3 perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
7 The section of the FAQ answers questions related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximated on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to an internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers. (See L<perlvar/"$#"> 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 If precision is important, such as when dealing with money, it's good
43 to work with integers and then divide at the last possible moment.
44 For example, work in pennies (1995) instead of dollars and cents
45 (19.95) and divide by 100 at the end.
47 To get rid of the superfluous digits, just use a format (eg,
48 C<printf("%.2f", 19.95)>) to get the required precision.
49 See L<perlop/"Floating-point Arithmetic">.
51 =head2 Why isn't my octal data interpreted correctly?
53 Perl only understands octal and hex numbers as such when they occur
54 as literals in your program. Octal literals in perl must start with
55 a leading "0" and hexadecimal literals must start with a leading "0x".
56 If they are read in from somewhere and assigned, no automatic
57 conversion takes place. You must explicitly use oct() or hex() if you
58 want the values converted to decimal. oct() interprets
59 both hex ("0x350") numbers and octal ones ("0350" or even without the
60 leading "0", like "377"), while hex() only converts hexadecimal ones,
61 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
62 The inverse mapping from decimal to octal can be done with either the
63 "%o" or "%O" sprintf() formats. To get from decimal to hex try either
64 the "%x" or the "%X" formats to sprintf().
66 This problem shows up most often when people try using chmod(), mkdir(),
67 umask(), or sysopen(), which by widespread tradition typically take
70 chmod(644, $file); # WRONG
71 chmod(0644, $file); # right
73 Note the mistake in the first line was specifying the decimal literal
74 644, rather than the intended octal literal 0644. The problem can
77 printf("%#o",644); # prints 01204
79 Surely you had not intended C<chmod(01204, $file);> - did you? If you
80 want to use numeric literals as arguments to chmod() et al. then please
81 try to express them as octal constants, that is with a leading zero and
82 with the following digits restricted to the set 0..7.
84 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
86 Remember that int() merely truncates toward 0. For rounding to a
87 certain number of digits, sprintf() or printf() is usually the easiest
90 printf("%.3f", 3.1415926535); # prints 3.142
92 The POSIX module (part of the standard Perl distribution) implements
93 ceil(), floor(), and a number of other mathematical and trigonometric
97 $ceil = ceil(3.5); # 4
98 $floor = floor(3.5); # 3
100 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
101 module. With 5.004, the Math::Trig module (part of the standard Perl
102 distribution) implements the trigonometric functions. Internally it
103 uses the Math::Complex module and some functions can break out from
104 the real axis into the complex plane, for example the inverse sine of
107 Rounding in financial applications can have serious implications, and
108 the rounding method used should be specified precisely. In these
109 cases, it probably pays not to trust whichever system rounding is
110 being used by Perl, but to instead implement the rounding function you
113 To see why, notice how you'll still have an issue on half-way-point
116 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
118 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
119 0.8 0.8 0.9 0.9 1.0 1.0
121 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
122 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
123 machines) will work pretty much like mathematical integers. Other numbers
126 =head2 How do I convert bits into ints?
128 To turn a string of 1s and 0s like C<10110110> into a scalar containing
129 its binary value, use the pack() and unpack() functions (documented in
130 L<perlfunc/"pack"> and L<perlfunc/"unpack">):
132 $decimal = unpack('c', pack('B8', '10110110'));
134 This packs the string C<10110110> into an eight bit binary structure.
135 This is then unpacked as a character, which returns its ordinal value.
137 This does the same thing:
139 $decimal = ord(pack('B8', '10110110'));
141 Here's an example of going the other way:
143 $binary_string = unpack('B*', "\x29");
145 =head2 Why doesn't & work the way I want it to?
147 The behavior of binary arithmetic operators depends on whether they're
148 used on numbers or strings. The operators treat a string as a series
149 of bits and work with that (the string C<"3"> is the bit pattern
150 C<00110011>). The operators work with the binary form of a number
151 (the number C<3> is treated as the bit pattern C<00000011>).
153 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
154 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
157 Most problems with C<&> and C<|> arise because the programmer thinks
158 they have a number but really it's a string. The rest arise because
161 if ("\020\020" & "\101\101") {
165 but a string consisting of two null bytes (the result of C<"\020\020"
166 & "\101\101">) is not a false value in Perl. You need:
168 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
172 =head2 How do I multiply matrices?
174 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
175 or the PDL extension (also available from CPAN).
177 =head2 How do I perform an operation on a series of integers?
179 To call a function on each element in an array, and collect the
182 @results = map { my_func($_) } @array;
186 @triple = map { 3 * $_ } @single;
188 To call a function on each element of an array, but ignore the
191 foreach $iterator (@array) {
192 some_func($iterator);
195 To call a function on each integer in a (small) range, you B<can> use:
197 @results = map { some_func($_) } (5 .. 25);
199 but you should be aware that the C<..> operator creates an array of
200 all integers in the range. This can take a lot of memory for large
204 for ($i=5; $i < 500_005; $i++) {
205 push(@results, some_func($i));
208 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
209 loop will iterate over the range, without creating the entire range.
211 for my $i (5 .. 500_005) {
212 push(@results, some_func($i));
215 will not create a list of 500,000 integers.
217 =head2 How can I output Roman numerals?
219 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
221 =head2 Why aren't my random numbers random?
223 If you're using a version of Perl before 5.004, you must call C<srand>
224 once at the start of your program to seed the random number generator.
225 5.004 and later automatically call C<srand> at the beginning. Don't
226 call C<srand> more than once--you make your numbers less random, rather
229 Computers are good at being predictable and bad at being random
230 (despite appearances caused by bugs in your programs :-).
231 http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
232 Phoenix, talks more about this. John von Neumann said, ``Anyone who
233 attempts to generate random numbers by deterministic means is, of
234 course, living in a state of sin.''
236 If you want numbers that are more random than C<rand> with C<srand>
237 provides, you should also check out the Math::TrulyRandom module from
238 CPAN. It uses the imperfections in your system's timer to generate
239 random numbers, but this takes quite a while. If you want a better
240 pseudorandom generator than comes with your operating system, look at
241 ``Numerical Recipes in C'' at http://www.nr.com/ .
245 =head2 How do I find the week-of-the-year/day-of-the-year?
247 The day of the year is in the array returned by localtime() (see
248 L<perlfunc/"localtime">):
250 $day_of_year = (localtime(time()))[7];
252 =head2 How do I find the current century or millennium?
254 Use the following simple functions:
257 return int((((localtime(shift || time))[5] + 1999))/100);
260 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
263 On some systems, you'll find that the POSIX module's strftime() function
264 has been extended in a non-standard way to use a C<%C> format, which they
265 sometimes claim is the "century". It isn't, because on most such systems,
266 this is only the first two digits of the four-digit year, and thus cannot
267 be used to reliably determine the current century or millennium.
269 =head2 How can I compare two dates and find the difference?
271 If you're storing your dates as epoch seconds then simply subtract one
272 from the other. If you've got a structured date (distinct year, day,
273 month, hour, minute, seconds values), then for reasons of accessibility,
274 simplicity, and efficiency, merely use either timelocal or timegm (from
275 the Time::Local module in the standard distribution) to reduce structured
276 dates to epoch seconds. However, if you don't know the precise format of
277 your dates, then you should probably use either of the Date::Manip and
278 Date::Calc modules from CPAN before you go hacking up your own parsing
279 routine to handle arbitrary date formats.
281 =head2 How can I take a string and turn it into epoch seconds?
283 If it's a regular enough string that it always has the same format,
284 you can split it up and pass the parts to C<timelocal> in the standard
285 Time::Local module. Otherwise, you should look into the Date::Calc
286 and Date::Manip modules from CPAN.
288 =head2 How can I find the Julian Day?
290 Use the Time::JulianDay module (part of the Time-modules bundle
291 available from CPAN.)
293 Before you immerse yourself too deeply in this, be sure to verify that
294 it is the I<Julian> Day you really want. Are you interested in a way
295 of getting serial days so that you just can tell how many days they
296 are apart or so that you can do also other date arithmetic? If you
297 are interested in performing date arithmetic, this can be done using
298 modules Date::Manip or Date::Calc.
300 There is too many details and much confusion on this issue to cover in
301 this FAQ, but the term is applied (correctly) to a calendar now
302 supplanted by the Gregorian Calendar, with the Julian Calendar failing
303 to adjust properly for leap years on centennial years (among other
304 annoyances). The term is also used (incorrectly) to mean: [1] days in
305 the Gregorian Calendar; and [2] days since a particular starting time
306 or `epoch', usually 1970 in the Unix world and 1980 in the
307 MS-DOS/Windows world. If you find that it is not the first meaning
308 that you really want, then check out the Date::Manip and Date::Calc
309 modules. (Thanks to David Cassell for most of this text.)
311 =head2 How do I find yesterday's date?
313 The C<time()> function returns the current time in seconds since the
314 epoch. Take twenty-four hours off that:
316 $yesterday = time() - ( 24 * 60 * 60 );
318 Then you can pass this to C<localtime()> and get the individual year,
319 month, day, hour, minute, seconds values.
321 Note very carefully that the code above assumes that your days are
322 twenty-four hours each. For most people, there are two days a year
323 when they aren't: the switch to and from summer time throws this off.
324 A solution to this issue is offered by Russ Allbery.
327 my $now = defined $_[0] ? $_[0] : time;
328 my $then = $now - 60 * 60 * 24;
329 my $ndst = (localtime $now)[8] > 0;
330 my $tdst = (localtime $then)[8] > 0;
331 $then - ($tdst - $ndst) * 60 * 60;
333 # Should give you "this time yesterday" in seconds since epoch relative to
334 # the first argument or the current time if no argument is given and
335 # suitable for passing to localtime or whatever else you need to do with
336 # it. $ndst is whether we're currently in daylight savings time; $tdst is
337 # whether the point 24 hours ago was in daylight savings time. If $tdst
338 # and $ndst are the same, a boundary wasn't crossed, and the correction
339 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
340 # from yesterday's time since we gained an extra hour while going off
341 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
342 # negative hour (add an hour) to yesterday's time since we lost an hour.
344 # All of this is because during those days when one switches off or onto
345 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
347 # The explicit settings of $ndst and $tdst are necessary because localtime
348 # only says it returns the system tm struct, and the system tm struct at
349 # least on Solaris doesn't guarantee any particular positive value (like,
350 # say, 1) for isdst, just a positive value. And that value can
351 # potentially be negative, if DST information isn't available (this sub
352 # just treats those cases like no DST).
354 # Note that between 2am and 3am on the day after the time zone switches
355 # off daylight savings time, the exact hour of "yesterday" corresponding
356 # to the current hour is not clearly defined. Note also that if used
357 # between 2am and 3am the day after the change to daylight savings time,
358 # the result will be between 3am and 4am of the previous day; it's
359 # arguable whether this is correct.
361 # This sub does not attempt to deal with leap seconds (most things don't).
363 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
364 # This code is in the public domain
366 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
368 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
369 Y2K compliant (whatever that means). The programmers you've hired to
370 use it, however, probably are not.
372 Long answer: The question belies a true understanding of the issue.
373 Perl is just as Y2K compliant as your pencil--no more, and no less.
374 Can you use your pencil to write a non-Y2K-compliant memo? Of course
375 you can. Is that the pencil's fault? Of course it isn't.
377 The date and time functions supplied with Perl (gmtime and localtime)
378 supply adequate information to determine the year well beyond 2000
379 (2038 is when trouble strikes for 32-bit machines). The year returned
380 by these functions when used in a list context is the year minus 1900.
381 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
382 number. To avoid the year 2000 problem simply do not treat the year as
383 a 2-digit number. It isn't.
385 When gmtime() and localtime() are used in scalar context they return
386 a timestamp string that contains a fully-expanded year. For example,
387 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
388 2001". There's no year 2000 problem here.
390 That doesn't mean that Perl can't be used to create non-Y2K compliant
391 programs. It can. But so can your pencil. It's the fault of the user,
392 not the language. At the risk of inflaming the NRA: ``Perl doesn't
393 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
398 =head2 How do I validate input?
400 The answer to this question is usually a regular expression, perhaps
401 with auxiliary logic. See the more specific questions (numbers, mail
402 addresses, etc.) for details.
404 =head2 How do I unescape a string?
406 It depends just what you mean by ``escape''. URL escapes are dealt
407 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
408 character are removed with
412 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
414 =head2 How do I remove consecutive pairs of characters?
416 To turn C<"abbcccd"> into C<"abccd">:
418 s/(.)\1/$1/g; # add /s to include newlines
420 Here's a solution that turns "abbcccd" to "abcd":
422 y///cs; # y == tr, but shorter :-)
424 =head2 How do I expand function calls in a string?
426 This is documented in L<perlref>. In general, this is fraught with
427 quoting and readability problems, but it is possible. To interpolate
428 a subroutine call (in list context) into a string:
430 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
432 If you prefer scalar context, similar chicanery is also useful for
433 arbitrary expressions:
435 print "That yields ${\($n + 5)} widgets\n";
437 Version 5.004 of Perl had a bug that gave list context to the
438 expression in C<${...}>, but this is fixed in version 5.005.
440 See also ``How can I expand variables in text strings?'' in this
443 =head2 How do I find matching/nesting anything?
445 This isn't something that can be done in one regular expression, no
446 matter how complicated. To find something between two single
447 characters, a pattern like C</x([^x]*)x/> will get the intervening
448 bits in $1. For multiple ones, then something more like
449 C</alpha(.*?)omega/> would be needed. But none of these deals with
450 nested patterns, nor can they. For that you'll have to write a
453 If you are serious about writing a parser, there are a number of
454 modules or oddities that will make your life a lot easier. There are
455 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
456 and the byacc program. Starting from perl 5.8 the Text::Balanced
457 is part of the standard distribution.
459 One simple destructive, inside-out approach that you might try is to
460 pull out the smallest nesting parts one at a time:
462 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
463 # do something with $1
466 A more complicated and sneaky approach is to make Perl's regular
467 expression engine do it for you. This is courtesy Dean Inada, and
468 rather has the nature of an Obfuscated Perl Contest entry, but it
471 # $_ contains the string to parse
472 # BEGIN and END are the opening and closing markers for the
477 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
478 @$ = (eval{/$re/},$@!~/unmatched/);
479 print join("\n",@$[0..$#$]) if( $$[-1] );
481 =head2 How do I reverse a string?
483 Use reverse() in scalar context, as documented in
486 $reversed = reverse $string;
488 =head2 How do I expand tabs in a string?
490 You can do it yourself:
492 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
494 Or you can just use the Text::Tabs module (part of the standard Perl
498 @expanded_lines = expand(@lines_with_tabs);
500 =head2 How do I reformat a paragraph?
502 Use Text::Wrap (part of the standard Perl distribution):
505 print wrap("\t", ' ', @paragraphs);
507 The paragraphs you give to Text::Wrap should not contain embedded
508 newlines. Text::Wrap doesn't justify the lines (flush-right).
510 =head2 How can I access/change the first N letters of a string?
512 There are many ways. If you just want to grab a copy, use
515 $first_byte = substr($a, 0, 1);
517 If you want to modify part of a string, the simplest way is often to
518 use substr() as an lvalue:
520 substr($a, 0, 3) = "Tom";
522 Although those with a pattern matching kind of thought process will
527 =head2 How do I change the Nth occurrence of something?
529 You have to keep track of N yourself. For example, let's say you want
530 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
531 C<"whosoever"> or C<"whomsoever">, case insensitively. These
532 all assume that $_ contains the string to be altered.
536 ++$count == 5 # is it the 5th?
537 ? "${2}soever" # yes, swap
538 : $1 # renege and leave it there
541 In the more general case, you can use the C</g> modifier in a C<while>
542 loop, keeping count of matches.
546 $_ = "One fish two fish red fish blue fish";
547 while (/(\w+)\s+fish\b/gi) {
548 if (++$count == $WANT) {
549 print "The third fish is a $1 one.\n";
553 That prints out: C<"The third fish is a red one."> You can also use a
554 repetition count and repeated pattern like this:
556 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
558 =head2 How can I count the number of occurrences of a substring within a string?
560 There are a number of ways, with varying efficiency. If you want a
561 count of a certain single character (X) within a string, you can use the
562 C<tr///> function like so:
564 $string = "ThisXlineXhasXsomeXx'sXinXit";
565 $count = ($string =~ tr/X//);
566 print "There are $count X characters in the string";
568 This is fine if you are just looking for a single character. However,
569 if you are trying to count multiple character substrings within a
570 larger string, C<tr///> won't work. What you can do is wrap a while()
571 loop around a global pattern match. For example, let's count negative
574 $string = "-9 55 48 -2 23 -76 4 14 -44";
575 while ($string =~ /-\d+/g) { $count++ }
576 print "There are $count negative numbers in the string";
578 =head2 How do I capitalize all the words on one line?
580 To make the first letter of each word upper case:
582 $line =~ s/\b(\w)/\U$1/g;
584 This has the strange effect of turning "C<don't do it>" into "C<Don'T
585 Do It>". Sometimes you might want this. Other times you might need a
586 more thorough solution (Suggested by brian d. foy):
589 (^\w) #at the beginning of the line
591 (\s\w) #preceded by whitespace
594 $string =~ /([\w']+)/\u\L$1/g;
596 To make the whole line upper case:
600 To force each word to be lower case, with the first letter upper case:
602 $line =~ s/(\w+)/\u\L$1/g;
604 You can (and probably should) enable locale awareness of those
605 characters by placing a C<use locale> pragma in your program.
606 See L<perllocale> for endless details on locales.
608 This is sometimes referred to as putting something into "title
609 case", but that's not quite accurate. Consider the proper
610 capitalization of the movie I<Dr. Strangelove or: How I Learned to
611 Stop Worrying and Love the Bomb>, for example.
613 =head2 How can I split a [character] delimited string except when inside
614 [character]? (Comma-separated files)
616 Take the example case of trying to split a string that is comma-separated
617 into its different fields. (We'll pretend you said comma-separated, not
618 comma-delimited, which is different and almost never what you mean.) You
619 can't use C<split(/,/)> because you shouldn't split if the comma is inside
620 quotes. For example, take a data line like this:
622 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
624 Due to the restriction of the quotes, this is a fairly complex
625 problem. Thankfully, we have Jeffrey Friedl, author of a highly
626 recommended book on regular expressions, to handle these for us. He
627 suggests (assuming your string is contained in $text):
630 push(@new, $+) while $text =~ m{
631 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
635 push(@new, undef) if substr($text,-1,1) eq ',';
637 If you want to represent quotation marks inside a
638 quotation-mark-delimited field, escape them with backslashes (eg,
639 C<"like \"this\"">. Unescaping them is a task addressed earlier in
642 Alternatively, the Text::ParseWords module (part of the standard Perl
643 distribution) lets you say:
645 use Text::ParseWords;
646 @new = quotewords(",", 0, $text);
648 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
650 =head2 How do I strip blank space from the beginning/end of a string?
652 Although the simplest approach would seem to be
654 $string =~ s/^\s*(.*?)\s*$/$1/;
656 not only is this unnecessarily slow and destructive, it also fails with
657 embedded newlines. It is much faster to do this operation in two steps:
662 Or more nicely written as:
669 This idiom takes advantage of the C<foreach> loop's aliasing
670 behavior to factor out common code. You can do this
671 on several strings at once, or arrays, or even the
672 values of a hash if you use a slice:
674 # trim whitespace in the scalar, the array,
675 # and all the values in the hash
676 foreach ($scalar, @array, @hash{keys %hash}) {
681 =head2 How do I pad a string with blanks or pad a number with zeroes?
683 (This answer contributed by Uri Guttman, with kibitzing from
686 In the following examples, C<$pad_len> is the length to which you wish
687 to pad the string, C<$text> or C<$num> contains the string to be padded,
688 and C<$pad_char> contains the padding character. You can use a single
689 character string constant instead of the C<$pad_char> variable if you
690 know what it is in advance. And in the same way you can use an integer in
691 place of C<$pad_len> if you know the pad length in advance.
693 The simplest method uses the C<sprintf> function. It can pad on the left
694 or right with blanks and on the left with zeroes and it will not
695 truncate the result. The C<pack> function can only pad strings on the
696 right with blanks and it will truncate the result to a maximum length of
699 # Left padding a string with blanks (no truncation):
700 $padded = sprintf("%${pad_len}s", $text);
702 # Right padding a string with blanks (no truncation):
703 $padded = sprintf("%-${pad_len}s", $text);
705 # Left padding a number with 0 (no truncation):
706 $padded = sprintf("%0${pad_len}d", $num);
708 # Right padding a string with blanks using pack (will truncate):
709 $padded = pack("A$pad_len",$text);
711 If you need to pad with a character other than blank or zero you can use
712 one of the following methods. They all generate a pad string with the
713 C<x> operator and combine that with C<$text>. These methods do
714 not truncate C<$text>.
716 Left and right padding with any character, creating a new string:
718 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
719 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
721 Left and right padding with any character, modifying C<$text> directly:
723 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
724 $text .= $pad_char x ( $pad_len - length( $text ) );
726 =head2 How do I extract selected columns from a string?
728 Use substr() or unpack(), both documented in L<perlfunc>.
729 If you prefer thinking in terms of columns instead of widths,
730 you can use this kind of thing:
732 # determine the unpack format needed to split Linux ps output
733 # arguments are cut columns
734 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
740 for my $place (@positions) {
741 $template .= "A" . ($place - $lastpos) . " ";
748 =head2 How do I find the soundex value of a string?
750 Use the standard Text::Soundex module distributed with Perl.
751 Before you do so, you may want to determine whether `soundex' is in
752 fact what you think it is. Knuth's soundex algorithm compresses words
753 into a small space, and so it does not necessarily distinguish between
754 two words which you might want to appear separately. For example, the
755 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
756 If Text::Soundex does not do what you are looking for, you might want
757 to consider the String::Approx module available at CPAN.
759 =head2 How can I expand variables in text strings?
761 Let's assume that you have a string like:
763 $text = 'this has a $foo in it and a $bar';
765 If those were both global variables, then this would
768 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
770 But since they are probably lexicals, or at least, they could
771 be, you'd have to do this:
773 $text =~ s/(\$\w+)/$1/eeg;
774 die if $@; # needed /ee, not /e
776 It's probably better in the general case to treat those
777 variables as entries in some special hash. For example:
783 $text =~ s/\$(\w+)/$user_defs{$1}/g;
785 See also ``How do I expand function calls in a string?'' in this section
788 =head2 What's wrong with always quoting "$vars"?
790 The problem is that those double-quotes force stringification--
791 coercing numbers and references into strings--even when you
792 don't want them to be strings. Think of it this way: double-quote
793 expansion is used to produce new strings. If you already
794 have a string, why do you need more?
796 If you get used to writing odd things like these:
800 somefunc("$var"); # BAD
802 You'll be in trouble. Those should (in 99.8% of the cases) be
803 the simpler and more direct:
809 Otherwise, besides slowing you down, you're going to break code when
810 the thing in the scalar is actually neither a string nor a number, but
816 my $oref = "$aref"; # WRONG
819 You can also get into subtle problems on those few operations in Perl
820 that actually do care about the difference between a string and a
821 number, such as the magical C<++> autoincrement operator or the
824 Stringification also destroys arrays.
827 print "@lines"; # WRONG - extra blanks
828 print @lines; # right
830 =head2 Why don't my <<HERE documents work?
832 Check for these three things:
836 =item 1. There must be no space after the << part.
838 =item 2. There (probably) should be a semicolon at the end.
840 =item 3. You can't (easily) have any space in front of the tag.
844 If you want to indent the text in the here document, you
848 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
853 But the HERE_TARGET must still be flush against the margin.
854 If you want that indented also, you'll have to quote
857 ($quote = <<' FINIS') =~ s/^\s+//gm;
858 ...we will have peace, when you and all your works have
859 perished--and the works of your dark master to whom you
860 would deliver us. You are a liar, Saruman, and a corrupter
861 of men's hearts. --Theoden in /usr/src/perl/taint.c
863 $quote =~ s/\s*--/\n--/;
865 A nice general-purpose fixer-upper function for indented here documents
866 follows. It expects to be called with a here document as its argument.
867 It looks to see whether each line begins with a common substring, and
868 if so, strips that substring off. Otherwise, it takes the amount of leading
869 whitespace found on the first line and removes that much off each
874 my ($white, $leader); # common whitespace and common leading string
875 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
876 ($white, $leader) = ($2, quotemeta($1));
878 ($white, $leader) = (/^(\s+)/, '');
880 s/^\s*?$leader(?:$white)?//gm;
884 This works with leading special strings, dynamically determined:
886 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
889 @@@ SAVEI32(runlevel);
891 @@@ while ( op = (*op->op_ppaddr)() );
895 MAIN_INTERPRETER_LOOP
897 Or with a fixed amount of leading whitespace, with remaining
898 indentation correctly preserved:
900 $poem = fix<<EVER_ON_AND_ON;
901 Now far ahead the Road has gone,
902 And I must follow, if I can,
903 Pursuing it with eager feet,
904 Until it joins some larger way
905 Where many paths and errands meet.
906 And whither then? I cannot say.
907 --Bilbo in /usr/src/perl/pp_ctl.c
912 =head2 What is the difference between a list and an array?
914 An array has a changeable length. A list does not. An array is something
915 you can push or pop, while a list is a set of values. Some people make
916 the distinction that a list is a value while an array is a variable.
917 Subroutines are passed and return lists, you put things into list
918 context, you initialize arrays with lists, and you foreach() across
919 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
920 in scalar context behave like the number of elements in them, subroutines
921 access their arguments through the array C<@_>, and push/pop/shift only work
924 As a side note, there's no such thing as a list in scalar context.
927 $scalar = (2, 5, 7, 9);
929 you're using the comma operator in scalar context, so it uses the scalar
930 comma operator. There never was a list there at all! This causes the
931 last value to be returned: 9.
933 =head2 What is the difference between $array[1] and @array[1]?
935 The former is a scalar value; the latter an array slice, making
936 it a list with one (scalar) value. You should use $ when you want a
937 scalar value (most of the time) and @ when you want a list with one
938 scalar value in it (very, very rarely; nearly never, in fact).
940 Sometimes it doesn't make a difference, but sometimes it does.
941 For example, compare:
943 $good[0] = `some program that outputs several lines`;
947 @bad[0] = `same program that outputs several lines`;
949 The C<use warnings> pragma and the B<-w> flag will warn you about these
952 =head2 How can I remove duplicate elements from a list or array?
954 There are several possible ways, depending on whether the array is
955 ordered and whether you wish to preserve the ordering.
961 If @in is sorted, and you want @out to be sorted:
962 (this assumes all true values in the array)
964 $prev = "not equal to $in[0]";
965 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
967 This is nice in that it doesn't use much extra memory, simulating
968 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
969 guarantees that the expression is true (so that grep picks it up)
970 even if the $_ is 0, "", or undef.
974 If you don't know whether @in is sorted:
977 @out = grep(!$saw{$_}++, @in);
981 Like (b), but @in contains only small integers:
983 @out = grep(!$saw[$_]++, @in);
987 A way to do (b) without any loops or greps:
991 @out = sort keys %saw; # remove sort if undesired
995 Like (d), but @in contains only small positive integers:
999 @out = grep {defined} @ary;
1003 But perhaps you should have been using a hash all along, eh?
1005 =head2 How can I tell whether a certain element is contained in a list or array?
1007 Hearing the word "in" is an I<in>dication that you probably should have
1008 used a hash, not a list or array, to store your data. Hashes are
1009 designed to answer this question quickly and efficiently. Arrays aren't.
1011 That being said, there are several ways to approach this. If you
1012 are going to make this query many times over arbitrary string values,
1013 the fastest way is probably to invert the original array and keep an
1014 associative array lying about whose keys are the first array's values.
1016 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1018 for (@blues) { $is_blue{$_} = 1 }
1020 Now you can check whether $is_blue{$some_color}. It might have been a
1021 good idea to keep the blues all in a hash in the first place.
1023 If the values are all small integers, you could use a simple indexed
1024 array. This kind of an array will take up less space:
1026 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1027 undef @is_tiny_prime;
1028 for (@primes) { $is_tiny_prime[$_] = 1 }
1029 # or simply @istiny_prime[@primes] = (1) x @primes;
1031 Now you check whether $is_tiny_prime[$some_number].
1033 If the values in question are integers instead of strings, you can save
1034 quite a lot of space by using bit strings instead:
1036 @articles = ( 1..10, 150..2000, 2017 );
1038 for (@articles) { vec($read,$_,1) = 1 }
1040 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1044 ($is_there) = grep $_ eq $whatever, @array;
1048 ($is_there) = grep /$whatever/, @array;
1050 These are slow (checks every element even if the first matches),
1051 inefficient (same reason), and potentially buggy (what if there are
1052 regex characters in $whatever?). If you're only testing once, then
1056 foreach $elt (@array) {
1057 if ($elt eq $elt_to_find) {
1062 if ($is_there) { ... }
1064 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1066 Use a hash. Here's code to do both and more. It assumes that
1067 each element is unique in a given array:
1069 @union = @intersection = @difference = ();
1071 foreach $element (@array1, @array2) { $count{$element}++ }
1072 foreach $element (keys %count) {
1073 push @union, $element;
1074 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1077 Note that this is the I<symmetric difference>, that is, all elements in
1078 either A or in B but not in both. Think of it as an xor operation.
1080 =head2 How do I test whether two arrays or hashes are equal?
1082 The following code works for single-level arrays. It uses a stringwise
1083 comparison, and does not distinguish defined versus undefined empty
1084 strings. Modify if you have other needs.
1086 $are_equal = compare_arrays(\@frogs, \@toads);
1088 sub compare_arrays {
1089 my ($first, $second) = @_;
1090 no warnings; # silence spurious -w undef complaints
1091 return 0 unless @$first == @$second;
1092 for (my $i = 0; $i < @$first; $i++) {
1093 return 0 if $first->[$i] ne $second->[$i];
1098 For multilevel structures, you may wish to use an approach more
1099 like this one. It uses the CPAN module FreezeThaw:
1101 use FreezeThaw qw(cmpStr);
1102 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1104 printf "a and b contain %s arrays\n",
1105 cmpStr(\@a, \@b) == 0
1109 This approach also works for comparing hashes. Here
1110 we'll demonstrate two different answers:
1112 use FreezeThaw qw(cmpStr cmpStrHard);
1114 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1118 printf "a and b contain %s hashes\n",
1119 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1121 printf "a and b contain %s hashes\n",
1122 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1125 The first reports that both those the hashes contain the same data,
1126 while the second reports that they do not. Which you prefer is left as
1127 an exercise to the reader.
1129 =head2 How do I find the first array element for which a condition is true?
1131 You can use this if you care about the index:
1133 for ($i= 0; $i < @array; $i++) {
1134 if ($array[$i] eq "Waldo") {
1140 Now C<$found_index> has what you want.
1142 =head2 How do I handle linked lists?
1144 In general, you usually don't need a linked list in Perl, since with
1145 regular arrays, you can push and pop or shift and unshift at either end,
1146 or you can use splice to add and/or remove arbitrary number of elements at
1147 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1148 dynamic arrays. In the absence of shifts and pops, push in general
1149 needs to reallocate on the order every log(N) times, and unshift will
1150 need to copy pointers each time.
1152 If you really, really wanted, you could use structures as described in
1153 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1154 to do. For example, imagine a list node like this:
1161 You could walk the list this way:
1164 for ($node = $head; $node; $node = $node->{LINK}) {
1165 print $node->{VALUE}, " ";
1169 You could add to the list this way:
1172 $tail = append($head, 1); # grow a new head
1173 for $value ( 2 .. 10 ) {
1174 $tail = append($tail, $value);
1178 my($list, $value) = @_;
1179 my $node = { VALUE => $value };
1181 $node->{LINK} = $list->{LINK};
1182 $list->{LINK} = $node;
1184 $_[0] = $node; # replace caller's version
1189 But again, Perl's built-in are virtually always good enough.
1191 =head2 How do I handle circular lists?
1193 Circular lists could be handled in the traditional fashion with linked
1194 lists, or you could just do something like this with an array:
1196 unshift(@array, pop(@array)); # the last shall be first
1197 push(@array, shift(@array)); # and vice versa
1199 =head2 How do I shuffle an array randomly?
1201 If you either have Perl 5.8.0 or later installed, or if you have
1202 Scalar-List-Utils 1.03 or later installed, you can say:
1204 use List::Util 'shuffle';
1206 @shuffled = shuffle(@list);
1208 If not, you can use this:
1210 # fisher_yates_shuffle( \@array ) :
1211 # generate a random permutation of @array in place
1212 sub fisher_yates_shuffle {
1215 for ($i = @$array; --$i; ) {
1216 my $j = int rand ($i+1);
1217 @$array[$i,$j] = @$array[$j,$i];
1221 fisher_yates_shuffle( \@array ); # permutes @array in place
1223 Note that the above implementation shuffles an array in place,
1224 unlike the List::Util::shuffle() which takes a list and returns
1225 a new shuffled list.
1227 You've probably seen shuffling algorithms that work using splice,
1228 randomly picking another element to swap the current element with
1232 @old = 1 .. 10; # just a demo
1234 push(@new, splice(@old, rand @old, 1));
1237 This is bad because splice is already O(N), and since you do it N times,
1238 you just invented a quadratic algorithm; that is, O(N**2). This does
1239 not scale, although Perl is so efficient that you probably won't notice
1240 this until you have rather largish arrays.
1242 =head2 How do I process/modify each element of an array?
1244 Use C<for>/C<foreach>:
1247 s/foo/bar/; # change that word
1248 y/XZ/ZX/; # swap those letters
1251 Here's another; let's compute spherical volumes:
1253 for (@volumes = @radii) { # @volumes has changed parts
1255 $_ *= (4/3) * 3.14159; # this will be constant folded
1258 If you want to do the same thing to modify the values of the hash,
1259 you may not use the C<values> function, oddly enough. You need a slice:
1261 for $orbit ( @orbits{keys %orbits} ) {
1262 ($orbit **= 3) *= (4/3) * 3.14159;
1265 =head2 How do I select a random element from an array?
1267 Use the rand() function (see L<perlfunc/rand>):
1269 # at the top of the program:
1270 srand; # not needed for 5.004 and later
1273 $index = rand @array;
1274 $element = $array[$index];
1276 Make sure you I<only call srand once per program, if then>.
1277 If you are calling it more than once (such as before each
1278 call to rand), you're almost certainly doing something wrong.
1280 =head2 How do I permute N elements of a list?
1282 Here's a little program that generates all permutations
1283 of all the words on each line of input. The algorithm embodied
1284 in the permute() function should work on any list:
1287 # tsc-permute: permute each word of input
1288 permute([split], []);
1290 my @items = @{ $_[0] };
1291 my @perms = @{ $_[1] };
1295 my(@newitems,@newperms,$i);
1296 foreach $i (0 .. $#items) {
1299 unshift(@newperms, splice(@newitems, $i, 1));
1300 permute([@newitems], [@newperms]);
1305 Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
1306 module from CPAN runs at least an order of magnitude faster. If you don't
1307 have a C compiler (or a binary distribution of Algorithm::Permute), then
1308 you can use List::Permutor which is written in pure Perl, and is still
1309 several times faster than the algorithm above.
1311 =head2 How do I sort an array by (anything)?
1313 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1315 @list = sort { $a <=> $b } @list;
1317 The default sort function is cmp, string comparison, which would
1318 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1319 the numerical comparison operator.
1321 If you have a complicated function needed to pull out the part you
1322 want to sort on, then don't do it inside the sort function. Pull it
1323 out first, because the sort BLOCK can be called many times for the
1324 same element. Here's an example of how to pull out the first word
1325 after the first number on each item, and then sort those words
1330 ($item) = /\d+\s*(\S+)/;
1331 push @idx, uc($item);
1333 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1335 which could also be written this way, using a trick
1336 that's come to be known as the Schwartzian Transform:
1338 @sorted = map { $_->[0] }
1339 sort { $a->[1] cmp $b->[1] }
1340 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1342 If you need to sort on several fields, the following paradigm is useful.
1344 @sorted = sort { field1($a) <=> field1($b) ||
1345 field2($a) cmp field2($b) ||
1346 field3($a) cmp field3($b)
1349 This can be conveniently combined with precalculation of keys as given
1352 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1355 See also the question below on sorting hashes.
1357 =head2 How do I manipulate arrays of bits?
1359 Use pack() and unpack(), or else vec() and the bitwise operations.
1361 For example, this sets $vec to have bit N set if $ints[N] was set:
1364 foreach(@ints) { vec($vec,$_,1) = 1 }
1366 And here's how, given a vector in $vec, you can
1367 get those bits into your @ints array:
1369 sub bitvec_to_list {
1372 # Find null-byte density then select best algorithm
1373 if ($vec =~ tr/\0// / length $vec > 0.95) {
1376 # This method is faster with mostly null-bytes
1377 while($vec =~ /[^\0]/g ) {
1378 $i = -9 + 8 * pos $vec;
1379 push @ints, $i if vec($vec, ++$i, 1);
1380 push @ints, $i if vec($vec, ++$i, 1);
1381 push @ints, $i if vec($vec, ++$i, 1);
1382 push @ints, $i if vec($vec, ++$i, 1);
1383 push @ints, $i if vec($vec, ++$i, 1);
1384 push @ints, $i if vec($vec, ++$i, 1);
1385 push @ints, $i if vec($vec, ++$i, 1);
1386 push @ints, $i if vec($vec, ++$i, 1);
1389 # This method is a fast general algorithm
1391 my $bits = unpack "b*", $vec;
1392 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1393 push @ints, pos $bits while($bits =~ /1/g);
1398 This method gets faster the more sparse the bit vector is.
1399 (Courtesy of Tim Bunce and Winfried Koenig.)
1401 Here's a demo on how to use vec():
1404 $vector = "\xff\x0f\xef\xfe";
1405 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1406 unpack("N", $vector), "\n";
1407 $is_set = vec($vector, 23, 1);
1408 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1426 my ($offset, $width, $value) = @_;
1428 vec($vector, $offset, $width) = $value;
1429 print "offset=$offset width=$width value=$value\n";
1435 my $bits = unpack("b*", $vector);
1439 print "vector length in bytes: ", length($vector), "\n";
1440 @bytes = unpack("A8" x length($vector), $bits);
1441 print "bits are: @bytes\n\n";
1444 =head2 Why does defined() return true on empty arrays and hashes?
1446 The short story is that you should probably only use defined on scalars or
1447 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1448 in the 5.004 release or later of Perl for more detail.
1450 =head1 Data: Hashes (Associative Arrays)
1452 =head2 How do I process an entire hash?
1454 Use the each() function (see L<perlfunc/each>) if you don't care
1455 whether it's sorted:
1457 while ( ($key, $value) = each %hash) {
1458 print "$key = $value\n";
1461 If you want it sorted, you'll have to use foreach() on the result of
1462 sorting the keys as shown in an earlier question.
1464 =head2 What happens if I add or remove keys from a hash while iterating over it?
1468 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1469 iterating over it. In Perl 5 you can delete from it, but you still
1470 can't add to it, because that might cause a doubling of the hash table,
1471 in which half the entries get copied up to the new top half of the
1472 table, at which point you've totally bamboozled the iterator code.
1473 Even if the table doesn't double, there's no telling whether your new
1474 entry will be inserted before or after the current iterator position.
1476 Either treasure up your changes and make them after the iterator finishes
1477 or use keys to fetch all the old keys at once, and iterate over the list
1480 =head2 How do I look up a hash element by value?
1482 Create a reverse hash:
1484 %by_value = reverse %by_key;
1485 $key = $by_value{$value};
1487 That's not particularly efficient. It would be more space-efficient
1490 while (($key, $value) = each %by_key) {
1491 $by_value{$value} = $key;
1494 If your hash could have repeated values, the methods above will only find
1495 one of the associated keys. This may or may not worry you. If it does
1496 worry you, you can always reverse the hash into a hash of arrays instead:
1498 while (($key, $value) = each %by_key) {
1499 push @{$key_list_by_value{$value}}, $key;
1502 =head2 How can I know how many entries are in a hash?
1504 If you mean how many keys, then all you have to do is
1505 take the scalar sense of the keys() function:
1507 $num_keys = scalar keys %hash;
1509 The keys() function also resets the iterator, which in void context is
1510 faster for tied hashes than would be iterating through the whole
1511 hash, one key-value pair at a time.
1513 =head2 How do I sort a hash (optionally by value instead of key)?
1515 Internally, hashes are stored in a way that prevents you from imposing
1516 an order on key-value pairs. Instead, you have to sort a list of the
1519 @keys = sort keys %hash; # sorted by key
1521 $hash{$a} cmp $hash{$b}
1522 } keys %hash; # and by value
1524 Here we'll do a reverse numeric sort by value, and if two keys are
1525 identical, sort by length of key, or if that fails, by straight ASCII
1526 comparison of the keys (well, possibly modified by your locale--see
1530 $hash{$b} <=> $hash{$a}
1532 length($b) <=> length($a)
1537 =head2 How can I always keep my hash sorted?
1539 You can look into using the DB_File module and tie() using the
1540 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1541 The Tie::IxHash module from CPAN might also be instructive.
1543 =head2 What's the difference between "delete" and "undef" with hashes?
1545 Hashes are pairs of scalars: the first is the key, the second is the
1546 value. The key will be coerced to a string, although the value can be
1547 any kind of scalar: string, number, or reference. If a key C<$key> is
1548 present in the array, C<exists($key)> will return true. The value for
1549 a given key can be C<undef>, in which case C<$array{$key}> will be
1550 C<undef> while C<$exists{$key}> will return true. This corresponds to
1551 (C<$key>, C<undef>) being in the hash.
1553 Pictures help... here's the C<%ary> table:
1563 And these conditions hold
1567 defined $ary{'d'} is true
1568 defined $ary{'a'} is true
1569 exists $ary{'a'} is true (Perl5 only)
1570 grep ($_ eq 'a', keys %ary) is true
1576 your table now reads:
1587 and these conditions now hold; changes in caps:
1591 defined $ary{'d'} is true
1592 defined $ary{'a'} is FALSE
1593 exists $ary{'a'} is true (Perl5 only)
1594 grep ($_ eq 'a', keys %ary) is true
1596 Notice the last two: you have an undef value, but a defined key!
1602 your table now reads:
1611 and these conditions now hold; changes in caps:
1615 defined $ary{'d'} is true
1616 defined $ary{'a'} is false
1617 exists $ary{'a'} is FALSE (Perl5 only)
1618 grep ($_ eq 'a', keys %ary) is FALSE
1620 See, the whole entry is gone!
1622 =head2 Why don't my tied hashes make the defined/exists distinction?
1624 They may or may not implement the EXISTS() and DEFINED() methods
1625 differently. For example, there isn't the concept of undef with hashes
1626 that are tied to DBM* files. This means the true/false tables above
1627 will give different results when used on such a hash. It also means
1628 that exists and defined do the same thing with a DBM* file, and what
1629 they end up doing is not what they do with ordinary hashes.
1631 =head2 How do I reset an each() operation part-way through?
1633 Using C<keys %hash> in scalar context returns the number of keys in
1634 the hash I<and> resets the iterator associated with the hash. You may
1635 need to do this if you use C<last> to exit a loop early so that when you
1636 re-enter it, the hash iterator has been reset.
1638 =head2 How can I get the unique keys from two hashes?
1640 First you extract the keys from the hashes into lists, then solve
1641 the "removing duplicates" problem described above. For example:
1644 for $element (keys(%foo), keys(%bar)) {
1651 @uniq = keys %{{%foo,%bar}};
1653 Or if you really want to save space:
1656 while (defined ($key = each %foo)) {
1659 while (defined ($key = each %bar)) {
1664 =head2 How can I store a multidimensional array in a DBM file?
1666 Either stringify the structure yourself (no fun), or else
1667 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1668 it on top of either DB_File or GDBM_File.
1670 =head2 How can I make my hash remember the order I put elements into it?
1672 Use the Tie::IxHash from CPAN.
1675 tie(%myhash, Tie::IxHash);
1676 for ($i=0; $i<20; $i++) {
1679 @keys = keys %myhash;
1680 # @keys = (0,1,2,3,...)
1682 =head2 Why does passing a subroutine an undefined element in a hash create it?
1684 If you say something like:
1686 somefunc($hash{"nonesuch key here"});
1688 Then that element "autovivifies"; that is, it springs into existence
1689 whether you store something there or not. That's because functions
1690 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1691 it has to be ready to write it back into the caller's version.
1693 This has been fixed as of Perl5.004.
1695 Normally, merely accessing a key's value for a nonexistent key does
1696 I<not> cause that key to be forever there. This is different than
1699 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1701 Usually a hash ref, perhaps like this:
1706 TITLE => "deputy peon",
1709 PALS => [ "Norbert", "Rhys", "Phineas"],
1712 References are documented in L<perlref> and the upcoming L<perlreftut>.
1713 Examples of complex data structures are given in L<perldsc> and
1714 L<perllol>. Examples of structures and object-oriented classes are
1717 =head2 How can I use a reference as a hash key?
1719 You can't do this directly, but you could use the standard Tie::Refhash
1720 module distributed with Perl.
1724 =head2 How do I handle binary data correctly?
1726 Perl is binary clean, so this shouldn't be a problem. For example,
1727 this works fine (assuming the files are found):
1729 if (`cat /vmunix` =~ /gzip/) {
1730 print "Your kernel is GNU-zip enabled!\n";
1733 On less elegant (read: Byzantine) systems, however, you have
1734 to play tedious games with "text" versus "binary" files. See
1735 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1736 systems are curses out of Microsoft, who seem to be committed to putting
1737 the backward into backward compatibility.
1739 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1741 If you want to deal with multibyte characters, however, there are
1742 some gotchas. See the section on Regular Expressions.
1744 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1746 Assuming that you don't care about IEEE notations like "NaN" or
1747 "Infinity", you probably just want to use a regular expression.
1749 if (/\D/) { print "has nondigits\n" }
1750 if (/^\d+$/) { print "is a whole number\n" }
1751 if (/^-?\d+$/) { print "is an integer\n" }
1752 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1753 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1754 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1755 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1756 { print "a C float" }
1758 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1759 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1760 wrapper function for more convenient access. This function takes
1761 a string and returns the number it found, or C<undef> for input that
1762 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1763 if you just want to say, ``Is this a float?''
1766 use POSIX qw(strtod);
1771 my($num, $unparsed) = strtod($str);
1772 if (($str eq '') || ($unparsed != 0) || $!) {
1779 sub is_numeric { defined getnum($_[0]) }
1781 Or you could check out the String::Scanf module on CPAN instead. The
1782 POSIX module (part of the standard Perl distribution) provides the
1783 C<strtod> and C<strtol> for converting strings to double and longs,
1786 =head2 How do I keep persistent data across program calls?
1788 For some specific applications, you can use one of the DBM modules.
1789 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1790 Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
1791 Storable is part of the standard distribution. Here's one example using
1792 Storable's C<store> and C<retrieve> functions:
1795 store(\%hash, "filename");
1798 $href = retrieve("filename"); # by ref
1799 %hash = %{ retrieve("filename") }; # direct to hash
1801 =head2 How do I print out or copy a recursive data structure?
1803 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1804 for printing out data structures. The Storable module, found on CPAN,
1805 provides a function called C<dclone> that recursively copies its argument.
1807 use Storable qw(dclone);
1810 Where $r1 can be a reference to any kind of data structure you'd like.
1811 It will be deeply copied. Because C<dclone> takes and returns references,
1812 you'd have to add extra punctuation if you had a hash of arrays that
1815 %newhash = %{ dclone(\%oldhash) };
1817 =head2 How do I define methods for every class/object?
1819 Use the UNIVERSAL class (see L<UNIVERSAL>).
1821 =head2 How do I verify a credit card checksum?
1823 Get the Business::CreditCard module from CPAN.
1825 =head2 How do I pack arrays of doubles or floats for XS code?
1827 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1828 If you're doing a lot of float or double processing, consider using
1829 the PDL module from CPAN instead--it makes number-crunching easy.
1831 =head1 AUTHOR AND COPYRIGHT
1833 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1834 All rights reserved.
1836 This documentation is free; you can redistribute it and/or modify it
1837 under the same terms as Perl itself.
1839 Irrespective of its distribution, all code examples in this file
1840 are hereby placed into the public domain. You are permitted and
1841 encouraged to use this code in your own programs for fun
1842 or for profit as you see fit. A simple comment in the code giving
1843 credit would be courteous but is not required.