3 perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
7 The section of the FAQ answers questions related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximated on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to an internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers. (See L<perlvar/"$#"> if you use
33 print. C<$#> has a different default value in Perl5 than it did in
34 Perl4. Changing C<$#> yourself is deprecated.)
36 This affects B<all> computer languages that represent decimal
37 floating-point numbers in binary, not just Perl. Perl provides
38 arbitrary-precision decimal numbers with the Math::BigFloat module
39 (part of the standard Perl distribution), but mathematical operations
40 are consequently slower.
42 If precision is important, such as when dealing with money, it's good
43 to work with integers and then divide at the last possible moment.
44 For example, work in pennies (1995) instead of dollars and cents
45 (19.95) and divide by 100 at the end.
47 To get rid of the superfluous digits, just use a format (eg,
48 C<printf("%.2f", 19.95)>) to get the required precision.
49 See L<perlop/"Floating-point Arithmetic">.
51 =head2 Why isn't my octal data interpreted correctly?
53 Perl only understands octal and hex numbers as such when they occur
54 as literals in your program. Octal literals in perl must start with
55 a leading "0" and hexadecimal literals must start with a leading "0x".
56 If they are read in from somewhere and assigned, no automatic
57 conversion takes place. You must explicitly use oct() or hex() if you
58 want the values converted to decimal. oct() interprets
59 both hex ("0x350") numbers and octal ones ("0350" or even without the
60 leading "0", like "377"), while hex() only converts hexadecimal ones,
61 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
62 The inverse mapping from decimal to octal can be done with either the
63 "%o" or "%O" sprintf() formats. To get from decimal to hex try either
64 the "%x" or the "%X" formats to sprintf().
66 This problem shows up most often when people try using chmod(), mkdir(),
67 umask(), or sysopen(), which by widespread tradition typically take
70 chmod(644, $file); # WRONG
71 chmod(0644, $file); # right
73 Note the mistake in the first line was specifying the decimal literal
74 644, rather than the intended octal literal 0644. The problem can
77 printf("%#o",644); # prints 01204
79 Surely you had not intended C<chmod(01204, $file);> - did you? If you
80 want to use numeric literals as arguments to chmod() et al. then please
81 try to express them as octal constants, that is with a leading zero and
82 with the following digits restricted to the set 0..7.
84 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
86 Remember that int() merely truncates toward 0. For rounding to a
87 certain number of digits, sprintf() or printf() is usually the easiest
90 printf("%.3f", 3.1415926535); # prints 3.142
92 The POSIX module (part of the standard Perl distribution) implements
93 ceil(), floor(), and a number of other mathematical and trigonometric
97 $ceil = ceil(3.5); # 4
98 $floor = floor(3.5); # 3
100 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
101 module. With 5.004, the Math::Trig module (part of the standard Perl
102 distribution) implements the trigonometric functions. Internally it
103 uses the Math::Complex module and some functions can break out from
104 the real axis into the complex plane, for example the inverse sine of
107 Rounding in financial applications can have serious implications, and
108 the rounding method used should be specified precisely. In these
109 cases, it probably pays not to trust whichever system rounding is
110 being used by Perl, but to instead implement the rounding function you
113 To see why, notice how you'll still have an issue on half-way-point
116 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
118 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
119 0.8 0.8 0.9 0.9 1.0 1.0
121 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
122 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
123 machines) will work pretty much like mathematical integers. Other numbers
126 =head2 How do I convert bits into ints?
128 To turn a string of 1s and 0s like C<10110110> into a scalar containing
129 its binary value, use the pack() and unpack() functions (documented in
130 L<perlfunc/"pack"> and L<perlfunc/"unpack">):
132 $decimal = unpack('c', pack('B8', '10110110'));
134 This packs the string C<10110110> into an eight bit binary structure.
135 This is then unpacked as a character, which returns its ordinal value.
137 This does the same thing:
139 $decimal = ord(pack('B8', '10110110'));
141 Here's an example of going the other way:
143 $binary_string = unpack('B*', "\x29");
145 =head2 Why doesn't & work the way I want it to?
147 The behavior of binary arithmetic operators depends on whether they're
148 used on numbers or strings. The operators treat a string as a series
149 of bits and work with that (the string C<"3"> is the bit pattern
150 C<00110011>). The operators work with the binary form of a number
151 (the number C<3> is treated as the bit pattern C<00000011>).
153 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
154 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
157 Most problems with C<&> and C<|> arise because the programmer thinks
158 they have a number but really it's a string. The rest arise because
161 if ("\020\020" & "\101\101") {
165 but a string consisting of two null bytes (the result of C<"\020\020"
166 & "\101\101">) is not a false value in Perl. You need:
168 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
172 =head2 How do I multiply matrices?
174 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
175 or the PDL extension (also available from CPAN).
177 =head2 How do I perform an operation on a series of integers?
179 To call a function on each element in an array, and collect the
182 @results = map { my_func($_) } @array;
186 @triple = map { 3 * $_ } @single;
188 To call a function on each element of an array, but ignore the
191 foreach $iterator (@array) {
192 some_func($iterator);
195 To call a function on each integer in a (small) range, you B<can> use:
197 @results = map { some_func($_) } (5 .. 25);
199 but you should be aware that the C<..> operator creates an array of
200 all integers in the range. This can take a lot of memory for large
204 for ($i=5; $i < 500_005; $i++) {
205 push(@results, some_func($i));
208 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
209 loop will iterate over the range, without creating the entire range.
211 for my $i (5 .. 500_005) {
212 push(@results, some_func($i));
215 will not create a list of 500,000 integers.
217 =head2 How can I output Roman numerals?
219 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
221 =head2 Why aren't my random numbers random?
223 If you're using a version of Perl before 5.004, you must call C<srand>
224 once at the start of your program to seed the random number generator.
225 5.004 and later automatically call C<srand> at the beginning. Don't
226 call C<srand> more than once--you make your numbers less random, rather
229 Computers are good at being predictable and bad at being random
230 (despite appearances caused by bugs in your programs :-).
231 http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
232 Phoenix, talks more about this. John von Neumann said, ``Anyone who
233 attempts to generate random numbers by deterministic means is, of
234 course, living in a state of sin.''
236 If you want numbers that are more random than C<rand> with C<srand>
237 provides, you should also check out the Math::TrulyRandom module from
238 CPAN. It uses the imperfections in your system's timer to generate
239 random numbers, but this takes quite a while. If you want a better
240 pseudorandom generator than comes with your operating system, look at
241 ``Numerical Recipes in C'' at http://www.nr.com/ .
245 =head2 How do I find the week-of-the-year/day-of-the-year?
247 The day of the year is in the array returned by localtime() (see
248 L<perlfunc/"localtime">):
250 $day_of_year = (localtime(time()))[7];
252 or more legibly (in 5.7.1 or higher):
255 $day_of_year = localtime->day_of_year();
257 You can find the week of the year by using Time::Piece's strftime():
259 $week_of_year = localtime->strftime("%U");
260 $iso_week = localtime->strftime("%V");
262 The difference between %U and %V is that %U assumes that the first day
263 of week 1 is the first Sunday of the year, whereas ISO 8601:1988 uses
264 the first week that has at least 4 days in the current year, and with
265 Monday as the first day of the week. You can also use %W, which will
266 return the week of the year with Monday as the first day of week 1. See
267 your strftime(3) man page for more details.
269 =head2 How do I find the current century or millennium?
271 Use the following simple functions:
274 return int((((localtime(shift || time))[5] + 1999))/100);
277 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
280 On some systems, you'll find that the POSIX module's strftime() function
281 has been extended in a non-standard way to use a C<%C> format, which they
282 sometimes claim is the "century". It isn't, because on most such systems,
283 this is only the first two digits of the four-digit year, and thus cannot
284 be used to reliably determine the current century or millennium.
286 =head2 How can I compare two dates and find the difference?
288 If you're storing your dates as epoch seconds then simply subtract one
289 from the other. If you've got a structured date (distinct year, day,
290 month, hour, minute, seconds values), then for reasons of accessibility,
291 simplicity, and efficiency, merely use either timelocal or timegm (from
292 the Time::Local module in the standard distribution) to reduce structured
293 dates to epoch seconds. However, if you don't know the precise format of
294 your dates, then you should probably use either of the Date::Manip and
295 Date::Calc modules from CPAN before you go hacking up your own parsing
296 routine to handle arbitrary date formats.
298 Also note that the core module Time::Piece overloads the addition and
299 subtraction operators to provide date calculation options. See
300 L<Time::Piece/Date Calculations>.
302 =head2 How can I take a string and turn it into epoch seconds?
304 If it's a regular enough string that it always has the same format,
305 you can split it up and pass the parts to C<timelocal> in the standard
306 Time::Local module. Otherwise, you should look into the Date::Calc
307 and Date::Manip modules from CPAN.
309 =head2 How can I find the Julian Day?
311 Use Time::Piece as follows:
314 my $julian_day = localtime->julian_day;
315 my $mjd = localtime->mjd; # modified julian day
317 Before you immerse yourself too deeply in this, be sure to verify that
318 it is the I<Julian> Day you really want. Are you interested in a way
319 of getting serial days so that you just can tell how many days they
320 are apart or so that you can do also other date arithmetic? If you
321 are interested in performing date arithmetic, this can be done using
322 Time::Piece (standard module since Perl 5.8), or by modules
323 Date::Manip or Date::Calc.
325 There is too many details and much confusion on this issue to cover in
326 this FAQ, but the term is applied (correctly) to a calendar now
327 supplanted by the Gregorian Calendar, with the Julian Calendar failing
328 to adjust properly for leap years on centennial years (among other
329 annoyances). The term is also used (incorrectly) to mean: [1] days in
330 the Gregorian Calendar; and [2] days since a particular starting time
331 or `epoch', usually 1970 in the Unix world and 1980 in the
332 MS-DOS/Windows world. If you find that it is not the first meaning
333 that you really want, then check out the Date::Manip and Date::Calc
334 modules. (Thanks to David Cassell for most of this text.)
336 =head2 How do I find yesterday's date?
338 The C<time()> function returns the current time in seconds since the
339 epoch. Take twenty-four hours off that:
341 $yesterday = time() - ( 24 * 60 * 60 );
343 Then you can pass this to C<localtime()> and get the individual year,
344 month, day, hour, minute, seconds values.
346 Alternatively, you can use Time::Piece to subtract a day from the value
347 returned from C<localtime()>:
350 use Time::Seconds; # imports seconds constants, like ONE_DAY
351 my $today = localtime();
352 my $yesterday = $today - ONE_DAY;
354 Note very carefully that the code above assumes that your days are
355 twenty-four hours each. For most people, there are two days a year
356 when they aren't: the switch to and from summer time throws this off.
357 A solution to this issue is offered by Russ Allbery.
360 my $now = defined $_[0] ? $_[0] : time;
361 my $then = $now - 60 * 60 * 24;
362 my $ndst = (localtime $now)[8] > 0;
363 my $tdst = (localtime $then)[8] > 0;
364 $then - ($tdst - $ndst) * 60 * 60;
366 # Should give you "this time yesterday" in seconds since epoch relative to
367 # the first argument or the current time if no argument is given and
368 # suitable for passing to localtime or whatever else you need to do with
369 # it. $ndst is whether we're currently in daylight savings time; $tdst is
370 # whether the point 24 hours ago was in daylight savings time. If $tdst
371 # and $ndst are the same, a boundary wasn't crossed, and the correction
372 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
373 # from yesterday's time since we gained an extra hour while going off
374 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
375 # negative hour (add an hour) to yesterday's time since we lost an hour.
377 # All of this is because during those days when one switches off or onto
378 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
380 # The explicit settings of $ndst and $tdst are necessary because localtime
381 # only says it returns the system tm struct, and the system tm struct at
382 # least on Solaris doesn't guarantee any particular positive value (like,
383 # say, 1) for isdst, just a positive value. And that value can
384 # potentially be negative, if DST information isn't available (this sub
385 # just treats those cases like no DST).
387 # Note that between 2am and 3am on the day after the time zone switches
388 # off daylight savings time, the exact hour of "yesterday" corresponding
389 # to the current hour is not clearly defined. Note also that if used
390 # between 2am and 3am the day after the change to daylight savings time,
391 # the result will be between 3am and 4am of the previous day; it's
392 # arguable whether this is correct.
394 # This sub does not attempt to deal with leap seconds (most things don't).
396 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
397 # This code is in the public domain
399 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
401 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
402 Y2K compliant (whatever that means). The programmers you've hired to
403 use it, however, probably are not.
405 Long answer: The question belies a true understanding of the issue.
406 Perl is just as Y2K compliant as your pencil--no more, and no less.
407 Can you use your pencil to write a non-Y2K-compliant memo? Of course
408 you can. Is that the pencil's fault? Of course it isn't.
410 The date and time functions supplied with Perl (gmtime and localtime)
411 supply adequate information to determine the year well beyond 2000
412 (2038 is when trouble strikes for 32-bit machines). The year returned
413 by these functions when used in a list context is the year minus 1900.
414 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
415 number. To avoid the year 2000 problem simply do not treat the year as
416 a 2-digit number. It isn't.
418 When gmtime() and localtime() are used in scalar context they return
419 a timestamp string that contains a fully-expanded year. For example,
420 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
421 2001". There's no year 2000 problem here.
423 That doesn't mean that Perl can't be used to create non-Y2K compliant
424 programs. It can. But so can your pencil. It's the fault of the user,
425 not the language. At the risk of inflaming the NRA: ``Perl doesn't
426 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
431 =head2 How do I validate input?
433 The answer to this question is usually a regular expression, perhaps
434 with auxiliary logic. See the more specific questions (numbers, mail
435 addresses, etc.) for details.
437 =head2 How do I unescape a string?
439 It depends just what you mean by ``escape''. URL escapes are dealt
440 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
441 character are removed with
445 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
447 =head2 How do I remove consecutive pairs of characters?
449 To turn C<"abbcccd"> into C<"abccd">:
451 s/(.)\1/$1/g; # add /s to include newlines
453 Here's a solution that turns "abbcccd" to "abcd":
455 y///cs; # y == tr, but shorter :-)
457 =head2 How do I expand function calls in a string?
459 This is documented in L<perlref>. In general, this is fraught with
460 quoting and readability problems, but it is possible. To interpolate
461 a subroutine call (in list context) into a string:
463 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
465 If you prefer scalar context, similar chicanery is also useful for
466 arbitrary expressions:
468 print "That yields ${\($n + 5)} widgets\n";
470 Version 5.004 of Perl had a bug that gave list context to the
471 expression in C<${...}>, but this is fixed in version 5.005.
473 See also ``How can I expand variables in text strings?'' in this
476 =head2 How do I find matching/nesting anything?
478 This isn't something that can be done in one regular expression, no
479 matter how complicated. To find something between two single
480 characters, a pattern like C</x([^x]*)x/> will get the intervening
481 bits in $1. For multiple ones, then something more like
482 C</alpha(.*?)omega/> would be needed. But none of these deals with
483 nested patterns, nor can they. For that you'll have to write a
486 If you are serious about writing a parser, there are a number of
487 modules or oddities that will make your life a lot easier. There are
488 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
489 and the byacc program. Starting from perl 5.8 the Text::Balanced
490 is part of the standard distribution.
492 One simple destructive, inside-out approach that you might try is to
493 pull out the smallest nesting parts one at a time:
495 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
496 # do something with $1
499 A more complicated and sneaky approach is to make Perl's regular
500 expression engine do it for you. This is courtesy Dean Inada, and
501 rather has the nature of an Obfuscated Perl Contest entry, but it
504 # $_ contains the string to parse
505 # BEGIN and END are the opening and closing markers for the
510 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
511 @$ = (eval{/$re/},$@!~/unmatched/);
512 print join("\n",@$[0..$#$]) if( $$[-1] );
514 =head2 How do I reverse a string?
516 Use reverse() in scalar context, as documented in
519 $reversed = reverse $string;
521 =head2 How do I expand tabs in a string?
523 You can do it yourself:
525 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
527 Or you can just use the Text::Tabs module (part of the standard Perl
531 @expanded_lines = expand(@lines_with_tabs);
533 =head2 How do I reformat a paragraph?
535 Use Text::Wrap (part of the standard Perl distribution):
538 print wrap("\t", ' ', @paragraphs);
540 The paragraphs you give to Text::Wrap should not contain embedded
541 newlines. Text::Wrap doesn't justify the lines (flush-right).
543 =head2 How can I access/change the first N letters of a string?
545 There are many ways. If you just want to grab a copy, use
548 $first_byte = substr($a, 0, 1);
550 If you want to modify part of a string, the simplest way is often to
551 use substr() as an lvalue:
553 substr($a, 0, 3) = "Tom";
555 Although those with a pattern matching kind of thought process will
560 =head2 How do I change the Nth occurrence of something?
562 You have to keep track of N yourself. For example, let's say you want
563 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
564 C<"whosoever"> or C<"whomsoever">, case insensitively. These
565 all assume that $_ contains the string to be altered.
569 ++$count == 5 # is it the 5th?
570 ? "${2}soever" # yes, swap
571 : $1 # renege and leave it there
574 In the more general case, you can use the C</g> modifier in a C<while>
575 loop, keeping count of matches.
579 $_ = "One fish two fish red fish blue fish";
580 while (/(\w+)\s+fish\b/gi) {
581 if (++$count == $WANT) {
582 print "The third fish is a $1 one.\n";
586 That prints out: C<"The third fish is a red one."> You can also use a
587 repetition count and repeated pattern like this:
589 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
591 =head2 How can I count the number of occurrences of a substring within a string?
593 There are a number of ways, with varying efficiency. If you want a
594 count of a certain single character (X) within a string, you can use the
595 C<tr///> function like so:
597 $string = "ThisXlineXhasXsomeXx'sXinXit";
598 $count = ($string =~ tr/X//);
599 print "There are $count X characters in the string";
601 This is fine if you are just looking for a single character. However,
602 if you are trying to count multiple character substrings within a
603 larger string, C<tr///> won't work. What you can do is wrap a while()
604 loop around a global pattern match. For example, let's count negative
607 $string = "-9 55 48 -2 23 -76 4 14 -44";
608 while ($string =~ /-\d+/g) { $count++ }
609 print "There are $count negative numbers in the string";
611 =head2 How do I capitalize all the words on one line?
613 To make the first letter of each word upper case:
615 $line =~ s/\b(\w)/\U$1/g;
617 This has the strange effect of turning "C<don't do it>" into "C<Don'T
618 Do It>". Sometimes you might want this. Other times you might need a
619 more thorough solution (Suggested by brian d. foy):
622 (^\w) #at the beginning of the line
624 (\s\w) #preceded by whitespace
627 $string =~ /([\w']+)/\u\L$1/g;
629 To make the whole line upper case:
633 To force each word to be lower case, with the first letter upper case:
635 $line =~ s/(\w+)/\u\L$1/g;
637 You can (and probably should) enable locale awareness of those
638 characters by placing a C<use locale> pragma in your program.
639 See L<perllocale> for endless details on locales.
641 This is sometimes referred to as putting something into "title
642 case", but that's not quite accurate. Consider the proper
643 capitalization of the movie I<Dr. Strangelove or: How I Learned to
644 Stop Worrying and Love the Bomb>, for example.
646 =head2 How can I split a [character] delimited string except when inside
647 [character]? (Comma-separated files)
649 Take the example case of trying to split a string that is comma-separated
650 into its different fields. (We'll pretend you said comma-separated, not
651 comma-delimited, which is different and almost never what you mean.) You
652 can't use C<split(/,/)> because you shouldn't split if the comma is inside
653 quotes. For example, take a data line like this:
655 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
657 Due to the restriction of the quotes, this is a fairly complex
658 problem. Thankfully, we have Jeffrey Friedl, author of a highly
659 recommended book on regular expressions, to handle these for us. He
660 suggests (assuming your string is contained in $text):
663 push(@new, $+) while $text =~ m{
664 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
668 push(@new, undef) if substr($text,-1,1) eq ',';
670 If you want to represent quotation marks inside a
671 quotation-mark-delimited field, escape them with backslashes (eg,
672 C<"like \"this\"">. Unescaping them is a task addressed earlier in
675 Alternatively, the Text::ParseWords module (part of the standard Perl
676 distribution) lets you say:
678 use Text::ParseWords;
679 @new = quotewords(",", 0, $text);
681 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
683 =head2 How do I strip blank space from the beginning/end of a string?
685 Although the simplest approach would seem to be
687 $string =~ s/^\s*(.*?)\s*$/$1/;
689 not only is this unnecessarily slow and destructive, it also fails with
690 embedded newlines. It is much faster to do this operation in two steps:
695 Or more nicely written as:
702 This idiom takes advantage of the C<foreach> loop's aliasing
703 behavior to factor out common code. You can do this
704 on several strings at once, or arrays, or even the
705 values of a hash if you use a slice:
707 # trim whitespace in the scalar, the array,
708 # and all the values in the hash
709 foreach ($scalar, @array, @hash{keys %hash}) {
714 =head2 How do I pad a string with blanks or pad a number with zeroes?
716 (This answer contributed by Uri Guttman, with kibitzing from
719 In the following examples, C<$pad_len> is the length to which you wish
720 to pad the string, C<$text> or C<$num> contains the string to be padded,
721 and C<$pad_char> contains the padding character. You can use a single
722 character string constant instead of the C<$pad_char> variable if you
723 know what it is in advance. And in the same way you can use an integer in
724 place of C<$pad_len> if you know the pad length in advance.
726 The simplest method uses the C<sprintf> function. It can pad on the left
727 or right with blanks and on the left with zeroes and it will not
728 truncate the result. The C<pack> function can only pad strings on the
729 right with blanks and it will truncate the result to a maximum length of
732 # Left padding a string with blanks (no truncation):
733 $padded = sprintf("%${pad_len}s", $text);
735 # Right padding a string with blanks (no truncation):
736 $padded = sprintf("%-${pad_len}s", $text);
738 # Left padding a number with 0 (no truncation):
739 $padded = sprintf("%0${pad_len}d", $num);
741 # Right padding a string with blanks using pack (will truncate):
742 $padded = pack("A$pad_len",$text);
744 If you need to pad with a character other than blank or zero you can use
745 one of the following methods. They all generate a pad string with the
746 C<x> operator and combine that with C<$text>. These methods do
747 not truncate C<$text>.
749 Left and right padding with any character, creating a new string:
751 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
752 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
754 Left and right padding with any character, modifying C<$text> directly:
756 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
757 $text .= $pad_char x ( $pad_len - length( $text ) );
759 =head2 How do I extract selected columns from a string?
761 Use substr() or unpack(), both documented in L<perlfunc>.
762 If you prefer thinking in terms of columns instead of widths,
763 you can use this kind of thing:
765 # determine the unpack format needed to split Linux ps output
766 # arguments are cut columns
767 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
773 for my $place (@positions) {
774 $template .= "A" . ($place - $lastpos) . " ";
781 =head2 How do I find the soundex value of a string?
783 Use the standard Text::Soundex module distributed with Perl.
784 Before you do so, you may want to determine whether `soundex' is in
785 fact what you think it is. Knuth's soundex algorithm compresses words
786 into a small space, and so it does not necessarily distinguish between
787 two words which you might want to appear separately. For example, the
788 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
789 If Text::Soundex does not do what you are looking for, you might want
790 to consider the String::Approx module available at CPAN.
792 =head2 How can I expand variables in text strings?
794 Let's assume that you have a string like:
796 $text = 'this has a $foo in it and a $bar';
798 If those were both global variables, then this would
801 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
803 But since they are probably lexicals, or at least, they could
804 be, you'd have to do this:
806 $text =~ s/(\$\w+)/$1/eeg;
807 die if $@; # needed /ee, not /e
809 It's probably better in the general case to treat those
810 variables as entries in some special hash. For example:
816 $text =~ s/\$(\w+)/$user_defs{$1}/g;
818 See also ``How do I expand function calls in a string?'' in this section
821 =head2 What's wrong with always quoting "$vars"?
823 The problem is that those double-quotes force stringification--
824 coercing numbers and references into strings--even when you
825 don't want them to be strings. Think of it this way: double-quote
826 expansion is used to produce new strings. If you already
827 have a string, why do you need more?
829 If you get used to writing odd things like these:
833 somefunc("$var"); # BAD
835 You'll be in trouble. Those should (in 99.8% of the cases) be
836 the simpler and more direct:
842 Otherwise, besides slowing you down, you're going to break code when
843 the thing in the scalar is actually neither a string nor a number, but
849 my $oref = "$aref"; # WRONG
852 You can also get into subtle problems on those few operations in Perl
853 that actually do care about the difference between a string and a
854 number, such as the magical C<++> autoincrement operator or the
857 Stringification also destroys arrays.
860 print "@lines"; # WRONG - extra blanks
861 print @lines; # right
863 =head2 Why don't my <<HERE documents work?
865 Check for these three things:
869 =item 1. There must be no space after the << part.
871 =item 2. There (probably) should be a semicolon at the end.
873 =item 3. You can't (easily) have any space in front of the tag.
877 If you want to indent the text in the here document, you
881 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
886 But the HERE_TARGET must still be flush against the margin.
887 If you want that indented also, you'll have to quote
890 ($quote = <<' FINIS') =~ s/^\s+//gm;
891 ...we will have peace, when you and all your works have
892 perished--and the works of your dark master to whom you
893 would deliver us. You are a liar, Saruman, and a corrupter
894 of men's hearts. --Theoden in /usr/src/perl/taint.c
896 $quote =~ s/\s*--/\n--/;
898 A nice general-purpose fixer-upper function for indented here documents
899 follows. It expects to be called with a here document as its argument.
900 It looks to see whether each line begins with a common substring, and
901 if so, strips that substring off. Otherwise, it takes the amount of leading
902 whitespace found on the first line and removes that much off each
907 my ($white, $leader); # common whitespace and common leading string
908 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
909 ($white, $leader) = ($2, quotemeta($1));
911 ($white, $leader) = (/^(\s+)/, '');
913 s/^\s*?$leader(?:$white)?//gm;
917 This works with leading special strings, dynamically determined:
919 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
922 @@@ SAVEI32(runlevel);
924 @@@ while ( op = (*op->op_ppaddr)() );
928 MAIN_INTERPRETER_LOOP
930 Or with a fixed amount of leading whitespace, with remaining
931 indentation correctly preserved:
933 $poem = fix<<EVER_ON_AND_ON;
934 Now far ahead the Road has gone,
935 And I must follow, if I can,
936 Pursuing it with eager feet,
937 Until it joins some larger way
938 Where many paths and errands meet.
939 And whither then? I cannot say.
940 --Bilbo in /usr/src/perl/pp_ctl.c
945 =head2 What is the difference between a list and an array?
947 An array has a changeable length. A list does not. An array is something
948 you can push or pop, while a list is a set of values. Some people make
949 the distinction that a list is a value while an array is a variable.
950 Subroutines are passed and return lists, you put things into list
951 context, you initialize arrays with lists, and you foreach() across
952 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
953 in scalar context behave like the number of elements in them, subroutines
954 access their arguments through the array C<@_>, and push/pop/shift only work
957 As a side note, there's no such thing as a list in scalar context.
960 $scalar = (2, 5, 7, 9);
962 you're using the comma operator in scalar context, so it uses the scalar
963 comma operator. There never was a list there at all! This causes the
964 last value to be returned: 9.
966 =head2 What is the difference between $array[1] and @array[1]?
968 The former is a scalar value; the latter an array slice, making
969 it a list with one (scalar) value. You should use $ when you want a
970 scalar value (most of the time) and @ when you want a list with one
971 scalar value in it (very, very rarely; nearly never, in fact).
973 Sometimes it doesn't make a difference, but sometimes it does.
974 For example, compare:
976 $good[0] = `some program that outputs several lines`;
980 @bad[0] = `same program that outputs several lines`;
982 The C<use warnings> pragma and the B<-w> flag will warn you about these
985 =head2 How can I remove duplicate elements from a list or array?
987 There are several possible ways, depending on whether the array is
988 ordered and whether you wish to preserve the ordering.
994 If @in is sorted, and you want @out to be sorted:
995 (this assumes all true values in the array)
997 $prev = "not equal to $in[0]";
998 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
1000 This is nice in that it doesn't use much extra memory, simulating
1001 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1002 guarantees that the expression is true (so that grep picks it up)
1003 even if the $_ is 0, "", or undef.
1007 If you don't know whether @in is sorted:
1010 @out = grep(!$saw{$_}++, @in);
1014 Like (b), but @in contains only small integers:
1016 @out = grep(!$saw[$_]++, @in);
1020 A way to do (b) without any loops or greps:
1024 @out = sort keys %saw; # remove sort if undesired
1028 Like (d), but @in contains only small positive integers:
1032 @out = grep {defined} @ary;
1036 But perhaps you should have been using a hash all along, eh?
1038 =head2 How can I tell whether a certain element is contained in a list or array?
1040 Hearing the word "in" is an I<in>dication that you probably should have
1041 used a hash, not a list or array, to store your data. Hashes are
1042 designed to answer this question quickly and efficiently. Arrays aren't.
1044 That being said, there are several ways to approach this. If you
1045 are going to make this query many times over arbitrary string values,
1046 the fastest way is probably to invert the original array and keep an
1047 associative array lying about whose keys are the first array's values.
1049 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1051 for (@blues) { $is_blue{$_} = 1 }
1053 Now you can check whether $is_blue{$some_color}. It might have been a
1054 good idea to keep the blues all in a hash in the first place.
1056 If the values are all small integers, you could use a simple indexed
1057 array. This kind of an array will take up less space:
1059 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1060 undef @is_tiny_prime;
1061 for (@primes) { $is_tiny_prime[$_] = 1 }
1062 # or simply @istiny_prime[@primes] = (1) x @primes;
1064 Now you check whether $is_tiny_prime[$some_number].
1066 If the values in question are integers instead of strings, you can save
1067 quite a lot of space by using bit strings instead:
1069 @articles = ( 1..10, 150..2000, 2017 );
1071 for (@articles) { vec($read,$_,1) = 1 }
1073 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1077 ($is_there) = grep $_ eq $whatever, @array;
1081 ($is_there) = grep /$whatever/, @array;
1083 These are slow (checks every element even if the first matches),
1084 inefficient (same reason), and potentially buggy (what if there are
1085 regex characters in $whatever?). If you're only testing once, then
1089 foreach $elt (@array) {
1090 if ($elt eq $elt_to_find) {
1095 if ($is_there) { ... }
1097 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1099 Use a hash. Here's code to do both and more. It assumes that
1100 each element is unique in a given array:
1102 @union = @intersection = @difference = ();
1104 foreach $element (@array1, @array2) { $count{$element}++ }
1105 foreach $element (keys %count) {
1106 push @union, $element;
1107 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1110 Note that this is the I<symmetric difference>, that is, all elements in
1111 either A or in B but not in both. Think of it as an xor operation.
1113 =head2 How do I test whether two arrays or hashes are equal?
1115 The following code works for single-level arrays. It uses a stringwise
1116 comparison, and does not distinguish defined versus undefined empty
1117 strings. Modify if you have other needs.
1119 $are_equal = compare_arrays(\@frogs, \@toads);
1121 sub compare_arrays {
1122 my ($first, $second) = @_;
1123 no warnings; # silence spurious -w undef complaints
1124 return 0 unless @$first == @$second;
1125 for (my $i = 0; $i < @$first; $i++) {
1126 return 0 if $first->[$i] ne $second->[$i];
1131 For multilevel structures, you may wish to use an approach more
1132 like this one. It uses the CPAN module FreezeThaw:
1134 use FreezeThaw qw(cmpStr);
1135 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1137 printf "a and b contain %s arrays\n",
1138 cmpStr(\@a, \@b) == 0
1142 This approach also works for comparing hashes. Here
1143 we'll demonstrate two different answers:
1145 use FreezeThaw qw(cmpStr cmpStrHard);
1147 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1151 printf "a and b contain %s hashes\n",
1152 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1154 printf "a and b contain %s hashes\n",
1155 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1158 The first reports that both those the hashes contain the same data,
1159 while the second reports that they do not. Which you prefer is left as
1160 an exercise to the reader.
1162 =head2 How do I find the first array element for which a condition is true?
1164 You can use this if you care about the index:
1166 for ($i= 0; $i < @array; $i++) {
1167 if ($array[$i] eq "Waldo") {
1173 Now C<$found_index> has what you want.
1175 =head2 How do I handle linked lists?
1177 In general, you usually don't need a linked list in Perl, since with
1178 regular arrays, you can push and pop or shift and unshift at either end,
1179 or you can use splice to add and/or remove arbitrary number of elements at
1180 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1181 dynamic arrays. In the absence of shifts and pops, push in general
1182 needs to reallocate on the order every log(N) times, and unshift will
1183 need to copy pointers each time.
1185 If you really, really wanted, you could use structures as described in
1186 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1187 to do. For example, imagine a list node like this:
1194 You could walk the list this way:
1197 for ($node = $head; $node; $node = $node->{LINK}) {
1198 print $node->{VALUE}, " ";
1202 You could add to the list this way:
1205 $tail = append($head, 1); # grow a new head
1206 for $value ( 2 .. 10 ) {
1207 $tail = append($tail, $value);
1211 my($list, $value) = @_;
1212 my $node = { VALUE => $value };
1214 $node->{LINK} = $list->{LINK};
1215 $list->{LINK} = $node;
1217 $_[0] = $node; # replace caller's version
1222 But again, Perl's built-in are virtually always good enough.
1224 =head2 How do I handle circular lists?
1226 Circular lists could be handled in the traditional fashion with linked
1227 lists, or you could just do something like this with an array:
1229 unshift(@array, pop(@array)); # the last shall be first
1230 push(@array, shift(@array)); # and vice versa
1232 =head2 How do I shuffle an array randomly?
1236 # fisher_yates_shuffle( \@array ) :
1237 # generate a random permutation of @array in place
1238 sub fisher_yates_shuffle {
1241 for ($i = @$array; --$i; ) {
1242 my $j = int rand ($i+1);
1243 @$array[$i,$j] = @$array[$j,$i];
1247 fisher_yates_shuffle( \@array ); # permutes @array in place
1249 You've probably seen shuffling algorithms that work using splice,
1250 randomly picking another element to swap the current element with
1254 @old = 1 .. 10; # just a demo
1256 push(@new, splice(@old, rand @old, 1));
1259 This is bad because splice is already O(N), and since you do it N times,
1260 you just invented a quadratic algorithm; that is, O(N**2). This does
1261 not scale, although Perl is so efficient that you probably won't notice
1262 this until you have rather largish arrays.
1264 =head2 How do I process/modify each element of an array?
1266 Use C<for>/C<foreach>:
1269 s/foo/bar/; # change that word
1270 y/XZ/ZX/; # swap those letters
1273 Here's another; let's compute spherical volumes:
1275 for (@volumes = @radii) { # @volumes has changed parts
1277 $_ *= (4/3) * 3.14159; # this will be constant folded
1280 If you want to do the same thing to modify the values of the hash,
1281 you may not use the C<values> function, oddly enough. You need a slice:
1283 for $orbit ( @orbits{keys %orbits} ) {
1284 ($orbit **= 3) *= (4/3) * 3.14159;
1287 =head2 How do I select a random element from an array?
1289 Use the rand() function (see L<perlfunc/rand>):
1291 # at the top of the program:
1292 srand; # not needed for 5.004 and later
1295 $index = rand @array;
1296 $element = $array[$index];
1298 Make sure you I<only call srand once per program, if then>.
1299 If you are calling it more than once (such as before each
1300 call to rand), you're almost certainly doing something wrong.
1302 =head2 How do I permute N elements of a list?
1304 Here's a little program that generates all permutations
1305 of all the words on each line of input. The algorithm embodied
1306 in the permute() function should work on any list:
1309 # tsc-permute: permute each word of input
1310 permute([split], []);
1312 my @items = @{ $_[0] };
1313 my @perms = @{ $_[1] };
1317 my(@newitems,@newperms,$i);
1318 foreach $i (0 .. $#items) {
1321 unshift(@newperms, splice(@newitems, $i, 1));
1322 permute([@newitems], [@newperms]);
1327 =head2 How do I sort an array by (anything)?
1329 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1331 @list = sort { $a <=> $b } @list;
1333 The default sort function is cmp, string comparison, which would
1334 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1335 the numerical comparison operator.
1337 If you have a complicated function needed to pull out the part you
1338 want to sort on, then don't do it inside the sort function. Pull it
1339 out first, because the sort BLOCK can be called many times for the
1340 same element. Here's an example of how to pull out the first word
1341 after the first number on each item, and then sort those words
1346 ($item) = /\d+\s*(\S+)/;
1347 push @idx, uc($item);
1349 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1351 which could also be written this way, using a trick
1352 that's come to be known as the Schwartzian Transform:
1354 @sorted = map { $_->[0] }
1355 sort { $a->[1] cmp $b->[1] }
1356 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1358 If you need to sort on several fields, the following paradigm is useful.
1360 @sorted = sort { field1($a) <=> field1($b) ||
1361 field2($a) cmp field2($b) ||
1362 field3($a) cmp field3($b)
1365 This can be conveniently combined with precalculation of keys as given
1368 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1371 See also the question below on sorting hashes.
1373 =head2 How do I manipulate arrays of bits?
1375 Use pack() and unpack(), or else vec() and the bitwise operations.
1377 For example, this sets $vec to have bit N set if $ints[N] was set:
1380 foreach(@ints) { vec($vec,$_,1) = 1 }
1382 And here's how, given a vector in $vec, you can
1383 get those bits into your @ints array:
1385 sub bitvec_to_list {
1388 # Find null-byte density then select best algorithm
1389 if ($vec =~ tr/\0// / length $vec > 0.95) {
1392 # This method is faster with mostly null-bytes
1393 while($vec =~ /[^\0]/g ) {
1394 $i = -9 + 8 * pos $vec;
1395 push @ints, $i if vec($vec, ++$i, 1);
1396 push @ints, $i if vec($vec, ++$i, 1);
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);
1405 # This method is a fast general algorithm
1407 my $bits = unpack "b*", $vec;
1408 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1409 push @ints, pos $bits while($bits =~ /1/g);
1414 This method gets faster the more sparse the bit vector is.
1415 (Courtesy of Tim Bunce and Winfried Koenig.)
1417 Here's a demo on how to use vec():
1420 $vector = "\xff\x0f\xef\xfe";
1421 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1422 unpack("N", $vector), "\n";
1423 $is_set = vec($vector, 23, 1);
1424 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1442 my ($offset, $width, $value) = @_;
1444 vec($vector, $offset, $width) = $value;
1445 print "offset=$offset width=$width value=$value\n";
1451 my $bits = unpack("b*", $vector);
1455 print "vector length in bytes: ", length($vector), "\n";
1456 @bytes = unpack("A8" x length($vector), $bits);
1457 print "bits are: @bytes\n\n";
1460 =head2 Why does defined() return true on empty arrays and hashes?
1462 The short story is that you should probably only use defined on scalars or
1463 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1464 in the 5.004 release or later of Perl for more detail.
1466 =head1 Data: Hashes (Associative Arrays)
1468 =head2 How do I process an entire hash?
1470 Use the each() function (see L<perlfunc/each>) if you don't care
1471 whether it's sorted:
1473 while ( ($key, $value) = each %hash) {
1474 print "$key = $value\n";
1477 If you want it sorted, you'll have to use foreach() on the result of
1478 sorting the keys as shown in an earlier question.
1480 =head2 What happens if I add or remove keys from a hash while iterating over it?
1484 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1485 iterating over it. In Perl 5 you can delete from it, but you still
1486 can't add to it, because that might cause a doubling of the hash table,
1487 in which half the entries get copied up to the new top half of the
1488 table, at which point you've totally bamboozled the iterator code.
1489 Even if the table doesn't double, there's no telling whether your new
1490 entry will be inserted before or after the current iterator position.
1492 Either treasure up your changes and make them after the iterator finishes
1493 or use keys to fetch all the old keys at once, and iterate over the list
1496 =head2 How do I look up a hash element by value?
1498 Create a reverse hash:
1500 %by_value = reverse %by_key;
1501 $key = $by_value{$value};
1503 That's not particularly efficient. It would be more space-efficient
1506 while (($key, $value) = each %by_key) {
1507 $by_value{$value} = $key;
1510 If your hash could have repeated values, the methods above will only find
1511 one of the associated keys. This may or may not worry you. If it does
1512 worry you, you can always reverse the hash into a hash of arrays instead:
1514 while (($key, $value) = each %by_key) {
1515 push @{$key_list_by_value{$value}}, $key;
1518 =head2 How can I know how many entries are in a hash?
1520 If you mean how many keys, then all you have to do is
1521 take the scalar sense of the keys() function:
1523 $num_keys = scalar keys %hash;
1525 The keys() function also resets the iterator, which in void context is
1526 faster for tied hashes than would be iterating through the whole
1527 hash, one key-value pair at a time.
1529 =head2 How do I sort a hash (optionally by value instead of key)?
1531 Internally, hashes are stored in a way that prevents you from imposing
1532 an order on key-value pairs. Instead, you have to sort a list of the
1535 @keys = sort keys %hash; # sorted by key
1537 $hash{$a} cmp $hash{$b}
1538 } keys %hash; # and by value
1540 Here we'll do a reverse numeric sort by value, and if two keys are
1541 identical, sort by length of key, or if that fails, by straight ASCII
1542 comparison of the keys (well, possibly modified by your locale--see
1546 $hash{$b} <=> $hash{$a}
1548 length($b) <=> length($a)
1553 =head2 How can I always keep my hash sorted?
1555 You can look into using the DB_File module and tie() using the
1556 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1557 The Tie::IxHash module from CPAN might also be instructive.
1559 =head2 What's the difference between "delete" and "undef" with hashes?
1561 Hashes are pairs of scalars: the first is the key, the second is the
1562 value. The key will be coerced to a string, although the value can be
1563 any kind of scalar: string, number, or reference. If a key C<$key> is
1564 present in the array, C<exists($key)> will return true. The value for
1565 a given key can be C<undef>, in which case C<$array{$key}> will be
1566 C<undef> while C<$exists{$key}> will return true. This corresponds to
1567 (C<$key>, C<undef>) being in the hash.
1569 Pictures help... here's the C<%ary> table:
1579 And these conditions hold
1583 defined $ary{'d'} is true
1584 defined $ary{'a'} is true
1585 exists $ary{'a'} is true (Perl5 only)
1586 grep ($_ eq 'a', keys %ary) is true
1592 your table now reads:
1603 and these conditions now hold; changes in caps:
1607 defined $ary{'d'} is true
1608 defined $ary{'a'} is FALSE
1609 exists $ary{'a'} is true (Perl5 only)
1610 grep ($_ eq 'a', keys %ary) is true
1612 Notice the last two: you have an undef value, but a defined key!
1618 your table now reads:
1627 and these conditions now hold; changes in caps:
1631 defined $ary{'d'} is true
1632 defined $ary{'a'} is false
1633 exists $ary{'a'} is FALSE (Perl5 only)
1634 grep ($_ eq 'a', keys %ary) is FALSE
1636 See, the whole entry is gone!
1638 =head2 Why don't my tied hashes make the defined/exists distinction?
1640 They may or may not implement the EXISTS() and DEFINED() methods
1641 differently. For example, there isn't the concept of undef with hashes
1642 that are tied to DBM* files. This means the true/false tables above
1643 will give different results when used on such a hash. It also means
1644 that exists and defined do the same thing with a DBM* file, and what
1645 they end up doing is not what they do with ordinary hashes.
1647 =head2 How do I reset an each() operation part-way through?
1649 Using C<keys %hash> in scalar context returns the number of keys in
1650 the hash I<and> resets the iterator associated with the hash. You may
1651 need to do this if you use C<last> to exit a loop early so that when you
1652 re-enter it, the hash iterator has been reset.
1654 =head2 How can I get the unique keys from two hashes?
1656 First you extract the keys from the hashes into lists, then solve
1657 the "removing duplicates" problem described above. For example:
1660 for $element (keys(%foo), keys(%bar)) {
1667 @uniq = keys %{{%foo,%bar}};
1669 Or if you really want to save space:
1672 while (defined ($key = each %foo)) {
1675 while (defined ($key = each %bar)) {
1680 =head2 How can I store a multidimensional array in a DBM file?
1682 Either stringify the structure yourself (no fun), or else
1683 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1684 it on top of either DB_File or GDBM_File.
1686 =head2 How can I make my hash remember the order I put elements into it?
1688 Use the Tie::IxHash from CPAN.
1691 tie(%myhash, Tie::IxHash);
1692 for ($i=0; $i<20; $i++) {
1695 @keys = keys %myhash;
1696 # @keys = (0,1,2,3,...)
1698 =head2 Why does passing a subroutine an undefined element in a hash create it?
1700 If you say something like:
1702 somefunc($hash{"nonesuch key here"});
1704 Then that element "autovivifies"; that is, it springs into existence
1705 whether you store something there or not. That's because functions
1706 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1707 it has to be ready to write it back into the caller's version.
1709 This has been fixed as of Perl5.004.
1711 Normally, merely accessing a key's value for a nonexistent key does
1712 I<not> cause that key to be forever there. This is different than
1715 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1717 Usually a hash ref, perhaps like this:
1722 TITLE => "deputy peon",
1725 PALS => [ "Norbert", "Rhys", "Phineas"],
1728 References are documented in L<perlref> and the upcoming L<perlreftut>.
1729 Examples of complex data structures are given in L<perldsc> and
1730 L<perllol>. Examples of structures and object-oriented classes are
1733 =head2 How can I use a reference as a hash key?
1735 You can't do this directly, but you could use the standard Tie::Refhash
1736 module distributed with Perl.
1740 =head2 How do I handle binary data correctly?
1742 Perl is binary clean, so this shouldn't be a problem. For example,
1743 this works fine (assuming the files are found):
1745 if (`cat /vmunix` =~ /gzip/) {
1746 print "Your kernel is GNU-zip enabled!\n";
1749 On less elegant (read: Byzantine) systems, however, you have
1750 to play tedious games with "text" versus "binary" files. See
1751 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1752 systems are curses out of Microsoft, who seem to be committed to putting
1753 the backward into backward compatibility.
1755 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1757 If you want to deal with multibyte characters, however, there are
1758 some gotchas. See the section on Regular Expressions.
1760 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1762 Assuming that you don't care about IEEE notations like "NaN" or
1763 "Infinity", you probably just want to use a regular expression.
1765 if (/\D/) { print "has nondigits\n" }
1766 if (/^\d+$/) { print "is a whole number\n" }
1767 if (/^-?\d+$/) { print "is an integer\n" }
1768 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1769 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1770 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1771 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1772 { print "a C float" }
1774 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1775 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1776 wrapper function for more convenient access. This function takes
1777 a string and returns the number it found, or C<undef> for input that
1778 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1779 if you just want to say, ``Is this a float?''
1782 use POSIX qw(strtod);
1787 my($num, $unparsed) = strtod($str);
1788 if (($str eq '') || ($unparsed != 0) || $!) {
1795 sub is_numeric { defined getnum($_[0]) }
1797 Or you could check out the String::Scanf module on CPAN instead. The
1798 POSIX module (part of the standard Perl distribution) provides the
1799 C<strtod> and C<strtol> for converting strings to double and longs,
1802 =head2 How do I keep persistent data across program calls?
1804 For some specific applications, you can use one of the DBM modules.
1805 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1806 Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
1807 Storable is part of the standard distribution. Here's one example using
1808 Storable's C<store> and C<retrieve> functions:
1811 store(\%hash, "filename");
1814 $href = retrieve("filename"); # by ref
1815 %hash = %{ retrieve("filename") }; # direct to hash
1817 =head2 How do I print out or copy a recursive data structure?
1819 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1820 for printing out data structures. The Storable module, found on CPAN,
1821 provides a function called C<dclone> that recursively copies its argument.
1823 use Storable qw(dclone);
1826 Where $r1 can be a reference to any kind of data structure you'd like.
1827 It will be deeply copied. Because C<dclone> takes and returns references,
1828 you'd have to add extra punctuation if you had a hash of arrays that
1831 %newhash = %{ dclone(\%oldhash) };
1833 =head2 How do I define methods for every class/object?
1835 Use the UNIVERSAL class (see L<UNIVERSAL>).
1837 =head2 How do I verify a credit card checksum?
1839 Get the Business::CreditCard module from CPAN.
1841 =head2 How do I pack arrays of doubles or floats for XS code?
1843 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1844 If you're doing a lot of float or double processing, consider using
1845 the PDL module from CPAN instead--it makes number-crunching easy.
1847 =head1 AUTHOR AND COPYRIGHT
1849 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1850 All rights reserved.
1852 This documentation is free; you can redistribute it and/or modify it
1853 under the same terms as Perl itself.
1855 Irrespective of its distribution, all code examples in this file
1856 are hereby placed into the public domain. You are permitted and
1857 encouraged to use this code in your own programs for fun
1858 or for profit as you see fit. A simple comment in the code giving
1859 credit would be courteous but is not required.