3 perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
7 The section of the FAQ answers question related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximate on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to the internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers (see L<perlvar/"$#"> if you use
33 print. C<$#> has a different default value in Perl5 than it did in
34 Perl4. Changing C<$#> yourself is deprecated.)
36 This affects B<all> computer languages that represent decimal
37 floating-point numbers in binary, not just Perl. Perl provides
38 arbitrary-precision decimal numbers with the Math::BigFloat module
39 (part of the standard Perl distribution), but mathematical operations
40 are consequently slower.
42 To get rid of the superfluous digits, just use a format (eg,
43 C<printf("%.2f", 19.95)>) to get the required precision.
44 See L<perlop/"Floating-point Arithmetic">.
46 =head2 Why isn't my octal data interpreted correctly?
48 Perl only understands octal and hex numbers as such when they occur
49 as literals in your program. If they are read in from somewhere and
50 assigned, no automatic conversion takes place. You must explicitly
51 use oct() or hex() if you want the values converted. oct() interprets
52 both hex ("0x350") numbers and octal ones ("0350" or even without the
53 leading "0", like "377"), while hex() only converts hexadecimal ones,
54 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
56 This problem shows up most often when people try using chmod(), mkdir(),
57 umask(), or sysopen(), which all want permissions in octal.
59 chmod(644, $file); # WRONG -- perl -w catches this
60 chmod(0644, $file); # right
62 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
64 Remember that int() merely truncates toward 0. For rounding to a
65 certain number of digits, sprintf() or printf() is usually the easiest
68 printf("%.3f", 3.1415926535); # prints 3.142
70 The POSIX module (part of the standard Perl distribution) implements
71 ceil(), floor(), and a number of other mathematical and trigonometric
75 $ceil = ceil(3.5); # 4
76 $floor = floor(3.5); # 3
78 In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex
79 module. With 5.004, the Math::Trig module (part of the standard Perl
80 distribution) implements the trigonometric functions. Internally it
81 uses the Math::Complex module and some functions can break out from
82 the real axis into the complex plane, for example the inverse sine of
85 Rounding in financial applications can have serious implications, and
86 the rounding method used should be specified precisely. In these
87 cases, it probably pays not to trust whichever system rounding is
88 being used by Perl, but to instead implement the rounding function you
91 To see why, notice how you'll still have an issue on half-way-point
94 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
96 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
97 0.8 0.8 0.9 0.9 1.0 1.0
99 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
100 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
101 machines) will work pretty much like mathematical integers. Other numbers
104 =head2 How do I convert bits into ints?
106 To turn a string of 1s and 0s like C<10110110> into a scalar containing
107 its binary value, use the pack() and unpack() functions (documented in
108 L<perlfunc/"pack"> and L<perlfunc/"unpack">):
110 $decimal = unpack('c', pack('B8', '10110110'));
112 This packs the string C<10110110> into an eight bit binary structure.
113 This is then unpacked as a character, which returns its ordinal value.
115 This does the same thing:
117 $decimal = ord(pack('B8', '10110110'));
119 Here's an example of going the other way:
121 $binary_string = unpack('B*', "\x29");
123 =head2 Why doesn't & work the way I want it to?
125 The behavior of binary arithmetic operators depends on whether they're
126 used on numbers or strings. The operators treat a string as a series
127 of bits and work with that (the string C<"3"> is the bit pattern
128 C<00110011>). The operators work with the binary form of a number
129 (the number C<3> is treated as the bit pattern C<00000011>).
131 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
132 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
135 Most problems with C<&> and C<|> arise because the programmer thinks
136 they have a number but really it's a string. The rest arise because
139 if ("\020\020" & "\101\101") {
143 but a string consisting of two null bytes (the result of C<"\020\020"
144 & "\101\101">) is not a false value in Perl. You need:
146 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
150 =head2 How do I multiply matrices?
152 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
153 or the PDL extension (also available from CPAN).
155 =head2 How do I perform an operation on a series of integers?
157 To call a function on each element in an array, and collect the
160 @results = map { my_func($_) } @array;
164 @triple = map { 3 * $_ } @single;
166 To call a function on each element of an array, but ignore the
169 foreach $iterator (@array) {
170 some_func($iterator);
173 To call a function on each integer in a (small) range, you B<can> use:
175 @results = map { some_func($_) } (5 .. 25);
177 but you should be aware that the C<..> operator creates an array of
178 all integers in the range. This can take a lot of memory for large
182 for ($i=5; $i < 500_005; $i++) {
183 push(@results, some_func($i));
186 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
187 loop will iterate over the range, without creating the entire range.
189 for my $i (5 .. 500_005) {
190 push(@results, some_func($i));
193 will not create a list of 500,000 integers.
195 =head2 How can I output Roman numerals?
197 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
199 =head2 Why aren't my random numbers random?
201 If you're using a version of Perl before 5.004, you must call C<srand>
202 once at the start of your program to seed the random number generator.
203 5.004 and later automatically call C<srand> at the beginning. Don't
204 call C<srand> more than once--you make your numbers less random, rather
207 Computers are good at being predictable and bad at being random
208 (despite appearances caused by bugs in your programs :-).
209 http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
210 Phoenix, talks more about this.. John von Neumann said, ``Anyone who
211 attempts to generate random numbers by deterministic means is, of
212 course, living in a state of sin.''
214 If you want numbers that are more random than C<rand> with C<srand>
215 provides, you should also check out the Math::TrulyRandom module from
216 CPAN. It uses the imperfections in your system's timer to generate
217 random numbers, but this takes quite a while. If you want a better
218 pseudorandom generator than comes with your operating system, look at
219 ``Numerical Recipes in C'' at http://www.nr.com/ .
223 =head2 How do I find the week-of-the-year/day-of-the-year?
225 The day of the year is in the array returned by localtime() (see
226 L<perlfunc/"localtime">):
228 $day_of_year = (localtime(time()))[7];
230 or more legibly (in 5.004 or higher):
233 $day_of_year = localtime(time())->yday;
235 You can find the week of the year by dividing this by 7:
237 $week_of_year = int($day_of_year / 7);
239 Of course, this believes that weeks start at zero. The Date::Calc
240 module from CPAN has a lot of date calculation functions, including
241 day of the year, week of the year, and so on. Note that not
242 all businesses consider ``week 1'' to be the same; for example,
243 American businesses often consider the first week with a Monday
244 in it to be Work Week #1, despite ISO 8601, which considers
245 WW1 to be the first week with a Thursday in it.
247 =head2 How do I find the current century or millennium?
249 Use the following simple functions:
252 return int((((localtime(shift || time))[5] + 1999))/100);
255 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
258 On some systems, you'll find that the POSIX module's strftime() function
259 has been extended in a non-standard way to use a C<%C> format, which they
260 sometimes claim is the "century". It isn't, because on most such systems,
261 this is only the first two digits of the four-digit year, and thus cannot
262 be used to reliably determine the current century or millennium.
264 =head2 How can I compare two dates and find the difference?
266 If you're storing your dates as epoch seconds then simply subtract one
267 from the other. If you've got a structured date (distinct year, day,
268 month, hour, minute, seconds values), then for reasons of accessibility,
269 simplicity, and efficiency, merely use either timelocal or timegm (from
270 the Time::Local module in the standard distribution) to reduce structured
271 dates to epoch seconds. However, if you don't know the precise format of
272 your dates, then you should probably use either of the Date::Manip and
273 Date::Calc modules from CPAN before you go hacking up your own parsing
274 routine to handle arbitrary date formats.
276 =head2 How can I take a string and turn it into epoch seconds?
278 If it's a regular enough string that it always has the same format,
279 you can split it up and pass the parts to C<timelocal> in the standard
280 Time::Local module. Otherwise, you should look into the Date::Calc
281 and Date::Manip modules from CPAN.
283 =head2 How can I find the Julian Day?
285 Use the Time::JulianDay module (part of the Time-modules bundle
286 available from CPAN.)
288 Before you immerse yourself too deeply in this, be sure to verify that it
289 is the I<Julian> Day you really want. Are they really just interested in
290 a way of getting serial days so that they can do date arithmetic? If you
291 are interested in performing date arithmetic, this can be done using
292 either Date::Manip or Date::Calc, without converting to Julian Day first.
294 There is too much confusion on this issue to cover in this FAQ, but the
295 term is applied (correctly) to a calendar now supplanted by the Gregorian
296 Calendar, with the Julian Calendar failing to adjust properly for leap
297 years on centennial years (among other annoyances). The term is also used
298 (incorrectly) to mean: [1] days in the Gregorian Calendar; and [2] days
299 since a particular starting time or `epoch', usually 1970 in the Unix
300 world and 1980 in the MS-DOS/Windows world. If you find that it is not
301 the first meaning that you really want, then check out the Date::Manip
302 and Date::Calc modules. (Thanks to David Cassell for most of this text.)
304 =head2 How do I find yesterday's date?
306 The C<time()> function returns the current time in seconds since the
307 epoch. Take twenty-four hours off that:
309 $yesterday = time() - ( 24 * 60 * 60 );
311 Then you can pass this to C<localtime()> and get the individual year,
312 month, day, hour, minute, seconds values.
314 Note very carefully that the code above assumes that your days are
315 twenty-four hours each. For most people, there are two days a year
316 when they aren't: the switch to and from summer time throws this off.
317 A solution to this issue is offered by Russ Allbery.
320 my $now = defined $_[0] ? $_[0] : time;
321 my $then = $now - 60 * 60 * 24;
322 my $ndst = (localtime $now)[8] > 0;
323 my $tdst = (localtime $then)[8] > 0;
324 $then - ($tdst - $ndst) * 60 * 60;
326 # Should give you "this time yesterday" in seconds since epoch relative to
327 # the first argument or the current time if no argument is given and
328 # suitable for passing to localtime or whatever else you need to do with
329 # it. $ndst is whether we're currently in daylight savings time; $tdst is
330 # whether the point 24 hours ago was in daylight savings time. If $tdst
331 # and $ndst are the same, a boundary wasn't crossed, and the correction
332 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
333 # from yesterday's time since we gained an extra hour while going off
334 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
335 # negative hour (add an hour) to yesterday's time since we lost an hour.
337 # All of this is because during those days when one switches off or onto
338 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
340 # The explicit settings of $ndst and $tdst are necessary because localtime
341 # only says it returns the system tm struct, and the system tm struct at
342 # least on Solaris doesn't guarantee any particular positive value (like,
343 # say, 1) for isdst, just a positive value. And that value can
344 # potentially be negative, if DST information isn't available (this sub
345 # just treats those cases like no DST).
347 # Note that between 2am and 3am on the day after the time zone switches
348 # off daylight savings time, the exact hour of "yesterday" corresponding
349 # to the current hour is not clearly defined. Note also that if used
350 # between 2am and 3am the day after the change to daylight savings time,
351 # the result will be between 3am and 4am of the previous day; it's
352 # arguable whether this is correct.
354 # This sub does not attempt to deal with leap seconds (most things don't).
356 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
357 # This code is in the public domain
359 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
361 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
362 Y2K compliant (whatever that means). The programmers you've hired to
363 use it, however, probably are not.
365 Long answer: The question belies a true understanding of the issue.
366 Perl is just as Y2K compliant as your pencil--no more, and no less.
367 Can you use your pencil to write a non-Y2K-compliant memo? Of course
368 you can. Is that the pencil's fault? Of course it isn't.
370 The date and time functions supplied with Perl (gmtime and localtime)
371 supply adequate information to determine the year well beyond 2000
372 (2038 is when trouble strikes for 32-bit machines). The year returned
373 by these functions when used in an array context is the year minus 1900.
374 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
375 number. To avoid the year 2000 problem simply do not treat the year as
376 a 2-digit number. It isn't.
378 When gmtime() and localtime() are used in scalar context they return
379 a timestamp string that contains a fully-expanded year. For example,
380 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
381 2001". There's no year 2000 problem here.
383 That doesn't mean that Perl can't be used to create non-Y2K compliant
384 programs. It can. But so can your pencil. It's the fault of the user,
385 not the language. At the risk of inflaming the NRA: ``Perl doesn't
386 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
391 =head2 How do I validate input?
393 The answer to this question is usually a regular expression, perhaps
394 with auxiliary logic. See the more specific questions (numbers, mail
395 addresses, etc.) for details.
397 =head2 How do I unescape a string?
399 It depends just what you mean by ``escape''. URL escapes are dealt
400 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
401 character are removed with:
405 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
407 =head2 How do I remove consecutive pairs of characters?
409 To turn C<"abbcccd"> into C<"abccd">:
411 s/(.)\1/$1/g; # add /s to include newlines
413 Here's a solution that turns "abbcccd" to "abcd":
415 y///cs; # y == tr, but shorter :-)
417 =head2 How do I expand function calls in a string?
419 This is documented in L<perlref>. In general, this is fraught with
420 quoting and readability problems, but it is possible. To interpolate
421 a subroutine call (in list context) into a string:
423 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
425 If you prefer scalar context, similar chicanery is also useful for
426 arbitrary expressions:
428 print "That yields ${\($n + 5)} widgets\n";
430 Version 5.004 of Perl had a bug that gave list context to the
431 expression in C<${...}>, but this is fixed in version 5.005.
433 See also ``How can I expand variables in text strings?'' in this
436 =head2 How do I find matching/nesting anything?
438 This isn't something that can be done in one regular expression, no
439 matter how complicated. To find something between two single
440 characters, a pattern like C</x([^x]*)x/> will get the intervening
441 bits in $1. For multiple ones, then something more like
442 C</alpha(.*?)omega/> would be needed. But none of these deals with
443 nested patterns, nor can they. For that you'll have to write a
446 If you are serious about writing a parser, there are a number of
447 modules or oddities that will make your life a lot easier. There is
448 the CPAN module Parse::RecDescent, the standard module Text::Balanced,
449 the byacc program, the CPAN module Parse::Yapp, and Mark-Jason
450 Dominus's excellent I<py> tool at http://www.plover.com/%7Emjd/perl/py/
453 One simple destructive, inside-out approach that you might try is to
454 pull out the smallest nesting parts one at a time:
456 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
457 # do something with $1
460 A more complicated and sneaky approach is to make Perl's regular
461 expression engine do it for you. This is courtesy Dean Inada, and
462 rather has the nature of an Obfuscated Perl Contest entry, but it
465 # $_ contains the string to parse
466 # BEGIN and END are the opening and closing markers for the
471 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
472 @$ = (eval{/$re/},$@!~/unmatched/);
473 print join("\n",@$[0..$#$]) if( $$[-1] );
475 =head2 How do I reverse a string?
477 Use reverse() in scalar context, as documented in
480 $reversed = reverse $string;
482 =head2 How do I expand tabs in a string?
484 You can do it yourself:
486 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
488 Or you can just use the Text::Tabs module (part of the standard Perl
492 @expanded_lines = expand(@lines_with_tabs);
494 =head2 How do I reformat a paragraph?
496 Use Text::Wrap (part of the standard Perl distribution):
499 print wrap("\t", ' ', @paragraphs);
501 The paragraphs you give to Text::Wrap should not contain embedded
502 newlines. Text::Wrap doesn't justify the lines (flush-right).
504 =head2 How can I access/change the first N letters of a string?
506 There are many ways. If you just want to grab a copy, use
509 $first_byte = substr($a, 0, 1);
511 If you want to modify part of a string, the simplest way is often to
512 use substr() as an lvalue:
514 substr($a, 0, 3) = "Tom";
516 Although those with a pattern matching kind of thought process will
521 =head2 How do I change the Nth occurrence of something?
523 You have to keep track of N yourself. For example, let's say you want
524 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
525 C<"whosoever"> or C<"whomsoever">, case insensitively. These
526 all assume that $_ contains the string to be altered.
530 ++$count == 5 # is it the 5th?
531 ? "${2}soever" # yes, swap
532 : $1 # renege and leave it there
535 In the more general case, you can use the C</g> modifier in a C<while>
536 loop, keeping count of matches.
540 $_ = "One fish two fish red fish blue fish";
541 while (/(\w+)\s+fish\b/gi) {
542 if (++$count == $WANT) {
543 print "The third fish is a $1 one.\n";
547 That prints out: C<"The third fish is a red one."> You can also use a
548 repetition count and repeated pattern like this:
550 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
552 =head2 How can I count the number of occurrences of a substring within a string?
554 There are a number of ways, with varying efficiency: If you want a
555 count of a certain single character (X) within a string, you can use the
556 C<tr///> function like so:
558 $string = "ThisXlineXhasXsomeXx'sXinXit";
559 $count = ($string =~ tr/X//);
560 print "There are $count X characters in the string";
562 This is fine if you are just looking for a single character. However,
563 if you are trying to count multiple character substrings within a
564 larger string, C<tr///> won't work. What you can do is wrap a while()
565 loop around a global pattern match. For example, let's count negative
568 $string = "-9 55 48 -2 23 -76 4 14 -44";
569 while ($string =~ /-\d+/g) { $count++ }
570 print "There are $count negative numbers in the string";
572 =head2 How do I capitalize all the words on one line?
574 To make the first letter of each word upper case:
576 $line =~ s/\b(\w)/\U$1/g;
578 This has the strange effect of turning "C<don't do it>" into "C<Don'T
579 Do It>". Sometimes you might want this, instead (Suggested by brian d.
583 (^\w) #at the beginning of the line
585 (\s\w) #preceded by whitespace
588 $string =~ /([\w']+)/\u\L$1/g;
590 To make the whole line upper case:
594 To force each word to be lower case, with the first letter upper case:
596 $line =~ s/(\w+)/\u\L$1/g;
598 You can (and probably should) enable locale awareness of those
599 characters by placing a C<use locale> pragma in your program.
600 See L<perllocale> for endless details on locales.
602 This is sometimes referred to as putting something into "title
603 case", but that's not quite accurate. Consider the proper
604 capitalization of the movie I<Dr. Strangelove or: How I Learned to
605 Stop Worrying and Love the Bomb>, for example.
607 =head2 How can I split a [character] delimited string except when inside
608 [character]? (Comma-separated files)
610 Take the example case of trying to split a string that is comma-separated
611 into its different fields. (We'll pretend you said comma-separated, not
612 comma-delimited, which is different and almost never what you mean.) You
613 can't use C<split(/,/)> because you shouldn't split if the comma is inside
614 quotes. For example, take a data line like this:
616 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
618 Due to the restriction of the quotes, this is a fairly complex
619 problem. Thankfully, we have Jeffrey Friedl, author of a highly
620 recommended book on regular expressions, to handle these for us. He
621 suggests (assuming your string is contained in $text):
624 push(@new, $+) while $text =~ m{
625 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
629 push(@new, undef) if substr($text,-1,1) eq ',';
631 If you want to represent quotation marks inside a
632 quotation-mark-delimited field, escape them with backslashes (eg,
633 C<"like \"this\"">. Unescaping them is a task addressed earlier in
636 Alternatively, the Text::ParseWords module (part of the standard Perl
637 distribution) lets you say:
639 use Text::ParseWords;
640 @new = quotewords(",", 0, $text);
642 There's also a Text::CSV module on CPAN.
644 =head2 How do I strip blank space from the beginning/end of a string?
646 Although the simplest approach would seem to be:
648 $string =~ s/^\s*(.*?)\s*$/$1/;
650 Not only is this unnecessarily slow and destructive, it also fails with
651 embedded newlines. It is much faster to do this operation in two steps:
656 Or more nicely written as:
663 This idiom takes advantage of the C<foreach> loop's aliasing
664 behavior to factor out common code. You can do this
665 on several strings at once, or arrays, or even the
666 values of a hash if you use a slice:
668 # trim whitespace in the scalar, the array,
669 # and all the values in the hash
670 foreach ($scalar, @array, @hash{keys %hash}) {
675 =head2 How do I pad a string with blanks or pad a number with zeroes?
677 (This answer contributed by Uri Guttman, with kibitzing from
680 In the following examples, C<$pad_len> is the length to which you wish
681 to pad the string, C<$text> or C<$num> contains the string to be padded,
682 and C<$pad_char> contains the padding character. You can use a single
683 character string constant instead of the C<$pad_char> variable if you
684 know what it is in advance. And in the same way you can use an integer in
685 place of C<$pad_len> if you know the pad length in advance.
687 The simplest method uses the C<sprintf> function. It can pad on the left
688 or right with blanks and on the left with zeroes and it will not
689 truncate the result. The C<pack> function can only pad strings on the
690 right with blanks and it will truncate the result to a maximum length of
693 # Left padding a string with blanks (no truncation):
694 $padded = sprintf("%${pad_len}s", $text);
696 # Right padding a string with blanks (no truncation):
697 $padded = sprintf("%-${pad_len}s", $text);
699 # Left padding a number with 0 (no truncation):
700 $padded = sprintf("%0${pad_len}d", $num);
702 # Right padding a string with blanks using pack (will truncate):
703 $padded = pack("A$pad_len",$text);
705 If you need to pad with a character other than blank or zero you can use
706 one of the following methods. They all generate a pad string with the
707 C<x> operator and combine that with C<$text>. These methods do
708 not truncate C<$text>.
710 Left and right padding with any character, creating a new string:
712 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
713 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
715 Left and right padding with any character, modifying C<$text> directly:
717 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
718 $text .= $pad_char x ( $pad_len - length( $text ) );
720 =head2 How do I extract selected columns from a string?
722 Use substr() or unpack(), both documented in L<perlfunc>.
723 If you prefer thinking in terms of columns instead of widths,
724 you can use this kind of thing:
726 # determine the unpack format needed to split Linux ps output
727 # arguments are cut columns
728 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
734 for my $place (@positions) {
735 $template .= "A" . ($place - $lastpos) . " ";
742 =head2 How do I find the soundex value of a string?
744 Use the standard Text::Soundex module distributed with Perl.
745 But before you do so, you may want to determine whether `soundex' is in
746 fact what you think it is. Knuth's soundex algorithm compresses words
747 into a small space, and so it does not necessarily distinguish between
748 two words which you might want to appear separately. For example, the
749 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
750 If Text::Soundex does not do what you are looking for, you might want
751 to consider the String::Approx module available at CPAN.
753 =head2 How can I expand variables in text strings?
755 Let's assume that you have a string like:
757 $text = 'this has a $foo in it and a $bar';
759 If those were both global variables, then this would
762 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
764 But since they are probably lexicals, or at least, they could
765 be, you'd have to do this:
767 $text =~ s/(\$\w+)/$1/eeg;
768 die if $@; # needed /ee, not /e
770 It's probably better in the general case to treat those
771 variables as entries in some special hash. For example:
777 $text =~ s/\$(\w+)/$user_defs{$1}/g;
779 See also ``How do I expand function calls in a string?'' in this section
782 =head2 What's wrong with always quoting "$vars"?
784 The problem is that those double-quotes force stringification,
785 coercing numbers and references into strings, even when you
786 don't want them to be. Think of it this way: double-quote
787 expansion is used to produce new strings. If you already
788 have a string, why do you need more?
790 If you get used to writing odd things like these:
794 somefunc("$var"); # BAD
796 You'll be in trouble. Those should (in 99.8% of the cases) be
797 the simpler and more direct:
803 Otherwise, besides slowing you down, you're going to break code when
804 the thing in the scalar is actually neither a string nor a number, but
810 my $oref = "$aref"; # WRONG
813 You can also get into subtle problems on those few operations in Perl
814 that actually do care about the difference between a string and a
815 number, such as the magical C<++> autoincrement operator or the
818 Stringification also destroys arrays.
821 print "@lines"; # WRONG - extra blanks
822 print @lines; # right
824 =head2 Why don't my E<lt>E<lt>HERE documents work?
826 Check for these three things:
830 =item 1. There must be no space after the << part.
832 =item 2. There (probably) should be a semicolon at the end.
834 =item 3. You can't (easily) have any space in front of the tag.
838 If you want to indent the text in the here document, you
842 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
847 But the HERE_TARGET must still be flush against the margin.
848 If you want that indented also, you'll have to quote
851 ($quote = <<' FINIS') =~ s/^\s+//gm;
852 ...we will have peace, when you and all your works have
853 perished--and the works of your dark master to whom you
854 would deliver us. You are a liar, Saruman, and a corrupter
855 of men's hearts. --Theoden in /usr/src/perl/taint.c
857 $quote =~ s/\s*--/\n--/;
859 A nice general-purpose fixer-upper function for indented here documents
860 follows. It expects to be called with a here document as its argument.
861 It looks to see whether each line begins with a common substring, and
862 if so, strips that off. Otherwise, it takes the amount of leading
863 white space found on the first line and removes that much off each
868 my ($white, $leader); # common white space and common leading string
869 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
870 ($white, $leader) = ($2, quotemeta($1));
872 ($white, $leader) = (/^(\s+)/, '');
874 s/^\s*?$leader(?:$white)?//gm;
878 This works with leading special strings, dynamically determined:
880 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
883 @@@ SAVEI32(runlevel);
885 @@@ while ( op = (*op->op_ppaddr)() );
889 MAIN_INTERPRETER_LOOP
891 Or with a fixed amount of leading white space, with remaining
892 indentation correctly preserved:
894 $poem = fix<<EVER_ON_AND_ON;
895 Now far ahead the Road has gone,
896 And I must follow, if I can,
897 Pursuing it with eager feet,
898 Until it joins some larger way
899 Where many paths and errands meet.
900 And whither then? I cannot say.
901 --Bilbo in /usr/src/perl/pp_ctl.c
906 =head2 What is the difference between a list and an array?
908 An array has a changeable length. A list does not. An array is something
909 you can push or pop, while a list is a set of values. Some people make
910 the distinction that a list is a value while an array is a variable.
911 Subroutines are passed and return lists, you put things into list
912 context, you initialize arrays with lists, and you foreach() across
913 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
914 in scalar context behave like the number of elements in them, subroutines
915 access their arguments through the array C<@_>, push/pop/shift only work
918 As a side note, there's no such thing as a list in scalar context.
921 $scalar = (2, 5, 7, 9);
923 you're using the comma operator in scalar context, so it uses the scalar
924 comma operator. There never was a list there at all! This causes the
925 last value to be returned: 9.
927 =head2 What is the difference between $array[1] and @array[1]?
929 The former is a scalar value, the latter an array slice, which makes
930 it a list with one (scalar) value. You should use $ when you want a
931 scalar value (most of the time) and @ when you want a list with one
932 scalar value in it (very, very rarely; nearly never, in fact).
934 Sometimes it doesn't make a difference, but sometimes it does.
935 For example, compare:
937 $good[0] = `some program that outputs several lines`;
941 @bad[0] = `same program that outputs several lines`;
943 The B<-w> flag will warn you about these matters.
945 =head2 How can I remove duplicate elements from a list or array?
947 There are several possible ways, depending on whether the array is
948 ordered and whether you wish to preserve the ordering.
952 =item a) If @in is sorted, and you want @out to be sorted:
953 (this assumes all true values in the array)
956 @out = grep($_ ne $prev && ($prev = $_), @in);
958 This is nice in that it doesn't use much extra memory, simulating
959 uniq(1)'s behavior of removing only adjacent duplicates. It's less
960 nice in that it won't work with false values like undef, 0, or "";
961 "0 but true" is OK, though.
963 =item b) If you don't know whether @in is sorted:
966 @out = grep(!$saw{$_}++, @in);
968 =item c) Like (b), but @in contains only small integers:
970 @out = grep(!$saw[$_]++, @in);
972 =item d) A way to do (b) without any loops or greps:
976 @out = sort keys %saw; # remove sort if undesired
978 =item e) Like (d), but @in contains only small positive integers:
982 @out = grep {defined} @ary;
986 But perhaps you should have been using a hash all along, eh?
988 =head2 How can I tell whether a list or array contains a certain element?
990 Hearing the word "in" is an I<in>dication that you probably should have
991 used a hash, not a list or array, to store your data. Hashes are
992 designed to answer this question quickly and efficiently. Arrays aren't.
994 That being said, there are several ways to approach this. If you
995 are going to make this query many times over arbitrary string values,
996 the fastest way is probably to invert the original array and keep an
997 associative array lying about whose keys are the first array's values.
999 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1001 for (@blues) { $is_blue{$_} = 1 }
1003 Now you can check whether $is_blue{$some_color}. It might have been a
1004 good idea to keep the blues all in a hash in the first place.
1006 If the values are all small integers, you could use a simple indexed
1007 array. This kind of an array will take up less space:
1009 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1010 undef @is_tiny_prime;
1011 for (@primes) { $is_tiny_prime[$_] = 1 }
1012 # or simply @istiny_prime[@primes] = (1) x @primes;
1014 Now you check whether $is_tiny_prime[$some_number].
1016 If the values in question are integers instead of strings, you can save
1017 quite a lot of space by using bit strings instead:
1019 @articles = ( 1..10, 150..2000, 2017 );
1021 for (@articles) { vec($read,$_,1) = 1 }
1023 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1027 $is_there = grep $_ eq $whatever, @array;
1031 $is_there = grep /$whatever/, @array;
1033 These are slow (checks every element even if the first matches),
1034 inefficient (same reason), and potentially buggy (what if there are
1035 regex characters in $whatever?). If you're only testing once, then
1039 foreach $elt (@array) {
1040 if ($elt eq $elt_to_find) {
1045 if ($is_there) { ... }
1047 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1049 Use a hash. Here's code to do both and more. It assumes that
1050 each element is unique in a given array:
1052 @union = @intersection = @difference = ();
1054 foreach $element (@array1, @array2) { $count{$element}++ }
1055 foreach $element (keys %count) {
1056 push @union, $element;
1057 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1060 Note that this is the I<symmetric difference>, that is, all elements in
1061 either A or in B, but not in both. Think of it as an xor operation.
1063 =head2 How do I test whether two arrays or hashes are equal?
1065 The following code works for single-level arrays. It uses a stringwise
1066 comparison, and does not distinguish defined versus undefined empty
1067 strings. Modify if you have other needs.
1069 $are_equal = compare_arrays(\@frogs, \@toads);
1071 sub compare_arrays {
1072 my ($first, $second) = @_;
1073 local $^W = 0; # silence spurious -w undef complaints
1074 return 0 unless @$first == @$second;
1075 for (my $i = 0; $i < @$first; $i++) {
1076 return 0 if $first->[$i] ne $second->[$i];
1081 For multilevel structures, you may wish to use an approach more
1082 like this one. It uses the CPAN module FreezeThaw:
1084 use FreezeThaw qw(cmpStr);
1085 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1087 printf "a and b contain %s arrays\n",
1088 cmpStr(\@a, \@b) == 0
1092 This approach also works for comparing hashes. Here
1093 we'll demonstrate two different answers:
1095 use FreezeThaw qw(cmpStr cmpStrHard);
1097 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1101 printf "a and b contain %s hashes\n",
1102 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1104 printf "a and b contain %s hashes\n",
1105 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1108 The first reports that both those the hashes contain the same data,
1109 while the second reports that they do not. Which you prefer is left as
1110 an exercise to the reader.
1112 =head2 How do I find the first array element for which a condition is true?
1114 You can use this if you care about the index:
1116 for ($i= 0; $i < @array; $i++) {
1117 if ($array[$i] eq "Waldo") {
1123 Now C<$found_index> has what you want.
1125 =head2 How do I handle linked lists?
1127 In general, you usually don't need a linked list in Perl, since with
1128 regular arrays, you can push and pop or shift and unshift at either end,
1129 or you can use splice to add and/or remove arbitrary number of elements at
1130 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1131 dynamic arrays. In the absence of shifts and pops, push in general
1132 needs to reallocate on the order every log(N) times, and unshift will
1133 need to copy pointers each time.
1135 If you really, really wanted, you could use structures as described in
1136 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1137 to do. For example, imagine a list node like this:
1144 You could walk the list this way:
1147 for ($node = $head; $node; $node = $node->{LINK}) {
1148 print $node->{VALUE}, " ";
1152 You could grow the list this way:
1155 $tail = append($head, 1); # grow a new head
1156 for $value ( 2 .. 10 ) {
1157 $tail = append($tail, $value);
1161 my($list, $value) = @_;
1162 my $node = { VALUE => $value };
1164 $node->{LINK} = $list->{LINK};
1165 $list->{LINK} = $node;
1167 $_[0] = $node; # replace caller's version
1172 But again, Perl's built-in are virtually always good enough.
1174 =head2 How do I handle circular lists?
1176 Circular lists could be handled in the traditional fashion with linked
1177 lists, or you could just do something like this with an array:
1179 unshift(@array, pop(@array)); # the last shall be first
1180 push(@array, shift(@array)); # and vice versa
1182 =head2 How do I shuffle an array randomly?
1186 # fisher_yates_shuffle( \@array ) :
1187 # generate a random permutation of @array in place
1188 sub fisher_yates_shuffle {
1191 for ($i = @$array; --$i; ) {
1192 my $j = int rand ($i+1);
1194 @$array[$i,$j] = @$array[$j,$i];
1198 fisher_yates_shuffle( \@array ); # permutes @array in place
1200 You've probably seen shuffling algorithms that work using splice,
1201 randomly picking another element to swap the current element with:
1205 @old = 1 .. 10; # just a demo
1207 push(@new, splice(@old, rand @old, 1));
1210 This is bad because splice is already O(N), and since you do it N times,
1211 you just invented a quadratic algorithm; that is, O(N**2). This does
1212 not scale, although Perl is so efficient that you probably won't notice
1213 this until you have rather largish arrays.
1215 =head2 How do I process/modify each element of an array?
1217 Use C<for>/C<foreach>:
1220 s/foo/bar/; # change that word
1221 y/XZ/ZX/; # swap those letters
1224 Here's another; let's compute spherical volumes:
1226 for (@volumes = @radii) { # @volumes has changed parts
1228 $_ *= (4/3) * 3.14159; # this will be constant folded
1231 If you want to do the same thing to modify the values of the hash,
1232 you may not use the C<values> function, oddly enough. You need a slice:
1234 for $orbit ( @orbits{keys %orbits} ) {
1235 ($orbit **= 3) *= (4/3) * 3.14159;
1238 =head2 How do I select a random element from an array?
1240 Use the rand() function (see L<perlfunc/rand>):
1242 # at the top of the program:
1243 srand; # not needed for 5.004 and later
1246 $index = rand @array;
1247 $element = $array[$index];
1249 Make sure you I<only call srand once per program, if then>.
1250 If you are calling it more than once (such as before each
1251 call to rand), you're almost certainly doing something wrong.
1253 =head2 How do I permute N elements of a list?
1255 Here's a little program that generates all permutations
1256 of all the words on each line of input. The algorithm embodied
1257 in the permute() function should work on any list:
1260 # tsc-permute: permute each word of input
1261 permute([split], []);
1263 my @items = @{ $_[0] };
1264 my @perms = @{ $_[1] };
1268 my(@newitems,@newperms,$i);
1269 foreach $i (0 .. $#items) {
1272 unshift(@newperms, splice(@newitems, $i, 1));
1273 permute([@newitems], [@newperms]);
1278 =head2 How do I sort an array by (anything)?
1280 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1282 @list = sort { $a <=> $b } @list;
1284 The default sort function is cmp, string comparison, which would
1285 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
1286 the numerical comparison operator.
1288 If you have a complicated function needed to pull out the part you
1289 want to sort on, then don't do it inside the sort function. Pull it
1290 out first, because the sort BLOCK can be called many times for the
1291 same element. Here's an example of how to pull out the first word
1292 after the first number on each item, and then sort those words
1297 ($item) = /\d+\s*(\S+)/;
1298 push @idx, uc($item);
1300 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1302 Which could also be written this way, using a trick
1303 that's come to be known as the Schwartzian Transform:
1305 @sorted = map { $_->[0] }
1306 sort { $a->[1] cmp $b->[1] }
1307 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1309 If you need to sort on several fields, the following paradigm is useful.
1311 @sorted = sort { field1($a) <=> field1($b) ||
1312 field2($a) cmp field2($b) ||
1313 field3($a) cmp field3($b)
1316 This can be conveniently combined with precalculation of keys as given
1319 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1322 See also the question below on sorting hashes.
1324 =head2 How do I manipulate arrays of bits?
1326 Use pack() and unpack(), or else vec() and the bitwise operations.
1328 For example, this sets $vec to have bit N set if $ints[N] was set:
1331 foreach(@ints) { vec($vec,$_,1) = 1 }
1333 And here's how, given a vector in $vec, you can
1334 get those bits into your @ints array:
1336 sub bitvec_to_list {
1339 # Find null-byte density then select best algorithm
1340 if ($vec =~ tr/\0// / length $vec > 0.95) {
1343 # This method is faster with mostly null-bytes
1344 while($vec =~ /[^\0]/g ) {
1345 $i = -9 + 8 * pos $vec;
1346 push @ints, $i if vec($vec, ++$i, 1);
1347 push @ints, $i if vec($vec, ++$i, 1);
1348 push @ints, $i if vec($vec, ++$i, 1);
1349 push @ints, $i if vec($vec, ++$i, 1);
1350 push @ints, $i if vec($vec, ++$i, 1);
1351 push @ints, $i if vec($vec, ++$i, 1);
1352 push @ints, $i if vec($vec, ++$i, 1);
1353 push @ints, $i if vec($vec, ++$i, 1);
1356 # This method is a fast general algorithm
1358 my $bits = unpack "b*", $vec;
1359 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1360 push @ints, pos $bits while($bits =~ /1/g);
1365 This method gets faster the more sparse the bit vector is.
1366 (Courtesy of Tim Bunce and Winfried Koenig.)
1368 Here's a demo on how to use vec():
1371 $vector = "\xff\x0f\xef\xfe";
1372 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1373 unpack("N", $vector), "\n";
1374 $is_set = vec($vector, 23, 1);
1375 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1393 my ($offset, $width, $value) = @_;
1395 vec($vector, $offset, $width) = $value;
1396 print "offset=$offset width=$width value=$value\n";
1402 my $bits = unpack("b*", $vector);
1406 print "vector length in bytes: ", length($vector), "\n";
1407 @bytes = unpack("A8" x length($vector), $bits);
1408 print "bits are: @bytes\n\n";
1411 =head2 Why does defined() return true on empty arrays and hashes?
1413 The short story is that you should probably only use defined on scalars or
1414 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1415 in the 5.004 release or later of Perl for more detail.
1417 =head1 Data: Hashes (Associative Arrays)
1419 =head2 How do I process an entire hash?
1421 Use the each() function (see L<perlfunc/each>) if you don't care
1422 whether it's sorted:
1424 while ( ($key, $value) = each %hash) {
1425 print "$key = $value\n";
1428 If you want it sorted, you'll have to use foreach() on the result of
1429 sorting the keys as shown in an earlier question.
1431 =head2 What happens if I add or remove keys from a hash while iterating over it?
1435 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1436 iterating over it. In Perl 5 you can delete from it, but you still
1437 can't add to it, because that might cause a doubling of the hash table,
1438 in which half the entries get copied up to the new top half of the
1439 table, at which point you've totally bamboozled the iterator code.
1440 Even if the table doesn't double, there's no telling whether your new
1441 entry will be inserted before or after the current iterator position.
1443 Either treasure up your changes and make them after the iterator finishes,
1444 or use keys to fetch all the old keys at once, and iterate over the list
1447 =head2 How do I look up a hash element by value?
1449 Create a reverse hash:
1451 %by_value = reverse %by_key;
1452 $key = $by_value{$value};
1454 That's not particularly efficient. It would be more space-efficient
1457 while (($key, $value) = each %by_key) {
1458 $by_value{$value} = $key;
1461 If your hash could have repeated values, the methods above will only find
1462 one of the associated keys. This may or may not worry you. If it does
1463 worry you, you can always reverse the hash into a hash of arrays instead:
1465 while (($key, $value) = each %by_key) {
1466 push @{$key_list_by_value{$value}}, $key;
1469 =head2 How can I know how many entries are in a hash?
1471 If you mean how many keys, then all you have to do is
1472 take the scalar sense of the keys() function:
1474 $num_keys = scalar keys %hash;
1476 In void context, the keys() function just resets the iterator, which is
1477 faster for tied hashes than would be iterating through the whole
1478 hash, one key-value pair at a time.
1480 =head2 How do I sort a hash (optionally by value instead of key)?
1482 Internally, hashes are stored in a way that prevents you from imposing
1483 an order on key-value pairs. Instead, you have to sort a list of the
1486 @keys = sort keys %hash; # sorted by key
1488 $hash{$a} cmp $hash{$b}
1489 } keys %hash; # and by value
1491 Here we'll do a reverse numeric sort by value, and if two keys are
1492 identical, sort by length of key, and if that fails, by straight ASCII
1493 comparison of the keys (well, possibly modified by your locale -- see
1497 $hash{$b} <=> $hash{$a}
1499 length($b) <=> length($a)
1504 =head2 How can I always keep my hash sorted?
1506 You can look into using the DB_File module and tie() using the
1507 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1508 The Tie::IxHash module from CPAN might also be instructive.
1510 =head2 What's the difference between "delete" and "undef" with hashes?
1512 Hashes are pairs of scalars: the first is the key, the second is the
1513 value. The key will be coerced to a string, although the value can be
1514 any kind of scalar: string, number, or reference. If a key C<$key> is
1515 present in the array, C<exists($key)> will return true. The value for
1516 a given key can be C<undef>, in which case C<$array{$key}> will be
1517 C<undef> while C<$exists{$key}> will return true. This corresponds to
1518 (C<$key>, C<undef>) being in the hash.
1520 Pictures help... here's the C<%ary> table:
1530 And these conditions hold
1534 defined $ary{'d'} is true
1535 defined $ary{'a'} is true
1536 exists $ary{'a'} is true (Perl5 only)
1537 grep ($_ eq 'a', keys %ary) is true
1543 your table now reads:
1554 and these conditions now hold; changes in caps:
1558 defined $ary{'d'} is true
1559 defined $ary{'a'} is FALSE
1560 exists $ary{'a'} is true (Perl5 only)
1561 grep ($_ eq 'a', keys %ary) is true
1563 Notice the last two: you have an undef value, but a defined key!
1569 your table now reads:
1578 and these conditions now hold; changes in caps:
1582 defined $ary{'d'} is true
1583 defined $ary{'a'} is false
1584 exists $ary{'a'} is FALSE (Perl5 only)
1585 grep ($_ eq 'a', keys %ary) is FALSE
1587 See, the whole entry is gone!
1589 =head2 Why don't my tied hashes make the defined/exists distinction?
1591 They may or may not implement the EXISTS() and DEFINED() methods
1592 differently. For example, there isn't the concept of undef with hashes
1593 that are tied to DBM* files. This means the true/false tables above
1594 will give different results when used on such a hash. It also means
1595 that exists and defined do the same thing with a DBM* file, and what
1596 they end up doing is not what they do with ordinary hashes.
1598 =head2 How do I reset an each() operation part-way through?
1600 Using C<keys %hash> in scalar context returns the number of keys in
1601 the hash I<and> resets the iterator associated with the hash. You may
1602 need to do this if you use C<last> to exit a loop early so that when you
1603 re-enter it, the hash iterator has been reset.
1605 =head2 How can I get the unique keys from two hashes?
1607 First you extract the keys from the hashes into lists, then solve
1608 the "removing duplicates" problem described above. For example:
1611 for $element (keys(%foo), keys(%bar)) {
1618 @uniq = keys %{{%foo,%bar}};
1620 Or if you really want to save space:
1623 while (defined ($key = each %foo)) {
1626 while (defined ($key = each %bar)) {
1631 =head2 How can I store a multidimensional array in a DBM file?
1633 Either stringify the structure yourself (no fun), or else
1634 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1635 it on top of either DB_File or GDBM_File.
1637 =head2 How can I make my hash remember the order I put elements into it?
1639 Use the Tie::IxHash from CPAN.
1642 tie(%myhash, Tie::IxHash);
1643 for ($i=0; $i<20; $i++) {
1646 @keys = keys %myhash;
1647 # @keys = (0,1,2,3,...)
1649 =head2 Why does passing a subroutine an undefined element in a hash create it?
1651 If you say something like:
1653 somefunc($hash{"nonesuch key here"});
1655 Then that element "autovivifies"; that is, it springs into existence
1656 whether you store something there or not. That's because functions
1657 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1658 it has to be ready to write it back into the caller's version.
1660 This has been fixed as of Perl5.004.
1662 Normally, merely accessing a key's value for a nonexistent key does
1663 I<not> cause that key to be forever there. This is different than
1666 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1668 Usually a hash ref, perhaps like this:
1673 TITLE => "deputy peon",
1676 PALS => [ "Norbert", "Rhys", "Phineas"],
1679 References are documented in L<perlref> and the upcoming L<perlreftut>.
1680 Examples of complex data structures are given in L<perldsc> and
1681 L<perllol>. Examples of structures and object-oriented classes are
1684 =head2 How can I use a reference as a hash key?
1686 You can't do this directly, but you could use the standard Tie::Refhash
1687 module distributed with Perl.
1691 =head2 How do I handle binary data correctly?
1693 Perl is binary clean, so this shouldn't be a problem. For example,
1694 this works fine (assuming the files are found):
1696 if (`cat /vmunix` =~ /gzip/) {
1697 print "Your kernel is GNU-zip enabled!\n";
1700 On less elegant (read: Byzantine) systems, however, you have
1701 to play tedious games with "text" versus "binary" files. See
1702 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1703 systems are curses out of Microsoft, who seem to be committed to putting
1704 the backward into backward compatibility.
1706 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1708 If you want to deal with multibyte characters, however, there are
1709 some gotchas. See the section on Regular Expressions.
1711 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1713 Assuming that you don't care about IEEE notations like "NaN" or
1714 "Infinity", you probably just want to use a regular expression.
1716 if (/\D/) { print "has nondigits\n" }
1717 if (/^\d+$/) { print "is a whole number\n" }
1718 if (/^-?\d+$/) { print "is an integer\n" }
1719 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1720 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1721 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1722 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1723 { print "a C float" }
1725 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1726 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1727 wrapper function for more convenient access. This function takes
1728 a string and returns the number it found, or C<undef> for input that
1729 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1730 if you just want to say, ``Is this a float?''
1733 use POSIX qw(strtod);
1738 my($num, $unparsed) = strtod($str);
1739 if (($str eq '') || ($unparsed != 0) || $!) {
1746 sub is_numeric { defined getnum($_[0]) }
1748 Or you could check out the String::Scanf module on CPAN instead. The
1749 POSIX module (part of the standard Perl distribution) provides the
1750 C<strtol> and C<strtod> for converting strings to double and longs,
1753 =head2 How do I keep persistent data across program calls?
1755 For some specific applications, you can use one of the DBM modules.
1756 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1757 Storable, or Class::Eroot modules from CPAN. Here's one example using
1758 Storable's C<store> and C<retrieve> functions:
1761 store(\%hash, "filename");
1764 $href = retrieve("filename"); # by ref
1765 %hash = %{ retrieve("filename") }; # direct to hash
1767 =head2 How do I print out or copy a recursive data structure?
1769 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1770 for printing out data structures. The Storable module, found on CPAN,
1771 provides a function called C<dclone> that recursively copies its argument.
1773 use Storable qw(dclone);
1776 Where $r1 can be a reference to any kind of data structure you'd like.
1777 It will be deeply copied. Because C<dclone> takes and returns references,
1778 you'd have to add extra punctuation if you had a hash of arrays that
1781 %newhash = %{ dclone(\%oldhash) };
1783 =head2 How do I define methods for every class/object?
1785 Use the UNIVERSAL class (see L<UNIVERSAL>).
1787 =head2 How do I verify a credit card checksum?
1789 Get the Business::CreditCard module from CPAN.
1791 =head2 How do I pack arrays of doubles or floats for XS code?
1793 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1794 If you're doing a lot of float or double processing, consider using
1795 the PDL module from CPAN instead--it makes number-crunching easy.
1797 =head1 AUTHOR AND COPYRIGHT
1799 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1800 All rights reserved.
1802 When included as part of the Standard Version of Perl, or as part of
1803 its complete documentation whether printed or otherwise, this work
1804 may be distributed only under the terms of Perl's Artistic License.
1805 Any distribution of this file or derivatives thereof I<outside>
1806 of that package require that special arrangements be made with
1809 Irrespective of its distribution, all code examples in this file
1810 are hereby placed into the public domain. You are permitted and
1811 encouraged to use this code in your own programs for fun
1812 or for profit as you see fit. A simple comment in the code giving
1813 credit would be courteous but is not required.