| 1 | =head1 NAME |
| 2 | |
| 3 | perlfaq4 - Data Manipulation ($Revision: 1.69 $, $Date: 2005/10/14 15:34:06 $) |
| 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 |
| 15 | in binary. Digital (as in powers of two) computers cannot |
| 16 | store all numbers exactly. Some real numbers lose precision |
| 17 | in the process. This is a problem with how computers store |
| 18 | numbers and affects all computer languages, not just Perl. |
| 19 | |
| 20 | L<perlnumber> show the gory details of number |
| 21 | representations and conversions. |
| 22 | |
| 23 | To limit the number of decimal places in your numbers, you |
| 24 | can use the printf or sprintf function. See the |
| 25 | L<"Floating Point 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 int() is most probably working just fine. It's the numbers that |
| 34 | aren't quite what you think. |
| 35 | |
| 36 | First, see the above item "Why am I getting long decimals |
| 37 | (eg, 19.9499999999999) instead of the numbers I should be getting |
| 38 | (eg, 19.95)?". |
| 39 | |
| 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 "0" and hexadecimal literals must start with a leading "0x". |
| 54 | If they are read in from somewhere and assigned, no automatic |
| 55 | conversion takes place. You must explicitly use oct() or hex() if you |
| 56 | want the values converted to decimal. oct() interprets hex ("0x350"), |
| 57 | octal ("0350" or even without the leading "0", like "377") and binary |
| 58 | ("0b1010") numbers, while hex() only converts hexadecimal ones, with |
| 59 | or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef". |
| 60 | The inverse mapping from decimal to octal can be done with either the |
| 61 | "%o" or "%O" sprintf() formats. |
| 62 | |
| 63 | This problem shows up most often when people try using chmod(), mkdir(), |
| 64 | umask(), or sysopen(), which by widespread tradition typically take |
| 65 | 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 | 644, rather than the intended octal literal 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 0..7. |
| 80 | |
| 81 | =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? |
| 82 | |
| 83 | Remember that int() merely truncates toward 0. For rounding to a |
| 84 | certain number of digits, sprintf() or printf() is usually the easiest |
| 85 | route. |
| 86 | |
| 87 | printf("%.3f", 3.1415926535); # prints 3.142 |
| 88 | |
| 89 | The POSIX module (part of the standard Perl distribution) implements |
| 90 | ceil(), floor(), and a number of other mathematical and trigonometric |
| 91 | functions. |
| 92 | |
| 93 | use POSIX; |
| 94 | $ceil = ceil(3.5); # 4 |
| 95 | $floor = floor(3.5); # 3 |
| 96 | |
| 97 | In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex |
| 98 | module. With 5.004, the Math::Trig module (part of the standard Perl |
| 99 | distribution) implements the trigonometric functions. Internally it |
| 100 | uses the Math::Complex module and some functions can break out from |
| 101 | the real axis into the complex plane, for example the inverse sine of |
| 102 | 2. |
| 103 | |
| 104 | Rounding in financial applications can have serious implications, and |
| 105 | the rounding method used should be specified precisely. In these |
| 106 | cases, it probably pays not to trust whichever system rounding is |
| 107 | being used by Perl, but to instead implement the rounding function you |
| 108 | need yourself. |
| 109 | |
| 110 | To see why, notice how you'll still have an issue on half-way-point |
| 111 | alternation: |
| 112 | |
| 113 | for ($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 this. |
| 119 | Perl numbers whose absolute values are integers under 2**31 (on 32 bit |
| 120 | machines) will work pretty much like mathematical integers. Other numbers |
| 121 | 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 |
| 126 | are a few examples of approaches to making common conversions |
| 127 | between number representations. This is intended to be representational |
| 128 | rather than exhaustive. |
| 129 | |
| 130 | Some of the examples below use the Bit::Vector module from CPAN. |
| 131 | The reason you might choose Bit::Vector over the perl built in |
| 132 | functions is that it works with numbers of ANY size, that it is |
| 133 | optimized for speed on some operations, and for at least some |
| 134 | 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 0x notation: |
| 141 | |
| 142 | $dec = 0xDEADBEEF; |
| 143 | |
| 144 | Using the hex function: |
| 145 | |
| 146 | $dec = hex("DEADBEEF"); |
| 147 | |
| 148 | Using pack: |
| 149 | |
| 150 | $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); |
| 151 | |
| 152 | Using the CPAN module 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 sprintf: |
| 161 | |
| 162 | $hex = sprintf("%X", 3735928559); # upper case A-F |
| 163 | $hex = sprintf("%x", 3735928559); # lower case a-f |
| 164 | |
| 165 | Using unpack: |
| 166 | |
| 167 | $hex = unpack("H*", pack("N", 3735928559)); |
| 168 | |
| 169 | Using Bit::Vector: |
| 170 | |
| 171 | use Bit::Vector; |
| 172 | $vec = Bit::Vector->new_Dec(32, -559038737); |
| 173 | $hex = $vec->to_Hex(); |
| 174 | |
| 175 | And 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 oct function: |
| 189 | |
| 190 | $dec = oct("33653337357"); |
| 191 | |
| 192 | Using 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 sprintf: |
| 202 | |
| 203 | $oct = sprintf("%o", 3735928559); |
| 204 | |
| 205 | Using 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 0b notation: |
| 215 | |
| 216 | $number = 0b10110110; |
| 217 | |
| 218 | Using oct: |
| 219 | |
| 220 | my $input = "10110110"; |
| 221 | $decimal = oct( "0b$input" ); |
| 222 | |
| 223 | Using pack and ord: |
| 224 | |
| 225 | $decimal = ord(pack('B8', '10110110')); |
| 226 | |
| 227 | Using pack and 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 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 sprintf (perl 5.6+): |
| 243 | |
| 244 | $bin = sprintf("%b", 3735928559); |
| 245 | |
| 246 | Using unpack: |
| 247 | |
| 248 | $bin = unpack("B*", pack("N", 3735928559)); |
| 249 | |
| 250 | Using 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 $] < 5.004 } |
| 343 | |
| 344 | 5.004 and later automatically call C<srand> at the beginning. Don't |
| 345 | call C<srand> more than once---you make your numbers less random, rather |
| 346 | than more. |
| 347 | |
| 348 | Computers are good at being predictable and bad at being random |
| 349 | (despite appearances caused by bugs in your programs :-). see the |
| 350 | F<random> article in the "Far More Than You Ever Wanted To Know" |
| 351 | collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of |
| 352 | Tom Phoenix, talks more about this. John von Neumann said, "Anyone |
| 353 | who attempts to generate random numbers by deterministic means is, of |
| 354 | course, living in a state of sin." |
| 355 | |
| 356 | If you want numbers that are more random than C<rand> with C<srand> |
| 357 | provides, you should also check out the Math::TrulyRandom module from |
| 358 | CPAN. It uses the imperfections in your system's timer to generate |
| 359 | random numbers, but this takes quite a while. If you want a better |
| 360 | pseudorandom generator than comes with your operating system, look at |
| 361 | "Numerical Recipes in C" at http://www.nr.com/ . |
| 362 | |
| 363 | =head2 How do I get a random number between X and Y? |
| 364 | |
| 365 | C<rand($x)> returns a number such that |
| 366 | C<< 0 <= rand($x) < $x >>. Thus what you want to have perl |
| 367 | figure out is a random number in the range from 0 to the |
| 368 | difference between your I<X> and I<Y>. |
| 369 | |
| 370 | That is, to get a number between 10 and 15, inclusive, you |
| 371 | want a random number between 0 and 5 that you can then add |
| 372 | to 10. |
| 373 | |
| 374 | my $number = 10 + int rand( 15-10+1 ); |
| 375 | |
| 376 | Hence you derive the following simple function to abstract |
| 377 | that. It selects a random integer between the two given |
| 378 | integers (inclusive), For example: C<random_int_in(50,120)>. |
| 379 | |
| 380 | sub random_int_in ($$) { |
| 381 | my($min, $max) = @_; |
| 382 | # Assumes that the two arguments are integers themselves! |
| 383 | return $min if $min == $max; |
| 384 | ($min, $max) = ($max, $min) if $min > $max; |
| 385 | return $min + int rand(1 + $max - $min); |
| 386 | } |
| 387 | |
| 388 | =head1 Data: Dates |
| 389 | |
| 390 | =head2 How do I find the day or week of the year? |
| 391 | |
| 392 | The localtime function returns the day of the year. Without an |
| 393 | argument localtime uses the current time. |
| 394 | |
| 395 | $day_of_year = (localtime)[7]; |
| 396 | |
| 397 | The POSIX module can also format a date as the day of the year or |
| 398 | week of the year. |
| 399 | |
| 400 | use POSIX qw/strftime/; |
| 401 | my $day_of_year = strftime "%j", localtime; |
| 402 | my $week_of_year = strftime "%W", localtime; |
| 403 | |
| 404 | To get the day of year for any date, use the Time::Local module to get |
| 405 | a time in epoch seconds for the argument to localtime. |
| 406 | |
| 407 | use POSIX qw/strftime/; |
| 408 | use Time::Local; |
| 409 | my $week_of_year = strftime "%W", |
| 410 | localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) ); |
| 411 | |
| 412 | The Date::Calc module provides two functions to calculate these. |
| 413 | |
| 414 | use Date::Calc; |
| 415 | my $day_of_year = Day_of_Year( 1987, 12, 18 ); |
| 416 | my $week_of_year = Week_of_Year( 1987, 12, 18 ); |
| 417 | |
| 418 | =head2 How do I find the current century or millennium? |
| 419 | |
| 420 | Use the following simple functions: |
| 421 | |
| 422 | sub get_century { |
| 423 | return int((((localtime(shift || time))[5] + 1999))/100); |
| 424 | } |
| 425 | |
| 426 | sub get_millennium { |
| 427 | return 1+int((((localtime(shift || time))[5] + 1899))/1000); |
| 428 | } |
| 429 | |
| 430 | On some systems, the POSIX module's strftime() function has |
| 431 | been extended in a non-standard way to use a C<%C> format, |
| 432 | which they sometimes claim is the "century". It isn't, |
| 433 | because on most such systems, this is only the first two |
| 434 | digits of the four-digit year, and thus cannot be used to |
| 435 | reliably determine the current century or millennium. |
| 436 | |
| 437 | =head2 How can I compare two dates and find the difference? |
| 438 | |
| 439 | (contributed by brian d foy) |
| 440 | |
| 441 | You could just store all your dates as a number and then subtract. Life |
| 442 | isn't always that simple though. If you want to work with formatted |
| 443 | dates, the Date::Manip, Date::Calc, or DateTime modules can help you. |
| 444 | |
| 445 | |
| 446 | =head2 How can I take a string and turn it into epoch seconds? |
| 447 | |
| 448 | If it's a regular enough string that it always has the same format, |
| 449 | you can split it up and pass the parts to C<timelocal> in the standard |
| 450 | Time::Local module. Otherwise, you should look into the Date::Calc |
| 451 | and Date::Manip modules from CPAN. |
| 452 | |
| 453 | =head2 How can I find the Julian Day? |
| 454 | |
| 455 | (contributed by brian d foy and Dave Cross) |
| 456 | |
| 457 | You can use the Time::JulianDay module available on CPAN. Ensure that |
| 458 | you really want to find a Julian day, though, as many people have |
| 459 | different ideas about Julian days. See |
| 460 | http://www.hermetic.ch/cal_stud/jdn.htm for instance. |
| 461 | |
| 462 | You can also try the DateTime module, which can convert a date/time |
| 463 | to a Julian Day. |
| 464 | |
| 465 | $ perl -MDateTime -le'print DateTime->today->jd' |
| 466 | 2453401.5 |
| 467 | |
| 468 | Or the modified Julian Day |
| 469 | |
| 470 | $ perl -MDateTime -le'print DateTime->today->mjd' |
| 471 | 53401 |
| 472 | |
| 473 | Or even the day of the year (which is what some people think of as a |
| 474 | Julian day) |
| 475 | |
| 476 | $ perl -MDateTime -le'print DateTime->today->doy' |
| 477 | 31 |
| 478 | |
| 479 | =head2 How do I find yesterday's date? |
| 480 | |
| 481 | (contributed by brian d foy) |
| 482 | |
| 483 | Use one of the Date modules. The C<DateTime> module makes it simple, and |
| 484 | give you the same time of day, only the day before. |
| 485 | |
| 486 | use DateTime; |
| 487 | |
| 488 | my $yesterday = DateTime->now->subtract( days => 1 ); |
| 489 | |
| 490 | print "Yesterday was $yesterday\n"; |
| 491 | |
| 492 | You can also use the C<Date::Calc> module using its Today_and_Now |
| 493 | function. |
| 494 | |
| 495 | use Date::Calc qw( Today_and_Now Add_Delta_DHMS ); |
| 496 | |
| 497 | my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 ); |
| 498 | |
| 499 | print "@date\n"; |
| 500 | |
| 501 | Most people try to use the time rather than the calendar to figure out |
| 502 | dates, but that assumes that days are twenty-four hours each. For |
| 503 | most people, there are two days a year when they aren't: the switch to |
| 504 | and from summer time throws this off. Let the modules do the work. |
| 505 | |
| 506 | =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? |
| 507 | |
| 508 | Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is |
| 509 | Y2K compliant (whatever that means). The programmers you've hired to |
| 510 | use it, however, probably are not. |
| 511 | |
| 512 | Long answer: The question belies a true understanding of the issue. |
| 513 | Perl is just as Y2K compliant as your pencil--no more, and no less. |
| 514 | Can you use your pencil to write a non-Y2K-compliant memo? Of course |
| 515 | you can. Is that the pencil's fault? Of course it isn't. |
| 516 | |
| 517 | The date and time functions supplied with Perl (gmtime and localtime) |
| 518 | supply adequate information to determine the year well beyond 2000 |
| 519 | (2038 is when trouble strikes for 32-bit machines). The year returned |
| 520 | by these functions when used in a list context is the year minus 1900. |
| 521 | For years between 1910 and 1999 this I<happens> to be a 2-digit decimal |
| 522 | number. To avoid the year 2000 problem simply do not treat the year as |
| 523 | a 2-digit number. It isn't. |
| 524 | |
| 525 | When gmtime() and localtime() are used in scalar context they return |
| 526 | a timestamp string that contains a fully-expanded year. For example, |
| 527 | C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 |
| 528 | 2001". There's no year 2000 problem here. |
| 529 | |
| 530 | That doesn't mean that Perl can't be used to create non-Y2K compliant |
| 531 | programs. It can. But so can your pencil. It's the fault of the user, |
| 532 | not the language. At the risk of inflaming the NRA: "Perl doesn't |
| 533 | break Y2K, people do." See http://www.perl.org/about/y2k.html for |
| 534 | a longer exposition. |
| 535 | |
| 536 | =head1 Data: Strings |
| 537 | |
| 538 | =head2 How do I validate input? |
| 539 | |
| 540 | (contributed by brian d foy) |
| 541 | |
| 542 | There are many ways to ensure that values are what you expect or |
| 543 | want to accept. Besides the specific examples that we cover in the |
| 544 | perlfaq, you can also look at the modules with "Assert" and "Validate" |
| 545 | in their names, along with other modules such as C<Regexp::Common>. |
| 546 | |
| 547 | Some modules have validation for particular types of input, such |
| 548 | as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>, |
| 549 | and C<Data::Validate::IP>. |
| 550 | |
| 551 | =head2 How do I unescape a string? |
| 552 | |
| 553 | It depends just what you mean by "escape". URL escapes are dealt |
| 554 | with in L<perlfaq9>. Shell escapes with the backslash (C<\>) |
| 555 | character are removed with |
| 556 | |
| 557 | s/\\(.)/$1/g; |
| 558 | |
| 559 | This won't expand C<"\n"> or C<"\t"> or any other special escapes. |
| 560 | |
| 561 | =head2 How do I remove consecutive pairs of characters? |
| 562 | |
| 563 | (contributed by brian d foy) |
| 564 | |
| 565 | You can use the substitution operator to find pairs of characters (or |
| 566 | runs of characters) and replace them with a single instance. In this |
| 567 | substitution, we find a character in C<(.)>. The memory parentheses |
| 568 | store the matched character in the back-reference C<\1> and we use |
| 569 | that to require that the same thing immediately follow it. We replace |
| 570 | that part of the string with the character in C<$1>. |
| 571 | |
| 572 | s/(.)\1/$1/g; |
| 573 | |
| 574 | We can also use the transliteration operator, C<tr///>. In this |
| 575 | example, the search list side of our C<tr///> contains nothing, but |
| 576 | the C<c> option complements that so it contains everything. The |
| 577 | replacement list also contains nothing, so the transliteration is |
| 578 | almost a no-op since it won't do any replacements (or more exactly, |
| 579 | replace the character with itself). However, the C<s> option squashes |
| 580 | duplicated and consecutive characters in the string so a character |
| 581 | does not show up next to itself |
| 582 | |
| 583 | my $str = 'Haarlem'; # in the Netherlands |
| 584 | $str =~ tr///cs; # Now Harlem, like in New York |
| 585 | |
| 586 | =head2 How do I expand function calls in a string? |
| 587 | |
| 588 | (contributed by brian d foy) |
| 589 | |
| 590 | This is documented in L<perlref>, and although it's not the easiest |
| 591 | thing to read, it does work. In each of these examples, we call the |
| 592 | function inside the braces of used to dereference a reference. If we |
| 593 | have a more than one return value, we can construct and dereference an |
| 594 | anonymous array. In this case, we call the function in list context. |
| 595 | |
| 596 | print "The time values are @{ [localtime] }.\n"; |
| 597 | |
| 598 | If we want to call the function in scalar context, we have to do a bit |
| 599 | more work. We can really have any code we like inside the braces, so |
| 600 | we simply have to end with the scalar reference, although how you do |
| 601 | that is up to you, and you can use code inside the braces. |
| 602 | |
| 603 | print "The time is ${\(scalar localtime)}.\n" |
| 604 | |
| 605 | print "The time is ${ my $x = localtime; \$x }.\n"; |
| 606 | |
| 607 | If your function already returns a reference, you don't need to create |
| 608 | the reference yourself. |
| 609 | |
| 610 | sub timestamp { my $t = localtime; \$t } |
| 611 | |
| 612 | print "The time is ${ timestamp() }.\n"; |
| 613 | |
| 614 | In most cases, it is probably easier to simply use string |
| 615 | concatenation, which also forces scalar context. |
| 616 | |
| 617 | print "The time is " . localtime . ".\n"; |
| 618 | |
| 619 | =head2 How do I find matching/nesting anything? |
| 620 | |
| 621 | This isn't something that can be done in one regular expression, no |
| 622 | matter how complicated. To find something between two single |
| 623 | characters, a pattern like C</x([^x]*)x/> will get the intervening |
| 624 | bits in $1. For multiple ones, then something more like |
| 625 | C</alpha(.*?)omega/> would be needed. But none of these deals with |
| 626 | nested patterns. For balanced expressions using C<(>, C<{>, C<[> or |
| 627 | C<< < >> as delimiters, use the CPAN module Regexp::Common, or see |
| 628 | L<perlre/(??{ code })>. For other cases, you'll have to write a |
| 629 | parser. |
| 630 | |
| 631 | If you are serious about writing a parser, there are a number of |
| 632 | modules or oddities that will make your life a lot easier. There are |
| 633 | the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; |
| 634 | and the byacc program. Starting from perl 5.8 the Text::Balanced is |
| 635 | part of the standard distribution. |
| 636 | |
| 637 | One simple destructive, inside-out approach that you might try is to |
| 638 | pull out the smallest nesting parts one at a time: |
| 639 | |
| 640 | while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { |
| 641 | # do something with $1 |
| 642 | } |
| 643 | |
| 644 | A more complicated and sneaky approach is to make Perl's regular |
| 645 | expression engine do it for you. This is courtesy Dean Inada, and |
| 646 | rather has the nature of an Obfuscated Perl Contest entry, but it |
| 647 | really does work: |
| 648 | |
| 649 | # $_ contains the string to parse |
| 650 | # BEGIN and END are the opening and closing markers for the |
| 651 | # nested text. |
| 652 | |
| 653 | @( = ('(',''); |
| 654 | @) = (')',''); |
| 655 | ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; |
| 656 | @$ = (eval{/$re/},$@!~/unmatched/i); |
| 657 | print join("\n",@$[0..$#$]) if( $$[-1] ); |
| 658 | |
| 659 | =head2 How do I reverse a string? |
| 660 | |
| 661 | Use reverse() in scalar context, as documented in |
| 662 | L<perlfunc/reverse>. |
| 663 | |
| 664 | $reversed = reverse $string; |
| 665 | |
| 666 | =head2 How do I expand tabs in a string? |
| 667 | |
| 668 | You can do it yourself: |
| 669 | |
| 670 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; |
| 671 | |
| 672 | Or you can just use the Text::Tabs module (part of the standard Perl |
| 673 | distribution). |
| 674 | |
| 675 | use Text::Tabs; |
| 676 | @expanded_lines = expand(@lines_with_tabs); |
| 677 | |
| 678 | =head2 How do I reformat a paragraph? |
| 679 | |
| 680 | Use Text::Wrap (part of the standard Perl distribution): |
| 681 | |
| 682 | use Text::Wrap; |
| 683 | print wrap("\t", ' ', @paragraphs); |
| 684 | |
| 685 | The paragraphs you give to Text::Wrap should not contain embedded |
| 686 | newlines. Text::Wrap doesn't justify the lines (flush-right). |
| 687 | |
| 688 | Or use the CPAN module Text::Autoformat. Formatting files can be easily |
| 689 | done by making a shell alias, like so: |
| 690 | |
| 691 | alias fmt="perl -i -MText::Autoformat -n0777 \ |
| 692 | -e 'print autoformat $_, {all=>1}' $*" |
| 693 | |
| 694 | See the documentation for Text::Autoformat to appreciate its many |
| 695 | capabilities. |
| 696 | |
| 697 | =head2 How can I access or change N characters of a string? |
| 698 | |
| 699 | You can access the first characters of a string with substr(). |
| 700 | To get the first character, for example, start at position 0 |
| 701 | and grab the string of length 1. |
| 702 | |
| 703 | |
| 704 | $string = "Just another Perl Hacker"; |
| 705 | $first_char = substr( $string, 0, 1 ); # 'J' |
| 706 | |
| 707 | To change part of a string, you can use the optional fourth |
| 708 | argument which is the replacement string. |
| 709 | |
| 710 | substr( $string, 13, 4, "Perl 5.8.0" ); |
| 711 | |
| 712 | You can also use substr() as an lvalue. |
| 713 | |
| 714 | substr( $string, 13, 4 ) = "Perl 5.8.0"; |
| 715 | |
| 716 | =head2 How do I change the Nth occurrence of something? |
| 717 | |
| 718 | You have to keep track of N yourself. For example, let's say you want |
| 719 | to change the fifth occurrence of C<"whoever"> or C<"whomever"> into |
| 720 | C<"whosoever"> or C<"whomsoever">, case insensitively. These |
| 721 | all assume that $_ contains the string to be altered. |
| 722 | |
| 723 | $count = 0; |
| 724 | s{((whom?)ever)}{ |
| 725 | ++$count == 5 # is it the 5th? |
| 726 | ? "${2}soever" # yes, swap |
| 727 | : $1 # renege and leave it there |
| 728 | }ige; |
| 729 | |
| 730 | In the more general case, you can use the C</g> modifier in a C<while> |
| 731 | loop, keeping count of matches. |
| 732 | |
| 733 | $WANT = 3; |
| 734 | $count = 0; |
| 735 | $_ = "One fish two fish red fish blue fish"; |
| 736 | while (/(\w+)\s+fish\b/gi) { |
| 737 | if (++$count == $WANT) { |
| 738 | print "The third fish is a $1 one.\n"; |
| 739 | } |
| 740 | } |
| 741 | |
| 742 | That prints out: C<"The third fish is a red one."> You can also use a |
| 743 | repetition count and repeated pattern like this: |
| 744 | |
| 745 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; |
| 746 | |
| 747 | =head2 How can I count the number of occurrences of a substring within a string? |
| 748 | |
| 749 | There are a number of ways, with varying efficiency. If you want a |
| 750 | count of a certain single character (X) within a string, you can use the |
| 751 | C<tr///> function like so: |
| 752 | |
| 753 | $string = "ThisXlineXhasXsomeXx'sXinXit"; |
| 754 | $count = ($string =~ tr/X//); |
| 755 | print "There are $count X characters in the string"; |
| 756 | |
| 757 | This is fine if you are just looking for a single character. However, |
| 758 | if you are trying to count multiple character substrings within a |
| 759 | larger string, C<tr///> won't work. What you can do is wrap a while() |
| 760 | loop around a global pattern match. For example, let's count negative |
| 761 | integers: |
| 762 | |
| 763 | $string = "-9 55 48 -2 23 -76 4 14 -44"; |
| 764 | while ($string =~ /-\d+/g) { $count++ } |
| 765 | print "There are $count negative numbers in the string"; |
| 766 | |
| 767 | Another version uses a global match in list context, then assigns the |
| 768 | result to a scalar, producing a count of the number of matches. |
| 769 | |
| 770 | $count = () = $string =~ /-\d+/g; |
| 771 | |
| 772 | =head2 How do I capitalize all the words on one line? |
| 773 | |
| 774 | To make the first letter of each word upper case: |
| 775 | |
| 776 | $line =~ s/\b(\w)/\U$1/g; |
| 777 | |
| 778 | This has the strange effect of turning "C<don't do it>" into "C<Don'T |
| 779 | Do It>". Sometimes you might want this. Other times you might need a |
| 780 | more thorough solution (Suggested by brian d foy): |
| 781 | |
| 782 | $string =~ s/ ( |
| 783 | (^\w) #at the beginning of the line |
| 784 | | # or |
| 785 | (\s\w) #preceded by whitespace |
| 786 | ) |
| 787 | /\U$1/xg; |
| 788 | $string =~ /([\w']+)/\u\L$1/g; |
| 789 | |
| 790 | To make the whole line upper case: |
| 791 | |
| 792 | $line = uc($line); |
| 793 | |
| 794 | To force each word to be lower case, with the first letter upper case: |
| 795 | |
| 796 | $line =~ s/(\w+)/\u\L$1/g; |
| 797 | |
| 798 | You can (and probably should) enable locale awareness of those |
| 799 | characters by placing a C<use locale> pragma in your program. |
| 800 | See L<perllocale> for endless details on locales. |
| 801 | |
| 802 | This is sometimes referred to as putting something into "title |
| 803 | case", but that's not quite accurate. Consider the proper |
| 804 | capitalization of the movie I<Dr. Strangelove or: How I Learned to |
| 805 | Stop Worrying and Love the Bomb>, for example. |
| 806 | |
| 807 | Damian Conway's L<Text::Autoformat> module provides some smart |
| 808 | case transformations: |
| 809 | |
| 810 | use Text::Autoformat; |
| 811 | my $x = "Dr. Strangelove or: How I Learned to Stop ". |
| 812 | "Worrying and Love the Bomb"; |
| 813 | |
| 814 | print $x, "\n"; |
| 815 | for my $style (qw( sentence title highlight )) |
| 816 | { |
| 817 | print autoformat($x, { case => $style }), "\n"; |
| 818 | } |
| 819 | |
| 820 | =head2 How can I split a [character] delimited string except when inside [character]? |
| 821 | |
| 822 | Several modules can handle this sort of pasing---Text::Balanced, |
| 823 | Text::CSV, Text::CSV_XS, and Text::ParseWords, among others. |
| 824 | |
| 825 | Take the example case of trying to split a string that is |
| 826 | comma-separated into its different fields. You can't use C<split(/,/)> |
| 827 | because you shouldn't split if the comma is inside quotes. For |
| 828 | example, take a data line like this: |
| 829 | |
| 830 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" |
| 831 | |
| 832 | Due to the restriction of the quotes, this is a fairly complex |
| 833 | problem. Thankfully, we have Jeffrey Friedl, author of |
| 834 | I<Mastering Regular Expressions>, to handle these for us. He |
| 835 | suggests (assuming your string is contained in $text): |
| 836 | |
| 837 | @new = (); |
| 838 | push(@new, $+) while $text =~ m{ |
| 839 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes |
| 840 | | ([^,]+),? |
| 841 | | , |
| 842 | }gx; |
| 843 | push(@new, undef) if substr($text,-1,1) eq ','; |
| 844 | |
| 845 | If you want to represent quotation marks inside a |
| 846 | quotation-mark-delimited field, escape them with backslashes (eg, |
| 847 | C<"like \"this\"">. |
| 848 | |
| 849 | Alternatively, the Text::ParseWords module (part of the standard Perl |
| 850 | distribution) lets you say: |
| 851 | |
| 852 | use Text::ParseWords; |
| 853 | @new = quotewords(",", 0, $text); |
| 854 | |
| 855 | There's also a Text::CSV (Comma-Separated Values) module on CPAN. |
| 856 | |
| 857 | =head2 How do I strip blank space from the beginning/end of a string? |
| 858 | |
| 859 | (contributed by brian d foy) |
| 860 | |
| 861 | A substitution can do this for you. For a single line, you want to |
| 862 | replace all the leading or trailing whitespace with nothing. You |
| 863 | can do that with a pair of substitutions. |
| 864 | |
| 865 | s/^\s+//; |
| 866 | s/\s+$//; |
| 867 | |
| 868 | You can also write that as a single substitution, although it turns |
| 869 | out the combined statement is slower than the separate ones. That |
| 870 | might not matter to you, though. |
| 871 | |
| 872 | s/^\s+|\s+$//g; |
| 873 | |
| 874 | In this regular expression, the alternation matches either at the |
| 875 | beginning or the end of the string since the anchors have a lower |
| 876 | precedence than the alternation. With the C</g> flag, the substitution |
| 877 | makes all possible matches, so it gets both. Remember, the trailing |
| 878 | newline matches the C<\s+>, and the C<$> anchor can match to the |
| 879 | physical end of the string, so the newline disappears too. Just add |
| 880 | the newline to the output, which has the added benefit of preserving |
| 881 | "blank" (consisting entirely of whitespace) lines which the C<^\s+> |
| 882 | would remove all by itself. |
| 883 | |
| 884 | while( <> ) |
| 885 | { |
| 886 | s/^\s+|\s+$//g; |
| 887 | print "$_\n"; |
| 888 | } |
| 889 | |
| 890 | For a multi-line string, you can apply the regular expression |
| 891 | to each logical line in the string by adding the C</m> flag (for |
| 892 | "multi-line"). With the C</m> flag, the C<$> matches I<before> an |
| 893 | embedded newline, so it doesn't remove it. It still removes the |
| 894 | newline at the end of the string. |
| 895 | |
| 896 | $string =~ s/^\s+|\s+$//gm; |
| 897 | |
| 898 | Remember that lines consisting entirely of whitespace will disappear, |
| 899 | since the first part of the alternation can match the entire string |
| 900 | and replace it with nothing. If need to keep embedded blank lines, |
| 901 | you have to do a little more work. Instead of matching any whitespace |
| 902 | (since that includes a newline), just match the other whitespace. |
| 903 | |
| 904 | $string =~ s/^[\t\f ]+|[\t\f ]+$//mg; |
| 905 | |
| 906 | =head2 How do I pad a string with blanks or pad a number with zeroes? |
| 907 | |
| 908 | In the following examples, C<$pad_len> is the length to which you wish |
| 909 | to pad the string, C<$text> or C<$num> contains the string to be padded, |
| 910 | and C<$pad_char> contains the padding character. You can use a single |
| 911 | character string constant instead of the C<$pad_char> variable if you |
| 912 | know what it is in advance. And in the same way you can use an integer in |
| 913 | place of C<$pad_len> if you know the pad length in advance. |
| 914 | |
| 915 | The simplest method uses the C<sprintf> function. It can pad on the left |
| 916 | or right with blanks and on the left with zeroes and it will not |
| 917 | truncate the result. The C<pack> function can only pad strings on the |
| 918 | right with blanks and it will truncate the result to a maximum length of |
| 919 | C<$pad_len>. |
| 920 | |
| 921 | # Left padding a string with blanks (no truncation): |
| 922 | $padded = sprintf("%${pad_len}s", $text); |
| 923 | $padded = sprintf("%*s", $pad_len, $text); # same thing |
| 924 | |
| 925 | # Right padding a string with blanks (no truncation): |
| 926 | $padded = sprintf("%-${pad_len}s", $text); |
| 927 | $padded = sprintf("%-*s", $pad_len, $text); # same thing |
| 928 | |
| 929 | # Left padding a number with 0 (no truncation): |
| 930 | $padded = sprintf("%0${pad_len}d", $num); |
| 931 | $padded = sprintf("%0*d", $pad_len, $num); # same thing |
| 932 | |
| 933 | # Right padding a string with blanks using pack (will truncate): |
| 934 | $padded = pack("A$pad_len",$text); |
| 935 | |
| 936 | If you need to pad with a character other than blank or zero you can use |
| 937 | one of the following methods. They all generate a pad string with the |
| 938 | C<x> operator and combine that with C<$text>. These methods do |
| 939 | not truncate C<$text>. |
| 940 | |
| 941 | Left and right padding with any character, creating a new string: |
| 942 | |
| 943 | $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; |
| 944 | $padded = $text . $pad_char x ( $pad_len - length( $text ) ); |
| 945 | |
| 946 | Left and right padding with any character, modifying C<$text> directly: |
| 947 | |
| 948 | substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); |
| 949 | $text .= $pad_char x ( $pad_len - length( $text ) ); |
| 950 | |
| 951 | =head2 How do I extract selected columns from a string? |
| 952 | |
| 953 | Use substr() or unpack(), both documented in L<perlfunc>. |
| 954 | If you prefer thinking in terms of columns instead of widths, |
| 955 | you can use this kind of thing: |
| 956 | |
| 957 | # determine the unpack format needed to split Linux ps output |
| 958 | # arguments are cut columns |
| 959 | my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72); |
| 960 | |
| 961 | sub cut2fmt { |
| 962 | my(@positions) = @_; |
| 963 | my $template = ''; |
| 964 | my $lastpos = 1; |
| 965 | for my $place (@positions) { |
| 966 | $template .= "A" . ($place - $lastpos) . " "; |
| 967 | $lastpos = $place; |
| 968 | } |
| 969 | $template .= "A*"; |
| 970 | return $template; |
| 971 | } |
| 972 | |
| 973 | =head2 How do I find the soundex value of a string? |
| 974 | |
| 975 | (contributed by brian d foy) |
| 976 | |
| 977 | You can use the Text::Soundex module. If you want to do fuzzy or close |
| 978 | matching, you might also try the String::Approx, and Text::Metaphone, |
| 979 | and Text::DoubleMetaphone modules. |
| 980 | |
| 981 | =head2 How can I expand variables in text strings? |
| 982 | |
| 983 | Let's assume that you have a string that contains placeholder |
| 984 | variables. |
| 985 | |
| 986 | $text = 'this has a $foo in it and a $bar'; |
| 987 | |
| 988 | You can use a substitution with a double evaluation. The |
| 989 | first /e turns C<$1> into C<$foo>, and the second /e turns |
| 990 | C<$foo> into its value. You may want to wrap this in an |
| 991 | C<eval>: if you try to get the value of an undeclared variable |
| 992 | while running under C<use strict>, you get a fatal error. |
| 993 | |
| 994 | eval { $text =~ s/(\$\w+)/$1/eeg }; |
| 995 | die if $@; |
| 996 | |
| 997 | It's probably better in the general case to treat those |
| 998 | variables as entries in some special hash. For example: |
| 999 | |
| 1000 | %user_defs = ( |
| 1001 | foo => 23, |
| 1002 | bar => 19, |
| 1003 | ); |
| 1004 | $text =~ s/\$(\w+)/$user_defs{$1}/g; |
| 1005 | |
| 1006 | =head2 What's wrong with always quoting "$vars"? |
| 1007 | |
| 1008 | The problem is that those double-quotes force stringification-- |
| 1009 | coercing numbers and references into strings--even when you |
| 1010 | don't want them to be strings. Think of it this way: double-quote |
| 1011 | expansion is used to produce new strings. If you already |
| 1012 | have a string, why do you need more? |
| 1013 | |
| 1014 | If you get used to writing odd things like these: |
| 1015 | |
| 1016 | print "$var"; # BAD |
| 1017 | $new = "$old"; # BAD |
| 1018 | somefunc("$var"); # BAD |
| 1019 | |
| 1020 | You'll be in trouble. Those should (in 99.8% of the cases) be |
| 1021 | the simpler and more direct: |
| 1022 | |
| 1023 | print $var; |
| 1024 | $new = $old; |
| 1025 | somefunc($var); |
| 1026 | |
| 1027 | Otherwise, besides slowing you down, you're going to break code when |
| 1028 | the thing in the scalar is actually neither a string nor a number, but |
| 1029 | a reference: |
| 1030 | |
| 1031 | func(\@array); |
| 1032 | sub func { |
| 1033 | my $aref = shift; |
| 1034 | my $oref = "$aref"; # WRONG |
| 1035 | } |
| 1036 | |
| 1037 | You can also get into subtle problems on those few operations in Perl |
| 1038 | that actually do care about the difference between a string and a |
| 1039 | number, such as the magical C<++> autoincrement operator or the |
| 1040 | syscall() function. |
| 1041 | |
| 1042 | Stringification also destroys arrays. |
| 1043 | |
| 1044 | @lines = `command`; |
| 1045 | print "@lines"; # WRONG - extra blanks |
| 1046 | print @lines; # right |
| 1047 | |
| 1048 | =head2 Why don't my E<lt>E<lt>HERE documents work? |
| 1049 | |
| 1050 | Check for these three things: |
| 1051 | |
| 1052 | =over 4 |
| 1053 | |
| 1054 | =item There must be no space after the E<lt>E<lt> part. |
| 1055 | |
| 1056 | =item There (probably) should be a semicolon at the end. |
| 1057 | |
| 1058 | =item You can't (easily) have any space in front of the tag. |
| 1059 | |
| 1060 | =back |
| 1061 | |
| 1062 | If you want to indent the text in the here document, you |
| 1063 | can do this: |
| 1064 | |
| 1065 | # all in one |
| 1066 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; |
| 1067 | your text |
| 1068 | goes here |
| 1069 | HERE_TARGET |
| 1070 | |
| 1071 | But the HERE_TARGET must still be flush against the margin. |
| 1072 | If you want that indented also, you'll have to quote |
| 1073 | in the indentation. |
| 1074 | |
| 1075 | ($quote = <<' FINIS') =~ s/^\s+//gm; |
| 1076 | ...we will have peace, when you and all your works have |
| 1077 | perished--and the works of your dark master to whom you |
| 1078 | would deliver us. You are a liar, Saruman, and a corrupter |
| 1079 | of men's hearts. --Theoden in /usr/src/perl/taint.c |
| 1080 | FINIS |
| 1081 | $quote =~ s/\s+--/\n--/; |
| 1082 | |
| 1083 | A nice general-purpose fixer-upper function for indented here documents |
| 1084 | follows. It expects to be called with a here document as its argument. |
| 1085 | It looks to see whether each line begins with a common substring, and |
| 1086 | if so, strips that substring off. Otherwise, it takes the amount of leading |
| 1087 | whitespace found on the first line and removes that much off each |
| 1088 | subsequent line. |
| 1089 | |
| 1090 | sub fix { |
| 1091 | local $_ = shift; |
| 1092 | my ($white, $leader); # common whitespace and common leading string |
| 1093 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { |
| 1094 | ($white, $leader) = ($2, quotemeta($1)); |
| 1095 | } else { |
| 1096 | ($white, $leader) = (/^(\s+)/, ''); |
| 1097 | } |
| 1098 | s/^\s*?$leader(?:$white)?//gm; |
| 1099 | return $_; |
| 1100 | } |
| 1101 | |
| 1102 | This works with leading special strings, dynamically determined: |
| 1103 | |
| 1104 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; |
| 1105 | @@@ int |
| 1106 | @@@ runops() { |
| 1107 | @@@ SAVEI32(runlevel); |
| 1108 | @@@ runlevel++; |
| 1109 | @@@ while ( op = (*op->op_ppaddr)() ); |
| 1110 | @@@ TAINT_NOT; |
| 1111 | @@@ return 0; |
| 1112 | @@@ } |
| 1113 | MAIN_INTERPRETER_LOOP |
| 1114 | |
| 1115 | Or with a fixed amount of leading whitespace, with remaining |
| 1116 | indentation correctly preserved: |
| 1117 | |
| 1118 | $poem = fix<<EVER_ON_AND_ON; |
| 1119 | Now far ahead the Road has gone, |
| 1120 | And I must follow, if I can, |
| 1121 | Pursuing it with eager feet, |
| 1122 | Until it joins some larger way |
| 1123 | Where many paths and errands meet. |
| 1124 | And whither then? I cannot say. |
| 1125 | --Bilbo in /usr/src/perl/pp_ctl.c |
| 1126 | EVER_ON_AND_ON |
| 1127 | |
| 1128 | =head1 Data: Arrays |
| 1129 | |
| 1130 | =head2 What is the difference between a list and an array? |
| 1131 | |
| 1132 | An array has a changeable length. A list does not. An array is something |
| 1133 | you can push or pop, while a list is a set of values. Some people make |
| 1134 | the distinction that a list is a value while an array is a variable. |
| 1135 | Subroutines are passed and return lists, you put things into list |
| 1136 | context, you initialize arrays with lists, and you foreach() across |
| 1137 | a list. C<@> variables are arrays, anonymous arrays are arrays, arrays |
| 1138 | in scalar context behave like the number of elements in them, subroutines |
| 1139 | access their arguments through the array C<@_>, and push/pop/shift only work |
| 1140 | on arrays. |
| 1141 | |
| 1142 | As a side note, there's no such thing as a list in scalar context. |
| 1143 | When you say |
| 1144 | |
| 1145 | $scalar = (2, 5, 7, 9); |
| 1146 | |
| 1147 | you're using the comma operator in scalar context, so it uses the scalar |
| 1148 | comma operator. There never was a list there at all! This causes the |
| 1149 | last value to be returned: 9. |
| 1150 | |
| 1151 | =head2 What is the difference between $array[1] and @array[1]? |
| 1152 | |
| 1153 | The former is a scalar value; the latter an array slice, making |
| 1154 | it a list with one (scalar) value. You should use $ when you want a |
| 1155 | scalar value (most of the time) and @ when you want a list with one |
| 1156 | scalar value in it (very, very rarely; nearly never, in fact). |
| 1157 | |
| 1158 | Sometimes it doesn't make a difference, but sometimes it does. |
| 1159 | For example, compare: |
| 1160 | |
| 1161 | $good[0] = `some program that outputs several lines`; |
| 1162 | |
| 1163 | with |
| 1164 | |
| 1165 | @bad[0] = `same program that outputs several lines`; |
| 1166 | |
| 1167 | The C<use warnings> pragma and the B<-w> flag will warn you about these |
| 1168 | matters. |
| 1169 | |
| 1170 | =head2 How can I remove duplicate elements from a list or array? |
| 1171 | |
| 1172 | (contributed by brian d foy) |
| 1173 | |
| 1174 | Use a hash. When you think the words "unique" or "duplicated", think |
| 1175 | "hash keys". |
| 1176 | |
| 1177 | If you don't care about the order of the elements, you could just |
| 1178 | create the hash then extract the keys. It's not important how you |
| 1179 | create that hash: just that you use C<keys> to get the unique |
| 1180 | elements. |
| 1181 | |
| 1182 | my %hash = map { $_, 1 } @array; |
| 1183 | # or a hash slice: @hash{ @array } = (); |
| 1184 | # or a foreach: $hash{$_} = 1 foreach ( @array ); |
| 1185 | |
| 1186 | my @unique = keys %hash; |
| 1187 | |
| 1188 | You can also go through each element and skip the ones you've seen |
| 1189 | before. Use a hash to keep track. The first time the loop sees an |
| 1190 | element, that element has no key in C<%Seen>. The C<next> statement |
| 1191 | creates the key and immediately uses its value, which is C<undef>, so |
| 1192 | the loop continues to the C<push> and increments the value for that |
| 1193 | key. The next time the loop sees that same element, its key exists in |
| 1194 | the hash I<and> the value for that key is true (since it's not 0 or |
| 1195 | undef), so the next skips that iteration and the loop goes to the next |
| 1196 | element. |
| 1197 | |
| 1198 | my @unique = (); |
| 1199 | my %seen = (); |
| 1200 | |
| 1201 | foreach my $elem ( @array ) |
| 1202 | { |
| 1203 | next if $seen{ $elem }++; |
| 1204 | push @unique, $elem; |
| 1205 | } |
| 1206 | |
| 1207 | You can write this more briefly using a grep, which does the |
| 1208 | same thing. |
| 1209 | |
| 1210 | my %seen = (); |
| 1211 | my @unique = grep { ! $seen{ $_ }++ } @array; |
| 1212 | |
| 1213 | =head2 How can I tell whether a certain element is contained in a list or array? |
| 1214 | |
| 1215 | (portions of this answer contributed by Anno Siegel) |
| 1216 | |
| 1217 | Hearing the word "in" is an I<in>dication that you probably should have |
| 1218 | used a hash, not a list or array, to store your data. Hashes are |
| 1219 | designed to answer this question quickly and efficiently. Arrays aren't. |
| 1220 | |
| 1221 | That being said, there are several ways to approach this. If you |
| 1222 | are going to make this query many times over arbitrary string values, |
| 1223 | the fastest way is probably to invert the original array and maintain a |
| 1224 | hash whose keys are the first array's values. |
| 1225 | |
| 1226 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; |
| 1227 | %is_blue = (); |
| 1228 | for (@blues) { $is_blue{$_} = 1 } |
| 1229 | |
| 1230 | Now you can check whether $is_blue{$some_color}. It might have been a |
| 1231 | good idea to keep the blues all in a hash in the first place. |
| 1232 | |
| 1233 | If the values are all small integers, you could use a simple indexed |
| 1234 | array. This kind of an array will take up less space: |
| 1235 | |
| 1236 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); |
| 1237 | @is_tiny_prime = (); |
| 1238 | for (@primes) { $is_tiny_prime[$_] = 1 } |
| 1239 | # or simply @istiny_prime[@primes] = (1) x @primes; |
| 1240 | |
| 1241 | Now you check whether $is_tiny_prime[$some_number]. |
| 1242 | |
| 1243 | If the values in question are integers instead of strings, you can save |
| 1244 | quite a lot of space by using bit strings instead: |
| 1245 | |
| 1246 | @articles = ( 1..10, 150..2000, 2017 ); |
| 1247 | undef $read; |
| 1248 | for (@articles) { vec($read,$_,1) = 1 } |
| 1249 | |
| 1250 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. |
| 1251 | |
| 1252 | These methods guarantee fast individual tests but require a re-organization |
| 1253 | of the original list or array. They only pay off if you have to test |
| 1254 | multiple values against the same array. |
| 1255 | |
| 1256 | If you are testing only once, the standard module List::Util exports |
| 1257 | the function C<first> for this purpose. It works by stopping once it |
| 1258 | finds the element. It's written in C for speed, and its Perl equivalant |
| 1259 | looks like this subroutine: |
| 1260 | |
| 1261 | sub first (&@) { |
| 1262 | my $code = shift; |
| 1263 | foreach (@_) { |
| 1264 | return $_ if &{$code}(); |
| 1265 | } |
| 1266 | undef; |
| 1267 | } |
| 1268 | |
| 1269 | If speed is of little concern, the common idiom uses grep in scalar context |
| 1270 | (which returns the number of items that passed its condition) to traverse the |
| 1271 | entire list. This does have the benefit of telling you how many matches it |
| 1272 | found, though. |
| 1273 | |
| 1274 | my $is_there = grep $_ eq $whatever, @array; |
| 1275 | |
| 1276 | If you want to actually extract the matching elements, simply use grep in |
| 1277 | list context. |
| 1278 | |
| 1279 | my @matches = grep $_ eq $whatever, @array; |
| 1280 | |
| 1281 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? |
| 1282 | |
| 1283 | Use a hash. Here's code to do both and more. It assumes that |
| 1284 | each element is unique in a given array: |
| 1285 | |
| 1286 | @union = @intersection = @difference = (); |
| 1287 | %count = (); |
| 1288 | foreach $element (@array1, @array2) { $count{$element}++ } |
| 1289 | foreach $element (keys %count) { |
| 1290 | push @union, $element; |
| 1291 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; |
| 1292 | } |
| 1293 | |
| 1294 | Note that this is the I<symmetric difference>, that is, all elements in |
| 1295 | either A or in B but not in both. Think of it as an xor operation. |
| 1296 | |
| 1297 | =head2 How do I test whether two arrays or hashes are equal? |
| 1298 | |
| 1299 | The following code works for single-level arrays. It uses a stringwise |
| 1300 | comparison, and does not distinguish defined versus undefined empty |
| 1301 | strings. Modify if you have other needs. |
| 1302 | |
| 1303 | $are_equal = compare_arrays(\@frogs, \@toads); |
| 1304 | |
| 1305 | sub compare_arrays { |
| 1306 | my ($first, $second) = @_; |
| 1307 | no warnings; # silence spurious -w undef complaints |
| 1308 | return 0 unless @$first == @$second; |
| 1309 | for (my $i = 0; $i < @$first; $i++) { |
| 1310 | return 0 if $first->[$i] ne $second->[$i]; |
| 1311 | } |
| 1312 | return 1; |
| 1313 | } |
| 1314 | |
| 1315 | For multilevel structures, you may wish to use an approach more |
| 1316 | like this one. It uses the CPAN module FreezeThaw: |
| 1317 | |
| 1318 | use FreezeThaw qw(cmpStr); |
| 1319 | @a = @b = ( "this", "that", [ "more", "stuff" ] ); |
| 1320 | |
| 1321 | printf "a and b contain %s arrays\n", |
| 1322 | cmpStr(\@a, \@b) == 0 |
| 1323 | ? "the same" |
| 1324 | : "different"; |
| 1325 | |
| 1326 | This approach also works for comparing hashes. Here |
| 1327 | we'll demonstrate two different answers: |
| 1328 | |
| 1329 | use FreezeThaw qw(cmpStr cmpStrHard); |
| 1330 | |
| 1331 | %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); |
| 1332 | $a{EXTRA} = \%b; |
| 1333 | $b{EXTRA} = \%a; |
| 1334 | |
| 1335 | printf "a and b contain %s hashes\n", |
| 1336 | cmpStr(\%a, \%b) == 0 ? "the same" : "different"; |
| 1337 | |
| 1338 | printf "a and b contain %s hashes\n", |
| 1339 | cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; |
| 1340 | |
| 1341 | |
| 1342 | The first reports that both those the hashes contain the same data, |
| 1343 | while the second reports that they do not. Which you prefer is left as |
| 1344 | an exercise to the reader. |
| 1345 | |
| 1346 | =head2 How do I find the first array element for which a condition is true? |
| 1347 | |
| 1348 | To find the first array element which satisfies a condition, you can |
| 1349 | use the first() function in the List::Util module, which comes with |
| 1350 | Perl 5.8. This example finds the first element that contains "Perl". |
| 1351 | |
| 1352 | use List::Util qw(first); |
| 1353 | |
| 1354 | my $element = first { /Perl/ } @array; |
| 1355 | |
| 1356 | If you cannot use List::Util, you can make your own loop to do the |
| 1357 | same thing. Once you find the element, you stop the loop with last. |
| 1358 | |
| 1359 | my $found; |
| 1360 | foreach ( @array ) |
| 1361 | { |
| 1362 | if( /Perl/ ) { $found = $_; last } |
| 1363 | } |
| 1364 | |
| 1365 | If you want the array index, you can iterate through the indices |
| 1366 | and check the array element at each index until you find one |
| 1367 | that satisfies the condition. |
| 1368 | |
| 1369 | my( $found, $index ) = ( undef, -1 ); |
| 1370 | for( $i = 0; $i < @array; $i++ ) |
| 1371 | { |
| 1372 | if( $array[$i] =~ /Perl/ ) |
| 1373 | { |
| 1374 | $found = $array[$i]; |
| 1375 | $index = $i; |
| 1376 | last; |
| 1377 | } |
| 1378 | } |
| 1379 | |
| 1380 | =head2 How do I handle linked lists? |
| 1381 | |
| 1382 | In general, you usually don't need a linked list in Perl, since with |
| 1383 | regular arrays, you can push and pop or shift and unshift at either end, |
| 1384 | or you can use splice to add and/or remove arbitrary number of elements at |
| 1385 | arbitrary points. Both pop and shift are both O(1) operations on Perl's |
| 1386 | dynamic arrays. In the absence of shifts and pops, push in general |
| 1387 | needs to reallocate on the order every log(N) times, and unshift will |
| 1388 | need to copy pointers each time. |
| 1389 | |
| 1390 | If you really, really wanted, you could use structures as described in |
| 1391 | L<perldsc> or L<perltoot> and do just what the algorithm book tells you |
| 1392 | to do. For example, imagine a list node like this: |
| 1393 | |
| 1394 | $node = { |
| 1395 | VALUE => 42, |
| 1396 | LINK => undef, |
| 1397 | }; |
| 1398 | |
| 1399 | You could walk the list this way: |
| 1400 | |
| 1401 | print "List: "; |
| 1402 | for ($node = $head; $node; $node = $node->{LINK}) { |
| 1403 | print $node->{VALUE}, " "; |
| 1404 | } |
| 1405 | print "\n"; |
| 1406 | |
| 1407 | You could add to the list this way: |
| 1408 | |
| 1409 | my ($head, $tail); |
| 1410 | $tail = append($head, 1); # grow a new head |
| 1411 | for $value ( 2 .. 10 ) { |
| 1412 | $tail = append($tail, $value); |
| 1413 | } |
| 1414 | |
| 1415 | sub append { |
| 1416 | my($list, $value) = @_; |
| 1417 | my $node = { VALUE => $value }; |
| 1418 | if ($list) { |
| 1419 | $node->{LINK} = $list->{LINK}; |
| 1420 | $list->{LINK} = $node; |
| 1421 | } else { |
| 1422 | $_[0] = $node; # replace caller's version |
| 1423 | } |
| 1424 | return $node; |
| 1425 | } |
| 1426 | |
| 1427 | But again, Perl's built-in are virtually always good enough. |
| 1428 | |
| 1429 | =head2 How do I handle circular lists? |
| 1430 | |
| 1431 | Circular lists could be handled in the traditional fashion with linked |
| 1432 | lists, or you could just do something like this with an array: |
| 1433 | |
| 1434 | unshift(@array, pop(@array)); # the last shall be first |
| 1435 | push(@array, shift(@array)); # and vice versa |
| 1436 | |
| 1437 | =head2 How do I shuffle an array randomly? |
| 1438 | |
| 1439 | If you either have Perl 5.8.0 or later installed, or if you have |
| 1440 | Scalar-List-Utils 1.03 or later installed, you can say: |
| 1441 | |
| 1442 | use List::Util 'shuffle'; |
| 1443 | |
| 1444 | @shuffled = shuffle(@list); |
| 1445 | |
| 1446 | If not, you can use a Fisher-Yates shuffle. |
| 1447 | |
| 1448 | sub fisher_yates_shuffle { |
| 1449 | my $deck = shift; # $deck is a reference to an array |
| 1450 | my $i = @$deck; |
| 1451 | while (--$i) { |
| 1452 | my $j = int rand ($i+1); |
| 1453 | @$deck[$i,$j] = @$deck[$j,$i]; |
| 1454 | } |
| 1455 | } |
| 1456 | |
| 1457 | # shuffle my mpeg collection |
| 1458 | # |
| 1459 | my @mpeg = <audio/*/*.mp3>; |
| 1460 | fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place |
| 1461 | print @mpeg; |
| 1462 | |
| 1463 | Note that the above implementation shuffles an array in place, |
| 1464 | unlike the List::Util::shuffle() which takes a list and returns |
| 1465 | a new shuffled list. |
| 1466 | |
| 1467 | You've probably seen shuffling algorithms that work using splice, |
| 1468 | randomly picking another element to swap the current element with |
| 1469 | |
| 1470 | srand; |
| 1471 | @new = (); |
| 1472 | @old = 1 .. 10; # just a demo |
| 1473 | while (@old) { |
| 1474 | push(@new, splice(@old, rand @old, 1)); |
| 1475 | } |
| 1476 | |
| 1477 | This is bad because splice is already O(N), and since you do it N times, |
| 1478 | you just invented a quadratic algorithm; that is, O(N**2). This does |
| 1479 | not scale, although Perl is so efficient that you probably won't notice |
| 1480 | this until you have rather largish arrays. |
| 1481 | |
| 1482 | =head2 How do I process/modify each element of an array? |
| 1483 | |
| 1484 | Use C<for>/C<foreach>: |
| 1485 | |
| 1486 | for (@lines) { |
| 1487 | s/foo/bar/; # change that word |
| 1488 | tr/XZ/ZX/; # swap those letters |
| 1489 | } |
| 1490 | |
| 1491 | Here's another; let's compute spherical volumes: |
| 1492 | |
| 1493 | for (@volumes = @radii) { # @volumes has changed parts |
| 1494 | $_ **= 3; |
| 1495 | $_ *= (4/3) * 3.14159; # this will be constant folded |
| 1496 | } |
| 1497 | |
| 1498 | which can also be done with map() which is made to transform |
| 1499 | one list into another: |
| 1500 | |
| 1501 | @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii; |
| 1502 | |
| 1503 | If you want to do the same thing to modify the values of the |
| 1504 | hash, you can use the C<values> function. As of Perl 5.6 |
| 1505 | the values are not copied, so if you modify $orbit (in this |
| 1506 | case), you modify the value. |
| 1507 | |
| 1508 | for $orbit ( values %orbits ) { |
| 1509 | ($orbit **= 3) *= (4/3) * 3.14159; |
| 1510 | } |
| 1511 | |
| 1512 | Prior to perl 5.6 C<values> returned copies of the values, |
| 1513 | so older perl code often contains constructions such as |
| 1514 | C<@orbits{keys %orbits}> instead of C<values %orbits> where |
| 1515 | the hash is to be modified. |
| 1516 | |
| 1517 | =head2 How do I select a random element from an array? |
| 1518 | |
| 1519 | Use the rand() function (see L<perlfunc/rand>): |
| 1520 | |
| 1521 | $index = rand @array; |
| 1522 | $element = $array[$index]; |
| 1523 | |
| 1524 | Or, simply: |
| 1525 | my $element = $array[ rand @array ]; |
| 1526 | |
| 1527 | =head2 How do I permute N elements of a list? |
| 1528 | |
| 1529 | Use the List::Permutor module on CPAN. If the list is |
| 1530 | actually an array, try the Algorithm::Permute module (also |
| 1531 | on CPAN). It's written in XS code and is very efficient. |
| 1532 | |
| 1533 | use Algorithm::Permute; |
| 1534 | my @array = 'a'..'d'; |
| 1535 | my $p_iterator = Algorithm::Permute->new ( \@array ); |
| 1536 | while (my @perm = $p_iterator->next) { |
| 1537 | print "next permutation: (@perm)\n"; |
| 1538 | } |
| 1539 | |
| 1540 | For even faster execution, you could do: |
| 1541 | |
| 1542 | use Algorithm::Permute; |
| 1543 | my @array = 'a'..'d'; |
| 1544 | Algorithm::Permute::permute { |
| 1545 | print "next permutation: (@array)\n"; |
| 1546 | } @array; |
| 1547 | |
| 1548 | Here's a little program that generates all permutations of |
| 1549 | all the words on each line of input. The algorithm embodied |
| 1550 | in the permute() function is discussed in Volume 4 (still |
| 1551 | unpublished) of Knuth's I<The Art of Computer Programming> |
| 1552 | and will work on any list: |
| 1553 | |
| 1554 | #!/usr/bin/perl -n |
| 1555 | # Fischer-Kause ordered permutation generator |
| 1556 | |
| 1557 | sub permute (&@) { |
| 1558 | my $code = shift; |
| 1559 | my @idx = 0..$#_; |
| 1560 | while ( $code->(@_[@idx]) ) { |
| 1561 | my $p = $#idx; |
| 1562 | --$p while $idx[$p-1] > $idx[$p]; |
| 1563 | my $q = $p or return; |
| 1564 | push @idx, reverse splice @idx, $p; |
| 1565 | ++$q while $idx[$p-1] > $idx[$q]; |
| 1566 | @idx[$p-1,$q]=@idx[$q,$p-1]; |
| 1567 | } |
| 1568 | } |
| 1569 | |
| 1570 | permute {print"@_\n"} split; |
| 1571 | |
| 1572 | =head2 How do I sort an array by (anything)? |
| 1573 | |
| 1574 | Supply a comparison function to sort() (described in L<perlfunc/sort>): |
| 1575 | |
| 1576 | @list = sort { $a <=> $b } @list; |
| 1577 | |
| 1578 | The default sort function is cmp, string comparison, which would |
| 1579 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is |
| 1580 | the numerical comparison operator. |
| 1581 | |
| 1582 | If you have a complicated function needed to pull out the part you |
| 1583 | want to sort on, then don't do it inside the sort function. Pull it |
| 1584 | out first, because the sort BLOCK can be called many times for the |
| 1585 | same element. Here's an example of how to pull out the first word |
| 1586 | after the first number on each item, and then sort those words |
| 1587 | case-insensitively. |
| 1588 | |
| 1589 | @idx = (); |
| 1590 | for (@data) { |
| 1591 | ($item) = /\d+\s*(\S+)/; |
| 1592 | push @idx, uc($item); |
| 1593 | } |
| 1594 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; |
| 1595 | |
| 1596 | which could also be written this way, using a trick |
| 1597 | that's come to be known as the Schwartzian Transform: |
| 1598 | |
| 1599 | @sorted = map { $_->[0] } |
| 1600 | sort { $a->[1] cmp $b->[1] } |
| 1601 | map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; |
| 1602 | |
| 1603 | If you need to sort on several fields, the following paradigm is useful. |
| 1604 | |
| 1605 | @sorted = sort { field1($a) <=> field1($b) || |
| 1606 | field2($a) cmp field2($b) || |
| 1607 | field3($a) cmp field3($b) |
| 1608 | } @data; |
| 1609 | |
| 1610 | This can be conveniently combined with precalculation of keys as given |
| 1611 | above. |
| 1612 | |
| 1613 | See the F<sort> article in the "Far More Than You Ever Wanted |
| 1614 | To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for |
| 1615 | more about this approach. |
| 1616 | |
| 1617 | See also the question below on sorting hashes. |
| 1618 | |
| 1619 | =head2 How do I manipulate arrays of bits? |
| 1620 | |
| 1621 | Use pack() and unpack(), or else vec() and the bitwise operations. |
| 1622 | |
| 1623 | For example, this sets $vec to have bit N set if $ints[N] was set: |
| 1624 | |
| 1625 | $vec = ''; |
| 1626 | foreach(@ints) { vec($vec,$_,1) = 1 } |
| 1627 | |
| 1628 | Here's how, given a vector in $vec, you can |
| 1629 | get those bits into your @ints array: |
| 1630 | |
| 1631 | sub bitvec_to_list { |
| 1632 | my $vec = shift; |
| 1633 | my @ints; |
| 1634 | # Find null-byte density then select best algorithm |
| 1635 | if ($vec =~ tr/\0// / length $vec > 0.95) { |
| 1636 | use integer; |
| 1637 | my $i; |
| 1638 | # This method is faster with mostly null-bytes |
| 1639 | while($vec =~ /[^\0]/g ) { |
| 1640 | $i = -9 + 8 * pos $vec; |
| 1641 | push @ints, $i if vec($vec, ++$i, 1); |
| 1642 | push @ints, $i if vec($vec, ++$i, 1); |
| 1643 | push @ints, $i if vec($vec, ++$i, 1); |
| 1644 | push @ints, $i if vec($vec, ++$i, 1); |
| 1645 | push @ints, $i if vec($vec, ++$i, 1); |
| 1646 | push @ints, $i if vec($vec, ++$i, 1); |
| 1647 | push @ints, $i if vec($vec, ++$i, 1); |
| 1648 | push @ints, $i if vec($vec, ++$i, 1); |
| 1649 | } |
| 1650 | } else { |
| 1651 | # This method is a fast general algorithm |
| 1652 | use integer; |
| 1653 | my $bits = unpack "b*", $vec; |
| 1654 | push @ints, 0 if $bits =~ s/^(\d)// && $1; |
| 1655 | push @ints, pos $bits while($bits =~ /1/g); |
| 1656 | } |
| 1657 | return \@ints; |
| 1658 | } |
| 1659 | |
| 1660 | This method gets faster the more sparse the bit vector is. |
| 1661 | (Courtesy of Tim Bunce and Winfried Koenig.) |
| 1662 | |
| 1663 | You can make the while loop a lot shorter with this suggestion |
| 1664 | from Benjamin Goldberg: |
| 1665 | |
| 1666 | while($vec =~ /[^\0]+/g ) { |
| 1667 | push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8; |
| 1668 | } |
| 1669 | |
| 1670 | Or use the CPAN module Bit::Vector: |
| 1671 | |
| 1672 | $vector = Bit::Vector->new($num_of_bits); |
| 1673 | $vector->Index_List_Store(@ints); |
| 1674 | @ints = $vector->Index_List_Read(); |
| 1675 | |
| 1676 | Bit::Vector provides efficient methods for bit vector, sets of small integers |
| 1677 | and "big int" math. |
| 1678 | |
| 1679 | Here's a more extensive illustration using vec(): |
| 1680 | |
| 1681 | # vec demo |
| 1682 | $vector = "\xff\x0f\xef\xfe"; |
| 1683 | print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", |
| 1684 | unpack("N", $vector), "\n"; |
| 1685 | $is_set = vec($vector, 23, 1); |
| 1686 | print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; |
| 1687 | pvec($vector); |
| 1688 | |
| 1689 | set_vec(1,1,1); |
| 1690 | set_vec(3,1,1); |
| 1691 | set_vec(23,1,1); |
| 1692 | |
| 1693 | set_vec(3,1,3); |
| 1694 | set_vec(3,2,3); |
| 1695 | set_vec(3,4,3); |
| 1696 | set_vec(3,4,7); |
| 1697 | set_vec(3,8,3); |
| 1698 | set_vec(3,8,7); |
| 1699 | |
| 1700 | set_vec(0,32,17); |
| 1701 | set_vec(1,32,17); |
| 1702 | |
| 1703 | sub set_vec { |
| 1704 | my ($offset, $width, $value) = @_; |
| 1705 | my $vector = ''; |
| 1706 | vec($vector, $offset, $width) = $value; |
| 1707 | print "offset=$offset width=$width value=$value\n"; |
| 1708 | pvec($vector); |
| 1709 | } |
| 1710 | |
| 1711 | sub pvec { |
| 1712 | my $vector = shift; |
| 1713 | my $bits = unpack("b*", $vector); |
| 1714 | my $i = 0; |
| 1715 | my $BASE = 8; |
| 1716 | |
| 1717 | print "vector length in bytes: ", length($vector), "\n"; |
| 1718 | @bytes = unpack("A8" x length($vector), $bits); |
| 1719 | print "bits are: @bytes\n\n"; |
| 1720 | } |
| 1721 | |
| 1722 | =head2 Why does defined() return true on empty arrays and hashes? |
| 1723 | |
| 1724 | The short story is that you should probably only use defined on scalars or |
| 1725 | functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> |
| 1726 | in the 5.004 release or later of Perl for more detail. |
| 1727 | |
| 1728 | =head1 Data: Hashes (Associative Arrays) |
| 1729 | |
| 1730 | =head2 How do I process an entire hash? |
| 1731 | |
| 1732 | Use the each() function (see L<perlfunc/each>) if you don't care |
| 1733 | whether it's sorted: |
| 1734 | |
| 1735 | while ( ($key, $value) = each %hash) { |
| 1736 | print "$key = $value\n"; |
| 1737 | } |
| 1738 | |
| 1739 | If you want it sorted, you'll have to use foreach() on the result of |
| 1740 | sorting the keys as shown in an earlier question. |
| 1741 | |
| 1742 | =head2 What happens if I add or remove keys from a hash while iterating over it? |
| 1743 | |
| 1744 | (contributed by brian d foy) |
| 1745 | |
| 1746 | The easy answer is "Don't do that!" |
| 1747 | |
| 1748 | If you iterate through the hash with each(), you can delete the key |
| 1749 | most recently returned without worrying about it. If you delete or add |
| 1750 | other keys, the iterator may skip or double up on them since perl |
| 1751 | may rearrange the hash table. See the |
| 1752 | entry for C<each()> in L<perlfunc>. |
| 1753 | |
| 1754 | =head2 How do I look up a hash element by value? |
| 1755 | |
| 1756 | Create a reverse hash: |
| 1757 | |
| 1758 | %by_value = reverse %by_key; |
| 1759 | $key = $by_value{$value}; |
| 1760 | |
| 1761 | That's not particularly efficient. It would be more space-efficient |
| 1762 | to use: |
| 1763 | |
| 1764 | while (($key, $value) = each %by_key) { |
| 1765 | $by_value{$value} = $key; |
| 1766 | } |
| 1767 | |
| 1768 | If your hash could have repeated values, the methods above will only find |
| 1769 | one of the associated keys. This may or may not worry you. If it does |
| 1770 | worry you, you can always reverse the hash into a hash of arrays instead: |
| 1771 | |
| 1772 | while (($key, $value) = each %by_key) { |
| 1773 | push @{$key_list_by_value{$value}}, $key; |
| 1774 | } |
| 1775 | |
| 1776 | =head2 How can I know how many entries are in a hash? |
| 1777 | |
| 1778 | If you mean how many keys, then all you have to do is |
| 1779 | use the keys() function in a scalar context: |
| 1780 | |
| 1781 | $num_keys = keys %hash; |
| 1782 | |
| 1783 | The keys() function also resets the iterator, which means that you may |
| 1784 | see strange results if you use this between uses of other hash operators |
| 1785 | such as each(). |
| 1786 | |
| 1787 | =head2 How do I sort a hash (optionally by value instead of key)? |
| 1788 | |
| 1789 | Internally, hashes are stored in a way that prevents you from imposing |
| 1790 | an order on key-value pairs. Instead, you have to sort a list of the |
| 1791 | keys or values: |
| 1792 | |
| 1793 | @keys = sort keys %hash; # sorted by key |
| 1794 | @keys = sort { |
| 1795 | $hash{$a} cmp $hash{$b} |
| 1796 | } keys %hash; # and by value |
| 1797 | |
| 1798 | Here we'll do a reverse numeric sort by value, and if two keys are |
| 1799 | identical, sort by length of key, or if that fails, by straight ASCII |
| 1800 | comparison of the keys (well, possibly modified by your locale--see |
| 1801 | L<perllocale>). |
| 1802 | |
| 1803 | @keys = sort { |
| 1804 | $hash{$b} <=> $hash{$a} |
| 1805 | || |
| 1806 | length($b) <=> length($a) |
| 1807 | || |
| 1808 | $a cmp $b |
| 1809 | } keys %hash; |
| 1810 | |
| 1811 | =head2 How can I always keep my hash sorted? |
| 1812 | |
| 1813 | You can look into using the DB_File module and tie() using the |
| 1814 | $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">. |
| 1815 | The Tie::IxHash module from CPAN might also be instructive. |
| 1816 | |
| 1817 | =head2 What's the difference between "delete" and "undef" with hashes? |
| 1818 | |
| 1819 | Hashes contain pairs of scalars: the first is the key, the |
| 1820 | second is the value. The key will be coerced to a string, |
| 1821 | although the value can be any kind of scalar: string, |
| 1822 | number, or reference. If a key $key is present in |
| 1823 | %hash, C<exists($hash{$key})> will return true. The value |
| 1824 | for a given key can be C<undef>, in which case |
| 1825 | C<$hash{$key}> will be C<undef> while C<exists $hash{$key}> |
| 1826 | will return true. This corresponds to (C<$key>, C<undef>) |
| 1827 | being in the hash. |
| 1828 | |
| 1829 | Pictures help... here's the %hash table: |
| 1830 | |
| 1831 | keys values |
| 1832 | +------+------+ |
| 1833 | | a | 3 | |
| 1834 | | x | 7 | |
| 1835 | | d | 0 | |
| 1836 | | e | 2 | |
| 1837 | +------+------+ |
| 1838 | |
| 1839 | And these conditions hold |
| 1840 | |
| 1841 | $hash{'a'} is true |
| 1842 | $hash{'d'} is false |
| 1843 | defined $hash{'d'} is true |
| 1844 | defined $hash{'a'} is true |
| 1845 | exists $hash{'a'} is true (Perl5 only) |
| 1846 | grep ($_ eq 'a', keys %hash) is true |
| 1847 | |
| 1848 | If you now say |
| 1849 | |
| 1850 | undef $hash{'a'} |
| 1851 | |
| 1852 | your table now reads: |
| 1853 | |
| 1854 | |
| 1855 | keys values |
| 1856 | +------+------+ |
| 1857 | | a | undef| |
| 1858 | | x | 7 | |
| 1859 | | d | 0 | |
| 1860 | | e | 2 | |
| 1861 | +------+------+ |
| 1862 | |
| 1863 | and these conditions now hold; changes in caps: |
| 1864 | |
| 1865 | $hash{'a'} is FALSE |
| 1866 | $hash{'d'} is false |
| 1867 | defined $hash{'d'} is true |
| 1868 | defined $hash{'a'} is FALSE |
| 1869 | exists $hash{'a'} is true (Perl5 only) |
| 1870 | grep ($_ eq 'a', keys %hash) is true |
| 1871 | |
| 1872 | Notice the last two: you have an undef value, but a defined key! |
| 1873 | |
| 1874 | Now, consider this: |
| 1875 | |
| 1876 | delete $hash{'a'} |
| 1877 | |
| 1878 | your table now reads: |
| 1879 | |
| 1880 | keys values |
| 1881 | +------+------+ |
| 1882 | | x | 7 | |
| 1883 | | d | 0 | |
| 1884 | | e | 2 | |
| 1885 | +------+------+ |
| 1886 | |
| 1887 | and these conditions now hold; changes in caps: |
| 1888 | |
| 1889 | $hash{'a'} is false |
| 1890 | $hash{'d'} is false |
| 1891 | defined $hash{'d'} is true |
| 1892 | defined $hash{'a'} is false |
| 1893 | exists $hash{'a'} is FALSE (Perl5 only) |
| 1894 | grep ($_ eq 'a', keys %hash) is FALSE |
| 1895 | |
| 1896 | See, the whole entry is gone! |
| 1897 | |
| 1898 | =head2 Why don't my tied hashes make the defined/exists distinction? |
| 1899 | |
| 1900 | This depends on the tied hash's implementation of EXISTS(). |
| 1901 | For example, there isn't the concept of undef with hashes |
| 1902 | that are tied to DBM* files. It also means that exists() and |
| 1903 | defined() do the same thing with a DBM* file, and what they |
| 1904 | end up doing is not what they do with ordinary hashes. |
| 1905 | |
| 1906 | =head2 How do I reset an each() operation part-way through? |
| 1907 | |
| 1908 | Using C<keys %hash> in scalar context returns the number of keys in |
| 1909 | the hash I<and> resets the iterator associated with the hash. You may |
| 1910 | need to do this if you use C<last> to exit a loop early so that when you |
| 1911 | re-enter it, the hash iterator has been reset. |
| 1912 | |
| 1913 | =head2 How can I get the unique keys from two hashes? |
| 1914 | |
| 1915 | First you extract the keys from the hashes into lists, then solve |
| 1916 | the "removing duplicates" problem described above. For example: |
| 1917 | |
| 1918 | %seen = (); |
| 1919 | for $element (keys(%foo), keys(%bar)) { |
| 1920 | $seen{$element}++; |
| 1921 | } |
| 1922 | @uniq = keys %seen; |
| 1923 | |
| 1924 | Or more succinctly: |
| 1925 | |
| 1926 | @uniq = keys %{{%foo,%bar}}; |
| 1927 | |
| 1928 | Or if you really want to save space: |
| 1929 | |
| 1930 | %seen = (); |
| 1931 | while (defined ($key = each %foo)) { |
| 1932 | $seen{$key}++; |
| 1933 | } |
| 1934 | while (defined ($key = each %bar)) { |
| 1935 | $seen{$key}++; |
| 1936 | } |
| 1937 | @uniq = keys %seen; |
| 1938 | |
| 1939 | =head2 How can I store a multidimensional array in a DBM file? |
| 1940 | |
| 1941 | Either stringify the structure yourself (no fun), or else |
| 1942 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer |
| 1943 | it on top of either DB_File or GDBM_File. |
| 1944 | |
| 1945 | =head2 How can I make my hash remember the order I put elements into it? |
| 1946 | |
| 1947 | Use the Tie::IxHash from CPAN. |
| 1948 | |
| 1949 | use Tie::IxHash; |
| 1950 | tie my %myhash, 'Tie::IxHash'; |
| 1951 | for (my $i=0; $i<20; $i++) { |
| 1952 | $myhash{$i} = 2*$i; |
| 1953 | } |
| 1954 | my @keys = keys %myhash; |
| 1955 | # @keys = (0,1,2,3,...) |
| 1956 | |
| 1957 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
| 1958 | |
| 1959 | If you say something like: |
| 1960 | |
| 1961 | somefunc($hash{"nonesuch key here"}); |
| 1962 | |
| 1963 | Then that element "autovivifies"; that is, it springs into existence |
| 1964 | whether you store something there or not. That's because functions |
| 1965 | get scalars passed in by reference. If somefunc() modifies C<$_[0]>, |
| 1966 | it has to be ready to write it back into the caller's version. |
| 1967 | |
| 1968 | This has been fixed as of Perl5.004. |
| 1969 | |
| 1970 | Normally, merely accessing a key's value for a nonexistent key does |
| 1971 | I<not> cause that key to be forever there. This is different than |
| 1972 | awk's behavior. |
| 1973 | |
| 1974 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
| 1975 | |
| 1976 | Usually a hash ref, perhaps like this: |
| 1977 | |
| 1978 | $record = { |
| 1979 | NAME => "Jason", |
| 1980 | EMPNO => 132, |
| 1981 | TITLE => "deputy peon", |
| 1982 | AGE => 23, |
| 1983 | SALARY => 37_000, |
| 1984 | PALS => [ "Norbert", "Rhys", "Phineas"], |
| 1985 | }; |
| 1986 | |
| 1987 | References are documented in L<perlref> and the upcoming L<perlreftut>. |
| 1988 | Examples of complex data structures are given in L<perldsc> and |
| 1989 | L<perllol>. Examples of structures and object-oriented classes are |
| 1990 | in L<perltoot>. |
| 1991 | |
| 1992 | =head2 How can I use a reference as a hash key? |
| 1993 | |
| 1994 | (contributed by brian d foy) |
| 1995 | |
| 1996 | Hash keys are strings, so you can't really use a reference as the key. |
| 1997 | When you try to do that, perl turns the reference into its stringified |
| 1998 | form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get back |
| 1999 | the reference from the stringified form, at least without doing some |
| 2000 | extra work on your own. Also remember that hash keys must be unique, but |
| 2001 | two different variables can store the same reference (and those variables |
| 2002 | can change later). |
| 2003 | |
| 2004 | The Tie::RefHash module, which is distributed with perl, might be what |
| 2005 | you want. It handles that extra work. |
| 2006 | |
| 2007 | =head1 Data: Misc |
| 2008 | |
| 2009 | =head2 How do I handle binary data correctly? |
| 2010 | |
| 2011 | Perl is binary clean, so this shouldn't be a problem. For example, |
| 2012 | this works fine (assuming the files are found): |
| 2013 | |
| 2014 | if (`cat /vmunix` =~ /gzip/) { |
| 2015 | print "Your kernel is GNU-zip enabled!\n"; |
| 2016 | } |
| 2017 | |
| 2018 | On less elegant (read: Byzantine) systems, however, you have |
| 2019 | to play tedious games with "text" versus "binary" files. See |
| 2020 | L<perlfunc/"binmode"> or L<perlopentut>. |
| 2021 | |
| 2022 | If you're concerned about 8-bit ASCII data, then see L<perllocale>. |
| 2023 | |
| 2024 | If you want to deal with multibyte characters, however, there are |
| 2025 | some gotchas. See the section on Regular Expressions. |
| 2026 | |
| 2027 | =head2 How do I determine whether a scalar is a number/whole/integer/float? |
| 2028 | |
| 2029 | Assuming that you don't care about IEEE notations like "NaN" or |
| 2030 | "Infinity", you probably just want to use a regular expression. |
| 2031 | |
| 2032 | if (/\D/) { print "has nondigits\n" } |
| 2033 | if (/^\d+$/) { print "is a whole number\n" } |
| 2034 | if (/^-?\d+$/) { print "is an integer\n" } |
| 2035 | if (/^[+-]?\d+$/) { print "is a +/- integer\n" } |
| 2036 | if (/^-?\d+\.?\d*$/) { print "is a real number\n" } |
| 2037 | if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" } |
| 2038 | if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) |
| 2039 | { print "a C float\n" } |
| 2040 | |
| 2041 | There are also some commonly used modules for the task. |
| 2042 | L<Scalar::Util> (distributed with 5.8) provides access to perl's |
| 2043 | internal function C<looks_like_number> for determining |
| 2044 | whether a variable looks like a number. L<Data::Types> |
| 2045 | exports functions that validate data types using both the |
| 2046 | above and other regular expressions. Thirdly, there is |
| 2047 | C<Regexp::Common> which has regular expressions to match |
| 2048 | various types of numbers. Those three modules are available |
| 2049 | from the CPAN. |
| 2050 | |
| 2051 | If you're on a POSIX system, Perl supports the C<POSIX::strtod> |
| 2052 | function. Its semantics are somewhat cumbersome, so here's a C<getnum> |
| 2053 | wrapper function for more convenient access. This function takes |
| 2054 | a string and returns the number it found, or C<undef> for input that |
| 2055 | isn't a C float. The C<is_numeric> function is a front end to C<getnum> |
| 2056 | if you just want to say, "Is this a float?" |
| 2057 | |
| 2058 | sub getnum { |
| 2059 | use POSIX qw(strtod); |
| 2060 | my $str = shift; |
| 2061 | $str =~ s/^\s+//; |
| 2062 | $str =~ s/\s+$//; |
| 2063 | $! = 0; |
| 2064 | my($num, $unparsed) = strtod($str); |
| 2065 | if (($str eq '') || ($unparsed != 0) || $!) { |
| 2066 | return undef; |
| 2067 | } else { |
| 2068 | return $num; |
| 2069 | } |
| 2070 | } |
| 2071 | |
| 2072 | sub is_numeric { defined getnum($_[0]) } |
| 2073 | |
| 2074 | Or you could check out the L<String::Scanf> module on the CPAN |
| 2075 | instead. The POSIX module (part of the standard Perl distribution) provides |
| 2076 | the C<strtod> and C<strtol> for converting strings to double and longs, |
| 2077 | respectively. |
| 2078 | |
| 2079 | =head2 How do I keep persistent data across program calls? |
| 2080 | |
| 2081 | For some specific applications, you can use one of the DBM modules. |
| 2082 | See L<AnyDBM_File>. More generically, you should consult the FreezeThaw |
| 2083 | or Storable modules from CPAN. Starting from Perl 5.8 Storable is part |
| 2084 | of the standard distribution. Here's one example using Storable's C<store> |
| 2085 | and C<retrieve> functions: |
| 2086 | |
| 2087 | use Storable; |
| 2088 | store(\%hash, "filename"); |
| 2089 | |
| 2090 | # later on... |
| 2091 | $href = retrieve("filename"); # by ref |
| 2092 | %hash = %{ retrieve("filename") }; # direct to hash |
| 2093 | |
| 2094 | =head2 How do I print out or copy a recursive data structure? |
| 2095 | |
| 2096 | The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great |
| 2097 | for printing out data structures. The Storable module on CPAN (or the |
| 2098 | 5.8 release of Perl), provides a function called C<dclone> that recursively |
| 2099 | copies its argument. |
| 2100 | |
| 2101 | use Storable qw(dclone); |
| 2102 | $r2 = dclone($r1); |
| 2103 | |
| 2104 | Where $r1 can be a reference to any kind of data structure you'd like. |
| 2105 | It will be deeply copied. Because C<dclone> takes and returns references, |
| 2106 | you'd have to add extra punctuation if you had a hash of arrays that |
| 2107 | you wanted to copy. |
| 2108 | |
| 2109 | %newhash = %{ dclone(\%oldhash) }; |
| 2110 | |
| 2111 | =head2 How do I define methods for every class/object? |
| 2112 | |
| 2113 | Use the UNIVERSAL class (see L<UNIVERSAL>). |
| 2114 | |
| 2115 | =head2 How do I verify a credit card checksum? |
| 2116 | |
| 2117 | Get the Business::CreditCard module from CPAN. |
| 2118 | |
| 2119 | =head2 How do I pack arrays of doubles or floats for XS code? |
| 2120 | |
| 2121 | The kgbpack.c code in the PGPLOT module on CPAN does just this. |
| 2122 | If you're doing a lot of float or double processing, consider using |
| 2123 | the PDL module from CPAN instead--it makes number-crunching easy. |
| 2124 | |
| 2125 | =head1 AUTHOR AND COPYRIGHT |
| 2126 | |
| 2127 | Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and |
| 2128 | other authors as noted. All rights reserved. |
| 2129 | |
| 2130 | This documentation is free; you can redistribute it and/or modify it |
| 2131 | under the same terms as Perl itself. |
| 2132 | |
| 2133 | Irrespective of its distribution, all code examples in this file |
| 2134 | are hereby placed into the public domain. You are permitted and |
| 2135 | encouraged to use this code in your own programs for fun |
| 2136 | or for profit as you see fit. A simple comment in the code giving |
| 2137 | credit would be courteous but is not required. |