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1=head1 NAME
2
d92eb7b0 3perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
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4
5=head1 DESCRIPTION
6
a6dd486b 7The section of the FAQ answers questions related to the manipulation
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8of data as numbers, dates, strings, arrays, hashes, and miscellaneous
9data issues.
10
11=head1 Data: Numbers
12
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13=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
14
5a964f20 15The infinite set that a mathematician thinks of as the real numbers can
a6dd486b 16only be approximated on a computer, since the computer only has a finite
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17number of bits to store an infinite number of, um, numbers.
18
46fc3d4c 19Internally, your computer represents floating-point numbers in binary.
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20Floating-point numbers read in from a file or appearing as literals
21in your program are converted from their decimal floating-point
a6dd486b 22representation (eg, 19.95) to an internal binary representation.
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23
24However, 19.95 can't be precisely represented as a binary
25floating-point number, just like 1/3 can't be exactly represented as a
26decimal floating-point number. The computer's binary representation
27of 19.95, therefore, isn't exactly 19.95.
28
29When a floating-point number gets printed, the binary floating-point
30representation is converted back to decimal. These decimal numbers
31are displayed in either the format you specify with printf(), or the
a6dd486b 32current output format for numbers. (See L<perlvar/"$#"> if you use
46fc3d4c 33print. C<$#> has a different default value in Perl5 than it did in
87275199 34Perl4. Changing C<$#> yourself is deprecated.)
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35
36This affects B<all> computer languages that represent decimal
37floating-point numbers in binary, not just Perl. Perl provides
38arbitrary-precision decimal numbers with the Math::BigFloat module
39(part of the standard Perl distribution), but mathematical operations
40are consequently slower.
41
80ba158a 42If precision is important, such as when dealing with money, it's good
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43to work with integers and then divide at the last possible moment.
44For example, work in pennies (1995) instead of dollars and cents
6b927632 45(19.95) and divide by 100 at the end.
1affb2ee 46
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47To get rid of the superfluous digits, just use a format (eg,
48C<printf("%.2f", 19.95)>) to get the required precision.
65acb1b1 49See L<perlop/"Floating-point Arithmetic">.
46fc3d4c 50
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51=head2 Why isn't my octal data interpreted correctly?
52
53Perl only understands octal and hex numbers as such when they occur
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54as literals in your program. Octal literals in perl must start with
55a leading "0" and hexadecimal literals must start with a leading "0x".
56If they are read in from somewhere and assigned, no automatic
57conversion takes place. You must explicitly use oct() or hex() if you
58want the values converted to decimal. oct() interprets
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59both hex ("0x350") numbers and octal ones ("0350" or even without the
60leading "0", like "377"), while hex() only converts hexadecimal ones,
61with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
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62The inverse mapping from decimal to octal can be done with either the
63"%o" or "%O" sprintf() formats. To get from decimal to hex try either
64the "%x" or the "%X" formats to sprintf().
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65
66This problem shows up most often when people try using chmod(), mkdir(),
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67umask(), or sysopen(), which by widespread tradition typically take
68permissions in octal.
68dc0745 69
33ce146f 70 chmod(644, $file); # WRONG
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71 chmod(0644, $file); # right
72
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73Note the mistake in the first line was specifying the decimal literal
74644, rather than the intended octal literal 0644. The problem can
75be seen with:
76
434f7166 77 printf("%#o",644); # prints 01204
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78
79Surely you had not intended C<chmod(01204, $file);> - did you? If you
80want to use numeric literals as arguments to chmod() et al. then please
81try to express them as octal constants, that is with a leading zero and
82with the following digits restricted to the set 0..7.
83
65acb1b1 84=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
68dc0745 85
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86Remember that int() merely truncates toward 0. For rounding to a
87certain number of digits, sprintf() or printf() is usually the easiest
88route.
89
90 printf("%.3f", 3.1415926535); # prints 3.142
68dc0745 91
87275199 92The POSIX module (part of the standard Perl distribution) implements
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93ceil(), floor(), and a number of other mathematical and trigonometric
94functions.
95
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96 use POSIX;
97 $ceil = ceil(3.5); # 4
98 $floor = floor(3.5); # 3
99
a6dd486b 100In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
87275199 101module. With 5.004, the Math::Trig module (part of the standard Perl
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102distribution) implements the trigonometric functions. Internally it
103uses the Math::Complex module and some functions can break out from
104the real axis into the complex plane, for example the inverse sine of
1052.
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106
107Rounding in financial applications can have serious implications, and
108the rounding method used should be specified precisely. In these
109cases, it probably pays not to trust whichever system rounding is
110being used by Perl, but to instead implement the rounding function you
111need yourself.
112
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113To see why, notice how you'll still have an issue on half-way-point
114alternation:
115
116 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
117
118 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
119 0.8 0.8 0.9 0.9 1.0 1.0
120
121Don't blame Perl. It's the same as in C. IEEE says we have to do this.
122Perl numbers whose absolute values are integers under 2**31 (on 32 bit
123machines) will work pretty much like mathematical integers. Other numbers
124are not guaranteed.
125
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126=head2 How do I convert bits into ints?
127
92c2ed05 128To turn a string of 1s and 0s like C<10110110> into a scalar containing
d92eb7b0 129its binary value, use the pack() and unpack() functions (documented in
87275199 130L<perlfunc/"pack"> and L<perlfunc/"unpack">):
68dc0745 131
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132 $decimal = unpack('c', pack('B8', '10110110'));
133
134This packs the string C<10110110> into an eight bit binary structure.
87275199 135This is then unpacked as a character, which returns its ordinal value.
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136
137This does the same thing:
138
139 $decimal = ord(pack('B8', '10110110'));
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140
141Here's an example of going the other way:
142
d92eb7b0 143 $binary_string = unpack('B*', "\x29");
68dc0745 144
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145=head2 Why doesn't & work the way I want it to?
146
147The behavior of binary arithmetic operators depends on whether they're
148used on numbers or strings. The operators treat a string as a series
149of bits and work with that (the string C<"3"> is the bit pattern
150C<00110011>). The operators work with the binary form of a number
151(the number C<3> is treated as the bit pattern C<00000011>).
152
153So, saying C<11 & 3> performs the "and" operation on numbers (yielding
154C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
155(yielding C<"1">).
156
157Most problems with C<&> and C<|> arise because the programmer thinks
158they have a number but really it's a string. The rest arise because
159the programmer says:
160
161 if ("\020\020" & "\101\101") {
162 # ...
163 }
164
165but a string consisting of two null bytes (the result of C<"\020\020"
166& "\101\101">) is not a false value in Perl. You need:
167
168 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
169 # ...
170 }
171
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172=head2 How do I multiply matrices?
173
174Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
175or the PDL extension (also available from CPAN).
176
177=head2 How do I perform an operation on a series of integers?
178
179To call a function on each element in an array, and collect the
180results, use:
181
182 @results = map { my_func($_) } @array;
183
184For example:
185
186 @triple = map { 3 * $_ } @single;
187
188To call a function on each element of an array, but ignore the
189results:
190
191 foreach $iterator (@array) {
65acb1b1 192 some_func($iterator);
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193 }
194
195To call a function on each integer in a (small) range, you B<can> use:
196
65acb1b1 197 @results = map { some_func($_) } (5 .. 25);
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198
199but you should be aware that the C<..> operator creates an array of
200all integers in the range. This can take a lot of memory for large
201ranges. Instead use:
202
203 @results = ();
204 for ($i=5; $i < 500_005; $i++) {
65acb1b1 205 push(@results, some_func($i));
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206 }
207
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208This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
209loop will iterate over the range, without creating the entire range.
210
211 for my $i (5 .. 500_005) {
212 push(@results, some_func($i));
213 }
214
215will not create a list of 500,000 integers.
216
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217=head2 How can I output Roman numerals?
218
219Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
220
221=head2 Why aren't my random numbers random?
222
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223If you're using a version of Perl before 5.004, you must call C<srand>
224once at the start of your program to seed the random number generator.
2255.004 and later automatically call C<srand> at the beginning. Don't
226call C<srand> more than once--you make your numbers less random, rather
227than more.
92c2ed05 228
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229Computers are good at being predictable and bad at being random
230(despite appearances caused by bugs in your programs :-).
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231http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
232Phoenix, talks more about this. John von Neumann said, ``Anyone who
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233attempts to generate random numbers by deterministic means is, of
234course, living in a state of sin.''
235
236If you want numbers that are more random than C<rand> with C<srand>
237provides, you should also check out the Math::TrulyRandom module from
238CPAN. It uses the imperfections in your system's timer to generate
239random numbers, but this takes quite a while. If you want a better
92c2ed05 240pseudorandom generator than comes with your operating system, look at
65acb1b1 241``Numerical Recipes in C'' at http://www.nr.com/ .
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242
243=head1 Data: Dates
244
245=head2 How do I find the week-of-the-year/day-of-the-year?
246
247The day of the year is in the array returned by localtime() (see
248L<perlfunc/"localtime">):
249
250 $day_of_year = (localtime(time()))[7];
251
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252=head2 How do I find the current century or millennium?
253
254Use the following simple functions:
255
256 sub get_century {
257 return int((((localtime(shift || time))[5] + 1999))/100);
258 }
259 sub get_millennium {
260 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
261 }
262
263On some systems, you'll find that the POSIX module's strftime() function
264has been extended in a non-standard way to use a C<%C> format, which they
265sometimes claim is the "century". It isn't, because on most such systems,
266this is only the first two digits of the four-digit year, and thus cannot
267be used to reliably determine the current century or millennium.
268
92c2ed05 269=head2 How can I compare two dates and find the difference?
68dc0745 270
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271If you're storing your dates as epoch seconds then simply subtract one
272from the other. If you've got a structured date (distinct year, day,
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273month, hour, minute, seconds values), then for reasons of accessibility,
274simplicity, and efficiency, merely use either timelocal or timegm (from
275the Time::Local module in the standard distribution) to reduce structured
276dates to epoch seconds. However, if you don't know the precise format of
277your dates, then you should probably use either of the Date::Manip and
278Date::Calc modules from CPAN before you go hacking up your own parsing
279routine to handle arbitrary date formats.
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280
281=head2 How can I take a string and turn it into epoch seconds?
282
283If it's a regular enough string that it always has the same format,
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284you can split it up and pass the parts to C<timelocal> in the standard
285Time::Local module. Otherwise, you should look into the Date::Calc
286and Date::Manip modules from CPAN.
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287
288=head2 How can I find the Julian Day?
289
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290Use the Time::JulianDay module (part of the Time-modules bundle
291available from CPAN.)
d92eb7b0 292
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293Before you immerse yourself too deeply in this, be sure to verify that
294it is the I<Julian> Day you really want. Are you interested in a way
295of getting serial days so that you just can tell how many days they
296are apart or so that you can do also other date arithmetic? If you
d92eb7b0 297are interested in performing date arithmetic, this can be done using
2a2bf5f4 298modules Date::Manip or Date::Calc.
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299
300There is too many details and much confusion on this issue to cover in
301this FAQ, but the term is applied (correctly) to a calendar now
302supplanted by the Gregorian Calendar, with the Julian Calendar failing
303to adjust properly for leap years on centennial years (among other
304annoyances). The term is also used (incorrectly) to mean: [1] days in
305the Gregorian Calendar; and [2] days since a particular starting time
306or `epoch', usually 1970 in the Unix world and 1980 in the
307MS-DOS/Windows world. If you find that it is not the first meaning
308that you really want, then check out the Date::Manip and Date::Calc
309modules. (Thanks to David Cassell for most of this text.)
be94a901 310
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311=head2 How do I find yesterday's date?
312
313The C<time()> function returns the current time in seconds since the
d92eb7b0 314epoch. Take twenty-four hours off that:
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315
316 $yesterday = time() - ( 24 * 60 * 60 );
317
318Then you can pass this to C<localtime()> and get the individual year,
319month, day, hour, minute, seconds values.
320
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321Note very carefully that the code above assumes that your days are
322twenty-four hours each. For most people, there are two days a year
323when they aren't: the switch to and from summer time throws this off.
324A solution to this issue is offered by Russ Allbery.
325
326 sub yesterday {
327 my $now = defined $_[0] ? $_[0] : time;
328 my $then = $now - 60 * 60 * 24;
329 my $ndst = (localtime $now)[8] > 0;
330 my $tdst = (localtime $then)[8] > 0;
331 $then - ($tdst - $ndst) * 60 * 60;
332 }
333 # Should give you "this time yesterday" in seconds since epoch relative to
334 # the first argument or the current time if no argument is given and
335 # suitable for passing to localtime or whatever else you need to do with
336 # it. $ndst is whether we're currently in daylight savings time; $tdst is
337 # whether the point 24 hours ago was in daylight savings time. If $tdst
338 # and $ndst are the same, a boundary wasn't crossed, and the correction
339 # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
340 # from yesterday's time since we gained an extra hour while going off
341 # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
342 # negative hour (add an hour) to yesterday's time since we lost an hour.
343 #
344 # All of this is because during those days when one switches off or onto
345 # DST, a "day" isn't 24 hours long; it's either 23 or 25.
346 #
347 # The explicit settings of $ndst and $tdst are necessary because localtime
348 # only says it returns the system tm struct, and the system tm struct at
87275199 349 # least on Solaris doesn't guarantee any particular positive value (like,
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350 # say, 1) for isdst, just a positive value. And that value can
351 # potentially be negative, if DST information isn't available (this sub
352 # just treats those cases like no DST).
353 #
354 # Note that between 2am and 3am on the day after the time zone switches
355 # off daylight savings time, the exact hour of "yesterday" corresponding
356 # to the current hour is not clearly defined. Note also that if used
357 # between 2am and 3am the day after the change to daylight savings time,
358 # the result will be between 3am and 4am of the previous day; it's
359 # arguable whether this is correct.
360 #
361 # This sub does not attempt to deal with leap seconds (most things don't).
362 #
363 # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
364 # This code is in the public domain
365
87275199 366=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
68dc0745 367
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368Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
369Y2K compliant (whatever that means). The programmers you've hired to
370use it, however, probably are not.
371
372Long answer: The question belies a true understanding of the issue.
373Perl is just as Y2K compliant as your pencil--no more, and no less.
374Can you use your pencil to write a non-Y2K-compliant memo? Of course
375you can. Is that the pencil's fault? Of course it isn't.
92c2ed05 376
87275199 377The date and time functions supplied with Perl (gmtime and localtime)
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378supply adequate information to determine the year well beyond 2000
379(2038 is when trouble strikes for 32-bit machines). The year returned
90fdbbb7 380by these functions when used in a list context is the year minus 1900.
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381For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
382number. To avoid the year 2000 problem simply do not treat the year as
383a 2-digit number. It isn't.
68dc0745 384
5a964f20 385When gmtime() and localtime() are used in scalar context they return
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386a timestamp string that contains a fully-expanded year. For example,
387C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
3882001". There's no year 2000 problem here.
389
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390That doesn't mean that Perl can't be used to create non-Y2K compliant
391programs. It can. But so can your pencil. It's the fault of the user,
392not the language. At the risk of inflaming the NRA: ``Perl doesn't
393break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
394a longer exposition.
395
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396=head1 Data: Strings
397
398=head2 How do I validate input?
399
400The answer to this question is usually a regular expression, perhaps
5a964f20 401with auxiliary logic. See the more specific questions (numbers, mail
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402addresses, etc.) for details.
403
404=head2 How do I unescape a string?
405
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406It depends just what you mean by ``escape''. URL escapes are dealt
407with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
a6dd486b 408character are removed with
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409
410 s/\\(.)/$1/g;
411
92c2ed05 412This won't expand C<"\n"> or C<"\t"> or any other special escapes.
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413
414=head2 How do I remove consecutive pairs of characters?
415
92c2ed05 416To turn C<"abbcccd"> into C<"abccd">:
68dc0745 417
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418 s/(.)\1/$1/g; # add /s to include newlines
419
420Here's a solution that turns "abbcccd" to "abcd":
421
422 y///cs; # y == tr, but shorter :-)
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423
424=head2 How do I expand function calls in a string?
425
426This is documented in L<perlref>. In general, this is fraught with
427quoting and readability problems, but it is possible. To interpolate
5a964f20 428a subroutine call (in list context) into a string:
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429
430 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
431
432If you prefer scalar context, similar chicanery is also useful for
433arbitrary expressions:
434
435 print "That yields ${\($n + 5)} widgets\n";
436
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437Version 5.004 of Perl had a bug that gave list context to the
438expression in C<${...}>, but this is fixed in version 5.005.
439
440See also ``How can I expand variables in text strings?'' in this
441section of the FAQ.
46fc3d4c 442
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443=head2 How do I find matching/nesting anything?
444
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445This isn't something that can be done in one regular expression, no
446matter how complicated. To find something between two single
447characters, a pattern like C</x([^x]*)x/> will get the intervening
448bits in $1. For multiple ones, then something more like
449C</alpha(.*?)omega/> would be needed. But none of these deals with
450nested patterns, nor can they. For that you'll have to write a
451parser.
452
453If you are serious about writing a parser, there are a number of
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454modules or oddities that will make your life a lot easier. There are
455the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
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456and the byacc program. Starting from perl 5.8 the Text::Balanced
457is part of the standard distribution.
68dc0745 458
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459One simple destructive, inside-out approach that you might try is to
460pull out the smallest nesting parts one at a time:
5a964f20 461
d92eb7b0 462 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
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463 # do something with $1
464 }
465
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466A more complicated and sneaky approach is to make Perl's regular
467expression engine do it for you. This is courtesy Dean Inada, and
468rather has the nature of an Obfuscated Perl Contest entry, but it
469really does work:
470
471 # $_ contains the string to parse
472 # BEGIN and END are the opening and closing markers for the
473 # nested text.
c47ff5f1 474
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475 @( = ('(','');
476 @) = (')','');
477 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
478 @$ = (eval{/$re/},$@!~/unmatched/);
479 print join("\n",@$[0..$#$]) if( $$[-1] );
480
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481=head2 How do I reverse a string?
482
5a964f20 483Use reverse() in scalar context, as documented in
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484L<perlfunc/reverse>.
485
486 $reversed = reverse $string;
487
488=head2 How do I expand tabs in a string?
489
5a964f20 490You can do it yourself:
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491
492 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
493
87275199 494Or you can just use the Text::Tabs module (part of the standard Perl
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495distribution).
496
497 use Text::Tabs;
498 @expanded_lines = expand(@lines_with_tabs);
499
500=head2 How do I reformat a paragraph?
501
87275199 502Use Text::Wrap (part of the standard Perl distribution):
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503
504 use Text::Wrap;
505 print wrap("\t", ' ', @paragraphs);
506
92c2ed05 507The paragraphs you give to Text::Wrap should not contain embedded
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508newlines. Text::Wrap doesn't justify the lines (flush-right).
509
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510=head2 How can I access/change the first N letters of a string?
511
512There are many ways. If you just want to grab a copy, use
92c2ed05 513substr():
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514
515 $first_byte = substr($a, 0, 1);
516
517If you want to modify part of a string, the simplest way is often to
518use substr() as an lvalue:
519
520 substr($a, 0, 3) = "Tom";
521
92c2ed05 522Although those with a pattern matching kind of thought process will
a6dd486b 523likely prefer
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524
525 $a =~ s/^.../Tom/;
526
527=head2 How do I change the Nth occurrence of something?
528
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529You have to keep track of N yourself. For example, let's say you want
530to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
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531C<"whosoever"> or C<"whomsoever">, case insensitively. These
532all assume that $_ contains the string to be altered.
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533
534 $count = 0;
535 s{((whom?)ever)}{
536 ++$count == 5 # is it the 5th?
537 ? "${2}soever" # yes, swap
538 : $1 # renege and leave it there
d92eb7b0 539 }ige;
68dc0745 540
5a964f20
TC
541In the more general case, you can use the C</g> modifier in a C<while>
542loop, keeping count of matches.
543
544 $WANT = 3;
545 $count = 0;
d92eb7b0 546 $_ = "One fish two fish red fish blue fish";
5a964f20
TC
547 while (/(\w+)\s+fish\b/gi) {
548 if (++$count == $WANT) {
549 print "The third fish is a $1 one.\n";
5a964f20
TC
550 }
551 }
552
92c2ed05 553That prints out: C<"The third fish is a red one."> You can also use a
5a964f20
TC
554repetition count and repeated pattern like this:
555
556 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
557
68dc0745
PP
558=head2 How can I count the number of occurrences of a substring within a string?
559
a6dd486b 560There are a number of ways, with varying efficiency. If you want a
68dc0745
PP
561count of a certain single character (X) within a string, you can use the
562C<tr///> function like so:
563
368c9434 564 $string = "ThisXlineXhasXsomeXx'sXinXit";
68dc0745 565 $count = ($string =~ tr/X//);
d92eb7b0 566 print "There are $count X characters in the string";
68dc0745
PP
567
568This is fine if you are just looking for a single character. However,
569if you are trying to count multiple character substrings within a
570larger string, C<tr///> won't work. What you can do is wrap a while()
571loop around a global pattern match. For example, let's count negative
572integers:
573
574 $string = "-9 55 48 -2 23 -76 4 14 -44";
575 while ($string =~ /-\d+/g) { $count++ }
576 print "There are $count negative numbers in the string";
577
578=head2 How do I capitalize all the words on one line?
579
580To make the first letter of each word upper case:
3fe9a6f1 581
68dc0745
PP
582 $line =~ s/\b(\w)/\U$1/g;
583
46fc3d4c 584This has the strange effect of turning "C<don't do it>" into "C<Don'T
a6dd486b
JB
585Do It>". Sometimes you might want this. Other times you might need a
586more thorough solution (Suggested by brian d. foy):
46fc3d4c
PP
587
588 $string =~ s/ (
589 (^\w) #at the beginning of the line
590 | # or
591 (\s\w) #preceded by whitespace
592 )
593 /\U$1/xg;
594 $string =~ /([\w']+)/\u\L$1/g;
595
68dc0745 596To make the whole line upper case:
3fe9a6f1 597
68dc0745
PP
598 $line = uc($line);
599
600To force each word to be lower case, with the first letter upper case:
3fe9a6f1 601
68dc0745
PP
602 $line =~ s/(\w+)/\u\L$1/g;
603
5a964f20
TC
604You can (and probably should) enable locale awareness of those
605characters by placing a C<use locale> pragma in your program.
92c2ed05 606See L<perllocale> for endless details on locales.
5a964f20 607
65acb1b1 608This is sometimes referred to as putting something into "title
d92eb7b0 609case", but that's not quite accurate. Consider the proper
65acb1b1
TC
610capitalization of the movie I<Dr. Strangelove or: How I Learned to
611Stop Worrying and Love the Bomb>, for example.
612
68dc0745
PP
613=head2 How can I split a [character] delimited string except when inside
614[character]? (Comma-separated files)
615
616Take the example case of trying to split a string that is comma-separated
617into its different fields. (We'll pretend you said comma-separated, not
618comma-delimited, which is different and almost never what you mean.) You
619can't use C<split(/,/)> because you shouldn't split if the comma is inside
620quotes. For example, take a data line like this:
621
622 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
623
624Due to the restriction of the quotes, this is a fairly complex
625problem. Thankfully, we have Jeffrey Friedl, author of a highly
626recommended book on regular expressions, to handle these for us. He
627suggests (assuming your string is contained in $text):
628
629 @new = ();
630 push(@new, $+) while $text =~ m{
631 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
632 | ([^,]+),?
633 | ,
634 }gx;
635 push(@new, undef) if substr($text,-1,1) eq ',';
636
46fc3d4c
PP
637If you want to represent quotation marks inside a
638quotation-mark-delimited field, escape them with backslashes (eg,
2ceaccd7 639C<"like \"this\"">. Unescaping them is a task addressed earlier in
46fc3d4c
PP
640this section.
641
87275199 642Alternatively, the Text::ParseWords module (part of the standard Perl
68dc0745
PP
643distribution) lets you say:
644
645 use Text::ParseWords;
646 @new = quotewords(",", 0, $text);
647
a6dd486b 648There's also a Text::CSV (Comma-Separated Values) module on CPAN.
65acb1b1 649
68dc0745
PP
650=head2 How do I strip blank space from the beginning/end of a string?
651
a6dd486b 652Although the simplest approach would seem to be
68dc0745
PP
653
654 $string =~ s/^\s*(.*?)\s*$/$1/;
655
a6dd486b 656not only is this unnecessarily slow and destructive, it also fails with
d92eb7b0 657embedded newlines. It is much faster to do this operation in two steps:
68dc0745
PP
658
659 $string =~ s/^\s+//;
660 $string =~ s/\s+$//;
661
662Or more nicely written as:
663
664 for ($string) {
665 s/^\s+//;
666 s/\s+$//;
667 }
668
5e3006a4 669This idiom takes advantage of the C<foreach> loop's aliasing
5a964f20
TC
670behavior to factor out common code. You can do this
671on several strings at once, or arrays, or even the
d92eb7b0 672values of a hash if you use a slice:
5a964f20
TC
673
674 # trim whitespace in the scalar, the array,
675 # and all the values in the hash
676 foreach ($scalar, @array, @hash{keys %hash}) {
677 s/^\s+//;
678 s/\s+$//;
679 }
680
65acb1b1
TC
681=head2 How do I pad a string with blanks or pad a number with zeroes?
682
d92eb7b0
GS
683(This answer contributed by Uri Guttman, with kibitzing from
684Bart Lateur.)
65acb1b1
TC
685
686In the following examples, C<$pad_len> is the length to which you wish
d92eb7b0
GS
687to pad the string, C<$text> or C<$num> contains the string to be padded,
688and C<$pad_char> contains the padding character. You can use a single
689character string constant instead of the C<$pad_char> variable if you
690know what it is in advance. And in the same way you can use an integer in
691place of C<$pad_len> if you know the pad length in advance.
65acb1b1 692
d92eb7b0
GS
693The simplest method uses the C<sprintf> function. It can pad on the left
694or right with blanks and on the left with zeroes and it will not
695truncate the result. The C<pack> function can only pad strings on the
696right with blanks and it will truncate the result to a maximum length of
697C<$pad_len>.
65acb1b1 698
d92eb7b0
GS
699 # Left padding a string with blanks (no truncation):
700 $padded = sprintf("%${pad_len}s", $text);
65acb1b1 701
d92eb7b0
GS
702 # Right padding a string with blanks (no truncation):
703 $padded = sprintf("%-${pad_len}s", $text);
65acb1b1 704
d92eb7b0
GS
705 # Left padding a number with 0 (no truncation):
706 $padded = sprintf("%0${pad_len}d", $num);
65acb1b1 707
d92eb7b0
GS
708 # Right padding a string with blanks using pack (will truncate):
709 $padded = pack("A$pad_len",$text);
65acb1b1 710
d92eb7b0
GS
711If you need to pad with a character other than blank or zero you can use
712one of the following methods. They all generate a pad string with the
713C<x> operator and combine that with C<$text>. These methods do
714not truncate C<$text>.
65acb1b1 715
d92eb7b0 716Left and right padding with any character, creating a new string:
65acb1b1 717
d92eb7b0
GS
718 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
719 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
65acb1b1 720
d92eb7b0 721Left and right padding with any character, modifying C<$text> directly:
65acb1b1 722
d92eb7b0
GS
723 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
724 $text .= $pad_char x ( $pad_len - length( $text ) );
65acb1b1 725
68dc0745
PP
726=head2 How do I extract selected columns from a string?
727
728Use substr() or unpack(), both documented in L<perlfunc>.
5a964f20
TC
729If you prefer thinking in terms of columns instead of widths,
730you can use this kind of thing:
731
732 # determine the unpack format needed to split Linux ps output
733 # arguments are cut columns
734 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
735
736 sub cut2fmt {
737 my(@positions) = @_;
738 my $template = '';
739 my $lastpos = 1;
740 for my $place (@positions) {
741 $template .= "A" . ($place - $lastpos) . " ";
742 $lastpos = $place;
743 }
744 $template .= "A*";
745 return $template;
746 }
68dc0745
PP
747
748=head2 How do I find the soundex value of a string?
749
87275199 750Use the standard Text::Soundex module distributed with Perl.
a6dd486b 751Before you do so, you may want to determine whether `soundex' is in
d92eb7b0
GS
752fact what you think it is. Knuth's soundex algorithm compresses words
753into a small space, and so it does not necessarily distinguish between
754two words which you might want to appear separately. For example, the
755last names `Knuth' and `Kant' are both mapped to the soundex code K530.
756If Text::Soundex does not do what you are looking for, you might want
757to consider the String::Approx module available at CPAN.
68dc0745
PP
758
759=head2 How can I expand variables in text strings?
760
761Let's assume that you have a string like:
762
763 $text = 'this has a $foo in it and a $bar';
5a964f20
TC
764
765If those were both global variables, then this would
766suffice:
767
65acb1b1 768 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
68dc0745 769
5a964f20
TC
770But since they are probably lexicals, or at least, they could
771be, you'd have to do this:
68dc0745
PP
772
773 $text =~ s/(\$\w+)/$1/eeg;
65acb1b1 774 die if $@; # needed /ee, not /e
68dc0745 775
5a964f20
TC
776It's probably better in the general case to treat those
777variables as entries in some special hash. For example:
778
779 %user_defs = (
780 foo => 23,
781 bar => 19,
782 );
783 $text =~ s/\$(\w+)/$user_defs{$1}/g;
68dc0745 784
92c2ed05 785See also ``How do I expand function calls in a string?'' in this section
46fc3d4c
PP
786of the FAQ.
787
68dc0745
PP
788=head2 What's wrong with always quoting "$vars"?
789
a6dd486b
JB
790The problem is that those double-quotes force stringification--
791coercing numbers and references into strings--even when you
792don't want them to be strings. Think of it this way: double-quote
65acb1b1
TC
793expansion is used to produce new strings. If you already
794have a string, why do you need more?
68dc0745
PP
795
796If you get used to writing odd things like these:
797
798 print "$var"; # BAD
799 $new = "$old"; # BAD
800 somefunc("$var"); # BAD
801
802You'll be in trouble. Those should (in 99.8% of the cases) be
803the simpler and more direct:
804
805 print $var;
806 $new = $old;
807 somefunc($var);
808
809Otherwise, besides slowing you down, you're going to break code when
810the thing in the scalar is actually neither a string nor a number, but
811a reference:
812
813 func(\@array);
814 sub func {
815 my $aref = shift;
816 my $oref = "$aref"; # WRONG
817 }
818
819You can also get into subtle problems on those few operations in Perl
820that actually do care about the difference between a string and a
821number, such as the magical C<++> autoincrement operator or the
822syscall() function.
823
5a964f20
TC
824Stringification also destroys arrays.
825
826 @lines = `command`;
827 print "@lines"; # WRONG - extra blanks
828 print @lines; # right
829
c47ff5f1 830=head2 Why don't my <<HERE documents work?
68dc0745
PP
831
832Check for these three things:
833
834=over 4
835
836=item 1. There must be no space after the << part.
837
838=item 2. There (probably) should be a semicolon at the end.
839
840=item 3. You can't (easily) have any space in front of the tag.
841
842=back
843
5a964f20
TC
844If you want to indent the text in the here document, you
845can do this:
846
847 # all in one
848 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
849 your text
850 goes here
851 HERE_TARGET
852
853But the HERE_TARGET must still be flush against the margin.
854If you want that indented also, you'll have to quote
855in the indentation.
856
857 ($quote = <<' FINIS') =~ s/^\s+//gm;
858 ...we will have peace, when you and all your works have
859 perished--and the works of your dark master to whom you
860 would deliver us. You are a liar, Saruman, and a corrupter
861 of men's hearts. --Theoden in /usr/src/perl/taint.c
862 FINIS
863 $quote =~ s/\s*--/\n--/;
864
865A nice general-purpose fixer-upper function for indented here documents
866follows. It expects to be called with a here document as its argument.
867It looks to see whether each line begins with a common substring, and
a6dd486b
JB
868if so, strips that substring off. Otherwise, it takes the amount of leading
869whitespace found on the first line and removes that much off each
5a964f20
TC
870subsequent line.
871
872 sub fix {
873 local $_ = shift;
a6dd486b 874 my ($white, $leader); # common whitespace and common leading string
5a964f20
TC
875 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
876 ($white, $leader) = ($2, quotemeta($1));
877 } else {
878 ($white, $leader) = (/^(\s+)/, '');
879 }
880 s/^\s*?$leader(?:$white)?//gm;
881 return $_;
882 }
883
c8db1d39 884This works with leading special strings, dynamically determined:
5a964f20
TC
885
886 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
887 @@@ int
888 @@@ runops() {
889 @@@ SAVEI32(runlevel);
890 @@@ runlevel++;
d92eb7b0 891 @@@ while ( op = (*op->op_ppaddr)() );
5a964f20
TC
892 @@@ TAINT_NOT;
893 @@@ return 0;
894 @@@ }
895 MAIN_INTERPRETER_LOOP
896
a6dd486b 897Or with a fixed amount of leading whitespace, with remaining
5a964f20
TC
898indentation correctly preserved:
899
900 $poem = fix<<EVER_ON_AND_ON;
901 Now far ahead the Road has gone,
902 And I must follow, if I can,
903 Pursuing it with eager feet,
904 Until it joins some larger way
905 Where many paths and errands meet.
906 And whither then? I cannot say.
907 --Bilbo in /usr/src/perl/pp_ctl.c
908 EVER_ON_AND_ON
909
68dc0745
PP
910=head1 Data: Arrays
911
65acb1b1
TC
912=head2 What is the difference between a list and an array?
913
914An array has a changeable length. A list does not. An array is something
915you can push or pop, while a list is a set of values. Some people make
916the distinction that a list is a value while an array is a variable.
917Subroutines are passed and return lists, you put things into list
918context, you initialize arrays with lists, and you foreach() across
919a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
920in scalar context behave like the number of elements in them, subroutines
a6dd486b 921access their arguments through the array C<@_>, and push/pop/shift only work
65acb1b1
TC
922on arrays.
923
924As a side note, there's no such thing as a list in scalar context.
925When you say
926
927 $scalar = (2, 5, 7, 9);
928
d92eb7b0
GS
929you're using the comma operator in scalar context, so it uses the scalar
930comma operator. There never was a list there at all! This causes the
931last value to be returned: 9.
65acb1b1 932
68dc0745
PP
933=head2 What is the difference between $array[1] and @array[1]?
934
a6dd486b 935The former is a scalar value; the latter an array slice, making
68dc0745
PP
936it a list with one (scalar) value. You should use $ when you want a
937scalar value (most of the time) and @ when you want a list with one
938scalar value in it (very, very rarely; nearly never, in fact).
939
940Sometimes it doesn't make a difference, but sometimes it does.
941For example, compare:
942
943 $good[0] = `some program that outputs several lines`;
944
945with
946
947 @bad[0] = `same program that outputs several lines`;
948
9f1b1f2d
GS
949The C<use warnings> pragma and the B<-w> flag will warn you about these
950matters.
68dc0745 951
d92eb7b0 952=head2 How can I remove duplicate elements from a list or array?
68dc0745
PP
953
954There are several possible ways, depending on whether the array is
955ordered and whether you wish to preserve the ordering.
956
957=over 4
958
551e1d92
RB
959=item a)
960
961If @in is sorted, and you want @out to be sorted:
5a964f20 962(this assumes all true values in the array)
68dc0745 963
a4341a65 964 $prev = "not equal to $in[0]";
3bc5ef3e 965 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
68dc0745 966
c8db1d39 967This is nice in that it doesn't use much extra memory, simulating
3bc5ef3e
HG
968uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
969guarantees that the expression is true (so that grep picks it up)
970even if the $_ is 0, "", or undef.
68dc0745 971
551e1d92
RB
972=item b)
973
974If you don't know whether @in is sorted:
68dc0745
PP
975
976 undef %saw;
977 @out = grep(!$saw{$_}++, @in);
978
551e1d92
RB
979=item c)
980
981Like (b), but @in contains only small integers:
68dc0745
PP
982
983 @out = grep(!$saw[$_]++, @in);
984
551e1d92
RB
985=item d)
986
987A way to do (b) without any loops or greps:
68dc0745
PP
988
989 undef %saw;
990 @saw{@in} = ();
991 @out = sort keys %saw; # remove sort if undesired
992
551e1d92
RB
993=item e)
994
995Like (d), but @in contains only small positive integers:
68dc0745
PP
996
997 undef @ary;
998 @ary[@in] = @in;
87275199 999 @out = grep {defined} @ary;
68dc0745
PP
1000
1001=back
1002
65acb1b1
TC
1003But perhaps you should have been using a hash all along, eh?
1004
ddbc1f16 1005=head2 How can I tell whether a certain element is contained in a list or array?
5a964f20
TC
1006
1007Hearing the word "in" is an I<in>dication that you probably should have
1008used a hash, not a list or array, to store your data. Hashes are
1009designed to answer this question quickly and efficiently. Arrays aren't.
68dc0745 1010
5a964f20
TC
1011That being said, there are several ways to approach this. If you
1012are going to make this query many times over arbitrary string values,
1013the fastest way is probably to invert the original array and keep an
68dc0745
PP
1014associative array lying about whose keys are the first array's values.
1015
1016 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1017 undef %is_blue;
1018 for (@blues) { $is_blue{$_} = 1 }
1019
1020Now you can check whether $is_blue{$some_color}. It might have been a
1021good idea to keep the blues all in a hash in the first place.
1022
1023If the values are all small integers, you could use a simple indexed
1024array. This kind of an array will take up less space:
1025
1026 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1027 undef @is_tiny_prime;
d92eb7b0
GS
1028 for (@primes) { $is_tiny_prime[$_] = 1 }
1029 # or simply @istiny_prime[@primes] = (1) x @primes;
68dc0745
PP
1030
1031Now you check whether $is_tiny_prime[$some_number].
1032
1033If the values in question are integers instead of strings, you can save
1034quite a lot of space by using bit strings instead:
1035
1036 @articles = ( 1..10, 150..2000, 2017 );
1037 undef $read;
7b8d334a 1038 for (@articles) { vec($read,$_,1) = 1 }
68dc0745
PP
1039
1040Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1041
1042Please do not use
1043
a6dd486b 1044 ($is_there) = grep $_ eq $whatever, @array;
68dc0745
PP
1045
1046or worse yet
1047
a6dd486b 1048 ($is_there) = grep /$whatever/, @array;
68dc0745
PP
1049
1050These are slow (checks every element even if the first matches),
1051inefficient (same reason), and potentially buggy (what if there are
d92eb7b0 1052regex characters in $whatever?). If you're only testing once, then
65acb1b1
TC
1053use:
1054
1055 $is_there = 0;
1056 foreach $elt (@array) {
1057 if ($elt eq $elt_to_find) {
1058 $is_there = 1;
1059 last;
1060 }
1061 }
1062 if ($is_there) { ... }
68dc0745
PP
1063
1064=head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1065
1066Use a hash. Here's code to do both and more. It assumes that
1067each element is unique in a given array:
1068
1069 @union = @intersection = @difference = ();
1070 %count = ();
1071 foreach $element (@array1, @array2) { $count{$element}++ }
1072 foreach $element (keys %count) {
1073 push @union, $element;
1074 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1075 }
1076
d92eb7b0 1077Note that this is the I<symmetric difference>, that is, all elements in
a6dd486b 1078either A or in B but not in both. Think of it as an xor operation.
d92eb7b0 1079
65acb1b1
TC
1080=head2 How do I test whether two arrays or hashes are equal?
1081
1082The following code works for single-level arrays. It uses a stringwise
1083comparison, and does not distinguish defined versus undefined empty
1084strings. Modify if you have other needs.
1085
1086 $are_equal = compare_arrays(\@frogs, \@toads);
1087
1088 sub compare_arrays {
1089 my ($first, $second) = @_;
9f1b1f2d 1090 no warnings; # silence spurious -w undef complaints
65acb1b1
TC
1091 return 0 unless @$first == @$second;
1092 for (my $i = 0; $i < @$first; $i++) {
1093 return 0 if $first->[$i] ne $second->[$i];
1094 }
1095 return 1;
1096 }
1097
1098For multilevel structures, you may wish to use an approach more
1099like this one. It uses the CPAN module FreezeThaw:
1100
1101 use FreezeThaw qw(cmpStr);
1102 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1103
1104 printf "a and b contain %s arrays\n",
1105 cmpStr(\@a, \@b) == 0
1106 ? "the same"
1107 : "different";
1108
1109This approach also works for comparing hashes. Here
1110we'll demonstrate two different answers:
1111
1112 use FreezeThaw qw(cmpStr cmpStrHard);
1113
1114 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1115 $a{EXTRA} = \%b;
1116 $b{EXTRA} = \%a;
1117
1118 printf "a and b contain %s hashes\n",
1119 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1120
1121 printf "a and b contain %s hashes\n",
1122 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1123
1124
1125The first reports that both those the hashes contain the same data,
1126while the second reports that they do not. Which you prefer is left as
1127an exercise to the reader.
1128
68dc0745
PP
1129=head2 How do I find the first array element for which a condition is true?
1130
1131You can use this if you care about the index:
1132
65acb1b1 1133 for ($i= 0; $i < @array; $i++) {
68dc0745
PP
1134 if ($array[$i] eq "Waldo") {
1135 $found_index = $i;
1136 last;
1137 }
1138 }
1139
1140Now C<$found_index> has what you want.
1141
1142=head2 How do I handle linked lists?
1143
1144In general, you usually don't need a linked list in Perl, since with
1145regular arrays, you can push and pop or shift and unshift at either end,
5a964f20 1146or you can use splice to add and/or remove arbitrary number of elements at
87275199 1147arbitrary points. Both pop and shift are both O(1) operations on Perl's
5a964f20
TC
1148dynamic arrays. In the absence of shifts and pops, push in general
1149needs to reallocate on the order every log(N) times, and unshift will
1150need to copy pointers each time.
68dc0745
PP
1151
1152If you really, really wanted, you could use structures as described in
1153L<perldsc> or L<perltoot> and do just what the algorithm book tells you
65acb1b1
TC
1154to do. For example, imagine a list node like this:
1155
1156 $node = {
1157 VALUE => 42,
1158 LINK => undef,
1159 };
1160
1161You could walk the list this way:
1162
1163 print "List: ";
1164 for ($node = $head; $node; $node = $node->{LINK}) {
1165 print $node->{VALUE}, " ";
1166 }
1167 print "\n";
1168
a6dd486b 1169You could add to the list this way:
65acb1b1
TC
1170
1171 my ($head, $tail);
1172 $tail = append($head, 1); # grow a new head
1173 for $value ( 2 .. 10 ) {
1174 $tail = append($tail, $value);
1175 }
1176
1177 sub append {
1178 my($list, $value) = @_;
1179 my $node = { VALUE => $value };
1180 if ($list) {
1181 $node->{LINK} = $list->{LINK};
1182 $list->{LINK} = $node;
1183 } else {
1184 $_[0] = $node; # replace caller's version
1185 }
1186 return $node;
1187 }
1188
1189But again, Perl's built-in are virtually always good enough.
68dc0745
PP
1190
1191=head2 How do I handle circular lists?
1192
1193Circular lists could be handled in the traditional fashion with linked
1194lists, or you could just do something like this with an array:
1195
1196 unshift(@array, pop(@array)); # the last shall be first
1197 push(@array, shift(@array)); # and vice versa
1198
1199=head2 How do I shuffle an array randomly?
1200
45bbf655
JH
1201If you either have Perl 5.8.0 or later installed, or if you have
1202Scalar-List-Utils 1.03 or later installed, you can say:
1203
1204 use List::Util 'shuffle';
1205
1206 @shuffled = shuffle(@list);
1207
1208If not, you can use this:
5a964f20
TC
1209
1210 # fisher_yates_shuffle( \@array ) :
1211 # generate a random permutation of @array in place
1212 sub fisher_yates_shuffle {
1213 my $array = shift;
8caf10e0
IK
1214 my $i = @$array;
1215 while (--$i) {
5a964f20 1216 my $j = int rand ($i+1);
5a964f20
TC
1217 @$array[$i,$j] = @$array[$j,$i];
1218 }
1219 }
1220
1221 fisher_yates_shuffle( \@array ); # permutes @array in place
1222
45bbf655
JH
1223Note that the above implementation shuffles an array in place,
1224unlike the List::Util::shuffle() which takes a list and returns
1225a new shuffled list.
1226
d92eb7b0 1227You've probably seen shuffling algorithms that work using splice,
a6dd486b 1228randomly picking another element to swap the current element with
68dc0745
PP
1229
1230 srand;
1231 @new = ();
1232 @old = 1 .. 10; # just a demo
1233 while (@old) {
1234 push(@new, splice(@old, rand @old, 1));
1235 }
1236
5a964f20
TC
1237This is bad because splice is already O(N), and since you do it N times,
1238you just invented a quadratic algorithm; that is, O(N**2). This does
1239not scale, although Perl is so efficient that you probably won't notice
1240this until you have rather largish arrays.
68dc0745
PP
1241
1242=head2 How do I process/modify each element of an array?
1243
1244Use C<for>/C<foreach>:
1245
1246 for (@lines) {
5a964f20
TC
1247 s/foo/bar/; # change that word
1248 y/XZ/ZX/; # swap those letters
68dc0745
PP
1249 }
1250
1251Here's another; let's compute spherical volumes:
1252
5a964f20 1253 for (@volumes = @radii) { # @volumes has changed parts
68dc0745
PP
1254 $_ **= 3;
1255 $_ *= (4/3) * 3.14159; # this will be constant folded
1256 }
1257
5a964f20
TC
1258If you want to do the same thing to modify the values of the hash,
1259you may not use the C<values> function, oddly enough. You need a slice:
1260
1261 for $orbit ( @orbits{keys %orbits} ) {
1262 ($orbit **= 3) *= (4/3) * 3.14159;
1263 }
1264
68dc0745
PP
1265=head2 How do I select a random element from an array?
1266
1267Use the rand() function (see L<perlfunc/rand>):
1268
5a964f20 1269 # at the top of the program:
68dc0745 1270 srand; # not needed for 5.004 and later
5a964f20
TC
1271
1272 # then later on
68dc0745
PP
1273 $index = rand @array;
1274 $element = $array[$index];
1275
5a964f20
TC
1276Make sure you I<only call srand once per program, if then>.
1277If you are calling it more than once (such as before each
1278call to rand), you're almost certainly doing something wrong.
1279
68dc0745
PP
1280=head2 How do I permute N elements of a list?
1281
1282Here's a little program that generates all permutations
1283of all the words on each line of input. The algorithm embodied
5a964f20 1284in the permute() function should work on any list:
68dc0745
PP
1285
1286 #!/usr/bin/perl -n
5a964f20
TC
1287 # tsc-permute: permute each word of input
1288 permute([split], []);
1289 sub permute {
1290 my @items = @{ $_[0] };
1291 my @perms = @{ $_[1] };
1292 unless (@items) {
1293 print "@perms\n";
68dc0745 1294 } else {
5a964f20
TC
1295 my(@newitems,@newperms,$i);
1296 foreach $i (0 .. $#items) {
1297 @newitems = @items;
1298 @newperms = @perms;
1299 unshift(@newperms, splice(@newitems, $i, 1));
1300 permute([@newitems], [@newperms]);
68dc0745
PP
1301 }
1302 }
1303 }
1304
b8d2732a
RH
1305Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
1306module from CPAN runs at least an order of magnitude faster. If you don't
1307have a C compiler (or a binary distribution of Algorithm::Permute), then
1308you can use List::Permutor which is written in pure Perl, and is still
f8620f40 1309several times faster than the algorithm above.
b8d2732a 1310
68dc0745
PP
1311=head2 How do I sort an array by (anything)?
1312
1313Supply a comparison function to sort() (described in L<perlfunc/sort>):
1314
1315 @list = sort { $a <=> $b } @list;
1316
1317The default sort function is cmp, string comparison, which would
c47ff5f1 1318sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
68dc0745
PP
1319the numerical comparison operator.
1320
1321If you have a complicated function needed to pull out the part you
1322want to sort on, then don't do it inside the sort function. Pull it
1323out first, because the sort BLOCK can be called many times for the
1324same element. Here's an example of how to pull out the first word
1325after the first number on each item, and then sort those words
1326case-insensitively.
1327
1328 @idx = ();
1329 for (@data) {
1330 ($item) = /\d+\s*(\S+)/;
1331 push @idx, uc($item);
1332 }
1333 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1334
a6dd486b 1335which could also be written this way, using a trick
68dc0745
PP
1336that's come to be known as the Schwartzian Transform:
1337
1338 @sorted = map { $_->[0] }
1339 sort { $a->[1] cmp $b->[1] }
d92eb7b0 1340 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
68dc0745
PP
1341
1342If you need to sort on several fields, the following paradigm is useful.
1343
1344 @sorted = sort { field1($a) <=> field1($b) ||
1345 field2($a) cmp field2($b) ||
1346 field3($a) cmp field3($b)
1347 } @data;
1348
1349This can be conveniently combined with precalculation of keys as given
1350above.
1351
1352See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
1353this approach.
1354
1355See also the question below on sorting hashes.
1356
1357=head2 How do I manipulate arrays of bits?
1358
1359Use pack() and unpack(), or else vec() and the bitwise operations.
1360
1361For example, this sets $vec to have bit N set if $ints[N] was set:
1362
1363 $vec = '';
1364 foreach(@ints) { vec($vec,$_,1) = 1 }
1365
1366And here's how, given a vector in $vec, you can
1367get those bits into your @ints array:
1368
1369 sub bitvec_to_list {
1370 my $vec = shift;
1371 my @ints;
1372 # Find null-byte density then select best algorithm
1373 if ($vec =~ tr/\0// / length $vec > 0.95) {
1374 use integer;
1375 my $i;
1376 # This method is faster with mostly null-bytes
1377 while($vec =~ /[^\0]/g ) {
1378 $i = -9 + 8 * pos $vec;
1379 push @ints, $i if vec($vec, ++$i, 1);
1380 push @ints, $i if vec($vec, ++$i, 1);
1381 push @ints, $i if vec($vec, ++$i, 1);
1382 push @ints, $i if vec($vec, ++$i, 1);
1383 push @ints, $i if vec($vec, ++$i, 1);
1384 push @ints, $i if vec($vec, ++$i, 1);
1385 push @ints, $i if vec($vec, ++$i, 1);
1386 push @ints, $i if vec($vec, ++$i, 1);
1387 }
1388 } else {
1389 # This method is a fast general algorithm
1390 use integer;
1391 my $bits = unpack "b*", $vec;
1392 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1393 push @ints, pos $bits while($bits =~ /1/g);
1394 }
1395 return \@ints;
1396 }
1397
1398This method gets faster the more sparse the bit vector is.
1399(Courtesy of Tim Bunce and Winfried Koenig.)
1400
65acb1b1
TC
1401Here's a demo on how to use vec():
1402
1403 # vec demo
1404 $vector = "\xff\x0f\xef\xfe";
1405 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
1406 unpack("N", $vector), "\n";
1407 $is_set = vec($vector, 23, 1);
1408 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1409 pvec($vector);
1410
1411 set_vec(1,1,1);
1412 set_vec(3,1,1);
1413 set_vec(23,1,1);
1414
1415 set_vec(3,1,3);
1416 set_vec(3,2,3);
1417 set_vec(3,4,3);
1418 set_vec(3,4,7);
1419 set_vec(3,8,3);
1420 set_vec(3,8,7);
1421
1422 set_vec(0,32,17);
1423 set_vec(1,32,17);
1424
1425 sub set_vec {
1426 my ($offset, $width, $value) = @_;
1427 my $vector = '';
1428 vec($vector, $offset, $width) = $value;
1429 print "offset=$offset width=$width value=$value\n";
1430 pvec($vector);
1431 }
1432
1433 sub pvec {
1434 my $vector = shift;
1435 my $bits = unpack("b*", $vector);
1436 my $i = 0;
1437 my $BASE = 8;
1438
1439 print "vector length in bytes: ", length($vector), "\n";
1440 @bytes = unpack("A8" x length($vector), $bits);
1441 print "bits are: @bytes\n\n";
1442 }
1443
68dc0745
PP
1444=head2 Why does defined() return true on empty arrays and hashes?
1445
65acb1b1
TC
1446The short story is that you should probably only use defined on scalars or
1447functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1448in the 5.004 release or later of Perl for more detail.
68dc0745
PP
1449
1450=head1 Data: Hashes (Associative Arrays)
1451
1452=head2 How do I process an entire hash?
1453
1454Use the each() function (see L<perlfunc/each>) if you don't care
1455whether it's sorted:
1456
5a964f20 1457 while ( ($key, $value) = each %hash) {
68dc0745
PP
1458 print "$key = $value\n";
1459 }
1460
1461If you want it sorted, you'll have to use foreach() on the result of
1462sorting the keys as shown in an earlier question.
1463
1464=head2 What happens if I add or remove keys from a hash while iterating over it?
1465
d92eb7b0
GS
1466Don't do that. :-)
1467
1468[lwall] In Perl 4, you were not allowed to modify a hash at all while
87275199 1469iterating over it. In Perl 5 you can delete from it, but you still
d92eb7b0
GS
1470can't add to it, because that might cause a doubling of the hash table,
1471in which half the entries get copied up to the new top half of the
87275199 1472table, at which point you've totally bamboozled the iterator code.
d92eb7b0
GS
1473Even if the table doesn't double, there's no telling whether your new
1474entry will be inserted before or after the current iterator position.
1475
a6dd486b 1476Either treasure up your changes and make them after the iterator finishes
d92eb7b0
GS
1477or use keys to fetch all the old keys at once, and iterate over the list
1478of keys.
68dc0745
PP
1479
1480=head2 How do I look up a hash element by value?
1481
1482Create a reverse hash:
1483
1484 %by_value = reverse %by_key;
1485 $key = $by_value{$value};
1486
1487That's not particularly efficient. It would be more space-efficient
1488to use:
1489
1490 while (($key, $value) = each %by_key) {
1491 $by_value{$value} = $key;
1492 }
1493
d92eb7b0
GS
1494If your hash could have repeated values, the methods above will only find
1495one of the associated keys. This may or may not worry you. If it does
1496worry you, you can always reverse the hash into a hash of arrays instead:
1497
1498 while (($key, $value) = each %by_key) {
1499 push @{$key_list_by_value{$value}}, $key;
1500 }
68dc0745
PP
1501
1502=head2 How can I know how many entries are in a hash?
1503
1504If you mean how many keys, then all you have to do is
1505take the scalar sense of the keys() function:
1506
3fe9a6f1 1507 $num_keys = scalar keys %hash;
68dc0745 1508
a6dd486b 1509The keys() function also resets the iterator, which in void context is
d92eb7b0
GS
1510faster for tied hashes than would be iterating through the whole
1511hash, one key-value pair at a time.
68dc0745
PP
1512
1513=head2 How do I sort a hash (optionally by value instead of key)?
1514
1515Internally, hashes are stored in a way that prevents you from imposing
1516an order on key-value pairs. Instead, you have to sort a list of the
1517keys or values:
1518
1519 @keys = sort keys %hash; # sorted by key
1520 @keys = sort {
1521 $hash{$a} cmp $hash{$b}
1522 } keys %hash; # and by value
1523
1524Here we'll do a reverse numeric sort by value, and if two keys are
a6dd486b
JB
1525identical, sort by length of key, or if that fails, by straight ASCII
1526comparison of the keys (well, possibly modified by your locale--see
68dc0745
PP
1527L<perllocale>).
1528
1529 @keys = sort {
1530 $hash{$b} <=> $hash{$a}
1531 ||
1532 length($b) <=> length($a)
1533 ||
1534 $a cmp $b
1535 } keys %hash;
1536
1537=head2 How can I always keep my hash sorted?
1538
1539You can look into using the DB_File module and tie() using the
1540$DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
5a964f20 1541The Tie::IxHash module from CPAN might also be instructive.
68dc0745
PP
1542
1543=head2 What's the difference between "delete" and "undef" with hashes?
1544
1545Hashes are pairs of scalars: the first is the key, the second is the
1546value. The key will be coerced to a string, although the value can be
1547any kind of scalar: string, number, or reference. If a key C<$key> is
1548present in the array, C<exists($key)> will return true. The value for
1549a given key can be C<undef>, in which case C<$array{$key}> will be
1550C<undef> while C<$exists{$key}> will return true. This corresponds to
1551(C<$key>, C<undef>) being in the hash.
1552
1553Pictures help... here's the C<%ary> table:
1554
1555 keys values
1556 +------+------+
1557 | a | 3 |
1558 | x | 7 |
1559 | d | 0 |
1560 | e | 2 |
1561 +------+------+
1562
1563And these conditions hold
1564
1565 $ary{'a'} is true
1566 $ary{'d'} is false
1567 defined $ary{'d'} is true
1568 defined $ary{'a'} is true
87275199 1569 exists $ary{'a'} is true (Perl5 only)
68dc0745
PP
1570 grep ($_ eq 'a', keys %ary) is true
1571
1572If you now say
1573
1574 undef $ary{'a'}
1575
1576your table now reads:
1577
1578
1579 keys values
1580 +------+------+
1581 | a | undef|
1582 | x | 7 |
1583 | d | 0 |
1584 | e | 2 |
1585 +------+------+
1586
1587and these conditions now hold; changes in caps:
1588
1589 $ary{'a'} is FALSE
1590 $ary{'d'} is false
1591 defined $ary{'d'} is true
1592 defined $ary{'a'} is FALSE
87275199 1593 exists $ary{'a'} is true (Perl5 only)
68dc0745
PP
1594 grep ($_ eq 'a', keys %ary) is true
1595
1596Notice the last two: you have an undef value, but a defined key!
1597
1598Now, consider this:
1599
1600 delete $ary{'a'}
1601
1602your table now reads:
1603
1604 keys values
1605 +------+------+
1606 | x | 7 |
1607 | d | 0 |
1608 | e | 2 |
1609 +------+------+
1610
1611and these conditions now hold; changes in caps:
1612
1613 $ary{'a'} is false
1614 $ary{'d'} is false
1615 defined $ary{'d'} is true
1616 defined $ary{'a'} is false
87275199 1617 exists $ary{'a'} is FALSE (Perl5 only)
68dc0745
PP
1618 grep ($_ eq 'a', keys %ary) is FALSE
1619
1620See, the whole entry is gone!
1621
1622=head2 Why don't my tied hashes make the defined/exists distinction?
1623
1624They may or may not implement the EXISTS() and DEFINED() methods
1625differently. For example, there isn't the concept of undef with hashes
1626that are tied to DBM* files. This means the true/false tables above
1627will give different results when used on such a hash. It also means
1628that exists and defined do the same thing with a DBM* file, and what
1629they end up doing is not what they do with ordinary hashes.
1630
1631=head2 How do I reset an each() operation part-way through?
1632
5a964f20 1633Using C<keys %hash> in scalar context returns the number of keys in
68dc0745
PP
1634the hash I<and> resets the iterator associated with the hash. You may
1635need to do this if you use C<last> to exit a loop early so that when you
46fc3d4c 1636re-enter it, the hash iterator has been reset.
68dc0745
PP
1637
1638=head2 How can I get the unique keys from two hashes?
1639
d92eb7b0
GS
1640First you extract the keys from the hashes into lists, then solve
1641the "removing duplicates" problem described above. For example:
68dc0745
PP
1642
1643 %seen = ();
1644 for $element (keys(%foo), keys(%bar)) {
1645 $seen{$element}++;
1646 }
1647 @uniq = keys %seen;
1648
1649Or more succinctly:
1650
1651 @uniq = keys %{{%foo,%bar}};
1652
1653Or if you really want to save space:
1654
1655 %seen = ();
1656 while (defined ($key = each %foo)) {
1657 $seen{$key}++;
1658 }
1659 while (defined ($key = each %bar)) {
1660 $seen{$key}++;
1661 }
1662 @uniq = keys %seen;
1663
1664=head2 How can I store a multidimensional array in a DBM file?
1665
1666Either stringify the structure yourself (no fun), or else
1667get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1668it on top of either DB_File or GDBM_File.
1669
1670=head2 How can I make my hash remember the order I put elements into it?
1671
1672Use the Tie::IxHash from CPAN.
1673
46fc3d4c
PP
1674 use Tie::IxHash;
1675 tie(%myhash, Tie::IxHash);
1676 for ($i=0; $i<20; $i++) {
1677 $myhash{$i} = 2*$i;
1678 }
1679 @keys = keys %myhash;
1680 # @keys = (0,1,2,3,...)
1681
68dc0745
PP
1682=head2 Why does passing a subroutine an undefined element in a hash create it?
1683
1684If you say something like:
1685
1686 somefunc($hash{"nonesuch key here"});
1687
1688Then that element "autovivifies"; that is, it springs into existence
1689whether you store something there or not. That's because functions
1690get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1691it has to be ready to write it back into the caller's version.
1692
87275199 1693This has been fixed as of Perl5.004.
68dc0745
PP
1694
1695Normally, merely accessing a key's value for a nonexistent key does
1696I<not> cause that key to be forever there. This is different than
1697awk's behavior.
1698
fc36a67e 1699=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
68dc0745 1700
65acb1b1
TC
1701Usually a hash ref, perhaps like this:
1702
1703 $record = {
1704 NAME => "Jason",
1705 EMPNO => 132,
1706 TITLE => "deputy peon",
1707 AGE => 23,
1708 SALARY => 37_000,
1709 PALS => [ "Norbert", "Rhys", "Phineas"],
1710 };
1711
1712References are documented in L<perlref> and the upcoming L<perlreftut>.
1713Examples of complex data structures are given in L<perldsc> and
1714L<perllol>. Examples of structures and object-oriented classes are
1715in L<perltoot>.
68dc0745
PP
1716
1717=head2 How can I use a reference as a hash key?
1718
1719You can't do this directly, but you could use the standard Tie::Refhash
87275199 1720module distributed with Perl.
68dc0745
PP
1721
1722=head1 Data: Misc
1723
1724=head2 How do I handle binary data correctly?
1725
1726Perl is binary clean, so this shouldn't be a problem. For example,
1727this works fine (assuming the files are found):
1728
1729 if (`cat /vmunix` =~ /gzip/) {
1730 print "Your kernel is GNU-zip enabled!\n";
1731 }
1732
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GS
1733On less elegant (read: Byzantine) systems, however, you have
1734to play tedious games with "text" versus "binary" files. See
1735L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
1736systems are curses out of Microsoft, who seem to be committed to putting
1737the backward into backward compatibility.
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1738
1739If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1740
54310121 1741If you want to deal with multibyte characters, however, there are
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PP
1742some gotchas. See the section on Regular Expressions.
1743
1744=head2 How do I determine whether a scalar is a number/whole/integer/float?
1745
1746Assuming that you don't care about IEEE notations like "NaN" or
1747"Infinity", you probably just want to use a regular expression.
1748
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TC
1749 if (/\D/) { print "has nondigits\n" }
1750 if (/^\d+$/) { print "is a whole number\n" }
1751 if (/^-?\d+$/) { print "is an integer\n" }
1752 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
1753 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
1754 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
1755 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
1756 { print "a C float" }
68dc0745 1757
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1758If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
1759function. Its semantics are somewhat cumbersome, so here's a C<getnum>
1760wrapper function for more convenient access. This function takes
1761a string and returns the number it found, or C<undef> for input that
1762isn't a C float. The C<is_numeric> function is a front end to C<getnum>
1763if you just want to say, ``Is this a float?''
1764
1765 sub getnum {
1766 use POSIX qw(strtod);
1767 my $str = shift;
1768 $str =~ s/^\s+//;
1769 $str =~ s/\s+$//;
1770 $! = 0;
1771 my($num, $unparsed) = strtod($str);
1772 if (($str eq '') || ($unparsed != 0) || $!) {
1773 return undef;
1774 } else {
1775 return $num;
1776 }
1777 }
1778
072dc14b 1779 sub is_numeric { defined getnum($_[0]) }
5a964f20 1780
6cecdcac
GS
1781Or you could check out the String::Scanf module on CPAN instead. The
1782POSIX module (part of the standard Perl distribution) provides the
bf4acbe4 1783C<strtod> and C<strtol> for converting strings to double and longs,
6cecdcac 1784respectively.
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1785
1786=head2 How do I keep persistent data across program calls?
1787
1788For some specific applications, you can use one of the DBM modules.
65acb1b1 1789See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
83df6a1d
JH
1790Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
1791Storable is part of the standard distribution. Here's one example using
65acb1b1
TC
1792Storable's C<store> and C<retrieve> functions:
1793
1794 use Storable;
1795 store(\%hash, "filename");
1796
1797 # later on...
1798 $href = retrieve("filename"); # by ref
1799 %hash = %{ retrieve("filename") }; # direct to hash
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1800
1801=head2 How do I print out or copy a recursive data structure?
1802
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TC
1803The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
1804for printing out data structures. The Storable module, found on CPAN,
1805provides a function called C<dclone> that recursively copies its argument.
1806
1807 use Storable qw(dclone);
1808 $r2 = dclone($r1);
68dc0745 1809
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TC
1810Where $r1 can be a reference to any kind of data structure you'd like.
1811It will be deeply copied. Because C<dclone> takes and returns references,
1812you'd have to add extra punctuation if you had a hash of arrays that
1813you wanted to copy.
68dc0745 1814
65acb1b1 1815 %newhash = %{ dclone(\%oldhash) };
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1816
1817=head2 How do I define methods for every class/object?
1818
1819Use the UNIVERSAL class (see L<UNIVERSAL>).
1820
1821=head2 How do I verify a credit card checksum?
1822
1823Get the Business::CreditCard module from CPAN.
1824
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TC
1825=head2 How do I pack arrays of doubles or floats for XS code?
1826
1827The kgbpack.c code in the PGPLOT module on CPAN does just this.
1828If you're doing a lot of float or double processing, consider using
1829the PDL module from CPAN instead--it makes number-crunching easy.
1830
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1831=head1 AUTHOR AND COPYRIGHT
1832
65acb1b1 1833Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
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1834All rights reserved.
1835
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JH
1836This documentation is free; you can redistribute it and/or modify it
1837under the same terms as Perl itself.
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1838
1839Irrespective of its distribution, all code examples in this file
1840are hereby placed into the public domain. You are permitted and
1841encouraged to use this code in your own programs for fun
1842or for profit as you see fit. A simple comment in the code giving
1843credit would be courteous but is not required.