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.
469 One simple destructive, inside-out approach that you might try is to
470 pull out the smallest nesting parts one at a time:
472 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
473 # do something with $1
476 A more complicated and sneaky approach is to make Perl's regular
477 expression engine do it for you. This is courtesy Dean Inada, and
478 rather has the nature of an Obfuscated Perl Contest entry, but it
481 # $_ contains the string to parse
482 # BEGIN and END are the opening and closing markers for the
487 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
488 @$ = (eval{/$re/},$@!~/unmatched/);
489 print join("\n",@$[0..$#$]) if( $$[-1] );
491 =head2 How do I reverse a string?
493 Use reverse() in scalar context, as documented in
496 $reversed = reverse $string;
498 =head2 How do I expand tabs in a string?
500 You can do it yourself:
502 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
504 Or you can just use the Text::Tabs module (part of the standard Perl
508 @expanded_lines = expand(@lines_with_tabs);
510 =head2 How do I reformat a paragraph?
512 Use Text::Wrap (part of the standard Perl distribution):
515 print wrap("\t", ' ', @paragraphs);
517 The paragraphs you give to Text::Wrap should not contain embedded
518 newlines. Text::Wrap doesn't justify the lines (flush-right).
520 =head2 How can I access/change the first N letters of a string?
522 There are many ways. If you just want to grab a copy, use
525 $first_byte = substr($a, 0, 1);
527 If you want to modify part of a string, the simplest way is often to
528 use substr() as an lvalue:
530 substr($a, 0, 3) = "Tom";
532 Although those with a pattern matching kind of thought process will
537 =head2 How do I change the Nth occurrence of something?
539 You have to keep track of N yourself. For example, let's say you want
540 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
541 C<"whosoever"> or C<"whomsoever">, case insensitively. These
542 all assume that $_ contains the string to be altered.
546 ++$count == 5 # is it the 5th?
547 ? "${2}soever" # yes, swap
548 : $1 # renege and leave it there
551 In the more general case, you can use the C</g> modifier in a C<while>
552 loop, keeping count of matches.
556 $_ = "One fish two fish red fish blue fish";
557 while (/(\w+)\s+fish\b/gi) {
558 if (++$count == $WANT) {
559 print "The third fish is a $1 one.\n";
563 That prints out: C<"The third fish is a red one."> You can also use a
564 repetition count and repeated pattern like this:
566 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
568 =head2 How can I count the number of occurrences of a substring within a string?
570 There are a number of ways, with varying efficiency. If you want a
571 count of a certain single character (X) within a string, you can use the
572 C<tr///> function like so:
574 $string = "ThisXlineXhasXsomeXx'sXinXit";
575 $count = ($string =~ tr/X//);
576 print "There are $count X characters in the string";
578 This is fine if you are just looking for a single character. However,
579 if you are trying to count multiple character substrings within a
580 larger string, C<tr///> won't work. What you can do is wrap a while()
581 loop around a global pattern match. For example, let's count negative
584 $string = "-9 55 48 -2 23 -76 4 14 -44";
585 while ($string =~ /-\d+/g) { $count++ }
586 print "There are $count negative numbers in the string";
588 =head2 How do I capitalize all the words on one line?
590 To make the first letter of each word upper case:
592 $line =~ s/\b(\w)/\U$1/g;
594 This has the strange effect of turning "C<don't do it>" into "C<Don'T
595 Do It>". Sometimes you might want this. Other times you might need a
596 more thorough solution (Suggested by brian d. foy):
599 (^\w) #at the beginning of the line
601 (\s\w) #preceded by whitespace
604 $string =~ /([\w']+)/\u\L$1/g;
606 To make the whole line upper case:
610 To force each word to be lower case, with the first letter upper case:
612 $line =~ s/(\w+)/\u\L$1/g;
614 You can (and probably should) enable locale awareness of those
615 characters by placing a C<use locale> pragma in your program.
616 See L<perllocale> for endless details on locales.
618 This is sometimes referred to as putting something into "title
619 case", but that's not quite accurate. Consider the proper
620 capitalization of the movie I<Dr. Strangelove or: How I Learned to
621 Stop Worrying and Love the Bomb>, for example.
623 =head2 How can I split a [character] delimited string except when inside
624 [character]? (Comma-separated files)
626 Take the example case of trying to split a string that is comma-separated
627 into its different fields. (We'll pretend you said comma-separated, not
628 comma-delimited, which is different and almost never what you mean.) You
629 can't use C<split(/,/)> because you shouldn't split if the comma is inside
630 quotes. For example, take a data line like this:
632 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
634 Due to the restriction of the quotes, this is a fairly complex
635 problem. Thankfully, we have Jeffrey Friedl, author of a highly
636 recommended book on regular expressions, to handle these for us. He
637 suggests (assuming your string is contained in $text):
640 push(@new, $+) while $text =~ m{
641 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
645 push(@new, undef) if substr($text,-1,1) eq ',';
647 If you want to represent quotation marks inside a
648 quotation-mark-delimited field, escape them with backslashes (eg,
649 C<"like \"this\"">. Unescaping them is a task addressed earlier in
652 Alternatively, the Text::ParseWords module (part of the standard Perl
653 distribution) lets you say:
655 use Text::ParseWords;
656 @new = quotewords(",", 0, $text);
658 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
660 =head2 How do I strip blank space from the beginning/end of a string?
662 Although the simplest approach would seem to be
664 $string =~ s/^\s*(.*?)\s*$/$1/;
666 not only is this unnecessarily slow and destructive, it also fails with
667 embedded newlines. It is much faster to do this operation in two steps:
672 Or more nicely written as:
679 This idiom takes advantage of the C<foreach> loop's aliasing
680 behavior to factor out common code. You can do this
681 on several strings at once, or arrays, or even the
682 values of a hash if you use a slice:
684 # trim whitespace in the scalar, the array,
685 # and all the values in the hash
686 foreach ($scalar, @array, @hash{keys %hash}) {
691 =head2 How do I pad a string with blanks or pad a number with zeroes?
693 (This answer contributed by Uri Guttman, with kibitzing from
696 In the following examples, C<$pad_len> is the length to which you wish
697 to pad the string, C<$text> or C<$num> contains the string to be padded,
698 and C<$pad_char> contains the padding character. You can use a single
699 character string constant instead of the C<$pad_char> variable if you
700 know what it is in advance. And in the same way you can use an integer in
701 place of C<$pad_len> if you know the pad length in advance.
703 The simplest method uses the C<sprintf> function. It can pad on the left
704 or right with blanks and on the left with zeroes and it will not
705 truncate the result. The C<pack> function can only pad strings on the
706 right with blanks and it will truncate the result to a maximum length of
709 # Left padding a string with blanks (no truncation):
710 $padded = sprintf("%${pad_len}s", $text);
712 # Right padding a string with blanks (no truncation):
713 $padded = sprintf("%-${pad_len}s", $text);
715 # Left padding a number with 0 (no truncation):
716 $padded = sprintf("%0${pad_len}d", $num);
718 # Right padding a string with blanks using pack (will truncate):
719 $padded = pack("A$pad_len",$text);
721 If you need to pad with a character other than blank or zero you can use
722 one of the following methods. They all generate a pad string with the
723 C<x> operator and combine that with C<$text>. These methods do
724 not truncate C<$text>.
726 Left and right padding with any character, creating a new string:
728 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
729 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
731 Left and right padding with any character, modifying C<$text> directly:
733 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
734 $text .= $pad_char x ( $pad_len - length( $text ) );
736 =head2 How do I extract selected columns from a string?
738 Use substr() or unpack(), both documented in L<perlfunc>.
739 If you prefer thinking in terms of columns instead of widths,
740 you can use this kind of thing:
742 # determine the unpack format needed to split Linux ps output
743 # arguments are cut columns
744 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
750 for my $place (@positions) {
751 $template .= "A" . ($place - $lastpos) . " ";
758 =head2 How do I find the soundex value of a string?
760 Use the standard Text::Soundex module distributed with Perl.
761 Before you do so, you may want to determine whether `soundex' is in
762 fact what you think it is. Knuth's soundex algorithm compresses words
763 into a small space, and so it does not necessarily distinguish between
764 two words which you might want to appear separately. For example, the
765 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
766 If Text::Soundex does not do what you are looking for, you might want
767 to consider the String::Approx module available at CPAN.
769 =head2 How can I expand variables in text strings?
771 Let's assume that you have a string like:
773 $text = 'this has a $foo in it and a $bar';
775 If those were both global variables, then this would
778 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
780 But since they are probably lexicals, or at least, they could
781 be, you'd have to do this:
783 $text =~ s/(\$\w+)/$1/eeg;
784 die if $@; # needed /ee, not /e
786 It's probably better in the general case to treat those
787 variables as entries in some special hash. For example:
793 $text =~ s/\$(\w+)/$user_defs{$1}/g;
795 See also ``How do I expand function calls in a string?'' in this section
798 =head2 What's wrong with always quoting "$vars"?
800 The problem is that those double-quotes force stringification--
801 coercing numbers and references into strings--even when you
802 don't want them to be strings. Think of it this way: double-quote
803 expansion is used to produce new strings. If you already
804 have a string, why do you need more?
806 If you get used to writing odd things like these:
810 somefunc("$var"); # BAD
812 You'll be in trouble. Those should (in 99.8% of the cases) be
813 the simpler and more direct:
819 Otherwise, besides slowing you down, you're going to break code when
820 the thing in the scalar is actually neither a string nor a number, but
826 my $oref = "$aref"; # WRONG
829 You can also get into subtle problems on those few operations in Perl
830 that actually do care about the difference between a string and a
831 number, such as the magical C<++> autoincrement operator or the
834 Stringification also destroys arrays.
837 print "@lines"; # WRONG - extra blanks
838 print @lines; # right
840 =head2 Why don't my <<HERE documents work?
842 Check for these three things:
846 =item 1. There must be no space after the << part.
848 =item 2. There (probably) should be a semicolon at the end.
850 =item 3. You can't (easily) have any space in front of the tag.
854 If you want to indent the text in the here document, you
858 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
863 But the HERE_TARGET must still be flush against the margin.
864 If you want that indented also, you'll have to quote
867 ($quote = <<' FINIS') =~ s/^\s+//gm;
868 ...we will have peace, when you and all your works have
869 perished--and the works of your dark master to whom you
870 would deliver us. You are a liar, Saruman, and a corrupter
871 of men's hearts. --Theoden in /usr/src/perl/taint.c
873 $quote =~ s/\s*--/\n--/;
875 A nice general-purpose fixer-upper function for indented here documents
876 follows. It expects to be called with a here document as its argument.
877 It looks to see whether each line begins with a common substring, and
878 if so, strips that substring off. Otherwise, it takes the amount of leading
879 whitespace found on the first line and removes that much off each
884 my ($white, $leader); # common whitespace and common leading string
885 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
886 ($white, $leader) = ($2, quotemeta($1));
888 ($white, $leader) = (/^(\s+)/, '');
890 s/^\s*?$leader(?:$white)?//gm;
894 This works with leading special strings, dynamically determined:
896 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
899 @@@ SAVEI32(runlevel);
901 @@@ while ( op = (*op->op_ppaddr)() );
905 MAIN_INTERPRETER_LOOP
907 Or with a fixed amount of leading whitespace, with remaining
908 indentation correctly preserved:
910 $poem = fix<<EVER_ON_AND_ON;
911 Now far ahead the Road has gone,
912 And I must follow, if I can,
913 Pursuing it with eager feet,
914 Until it joins some larger way
915 Where many paths and errands meet.
916 And whither then? I cannot say.
917 --Bilbo in /usr/src/perl/pp_ctl.c
922 =head2 What is the difference between a list and an array?
924 An array has a changeable length. A list does not. An array is something
925 you can push or pop, while a list is a set of values. Some people make
926 the distinction that a list is a value while an array is a variable.
927 Subroutines are passed and return lists, you put things into list
928 context, you initialize arrays with lists, and you foreach() across
929 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
930 in scalar context behave like the number of elements in them, subroutines
931 access their arguments through the array C<@_>, and push/pop/shift only work
934 As a side note, there's no such thing as a list in scalar context.
937 $scalar = (2, 5, 7, 9);
939 you're using the comma operator in scalar context, so it uses the scalar
940 comma operator. There never was a list there at all! This causes the
941 last value to be returned: 9.
943 =head2 What is the difference between $array[1] and @array[1]?
945 The former is a scalar value; the latter an array slice, making
946 it a list with one (scalar) value. You should use $ when you want a
947 scalar value (most of the time) and @ when you want a list with one
948 scalar value in it (very, very rarely; nearly never, in fact).
950 Sometimes it doesn't make a difference, but sometimes it does.
951 For example, compare:
953 $good[0] = `some program that outputs several lines`;
957 @bad[0] = `same program that outputs several lines`;
959 The C<use warnings> pragma and the B<-w> flag will warn you about these
962 =head2 How can I remove duplicate elements from a list or array?
964 There are several possible ways, depending on whether the array is
965 ordered and whether you wish to preserve the ordering.
971 If @in is sorted, and you want @out to be sorted:
972 (this assumes all true values in the array)
974 $prev = "not equal to $in[0]";
975 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
977 This is nice in that it doesn't use much extra memory, simulating
978 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
979 guarantees that the expression is true (so that grep picks it up)
980 even if the $_ is 0, "", or undef.
984 If you don't know whether @in is sorted:
987 @out = grep(!$saw{$_}++, @in);
991 Like (b), but @in contains only small integers:
993 @out = grep(!$saw[$_]++, @in);
997 A way to do (b) without any loops or greps:
1001 @out = sort keys %saw; # remove sort if undesired
1005 Like (d), but @in contains only small positive integers:
1009 @out = grep {defined} @ary;
1013 But perhaps you should have been using a hash all along, eh?
1015 =head2 How can I tell whether a list or array contains a certain element?
1017 Hearing the word "in" is an I<in>dication that you probably should have
1018 used a hash, not a list or array, to store your data. Hashes are
1019 designed to answer this question quickly and efficiently. Arrays aren't.
1021 That being said, there are several ways to approach this. If you
1022 are going to make this query many times over arbitrary string values,
1023 the fastest way is probably to invert the original array and keep an
1024 associative array lying about whose keys are the first array's values.
1026 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1028 for (@blues) { $is_blue{$_} = 1 }
1030 Now you can check whether $is_blue{$some_color}. It might have been a
1031 good idea to keep the blues all in a hash in the first place.
1033 If the values are all small integers, you could use a simple indexed
1034 array. This kind of an array will take up less space:
1036 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1037 undef @is_tiny_prime;
1038 for (@primes) { $is_tiny_prime[$_] = 1 }
1039 # or simply @istiny_prime[@primes] = (1) x @primes;
1041 Now you check whether $is_tiny_prime[$some_number].
1043 If the values in question are integers instead of strings, you can save
1044 quite a lot of space by using bit strings instead:
1046 @articles = ( 1..10, 150..2000, 2017 );
1048 for (@articles) { vec($read,$_,1) = 1 }
1050 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1054 ($is_there) = grep $_ eq $whatever, @array;
1058 ($is_there) = grep /$whatever/, @array;
1060 These are slow (checks every element even if the first matches),
1061 inefficient (same reason), and potentially buggy (what if there are
1062 regex characters in $whatever?). If you're only testing once, then
1066 foreach $elt (@array) {
1067 if ($elt eq $elt_to_find) {
1072 if ($is_there) { ... }
1074 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1076 Use a hash. Here's code to do both and more. It assumes that
1077 each element is unique in a given array:
1079 @union = @intersection = @difference = ();
1081 foreach $element (@array1, @array2) { $count{$element}++ }
1082 foreach $element (keys %count) {
1083 push @union, $element;
1084 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1087 Note that this is the I<symmetric difference>, that is, all elements in
1088 either A or in B but not in both. Think of it as an xor operation.
1090 =head2 How do I test whether two arrays or hashes are equal?
1092 The following code works for single-level arrays. It uses a stringwise
1093 comparison, and does not distinguish defined versus undefined empty
1094 strings. Modify if you have other needs.
1096 $are_equal = compare_arrays(\@frogs, \@toads);
1098 sub compare_arrays {
1099 my ($first, $second) = @_;
1100 no warnings; # silence spurious -w undef complaints
1101 return 0 unless @$first == @$second;
1102 for (my $i = 0; $i < @$first; $i++) {
1103 return 0 if $first->[$i] ne $second->[$i];
1108 For multilevel structures, you may wish to use an approach more
1109 like this one. It uses the CPAN module FreezeThaw:
1111 use FreezeThaw qw(cmpStr);
1112 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1114 printf "a and b contain %s arrays\n",
1115 cmpStr(\@a, \@b) == 0
1119 This approach also works for comparing hashes. Here
1120 we'll demonstrate two different answers:
1122 use FreezeThaw qw(cmpStr cmpStrHard);
1124 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1128 printf "a and b contain %s hashes\n",
1129 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1131 printf "a and b contain %s hashes\n",
1132 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1135 The first reports that both those the hashes contain the same data,
1136 while the second reports that they do not. Which you prefer is left as
1137 an exercise to the reader.
1139 =head2 How do I find the first array element for which a condition is true?
1141 You can use this if you care about the index:
1143 for ($i= 0; $i < @array; $i++) {
1144 if ($array[$i] eq "Waldo") {
1150 Now C<$found_index> has what you want.
1152 =head2 How do I handle linked lists?
1154 In general, you usually don't need a linked list in Perl, since with
1155 regular arrays, you can push and pop or shift and unshift at either end,
1156 or you can use splice to add and/or remove arbitrary number of elements at
1157 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1158 dynamic arrays. In the absence of shifts and pops, push in general
1159 needs to reallocate on the order every log(N) times, and unshift will
1160 need to copy pointers each time.
1162 If you really, really wanted, you could use structures as described in
1163 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1164 to do. For example, imagine a list node like this:
1171 You could walk the list this way:
1174 for ($node = $head; $node; $node = $node->{LINK}) {
1175 print $node->{VALUE}, " ";
1179 You could add to the list this way:
1182 $tail = append($head, 1); # grow a new head
1183 for $value ( 2 .. 10 ) {
1184 $tail = append($tail, $value);
1188 my($list, $value) = @_;
1189 my $node = { VALUE => $value };
1191 $node->{LINK} = $list->{LINK};
1192 $list->{LINK} = $node;
1194 $_[0] = $node; # replace caller's version
1199 But again, Perl's built-in are virtually always good enough.
1201 =head2 How do I handle circular lists?
1203 Circular lists could be handled in the traditional fashion with linked
1204 lists, or you could just do something like this with an array:
1206 unshift(@array, pop(@array)); # the last shall be first
1207 push(@array, shift(@array)); # and vice versa
1209 =head2 How do I shuffle an array randomly?
1213 # fisher_yates_shuffle( \@array ) :
1214 # generate a random permutation of @array in place
1215 sub fisher_yates_shuffle {
1218 for ($i = @$array; --$i; ) {
1219 my $j = int rand ($i+1);
1220 @$array[$i,$j] = @$array[$j,$i];
1224 fisher_yates_shuffle( \@array ); # permutes @array in place
1226 You've probably seen shuffling algorithms that work using splice,
1227 randomly picking another element to swap the current element with
1231 @old = 1 .. 10; # just a demo
1233 push(@new, splice(@old, rand @old, 1));
1236 This is bad because splice is already O(N), and since you do it N times,
1237 you just invented a quadratic algorithm; that is, O(N**2). This does
1238 not scale, although Perl is so efficient that you probably won't notice
1239 this until you have rather largish arrays.
1241 =head2 How do I process/modify each element of an array?
1243 Use C<for>/C<foreach>:
1246 s/foo/bar/; # change that word
1247 y/XZ/ZX/; # swap those letters
1250 Here's another; let's compute spherical volumes:
1252 for (@volumes = @radii) { # @volumes has changed parts
1254 $_ *= (4/3) * 3.14159; # this will be constant folded
1257 If you want to do the same thing to modify the values of the hash,
1258 you may not use the C<values> function, oddly enough. You need a slice:
1260 for $orbit ( @orbits{keys %orbits} ) {
1261 ($orbit **= 3) *= (4/3) * 3.14159;
1264 =head2 How do I select a random element from an array?
1266 Use the rand() function (see L<perlfunc/rand>):
1268 # at the top of the program:
1269 srand; # not needed for 5.004 and later
1272 $index = rand @array;
1273 $element = $array[$index];
1275 Make sure you I<only call srand once per program, if then>.
1276 If you are calling it more than once (such as before each
1277 call to rand), you're almost certainly doing something wrong.
1279 =head2 How do I permute N elements of a list?
1281 Here's a little program that generates all permutations
1282 of all the words on each line of input. The algorithm embodied
1283 in the permute() function should work on any list:
1286 # tsc-permute: permute each word of input
1287 permute([split], []);
1289 my @items = @{ $_[0] };
1290 my @perms = @{ $_[1] };
1294 my(@newitems,@newperms,$i);
1295 foreach $i (0 .. $#items) {
1298 unshift(@newperms, splice(@newitems, $i, 1));
1299 permute([@newitems], [@newperms]);
1304 =head2 How do I sort an array by (anything)?
1306 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1308 @list = sort { $a <=> $b } @list;
1310 The default sort function is cmp, string comparison, which would
1311 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1312 the numerical comparison operator.
1314 If you have a complicated function needed to pull out the part you
1315 want to sort on, then don't do it inside the sort function. Pull it
1316 out first, because the sort BLOCK can be called many times for the
1317 same element. Here's an example of how to pull out the first word
1318 after the first number on each item, and then sort those words
1323 ($item) = /\d+\s*(\S+)/;
1324 push @idx, uc($item);
1326 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1328 which could also be written this way, using a trick
1329 that's come to be known as the Schwartzian Transform:
1331 @sorted = map { $_->[0] }
1332 sort { $a->[1] cmp $b->[1] }
1333 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1335 If you need to sort on several fields, the following paradigm is useful.
1337 @sorted = sort { field1($a) <=> field1($b) ||
1338 field2($a) cmp field2($b) ||
1339 field3($a) cmp field3($b)
1342 This can be conveniently combined with precalculation of keys as given
1345 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1348 See also the question below on sorting hashes.
1350 =head2 How do I manipulate arrays of bits?
1352 Use pack() and unpack(), or else vec() and the bitwise operations.
1354 For example, this sets $vec to have bit N set if $ints[N] was set:
1357 foreach(@ints) { vec($vec,$_,1) = 1 }
1359 And here's how, given a vector in $vec, you can
1360 get those bits into your @ints array:
1362 sub bitvec_to_list {
1365 # Find null-byte density then select best algorithm
1366 if ($vec =~ tr/\0// / length $vec > 0.95) {
1369 # This method is faster with mostly null-bytes
1370 while($vec =~ /[^\0]/g ) {
1371 $i = -9 + 8 * pos $vec;
1372 push @ints, $i if vec($vec, ++$i, 1);
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);
1382 # This method is a fast general algorithm
1384 my $bits = unpack "b*", $vec;
1385 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1386 push @ints, pos $bits while($bits =~ /1/g);
1391 This method gets faster the more sparse the bit vector is.
1392 (Courtesy of Tim Bunce and Winfried Koenig.)
1394 Here's a demo on how to use vec():
1397 $vector = "\xff\x0f\xef\xfe";
1398 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1399 unpack("N", $vector), "\n";
1400 $is_set = vec($vector, 23, 1);
1401 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1419 my ($offset, $width, $value) = @_;
1421 vec($vector, $offset, $width) = $value;
1422 print "offset=$offset width=$width value=$value\n";
1428 my $bits = unpack("b*", $vector);
1432 print "vector length in bytes: ", length($vector), "\n";
1433 @bytes = unpack("A8" x length($vector), $bits);
1434 print "bits are: @bytes\n\n";
1437 =head2 Why does defined() return true on empty arrays and hashes?
1439 The short story is that you should probably only use defined on scalars or
1440 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1441 in the 5.004 release or later of Perl for more detail.
1443 =head1 Data: Hashes (Associative Arrays)
1445 =head2 How do I process an entire hash?
1447 Use the each() function (see L<perlfunc/each>) if you don't care
1448 whether it's sorted:
1450 while ( ($key, $value) = each %hash) {
1451 print "$key = $value\n";
1454 If you want it sorted, you'll have to use foreach() on the result of
1455 sorting the keys as shown in an earlier question.
1457 =head2 What happens if I add or remove keys from a hash while iterating over it?
1461 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1462 iterating over it. In Perl 5 you can delete from it, but you still
1463 can't add to it, because that might cause a doubling of the hash table,
1464 in which half the entries get copied up to the new top half of the
1465 table, at which point you've totally bamboozled the iterator code.
1466 Even if the table doesn't double, there's no telling whether your new
1467 entry will be inserted before or after the current iterator position.
1469 Either treasure up your changes and make them after the iterator finishes
1470 or use keys to fetch all the old keys at once, and iterate over the list
1473 =head2 How do I look up a hash element by value?
1475 Create a reverse hash:
1477 %by_value = reverse %by_key;
1478 $key = $by_value{$value};
1480 That's not particularly efficient. It would be more space-efficient
1483 while (($key, $value) = each %by_key) {
1484 $by_value{$value} = $key;
1487 If your hash could have repeated values, the methods above will only find
1488 one of the associated keys. This may or may not worry you. If it does
1489 worry you, you can always reverse the hash into a hash of arrays instead:
1491 while (($key, $value) = each %by_key) {
1492 push @{$key_list_by_value{$value}}, $key;
1495 =head2 How can I know how many entries are in a hash?
1497 If you mean how many keys, then all you have to do is
1498 take the scalar sense of the keys() function:
1500 $num_keys = scalar keys %hash;
1502 The keys() function also resets the iterator, which in void context is
1503 faster for tied hashes than would be iterating through the whole
1504 hash, one key-value pair at a time.
1506 =head2 How do I sort a hash (optionally by value instead of key)?
1508 Internally, hashes are stored in a way that prevents you from imposing
1509 an order on key-value pairs. Instead, you have to sort a list of the
1512 @keys = sort keys %hash; # sorted by key
1514 $hash{$a} cmp $hash{$b}
1515 } keys %hash; # and by value
1517 Here we'll do a reverse numeric sort by value, and if two keys are
1518 identical, sort by length of key, or if that fails, by straight ASCII
1519 comparison of the keys (well, possibly modified by your locale--see
1523 $hash{$b} <=> $hash{$a}
1525 length($b) <=> length($a)
1530 =head2 How can I always keep my hash sorted?
1532 You can look into using the DB_File module and tie() using the
1533 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1534 The Tie::IxHash module from CPAN might also be instructive.
1536 =head2 What's the difference between "delete" and "undef" with hashes?
1538 Hashes are pairs of scalars: the first is the key, the second is the
1539 value. The key will be coerced to a string, although the value can be
1540 any kind of scalar: string, number, or reference. If a key C<$key> is
1541 present in the array, C<exists($key)> will return true. The value for
1542 a given key can be C<undef>, in which case C<$array{$key}> will be
1543 C<undef> while C<$exists{$key}> will return true. This corresponds to
1544 (C<$key>, C<undef>) being in the hash.
1546 Pictures help... here's the C<%ary> table:
1556 And these conditions hold
1560 defined $ary{'d'} is true
1561 defined $ary{'a'} is true
1562 exists $ary{'a'} is true (Perl5 only)
1563 grep ($_ eq 'a', keys %ary) is true
1569 your table now reads:
1580 and these conditions now hold; changes in caps:
1584 defined $ary{'d'} is true
1585 defined $ary{'a'} is FALSE
1586 exists $ary{'a'} is true (Perl5 only)
1587 grep ($_ eq 'a', keys %ary) is true
1589 Notice the last two: you have an undef value, but a defined key!
1595 your table now reads:
1604 and these conditions now hold; changes in caps:
1608 defined $ary{'d'} is true
1609 defined $ary{'a'} is false
1610 exists $ary{'a'} is FALSE (Perl5 only)
1611 grep ($_ eq 'a', keys %ary) is FALSE
1613 See, the whole entry is gone!
1615 =head2 Why don't my tied hashes make the defined/exists distinction?
1617 They may or may not implement the EXISTS() and DEFINED() methods
1618 differently. For example, there isn't the concept of undef with hashes
1619 that are tied to DBM* files. This means the true/false tables above
1620 will give different results when used on such a hash. It also means
1621 that exists and defined do the same thing with a DBM* file, and what
1622 they end up doing is not what they do with ordinary hashes.
1624 =head2 How do I reset an each() operation part-way through?
1626 Using C<keys %hash> in scalar context returns the number of keys in
1627 the hash I<and> resets the iterator associated with the hash. You may
1628 need to do this if you use C<last> to exit a loop early so that when you
1629 re-enter it, the hash iterator has been reset.
1631 =head2 How can I get the unique keys from two hashes?
1633 First you extract the keys from the hashes into lists, then solve
1634 the "removing duplicates" problem described above. For example:
1637 for $element (keys(%foo), keys(%bar)) {
1644 @uniq = keys %{{%foo,%bar}};
1646 Or if you really want to save space:
1649 while (defined ($key = each %foo)) {
1652 while (defined ($key = each %bar)) {
1657 =head2 How can I store a multidimensional array in a DBM file?
1659 Either stringify the structure yourself (no fun), or else
1660 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1661 it on top of either DB_File or GDBM_File.
1663 =head2 How can I make my hash remember the order I put elements into it?
1665 Use the Tie::IxHash from CPAN.
1668 tie(%myhash, Tie::IxHash);
1669 for ($i=0; $i<20; $i++) {
1672 @keys = keys %myhash;
1673 # @keys = (0,1,2,3,...)
1675 =head2 Why does passing a subroutine an undefined element in a hash create it?
1677 If you say something like:
1679 somefunc($hash{"nonesuch key here"});
1681 Then that element "autovivifies"; that is, it springs into existence
1682 whether you store something there or not. That's because functions
1683 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1684 it has to be ready to write it back into the caller's version.
1686 This has been fixed as of Perl5.004.
1688 Normally, merely accessing a key's value for a nonexistent key does
1689 I<not> cause that key to be forever there. This is different than
1692 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1694 Usually a hash ref, perhaps like this:
1699 TITLE => "deputy peon",
1702 PALS => [ "Norbert", "Rhys", "Phineas"],
1705 References are documented in L<perlref> and the upcoming L<perlreftut>.
1706 Examples of complex data structures are given in L<perldsc> and
1707 L<perllol>. Examples of structures and object-oriented classes are
1710 =head2 How can I use a reference as a hash key?
1712 You can't do this directly, but you could use the standard Tie::Refhash
1713 module distributed with Perl.
1717 =head2 How do I handle binary data correctly?
1719 Perl is binary clean, so this shouldn't be a problem. For example,
1720 this works fine (assuming the files are found):
1722 if (`cat /vmunix` =~ /gzip/) {
1723 print "Your kernel is GNU-zip enabled!\n";
1726 On less elegant (read: Byzantine) systems, however, you have
1727 to play tedious games with "text" versus "binary" files. See
1728 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1729 systems are curses out of Microsoft, who seem to be committed to putting
1730 the backward into backward compatibility.
1732 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1734 If you want to deal with multibyte characters, however, there are
1735 some gotchas. See the section on Regular Expressions.
1737 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1739 Assuming that you don't care about IEEE notations like "NaN" or
1740 "Infinity", you probably just want to use a regular expression.
1742 if (/\D/) { print "has nondigits\n" }
1743 if (/^\d+$/) { print "is a whole number\n" }
1744 if (/^-?\d+$/) { print "is an integer\n" }
1745 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1746 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1747 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1748 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1749 { print "a C float" }
1751 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1752 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1753 wrapper function for more convenient access. This function takes
1754 a string and returns the number it found, or C<undef> for input that
1755 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1756 if you just want to say, ``Is this a float?''
1759 use POSIX qw(strtod);
1764 my($num, $unparsed) = strtod($str);
1765 if (($str eq '') || ($unparsed != 0) || $!) {
1772 sub is_numeric { defined getnum($_[0]) }
1774 Or you could check out the String::Scanf module on CPAN instead. The
1775 POSIX module (part of the standard Perl distribution) provides the
1776 C<strtod> and C<strtol> for converting strings to double and longs,
1779 =head2 How do I keep persistent data across program calls?
1781 For some specific applications, you can use one of the DBM modules.
1782 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1783 Storable, or Class::Eroot modules from CPAN. Here's one example using
1784 Storable's C<store> and C<retrieve> functions:
1787 store(\%hash, "filename");
1790 $href = retrieve("filename"); # by ref
1791 %hash = %{ retrieve("filename") }; # direct to hash
1793 =head2 How do I print out or copy a recursive data structure?
1795 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1796 for printing out data structures. The Storable module, found on CPAN,
1797 provides a function called C<dclone> that recursively copies its argument.
1799 use Storable qw(dclone);
1802 Where $r1 can be a reference to any kind of data structure you'd like.
1803 It will be deeply copied. Because C<dclone> takes and returns references,
1804 you'd have to add extra punctuation if you had a hash of arrays that
1807 %newhash = %{ dclone(\%oldhash) };
1809 =head2 How do I define methods for every class/object?
1811 Use the UNIVERSAL class (see L<UNIVERSAL>).
1813 =head2 How do I verify a credit card checksum?
1815 Get the Business::CreditCard module from CPAN.
1817 =head2 How do I pack arrays of doubles or floats for XS code?
1819 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1820 If you're doing a lot of float or double processing, consider using
1821 the PDL module from CPAN instead--it makes number-crunching easy.
1823 =head1 AUTHOR AND COPYRIGHT
1825 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1826 All rights reserved.
1828 When included as part of the Standard Version of Perl, or as part of
1829 its complete documentation whether printed or otherwise, this work
1830 may be distributed only under the terms of Perl's Artistic License.
1831 Any distribution of this file or derivatives thereof I<outside>
1832 of that package require that special arrangements be made with
1835 Irrespective of its distribution, all code examples in this file
1836 are hereby placed into the public domain. You are permitted and
1837 encouraged to use this code in your own programs for fun
1838 or for profit as you see fit. A simple comment in the code giving
1839 credit would be courteous but is not required.