3 perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
7 The section of the FAQ answers questions related to the manipulation
8 of data as numbers, dates, strings, arrays, hashes, and miscellaneous
13 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
15 The infinite set that a mathematician thinks of as the real numbers can
16 only be approximated on a computer, since the computer only has a finite
17 number of bits to store an infinite number of, um, numbers.
19 Internally, your computer represents floating-point numbers in binary.
20 Floating-point numbers read in from a file or appearing as literals
21 in your program are converted from their decimal floating-point
22 representation (eg, 19.95) to an internal binary representation.
24 However, 19.95 can't be precisely represented as a binary
25 floating-point number, just like 1/3 can't be exactly represented as a
26 decimal floating-point number. The computer's binary representation
27 of 19.95, therefore, isn't exactly 19.95.
29 When a floating-point number gets printed, the binary floating-point
30 representation is converted back to decimal. These decimal numbers
31 are displayed in either the format you specify with printf(), or the
32 current output format for numbers. (See L<perlvar/"$#"> if you use
33 print. C<$#> has a different default value in Perl5 than it did in
34 Perl4. Changing C<$#> yourself is deprecated.)
36 This affects B<all> computer languages that represent decimal
37 floating-point numbers in binary, not just Perl. Perl provides
38 arbitrary-precision decimal numbers with the Math::BigFloat module
39 (part of the standard Perl distribution), but mathematical operations
40 are consequently slower.
42 If precision is important, such as when dealing with money, it's good
43 to work with integers and then divide at the last possible moment.
44 For example, work in pennies (1995) instead of dollars and cents
45 (19.95) and divide by 100 at the end. In fact, if you are dividing by
46 100, you don't even need to really divide-- just split of the
47 fractional parts and insert the '.' (or whichever is your decimal
48 separator) in between, e.g.
51 $_[0] =~ /(.*?)(.(?:.)?)$/;
52 sprintf("%d.%02d", $1||0, $2);
55 and then display all your numbers like this: C<d100($number)>
57 To get rid of the superfluous digits, just use a format (eg,
58 C<printf("%.2f", 19.95)>) to get the required precision.
59 See L<perlop/"Floating-point Arithmetic">.
61 =head2 Why isn't my octal data interpreted correctly?
63 Perl only understands octal and hex numbers as such when they occur
64 as literals in your program. If they are read in from somewhere and
65 assigned, no automatic conversion takes place. You must explicitly
66 use oct() or hex() if you want the values converted. oct() interprets
67 both hex ("0x350") numbers and octal ones ("0350" or even without the
68 leading "0", like "377"), while hex() only converts hexadecimal ones,
69 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
71 This problem shows up most often when people try using chmod(), mkdir(),
72 umask(), or sysopen(), which all want permissions in octal.
74 chmod(644, $file); # WRONG -- perl -w catches this
75 chmod(0644, $file); # right
77 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
79 Remember that int() merely truncates toward 0. For rounding to a
80 certain number of digits, sprintf() or printf() is usually the easiest
83 printf("%.3f", 3.1415926535); # prints 3.142
85 The POSIX module (part of the standard Perl distribution) implements
86 ceil(), floor(), and a number of other mathematical and trigonometric
90 $ceil = ceil(3.5); # 4
91 $floor = floor(3.5); # 3
93 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
94 module. With 5.004, the Math::Trig module (part of the standard Perl
95 distribution) implements the trigonometric functions. Internally it
96 uses the Math::Complex module and some functions can break out from
97 the real axis into the complex plane, for example the inverse sine of
100 Rounding in financial applications can have serious implications, and
101 the rounding method used should be specified precisely. In these
102 cases, it probably pays not to trust whichever system rounding is
103 being used by Perl, but to instead implement the rounding function you
106 To see why, notice how you'll still have an issue on half-way-point
109 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
111 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
112 0.8 0.8 0.9 0.9 1.0 1.0
114 Don't blame Perl. It's the same as in C. IEEE says we have to do this.
115 Perl numbers whose absolute values are integers under 2**31 (on 32 bit
116 machines) will work pretty much like mathematical integers. Other numbers
119 =head2 How do I convert bits into ints?
121 To turn a string of 1s and 0s like C<10110110> into a scalar containing
122 its binary value, use the pack() and unpack() functions (documented in
123 L<perlfunc/"pack"> and L<perlfunc/"unpack">):
125 $decimal = unpack('c', pack('B8', '10110110'));
127 This packs the string C<10110110> into an eight bit binary structure.
128 This is then unpacked as a character, which returns its ordinal value.
130 This does the same thing:
132 $decimal = ord(pack('B8', '10110110'));
134 Here's an example of going the other way:
136 $binary_string = unpack('B*', "\x29");
138 =head2 Why doesn't & work the way I want it to?
140 The behavior of binary arithmetic operators depends on whether they're
141 used on numbers or strings. The operators treat a string as a series
142 of bits and work with that (the string C<"3"> is the bit pattern
143 C<00110011>). The operators work with the binary form of a number
144 (the number C<3> is treated as the bit pattern C<00000011>).
146 So, saying C<11 & 3> performs the "and" operation on numbers (yielding
147 C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
150 Most problems with C<&> and C<|> arise because the programmer thinks
151 they have a number but really it's a string. The rest arise because
154 if ("\020\020" & "\101\101") {
158 but a string consisting of two null bytes (the result of C<"\020\020"
159 & "\101\101">) is not a false value in Perl. You need:
161 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
165 =head2 How do I multiply matrices?
167 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
168 or the PDL extension (also available from CPAN).
170 =head2 How do I perform an operation on a series of integers?
172 To call a function on each element in an array, and collect the
175 @results = map { my_func($_) } @array;
179 @triple = map { 3 * $_ } @single;
181 To call a function on each element of an array, but ignore the
184 foreach $iterator (@array) {
185 some_func($iterator);
188 To call a function on each integer in a (small) range, you B<can> use:
190 @results = map { some_func($_) } (5 .. 25);
192 but you should be aware that the C<..> operator creates an array of
193 all integers in the range. This can take a lot of memory for large
197 for ($i=5; $i < 500_005; $i++) {
198 push(@results, some_func($i));
201 This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
202 loop will iterate over the range, without creating the entire range.
204 for my $i (5 .. 500_005) {
205 push(@results, some_func($i));
208 will not create a list of 500,000 integers.
210 =head2 How can I output Roman numerals?
212 Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
214 =head2 Why aren't my random numbers random?
216 If you're using a version of Perl before 5.004, you must call C<srand>
217 once at the start of your program to seed the random number generator.
218 5.004 and later automatically call C<srand> at the beginning. Don't
219 call C<srand> more than once--you make your numbers less random, rather
222 Computers are good at being predictable and bad at being random
223 (despite appearances caused by bugs in your programs :-).
224 http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
225 Phoenix, talks more about this. John von Neumann said, ``Anyone who
226 attempts to generate random numbers by deterministic means is, of
227 course, living in a state of sin.''
229 If you want numbers that are more random than C<rand> with C<srand>
230 provides, you should also check out the Math::TrulyRandom module from
231 CPAN. It uses the imperfections in your system's timer to generate
232 random numbers, but this takes quite a while. If you want a better
233 pseudorandom generator than comes with your operating system, look at
234 ``Numerical Recipes in C'' at http://www.nr.com/ .
238 =head2 How do I find the week-of-the-year/day-of-the-year?
240 The day of the year is in the array returned by localtime() (see
241 L<perlfunc/"localtime">):
243 $day_of_year = (localtime(time()))[7];
245 or more legibly (in 5.7.1 or higher):
248 $day_of_year = localtime->day_of_year();
250 You can find the week of the year by using Time::Piece's strftime():
252 $week_of_year = localtime->strftime("%U");
253 $iso_week = localtime->strftime("%V");
255 The difference between %U and %V is that %U assumes that the first day
256 of week 1 is the first Sunday of the year, whereas ISO 8601:1988 uses
257 the first week that has at least 4 days in the current year, and with
258 Monday as the first day of the week. You can also use %W, which will
259 return the week of the year with Monday as the first day of week 1. See
260 your strftime(3) man page for more details.
262 =head2 How do I find the current century or millennium?
264 Use the following simple functions:
267 return int((((localtime(shift || time))[5] + 1999))/100);
270 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
273 On some systems, you'll find that the POSIX module's strftime() function
274 has been extended in a non-standard way to use a C<%C> format, which they
275 sometimes claim is the "century". It isn't, because on most such systems,
276 this is only the first two digits of the four-digit year, and thus cannot
277 be used to reliably determine the current century or millennium.
279 =head2 How can I compare two dates and find the difference?
281 If you're storing your dates as epoch seconds then simply subtract one
282 from the other. If you've got a structured date (distinct year, day,
283 month, hour, minute, seconds values), then for reasons of accessibility,
284 simplicity, and efficiency, merely use either timelocal or timegm (from
285 the Time::Local module in the standard distribution) to reduce structured
286 dates to epoch seconds. However, if you don't know the precise format of
287 your dates, then you should probably use either of the Date::Manip and
288 Date::Calc modules from CPAN before you go hacking up your own parsing
289 routine to handle arbitrary date formats.
291 Also note that the core module Time::Piece overloads the addition and
292 subtraction operators to provide date calculation options. See
293 L<Time::Piece/Date Calculations>.
295 =head2 How can I take a string and turn it into epoch seconds?
297 If it's a regular enough string that it always has the same format,
298 you can split it up and pass the parts to C<timelocal> in the standard
299 Time::Local module. Otherwise, you should look into the Date::Calc
300 and Date::Manip modules from CPAN.
302 =head2 How can I find the Julian Day?
304 Use Time::Piece as follows:
307 my $julian_day = localtime->julian_day;
308 my $mjd = localtime->mjd; # modified julian day
310 Before you immerse yourself too deeply in this, be sure to verify that
311 it is the I<Julian> Day you really want. Are you interested in a way
312 of getting serial days so that you just can tell how many days they
313 are apart or so that you can do also other date arithmetic? If you
314 are interested in performing date arithmetic, this can be done using
315 Time::Piece (standard module since Perl 5.8), or by modules
316 Date::Manip or Date::Calc.
318 There is too many details and much confusion on this issue to cover in
319 this FAQ, but the term is applied (correctly) to a calendar now
320 supplanted by the Gregorian Calendar, with the Julian Calendar failing
321 to adjust properly for leap years on centennial years (among other
322 annoyances). The term is also used (incorrectly) to mean: [1] days in
323 the Gregorian Calendar; and [2] days since a particular starting time
324 or `epoch', usually 1970 in the Unix world and 1980 in the
325 MS-DOS/Windows world. If you find that it is not the first meaning
326 that you really want, then check out the Date::Manip and Date::Calc
327 modules. (Thanks to David Cassell for most of this text.)
329 =head2 How do I find yesterday's date?
331 The C<time()> function returns the current time in seconds since the
332 epoch. Take twenty-four hours off that:
334 $yesterday = time() - ( 24 * 60 * 60 );
336 Then you can pass this to C<localtime()> and get the individual year,
337 month, day, hour, minute, seconds values.
339 Alternatively, you can use Time::Piece to subtract a day from the value
340 returned from C<localtime()>:
343 use Time::Seconds; # imports seconds constants, like ONE_DAY
344 my $today = localtime();
345 my $yesterday = $today - ONE_DAY;
347 Note very carefully that the code above assumes that your days are
348 twenty-four hours each. For most people, there are two days a year
349 when they aren't: the switch to and from summer time throws this off.
350 A solution to this issue is offered by Russ Allbery.
353 my $now = defined $_[0] ? $_[0] : time;
354 my $then = $now - 60 * 60 * 24;
355 my $ndst = (localtime $now)[8] > 0;
356 my $tdst = (localtime $then)[8] > 0;
357 $then - ($tdst - $ndst) * 60 * 60;
359 # Should give you "this time yesterday" in seconds since epoch relative to
360 # the first argument or the current time if no argument is given and
361 # suitable for passing to localtime or whatever else you need to do with
362 # it. $ndst is whether we're currently in daylight savings time; $tdst is
363 # whether the point 24 hours ago was in daylight savings time. If $tdst
364 # and $ndst are the same, a boundary wasn't crossed, and the correction
365 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
366 # from yesterday's time since we gained an extra hour while going off
367 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
368 # negative hour (add an hour) to yesterday's time since we lost an hour.
370 # All of this is because during those days when one switches off or onto
371 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
373 # The explicit settings of $ndst and $tdst are necessary because localtime
374 # only says it returns the system tm struct, and the system tm struct at
375 # least on Solaris doesn't guarantee any particular positive value (like,
376 # say, 1) for isdst, just a positive value. And that value can
377 # potentially be negative, if DST information isn't available (this sub
378 # just treats those cases like no DST).
380 # Note that between 2am and 3am on the day after the time zone switches
381 # off daylight savings time, the exact hour of "yesterday" corresponding
382 # to the current hour is not clearly defined. Note also that if used
383 # between 2am and 3am the day after the change to daylight savings time,
384 # the result will be between 3am and 4am of the previous day; it's
385 # arguable whether this is correct.
387 # This sub does not attempt to deal with leap seconds (most things don't).
389 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
390 # This code is in the public domain
392 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
394 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
395 Y2K compliant (whatever that means). The programmers you've hired to
396 use it, however, probably are not.
398 Long answer: The question belies a true understanding of the issue.
399 Perl is just as Y2K compliant as your pencil--no more, and no less.
400 Can you use your pencil to write a non-Y2K-compliant memo? Of course
401 you can. Is that the pencil's fault? Of course it isn't.
403 The date and time functions supplied with Perl (gmtime and localtime)
404 supply adequate information to determine the year well beyond 2000
405 (2038 is when trouble strikes for 32-bit machines). The year returned
406 by these functions when used in a list context is the year minus 1900.
407 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
408 number. To avoid the year 2000 problem simply do not treat the year as
409 a 2-digit number. It isn't.
411 When gmtime() and localtime() are used in scalar context they return
412 a timestamp string that contains a fully-expanded year. For example,
413 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
414 2001". There's no year 2000 problem here.
416 That doesn't mean that Perl can't be used to create non-Y2K compliant
417 programs. It can. But so can your pencil. It's the fault of the user,
418 not the language. At the risk of inflaming the NRA: ``Perl doesn't
419 break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
424 =head2 How do I validate input?
426 The answer to this question is usually a regular expression, perhaps
427 with auxiliary logic. See the more specific questions (numbers, mail
428 addresses, etc.) for details.
430 =head2 How do I unescape a string?
432 It depends just what you mean by ``escape''. URL escapes are dealt
433 with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
434 character are removed with
438 This won't expand C<"\n"> or C<"\t"> or any other special escapes.
440 =head2 How do I remove consecutive pairs of characters?
442 To turn C<"abbcccd"> into C<"abccd">:
444 s/(.)\1/$1/g; # add /s to include newlines
446 Here's a solution that turns "abbcccd" to "abcd":
448 y///cs; # y == tr, but shorter :-)
450 =head2 How do I expand function calls in a string?
452 This is documented in L<perlref>. In general, this is fraught with
453 quoting and readability problems, but it is possible. To interpolate
454 a subroutine call (in list context) into a string:
456 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
458 If you prefer scalar context, similar chicanery is also useful for
459 arbitrary expressions:
461 print "That yields ${\($n + 5)} widgets\n";
463 Version 5.004 of Perl had a bug that gave list context to the
464 expression in C<${...}>, but this is fixed in version 5.005.
466 See also ``How can I expand variables in text strings?'' in this
469 =head2 How do I find matching/nesting anything?
471 This isn't something that can be done in one regular expression, no
472 matter how complicated. To find something between two single
473 characters, a pattern like C</x([^x]*)x/> will get the intervening
474 bits in $1. For multiple ones, then something more like
475 C</alpha(.*?)omega/> would be needed. But none of these deals with
476 nested patterns, nor can they. For that you'll have to write a
479 If you are serious about writing a parser, there are a number of
480 modules or oddities that will make your life a lot easier. There are
481 the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
482 and the byacc program. Starting from perl 5.8 the Text::Balanced
483 is part of the standard distribution.
485 One simple destructive, inside-out approach that you might try is to
486 pull out the smallest nesting parts one at a time:
488 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
489 # do something with $1
492 A more complicated and sneaky approach is to make Perl's regular
493 expression engine do it for you. This is courtesy Dean Inada, and
494 rather has the nature of an Obfuscated Perl Contest entry, but it
497 # $_ contains the string to parse
498 # BEGIN and END are the opening and closing markers for the
503 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
504 @$ = (eval{/$re/},$@!~/unmatched/);
505 print join("\n",@$[0..$#$]) if( $$[-1] );
507 =head2 How do I reverse a string?
509 Use reverse() in scalar context, as documented in
512 $reversed = reverse $string;
514 =head2 How do I expand tabs in a string?
516 You can do it yourself:
518 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
520 Or you can just use the Text::Tabs module (part of the standard Perl
524 @expanded_lines = expand(@lines_with_tabs);
526 =head2 How do I reformat a paragraph?
528 Use Text::Wrap (part of the standard Perl distribution):
531 print wrap("\t", ' ', @paragraphs);
533 The paragraphs you give to Text::Wrap should not contain embedded
534 newlines. Text::Wrap doesn't justify the lines (flush-right).
536 =head2 How can I access/change the first N letters of a string?
538 There are many ways. If you just want to grab a copy, use
541 $first_byte = substr($a, 0, 1);
543 If you want to modify part of a string, the simplest way is often to
544 use substr() as an lvalue:
546 substr($a, 0, 3) = "Tom";
548 Although those with a pattern matching kind of thought process will
553 =head2 How do I change the Nth occurrence of something?
555 You have to keep track of N yourself. For example, let's say you want
556 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
557 C<"whosoever"> or C<"whomsoever">, case insensitively. These
558 all assume that $_ contains the string to be altered.
562 ++$count == 5 # is it the 5th?
563 ? "${2}soever" # yes, swap
564 : $1 # renege and leave it there
567 In the more general case, you can use the C</g> modifier in a C<while>
568 loop, keeping count of matches.
572 $_ = "One fish two fish red fish blue fish";
573 while (/(\w+)\s+fish\b/gi) {
574 if (++$count == $WANT) {
575 print "The third fish is a $1 one.\n";
579 That prints out: C<"The third fish is a red one."> You can also use a
580 repetition count and repeated pattern like this:
582 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
584 =head2 How can I count the number of occurrences of a substring within a string?
586 There are a number of ways, with varying efficiency. If you want a
587 count of a certain single character (X) within a string, you can use the
588 C<tr///> function like so:
590 $string = "ThisXlineXhasXsomeXx'sXinXit";
591 $count = ($string =~ tr/X//);
592 print "There are $count X characters in the string";
594 This is fine if you are just looking for a single character. However,
595 if you are trying to count multiple character substrings within a
596 larger string, C<tr///> won't work. What you can do is wrap a while()
597 loop around a global pattern match. For example, let's count negative
600 $string = "-9 55 48 -2 23 -76 4 14 -44";
601 while ($string =~ /-\d+/g) { $count++ }
602 print "There are $count negative numbers in the string";
604 =head2 How do I capitalize all the words on one line?
606 To make the first letter of each word upper case:
608 $line =~ s/\b(\w)/\U$1/g;
610 This has the strange effect of turning "C<don't do it>" into "C<Don'T
611 Do It>". Sometimes you might want this. Other times you might need a
612 more thorough solution (Suggested by brian d. foy):
615 (^\w) #at the beginning of the line
617 (\s\w) #preceded by whitespace
620 $string =~ /([\w']+)/\u\L$1/g;
622 To make the whole line upper case:
626 To force each word to be lower case, with the first letter upper case:
628 $line =~ s/(\w+)/\u\L$1/g;
630 You can (and probably should) enable locale awareness of those
631 characters by placing a C<use locale> pragma in your program.
632 See L<perllocale> for endless details on locales.
634 This is sometimes referred to as putting something into "title
635 case", but that's not quite accurate. Consider the proper
636 capitalization of the movie I<Dr. Strangelove or: How I Learned to
637 Stop Worrying and Love the Bomb>, for example.
639 =head2 How can I split a [character] delimited string except when inside
640 [character]? (Comma-separated files)
642 Take the example case of trying to split a string that is comma-separated
643 into its different fields. (We'll pretend you said comma-separated, not
644 comma-delimited, which is different and almost never what you mean.) You
645 can't use C<split(/,/)> because you shouldn't split if the comma is inside
646 quotes. For example, take a data line like this:
648 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
650 Due to the restriction of the quotes, this is a fairly complex
651 problem. Thankfully, we have Jeffrey Friedl, author of a highly
652 recommended book on regular expressions, to handle these for us. He
653 suggests (assuming your string is contained in $text):
656 push(@new, $+) while $text =~ m{
657 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
661 push(@new, undef) if substr($text,-1,1) eq ',';
663 If you want to represent quotation marks inside a
664 quotation-mark-delimited field, escape them with backslashes (eg,
665 C<"like \"this\"">. Unescaping them is a task addressed earlier in
668 Alternatively, the Text::ParseWords module (part of the standard Perl
669 distribution) lets you say:
671 use Text::ParseWords;
672 @new = quotewords(",", 0, $text);
674 There's also a Text::CSV (Comma-Separated Values) module on CPAN.
676 =head2 How do I strip blank space from the beginning/end of a string?
678 Although the simplest approach would seem to be
680 $string =~ s/^\s*(.*?)\s*$/$1/;
682 not only is this unnecessarily slow and destructive, it also fails with
683 embedded newlines. It is much faster to do this operation in two steps:
688 Or more nicely written as:
695 This idiom takes advantage of the C<foreach> loop's aliasing
696 behavior to factor out common code. You can do this
697 on several strings at once, or arrays, or even the
698 values of a hash if you use a slice:
700 # trim whitespace in the scalar, the array,
701 # and all the values in the hash
702 foreach ($scalar, @array, @hash{keys %hash}) {
707 =head2 How do I pad a string with blanks or pad a number with zeroes?
709 (This answer contributed by Uri Guttman, with kibitzing from
712 In the following examples, C<$pad_len> is the length to which you wish
713 to pad the string, C<$text> or C<$num> contains the string to be padded,
714 and C<$pad_char> contains the padding character. You can use a single
715 character string constant instead of the C<$pad_char> variable if you
716 know what it is in advance. And in the same way you can use an integer in
717 place of C<$pad_len> if you know the pad length in advance.
719 The simplest method uses the C<sprintf> function. It can pad on the left
720 or right with blanks and on the left with zeroes and it will not
721 truncate the result. The C<pack> function can only pad strings on the
722 right with blanks and it will truncate the result to a maximum length of
725 # Left padding a string with blanks (no truncation):
726 $padded = sprintf("%${pad_len}s", $text);
728 # Right padding a string with blanks (no truncation):
729 $padded = sprintf("%-${pad_len}s", $text);
731 # Left padding a number with 0 (no truncation):
732 $padded = sprintf("%0${pad_len}d", $num);
734 # Right padding a string with blanks using pack (will truncate):
735 $padded = pack("A$pad_len",$text);
737 If you need to pad with a character other than blank or zero you can use
738 one of the following methods. They all generate a pad string with the
739 C<x> operator and combine that with C<$text>. These methods do
740 not truncate C<$text>.
742 Left and right padding with any character, creating a new string:
744 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
745 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
747 Left and right padding with any character, modifying C<$text> directly:
749 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
750 $text .= $pad_char x ( $pad_len - length( $text ) );
752 =head2 How do I extract selected columns from a string?
754 Use substr() or unpack(), both documented in L<perlfunc>.
755 If you prefer thinking in terms of columns instead of widths,
756 you can use this kind of thing:
758 # determine the unpack format needed to split Linux ps output
759 # arguments are cut columns
760 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
766 for my $place (@positions) {
767 $template .= "A" . ($place - $lastpos) . " ";
774 =head2 How do I find the soundex value of a string?
776 Use the standard Text::Soundex module distributed with Perl.
777 Before you do so, you may want to determine whether `soundex' is in
778 fact what you think it is. Knuth's soundex algorithm compresses words
779 into a small space, and so it does not necessarily distinguish between
780 two words which you might want to appear separately. For example, the
781 last names `Knuth' and `Kant' are both mapped to the soundex code K530.
782 If Text::Soundex does not do what you are looking for, you might want
783 to consider the String::Approx module available at CPAN.
785 =head2 How can I expand variables in text strings?
787 Let's assume that you have a string like:
789 $text = 'this has a $foo in it and a $bar';
791 If those were both global variables, then this would
794 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
796 But since they are probably lexicals, or at least, they could
797 be, you'd have to do this:
799 $text =~ s/(\$\w+)/$1/eeg;
800 die if $@; # needed /ee, not /e
802 It's probably better in the general case to treat those
803 variables as entries in some special hash. For example:
809 $text =~ s/\$(\w+)/$user_defs{$1}/g;
811 See also ``How do I expand function calls in a string?'' in this section
814 =head2 What's wrong with always quoting "$vars"?
816 The problem is that those double-quotes force stringification--
817 coercing numbers and references into strings--even when you
818 don't want them to be strings. Think of it this way: double-quote
819 expansion is used to produce new strings. If you already
820 have a string, why do you need more?
822 If you get used to writing odd things like these:
826 somefunc("$var"); # BAD
828 You'll be in trouble. Those should (in 99.8% of the cases) be
829 the simpler and more direct:
835 Otherwise, besides slowing you down, you're going to break code when
836 the thing in the scalar is actually neither a string nor a number, but
842 my $oref = "$aref"; # WRONG
845 You can also get into subtle problems on those few operations in Perl
846 that actually do care about the difference between a string and a
847 number, such as the magical C<++> autoincrement operator or the
850 Stringification also destroys arrays.
853 print "@lines"; # WRONG - extra blanks
854 print @lines; # right
856 =head2 Why don't my <<HERE documents work?
858 Check for these three things:
862 =item 1. There must be no space after the << part.
864 =item 2. There (probably) should be a semicolon at the end.
866 =item 3. You can't (easily) have any space in front of the tag.
870 If you want to indent the text in the here document, you
874 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
879 But the HERE_TARGET must still be flush against the margin.
880 If you want that indented also, you'll have to quote
883 ($quote = <<' FINIS') =~ s/^\s+//gm;
884 ...we will have peace, when you and all your works have
885 perished--and the works of your dark master to whom you
886 would deliver us. You are a liar, Saruman, and a corrupter
887 of men's hearts. --Theoden in /usr/src/perl/taint.c
889 $quote =~ s/\s*--/\n--/;
891 A nice general-purpose fixer-upper function for indented here documents
892 follows. It expects to be called with a here document as its argument.
893 It looks to see whether each line begins with a common substring, and
894 if so, strips that substring off. Otherwise, it takes the amount of leading
895 whitespace found on the first line and removes that much off each
900 my ($white, $leader); # common whitespace and common leading string
901 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
902 ($white, $leader) = ($2, quotemeta($1));
904 ($white, $leader) = (/^(\s+)/, '');
906 s/^\s*?$leader(?:$white)?//gm;
910 This works with leading special strings, dynamically determined:
912 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
915 @@@ SAVEI32(runlevel);
917 @@@ while ( op = (*op->op_ppaddr)() );
921 MAIN_INTERPRETER_LOOP
923 Or with a fixed amount of leading whitespace, with remaining
924 indentation correctly preserved:
926 $poem = fix<<EVER_ON_AND_ON;
927 Now far ahead the Road has gone,
928 And I must follow, if I can,
929 Pursuing it with eager feet,
930 Until it joins some larger way
931 Where many paths and errands meet.
932 And whither then? I cannot say.
933 --Bilbo in /usr/src/perl/pp_ctl.c
938 =head2 What is the difference between a list and an array?
940 An array has a changeable length. A list does not. An array is something
941 you can push or pop, while a list is a set of values. Some people make
942 the distinction that a list is a value while an array is a variable.
943 Subroutines are passed and return lists, you put things into list
944 context, you initialize arrays with lists, and you foreach() across
945 a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
946 in scalar context behave like the number of elements in them, subroutines
947 access their arguments through the array C<@_>, and push/pop/shift only work
950 As a side note, there's no such thing as a list in scalar context.
953 $scalar = (2, 5, 7, 9);
955 you're using the comma operator in scalar context, so it uses the scalar
956 comma operator. There never was a list there at all! This causes the
957 last value to be returned: 9.
959 =head2 What is the difference between $array[1] and @array[1]?
961 The former is a scalar value; the latter an array slice, making
962 it a list with one (scalar) value. You should use $ when you want a
963 scalar value (most of the time) and @ when you want a list with one
964 scalar value in it (very, very rarely; nearly never, in fact).
966 Sometimes it doesn't make a difference, but sometimes it does.
967 For example, compare:
969 $good[0] = `some program that outputs several lines`;
973 @bad[0] = `same program that outputs several lines`;
975 The C<use warnings> pragma and the B<-w> flag will warn you about these
978 =head2 How can I remove duplicate elements from a list or array?
980 There are several possible ways, depending on whether the array is
981 ordered and whether you wish to preserve the ordering.
987 If @in is sorted, and you want @out to be sorted:
988 (this assumes all true values in the array)
990 $prev = "not equal to $in[0]";
991 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
993 This is nice in that it doesn't use much extra memory, simulating
994 uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
995 guarantees that the expression is true (so that grep picks it up)
996 even if the $_ is 0, "", or undef.
1000 If you don't know whether @in is sorted:
1003 @out = grep(!$saw{$_}++, @in);
1007 Like (b), but @in contains only small integers:
1009 @out = grep(!$saw[$_]++, @in);
1013 A way to do (b) without any loops or greps:
1017 @out = sort keys %saw; # remove sort if undesired
1021 Like (d), but @in contains only small positive integers:
1025 @out = grep {defined} @ary;
1029 But perhaps you should have been using a hash all along, eh?
1031 =head2 How can I tell whether a list or array contains a certain element?
1033 Hearing the word "in" is an I<in>dication that you probably should have
1034 used a hash, not a list or array, to store your data. Hashes are
1035 designed to answer this question quickly and efficiently. Arrays aren't.
1037 That being said, there are several ways to approach this. If you
1038 are going to make this query many times over arbitrary string values,
1039 the fastest way is probably to invert the original array and keep an
1040 associative array lying about whose keys are the first array's values.
1042 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1044 for (@blues) { $is_blue{$_} = 1 }
1046 Now you can check whether $is_blue{$some_color}. It might have been a
1047 good idea to keep the blues all in a hash in the first place.
1049 If the values are all small integers, you could use a simple indexed
1050 array. This kind of an array will take up less space:
1052 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1053 undef @is_tiny_prime;
1054 for (@primes) { $is_tiny_prime[$_] = 1 }
1055 # or simply @istiny_prime[@primes] = (1) x @primes;
1057 Now you check whether $is_tiny_prime[$some_number].
1059 If the values in question are integers instead of strings, you can save
1060 quite a lot of space by using bit strings instead:
1062 @articles = ( 1..10, 150..2000, 2017 );
1064 for (@articles) { vec($read,$_,1) = 1 }
1066 Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1070 ($is_there) = grep $_ eq $whatever, @array;
1074 ($is_there) = grep /$whatever/, @array;
1076 These are slow (checks every element even if the first matches),
1077 inefficient (same reason), and potentially buggy (what if there are
1078 regex characters in $whatever?). If you're only testing once, then
1082 foreach $elt (@array) {
1083 if ($elt eq $elt_to_find) {
1088 if ($is_there) { ... }
1090 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1092 Use a hash. Here's code to do both and more. It assumes that
1093 each element is unique in a given array:
1095 @union = @intersection = @difference = ();
1097 foreach $element (@array1, @array2) { $count{$element}++ }
1098 foreach $element (keys %count) {
1099 push @union, $element;
1100 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1103 Note that this is the I<symmetric difference>, that is, all elements in
1104 either A or in B but not in both. Think of it as an xor operation.
1106 =head2 How do I test whether two arrays or hashes are equal?
1108 The following code works for single-level arrays. It uses a stringwise
1109 comparison, and does not distinguish defined versus undefined empty
1110 strings. Modify if you have other needs.
1112 $are_equal = compare_arrays(\@frogs, \@toads);
1114 sub compare_arrays {
1115 my ($first, $second) = @_;
1116 no warnings; # silence spurious -w undef complaints
1117 return 0 unless @$first == @$second;
1118 for (my $i = 0; $i < @$first; $i++) {
1119 return 0 if $first->[$i] ne $second->[$i];
1124 For multilevel structures, you may wish to use an approach more
1125 like this one. It uses the CPAN module FreezeThaw:
1127 use FreezeThaw qw(cmpStr);
1128 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1130 printf "a and b contain %s arrays\n",
1131 cmpStr(\@a, \@b) == 0
1135 This approach also works for comparing hashes. Here
1136 we'll demonstrate two different answers:
1138 use FreezeThaw qw(cmpStr cmpStrHard);
1140 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1144 printf "a and b contain %s hashes\n",
1145 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1147 printf "a and b contain %s hashes\n",
1148 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1151 The first reports that both those the hashes contain the same data,
1152 while the second reports that they do not. Which you prefer is left as
1153 an exercise to the reader.
1155 =head2 How do I find the first array element for which a condition is true?
1157 You can use this if you care about the index:
1159 for ($i= 0; $i < @array; $i++) {
1160 if ($array[$i] eq "Waldo") {
1166 Now C<$found_index> has what you want.
1168 =head2 How do I handle linked lists?
1170 In general, you usually don't need a linked list in Perl, since with
1171 regular arrays, you can push and pop or shift and unshift at either end,
1172 or you can use splice to add and/or remove arbitrary number of elements at
1173 arbitrary points. Both pop and shift are both O(1) operations on Perl's
1174 dynamic arrays. In the absence of shifts and pops, push in general
1175 needs to reallocate on the order every log(N) times, and unshift will
1176 need to copy pointers each time.
1178 If you really, really wanted, you could use structures as described in
1179 L<perldsc> or L<perltoot> and do just what the algorithm book tells you
1180 to do. For example, imagine a list node like this:
1187 You could walk the list this way:
1190 for ($node = $head; $node; $node = $node->{LINK}) {
1191 print $node->{VALUE}, " ";
1195 You could add to the list this way:
1198 $tail = append($head, 1); # grow a new head
1199 for $value ( 2 .. 10 ) {
1200 $tail = append($tail, $value);
1204 my($list, $value) = @_;
1205 my $node = { VALUE => $value };
1207 $node->{LINK} = $list->{LINK};
1208 $list->{LINK} = $node;
1210 $_[0] = $node; # replace caller's version
1215 But again, Perl's built-in are virtually always good enough.
1217 =head2 How do I handle circular lists?
1219 Circular lists could be handled in the traditional fashion with linked
1220 lists, or you could just do something like this with an array:
1222 unshift(@array, pop(@array)); # the last shall be first
1223 push(@array, shift(@array)); # and vice versa
1225 =head2 How do I shuffle an array randomly?
1229 # fisher_yates_shuffle( \@array ) :
1230 # generate a random permutation of @array in place
1231 sub fisher_yates_shuffle {
1234 for ($i = @$array; --$i; ) {
1235 my $j = int rand ($i+1);
1236 @$array[$i,$j] = @$array[$j,$i];
1240 fisher_yates_shuffle( \@array ); # permutes @array in place
1242 You've probably seen shuffling algorithms that work using splice,
1243 randomly picking another element to swap the current element with
1247 @old = 1 .. 10; # just a demo
1249 push(@new, splice(@old, rand @old, 1));
1252 This is bad because splice is already O(N), and since you do it N times,
1253 you just invented a quadratic algorithm; that is, O(N**2). This does
1254 not scale, although Perl is so efficient that you probably won't notice
1255 this until you have rather largish arrays.
1257 =head2 How do I process/modify each element of an array?
1259 Use C<for>/C<foreach>:
1262 s/foo/bar/; # change that word
1263 y/XZ/ZX/; # swap those letters
1266 Here's another; let's compute spherical volumes:
1268 for (@volumes = @radii) { # @volumes has changed parts
1270 $_ *= (4/3) * 3.14159; # this will be constant folded
1273 If you want to do the same thing to modify the values of the hash,
1274 you may not use the C<values> function, oddly enough. You need a slice:
1276 for $orbit ( @orbits{keys %orbits} ) {
1277 ($orbit **= 3) *= (4/3) * 3.14159;
1280 =head2 How do I select a random element from an array?
1282 Use the rand() function (see L<perlfunc/rand>):
1284 # at the top of the program:
1285 srand; # not needed for 5.004 and later
1288 $index = rand @array;
1289 $element = $array[$index];
1291 Make sure you I<only call srand once per program, if then>.
1292 If you are calling it more than once (such as before each
1293 call to rand), you're almost certainly doing something wrong.
1295 =head2 How do I permute N elements of a list?
1297 Here's a little program that generates all permutations
1298 of all the words on each line of input. The algorithm embodied
1299 in the permute() function should work on any list:
1302 # tsc-permute: permute each word of input
1303 permute([split], []);
1305 my @items = @{ $_[0] };
1306 my @perms = @{ $_[1] };
1310 my(@newitems,@newperms,$i);
1311 foreach $i (0 .. $#items) {
1314 unshift(@newperms, splice(@newitems, $i, 1));
1315 permute([@newitems], [@newperms]);
1320 =head2 How do I sort an array by (anything)?
1322 Supply a comparison function to sort() (described in L<perlfunc/sort>):
1324 @list = sort { $a <=> $b } @list;
1326 The default sort function is cmp, string comparison, which would
1327 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
1328 the numerical comparison operator.
1330 If you have a complicated function needed to pull out the part you
1331 want to sort on, then don't do it inside the sort function. Pull it
1332 out first, because the sort BLOCK can be called many times for the
1333 same element. Here's an example of how to pull out the first word
1334 after the first number on each item, and then sort those words
1339 ($item) = /\d+\s*(\S+)/;
1340 push @idx, uc($item);
1342 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1344 which could also be written this way, using a trick
1345 that's come to be known as the Schwartzian Transform:
1347 @sorted = map { $_->[0] }
1348 sort { $a->[1] cmp $b->[1] }
1349 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
1351 If you need to sort on several fields, the following paradigm is useful.
1353 @sorted = sort { field1($a) <=> field1($b) ||
1354 field2($a) cmp field2($b) ||
1355 field3($a) cmp field3($b)
1358 This can be conveniently combined with precalculation of keys as given
1361 See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1364 See also the question below on sorting hashes.
1366 =head2 How do I manipulate arrays of bits?
1368 Use pack() and unpack(), or else vec() and the bitwise operations.
1370 For example, this sets $vec to have bit N set if $ints[N] was set:
1373 foreach(@ints) { vec($vec,$_,1) = 1 }
1375 And here's how, given a vector in $vec, you can
1376 get those bits into your @ints array:
1378 sub bitvec_to_list {
1381 # Find null-byte density then select best algorithm
1382 if ($vec =~ tr/\0// / length $vec > 0.95) {
1385 # This method is faster with mostly null-bytes
1386 while($vec =~ /[^\0]/g ) {
1387 $i = -9 + 8 * pos $vec;
1388 push @ints, $i if vec($vec, ++$i, 1);
1389 push @ints, $i if vec($vec, ++$i, 1);
1390 push @ints, $i if vec($vec, ++$i, 1);
1391 push @ints, $i if vec($vec, ++$i, 1);
1392 push @ints, $i if vec($vec, ++$i, 1);
1393 push @ints, $i if vec($vec, ++$i, 1);
1394 push @ints, $i if vec($vec, ++$i, 1);
1395 push @ints, $i if vec($vec, ++$i, 1);
1398 # This method is a fast general algorithm
1400 my $bits = unpack "b*", $vec;
1401 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1402 push @ints, pos $bits while($bits =~ /1/g);
1407 This method gets faster the more sparse the bit vector is.
1408 (Courtesy of Tim Bunce and Winfried Koenig.)
1410 Here's a demo on how to use vec():
1413 $vector = "\xff\x0f\xef\xfe";
1414 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1415 unpack("N", $vector), "\n";
1416 $is_set = vec($vector, 23, 1);
1417 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1435 my ($offset, $width, $value) = @_;
1437 vec($vector, $offset, $width) = $value;
1438 print "offset=$offset width=$width value=$value\n";
1444 my $bits = unpack("b*", $vector);
1448 print "vector length in bytes: ", length($vector), "\n";
1449 @bytes = unpack("A8" x length($vector), $bits);
1450 print "bits are: @bytes\n\n";
1453 =head2 Why does defined() return true on empty arrays and hashes?
1455 The short story is that you should probably only use defined on scalars or
1456 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1457 in the 5.004 release or later of Perl for more detail.
1459 =head1 Data: Hashes (Associative Arrays)
1461 =head2 How do I process an entire hash?
1463 Use the each() function (see L<perlfunc/each>) if you don't care
1464 whether it's sorted:
1466 while ( ($key, $value) = each %hash) {
1467 print "$key = $value\n";
1470 If you want it sorted, you'll have to use foreach() on the result of
1471 sorting the keys as shown in an earlier question.
1473 =head2 What happens if I add or remove keys from a hash while iterating over it?
1477 [lwall] In Perl 4, you were not allowed to modify a hash at all while
1478 iterating over it. In Perl 5 you can delete from it, but you still
1479 can't add to it, because that might cause a doubling of the hash table,
1480 in which half the entries get copied up to the new top half of the
1481 table, at which point you've totally bamboozled the iterator code.
1482 Even if the table doesn't double, there's no telling whether your new
1483 entry will be inserted before or after the current iterator position.
1485 Either treasure up your changes and make them after the iterator finishes
1486 or use keys to fetch all the old keys at once, and iterate over the list
1489 =head2 How do I look up a hash element by value?
1491 Create a reverse hash:
1493 %by_value = reverse %by_key;
1494 $key = $by_value{$value};
1496 That's not particularly efficient. It would be more space-efficient
1499 while (($key, $value) = each %by_key) {
1500 $by_value{$value} = $key;
1503 If your hash could have repeated values, the methods above will only find
1504 one of the associated keys. This may or may not worry you. If it does
1505 worry you, you can always reverse the hash into a hash of arrays instead:
1507 while (($key, $value) = each %by_key) {
1508 push @{$key_list_by_value{$value}}, $key;
1511 =head2 How can I know how many entries are in a hash?
1513 If you mean how many keys, then all you have to do is
1514 take the scalar sense of the keys() function:
1516 $num_keys = scalar keys %hash;
1518 The keys() function also resets the iterator, which in void context is
1519 faster for tied hashes than would be iterating through the whole
1520 hash, one key-value pair at a time.
1522 =head2 How do I sort a hash (optionally by value instead of key)?
1524 Internally, hashes are stored in a way that prevents you from imposing
1525 an order on key-value pairs. Instead, you have to sort a list of the
1528 @keys = sort keys %hash; # sorted by key
1530 $hash{$a} cmp $hash{$b}
1531 } keys %hash; # and by value
1533 Here we'll do a reverse numeric sort by value, and if two keys are
1534 identical, sort by length of key, or if that fails, by straight ASCII
1535 comparison of the keys (well, possibly modified by your locale--see
1539 $hash{$b} <=> $hash{$a}
1541 length($b) <=> length($a)
1546 =head2 How can I always keep my hash sorted?
1548 You can look into using the DB_File module and tie() using the
1549 $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
1550 The Tie::IxHash module from CPAN might also be instructive.
1552 =head2 What's the difference between "delete" and "undef" with hashes?
1554 Hashes are pairs of scalars: the first is the key, the second is the
1555 value. The key will be coerced to a string, although the value can be
1556 any kind of scalar: string, number, or reference. If a key C<$key> is
1557 present in the array, C<exists($key)> will return true. The value for
1558 a given key can be C<undef>, in which case C<$array{$key}> will be
1559 C<undef> while C<$exists{$key}> will return true. This corresponds to
1560 (C<$key>, C<undef>) being in the hash.
1562 Pictures help... here's the C<%ary> table:
1572 And these conditions hold
1576 defined $ary{'d'} is true
1577 defined $ary{'a'} is true
1578 exists $ary{'a'} is true (Perl5 only)
1579 grep ($_ eq 'a', keys %ary) is true
1585 your table now reads:
1596 and these conditions now hold; changes in caps:
1600 defined $ary{'d'} is true
1601 defined $ary{'a'} is FALSE
1602 exists $ary{'a'} is true (Perl5 only)
1603 grep ($_ eq 'a', keys %ary) is true
1605 Notice the last two: you have an undef value, but a defined key!
1611 your table now reads:
1620 and these conditions now hold; changes in caps:
1624 defined $ary{'d'} is true
1625 defined $ary{'a'} is false
1626 exists $ary{'a'} is FALSE (Perl5 only)
1627 grep ($_ eq 'a', keys %ary) is FALSE
1629 See, the whole entry is gone!
1631 =head2 Why don't my tied hashes make the defined/exists distinction?
1633 They may or may not implement the EXISTS() and DEFINED() methods
1634 differently. For example, there isn't the concept of undef with hashes
1635 that are tied to DBM* files. This means the true/false tables above
1636 will give different results when used on such a hash. It also means
1637 that exists and defined do the same thing with a DBM* file, and what
1638 they end up doing is not what they do with ordinary hashes.
1640 =head2 How do I reset an each() operation part-way through?
1642 Using C<keys %hash> in scalar context returns the number of keys in
1643 the hash I<and> resets the iterator associated with the hash. You may
1644 need to do this if you use C<last> to exit a loop early so that when you
1645 re-enter it, the hash iterator has been reset.
1647 =head2 How can I get the unique keys from two hashes?
1649 First you extract the keys from the hashes into lists, then solve
1650 the "removing duplicates" problem described above. For example:
1653 for $element (keys(%foo), keys(%bar)) {
1660 @uniq = keys %{{%foo,%bar}};
1662 Or if you really want to save space:
1665 while (defined ($key = each %foo)) {
1668 while (defined ($key = each %bar)) {
1673 =head2 How can I store a multidimensional array in a DBM file?
1675 Either stringify the structure yourself (no fun), or else
1676 get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1677 it on top of either DB_File or GDBM_File.
1679 =head2 How can I make my hash remember the order I put elements into it?
1681 Use the Tie::IxHash from CPAN.
1684 tie(%myhash, Tie::IxHash);
1685 for ($i=0; $i<20; $i++) {
1688 @keys = keys %myhash;
1689 # @keys = (0,1,2,3,...)
1691 =head2 Why does passing a subroutine an undefined element in a hash create it?
1693 If you say something like:
1695 somefunc($hash{"nonesuch key here"});
1697 Then that element "autovivifies"; that is, it springs into existence
1698 whether you store something there or not. That's because functions
1699 get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1700 it has to be ready to write it back into the caller's version.
1702 This has been fixed as of Perl5.004.
1704 Normally, merely accessing a key's value for a nonexistent key does
1705 I<not> cause that key to be forever there. This is different than
1708 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
1710 Usually a hash ref, perhaps like this:
1715 TITLE => "deputy peon",
1718 PALS => [ "Norbert", "Rhys", "Phineas"],
1721 References are documented in L<perlref> and the upcoming L<perlreftut>.
1722 Examples of complex data structures are given in L<perldsc> and
1723 L<perllol>. Examples of structures and object-oriented classes are
1726 =head2 How can I use a reference as a hash key?
1728 You can't do this directly, but you could use the standard Tie::Refhash
1729 module distributed with Perl.
1733 =head2 How do I handle binary data correctly?
1735 Perl is binary clean, so this shouldn't be a problem. For example,
1736 this works fine (assuming the files are found):
1738 if (`cat /vmunix` =~ /gzip/) {
1739 print "Your kernel is GNU-zip enabled!\n";
1742 On less elegant (read: Byzantine) systems, however, you have
1743 to play tedious games with "text" versus "binary" files. See
1744 L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1745 systems are curses out of Microsoft, who seem to be committed to putting
1746 the backward into backward compatibility.
1748 If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1750 If you want to deal with multibyte characters, however, there are
1751 some gotchas. See the section on Regular Expressions.
1753 =head2 How do I determine whether a scalar is a number/whole/integer/float?
1755 Assuming that you don't care about IEEE notations like "NaN" or
1756 "Infinity", you probably just want to use a regular expression.
1758 if (/\D/) { print "has nondigits\n" }
1759 if (/^\d+$/) { print "is a whole number\n" }
1760 if (/^-?\d+$/) { print "is an integer\n" }
1761 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1762 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1763 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1764 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1765 { print "a C float" }
1767 If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1768 function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1769 wrapper function for more convenient access. This function takes
1770 a string and returns the number it found, or C<undef> for input that
1771 isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1772 if you just want to say, ``Is this a float?''
1775 use POSIX qw(strtod);
1780 my($num, $unparsed) = strtod($str);
1781 if (($str eq '') || ($unparsed != 0) || $!) {
1788 sub is_numeric { defined getnum($_[0]) }
1790 Or you could check out the String::Scanf module on CPAN instead. The
1791 POSIX module (part of the standard Perl distribution) provides the
1792 C<strtod> and C<strtol> for converting strings to double and longs,
1795 =head2 How do I keep persistent data across program calls?
1797 For some specific applications, you can use one of the DBM modules.
1798 See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
1799 Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
1800 Storable is part of the standard distribution. Here's one example using
1801 Storable's C<store> and C<retrieve> functions:
1804 store(\%hash, "filename");
1807 $href = retrieve("filename"); # by ref
1808 %hash = %{ retrieve("filename") }; # direct to hash
1810 =head2 How do I print out or copy a recursive data structure?
1812 The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1813 for printing out data structures. The Storable module, found on CPAN,
1814 provides a function called C<dclone> that recursively copies its argument.
1816 use Storable qw(dclone);
1819 Where $r1 can be a reference to any kind of data structure you'd like.
1820 It will be deeply copied. Because C<dclone> takes and returns references,
1821 you'd have to add extra punctuation if you had a hash of arrays that
1824 %newhash = %{ dclone(\%oldhash) };
1826 =head2 How do I define methods for every class/object?
1828 Use the UNIVERSAL class (see L<UNIVERSAL>).
1830 =head2 How do I verify a credit card checksum?
1832 Get the Business::CreditCard module from CPAN.
1834 =head2 How do I pack arrays of doubles or floats for XS code?
1836 The kgbpack.c code in the PGPLOT module on CPAN does just this.
1837 If you're doing a lot of float or double processing, consider using
1838 the PDL module from CPAN instead--it makes number-crunching easy.
1840 =head1 AUTHOR AND COPYRIGHT
1842 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1843 All rights reserved.
1845 When included as part of the Standard Version of Perl, or as part of
1846 its complete documentation whether printed or otherwise, this work
1847 may be distributed only under the terms of Perl's Artistic License.
1848 Any distribution of this file or derivatives thereof I<outside>
1849 of that package require that special arrangements be made with
1852 Irrespective of its distribution, all code examples in this file
1853 are hereby placed into the public domain. You are permitted and
1854 encouraged to use this code in your own programs for fun
1855 or for profit as you see fit. A simple comment in the code giving
1856 credit would be courteous but is not required.