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1=head1 NAME
2
6f0efb17 3perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 2003/09/20 06:37:43 $)
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4
5=head1 DESCRIPTION
6
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7This section of the FAQ answers questions related to manipulating
8numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
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9
10=head1 Data: Numbers
11
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12=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
13
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14Internally, your computer represents floating-point numbers
15in binary. Digital (as in powers of two) computers cannot
16store all numbers exactly. Some real numbers lose precision
17in the process. This is a problem with how computers store
18numbers and affects all computer languages, not just Perl.
46fc3d4c 19
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20L<perlnumber> show the gory details of number
21representations and conversions.
22
23To limit the number of decimal places in your numbers, you
24can use the printf or sprintf function. See the
197aec24 25L<"Floating Point Arithmetic"|perlop> for more details.
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26
27 printf "%.2f", 10/3;
197aec24 28
49d635f9 29 my $number = sprintf "%.2f", 10/3;
197aec24 30
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31=head2 Why is int() broken?
32
33Your int() is most probably working just fine. It's the numbers that
34aren't quite what you think.
35
36First, see the above item "Why am I getting long decimals
37(eg, 19.9499999999999) instead of the numbers I should be getting
38(eg, 19.95)?".
39
40For example, this
41
42 print int(0.6/0.2-2), "\n";
43
44will in most computers print 0, not 1, because even such simple
45numbers as 0.6 and 0.2 cannot be presented exactly by floating-point
46numbers. What you think in the above as 'three' is really more like
472.9999999999999995559.
48
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49=head2 Why isn't my octal data interpreted correctly?
50
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51Perl only understands octal and hex numbers as such when they occur as
52literals in your program. Octal literals in perl must start with a
53leading "0" and hexadecimal literals must start with a leading "0x".
54If they are read in from somewhere and assigned, no automatic
55conversion takes place. You must explicitly use oct() or hex() if you
56want the values converted to decimal. oct() interprets hex ("0x350"),
57octal ("0350" or even without the leading "0", like "377") and binary
58("0b1010") numbers, while hex() only converts hexadecimal ones, with
59or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
33ce146f 60The inverse mapping from decimal to octal can be done with either the
49d635f9 61"%o" or "%O" sprintf() formats.
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62
63This problem shows up most often when people try using chmod(), mkdir(),
197aec24 64umask(), or sysopen(), which by widespread tradition typically take
33ce146f 65permissions in octal.
68dc0745 66
33ce146f 67 chmod(644, $file); # WRONG
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68 chmod(0644, $file); # right
69
197aec24 70Note the mistake in the first line was specifying the decimal literal
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71644, rather than the intended octal literal 0644. The problem can
72be seen with:
73
434f7166 74 printf("%#o",644); # prints 01204
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75
76Surely you had not intended C<chmod(01204, $file);> - did you? If you
77want to use numeric literals as arguments to chmod() et al. then please
197aec24 78try to express them as octal constants, that is with a leading zero and
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79with the following digits restricted to the set 0..7.
80
65acb1b1 81=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
68dc0745 82
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83Remember that int() merely truncates toward 0. For rounding to a
84certain number of digits, sprintf() or printf() is usually the easiest
85route.
86
87 printf("%.3f", 3.1415926535); # prints 3.142
68dc0745 88
87275199 89The POSIX module (part of the standard Perl distribution) implements
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90ceil(), floor(), and a number of other mathematical and trigonometric
91functions.
92
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93 use POSIX;
94 $ceil = ceil(3.5); # 4
95 $floor = floor(3.5); # 3
96
a6dd486b 97In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
87275199 98module. With 5.004, the Math::Trig module (part of the standard Perl
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99distribution) implements the trigonometric functions. Internally it
100uses the Math::Complex module and some functions can break out from
101the real axis into the complex plane, for example the inverse sine of
1022.
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103
104Rounding in financial applications can have serious implications, and
105the rounding method used should be specified precisely. In these
106cases, it probably pays not to trust whichever system rounding is
107being used by Perl, but to instead implement the rounding function you
108need yourself.
109
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110To see why, notice how you'll still have an issue on half-way-point
111alternation:
112
113 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
114
197aec24 115 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
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116 0.8 0.8 0.9 0.9 1.0 1.0
117
118Don't blame Perl. It's the same as in C. IEEE says we have to do this.
119Perl numbers whose absolute values are integers under 2**31 (on 32 bit
120machines) will work pretty much like mathematical integers. Other numbers
121are not guaranteed.
122
6f0efb17 123=head2 How do I convert between numeric representations/bases/radixes?
68dc0745 124
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125As always with Perl there is more than one way to do it. Below
126are a few examples of approaches to making common conversions
127between number representations. This is intended to be representational
128rather than exhaustive.
68dc0745 129
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130Some of the examples below use the Bit::Vector module from CPAN.
131The reason you might choose Bit::Vector over the perl built in
132functions is that it works with numbers of ANY size, that it is
133optimized for speed on some operations, and for at least some
134programmers the notation might be familiar.
d92eb7b0 135
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136=over 4
137
138=item How do I convert hexadecimal into decimal
d92eb7b0 139
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140Using perl's built in conversion of 0x notation:
141
6f0efb17 142 $dec = 0xDEADBEEF;
7207e29d 143
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144Using the hex function:
145
6f0efb17 146 $dec = hex("DEADBEEF");
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147
148Using pack:
149
6f0efb17 150 $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
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151
152Using the CPAN module Bit::Vector:
153
154 use Bit::Vector;
155 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
156 $dec = $vec->to_Dec();
157
818c4caa 158=item How do I convert from decimal to hexadecimal
6761e064 159
04d666b1 160Using sprintf:
6761e064 161
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162 $hex = sprintf("%X", 3735928559); # upper case A-F
163 $hex = sprintf("%x", 3735928559); # lower case a-f
6761e064 164
6f0efb17 165Using unpack:
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166
167 $hex = unpack("H*", pack("N", 3735928559));
168
6f0efb17 169Using Bit::Vector:
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170
171 use Bit::Vector;
172 $vec = Bit::Vector->new_Dec(32, -559038737);
173 $hex = $vec->to_Hex();
174
175And Bit::Vector supports odd bit counts:
176
177 use Bit::Vector;
178 $vec = Bit::Vector->new_Dec(33, 3735928559);
179 $vec->Resize(32); # suppress leading 0 if unwanted
180 $hex = $vec->to_Hex();
181
818c4caa 182=item How do I convert from octal to decimal
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183
184Using Perl's built in conversion of numbers with leading zeros:
185
6f0efb17 186 $dec = 033653337357; # note the leading 0!
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187
188Using the oct function:
189
6f0efb17 190 $dec = oct("33653337357");
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191
192Using Bit::Vector:
193
194 use Bit::Vector;
195 $vec = Bit::Vector->new(32);
196 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
197 $dec = $vec->to_Dec();
198
818c4caa 199=item How do I convert from decimal to octal
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200
201Using sprintf:
202
203 $oct = sprintf("%o", 3735928559);
204
6f0efb17 205Using Bit::Vector:
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206
207 use Bit::Vector;
208 $vec = Bit::Vector->new_Dec(32, -559038737);
209 $oct = reverse join('', $vec->Chunk_List_Read(3));
210
818c4caa 211=item How do I convert from binary to decimal
6761e064 212
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213Perl 5.6 lets you write binary numbers directly with
214the 0b notation:
215
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216 $number = 0b10110110;
217
218Using oct:
219
220 my $input = "10110110";
221 $decimal = oct( "0b$input" );
2c646907 222
6f0efb17 223Using pack and ord:
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224
225 $decimal = ord(pack('B8', '10110110'));
68dc0745 226
6f0efb17 227Using pack and unpack for larger strings:
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228
229 $int = unpack("N", pack("B32",
230 substr("0" x 32 . "11110101011011011111011101111", -32)));
231 $dec = sprintf("%d", $int);
232
5efd7060 233 # substr() is used to left pad a 32 character string with zeros.
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234
235Using Bit::Vector:
236
237 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
238 $dec = $vec->to_Dec();
239
818c4caa 240=item How do I convert from decimal to binary
6761e064 241
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242Using sprintf (perl 5.6+):
243
244 $bin = sprintf("%b", 3735928559);
245
246Using unpack:
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247
248 $bin = unpack("B*", pack("N", 3735928559));
249
250Using Bit::Vector:
251
252 use Bit::Vector;
253 $vec = Bit::Vector->new_Dec(32, -559038737);
254 $bin = $vec->to_Bin();
255
256The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
257are left as an exercise to the inclined reader.
68dc0745 258
818c4caa 259=back
68dc0745 260
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261=head2 Why doesn't & work the way I want it to?
262
263The behavior of binary arithmetic operators depends on whether they're
264used on numbers or strings. The operators treat a string as a series
265of bits and work with that (the string C<"3"> is the bit pattern
266C<00110011>). The operators work with the binary form of a number
267(the number C<3> is treated as the bit pattern C<00000011>).
268
269So, saying C<11 & 3> performs the "and" operation on numbers (yielding
49d635f9 270C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
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271(yielding C<"1">).
272
273Most problems with C<&> and C<|> arise because the programmer thinks
274they have a number but really it's a string. The rest arise because
275the programmer says:
276
277 if ("\020\020" & "\101\101") {
278 # ...
279 }
280
281but a string consisting of two null bytes (the result of C<"\020\020"
282& "\101\101">) is not a false value in Perl. You need:
283
284 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
285 # ...
286 }
287
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288=head2 How do I multiply matrices?
289
290Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
291or the PDL extension (also available from CPAN).
292
293=head2 How do I perform an operation on a series of integers?
294
295To call a function on each element in an array, and collect the
296results, use:
297
298 @results = map { my_func($_) } @array;
299
300For example:
301
302 @triple = map { 3 * $_ } @single;
303
304To call a function on each element of an array, but ignore the
305results:
306
307 foreach $iterator (@array) {
65acb1b1 308 some_func($iterator);
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309 }
310
311To call a function on each integer in a (small) range, you B<can> use:
312
65acb1b1 313 @results = map { some_func($_) } (5 .. 25);
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314
315but you should be aware that the C<..> operator creates an array of
316all integers in the range. This can take a lot of memory for large
317ranges. Instead use:
318
319 @results = ();
320 for ($i=5; $i < 500_005; $i++) {
65acb1b1 321 push(@results, some_func($i));
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322 }
323
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324This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
325loop will iterate over the range, without creating the entire range.
326
327 for my $i (5 .. 500_005) {
328 push(@results, some_func($i));
329 }
330
331will not create a list of 500,000 integers.
332
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333=head2 How can I output Roman numerals?
334
a93751fa 335Get the http://www.cpan.org/modules/by-module/Roman module.
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336
337=head2 Why aren't my random numbers random?
338
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339If you're using a version of Perl before 5.004, you must call C<srand>
340once at the start of your program to seed the random number generator.
49d635f9 341
5cd0b561 342 BEGIN { srand() if $] < 5.004 }
49d635f9 343
65acb1b1 3445.004 and later automatically call C<srand> at the beginning. Don't
49d635f9 345call C<srand> more than once---you make your numbers less random, rather
65acb1b1 346than more.
92c2ed05 347
65acb1b1 348Computers are good at being predictable and bad at being random
06a5f41f 349(despite appearances caused by bugs in your programs :-). see the
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350F<random> article in the "Far More Than You Ever Wanted To Know"
351collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
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352Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
353who attempts to generate random numbers by deterministic means is, of
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354course, living in a state of sin.''
355
356If you want numbers that are more random than C<rand> with C<srand>
357provides, you should also check out the Math::TrulyRandom module from
358CPAN. It uses the imperfections in your system's timer to generate
359random numbers, but this takes quite a while. If you want a better
92c2ed05 360pseudorandom generator than comes with your operating system, look at
65acb1b1 361``Numerical Recipes in C'' at http://www.nr.com/ .
68dc0745 362
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363=head2 How do I get a random number between X and Y?
364
365Use the following simple function. It selects a random integer between
366(and possibly including!) the two given integers, e.g.,
367C<random_int_in(50,120)>
368
369 sub random_int_in ($$) {
370 my($min, $max) = @_;
371 # Assumes that the two arguments are integers themselves!
372 return $min if $min == $max;
373 ($min, $max) = ($max, $min) if $min > $max;
374 return $min + int rand(1 + $max - $min);
375 }
376
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377=head1 Data: Dates
378
5cd0b561 379=head2 How do I find the day or week of the year?
68dc0745 380
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381The localtime function returns the day of the week. Without an
382argument localtime uses the current time.
68dc0745 383
5cd0b561 384 $day_of_year = (localtime)[7];
ffc145e8 385
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386The POSIX module can also format a date as the day of the year or
387week of the year.
68dc0745 388
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389 use POSIX qw/strftime/;
390 my $day_of_year = strftime "%j", localtime;
391 my $week_of_year = strftime "%W", localtime;
392
393To get the day of year for any date, use the Time::Local module to get
394a time in epoch seconds for the argument to localtime.
ffc145e8 395
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396 use POSIX qw/strftime/;
397 use Time::Local;
398 my $week_of_year = strftime "%W",
399 localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
400
401The Date::Calc module provides two functions for to calculate these.
402
403 use Date::Calc;
404 my $day_of_year = Day_of_Year( 1987, 12, 18 );
405 my $week_of_year = Week_of_Year( 1987, 12, 18 );
ffc145e8 406
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407=head2 How do I find the current century or millennium?
408
409Use the following simple functions:
410
197aec24 411 sub get_century {
d92eb7b0 412 return int((((localtime(shift || time))[5] + 1999))/100);
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413 }
414 sub get_millennium {
d92eb7b0 415 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
197aec24 416 }
d92eb7b0 417
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418You can also use the POSIX strftime() function which may be a bit
419slower but is easier to read and maintain.
420
421 use POSIX qw/strftime/;
197aec24 422
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423 my $week_of_the_year = strftime "%W", localtime;
424 my $day_of_the_year = strftime "%j", localtime;
425
426On some systems, the POSIX module's strftime() function has
427been extended in a non-standard way to use a C<%C> format,
428which they sometimes claim is the "century". It isn't,
429because on most such systems, this is only the first two
430digits of the four-digit year, and thus cannot be used to
431reliably determine the current century or millennium.
d92eb7b0 432
92c2ed05 433=head2 How can I compare two dates and find the difference?
68dc0745 434
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435If you're storing your dates as epoch seconds then simply subtract one
436from the other. If you've got a structured date (distinct year, day,
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437month, hour, minute, seconds values), then for reasons of accessibility,
438simplicity, and efficiency, merely use either timelocal or timegm (from
439the Time::Local module in the standard distribution) to reduce structured
440dates to epoch seconds. However, if you don't know the precise format of
441your dates, then you should probably use either of the Date::Manip and
442Date::Calc modules from CPAN before you go hacking up your own parsing
443routine to handle arbitrary date formats.
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444
445=head2 How can I take a string and turn it into epoch seconds?
446
447If it's a regular enough string that it always has the same format,
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448you can split it up and pass the parts to C<timelocal> in the standard
449Time::Local module. Otherwise, you should look into the Date::Calc
450and Date::Manip modules from CPAN.
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451
452=head2 How can I find the Julian Day?
453
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454Use the Time::JulianDay module (part of the Time-modules bundle
455available from CPAN.)
d92eb7b0 456
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457Before you immerse yourself too deeply in this, be sure to verify that
458it is the I<Julian> Day you really want. Are you interested in a way
459of getting serial days so that you just can tell how many days they
460are apart or so that you can do also other date arithmetic? If you
d92eb7b0 461are interested in performing date arithmetic, this can be done using
2a2bf5f4 462modules Date::Manip or Date::Calc.
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463
464There is too many details and much confusion on this issue to cover in
465this FAQ, but the term is applied (correctly) to a calendar now
466supplanted by the Gregorian Calendar, with the Julian Calendar failing
467to adjust properly for leap years on centennial years (among other
468annoyances). The term is also used (incorrectly) to mean: [1] days in
469the Gregorian Calendar; and [2] days since a particular starting time
470or `epoch', usually 1970 in the Unix world and 1980 in the
471MS-DOS/Windows world. If you find that it is not the first meaning
472that you really want, then check out the Date::Manip and Date::Calc
473modules. (Thanks to David Cassell for most of this text.)
be94a901 474
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475=head2 How do I find yesterday's date?
476
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477If you only need to find the date (and not the same time), you
478can use the Date::Calc module.
65acb1b1 479
49d635f9 480 use Date::Calc qw(Today Add_Delta_Days);
197aec24 481
49d635f9 482 my @date = Add_Delta_Days( Today(), -1 );
197aec24 483
49d635f9 484 print "@date\n";
65acb1b1 485
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486Most people try to use the time rather than the calendar to
487figure out dates, but that assumes that your days are
488twenty-four hours each. For most people, there are two days
489a year when they aren't: the switch to and from summer time
490throws this off. Russ Allbery offers this solution.
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491
492 sub yesterday {
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493 my $now = defined $_[0] ? $_[0] : time;
494 my $then = $now - 60 * 60 * 24;
495 my $ndst = (localtime $now)[8] > 0;
496 my $tdst = (localtime $then)[8] > 0;
497 $then - ($tdst - $ndst) * 60 * 60;
498 }
197aec24 499
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500Should give you "this time yesterday" in seconds since epoch relative to
501the first argument or the current time if no argument is given and
502suitable for passing to localtime or whatever else you need to do with
503it. $ndst is whether we're currently in daylight savings time; $tdst is
504whether the point 24 hours ago was in daylight savings time. If $tdst
505and $ndst are the same, a boundary wasn't crossed, and the correction
506will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
507from yesterday's time since we gained an extra hour while going off
508daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
509negative hour (add an hour) to yesterday's time since we lost an hour.
510
511All of this is because during those days when one switches off or onto
512DST, a "day" isn't 24 hours long; it's either 23 or 25.
513
514The explicit settings of $ndst and $tdst are necessary because localtime
515only says it returns the system tm struct, and the system tm struct at
516least on Solaris doesn't guarantee any particular positive value (like,
517say, 1) for isdst, just a positive value. And that value can
518potentially be negative, if DST information isn't available (this sub
519just treats those cases like no DST).
520
521Note that between 2am and 3am on the day after the time zone switches
522off daylight savings time, the exact hour of "yesterday" corresponding
523to the current hour is not clearly defined. Note also that if used
524between 2am and 3am the day after the change to daylight savings time,
525the result will be between 3am and 4am of the previous day; it's
526arguable whether this is correct.
527
528This sub does not attempt to deal with leap seconds (most things don't).
529
530
d92eb7b0 531
87275199 532=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
68dc0745 533
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534Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
535Y2K compliant (whatever that means). The programmers you've hired to
536use it, however, probably are not.
537
538Long answer: The question belies a true understanding of the issue.
539Perl is just as Y2K compliant as your pencil--no more, and no less.
540Can you use your pencil to write a non-Y2K-compliant memo? Of course
541you can. Is that the pencil's fault? Of course it isn't.
92c2ed05 542
87275199 543The date and time functions supplied with Perl (gmtime and localtime)
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544supply adequate information to determine the year well beyond 2000
545(2038 is when trouble strikes for 32-bit machines). The year returned
90fdbbb7 546by these functions when used in a list context is the year minus 1900.
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547For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
548number. To avoid the year 2000 problem simply do not treat the year as
549a 2-digit number. It isn't.
68dc0745 550
5a964f20 551When gmtime() and localtime() are used in scalar context they return
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552a timestamp string that contains a fully-expanded year. For example,
553C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
5542001". There's no year 2000 problem here.
555
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TC
556That doesn't mean that Perl can't be used to create non-Y2K compliant
557programs. It can. But so can your pencil. It's the fault of the user,
558not the language. At the risk of inflaming the NRA: ``Perl doesn't
559break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
560a longer exposition.
561
68dc0745
PP
562=head1 Data: Strings
563
564=head2 How do I validate input?
565
566The answer to this question is usually a regular expression, perhaps
5a964f20 567with auxiliary logic. See the more specific questions (numbers, mail
68dc0745
PP
568addresses, etc.) for details.
569
570=head2 How do I unescape a string?
571
92c2ed05
GS
572It depends just what you mean by ``escape''. URL escapes are dealt
573with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
a6dd486b 574character are removed with
68dc0745
PP
575
576 s/\\(.)/$1/g;
577
92c2ed05 578This won't expand C<"\n"> or C<"\t"> or any other special escapes.
68dc0745
PP
579
580=head2 How do I remove consecutive pairs of characters?
581
92c2ed05 582To turn C<"abbcccd"> into C<"abccd">:
68dc0745 583
d92eb7b0
GS
584 s/(.)\1/$1/g; # add /s to include newlines
585
586Here's a solution that turns "abbcccd" to "abcd":
587
588 y///cs; # y == tr, but shorter :-)
68dc0745
PP
589
590=head2 How do I expand function calls in a string?
591
592This is documented in L<perlref>. In general, this is fraught with
593quoting and readability problems, but it is possible. To interpolate
5a964f20 594a subroutine call (in list context) into a string:
68dc0745
PP
595
596 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
597
92c2ed05
GS
598See also ``How can I expand variables in text strings?'' in this
599section of the FAQ.
46fc3d4c 600
68dc0745
PP
601=head2 How do I find matching/nesting anything?
602
92c2ed05
GS
603This isn't something that can be done in one regular expression, no
604matter how complicated. To find something between two single
605characters, a pattern like C</x([^x]*)x/> will get the intervening
606bits in $1. For multiple ones, then something more like
607C</alpha(.*?)omega/> would be needed. But none of these deals with
f0f835c2
A
608nested patterns. For balanced expressions using C<(>, C<{>, C<[>
609or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
610L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
92c2ed05
GS
611
612If you are serious about writing a parser, there are a number of
6a2af475
GS
613modules or oddities that will make your life a lot easier. There are
614the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
83df6a1d
JH
615and the byacc program. Starting from perl 5.8 the Text::Balanced
616is part of the standard distribution.
68dc0745 617
92c2ed05
GS
618One simple destructive, inside-out approach that you might try is to
619pull out the smallest nesting parts one at a time:
5a964f20 620
d92eb7b0 621 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
5a964f20 622 # do something with $1
197aec24 623 }
5a964f20 624
65acb1b1
TC
625A more complicated and sneaky approach is to make Perl's regular
626expression engine do it for you. This is courtesy Dean Inada, and
627rather has the nature of an Obfuscated Perl Contest entry, but it
628really does work:
629
630 # $_ contains the string to parse
631 # BEGIN and END are the opening and closing markers for the
632 # nested text.
c47ff5f1 633
65acb1b1
TC
634 @( = ('(','');
635 @) = (')','');
636 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
5ed30e05 637 @$ = (eval{/$re/},$@!~/unmatched/i);
65acb1b1
TC
638 print join("\n",@$[0..$#$]) if( $$[-1] );
639
68dc0745
PP
640=head2 How do I reverse a string?
641
5a964f20 642Use reverse() in scalar context, as documented in
68dc0745
PP
643L<perlfunc/reverse>.
644
645 $reversed = reverse $string;
646
647=head2 How do I expand tabs in a string?
648
5a964f20 649You can do it yourself:
68dc0745
PP
650
651 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
652
87275199 653Or you can just use the Text::Tabs module (part of the standard Perl
68dc0745
PP
654distribution).
655
656 use Text::Tabs;
657 @expanded_lines = expand(@lines_with_tabs);
658
659=head2 How do I reformat a paragraph?
660
87275199 661Use Text::Wrap (part of the standard Perl distribution):
68dc0745
PP
662
663 use Text::Wrap;
664 print wrap("\t", ' ', @paragraphs);
665
92c2ed05 666The paragraphs you give to Text::Wrap should not contain embedded
46fc3d4c
PP
667newlines. Text::Wrap doesn't justify the lines (flush-right).
668
bc06af74
JH
669Or use the CPAN module Text::Autoformat. Formatting files can be easily
670done by making a shell alias, like so:
671
672 alias fmt="perl -i -MText::Autoformat -n0777 \
673 -e 'print autoformat $_, {all=>1}' $*"
674
675See the documentation for Text::Autoformat to appreciate its many
676capabilities.
677
49d635f9 678=head2 How can I access or change N characters of a string?
68dc0745 679
49d635f9
RGS
680You can access the first characters of a string with substr().
681To get the first character, for example, start at position 0
197aec24 682and grab the string of length 1.
68dc0745 683
68dc0745 684
49d635f9
RGS
685 $string = "Just another Perl Hacker";
686 $first_char = substr( $string, 0, 1 ); # 'J'
68dc0745 687
49d635f9
RGS
688To change part of a string, you can use the optional fourth
689argument which is the replacement string.
68dc0745 690
49d635f9 691 substr( $string, 13, 4, "Perl 5.8.0" );
197aec24 692
49d635f9 693You can also use substr() as an lvalue.
68dc0745 694
49d635f9 695 substr( $string, 13, 4 ) = "Perl 5.8.0";
197aec24 696
68dc0745
PP
697=head2 How do I change the Nth occurrence of something?
698
92c2ed05
GS
699You have to keep track of N yourself. For example, let's say you want
700to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
d92eb7b0
GS
701C<"whosoever"> or C<"whomsoever">, case insensitively. These
702all assume that $_ contains the string to be altered.
68dc0745
PP
703
704 $count = 0;
705 s{((whom?)ever)}{
706 ++$count == 5 # is it the 5th?
707 ? "${2}soever" # yes, swap
708 : $1 # renege and leave it there
d92eb7b0 709 }ige;
68dc0745 710
5a964f20
TC
711In the more general case, you can use the C</g> modifier in a C<while>
712loop, keeping count of matches.
713
714 $WANT = 3;
715 $count = 0;
d92eb7b0 716 $_ = "One fish two fish red fish blue fish";
5a964f20
TC
717 while (/(\w+)\s+fish\b/gi) {
718 if (++$count == $WANT) {
719 print "The third fish is a $1 one.\n";
5a964f20
TC
720 }
721 }
722
92c2ed05 723That prints out: C<"The third fish is a red one."> You can also use a
5a964f20
TC
724repetition count and repeated pattern like this:
725
726 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
727
68dc0745
PP
728=head2 How can I count the number of occurrences of a substring within a string?
729
a6dd486b 730There are a number of ways, with varying efficiency. If you want a
68dc0745
PP
731count of a certain single character (X) within a string, you can use the
732C<tr///> function like so:
733
368c9434 734 $string = "ThisXlineXhasXsomeXx'sXinXit";
68dc0745 735 $count = ($string =~ tr/X//);
d92eb7b0 736 print "There are $count X characters in the string";
68dc0745
PP
737
738This is fine if you are just looking for a single character. However,
739if you are trying to count multiple character substrings within a
740larger string, C<tr///> won't work. What you can do is wrap a while()
741loop around a global pattern match. For example, let's count negative
742integers:
743
744 $string = "-9 55 48 -2 23 -76 4 14 -44";
745 while ($string =~ /-\d+/g) { $count++ }
746 print "There are $count negative numbers in the string";
747
881bdbd4
JH
748Another version uses a global match in list context, then assigns the
749result to a scalar, producing a count of the number of matches.
750
751 $count = () = $string =~ /-\d+/g;
752
68dc0745
PP
753=head2 How do I capitalize all the words on one line?
754
755To make the first letter of each word upper case:
3fe9a6f1 756
68dc0745
PP
757 $line =~ s/\b(\w)/\U$1/g;
758
46fc3d4c 759This has the strange effect of turning "C<don't do it>" into "C<Don'T
a6dd486b 760Do It>". Sometimes you might want this. Other times you might need a
24f1ba9b 761more thorough solution (Suggested by brian d foy):
46fc3d4c
PP
762
763 $string =~ s/ (
764 (^\w) #at the beginning of the line
765 | # or
766 (\s\w) #preceded by whitespace
767 )
768 /\U$1/xg;
769 $string =~ /([\w']+)/\u\L$1/g;
770
68dc0745 771To make the whole line upper case:
3fe9a6f1 772
68dc0745
PP
773 $line = uc($line);
774
775To force each word to be lower case, with the first letter upper case:
3fe9a6f1 776
68dc0745
PP
777 $line =~ s/(\w+)/\u\L$1/g;
778
5a964f20
TC
779You can (and probably should) enable locale awareness of those
780characters by placing a C<use locale> pragma in your program.
92c2ed05 781See L<perllocale> for endless details on locales.
5a964f20 782
65acb1b1 783This is sometimes referred to as putting something into "title
d92eb7b0 784case", but that's not quite accurate. Consider the proper
65acb1b1
TC
785capitalization of the movie I<Dr. Strangelove or: How I Learned to
786Stop Worrying and Love the Bomb>, for example.
787
369b44b4
RGS
788Damian Conway's L<Text::Autoformat> module provides some smart
789case transformations:
790
791 use Text::Autoformat;
792 my $x = "Dr. Strangelove or: How I Learned to Stop ".
793 "Worrying and Love the Bomb";
794
795 print $x, "\n";
796 for my $style (qw( sentence title highlight ))
797 {
798 print autoformat($x, { case => $style }), "\n";
799 }
800
49d635f9 801=head2 How can I split a [character] delimited string except when inside [character]?
68dc0745 802
49d635f9
RGS
803Several modules can handle this sort of pasing---Text::Balanced,
804Text::CVS, Text::CVS_XS, and Text::ParseWords, among others.
805
806Take the example case of trying to split a string that is
807comma-separated into its different fields. You can't use C<split(/,/)>
808because you shouldn't split if the comma is inside quotes. For
809example, take a data line like this:
68dc0745
PP
810
811 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
812
813Due to the restriction of the quotes, this is a fairly complex
197aec24 814problem. Thankfully, we have Jeffrey Friedl, author of
49d635f9 815I<Mastering Regular Expressions>, to handle these for us. He
68dc0745
PP
816suggests (assuming your string is contained in $text):
817
818 @new = ();
819 push(@new, $+) while $text =~ m{
820 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
821 | ([^,]+),?
822 | ,
823 }gx;
824 push(@new, undef) if substr($text,-1,1) eq ',';
825
46fc3d4c
PP
826If you want to represent quotation marks inside a
827quotation-mark-delimited field, escape them with backslashes (eg,
49d635f9 828C<"like \"this\"">.
46fc3d4c 829
87275199 830Alternatively, the Text::ParseWords module (part of the standard Perl
68dc0745
PP
831distribution) lets you say:
832
833 use Text::ParseWords;
834 @new = quotewords(",", 0, $text);
835
a6dd486b 836There's also a Text::CSV (Comma-Separated Values) module on CPAN.
65acb1b1 837
68dc0745
PP
838=head2 How do I strip blank space from the beginning/end of a string?
839
a6dd486b 840Although the simplest approach would seem to be
68dc0745
PP
841
842 $string =~ s/^\s*(.*?)\s*$/$1/;
843
a6dd486b 844not only is this unnecessarily slow and destructive, it also fails with
d92eb7b0 845embedded newlines. It is much faster to do this operation in two steps:
68dc0745
PP
846
847 $string =~ s/^\s+//;
848 $string =~ s/\s+$//;
849
850Or more nicely written as:
851
852 for ($string) {
853 s/^\s+//;
854 s/\s+$//;
855 }
856
5e3006a4 857This idiom takes advantage of the C<foreach> loop's aliasing
5a964f20 858behavior to factor out common code. You can do this
197aec24 859on several strings at once, or arrays, or even the
d92eb7b0 860values of a hash if you use a slice:
5a964f20 861
197aec24 862 # trim whitespace in the scalar, the array,
5a964f20
TC
863 # and all the values in the hash
864 foreach ($scalar, @array, @hash{keys %hash}) {
865 s/^\s+//;
866 s/\s+$//;
867 }
868
65acb1b1
TC
869=head2 How do I pad a string with blanks or pad a number with zeroes?
870
65acb1b1 871In the following examples, C<$pad_len> is the length to which you wish
d92eb7b0
GS
872to pad the string, C<$text> or C<$num> contains the string to be padded,
873and C<$pad_char> contains the padding character. You can use a single
874character string constant instead of the C<$pad_char> variable if you
875know what it is in advance. And in the same way you can use an integer in
876place of C<$pad_len> if you know the pad length in advance.
65acb1b1 877
d92eb7b0
GS
878The simplest method uses the C<sprintf> function. It can pad on the left
879or right with blanks and on the left with zeroes and it will not
880truncate the result. The C<pack> function can only pad strings on the
881right with blanks and it will truncate the result to a maximum length of
882C<$pad_len>.
65acb1b1 883
d92eb7b0 884 # Left padding a string with blanks (no truncation):
04d666b1
RGS
885 $padded = sprintf("%${pad_len}s", $text);
886 $padded = sprintf("%*s", $pad_len, $text); # same thing
65acb1b1 887
d92eb7b0 888 # Right padding a string with blanks (no truncation):
04d666b1
RGS
889 $padded = sprintf("%-${pad_len}s", $text);
890 $padded = sprintf("%-*s", $pad_len, $text); # same thing
65acb1b1 891
197aec24 892 # Left padding a number with 0 (no truncation):
04d666b1
RGS
893 $padded = sprintf("%0${pad_len}d", $num);
894 $padded = sprintf("%0*d", $pad_len, $num); # same thing
65acb1b1 895
d92eb7b0
GS
896 # Right padding a string with blanks using pack (will truncate):
897 $padded = pack("A$pad_len",$text);
65acb1b1 898
d92eb7b0
GS
899If you need to pad with a character other than blank or zero you can use
900one of the following methods. They all generate a pad string with the
901C<x> operator and combine that with C<$text>. These methods do
902not truncate C<$text>.
65acb1b1 903
d92eb7b0 904Left and right padding with any character, creating a new string:
65acb1b1 905
d92eb7b0
GS
906 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
907 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
65acb1b1 908
d92eb7b0 909Left and right padding with any character, modifying C<$text> directly:
65acb1b1 910
d92eb7b0
GS
911 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
912 $text .= $pad_char x ( $pad_len - length( $text ) );
65acb1b1 913
68dc0745
PP
914=head2 How do I extract selected columns from a string?
915
916Use substr() or unpack(), both documented in L<perlfunc>.
197aec24 917If you prefer thinking in terms of columns instead of widths,
5a964f20
TC
918you can use this kind of thing:
919
920 # determine the unpack format needed to split Linux ps output
921 # arguments are cut columns
922 my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
923
197aec24 924 sub cut2fmt {
5a964f20
TC
925 my(@positions) = @_;
926 my $template = '';
927 my $lastpos = 1;
928 for my $place (@positions) {
197aec24 929 $template .= "A" . ($place - $lastpos) . " ";
5a964f20
TC
930 $lastpos = $place;
931 }
932 $template .= "A*";
933 return $template;
934 }
68dc0745
PP
935
936=head2 How do I find the soundex value of a string?
937
87275199 938Use the standard Text::Soundex module distributed with Perl.
a6dd486b 939Before you do so, you may want to determine whether `soundex' is in
d92eb7b0
GS
940fact what you think it is. Knuth's soundex algorithm compresses words
941into a small space, and so it does not necessarily distinguish between
942two words which you might want to appear separately. For example, the
943last names `Knuth' and `Kant' are both mapped to the soundex code K530.
944If Text::Soundex does not do what you are looking for, you might want
945to consider the String::Approx module available at CPAN.
68dc0745
PP
946
947=head2 How can I expand variables in text strings?
948
949Let's assume that you have a string like:
950
951 $text = 'this has a $foo in it and a $bar';
5a964f20
TC
952
953If those were both global variables, then this would
954suffice:
955
65acb1b1 956 $text =~ s/\$(\w+)/${$1}/g; # no /e needed
68dc0745 957
5a964f20
TC
958But since they are probably lexicals, or at least, they could
959be, you'd have to do this:
68dc0745
PP
960
961 $text =~ s/(\$\w+)/$1/eeg;
65acb1b1 962 die if $@; # needed /ee, not /e
68dc0745 963
5a964f20
TC
964It's probably better in the general case to treat those
965variables as entries in some special hash. For example:
966
197aec24 967 %user_defs = (
5a964f20
TC
968 foo => 23,
969 bar => 19,
970 );
971 $text =~ s/\$(\w+)/$user_defs{$1}/g;
68dc0745 972
92c2ed05 973See also ``How do I expand function calls in a string?'' in this section
46fc3d4c
PP
974of the FAQ.
975
68dc0745
PP
976=head2 What's wrong with always quoting "$vars"?
977
a6dd486b
JB
978The problem is that those double-quotes force stringification--
979coercing numbers and references into strings--even when you
980don't want them to be strings. Think of it this way: double-quote
197aec24 981expansion is used to produce new strings. If you already
65acb1b1 982have a string, why do you need more?
68dc0745
PP
983
984If you get used to writing odd things like these:
985
986 print "$var"; # BAD
987 $new = "$old"; # BAD
988 somefunc("$var"); # BAD
989
990You'll be in trouble. Those should (in 99.8% of the cases) be
991the simpler and more direct:
992
993 print $var;
994 $new = $old;
995 somefunc($var);
996
997Otherwise, besides slowing you down, you're going to break code when
998the thing in the scalar is actually neither a string nor a number, but
999a reference:
1000
1001 func(\@array);
1002 sub func {
1003 my $aref = shift;
1004 my $oref = "$aref"; # WRONG
1005 }
1006
1007You can also get into subtle problems on those few operations in Perl
1008that actually do care about the difference between a string and a
1009number, such as the magical C<++> autoincrement operator or the
1010syscall() function.
1011
197aec24 1012Stringification also destroys arrays.
5a964f20
TC
1013
1014 @lines = `command`;
1015 print "@lines"; # WRONG - extra blanks
1016 print @lines; # right
1017
04d666b1 1018=head2 Why don't my E<lt>E<lt>HERE documents work?
68dc0745
PP
1019
1020Check for these three things:
1021
1022=over 4
1023
04d666b1 1024=item There must be no space after the E<lt>E<lt> part.
68dc0745 1025
197aec24 1026=item There (probably) should be a semicolon at the end.
68dc0745 1027
197aec24 1028=item You can't (easily) have any space in front of the tag.
68dc0745
PP
1029
1030=back
1031
197aec24 1032If you want to indent the text in the here document, you
5a964f20
TC
1033can do this:
1034
1035 # all in one
1036 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1037 your text
1038 goes here
1039 HERE_TARGET
1040
1041But the HERE_TARGET must still be flush against the margin.
197aec24 1042If you want that indented also, you'll have to quote
5a964f20
TC
1043in the indentation.
1044
1045 ($quote = <<' FINIS') =~ s/^\s+//gm;
1046 ...we will have peace, when you and all your works have
1047 perished--and the works of your dark master to whom you
1048 would deliver us. You are a liar, Saruman, and a corrupter
1049 of men's hearts. --Theoden in /usr/src/perl/taint.c
1050 FINIS
83ded9ee 1051 $quote =~ s/\s+--/\n--/;
5a964f20
TC
1052
1053A nice general-purpose fixer-upper function for indented here documents
1054follows. It expects to be called with a here document as its argument.
1055It looks to see whether each line begins with a common substring, and
a6dd486b
JB
1056if so, strips that substring off. Otherwise, it takes the amount of leading
1057whitespace found on the first line and removes that much off each
5a964f20
TC
1058subsequent line.
1059
1060 sub fix {
1061 local $_ = shift;
a6dd486b 1062 my ($white, $leader); # common whitespace and common leading string
5a964f20
TC
1063 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1064 ($white, $leader) = ($2, quotemeta($1));
1065 } else {
1066 ($white, $leader) = (/^(\s+)/, '');
1067 }
1068 s/^\s*?$leader(?:$white)?//gm;
1069 return $_;
1070 }
1071
c8db1d39 1072This works with leading special strings, dynamically determined:
5a964f20
TC
1073
1074 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
1075 @@@ int
1076 @@@ runops() {
1077 @@@ SAVEI32(runlevel);
1078 @@@ runlevel++;
d92eb7b0 1079 @@@ while ( op = (*op->op_ppaddr)() );
5a964f20
TC
1080 @@@ TAINT_NOT;
1081 @@@ return 0;
1082 @@@ }
1083 MAIN_INTERPRETER_LOOP
1084
a6dd486b 1085Or with a fixed amount of leading whitespace, with remaining
5a964f20
TC
1086indentation correctly preserved:
1087
1088 $poem = fix<<EVER_ON_AND_ON;
1089 Now far ahead the Road has gone,
1090 And I must follow, if I can,
1091 Pursuing it with eager feet,
1092 Until it joins some larger way
1093 Where many paths and errands meet.
1094 And whither then? I cannot say.
1095 --Bilbo in /usr/src/perl/pp_ctl.c
1096 EVER_ON_AND_ON
1097
68dc0745
PP
1098=head1 Data: Arrays
1099
65acb1b1
TC
1100=head2 What is the difference between a list and an array?
1101
1102An array has a changeable length. A list does not. An array is something
1103you can push or pop, while a list is a set of values. Some people make
1104the distinction that a list is a value while an array is a variable.
1105Subroutines are passed and return lists, you put things into list
1106context, you initialize arrays with lists, and you foreach() across
1107a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
1108in scalar context behave like the number of elements in them, subroutines
a6dd486b 1109access their arguments through the array C<@_>, and push/pop/shift only work
65acb1b1
TC
1110on arrays.
1111
1112As a side note, there's no such thing as a list in scalar context.
1113When you say
1114
1115 $scalar = (2, 5, 7, 9);
1116
d92eb7b0
GS
1117you're using the comma operator in scalar context, so it uses the scalar
1118comma operator. There never was a list there at all! This causes the
1119last value to be returned: 9.
65acb1b1 1120
68dc0745
PP
1121=head2 What is the difference between $array[1] and @array[1]?
1122
a6dd486b 1123The former is a scalar value; the latter an array slice, making
68dc0745
PP
1124it a list with one (scalar) value. You should use $ when you want a
1125scalar value (most of the time) and @ when you want a list with one
1126scalar value in it (very, very rarely; nearly never, in fact).
1127
1128Sometimes it doesn't make a difference, but sometimes it does.
1129For example, compare:
1130
1131 $good[0] = `some program that outputs several lines`;
1132
1133with
1134
1135 @bad[0] = `same program that outputs several lines`;
1136
197aec24 1137The C<use warnings> pragma and the B<-w> flag will warn you about these
9f1b1f2d 1138matters.
68dc0745 1139
d92eb7b0 1140=head2 How can I remove duplicate elements from a list or array?
68dc0745
PP
1141
1142There are several possible ways, depending on whether the array is
1143ordered and whether you wish to preserve the ordering.
1144
1145=over 4
1146
551e1d92
RB
1147=item a)
1148
1149If @in is sorted, and you want @out to be sorted:
5a964f20 1150(this assumes all true values in the array)
68dc0745 1151
a4341a65 1152 $prev = "not equal to $in[0]";
3bc5ef3e 1153 @out = grep($_ ne $prev && ($prev = $_, 1), @in);
68dc0745 1154
c8db1d39 1155This is nice in that it doesn't use much extra memory, simulating
3bc5ef3e
HG
1156uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
1157guarantees that the expression is true (so that grep picks it up)
1158even if the $_ is 0, "", or undef.
68dc0745 1159
551e1d92
RB
1160=item b)
1161
1162If you don't know whether @in is sorted:
68dc0745
PP
1163
1164 undef %saw;
1165 @out = grep(!$saw{$_}++, @in);
1166
551e1d92
RB
1167=item c)
1168
1169Like (b), but @in contains only small integers:
68dc0745
PP
1170
1171 @out = grep(!$saw[$_]++, @in);
1172
551e1d92
RB
1173=item d)
1174
1175A way to do (b) without any loops or greps:
68dc0745
PP
1176
1177 undef %saw;
1178 @saw{@in} = ();
1179 @out = sort keys %saw; # remove sort if undesired
1180
551e1d92
RB
1181=item e)
1182
1183Like (d), but @in contains only small positive integers:
68dc0745
PP
1184
1185 undef @ary;
1186 @ary[@in] = @in;
87275199 1187 @out = grep {defined} @ary;
68dc0745
PP
1188
1189=back
1190
65acb1b1
TC
1191But perhaps you should have been using a hash all along, eh?
1192
ddbc1f16 1193=head2 How can I tell whether a certain element is contained in a list or array?
5a964f20
TC
1194
1195Hearing the word "in" is an I<in>dication that you probably should have
1196used a hash, not a list or array, to store your data. Hashes are
1197designed to answer this question quickly and efficiently. Arrays aren't.
68dc0745 1198
5a964f20
TC
1199That being said, there are several ways to approach this. If you
1200are going to make this query many times over arbitrary string values,
881bdbd4
JH
1201the fastest way is probably to invert the original array and maintain a
1202hash whose keys are the first array's values.
68dc0745
PP
1203
1204 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
881bdbd4 1205 %is_blue = ();
68dc0745
PP
1206 for (@blues) { $is_blue{$_} = 1 }
1207
1208Now you can check whether $is_blue{$some_color}. It might have been a
1209good idea to keep the blues all in a hash in the first place.
1210
1211If the values are all small integers, you could use a simple indexed
1212array. This kind of an array will take up less space:
1213
1214 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
881bdbd4 1215 @is_tiny_prime = ();
d92eb7b0
GS
1216 for (@primes) { $is_tiny_prime[$_] = 1 }
1217 # or simply @istiny_prime[@primes] = (1) x @primes;
68dc0745
PP
1218
1219Now you check whether $is_tiny_prime[$some_number].
1220
1221If the values in question are integers instead of strings, you can save
1222quite a lot of space by using bit strings instead:
1223
1224 @articles = ( 1..10, 150..2000, 2017 );
1225 undef $read;
7b8d334a 1226 for (@articles) { vec($read,$_,1) = 1 }
68dc0745
PP
1227
1228Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1229
1230Please do not use
1231
a6dd486b 1232 ($is_there) = grep $_ eq $whatever, @array;
68dc0745
PP
1233
1234or worse yet
1235
a6dd486b 1236 ($is_there) = grep /$whatever/, @array;
68dc0745
PP
1237
1238These are slow (checks every element even if the first matches),
1239inefficient (same reason), and potentially buggy (what if there are
d92eb7b0 1240regex characters in $whatever?). If you're only testing once, then
65acb1b1
TC
1241use:
1242
1243 $is_there = 0;
1244 foreach $elt (@array) {
1245 if ($elt eq $elt_to_find) {
1246 $is_there = 1;
1247 last;
1248 }
1249 }
1250 if ($is_there) { ... }
68dc0745
PP
1251
1252=head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1253
1254Use a hash. Here's code to do both and more. It assumes that
1255each element is unique in a given array:
1256
1257 @union = @intersection = @difference = ();
1258 %count = ();
1259 foreach $element (@array1, @array2) { $count{$element}++ }
1260 foreach $element (keys %count) {
1261 push @union, $element;
1262 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1263 }
1264
d92eb7b0 1265Note that this is the I<symmetric difference>, that is, all elements in
a6dd486b 1266either A or in B but not in both. Think of it as an xor operation.
d92eb7b0 1267
65acb1b1
TC
1268=head2 How do I test whether two arrays or hashes are equal?
1269
1270The following code works for single-level arrays. It uses a stringwise
1271comparison, and does not distinguish defined versus undefined empty
1272strings. Modify if you have other needs.
1273
1274 $are_equal = compare_arrays(\@frogs, \@toads);
1275
1276 sub compare_arrays {
1277 my ($first, $second) = @_;
9f1b1f2d 1278 no warnings; # silence spurious -w undef complaints
65acb1b1
TC
1279 return 0 unless @$first == @$second;
1280 for (my $i = 0; $i < @$first; $i++) {
1281 return 0 if $first->[$i] ne $second->[$i];
1282 }
1283 return 1;
1284 }
1285
1286For multilevel structures, you may wish to use an approach more
1287like this one. It uses the CPAN module FreezeThaw:
1288
1289 use FreezeThaw qw(cmpStr);
1290 @a = @b = ( "this", "that", [ "more", "stuff" ] );
1291
1292 printf "a and b contain %s arrays\n",
197aec24
RGS
1293 cmpStr(\@a, \@b) == 0
1294 ? "the same"
65acb1b1
TC
1295 : "different";
1296
1297This approach also works for comparing hashes. Here
1298we'll demonstrate two different answers:
1299
1300 use FreezeThaw qw(cmpStr cmpStrHard);
1301
1302 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1303 $a{EXTRA} = \%b;
197aec24 1304 $b{EXTRA} = \%a;
65acb1b1
TC
1305
1306 printf "a and b contain %s hashes\n",
1307 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1308
1309 printf "a and b contain %s hashes\n",
1310 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1311
1312
1313The first reports that both those the hashes contain the same data,
1314while the second reports that they do not. Which you prefer is left as
1315an exercise to the reader.
1316
68dc0745
PP
1317=head2 How do I find the first array element for which a condition is true?
1318
49d635f9
RGS
1319To find the first array element which satisfies a condition, you can
1320use the first() function in the List::Util module, which comes with
1321Perl 5.8. This example finds the first element that contains "Perl".
1322
1323 use List::Util qw(first);
197aec24 1324
49d635f9 1325 my $element = first { /Perl/ } @array;
197aec24 1326
49d635f9
RGS
1327If you cannot use List::Util, you can make your own loop to do the
1328same thing. Once you find the element, you stop the loop with last.
1329
1330 my $found;
1331 foreach my $element ( @array )
1332 {
1333 if( /Perl/ ) { $found = $element; last }
1334 }
1335
1336If you want the array index, you can iterate through the indices
1337and check the array element at each index until you find one
1338that satisfies the condition.
1339
197aec24
RGS
1340 my( $found, $index ) = ( undef, -1 );
1341 for( $i = 0; $i < @array; $i++ )
49d635f9 1342 {
197aec24
RGS
1343 if( $array[$i] =~ /Perl/ )
1344 {
49d635f9 1345 $found = $array[$i];
197aec24 1346 $index = $i;
49d635f9
RGS
1347 last;
1348 }
68dc0745 1349 }
68dc0745
PP
1350
1351=head2 How do I handle linked lists?
1352
1353In general, you usually don't need a linked list in Perl, since with
1354regular arrays, you can push and pop or shift and unshift at either end,
5a964f20 1355or you can use splice to add and/or remove arbitrary number of elements at
87275199 1356arbitrary points. Both pop and shift are both O(1) operations on Perl's
5a964f20
TC
1357dynamic arrays. In the absence of shifts and pops, push in general
1358needs to reallocate on the order every log(N) times, and unshift will
1359need to copy pointers each time.
68dc0745
PP
1360
1361If you really, really wanted, you could use structures as described in
1362L<perldsc> or L<perltoot> and do just what the algorithm book tells you
65acb1b1
TC
1363to do. For example, imagine a list node like this:
1364
1365 $node = {
1366 VALUE => 42,
1367 LINK => undef,
1368 };
1369
1370You could walk the list this way:
1371
1372 print "List: ";
1373 for ($node = $head; $node; $node = $node->{LINK}) {
1374 print $node->{VALUE}, " ";
1375 }
1376 print "\n";
1377
a6dd486b 1378You could add to the list this way:
65acb1b1
TC
1379
1380 my ($head, $tail);
1381 $tail = append($head, 1); # grow a new head
1382 for $value ( 2 .. 10 ) {
1383 $tail = append($tail, $value);
1384 }
1385
1386 sub append {
1387 my($list, $value) = @_;
1388 my $node = { VALUE => $value };
1389 if ($list) {
1390 $node->{LINK} = $list->{LINK};
1391 $list->{LINK} = $node;
1392 } else {
1393 $_[0] = $node; # replace caller's version
1394 }
1395 return $node;
1396 }
1397
1398But again, Perl's built-in are virtually always good enough.
68dc0745
PP
1399
1400=head2 How do I handle circular lists?
1401
1402Circular lists could be handled in the traditional fashion with linked
1403lists, or you could just do something like this with an array:
1404
1405 unshift(@array, pop(@array)); # the last shall be first
1406 push(@array, shift(@array)); # and vice versa
1407
1408=head2 How do I shuffle an array randomly?
1409
45bbf655
JH
1410If you either have Perl 5.8.0 or later installed, or if you have
1411Scalar-List-Utils 1.03 or later installed, you can say:
1412
f05bbc40 1413 use List::Util 'shuffle';
45bbf655
JH
1414
1415 @shuffled = shuffle(@list);
1416
f05bbc40 1417If not, you can use a Fisher-Yates shuffle.
5a964f20 1418
5a964f20 1419 sub fisher_yates_shuffle {
cc30d1a7
JH
1420 my $deck = shift; # $deck is a reference to an array
1421 my $i = @$deck;
f05bbc40 1422 while ($i--) {
5a964f20 1423 my $j = int rand ($i+1);
cc30d1a7 1424 @$deck[$i,$j] = @$deck[$j,$i];
5a964f20
TC
1425 }
1426 }
1427
cc30d1a7
JH
1428 # shuffle my mpeg collection
1429 #
1430 my @mpeg = <audio/*/*.mp3>;
1431 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1432 print @mpeg;
5a964f20 1433
45bbf655
JH
1434Note that the above implementation shuffles an array in place,
1435unlike the List::Util::shuffle() which takes a list and returns
1436a new shuffled list.
1437
d92eb7b0 1438You've probably seen shuffling algorithms that work using splice,
a6dd486b 1439randomly picking another element to swap the current element with
68dc0745
PP
1440
1441 srand;
1442 @new = ();
1443 @old = 1 .. 10; # just a demo
1444 while (@old) {
1445 push(@new, splice(@old, rand @old, 1));
1446 }
1447
5a964f20
TC
1448This is bad because splice is already O(N), and since you do it N times,
1449you just invented a quadratic algorithm; that is, O(N**2). This does
1450not scale, although Perl is so efficient that you probably won't notice
1451this until you have rather largish arrays.
68dc0745
PP
1452
1453=head2 How do I process/modify each element of an array?
1454
1455Use C<for>/C<foreach>:
1456
1457 for (@lines) {
5a964f20
TC
1458 s/foo/bar/; # change that word
1459 y/XZ/ZX/; # swap those letters
68dc0745
PP
1460 }
1461
1462Here's another; let's compute spherical volumes:
1463
5a964f20 1464 for (@volumes = @radii) { # @volumes has changed parts
68dc0745
PP
1465 $_ **= 3;
1466 $_ *= (4/3) * 3.14159; # this will be constant folded
1467 }
197aec24 1468
49d635f9
RGS
1469which can also be done with map() which is made to transform
1470one list into another:
1471
1472 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
68dc0745 1473
76817d6d
JH
1474If you want to do the same thing to modify the values of the
1475hash, you can use the C<values> function. As of Perl 5.6
1476the values are not copied, so if you modify $orbit (in this
1477case), you modify the value.
5a964f20 1478
76817d6d 1479 for $orbit ( values %orbits ) {
197aec24 1480 ($orbit **= 3) *= (4/3) * 3.14159;
5a964f20 1481 }
818c4caa 1482
76817d6d
JH
1483Prior to perl 5.6 C<values> returned copies of the values,
1484so older perl code often contains constructions such as
1485C<@orbits{keys %orbits}> instead of C<values %orbits> where
1486the hash is to be modified.
818c4caa 1487
68dc0745
PP
1488=head2 How do I select a random element from an array?
1489
1490Use the rand() function (see L<perlfunc/rand>):
1491
5a964f20 1492 # at the top of the program:
68dc0745 1493 srand; # not needed for 5.004 and later
5a964f20
TC
1494
1495 # then later on
68dc0745
PP
1496 $index = rand @array;
1497 $element = $array[$index];
1498
5a964f20 1499Make sure you I<only call srand once per program, if then>.
197aec24 1500If you are calling it more than once (such as before each
5a964f20
TC
1501call to rand), you're almost certainly doing something wrong.
1502
68dc0745
PP
1503=head2 How do I permute N elements of a list?
1504
49d635f9
RGS
1505Use the List::Permutor module on CPAN. If the list is
1506actually an array, try the Algorithm::Permute module (also
1507on CPAN). It's written in XS code and is very efficient.
1508
1509 use Algorithm::Permute;
1510 my @array = 'a'..'d';
1511 my $p_iterator = Algorithm::Permute->new ( \@array );
1512 while (my @perm = $p_iterator->next) {
1513 print "next permutation: (@perm)\n";
1514 }
1515
197aec24
RGS
1516For even faster execution, you could do:
1517
1518 use Algorithm::Permute;
1519 my @array = 'a'..'d';
1520 Algorithm::Permute::permute {
1521 print "next permutation: (@array)\n";
1522 } @array;
1523
49d635f9
RGS
1524Here's a little program that generates all permutations of
1525all the words on each line of input. The algorithm embodied
1526in the permute() function is discussed in Volume 4 (still
1527unpublished) of Knuth's I<The Art of Computer Programming>
1528and will work on any list:
1529
1530 #!/usr/bin/perl -n
1531 # Fischer-Kause ordered permutation generator
1532
1533 sub permute (&@) {
1534 my $code = shift;
1535 my @idx = 0..$#_;
1536 while ( $code->(@_[@idx]) ) {
1537 my $p = $#idx;
1538 --$p while $idx[$p-1] > $idx[$p];
1539 my $q = $p or return;
1540 push @idx, reverse splice @idx, $p;
1541 ++$q while $idx[$p-1] > $idx[$q];
1542 @idx[$p-1,$q]=@idx[$q,$p-1];
1543 }
68dc0745 1544 }
68dc0745 1545
49d635f9 1546 permute {print"@_\n"} split;
b8d2732a 1547
68dc0745
PP
1548=head2 How do I sort an array by (anything)?
1549
1550Supply a comparison function to sort() (described in L<perlfunc/sort>):
1551
1552 @list = sort { $a <=> $b } @list;
1553
1554The default sort function is cmp, string comparison, which would
c47ff5f1 1555sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
68dc0745
PP
1556the numerical comparison operator.
1557
1558If you have a complicated function needed to pull out the part you
1559want to sort on, then don't do it inside the sort function. Pull it
1560out first, because the sort BLOCK can be called many times for the
1561same element. Here's an example of how to pull out the first word
1562after the first number on each item, and then sort those words
1563case-insensitively.
1564
1565 @idx = ();
1566 for (@data) {
1567 ($item) = /\d+\s*(\S+)/;
1568 push @idx, uc($item);
1569 }
1570 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
1571
a6dd486b 1572which could also be written this way, using a trick
68dc0745
PP
1573that's come to be known as the Schwartzian Transform:
1574
1575 @sorted = map { $_->[0] }
1576 sort { $a->[1] cmp $b->[1] }
d92eb7b0 1577 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
68dc0745
PP
1578
1579If you need to sort on several fields, the following paradigm is useful.
1580
1581 @sorted = sort { field1($a) <=> field1($b) ||
1582 field2($a) cmp field2($b) ||
1583 field3($a) cmp field3($b)
1584 } @data;
1585
1586This can be conveniently combined with precalculation of keys as given
1587above.
1588
06a5f41f 1589See the F<sort> artitcle article in the "Far More Than You Ever Wanted
49d635f9 1590To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
06a5f41f 1591more about this approach.
68dc0745
PP
1592
1593See also the question below on sorting hashes.
1594
1595=head2 How do I manipulate arrays of bits?
1596
1597Use pack() and unpack(), or else vec() and the bitwise operations.
1598
1599For example, this sets $vec to have bit N set if $ints[N] was set:
1600
1601 $vec = '';
1602 foreach(@ints) { vec($vec,$_,1) = 1 }
1603
cc30d1a7 1604Here's how, given a vector in $vec, you can
68dc0745
PP
1605get those bits into your @ints array:
1606
1607 sub bitvec_to_list {
1608 my $vec = shift;
1609 my @ints;
1610 # Find null-byte density then select best algorithm
1611 if ($vec =~ tr/\0// / length $vec > 0.95) {
1612 use integer;
1613 my $i;
1614 # This method is faster with mostly null-bytes
1615 while($vec =~ /[^\0]/g ) {
1616 $i = -9 + 8 * pos $vec;
1617 push @ints, $i if vec($vec, ++$i, 1);
1618 push @ints, $i if vec($vec, ++$i, 1);
1619 push @ints, $i if vec($vec, ++$i, 1);
1620 push @ints, $i if vec($vec, ++$i, 1);
1621 push @ints, $i if vec($vec, ++$i, 1);
1622 push @ints, $i if vec($vec, ++$i, 1);
1623 push @ints, $i if vec($vec, ++$i, 1);
1624 push @ints, $i if vec($vec, ++$i, 1);
1625 }
1626 } else {
1627 # This method is a fast general algorithm
1628 use integer;
1629 my $bits = unpack "b*", $vec;
1630 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1631 push @ints, pos $bits while($bits =~ /1/g);
1632 }
1633 return \@ints;
1634 }
1635
1636This method gets faster the more sparse the bit vector is.
1637(Courtesy of Tim Bunce and Winfried Koenig.)
1638
76817d6d
JH
1639You can make the while loop a lot shorter with this suggestion
1640from Benjamin Goldberg:
1641
1642 while($vec =~ /[^\0]+/g ) {
1643 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1644 }
1645
cc30d1a7
JH
1646Or use the CPAN module Bit::Vector:
1647
1648 $vector = Bit::Vector->new($num_of_bits);
1649 $vector->Index_List_Store(@ints);
1650 @ints = $vector->Index_List_Read();
1651
1652Bit::Vector provides efficient methods for bit vector, sets of small integers
197aec24 1653and "big int" math.
cc30d1a7
JH
1654
1655Here's a more extensive illustration using vec():
65acb1b1
TC
1656
1657 # vec demo
1658 $vector = "\xff\x0f\xef\xfe";
197aec24 1659 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
65acb1b1
TC
1660 unpack("N", $vector), "\n";
1661 $is_set = vec($vector, 23, 1);
1662 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
1663 pvec($vector);
1664
1665 set_vec(1,1,1);
1666 set_vec(3,1,1);
1667 set_vec(23,1,1);
1668
1669 set_vec(3,1,3);
1670 set_vec(3,2,3);
1671 set_vec(3,4,3);
1672 set_vec(3,4,7);
1673 set_vec(3,8,3);
1674 set_vec(3,8,7);
1675
1676 set_vec(0,32,17);
1677 set_vec(1,32,17);
1678
197aec24 1679 sub set_vec {
65acb1b1
TC
1680 my ($offset, $width, $value) = @_;
1681 my $vector = '';
1682 vec($vector, $offset, $width) = $value;
1683 print "offset=$offset width=$width value=$value\n";
1684 pvec($vector);
1685 }
1686
1687 sub pvec {
1688 my $vector = shift;
1689 my $bits = unpack("b*", $vector);
1690 my $i = 0;
1691 my $BASE = 8;
1692
1693 print "vector length in bytes: ", length($vector), "\n";
1694 @bytes = unpack("A8" x length($vector), $bits);
1695 print "bits are: @bytes\n\n";
197aec24 1696 }
65acb1b1 1697
68dc0745
PP
1698=head2 Why does defined() return true on empty arrays and hashes?
1699
65acb1b1
TC
1700The short story is that you should probably only use defined on scalars or
1701functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1702in the 5.004 release or later of Perl for more detail.
68dc0745
PP
1703
1704=head1 Data: Hashes (Associative Arrays)
1705
1706=head2 How do I process an entire hash?
1707
1708Use the each() function (see L<perlfunc/each>) if you don't care
1709whether it's sorted:
1710
5a964f20 1711 while ( ($key, $value) = each %hash) {
68dc0745
PP
1712 print "$key = $value\n";
1713 }
1714
1715If you want it sorted, you'll have to use foreach() on the result of
1716sorting the keys as shown in an earlier question.
1717
1718=head2 What happens if I add or remove keys from a hash while iterating over it?
1719
d92eb7b0
GS
1720Don't do that. :-)
1721
1722[lwall] In Perl 4, you were not allowed to modify a hash at all while
87275199 1723iterating over it. In Perl 5 you can delete from it, but you still
d92eb7b0
GS
1724can't add to it, because that might cause a doubling of the hash table,
1725in which half the entries get copied up to the new top half of the
87275199 1726table, at which point you've totally bamboozled the iterator code.
d92eb7b0
GS
1727Even if the table doesn't double, there's no telling whether your new
1728entry will be inserted before or after the current iterator position.
1729
a6dd486b 1730Either treasure up your changes and make them after the iterator finishes
d92eb7b0
GS
1731or use keys to fetch all the old keys at once, and iterate over the list
1732of keys.
68dc0745
PP
1733
1734=head2 How do I look up a hash element by value?
1735
1736Create a reverse hash:
1737
1738 %by_value = reverse %by_key;
1739 $key = $by_value{$value};
1740
1741That's not particularly efficient. It would be more space-efficient
1742to use:
1743
1744 while (($key, $value) = each %by_key) {
1745 $by_value{$value} = $key;
1746 }
1747
d92eb7b0
GS
1748If your hash could have repeated values, the methods above will only find
1749one of the associated keys. This may or may not worry you. If it does
1750worry you, you can always reverse the hash into a hash of arrays instead:
1751
1752 while (($key, $value) = each %by_key) {
1753 push @{$key_list_by_value{$value}}, $key;
1754 }
68dc0745
PP
1755
1756=head2 How can I know how many entries are in a hash?
1757
1758If you mean how many keys, then all you have to do is
875e5c2f 1759use the keys() function in a scalar context:
68dc0745 1760
875e5c2f 1761 $num_keys = keys %hash;
68dc0745 1762
197aec24
RGS
1763The keys() function also resets the iterator, which means that you may
1764see strange results if you use this between uses of other hash operators
875e5c2f 1765such as each().
68dc0745
PP
1766
1767=head2 How do I sort a hash (optionally by value instead of key)?
1768
1769Internally, hashes are stored in a way that prevents you from imposing
1770an order on key-value pairs. Instead, you have to sort a list of the
1771keys or values:
1772
1773 @keys = sort keys %hash; # sorted by key
1774 @keys = sort {
1775 $hash{$a} cmp $hash{$b}
1776 } keys %hash; # and by value
1777
1778Here we'll do a reverse numeric sort by value, and if two keys are
a6dd486b
JB
1779identical, sort by length of key, or if that fails, by straight ASCII
1780comparison of the keys (well, possibly modified by your locale--see
68dc0745
PP
1781L<perllocale>).
1782
1783 @keys = sort {
1784 $hash{$b} <=> $hash{$a}
1785 ||
1786 length($b) <=> length($a)
1787 ||
1788 $a cmp $b
1789 } keys %hash;
1790
1791=head2 How can I always keep my hash sorted?
1792
1793You can look into using the DB_File module and tie() using the
1794$DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
5a964f20 1795The Tie::IxHash module from CPAN might also be instructive.
68dc0745
PP
1796
1797=head2 What's the difference between "delete" and "undef" with hashes?
1798
92993692
JH
1799Hashes contain pairs of scalars: the first is the key, the
1800second is the value. The key will be coerced to a string,
1801although the value can be any kind of scalar: string,
1802number, or reference. If a key $key is present in
1803%hash, C<exists($hash{$key})> will return true. The value
1804for a given key can be C<undef>, in which case
1805C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
1806will return true. This corresponds to (C<$key>, C<undef>)
1807being in the hash.
68dc0745 1808
92993692 1809Pictures help... here's the %hash table:
68dc0745
PP
1810
1811 keys values
1812 +------+------+
1813 | a | 3 |
1814 | x | 7 |
1815 | d | 0 |
1816 | e | 2 |
1817 +------+------+
1818
1819And these conditions hold
1820
92993692
JH
1821 $hash{'a'} is true
1822 $hash{'d'} is false
1823 defined $hash{'d'} is true
1824 defined $hash{'a'} is true
1825 exists $hash{'a'} is true (Perl5 only)
1826 grep ($_ eq 'a', keys %hash) is true
68dc0745
PP
1827
1828If you now say
1829
92993692 1830 undef $hash{'a'}
68dc0745
PP
1831
1832your table now reads:
1833
1834
1835 keys values
1836 +------+------+
1837 | a | undef|
1838 | x | 7 |
1839 | d | 0 |
1840 | e | 2 |
1841 +------+------+
1842
1843and these conditions now hold; changes in caps:
1844
92993692
JH
1845 $hash{'a'} is FALSE
1846 $hash{'d'} is false
1847 defined $hash{'d'} is true
1848 defined $hash{'a'} is FALSE
1849 exists $hash{'a'} is true (Perl5 only)
1850 grep ($_ eq 'a', keys %hash) is true
68dc0745
PP
1851
1852Notice the last two: you have an undef value, but a defined key!
1853
1854Now, consider this:
1855
92993692 1856 delete $hash{'a'}
68dc0745
PP
1857
1858your table now reads:
1859
1860 keys values
1861 +------+------+
1862 | x | 7 |
1863 | d | 0 |
1864 | e | 2 |
1865 +------+------+
1866
1867and these conditions now hold; changes in caps:
1868
92993692
JH
1869 $hash{'a'} is false
1870 $hash{'d'} is false
1871 defined $hash{'d'} is true
1872 defined $hash{'a'} is false
1873 exists $hash{'a'} is FALSE (Perl5 only)
1874 grep ($_ eq 'a', keys %hash) is FALSE
68dc0745
PP
1875
1876See, the whole entry is gone!
1877
1878=head2 Why don't my tied hashes make the defined/exists distinction?
1879
92993692
JH
1880This depends on the tied hash's implementation of EXISTS().
1881For example, there isn't the concept of undef with hashes
1882that are tied to DBM* files. It also means that exists() and
1883defined() do the same thing with a DBM* file, and what they
1884end up doing is not what they do with ordinary hashes.
68dc0745
PP
1885
1886=head2 How do I reset an each() operation part-way through?
1887
5a964f20 1888Using C<keys %hash> in scalar context returns the number of keys in
68dc0745
PP
1889the hash I<and> resets the iterator associated with the hash. You may
1890need to do this if you use C<last> to exit a loop early so that when you
46fc3d4c 1891re-enter it, the hash iterator has been reset.
68dc0745
PP
1892
1893=head2 How can I get the unique keys from two hashes?
1894
d92eb7b0
GS
1895First you extract the keys from the hashes into lists, then solve
1896the "removing duplicates" problem described above. For example:
68dc0745
PP
1897
1898 %seen = ();
1899 for $element (keys(%foo), keys(%bar)) {
1900 $seen{$element}++;
1901 }
1902 @uniq = keys %seen;
1903
1904Or more succinctly:
1905
1906 @uniq = keys %{{%foo,%bar}};
1907
1908Or if you really want to save space:
1909
1910 %seen = ();
1911 while (defined ($key = each %foo)) {
1912 $seen{$key}++;
1913 }
1914 while (defined ($key = each %bar)) {
1915 $seen{$key}++;
1916 }
1917 @uniq = keys %seen;
1918
1919=head2 How can I store a multidimensional array in a DBM file?
1920
1921Either stringify the structure yourself (no fun), or else
1922get the MLDBM (which uses Data::Dumper) module from CPAN and layer
1923it on top of either DB_File or GDBM_File.
1924
1925=head2 How can I make my hash remember the order I put elements into it?
1926
1927Use the Tie::IxHash from CPAN.
1928
46fc3d4c 1929 use Tie::IxHash;
5f8d77f1 1930 tie my %myhash, 'Tie::IxHash';
49d635f9 1931 for (my $i=0; $i<20; $i++) {
46fc3d4c
PP
1932 $myhash{$i} = 2*$i;
1933 }
49d635f9 1934 my @keys = keys %myhash;
46fc3d4c
PP
1935 # @keys = (0,1,2,3,...)
1936
68dc0745
PP
1937=head2 Why does passing a subroutine an undefined element in a hash create it?
1938
1939If you say something like:
1940
1941 somefunc($hash{"nonesuch key here"});
1942
1943Then that element "autovivifies"; that is, it springs into existence
1944whether you store something there or not. That's because functions
1945get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
1946it has to be ready to write it back into the caller's version.
1947
87275199 1948This has been fixed as of Perl5.004.
68dc0745
PP
1949
1950Normally, merely accessing a key's value for a nonexistent key does
1951I<not> cause that key to be forever there. This is different than
1952awk's behavior.
1953
fc36a67e 1954=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
68dc0745 1955
65acb1b1
TC
1956Usually a hash ref, perhaps like this:
1957
1958 $record = {
1959 NAME => "Jason",
1960 EMPNO => 132,
1961 TITLE => "deputy peon",
1962 AGE => 23,
1963 SALARY => 37_000,
1964 PALS => [ "Norbert", "Rhys", "Phineas"],
1965 };
1966
1967References are documented in L<perlref> and the upcoming L<perlreftut>.
1968Examples of complex data structures are given in L<perldsc> and
1969L<perllol>. Examples of structures and object-oriented classes are
1970in L<perltoot>.
68dc0745
PP
1971
1972=head2 How can I use a reference as a hash key?
1973
fe854a6f 1974You can't do this directly, but you could use the standard Tie::RefHash
87275199 1975module distributed with Perl.
68dc0745
PP
1976
1977=head1 Data: Misc
1978
1979=head2 How do I handle binary data correctly?
1980
1981Perl is binary clean, so this shouldn't be a problem. For example,
1982this works fine (assuming the files are found):
1983
1984 if (`cat /vmunix` =~ /gzip/) {
1985 print "Your kernel is GNU-zip enabled!\n";
1986 }
1987
d92eb7b0
GS
1988On less elegant (read: Byzantine) systems, however, you have
1989to play tedious games with "text" versus "binary" files. See
49d635f9 1990L<perlfunc/"binmode"> or L<perlopentut>.
68dc0745
PP
1991
1992If you're concerned about 8-bit ASCII data, then see L<perllocale>.
1993
54310121 1994If you want to deal with multibyte characters, however, there are
68dc0745
PP
1995some gotchas. See the section on Regular Expressions.
1996
1997=head2 How do I determine whether a scalar is a number/whole/integer/float?
1998
1999Assuming that you don't care about IEEE notations like "NaN" or
2000"Infinity", you probably just want to use a regular expression.
2001
65acb1b1
TC
2002 if (/\D/) { print "has nondigits\n" }
2003 if (/^\d+$/) { print "is a whole number\n" }
2004 if (/^-?\d+$/) { print "is an integer\n" }
2005 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2006 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
881bdbd4 2007 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
65acb1b1 2008 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
881bdbd4 2009 { print "a C float\n" }
68dc0745 2010
f0d19b68
RGS
2011There are also some commonly used modules for the task.
2012L<Scalar::Util> (distributed with 5.8) provides access to perl's
2013internal function C<looks_like_number> for determining
2014whether a variable looks like a number. L<Data::Types>
2015exports functions that validate data types using both the
2016above and other regular expressions. Thirdly, there is
2017C<Regexp::Common> which has regular expressions to match
2018various types of numbers. Those three modules are available
2019from the CPAN.
2020
2021If you're on a POSIX system, Perl supports the C<POSIX::strtod>
5a964f20
TC
2022function. Its semantics are somewhat cumbersome, so here's a C<getnum>
2023wrapper function for more convenient access. This function takes
2024a string and returns the number it found, or C<undef> for input that
2025isn't a C float. The C<is_numeric> function is a front end to C<getnum>
2026if you just want to say, ``Is this a float?''
2027
2028 sub getnum {
2029 use POSIX qw(strtod);
2030 my $str = shift;
2031 $str =~ s/^\s+//;
2032 $str =~ s/\s+$//;
2033 $! = 0;
2034 my($num, $unparsed) = strtod($str);
2035 if (($str eq '') || ($unparsed != 0) || $!) {
2036 return undef;
2037 } else {
2038 return $num;
197aec24
RGS
2039 }
2040 }
5a964f20 2041
197aec24 2042 sub is_numeric { defined getnum($_[0]) }
5a964f20 2043
f0d19b68 2044Or you could check out the L<String::Scanf> module on the CPAN
b5b6f210
JH
2045instead. The POSIX module (part of the standard Perl distribution) provides
2046the C<strtod> and C<strtol> for converting strings to double and longs,
6cecdcac 2047respectively.
68dc0745
PP
2048
2049=head2 How do I keep persistent data across program calls?
2050
2051For some specific applications, you can use one of the DBM modules.
fe854a6f
AT
2052See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
2053or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
2054of the standard distribution. Here's one example using Storable's C<store>
2055and C<retrieve> functions:
65acb1b1 2056
197aec24 2057 use Storable;
65acb1b1
TC
2058 store(\%hash, "filename");
2059
197aec24 2060 # later on...
65acb1b1
TC
2061 $href = retrieve("filename"); # by ref
2062 %hash = %{ retrieve("filename") }; # direct to hash
68dc0745
PP
2063
2064=head2 How do I print out or copy a recursive data structure?
2065
65acb1b1 2066The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
6f82c03a
EM
2067for printing out data structures. The Storable module on CPAN (or the
20685.8 release of Perl), provides a function called C<dclone> that recursively
2069copies its argument.
65acb1b1 2070
197aec24 2071 use Storable qw(dclone);
65acb1b1 2072 $r2 = dclone($r1);
68dc0745 2073
65acb1b1
TC
2074Where $r1 can be a reference to any kind of data structure you'd like.
2075It will be deeply copied. Because C<dclone> takes and returns references,
2076you'd have to add extra punctuation if you had a hash of arrays that
2077you wanted to copy.
68dc0745 2078
65acb1b1 2079 %newhash = %{ dclone(\%oldhash) };
68dc0745
PP
2080
2081=head2 How do I define methods for every class/object?
2082
2083Use the UNIVERSAL class (see L<UNIVERSAL>).
2084
2085=head2 How do I verify a credit card checksum?
2086
2087Get the Business::CreditCard module from CPAN.
2088
65acb1b1
TC
2089=head2 How do I pack arrays of doubles or floats for XS code?
2090
2091The kgbpack.c code in the PGPLOT module on CPAN does just this.
2092If you're doing a lot of float or double processing, consider using
2093the PDL module from CPAN instead--it makes number-crunching easy.
2094
68dc0745
PP
2095=head1 AUTHOR AND COPYRIGHT
2096
0bc0ad85 2097Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
5a964f20
TC
2098All rights reserved.
2099
5a7beb56
JH
2100This documentation is free; you can redistribute it and/or modify it
2101under the same terms as Perl itself.
5a964f20
TC
2102
2103Irrespective of its distribution, all code examples in this file
2104are hereby placed into the public domain. You are permitted and
2105encouraged to use this code in your own programs for fun
2106or for profit as you see fit. A simple comment in the code giving
2107credit would be courteous but is not required.