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 To get rid of the superfluous digits, just use a format (eg,
43 C<printf("%.2f", 19.95)>) to get the required precision.
44 See L<perlop/"Floating-point Arithmetic">.
46 =head2 Why isn't my octal data interpreted correctly?
48 Perl only understands octal and hex numbers as such when they occur
49 as literals in your program. If they are read in from somewhere and
50 assigned, no automatic conversion takes place. You must explicitly
51 use oct() or hex() if you want the values converted. oct() interprets
52 both hex ("0x350") numbers and octal ones ("0350" or even without the
53 leading "0", like "377"), while hex() only converts hexadecimal ones,
54 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
56 This problem shows up most often when people try using chmod(), mkdir(),
57 umask(), or sysopen(), which all want permissions in octal.
59 chmod(644, $file); # WRONG -- perl -w catches this
60 chmod(0644, $file); # right
62 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
64 Remember that int() merely truncates toward 0. For rounding to a
65 certain number of digits, sprintf() or printf() is usually the easiest
68 printf("%.3f", 3.1415926535); # prints 3.142
70 The POSIX module (part of the standard Perl distribution) implements
71 ceil(), floor(), and a number of other mathematical and trigonometric
75 $ceil = ceil(3.5); # 4
76 $floor = floor(3.5); # 3
78 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
79 module. With 5.004, the Math::Trig module (part of the standard Perl
80 distribution) implements the trigonometric functions. Internally it
81 uses the Math::Complex module and some functions can break out from
82 the real axis into the complex plane, for example the inverse sine of
85 Rounding in financial applications can have serious implications, and
86 the rounding method used should be specified precisely. In these
87 cases, it probably pays not to trust whichever system rounding is
88 being used by Perl, but to instead implement the rounding function you
91 To see why, notice how you'll still have an issue on half-way-point
94 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
96 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
97 0.8 0.8 0.9 0.9 1.0 1.0
99 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
100 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
101 machines) will work pretty much like mathematical integers. Other numbers
104 =head2 How do I convert bits into ints?
106 To turn a string of 1s and 0s like C<10110110> into a scalar containing
107 its binary value, use the pack() and unpack() functions (documented in
108 L<perlfunc/"pack"> and L<perlfunc/"unpack">):
110 $decimal = unpack('c', pack('B8', '10110110'));
112 This packs the string C<10110110> into an eight bit binary structure.
113 This is then unpacked as a character, which returns its ordinal value.
115 This does the same thing:
117 $decimal = ord(pack('B8', '10110110'));
119 Here's an example of going the other way:
121 $binary_string = unpack('B*', "\x29");
123 =head2 Why doesn't & work the way I want it to?
125 The behavior of binary arithmetic operators depends on whether they're
126 used on numbers or strings. The operators treat a string as a series
127 of bits and work with that (the string C<"3"> is the bit pattern
128 C<00110011>). The operators work with the binary form of a number
129 (the number C<3> is treated as the bit pattern C<00000011>).
131 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
132 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
135 Most problems with C<&> and C<|> arise because the programmer thinks
136 they have a number but really it's a string. The rest arise because
139 if ("\020\020" & "\101\101") {
143 but a string consisting of two null bytes (the result of C<"\020\020"
144 & "\101\101">) is not a false value in Perl. You need:
146 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
150 =head2 How do I multiply matrices?
152 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
153 or the PDL extension (also available from CPAN).
155 =head2 How do I perform an operation on a series of integers?
157 To call a function on each element in an array, and collect the
160 @results = map { my_func($_) } @array;
164 @triple = map { 3 * $_ } @single;
166 To call a function on each element of an array, but ignore the
169 foreach $iterator (@array) {
170 some_func($iterator);
173 To call a function on each integer in a (small) range, you B<can> use:
175 @results = map { some_func($_) } (5 .. 25);
177 but you should be aware that the C<..> operator creates an array of
178 all integers in the range. This can take a lot of memory for large
182 for ($i=5; $i < 500_005; $i++) {
183 push(@results, some_func($i));
186 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
187 loop will iterate over the range, without creating the entire range.
189 for my $i (5 .. 500_005) {
190 push(@results, some_func($i));
193 will not create a list of 500,000 integers.
195 =head2 How can I output Roman numerals?
197 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
199 =head2 Why aren't my random numbers random?
201 If you're using a version of Perl before 5.004, you must call C<srand>
202 once at the start of your program to seed the random number generator.
203 5.004 and later automatically call C<srand> at the beginning. Don't
204 call C<srand> more than once--you make your numbers less random, rather
207 Computers are good at being predictable and bad at being random
208 (despite appearances caused by bugs in your programs :-).
209 http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
210 Phoenix, talks more about this. John von Neumann said, ``Anyone who
211 attempts to generate random numbers by deterministic means is, of
212 course, living in a state of sin.''
214 If you want numbers that are more random than C<rand> with C<srand>
215 provides, you should also check out the Math::TrulyRandom module from
216 CPAN. It uses the imperfections in your system's timer to generate
217 random numbers, but this takes quite a while. If you want a better
218 pseudorandom generator than comes with your operating system, look at
219 ``Numerical Recipes in C'' at http://www.nr.com/ .
223 =head2 How do I find the week-of-the-year/day-of-the-year?
225 The day of the year is in the array returned by localtime() (see
226 L<perlfunc/"localtime">):
228 $day_of_year = (localtime(time()))[7];
230 or more legibly (in 5.7.1 or higher):
233 $day_of_year = localtime->day_of_year();
235 You can find the week of the year by using Time::Piece's strftime():
237 $week_of_year = localtime->strftime("%U");
238 $iso_week = localtime->strftime("%V");
240 The difference between %U and %V is that %U assumes that the first day
241 of week 1 is the first Sunday of the year, whereas ISO 8601:1988 uses
242 the first week that has at least 4 days in the current year, and with
243 Monday as the first day of the week. You can also use %W, which will
244 return the week of the year with Monday as the first day of week 1. See
245 your strftime(3) man page for more details.
247 =head2 How do I find the current century or millennium?
249 Use the following simple functions:
252 return int((((localtime(shift || time))[5] + 1999))/100);
255 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
258 On some systems, you'll find that the POSIX module's strftime() function
259 has been extended in a non-standard way to use a C<%C> format, which they
260 sometimes claim is the "century". It isn't, because on most such systems,
261 this is only the first two digits of the four-digit year, and thus cannot
262 be used to reliably determine the current century or millennium.
264 =head2 How can I compare two dates and find the difference?
266 If you're storing your dates as epoch seconds then simply subtract one
267 from the other. If you've got a structured date (distinct year, day,
268 month, hour, minute, seconds values), then for reasons of accessibility,
269 simplicity, and efficiency, merely use either timelocal or timegm (from
270 the Time::Local module in the standard distribution) to reduce structured
271 dates to epoch seconds. However, if you don't know the precise format of
272 your dates, then you should probably use either of the Date::Manip and
273 Date::Calc modules from CPAN before you go hacking up your own parsing
274 routine to handle arbitrary date formats.
276 Also note that the core module Time::Piece overloads the addition and
277 subtraction operators to provide date calculation options. See
278 L<Time::Piece/Date Calculations>.
280 =head2 How can I take a string and turn it into epoch seconds?
282 If it's a regular enough string that it always has the same format,
283 you can split it up and pass the parts to C<timelocal> in the standard
284 Time::Local module. Otherwise, you should look into the Date::Calc
285 and Date::Manip modules from CPAN.
287 =head2 How can I find the Julian Day?
289 Use Time::Piece as follows:
292 my $julian_day = localtime->julian_day;
293 my $mjd = localtime->mjd; # modified julian day
295 Before you immerse yourself too deeply in this, be sure to verify that
296 it is the I<Julian> Day you really want. Are you interested in a way
297 of getting serial days so that you just can tell how many days they
298 are apart or so that you can do also other date arithmetic? If you
299 are interested in performing date arithmetic, this can be done using
300 Time::Piece (standard module since Perl 5.8), or by modules
301 Date::Manip or Date::Calc.
303 There is too many details and much confusion on this issue to cover in
304 this FAQ, but the term is applied (correctly) to a calendar now
305 supplanted by the Gregorian Calendar, with the Julian Calendar failing
306 to adjust properly for leap years on centennial years (among other
307 annoyances). The term is also used (incorrectly) to mean: [1] days in
308 the Gregorian Calendar; and [2] days since a particular starting time
309 or `epoch', usually 1970 in the Unix world and 1980 in the
310 MS-DOS/Windows world. If you find that it is not the first meaning
311 that you really want, then check out the Date::Manip and Date::Calc
312 modules. (Thanks to David Cassell for most of this text.)
314 =head2 How do I find yesterday's date?
316 The C<time()> function returns the current time in seconds since the
317 epoch. Take twenty-four hours off that:
319 $yesterday = time() - ( 24 * 60 * 60 );
321 Then you can pass this to C<localtime()> and get the individual year,
322 month, day, hour, minute, seconds values.
324 Alternatively, you can use Time::Piece to subtract a day from the value
325 returned from C<localtime()>:
328 use Time::Seconds; # imports seconds constants, like ONE_DAY
329 my $today = localtime();
330 my $yesterday = $today - ONE_DAY;
332 Note very carefully that the code above assumes that your days are
333 twenty-four hours each. For most people, there are two days a year
334 when they aren't: the switch to and from summer time throws this off.
335 A solution to this issue is offered by Russ Allbery.
338 my $now = defined $_[0] ? $_[0] : time;
339 my $then = $now - 60 * 60 * 24;
340 my $ndst = (localtime $now)[8] > 0;
341 my $tdst = (localtime $then)[8] > 0;
342 $then - ($tdst - $ndst) * 60 * 60;
344 # Should give you "this time yesterday" in seconds since epoch relative to
345 # the first argument or the current time if no argument is given and
346 # suitable for passing to localtime or whatever else you need to do with
347 # it. $ndst is whether we're currently in daylight savings time; $tdst is
348 # whether the point 24 hours ago was in daylight savings time. If $tdst
349 # and $ndst are the same, a boundary wasn't crossed, and the correction
350 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
351 # from yesterday's time since we gained an extra hour while going off
352 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
353 # negative hour (add an hour) to yesterday's time since we lost an hour.
355 # All of this is because during those days when one switches off or onto
356 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
358 # The explicit settings of $ndst and $tdst are necessary because localtime
359 # only says it returns the system tm struct, and the system tm struct at
360 # least on Solaris doesn't guarantee any particular positive value (like,
361 # say, 1) for isdst, just a positive value. And that value can
362 # potentially be negative, if DST information isn't available (this sub
363 # just treats those cases like no DST).
365 # Note that between 2am and 3am on the day after the time zone switches
366 # off daylight savings time, the exact hour of "yesterday" corresponding
367 # to the current hour is not clearly defined. Note also that if used
368 # between 2am and 3am the day after the change to daylight savings time,
369 # the result will be between 3am and 4am of the previous day; it's
370 # arguable whether this is correct.
372 # This sub does not attempt to deal with leap seconds (most things don't).
374 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
375 # This code is in the public domain
377 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
379 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
380 Y2K compliant (whatever that means). The programmers you've hired to
381 use it, however, probably are not.
383 Long answer: The question belies a true understanding of the issue.
384 Perl is just as Y2K compliant as your pencil--no more, and no less.
385 Can you use your pencil to write a non-Y2K-compliant memo? Of course
386 you can. Is that the pencil's fault? Of course it isn't.
388 The date and time functions supplied with Perl (gmtime and localtime)
389 supply adequate information to determine the year well beyond 2000
390 (2038 is when trouble strikes for 32-bit machines). The year returned
391 by these functions when used in a list context is the year minus 1900.
392 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
393 number. To avoid the year 2000 problem simply do not treat the year as
394 a 2-digit number. It isn't.
396 When gmtime() and localtime() are used in scalar context they return
397 a timestamp string that contains a fully-expanded year. For example,
398 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
399 2001". There's no year 2000 problem here.
401 That doesn't mean that Perl can't be used to create non-Y2K compliant
402 programs. It can. But so can your pencil. It's the fault of the user,
403 not the language. At the risk of inflaming the NRA: ``Perl doesn't
404 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
409 =head2 How do I validate input?
411 The answer to this question is usually a regular expression, perhaps
412 with auxiliary logic. See the more specific questions (numbers, mail
413 addresses, etc.) for details.
415 =head2 How do I unescape a string?
417 It depends just what you mean by ``escape''. URL escapes are dealt
418 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
419 character are removed with
423 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
425 =head2 How do I remove consecutive pairs of characters?
427 To turn C<"abbcccd"> into C<"abccd">:
429 s/(.)\1/$1/g; # add /s to include newlines
431 Here's a solution that turns "abbcccd" to "abcd":
433 y///cs; # y == tr, but shorter :-)
435 =head2 How do I expand function calls in a string?
437 This is documented in L<perlref>. In general, this is fraught with
438 quoting and readability problems, but it is possible. To interpolate
439 a subroutine call (in list context) into a string:
441 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
443 If you prefer scalar context, similar chicanery is also useful for
444 arbitrary expressions:
446 print "That yields ${\($n + 5)} widgets\n";
448 Version 5.004 of Perl had a bug that gave list context to the
449 expression in C<${...}>, but this is fixed in version 5.005.
451 See also ``How can I expand variables in text strings?'' in this
454 =head2 How do I find matching/nesting anything?
456 This isn't something that can be done in one regular expression, no
457 matter how complicated. To find something between two single
458 characters, a pattern like C</x([^x]*)x/> will get the intervening
459 bits in $1. For multiple ones, then something more like
460 C</alpha(.*?)omega/> would be needed. But none of these deals with
461 nested patterns, nor can they. For that you'll have to write a
464 If you are serious about writing a parser, there are a number of
465 modules or oddities that will make your life a lot easier. There are
466 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
467 and the byacc program. Starting from perl 5.8 the Text::Balanced
468 is part of the standard distribution.
470 One simple destructive, inside-out approach that you might try is to
471 pull out the smallest nesting parts one at a time:
473 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
474 # do something with $1
477 A more complicated and sneaky approach is to make Perl's regular
478 expression engine do it for you. This is courtesy Dean Inada, and
479 rather has the nature of an Obfuscated Perl Contest entry, but it
482 # $_ contains the string to parse
483 # BEGIN and END are the opening and closing markers for the
488 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
489 @$ = (eval{/$re/},$@!~/unmatched/);
490 print join("\n",@$[0..$#$]) if( $$[-1] );
492 =head2 How do I reverse a string?
494 Use reverse() in scalar context, as documented in
497 $reversed = reverse $string;
499 =head2 How do I expand tabs in a string?
501 You can do it yourself:
503 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
505 Or you can just use the Text::Tabs module (part of the standard Perl
509 @expanded_lines = expand(@lines_with_tabs);
511 =head2 How do I reformat a paragraph?
513 Use Text::Wrap (part of the standard Perl distribution):
516 print wrap("\t", ' ', @paragraphs);
518 The paragraphs you give to Text::Wrap should not contain embedded
519 newlines. Text::Wrap doesn't justify the lines (flush-right).
521 =head2 How can I access/change the first N letters of a string?
523 There are many ways. If you just want to grab a copy, use
526 $first_byte = substr($a, 0, 1);
528 If you want to modify part of a string, the simplest way is often to
529 use substr() as an lvalue:
531 substr($a, 0, 3) = "Tom";
533 Although those with a pattern matching kind of thought process will
538 =head2 How do I change the Nth occurrence of something?
540 You have to keep track of N yourself. For example, let's say you want
541 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
542 C<"whosoever"> or C<"whomsoever">, case insensitively. These
543 all assume that $_ contains the string to be altered.
547 ++$count == 5 # is it the 5th?
548 ? "${2}soever" # yes, swap
549 : $1 # renege and leave it there
552 In the more general case, you can use the C</g> modifier in a C<while>
553 loop, keeping count of matches.
557 $_ = "One fish two fish red fish blue fish";
558 while (/(\w+)\s+fish\b/gi) {
559 if (++$count == $WANT) {
560 print "The third fish is a $1 one.\n";
564 That prints out: C<"The third fish is a red one."> You can also use a
565 repetition count and repeated pattern like this:
567 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
569 =head2 How can I count the number of occurrences of a substring within a string?
571 There are a number of ways, with varying efficiency. If you want a
572 count of a certain single character (X) within a string, you can use the
573 C<tr///> function like so:
575 $string = "ThisXlineXhasXsomeXx'sXinXit";
576 $count = ($string =~ tr/X//);
577 print "There are $count X characters in the string";
579 This is fine if you are just looking for a single character. However,
580 if you are trying to count multiple character substrings within a
581 larger string, C<tr///> won't work. What you can do is wrap a while()
582 loop around a global pattern match. For example, let's count negative
585 $string = "-9 55 48 -2 23 -76 4 14 -44";
586 while ($string =~ /-\d+/g) { $count++ }
587 print "There are $count negative numbers in the string";
589 =head2 How do I capitalize all the words on one line?
591 To make the first letter of each word upper case:
593 $line =~ s/\b(\w)/\U$1/g;
595 This has the strange effect of turning "C<don't do it>" into "C<Don'T
596 Do It>". Sometimes you might want this. Other times you might need a
597 more thorough solution (Suggested by brian d. foy):
600 (^\w) #at the beginning of the line
602 (\s\w) #preceded by whitespace
605 $string =~ /([\w']+)/\u\L$1/g;
607 To make the whole line upper case:
611 To force each word to be lower case, with the first letter upper case:
613 $line =~ s/(\w+)/\u\L$1/g;
615 You can (and probably should) enable locale awareness of those
616 characters by placing a C<use locale> pragma in your program.
617 See L<perllocale> for endless details on locales.
619 This is sometimes referred to as putting something into "title
620 case", but that's not quite accurate. Consider the proper
621 capitalization of the movie I<Dr. Strangelove or: How I Learned to
622 Stop Worrying and Love the Bomb>, for example.
624 =head2 How can I split a [character] delimited string except when inside
625 [character]? (Comma-separated files)
627 Take the example case of trying to split a string that is comma-separated
628 into its different fields. (We'll pretend you said comma-separated, not
629 comma-delimited, which is different and almost never what you mean.) You
630 can't use C<split(/,/)> because you shouldn't split if the comma is inside
631 quotes. For example, take a data line like this:
633 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
635 Due to the restriction of the quotes, this is a fairly complex
636 problem. Thankfully, we have Jeffrey Friedl, author of a highly
637 recommended book on regular expressions, to handle these for us. He
638 suggests (assuming your string is contained in $text):
641 push(@new, $+) while $text =~ m{
642 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
646 push(@new, undef) if substr($text,-1,1) eq ',';
648 If you want to represent quotation marks inside a
649 quotation-mark-delimited field, escape them with backslashes (eg,
650 C<"like \"this\"">. Unescaping them is a task addressed earlier in
653 Alternatively, the Text::ParseWords module (part of the standard Perl
654 distribution) lets you say:
656 use Text::ParseWords;
657 @new = quotewords(",", 0, $text);
659 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
661 =head2 How do I strip blank space from the beginning/end of a string?
663 Although the simplest approach would seem to be
665 $string =~ s/^\s*(.*?)\s*$/$1/;
667 not only is this unnecessarily slow and destructive, it also fails with
668 embedded newlines. It is much faster to do this operation in two steps:
673 Or more nicely written as:
680 This idiom takes advantage of the C<foreach> loop's aliasing
681 behavior to factor out common code. You can do this
682 on several strings at once, or arrays, or even the
683 values of a hash if you use a slice:
685 # trim whitespace in the scalar, the array,
686 # and all the values in the hash
687 foreach ($scalar, @array, @hash{keys %hash}) {
692 =head2 How do I pad a string with blanks or pad a number with zeroes?
694 (This answer contributed by Uri Guttman, with kibitzing from
697 In the following examples, C<$pad_len> is the length to which you wish
698 to pad the string, C<$text> or C<$num> contains the string to be padded,
699 and C<$pad_char> contains the padding character. You can use a single
700 character string constant instead of the C<$pad_char> variable if you
701 know what it is in advance. And in the same way you can use an integer in
702 place of C<$pad_len> if you know the pad length in advance.
704 The simplest method uses the C<sprintf> function. It can pad on the left
705 or right with blanks and on the left with zeroes and it will not
706 truncate the result. The C<pack> function can only pad strings on the
707 right with blanks and it will truncate the result to a maximum length of
710 # Left padding a string with blanks (no truncation):
711 $padded = sprintf("%${pad_len}s", $text);
713 # Right padding a string with blanks (no truncation):
714 $padded = sprintf("%-${pad_len}s", $text);
716 # Left padding a number with 0 (no truncation):
717 $padded = sprintf("%0${pad_len}d", $num);
719 # Right padding a string with blanks using pack (will truncate):
720 $padded = pack("A$pad_len",$text);
722 If you need to pad with a character other than blank or zero you can use
723 one of the following methods. They all generate a pad string with the
724 C<x> operator and combine that with C<$text>. These methods do
725 not truncate C<$text>.
727 Left and right padding with any character, creating a new string:
729 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
730 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
732 Left and right padding with any character, modifying C<$text> directly:
734 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
735 $text .= $pad_char x ( $pad_len - length( $text ) );
737 =head2 How do I extract selected columns from a string?
739 Use substr() or unpack(), both documented in L<perlfunc>.
740 If you prefer thinking in terms of columns instead of widths,
741 you can use this kind of thing:
743 # determine the unpack format needed to split Linux ps output
744 # arguments are cut columns
745 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
751 for my $place (@positions) {
752 $template .= "A" . ($place - $lastpos) . " ";
759 =head2 How do I find the soundex value of a string?
761 Use the standard Text::Soundex module distributed with Perl.
762 Before you do so, you may want to determine whether `soundex' is in
763 fact what you think it is. Knuth's soundex algorithm compresses words
764 into a small space, and so it does not necessarily distinguish between
765 two words which you might want to appear separately. For example, the
766 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
767 If Text::Soundex does not do what you are looking for, you might want
768 to consider the String::Approx module available at CPAN.
770 =head2 How can I expand variables in text strings?
772 Let's assume that you have a string like:
774 $text = 'this has a $foo in it and a $bar';
776 If those were both global variables, then this would
779 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
781 But since they are probably lexicals, or at least, they could
782 be, you'd have to do this:
784 $text =~ s/(\$\w+)/$1/eeg;
785 die if $@; # needed /ee, not /e
787 It's probably better in the general case to treat those
788 variables as entries in some special hash. For example:
794 $text =~ s/\$(\w+)/$user_defs{$1}/g;
796 See also ``How do I expand function calls in a string?'' in this section
799 =head2 What's wrong with always quoting "$vars"?
801 The problem is that those double-quotes force stringification--
802 coercing numbers and references into strings--even when you
803 don't want them to be strings. Think of it this way: double-quote
804 expansion is used to produce new strings. If you already
805 have a string, why do you need more?
807 If you get used to writing odd things like these:
811 somefunc("$var"); # BAD
813 You'll be in trouble. Those should (in 99.8% of the cases) be
814 the simpler and more direct:
820 Otherwise, besides slowing you down, you're going to break code when
821 the thing in the scalar is actually neither a string nor a number, but
827 my $oref = "$aref"; # WRONG
830 You can also get into subtle problems on those few operations in Perl
831 that actually do care about the difference between a string and a
832 number, such as the magical C<++> autoincrement operator or the
835 Stringification also destroys arrays.
838 print "@lines"; # WRONG - extra blanks
839 print @lines; # right
841 =head2 Why don't my <<HERE documents work?
843 Check for these three things:
847 =item 1. There must be no space after the << part.
849 =item 2. There (probably) should be a semicolon at the end.
851 =item 3. You can't (easily) have any space in front of the tag.
855 If you want to indent the text in the here document, you
859 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
864 But the HERE_TARGET must still be flush against the margin.
865 If you want that indented also, you'll have to quote
868 ($quote = <<' FINIS') =~ s/^\s+//gm;
869 ...we will have peace, when you and all your works have
870 perished--and the works of your dark master to whom you
871 would deliver us. You are a liar, Saruman, and a corrupter
872 of men's hearts. --Theoden in /usr/src/perl/taint.c
874 $quote =~ s/\s*--/\n--/;
876 A nice general-purpose fixer-upper function for indented here documents
877 follows. It expects to be called with a here document as its argument.
878 It looks to see whether each line begins with a common substring, and
879 if so, strips that substring off. Otherwise, it takes the amount of leading
880 whitespace found on the first line and removes that much off each
885 my ($white, $leader); # common whitespace and common leading string
886 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
887 ($white, $leader) = ($2, quotemeta($1));
889 ($white, $leader) = (/^(\s+)/, '');
891 s/^\s*?$leader(?:$white)?//gm;
895 This works with leading special strings, dynamically determined:
897 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
900 @@@ SAVEI32(runlevel);
902 @@@ while ( op = (*op->op_ppaddr)() );
906 MAIN_INTERPRETER_LOOP
908 Or with a fixed amount of leading whitespace, with remaining
909 indentation correctly preserved:
911 $poem = fix<<EVER_ON_AND_ON;
912 Now far ahead the Road has gone,
913 And I must follow, if I can,
914 Pursuing it with eager feet,
915 Until it joins some larger way
916 Where many paths and errands meet.
917 And whither then? I cannot say.
918 --Bilbo in /usr/src/perl/pp_ctl.c
923 =head2 What is the difference between a list and an array?
925 An array has a changeable length. A list does not. An array is something
926 you can push or pop, while a list is a set of values. Some people make
927 the distinction that a list is a value while an array is a variable.
928 Subroutines are passed and return lists, you put things into list
929 context, you initialize arrays with lists, and you foreach() across
930 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
931 in scalar context behave like the number of elements in them, subroutines
932 access their arguments through the array C<@_>, and push/pop/shift only work
935 As a side note, there's no such thing as a list in scalar context.
938 $scalar = (2, 5, 7, 9);
940 you're using the comma operator in scalar context, so it uses the scalar
941 comma operator. There never was a list there at all! This causes the
942 last value to be returned: 9.
944 =head2 What is the difference between $array[1] and @array[1]?
946 The former is a scalar value; the latter an array slice, making
947 it a list with one (scalar) value. You should use $ when you want a
948 scalar value (most of the time) and @ when you want a list with one
949 scalar value in it (very, very rarely; nearly never, in fact).
951 Sometimes it doesn't make a difference, but sometimes it does.
952 For example, compare:
954 $good[0] = `some program that outputs several lines`;
958 @bad[0] = `same program that outputs several lines`;
960 The C<use warnings> pragma and the B<-w> flag will warn you about these
963 =head2 How can I remove duplicate elements from a list or array?
965 There are several possible ways, depending on whether the array is
966 ordered and whether you wish to preserve the ordering.
972 If @in is sorted, and you want @out to be sorted:
973 (this assumes all true values in the array)
975 $prev = "not equal to $in[0]";
976 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
978 This is nice in that it doesn't use much extra memory, simulating
979 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
980 guarantees that the expression is true (so that grep picks it up)
981 even if the $_ is 0, "", or undef.
985 If you don't know whether @in is sorted:
988 @out = grep(!$saw{$_}++, @in);
992 Like (b), but @in contains only small integers:
994 @out = grep(!$saw[$_]++, @in);
998 A way to do (b) without any loops or greps:
1002 @out = sort keys %saw; # remove sort if undesired
1006 Like (d), but @in contains only small positive integers:
1010 @out = grep {defined} @ary;
1014 But perhaps you should have been using a hash all along, eh?
1016 =head2 How can I tell whether a list or array contains a certain element?
1018 Hearing the word "in" is an I<in>dication that you probably should have
1019 used a hash, not a list or array, to store your data. Hashes are
1020 designed to answer this question quickly and efficiently. Arrays aren't.
1022 That being said, there are several ways to approach this. If you
1023 are going to make this query many times over arbitrary string values,
1024 the fastest way is probably to invert the original array and keep an
1025 associative array lying about whose keys are the first array's values.
1027 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1029 for (@blues) { $is_blue{$_} = 1 }
1031 Now you can check whether $is_blue{$some_color}. It might have been a
1032 good idea to keep the blues all in a hash in the first place.
1034 If the values are all small integers, you could use a simple indexed
1035 array. This kind of an array will take up less space:
1037 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1038 undef @is_tiny_prime;
1039 for (@primes) { $is_tiny_prime[$_] = 1 }
1040 # or simply @istiny_prime[@primes] = (1) x @primes;
1042 Now you check whether $is_tiny_prime[$some_number].
1044 If the values in question are integers instead of strings, you can save
1045 quite a lot of space by using bit strings instead:
1047 @articles = ( 1..10, 150..2000, 2017 );
1049 for (@articles) { vec($read,$_,1) = 1 }
1051 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1055 ($is_there) = grep $_ eq $whatever, @array;
1059 ($is_there) = grep /$whatever/, @array;
1061 These are slow (checks every element even if the first matches),
1062 inefficient (same reason), and potentially buggy (what if there are
1063 regex characters in $whatever?). If you're only testing once, then
1067 foreach $elt (@array) {
1068 if ($elt eq $elt_to_find) {
1073 if ($is_there) { ... }
1075 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1077 Use a hash. Here's code to do both and more. It assumes that
1078 each element is unique in a given array:
1080 @union = @intersection = @difference = ();
1082 foreach $element (@array1, @array2) { $count{$element}++ }
1083 foreach $element (keys %count) {
1084 push @union, $element;
1085 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1088 Note that this is the I<symmetric difference>, that is, all elements in
1089 either A or in B but not in both. Think of it as an xor operation.
1091 =head2 How do I test whether two arrays or hashes are equal?
1093 The following code works for single-level arrays. It uses a stringwise
1094 comparison, and does not distinguish defined versus undefined empty
1095 strings. Modify if you have other needs.
1097 $are_equal = compare_arrays(\@frogs, \@toads);
1099 sub compare_arrays {
1100 my ($first, $second) = @_;
1101 no warnings; # silence spurious -w undef complaints
1102 return 0 unless @$first == @$second;
1103 for (my $i = 0; $i < @$first; $i++) {
1104 return 0 if $first->[$i] ne $second->[$i];
1109 For multilevel structures, you may wish to use an approach more
1110 like this one. It uses the CPAN module FreezeThaw:
1112 use FreezeThaw qw(cmpStr);
1113 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1115 printf "a and b contain %s arrays\n",
1116 cmpStr(\@a, \@b) == 0
1120 This approach also works for comparing hashes. Here
1121 we'll demonstrate two different answers:
1123 use FreezeThaw qw(cmpStr cmpStrHard);
1125 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1129 printf "a and b contain %s hashes\n",
1130 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1132 printf "a and b contain %s hashes\n",
1133 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1136 The first reports that both those the hashes contain the same data,
1137 while the second reports that they do not. Which you prefer is left as
1138 an exercise to the reader.
1140 =head2 How do I find the first array element for which a condition is true?
1142 You can use this if you care about the index:
1144 for ($i= 0; $i < @array; $i++) {
1145 if ($array[$i] eq "Waldo") {
1151 Now C<$found_index> has what you want.
1153 =head2 How do I handle linked lists?
1155 In general, you usually don't need a linked list in Perl, since with
1156 regular arrays, you can push and pop or shift and unshift at either end,
1157 or you can use splice to add and/or remove arbitrary number of elements at
1158 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1159 dynamic arrays. In the absence of shifts and pops, push in general
1160 needs to reallocate on the order every log(N) times, and unshift will
1161 need to copy pointers each time.
1163 If you really, really wanted, you could use structures as described in
1164 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1165 to do. For example, imagine a list node like this:
1172 You could walk the list this way:
1175 for ($node = $head; $node; $node = $node->{LINK}) {
1176 print $node->{VALUE}, " ";
1180 You could add to the list this way:
1183 $tail = append($head, 1); # grow a new head
1184 for $value ( 2 .. 10 ) {
1185 $tail = append($tail, $value);
1189 my($list, $value) = @_;
1190 my $node = { VALUE => $value };
1192 $node->{LINK} = $list->{LINK};
1193 $list->{LINK} = $node;
1195 $_[0] = $node; # replace caller's version
1200 But again, Perl's built-in are virtually always good enough.
1202 =head2 How do I handle circular lists?
1204 Circular lists could be handled in the traditional fashion with linked
1205 lists, or you could just do something like this with an array:
1207 unshift(@array, pop(@array)); # the last shall be first
1208 push(@array, shift(@array)); # and vice versa
1210 =head2 How do I shuffle an array randomly?
1214 # fisher_yates_shuffle( \@array ) :
1215 # generate a random permutation of @array in place
1216 sub fisher_yates_shuffle {
1219 for ($i = @$array; --$i; ) {
1220 my $j = int rand ($i+1);
1221 @$array[$i,$j] = @$array[$j,$i];
1225 fisher_yates_shuffle( \@array ); # permutes @array in place
1227 You've probably seen shuffling algorithms that work using splice,
1228 randomly picking another element to swap the current element with
1232 @old = 1 .. 10; # just a demo
1234 push(@new, splice(@old, rand @old, 1));
1237 This is bad because splice is already O(N), and since you do it N times,
1238 you just invented a quadratic algorithm; that is, O(N**2). This does
1239 not scale, although Perl is so efficient that you probably won't notice
1240 this until you have rather largish arrays.
1242 =head2 How do I process/modify each element of an array?
1244 Use C<for>/C<foreach>:
1247 s/foo/bar/; # change that word
1248 y/XZ/ZX/; # swap those letters
1251 Here's another; let's compute spherical volumes:
1253 for (@volumes = @radii) { # @volumes has changed parts
1255 $_ *= (4/3) * 3.14159; # this will be constant folded
1258 If you want to do the same thing to modify the values of the hash,
1259 you may not use the C<values> function, oddly enough. You need a slice:
1261 for $orbit ( @orbits{keys %orbits} ) {
1262 ($orbit **= 3) *= (4/3) * 3.14159;
1265 =head2 How do I select a random element from an array?
1267 Use the rand() function (see L<perlfunc/rand>):
1269 # at the top of the program:
1270 srand; # not needed for 5.004 and later
1273 $index = rand @array;
1274 $element = $array[$index];
1276 Make sure you I<only call srand once per program, if then>.
1277 If you are calling it more than once (such as before each
1278 call to rand), you're almost certainly doing something wrong.
1280 =head2 How do I permute N elements of a list?
1282 Here's a little program that generates all permutations
1283 of all the words on each line of input. The algorithm embodied
1284 in the permute() function should work on any list:
1287 # tsc-permute: permute each word of input
1288 permute([split], []);
1290 my @items = @{ $_[0] };
1291 my @perms = @{ $_[1] };
1295 my(@newitems,@newperms,$i);
1296 foreach $i (0 .. $#items) {
1299 unshift(@newperms, splice(@newitems, $i, 1));
1300 permute([@newitems], [@newperms]);
1305 =head2 How do I sort an array by (anything)?
1307 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1309 @list = sort { $a <=> $b } @list;
1311 The default sort function is cmp, string comparison, which would
1312 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1313 the numerical comparison operator.
1315 If you have a complicated function needed to pull out the part you
1316 want to sort on, then don't do it inside the sort function. Pull it
1317 out first, because the sort BLOCK can be called many times for the
1318 same element. Here's an example of how to pull out the first word
1319 after the first number on each item, and then sort those words
1324 ($item) = /\d+\s*(\S+)/;
1325 push @idx, uc($item);
1327 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1329 which could also be written this way, using a trick
1330 that's come to be known as the Schwartzian Transform:
1332 @sorted = map { $_->[0] }
1333 sort { $a->[1] cmp $b->[1] }
1334 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1336 If you need to sort on several fields, the following paradigm is useful.
1338 @sorted = sort { field1($a) <=> field1($b) ||
1339 field2($a) cmp field2($b) ||
1340 field3($a) cmp field3($b)
1343 This can be conveniently combined with precalculation of keys as given
1346 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1349 See also the question below on sorting hashes.
1351 =head2 How do I manipulate arrays of bits?
1353 Use pack() and unpack(), or else vec() and the bitwise operations.
1355 For example, this sets $vec to have bit N set if $ints[N] was set:
1358 foreach(@ints) { vec($vec,$_,1) = 1 }
1360 And here's how, given a vector in $vec, you can
1361 get those bits into your @ints array:
1363 sub bitvec_to_list {
1366 # Find null-byte density then select best algorithm
1367 if ($vec =~ tr/\0// / length $vec > 0.95) {
1370 # This method is faster with mostly null-bytes
1371 while($vec =~ /[^\0]/g ) {
1372 $i = -9 + 8 * pos $vec;
1373 push @ints, $i if vec($vec, ++$i, 1);
1374 push @ints, $i if vec($vec, ++$i, 1);
1375 push @ints, $i if vec($vec, ++$i, 1);
1376 push @ints, $i if vec($vec, ++$i, 1);
1377 push @ints, $i if vec($vec, ++$i, 1);
1378 push @ints, $i if vec($vec, ++$i, 1);
1379 push @ints, $i if vec($vec, ++$i, 1);
1380 push @ints, $i if vec($vec, ++$i, 1);
1383 # This method is a fast general algorithm
1385 my $bits = unpack "b*", $vec;
1386 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1387 push @ints, pos $bits while($bits =~ /1/g);
1392 This method gets faster the more sparse the bit vector is.
1393 (Courtesy of Tim Bunce and Winfried Koenig.)
1395 Here's a demo on how to use vec():
1398 $vector = "\xff\x0f\xef\xfe";
1399 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1400 unpack("N", $vector), "\n";
1401 $is_set = vec($vector, 23, 1);
1402 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1420 my ($offset, $width, $value) = @_;
1422 vec($vector, $offset, $width) = $value;
1423 print "offset=$offset width=$width value=$value\n";
1429 my $bits = unpack("b*", $vector);
1433 print "vector length in bytes: ", length($vector), "\n";
1434 @bytes = unpack("A8" x length($vector), $bits);
1435 print "bits are: @bytes\n\n";
1438 =head2 Why does defined() return true on empty arrays and hashes?
1440 The short story is that you should probably only use defined on scalars or
1441 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1442 in the 5.004 release or later of Perl for more detail.
1444 =head1 Data: Hashes (Associative Arrays)
1446 =head2 How do I process an entire hash?
1448 Use the each() function (see L<perlfunc/each>) if you don't care
1449 whether it's sorted:
1451 while ( ($key, $value) = each %hash) {
1452 print "$key = $value\n";
1455 If you want it sorted, you'll have to use foreach() on the result of
1456 sorting the keys as shown in an earlier question.
1458 =head2 What happens if I add or remove keys from a hash while iterating over it?
1462 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1463 iterating over it. In Perl 5 you can delete from it, but you still
1464 can't add to it, because that might cause a doubling of the hash table,
1465 in which half the entries get copied up to the new top half of the
1466 table, at which point you've totally bamboozled the iterator code.
1467 Even if the table doesn't double, there's no telling whether your new
1468 entry will be inserted before or after the current iterator position.
1470 Either treasure up your changes and make them after the iterator finishes
1471 or use keys to fetch all the old keys at once, and iterate over the list
1474 =head2 How do I look up a hash element by value?
1476 Create a reverse hash:
1478 %by_value = reverse %by_key;
1479 $key = $by_value{$value};
1481 That's not particularly efficient. It would be more space-efficient
1484 while (($key, $value) = each %by_key) {
1485 $by_value{$value} = $key;
1488 If your hash could have repeated values, the methods above will only find
1489 one of the associated keys. This may or may not worry you. If it does
1490 worry you, you can always reverse the hash into a hash of arrays instead:
1492 while (($key, $value) = each %by_key) {
1493 push @{$key_list_by_value{$value}}, $key;
1496 =head2 How can I know how many entries are in a hash?
1498 If you mean how many keys, then all you have to do is
1499 take the scalar sense of the keys() function:
1501 $num_keys = scalar keys %hash;
1503 The keys() function also resets the iterator, which in void context is
1504 faster for tied hashes than would be iterating through the whole
1505 hash, one key-value pair at a time.
1507 =head2 How do I sort a hash (optionally by value instead of key)?
1509 Internally, hashes are stored in a way that prevents you from imposing
1510 an order on key-value pairs. Instead, you have to sort a list of the
1513 @keys = sort keys %hash; # sorted by key
1515 $hash{$a} cmp $hash{$b}
1516 } keys %hash; # and by value
1518 Here we'll do a reverse numeric sort by value, and if two keys are
1519 identical, sort by length of key, or if that fails, by straight ASCII
1520 comparison of the keys (well, possibly modified by your locale--see
1524 $hash{$b} <=> $hash{$a}
1526 length($b) <=> length($a)
1531 =head2 How can I always keep my hash sorted?
1533 You can look into using the DB_File module and tie() using the
1534 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1535 The Tie::IxHash module from CPAN might also be instructive.
1537 =head2 What's the difference between "delete" and "undef" with hashes?
1539 Hashes are pairs of scalars: the first is the key, the second is the
1540 value. The key will be coerced to a string, although the value can be
1541 any kind of scalar: string, number, or reference. If a key C<$key> is
1542 present in the array, C<exists($key)> will return true. The value for
1543 a given key can be C<undef>, in which case C<$array{$key}> will be
1544 C<undef> while C<$exists{$key}> will return true. This corresponds to
1545 (C<$key>, C<undef>) being in the hash.
1547 Pictures help... here's the C<%ary> table:
1557 And these conditions hold
1561 defined $ary{'d'} is true
1562 defined $ary{'a'} is true
1563 exists $ary{'a'} is true (Perl5 only)
1564 grep ($_ eq 'a', keys %ary) is true
1570 your table now reads:
1581 and these conditions now hold; changes in caps:
1585 defined $ary{'d'} is true
1586 defined $ary{'a'} is FALSE
1587 exists $ary{'a'} is true (Perl5 only)
1588 grep ($_ eq 'a', keys %ary) is true
1590 Notice the last two: you have an undef value, but a defined key!
1596 your table now reads:
1605 and these conditions now hold; changes in caps:
1609 defined $ary{'d'} is true
1610 defined $ary{'a'} is false
1611 exists $ary{'a'} is FALSE (Perl5 only)
1612 grep ($_ eq 'a', keys %ary) is FALSE
1614 See, the whole entry is gone!
1616 =head2 Why don't my tied hashes make the defined/exists distinction?
1618 They may or may not implement the EXISTS() and DEFINED() methods
1619 differently. For example, there isn't the concept of undef with hashes
1620 that are tied to DBM* files. This means the true/false tables above
1621 will give different results when used on such a hash. It also means
1622 that exists and defined do the same thing with a DBM* file, and what
1623 they end up doing is not what they do with ordinary hashes.
1625 =head2 How do I reset an each() operation part-way through?
1627 Using C<keys %hash> in scalar context returns the number of keys in
1628 the hash I<and> resets the iterator associated with the hash. You may
1629 need to do this if you use C<last> to exit a loop early so that when you
1630 re-enter it, the hash iterator has been reset.
1632 =head2 How can I get the unique keys from two hashes?
1634 First you extract the keys from the hashes into lists, then solve
1635 the "removing duplicates" problem described above. For example:
1638 for $element (keys(%foo), keys(%bar)) {
1645 @uniq = keys %{{%foo,%bar}};
1647 Or if you really want to save space:
1650 while (defined ($key = each %foo)) {
1653 while (defined ($key = each %bar)) {
1658 =head2 How can I store a multidimensional array in a DBM file?
1660 Either stringify the structure yourself (no fun), or else
1661 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1662 it on top of either DB_File or GDBM_File.
1664 =head2 How can I make my hash remember the order I put elements into it?
1666 Use the Tie::IxHash from CPAN.
1669 tie(%myhash, Tie::IxHash);
1670 for ($i=0; $i<20; $i++) {
1673 @keys = keys %myhash;
1674 # @keys = (0,1,2,3,...)
1676 =head2 Why does passing a subroutine an undefined element in a hash create it?
1678 If you say something like:
1680 somefunc($hash{"nonesuch key here"});
1682 Then that element "autovivifies"; that is, it springs into existence
1683 whether you store something there or not. That's because functions
1684 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1685 it has to be ready to write it back into the caller's version.
1687 This has been fixed as of Perl5.004.
1689 Normally, merely accessing a key's value for a nonexistent key does
1690 I<not> cause that key to be forever there. This is different than
1693 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1695 Usually a hash ref, perhaps like this:
1700 TITLE => "deputy peon",
1703 PALS => [ "Norbert", "Rhys", "Phineas"],
1706 References are documented in L<perlref> and the upcoming L<perlreftut>.
1707 Examples of complex data structures are given in L<perldsc> and
1708 L<perllol>. Examples of structures and object-oriented classes are
1711 =head2 How can I use a reference as a hash key?
1713 You can't do this directly, but you could use the standard Tie::Refhash
1714 module distributed with Perl.
1718 =head2 How do I handle binary data correctly?
1720 Perl is binary clean, so this shouldn't be a problem. For example,
1721 this works fine (assuming the files are found):
1723 if (`cat /vmunix` =~ /gzip/) {
1724 print "Your kernel is GNU-zip enabled!\n";
1727 On less elegant (read: Byzantine) systems, however, you have
1728 to play tedious games with "text" versus "binary" files. See
1729 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1730 systems are curses out of Microsoft, who seem to be committed to putting
1731 the backward into backward compatibility.
1733 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1735 If you want to deal with multibyte characters, however, there are
1736 some gotchas. See the section on Regular Expressions.
1738 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1740 Assuming that you don't care about IEEE notations like "NaN" or
1741 "Infinity", you probably just want to use a regular expression.
1743 if (/\D/) { print "has nondigits\n" }
1744 if (/^\d+$/) { print "is a whole number\n" }
1745 if (/^-?\d+$/) { print "is an integer\n" }
1746 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1747 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1748 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1749 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1750 { print "a C float" }
1752 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1753 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1754 wrapper function for more convenient access. This function takes
1755 a string and returns the number it found, or C<undef> for input that
1756 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1757 if you just want to say, ``Is this a float?''
1760 use POSIX qw(strtod);
1765 my($num, $unparsed) = strtod($str);
1766 if (($str eq '') || ($unparsed != 0) || $!) {
1773 sub is_numeric { defined getnum($_[0]) }
1775 Or you could check out the String::Scanf module on CPAN instead. The
1776 POSIX module (part of the standard Perl distribution) provides the
1777 C<strtod> and C<strtol> for converting strings to double and longs,
1780 =head2 How do I keep persistent data across program calls?
1782 For some specific applications, you can use one of the DBM modules.
1783 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1784 Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
1785 Storable is part of the standard distribution. Here's one example using
1786 Storable's C<store> and C<retrieve> functions:
1789 store(\%hash, "filename");
1792 $href = retrieve("filename"); # by ref
1793 %hash = %{ retrieve("filename") }; # direct to hash
1795 =head2 How do I print out or copy a recursive data structure?
1797 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1798 for printing out data structures. The Storable module, found on CPAN,
1799 provides a function called C<dclone> that recursively copies its argument.
1801 use Storable qw(dclone);
1804 Where $r1 can be a reference to any kind of data structure you'd like.
1805 It will be deeply copied. Because C<dclone> takes and returns references,
1806 you'd have to add extra punctuation if you had a hash of arrays that
1809 %newhash = %{ dclone(\%oldhash) };
1811 =head2 How do I define methods for every class/object?
1813 Use the UNIVERSAL class (see L<UNIVERSAL>).
1815 =head2 How do I verify a credit card checksum?
1817 Get the Business::CreditCard module from CPAN.
1819 =head2 How do I pack arrays of doubles or floats for XS code?
1821 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1822 If you're doing a lot of float or double processing, consider using
1823 the PDL module from CPAN instead--it makes number-crunching easy.
1825 =head1 AUTHOR AND COPYRIGHT
1827 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1828 All rights reserved.
1830 When included as part of the Standard Version of Perl, or as part of
1831 its complete documentation whether printed or otherwise, this work
1832 may be distributed only under the terms of Perl's Artistic License.
1833 Any distribution of this file or derivatives thereof I<outside>
1834 of that package require that special arrangements be made with
1837 Irrespective of its distribution, all code examples in this file
1838 are hereby placed into the public domain. You are permitted and
1839 encouraged to use this code in your own programs for fun
1840 or for profit as you see fit. A simple comment in the code giving
1841 credit would be courteous but is not required.