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" 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 unpack 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 =head2 How can I output Roman numerals?
188 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
190 =head2 Why aren't my random numbers random?
192 If you're using a version of Perl before 5.004, you must call C<srand>
193 once at the start of your program to seed the random number generator.
194 5.004 and later automatically call C<srand> at the beginning. Don't
195 call C<srand> more than once--you make your numbers less random, rather
198 Computers are good at being predictable and bad at being random
199 (despite appearances caused by bugs in your programs :-).
200 http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
201 Phoenix, talks more about this.. John von Neumann said, ``Anyone who
202 attempts to generate random numbers by deterministic means is, of
203 course, living in a state of sin.''
205 If you want numbers that are more random than C<rand> with C<srand>
206 provides, you should also check out the Math::TrulyRandom module from
207 CPAN. It uses the imperfections in your system's timer to generate
208 random numbers, but this takes quite a while. If you want a better
209 pseudorandom generator than comes with your operating system, look at
210 ``Numerical Recipes in C'' at http://www.nr.com/ .
214 =head2 How do I find the week-of-the-year/day-of-the-year?
216 The day of the year is in the array returned by localtime() (see
217 L<perlfunc/"localtime">):
219 $day_of_year = (localtime(time()))[7];
221 or more legibly (in 5.004 or higher):
224 $day_of_year = localtime(time())->yday;
226 You can find the week of the year by dividing this by 7:
228 $week_of_year = int($day_of_year / 7);
230 Of course, this believes that weeks start at zero. The Date::Calc
231 module from CPAN has a lot of date calculation functions, including
232 day of the year, week of the year, and so on. Note that not
233 all businesses consider ``week 1'' to be the same; for example,
234 American businesses often consider the first week with a Monday
235 in it to be Work Week #1, despite ISO 8601, which considers
236 WW1 to be the first week with a Thursday in it.
238 =head2 How do I find the current century or millennium?
240 Use the following simple functions:
243 return int((((localtime(shift || time))[5] + 1999))/100);
246 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
249 On some systems, you'll find that the POSIX module's strftime() function
250 has been extended in a non-standard way to use a C<%C> format, which they
251 sometimes claim is the "century". It isn't, because on most such systems,
252 this is only the first two digits of the four-digit year, and thus cannot
253 be used to reliably determine the current century or millennium.
255 =head2 How can I compare two dates and find the difference?
257 If you're storing your dates as epoch seconds then simply subtract one
258 from the other. If you've got a structured date (distinct year, day,
259 month, hour, minute, seconds values), then for reasons of accessibility,
260 simplicity, and efficiency, merely use either timelocal or timegm (from
261 the Time::Local module in the standard distribution) to reduce structured
262 dates to epoch seconds. However, if you don't know the precise format of
263 your dates, then you should probably use either of the Date::Manip and
264 Date::Calc modules from CPAN before you go hacking up your own parsing
265 routine to handle arbitrary date formats.
267 =head2 How can I take a string and turn it into epoch seconds?
269 If it's a regular enough string that it always has the same format,
270 you can split it up and pass the parts to C<timelocal> in the standard
271 Time::Local module. Otherwise, you should look into the Date::Calc
272 and Date::Manip modules from CPAN.
274 =head2 How can I find the Julian Day?
276 You could use Date::Calc's Delta_Days function and calculate the number
277 of days from there. Assuming that's what you really want, that is.
279 Before you immerse yourself too deeply in this, be sure to verify that it
280 is the I<Julian> Day you really want. Are they really just interested in
281 a way of getting serial days so that they can do date arithmetic? If you
282 are interested in performing date arithmetic, this can be done using
283 either Date::Manip or Date::Calc, without converting to Julian Day first.
285 There is too much confusion on this issue to cover in this FAQ, but the
286 term is applied (correctly) to a calendar now supplanted by the Gregorian
287 Calendar, with the Julian Calendar failing to adjust properly for leap
288 years on centennial years (among other annoyances). The term is also used
289 (incorrectly) to mean: [1] days in the Gregorian Calendar; and [2] days
290 since a particular starting time or `epoch', usually 1970 in the Unix
291 world and 1980 in the MS-DOS/Windows world. If you find that it is not
292 the first meaning that you really want, then check out the Date::Manip
293 and Date::Calc modules. (Thanks to David Cassell for most of this text.)
295 There is also an example of Julian date calculation that should help you in
296 http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz
298 =head2 How do I find yesterday's date?
300 The C<time()> function returns the current time in seconds since the
301 epoch. Take twenty-four hours off that:
303 $yesterday = time() - ( 24 * 60 * 60 );
305 Then you can pass this to C<localtime()> and get the individual year,
306 month, day, hour, minute, seconds values.
308 Note very carefully that the code above assumes that your days are
309 twenty-four hours each. For most people, there are two days a year
310 when they aren't: the switch to and from summer time throws this off.
311 A solution to this issue is offered by Russ Allbery.
314 my $now = defined $_[0] ? $_[0] : time;
315 my $then = $now - 60 * 60 * 24;
316 my $ndst = (localtime $now)[8] > 0;
317 my $tdst = (localtime $then)[8] > 0;
318 $then - ($tdst - $ndst) * 60 * 60;
320 # Should give you "this time yesterday" in seconds since epoch relative to
321 # the first argument or the current time if no argument is given and
322 # suitable for passing to localtime or whatever else you need to do with
323 # it. $ndst is whether we're currently in daylight savings time; $tdst is
324 # whether the point 24 hours ago was in daylight savings time. If $tdst
325 # and $ndst are the same, a boundary wasn't crossed, and the correction
326 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
327 # from yesterday's time since we gained an extra hour while going off
328 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
329 # negative hour (add an hour) to yesterday's time since we lost an hour.
331 # All of this is because during those days when one switches off or onto
332 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
334 # The explicit settings of $ndst and $tdst are necessary because localtime
335 # only says it returns the system tm struct, and the system tm struct at
336 # least on Solaris doesn't guarantee any particuliar positive value (like,
337 # say, 1) for isdst, just a positive value. And that value can
338 # potentially be negative, if DST information isn't available (this sub
339 # just treats those cases like no DST).
341 # Note that between 2am and 3am on the day after the time zone switches
342 # off daylight savings time, the exact hour of "yesterday" corresponding
343 # to the current hour is not clearly defined. Note also that if used
344 # between 2am and 3am the day after the change to daylight savings time,
345 # the result will be between 3am and 4am of the previous day; it's
346 # arguable whether this is correct.
348 # This sub does not attempt to deal with leap seconds (most things don't).
350 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
351 # This code is in the public domain
353 =head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
355 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
356 Y2K compliant (whatever that means). The programmers you've hired to
357 use it, however, probably are not.
359 Long answer: The question belies a true understanding of the issue.
360 Perl is just as Y2K compliant as your pencil--no more, and no less.
361 Can you use your pencil to write a non-Y2K-compliant memo? Of course
362 you can. Is that the pencil's fault? Of course it isn't.
364 The date and time functions supplied with perl (gmtime and localtime)
365 supply adequate information to determine the year well beyond 2000
366 (2038 is when trouble strikes for 32-bit machines). The year returned
367 by these functions when used in an array context is the year minus 1900.
368 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
369 number. To avoid the year 2000 problem simply do not treat the year as
370 a 2-digit number. It isn't.
372 When gmtime() and localtime() are used in scalar context they return
373 a timestamp string that contains a fully-expanded year. For example,
374 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
375 2001". There's no year 2000 problem here.
377 That doesn't mean that Perl can't be used to create non-Y2K compliant
378 programs. It can. But so can your pencil. It's the fault of the user,
379 not the language. At the risk of inflaming the NRA: ``Perl doesn't
380 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
385 =head2 How do I validate input?
387 The answer to this question is usually a regular expression, perhaps
388 with auxiliary logic. See the more specific questions (numbers, mail
389 addresses, etc.) for details.
391 =head2 How do I unescape a string?
393 It depends just what you mean by ``escape''. URL escapes are dealt
394 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
395 character are removed with:
399 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
401 =head2 How do I remove consecutive pairs of characters?
403 To turn C<"abbcccd"> into C<"abccd">:
405 s/(.)\1/$1/g; # add /s to include newlines
407 Here's a solution that turns "abbcccd" to "abcd":
409 y///cs; # y == tr, but shorter :-)
411 =head2 How do I expand function calls in a string?
413 This is documented in L<perlref>. In general, this is fraught with
414 quoting and readability problems, but it is possible. To interpolate
415 a subroutine call (in list context) into a string:
417 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
419 If you prefer scalar context, similar chicanery is also useful for
420 arbitrary expressions:
422 print "That yields ${\($n + 5)} widgets\n";
424 Version 5.004 of Perl had a bug that gave list context to the
425 expression in C<${...}>, but this is fixed in version 5.005.
427 See also ``How can I expand variables in text strings?'' in this
430 =head2 How do I find matching/nesting anything?
432 This isn't something that can be done in one regular expression, no
433 matter how complicated. To find something between two single
434 characters, a pattern like C</x([^x]*)x/> will get the intervening
435 bits in $1. For multiple ones, then something more like
436 C</alpha(.*?)omega/> would be needed. But none of these deals with
437 nested patterns, nor can they. For that you'll have to write a
440 If you are serious about writing a parser, there are a number of
441 modules or oddities that will make your life a lot easier. There is
442 the CPAN module Parse::RecDescent, the standard module Text::Balanced,
443 the byacc program, the CPAN module Parse::Yapp, and Mark-Jason
444 Dominus's excellent I<py> tool at http://www.plover.com/~mjd/perl/py/
447 One simple destructive, inside-out approach that you might try is to
448 pull out the smallest nesting parts one at a time:
450 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
451 # do something with $1
454 A more complicated and sneaky approach is to make Perl's regular
455 expression engine do it for you. This is courtesy Dean Inada, and
456 rather has the nature of an Obfuscated Perl Contest entry, but it
459 # $_ contains the string to parse
460 # BEGIN and END are the opening and closing markers for the
465 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
466 @$ = (eval{/$re/},$@!~/unmatched/);
467 print join("\n",@$[0..$#$]) if( $$[-1] );
469 =head2 How do I reverse a string?
471 Use reverse() in scalar context, as documented in
474 $reversed = reverse $string;
476 =head2 How do I expand tabs in a string?
478 You can do it yourself:
480 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
482 Or you can just use the Text::Tabs module (part of the standard perl
486 @expanded_lines = expand(@lines_with_tabs);
488 =head2 How do I reformat a paragraph?
490 Use Text::Wrap (part of the standard perl distribution):
493 print wrap("\t", ' ', @paragraphs);
495 The paragraphs you give to Text::Wrap should not contain embedded
496 newlines. Text::Wrap doesn't justify the lines (flush-right).
498 =head2 How can I access/change the first N letters of a string?
500 There are many ways. If you just want to grab a copy, use
503 $first_byte = substr($a, 0, 1);
505 If you want to modify part of a string, the simplest way is often to
506 use substr() as an lvalue:
508 substr($a, 0, 3) = "Tom";
510 Although those with a pattern matching kind of thought process will
515 =head2 How do I change the Nth occurrence of something?
517 You have to keep track of N yourself. For example, let's say you want
518 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
519 C<"whosoever"> or C<"whomsoever">, case insensitively. These
520 all assume that $_ contains the string to be altered.
524 ++$count == 5 # is it the 5th?
525 ? "${2}soever" # yes, swap
526 : $1 # renege and leave it there
529 In the more general case, you can use the C</g> modifier in a C<while>
530 loop, keeping count of matches.
534 $_ = "One fish two fish red fish blue fish";
535 while (/(\w+)\s+fish\b/gi) {
536 if (++$count == $WANT) {
537 print "The third fish is a $1 one.\n";
541 That prints out: C<"The third fish is a red one."> You can also use a
542 repetition count and repeated pattern like this:
544 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
546 =head2 How can I count the number of occurrences of a substring within a string?
548 There are a number of ways, with varying efficiency: If you want a
549 count of a certain single character (X) within a string, you can use the
550 C<tr///> function like so:
552 $string = "ThisXlineXhasXsomeXx'sXinXit";
553 $count = ($string =~ tr/X//);
554 print "There are $count X characters in the string";
556 This is fine if you are just looking for a single character. However,
557 if you are trying to count multiple character substrings within a
558 larger string, C<tr///> won't work. What you can do is wrap a while()
559 loop around a global pattern match. For example, let's count negative
562 $string = "-9 55 48 -2 23 -76 4 14 -44";
563 while ($string =~ /-\d+/g) { $count++ }
564 print "There are $count negative numbers in the string";
566 =head2 How do I capitalize all the words on one line?
568 To make the first letter of each word upper case:
570 $line =~ s/\b(\w)/\U$1/g;
572 This has the strange effect of turning "C<don't do it>" into "C<Don'T
573 Do It>". Sometimes you might want this, instead (Suggested by Brian
577 (^\w) #at the beginning of the line
579 (\s\w) #preceded by whitespace
582 $string =~ /([\w']+)/\u\L$1/g;
584 To make the whole line upper case:
588 To force each word to be lower case, with the first letter upper case:
590 $line =~ s/(\w+)/\u\L$1/g;
592 You can (and probably should) enable locale awareness of those
593 characters by placing a C<use locale> pragma in your program.
594 See L<perllocale> for endless details on locales.
596 This is sometimes referred to as putting something into "title
597 case", but that's not quite accurate. Consider the proper
598 capitalization of the movie I<Dr. Strangelove or: How I Learned to
599 Stop Worrying and Love the Bomb>, for example.
601 =head2 How can I split a [character] delimited string except when inside
602 [character]? (Comma-separated files)
604 Take the example case of trying to split a string that is comma-separated
605 into its different fields. (We'll pretend you said comma-separated, not
606 comma-delimited, which is different and almost never what you mean.) You
607 can't use C<split(/,/)> because you shouldn't split if the comma is inside
608 quotes. For example, take a data line like this:
610 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
612 Due to the restriction of the quotes, this is a fairly complex
613 problem. Thankfully, we have Jeffrey Friedl, author of a highly
614 recommended book on regular expressions, to handle these for us. He
615 suggests (assuming your string is contained in $text):
618 push(@new, $+) while $text =~ m{
619 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
623 push(@new, undef) if substr($text,-1,1) eq ',';
625 If you want to represent quotation marks inside a
626 quotation-mark-delimited field, escape them with backslashes (eg,
627 C<"like \"this\"">. Unescaping them is a task addressed earlier in
630 Alternatively, the Text::ParseWords module (part of the standard perl
631 distribution) lets you say:
633 use Text::ParseWords;
634 @new = quotewords(",", 0, $text);
636 There's also a Text::CSV module on CPAN.
638 =head2 How do I strip blank space from the beginning/end of a string?
640 Although the simplest approach would seem to be:
642 $string =~ s/^\s*(.*?)\s*$/$1/;
644 Not only is this unnecessarily slow and destructive, it also fails with
645 embedded newlines. It is much faster to do this operation in two steps:
650 Or more nicely written as:
657 This idiom takes advantage of the C<foreach> loop's aliasing
658 behavior to factor out common code. You can do this
659 on several strings at once, or arrays, or even the
660 values of a hash if you use a slice:
662 # trim whitespace in the scalar, the array,
663 # and all the values in the hash
664 foreach ($scalar, @array, @hash{keys %hash}) {
669 =head2 How do I pad a string with blanks or pad a number with zeroes?
671 (This answer contributed by Uri Guttman, with kibitzing from
674 In the following examples, C<$pad_len> is the length to which you wish
675 to pad the string, C<$text> or C<$num> contains the string to be padded,
676 and C<$pad_char> contains the padding character. You can use a single
677 character string constant instead of the C<$pad_char> variable if you
678 know what it is in advance. And in the same way you can use an integer in
679 place of C<$pad_len> if you know the pad length in advance.
681 The simplest method uses the C<sprintf> function. It can pad on the left
682 or right with blanks and on the left with zeroes and it will not
683 truncate the result. The C<pack> function can only pad strings on the
684 right with blanks and it will truncate the result to a maximum length of
687 # Left padding a string with blanks (no truncation):
688 $padded = sprintf("%${pad_len}s", $text);
690 # Right padding a string with blanks (no truncation):
691 $padded = sprintf("%-${pad_len}s", $text);
693 # Left padding a number with 0 (no truncation):
694 $padded = sprintf("%0${pad_len}d", $num);
696 # Right padding a string with blanks using pack (will truncate):
697 $padded = pack("A$pad_len",$text);
699 If you need to pad with a character other than blank or zero you can use
700 one of the following methods. They all generate a pad string with the
701 C<x> operator and combine that with C<$text>. These methods do
702 not truncate C<$text>.
704 Left and right padding with any character, creating a new string:
706 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
707 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
709 Left and right padding with any character, modifying C<$text> directly:
711 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
712 $text .= $pad_char x ( $pad_len - length( $text ) );
714 =head2 How do I extract selected columns from a string?
716 Use substr() or unpack(), both documented in L<perlfunc>.
717 If you prefer thinking in terms of columns instead of widths,
718 you can use this kind of thing:
720 # determine the unpack format needed to split Linux ps output
721 # arguments are cut columns
722 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
728 for my $place (@positions) {
729 $template .= "A" . ($place - $lastpos) . " ";
736 =head2 How do I find the soundex value of a string?
738 Use the standard Text::Soundex module distributed with perl.
739 But before you do so, you may want to determine whether `soundex' is in
740 fact what you think it is. Knuth's soundex algorithm compresses words
741 into a small space, and so it does not necessarily distinguish between
742 two words which you might want to appear separately. For example, the
743 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
744 If Text::Soundex does not do what you are looking for, you might want
745 to consider the String::Approx module available at CPAN.
747 =head2 How can I expand variables in text strings?
749 Let's assume that you have a string like:
751 $text = 'this has a $foo in it and a $bar';
753 If those were both global variables, then this would
756 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
758 But since they are probably lexicals, or at least, they could
759 be, you'd have to do this:
761 $text =~ s/(\$\w+)/$1/eeg;
762 die if $@; # needed /ee, not /e
764 It's probably better in the general case to treat those
765 variables as entries in some special hash. For example:
771 $text =~ s/\$(\w+)/$user_defs{$1}/g;
773 See also ``How do I expand function calls in a string?'' in this section
776 =head2 What's wrong with always quoting "$vars"?
778 The problem is that those double-quotes force stringification,
779 coercing numbers and references into strings, even when you
780 don't want them to be. Think of it this way: double-quote
781 expansion is used to produce new strings. If you already
782 have a string, why do you need more?
784 If you get used to writing odd things like these:
788 somefunc("$var"); # BAD
790 You'll be in trouble. Those should (in 99.8% of the cases) be
791 the simpler and more direct:
797 Otherwise, besides slowing you down, you're going to break code when
798 the thing in the scalar is actually neither a string nor a number, but
804 my $oref = "$aref"; # WRONG
807 You can also get into subtle problems on those few operations in Perl
808 that actually do care about the difference between a string and a
809 number, such as the magical C<++> autoincrement operator or the
812 Stringification also destroys arrays.
815 print "@lines"; # WRONG - extra blanks
816 print @lines; # right
818 =head2 Why don't my E<lt>E<lt>HERE documents work?
820 Check for these three things:
824 =item 1. There must be no space after the << part.
826 =item 2. There (probably) should be a semicolon at the end.
828 =item 3. You can't (easily) have any space in front of the tag.
832 If you want to indent the text in the here document, you
836 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
841 But the HERE_TARGET must still be flush against the margin.
842 If you want that indented also, you'll have to quote
845 ($quote = <<' FINIS') =~ s/^\s+//gm;
846 ...we will have peace, when you and all your works have
847 perished--and the works of your dark master to whom you
848 would deliver us. You are a liar, Saruman, and a corrupter
849 of men's hearts. --Theoden in /usr/src/perl/taint.c
851 $quote =~ s/\s*--/\n--/;
853 A nice general-purpose fixer-upper function for indented here documents
854 follows. It expects to be called with a here document as its argument.
855 It looks to see whether each line begins with a common substring, and
856 if so, strips that off. Otherwise, it takes the amount of leading
857 white space found on the first line and removes that much off each
862 my ($white, $leader); # common white space and common leading string
863 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
864 ($white, $leader) = ($2, quotemeta($1));
866 ($white, $leader) = (/^(\s+)/, '');
868 s/^\s*?$leader(?:$white)?//gm;
872 This works with leading special strings, dynamically determined:
874 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
877 @@@ SAVEI32(runlevel);
879 @@@ while ( op = (*op->op_ppaddr)() );
883 MAIN_INTERPRETER_LOOP
885 Or with a fixed amount of leading white space, with remaining
886 indentation correctly preserved:
888 $poem = fix<<EVER_ON_AND_ON;
889 Now far ahead the Road has gone,
890 And I must follow, if I can,
891 Pursuing it with eager feet,
892 Until it joins some larger way
893 Where many paths and errands meet.
894 And whither then? I cannot say.
895 --Bilbo in /usr/src/perl/pp_ctl.c
900 =head2 What is the difference between a list and an array?
902 An array has a changeable length. A list does not. An array is something
903 you can push or pop, while a list is a set of values. Some people make
904 the distinction that a list is a value while an array is a variable.
905 Subroutines are passed and return lists, you put things into list
906 context, you initialize arrays with lists, and you foreach() across
907 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
908 in scalar context behave like the number of elements in them, subroutines
909 access their arguments through the array C<@_>, push/pop/shift only work
912 As a side note, there's no such thing as a list in scalar context.
915 $scalar = (2, 5, 7, 9);
917 you're using the comma operator in scalar context, so it uses the scalar
918 comma operator. There never was a list there at all! This causes the
919 last value to be returned: 9.
921 =head2 What is the difference between $array[1] and @array[1]?
923 The former is a scalar value, the latter an array slice, which makes
924 it a list with one (scalar) value. You should use $ when you want a
925 scalar value (most of the time) and @ when you want a list with one
926 scalar value in it (very, very rarely; nearly never, in fact).
928 Sometimes it doesn't make a difference, but sometimes it does.
929 For example, compare:
931 $good[0] = `some program that outputs several lines`;
935 @bad[0] = `same program that outputs several lines`;
937 The B<-w> flag will warn you about these matters.
939 =head2 How can I remove duplicate elements from a list or array?
941 There are several possible ways, depending on whether the array is
942 ordered and whether you wish to preserve the ordering.
946 =item a) If @in is sorted, and you want @out to be sorted:
947 (this assumes all true values in the array)
950 @out = grep($_ ne $prev && ($prev = $_), @in);
952 This is nice in that it doesn't use much extra memory, simulating
953 uniq(1)'s behavior of removing only adjacent duplicates. It's less
954 nice in that it won't work with false values like undef, 0, or "";
955 "0 but true" is ok, though.
957 =item b) If you don't know whether @in is sorted:
960 @out = grep(!$saw{$_}++, @in);
962 =item c) Like (b), but @in contains only small integers:
964 @out = grep(!$saw[$_]++, @in);
966 =item d) A way to do (b) without any loops or greps:
970 @out = sort keys %saw; # remove sort if undesired
972 =item e) Like (d), but @in contains only small positive integers:
980 But perhaps you should have been using a hash all along, eh?
982 =head2 How can I tell whether a list or array contains a certain element?
984 Hearing the word "in" is an I<in>dication that you probably should have
985 used a hash, not a list or array, to store your data. Hashes are
986 designed to answer this question quickly and efficiently. Arrays aren't.
988 That being said, there are several ways to approach this. If you
989 are going to make this query many times over arbitrary string values,
990 the fastest way is probably to invert the original array and keep an
991 associative array lying about whose keys are the first array's values.
993 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
995 for (@blues) { $is_blue{$_} = 1 }
997 Now you can check whether $is_blue{$some_color}. It might have been a
998 good idea to keep the blues all in a hash in the first place.
1000 If the values are all small integers, you could use a simple indexed
1001 array. This kind of an array will take up less space:
1003 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1004 undef @is_tiny_prime;
1005 for (@primes) { $is_tiny_prime[$_] = 1 }
1006 # or simply @istiny_prime[@primes] = (1) x @primes;
1008 Now you check whether $is_tiny_prime[$some_number].
1010 If the values in question are integers instead of strings, you can save
1011 quite a lot of space by using bit strings instead:
1013 @articles = ( 1..10, 150..2000, 2017 );
1015 for (@articles) { vec($read,$_,1) = 1 }
1017 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1021 $is_there = grep $_ eq $whatever, @array;
1025 $is_there = grep /$whatever/, @array;
1027 These are slow (checks every element even if the first matches),
1028 inefficient (same reason), and potentially buggy (what if there are
1029 regex characters in $whatever?). If you're only testing once, then
1033 foreach $elt (@array) {
1034 if ($elt eq $elt_to_find) {
1039 if ($is_there) { ... }
1041 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1043 Use a hash. Here's code to do both and more. It assumes that
1044 each element is unique in a given array:
1046 @union = @intersection = @difference = ();
1048 foreach $element (@array1, @array2) { $count{$element}++ }
1049 foreach $element (keys %count) {
1050 push @union, $element;
1051 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1054 Note that this is the I<symmetric difference>, that is, all elements in
1055 either A or in B, but not in both. Think of it as an xor operation.
1057 =head2 How do I test whether two arrays or hashes are equal?
1059 The following code works for single-level arrays. It uses a stringwise
1060 comparison, and does not distinguish defined versus undefined empty
1061 strings. Modify if you have other needs.
1063 $are_equal = compare_arrays(\@frogs, \@toads);
1065 sub compare_arrays {
1066 my ($first, $second) = @_;
1067 local $^W = 0; # silence spurious -w undef complaints
1068 return 0 unless @$first == @$second;
1069 for (my $i = 0; $i < @$first; $i++) {
1070 return 0 if $first->[$i] ne $second->[$i];
1075 For multilevel structures, you may wish to use an approach more
1076 like this one. It uses the CPAN module FreezeThaw:
1078 use FreezeThaw qw(cmpStr);
1079 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1081 printf "a and b contain %s arrays\n",
1082 cmpStr(\@a, \@b) == 0
1086 This approach also works for comparing hashes. Here
1087 we'll demonstrate two different answers:
1089 use FreezeThaw qw(cmpStr cmpStrHard);
1091 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1095 printf "a and b contain %s hashes\n",
1096 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1098 printf "a and b contain %s hashes\n",
1099 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1102 The first reports that both those the hashes contain the same data,
1103 while the second reports that they do not. Which you prefer is left as
1104 an exercise to the reader.
1106 =head2 How do I find the first array element for which a condition is true?
1108 You can use this if you care about the index:
1110 for ($i= 0; $i < @array; $i++) {
1111 if ($array[$i] eq "Waldo") {
1117 Now C<$found_index> has what you want.
1119 =head2 How do I handle linked lists?
1121 In general, you usually don't need a linked list in Perl, since with
1122 regular arrays, you can push and pop or shift and unshift at either end,
1123 or you can use splice to add and/or remove arbitrary number of elements at
1124 arbitrary points. Both pop and shift are both O(1) operations on perl's
1125 dynamic arrays. In the absence of shifts and pops, push in general
1126 needs to reallocate on the order every log(N) times, and unshift will
1127 need to copy pointers each time.
1129 If you really, really wanted, you could use structures as described in
1130 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1131 to do. For example, imagine a list node like this:
1138 You could walk the list this way:
1141 for ($node = $head; $node; $node = $node->{LINK}) {
1142 print $node->{VALUE}, " ";
1146 You could grow the list this way:
1149 $tail = append($head, 1); # grow a new head
1150 for $value ( 2 .. 10 ) {
1151 $tail = append($tail, $value);
1155 my($list, $value) = @_;
1156 my $node = { VALUE => $value };
1158 $node->{LINK} = $list->{LINK};
1159 $list->{LINK} = $node;
1161 $_[0] = $node; # replace caller's version
1166 But again, Perl's built-in are virtually always good enough.
1168 =head2 How do I handle circular lists?
1170 Circular lists could be handled in the traditional fashion with linked
1171 lists, or you could just do something like this with an array:
1173 unshift(@array, pop(@array)); # the last shall be first
1174 push(@array, shift(@array)); # and vice versa
1176 =head2 How do I shuffle an array randomly?
1180 # fisher_yates_shuffle( \@array ) :
1181 # generate a random permutation of @array in place
1182 sub fisher_yates_shuffle {
1185 for ($i = @$array; --$i; ) {
1186 my $j = int rand ($i+1);
1188 @$array[$i,$j] = @$array[$j,$i];
1192 fisher_yates_shuffle( \@array ); # permutes @array in place
1194 You've probably seen shuffling algorithms that work using splice,
1195 randomly picking another element to swap the current element with:
1199 @old = 1 .. 10; # just a demo
1201 push(@new, splice(@old, rand @old, 1));
1204 This is bad because splice is already O(N), and since you do it N times,
1205 you just invented a quadratic algorithm; that is, O(N**2). This does
1206 not scale, although Perl is so efficient that you probably won't notice
1207 this until you have rather largish arrays.
1209 =head2 How do I process/modify each element of an array?
1211 Use C<for>/C<foreach>:
1214 s/foo/bar/; # change that word
1215 y/XZ/ZX/; # swap those letters
1218 Here's another; let's compute spherical volumes:
1220 for (@volumes = @radii) { # @volumes has changed parts
1222 $_ *= (4/3) * 3.14159; # this will be constant folded
1225 If you want to do the same thing to modify the values of the hash,
1226 you may not use the C<values> function, oddly enough. You need a slice:
1228 for $orbit ( @orbits{keys %orbits} ) {
1229 ($orbit **= 3) *= (4/3) * 3.14159;
1232 =head2 How do I select a random element from an array?
1234 Use the rand() function (see L<perlfunc/rand>):
1236 # at the top of the program:
1237 srand; # not needed for 5.004 and later
1240 $index = rand @array;
1241 $element = $array[$index];
1243 Make sure you I<only call srand once per program, if then>.
1244 If you are calling it more than once (such as before each
1245 call to rand), you're almost certainly doing something wrong.
1247 =head2 How do I permute N elements of a list?
1249 Here's a little program that generates all permutations
1250 of all the words on each line of input. The algorithm embodied
1251 in the permute() function should work on any list:
1254 # tsc-permute: permute each word of input
1255 permute([split], []);
1257 my @items = @{ $_[0] };
1258 my @perms = @{ $_[1] };
1262 my(@newitems,@newperms,$i);
1263 foreach $i (0 .. $#items) {
1266 unshift(@newperms, splice(@newitems, $i, 1));
1267 permute([@newitems], [@newperms]);
1272 =head2 How do I sort an array by (anything)?
1274 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1276 @list = sort { $a <=> $b } @list;
1278 The default sort function is cmp, string comparison, which would
1279 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
1280 the numerical comparison operator.
1282 If you have a complicated function needed to pull out the part you
1283 want to sort on, then don't do it inside the sort function. Pull it
1284 out first, because the sort BLOCK can be called many times for the
1285 same element. Here's an example of how to pull out the first word
1286 after the first number on each item, and then sort those words
1291 ($item) = /\d+\s*(\S+)/;
1292 push @idx, uc($item);
1294 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1296 Which could also be written this way, using a trick
1297 that's come to be known as the Schwartzian Transform:
1299 @sorted = map { $_->[0] }
1300 sort { $a->[1] cmp $b->[1] }
1301 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1303 If you need to sort on several fields, the following paradigm is useful.
1305 @sorted = sort { field1($a) <=> field1($b) ||
1306 field2($a) cmp field2($b) ||
1307 field3($a) cmp field3($b)
1310 This can be conveniently combined with precalculation of keys as given
1313 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1316 See also the question below on sorting hashes.
1318 =head2 How do I manipulate arrays of bits?
1320 Use pack() and unpack(), or else vec() and the bitwise operations.
1322 For example, this sets $vec to have bit N set if $ints[N] was set:
1325 foreach(@ints) { vec($vec,$_,1) = 1 }
1327 And here's how, given a vector in $vec, you can
1328 get those bits into your @ints array:
1330 sub bitvec_to_list {
1333 # Find null-byte density then select best algorithm
1334 if ($vec =~ tr/\0// / length $vec > 0.95) {
1337 # This method is faster with mostly null-bytes
1338 while($vec =~ /[^\0]/g ) {
1339 $i = -9 + 8 * pos $vec;
1340 push @ints, $i if vec($vec, ++$i, 1);
1341 push @ints, $i if vec($vec, ++$i, 1);
1342 push @ints, $i if vec($vec, ++$i, 1);
1343 push @ints, $i if vec($vec, ++$i, 1);
1344 push @ints, $i if vec($vec, ++$i, 1);
1345 push @ints, $i if vec($vec, ++$i, 1);
1346 push @ints, $i if vec($vec, ++$i, 1);
1347 push @ints, $i if vec($vec, ++$i, 1);
1350 # This method is a fast general algorithm
1352 my $bits = unpack "b*", $vec;
1353 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1354 push @ints, pos $bits while($bits =~ /1/g);
1359 This method gets faster the more sparse the bit vector is.
1360 (Courtesy of Tim Bunce and Winfried Koenig.)
1362 Here's a demo on how to use vec():
1365 $vector = "\xff\x0f\xef\xfe";
1366 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1367 unpack("N", $vector), "\n";
1368 $is_set = vec($vector, 23, 1);
1369 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1387 my ($offset, $width, $value) = @_;
1389 vec($vector, $offset, $width) = $value;
1390 print "offset=$offset width=$width value=$value\n";
1396 my $bits = unpack("b*", $vector);
1400 print "vector length in bytes: ", length($vector), "\n";
1401 @bytes = unpack("A8" x length($vector), $bits);
1402 print "bits are: @bytes\n\n";
1405 =head2 Why does defined() return true on empty arrays and hashes?
1407 The short story is that you should probably only use defined on scalars or
1408 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1409 in the 5.004 release or later of Perl for more detail.
1411 =head1 Data: Hashes (Associative Arrays)
1413 =head2 How do I process an entire hash?
1415 Use the each() function (see L<perlfunc/each>) if you don't care
1416 whether it's sorted:
1418 while ( ($key, $value) = each %hash) {
1419 print "$key = $value\n";
1422 If you want it sorted, you'll have to use foreach() on the result of
1423 sorting the keys as shown in an earlier question.
1425 =head2 What happens if I add or remove keys from a hash while iterating over it?
1429 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1430 interating over it. In Perl 5 you can delete from it, but you still
1431 can't add to it, because that might cause a doubling of the hash table,
1432 in which half the entries get copied up to the new top half of the
1433 table, at which point you've totally bamboozled the interator code.
1434 Even if the table doesn't double, there's no telling whether your new
1435 entry will be inserted before or after the current iterator position.
1437 Either treasure up your changes and make them after the iterator finishes,
1438 or use keys to fetch all the old keys at once, and iterate over the list
1441 =head2 How do I look up a hash element by value?
1443 Create a reverse hash:
1445 %by_value = reverse %by_key;
1446 $key = $by_value{$value};
1448 That's not particularly efficient. It would be more space-efficient
1451 while (($key, $value) = each %by_key) {
1452 $by_value{$value} = $key;
1455 If your hash could have repeated values, the methods above will only find
1456 one of the associated keys. This may or may not worry you. If it does
1457 worry you, you can always reverse the hash into a hash of arrays instead:
1459 while (($key, $value) = each %by_key) {
1460 push @{$key_list_by_value{$value}}, $key;
1463 =head2 How can I know how many entries are in a hash?
1465 If you mean how many keys, then all you have to do is
1466 take the scalar sense of the keys() function:
1468 $num_keys = scalar keys %hash;
1470 In void context, the keys() function just resets the iterator, which is
1471 faster for tied hashes than would be iterating through the whole
1472 hash, one key-value pair at a time.
1474 =head2 How do I sort a hash (optionally by value instead of key)?
1476 Internally, hashes are stored in a way that prevents you from imposing
1477 an order on key-value pairs. Instead, you have to sort a list of the
1480 @keys = sort keys %hash; # sorted by key
1482 $hash{$a} cmp $hash{$b}
1483 } keys %hash; # and by value
1485 Here we'll do a reverse numeric sort by value, and if two keys are
1486 identical, sort by length of key, and if that fails, by straight ASCII
1487 comparison of the keys (well, possibly modified by your locale -- see
1491 $hash{$b} <=> $hash{$a}
1493 length($b) <=> length($a)
1498 =head2 How can I always keep my hash sorted?
1500 You can look into using the DB_File module and tie() using the
1501 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1502 The Tie::IxHash module from CPAN might also be instructive.
1504 =head2 What's the difference between "delete" and "undef" with hashes?
1506 Hashes are pairs of scalars: the first is the key, the second is the
1507 value. The key will be coerced to a string, although the value can be
1508 any kind of scalar: string, number, or reference. If a key C<$key> is
1509 present in the array, C<exists($key)> will return true. The value for
1510 a given key can be C<undef>, in which case C<$array{$key}> will be
1511 C<undef> while C<$exists{$key}> will return true. This corresponds to
1512 (C<$key>, C<undef>) being in the hash.
1514 Pictures help... here's the C<%ary> table:
1524 And these conditions hold
1528 defined $ary{'d'} is true
1529 defined $ary{'a'} is true
1530 exists $ary{'a'} is true (perl5 only)
1531 grep ($_ eq 'a', keys %ary) is true
1537 your table now reads:
1548 and these conditions now hold; changes in caps:
1552 defined $ary{'d'} is true
1553 defined $ary{'a'} is FALSE
1554 exists $ary{'a'} is true (perl5 only)
1555 grep ($_ eq 'a', keys %ary) is true
1557 Notice the last two: you have an undef value, but a defined key!
1563 your table now reads:
1572 and these conditions now hold; changes in caps:
1576 defined $ary{'d'} is true
1577 defined $ary{'a'} is false
1578 exists $ary{'a'} is FALSE (perl5 only)
1579 grep ($_ eq 'a', keys %ary) is FALSE
1581 See, the whole entry is gone!
1583 =head2 Why don't my tied hashes make the defined/exists distinction?
1585 They may or may not implement the EXISTS() and DEFINED() methods
1586 differently. For example, there isn't the concept of undef with hashes
1587 that are tied to DBM* files. This means the true/false tables above
1588 will give different results when used on such a hash. It also means
1589 that exists and defined do the same thing with a DBM* file, and what
1590 they end up doing is not what they do with ordinary hashes.
1592 =head2 How do I reset an each() operation part-way through?
1594 Using C<keys %hash> in scalar context returns the number of keys in
1595 the hash I<and> resets the iterator associated with the hash. You may
1596 need to do this if you use C<last> to exit a loop early so that when you
1597 re-enter it, the hash iterator has been reset.
1599 =head2 How can I get the unique keys from two hashes?
1601 First you extract the keys from the hashes into lists, then solve
1602 the "removing duplicates" problem described above. For example:
1605 for $element (keys(%foo), keys(%bar)) {
1612 @uniq = keys %{{%foo,%bar}};
1614 Or if you really want to save space:
1617 while (defined ($key = each %foo)) {
1620 while (defined ($key = each %bar)) {
1625 =head2 How can I store a multidimensional array in a DBM file?
1627 Either stringify the structure yourself (no fun), or else
1628 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1629 it on top of either DB_File or GDBM_File.
1631 =head2 How can I make my hash remember the order I put elements into it?
1633 Use the Tie::IxHash from CPAN.
1636 tie(%myhash, Tie::IxHash);
1637 for ($i=0; $i<20; $i++) {
1640 @keys = keys %myhash;
1641 # @keys = (0,1,2,3,...)
1643 =head2 Why does passing a subroutine an undefined element in a hash create it?
1645 If you say something like:
1647 somefunc($hash{"nonesuch key here"});
1649 Then that element "autovivifies"; that is, it springs into existence
1650 whether you store something there or not. That's because functions
1651 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1652 it has to be ready to write it back into the caller's version.
1654 This has been fixed as of perl5.004.
1656 Normally, merely accessing a key's value for a nonexistent key does
1657 I<not> cause that key to be forever there. This is different than
1660 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1662 Usually a hash ref, perhaps like this:
1667 TITLE => "deputy peon",
1670 PALS => [ "Norbert", "Rhys", "Phineas"],
1673 References are documented in L<perlref> and the upcoming L<perlreftut>.
1674 Examples of complex data structures are given in L<perldsc> and
1675 L<perllol>. Examples of structures and object-oriented classes are
1678 =head2 How can I use a reference as a hash key?
1680 You can't do this directly, but you could use the standard Tie::Refhash
1681 module distributed with perl.
1685 =head2 How do I handle binary data correctly?
1687 Perl is binary clean, so this shouldn't be a problem. For example,
1688 this works fine (assuming the files are found):
1690 if (`cat /vmunix` =~ /gzip/) {
1691 print "Your kernel is GNU-zip enabled!\n";
1694 On less elegant (read: Byzantine) systems, however, you have
1695 to play tedious games with "text" versus "binary" files. See
1696 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1697 systems are curses out of Microsoft, who seem to be committed to putting
1698 the backward into backward compatibility.
1700 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1702 If you want to deal with multibyte characters, however, there are
1703 some gotchas. See the section on Regular Expressions.
1705 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1707 Assuming that you don't care about IEEE notations like "NaN" or
1708 "Infinity", you probably just want to use a regular expression.
1710 if (/\D/) { print "has nondigits\n" }
1711 if (/^\d+$/) { print "is a whole number\n" }
1712 if (/^-?\d+$/) { print "is an integer\n" }
1713 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1714 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1715 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1716 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1717 { print "a C float" }
1719 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1720 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1721 wrapper function for more convenient access. This function takes
1722 a string and returns the number it found, or C<undef> for input that
1723 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1724 if you just want to say, ``Is this a float?''
1727 use POSIX qw(strtod);
1732 my($num, $unparsed) = strtod($str);
1733 if (($str eq '') || ($unparsed != 0) || $!) {
1740 sub is_numeric { defined &getnum }
1742 Or you could check out
1743 http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
1744 instead. The POSIX module (part of the standard Perl distribution)
1745 provides the C<strtol> and C<strtod> for converting strings to double
1746 and longs, respectively.
1748 =head2 How do I keep persistent data across program calls?
1750 For some specific applications, you can use one of the DBM modules.
1751 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1752 Storable, or Class::Eroot modules from CPAN. Here's one example using
1753 Storable's C<store> and C<retrieve> functions:
1756 store(\%hash, "filename");
1759 $href = retrieve("filename"); # by ref
1760 %hash = %{ retrieve("filename") }; # direct to hash
1762 =head2 How do I print out or copy a recursive data structure?
1764 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1765 for printing out data structures. The Storable module, found on CPAN,
1766 provides a function called C<dclone> that recursively copies its argument.
1768 use Storable qw(dclone);
1771 Where $r1 can be a reference to any kind of data structure you'd like.
1772 It will be deeply copied. Because C<dclone> takes and returns references,
1773 you'd have to add extra punctuation if you had a hash of arrays that
1776 %newhash = %{ dclone(\%oldhash) };
1778 =head2 How do I define methods for every class/object?
1780 Use the UNIVERSAL class (see L<UNIVERSAL>).
1782 =head2 How do I verify a credit card checksum?
1784 Get the Business::CreditCard module from CPAN.
1786 =head2 How do I pack arrays of doubles or floats for XS code?
1788 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1789 If you're doing a lot of float or double processing, consider using
1790 the PDL module from CPAN instead--it makes number-crunching easy.
1792 =head1 AUTHOR AND COPYRIGHT
1794 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1795 All rights reserved.
1797 When included as part of the Standard Version of Perl, or as part of
1798 its complete documentation whether printed or otherwise, this work
1799 may be distributed only under the terms of Perl's Artistic License.
1800 Any distribution of this file or derivatives thereof I<outside>
1801 of that package require that special arrangements be made with
1804 Irrespective of its distribution, all code examples in this file
1805 are hereby placed into the public domain. You are permitted and
1806 encouraged to use this code in your own programs for fun
1807 or for profit as you see fit. A simple comment in the code giving
1808 credit would be courteous but is not required.