| [ Index ] |
PHP Cross Reference of Unnamed Project |
[Summary view] [Print] [Text view]
1 =head1 NAME 2 3 perlfaq4 - Data Manipulation ($Revision: 10394 $) 4 5 =head1 DESCRIPTION 6 7 This section of the FAQ answers questions related to manipulating 8 numbers, dates, strings, arrays, hashes, and miscellaneous data issues. 9 10 =head1 Data: Numbers 11 12 =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? 13 14 Internally, your computer represents floating-point numbers in binary. 15 Digital (as in powers of two) computers cannot store all numbers 16 exactly. Some real numbers lose precision in the process. This is a 17 problem with how computers store numbers and affects all computer 18 languages, not just Perl. 19 20 L<perlnumber> shows the gory details of number representations and 21 conversions. 22 23 To limit the number of decimal places in your numbers, you can use the 24 printf or sprintf function. See the L<"Floating Point 25 Arithmetic"|perlop> for more details. 26 27 printf "%.2f", 10/3; 28 29 my $number = sprintf "%.2f", 10/3; 30 31 =head2 Why is int() broken? 32 33 Your C<int()> is most probably working just fine. It's the numbers that 34 aren't quite what you think. 35 36 First, see the answer to "Why am I getting long decimals 37 (eg, 19.9499999999999) instead of the numbers I should be getting 38 (eg, 19.95)?". 39 40 For example, this 41 42 print int(0.6/0.2-2), "\n"; 43 44 will in most computers print 0, not 1, because even such simple 45 numbers as 0.6 and 0.2 cannot be presented exactly by floating-point 46 numbers. What you think in the above as 'three' is really more like 47 2.9999999999999995559. 48 49 =head2 Why isn't my octal data interpreted correctly? 50 51 Perl only understands octal and hex numbers as such when they occur as 52 literals in your program. Octal literals in perl must start with a 53 leading C<0> and hexadecimal literals must start with a leading C<0x>. 54 If they are read in from somewhere and assigned, no automatic 55 conversion takes place. You must explicitly use C<oct()> or C<hex()> if you 56 want the values converted to decimal. C<oct()> interprets hexadecimal (C<0x350>), 57 octal (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 59 or without a leading C<0x>, such as C<0x255>, C<3A>, C<ff>, or C<deadbeef>. 60 The inverse mapping from decimal to octal can be done with either the 61 <%o> or C<%O> C<sprintf()> formats. 62 63 This problem shows up most often when people try using C<chmod()>, 64 C<mkdir()>, C<umask()>, or C<sysopen()>, which by widespread tradition 65 typically take permissions in octal. 66 67 chmod(644, $file); # WRONG 68 chmod(0644, $file); # right 69 70 Note the mistake in the first line was specifying the decimal literal 71 C<644>, rather than the intended octal literal C<0644>. The problem can 72 be seen with: 73 74 printf("%#o",644); # prints 01204 75 76 Surely you had not intended C<chmod(01204, $file);> - did you? If you 77 want to use numeric literals as arguments to chmod() et al. then please 78 try to express them as octal constants, that is with a leading zero and 79 with the following digits restricted to the set C<0..7>. 80 81 =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? 82 83 Remember that C<int()> merely truncates toward 0. For rounding to a 84 certain number of digits, C<sprintf()> or C<printf()> is usually the 85 easiest route. 86 87 printf("%.3f", 3.1415926535); # prints 3.142 88 89 The C<POSIX> module (part of the standard Perl distribution) 90 implements C<ceil()>, C<floor()>, and a number of other mathematical 91 and trigonometric functions. 92 93 use POSIX; 94 $ceil = ceil(3.5); # 4 95 $floor = floor(3.5); # 3 96 97 In 5.000 to 5.003 perls, trigonometry was done in the C<Math::Complex> 98 module. With 5.004, the C<Math::Trig> module (part of the standard Perl 99 distribution) implements the trigonometric functions. Internally it 100 uses the C<Math::Complex> module and some functions can break out from 101 the real axis into the complex plane, for example the inverse sine of 102 2. 103 104 Rounding in financial applications can have serious implications, and 105 the rounding method used should be specified precisely. In these 106 cases, it probably pays not to trust whichever system rounding is 107 being used by Perl, but to instead implement the rounding function you 108 need yourself. 109 110 To see why, notice how you'll still have an issue on half-way-point 111 alternation: 112 113 for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} 114 115 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 1.0 117 118 Don't blame Perl. It's the same as in C. IEEE says we have to do 119 this. Perl numbers whose absolute values are integers under 2**31 (on 120 32 bit machines) will work pretty much like mathematical integers. 121 Other numbers are not guaranteed. 122 123 =head2 How do I convert between numeric representations/bases/radixes? 124 125 As always with Perl there is more than one way to do it. Below are a 126 few examples of approaches to making common conversions between number 127 representations. This is intended to be representational rather than 128 exhaustive. 129 130 Some of the examples later in L<perlfaq4> use the C<Bit::Vector> 131 module from CPAN. The reason you might choose C<Bit::Vector> over the 132 perl built in functions is that it works with numbers of ANY size, 133 that it is optimized for speed on some operations, and for at least 134 some programmers the notation might be familiar. 135 136 =over 4 137 138 =item How do I convert hexadecimal into decimal 139 140 Using perl's built in conversion of C<0x> notation: 141 142 $dec = 0xDEADBEEF; 143 144 Using the C<hex> function: 145 146 $dec = hex("DEADBEEF"); 147 148 Using C<pack>: 149 150 $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); 151 152 Using the CPAN module C<Bit::Vector>: 153 154 use Bit::Vector; 155 $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); 156 $dec = $vec->to_Dec(); 157 158 =item How do I convert from decimal to hexadecimal 159 160 Using C<sprintf>: 161 162 $hex = sprintf("%X", 3735928559); # upper case A-F 163 $hex = sprintf("%x", 3735928559); # lower case a-f 164 165 Using C<unpack>: 166 167 $hex = unpack("H*", pack("N", 3735928559)); 168 169 Using C<Bit::Vector>: 170 171 use Bit::Vector; 172 $vec = Bit::Vector->new_Dec(32, -559038737); 173 $hex = $vec->to_Hex(); 174 175 And C<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(); 181 182 =item How do I convert from octal to decimal 183 184 Using Perl's built in conversion of numbers with leading zeros: 185 186 $dec = 033653337357; # note the leading 0! 187 188 Using the C<oct> function: 189 190 $dec = oct("33653337357"); 191 192 Using C<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(); 198 199 =item How do I convert from decimal to octal 200 201 Using C<sprintf>: 202 203 $oct = sprintf("%o", 3735928559); 204 205 Using C<Bit::Vector>: 206 207 use Bit::Vector; 208 $vec = Bit::Vector->new_Dec(32, -559038737); 209 $oct = reverse join('', $vec->Chunk_List_Read(3)); 210 211 =item How do I convert from binary to decimal 212 213 Perl 5.6 lets you write binary numbers directly with 214 the C<0b> notation: 215 216 $number = 0b10110110; 217 218 Using C<oct>: 219 220 my $input = "10110110"; 221 $decimal = oct( "0b$input" ); 222 223 Using C<pack> and C<ord>: 224 225 $decimal = ord(pack('B8', '10110110')); 226 227 Using C<pack> and C<unpack> for larger strings: 228 229 $int = unpack("N", pack("B32", 230 substr("0" x 32 . "11110101011011011111011101111", -32))); 231 $dec = sprintf("%d", $int); 232 233 # substr() is used to left pad a 32 character string with zeros. 234 235 Using C<Bit::Vector>: 236 237 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); 238 $dec = $vec->to_Dec(); 239 240 =item How do I convert from decimal to binary 241 242 Using C<sprintf> (perl 5.6+): 243 244 $bin = sprintf("%b", 3735928559); 245 246 Using C<unpack>: 247 248 $bin = unpack("B*", pack("N", 3735928559)); 249 250 Using C<Bit::Vector>: 251 252 use Bit::Vector; 253 $vec = Bit::Vector->new_Dec(32, -559038737); 254 $bin = $vec->to_Bin(); 255 256 The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) 257 are left as an exercise to the inclined reader. 258 259 =back 260 261 =head2 Why doesn't & work the way I want it to? 262 263 The behavior of binary arithmetic operators depends on whether they're 264 used on numbers or strings. The operators treat a string as a series 265 of bits and work with that (the string C<"3"> is the bit pattern 266 C<00110011>). The operators work with the binary form of a number 267 (the number C<3> is treated as the bit pattern C<00000011>). 268 269 So, saying C<11 & 3> performs the "and" operation on numbers (yielding 270 C<3>). Saying C<"11" & "3"> performs the "and" operation on strings 271 (yielding C<"1">). 272 273 Most problems with C<&> and C<|> arise because the programmer thinks 274 they have a number but really it's a string. The rest arise because 275 the programmer says: 276 277 if ("\020\020" & "\101\101") { 278 # ... 279 } 280 281 but a string consisting of two null bytes (the result of C<"\020\020" 282 & "\101\101">) is not a false value in Perl. You need: 283 284 if ( ("\020\020" & "\101\101") !~ /[^\000]/) { 285 # ... 286 } 287 288 =head2 How do I multiply matrices? 289 290 Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) 291 or the PDL extension (also available from CPAN). 292 293 =head2 How do I perform an operation on a series of integers? 294 295 To call a function on each element in an array, and collect the 296 results, use: 297 298 @results = map { my_func($_) } @array; 299 300 For example: 301 302 @triple = map { 3 * $_ } @single; 303 304 To call a function on each element of an array, but ignore the 305 results: 306 307 foreach $iterator (@array) { 308 some_func($iterator); 309 } 310 311 To call a function on each integer in a (small) range, you B<can> use: 312 313 @results = map { some_func($_) } (5 .. 25); 314 315 but you should be aware that the C<..> operator creates an array of 316 all integers in the range. This can take a lot of memory for large 317 ranges. Instead use: 318 319 @results = (); 320 for ($i=5; $i < 500_005; $i++) { 321 push(@results, some_func($i)); 322 } 323 324 This situation has been fixed in Perl5.005. Use of C<..> in a C<for> 325 loop 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 } 330 331 will not create a list of 500,000 integers. 332 333 =head2 How can I output Roman numerals? 334 335 Get the http://www.cpan.org/modules/by-module/Roman module. 336 337 =head2 Why aren't my random numbers random? 338 339 If you're using a version of Perl before 5.004, you must call C<srand> 340 once at the start of your program to seed the random number generator. 341 342 BEGIN { srand() if $] < 5.004 } 343 344 5.004 and later automatically call C<srand> at the beginning. Don't 345 call C<srand> more than once--you make your numbers less random, 346 rather than more. 347 348 Computers are good at being predictable and bad at being random 349 (despite appearances caused by bugs in your programs :-). see the 350 F<random> article in the "Far More Than You Ever Wanted To Know" 351 collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy 352 of Tom Phoenix, talks more about this. John von Neumann said, "Anyone 353 who attempts to generate random numbers by deterministic means is, of 354 course, living in a state of sin." 355 356 If you want numbers that are more random than C<rand> with C<srand> 357 provides, you should also check out the C<Math::TrulyRandom> module from 358 CPAN. It uses the imperfections in your system's timer to generate 359 random numbers, but this takes quite a while. If you want a better 360 pseudorandom generator than comes with your operating system, look at 361 "Numerical Recipes in C" at http://www.nr.com/ . 362 363 =head2 How do I get a random number between X and Y? 364 365 To get a random number between two values, you can use the C<rand()> 366 builtin to get a random number between 0 and 1. From there, you shift 367 that into the range that you want. 368 369 C<rand($x)> returns a number such that C<< 0 <= rand($x) < $x >>. Thus 370 what you want to have perl figure out is a random number in the range 371 from 0 to the difference between your I<X> and I<Y>. 372 373 That is, to get a number between 10 and 15, inclusive, you want a 374 random number between 0 and 5 that you can then add to 10. 375 376 my $number = 10 + int rand( 15-10+1 ); 377 378 Hence you derive the following simple function to abstract 379 that. It selects a random integer between the two given 380 integers (inclusive), For example: C<random_int_between(50,120)>. 381 382 sub random_int_between { 383 my($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 } 389 390 =head1 Data: Dates 391 392 =head2 How do I find the day or week of the year? 393 394 The localtime function returns the day of the year. Without an 395 argument localtime uses the current time. 396 397 $day_of_year = (localtime)[7]; 398 399 The C<POSIX> module can also format a date as the day of the year or 400 week of the year. 401 402 use POSIX qw/strftime/; 403 my $day_of_year = strftime "%j", localtime; 404 my $week_of_year = strftime "%W", localtime; 405 406 To get the day of year for any date, use C<POSIX>'s C<mktime> to get 407 a time in epoch seconds for the argument to localtime. 408 409 use POSIX qw/mktime strftime/; 410 my $week_of_year = strftime "%W", 411 localtime( mktime( 0, 0, 0, 18, 11, 87 ) ); 412 413 The C<Date::Calc> module provides two functions to calculate these. 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 ); 418 419 =head2 How do I find the current century or millennium? 420 421 Use the following simple functions: 422 423 sub get_century { 424 return int((((localtime(shift || time))[5] + 1999))/100); 425 } 426 427 sub get_millennium { 428 return 1+int((((localtime(shift || time))[5] + 1899))/1000); 429 } 430 431 On some systems, the C<POSIX> module's C<strftime()> function has been 432 extended in a non-standard way to use a C<%C> format, which they 433 sometimes claim is the "century". It isn't, because on most such 434 systems, this is only the first two digits of the four-digit year, and 435 thus cannot be used to reliably determine the current century or 436 millennium. 437 438 =head2 How can I compare two dates and find the difference? 439 440 (contributed by brian d foy) 441 442 You could just store all your dates as a number and then subtract. 443 Life isn't always that simple though. If you want to work with 444 formatted dates, the C<Date::Manip>, C<Date::Calc>, or C<DateTime> 445 modules can help you. 446 447 =head2 How can I take a string and turn it into epoch seconds? 448 449 If it's a regular enough string that it always has the same format, 450 you can split it up and pass the parts to C<timelocal> in the standard 451 C<Time::Local> module. Otherwise, you should look into the C<Date::Calc> 452 and C<Date::Manip> modules from CPAN. 453 454 =head2 How can I find the Julian Day? 455 456 (contributed by brian d foy and Dave Cross) 457 458 You can use the C<Time::JulianDay> module available on CPAN. Ensure 459 that you really want to find a Julian day, though, as many people have 460 different ideas about Julian days. See 461 http://www.hermetic.ch/cal_stud/jdn.htm for instance. 462 463 You can also try the C<DateTime> module, which can convert a date/time 464 to a Julian Day. 465 466 $ perl -MDateTime -le'print DateTime->today->jd' 467 2453401.5 468 469 Or the modified Julian Day 470 471 $ perl -MDateTime -le'print DateTime->today->mjd' 472 53401 473 474 Or even the day of the year (which is what some people think of as a 475 Julian day) 476 477 $ perl -MDateTime -le'print DateTime->today->doy' 478 31 479 480 =head2 How do I find yesterday's date? 481 482 (contributed by brian d foy) 483 484 Use one of the Date modules. The C<DateTime> module makes it simple, and 485 give you the same time of day, only the day before. 486 487 use DateTime; 488 489 my $yesterday = DateTime->now->subtract( days => 1 ); 490 491 print "Yesterday was $yesterday\n"; 492 493 You can also use the C<Date::Calc> module using its C<Today_and_Now> 494 function. 495 496 use Date::Calc qw( Today_and_Now Add_Delta_DHMS ); 497 498 my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 ); 499 500 print "@date_time\n"; 501 502 Most people try to use the time rather than the calendar to figure out 503 dates, but that assumes that days are twenty-four hours each. For 504 most people, there are two days a year when they aren't: the switch to 505 and from summer time throws this off. Let the modules do the work. 506 507 =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? 508 509 Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is 510 Y2K compliant (whatever that means). The programmers you've hired to 511 use it, however, probably are not. 512 513 Long answer: The question belies a true understanding of the issue. 514 Perl is just as Y2K compliant as your pencil--no more, and no less. 515 Can you use your pencil to write a non-Y2K-compliant memo? Of course 516 you can. Is that the pencil's fault? Of course it isn't. 517 518 The date and time functions supplied with Perl (gmtime and localtime) 519 supply adequate information to determine the year well beyond 2000 520 (2038 is when trouble strikes for 32-bit machines). The year returned 521 by these functions when used in a list context is the year minus 1900. 522 For years between 1910 and 1999 this I<happens> to be a 2-digit decimal 523 number. To avoid the year 2000 problem simply do not treat the year as 524 a 2-digit number. It isn't. 525 526 When gmtime() and localtime() are used in scalar context they return 527 a timestamp string that contains a fully-expanded year. For example, 528 C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 529 2001". There's no year 2000 problem here. 530 531 That doesn't mean that Perl can't be used to create non-Y2K compliant 532 programs. It can. But so can your pencil. It's the fault of the user, 533 not the language. At the risk of inflaming the NRA: "Perl doesn't 534 break Y2K, people do." See http://www.perl.org/about/y2k.html for 535 a longer exposition. 536 537 =head1 Data: Strings 538 539 =head2 How do I validate input? 540 541 (contributed by brian d foy) 542 543 There are many ways to ensure that values are what you expect or 544 want to accept. Besides the specific examples that we cover in the 545 perlfaq, you can also look at the modules with "Assert" and "Validate" 546 in their names, along with other modules such as C<Regexp::Common>. 547 548 Some modules have validation for particular types of input, such 549 as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>, 550 and C<Data::Validate::IP>. 551 552 =head2 How do I unescape a string? 553 554 It depends just what you mean by "escape". URL escapes are dealt 555 with in L<perlfaq9>. Shell escapes with the backslash (C<\>) 556 character are removed with 557 558 s/\\(.)/$1/g; 559 560 This won't expand C<"\n"> or C<"\t"> or any other special escapes. 561 562 =head2 How do I remove consecutive pairs of characters? 563 564 (contributed by brian d foy) 565 566 You can use the substitution operator to find pairs of characters (or 567 runs of characters) and replace them with a single instance. In this 568 substitution, we find a character in C<(.)>. The memory parentheses 569 store the matched character in the back-reference C<\1> and we use 570 that to require that the same thing immediately follow it. We replace 571 that part of the string with the character in C<$1>. 572 573 s/(.)\1/$1/g; 574 575 We can also use the transliteration operator, C<tr///>. In this 576 example, the search list side of our C<tr///> contains nothing, but 577 the C<c> option complements that so it contains everything. The 578 replacement list also contains nothing, so the transliteration is 579 almost a no-op since it won't do any replacements (or more exactly, 580 replace the character with itself). However, the C<s> option squashes 581 duplicated and consecutive characters in the string so a character 582 does not show up next to itself 583 584 my $str = 'Haarlem'; # in the Netherlands 585 $str =~ tr///cs; # Now Harlem, like in New York 586 587 =head2 How do I expand function calls in a string? 588 589 (contributed by brian d foy) 590 591 This is documented in L<perlref>, and although it's not the easiest 592 thing to read, it does work. In each of these examples, we call the 593 function inside the braces used to dereference a reference. If we 594 have more than one return value, we can construct and dereference an 595 anonymous array. In this case, we call the function in list context. 596 597 print "The time values are @{ [localtime] }.\n"; 598 599 If we want to call the function in scalar context, we have to do a bit 600 more work. We can really have any code we like inside the braces, so 601 we simply have to end with the scalar reference, although how you do 602 that is up to you, and you can use code inside the braces. Note that 603 the use of parens creates a list context, so we need C<scalar> to 604 force the scalar context on the function: 605 606 print "The time is ${\(scalar localtime)}.\n" 607 608 print "The time is ${ my $x = localtime; \$x }.\n"; 609 610 If your function already returns a reference, you don't need to create 611 the reference yourself. 612 613 sub timestamp { my $t = localtime; \$t } 614 615 print "The time is ${ timestamp() }.\n"; 616 617 The C<Interpolation> module can also do a lot of magic for you. You can 618 specify a variable name, in this case C<E>, to set up a tied hash that 619 does the interpolation for you. It has several other methods to do this 620 as well. 621 622 use Interpolation E => 'eval'; 623 print "The time values are $E{localtime()}.\n"; 624 625 In most cases, it is probably easier to simply use string concatenation, 626 which also forces scalar context. 627 628 print "The time is " . localtime() . ".\n"; 629 630 =head2 How do I find matching/nesting anything? 631 632 This isn't something that can be done in one regular expression, no 633 matter how complicated. To find something between two single 634 characters, a pattern like C</x([^x]*)x/> will get the intervening 635 bits in $1. For multiple ones, then something more like 636 C</alpha(.*?)omega/> would be needed. But none of these deals with 637 nested patterns. For balanced expressions using C<(>, C<{>, C<[> or 638 C<< < >> as delimiters, use the CPAN module Regexp::Common, or see 639 L<perlre/(??{ code })>. For other cases, you'll have to write a 640 parser. 641 642 If you are serious about writing a parser, there are a number of 643 modules or oddities that will make your life a lot easier. There are 644 the CPAN modules C<Parse::RecDescent>, C<Parse::Yapp>, and 645 C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8 646 the C<Text::Balanced> is part of the standard distribution. 647 648 One simple destructive, inside-out approach that you might try is to 649 pull out the smallest nesting parts one at a time: 650 651 while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { 652 # do something with $1 653 } 654 655 A more complicated and sneaky approach is to make Perl's regular 656 expression engine do it for you. This is courtesy Dean Inada, and 657 rather has the nature of an Obfuscated Perl Contest entry, but it 658 really does work: 659 660 # $_ contains the string to parse 661 # BEGIN and END are the opening and closing markers for the 662 # nested text. 663 664 @( = ('(',''); 665 @) = (')',''); 666 ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; 667 @$ = (eval{/$re/},$@!~/unmatched/i); 668 print join("\n",@$[0..$#$]) if( $$[-1] ); 669 670 =head2 How do I reverse a string? 671 672 Use C<reverse()> in scalar context, as documented in 673 L<perlfunc/reverse>. 674 675 $reversed = reverse $string; 676 677 =head2 How do I expand tabs in a string? 678 679 You can do it yourself: 680 681 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; 682 683 Or you can just use the C<Text::Tabs> module (part of the standard Perl 684 distribution). 685 686 use Text::Tabs; 687 @expanded_lines = expand(@lines_with_tabs); 688 689 =head2 How do I reformat a paragraph? 690 691 Use C<Text::Wrap> (part of the standard Perl distribution): 692 693 use Text::Wrap; 694 print wrap("\t", ' ', @paragraphs); 695 696 The paragraphs you give to C<Text::Wrap> should not contain embedded 697 newlines. C<Text::Wrap> doesn't justify the lines (flush-right). 698 699 Or use the CPAN module C<Text::Autoformat>. Formatting files can be 700 easily done by making a shell alias, like so: 701 702 alias fmt="perl -i -MText::Autoformat -n0777 \ 703 -e 'print autoformat $_, {all=>1}' $*" 704 705 See the documentation for C<Text::Autoformat> to appreciate its many 706 capabilities. 707 708 =head2 How can I access or change N characters of a string? 709 710 You can access the first characters of a string with substr(). 711 To get the first character, for example, start at position 0 712 and grab the string of length 1. 713 714 715 $string = "Just another Perl Hacker"; 716 $first_char = substr( $string, 0, 1 ); # 'J' 717 718 To change part of a string, you can use the optional fourth 719 argument which is the replacement string. 720 721 substr( $string, 13, 4, "Perl 5.8.0" ); 722 723 You can also use substr() as an lvalue. 724 725 substr( $string, 13, 4 ) = "Perl 5.8.0"; 726 727 =head2 How do I change the Nth occurrence of something? 728 729 You have to keep track of N yourself. For example, let's say you want 730 to change the fifth occurrence of C<"whoever"> or C<"whomever"> into 731 C<"whosoever"> or C<"whomsoever">, case insensitively. These 732 all assume that $_ contains the string to be altered. 733 734 $count = 0; 735 s{((whom?)ever)}{ 736 ++$count == 5 # is it the 5th? 737 ? "$2}soever" # yes, swap 738 : $1 # renege and leave it there 739 }ige; 740 741 In the more general case, you can use the C</g> modifier in a C<while> 742 loop, keeping count of matches. 743 744 $WANT = 3; 745 $count = 0; 746 $_ = "One fish two fish red fish blue fish"; 747 while (/(\w+)\s+fish\b/gi) { 748 if (++$count == $WANT) { 749 print "The third fish is a $1 one.\n"; 750 } 751 } 752 753 That prints out: C<"The third fish is a red one."> You can also use a 754 repetition count and repeated pattern like this: 755 756 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; 757 758 =head2 How can I count the number of occurrences of a substring within a string? 759 760 There are a number of ways, with varying efficiency. If you want a 761 count of a certain single character (X) within a string, you can use the 762 C<tr///> function like so: 763 764 $string = "ThisXlineXhasXsomeXx'sXinXit"; 765 $count = ($string =~ tr/X//); 766 print "There are $count X characters in the string"; 767 768 This is fine if you are just looking for a single character. However, 769 if you are trying to count multiple character substrings within a 770 larger string, C<tr///> won't work. What you can do is wrap a while() 771 loop around a global pattern match. For example, let's count negative 772 integers: 773 774 $string = "-9 55 48 -2 23 -76 4 14 -44"; 775 while ($string =~ /-\d+/g) { $count++ } 776 print "There are $count negative numbers in the string"; 777 778 Another version uses a global match in list context, then assigns the 779 result to a scalar, producing a count of the number of matches. 780 781 $count = () = $string =~ /-\d+/g; 782 783 =head2 How do I capitalize all the words on one line? 784 785 To make the first letter of each word upper case: 786 787 $line =~ s/\b(\w)/\U$1/g; 788 789 This has the strange effect of turning "C<don't do it>" into "C<Don'T 790 Do It>". Sometimes you might want this. Other times you might need a 791 more thorough solution (Suggested by brian d foy): 792 793 $string =~ s/ ( 794 (^\w) #at the beginning of the line 795 | # or 796 (\s\w) #preceded by whitespace 797 ) 798 /\U$1/xg; 799 800 $string =~ s/([\w']+)/\u\L$1/g; 801 802 To make the whole line upper case: 803 804 $line = uc($line); 805 806 To force each word to be lower case, with the first letter upper case: 807 808 $line =~ s/(\w+)/\u\L$1/g; 809 810 You can (and probably should) enable locale awareness of those 811 characters by placing a C<use locale> pragma in your program. 812 See L<perllocale> for endless details on locales. 813 814 This is sometimes referred to as putting something into "title 815 case", but that's not quite accurate. Consider the proper 816 capitalization of the movie I<Dr. Strangelove or: How I Learned to 817 Stop Worrying and Love the Bomb>, for example. 818 819 Damian Conway's L<Text::Autoformat> module provides some smart 820 case transformations: 821 822 use Text::Autoformat; 823 my $x = "Dr. Strangelove or: How I Learned to Stop ". 824 "Worrying and Love the Bomb"; 825 826 print $x, "\n"; 827 for my $style (qw( sentence title highlight )) { 828 print autoformat($x, { case => $style }), "\n"; 829 } 830 831 =head2 How can I split a [character] delimited string except when inside [character]? 832 833 Several modules can handle this sort of parsing--C<Text::Balanced>, 834 C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others. 835 836 Take the example case of trying to split a string that is 837 comma-separated into its different fields. You can't use C<split(/,/)> 838 because you shouldn't split if the comma is inside quotes. For 839 example, take a data line like this: 840 841 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" 842 843 Due to the restriction of the quotes, this is a fairly complex 844 problem. Thankfully, we have Jeffrey Friedl, author of 845 I<Mastering Regular Expressions>, to handle these for us. He 846 suggests (assuming your string is contained in C<$text>): 847 848 @new = (); 849 push(@new, $+) while $text =~ m{ 850 "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes 851 | ([^,]+),? 852 | , 853 }gx; 854 push(@new, undef) if substr($text,-1,1) eq ','; 855 856 If you want to represent quotation marks inside a 857 quotation-mark-delimited field, escape them with backslashes (eg, 858 C<"like \"this\"">. 859 860 Alternatively, the C<Text::ParseWords> module (part of the standard 861 Perl distribution) lets you say: 862 863 use Text::ParseWords; 864 @new = quotewords(",", 0, $text); 865 866 =head2 How do I strip blank space from the beginning/end of a string? 867 868 (contributed by brian d foy) 869 870 A substitution can do this for you. For a single line, you want to 871 replace all the leading or trailing whitespace with nothing. You 872 can do that with a pair of substitutions. 873 874 s/^\s+//; 875 s/\s+$//; 876 877 You can also write that as a single substitution, although it turns 878 out the combined statement is slower than the separate ones. That 879 might not matter to you, though. 880 881 s/^\s+|\s+$//g; 882 883 In this regular expression, the alternation matches either at the 884 beginning or the end of the string since the anchors have a lower 885 precedence than the alternation. With the C</g> flag, the substitution 886 makes all possible matches, so it gets both. Remember, the trailing 887 newline matches the C<\s+>, and the C<$> anchor can match to the 888 physical end of the string, so the newline disappears too. Just add 889 the newline to the output, which has the added benefit of preserving 890 "blank" (consisting entirely of whitespace) lines which the C<^\s+> 891 would remove all by itself. 892 893 while( <> ) 894 { 895 s/^\s+|\s+$//g; 896 print "$_\n"; 897 } 898 899 For a multi-line string, you can apply the regular expression 900 to each logical line in the string by adding the C</m> flag (for 901 "multi-line"). With the C</m> flag, the C<$> matches I<before> an 902 embedded newline, so it doesn't remove it. It still removes the 903 newline at the end of the string. 904 905 $string =~ s/^\s+|\s+$//gm; 906 907 Remember that lines consisting entirely of whitespace will disappear, 908 since the first part of the alternation can match the entire string 909 and replace it with nothing. If need to keep embedded blank lines, 910 you have to do a little more work. Instead of matching any whitespace 911 (since that includes a newline), just match the other whitespace. 912 913 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg; 914 915 =head2 How do I pad a string with blanks or pad a number with zeroes? 916 917 In the following examples, C<$pad_len> is the length to which you wish 918 to pad the string, C<$text> or C<$num> contains the string to be padded, 919 and C<$pad_char> contains the padding character. You can use a single 920 character string constant instead of the C<$pad_char> variable if you 921 know what it is in advance. And in the same way you can use an integer in 922 place of C<$pad_len> if you know the pad length in advance. 923 924 The simplest method uses the C<sprintf> function. It can pad on the left 925 or right with blanks and on the left with zeroes and it will not 926 truncate the result. The C<pack> function can only pad strings on the 927 right with blanks and it will truncate the result to a maximum length of 928 C<$pad_len>. 929 930 # Left padding a string with blanks (no truncation): 931 $padded = sprintf("%$pad_len}s", $text); 932 $padded = sprintf("%*s", $pad_len, $text); # same thing 933 934 # Right padding a string with blanks (no truncation): 935 $padded = sprintf("%-$pad_len}s", $text); 936 $padded = sprintf("%-*s", $pad_len, $text); # same thing 937 938 # Left padding a number with 0 (no truncation): 939 $padded = sprintf("%0$pad_len}d", $num); 940 $padded = sprintf("%0*d", $pad_len, $num); # same thing 941 942 # Right padding a string with blanks using pack (will truncate): 943 $padded = pack("A$pad_len",$text); 944 945 If you need to pad with a character other than blank or zero you can use 946 one of the following methods. They all generate a pad string with the 947 C<x> operator and combine that with C<$text>. These methods do 948 not truncate C<$text>. 949 950 Left and right padding with any character, creating a new string: 951 952 $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; 953 $padded = $text . $pad_char x ( $pad_len - length( $text ) ); 954 955 Left and right padding with any character, modifying C<$text> directly: 956 957 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); 958 $text .= $pad_char x ( $pad_len - length( $text ) ); 959 960 =head2 How do I extract selected columns from a string? 961 962 (contributed by brian d foy) 963 964 If you know where the columns that contain the data, you can 965 use C<substr> to extract a single column. 966 967 my $column = substr( $line, $start_column, $length ); 968 969 You can use C<split> if the columns are separated by whitespace or 970 some other delimiter, as long as whitespace or the delimiter cannot 971 appear as part of the data. 972 973 my $line = ' fred barney betty '; 974 my @columns = split /\s+/, $line; 975 # ( '', 'fred', 'barney', 'betty' ); 976 977 my $line = 'fred||barney||betty'; 978 my @columns = split /\|/, $line; 979 # ( 'fred', '', 'barney', '', 'betty' ); 980 981 If you want to work with comma-separated values, don't do this since 982 that format is a bit more complicated. Use one of the modules that 983 handle that fornat, such as C<Text::CSV>, C<Text::CSV_XS>, or 984 C<Text::CSV_PP>. 985 986 If you want to break apart an entire line of fixed columns, you can use 987 C<unpack> with the A (ASCII) format. by using a number after the format 988 specifier, you can denote the column width. See the C<pack> and C<unpack> 989 entries in L<perlfunc> for more details. 990 991 my @fields = unpack( $line, "A8 A8 A8 A16 A4" ); 992 993 Note that spaces in the format argument to C<unpack> do not denote literal 994 spaces. If you have space separated data, you may want C<split> instead. 995 996 =head2 How do I find the soundex value of a string? 997 998 (contributed by brian d foy) 999 1000 You can use the Text::Soundex module. If you want to do fuzzy or close 1001 matching, you might also try the C<String::Approx>, and 1002 C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules. 1003 1004 =head2 How can I expand variables in text strings? 1005 1006 (contributed by brian d foy) 1007 1008 If you can avoid it, don't, or if you can use a templating system, 1009 such as C<Text::Template> or C<Template> Toolkit, do that instead. You 1010 might even be able to get the job done with C<sprintf> or C<printf>: 1011 1012 my $string = sprintf 'Say hello to %s and %s', $foo, $bar; 1013 1014 However, for the one-off simple case where I don't want to pull out a 1015 full templating system, I'll use a string that has two Perl scalar 1016 variables in it. In this example, I want to expand C<$foo> and C<$bar> 1017 to their variable's values: 1018 1019 my $foo = 'Fred'; 1020 my $bar = 'Barney'; 1021 $string = 'Say hello to $foo and $bar'; 1022 1023 One way I can do this involves the substitution operator and a double 1024 C</e> flag. The first C</e> evaluates C<$1> on the replacement side and 1025 turns it into C<$foo>. The second /e starts with C<$foo> and replaces 1026 it with its value. C<$foo>, then, turns into 'Fred', and that's finally 1027 what's left in the string: 1028 1029 $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney' 1030 1031 The C</e> will also silently ignore violations of strict, replacing 1032 undefined variable names with the empty string. Since I'm using the 1033 C</e> flag (twice even!), I have all of the same security problems I 1034 have with C<eval> in its string form. If there's something odd in 1035 C<$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then 1036 I could get myself in trouble. 1037 1038 To get around the security problem, I could also pull the values from 1039 a hash instead of evaluating variable names. Using a single C</e>, I 1040 can check the hash to ensure the value exists, and if it doesn't, I 1041 can replace the missing value with a marker, in this case C<???> to 1042 signal that I missed something: 1043 1044 my $string = 'This has $foo and $bar'; 1045 1046 my %Replacements = ( 1047 foo => 'Fred', 1048 ); 1049 1050 # $string =~ s/\$(\w+)/$Replacements{$1}/g; 1051 $string =~ s/\$(\w+)/ 1052 exists $Replacements{$1} ? $Replacements{$1} : '???' 1053 /eg; 1054 1055 print $string; 1056 1057 =head2 What's wrong with always quoting "$vars"? 1058 1059 The problem is that those double-quotes force 1060 stringification--coercing numbers and references into strings--even 1061 when you don't want them to be strings. Think of it this way: 1062 double-quote expansion is used to produce new strings. If you already 1063 have a string, why do you need more? 1064 1065 If you get used to writing odd things like these: 1066 1067 print "$var"; # BAD 1068 $new = "$old"; # BAD 1069 somefunc("$var"); # BAD 1070 1071 You'll be in trouble. Those should (in 99.8% of the cases) be 1072 the simpler and more direct: 1073 1074 print $var; 1075 $new = $old; 1076 somefunc($var); 1077 1078 Otherwise, besides slowing you down, you're going to break code when 1079 the thing in the scalar is actually neither a string nor a number, but 1080 a reference: 1081 1082 func(\@array); 1083 sub func { 1084 my $aref = shift; 1085 my $oref = "$aref"; # WRONG 1086 } 1087 1088 You can also get into subtle problems on those few operations in Perl 1089 that actually do care about the difference between a string and a 1090 number, such as the magical C<++> autoincrement operator or the 1091 syscall() function. 1092 1093 Stringification also destroys arrays. 1094 1095 @lines = `command`; 1096 print "@lines"; # WRONG - extra blanks 1097 print @lines; # right 1098 1099 =head2 Why don't my E<lt>E<lt>HERE documents work? 1100 1101 Check for these three things: 1102 1103 =over 4 1104 1105 =item There must be no space after the E<lt>E<lt> part. 1106 1107 =item There (probably) should be a semicolon at the end. 1108 1109 =item You can't (easily) have any space in front of the tag. 1110 1111 =back 1112 1113 If you want to indent the text in the here document, you 1114 can do this: 1115 1116 # all in one 1117 ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; 1118 your text 1119 goes here 1120 HERE_TARGET 1121 1122 But the HERE_TARGET must still be flush against the margin. 1123 If you want that indented also, you'll have to quote 1124 in the indentation. 1125 1126 ($quote = <<' FINIS') =~ s/^\s+//gm; 1127 ...we will have peace, when you and all your works have 1128 perished--and the works of your dark master to whom you 1129 would deliver us. You are a liar, Saruman, and a corrupter 1130 of men's hearts. --Theoden in /usr/src/perl/taint.c 1131 FINIS 1132 $quote =~ s/\s+--/\n--/; 1133 1134 A nice general-purpose fixer-upper function for indented here documents 1135 follows. It expects to be called with a here document as its argument. 1136 It looks to see whether each line begins with a common substring, and 1137 if so, strips that substring off. Otherwise, it takes the amount of leading 1138 whitespace found on the first line and removes that much off each 1139 subsequent line. 1140 1141 sub fix { 1142 local $_ = shift; 1143 my ($white, $leader); # common whitespace and common leading string 1144 if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { 1145 ($white, $leader) = ($2, quotemeta($1)); 1146 } else { 1147 ($white, $leader) = (/^(\s+)/, ''); 1148 } 1149 s/^\s*?$leader(?:$white)?//gm; 1150 return $_; 1151 } 1152 1153 This works with leading special strings, dynamically determined: 1154 1155 $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; 1156 @@@ int 1157 @@@ runops() { 1158 @@@ SAVEI32(runlevel); 1159 @@@ runlevel++; 1160 @@@ while ( op = (*op->op_ppaddr)() ); 1161 @@@ TAINT_NOT; 1162 @@@ return 0; 1163 @@@ } 1164 MAIN_INTERPRETER_LOOP 1165 1166 Or with a fixed amount of leading whitespace, with remaining 1167 indentation correctly preserved: 1168 1169 $poem = fix<<EVER_ON_AND_ON; 1170 Now far ahead the Road has gone, 1171 And I must follow, if I can, 1172 Pursuing it with eager feet, 1173 Until it joins some larger way 1174 Where many paths and errands meet. 1175 And whither then? I cannot say. 1176 --Bilbo in /usr/src/perl/pp_ctl.c 1177 EVER_ON_AND_ON 1178 1179 =head1 Data: Arrays 1180 1181 =head2 What is the difference between a list and an array? 1182 1183 An array has a changeable length. A list does not. An array is 1184 something you can push or pop, while a list is a set of values. Some 1185 people make the distinction that a list is a value while an array is a 1186 variable. Subroutines are passed and return lists, you put things into 1187 list context, you initialize arrays with lists, and you C<foreach()> 1188 across a list. C<@> variables are arrays, anonymous arrays are 1189 arrays, arrays in scalar context behave like the number of elements in 1190 them, subroutines access their arguments through the array C<@_>, and 1191 C<push>/C<pop>/C<shift> only work on arrays. 1192 1193 As a side note, there's no such thing as a list in scalar context. 1194 When you say 1195 1196 $scalar = (2, 5, 7, 9); 1197 1198 you're using the comma operator in scalar context, so it uses the scalar 1199 comma operator. There never was a list there at all! This causes the 1200 last value to be returned: 9. 1201 1202 =head2 What is the difference between $array[1] and @array[1]? 1203 1204 The former is a scalar value; the latter an array slice, making 1205 it a list with one (scalar) value. You should use $ when you want a 1206 scalar value (most of the time) and @ when you want a list with one 1207 scalar value in it (very, very rarely; nearly never, in fact). 1208 1209 Sometimes it doesn't make a difference, but sometimes it does. 1210 For example, compare: 1211 1212 $good[0] = `some program that outputs several lines`; 1213 1214 with 1215 1216 @bad[0] = `same program that outputs several lines`; 1217 1218 The C<use warnings> pragma and the B<-w> flag will warn you about these 1219 matters. 1220 1221 =head2 How can I remove duplicate elements from a list or array? 1222 1223 (contributed by brian d foy) 1224 1225 Use a hash. When you think the words "unique" or "duplicated", think 1226 "hash keys". 1227 1228 If you don't care about the order of the elements, you could just 1229 create the hash then extract the keys. It's not important how you 1230 create that hash: just that you use C<keys> to get the unique 1231 elements. 1232 1233 my %hash = map { $_, 1 } @array; 1234 # or a hash slice: @hash{ @array } = (); 1235 # or a foreach: $hash{$_} = 1 foreach ( @array ); 1236 1237 my @unique = keys %hash; 1238 1239 If you want to use a module, try the C<uniq> function from 1240 C<List::MoreUtils>. In list context it returns the unique elements, 1241 preserving their order in the list. In scalar context, it returns the 1242 number of unique elements. 1243 1244 use List::MoreUtils qw(uniq); 1245 1246 my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7 1247 my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7 1248 1249 You can also go through each element and skip the ones you've seen 1250 before. Use a hash to keep track. The first time the loop sees an 1251 element, that element has no key in C<%Seen>. The C<next> statement 1252 creates the key and immediately uses its value, which is C<undef>, so 1253 the loop continues to the C<push> and increments the value for that 1254 key. The next time the loop sees that same element, its key exists in 1255 the hash I<and> the value for that key is true (since it's not 0 or 1256 C<undef>), so the next skips that iteration and the loop goes to the 1257 next element. 1258 1259 my @unique = (); 1260 my %seen = (); 1261 1262 foreach my $elem ( @array ) 1263 { 1264 next if $seen{ $elem }++; 1265 push @unique, $elem; 1266 } 1267 1268 You can write this more briefly using a grep, which does the 1269 same thing. 1270 1271 my %seen = (); 1272 my @unique = grep { ! $seen{ $_ }++ } @array; 1273 1274 =head2 How can I tell whether a certain element is contained in a list or array? 1275 1276 (portions of this answer contributed by Anno Siegel) 1277 1278 Hearing the word "in" is an I<in>dication that you probably should have 1279 used a hash, not a list or array, to store your data. Hashes are 1280 designed to answer this question quickly and efficiently. Arrays aren't. 1281 1282 That being said, there are several ways to approach this. If you 1283 are going to make this query many times over arbitrary string values, 1284 the fastest way is probably to invert the original array and maintain a 1285 hash whose keys are the first array's values. 1286 1287 @blues = qw/azure cerulean teal turquoise lapis-lazuli/; 1288 %is_blue = (); 1289 for (@blues) { $is_blue{$_} = 1 } 1290 1291 Now you can check whether C<$is_blue{$some_color}>. It might have 1292 been a good idea to keep the blues all in a hash in the first place. 1293 1294 If the values are all small integers, you could use a simple indexed 1295 array. This kind of an array will take up less space: 1296 1297 @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); 1298 @is_tiny_prime = (); 1299 for (@primes) { $is_tiny_prime[$_] = 1 } 1300 # or simply @istiny_prime[@primes] = (1) x @primes; 1301 1302 Now you check whether $is_tiny_prime[$some_number]. 1303 1304 If the values in question are integers instead of strings, you can save 1305 quite a lot of space by using bit strings instead: 1306 1307 @articles = ( 1..10, 150..2000, 2017 ); 1308 undef $read; 1309 for (@articles) { vec($read,$_,1) = 1 } 1310 1311 Now check whether C<vec($read,$n,1)> is true for some C<$n>. 1312 1313 These methods guarantee fast individual tests but require a re-organization 1314 of the original list or array. They only pay off if you have to test 1315 multiple values against the same array. 1316 1317 If you are testing only once, the standard module C<List::Util> exports 1318 the function C<first> for this purpose. It works by stopping once it 1319 finds the element. It's written in C for speed, and its Perl equivalent 1320 looks like this subroutine: 1321 1322 sub first (&@) { 1323 my $code = shift; 1324 foreach (@_) { 1325 return $_ if &{$code}(); 1326 } 1327 undef; 1328 } 1329 1330 If speed is of little concern, the common idiom uses grep in scalar context 1331 (which returns the number of items that passed its condition) to traverse the 1332 entire list. This does have the benefit of telling you how many matches it 1333 found, though. 1334 1335 my $is_there = grep $_ eq $whatever, @array; 1336 1337 If you want to actually extract the matching elements, simply use grep in 1338 list context. 1339 1340 my @matches = grep $_ eq $whatever, @array; 1341 1342 =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? 1343 1344 Use a hash. Here's code to do both and more. It assumes that each 1345 element is unique in a given array: 1346 1347 @union = @intersection = @difference = (); 1348 %count = (); 1349 foreach $element (@array1, @array2) { $count{$element}++ } 1350 foreach $element (keys %count) { 1351 push @union, $element; 1352 push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; 1353 } 1354 1355 Note that this is the I<symmetric difference>, that is, all elements 1356 in either A or in B but not in both. Think of it as an xor operation. 1357 1358 =head2 How do I test whether two arrays or hashes are equal? 1359 1360 The following code works for single-level arrays. It uses a 1361 stringwise comparison, and does not distinguish defined versus 1362 undefined empty strings. Modify if you have other needs. 1363 1364 $are_equal = compare_arrays(\@frogs, \@toads); 1365 1366 sub compare_arrays { 1367 my ($first, $second) = @_; 1368 no warnings; # silence spurious -w undef complaints 1369 return 0 unless @$first == @$second; 1370 for (my $i = 0; $i < @$first; $i++) { 1371 return 0 if $first->[$i] ne $second->[$i]; 1372 } 1373 return 1; 1374 } 1375 1376 For multilevel structures, you may wish to use an approach more 1377 like this one. It uses the CPAN module C<FreezeThaw>: 1378 1379 use FreezeThaw qw(cmpStr); 1380 @a = @b = ( "this", "that", [ "more", "stuff" ] ); 1381 1382 printf "a and b contain %s arrays\n", 1383 cmpStr(\@a, \@b) == 0 1384 ? "the same" 1385 : "different"; 1386 1387 This approach also works for comparing hashes. Here we'll demonstrate 1388 two different answers: 1389 1390 use FreezeThaw qw(cmpStr cmpStrHard); 1391 1392 %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); 1393 $a{EXTRA} = \%b; 1394 $b{EXTRA} = \%a; 1395 1396 printf "a and b contain %s hashes\n", 1397 cmpStr(\%a, \%b) == 0 ? "the same" : "different"; 1398 1399 printf "a and b contain %s hashes\n", 1400 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; 1401 1402 1403 The first reports that both those the hashes contain the same data, 1404 while the second reports that they do not. Which you prefer is left as 1405 an exercise to the reader. 1406 1407 =head2 How do I find the first array element for which a condition is true? 1408 1409 To find the first array element which satisfies a condition, you can 1410 use the C<first()> function in the C<List::Util> module, which comes 1411 with Perl 5.8. This example finds the first element that contains 1412 "Perl". 1413 1414 use List::Util qw(first); 1415 1416 my $element = first { /Perl/ } @array; 1417 1418 If you cannot use C<List::Util>, you can make your own loop to do the 1419 same thing. Once you find the element, you stop the loop with last. 1420 1421 my $found; 1422 foreach ( @array ) { 1423 if( /Perl/ ) { $found = $_; last } 1424 } 1425 1426 If you want the array index, you can iterate through the indices 1427 and check the array element at each index until you find one 1428 that satisfies the condition. 1429 1430 my( $found, $index ) = ( undef, -1 ); 1431 for( $i = 0; $i < @array; $i++ ) { 1432 if( $array[$i] =~ /Perl/ ) { 1433 $found = $array[$i]; 1434 $index = $i; 1435 last; 1436 } 1437 } 1438 1439 =head2 How do I handle linked lists? 1440 1441 In general, you usually don't need a linked list in Perl, since with 1442 regular arrays, you can push and pop or shift and unshift at either 1443 end, or you can use splice to add and/or remove arbitrary number of 1444 elements at arbitrary points. Both pop and shift are O(1) 1445 operations on Perl's dynamic arrays. In the absence of shifts and 1446 pops, push in general needs to reallocate on the order every log(N) 1447 times, and unshift will need to copy pointers each time. 1448 1449 If you really, really wanted, you could use structures as described in 1450 L<perldsc> or L<perltoot> and do just what the algorithm book tells 1451 you to do. For example, imagine a list node like this: 1452 1453 $node = { 1454 VALUE => 42, 1455 LINK => undef, 1456 }; 1457 1458 You could walk the list this way: 1459 1460 print "List: "; 1461 for ($node = $head; $node; $node = $node->{LINK}) { 1462 print $node->{VALUE}, " "; 1463 } 1464 print "\n"; 1465 1466 You could add to the list this way: 1467 1468 my ($head, $tail); 1469 $tail = append($head, 1); # grow a new head 1470 for $value ( 2 .. 10 ) { 1471 $tail = append($tail, $value); 1472 } 1473 1474 sub append { 1475 my($list, $value) = @_; 1476 my $node = { VALUE => $value }; 1477 if ($list) { 1478 $node->{LINK} = $list->{LINK}; 1479 $list->{LINK} = $node; 1480 } 1481 else { 1482 $_[0] = $node; # replace caller's version 1483 } 1484 return $node; 1485 } 1486 1487 But again, Perl's built-in are virtually always good enough. 1488 1489 =head2 How do I handle circular lists? 1490 1491 Circular lists could be handled in the traditional fashion with linked 1492 lists, or you could just do something like this with an array: 1493 1494 unshift(@array, pop(@array)); # the last shall be first 1495 push(@array, shift(@array)); # and vice versa 1496 1497 You can also use C<Tie::Cycle>: 1498 1499 use Tie::Cycle; 1500 1501 tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ]; 1502 1503 print $cycle; # FFFFFF 1504 print $cycle; # 000000 1505 print $cycle; # FFFF00 1506 1507 =head2 How do I shuffle an array randomly? 1508 1509 If you either have Perl 5.8.0 or later installed, or if you have 1510 Scalar-List-Utils 1.03 or later installed, you can say: 1511 1512 use List::Util 'shuffle'; 1513 1514 @shuffled = shuffle(@list); 1515 1516 If not, you can use a Fisher-Yates shuffle. 1517 1518 sub fisher_yates_shuffle { 1519 my $deck = shift; # $deck is a reference to an array 1520 my $i = @$deck; 1521 while (--$i) { 1522 my $j = int rand ($i+1); 1523 @$deck[$i,$j] = @$deck[$j,$i]; 1524 } 1525 } 1526 1527 # shuffle my mpeg collection 1528 # 1529 my @mpeg = <audio/*/*.mp3>; 1530 fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place 1531 print @mpeg; 1532 1533 Note that the above implementation shuffles an array in place, 1534 unlike the C<List::Util::shuffle()> which takes a list and returns 1535 a new shuffled list. 1536 1537 You've probably seen shuffling algorithms that work using splice, 1538 randomly picking another element to swap the current element with 1539 1540 srand; 1541 @new = (); 1542 @old = 1 .. 10; # just a demo 1543 while (@old) { 1544 push(@new, splice(@old, rand @old, 1)); 1545 } 1546 1547 This is bad because splice is already O(N), and since you do it N 1548 times, you just invented a quadratic algorithm; that is, O(N**2). 1549 This does not scale, although Perl is so efficient that you probably 1550 won't notice this until you have rather largish arrays. 1551 1552 =head2 How do I process/modify each element of an array? 1553 1554 Use C<for>/C<foreach>: 1555 1556 for (@lines) { 1557 s/foo/bar/; # change that word 1558 tr/XZ/ZX/; # swap those letters 1559 } 1560 1561 Here's another; let's compute spherical volumes: 1562 1563 for (@volumes = @radii) { # @volumes has changed parts 1564 $_ **= 3; 1565 $_ *= (4/3) * 3.14159; # this will be constant folded 1566 } 1567 1568 which can also be done with C<map()> which is made to transform 1569 one list into another: 1570 1571 @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii; 1572 1573 If you want to do the same thing to modify the values of the 1574 hash, you can use the C<values> function. As of Perl 5.6 1575 the values are not copied, so if you modify $orbit (in this 1576 case), you modify the value. 1577 1578 for $orbit ( values %orbits ) { 1579 ($orbit **= 3) *= (4/3) * 3.14159; 1580 } 1581 1582 Prior to perl 5.6 C<values> returned copies of the values, 1583 so older perl code often contains constructions such as 1584 C<@orbits{keys %orbits}> instead of C<values %orbits> where 1585 the hash is to be modified. 1586 1587 =head2 How do I select a random element from an array? 1588 1589 Use the C<rand()> function (see L<perlfunc/rand>): 1590 1591 $index = rand @array; 1592 $element = $array[$index]; 1593 1594 Or, simply: 1595 1596 my $element = $array[ rand @array ]; 1597 1598 =head2 How do I permute N elements of a list? 1599 X<List::Permuter> X<permute> X<Algorithm::Loops> X<Knuth> 1600 X<The Art of Computer Programming> X<Fischer-Krause> 1601 1602 Use the C<List::Permutor> module on CPAN. If the list is actually an 1603 array, try the C<Algorithm::Permute> module (also on CPAN). It's 1604 written in XS code and is very efficient: 1605 1606 use Algorithm::Permute; 1607 1608 my @array = 'a'..'d'; 1609 my $p_iterator = Algorithm::Permute->new ( \@array ); 1610 1611 while (my @perm = $p_iterator->next) { 1612 print "next permutation: (@perm)\n"; 1613 } 1614 1615 For even faster execution, you could do: 1616 1617 use Algorithm::Permute; 1618 1619 my @array = 'a'..'d'; 1620 1621 Algorithm::Permute::permute { 1622 print "next permutation: (@array)\n"; 1623 } @array; 1624 1625 Here's a little program that generates all permutations of all the 1626 words on each line of input. The algorithm embodied in the 1627 C<permute()> function is discussed in Volume 4 (still unpublished) of 1628 Knuth's I<The Art of Computer Programming> and will work on any list: 1629 1630 #!/usr/bin/perl -n 1631 # Fischer-Krause ordered permutation generator 1632 1633 sub permute (&@) { 1634 my $code = shift; 1635 my @idx = 0..$#_; 1636 while ( $code->(@_[@idx]) ) { 1637 my $p = $#idx; 1638 --$p while $idx[$p-1] > $idx[$p]; 1639 my $q = $p or return; 1640 push @idx, reverse splice @idx, $p; 1641 ++$q while $idx[$p-1] > $idx[$q]; 1642 @idx[$p-1,$q]=@idx[$q,$p-1]; 1643 } 1644 } 1645 1646 permute { print "@_\n" } split; 1647 1648 The C<Algorithm::Loops> module also provides the C<NextPermute> and 1649 C<NextPermuteNum> functions which efficiently find all unique permutations 1650 of an array, even if it contains duplicate values, modifying it in-place: 1651 if its elements are in reverse-sorted order then the array is reversed, 1652 making it sorted, and it returns false; otherwise the next 1653 permutation is returned. 1654 1655 C<NextPermute> uses string order and C<NextPermuteNum> numeric order, so 1656 you can enumerate all the permutations of C<0..9> like this: 1657 1658 use Algorithm::Loops qw(NextPermuteNum); 1659 1660 my @list= 0..9; 1661 do { print "@list\n" } while NextPermuteNum @list; 1662 1663 =head2 How do I sort an array by (anything)? 1664 1665 Supply a comparison function to sort() (described in L<perlfunc/sort>): 1666 1667 @list = sort { $a <=> $b } @list; 1668 1669 The default sort function is cmp, string comparison, which would 1670 sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is 1671 the numerical comparison operator. 1672 1673 If you have a complicated function needed to pull out the part you 1674 want to sort on, then don't do it inside the sort function. Pull it 1675 out first, because the sort BLOCK can be called many times for the 1676 same element. Here's an example of how to pull out the first word 1677 after the first number on each item, and then sort those words 1678 case-insensitively. 1679 1680 @idx = (); 1681 for (@data) { 1682 ($item) = /\d+\s*(\S+)/; 1683 push @idx, uc($item); 1684 } 1685 @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; 1686 1687 which could also be written this way, using a trick 1688 that's come to be known as the Schwartzian Transform: 1689 1690 @sorted = map { $_->[0] } 1691 sort { $a->[1] cmp $b->[1] } 1692 map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; 1693 1694 If you need to sort on several fields, the following paradigm is useful. 1695 1696 @sorted = sort { 1697 field1($a) <=> field1($b) || 1698 field2($a) cmp field2($b) || 1699 field3($a) cmp field3($b) 1700 } @data; 1701 1702 This can be conveniently combined with precalculation of keys as given 1703 above. 1704 1705 See the F<sort> article in the "Far More Than You Ever Wanted 1706 To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for 1707 more about this approach. 1708 1709 See also the question later in L<perlfaq4> on sorting hashes. 1710 1711 =head2 How do I manipulate arrays of bits? 1712 1713 Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise 1714 operations. 1715 1716 For example, this sets C<$vec> to have bit N set if C<$ints[N]> was 1717 set: 1718 1719 $vec = ''; 1720 foreach(@ints) { vec($vec,$_,1) = 1 } 1721 1722 Here's how, given a vector in C<$vec>, you can get those bits into your 1723 C<@ints> array: 1724 1725 sub bitvec_to_list { 1726 my $vec = shift; 1727 my @ints; 1728 # Find null-byte density then select best algorithm 1729 if ($vec =~ tr/\0// / length $vec > 0.95) { 1730 use integer; 1731 my $i; 1732 1733 # This method is faster with mostly null-bytes 1734 while($vec =~ /[^\0]/g ) { 1735 $i = -9 + 8 * pos $vec; 1736 push @ints, $i if vec($vec, ++$i, 1); 1737 push @ints, $i if vec($vec, ++$i, 1); 1738 push @ints, $i if vec($vec, ++$i, 1); 1739 push @ints, $i if vec($vec, ++$i, 1); 1740 push @ints, $i if vec($vec, ++$i, 1); 1741 push @ints, $i if vec($vec, ++$i, 1); 1742 push @ints, $i if vec($vec, ++$i, 1); 1743 push @ints, $i if vec($vec, ++$i, 1); 1744 } 1745 } 1746 else { 1747 # This method is a fast general algorithm 1748 use integer; 1749 my $bits = unpack "b*", $vec; 1750 push @ints, 0 if $bits =~ s/^(\d)// && $1; 1751 push @ints, pos $bits while($bits =~ /1/g); 1752 } 1753 1754 return \@ints; 1755 } 1756 1757 This method gets faster the more sparse the bit vector is. 1758 (Courtesy of Tim Bunce and Winfried Koenig.) 1759 1760 You can make the while loop a lot shorter with this suggestion 1761 from Benjamin Goldberg: 1762 1763 while($vec =~ /[^\0]+/g ) { 1764 push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8; 1765 } 1766 1767 Or use the CPAN module C<Bit::Vector>: 1768 1769 $vector = Bit::Vector->new($num_of_bits); 1770 $vector->Index_List_Store(@ints); 1771 @ints = $vector->Index_List_Read(); 1772 1773 C<Bit::Vector> provides efficient methods for bit vector, sets of 1774 small integers and "big int" math. 1775 1776 Here's a more extensive illustration using vec(): 1777 1778 # vec demo 1779 $vector = "\xff\x0f\xef\xfe"; 1780 print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", 1781 unpack("N", $vector), "\n"; 1782 $is_set = vec($vector, 23, 1); 1783 print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; 1784 pvec($vector); 1785 1786 set_vec(1,1,1); 1787 set_vec(3,1,1); 1788 set_vec(23,1,1); 1789 1790 set_vec(3,1,3); 1791 set_vec(3,2,3); 1792 set_vec(3,4,3); 1793 set_vec(3,4,7); 1794 set_vec(3,8,3); 1795 set_vec(3,8,7); 1796 1797 set_vec(0,32,17); 1798 set_vec(1,32,17); 1799 1800 sub set_vec { 1801 my ($offset, $width, $value) = @_; 1802 my $vector = ''; 1803 vec($vector, $offset, $width) = $value; 1804 print "offset=$offset width=$width value=$value\n"; 1805 pvec($vector); 1806 } 1807 1808 sub pvec { 1809 my $vector = shift; 1810 my $bits = unpack("b*", $vector); 1811 my $i = 0; 1812 my $BASE = 8; 1813 1814 print "vector length in bytes: ", length($vector), "\n"; 1815 @bytes = unpack("A8" x length($vector), $bits); 1816 print "bits are: @bytes\n\n"; 1817 } 1818 1819 =head2 Why does defined() return true on empty arrays and hashes? 1820 1821 The short story is that you should probably only use defined on scalars or 1822 functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> 1823 in the 5.004 release or later of Perl for more detail. 1824 1825 =head1 Data: Hashes (Associative Arrays) 1826 1827 =head2 How do I process an entire hash? 1828 1829 (contributed by brian d foy) 1830 1831 There are a couple of ways that you can process an entire hash. You 1832 can get a list of keys, then go through each key, or grab a one 1833 key-value pair at a time. 1834 1835 To go through all of the keys, use the C<keys> function. This extracts 1836 all of the keys of the hash and gives them back to you as a list. You 1837 can then get the value through the particular key you're processing: 1838 1839 foreach my $key ( keys %hash ) { 1840 my $value = $hash{$key} 1841 ... 1842 } 1843 1844 Once you have the list of keys, you can process that list before you 1845 process the hashh elements. For instance, you can sort the keys so you 1846 can process them in lexical order: 1847 1848 foreach my $key ( sort keys %hash ) { 1849 my $value = $hash{$key} 1850 ... 1851 } 1852 1853 Or, you might want to only process some of the items. If you only want 1854 to deal with the keys that start with C<text:>, you can select just 1855 those using C<grep>: 1856 1857 foreach my $key ( grep /^text:/, keys %hash ) { 1858 my $value = $hash{$key} 1859 ... 1860 } 1861 1862 If the hash is very large, you might not want to create a long list of 1863 keys. To save some memory, you can grab on key-value pair at a time using 1864 C<each()>, which returns a pair you haven't seen yet: 1865 1866 while( my( $key, $value ) = each( %hash ) ) { 1867 ... 1868 } 1869 1870 The C<each> operator returns the pairs in apparently random order, so if 1871 ordering matters to you, you'll have to stick with the C<keys> method. 1872 1873 The C<each()> operator can be a bit tricky though. You can't add or 1874 delete keys of the hash while you're using it without possibly 1875 skipping or re-processing some pairs after Perl internally rehashes 1876 all of the elements. Additionally, a hash has only one iterator, so if 1877 you use C<keys>, C<values>, or C<each> on the same hash, you can reset 1878 the iterator and mess up your processing. See the C<each> entry in 1879 L<perlfunc> for more details. 1880 1881 =head2 What happens if I add or remove keys from a hash while iterating over it? 1882 1883 (contributed by brian d foy) 1884 1885 The easy answer is "Don't do that!" 1886 1887 If you iterate through the hash with each(), you can delete the key 1888 most recently returned without worrying about it. If you delete or add 1889 other keys, the iterator may skip or double up on them since perl 1890 may rearrange the hash table. See the 1891 entry for C<each()> in L<perlfunc>. 1892 1893 =head2 How do I look up a hash element by value? 1894 1895 Create a reverse hash: 1896 1897 %by_value = reverse %by_key; 1898 $key = $by_value{$value}; 1899 1900 That's not particularly efficient. It would be more space-efficient 1901 to use: 1902 1903 while (($key, $value) = each %by_key) { 1904 $by_value{$value} = $key; 1905 } 1906 1907 If your hash could have repeated values, the methods above will only find 1908 one of the associated keys. This may or may not worry you. If it does 1909 worry you, you can always reverse the hash into a hash of arrays instead: 1910 1911 while (($key, $value) = each %by_key) { 1912 push @{$key_list_by_value{$value}}, $key; 1913 } 1914 1915 =head2 How can I know how many entries are in a hash? 1916 1917 If you mean how many keys, then all you have to do is 1918 use the keys() function in a scalar context: 1919 1920 $num_keys = keys %hash; 1921 1922 The keys() function also resets the iterator, which means that you may 1923 see strange results if you use this between uses of other hash operators 1924 such as each(). 1925 1926 =head2 How do I sort a hash (optionally by value instead of key)? 1927 1928 (contributed by brian d foy) 1929 1930 To sort a hash, start with the keys. In this example, we give the list of 1931 keys to the sort function which then compares them ASCIIbetically (which 1932 might be affected by your locale settings). The output list has the keys 1933 in ASCIIbetical order. Once we have the keys, we can go through them to 1934 create a report which lists the keys in ASCIIbetical order. 1935 1936 my @keys = sort { $a cmp $b } keys %hash; 1937 1938 foreach my $key ( @keys ) 1939 { 1940 printf "%-20s %6d\n", $key, $hash{$value}; 1941 } 1942 1943 We could get more fancy in the C<sort()> block though. Instead of 1944 comparing the keys, we can compute a value with them and use that 1945 value as the comparison. 1946 1947 For instance, to make our report order case-insensitive, we use 1948 the C<\L> sequence in a double-quoted string to make everything 1949 lowercase. The C<sort()> block then compares the lowercased 1950 values to determine in which order to put the keys. 1951 1952 my @keys = sort { "\L$a" cmp "\L$b" } keys %hash; 1953 1954 Note: if the computation is expensive or the hash has many elements, 1955 you may want to look at the Schwartzian Transform to cache the 1956 computation results. 1957 1958 If we want to sort by the hash value instead, we use the hash key 1959 to look it up. We still get out a list of keys, but this time they 1960 are ordered by their value. 1961 1962 my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash; 1963 1964 From there we can get more complex. If the hash values are the same, 1965 we can provide a secondary sort on the hash key. 1966 1967 my @keys = sort { 1968 $hash{$a} <=> $hash{$b} 1969 or 1970 "\L$a" cmp "\L$b" 1971 } keys %hash; 1972 1973 =head2 How can I always keep my hash sorted? 1974 X<hash tie sort DB_File Tie::IxHash> 1975 1976 You can look into using the C<DB_File> module and C<tie()> using the 1977 C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory 1978 Databases">. The C<Tie::IxHash> module from CPAN might also be 1979 instructive. Although this does keep your hash sorted, you might not 1980 like the slow down you suffer from the tie interface. Are you sure you 1981 need to do this? :) 1982 1983 =head2 What's the difference between "delete" and "undef" with hashes? 1984 1985 Hashes contain pairs of scalars: the first is the key, the 1986 second is the value. The key will be coerced to a string, 1987 although the value can be any kind of scalar: string, 1988 number, or reference. If a key C<$key> is present in 1989 %hash, C<exists($hash{$key})> will return true. The value 1990 for a given key can be C<undef>, in which case 1991 C<$hash{$key}> will be C<undef> while C<exists $hash{$key}> 1992 will return true. This corresponds to (C<$key>, C<undef>) 1993 being in the hash. 1994 1995 Pictures help... here's the C<%hash> table: 1996 1997 keys values 1998 +------+------+ 1999 | a | 3 | 2000 | x | 7 | 2001 | d | 0 | 2002 | e | 2 | 2003 +------+------+ 2004 2005 And these conditions hold 2006 2007 $hash{'a'} is true 2008 $hash{'d'} is false 2009 defined $hash{'d'} is true 2010 defined $hash{'a'} is true 2011 exists $hash{'a'} is true (Perl 5 only) 2012 grep ($_ eq 'a', keys %hash) is true 2013 2014 If you now say 2015 2016 undef $hash{'a'} 2017 2018 your table now reads: 2019 2020 2021 keys values 2022 +------+------+ 2023 | a | undef| 2024 | x | 7 | 2025 | d | 0 | 2026 | e | 2 | 2027 +------+------+ 2028 2029 and these conditions now hold; changes in caps: 2030 2031 $hash{'a'} is FALSE 2032 $hash{'d'} is false 2033 defined $hash{'d'} is true 2034 defined $hash{'a'} is FALSE 2035 exists $hash{'a'} is true (Perl 5 only) 2036 grep ($_ eq 'a', keys %hash) is true 2037 2038 Notice the last two: you have an undef value, but a defined key! 2039 2040 Now, consider this: 2041 2042 delete $hash{'a'} 2043 2044 your table now reads: 2045 2046 keys values 2047 +------+------+ 2048 | x | 7 | 2049 | d | 0 | 2050 | e | 2 | 2051 +------+------+ 2052 2053 and these conditions now hold; changes in caps: 2054 2055 $hash{'a'} is false 2056 $hash{'d'} is false 2057 defined $hash{'d'} is true 2058 defined $hash{'a'} is false 2059 exists $hash{'a'} is FALSE (Perl 5 only) 2060 grep ($_ eq 'a', keys %hash) is FALSE 2061 2062 See, the whole entry is gone! 2063 2064 =head2 Why don't my tied hashes make the defined/exists distinction? 2065 2066 This depends on the tied hash's implementation of EXISTS(). 2067 For example, there isn't the concept of undef with hashes 2068 that are tied to DBM* files. It also means that exists() and 2069 defined() do the same thing with a DBM* file, and what they 2070 end up doing is not what they do with ordinary hashes. 2071 2072 =head2 How do I reset an each() operation part-way through? 2073 2074 (contributed by brian d foy) 2075 2076 You can use the C<keys> or C<values> functions to reset C<each>. To 2077 simply reset the iterator used by C<each> without doing anything else, 2078 use one of them in void context: 2079 2080 keys %hash; # resets iterator, nothing else. 2081 values %hash; # resets iterator, nothing else. 2082 2083 See the documentation for C<each> in L<perlfunc>. 2084 2085 =head2 How can I get the unique keys from two hashes? 2086 2087 First you extract the keys from the hashes into lists, then solve 2088 the "removing duplicates" problem described above. For example: 2089 2090 %seen = (); 2091 for $element (keys(%foo), keys(%bar)) { 2092 $seen{$element}++; 2093 } 2094 @uniq = keys %seen; 2095 2096 Or more succinctly: 2097 2098 @uniq = keys %{{%foo,%bar}}; 2099 2100 Or if you really want to save space: 2101 2102 %seen = (); 2103 while (defined ($key = each %foo)) { 2104 $seen{$key}++; 2105 } 2106 while (defined ($key = each %bar)) { 2107 $seen{$key}++; 2108 } 2109 @uniq = keys %seen; 2110 2111 =head2 How can I store a multidimensional array in a DBM file? 2112 2113 Either stringify the structure yourself (no fun), or else 2114 get the MLDBM (which uses Data::Dumper) module from CPAN and layer 2115 it on top of either DB_File or GDBM_File. 2116 2117 =head2 How can I make my hash remember the order I put elements into it? 2118 2119 Use the C<Tie::IxHash> from CPAN. 2120 2121 use Tie::IxHash; 2122 2123 tie my %myhash, 'Tie::IxHash'; 2124 2125 for (my $i=0; $i<20; $i++) { 2126 $myhash{$i} = 2*$i; 2127 } 2128 2129 my @keys = keys %myhash; 2130 # @keys = (0,1,2,3,...) 2131 2132 =head2 Why does passing a subroutine an undefined element in a hash create it? 2133 2134 If you say something like: 2135 2136 somefunc($hash{"nonesuch key here"}); 2137 2138 Then that element "autovivifies"; that is, it springs into existence 2139 whether you store something there or not. That's because functions 2140 get scalars passed in by reference. If somefunc() modifies C<$_[0]>, 2141 it has to be ready to write it back into the caller's version. 2142 2143 This has been fixed as of Perl5.004. 2144 2145 Normally, merely accessing a key's value for a nonexistent key does 2146 I<not> cause that key to be forever there. This is different than 2147 awk's behavior. 2148 2149 =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? 2150 2151 Usually a hash ref, perhaps like this: 2152 2153 $record = { 2154 NAME => "Jason", 2155 EMPNO => 132, 2156 TITLE => "deputy peon", 2157 AGE => 23, 2158 SALARY => 37_000, 2159 PALS => [ "Norbert", "Rhys", "Phineas"], 2160 }; 2161 2162 References are documented in L<perlref> and the upcoming L<perlreftut>. 2163 Examples of complex data structures are given in L<perldsc> and 2164 L<perllol>. Examples of structures and object-oriented classes are 2165 in L<perltoot>. 2166 2167 =head2 How can I use a reference as a hash key? 2168 2169 (contributed by brian d foy) 2170 2171 Hash keys are strings, so you can't really use a reference as the key. 2172 When you try to do that, perl turns the reference into its stringified 2173 form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get 2174 back the reference from the stringified form, at least without doing 2175 some extra work on your own. Also remember that hash keys must be 2176 unique, but two different variables can store the same reference (and 2177 those variables can change later). 2178 2179 The C<Tie::RefHash> module, which is distributed with perl, might be 2180 what you want. It handles that extra work. 2181 2182 =head1 Data: Misc 2183 2184 =head2 How do I handle binary data correctly? 2185 2186 Perl is binary clean, so it can handle binary data just fine. 2187 On Windows or DOS, however, you have to use C<binmode> for binary 2188 files to avoid conversions for line endings. In general, you should 2189 use C<binmode> any time you want to work with binary data. 2190 2191 Also see L<perlfunc/"binmode"> or L<perlopentut>. 2192 2193 If you're concerned about 8-bit textual data then see L<perllocale>. 2194 If you want to deal with multibyte characters, however, there are 2195 some gotchas. See the section on Regular Expressions. 2196 2197 =head2 How do I determine whether a scalar is a number/whole/integer/float? 2198 2199 Assuming that you don't care about IEEE notations like "NaN" or 2200 "Infinity", you probably just want to use a regular expression. 2201 2202 if (/\D/) { print "has nondigits\n" } 2203 if (/^\d+$/) { print "is a whole number\n" } 2204 if (/^-?\d+$/) { print "is an integer\n" } 2205 if (/^[+-]?\d+$/) { print "is a +/- integer\n" } 2206 if (/^-?\d+\.?\d*$/) { print "is a real number\n" } 2207 if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" } 2208 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) 2209 { print "a C float\n" } 2210 2211 There are also some commonly used modules for the task. 2212 L<Scalar::Util> (distributed with 5.8) provides access to perl's 2213 internal function C<looks_like_number> for determining whether a 2214 variable looks like a number. L<Data::Types> exports functions that 2215 validate data types using both the above and other regular 2216 expressions. Thirdly, there is C<Regexp::Common> which has regular 2217 expressions to match various types of numbers. Those three modules are 2218 available from the CPAN. 2219 2220 If you're on a POSIX system, Perl supports the C<POSIX::strtod> 2221 function. Its semantics are somewhat cumbersome, so here's a 2222 C<getnum> wrapper function for more convenient access. This function 2223 takes a string and returns the number it found, or C<undef> for input 2224 that isn't a C float. The C<is_numeric> function is a front end to 2225 C<getnum> if you just want to say, "Is this a float?" 2226 2227 sub getnum { 2228 use POSIX qw(strtod); 2229 my $str = shift; 2230 $str =~ s/^\s+//; 2231 $str =~ s/\s+$//; 2232 $! = 0; 2233 my($num, $unparsed) = strtod($str); 2234 if (($str eq '') || ($unparsed != 0) || $!) { 2235 return undef; 2236 } 2237 else { 2238 return $num; 2239 } 2240 } 2241 2242 sub is_numeric { defined getnum($_[0]) } 2243 2244 Or you could check out the L<String::Scanf> module on the CPAN 2245 instead. The C<POSIX> module (part of the standard Perl distribution) 2246 provides the C<strtod> and C<strtol> for converting strings to double 2247 and longs, respectively. 2248 2249 =head2 How do I keep persistent data across program calls? 2250 2251 For some specific applications, you can use one of the DBM modules. 2252 See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw> 2253 or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part 2254 of the standard distribution. Here's one example using C<Storable>'s C<store> 2255 and C<retrieve> functions: 2256 2257 use Storable; 2258 store(\%hash, "filename"); 2259 2260 # later on... 2261 $href = retrieve("filename"); # by ref 2262 %hash = %{ retrieve("filename") }; # direct to hash 2263 2264 =head2 How do I print out or copy a recursive data structure? 2265 2266 The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great 2267 for printing out data structures. The C<Storable> module on CPAN (or the 2268 5.8 release of Perl), provides a function called C<dclone> that recursively 2269 copies its argument. 2270 2271 use Storable qw(dclone); 2272 $r2 = dclone($r1); 2273 2274 Where C<$r1> can be a reference to any kind of data structure you'd like. 2275 It will be deeply copied. Because C<dclone> takes and returns references, 2276 you'd have to add extra punctuation if you had a hash of arrays that 2277 you wanted to copy. 2278 2279 %newhash = %{ dclone(\%oldhash) }; 2280 2281 =head2 How do I define methods for every class/object? 2282 2283 Use the C<UNIVERSAL> class (see L<UNIVERSAL>). 2284 2285 =head2 How do I verify a credit card checksum? 2286 2287 Get the C<Business::CreditCard> module from CPAN. 2288 2289 =head2 How do I pack arrays of doubles or floats for XS code? 2290 2291 The kgbpack.c code in the C<PGPLOT> module on CPAN does just this. 2292 If you're doing a lot of float or double processing, consider using 2293 the C<PDL> module from CPAN instead--it makes number-crunching easy. 2294 2295 =head1 REVISION 2296 2297 Revision: $Revision: 10394 $ 2298 2299 Date: $Date: 2007-12-09 18:47:15 +0100 (Sun, 09 Dec 2007) $ 2300 2301 See L<perlfaq> for source control details and availability. 2302 2303 =head1 AUTHOR AND COPYRIGHT 2304 2305 Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and 2306 other authors as noted. All rights reserved. 2307 2308 This documentation is free; you can redistribute it and/or modify it 2309 under the same terms as Perl itself. 2310 2311 Irrespective of its distribution, all code examples in this file 2312 are hereby placed into the public domain. You are permitted and 2313 encouraged to use this code in your own programs for fun 2314 or for profit as you see fit. A simple comment in the code giving 2315 credit would be courteous but is not required.
title
Description
Body
title
Description
Body
title
Description
Body
title
Body
| Generated: Tue Mar 17 22:47:18 2015 | Cross-referenced by PHPXref 0.7.1 |