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. If they are read in from somewhere and
55 assigned, no automatic conversion takes place. You must explicitly
56 use oct() or hex() if you want the values converted. oct() interprets
57 both hex ("0x350") numbers and octal ones ("0350" or even without the
58 leading "0", like "377"), while hex() only converts hexadecimal ones,
59 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
61 This problem shows up most often when people try using chmod(), mkdir(),
62 umask(), or sysopen(), which all want permissions in octal.
64 chmod(644, $file); # WRONG -- perl -w catches this
65 chmod(0644, $file); # right
67 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
69 Remember that int() merely truncates toward 0. For rounding to a
70 certain number of digits, sprintf() or printf() is usually the easiest
73 printf("%.3f", 3.1415926535); # prints 3.142
75 The POSIX module (part of the standard Perl distribution) implements
76 ceil(), floor(), and a number of other mathematical and trigonometric
80 $ceil = ceil(3.5); # 4
81 $floor = floor(3.5); # 3
83 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
84 module. With 5.004, the Math::Trig module (part of the standard Perl
85 distribution) implements the trigonometric functions. Internally it
86 uses the Math::Complex module and some functions can break out from
87 the real axis into the complex plane, for example the inverse sine of
90 Rounding in financial applications can have serious implications, and
91 the rounding method used should be specified precisely. In these
92 cases, it probably pays not to trust whichever system rounding is
93 being used by Perl, but to instead implement the rounding function you
96 To see why, notice how you'll still have an issue on half-way-point
99 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
101 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
102 0.8 0.8 0.9 0.9 1.0 1.0
104 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
105 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
106 machines) will work pretty much like mathematical integers. Other numbers
109 =head2 How do I convert bits into ints?
111 To turn a string of 1s and 0s like C<10110110> into a scalar containing
112 its binary value, use the pack() and unpack() functions (documented in
113 L<perlfunc/"pack"> and L<perlfunc/"unpack">):
115 $decimal = unpack('c', pack('B8', '10110110'));
117 This packs the string C<10110110> into an eight bit binary structure.
118 This is then unpacked as a character, which returns its ordinal value.
120 This does the same thing:
122 $decimal = ord(pack('B8', '10110110'));
124 Here's an example of going the other way:
126 $binary_string = unpack('B*', "\x29");
128 =head2 Why doesn't & work the way I want it to?
130 The behavior of binary arithmetic operators depends on whether they're
131 used on numbers or strings. The operators treat a string as a series
132 of bits and work with that (the string C<"3"> is the bit pattern
133 C<00110011>). The operators work with the binary form of a number
134 (the number C<3> is treated as the bit pattern C<00000011>).
136 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
137 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
140 Most problems with C<&> and C<|> arise because the programmer thinks
141 they have a number but really it's a string. The rest arise because
144 if ("\020\020" & "\101\101") {
148 but a string consisting of two null bytes (the result of C<"\020\020"
149 & "\101\101">) is not a false value in Perl. You need:
151 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
155 =head2 How do I multiply matrices?
157 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
158 or the PDL extension (also available from CPAN).
160 =head2 How do I perform an operation on a series of integers?
162 To call a function on each element in an array, and collect the
165 @results = map { my_func($_) } @array;
169 @triple = map { 3 * $_ } @single;
171 To call a function on each element of an array, but ignore the
174 foreach $iterator (@array) {
175 some_func($iterator);
178 To call a function on each integer in a (small) range, you B<can> use:
180 @results = map { some_func($_) } (5 .. 25);
182 but you should be aware that the C<..> operator creates an array of
183 all integers in the range. This can take a lot of memory for large
187 for ($i=5; $i < 500_005; $i++) {
188 push(@results, some_func($i));
191 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
192 loop will iterate over the range, without creating the entire range.
194 for my $i (5 .. 500_005) {
195 push(@results, some_func($i));
198 will not create a list of 500,000 integers.
200 =head2 How can I output Roman numerals?
202 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
204 =head2 Why aren't my random numbers random?
206 If you're using a version of Perl before 5.004, you must call C<srand>
207 once at the start of your program to seed the random number generator.
208 5.004 and later automatically call C<srand> at the beginning. Don't
209 call C<srand> more than once--you make your numbers less random, rather
212 Computers are good at being predictable and bad at being random
213 (despite appearances caused by bugs in your programs :-).
214 http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
215 Phoenix, talks more about this. John von Neumann said, ``Anyone who
216 attempts to generate random numbers by deterministic means is, of
217 course, living in a state of sin.''
219 If you want numbers that are more random than C<rand> with C<srand>
220 provides, you should also check out the Math::TrulyRandom module from
221 CPAN. It uses the imperfections in your system's timer to generate
222 random numbers, but this takes quite a while. If you want a better
223 pseudorandom generator than comes with your operating system, look at
224 ``Numerical Recipes in C'' at http://www.nr.com/ .
228 =head2 How do I find the week-of-the-year/day-of-the-year?
230 The day of the year is in the array returned by localtime() (see
231 L<perlfunc/"localtime">):
233 $day_of_year = (localtime(time()))[7];
235 or more legibly (in 5.7.1 or higher):
238 $day_of_year = localtime->day_of_year();
240 You can find the week of the year by using Time::Piece's strftime():
242 $week_of_year = localtime->strftime("%U");
243 $iso_week = localtime->strftime("%V");
245 The difference between %U and %V is that %U assumes that the first day
246 of week 1 is the first Sunday of the year, whereas ISO 8601:1988 uses
247 the first week that has at least 4 days in the current year, and with
248 Monday as the first day of the week. You can also use %W, which will
249 return the week of the year with Monday as the first day of week 1. See
250 your strftime(3) man page for more details.
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 Also note that the core module Time::Piece overloads the addition and
282 subtraction operators to provide date calculation options. See
283 L<Time::Piece/Date Calculations>.
285 =head2 How can I take a string and turn it into epoch seconds?
287 If it's a regular enough string that it always has the same format,
288 you can split it up and pass the parts to C<timelocal> in the standard
289 Time::Local module. Otherwise, you should look into the Date::Calc
290 and Date::Manip modules from CPAN.
292 =head2 How can I find the Julian Day?
294 Use Time::Piece as follows:
297 my $julian_day = localtime->julian_day;
298 my $mjd = localtime->mjd; # modified julian day
300 Before you immerse yourself too deeply in this, be sure to verify that
301 it is the I<Julian> Day you really want. Are you interested in a way
302 of getting serial days so that you just can tell how many days they
303 are apart or so that you can do also other date arithmetic? If you
304 are interested in performing date arithmetic, this can be done using
305 Time::Piece (standard module since Perl 5.8), or by modules
306 Date::Manip or Date::Calc.
308 There is too many details and much confusion on this issue to cover in
309 this FAQ, but the term is applied (correctly) to a calendar now
310 supplanted by the Gregorian Calendar, with the Julian Calendar failing
311 to adjust properly for leap years on centennial years (among other
312 annoyances). The term is also used (incorrectly) to mean: [1] days in
313 the Gregorian Calendar; and [2] days since a particular starting time
314 or `epoch', usually 1970 in the Unix world and 1980 in the
315 MS-DOS/Windows world. If you find that it is not the first meaning
316 that you really want, then check out the Date::Manip and Date::Calc
317 modules. (Thanks to David Cassell for most of this text.)
319 =head2 How do I find yesterday's date?
321 The C<time()> function returns the current time in seconds since the
322 epoch. Take twenty-four hours off that:
324 $yesterday = time() - ( 24 * 60 * 60 );
326 Then you can pass this to C<localtime()> and get the individual year,
327 month, day, hour, minute, seconds values.
329 Alternatively, you can use Time::Piece to subtract a day from the value
330 returned from C<localtime()>:
333 use Time::Seconds; # imports seconds constants, like ONE_DAY
334 my $today = localtime();
335 my $yesterday = $today - ONE_DAY;
337 Note very carefully that the code above assumes that your days are
338 twenty-four hours each. For most people, there are two days a year
339 when they aren't: the switch to and from summer time throws this off.
340 A solution to this issue is offered by Russ Allbery.
343 my $now = defined $_[0] ? $_[0] : time;
344 my $then = $now - 60 * 60 * 24;
345 my $ndst = (localtime $now)[8] > 0;
346 my $tdst = (localtime $then)[8] > 0;
347 $then - ($tdst - $ndst) * 60 * 60;
349 # Should give you "this time yesterday" in seconds since epoch relative to
350 # the first argument or the current time if no argument is given and
351 # suitable for passing to localtime or whatever else you need to do with
352 # it. $ndst is whether we're currently in daylight savings time; $tdst is
353 # whether the point 24 hours ago was in daylight savings time. If $tdst
354 # and $ndst are the same, a boundary wasn't crossed, and the correction
355 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
356 # from yesterday's time since we gained an extra hour while going off
357 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
358 # negative hour (add an hour) to yesterday's time since we lost an hour.
360 # All of this is because during those days when one switches off or onto
361 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
363 # The explicit settings of $ndst and $tdst are necessary because localtime
364 # only says it returns the system tm struct, and the system tm struct at
365 # least on Solaris doesn't guarantee any particular positive value (like,
366 # say, 1) for isdst, just a positive value. And that value can
367 # potentially be negative, if DST information isn't available (this sub
368 # just treats those cases like no DST).
370 # Note that between 2am and 3am on the day after the time zone switches
371 # off daylight savings time, the exact hour of "yesterday" corresponding
372 # to the current hour is not clearly defined. Note also that if used
373 # between 2am and 3am the day after the change to daylight savings time,
374 # the result will be between 3am and 4am of the previous day; it's
375 # arguable whether this is correct.
377 # This sub does not attempt to deal with leap seconds (most things don't).
379 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
380 # This code is in the public domain
382 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
384 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
385 Y2K compliant (whatever that means). The programmers you've hired to
386 use it, however, probably are not.
388 Long answer: The question belies a true understanding of the issue.
389 Perl is just as Y2K compliant as your pencil--no more, and no less.
390 Can you use your pencil to write a non-Y2K-compliant memo? Of course
391 you can. Is that the pencil's fault? Of course it isn't.
393 The date and time functions supplied with Perl (gmtime and localtime)
394 supply adequate information to determine the year well beyond 2000
395 (2038 is when trouble strikes for 32-bit machines). The year returned
396 by these functions when used in a list context is the year minus 1900.
397 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
398 number. To avoid the year 2000 problem simply do not treat the year as
399 a 2-digit number. It isn't.
401 When gmtime() and localtime() are used in scalar context they return
402 a timestamp string that contains a fully-expanded year. For example,
403 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
404 2001". There's no year 2000 problem here.
406 That doesn't mean that Perl can't be used to create non-Y2K compliant
407 programs. It can. But so can your pencil. It's the fault of the user,
408 not the language. At the risk of inflaming the NRA: ``Perl doesn't
409 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
414 =head2 How do I validate input?
416 The answer to this question is usually a regular expression, perhaps
417 with auxiliary logic. See the more specific questions (numbers, mail
418 addresses, etc.) for details.
420 =head2 How do I unescape a string?
422 It depends just what you mean by ``escape''. URL escapes are dealt
423 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
424 character are removed with
428 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
430 =head2 How do I remove consecutive pairs of characters?
432 To turn C<"abbcccd"> into C<"abccd">:
434 s/(.)\1/$1/g; # add /s to include newlines
436 Here's a solution that turns "abbcccd" to "abcd":
438 y///cs; # y == tr, but shorter :-)
440 =head2 How do I expand function calls in a string?
442 This is documented in L<perlref>. In general, this is fraught with
443 quoting and readability problems, but it is possible. To interpolate
444 a subroutine call (in list context) into a string:
446 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
448 If you prefer scalar context, similar chicanery is also useful for
449 arbitrary expressions:
451 print "That yields ${\($n + 5)} widgets\n";
453 Version 5.004 of Perl had a bug that gave list context to the
454 expression in C<${...}>, but this is fixed in version 5.005.
456 See also ``How can I expand variables in text strings?'' in this
459 =head2 How do I find matching/nesting anything?
461 This isn't something that can be done in one regular expression, no
462 matter how complicated. To find something between two single
463 characters, a pattern like C</x([^x]*)x/> will get the intervening
464 bits in $1. For multiple ones, then something more like
465 C</alpha(.*?)omega/> would be needed. But none of these deals with
466 nested patterns, nor can they. For that you'll have to write a
469 If you are serious about writing a parser, there are a number of
470 modules or oddities that will make your life a lot easier. There are
471 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
472 and the byacc program. Starting from perl 5.8 the Text::Balanced
473 is part of the standard distribution.
475 One simple destructive, inside-out approach that you might try is to
476 pull out the smallest nesting parts one at a time:
478 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
479 # do something with $1
482 A more complicated and sneaky approach is to make Perl's regular
483 expression engine do it for you. This is courtesy Dean Inada, and
484 rather has the nature of an Obfuscated Perl Contest entry, but it
487 # $_ contains the string to parse
488 # BEGIN and END are the opening and closing markers for the
493 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
494 @$ = (eval{/$re/},$@!~/unmatched/);
495 print join("\n",@$[0..$#$]) if( $$[-1] );
497 =head2 How do I reverse a string?
499 Use reverse() in scalar context, as documented in
502 $reversed = reverse $string;
504 =head2 How do I expand tabs in a string?
506 You can do it yourself:
508 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
510 Or you can just use the Text::Tabs module (part of the standard Perl
514 @expanded_lines = expand(@lines_with_tabs);
516 =head2 How do I reformat a paragraph?
518 Use Text::Wrap (part of the standard Perl distribution):
521 print wrap("\t", ' ', @paragraphs);
523 The paragraphs you give to Text::Wrap should not contain embedded
524 newlines. Text::Wrap doesn't justify the lines (flush-right).
526 =head2 How can I access/change the first N letters of a string?
528 There are many ways. If you just want to grab a copy, use
531 $first_byte = substr($a, 0, 1);
533 If you want to modify part of a string, the simplest way is often to
534 use substr() as an lvalue:
536 substr($a, 0, 3) = "Tom";
538 Although those with a pattern matching kind of thought process will
543 =head2 How do I change the Nth occurrence of something?
545 You have to keep track of N yourself. For example, let's say you want
546 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
547 C<"whosoever"> or C<"whomsoever">, case insensitively. These
548 all assume that $_ contains the string to be altered.
552 ++$count == 5 # is it the 5th?
553 ? "${2}soever" # yes, swap
554 : $1 # renege and leave it there
557 In the more general case, you can use the C</g> modifier in a C<while>
558 loop, keeping count of matches.
562 $_ = "One fish two fish red fish blue fish";
563 while (/(\w+)\s+fish\b/gi) {
564 if (++$count == $WANT) {
565 print "The third fish is a $1 one.\n";
569 That prints out: C<"The third fish is a red one."> You can also use a
570 repetition count and repeated pattern like this:
572 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
574 =head2 How can I count the number of occurrences of a substring within a string?
576 There are a number of ways, with varying efficiency. If you want a
577 count of a certain single character (X) within a string, you can use the
578 C<tr///> function like so:
580 $string = "ThisXlineXhasXsomeXx'sXinXit";
581 $count = ($string =~ tr/X//);
582 print "There are $count X characters in the string";
584 This is fine if you are just looking for a single character. However,
585 if you are trying to count multiple character substrings within a
586 larger string, C<tr///> won't work. What you can do is wrap a while()
587 loop around a global pattern match. For example, let's count negative
590 $string = "-9 55 48 -2 23 -76 4 14 -44";
591 while ($string =~ /-\d+/g) { $count++ }
592 print "There are $count negative numbers in the string";
594 =head2 How do I capitalize all the words on one line?
596 To make the first letter of each word upper case:
598 $line =~ s/\b(\w)/\U$1/g;
600 This has the strange effect of turning "C<don't do it>" into "C<Don'T
601 Do It>". Sometimes you might want this. Other times you might need a
602 more thorough solution (Suggested by brian d. foy):
605 (^\w) #at the beginning of the line
607 (\s\w) #preceded by whitespace
610 $string =~ /([\w']+)/\u\L$1/g;
612 To make the whole line upper case:
616 To force each word to be lower case, with the first letter upper case:
618 $line =~ s/(\w+)/\u\L$1/g;
620 You can (and probably should) enable locale awareness of those
621 characters by placing a C<use locale> pragma in your program.
622 See L<perllocale> for endless details on locales.
624 This is sometimes referred to as putting something into "title
625 case", but that's not quite accurate. Consider the proper
626 capitalization of the movie I<Dr. Strangelove or: How I Learned to
627 Stop Worrying and Love the Bomb>, for example.
629 =head2 How can I split a [character] delimited string except when inside
630 [character]? (Comma-separated files)
632 Take the example case of trying to split a string that is comma-separated
633 into its different fields. (We'll pretend you said comma-separated, not
634 comma-delimited, which is different and almost never what you mean.) You
635 can't use C<split(/,/)> because you shouldn't split if the comma is inside
636 quotes. For example, take a data line like this:
638 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
640 Due to the restriction of the quotes, this is a fairly complex
641 problem. Thankfully, we have Jeffrey Friedl, author of a highly
642 recommended book on regular expressions, to handle these for us. He
643 suggests (assuming your string is contained in $text):
646 push(@new, $+) while $text =~ m{
647 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
651 push(@new, undef) if substr($text,-1,1) eq ',';
653 If you want to represent quotation marks inside a
654 quotation-mark-delimited field, escape them with backslashes (eg,
655 C<"like \"this\"">. Unescaping them is a task addressed earlier in
658 Alternatively, the Text::ParseWords module (part of the standard Perl
659 distribution) lets you say:
661 use Text::ParseWords;
662 @new = quotewords(",", 0, $text);
664 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
666 =head2 How do I strip blank space from the beginning/end of a string?
668 Although the simplest approach would seem to be
670 $string =~ s/^\s*(.*?)\s*$/$1/;
672 not only is this unnecessarily slow and destructive, it also fails with
673 embedded newlines. It is much faster to do this operation in two steps:
678 Or more nicely written as:
685 This idiom takes advantage of the C<foreach> loop's aliasing
686 behavior to factor out common code. You can do this
687 on several strings at once, or arrays, or even the
688 values of a hash if you use a slice:
690 # trim whitespace in the scalar, the array,
691 # and all the values in the hash
692 foreach ($scalar, @array, @hash{keys %hash}) {
697 =head2 How do I pad a string with blanks or pad a number with zeroes?
699 (This answer contributed by Uri Guttman, with kibitzing from
702 In the following examples, C<$pad_len> is the length to which you wish
703 to pad the string, C<$text> or C<$num> contains the string to be padded,
704 and C<$pad_char> contains the padding character. You can use a single
705 character string constant instead of the C<$pad_char> variable if you
706 know what it is in advance. And in the same way you can use an integer in
707 place of C<$pad_len> if you know the pad length in advance.
709 The simplest method uses the C<sprintf> function. It can pad on the left
710 or right with blanks and on the left with zeroes and it will not
711 truncate the result. The C<pack> function can only pad strings on the
712 right with blanks and it will truncate the result to a maximum length of
715 # Left padding a string with blanks (no truncation):
716 $padded = sprintf("%${pad_len}s", $text);
718 # Right padding a string with blanks (no truncation):
719 $padded = sprintf("%-${pad_len}s", $text);
721 # Left padding a number with 0 (no truncation):
722 $padded = sprintf("%0${pad_len}d", $num);
724 # Right padding a string with blanks using pack (will truncate):
725 $padded = pack("A$pad_len",$text);
727 If you need to pad with a character other than blank or zero you can use
728 one of the following methods. They all generate a pad string with the
729 C<x> operator and combine that with C<$text>. These methods do
730 not truncate C<$text>.
732 Left and right padding with any character, creating a new string:
734 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
735 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
737 Left and right padding with any character, modifying C<$text> directly:
739 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
740 $text .= $pad_char x ( $pad_len - length( $text ) );
742 =head2 How do I extract selected columns from a string?
744 Use substr() or unpack(), both documented in L<perlfunc>.
745 If you prefer thinking in terms of columns instead of widths,
746 you can use this kind of thing:
748 # determine the unpack format needed to split Linux ps output
749 # arguments are cut columns
750 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
756 for my $place (@positions) {
757 $template .= "A" . ($place - $lastpos) . " ";
764 =head2 How do I find the soundex value of a string?
766 Use the standard Text::Soundex module distributed with Perl.
767 Before you do so, you may want to determine whether `soundex' is in
768 fact what you think it is. Knuth's soundex algorithm compresses words
769 into a small space, and so it does not necessarily distinguish between
770 two words which you might want to appear separately. For example, the
771 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
772 If Text::Soundex does not do what you are looking for, you might want
773 to consider the String::Approx module available at CPAN.
775 =head2 How can I expand variables in text strings?
777 Let's assume that you have a string like:
779 $text = 'this has a $foo in it and a $bar';
781 If those were both global variables, then this would
784 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
786 But since they are probably lexicals, or at least, they could
787 be, you'd have to do this:
789 $text =~ s/(\$\w+)/$1/eeg;
790 die if $@; # needed /ee, not /e
792 It's probably better in the general case to treat those
793 variables as entries in some special hash. For example:
799 $text =~ s/\$(\w+)/$user_defs{$1}/g;
801 See also ``How do I expand function calls in a string?'' in this section
804 =head2 What's wrong with always quoting "$vars"?
806 The problem is that those double-quotes force stringification--
807 coercing numbers and references into strings--even when you
808 don't want them to be strings. Think of it this way: double-quote
809 expansion is used to produce new strings. If you already
810 have a string, why do you need more?
812 If you get used to writing odd things like these:
816 somefunc("$var"); # BAD
818 You'll be in trouble. Those should (in 99.8% of the cases) be
819 the simpler and more direct:
825 Otherwise, besides slowing you down, you're going to break code when
826 the thing in the scalar is actually neither a string nor a number, but
832 my $oref = "$aref"; # WRONG
835 You can also get into subtle problems on those few operations in Perl
836 that actually do care about the difference between a string and a
837 number, such as the magical C<++> autoincrement operator or the
840 Stringification also destroys arrays.
843 print "@lines"; # WRONG - extra blanks
844 print @lines; # right
846 =head2 Why don't my <<HERE documents work?
848 Check for these three things:
852 =item 1. There must be no space after the << part.
854 =item 2. There (probably) should be a semicolon at the end.
856 =item 3. You can't (easily) have any space in front of the tag.
860 If you want to indent the text in the here document, you
864 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
869 But the HERE_TARGET must still be flush against the margin.
870 If you want that indented also, you'll have to quote
873 ($quote = <<' FINIS') =~ s/^\s+//gm;
874 ...we will have peace, when you and all your works have
875 perished--and the works of your dark master to whom you
876 would deliver us. You are a liar, Saruman, and a corrupter
877 of men's hearts. --Theoden in /usr/src/perl/taint.c
879 $quote =~ s/\s*--/\n--/;
881 A nice general-purpose fixer-upper function for indented here documents
882 follows. It expects to be called with a here document as its argument.
883 It looks to see whether each line begins with a common substring, and
884 if so, strips that substring off. Otherwise, it takes the amount of leading
885 whitespace found on the first line and removes that much off each
890 my ($white, $leader); # common whitespace and common leading string
891 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
892 ($white, $leader) = ($2, quotemeta($1));
894 ($white, $leader) = (/^(\s+)/, '');
896 s/^\s*?$leader(?:$white)?//gm;
900 This works with leading special strings, dynamically determined:
902 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
905 @@@ SAVEI32(runlevel);
907 @@@ while ( op = (*op->op_ppaddr)() );
911 MAIN_INTERPRETER_LOOP
913 Or with a fixed amount of leading whitespace, with remaining
914 indentation correctly preserved:
916 $poem = fix<<EVER_ON_AND_ON;
917 Now far ahead the Road has gone,
918 And I must follow, if I can,
919 Pursuing it with eager feet,
920 Until it joins some larger way
921 Where many paths and errands meet.
922 And whither then? I cannot say.
923 --Bilbo in /usr/src/perl/pp_ctl.c
928 =head2 What is the difference between a list and an array?
930 An array has a changeable length. A list does not. An array is something
931 you can push or pop, while a list is a set of values. Some people make
932 the distinction that a list is a value while an array is a variable.
933 Subroutines are passed and return lists, you put things into list
934 context, you initialize arrays with lists, and you foreach() across
935 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
936 in scalar context behave like the number of elements in them, subroutines
937 access their arguments through the array C<@_>, and push/pop/shift only work
940 As a side note, there's no such thing as a list in scalar context.
943 $scalar = (2, 5, 7, 9);
945 you're using the comma operator in scalar context, so it uses the scalar
946 comma operator. There never was a list there at all! This causes the
947 last value to be returned: 9.
949 =head2 What is the difference between $array[1] and @array[1]?
951 The former is a scalar value; the latter an array slice, making
952 it a list with one (scalar) value. You should use $ when you want a
953 scalar value (most of the time) and @ when you want a list with one
954 scalar value in it (very, very rarely; nearly never, in fact).
956 Sometimes it doesn't make a difference, but sometimes it does.
957 For example, compare:
959 $good[0] = `some program that outputs several lines`;
963 @bad[0] = `same program that outputs several lines`;
965 The C<use warnings> pragma and the B<-w> flag will warn you about these
968 =head2 How can I remove duplicate elements from a list or array?
970 There are several possible ways, depending on whether the array is
971 ordered and whether you wish to preserve the ordering.
977 If @in is sorted, and you want @out to be sorted:
978 (this assumes all true values in the array)
980 $prev = "not equal to $in[0]";
981 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
983 This is nice in that it doesn't use much extra memory, simulating
984 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
985 guarantees that the expression is true (so that grep picks it up)
986 even if the $_ is 0, "", or undef.
990 If you don't know whether @in is sorted:
993 @out = grep(!$saw{$_}++, @in);
997 Like (b), but @in contains only small integers:
999 @out = grep(!$saw[$_]++, @in);
1003 A way to do (b) without any loops or greps:
1007 @out = sort keys %saw; # remove sort if undesired
1011 Like (d), but @in contains only small positive integers:
1015 @out = grep {defined} @ary;
1019 But perhaps you should have been using a hash all along, eh?
1021 =head2 How can I tell whether a list or array contains a certain element?
1023 Hearing the word "in" is an I<in>dication that you probably should have
1024 used a hash, not a list or array, to store your data. Hashes are
1025 designed to answer this question quickly and efficiently. Arrays aren't.
1027 That being said, there are several ways to approach this. If you
1028 are going to make this query many times over arbitrary string values,
1029 the fastest way is probably to invert the original array and keep an
1030 associative array lying about whose keys are the first array's values.
1032 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1034 for (@blues) { $is_blue{$_} = 1 }
1036 Now you can check whether $is_blue{$some_color}. It might have been a
1037 good idea to keep the blues all in a hash in the first place.
1039 If the values are all small integers, you could use a simple indexed
1040 array. This kind of an array will take up less space:
1042 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1043 undef @is_tiny_prime;
1044 for (@primes) { $is_tiny_prime[$_] = 1 }
1045 # or simply @istiny_prime[@primes] = (1) x @primes;
1047 Now you check whether $is_tiny_prime[$some_number].
1049 If the values in question are integers instead of strings, you can save
1050 quite a lot of space by using bit strings instead:
1052 @articles = ( 1..10, 150..2000, 2017 );
1054 for (@articles) { vec($read,$_,1) = 1 }
1056 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1060 ($is_there) = grep $_ eq $whatever, @array;
1064 ($is_there) = grep /$whatever/, @array;
1066 These are slow (checks every element even if the first matches),
1067 inefficient (same reason), and potentially buggy (what if there are
1068 regex characters in $whatever?). If you're only testing once, then
1072 foreach $elt (@array) {
1073 if ($elt eq $elt_to_find) {
1078 if ($is_there) { ... }
1080 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1082 Use a hash. Here's code to do both and more. It assumes that
1083 each element is unique in a given array:
1085 @union = @intersection = @difference = ();
1087 foreach $element (@array1, @array2) { $count{$element}++ }
1088 foreach $element (keys %count) {
1089 push @union, $element;
1090 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1093 Note that this is the I<symmetric difference>, that is, all elements in
1094 either A or in B but not in both. Think of it as an xor operation.
1096 =head2 How do I test whether two arrays or hashes are equal?
1098 The following code works for single-level arrays. It uses a stringwise
1099 comparison, and does not distinguish defined versus undefined empty
1100 strings. Modify if you have other needs.
1102 $are_equal = compare_arrays(\@frogs, \@toads);
1104 sub compare_arrays {
1105 my ($first, $second) = @_;
1106 no warnings; # silence spurious -w undef complaints
1107 return 0 unless @$first == @$second;
1108 for (my $i = 0; $i < @$first; $i++) {
1109 return 0 if $first->[$i] ne $second->[$i];
1114 For multilevel structures, you may wish to use an approach more
1115 like this one. It uses the CPAN module FreezeThaw:
1117 use FreezeThaw qw(cmpStr);
1118 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1120 printf "a and b contain %s arrays\n",
1121 cmpStr(\@a, \@b) == 0
1125 This approach also works for comparing hashes. Here
1126 we'll demonstrate two different answers:
1128 use FreezeThaw qw(cmpStr cmpStrHard);
1130 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1134 printf "a and b contain %s hashes\n",
1135 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1137 printf "a and b contain %s hashes\n",
1138 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1141 The first reports that both those the hashes contain the same data,
1142 while the second reports that they do not. Which you prefer is left as
1143 an exercise to the reader.
1145 =head2 How do I find the first array element for which a condition is true?
1147 You can use this if you care about the index:
1149 for ($i= 0; $i < @array; $i++) {
1150 if ($array[$i] eq "Waldo") {
1156 Now C<$found_index> has what you want.
1158 =head2 How do I handle linked lists?
1160 In general, you usually don't need a linked list in Perl, since with
1161 regular arrays, you can push and pop or shift and unshift at either end,
1162 or you can use splice to add and/or remove arbitrary number of elements at
1163 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1164 dynamic arrays. In the absence of shifts and pops, push in general
1165 needs to reallocate on the order every log(N) times, and unshift will
1166 need to copy pointers each time.
1168 If you really, really wanted, you could use structures as described in
1169 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1170 to do. For example, imagine a list node like this:
1177 You could walk the list this way:
1180 for ($node = $head; $node; $node = $node->{LINK}) {
1181 print $node->{VALUE}, " ";
1185 You could add to the list this way:
1188 $tail = append($head, 1); # grow a new head
1189 for $value ( 2 .. 10 ) {
1190 $tail = append($tail, $value);
1194 my($list, $value) = @_;
1195 my $node = { VALUE => $value };
1197 $node->{LINK} = $list->{LINK};
1198 $list->{LINK} = $node;
1200 $_[0] = $node; # replace caller's version
1205 But again, Perl's built-in are virtually always good enough.
1207 =head2 How do I handle circular lists?
1209 Circular lists could be handled in the traditional fashion with linked
1210 lists, or you could just do something like this with an array:
1212 unshift(@array, pop(@array)); # the last shall be first
1213 push(@array, shift(@array)); # and vice versa
1215 =head2 How do I shuffle an array randomly?
1219 # fisher_yates_shuffle( \@array ) :
1220 # generate a random permutation of @array in place
1221 sub fisher_yates_shuffle {
1224 for ($i = @$array; --$i; ) {
1225 my $j = int rand ($i+1);
1226 @$array[$i,$j] = @$array[$j,$i];
1230 fisher_yates_shuffle( \@array ); # permutes @array in place
1232 You've probably seen shuffling algorithms that work using splice,
1233 randomly picking another element to swap the current element with
1237 @old = 1 .. 10; # just a demo
1239 push(@new, splice(@old, rand @old, 1));
1242 This is bad because splice is already O(N), and since you do it N times,
1243 you just invented a quadratic algorithm; that is, O(N**2). This does
1244 not scale, although Perl is so efficient that you probably won't notice
1245 this until you have rather largish arrays.
1247 =head2 How do I process/modify each element of an array?
1249 Use C<for>/C<foreach>:
1252 s/foo/bar/; # change that word
1253 y/XZ/ZX/; # swap those letters
1256 Here's another; let's compute spherical volumes:
1258 for (@volumes = @radii) { # @volumes has changed parts
1260 $_ *= (4/3) * 3.14159; # this will be constant folded
1263 If you want to do the same thing to modify the values of the hash,
1264 you may not use the C<values> function, oddly enough. You need a slice:
1266 for $orbit ( @orbits{keys %orbits} ) {
1267 ($orbit **= 3) *= (4/3) * 3.14159;
1270 =head2 How do I select a random element from an array?
1272 Use the rand() function (see L<perlfunc/rand>):
1274 # at the top of the program:
1275 srand; # not needed for 5.004 and later
1278 $index = rand @array;
1279 $element = $array[$index];
1281 Make sure you I<only call srand once per program, if then>.
1282 If you are calling it more than once (such as before each
1283 call to rand), you're almost certainly doing something wrong.
1285 =head2 How do I permute N elements of a list?
1287 Here's a little program that generates all permutations
1288 of all the words on each line of input. The algorithm embodied
1289 in the permute() function should work on any list:
1292 # tsc-permute: permute each word of input
1293 permute([split], []);
1295 my @items = @{ $_[0] };
1296 my @perms = @{ $_[1] };
1300 my(@newitems,@newperms,$i);
1301 foreach $i (0 .. $#items) {
1304 unshift(@newperms, splice(@newitems, $i, 1));
1305 permute([@newitems], [@newperms]);
1310 =head2 How do I sort an array by (anything)?
1312 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1314 @list = sort { $a <=> $b } @list;
1316 The default sort function is cmp, string comparison, which would
1317 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1318 the numerical comparison operator.
1320 If you have a complicated function needed to pull out the part you
1321 want to sort on, then don't do it inside the sort function. Pull it
1322 out first, because the sort BLOCK can be called many times for the
1323 same element. Here's an example of how to pull out the first word
1324 after the first number on each item, and then sort those words
1329 ($item) = /\d+\s*(\S+)/;
1330 push @idx, uc($item);
1332 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1334 which could also be written this way, using a trick
1335 that's come to be known as the Schwartzian Transform:
1337 @sorted = map { $_->[0] }
1338 sort { $a->[1] cmp $b->[1] }
1339 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1341 If you need to sort on several fields, the following paradigm is useful.
1343 @sorted = sort { field1($a) <=> field1($b) ||
1344 field2($a) cmp field2($b) ||
1345 field3($a) cmp field3($b)
1348 This can be conveniently combined with precalculation of keys as given
1351 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1354 See also the question below on sorting hashes.
1356 =head2 How do I manipulate arrays of bits?
1358 Use pack() and unpack(), or else vec() and the bitwise operations.
1360 For example, this sets $vec to have bit N set if $ints[N] was set:
1363 foreach(@ints) { vec($vec,$_,1) = 1 }
1365 And here's how, given a vector in $vec, you can
1366 get those bits into your @ints array:
1368 sub bitvec_to_list {
1371 # Find null-byte density then select best algorithm
1372 if ($vec =~ tr/\0// / length $vec > 0.95) {
1375 # This method is faster with mostly null-bytes
1376 while($vec =~ /[^\0]/g ) {
1377 $i = -9 + 8 * pos $vec;
1378 push @ints, $i if vec($vec, ++$i, 1);
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);
1388 # This method is a fast general algorithm
1390 my $bits = unpack "b*", $vec;
1391 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1392 push @ints, pos $bits while($bits =~ /1/g);
1397 This method gets faster the more sparse the bit vector is.
1398 (Courtesy of Tim Bunce and Winfried Koenig.)
1400 Here's a demo on how to use vec():
1403 $vector = "\xff\x0f\xef\xfe";
1404 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1405 unpack("N", $vector), "\n";
1406 $is_set = vec($vector, 23, 1);
1407 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1425 my ($offset, $width, $value) = @_;
1427 vec($vector, $offset, $width) = $value;
1428 print "offset=$offset width=$width value=$value\n";
1434 my $bits = unpack("b*", $vector);
1438 print "vector length in bytes: ", length($vector), "\n";
1439 @bytes = unpack("A8" x length($vector), $bits);
1440 print "bits are: @bytes\n\n";
1443 =head2 Why does defined() return true on empty arrays and hashes?
1445 The short story is that you should probably only use defined on scalars or
1446 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1447 in the 5.004 release or later of Perl for more detail.
1449 =head1 Data: Hashes (Associative Arrays)
1451 =head2 How do I process an entire hash?
1453 Use the each() function (see L<perlfunc/each>) if you don't care
1454 whether it's sorted:
1456 while ( ($key, $value) = each %hash) {
1457 print "$key = $value\n";
1460 If you want it sorted, you'll have to use foreach() on the result of
1461 sorting the keys as shown in an earlier question.
1463 =head2 What happens if I add or remove keys from a hash while iterating over it?
1467 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1468 iterating over it. In Perl 5 you can delete from it, but you still
1469 can't add to it, because that might cause a doubling of the hash table,
1470 in which half the entries get copied up to the new top half of the
1471 table, at which point you've totally bamboozled the iterator code.
1472 Even if the table doesn't double, there's no telling whether your new
1473 entry will be inserted before or after the current iterator position.
1475 Either treasure up your changes and make them after the iterator finishes
1476 or use keys to fetch all the old keys at once, and iterate over the list
1479 =head2 How do I look up a hash element by value?
1481 Create a reverse hash:
1483 %by_value = reverse %by_key;
1484 $key = $by_value{$value};
1486 That's not particularly efficient. It would be more space-efficient
1489 while (($key, $value) = each %by_key) {
1490 $by_value{$value} = $key;
1493 If your hash could have repeated values, the methods above will only find
1494 one of the associated keys. This may or may not worry you. If it does
1495 worry you, you can always reverse the hash into a hash of arrays instead:
1497 while (($key, $value) = each %by_key) {
1498 push @{$key_list_by_value{$value}}, $key;
1501 =head2 How can I know how many entries are in a hash?
1503 If you mean how many keys, then all you have to do is
1504 take the scalar sense of the keys() function:
1506 $num_keys = scalar keys %hash;
1508 The keys() function also resets the iterator, which in void context is
1509 faster for tied hashes than would be iterating through the whole
1510 hash, one key-value pair at a time.
1512 =head2 How do I sort a hash (optionally by value instead of key)?
1514 Internally, hashes are stored in a way that prevents you from imposing
1515 an order on key-value pairs. Instead, you have to sort a list of the
1518 @keys = sort keys %hash; # sorted by key
1520 $hash{$a} cmp $hash{$b}
1521 } keys %hash; # and by value
1523 Here we'll do a reverse numeric sort by value, and if two keys are
1524 identical, sort by length of key, or if that fails, by straight ASCII
1525 comparison of the keys (well, possibly modified by your locale--see
1529 $hash{$b} <=> $hash{$a}
1531 length($b) <=> length($a)
1536 =head2 How can I always keep my hash sorted?
1538 You can look into using the DB_File module and tie() using the
1539 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1540 The Tie::IxHash module from CPAN might also be instructive.
1542 =head2 What's the difference between "delete" and "undef" with hashes?
1544 Hashes are pairs of scalars: the first is the key, the second is the
1545 value. The key will be coerced to a string, although the value can be
1546 any kind of scalar: string, number, or reference. If a key C<$key> is
1547 present in the array, C<exists($key)> will return true. The value for
1548 a given key can be C<undef>, in which case C<$array{$key}> will be
1549 C<undef> while C<$exists{$key}> will return true. This corresponds to
1550 (C<$key>, C<undef>) being in the hash.
1552 Pictures help... here's the C<%ary> table:
1562 And these conditions hold
1566 defined $ary{'d'} is true
1567 defined $ary{'a'} is true
1568 exists $ary{'a'} is true (Perl5 only)
1569 grep ($_ eq 'a', keys %ary) is true
1575 your table now reads:
1586 and these conditions now hold; changes in caps:
1590 defined $ary{'d'} is true
1591 defined $ary{'a'} is FALSE
1592 exists $ary{'a'} is true (Perl5 only)
1593 grep ($_ eq 'a', keys %ary) is true
1595 Notice the last two: you have an undef value, but a defined key!
1601 your table now reads:
1610 and these conditions now hold; changes in caps:
1614 defined $ary{'d'} is true
1615 defined $ary{'a'} is false
1616 exists $ary{'a'} is FALSE (Perl5 only)
1617 grep ($_ eq 'a', keys %ary) is FALSE
1619 See, the whole entry is gone!
1621 =head2 Why don't my tied hashes make the defined/exists distinction?
1623 They may or may not implement the EXISTS() and DEFINED() methods
1624 differently. For example, there isn't the concept of undef with hashes
1625 that are tied to DBM* files. This means the true/false tables above
1626 will give different results when used on such a hash. It also means
1627 that exists and defined do the same thing with a DBM* file, and what
1628 they end up doing is not what they do with ordinary hashes.
1630 =head2 How do I reset an each() operation part-way through?
1632 Using C<keys %hash> in scalar context returns the number of keys in
1633 the hash I<and> resets the iterator associated with the hash. You may
1634 need to do this if you use C<last> to exit a loop early so that when you
1635 re-enter it, the hash iterator has been reset.
1637 =head2 How can I get the unique keys from two hashes?
1639 First you extract the keys from the hashes into lists, then solve
1640 the "removing duplicates" problem described above. For example:
1643 for $element (keys(%foo), keys(%bar)) {
1650 @uniq = keys %{{%foo,%bar}};
1652 Or if you really want to save space:
1655 while (defined ($key = each %foo)) {
1658 while (defined ($key = each %bar)) {
1663 =head2 How can I store a multidimensional array in a DBM file?
1665 Either stringify the structure yourself (no fun), or else
1666 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1667 it on top of either DB_File or GDBM_File.
1669 =head2 How can I make my hash remember the order I put elements into it?
1671 Use the Tie::IxHash from CPAN.
1674 tie(%myhash, Tie::IxHash);
1675 for ($i=0; $i<20; $i++) {
1678 @keys = keys %myhash;
1679 # @keys = (0,1,2,3,...)
1681 =head2 Why does passing a subroutine an undefined element in a hash create it?
1683 If you say something like:
1685 somefunc($hash{"nonesuch key here"});
1687 Then that element "autovivifies"; that is, it springs into existence
1688 whether you store something there or not. That's because functions
1689 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1690 it has to be ready to write it back into the caller's version.
1692 This has been fixed as of Perl5.004.
1694 Normally, merely accessing a key's value for a nonexistent key does
1695 I<not> cause that key to be forever there. This is different than
1698 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1700 Usually a hash ref, perhaps like this:
1705 TITLE => "deputy peon",
1708 PALS => [ "Norbert", "Rhys", "Phineas"],
1711 References are documented in L<perlref> and the upcoming L<perlreftut>.
1712 Examples of complex data structures are given in L<perldsc> and
1713 L<perllol>. Examples of structures and object-oriented classes are
1716 =head2 How can I use a reference as a hash key?
1718 You can't do this directly, but you could use the standard Tie::Refhash
1719 module distributed with Perl.
1723 =head2 How do I handle binary data correctly?
1725 Perl is binary clean, so this shouldn't be a problem. For example,
1726 this works fine (assuming the files are found):
1728 if (`cat /vmunix` =~ /gzip/) {
1729 print "Your kernel is GNU-zip enabled!\n";
1732 On less elegant (read: Byzantine) systems, however, you have
1733 to play tedious games with "text" versus "binary" files. See
1734 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1735 systems are curses out of Microsoft, who seem to be committed to putting
1736 the backward into backward compatibility.
1738 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1740 If you want to deal with multibyte characters, however, there are
1741 some gotchas. See the section on Regular Expressions.
1743 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1745 Assuming that you don't care about IEEE notations like "NaN" or
1746 "Infinity", you probably just want to use a regular expression.
1748 if (/\D/) { print "has nondigits\n" }
1749 if (/^\d+$/) { print "is a whole number\n" }
1750 if (/^-?\d+$/) { print "is an integer\n" }
1751 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1752 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1753 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1754 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1755 { print "a C float" }
1757 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1758 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1759 wrapper function for more convenient access. This function takes
1760 a string and returns the number it found, or C<undef> for input that
1761 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1762 if you just want to say, ``Is this a float?''
1765 use POSIX qw(strtod);
1770 my($num, $unparsed) = strtod($str);
1771 if (($str eq '') || ($unparsed != 0) || $!) {
1778 sub is_numeric { defined getnum($_[0]) }
1780 Or you could check out the String::Scanf module on CPAN instead. The
1781 POSIX module (part of the standard Perl distribution) provides the
1782 C<strtod> and C<strtol> for converting strings to double and longs,
1785 =head2 How do I keep persistent data across program calls?
1787 For some specific applications, you can use one of the DBM modules.
1788 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1789 Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
1790 Storable is part of the standard distribution. Here's one example using
1791 Storable's C<store> and C<retrieve> functions:
1794 store(\%hash, "filename");
1797 $href = retrieve("filename"); # by ref
1798 %hash = %{ retrieve("filename") }; # direct to hash
1800 =head2 How do I print out or copy a recursive data structure?
1802 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1803 for printing out data structures. The Storable module, found on CPAN,
1804 provides a function called C<dclone> that recursively copies its argument.
1806 use Storable qw(dclone);
1809 Where $r1 can be a reference to any kind of data structure you'd like.
1810 It will be deeply copied. Because C<dclone> takes and returns references,
1811 you'd have to add extra punctuation if you had a hash of arrays that
1814 %newhash = %{ dclone(\%oldhash) };
1816 =head2 How do I define methods for every class/object?
1818 Use the UNIVERSAL class (see L<UNIVERSAL>).
1820 =head2 How do I verify a credit card checksum?
1822 Get the Business::CreditCard module from CPAN.
1824 =head2 How do I pack arrays of doubles or floats for XS code?
1826 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1827 If you're doing a lot of float or double processing, consider using
1828 the PDL module from CPAN instead--it makes number-crunching easy.
1830 =head1 AUTHOR AND COPYRIGHT
1832 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1833 All rights reserved.
1835 When included as part of the Standard Version of Perl, or as part of
1836 its complete documentation whether printed or otherwise, this work
1837 may be distributed only under the terms of Perl's Artistic License.
1838 Any distribution of this file or derivatives thereof I<outside>
1839 of that package require that special arrangements be made with
1842 Irrespective of its distribution, all code examples in this file
1843 are hereby placed into the public domain. You are permitted and
1844 encouraged to use this code in your own programs for fun
1845 or for profit as you see fit. A simple comment in the code giving
1846 credit would be courteous but is not required.