3 perlfaq4 - Data Manipulation ($Revision: 1.5 $, $Date: 2001/10/12 15:20:13 $)
7 The section of the FAQ answers questions related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximated on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to an internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers. (See L<perlvar/"$#"> 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 Or use the CPAN module Text::Autoformat. Formatting files can be easily
511 done by making a shell alias, like so:
513 alias fmt="perl -i -MText::Autoformat -n0777 \
514 -e 'print autoformat $_, {all=>1}' $*"
516 See the documentation for Text::Autoformat to appreciate its many
519 =head2 How can I access/change the first N letters of a string?
521 There are many ways. If you just want to grab a copy, use
524 $first_byte = substr($a, 0, 1);
526 If you want to modify part of a string, the simplest way is often to
527 use substr() as an lvalue:
529 substr($a, 0, 3) = "Tom";
531 Although those with a pattern matching kind of thought process will
536 =head2 How do I change the Nth occurrence of something?
538 You have to keep track of N yourself. For example, let's say you want
539 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
540 C<"whosoever"> or C<"whomsoever">, case insensitively. These
541 all assume that $_ contains the string to be altered.
545 ++$count == 5 # is it the 5th?
546 ? "${2}soever" # yes, swap
547 : $1 # renege and leave it there
550 In the more general case, you can use the C</g> modifier in a C<while>
551 loop, keeping count of matches.
555 $_ = "One fish two fish red fish blue fish";
556 while (/(\w+)\s+fish\b/gi) {
557 if (++$count == $WANT) {
558 print "The third fish is a $1 one.\n";
562 That prints out: C<"The third fish is a red one."> You can also use a
563 repetition count and repeated pattern like this:
565 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
567 =head2 How can I count the number of occurrences of a substring within a string?
569 There are a number of ways, with varying efficiency. If you want a
570 count of a certain single character (X) within a string, you can use the
571 C<tr///> function like so:
573 $string = "ThisXlineXhasXsomeXx'sXinXit";
574 $count = ($string =~ tr/X//);
575 print "There are $count X characters in the string";
577 This is fine if you are just looking for a single character. However,
578 if you are trying to count multiple character substrings within a
579 larger string, C<tr///> won't work. What you can do is wrap a while()
580 loop around a global pattern match. For example, let's count negative
583 $string = "-9 55 48 -2 23 -76 4 14 -44";
584 while ($string =~ /-\d+/g) { $count++ }
585 print "There are $count negative numbers in the string";
587 =head2 How do I capitalize all the words on one line?
589 To make the first letter of each word upper case:
591 $line =~ s/\b(\w)/\U$1/g;
593 This has the strange effect of turning "C<don't do it>" into "C<Don'T
594 Do It>". Sometimes you might want this. Other times you might need a
595 more thorough solution (Suggested by brian d foy):
598 (^\w) #at the beginning of the line
600 (\s\w) #preceded by whitespace
603 $string =~ /([\w']+)/\u\L$1/g;
605 To make the whole line upper case:
609 To force each word to be lower case, with the first letter upper case:
611 $line =~ s/(\w+)/\u\L$1/g;
613 You can (and probably should) enable locale awareness of those
614 characters by placing a C<use locale> pragma in your program.
615 See L<perllocale> for endless details on locales.
617 This is sometimes referred to as putting something into "title
618 case", but that's not quite accurate. Consider the proper
619 capitalization of the movie I<Dr. Strangelove or: How I Learned to
620 Stop Worrying and Love the Bomb>, for example.
622 =head2 How can I split a [character] delimited string except when inside
623 [character]? (Comma-separated files)
625 Take the example case of trying to split a string that is comma-separated
626 into its different fields. (We'll pretend you said comma-separated, not
627 comma-delimited, which is different and almost never what you mean.) You
628 can't use C<split(/,/)> because you shouldn't split if the comma is inside
629 quotes. For example, take a data line like this:
631 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
633 Due to the restriction of the quotes, this is a fairly complex
634 problem. Thankfully, we have Jeffrey Friedl, author of a highly
635 recommended book on regular expressions, to handle these for us. He
636 suggests (assuming your string is contained in $text):
639 push(@new, $+) while $text =~ m{
640 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
644 push(@new, undef) if substr($text,-1,1) eq ',';
646 If you want to represent quotation marks inside a
647 quotation-mark-delimited field, escape them with backslashes (eg,
648 C<"like \"this\"">. Unescaping them is a task addressed earlier in
651 Alternatively, the Text::ParseWords module (part of the standard Perl
652 distribution) lets you say:
654 use Text::ParseWords;
655 @new = quotewords(",", 0, $text);
657 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
659 =head2 How do I strip blank space from the beginning/end of a string?
661 Although the simplest approach would seem to be
663 $string =~ s/^\s*(.*?)\s*$/$1/;
665 not only is this unnecessarily slow and destructive, it also fails with
666 embedded newlines. It is much faster to do this operation in two steps:
671 Or more nicely written as:
678 This idiom takes advantage of the C<foreach> loop's aliasing
679 behavior to factor out common code. You can do this
680 on several strings at once, or arrays, or even the
681 values of a hash if you use a slice:
683 # trim whitespace in the scalar, the array,
684 # and all the values in the hash
685 foreach ($scalar, @array, @hash{keys %hash}) {
690 =head2 How do I pad a string with blanks or pad a number with zeroes?
692 (This answer contributed by Uri Guttman, with kibitzing from
695 In the following examples, C<$pad_len> is the length to which you wish
696 to pad the string, C<$text> or C<$num> contains the string to be padded,
697 and C<$pad_char> contains the padding character. You can use a single
698 character string constant instead of the C<$pad_char> variable if you
699 know what it is in advance. And in the same way you can use an integer in
700 place of C<$pad_len> if you know the pad length in advance.
702 The simplest method uses the C<sprintf> function. It can pad on the left
703 or right with blanks and on the left with zeroes and it will not
704 truncate the result. The C<pack> function can only pad strings on the
705 right with blanks and it will truncate the result to a maximum length of
708 # Left padding a string with blanks (no truncation):
709 $padded = sprintf("%${pad_len}s", $text);
711 # Right padding a string with blanks (no truncation):
712 $padded = sprintf("%-${pad_len}s", $text);
714 # Left padding a number with 0 (no truncation):
715 $padded = sprintf("%0${pad_len}d", $num);
717 # Right padding a string with blanks using pack (will truncate):
718 $padded = pack("A$pad_len",$text);
720 If you need to pad with a character other than blank or zero you can use
721 one of the following methods. They all generate a pad string with the
722 C<x> operator and combine that with C<$text>. These methods do
723 not truncate C<$text>.
725 Left and right padding with any character, creating a new string:
727 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
728 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
730 Left and right padding with any character, modifying C<$text> directly:
732 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
733 $text .= $pad_char x ( $pad_len - length( $text ) );
735 =head2 How do I extract selected columns from a string?
737 Use substr() or unpack(), both documented in L<perlfunc>.
738 If you prefer thinking in terms of columns instead of widths,
739 you can use this kind of thing:
741 # determine the unpack format needed to split Linux ps output
742 # arguments are cut columns
743 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
749 for my $place (@positions) {
750 $template .= "A" . ($place - $lastpos) . " ";
757 =head2 How do I find the soundex value of a string?
759 Use the standard Text::Soundex module distributed with Perl.
760 Before you do so, you may want to determine whether `soundex' is in
761 fact what you think it is. Knuth's soundex algorithm compresses words
762 into a small space, and so it does not necessarily distinguish between
763 two words which you might want to appear separately. For example, the
764 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
765 If Text::Soundex does not do what you are looking for, you might want
766 to consider the String::Approx module available at CPAN.
768 =head2 How can I expand variables in text strings?
770 Let's assume that you have a string like:
772 $text = 'this has a $foo in it and a $bar';
774 If those were both global variables, then this would
777 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
779 But since they are probably lexicals, or at least, they could
780 be, you'd have to do this:
782 $text =~ s/(\$\w+)/$1/eeg;
783 die if $@; # needed /ee, not /e
785 It's probably better in the general case to treat those
786 variables as entries in some special hash. For example:
792 $text =~ s/\$(\w+)/$user_defs{$1}/g;
794 See also ``How do I expand function calls in a string?'' in this section
797 =head2 What's wrong with always quoting "$vars"?
799 The problem is that those double-quotes force stringification--
800 coercing numbers and references into strings--even when you
801 don't want them to be strings. Think of it this way: double-quote
802 expansion is used to produce new strings. If you already
803 have a string, why do you need more?
805 If you get used to writing odd things like these:
809 somefunc("$var"); # BAD
811 You'll be in trouble. Those should (in 99.8% of the cases) be
812 the simpler and more direct:
818 Otherwise, besides slowing you down, you're going to break code when
819 the thing in the scalar is actually neither a string nor a number, but
825 my $oref = "$aref"; # WRONG
828 You can also get into subtle problems on those few operations in Perl
829 that actually do care about the difference between a string and a
830 number, such as the magical C<++> autoincrement operator or the
833 Stringification also destroys arrays.
836 print "@lines"; # WRONG - extra blanks
837 print @lines; # right
839 =head2 Why don't my <<HERE documents work?
841 Check for these three things:
845 =item 1. There must be no space after the << part.
847 =item 2. There (probably) should be a semicolon at the end.
849 =item 3. You can't (easily) have any space in front of the tag.
853 If you want to indent the text in the here document, you
857 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
862 But the HERE_TARGET must still be flush against the margin.
863 If you want that indented also, you'll have to quote
866 ($quote = <<' FINIS') =~ s/^\s+//gm;
867 ...we will have peace, when you and all your works have
868 perished--and the works of your dark master to whom you
869 would deliver us. You are a liar, Saruman, and a corrupter
870 of men's hearts. --Theoden in /usr/src/perl/taint.c
872 $quote =~ s/\s*--/\n--/;
874 A nice general-purpose fixer-upper function for indented here documents
875 follows. It expects to be called with a here document as its argument.
876 It looks to see whether each line begins with a common substring, and
877 if so, strips that substring off. Otherwise, it takes the amount of leading
878 whitespace found on the first line and removes that much off each
883 my ($white, $leader); # common whitespace and common leading string
884 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
885 ($white, $leader) = ($2, quotemeta($1));
887 ($white, $leader) = (/^(\s+)/, '');
889 s/^\s*?$leader(?:$white)?//gm;
893 This works with leading special strings, dynamically determined:
895 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
898 @@@ SAVEI32(runlevel);
900 @@@ while ( op = (*op->op_ppaddr)() );
904 MAIN_INTERPRETER_LOOP
906 Or with a fixed amount of leading whitespace, with remaining
907 indentation correctly preserved:
909 $poem = fix<<EVER_ON_AND_ON;
910 Now far ahead the Road has gone,
911 And I must follow, if I can,
912 Pursuing it with eager feet,
913 Until it joins some larger way
914 Where many paths and errands meet.
915 And whither then? I cannot say.
916 --Bilbo in /usr/src/perl/pp_ctl.c
921 =head2 What is the difference between a list and an array?
923 An array has a changeable length. A list does not. An array is something
924 you can push or pop, while a list is a set of values. Some people make
925 the distinction that a list is a value while an array is a variable.
926 Subroutines are passed and return lists, you put things into list
927 context, you initialize arrays with lists, and you foreach() across
928 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
929 in scalar context behave like the number of elements in them, subroutines
930 access their arguments through the array C<@_>, and push/pop/shift only work
933 As a side note, there's no such thing as a list in scalar context.
936 $scalar = (2, 5, 7, 9);
938 you're using the comma operator in scalar context, so it uses the scalar
939 comma operator. There never was a list there at all! This causes the
940 last value to be returned: 9.
942 =head2 What is the difference between $array[1] and @array[1]?
944 The former is a scalar value; the latter an array slice, making
945 it a list with one (scalar) value. You should use $ when you want a
946 scalar value (most of the time) and @ when you want a list with one
947 scalar value in it (very, very rarely; nearly never, in fact).
949 Sometimes it doesn't make a difference, but sometimes it does.
950 For example, compare:
952 $good[0] = `some program that outputs several lines`;
956 @bad[0] = `same program that outputs several lines`;
958 The C<use warnings> pragma and the B<-w> flag will warn you about these
961 =head2 How can I remove duplicate elements from a list or array?
963 There are several possible ways, depending on whether the array is
964 ordered and whether you wish to preserve the ordering.
970 If @in is sorted, and you want @out to be sorted:
971 (this assumes all true values in the array)
973 $prev = "not equal to $in[0]";
974 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
976 This is nice in that it doesn't use much extra memory, simulating
977 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
978 guarantees that the expression is true (so that grep picks it up)
979 even if the $_ is 0, "", or undef.
983 If you don't know whether @in is sorted:
986 @out = grep(!$saw{$_}++, @in);
990 Like (b), but @in contains only small integers:
992 @out = grep(!$saw[$_]++, @in);
996 A way to do (b) without any loops or greps:
1000 @out = sort keys %saw; # remove sort if undesired
1004 Like (d), but @in contains only small positive integers:
1008 @out = grep {defined} @ary;
1012 But perhaps you should have been using a hash all along, eh?
1014 =head2 How can I tell whether a certain element is contained in a list or array?
1016 Hearing the word "in" is an I<in>dication that you probably should have
1017 used a hash, not a list or array, to store your data. Hashes are
1018 designed to answer this question quickly and efficiently. Arrays aren't.
1020 That being said, there are several ways to approach this. If you
1021 are going to make this query many times over arbitrary string values,
1022 the fastest way is probably to invert the original array and keep an
1023 associative array lying about whose keys are the first array's values.
1025 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1027 for (@blues) { $is_blue{$_} = 1 }
1029 Now you can check whether $is_blue{$some_color}. It might have been a
1030 good idea to keep the blues all in a hash in the first place.
1032 If the values are all small integers, you could use a simple indexed
1033 array. This kind of an array will take up less space:
1035 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1036 undef @is_tiny_prime;
1037 for (@primes) { $is_tiny_prime[$_] = 1 }
1038 # or simply @istiny_prime[@primes] = (1) x @primes;
1040 Now you check whether $is_tiny_prime[$some_number].
1042 If the values in question are integers instead of strings, you can save
1043 quite a lot of space by using bit strings instead:
1045 @articles = ( 1..10, 150..2000, 2017 );
1047 for (@articles) { vec($read,$_,1) = 1 }
1049 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1053 ($is_there) = grep $_ eq $whatever, @array;
1057 ($is_there) = grep /$whatever/, @array;
1059 These are slow (checks every element even if the first matches),
1060 inefficient (same reason), and potentially buggy (what if there are
1061 regex characters in $whatever?). If you're only testing once, then
1065 foreach $elt (@array) {
1066 if ($elt eq $elt_to_find) {
1071 if ($is_there) { ... }
1073 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1075 Use a hash. Here's code to do both and more. It assumes that
1076 each element is unique in a given array:
1078 @union = @intersection = @difference = ();
1080 foreach $element (@array1, @array2) { $count{$element}++ }
1081 foreach $element (keys %count) {
1082 push @union, $element;
1083 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1086 Note that this is the I<symmetric difference>, that is, all elements in
1087 either A or in B but not in both. Think of it as an xor operation.
1089 =head2 How do I test whether two arrays or hashes are equal?
1091 The following code works for single-level arrays. It uses a stringwise
1092 comparison, and does not distinguish defined versus undefined empty
1093 strings. Modify if you have other needs.
1095 $are_equal = compare_arrays(\@frogs, \@toads);
1097 sub compare_arrays {
1098 my ($first, $second) = @_;
1099 no warnings; # silence spurious -w undef complaints
1100 return 0 unless @$first == @$second;
1101 for (my $i = 0; $i < @$first; $i++) {
1102 return 0 if $first->[$i] ne $second->[$i];
1107 For multilevel structures, you may wish to use an approach more
1108 like this one. It uses the CPAN module FreezeThaw:
1110 use FreezeThaw qw(cmpStr);
1111 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1113 printf "a and b contain %s arrays\n",
1114 cmpStr(\@a, \@b) == 0
1118 This approach also works for comparing hashes. Here
1119 we'll demonstrate two different answers:
1121 use FreezeThaw qw(cmpStr cmpStrHard);
1123 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1127 printf "a and b contain %s hashes\n",
1128 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1130 printf "a and b contain %s hashes\n",
1131 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1134 The first reports that both those the hashes contain the same data,
1135 while the second reports that they do not. Which you prefer is left as
1136 an exercise to the reader.
1138 =head2 How do I find the first array element for which a condition is true?
1140 You can use this if you care about the index:
1142 for ($i= 0; $i < @array; $i++) {
1143 if ($array[$i] eq "Waldo") {
1149 Now C<$found_index> has what you want.
1151 =head2 How do I handle linked lists?
1153 In general, you usually don't need a linked list in Perl, since with
1154 regular arrays, you can push and pop or shift and unshift at either end,
1155 or you can use splice to add and/or remove arbitrary number of elements at
1156 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1157 dynamic arrays. In the absence of shifts and pops, push in general
1158 needs to reallocate on the order every log(N) times, and unshift will
1159 need to copy pointers each time.
1161 If you really, really wanted, you could use structures as described in
1162 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1163 to do. For example, imagine a list node like this:
1170 You could walk the list this way:
1173 for ($node = $head; $node; $node = $node->{LINK}) {
1174 print $node->{VALUE}, " ";
1178 You could add to the list this way:
1181 $tail = append($head, 1); # grow a new head
1182 for $value ( 2 .. 10 ) {
1183 $tail = append($tail, $value);
1187 my($list, $value) = @_;
1188 my $node = { VALUE => $value };
1190 $node->{LINK} = $list->{LINK};
1191 $list->{LINK} = $node;
1193 $_[0] = $node; # replace caller's version
1198 But again, Perl's built-in are virtually always good enough.
1200 =head2 How do I handle circular lists?
1202 Circular lists could be handled in the traditional fashion with linked
1203 lists, or you could just do something like this with an array:
1205 unshift(@array, pop(@array)); # the last shall be first
1206 push(@array, shift(@array)); # and vice versa
1208 =head2 How do I shuffle an array randomly?
1210 If you either have Perl 5.8.0 or later installed, or if you have
1211 Scalar-List-Utils 1.03 or later installed, you can say:
1213 use List::Util 'shuffle';
1215 @shuffled = shuffle(@list);
1217 If not, you can use this:
1219 # fisher_yates_shuffle
1220 # generate a random permutation of an array in place
1221 # As in shuffling a deck of cards
1223 sub fisher_yates_shuffle {
1224 my $deck = shift; # $deck is a reference to an array
1227 my $j = int rand ($i+1);
1228 @$deck[$i,$j] = @$deck[$j,$i];
1232 And here is an example of using it:
1235 # shuffle my mpeg collection
1237 my @mpeg = <audio/*/*.mp3>;
1238 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1241 Note that the above implementation shuffles an array in place,
1242 unlike the List::Util::shuffle() which takes a list and returns
1243 a new shuffled list.
1245 You've probably seen shuffling algorithms that work using splice,
1246 randomly picking another element to swap the current element with
1250 @old = 1 .. 10; # just a demo
1252 push(@new, splice(@old, rand @old, 1));
1255 This is bad because splice is already O(N), and since you do it N times,
1256 you just invented a quadratic algorithm; that is, O(N**2). This does
1257 not scale, although Perl is so efficient that you probably won't notice
1258 this until you have rather largish arrays.
1260 =head2 How do I process/modify each element of an array?
1262 Use C<for>/C<foreach>:
1265 s/foo/bar/; # change that word
1266 y/XZ/ZX/; # swap those letters
1269 Here's another; let's compute spherical volumes:
1271 for (@volumes = @radii) { # @volumes has changed parts
1273 $_ *= (4/3) * 3.14159; # this will be constant folded
1276 If you want to do the same thing to modify the values of the hash,
1277 you may not use the C<values> function, oddly enough. You need a slice:
1279 for $orbit ( @orbits{keys %orbits} ) {
1280 ($orbit **= 3) *= (4/3) * 3.14159;
1283 =head2 How do I select a random element from an array?
1285 Use the rand() function (see L<perlfunc/rand>):
1287 # at the top of the program:
1288 srand; # not needed for 5.004 and later
1291 $index = rand @array;
1292 $element = $array[$index];
1294 Make sure you I<only call srand once per program, if then>.
1295 If you are calling it more than once (such as before each
1296 call to rand), you're almost certainly doing something wrong.
1298 =head2 How do I permute N elements of a list?
1300 Here's a little program that generates all permutations
1301 of all the words on each line of input. The algorithm embodied
1302 in the permute() function should work on any list:
1305 # tsc-permute: permute each word of input
1306 permute([split], []);
1308 my @items = @{ $_[0] };
1309 my @perms = @{ $_[1] };
1313 my(@newitems,@newperms,$i);
1314 foreach $i (0 .. $#items) {
1317 unshift(@newperms, splice(@newitems, $i, 1));
1318 permute([@newitems], [@newperms]);
1323 Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
1324 module from CPAN runs at least an order of magnitude faster. If you don't
1325 have a C compiler (or a binary distribution of Algorithm::Permute), then
1326 you can use List::Permutor which is written in pure Perl, and is still
1327 several times faster than the algorithm above.
1329 =head2 How do I sort an array by (anything)?
1331 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1333 @list = sort { $a <=> $b } @list;
1335 The default sort function is cmp, string comparison, which would
1336 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1337 the numerical comparison operator.
1339 If you have a complicated function needed to pull out the part you
1340 want to sort on, then don't do it inside the sort function. Pull it
1341 out first, because the sort BLOCK can be called many times for the
1342 same element. Here's an example of how to pull out the first word
1343 after the first number on each item, and then sort those words
1348 ($item) = /\d+\s*(\S+)/;
1349 push @idx, uc($item);
1351 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1353 which could also be written this way, using a trick
1354 that's come to be known as the Schwartzian Transform:
1356 @sorted = map { $_->[0] }
1357 sort { $a->[1] cmp $b->[1] }
1358 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1360 If you need to sort on several fields, the following paradigm is useful.
1362 @sorted = sort { field1($a) <=> field1($b) ||
1363 field2($a) cmp field2($b) ||
1364 field3($a) cmp field3($b)
1367 This can be conveniently combined with precalculation of keys as given
1370 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1373 See also the question below on sorting hashes.
1375 =head2 How do I manipulate arrays of bits?
1377 Use pack() and unpack(), or else vec() and the bitwise operations.
1379 For example, this sets $vec to have bit N set if $ints[N] was set:
1382 foreach(@ints) { vec($vec,$_,1) = 1 }
1384 Here's how, given a vector in $vec, you can
1385 get those bits into your @ints array:
1387 sub bitvec_to_list {
1390 # Find null-byte density then select best algorithm
1391 if ($vec =~ tr/\0// / length $vec > 0.95) {
1394 # This method is faster with mostly null-bytes
1395 while($vec =~ /[^\0]/g ) {
1396 $i = -9 + 8 * pos $vec;
1397 push @ints, $i if vec($vec, ++$i, 1);
1398 push @ints, $i if vec($vec, ++$i, 1);
1399 push @ints, $i if vec($vec, ++$i, 1);
1400 push @ints, $i if vec($vec, ++$i, 1);
1401 push @ints, $i if vec($vec, ++$i, 1);
1402 push @ints, $i if vec($vec, ++$i, 1);
1403 push @ints, $i if vec($vec, ++$i, 1);
1404 push @ints, $i if vec($vec, ++$i, 1);
1407 # This method is a fast general algorithm
1409 my $bits = unpack "b*", $vec;
1410 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1411 push @ints, pos $bits while($bits =~ /1/g);
1416 This method gets faster the more sparse the bit vector is.
1417 (Courtesy of Tim Bunce and Winfried Koenig.)
1419 Or use the CPAN module Bit::Vector:
1421 $vector = Bit::Vector->new($num_of_bits);
1422 $vector->Index_List_Store(@ints);
1423 @ints = $vector->Index_List_Read();
1425 Bit::Vector provides efficient methods for bit vector, sets of small integers
1428 Here's a more extensive illustration using vec():
1431 $vector = "\xff\x0f\xef\xfe";
1432 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1433 unpack("N", $vector), "\n";
1434 $is_set = vec($vector, 23, 1);
1435 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1453 my ($offset, $width, $value) = @_;
1455 vec($vector, $offset, $width) = $value;
1456 print "offset=$offset width=$width value=$value\n";
1462 my $bits = unpack("b*", $vector);
1466 print "vector length in bytes: ", length($vector), "\n";
1467 @bytes = unpack("A8" x length($vector), $bits);
1468 print "bits are: @bytes\n\n";
1471 =head2 Why does defined() return true on empty arrays and hashes?
1473 The short story is that you should probably only use defined on scalars or
1474 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1475 in the 5.004 release or later of Perl for more detail.
1477 =head1 Data: Hashes (Associative Arrays)
1479 =head2 How do I process an entire hash?
1481 Use the each() function (see L<perlfunc/each>) if you don't care
1482 whether it's sorted:
1484 while ( ($key, $value) = each %hash) {
1485 print "$key = $value\n";
1488 If you want it sorted, you'll have to use foreach() on the result of
1489 sorting the keys as shown in an earlier question.
1491 =head2 What happens if I add or remove keys from a hash while iterating over it?
1495 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1496 iterating over it. In Perl 5 you can delete from it, but you still
1497 can't add to it, because that might cause a doubling of the hash table,
1498 in which half the entries get copied up to the new top half of the
1499 table, at which point you've totally bamboozled the iterator code.
1500 Even if the table doesn't double, there's no telling whether your new
1501 entry will be inserted before or after the current iterator position.
1503 Either treasure up your changes and make them after the iterator finishes
1504 or use keys to fetch all the old keys at once, and iterate over the list
1507 =head2 How do I look up a hash element by value?
1509 Create a reverse hash:
1511 %by_value = reverse %by_key;
1512 $key = $by_value{$value};
1514 That's not particularly efficient. It would be more space-efficient
1517 while (($key, $value) = each %by_key) {
1518 $by_value{$value} = $key;
1521 If your hash could have repeated values, the methods above will only find
1522 one of the associated keys. This may or may not worry you. If it does
1523 worry you, you can always reverse the hash into a hash of arrays instead:
1525 while (($key, $value) = each %by_key) {
1526 push @{$key_list_by_value{$value}}, $key;
1529 =head2 How can I know how many entries are in a hash?
1531 If you mean how many keys, then all you have to do is
1532 take the scalar sense of the keys() function:
1534 $num_keys = scalar keys %hash;
1536 The keys() function also resets the iterator, which in void context is
1537 faster for tied hashes than would be iterating through the whole
1538 hash, one key-value pair at a time.
1540 =head2 How do I sort a hash (optionally by value instead of key)?
1542 Internally, hashes are stored in a way that prevents you from imposing
1543 an order on key-value pairs. Instead, you have to sort a list of the
1546 @keys = sort keys %hash; # sorted by key
1548 $hash{$a} cmp $hash{$b}
1549 } keys %hash; # and by value
1551 Here we'll do a reverse numeric sort by value, and if two keys are
1552 identical, sort by length of key, or if that fails, by straight ASCII
1553 comparison of the keys (well, possibly modified by your locale--see
1557 $hash{$b} <=> $hash{$a}
1559 length($b) <=> length($a)
1564 =head2 How can I always keep my hash sorted?
1566 You can look into using the DB_File module and tie() using the
1567 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1568 The Tie::IxHash module from CPAN might also be instructive.
1570 =head2 What's the difference between "delete" and "undef" with hashes?
1572 Hashes are pairs of scalars: the first is the key, the second is the
1573 value. The key will be coerced to a string, although the value can be
1574 any kind of scalar: string, number, or reference. If a key C<$key> is
1575 present in the array, C<exists($key)> will return true. The value for
1576 a given key can be C<undef>, in which case C<$array{$key}> will be
1577 C<undef> while C<$exists{$key}> will return true. This corresponds to
1578 (C<$key>, C<undef>) being in the hash.
1580 Pictures help... here's the C<%ary> table:
1590 And these conditions hold
1594 defined $ary{'d'} is true
1595 defined $ary{'a'} is true
1596 exists $ary{'a'} is true (Perl5 only)
1597 grep ($_ eq 'a', keys %ary) is true
1603 your table now reads:
1614 and these conditions now hold; changes in caps:
1618 defined $ary{'d'} is true
1619 defined $ary{'a'} is FALSE
1620 exists $ary{'a'} is true (Perl5 only)
1621 grep ($_ eq 'a', keys %ary) is true
1623 Notice the last two: you have an undef value, but a defined key!
1629 your table now reads:
1638 and these conditions now hold; changes in caps:
1642 defined $ary{'d'} is true
1643 defined $ary{'a'} is false
1644 exists $ary{'a'} is FALSE (Perl5 only)
1645 grep ($_ eq 'a', keys %ary) is FALSE
1647 See, the whole entry is gone!
1649 =head2 Why don't my tied hashes make the defined/exists distinction?
1651 They may or may not implement the EXISTS() and DEFINED() methods
1652 differently. For example, there isn't the concept of undef with hashes
1653 that are tied to DBM* files. This means the true/false tables above
1654 will give different results when used on such a hash. It also means
1655 that exists and defined do the same thing with a DBM* file, and what
1656 they end up doing is not what they do with ordinary hashes.
1658 =head2 How do I reset an each() operation part-way through?
1660 Using C<keys %hash> in scalar context returns the number of keys in
1661 the hash I<and> resets the iterator associated with the hash. You may
1662 need to do this if you use C<last> to exit a loop early so that when you
1663 re-enter it, the hash iterator has been reset.
1665 =head2 How can I get the unique keys from two hashes?
1667 First you extract the keys from the hashes into lists, then solve
1668 the "removing duplicates" problem described above. For example:
1671 for $element (keys(%foo), keys(%bar)) {
1678 @uniq = keys %{{%foo,%bar}};
1680 Or if you really want to save space:
1683 while (defined ($key = each %foo)) {
1686 while (defined ($key = each %bar)) {
1691 =head2 How can I store a multidimensional array in a DBM file?
1693 Either stringify the structure yourself (no fun), or else
1694 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1695 it on top of either DB_File or GDBM_File.
1697 =head2 How can I make my hash remember the order I put elements into it?
1699 Use the Tie::IxHash from CPAN.
1702 tie(%myhash, Tie::IxHash);
1703 for ($i=0; $i<20; $i++) {
1706 @keys = keys %myhash;
1707 # @keys = (0,1,2,3,...)
1709 =head2 Why does passing a subroutine an undefined element in a hash create it?
1711 If you say something like:
1713 somefunc($hash{"nonesuch key here"});
1715 Then that element "autovivifies"; that is, it springs into existence
1716 whether you store something there or not. That's because functions
1717 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1718 it has to be ready to write it back into the caller's version.
1720 This has been fixed as of Perl5.004.
1722 Normally, merely accessing a key's value for a nonexistent key does
1723 I<not> cause that key to be forever there. This is different than
1726 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1728 Usually a hash ref, perhaps like this:
1733 TITLE => "deputy peon",
1736 PALS => [ "Norbert", "Rhys", "Phineas"],
1739 References are documented in L<perlref> and the upcoming L<perlreftut>.
1740 Examples of complex data structures are given in L<perldsc> and
1741 L<perllol>. Examples of structures and object-oriented classes are
1744 =head2 How can I use a reference as a hash key?
1746 You can't do this directly, but you could use the standard Tie::Refhash
1747 module distributed with Perl.
1751 =head2 How do I handle binary data correctly?
1753 Perl is binary clean, so this shouldn't be a problem. For example,
1754 this works fine (assuming the files are found):
1756 if (`cat /vmunix` =~ /gzip/) {
1757 print "Your kernel is GNU-zip enabled!\n";
1760 On less elegant (read: Byzantine) systems, however, you have
1761 to play tedious games with "text" versus "binary" files. See
1762 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1763 systems are curses out of Microsoft, who seem to be committed to putting
1764 the backward into backward compatibility.
1766 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1768 If you want to deal with multibyte characters, however, there are
1769 some gotchas. See the section on Regular Expressions.
1771 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1773 Assuming that you don't care about IEEE notations like "NaN" or
1774 "Infinity", you probably just want to use a regular expression.
1776 if (/\D/) { print "has nondigits\n" }
1777 if (/^\d+$/) { print "is a whole number\n" }
1778 if (/^-?\d+$/) { print "is an integer\n" }
1779 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1780 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1781 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1782 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1783 { print "a C float" }
1785 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1786 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1787 wrapper function for more convenient access. This function takes
1788 a string and returns the number it found, or C<undef> for input that
1789 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1790 if you just want to say, ``Is this a float?''
1793 use POSIX qw(strtod);
1798 my($num, $unparsed) = strtod($str);
1799 if (($str eq '') || ($unparsed != 0) || $!) {
1806 sub is_numeric { defined getnum($_[0]) }
1808 Or you could check out the String::Scanf module on CPAN instead. The
1809 POSIX module (part of the standard Perl distribution) provides the
1810 C<strtod> and C<strtol> for converting strings to double and longs,
1813 =head2 How do I keep persistent data across program calls?
1815 For some specific applications, you can use one of the DBM modules.
1816 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1817 Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
1818 Storable is part of the standard distribution. Here's one example using
1819 Storable's C<store> and C<retrieve> functions:
1822 store(\%hash, "filename");
1825 $href = retrieve("filename"); # by ref
1826 %hash = %{ retrieve("filename") }; # direct to hash
1828 =head2 How do I print out or copy a recursive data structure?
1830 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1831 for printing out data structures. The Storable module, found on CPAN,
1832 provides a function called C<dclone> that recursively copies its argument.
1834 use Storable qw(dclone);
1837 Where $r1 can be a reference to any kind of data structure you'd like.
1838 It will be deeply copied. Because C<dclone> takes and returns references,
1839 you'd have to add extra punctuation if you had a hash of arrays that
1842 %newhash = %{ dclone(\%oldhash) };
1844 =head2 How do I define methods for every class/object?
1846 Use the UNIVERSAL class (see L<UNIVERSAL>).
1848 =head2 How do I verify a credit card checksum?
1850 Get the Business::CreditCard module from CPAN.
1852 =head2 How do I pack arrays of doubles or floats for XS code?
1854 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1855 If you're doing a lot of float or double processing, consider using
1856 the PDL module from CPAN instead--it makes number-crunching easy.
1858 =head1 AUTHOR AND COPYRIGHT
1860 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1861 All rights reserved.
1863 This documentation is free; you can redistribute it and/or modify it
1864 under the same terms as Perl itself.
1866 Irrespective of its distribution, all code examples in this file
1867 are hereby placed into the public domain. You are permitted and
1868 encouraged to use this code in your own programs for fun
1869 or for profit as you see fit. A simple comment in the code giving
1870 credit would be courteous but is not required.