalization of the movie I<Dr. Strangelove or: How I Learned to
754 Stop Worrying and Love the Bomb>, for example.
756 =head2 How can I split a [character] delimited string except when inside
757 [character]? (Comma-separated files)
759 Take the example case of trying to split a string that is comma-separated
760 into its different fields. (We'll pretend you said comma-separated, not
761 comma-delimited, which is different and almost never what you mean.) You
762 can't use C<split(/,/)> because you shouldn't split if the comma is inside
763 quotes. For example, take a data line like this:
765 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
767 Due to the restriction of the quotes, this is a fairly complex
768 problem. Thankfully, we have Jeffrey Friedl, author of a highly
769 recommended book on regular expressions, to handle these for us. He
770 suggests (assuming your string is contained in $text):
773 push(@new, $+) while $text =~ m{
774 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
778 push(@new, undef) if substr($text,-1,1) eq ',';
780 If you want to represent quotation marks inside a
781 quotation-mark-delimited field, escape them with backslashes (eg,
782 C<"like \"this\"">. Unescaping them is a task addressed earlier in
785 Alternatively, the Text::ParseWords module (part of the standard Perl
786 distribution) lets you say:
788 use Text::ParseWords;
789 @new = quotewords(",", 0, $text);
791 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
793 =head2 How do I strip blank space from the beginning/end of a string?
795 Although the simplest approach would seem to be
797 $string =~ s/^\s*(.*?)\s*$/$1/;
799 not only is this unnecessarily slow and destructive, it also fails with
800 embedded newlines. It is much faster to do this operation in two steps:
805 Or more nicely written as:
812 This idiom takes advantage of the C<foreach> loop's aliasing
813 behavior to factor out common code. You can do this
814 on several strings at once, or arrays, or even the
815 values of a hash if you use a slice:
817 # trim whitespace in the scalar, the array,
818 # and all the values in the hash
819 foreach ($scalar, @array, @hash{keys %hash}) {
824 =head2 How do I pad a string with blanks or pad a number with zeroes?
826 (This answer contributed by Uri Guttman, with kibitzing from
829 In the following examples, C<$pad_len> is the length to which you wish
830 to pad the string, C<$text> or C<$num> contains the string to be padded,
831 and C<$pad_char> contains the padding character. You can use a single
832 character string constant instead of the C<$pad_char> variable if you
833 know what it is in advance. And in the same way you can use an integer in
834 place of C<$pad_len> if you know the pad length in advance.
836 The simplest method uses the C<sprintf> function. It can pad on the left
837 or right with blanks and on the left with zeroes and it will not
838 truncate the result. The C<pack> function can only pad strings on the
839 right with blanks and it will truncate the result to a maximum length of
842 # Left padding a string with blanks (no truncation):
843 $padded = sprintf("%${pad_len}s", $text);
845 # Right padding a string with blanks (no truncation):
846 $padded = sprintf("%-${pad_len}s", $text);
848 # Left padding a number with 0 (no truncation):
849 $padded = sprintf("%0${pad_len}d", $num);
851 # Right padding a string with blanks using pack (will truncate):
852 $padded = pack("A$pad_len",$text);
854 If you need to pad with a character other than blank or zero you can use
855 one of the following methods. They all generate a pad string with the
856 C<x> operator and combine that with C<$text>. These methods do
857 not truncate C<$text>.
859 Left and right padding with any character, creating a new string:
861 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
862 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
864 Left and right padding with any character, modifying C<$text> directly:
866 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
867 $text .= $pad_char x ( $pad_len - length( $text ) );
869 =head2 How do I extract selected columns from a string?
871 Use substr() or unpack(), both documented in L<perlfunc>.
872 If you prefer thinking in terms of columns instead of widths,
873 you can use this kind of thing:
875 # determine the unpack format needed to split Linux ps output
876 # arguments are cut columns
877 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
883 for my $place (@positions) {
884 $template .= "A" . ($place - $lastpos) . " ";
891 =head2 How do I find the soundex value of a string?
893 Use the standard Text::Soundex module distributed with Perl.
894 Before you do so, you may want to determine whether `soundex' is in
895 fact what you think it is. Knuth's soundex algorithm compresses words
896 into a small space, and so it does not necessarily distinguish between
897 two words which you might want to appear separately. For example, the
898 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
899 If Text::Soundex does not do what you are looking for, you might want
900 to consider the String::Approx module available at CPAN.
902 =head2 How can I expand variables in text strings?
904 Let's assume that you have a string like:
906 $text = 'this has a $foo in it and a $bar';
908 If those were both global variables, then this would
911 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
913 But since they are probably lexicals, or at least, they could
914 be, you'd have to do this:
916 $text =~ s/(\$\w+)/$1/eeg;
917 die if $@; # needed /ee, not /e
919 It's probably better in the general case to treat those
920 variables as entries in some special hash. For example:
926 $text =~ s/\$(\w+)/$user_defs{$1}/g;
928 See also ``How do I expand function calls in a string?'' in this section
931 =head2 What's wrong with always quoting "$vars"?
933 The problem is that those double-quotes force stringification--
934 coercing numbers and references into strings--even when you
935 don't want them to be strings. Think of it this way: double-quote
936 expansion is used to produce new strings. If you already
937 have a string, why do you need more?
939 If you get used to writing odd things like these:
943 somefunc("$var"); # BAD
945 You'll be in trouble. Those should (in 99.8% of the cases) be
946 the simpler and more direct:
952 Otherwise, besides slowing you down, you're going to break code when
953 the thing in the scalar is actually neither a string nor a number, but
959 my $oref = "$aref"; # WRONG
962 You can also get into subtle problems on those few operations in Perl
963 that actually do care about the difference between a string and a
964 number, such as the magical C<++> autoincrement operator or the
967 Stringification also destroys arrays.
970 print "@lines"; # WRONG - extra blanks
971 print @lines; # right
973 =head2 Why don't my <<HERE documents work?
975 Check for these three things:
979 =item 1. There must be no space after the << part.
981 =item 2. There (probably) should be a semicolon at the end.
983 =item 3. You can't (easily) have any space in front of the tag.
987 If you want to indent the text in the here document, you
991 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
996 But the HERE_TARGET must still be flush against the margin.
997 If you want that indented also, you'll have to quote
1000 ($quote = <<' FINIS') =~ s/^\s+//gm;
1001 ...we will have peace, when you and all your works have
1002 perished--and the works of your dark master to whom you
1003 would deliver us. You are a liar, Saruman, and a corrupter
1004 of men's hearts. --Theoden in /usr/src/perl/taint.c
1006 $quote =~ s/\s+--/\n--/;
1008 A nice general-purpose fixer-upper function for indented here documents
1009 follows. It expects to be called with a here document as its argument.
1010 It looks to see whether each line begins with a common substring, and
1011 if so, strips that substring off. Otherwise, it takes the amount of leading
1012 whitespace found on the first line and removes that much off each
1017 my ($white, $leader); # common whitespace and common leading string
1018 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1019 ($white, $leader) = ($2, quotemeta($1));
1021 ($white, $leader) = (/^(\s+)/, '');
1023 s/^\s*?$leader(?:$white)?//gm;
1027 This works with leading special strings, dynamically determined:
1029 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1032 @@@ SAVEI32(runlevel);
1034 @@@ while ( op = (*op->op_ppaddr)() );
1038 MAIN_INTERPRETER_LOOP
1040 Or with a fixed amount of leading whitespace, with remaining
1041 indentation correctly preserved:
1043 $poem = fix<<EVER_ON_AND_ON;
1044 Now far ahead the Road has gone,
1045 And I must follow, if I can,
1046 Pursuing it with eager feet,
1047 Until it joins some larger way
1048 Where many paths and errands meet.
1049 And whither then? I cannot say.
1050 --Bilbo in /usr/src/perl/pp_ctl.c
1055 =head2 What is the difference between a list and an array?
1057 An array has a changeable length. A list does not. An array is something
1058 you can push or pop, while a list is a set of values. Some people make
1059 the distinction that a list is a value while an array is a variable.
1060 Subroutines are passed and return lists, you put things into list
1061 context, you initialize arrays with lists, and you foreach() across
1062 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1063 in scalar context behave like the number of elements in them, subroutines
1064 access their arguments through the array C<@_>, and push/pop/shift only work
1067 As a side note, there's no such thing as a list in scalar context.
1070 $scalar = (2, 5, 7, 9);
1072 you're using the comma operator in scalar context, so it uses the scalar
1073 comma operator. There never was a list there at all! This causes the
1074 last value to be returned: 9.
1076 =head2 What is the difference between $array[1] and @array[1]?
1078 The former is a scalar value; the latter an array slice, making
1079 it a list with one (scalar) value. You should use $ when you want a
1080 scalar value (most of the time) and @ when you want a list with one
1081 scalar value in it (very, very rarely; nearly never, in fact).
1083 Sometimes it doesn't make a difference, but sometimes it does.
1084 For example, compare:
1086 $good[0] = `some program that outputs several lines`;
1090 @bad[0] = `same program that outputs several lines`;
1092 The C<use warnings> pragma and the B<-w> flag will warn you about these
1095 =head2 How can I remove duplicate elements from a list or array?
1097 There are several possible ways, depending on whether the array is
1098 ordered and whether you wish to preserve the ordering.
1104 If @in is sorted, and you want @out to be sorted:
1105 (this assumes all true values in the array)
1107 $prev = "not equal to $in[0]";
1108 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1110 This is nice in that it doesn't use much extra memory, simulating
1111 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1112 guarantees that the expression is true (so that grep picks it up)
1113 even if the $_ is 0, "", or undef.
1117 If you don't know whether @in is sorted:
1120 @out = grep(!$saw{$_}++, @in);
1124 Like (b), but @in contains only small integers:
1126 @out = grep(!$saw[$_]++, @in);
1130 A way to do (b) without any loops or greps:
1134 @out = sort keys %saw; # remove sort if undesired
1138 Like (d), but @in contains only small positive integers:
1142 @out = grep {defined} @ary;
1146 But perhaps you should have been using a hash all along, eh?
1148 =head2 How can I tell whether a certain element is contained in a list or array?
1150 Hearing the word "in" is an I<in>dication that you probably should have
1151 used a hash, not a list or array, to store your data. Hashes are
1152 designed to answer this question quickly and efficiently. Arrays aren't.
1154 That being said, there are several ways to approach this. If you
1155 are going to make this query many times over arbitrary string values,
1156 the fastest way is probably to invert the original array and maintain a
1157 hash whose keys are the first array's values.
1159 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1161 for (@blues) { $is_blue{$_} = 1 }
1163 Now you can check whether $is_blue{$some_color}. It might have been a
1164 good idea to keep the blues all in a hash in the first place.
1166 If the values are all small integers, you could use a simple indexed
1167 array. This kind of an array will take up less space:
1169 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1170 @is_tiny_prime = ();
1171 for (@primes) { $is_tiny_prime[$_] = 1 }
1172 # or simply @istiny_prime[@primes] = (1) x @primes;
1174 Now you check whether $is_tiny_prime[$some_number].
1176 If the values in question are integers instead of strings, you can save
1177 quite a lot of space by using bit strings instead:
1179 @articles = ( 1..10, 150..2000, 2017 );
1181 for (@articles) { vec($read,$_,1) = 1 }
1183 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1187 ($is_there) = grep $_ eq $whatever, @array;
1191 ($is_there) = grep /$whatever/, @array;
1193 These are slow (checks every element even if the first matches),
1194 inefficient (same reason), and potentially buggy (what if there are
1195 regex characters in $whatever?). If you're only testing once, then
1199 foreach $elt (@array) {
1200 if ($elt eq $elt_to_find) {
1205 if ($is_there) { ... }
1207 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1209 Use a hash. Here's code to do both and more. It assumes that
1210 each element is unique in a given array:
1212 @union = @intersection = @difference = ();
1214 foreach $element (@array1, @array2) { $count{$element}++ }
1215 foreach $element (keys %count) {
1216 push @union, $element;
1217 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1220 Note that this is the I<symmetric difference>, that is, all elements in
1221 either A or in B but not in both. Think of it as an xor operation.
1223 =head2 How do I test whether two arrays or hashes are equal?
1225 The following code works for single-level arrays. It uses a stringwise
1226 comparison, and does not distinguish defined versus undefined empty
1227 strings. Modify if you have other needs.
1229 $are_equal = compare_arrays(\@frogs, \@toads);
1231 sub compare_arrays {
1232 my ($first, $second) = @_;
1233 no warnings; # silence spurious -w undef complaints
1234 return 0 unless @$first == @$second;
1235 for (my $i = 0; $i < @$first; $i++) {
1236 return 0 if $first->[$i] ne $second->[$i];
1241 For multilevel structures, you may wish to use an approach more
1242 like this one. It uses the CPAN module FreezeThaw:
1244 use FreezeThaw qw(cmpStr);
1245 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1247 printf "a and b contain %s arrays\n",
1248 cmpStr(\@a, \@b) == 0
1252 This approach also works for comparing hashes. Here
1253 we'll demonstrate two different answers:
1255 use FreezeThaw qw(cmpStr cmpStrHard);
1257 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1261 printf "a and b contain %s hashes\n",
1262 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1264 printf "a and b contain %s hashes\n",
1265 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1268 The first reports that both those the hashes contain the same data,
1269 while the second reports that they do not. Which you prefer is left as
1270 an exercise to the reader.
1272 =head2 How do I find the first array element for which a condition is true?
1274 You can use this if you care about the index:
1276 for ($i= 0; $i < @array; $i++) {
1277 if ($array[$i] eq "Waldo") {
1283 Now C<$found_index> has what you want.
1285 =head2 How do I handle linked lists?
1287 In general, you usually don't need a linked list in Perl, since with
1288 regular arrays, you can push and pop or shift and unshift at either end,
1289 or you can use splice to add and/or remove arbitrary number of elements at
1290 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1291 dynamic arrays. In the absence of shifts and pops, push in general
1292 needs to reallocate on the order every log(N) times, and unshift will
1293 need to copy pointers each time.
1295 If you really, really wanted, you could use structures as described in
1296 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1297 to do. For example, imagine a list node like this:
1304 You could walk the list this way:
1307 for ($node = $head; $node; $node = $node->{LINK}) {
1308 print $node->{VALUE}, " ";
1312 You could add to the list this way:
1315 $tail = append($head, 1); # grow a new head
1316 for $value ( 2 .. 10 ) {
1317 $tail = append($tail, $value);
1321 my($list, $value) = @_;
1322 my $node = { VALUE => $value };
1324 $node->{LINK} = $list->{LINK};
1325 $list->{LINK} = $node;
1327 $_[0] = $node; # replace caller's version
1332 But again, Perl's built-in are virtually always good enough.
1334 =head2 How do I handle circular lists?
1336 Circular lists could be handled in the traditional fashion with linked
1337 lists, or you could just do something like this with an array:
1339 unshift(@array, pop(@array)); # the last shall be first
1340 push(@array, shift(@array)); # and vice versa
1342 =head2 How do I shuffle an array randomly?
1344 If you either have Perl 5.8.0 or later installed, or if you have
1345 Scalar-List-Utils 1.03 or later installed, you can say:
1347 use List::Util 'shuffle';
1349 @shuffled = shuffle(@list);
1351 If not, you can use a Fisher-Yates shuffle.
1353 sub fisher_yates_shuffle {
1354 my $deck = shift; # $deck is a reference to an array
1357 my $j = int rand ($i+1);
1358 @$deck[$i,$j] = @$deck[$j,$i];
1362 # shuffle my mpeg collection
1364 my @mpeg = <audio/*/*.mp3>;
1365 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1368 Note that the above implementation shuffles an array in place,
1369 unlike the List::Util::shuffle() which takes a list and returns
1370 a new shuffled list.
1372 You've probably seen shuffling algorithms that work using splice,
1373 randomly picking another element to swap the current element with
1377 @old = 1 .. 10; # just a demo
1379 push(@new, splice(@old, rand @old, 1));
1382 This is bad because splice is already O(N), and since you do it N times,
1383 you just invented a quadratic algorithm; that is, O(N**2). This does
1384 not scale, although Perl is so efficient that you probably won't notice
1385 this until you have rather largish arrays.
1387 =head2 How do I process/modify each element of an array?
1389 Use C<for>/C<foreach>:
1392 s/foo/bar/; # change that word
1393 y/XZ/ZX/; # swap those letters
1396 Here's another; let's compute spherical volumes:
1398 for (@volumes = @radii) { # @volumes has changed parts
1400 $_ *= (4/3) * 3.14159; # this will be constant folded
1403 If you want to do the same thing to modify the values of the
1404 hash, you can use the C<values> function. As of Perl 5.6
1405 the values are not copied, so if you modify $orbit (in this
1406 case), you modify the value.
1408 for $orbit ( values %orbits ) {
1409 ($orbit **= 3) *= (4/3) * 3.14159;
1412 Prior to perl 5.6 C<values> returned copies of the values,
1413 so older perl code often contains constructions such as
1414 C<@orbits{keys %orbits}> instead of C<values %orbits> where
1415 the hash is to be modified.
1417 =head2 How do I select a random element from an array?
1419 Use the rand() function (see L<perlfunc/rand>):
1421 # at the top of the program:
1422 srand; # not needed for 5.004 and later
1425 $index = rand @array;
1426 $element = $array[$index];
1428 Make sure you I<only call srand once per program, if then>.
1429 If you are calling it more than once (such as before each
1430 call to rand), you're almost certainly doing something wrong.
1432 =head2 How do I permute N elements of a list?
1434 Here's a little program that generates all permutations
1435 of all the words on each line of input. The algorithm embodied
1436 in the permute() function should work on any list:
1439 # tsc-permute: permute each word of input
1440 permute([split], []);
1442 my @items = @{ $_[0] };
1443 my @perms = @{ $_[1] };
1447 my(@newitems,@newperms,$i);
1448 foreach $i (0 .. $#items) {
1451 unshift(@newperms, splice(@newitems, $i, 1));
1452 permute([@newitems], [@newperms]);
1457 Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
1458 module from CPAN runs at least an order of magnitude faster. If you don't
1459 have a C compiler (or a binary distribution of Algorithm::Permute), then
1460 you can use List::Permutor which is written in pure Perl, and is still
1461 several times faster than the algorithm above.
1463 =head2 How do I sort an array by (anything)?
1465 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1467 @list = sort { $a <=> $b } @list;
1469 The default sort function is cmp, string comparison, which would
1470 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1471 the numerical comparison operator.
1473 If you have a complicated function needed to pull out the part you
1474 want to sort on, then don't do it inside the sort function. Pull it
1475 out first, because the sort BLOCK can be called many times for the
1476 same element. Here's an example of how to pull out the first word
1477 after the first number on each item, and then sort those words
1482 ($item) = /\d+\s*(\S+)/;
1483 push @idx, uc($item);
1485 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1487 which could also be written this way, using a trick
1488 that's come to be known as the Schwartzian Transform:
1490 @sorted = map { $_->[0] }
1491 sort { $a->[1] cmp $b->[1] }
1492 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1494 If you need to sort on several fields, the following paradigm is useful.
1496 @sorted = sort { field1($a) <=> field1($b) ||
1497 field2($a) cmp field2($b) ||
1498 field3($a) cmp field3($b)
1501 This can be conveniently combined with precalculation of keys as given
1504 See the F<sort> artitcle article in the "Far More Than You Ever Wanted
1505 To Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz for
1506 more about this approach.
1508 See also the question below on sorting hashes.
1510 =head2 How do I manipulate arrays of bits?
1512 Use pack() and unpack(), or else vec() and the bitwise operations.
1514 For example, this sets $vec to have bit N set if $ints[N] was set:
1517 foreach(@ints) { vec($vec,$_,1) = 1 }
1519 Here's how, given a vector in $vec, you can
1520 get those bits into your @ints array:
1522 sub bitvec_to_list {
1525 # Find null-byte density then select best algorithm
1526 if ($vec =~ tr/\0// / length $vec > 0.95) {
1529 # This method is faster with mostly null-bytes
1530 while($vec =~ /[^\0]/g ) {
1531 $i = -9 + 8 * pos $vec;
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);
1539 push @ints, $i if vec($vec, ++$i, 1);
1542 # This method is a fast general algorithm
1544 my $bits = unpack "b*", $vec;
1545 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1546 push @ints, pos $bits while($bits =~ /1/g);
1551 This method gets faster the more sparse the bit vector is.
1552 (Courtesy of Tim Bunce and Winfried Koenig.)
1554 You can make the while loop a lot shorter with this suggestion
1555 from Benjamin Goldberg:
1557 while($vec =~ /[^\0]+/g ) {
1558 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1561 Or use the CPAN module Bit::Vector:
1563 $vector = Bit::Vector->new($num_of_bits);
1564 $vector->Index_List_Store(@ints);
1565 @ints = $vector->Index_List_Read();
1567 Bit::Vector provides efficient methods for bit vector, sets of small integers
1570 Here's a more extensive illustration using vec():
1573 $vector = "\xff\x0f\xef\xfe";
1574 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1575 unpack("N", $vector), "\n";
1576 $is_set = vec($vector, 23, 1);
1577 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1595 my ($offset, $width, $value) = @_;
1597 vec($vector, $offset, $width) = $value;
1598 print "offset=$offset width=$width value=$value\n";
1604 my $bits = unpack("b*", $vector);
1608 print "vector length in bytes: ", length($vector), "\n";
1609 @bytes = unpack("A8" x length($vector), $bits);
1610 print "bits are: @bytes\n\n";
1613 =head2 Why does defined() return true on empty arrays and hashes?
1615 The short story is that you should probably only use defined on scalars or
1616 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1617 in the 5.004 release or later of Perl for more detail.
1619 =head1 Data: Hashes (Associative Arrays)
1621 =head2 How do I process an entire hash?
1623 Use the each() function (see L<perlfunc/each>) if you don't care
1624 whether it's sorted:
1626 while ( ($key, $value) = each %hash) {
1627 print "$key = $value\n";
1630 If you want it sorted, you'll have to use foreach() on the result of
1631 sorting the keys as shown in an earlier question.
1633 =head2 What happens if I add or remove keys from a hash while iterating over it?
1637 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1638 iterating over it. In Perl 5 you can delete from it, but you still
1639 can't add to it, because that might cause a doubling of the hash table,
1640 in which half the entries get copied up to the new top half of the
1641 table, at which point you've totally bamboozled the iterator code.
1642 Even if the table doesn't double, there's no telling whether your new
1643 entry will be inserted before or after the current iterator position.
1645 Either treasure up your changes and make them after the iterator finishes
1646 or use keys to fetch all the old keys at once, and iterate over the list
1649 =head2 How do I look up a hash element by value?
1651 Create a reverse hash:
1653 %by_value = reverse %by_key;
1654 $key = $by_value{$value};
1656 That's not particularly efficient. It would be more space-efficient
1659 while (($key, $value) = each %by_key) {
1660 $by_value{$value} = $key;
1663 If your hash could have repeated values, the methods above will only find
1664 one of the associated keys. This may or may not worry you. If it does
1665 worry you, you can always reverse the hash into a hash of arrays instead:
1667 while (($key, $value) = each %by_key) {
1668 push @{$key_list_by_value{$value}}, $key;
1671 =head2 How can I know how many entries are in a hash?
1673 If you mean how many keys, then all you have to do is
1674 use the keys() function in a scalar context:
1676 $num_keys = keys %hash;
1678 The keys() function also resets the iterator, which means that you may
1679 see strange results if you use this between uses of other hash operators
1682 =head2 How do I sort a hash (optionally by value instead of key)?
1684 Internally, hashes are stored in a way that prevents you from imposing
1685 an order on key-value pairs. Instead, you have to sort a list of the
1688 @keys = sort keys %hash; # sorted by key
1690 $hash{$a} cmp $hash{$b}
1691 } keys %hash; # and by value
1693 Here we'll do a reverse numeric sort by value, and if two keys are
1694 identical, sort by length of key, or if that fails, by straight ASCII
1695 comparison of the keys (well, possibly modified by your locale--see
1699 $hash{$b} <=> $hash{$a}
1701 length($b) <=> length($a)
1706 =head2 How can I always keep my hash sorted?
1708 You can look into using the DB_File module and tie() using the
1709 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1710 The Tie::IxHash module from CPAN might also be instructive.
1712 =head2 What's the difference between "delete" and "undef" with hashes?
1714 Hashes contain pairs of scalars: the first is the key, the
1715 second is the value. The key will be coerced to a string,
1716 although the value can be any kind of scalar: string,
1717 number, or reference. If a key $key is present in
1718 %hash, C<exists($hash{$key})> will return true. The value
1719 for a given key can be C<undef>, in which case
1720 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1721 will return true. This corresponds to (C<$key>, C<undef>)
1724 Pictures help... here's the %hash table:
1734 And these conditions hold
1738 defined $hash{'d'} is true
1739 defined $hash{'a'} is true
1740 exists $hash{'a'} is true (Perl5 only)
1741 grep ($_ eq 'a', keys %hash) is true
1747 your table now reads:
1758 and these conditions now hold; changes in caps:
1762 defined $hash{'d'} is true
1763 defined $hash{'a'} is FALSE
1764 exists $hash{'a'} is true (Perl5 only)
1765 grep ($_ eq 'a', keys %hash) is true
1767 Notice the last two: you have an undef value, but a defined key!
1773 your table now reads:
1782 and these conditions now hold; changes in caps:
1786 defined $hash{'d'} is true
1787 defined $hash{'a'} is false
1788 exists $hash{'a'} is FALSE (Perl5 only)
1789 grep ($_ eq 'a', keys %hash) is FALSE
1791 See, the whole entry is gone!
1793 =head2 Why don't my tied hashes make the defined/exists distinction?
1795 This depends on the tied hash's implementation of EXISTS().
1796 For example, there isn't the concept of undef with hashes
1797 that are tied to DBM* files. It also means that exists() and
1798 defined() do the same thing with a DBM* file, and what they
1799 end up doing is not what they do with ordinary hashes.
1801 =head2 How do I reset an each() operation part-way through?
1803 Using C<keys %hash> in scalar context returns the number of keys in
1804 the hash I<and> resets the iterator associated with the hash. You may
1805 need to do this if you use C<last> to exit a loop early so that when you
1806 re-enter it, the hash iterator has been reset.
1808 =head2 How can I get the unique keys from two hashes?
1810 First you extract the keys from the hashes into lists, then solve
1811 the "removing duplicates" problem described above. For example:
1814 for $element (keys(%foo), keys(%bar)) {
1821 @uniq = keys %{{%foo,%bar}};
1823 Or if you really want to save space:
1826 while (defined ($key = each %foo)) {
1829 while (defined ($key = each %bar)) {
1834 =head2 How can I store a multidimensional array in a DBM file?
1836 Either stringify the structure yourself (no fun), or else
1837 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1838 it on top of either DB_File or GDBM_File.
1840 =head2 How can I make my hash remember the order I put elements into it?
1842 Use the Tie::IxHash from CPAN.
1845 tie(%myhash, Tie::IxHash);
1846 for ($i=0; $i<20; $i++) {
1849 @keys = keys %myhash;
1850 # @keys = (0,1,2,3,...)
1852 =head2 Why does passing a subroutine an undefined element in a hash create it?
1854 If you say something like:
1856 somefunc($hash{"nonesuch key here"});
1858 Then that element "autovivifies"; that is, it springs into existence
1859 whether you store something there or not. That's because functions
1860 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1861 it has to be ready to write it back into the caller's version.
1863 This has been fixed as of Perl5.004.
1865 Normally, merely accessing a key's value for a nonexistent key does
1866 I<not> cause that key to be forever there. This is different than
1869 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1871 Usually a hash ref, perhaps like this:
1876 TITLE => "deputy peon",
1879 PALS => [ "Norbert", "Rhys", "Phineas"],
1882 References are documented in L<perlref> and the upcoming L<perlreftut>.
1883 Examples of complex data structures are given in L<perldsc> and
1884 L<perllol>. Examples of structures and object-oriented classes are
1887 =head2 How can I use a reference as a hash key?
1889 You can't do this directly, but you could use the standard Tie::RefHash
1890 module distributed with Perl.
1894 =head2 How do I handle binary data correctly?
1896 Perl is binary clean, so this shouldn't be a problem. For example,
1897 this works fine (assuming the files are found):
1899 if (`cat /vmunix` =~ /gzip/) {
1900 print "Your kernel is GNU-zip enabled!\n";
1903 On less elegant (read: Byzantine) systems, however, you have
1904 to play tedious games with "text" versus "binary" files. See
1905 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1906 systems are curses out of Microsoft, who seem to be committed to putting
1907 the backward into backward compatibility.
1909 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1911 If you want to deal with multibyte characters, however, there are
1912 some gotchas. See the section on Regular Expressions.
1914 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1916 Assuming that you don't care about IEEE notations like "NaN" or
1917 "Infinity", you probably just want to use a regular expression.
1919 if (/\D/) { print "has nondigits\n" }
1920 if (/^\d+$/) { print "is a whole number\n" }
1921 if (/^-?\d+$/) { print "is an integer\n" }
1922 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1923 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1924 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
1925 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1926 { print "a C float\n" }
1928 You can also use the L<Data::Types|Data::Types> module on
1929 the CPAN, which exports functions that validate data types
1930 using these and other regular expressions, or you can use
1931 the C<Regexp::Common> module from CPAN which has regular
1932 expressions to match various types of numbers.
1934 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1935 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1936 wrapper function for more convenient access. This function takes
1937 a string and returns the number it found, or C<undef> for input that
1938 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1939 if you just want to say, ``Is this a float?''
1942 use POSIX qw(strtod);
1947 my($num, $unparsed) = strtod($str);
1948 if (($str eq '') || ($unparsed != 0) || $!) {
1955 sub is_numeric { defined getnum($_[0]) }
1957 Or you could check out the L<String::Scanf|String::Scanf> module on the CPAN
1958 instead. The POSIX module (part of the standard Perl distribution) provides
1959 the C<strtod> and C<strtol> for converting strings to double and longs,
1962 =head2 How do I keep persistent data across program calls?
1964 For some specific applications, you can use one of the DBM modules.
1965 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
1966 or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
1967 of the standard distribution. Here's one example using Storable's C<store>
1968 and C<retrieve> functions:
1971 store(\%hash, "filename");
1974 $href = retrieve("filename"); # by ref
1975 %hash = %{ retrieve("filename") }; # direct to hash
1977 =head2 How do I print out or copy a recursive data structure?
1979 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1980 for printing out data structures. The Storable module, found on CPAN,
1981 provides a function called C<dclone> that recursively copies its argument.
1983 use Storable qw(dclone);
1986 Where $r1 can be a reference to any kind of data structure you'd like.
1987 It will be deeply copied. Because C<dclone> takes and returns references,
1988 you'd have to add extra punctuation if you had a hash of arrays that
1991 %newhash = %{ dclone(\%oldhash) };
1993 =head2 How do I define methods for every class/object?
1995 Use the UNIVERSAL class (see L<UNIVERSAL>).
1997 =head2 How do I verify a credit card checksum?
1999 Get the Business::CreditCard module from CPAN.
2001 =head2 How do I pack arrays of doubles or floats for XS code?
2003 The kgbpack.c code in the PGPLOT module on CPAN does just this.
2004 If you're doing a lot of float or double processing, consider using
2005 the PDL module from CPAN instead--it makes number-crunching easy.
2007 =head1 AUTHOR AND COPYRIGHT
2009 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
2010 All rights reserved.
2012 This documentation is free; you can redistribute it and/or modify it
2013 under the same terms as Perl itself.
2015 Irrespective of its distribution, all code examples in this file
2016 are hereby placed into the public domain. You are permitted and
2017 encouraged to use this code in your own programs for fun
2018 or for profit as you see fit. A simple comment in the code giving
2019 credit would be courteous but is not required.