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68dc0745 1=head1 NAME
2
fb2fe781 3perlfaq4 - Data Manipulation ($Revision: 10394 $)
68dc0745 4
5=head1 DESCRIPTION
6
ae3d0b9f 7This section of the FAQ answers questions related to manipulating
8numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
68dc0745 9
10=head1 Data: Numbers
11
46fc3d4c 12=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
13
ac9dac7f 14Internally, your computer represents floating-point numbers in binary.
15Digital (as in powers of two) computers cannot store all numbers
16exactly. Some real numbers lose precision in the process. This is a
17problem with how computers store numbers and affects all computer
18languages, not just Perl.
46fc3d4c 19
ee891a00 20L<perlnumber> shows the gory details of number representations and
ac9dac7f 21conversions.
49d635f9 22
ac9dac7f 23To limit the number of decimal places in your numbers, you can use the
24printf or sprintf function. See the L<"Floating Point
25Arithmetic"|perlop> for more details.
49d635f9 26
27 printf "%.2f", 10/3;
197aec24 28
49d635f9 29 my $number = sprintf "%.2f", 10/3;
197aec24 30
32969b6e 31=head2 Why is int() broken?
32
ac9dac7f 33Your C<int()> is most probably working just fine. It's the numbers that
32969b6e 34aren't quite what you think.
35
ac9dac7f 36First, see the answer to "Why am I getting long decimals
32969b6e 37(eg, 19.9499999999999) instead of the numbers I should be getting
38(eg, 19.95)?".
39
40For example, this
41
ac9dac7f 42 print int(0.6/0.2-2), "\n";
32969b6e 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
68dc0745 49=head2 Why isn't my octal data interpreted correctly?
50
49d635f9 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
ac9dac7f 53leading C<0> and hexadecimal literals must start with a leading C<0x>.
49d635f9 54If they are read in from somewhere and assigned, no automatic
ac9dac7f 55conversion takes place. You must explicitly use C<oct()> or C<hex()> if you
56want the values converted to decimal. C<oct()> interprets hexadecimal (C<0x350>),
57octal (C<0350> or even without the leading C<0>, like C<377>) and binary
58(C<0b1010>) numbers, while C<hex()> only converts hexadecimal ones, with
59or without a leading C<0x>, such as C<0x255>, C<3A>, C<ff>, or C<deadbeef>.
33ce146f 60The inverse mapping from decimal to octal can be done with either the
ac9dac7f 61<%o> or C<%O> C<sprintf()> formats.
68dc0745 62
ac9dac7f 63This problem shows up most often when people try using C<chmod()>,
64C<mkdir()>, C<umask()>, or C<sysopen()>, which by widespread tradition
65typically take permissions in octal.
68dc0745 66
ac9dac7f 67 chmod(644, $file); # WRONG
68 chmod(0644, $file); # right
68dc0745 69
197aec24 70Note the mistake in the first line was specifying the decimal literal
ac9dac7f 71C<644>, rather than the intended octal literal C<0644>. The problem can
33ce146f 72be seen with:
73
ac9dac7f 74 printf("%#o",644); # prints 01204
33ce146f 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
ac9dac7f 79with the following digits restricted to the set C<0..7>.
33ce146f 80
65acb1b1 81=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
68dc0745 82
ac9dac7f 83Remember that C<int()> merely truncates toward 0. For rounding to a
84certain number of digits, C<sprintf()> or C<printf()> is usually the
85easiest route.
92c2ed05 86
ac9dac7f 87 printf("%.3f", 3.1415926535); # prints 3.142
68dc0745 88
ac9dac7f 89The C<POSIX> module (part of the standard Perl distribution)
90implements C<ceil()>, C<floor()>, and a number of other mathematical
91and trigonometric functions.
68dc0745 92
ac9dac7f 93 use POSIX;
94 $ceil = ceil(3.5); # 4
95 $floor = floor(3.5); # 3
92c2ed05 96
ac9dac7f 97In 5.000 to 5.003 perls, trigonometry was done in the C<Math::Complex>
98module. With 5.004, the C<Math::Trig> module (part of the standard Perl
46fc3d4c 99distribution) implements the trigonometric functions. Internally it
ac9dac7f 100uses the C<Math::Complex> module and some functions can break out from
46fc3d4c 101the real axis into the complex plane, for example the inverse sine of
1022.
68dc0745 103
104Rounding in financial applications can have serious implications, and
105the rounding method used should be specified precisely. In these
106cases, it probably pays not to trust whichever system rounding is
107being used by Perl, but to instead implement the rounding function you
108need yourself.
109
65acb1b1 110To see why, notice how you'll still have an issue on half-way-point
111alternation:
112
ac9dac7f 113 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
65acb1b1 114
ac9dac7f 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
116 0.8 0.8 0.9 0.9 1.0 1.0
65acb1b1 117
ac9dac7f 118Don't blame Perl. It's the same as in C. IEEE says we have to do
119this. Perl numbers whose absolute values are integers under 2**31 (on
12032 bit machines) will work pretty much like mathematical integers.
121Other numbers are not guaranteed.
65acb1b1 122
6f0efb17 123=head2 How do I convert between numeric representations/bases/radixes?
68dc0745 124
ac9dac7f 125As always with Perl there is more than one way to do it. Below are a
126few examples of approaches to making common conversions between number
127representations. This is intended to be representational rather than
128exhaustive.
68dc0745 129
ac9dac7f 130Some of the examples later in L<perlfaq4> use the C<Bit::Vector>
131module from CPAN. The reason you might choose C<Bit::Vector> over the
132perl built in functions is that it works with numbers of ANY size,
133that it is optimized for speed on some operations, and for at least
134some programmers the notation might be familiar.
d92eb7b0 135
818c4caa 136=over 4
137
138=item How do I convert hexadecimal into decimal
d92eb7b0 139
ac9dac7f 140Using perl's built in conversion of C<0x> notation:
6761e064 141
ac9dac7f 142 $dec = 0xDEADBEEF;
7207e29d 143
ac9dac7f 144Using the C<hex> function:
6761e064 145
ac9dac7f 146 $dec = hex("DEADBEEF");
6761e064 147
ac9dac7f 148Using C<pack>:
6761e064 149
ac9dac7f 150 $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
6761e064 151
ac9dac7f 152Using the CPAN module C<Bit::Vector>:
6761e064 153
ac9dac7f 154 use Bit::Vector;
155 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
156 $dec = $vec->to_Dec();
6761e064 157
818c4caa 158=item How do I convert from decimal to hexadecimal
6761e064 159
ac9dac7f 160Using C<sprintf>:
6761e064 161
ac9dac7f 162 $hex = sprintf("%X", 3735928559); # upper case A-F
163 $hex = sprintf("%x", 3735928559); # lower case a-f
6761e064 164
ac9dac7f 165Using C<unpack>:
6761e064 166
ac9dac7f 167 $hex = unpack("H*", pack("N", 3735928559));
6761e064 168
ac9dac7f 169Using C<Bit::Vector>:
6761e064 170
ac9dac7f 171 use Bit::Vector;
172 $vec = Bit::Vector->new_Dec(32, -559038737);
173 $hex = $vec->to_Hex();
6761e064 174
ac9dac7f 175And C<Bit::Vector> supports odd bit counts:
6761e064 176
ac9dac7f 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();
6761e064 181
818c4caa 182=item How do I convert from octal to decimal
6761e064 183
184Using Perl's built in conversion of numbers with leading zeros:
185
ac9dac7f 186 $dec = 033653337357; # note the leading 0!
6761e064 187
ac9dac7f 188Using the C<oct> function:
6761e064 189
ac9dac7f 190 $dec = oct("33653337357");
6761e064 191
ac9dac7f 192Using C<Bit::Vector>:
6761e064 193
ac9dac7f 194 use Bit::Vector;
195 $vec = Bit::Vector->new(32);
196 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
197 $dec = $vec->to_Dec();
6761e064 198
818c4caa 199=item How do I convert from decimal to octal
6761e064 200
ac9dac7f 201Using C<sprintf>:
6761e064 202
ac9dac7f 203 $oct = sprintf("%o", 3735928559);
6761e064 204
ac9dac7f 205Using C<Bit::Vector>:
6761e064 206
ac9dac7f 207 use Bit::Vector;
208 $vec = Bit::Vector->new_Dec(32, -559038737);
209 $oct = reverse join('', $vec->Chunk_List_Read(3));
6761e064 210
818c4caa 211=item How do I convert from binary to decimal
6761e064 212
2c646907 213Perl 5.6 lets you write binary numbers directly with
ac9dac7f 214the C<0b> notation:
2c646907 215
ac9dac7f 216 $number = 0b10110110;
6f0efb17 217
ac9dac7f 218Using C<oct>:
6f0efb17 219
ac9dac7f 220 my $input = "10110110";
221 $decimal = oct( "0b$input" );
2c646907 222
ac9dac7f 223Using C<pack> and C<ord>:
d92eb7b0 224
ac9dac7f 225 $decimal = ord(pack('B8', '10110110'));
68dc0745 226
ac9dac7f 227Using C<pack> and C<unpack> for larger strings:
6761e064 228
ac9dac7f 229 $int = unpack("N", pack("B32",
6761e064 230 substr("0" x 32 . "11110101011011011111011101111", -32)));
ac9dac7f 231 $dec = sprintf("%d", $int);
6761e064 232
ac9dac7f 233 # substr() is used to left pad a 32 character string with zeros.
6761e064 234
ac9dac7f 235Using C<Bit::Vector>:
6761e064 236
ac9dac7f 237 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
238 $dec = $vec->to_Dec();
6761e064 239
818c4caa 240=item How do I convert from decimal to binary
6761e064 241
ac9dac7f 242Using C<sprintf> (perl 5.6+):
4dfcc30b 243
ac9dac7f 244 $bin = sprintf("%b", 3735928559);
4dfcc30b 245
ac9dac7f 246Using C<unpack>:
6761e064 247
ac9dac7f 248 $bin = unpack("B*", pack("N", 3735928559));
6761e064 249
ac9dac7f 250Using C<Bit::Vector>:
6761e064 251
ac9dac7f 252 use Bit::Vector;
253 $vec = Bit::Vector->new_Dec(32, -559038737);
254 $bin = $vec->to_Bin();
6761e064 255
256The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
257are left as an exercise to the inclined reader.
68dc0745 258
818c4caa 259=back
68dc0745 260
65acb1b1 261=head2 Why doesn't & work the way I want it to?
262
263The behavior of binary arithmetic operators depends on whether they're
264used on numbers or strings. The operators treat a string as a series
265of bits and work with that (the string C<"3"> is the bit pattern
266C<00110011>). The operators work with the binary form of a number
267(the number C<3> is treated as the bit pattern C<00000011>).
268
269So, saying C<11 & 3> performs the "and" operation on numbers (yielding
49d635f9 270C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
65acb1b1 271(yielding C<"1">).
272
273Most problems with C<&> and C<|> arise because the programmer thinks
274they have a number but really it's a string. The rest arise because
275the programmer says:
276
ac9dac7f 277 if ("\020\020" & "\101\101") {
278 # ...
279 }
65acb1b1 280
281but a string consisting of two null bytes (the result of C<"\020\020"
282& "\101\101">) is not a false value in Perl. You need:
283
ac9dac7f 284 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
285 # ...
286 }
65acb1b1 287
68dc0745 288=head2 How do I multiply matrices?
289
290Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
291or the PDL extension (also available from CPAN).
292
293=head2 How do I perform an operation on a series of integers?
294
295To call a function on each element in an array, and collect the
296results, use:
297
ac9dac7f 298 @results = map { my_func($_) } @array;
68dc0745 299
300For example:
301
ac9dac7f 302 @triple = map { 3 * $_ } @single;
68dc0745 303
304To call a function on each element of an array, but ignore the
305results:
306
ac9dac7f 307 foreach $iterator (@array) {
308 some_func($iterator);
309 }
68dc0745 310
311To call a function on each integer in a (small) range, you B<can> use:
312
ac9dac7f 313 @results = map { some_func($_) } (5 .. 25);
68dc0745 314
315but you should be aware that the C<..> operator creates an array of
316all integers in the range. This can take a lot of memory for large
317ranges. Instead use:
318
ac9dac7f 319 @results = ();
320 for ($i=5; $i < 500_005; $i++) {
321 push(@results, some_func($i));
322 }
68dc0745 323
87275199 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
ac9dac7f 327 for my $i (5 .. 500_005) {
328 push(@results, some_func($i));
329 }
87275199 330
331will not create a list of 500,000 integers.
332
68dc0745 333=head2 How can I output Roman numerals?
334
a93751fa 335Get the http://www.cpan.org/modules/by-module/Roman module.
68dc0745 336
337=head2 Why aren't my random numbers random?
338
65acb1b1 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 $] < 5.004 }
49d635f9 343
65acb1b1 3445.004 and later automatically call C<srand> at the beginning. Don't
ac9dac7f 345call C<srand> more than once--you make your numbers less random,
346rather than more.
92c2ed05 347
65acb1b1 348Computers are good at being predictable and bad at being random
06a5f41f 349(despite appearances caused by bugs in your programs :-). see the
49d635f9 350F<random> article in the "Far More Than You Ever Wanted To Know"
ac9dac7f 351collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy
352of Tom Phoenix, talks more about this. John von Neumann said, "Anyone
06a5f41f 353who attempts to generate random numbers by deterministic means is, of
b432a672 354course, living in a state of sin."
65acb1b1 355
356If you want numbers that are more random than C<rand> with C<srand>
ac9dac7f 357provides, you should also check out the C<Math::TrulyRandom> module from
65acb1b1 358CPAN. It uses the imperfections in your system's timer to generate
359random numbers, but this takes quite a while. If you want a better
92c2ed05 360pseudorandom generator than comes with your operating system, look at
b432a672 361"Numerical Recipes in C" at http://www.nr.com/ .
68dc0745 362
881bdbd4 363=head2 How do I get a random number between X and Y?
364
ee891a00 365To get a random number between two values, you can use the C<rand()>
366builtin to get a random number between 0 and 1. From there, you shift
367that into the range that you want.
500071f4 368
ee891a00 369C<rand($x)> returns a number such that C<< 0 <= rand($x) < $x >>. Thus
370what you want to have perl figure out is a random number in the range
371from 0 to the difference between your I<X> and I<Y>.
793f5136 372
ee891a00 373That is, to get a number between 10 and 15, inclusive, you want a
374random number between 0 and 5 that you can then add to 10.
793f5136 375
500071f4 376 my $number = 10 + int rand( 15-10+1 );
793f5136 377
378Hence you derive the following simple function to abstract
379that. It selects a random integer between the two given
500071f4 380integers (inclusive), For example: C<random_int_between(50,120)>.
381
ac9dac7f 382 sub random_int_between {
500071f4 383 my($min, $max) = @_;
384 # Assumes that the two arguments are integers themselves!
385 return $min if $min == $max;
386 ($min, $max) = ($max, $min) if $min > $max;
387 return $min + int rand(1 + $max - $min);
388 }
881bdbd4 389
68dc0745 390=head1 Data: Dates
391
5cd0b561 392=head2 How do I find the day or week of the year?
68dc0745 393
571e049f 394The localtime function returns the day of the year. Without an
5cd0b561 395argument localtime uses the current time.
68dc0745 396
a05e4845 397 $day_of_year = (localtime)[7];
ffc145e8 398
ac9dac7f 399The C<POSIX> module can also format a date as the day of the year or
5cd0b561 400week of the year.
68dc0745 401
5cd0b561 402 use POSIX qw/strftime/;
403 my $day_of_year = strftime "%j", localtime;
404 my $week_of_year = strftime "%W", localtime;
405
ac9dac7f 406To get the day of year for any date, use C<POSIX>'s C<mktime> to get
5cd0b561 407a time in epoch seconds for the argument to localtime.
ffc145e8 408
ac9dac7f 409 use POSIX qw/mktime strftime/;
6670e5e7 410 my $week_of_year = strftime "%W",
ac9dac7f 411 localtime( mktime( 0, 0, 0, 18, 11, 87 ) );
5cd0b561 412
ac9dac7f 413The C<Date::Calc> module provides two functions to calculate these.
5cd0b561 414
415 use Date::Calc;
416 my $day_of_year = Day_of_Year( 1987, 12, 18 );
417 my $week_of_year = Week_of_Year( 1987, 12, 18 );
ffc145e8 418
d92eb7b0 419=head2 How do I find the current century or millennium?
420
421Use the following simple functions:
422
ac9dac7f 423 sub get_century {
424 return int((((localtime(shift || time))[5] + 1999))/100);
425 }
6670e5e7 426
ac9dac7f 427 sub get_millennium {
428 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
429 }
d92eb7b0 430
ac9dac7f 431On some systems, the C<POSIX> module's C<strftime()> function has been
432extended in a non-standard way to use a C<%C> format, which they
433sometimes claim is the "century". It isn't, because on most such
434systems, this is only the first two digits of the four-digit year, and
435thus cannot be used to reliably determine the current century or
436millennium.
d92eb7b0 437
92c2ed05 438=head2 How can I compare two dates and find the difference?
68dc0745 439
b68463f7 440(contributed by brian d foy)
441
ac9dac7f 442You could just store all your dates as a number and then subtract.
443Life isn't always that simple though. If you want to work with
444formatted dates, the C<Date::Manip>, C<Date::Calc>, or C<DateTime>
445modules can help you.
68dc0745 446
447=head2 How can I take a string and turn it into epoch seconds?
448
449If it's a regular enough string that it always has the same format,
92c2ed05 450you can split it up and pass the parts to C<timelocal> in the standard
ac9dac7f 451C<Time::Local> module. Otherwise, you should look into the C<Date::Calc>
452and C<Date::Manip> modules from CPAN.
68dc0745 453
454=head2 How can I find the Julian Day?
455
7678cced 456(contributed by brian d foy and Dave Cross)
457
ac9dac7f 458You can use the C<Time::JulianDay> module available on CPAN. Ensure
459that you really want to find a Julian day, though, as many people have
7678cced 460different ideas about Julian days. See
461http://www.hermetic.ch/cal_stud/jdn.htm for instance.
462
ac9dac7f 463You can also try the C<DateTime> module, which can convert a date/time
7678cced 464to a Julian Day.
465
ac9dac7f 466 $ perl -MDateTime -le'print DateTime->today->jd'
467 2453401.5
7678cced 468
469Or the modified Julian Day
470
ac9dac7f 471 $ perl -MDateTime -le'print DateTime->today->mjd'
472 53401
7678cced 473
474Or even the day of the year (which is what some people think of as a
475Julian day)
476
ac9dac7f 477 $ perl -MDateTime -le'print DateTime->today->doy'
478 31
be94a901 479
65acb1b1 480=head2 How do I find yesterday's date?
481
6670e5e7 482(contributed by brian d foy)
49d635f9 483
6670e5e7 484Use one of the Date modules. The C<DateTime> module makes it simple, and
485give you the same time of day, only the day before.
49d635f9 486
6670e5e7 487 use DateTime;
58103a2e 488
6670e5e7 489 my $yesterday = DateTime->now->subtract( days => 1 );
58103a2e 490
6670e5e7 491 print "Yesterday was $yesterday\n";
49d635f9 492
ee891a00 493You can also use the C<Date::Calc> module using its C<Today_and_Now>
6670e5e7 494function.
49d635f9 495
6670e5e7 496 use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
58103a2e 497
6670e5e7 498 my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
58103a2e 499
ee891a00 500 print "@date_time\n";
58103a2e 501
6670e5e7 502Most people try to use the time rather than the calendar to figure out
503dates, but that assumes that days are twenty-four hours each. For
504most people, there are two days a year when they aren't: the switch to
505and from summer time throws this off. Let the modules do the work.
d92eb7b0 506
ac9dac7f 507=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
68dc0745 508
65acb1b1 509Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
ac9dac7f 510Y2K compliant (whatever that means). The programmers you've hired to
65acb1b1 511use it, however, probably are not.
512
513Long answer: The question belies a true understanding of the issue.
514Perl is just as Y2K compliant as your pencil--no more, and no less.
515Can you use your pencil to write a non-Y2K-compliant memo? Of course
516you can. Is that the pencil's fault? Of course it isn't.
92c2ed05 517
87275199 518The date and time functions supplied with Perl (gmtime and localtime)
dc164757 519supply adequate information to determine the year well beyond 2000 and
5202038. The year returned by these functions when used in a list
521context is the year minus 1900. For years between 1910 and 1999 this
522I<happens> to be a 2-digit decimal number. To avoid the year 2000
523problem simply do not treat the year as a 2-digit number. It isn't.
68dc0745 524
5a964f20 525When gmtime() and localtime() are used in scalar context they return
68dc0745 526a timestamp string that contains a fully-expanded year. For example,
527C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
5282001". There's no year 2000 problem here.
529
5a964f20 530That doesn't mean that Perl can't be used to create non-Y2K compliant
531programs. It can. But so can your pencil. It's the fault of the user,
b432a672 532not the language. At the risk of inflaming the NRA: "Perl doesn't
533break Y2K, people do." See http://www.perl.org/about/y2k.html for
5a964f20 534a longer exposition.
535
dc164757 536=head2 Does Perl have a Year 2038 problem?
537
538No, all of Perl's built in date and time functions and modules will
539work to about 2 billion years before and after 1970.
540
541Many systems cannot count time past the year 2038. Older versions of
542Perl were dependent on the system to do date calculation and thus
543shared their 2038 bug.
544
68dc0745 545=head1 Data: Strings
546
547=head2 How do I validate input?
548
6670e5e7 549(contributed by brian d foy)
550
551There are many ways to ensure that values are what you expect or
552want to accept. Besides the specific examples that we cover in the
553perlfaq, you can also look at the modules with "Assert" and "Validate"
554in their names, along with other modules such as C<Regexp::Common>.
555
556Some modules have validation for particular types of input, such
557as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>,
558and C<Data::Validate::IP>.
68dc0745 559
560=head2 How do I unescape a string?
561
b432a672 562It depends just what you mean by "escape". URL escapes are dealt
92c2ed05 563with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
a6dd486b 564character are removed with
68dc0745 565
ac9dac7f 566 s/\\(.)/$1/g;
68dc0745 567
92c2ed05 568This won't expand C<"\n"> or C<"\t"> or any other special escapes.
68dc0745 569
570=head2 How do I remove consecutive pairs of characters?
571
6670e5e7 572(contributed by brian d foy)
573
574You can use the substitution operator to find pairs of characters (or
575runs of characters) and replace them with a single instance. In this
576substitution, we find a character in C<(.)>. The memory parentheses
577store the matched character in the back-reference C<\1> and we use
578that to require that the same thing immediately follow it. We replace
579that part of the string with the character in C<$1>.
68dc0745 580
ac9dac7f 581 s/(.)\1/$1/g;
d92eb7b0 582
6670e5e7 583We can also use the transliteration operator, C<tr///>. In this
584example, the search list side of our C<tr///> contains nothing, but
585the C<c> option complements that so it contains everything. The
586replacement list also contains nothing, so the transliteration is
587almost a no-op since it won't do any replacements (or more exactly,
588replace the character with itself). However, the C<s> option squashes
589duplicated and consecutive characters in the string so a character
590does not show up next to itself
d92eb7b0 591
6670e5e7 592 my $str = 'Haarlem'; # in the Netherlands
ac9dac7f 593 $str =~ tr///cs; # Now Harlem, like in New York
68dc0745 594
595=head2 How do I expand function calls in a string?
596
6670e5e7 597(contributed by brian d foy)
598
599This is documented in L<perlref>, and although it's not the easiest
600thing to read, it does work. In each of these examples, we call the
58103a2e 601function inside the braces used to dereference a reference. If we
5ae37c3f 602have more than one return value, we can construct and dereference an
6670e5e7 603anonymous array. In this case, we call the function in list context.
604
58103a2e 605 print "The time values are @{ [localtime] }.\n";
6670e5e7 606
607If we want to call the function in scalar context, we have to do a bit
608more work. We can really have any code we like inside the braces, so
609we simply have to end with the scalar reference, although how you do
e573f903 610that is up to you, and you can use code inside the braces. Note that
611the use of parens creates a list context, so we need C<scalar> to
612force the scalar context on the function:
68dc0745 613
6670e5e7 614 print "The time is ${\(scalar localtime)}.\n"
58103a2e 615
6670e5e7 616 print "The time is ${ my $x = localtime; \$x }.\n";
58103a2e 617
6670e5e7 618If your function already returns a reference, you don't need to create
619the reference yourself.
620
621 sub timestamp { my $t = localtime; \$t }
58103a2e 622
6670e5e7 623 print "The time is ${ timestamp() }.\n";
58103a2e 624
625The C<Interpolation> module can also do a lot of magic for you. You can
626specify a variable name, in this case C<E>, to set up a tied hash that
627does the interpolation for you. It has several other methods to do this
628as well.
629
630 use Interpolation E => 'eval';
631 print "The time values are $E{localtime()}.\n";
632
633In most cases, it is probably easier to simply use string concatenation,
634which also forces scalar context.
6670e5e7 635
ac9dac7f 636 print "The time is " . localtime() . ".\n";
68dc0745 637
68dc0745 638=head2 How do I find matching/nesting anything?
639
92c2ed05 640This isn't something that can be done in one regular expression, no
641matter how complicated. To find something between two single
642characters, a pattern like C</x([^x]*)x/> will get the intervening
643bits in $1. For multiple ones, then something more like
ac9dac7f 644C</alpha(.*?)omega/> would be needed. But none of these deals with
6670e5e7 645nested patterns. For balanced expressions using C<(>, C<{>, C<[> or
646C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
647L<perlre/(??{ code })>. For other cases, you'll have to write a
648parser.
92c2ed05 649
650If you are serious about writing a parser, there are a number of
6a2af475 651modules or oddities that will make your life a lot easier. There are
ac9dac7f 652the CPAN modules C<Parse::RecDescent>, C<Parse::Yapp>, and
653C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8
654the C<Text::Balanced> is part of the standard distribution.
68dc0745 655
92c2ed05 656One simple destructive, inside-out approach that you might try is to
657pull out the smallest nesting parts one at a time:
5a964f20 658
ac9dac7f 659 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
660 # do something with $1
661 }
5a964f20 662
65acb1b1 663A more complicated and sneaky approach is to make Perl's regular
664expression engine do it for you. This is courtesy Dean Inada, and
665rather has the nature of an Obfuscated Perl Contest entry, but it
666really does work:
667
ac9dac7f 668 # $_ contains the string to parse
669 # BEGIN and END are the opening and closing markers for the
670 # nested text.
c47ff5f1 671
ac9dac7f 672 @( = ('(','');
673 @) = (')','');
674 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
675 @$ = (eval{/$re/},$@!~/unmatched/i);
676 print join("\n",@$[0..$#$]) if( $$[-1] );
65acb1b1 677
68dc0745 678=head2 How do I reverse a string?
679
ac9dac7f 680Use C<reverse()> in scalar context, as documented in
68dc0745 681L<perlfunc/reverse>.
682
ac9dac7f 683 $reversed = reverse $string;
68dc0745 684
685=head2 How do I expand tabs in a string?
686
5a964f20 687You can do it yourself:
68dc0745 688
ac9dac7f 689 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
68dc0745 690
ac9dac7f 691Or you can just use the C<Text::Tabs> module (part of the standard Perl
68dc0745 692distribution).
693
ac9dac7f 694 use Text::Tabs;
695 @expanded_lines = expand(@lines_with_tabs);
68dc0745 696
697=head2 How do I reformat a paragraph?
698
ac9dac7f 699Use C<Text::Wrap> (part of the standard Perl distribution):
68dc0745 700
ac9dac7f 701 use Text::Wrap;
702 print wrap("\t", ' ', @paragraphs);
68dc0745 703
ac9dac7f 704The paragraphs you give to C<Text::Wrap> should not contain embedded
705newlines. C<Text::Wrap> doesn't justify the lines (flush-right).
46fc3d4c 706
ac9dac7f 707Or use the CPAN module C<Text::Autoformat>. Formatting files can be
708easily done by making a shell alias, like so:
bc06af74 709
ac9dac7f 710 alias fmt="perl -i -MText::Autoformat -n0777 \
711 -e 'print autoformat $_, {all=>1}' $*"
bc06af74 712
ac9dac7f 713See the documentation for C<Text::Autoformat> to appreciate its many
bc06af74 714capabilities.
715
49d635f9 716=head2 How can I access or change N characters of a string?
68dc0745 717
49d635f9 718You can access the first characters of a string with substr().
719To get the first character, for example, start at position 0
197aec24 720and grab the string of length 1.
68dc0745 721
68dc0745 722
49d635f9 723 $string = "Just another Perl Hacker";
ac9dac7f 724 $first_char = substr( $string, 0, 1 ); # 'J'
68dc0745 725
49d635f9 726To change part of a string, you can use the optional fourth
727argument which is the replacement string.
68dc0745 728
ac9dac7f 729 substr( $string, 13, 4, "Perl 5.8.0" );
197aec24 730
49d635f9 731You can also use substr() as an lvalue.
68dc0745 732
ac9dac7f 733 substr( $string, 13, 4 ) = "Perl 5.8.0";
197aec24 734
68dc0745 735=head2 How do I change the Nth occurrence of something?
736
92c2ed05 737You have to keep track of N yourself. For example, let's say you want
738to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
d92eb7b0 739C<"whosoever"> or C<"whomsoever">, case insensitively. These
740all assume that $_ contains the string to be altered.
68dc0745 741
ac9dac7f 742 $count = 0;
743 s{((whom?)ever)}{
744 ++$count == 5 # is it the 5th?
745 ? "${2}soever" # yes, swap
746 : $1 # renege and leave it there
747 }ige;
68dc0745 748
5a964f20 749In the more general case, you can use the C</g> modifier in a C<while>
750loop, keeping count of matches.
751
ac9dac7f 752 $WANT = 3;
753 $count = 0;
754 $_ = "One fish two fish red fish blue fish";
755 while (/(\w+)\s+fish\b/gi) {
756 if (++$count == $WANT) {
757 print "The third fish is a $1 one.\n";
758 }
759 }
5a964f20 760
92c2ed05 761That prints out: C<"The third fish is a red one."> You can also use a
5a964f20 762repetition count and repeated pattern like this:
763
ac9dac7f 764 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
5a964f20 765
68dc0745 766=head2 How can I count the number of occurrences of a substring within a string?
767
a6dd486b 768There are a number of ways, with varying efficiency. If you want a
68dc0745 769count of a certain single character (X) within a string, you can use the
770C<tr///> function like so:
771
ac9dac7f 772 $string = "ThisXlineXhasXsomeXx'sXinXit";
773 $count = ($string =~ tr/X//);
774 print "There are $count X characters in the string";
68dc0745 775
776This is fine if you are just looking for a single character. However,
777if you are trying to count multiple character substrings within a
778larger string, C<tr///> won't work. What you can do is wrap a while()
779loop around a global pattern match. For example, let's count negative
780integers:
781
ac9dac7f 782 $string = "-9 55 48 -2 23 -76 4 14 -44";
783 while ($string =~ /-\d+/g) { $count++ }
784 print "There are $count negative numbers in the string";
68dc0745 785
881bdbd4 786Another version uses a global match in list context, then assigns the
787result to a scalar, producing a count of the number of matches.
788
789 $count = () = $string =~ /-\d+/g;
790
68dc0745 791=head2 How do I capitalize all the words on one line?
792
793To make the first letter of each word upper case:
3fe9a6f1 794
ac9dac7f 795 $line =~ s/\b(\w)/\U$1/g;
68dc0745 796
46fc3d4c 797This has the strange effect of turning "C<don't do it>" into "C<Don'T
a6dd486b 798Do It>". Sometimes you might want this. Other times you might need a
24f1ba9b 799more thorough solution (Suggested by brian d foy):
46fc3d4c 800
ac9dac7f 801 $string =~ s/ (
802 (^\w) #at the beginning of the line
803 | # or
804 (\s\w) #preceded by whitespace
805 )
806 /\U$1/xg;
807
808 $string =~ s/([\w']+)/\u\L$1/g;
46fc3d4c 809
68dc0745 810To make the whole line upper case:
3fe9a6f1 811
ac9dac7f 812 $line = uc($line);
68dc0745 813
814To force each word to be lower case, with the first letter upper case:
3fe9a6f1 815
ac9dac7f 816 $line =~ s/(\w+)/\u\L$1/g;
68dc0745 817
5a964f20 818You can (and probably should) enable locale awareness of those
819characters by placing a C<use locale> pragma in your program.
92c2ed05 820See L<perllocale> for endless details on locales.
5a964f20 821
65acb1b1 822This is sometimes referred to as putting something into "title
d92eb7b0 823case", but that's not quite accurate. Consider the proper
65acb1b1 824capitalization of the movie I<Dr. Strangelove or: How I Learned to
825Stop Worrying and Love the Bomb>, for example.
826
369b44b4 827Damian Conway's L<Text::Autoformat> module provides some smart
828case transformations:
829
ac9dac7f 830 use Text::Autoformat;
831 my $x = "Dr. Strangelove or: How I Learned to Stop ".
832 "Worrying and Love the Bomb";
369b44b4 833
ac9dac7f 834 print $x, "\n";
835 for my $style (qw( sentence title highlight )) {
836 print autoformat($x, { case => $style }), "\n";
837 }
369b44b4 838
49d635f9 839=head2 How can I split a [character] delimited string except when inside [character]?
68dc0745 840
ac9dac7f 841Several modules can handle this sort of parsing--C<Text::Balanced>,
842C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others.
49d635f9 843
844Take the example case of trying to split a string that is
845comma-separated into its different fields. You can't use C<split(/,/)>
846because you shouldn't split if the comma is inside quotes. For
847example, take a data line like this:
68dc0745 848
ac9dac7f 849 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
68dc0745 850
851Due to the restriction of the quotes, this is a fairly complex
197aec24 852problem. Thankfully, we have Jeffrey Friedl, author of
49d635f9 853I<Mastering Regular Expressions>, to handle these for us. He
ac9dac7f 854suggests (assuming your string is contained in C<$text>):
68dc0745 855
ac9dac7f 856 @new = ();
857 push(@new, $+) while $text =~ m{
858 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
859 | ([^,]+),?
860 | ,
861 }gx;
862 push(@new, undef) if substr($text,-1,1) eq ',';
68dc0745 863
46fc3d4c 864If you want to represent quotation marks inside a
865quotation-mark-delimited field, escape them with backslashes (eg,
49d635f9 866C<"like \"this\"">.
46fc3d4c 867
ac9dac7f 868Alternatively, the C<Text::ParseWords> module (part of the standard
869Perl distribution) lets you say:
68dc0745 870
ac9dac7f 871 use Text::ParseWords;
872 @new = quotewords(",", 0, $text);
65acb1b1 873
68dc0745 874=head2 How do I strip blank space from the beginning/end of a string?
875
6670e5e7 876(contributed by brian d foy)
68dc0745 877
6670e5e7 878A substitution can do this for you. For a single line, you want to
879replace all the leading or trailing whitespace with nothing. You
880can do that with a pair of substitutions.
68dc0745 881
6670e5e7 882 s/^\s+//;
883 s/\s+$//;
68dc0745 884
6670e5e7 885You can also write that as a single substitution, although it turns
886out the combined statement is slower than the separate ones. That
887might not matter to you, though.
68dc0745 888
6670e5e7 889 s/^\s+|\s+$//g;
68dc0745 890
6670e5e7 891In this regular expression, the alternation matches either at the
892beginning or the end of the string since the anchors have a lower
893precedence than the alternation. With the C</g> flag, the substitution
894makes all possible matches, so it gets both. Remember, the trailing
895newline matches the C<\s+>, and the C<$> anchor can match to the
896physical end of the string, so the newline disappears too. Just add
897the newline to the output, which has the added benefit of preserving
898"blank" (consisting entirely of whitespace) lines which the C<^\s+>
899would remove all by itself.
68dc0745 900
6670e5e7 901 while( <> )
902 {
903 s/^\s+|\s+$//g;
904 print "$_\n";
905 }
5a964f20 906
6670e5e7 907For a multi-line string, you can apply the regular expression
908to each logical line in the string by adding the C</m> flag (for
909"multi-line"). With the C</m> flag, the C<$> matches I<before> an
910embedded newline, so it doesn't remove it. It still removes the
911newline at the end of the string.
912
ac9dac7f 913 $string =~ s/^\s+|\s+$//gm;
6670e5e7 914
915Remember that lines consisting entirely of whitespace will disappear,
916since the first part of the alternation can match the entire string
917and replace it with nothing. If need to keep embedded blank lines,
918you have to do a little more work. Instead of matching any whitespace
919(since that includes a newline), just match the other whitespace.
920
921 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
5a964f20 922
65acb1b1 923=head2 How do I pad a string with blanks or pad a number with zeroes?
924
65acb1b1 925In the following examples, C<$pad_len> is the length to which you wish
d92eb7b0 926to pad the string, C<$text> or C<$num> contains the string to be padded,
927and C<$pad_char> contains the padding character. You can use a single
928character string constant instead of the C<$pad_char> variable if you
929know what it is in advance. And in the same way you can use an integer in
930place of C<$pad_len> if you know the pad length in advance.
65acb1b1 931
d92eb7b0 932The simplest method uses the C<sprintf> function. It can pad on the left
933or right with blanks and on the left with zeroes and it will not
934truncate the result. The C<pack> function can only pad strings on the
935right with blanks and it will truncate the result to a maximum length of
936C<$pad_len>.
65acb1b1 937
ac9dac7f 938 # Left padding a string with blanks (no truncation):
04d666b1 939 $padded = sprintf("%${pad_len}s", $text);
940 $padded = sprintf("%*s", $pad_len, $text); # same thing
65acb1b1 941
ac9dac7f 942 # Right padding a string with blanks (no truncation):
04d666b1 943 $padded = sprintf("%-${pad_len}s", $text);
944 $padded = sprintf("%-*s", $pad_len, $text); # same thing
65acb1b1 945
ac9dac7f 946 # Left padding a number with 0 (no truncation):
04d666b1 947 $padded = sprintf("%0${pad_len}d", $num);
948 $padded = sprintf("%0*d", $pad_len, $num); # same thing
65acb1b1 949
ac9dac7f 950 # Right padding a string with blanks using pack (will truncate):
951 $padded = pack("A$pad_len",$text);
65acb1b1 952
d92eb7b0 953If you need to pad with a character other than blank or zero you can use
954one of the following methods. They all generate a pad string with the
955C<x> operator and combine that with C<$text>. These methods do
956not truncate C<$text>.
65acb1b1 957
d92eb7b0 958Left and right padding with any character, creating a new string:
65acb1b1 959
ac9dac7f 960 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
961 $padded = $text . $pad_char x ( $pad_len - length( $text ) );
65acb1b1 962
d92eb7b0 963Left and right padding with any character, modifying C<$text> directly:
65acb1b1 964
ac9dac7f 965 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
966 $text .= $pad_char x ( $pad_len - length( $text ) );
65acb1b1 967
68dc0745 968=head2 How do I extract selected columns from a string?
969
e573f903 970(contributed by brian d foy)
971
972If you know where the columns that contain the data, you can
973use C<substr> to extract a single column.
974
975 my $column = substr( $line, $start_column, $length );
976
977You can use C<split> if the columns are separated by whitespace or
978some other delimiter, as long as whitespace or the delimiter cannot
979appear as part of the data.
980
981 my $line = ' fred barney betty ';
982 my @columns = split /\s+/, $line;
983 # ( '', 'fred', 'barney', 'betty' );
984
985 my $line = 'fred||barney||betty';
986 my @columns = split /\|/, $line;
987 # ( 'fred', '', 'barney', '', 'betty' );
988
989If you want to work with comma-separated values, don't do this since
990that format is a bit more complicated. Use one of the modules that
991handle that fornat, such as C<Text::CSV>, C<Text::CSV_XS>, or
992C<Text::CSV_PP>.
993
994If you want to break apart an entire line of fixed columns, you can use
995C<unpack> with the A (ASCII) format. by using a number after the format
996specifier, you can denote the column width. See the C<pack> and C<unpack>
997entries in L<perlfunc> for more details.
998
999 my @fields = unpack( $line, "A8 A8 A8 A16 A4" );
1000
1001Note that spaces in the format argument to C<unpack> do not denote literal
1002spaces. If you have space separated data, you may want C<split> instead.
68dc0745 1003
1004=head2 How do I find the soundex value of a string?
1005
7678cced 1006(contributed by brian d foy)
1007
1008You can use the Text::Soundex module. If you want to do fuzzy or close
ac9dac7f 1009matching, you might also try the C<String::Approx>, and
1010C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules.
68dc0745 1011
1012=head2 How can I expand variables in text strings?
1013
e573f903 1014(contributed by brian d foy)
5a964f20 1015
322be77c 1016If you can avoid it, don't, or if you can use a templating system,
c195e131 1017such as C<Text::Template> or C<Template> Toolkit, do that instead. You
1018might even be able to get the job done with C<sprintf> or C<printf>:
1019
1020 my $string = sprintf 'Say hello to %s and %s', $foo, $bar;
322be77c 1021
1022However, for the one-off simple case where I don't want to pull out a
1023full templating system, I'll use a string that has two Perl scalar
1024variables in it. In this example, I want to expand C<$foo> and C<$bar>
c195e131 1025to their variable's values:
e573f903 1026
1027 my $foo = 'Fred';
1028 my $bar = 'Barney';
1029 $string = 'Say hello to $foo and $bar';
1030
1031One way I can do this involves the substitution operator and a double
1032C</e> flag. The first C</e> evaluates C<$1> on the replacement side and
1033turns it into C<$foo>. The second /e starts with C<$foo> and replaces
1034it with its value. C<$foo>, then, turns into 'Fred', and that's finally
c195e131 1035what's left in the string:
e573f903 1036
1037 $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'
322be77c 1038
e573f903 1039The C</e> will also silently ignore violations of strict, replacing
c195e131 1040undefined variable names with the empty string. Since I'm using the
1041C</e> flag (twice even!), I have all of the same security problems I
1042have with C<eval> in its string form. If there's something odd in
1043C<$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then
1044I could get myself in trouble.
1045
1046To get around the security problem, I could also pull the values from
1047a hash instead of evaluating variable names. Using a single C</e>, I
1048can check the hash to ensure the value exists, and if it doesn't, I
1049can replace the missing value with a marker, in this case C<???> to
1050signal that I missed something:
e573f903 1051
1052 my $string = 'This has $foo and $bar';
1053
1054 my %Replacements = (
1055 foo => 'Fred',
ac9dac7f 1056 );
322be77c 1057
e573f903 1058 # $string =~ s/\$(\w+)/$Replacements{$1}/g;
1059 $string =~ s/\$(\w+)/
1060 exists $Replacements{$1} ? $Replacements{$1} : '???'
1061 /eg;
322be77c 1062
e573f903 1063 print $string;
322be77c 1064
68dc0745 1065=head2 What's wrong with always quoting "$vars"?
1066
ac9dac7f 1067The problem is that those double-quotes force
e573f903 1068stringification--coercing numbers and references into strings--even
1069when you don't want them to be strings. Think of it this way:
1070double-quote expansion is used to produce new strings. If you already
1071have a string, why do you need more?
68dc0745 1072
1073If you get used to writing odd things like these:
1074
ac9dac7f 1075 print "$var"; # BAD
1076 $new = "$old"; # BAD
1077 somefunc("$var"); # BAD
68dc0745 1078
1079You'll be in trouble. Those should (in 99.8% of the cases) be
1080the simpler and more direct:
1081
ac9dac7f 1082 print $var;
1083 $new = $old;
1084 somefunc($var);
68dc0745 1085
1086Otherwise, besides slowing you down, you're going to break code when
1087the thing in the scalar is actually neither a string nor a number, but
1088a reference:
1089
ac9dac7f 1090 func(\@array);
1091 sub func {
1092 my $aref = shift;
1093 my $oref = "$aref"; # WRONG
1094 }
68dc0745 1095
1096You can also get into subtle problems on those few operations in Perl
1097that actually do care about the difference between a string and a
1098number, such as the magical C<++> autoincrement operator or the
1099syscall() function.
1100
197aec24 1101Stringification also destroys arrays.
5a964f20 1102
ac9dac7f 1103 @lines = `command`;
1104 print "@lines"; # WRONG - extra blanks
1105 print @lines; # right
5a964f20 1106
04d666b1 1107=head2 Why don't my E<lt>E<lt>HERE documents work?
68dc0745 1108
1109Check for these three things:
1110
1111=over 4
1112
04d666b1 1113=item There must be no space after the E<lt>E<lt> part.
68dc0745 1114
197aec24 1115=item There (probably) should be a semicolon at the end.
68dc0745 1116
197aec24 1117=item You can't (easily) have any space in front of the tag.
68dc0745 1118
1119=back
1120
197aec24 1121If you want to indent the text in the here document, you
5a964f20 1122can do this:
1123
1124 # all in one
1125 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
1126 your text
1127 goes here
1128 HERE_TARGET
1129
1130But the HERE_TARGET must still be flush against the margin.
197aec24 1131If you want that indented also, you'll have to quote
5a964f20 1132in the indentation.
1133
1134 ($quote = <<' FINIS') =~ s/^\s+//gm;
1135 ...we will have peace, when you and all your works have
1136 perished--and the works of your dark master to whom you
1137 would deliver us. You are a liar, Saruman, and a corrupter
1138 of men's hearts. --Theoden in /usr/src/perl/taint.c
1139 FINIS
83ded9ee 1140 $quote =~ s/\s+--/\n--/;
5a964f20 1141
1142A nice general-purpose fixer-upper function for indented here documents
1143follows. It expects to be called with a here document as its argument.
1144It looks to see whether each line begins with a common substring, and
a6dd486b 1145if so, strips that substring off. Otherwise, it takes the amount of leading
1146whitespace found on the first line and removes that much off each
5a964f20 1147subsequent line.
1148
1149 sub fix {
1150 local $_ = shift;
a6dd486b 1151 my ($white, $leader); # common whitespace and common leading string
5a964f20 1152 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
1153 ($white, $leader) = ($2, quotemeta($1));
1154 } else {
1155 ($white, $leader) = (/^(\s+)/, '');
1156 }
1157 s/^\s*?$leader(?:$white)?//gm;
1158 return $_;
1159 }
1160
c8db1d39 1161This works with leading special strings, dynamically determined:
5a964f20 1162
ac9dac7f 1163 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
5a964f20 1164 @@@ int
1165 @@@ runops() {
1166 @@@ SAVEI32(runlevel);
1167 @@@ runlevel++;
d92eb7b0 1168 @@@ while ( op = (*op->op_ppaddr)() );
5a964f20 1169 @@@ TAINT_NOT;
1170 @@@ return 0;
1171 @@@ }
ac9dac7f 1172 MAIN_INTERPRETER_LOOP
5a964f20 1173
a6dd486b 1174Or with a fixed amount of leading whitespace, with remaining
5a964f20 1175indentation correctly preserved:
1176
ac9dac7f 1177 $poem = fix<<EVER_ON_AND_ON;
5a964f20 1178 Now far ahead the Road has gone,
1179 And I must follow, if I can,
1180 Pursuing it with eager feet,
1181 Until it joins some larger way
1182 Where many paths and errands meet.
1183 And whither then? I cannot say.
1184 --Bilbo in /usr/src/perl/pp_ctl.c
ac9dac7f 1185 EVER_ON_AND_ON
5a964f20 1186
68dc0745 1187=head1 Data: Arrays
1188
65acb1b1 1189=head2 What is the difference between a list and an array?
1190
ac9dac7f 1191An array has a changeable length. A list does not. An array is
1192something you can push or pop, while a list is a set of values. Some
1193people make the distinction that a list is a value while an array is a
1194variable. Subroutines are passed and return lists, you put things into
1195list context, you initialize arrays with lists, and you C<foreach()>
1196across a list. C<@> variables are arrays, anonymous arrays are
1197arrays, arrays in scalar context behave like the number of elements in
1198them, subroutines access their arguments through the array C<@_>, and
1199C<push>/C<pop>/C<shift> only work on arrays.
65acb1b1 1200
1201As a side note, there's no such thing as a list in scalar context.
1202When you say
1203
ac9dac7f 1204 $scalar = (2, 5, 7, 9);
65acb1b1 1205
d92eb7b0 1206you're using the comma operator in scalar context, so it uses the scalar
ac9dac7f 1207comma operator. There never was a list there at all! This causes the
d92eb7b0 1208last value to be returned: 9.
65acb1b1 1209
68dc0745 1210=head2 What is the difference between $array[1] and @array[1]?
1211
a6dd486b 1212The former is a scalar value; the latter an array slice, making
68dc0745 1213it a list with one (scalar) value. You should use $ when you want a
1214scalar value (most of the time) and @ when you want a list with one
1215scalar value in it (very, very rarely; nearly never, in fact).
1216
1217Sometimes it doesn't make a difference, but sometimes it does.
1218For example, compare:
1219
ac9dac7f 1220 $good[0] = `some program that outputs several lines`;
68dc0745 1221
1222with
1223
ac9dac7f 1224 @bad[0] = `same program that outputs several lines`;
68dc0745 1225
197aec24 1226The C<use warnings> pragma and the B<-w> flag will warn you about these
9f1b1f2d 1227matters.
68dc0745 1228
d92eb7b0 1229=head2 How can I remove duplicate elements from a list or array?
68dc0745 1230
6670e5e7 1231(contributed by brian d foy)
68dc0745 1232
6670e5e7 1233Use a hash. When you think the words "unique" or "duplicated", think
1234"hash keys".
68dc0745 1235
6670e5e7 1236If you don't care about the order of the elements, you could just
1237create the hash then extract the keys. It's not important how you
1238create that hash: just that you use C<keys> to get the unique
1239elements.
551e1d92 1240
ac9dac7f 1241 my %hash = map { $_, 1 } @array;
1242 # or a hash slice: @hash{ @array } = ();
1243 # or a foreach: $hash{$_} = 1 foreach ( @array );
1244
1245 my @unique = keys %hash;
68dc0745 1246
ac9dac7f 1247If you want to use a module, try the C<uniq> function from
1248C<List::MoreUtils>. In list context it returns the unique elements,
1249preserving their order in the list. In scalar context, it returns the
1250number of unique elements.
1251
1252 use List::MoreUtils qw(uniq);
1253
1254 my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
1255 my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7
68dc0745 1256
6670e5e7 1257You can also go through each element and skip the ones you've seen
1258before. Use a hash to keep track. The first time the loop sees an
1259element, that element has no key in C<%Seen>. The C<next> statement
1260creates the key and immediately uses its value, which is C<undef>, so
1261the loop continues to the C<push> and increments the value for that
1262key. The next time the loop sees that same element, its key exists in
1263the hash I<and> the value for that key is true (since it's not 0 or
ac9dac7f 1264C<undef>), so the next skips that iteration and the loop goes to the
1265next element.
551e1d92 1266
6670e5e7 1267 my @unique = ();
1268 my %seen = ();
68dc0745 1269
6670e5e7 1270 foreach my $elem ( @array )
1271 {
1272 next if $seen{ $elem }++;
1273 push @unique, $elem;
1274 }
68dc0745 1275
6670e5e7 1276You can write this more briefly using a grep, which does the
1277same thing.
68dc0745 1278
ac9dac7f 1279 my %seen = ();
1280 my @unique = grep { ! $seen{ $_ }++ } @array;
65acb1b1 1281
ddbc1f16 1282=head2 How can I tell whether a certain element is contained in a list or array?
5a964f20 1283
9e72e4c6 1284(portions of this answer contributed by Anno Siegel)
1285
5a964f20 1286Hearing the word "in" is an I<in>dication that you probably should have
1287used a hash, not a list or array, to store your data. Hashes are
1288designed to answer this question quickly and efficiently. Arrays aren't.
68dc0745 1289
5a964f20 1290That being said, there are several ways to approach this. If you
1291are going to make this query many times over arbitrary string values,
881bdbd4 1292the fastest way is probably to invert the original array and maintain a
1293hash whose keys are the first array's values.
68dc0745 1294
ac9dac7f 1295 @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
1296 %is_blue = ();
1297 for (@blues) { $is_blue{$_} = 1 }
68dc0745 1298
ac9dac7f 1299Now you can check whether C<$is_blue{$some_color}>. It might have
1300been a good idea to keep the blues all in a hash in the first place.
68dc0745 1301
1302If the values are all small integers, you could use a simple indexed
1303array. This kind of an array will take up less space:
1304
ac9dac7f 1305 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
1306 @is_tiny_prime = ();
1307 for (@primes) { $is_tiny_prime[$_] = 1 }
1308 # or simply @istiny_prime[@primes] = (1) x @primes;
68dc0745 1309
1310Now you check whether $is_tiny_prime[$some_number].
1311
1312If the values in question are integers instead of strings, you can save
1313quite a lot of space by using bit strings instead:
1314
ac9dac7f 1315 @articles = ( 1..10, 150..2000, 2017 );
1316 undef $read;
1317 for (@articles) { vec($read,$_,1) = 1 }
68dc0745 1318
1319Now check whether C<vec($read,$n,1)> is true for some C<$n>.
1320
9e72e4c6 1321These methods guarantee fast individual tests but require a re-organization
1322of the original list or array. They only pay off if you have to test
1323multiple values against the same array.
68dc0745 1324
ac9dac7f 1325If you are testing only once, the standard module C<List::Util> exports
9e72e4c6 1326the function C<first> for this purpose. It works by stopping once it
c195e131 1327finds the element. It's written in C for speed, and its Perl equivalent
9e72e4c6 1328looks like this subroutine:
68dc0745 1329
9e72e4c6 1330 sub first (&@) {
1331 my $code = shift;
1332 foreach (@_) {
1333 return $_ if &{$code}();
1334 }
1335 undef;
1336 }
68dc0745 1337
9e72e4c6 1338If speed is of little concern, the common idiom uses grep in scalar context
1339(which returns the number of items that passed its condition) to traverse the
1340entire list. This does have the benefit of telling you how many matches it
1341found, though.
68dc0745 1342
9e72e4c6 1343 my $is_there = grep $_ eq $whatever, @array;
65acb1b1 1344
9e72e4c6 1345If you want to actually extract the matching elements, simply use grep in
1346list context.
68dc0745 1347
9e72e4c6 1348 my @matches = grep $_ eq $whatever, @array;
58103a2e 1349
68dc0745 1350=head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
1351
ac9dac7f 1352Use a hash. Here's code to do both and more. It assumes that each
1353element is unique in a given array:
68dc0745 1354
ac9dac7f 1355 @union = @intersection = @difference = ();
1356 %count = ();
1357 foreach $element (@array1, @array2) { $count{$element}++ }
1358 foreach $element (keys %count) {
1359 push @union, $element;
1360 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
1361 }
68dc0745 1362
ac9dac7f 1363Note that this is the I<symmetric difference>, that is, all elements
1364in either A or in B but not in both. Think of it as an xor operation.
d92eb7b0 1365
65acb1b1 1366=head2 How do I test whether two arrays or hashes are equal?
1367
ac9dac7f 1368The following code works for single-level arrays. It uses a
1369stringwise comparison, and does not distinguish defined versus
1370undefined empty strings. Modify if you have other needs.
65acb1b1 1371
ac9dac7f 1372 $are_equal = compare_arrays(\@frogs, \@toads);
65acb1b1 1373
ac9dac7f 1374 sub compare_arrays {
1375 my ($first, $second) = @_;
1376 no warnings; # silence spurious -w undef complaints
1377 return 0 unless @$first == @$second;
1378 for (my $i = 0; $i < @$first; $i++) {
1379 return 0 if $first->[$i] ne $second->[$i];
1380 }
1381 return 1;
1382 }
65acb1b1 1383
1384For multilevel structures, you may wish to use an approach more
ac9dac7f 1385like this one. It uses the CPAN module C<FreezeThaw>:
65acb1b1 1386
ac9dac7f 1387 use FreezeThaw qw(cmpStr);
1388 @a = @b = ( "this", "that", [ "more", "stuff" ] );
65acb1b1 1389
ac9dac7f 1390 printf "a and b contain %s arrays\n",
1391 cmpStr(\@a, \@b) == 0
1392 ? "the same"
1393 : "different";
65acb1b1 1394
ac9dac7f 1395This approach also works for comparing hashes. Here we'll demonstrate
1396two different answers:
65acb1b1 1397
ac9dac7f 1398 use FreezeThaw qw(cmpStr cmpStrHard);
65acb1b1 1399
ac9dac7f 1400 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
1401 $a{EXTRA} = \%b;
1402 $b{EXTRA} = \%a;
65acb1b1 1403
ac9dac7f 1404 printf "a and b contain %s hashes\n",
65acb1b1 1405 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
1406
ac9dac7f 1407 printf "a and b contain %s hashes\n",
65acb1b1 1408 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
1409
1410
1411The first reports that both those the hashes contain the same data,
1412while the second reports that they do not. Which you prefer is left as
1413an exercise to the reader.
1414
68dc0745 1415=head2 How do I find the first array element for which a condition is true?
1416
49d635f9 1417To find the first array element which satisfies a condition, you can
ac9dac7f 1418use the C<first()> function in the C<List::Util> module, which comes
1419with Perl 5.8. This example finds the first element that contains
1420"Perl".
49d635f9 1421
1422 use List::Util qw(first);
197aec24 1423
49d635f9 1424 my $element = first { /Perl/ } @array;
197aec24 1425
ac9dac7f 1426If you cannot use C<List::Util>, you can make your own loop to do the
49d635f9 1427same thing. Once you find the element, you stop the loop with last.
1428
1429 my $found;
ac9dac7f 1430 foreach ( @array ) {
6670e5e7 1431 if( /Perl/ ) { $found = $_; last }
49d635f9 1432 }
1433
1434If you want the array index, you can iterate through the indices
1435and check the array element at each index until you find one
1436that satisfies the condition.
1437
197aec24 1438 my( $found, $index ) = ( undef, -1 );
ac9dac7f 1439 for( $i = 0; $i < @array; $i++ ) {
1440 if( $array[$i] =~ /Perl/ ) {
6670e5e7 1441 $found = $array[$i];
1442 $index = $i;
1443 last;
1444 }
1445 }
68dc0745 1446
1447=head2 How do I handle linked lists?
1448
1449In general, you usually don't need a linked list in Perl, since with
ac9dac7f 1450regular arrays, you can push and pop or shift and unshift at either
1451end, or you can use splice to add and/or remove arbitrary number of
ac003c96 1452elements at arbitrary points. Both pop and shift are O(1)
ac9dac7f 1453operations on Perl's dynamic arrays. In the absence of shifts and
1454pops, push in general needs to reallocate on the order every log(N)
1455times, and unshift will need to copy pointers each time.
68dc0745 1456
1457If you really, really wanted, you could use structures as described in
ac9dac7f 1458L<perldsc> or L<perltoot> and do just what the algorithm book tells
1459you to do. For example, imagine a list node like this:
65acb1b1 1460
ac9dac7f 1461 $node = {
1462 VALUE => 42,
1463 LINK => undef,
1464 };
65acb1b1 1465
1466You could walk the list this way:
1467
ac9dac7f 1468 print "List: ";
1469 for ($node = $head; $node; $node = $node->{LINK}) {
1470 print $node->{VALUE}, " ";
1471 }
1472 print "\n";
65acb1b1 1473
a6dd486b 1474You could add to the list this way:
65acb1b1 1475
ac9dac7f 1476 my ($head, $tail);
1477 $tail = append($head, 1); # grow a new head
1478 for $value ( 2 .. 10 ) {
1479 $tail = append($tail, $value);
1480 }
65acb1b1 1481
ac9dac7f 1482 sub append {
1483 my($list, $value) = @_;
1484 my $node = { VALUE => $value };
1485 if ($list) {
1486 $node->{LINK} = $list->{LINK};
1487 $list->{LINK} = $node;
1488 }
1489 else {
1490 $_[0] = $node; # replace caller's version
1491 }
1492 return $node;
1493 }
65acb1b1 1494
1495But again, Perl's built-in are virtually always good enough.
68dc0745 1496
1497=head2 How do I handle circular lists?
1498
1499Circular lists could be handled in the traditional fashion with linked
1500lists, or you could just do something like this with an array:
1501
ac9dac7f 1502 unshift(@array, pop(@array)); # the last shall be first
1503 push(@array, shift(@array)); # and vice versa
1504
1505You can also use C<Tie::Cycle>:
1506
1507 use Tie::Cycle;
1508
1509 tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
1510
1511 print $cycle; # FFFFFF
1512 print $cycle; # 000000
1513 print $cycle; # FFFF00
68dc0745 1514
1515=head2 How do I shuffle an array randomly?
1516
45bbf655 1517If you either have Perl 5.8.0 or later installed, or if you have
1518Scalar-List-Utils 1.03 or later installed, you can say:
1519
ac9dac7f 1520 use List::Util 'shuffle';
45bbf655 1521
1522 @shuffled = shuffle(@list);
1523
f05bbc40 1524If not, you can use a Fisher-Yates shuffle.
5a964f20 1525
ac9dac7f 1526 sub fisher_yates_shuffle {
1527 my $deck = shift; # $deck is a reference to an array
1528 my $i = @$deck;
1529 while (--$i) {
1530 my $j = int rand ($i+1);
1531 @$deck[$i,$j] = @$deck[$j,$i];
1532 }
1533 }
5a964f20 1534
ac9dac7f 1535 # shuffle my mpeg collection
1536 #
1537 my @mpeg = <audio/*/*.mp3>;
1538 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
1539 print @mpeg;
5a964f20 1540
45bbf655 1541Note that the above implementation shuffles an array in place,
ac9dac7f 1542unlike the C<List::Util::shuffle()> which takes a list and returns
45bbf655 1543a new shuffled list.
1544
d92eb7b0 1545You've probably seen shuffling algorithms that work using splice,
a6dd486b 1546randomly picking another element to swap the current element with
68dc0745 1547
ac9dac7f 1548 srand;
1549 @new = ();
1550 @old = 1 .. 10; # just a demo
1551 while (@old) {
1552 push(@new, splice(@old, rand @old, 1));
1553 }
68dc0745 1554
ac9dac7f 1555This is bad because splice is already O(N), and since you do it N
1556times, you just invented a quadratic algorithm; that is, O(N**2).
1557This does not scale, although Perl is so efficient that you probably
1558won't notice this until you have rather largish arrays.
68dc0745 1559
1560=head2 How do I process/modify each element of an array?
1561
1562Use C<for>/C<foreach>:
1563
ac9dac7f 1564 for (@lines) {
6670e5e7 1565 s/foo/bar/; # change that word
1566 tr/XZ/ZX/; # swap those letters
ac9dac7f 1567 }
68dc0745 1568
1569Here's another; let's compute spherical volumes:
1570
ac9dac7f 1571 for (@volumes = @radii) { # @volumes has changed parts
6670e5e7 1572 $_ **= 3;
1573 $_ *= (4/3) * 3.14159; # this will be constant folded
ac9dac7f 1574 }
197aec24 1575
ac9dac7f 1576which can also be done with C<map()> which is made to transform
49d635f9 1577one list into another:
1578
1579 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
68dc0745 1580
76817d6d 1581If you want to do the same thing to modify the values of the
1582hash, you can use the C<values> function. As of Perl 5.6
1583the values are not copied, so if you modify $orbit (in this
1584case), you modify the value.
5a964f20 1585
ac9dac7f 1586 for $orbit ( values %orbits ) {
6670e5e7 1587 ($orbit **= 3) *= (4/3) * 3.14159;
ac9dac7f 1588 }
818c4caa 1589
76817d6d 1590Prior to perl 5.6 C<values> returned copies of the values,
1591so older perl code often contains constructions such as
1592C<@orbits{keys %orbits}> instead of C<values %orbits> where
1593the hash is to be modified.
818c4caa 1594
68dc0745 1595=head2 How do I select a random element from an array?
1596
ac9dac7f 1597Use the C<rand()> function (see L<perlfunc/rand>):
68dc0745 1598
ac9dac7f 1599 $index = rand @array;
1600 $element = $array[$index];
68dc0745 1601
793f5136 1602Or, simply:
ac9dac7f 1603
1604 my $element = $array[ rand @array ];
5a964f20 1605
68dc0745 1606=head2 How do I permute N elements of a list?
c195e131 1607X<List::Permuter> X<permute> X<Algorithm::Loops> X<Knuth>
1608X<The Art of Computer Programming> X<Fischer-Krause>
68dc0745 1609
c195e131 1610Use the C<List::Permutor> module on CPAN. If the list is actually an
ac9dac7f 1611array, try the C<Algorithm::Permute> module (also on CPAN). It's
c195e131 1612written in XS code and is very efficient:
49d635f9 1613
1614 use Algorithm::Permute;
c195e131 1615
49d635f9 1616 my @array = 'a'..'d';
1617 my $p_iterator = Algorithm::Permute->new ( \@array );
c195e131 1618
49d635f9 1619 while (my @perm = $p_iterator->next) {
1620 print "next permutation: (@perm)\n";
ac9dac7f 1621 }
49d635f9 1622
197aec24 1623For even faster execution, you could do:
1624
ac9dac7f 1625 use Algorithm::Permute;
c195e131 1626
ac9dac7f 1627 my @array = 'a'..'d';
c195e131 1628
ac9dac7f 1629 Algorithm::Permute::permute {
1630 print "next permutation: (@array)\n";
1631 } @array;
197aec24 1632
c195e131 1633Here's a little program that generates all permutations of all the
1634words on each line of input. The algorithm embodied in the
1635C<permute()> function is discussed in Volume 4 (still unpublished) of
1636Knuth's I<The Art of Computer Programming> and will work on any list:
49d635f9 1637
1638 #!/usr/bin/perl -n
ac003c96 1639 # Fischer-Krause ordered permutation generator
49d635f9 1640
1641 sub permute (&@) {
1642 my $code = shift;
1643 my @idx = 0..$#_;
1644 while ( $code->(@_[@idx]) ) {
1645 my $p = $#idx;
1646 --$p while $idx[$p-1] > $idx[$p];
1647 my $q = $p or return;
1648 push @idx, reverse splice @idx, $p;
1649 ++$q while $idx[$p-1] > $idx[$q];
1650 @idx[$p-1,$q]=@idx[$q,$p-1];
1651 }
68dc0745 1652 }
68dc0745 1653
c195e131 1654 permute { print "@_\n" } split;
1655
1656The C<Algorithm::Loops> module also provides the C<NextPermute> and
1657C<NextPermuteNum> functions which efficiently find all unique permutations
1658of an array, even if it contains duplicate values, modifying it in-place:
1659if its elements are in reverse-sorted order then the array is reversed,
1660making it sorted, and it returns false; otherwise the next
1661permutation is returned.
1662
1663C<NextPermute> uses string order and C<NextPermuteNum> numeric order, so
1664you can enumerate all the permutations of C<0..9> like this:
1665
1666 use Algorithm::Loops qw(NextPermuteNum);
1667
1668 my @list= 0..9;
1669 do { print "@list\n" } while NextPermuteNum @list;
b8d2732a 1670
68dc0745 1671=head2 How do I sort an array by (anything)?
1672
1673Supply a comparison function to sort() (described in L<perlfunc/sort>):
1674
ac9dac7f 1675 @list = sort { $a <=> $b } @list;
68dc0745 1676
1677The default sort function is cmp, string comparison, which would
c47ff5f1 1678sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
68dc0745 1679the numerical comparison operator.
1680
1681If you have a complicated function needed to pull out the part you
1682want to sort on, then don't do it inside the sort function. Pull it
1683out first, because the sort BLOCK can be called many times for the
1684same element. Here's an example of how to pull out the first word
1685after the first number on each item, and then sort those words
1686case-insensitively.
1687
ac9dac7f 1688 @idx = ();
1689 for (@data) {
1690 ($item) = /\d+\s*(\S+)/;
1691 push @idx, uc($item);
1692 }
1693 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
68dc0745 1694
a6dd486b 1695which could also be written this way, using a trick
68dc0745 1696that's come to be known as the Schwartzian Transform:
1697
ac9dac7f 1698 @sorted = map { $_->[0] }
1699 sort { $a->[1] cmp $b->[1] }
1700 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
68dc0745 1701
1702If you need to sort on several fields, the following paradigm is useful.
1703
ac9dac7f 1704 @sorted = sort {
1705 field1($a) <=> field1($b) ||
1706 field2($a) cmp field2($b) ||
1707 field3($a) cmp field3($b)
1708 } @data;
68dc0745 1709
1710This can be conveniently combined with precalculation of keys as given
1711above.
1712
379e39d7 1713See the F<sort> article in the "Far More Than You Ever Wanted
49d635f9 1714To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
06a5f41f 1715more about this approach.
68dc0745 1716
ac9dac7f 1717See also the question later in L<perlfaq4> on sorting hashes.
68dc0745 1718
1719=head2 How do I manipulate arrays of bits?
1720
ac9dac7f 1721Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise
1722operations.
1723
1724For example, this sets C<$vec> to have bit N set if C<$ints[N]> was
1725set:
1726
1727 $vec = '';
1728 foreach(@ints) { vec($vec,$_,1) = 1 }
1729
1730Here's how, given a vector in C<$vec>, you can get those bits into your
1731C<@ints> array:
1732
1733 sub bitvec_to_list {
1734 my $vec = shift;
1735 my @ints;
1736 # Find null-byte density then select best algorithm
1737 if ($vec =~ tr/\0// / length $vec > 0.95) {
1738 use integer;
1739 my $i;
1740
1741 # This method is faster with mostly null-bytes
1742 while($vec =~ /[^\0]/g ) {
1743 $i = -9 + 8 * pos $vec;
1744 push @ints, $i if vec($vec, ++$i, 1);
1745 push @ints, $i if vec($vec, ++$i, 1);
1746 push @ints, $i if vec($vec, ++$i, 1);
1747 push @ints, $i if vec($vec, ++$i, 1);
1748 push @ints, $i if vec($vec, ++$i, 1);
1749 push @ints, $i if vec($vec, ++$i, 1);
1750 push @ints, $i if vec($vec, ++$i, 1);
1751 push @ints, $i if vec($vec, ++$i, 1);
1752 }
1753 }
1754 else {
1755 # This method is a fast general algorithm
1756 use integer;
1757 my $bits = unpack "b*", $vec;
1758 push @ints, 0 if $bits =~ s/^(\d)// && $1;
1759 push @ints, pos $bits while($bits =~ /1/g);
1760 }
1761
1762 return \@ints;
1763 }
68dc0745 1764
1765This method gets faster the more sparse the bit vector is.
1766(Courtesy of Tim Bunce and Winfried Koenig.)
1767
76817d6d 1768You can make the while loop a lot shorter with this suggestion
1769from Benjamin Goldberg:
1770
1771 while($vec =~ /[^\0]+/g ) {
ac9dac7f 1772 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
1773 }
76817d6d 1774
ac9dac7f 1775Or use the CPAN module C<Bit::Vector>:
cc30d1a7 1776
ac9dac7f 1777 $vector = Bit::Vector->new($num_of_bits);
1778 $vector->Index_List_Store(@ints);
1779 @ints = $vector->Index_List_Read();
cc30d1a7 1780
ac9dac7f 1781C<Bit::Vector> provides efficient methods for bit vector, sets of
1782small integers and "big int" math.
cc30d1a7 1783
1784Here's a more extensive illustration using vec():
65acb1b1 1785
ac9dac7f 1786 # vec demo
1787 $vector = "\xff\x0f\xef\xfe";
1788 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
65acb1b1 1789 unpack("N", $vector), "\n";
ac9dac7f 1790 $is_set = vec($vector, 23, 1);
1791 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
65acb1b1 1792 pvec($vector);
65acb1b1 1793
ac9dac7f 1794 set_vec(1,1,1);
1795 set_vec(3,1,1);
1796 set_vec(23,1,1);
1797
1798 set_vec(3,1,3);
1799 set_vec(3,2,3);
1800 set_vec(3,4,3);
1801 set_vec(3,4,7);
1802 set_vec(3,8,3);
1803 set_vec(3,8,7);
1804
1805 set_vec(0,32,17);
1806 set_vec(1,32,17);
1807
1808 sub set_vec {
1809 my ($offset, $width, $value) = @_;
1810 my $vector = '';
1811 vec($vector, $offset, $width) = $value;
1812 print "offset=$offset width=$width value=$value\n";
1813 pvec($vector);
1814 }
65acb1b1 1815
ac9dac7f 1816 sub pvec {
1817 my $vector = shift;
1818 my $bits = unpack("b*", $vector);
1819 my $i = 0;
1820 my $BASE = 8;
1821
1822 print "vector length in bytes: ", length($vector), "\n";
1823 @bytes = unpack("A8" x length($vector), $bits);
1824 print "bits are: @bytes\n\n";
1825 }
65acb1b1 1826
68dc0745 1827=head2 Why does defined() return true on empty arrays and hashes?
1828
65acb1b1 1829The short story is that you should probably only use defined on scalars or
1830functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
1831in the 5.004 release or later of Perl for more detail.
68dc0745 1832
1833=head1 Data: Hashes (Associative Arrays)
1834
1835=head2 How do I process an entire hash?
1836
ee891a00 1837(contributed by brian d foy)
1838
1839There are a couple of ways that you can process an entire hash. You
1840can get a list of keys, then go through each key, or grab a one
1841key-value pair at a time.
68dc0745 1842
ee891a00 1843To go through all of the keys, use the C<keys> function. This extracts
1844all of the keys of the hash and gives them back to you as a list. You
1845can then get the value through the particular key you're processing:
1846
1847 foreach my $key ( keys %hash ) {
1848 my $value = $hash{$key}
1849 ...
ac9dac7f 1850 }
68dc0745 1851
ee891a00 1852Once you have the list of keys, you can process that list before you
1853process the hashh elements. For instance, you can sort the keys so you
1854can process them in lexical order:
1855
1856 foreach my $key ( sort keys %hash ) {
1857 my $value = $hash{$key}
1858 ...
1859 }
1860
1861Or, you might want to only process some of the items. If you only want
1862to deal with the keys that start with C<text:>, you can select just
1863those using C<grep>:
1864
1865 foreach my $key ( grep /^text:/, keys %hash ) {
1866 my $value = $hash{$key}
1867 ...
1868 }
1869
1870If the hash is very large, you might not want to create a long list of
1871keys. To save some memory, you can grab on key-value pair at a time using
1872C<each()>, which returns a pair you haven't seen yet:
1873
1874 while( my( $key, $value ) = each( %hash ) ) {
1875 ...
1876 }
1877
1878The C<each> operator returns the pairs in apparently random order, so if
1879ordering matters to you, you'll have to stick with the C<keys> method.
1880
1881The C<each()> operator can be a bit tricky though. You can't add or
1882delete keys of the hash while you're using it without possibly
1883skipping or re-processing some pairs after Perl internally rehashes
1884all of the elements. Additionally, a hash has only one iterator, so if
1885you use C<keys>, C<values>, or C<each> on the same hash, you can reset
1886the iterator and mess up your processing. See the C<each> entry in
1887L<perlfunc> for more details.
68dc0745 1888
1889=head2 What happens if I add or remove keys from a hash while iterating over it?
1890
28b41a80 1891(contributed by brian d foy)
d92eb7b0 1892
28b41a80 1893The easy answer is "Don't do that!"
d92eb7b0 1894
28b41a80 1895If you iterate through the hash with each(), you can delete the key
1896most recently returned without worrying about it. If you delete or add
1897other keys, the iterator may skip or double up on them since perl
1898may rearrange the hash table. See the
1899entry for C<each()> in L<perlfunc>.
68dc0745 1900
1901=head2 How do I look up a hash element by value?
1902
1903Create a reverse hash:
1904
ac9dac7f 1905 %by_value = reverse %by_key;
1906 $key = $by_value{$value};
68dc0745 1907
1908That's not particularly efficient. It would be more space-efficient
1909to use:
1910
ac9dac7f 1911 while (($key, $value) = each %by_key) {
1912 $by_value{$value} = $key;
1913 }
68dc0745 1914
d92eb7b0 1915If your hash could have repeated values, the methods above will only find
1916one of the associated keys. This may or may not worry you. If it does
1917worry you, you can always reverse the hash into a hash of arrays instead:
1918
ac9dac7f 1919 while (($key, $value) = each %by_key) {
1920 push @{$key_list_by_value{$value}}, $key;
1921 }
68dc0745 1922
1923=head2 How can I know how many entries are in a hash?
1924
1925If you mean how many keys, then all you have to do is
875e5c2f 1926use the keys() function in a scalar context:
68dc0745 1927
875e5c2f 1928 $num_keys = keys %hash;
68dc0745 1929
197aec24 1930The keys() function also resets the iterator, which means that you may
1931see strange results if you use this between uses of other hash operators
875e5c2f 1932such as each().
68dc0745 1933
1934=head2 How do I sort a hash (optionally by value instead of key)?
1935
a05e4845 1936(contributed by brian d foy)
1937
1938To sort a hash, start with the keys. In this example, we give the list of
1939keys to the sort function which then compares them ASCIIbetically (which
1940might be affected by your locale settings). The output list has the keys
1941in ASCIIbetical order. Once we have the keys, we can go through them to
1942create a report which lists the keys in ASCIIbetical order.
1943
1944 my @keys = sort { $a cmp $b } keys %hash;
58103a2e 1945
a05e4845 1946 foreach my $key ( @keys )
1947 {
1948 printf "%-20s %6d\n", $key, $hash{$value};
1949 }
1950
58103a2e 1951We could get more fancy in the C<sort()> block though. Instead of
a05e4845 1952comparing the keys, we can compute a value with them and use that
58103a2e 1953value as the comparison.
a05e4845 1954
1955For instance, to make our report order case-insensitive, we use
58103a2e 1956the C<\L> sequence in a double-quoted string to make everything
a05e4845 1957lowercase. The C<sort()> block then compares the lowercased
1958values to determine in which order to put the keys.
1959
1960 my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
58103a2e 1961
a05e4845 1962Note: if the computation is expensive or the hash has many elements,
58103a2e 1963you may want to look at the Schwartzian Transform to cache the
a05e4845 1964computation results.
1965
1966If we want to sort by the hash value instead, we use the hash key
1967to look it up. We still get out a list of keys, but this time they
1968are ordered by their value.
1969
1970 my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
1971
1972From there we can get more complex. If the hash values are the same,
1973we can provide a secondary sort on the hash key.
1974
58103a2e 1975 my @keys = sort {
1976 $hash{$a} <=> $hash{$b}
a05e4845 1977 or
1978 "\L$a" cmp "\L$b"
1979 } keys %hash;
68dc0745 1980
1981=head2 How can I always keep my hash sorted?
ac9dac7f 1982X<hash tie sort DB_File Tie::IxHash>
68dc0745 1983
ac9dac7f 1984You can look into using the C<DB_File> module and C<tie()> using the
1985C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory
1986Databases">. The C<Tie::IxHash> module from CPAN might also be
1987instructive. Although this does keep your hash sorted, you might not
1988like the slow down you suffer from the tie interface. Are you sure you
1989need to do this? :)
68dc0745 1990
1991=head2 What's the difference between "delete" and "undef" with hashes?
1992
92993692 1993Hashes contain pairs of scalars: the first is the key, the
1994second is the value. The key will be coerced to a string,
1995although the value can be any kind of scalar: string,
ac9dac7f 1996number, or reference. If a key C<$key> is present in
92993692 1997%hash, C<exists($hash{$key})> will return true. The value
1998for a given key can be C<undef>, in which case
1999C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
2000will return true. This corresponds to (C<$key>, C<undef>)
2001being in the hash.
68dc0745 2002
ac9dac7f 2003Pictures help... here's the C<%hash> table:
68dc0745 2004
2005 keys values
2006 +------+------+
2007 | a | 3 |
2008 | x | 7 |
2009 | d | 0 |
2010 | e | 2 |
2011 +------+------+
2012
2013And these conditions hold
2014
92993692 2015 $hash{'a'} is true
2016 $hash{'d'} is false
2017 defined $hash{'d'} is true
2018 defined $hash{'a'} is true
e9d185f8 2019 exists $hash{'a'} is true (Perl 5 only)
92993692 2020 grep ($_ eq 'a', keys %hash) is true
68dc0745 2021
2022If you now say
2023
92993692 2024 undef $hash{'a'}
68dc0745 2025
2026your table now reads:
2027
2028
2029 keys values
2030 +------+------+
2031 | a | undef|
2032 | x | 7 |
2033 | d | 0 |
2034 | e | 2 |
2035 +------+------+
2036
2037and these conditions now hold; changes in caps:
2038
92993692 2039 $hash{'a'} is FALSE
2040 $hash{'d'} is false
2041 defined $hash{'d'} is true
2042 defined $hash{'a'} is FALSE
e9d185f8 2043 exists $hash{'a'} is true (Perl 5 only)
92993692 2044 grep ($_ eq 'a', keys %hash) is true
68dc0745 2045
2046Notice the last two: you have an undef value, but a defined key!
2047
2048Now, consider this:
2049
92993692 2050 delete $hash{'a'}
68dc0745 2051
2052your table now reads:
2053
2054 keys values
2055 +------+------+
2056 | x | 7 |
2057 | d | 0 |
2058 | e | 2 |
2059 +------+------+
2060
2061and these conditions now hold; changes in caps:
2062
92993692 2063 $hash{'a'} is false
2064 $hash{'d'} is false
2065 defined $hash{'d'} is true
2066 defined $hash{'a'} is false
e9d185f8 2067 exists $hash{'a'} is FALSE (Perl 5 only)
92993692 2068 grep ($_ eq 'a', keys %hash) is FALSE
68dc0745 2069
2070See, the whole entry is gone!
2071
2072=head2 Why don't my tied hashes make the defined/exists distinction?
2073
92993692 2074This depends on the tied hash's implementation of EXISTS().
2075For example, there isn't the concept of undef with hashes
2076that are tied to DBM* files. It also means that exists() and
2077defined() do the same thing with a DBM* file, and what they
2078end up doing is not what they do with ordinary hashes.
68dc0745 2079
2080=head2 How do I reset an each() operation part-way through?
2081
fb2fe781 2082(contributed by brian d foy)
2083
2084You can use the C<keys> or C<values> functions to reset C<each>. To
2085simply reset the iterator used by C<each> without doing anything else,
2086use one of them in void context:
2087
2088 keys %hash; # resets iterator, nothing else.
2089 values %hash; # resets iterator, nothing else.
2090
2091See the documentation for C<each> in L<perlfunc>.
68dc0745 2092
2093=head2 How can I get the unique keys from two hashes?
2094
d92eb7b0 2095First you extract the keys from the hashes into lists, then solve
2096the "removing duplicates" problem described above. For example:
68dc0745 2097
ac9dac7f 2098 %seen = ();
2099 for $element (keys(%foo), keys(%bar)) {
2100 $seen{$element}++;
2101 }
2102 @uniq = keys %seen;
68dc0745 2103
2104Or more succinctly:
2105
ac9dac7f 2106 @uniq = keys %{{%foo,%bar}};
68dc0745 2107
2108Or if you really want to save space:
2109
ac9dac7f 2110 %seen = ();
2111 while (defined ($key = each %foo)) {
2112 $seen{$key}++;
2113 }
2114 while (defined ($key = each %bar)) {
2115 $seen{$key}++;
2116 }
2117 @uniq = keys %seen;
68dc0745 2118
2119=head2 How can I store a multidimensional array in a DBM file?
2120
2121Either stringify the structure yourself (no fun), or else
2122get the MLDBM (which uses Data::Dumper) module from CPAN and layer
2123it on top of either DB_File or GDBM_File.
2124
2125=head2 How can I make my hash remember the order I put elements into it?
2126
ac9dac7f 2127Use the C<Tie::IxHash> from CPAN.
68dc0745 2128
ac9dac7f 2129 use Tie::IxHash;
2130
2131 tie my %myhash, 'Tie::IxHash';
2132
2133 for (my $i=0; $i<20; $i++) {
2134 $myhash{$i} = 2*$i;
2135 }
2136
2137 my @keys = keys %myhash;
2138 # @keys = (0,1,2,3,...)
46fc3d4c 2139
68dc0745 2140=head2 Why does passing a subroutine an undefined element in a hash create it?
2141
2142If you say something like:
2143
ac9dac7f 2144 somefunc($hash{"nonesuch key here"});
68dc0745 2145
2146Then that element "autovivifies"; that is, it springs into existence
2147whether you store something there or not. That's because functions
2148get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
2149it has to be ready to write it back into the caller's version.
2150
87275199 2151This has been fixed as of Perl5.004.
68dc0745 2152
2153Normally, merely accessing a key's value for a nonexistent key does
2154I<not> cause that key to be forever there. This is different than
2155awk's behavior.
2156
fc36a67e 2157=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
68dc0745 2158
65acb1b1 2159Usually a hash ref, perhaps like this:
2160
ac9dac7f 2161 $record = {
2162 NAME => "Jason",
2163 EMPNO => 132,
2164 TITLE => "deputy peon",
2165 AGE => 23,
2166 SALARY => 37_000,
2167 PALS => [ "Norbert", "Rhys", "Phineas"],
2168 };
65acb1b1 2169
2170References are documented in L<perlref> and the upcoming L<perlreftut>.
2171Examples of complex data structures are given in L<perldsc> and
2172L<perllol>. Examples of structures and object-oriented classes are
2173in L<perltoot>.
68dc0745 2174
2175=head2 How can I use a reference as a hash key?
2176
9e72e4c6 2177(contributed by brian d foy)
2178
2179Hash keys are strings, so you can't really use a reference as the key.
2180When you try to do that, perl turns the reference into its stringified
ac9dac7f 2181form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get
2182back the reference from the stringified form, at least without doing
2183some extra work on your own. Also remember that hash keys must be
2184unique, but two different variables can store the same reference (and
2185those variables can change later).
9e72e4c6 2186
ac9dac7f 2187The C<Tie::RefHash> module, which is distributed with perl, might be
2188what you want. It handles that extra work.
68dc0745 2189
2190=head1 Data: Misc
2191
2192=head2 How do I handle binary data correctly?
2193
ac9dac7f 2194Perl is binary clean, so it can handle binary data just fine.
e573f903 2195On Windows or DOS, however, you have to use C<binmode> for binary
ac9dac7f 2196files to avoid conversions for line endings. In general, you should
2197use C<binmode> any time you want to work with binary data.
68dc0745 2198
ac9dac7f 2199Also see L<perlfunc/"binmode"> or L<perlopentut>.
68dc0745 2200
ac9dac7f 2201If you're concerned about 8-bit textual data then see L<perllocale>.
54310121 2202If you want to deal with multibyte characters, however, there are
68dc0745 2203some gotchas. See the section on Regular Expressions.
2204
2205=head2 How do I determine whether a scalar is a number/whole/integer/float?
2206
2207Assuming that you don't care about IEEE notations like "NaN" or
2208"Infinity", you probably just want to use a regular expression.
2209
ac9dac7f 2210 if (/\D/) { print "has nondigits\n" }
2211 if (/^\d+$/) { print "is a whole number\n" }
2212 if (/^-?\d+$/) { print "is an integer\n" }
2213 if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
2214 if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
2215 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
2216 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
881bdbd4 2217 { print "a C float\n" }
68dc0745 2218
f0d19b68 2219There are also some commonly used modules for the task.
2220L<Scalar::Util> (distributed with 5.8) provides access to perl's
ac9dac7f 2221internal function C<looks_like_number> for determining whether a
2222variable looks like a number. L<Data::Types> exports functions that
2223validate data types using both the above and other regular
2224expressions. Thirdly, there is C<Regexp::Common> which has regular
2225expressions to match various types of numbers. Those three modules are
2226available from the CPAN.
f0d19b68 2227
2228If you're on a POSIX system, Perl supports the C<POSIX::strtod>
ac9dac7f 2229function. Its semantics are somewhat cumbersome, so here's a
2230C<getnum> wrapper function for more convenient access. This function
2231takes a string and returns the number it found, or C<undef> for input
2232that isn't a C float. The C<is_numeric> function is a front end to
2233C<getnum> if you just want to say, "Is this a float?"
2234
2235 sub getnum {
2236 use POSIX qw(strtod);
2237 my $str = shift;
2238 $str =~ s/^\s+//;
2239 $str =~ s/\s+$//;
2240 $! = 0;
2241 my($num, $unparsed) = strtod($str);
2242 if (($str eq '') || ($unparsed != 0) || $!) {
2243 return undef;
2244 }
2245 else {
2246 return $num;
2247 }
2248 }
5a964f20 2249
ac9dac7f 2250 sub is_numeric { defined getnum($_[0]) }
5a964f20 2251
f0d19b68 2252Or you could check out the L<String::Scanf> module on the CPAN
ac9dac7f 2253instead. The C<POSIX> module (part of the standard Perl distribution)
2254provides the C<strtod> and C<strtol> for converting strings to double
2255and longs, respectively.
68dc0745 2256
2257=head2 How do I keep persistent data across program calls?
2258
2259For some specific applications, you can use one of the DBM modules.
ac9dac7f 2260See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw>
2261or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part
2262of the standard distribution. Here's one example using C<Storable>'s C<store>
fe854a6f 2263and C<retrieve> functions:
65acb1b1 2264
ac9dac7f 2265 use Storable;
2266 store(\%hash, "filename");
65acb1b1 2267
ac9dac7f 2268 # later on...
2269 $href = retrieve("filename"); # by ref
2270 %hash = %{ retrieve("filename") }; # direct to hash
68dc0745 2271
2272=head2 How do I print out or copy a recursive data structure?
2273
ac9dac7f 2274The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great
2275for printing out data structures. The C<Storable> module on CPAN (or the
6f82c03a 22765.8 release of Perl), provides a function called C<dclone> that recursively
2277copies its argument.
65acb1b1 2278
ac9dac7f 2279 use Storable qw(dclone);
2280 $r2 = dclone($r1);
68dc0745 2281
ac9dac7f 2282Where C<$r1> can be a reference to any kind of data structure you'd like.
65acb1b1 2283It will be deeply copied. Because C<dclone> takes and returns references,
2284you'd have to add extra punctuation if you had a hash of arrays that
2285you wanted to copy.
68dc0745 2286
ac9dac7f 2287 %newhash = %{ dclone(\%oldhash) };
68dc0745 2288
2289=head2 How do I define methods for every class/object?
2290
ac9dac7f 2291Use the C<UNIVERSAL> class (see L<UNIVERSAL>).
68dc0745 2292
2293=head2 How do I verify a credit card checksum?
2294
ac9dac7f 2295Get the C<Business::CreditCard> module from CPAN.
68dc0745 2296
65acb1b1 2297=head2 How do I pack arrays of doubles or floats for XS code?
2298
ac9dac7f 2299The kgbpack.c code in the C<PGPLOT> module on CPAN does just this.
65acb1b1 2300If you're doing a lot of float or double processing, consider using
ac9dac7f 2301the C<PDL> module from CPAN instead--it makes number-crunching easy.
65acb1b1 2302
500071f4 2303=head1 REVISION
2304
fb2fe781 2305Revision: $Revision: 10394 $
500071f4 2306
fb2fe781 2307Date: $Date: 2007-12-09 18:47:15 +0100 (Sun, 09 Dec 2007) $
500071f4 2308
2309See L<perlfaq> for source control details and availability.
2310
68dc0745 2311=head1 AUTHOR AND COPYRIGHT
2312
ee891a00 2313Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
7678cced 2314other authors as noted. All rights reserved.
5a964f20 2315
5a7beb56 2316This documentation is free; you can redistribute it and/or modify it
2317under the same terms as Perl itself.
5a964f20 2318
2319Irrespective of its distribution, all code examples in this file
2320are hereby placed into the public domain. You are permitted and
2321encouraged to use this code in your own programs for fun
2322or for profit as you see fit. A simple comment in the code giving
2323credit would be courteous but is not required.