| [ Index ] |
PHP Cross Reference of Unnamed Project |
[Summary view] [Print] [Text view]
1 =head1 NAME 2 3 perldata - Perl data types 4 5 =head1 DESCRIPTION 6 7 =head2 Variable names 8 X<variable, name> X<variable name> X<data type> X<type> 9 10 Perl has three built-in data types: scalars, arrays of scalars, and 11 associative arrays of scalars, known as "hashes". A scalar is a 12 single string (of any size, limited only by the available memory), 13 number, or a reference to something (which will be discussed 14 in L<perlref>). Normal arrays are ordered lists of scalars indexed 15 by number, starting with 0. Hashes are unordered collections of scalar 16 values indexed by their associated string key. 17 18 Values are usually referred to by name, or through a named reference. 19 The first character of the name tells you to what sort of data 20 structure it refers. The rest of the name tells you the particular 21 value to which it refers. Usually this name is a single I<identifier>, 22 that is, a string beginning with a letter or underscore, and 23 containing letters, underscores, and digits. In some cases, it may 24 be a chain of identifiers, separated by C<::> (or by the slightly 25 archaic C<'>); all but the last are interpreted as names of packages, 26 to locate the namespace in which to look up the final identifier 27 (see L<perlmod/Packages> for details). It's possible to substitute 28 for a simple identifier, an expression that produces a reference 29 to the value at runtime. This is described in more detail below 30 and in L<perlref>. 31 X<identifier> 32 33 Perl also has its own built-in variables whose names don't follow 34 these rules. They have strange names so they don't accidentally 35 collide with one of your normal variables. Strings that match 36 parenthesized parts of a regular expression are saved under names 37 containing only digits after the C<$> (see L<perlop> and L<perlre>). 38 In addition, several special variables that provide windows into 39 the inner working of Perl have names containing punctuation characters 40 and control characters. These are documented in L<perlvar>. 41 X<variable, built-in> 42 43 Scalar values are always named with '$', even when referring to a 44 scalar that is part of an array or a hash. The '$' symbol works 45 semantically like the English word "the" in that it indicates a 46 single value is expected. 47 X<scalar> 48 49 $days # the simple scalar value "days" 50 $days[28] # the 29th element of array @days 51 $days{'Feb'} # the 'Feb' value from hash %days 52 $#days # the last index of array @days 53 54 Entire arrays (and slices of arrays and hashes) are denoted by '@', 55 which works much like the word "these" or "those" does in English, 56 in that it indicates multiple values are expected. 57 X<array> 58 59 @days # ($days[0], $days[1],... $days[n]) 60 @days[3,4,5] # same as ($days[3],$days[4],$days[5]) 61 @days{'a','c'} # same as ($days{'a'},$days{'c'}) 62 63 Entire hashes are denoted by '%': 64 X<hash> 65 66 %days # (key1, val1, key2, val2 ...) 67 68 In addition, subroutines are named with an initial '&', though this 69 is optional when unambiguous, just as the word "do" is often redundant 70 in English. Symbol table entries can be named with an initial '*', 71 but you don't really care about that yet (if ever :-). 72 73 Every variable type has its own namespace, as do several 74 non-variable identifiers. This means that you can, without fear 75 of conflict, use the same name for a scalar variable, an array, or 76 a hash--or, for that matter, for a filehandle, a directory handle, a 77 subroutine name, a format name, or a label. This means that $foo 78 and @foo are two different variables. It also means that C<$foo[1]> 79 is a part of @foo, not a part of $foo. This may seem a bit weird, 80 but that's okay, because it is weird. 81 X<namespace> 82 83 Because variable references always start with '$', '@', or '%', the 84 "reserved" words aren't in fact reserved with respect to variable 85 names. They I<are> reserved with respect to labels and filehandles, 86 however, which don't have an initial special character. You can't 87 have a filehandle named "log", for instance. Hint: you could say 88 C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using 89 uppercase filehandles also improves readability and protects you 90 from conflict with future reserved words. Case I<is> significant--"FOO", 91 "Foo", and "foo" are all different names. Names that start with a 92 letter or underscore may also contain digits and underscores. 93 X<identifier, case sensitivity> 94 X<case> 95 96 It is possible to replace such an alphanumeric name with an expression 97 that returns a reference to the appropriate type. For a description 98 of this, see L<perlref>. 99 100 Names that start with a digit may contain only more digits. Names 101 that do not start with a letter, underscore, digit or a caret (i.e. 102 a control character) are limited to one character, e.g., C<$%> or 103 C<$$>. (Most of these one character names have a predefined 104 significance to Perl. For instance, C<$$> is the current process 105 id.) 106 107 =head2 Context 108 X<context> X<scalar context> X<list context> 109 110 The interpretation of operations and values in Perl sometimes depends 111 on the requirements of the context around the operation or value. 112 There are two major contexts: list and scalar. Certain operations 113 return list values in contexts wanting a list, and scalar values 114 otherwise. If this is true of an operation it will be mentioned in 115 the documentation for that operation. In other words, Perl overloads 116 certain operations based on whether the expected return value is 117 singular or plural. Some words in English work this way, like "fish" 118 and "sheep". 119 120 In a reciprocal fashion, an operation provides either a scalar or a 121 list context to each of its arguments. For example, if you say 122 123 int( <STDIN> ) 124 125 the integer operation provides scalar context for the <> 126 operator, which responds by reading one line from STDIN and passing it 127 back to the integer operation, which will then find the integer value 128 of that line and return that. If, on the other hand, you say 129 130 sort( <STDIN> ) 131 132 then the sort operation provides list context for <>, which 133 will proceed to read every line available up to the end of file, and 134 pass that list of lines back to the sort routine, which will then 135 sort those lines and return them as a list to whatever the context 136 of the sort was. 137 138 Assignment is a little bit special in that it uses its left argument 139 to determine the context for the right argument. Assignment to a 140 scalar evaluates the right-hand side in scalar context, while 141 assignment to an array or hash evaluates the righthand side in list 142 context. Assignment to a list (or slice, which is just a list 143 anyway) also evaluates the righthand side in list context. 144 145 When you use the C<use warnings> pragma or Perl's B<-w> command-line 146 option, you may see warnings 147 about useless uses of constants or functions in "void context". 148 Void context just means the value has been discarded, such as a 149 statement containing only C<"fred";> or C<getpwuid(0);>. It still 150 counts as scalar context for functions that care whether or not 151 they're being called in list context. 152 153 User-defined subroutines may choose to care whether they are being 154 called in a void, scalar, or list context. Most subroutines do not 155 need to bother, though. That's because both scalars and lists are 156 automatically interpolated into lists. See L<perlfunc/wantarray> 157 for how you would dynamically discern your function's calling 158 context. 159 160 =head2 Scalar values 161 X<scalar> X<number> X<string> X<reference> 162 163 All data in Perl is a scalar, an array of scalars, or a hash of 164 scalars. A scalar may contain one single value in any of three 165 different flavors: a number, a string, or a reference. In general, 166 conversion from one form to another is transparent. Although a 167 scalar may not directly hold multiple values, it may contain a 168 reference to an array or hash which in turn contains multiple values. 169 170 Scalars aren't necessarily one thing or another. There's no place 171 to declare a scalar variable to be of type "string", type "number", 172 type "reference", or anything else. Because of the automatic 173 conversion of scalars, operations that return scalars don't need 174 to care (and in fact, cannot care) whether their caller is looking 175 for a string, a number, or a reference. Perl is a contextually 176 polymorphic language whose scalars can be strings, numbers, or 177 references (which includes objects). Although strings and numbers 178 are considered pretty much the same thing for nearly all purposes, 179 references are strongly-typed, uncastable pointers with builtin 180 reference-counting and destructor invocation. 181 182 A scalar value is interpreted as TRUE in the Boolean sense if it is not 183 the null string or the number 0 (or its string equivalent, "0"). The 184 Boolean context is just a special kind of scalar context where no 185 conversion to a string or a number is ever performed. 186 X<boolean> X<bool> X<true> X<false> X<truth> 187 188 There are actually two varieties of null strings (sometimes referred 189 to as "empty" strings), a defined one and an undefined one. The 190 defined version is just a string of length zero, such as C<"">. 191 The undefined version is the value that indicates that there is 192 no real value for something, such as when there was an error, or 193 at end of file, or when you refer to an uninitialized variable or 194 element of an array or hash. Although in early versions of Perl, 195 an undefined scalar could become defined when first used in a 196 place expecting a defined value, this no longer happens except for 197 rare cases of autovivification as explained in L<perlref>. You can 198 use the defined() operator to determine whether a scalar value is 199 defined (this has no meaning on arrays or hashes), and the undef() 200 operator to produce an undefined value. 201 X<defined> X<undefined> X<undef> X<null> X<string, null> 202 203 To find out whether a given string is a valid non-zero number, it's 204 sometimes enough to test it against both numeric 0 and also lexical 205 "0" (although this will cause noises if warnings are on). That's 206 because strings that aren't numbers count as 0, just as they do in B<awk>: 207 208 if ($str == 0 && $str ne "0") { 209 warn "That doesn't look like a number"; 210 } 211 212 That method may be best because otherwise you won't treat IEEE 213 notations like C<NaN> or C<Infinity> properly. At other times, you 214 might prefer to determine whether string data can be used numerically 215 by calling the POSIX::strtod() function or by inspecting your string 216 with a regular expression (as documented in L<perlre>). 217 218 warn "has nondigits" if /\D/; 219 warn "not a natural number" unless /^\d+$/; # rejects -3 220 warn "not an integer" unless /^-?\d+$/; # rejects +3 221 warn "not an integer" unless /^[+-]?\d+$/; 222 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2 223 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/; 224 warn "not a C float" 225 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/; 226 227 The length of an array is a scalar value. You may find the length 228 of array @days by evaluating C<$#days>, as in B<csh>. However, this 229 isn't the length of the array; it's the subscript of the last element, 230 which is a different value since there is ordinarily a 0th element. 231 Assigning to C<$#days> actually changes the length of the array. 232 Shortening an array this way destroys intervening values. Lengthening 233 an array that was previously shortened does not recover values 234 that were in those elements. (It used to do so in Perl 4, but we 235 had to break this to make sure destructors were called when expected.) 236 X<$#> X<array, length> 237 238 You can also gain some minuscule measure of efficiency by pre-extending 239 an array that is going to get big. You can also extend an array 240 by assigning to an element that is off the end of the array. You 241 can truncate an array down to nothing by assigning the null list 242 () to it. The following are equivalent: 243 244 @whatever = (); 245 $#whatever = -1; 246 247 If you evaluate an array in scalar context, it returns the length 248 of the array. (Note that this is not true of lists, which return 249 the last value, like the C comma operator, nor of built-in functions, 250 which return whatever they feel like returning.) The following is 251 always true: 252 X<array, length> 253 254 scalar(@whatever) == $#whatever - $[ + 1; 255 256 Version 5 of Perl changed the semantics of C<$[>: files that don't set 257 the value of C<$[> no longer need to worry about whether another 258 file changed its value. (In other words, use of C<$[> is deprecated.) 259 So in general you can assume that 260 X<$[> 261 262 scalar(@whatever) == $#whatever + 1; 263 264 Some programmers choose to use an explicit conversion so as to 265 leave nothing to doubt: 266 267 $element_count = scalar(@whatever); 268 269 If you evaluate a hash in scalar context, it returns false if the 270 hash is empty. If there are any key/value pairs, it returns true; 271 more precisely, the value returned is a string consisting of the 272 number of used buckets and the number of allocated buckets, separated 273 by a slash. This is pretty much useful only to find out whether 274 Perl's internal hashing algorithm is performing poorly on your data 275 set. For example, you stick 10,000 things in a hash, but evaluating 276 %HASH in scalar context reveals C<"1/16">, which means only one out 277 of sixteen buckets has been touched, and presumably contains all 278 10,000 of your items. This isn't supposed to happen. If a tied hash 279 is evaluated in scalar context, a fatal error will result, since this 280 bucket usage information is currently not available for tied hashes. 281 X<hash, scalar context> X<hash, bucket> X<bucket> 282 283 You can preallocate space for a hash by assigning to the keys() function. 284 This rounds up the allocated buckets to the next power of two: 285 286 keys(%users) = 1000; # allocate 1024 buckets 287 288 =head2 Scalar value constructors 289 X<scalar, literal> X<scalar, constant> 290 291 Numeric literals are specified in any of the following floating point or 292 integer formats: 293 294 12345 295 12345.67 296 .23E-10 # a very small number 297 3.14_15_92 # a very important number 298 4_294_967_296 # underscore for legibility 299 0xff # hex 300 0xdead_beef # more hex 301 0377 # octal (only numbers, begins with 0) 302 0b011011 # binary 303 304 You are allowed to use underscores (underbars) in numeric literals 305 between digits for legibility. You could, for example, group binary 306 digits by threes (as for a Unix-style mode argument such as 0b110_100_100) 307 or by fours (to represent nibbles, as in 0b1010_0110) or in other groups. 308 X<number, literal> 309 310 String literals are usually delimited by either single or double 311 quotes. They work much like quotes in the standard Unix shells: 312 double-quoted string literals are subject to backslash and variable 313 substitution; single-quoted strings are not (except for C<\'> and 314 C<\\>). The usual C-style backslash rules apply for making 315 characters such as newline, tab, etc., as well as some more exotic 316 forms. See L<perlop/"Quote and Quote-like Operators"> for a list. 317 X<string, literal> 318 319 Hexadecimal, octal, or binary, representations in string literals 320 (e.g. '0xff') are not automatically converted to their integer 321 representation. The hex() and oct() functions make these conversions 322 for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details. 323 324 You can also embed newlines directly in your strings, i.e., they can end 325 on a different line than they begin. This is nice, but if you forget 326 your trailing quote, the error will not be reported until Perl finds 327 another line containing the quote character, which may be much further 328 on in the script. Variable substitution inside strings is limited to 329 scalar variables, arrays, and array or hash slices. (In other words, 330 names beginning with $ or @, followed by an optional bracketed 331 expression as a subscript.) The following code segment prints out "The 332 price is $Z<>100." 333 X<interpolation> 334 335 $Price = '$100'; # not interpolated 336 print "The price is $Price.\n"; # interpolated 337 338 There is no double interpolation in Perl, so the C<$100> is left as is. 339 340 By default floating point numbers substituted inside strings use the 341 dot (".") as the decimal separator. If C<use locale> is in effect, 342 and POSIX::setlocale() has been called, the character used for the 343 decimal separator is affected by the LC_NUMERIC locale. 344 See L<perllocale> and L<POSIX>. 345 346 As in some shells, you can enclose the variable name in braces to 347 disambiguate it from following alphanumerics (and underscores). 348 You must also do 349 this when interpolating a variable into a string to separate the 350 variable name from a following double-colon or an apostrophe, since 351 these would be otherwise treated as a package separator: 352 X<interpolation> 353 354 $who = "Larry"; 355 print PASSWD "$who}::0:0:Superuser:/:/bin/perl\n"; 356 print "We use $who}speak when $who}'s here.\n"; 357 358 Without the braces, Perl would have looked for a $whospeak, a 359 C<$who::0>, and a C<$who's> variable. The last two would be the 360 $0 and the $s variables in the (presumably) non-existent package 361 C<who>. 362 363 In fact, an identifier within such curlies is forced to be a string, 364 as is any simple identifier within a hash subscript. Neither need 365 quoting. Our earlier example, C<$days{'Feb'}> can be written as 366 C<$days{Feb}> and the quotes will be assumed automatically. But 367 anything more complicated in the subscript will be interpreted as an 368 expression. This means for example that C<$version{2.0}++> is 369 equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>. 370 371 =head3 Version Strings 372 X<version string> X<vstring> X<v-string> 373 374 B<Note:> Version Strings (v-strings) have been deprecated. They will 375 be removed in some future release after Perl 5.8.1. The marginal 376 benefits of v-strings were greatly outweighed by the potential for 377 Surprise and Confusion. 378 379 A literal of the form C<v1.20.300.4000> is parsed as a string composed 380 of characters with the specified ordinals. This form, known as 381 v-strings, provides an alternative, more readable way to construct 382 strings, rather than use the somewhat less readable interpolation form 383 C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing 384 Unicode strings, and for comparing version "numbers" using the string 385 comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or 386 more dots in the literal, the leading C<v> may be omitted. 387 388 print v9786; # prints SMILEY, "\x{263a}" 389 print v102.111.111; # prints "foo" 390 print 102.111.111; # same 391 392 Such literals are accepted by both C<require> and C<use> for 393 doing a version check. Note that using the v-strings for IPv4 394 addresses is not portable unless you also use the 395 inet_aton()/inet_ntoa() routines of the Socket package. 396 397 Note that since Perl 5.8.1 the single-number v-strings (like C<v65>) 398 are not v-strings before the C<< => >> operator (which is usually used 399 to separate a hash key from a hash value), instead they are interpreted 400 as literal strings ('v65'). They were v-strings from Perl 5.6.0 to 401 Perl 5.8.0, but that caused more confusion and breakage than good. 402 Multi-number v-strings like C<v65.66> and C<65.66.67> continue to 403 be v-strings always. 404 405 =head3 Special Literals 406 X<special literal> X<__END__> X<__DATA__> X<END> X<DATA> 407 X<end> X<data> X<^D> X<^Z> 408 409 The special literals __FILE__, __LINE__, and __PACKAGE__ 410 represent the current filename, line number, and package name at that 411 point in your program. They may be used only as separate tokens; they 412 will not be interpolated into strings. If there is no current package 413 (due to an empty C<package;> directive), __PACKAGE__ is the undefined 414 value. 415 X<__FILE__> X<__LINE__> X<__PACKAGE__> X<line> X<file> X<package> 416 417 The two control characters ^D and ^Z, and the tokens __END__ and __DATA__ 418 may be used to indicate the logical end of the script before the actual 419 end of file. Any following text is ignored. 420 421 Text after __DATA__ but may be read via the filehandle C<PACKNAME::DATA>, 422 where C<PACKNAME> is the package that was current when the __DATA__ 423 token was encountered. The filehandle is left open pointing to the 424 contents after __DATA__. It is the program's responsibility to 425 C<close DATA> when it is done reading from it. For compatibility with 426 older scripts written before __DATA__ was introduced, __END__ behaves 427 like __DATA__ in the top level script (but not in files loaded with 428 C<require> or C<do>) and leaves the remaining contents of the 429 file accessible via C<main::DATA>. 430 431 See L<SelfLoader> for more description of __DATA__, and 432 an example of its use. Note that you cannot read from the DATA 433 filehandle in a BEGIN block: the BEGIN block is executed as soon 434 as it is seen (during compilation), at which point the corresponding 435 __DATA__ (or __END__) token has not yet been seen. 436 437 =head3 Barewords 438 X<bareword> 439 440 A word that has no other interpretation in the grammar will 441 be treated as if it were a quoted string. These are known as 442 "barewords". As with filehandles and labels, a bareword that consists 443 entirely of lowercase letters risks conflict with future reserved 444 words, and if you use the C<use warnings> pragma or the B<-w> switch, 445 Perl will warn you about any 446 such words. Some people may wish to outlaw barewords entirely. If you 447 say 448 449 use strict 'subs'; 450 451 then any bareword that would NOT be interpreted as a subroutine call 452 produces a compile-time error instead. The restriction lasts to the 453 end of the enclosing block. An inner block may countermand this 454 by saying C<no strict 'subs'>. 455 456 =head3 Array Joining Delimiter 457 X<array, interpolation> X<interpolation, array> X<$"> 458 459 Arrays and slices are interpolated into double-quoted strings 460 by joining the elements with the delimiter specified in the C<$"> 461 variable (C<$LIST_SEPARATOR> if "use English;" is specified), 462 space by default. The following are equivalent: 463 464 $temp = join($", @ARGV); 465 system "echo $temp"; 466 467 system "echo @ARGV"; 468 469 Within search patterns (which also undergo double-quotish substitution) 470 there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as 471 C</$foo}[bar]/> (where C<[bar]> is a character class for the regular 472 expression) or as C</$foo[bar]}/> (where C<[bar]> is the subscript to array 473 @foo)? If @foo doesn't otherwise exist, then it's obviously a 474 character class. If @foo exists, Perl takes a good guess about C<[bar]>, 475 and is almost always right. If it does guess wrong, or if you're just 476 plain paranoid, you can force the correct interpretation with curly 477 braces as above. 478 479 If you're looking for the information on how to use here-documents, 480 which used to be here, that's been moved to 481 L<perlop/Quote and Quote-like Operators>. 482 483 =head2 List value constructors 484 X<list> 485 486 List values are denoted by separating individual values by commas 487 (and enclosing the list in parentheses where precedence requires it): 488 489 (LIST) 490 491 In a context not requiring a list value, the value of what appears 492 to be a list literal is simply the value of the final element, as 493 with the C comma operator. For example, 494 495 @foo = ('cc', '-E', $bar); 496 497 assigns the entire list value to array @foo, but 498 499 $foo = ('cc', '-E', $bar); 500 501 assigns the value of variable $bar to the scalar variable $foo. 502 Note that the value of an actual array in scalar context is the 503 length of the array; the following assigns the value 3 to $foo: 504 505 @foo = ('cc', '-E', $bar); 506 $foo = @foo; # $foo gets 3 507 508 You may have an optional comma before the closing parenthesis of a 509 list literal, so that you can say: 510 511 @foo = ( 512 1, 513 2, 514 3, 515 ); 516 517 To use a here-document to assign an array, one line per element, 518 you might use an approach like this: 519 520 @sauces = <<End_Lines =~ m/(\S.*\S)/g; 521 normal tomato 522 spicy tomato 523 green chile 524 pesto 525 white wine 526 End_Lines 527 528 LISTs do automatic interpolation of sublists. That is, when a LIST is 529 evaluated, each element of the list is evaluated in list context, and 530 the resulting list value is interpolated into LIST just as if each 531 individual element were a member of LIST. Thus arrays and hashes lose their 532 identity in a LIST--the list 533 534 (@foo,@bar,&SomeSub,%glarch) 535 536 contains all the elements of @foo followed by all the elements of @bar, 537 followed by all the elements returned by the subroutine named SomeSub 538 called in list context, followed by the key/value pairs of %glarch. 539 To make a list reference that does I<NOT> interpolate, see L<perlref>. 540 541 The null list is represented by (). Interpolating it in a list 542 has no effect. Thus ((),(),()) is equivalent to (). Similarly, 543 interpolating an array with no elements is the same as if no 544 array had been interpolated at that point. 545 546 This interpolation combines with the facts that the opening 547 and closing parentheses are optional (except when necessary for 548 precedence) and lists may end with an optional comma to mean that 549 multiple commas within lists are legal syntax. The list C<1,,3> is a 550 concatenation of two lists, C<1,> and C<3>, the first of which ends 551 with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And 552 similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that 553 we'd advise you to use this obfuscation. 554 555 A list value may also be subscripted like a normal array. You must 556 put the list in parentheses to avoid ambiguity. For example: 557 558 # Stat returns list value. 559 $time = (stat($file))[8]; 560 561 # SYNTAX ERROR HERE. 562 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES 563 564 # Find a hex digit. 565 $hexdigit = ('a','b','c','d','e','f')[$digit-10]; 566 567 # A "reverse comma operator". 568 return (pop(@foo),pop(@foo))[0]; 569 570 Lists may be assigned to only when each element of the list 571 is itself legal to assign to: 572 573 ($a, $b, $c) = (1, 2, 3); 574 575 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00); 576 577 An exception to this is that you may assign to C<undef> in a list. 578 This is useful for throwing away some of the return values of a 579 function: 580 581 ($dev, $ino, undef, undef, $uid, $gid) = stat($file); 582 583 List assignment in scalar context returns the number of elements 584 produced by the expression on the right side of the assignment: 585 586 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2 587 $x = (($foo,$bar) = f()); # set $x to f()'s return count 588 589 This is handy when you want to do a list assignment in a Boolean 590 context, because most list functions return a null list when finished, 591 which when assigned produces a 0, which is interpreted as FALSE. 592 593 It's also the source of a useful idiom for executing a function or 594 performing an operation in list context and then counting the number of 595 return values, by assigning to an empty list and then using that 596 assignment in scalar context. For example, this code: 597 598 $count = () = $string =~ /\d+/g; 599 600 will place into $count the number of digit groups found in $string. 601 This happens because the pattern match is in list context (since it 602 is being assigned to the empty list), and will therefore return a list 603 of all matching parts of the string. The list assignment in scalar 604 context will translate that into the number of elements (here, the 605 number of times the pattern matched) and assign that to $count. Note 606 that simply using 607 608 $count = $string =~ /\d+/g; 609 610 would not have worked, since a pattern match in scalar context will 611 only return true or false, rather than a count of matches. 612 613 The final element of a list assignment may be an array or a hash: 614 615 ($a, $b, @rest) = split; 616 my($a, $b, %rest) = @_; 617 618 You can actually put an array or hash anywhere in the list, but the first one 619 in the list will soak up all the values, and anything after it will become 620 undefined. This may be useful in a my() or local(). 621 622 A hash can be initialized using a literal list holding pairs of 623 items to be interpreted as a key and a value: 624 625 # same as map assignment above 626 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00); 627 628 While literal lists and named arrays are often interchangeable, that's 629 not the case for hashes. Just because you can subscript a list value like 630 a normal array does not mean that you can subscript a list value as a 631 hash. Likewise, hashes included as parts of other lists (including 632 parameters lists and return lists from functions) always flatten out into 633 key/value pairs. That's why it's good to use references sometimes. 634 635 It is often more readable to use the C<< => >> operator between key/value 636 pairs. The C<< => >> operator is mostly just a more visually distinctive 637 synonym for a comma, but it also arranges for its left-hand operand to be 638 interpreted as a string -- if it's a bareword that would be a legal simple 639 identifier (C<< => >> doesn't quote compound identifiers, that contain 640 double colons). This makes it nice for initializing hashes: 641 642 %map = ( 643 red => 0x00f, 644 blue => 0x0f0, 645 green => 0xf00, 646 ); 647 648 or for initializing hash references to be used as records: 649 650 $rec = { 651 witch => 'Mable the Merciless', 652 cat => 'Fluffy the Ferocious', 653 date => '10/31/1776', 654 }; 655 656 or for using call-by-named-parameter to complicated functions: 657 658 $field = $query->radio_group( 659 name => 'group_name', 660 values => ['eenie','meenie','minie'], 661 default => 'meenie', 662 linebreak => 'true', 663 labels => \%labels 664 ); 665 666 Note that just because a hash is initialized in that order doesn't 667 mean that it comes out in that order. See L<perlfunc/sort> for examples 668 of how to arrange for an output ordering. 669 670 =head2 Subscripts 671 672 An array is subscripted by specifying a dollar sign (C<$>), then the 673 name of the array (without the leading C<@>), then the subscript inside 674 square brackets. For example: 675 676 @myarray = (5, 50, 500, 5000); 677 print "Element Number 2 is", $myarray[2], "\n"; 678 679 The array indices start with 0. A negative subscript retrieves its 680 value from the end. In our example, C<$myarray[-1]> would have been 681 5000, and C<$myarray[-2]> would have been 500. 682 683 Hash subscripts are similar, only instead of square brackets curly brackets 684 are used. For example: 685 686 %scientists = 687 ( 688 "Newton" => "Isaac", 689 "Einstein" => "Albert", 690 "Darwin" => "Charles", 691 "Feynman" => "Richard", 692 ); 693 694 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n"; 695 696 =head2 Slices 697 X<slice> X<array, slice> X<hash, slice> 698 699 A common way to access an array or a hash is one scalar element at a 700 time. You can also subscript a list to get a single element from it. 701 702 $whoami = $ENV{"USER"}; # one element from the hash 703 $parent = $ISA[0]; # one element from the array 704 $dir = (getpwnam("daemon"))[7]; # likewise, but with list 705 706 A slice accesses several elements of a list, an array, or a hash 707 simultaneously using a list of subscripts. It's more convenient 708 than writing out the individual elements as a list of separate 709 scalar values. 710 711 ($him, $her) = @folks[0,-1]; # array slice 712 @them = @folks[0 .. 3]; # array slice 713 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice 714 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice 715 716 Since you can assign to a list of variables, you can also assign to 717 an array or hash slice. 718 719 @days[3..5] = qw/Wed Thu Fri/; 720 @colors{'red','blue','green'} 721 = (0xff0000, 0x0000ff, 0x00ff00); 722 @folks[0, -1] = @folks[-1, 0]; 723 724 The previous assignments are exactly equivalent to 725 726 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/; 727 ($colors{'red'}, $colors{'blue'}, $colors{'green'}) 728 = (0xff0000, 0x0000ff, 0x00ff00); 729 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]); 730 731 Since changing a slice changes the original array or hash that it's 732 slicing, a C<foreach> construct will alter some--or even all--of the 733 values of the array or hash. 734 735 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ } 736 737 foreach (@hash{qw[key1 key2]}) { 738 s/^\s+//; # trim leading whitespace 739 s/\s+$//; # trim trailing whitespace 740 s/(\w+)/\u\L$1/g; # "titlecase" words 741 } 742 743 A slice of an empty list is still an empty list. Thus: 744 745 @a = ()[1,0]; # @a has no elements 746 @b = (@a)[0,1]; # @b has no elements 747 @c = (0,1)[2,3]; # @c has no elements 748 749 But: 750 751 @a = (1)[1,0]; # @a has two elements 752 @b = (1,undef)[1,0,2]; # @b has three elements 753 754 This makes it easy to write loops that terminate when a null list 755 is returned: 756 757 while ( ($home, $user) = (getpwent)[7,0]) { 758 printf "%-8s %s\n", $user, $home; 759 } 760 761 As noted earlier in this document, the scalar sense of list assignment 762 is the number of elements on the right-hand side of the assignment. 763 The null list contains no elements, so when the password file is 764 exhausted, the result is 0, not 2. 765 766 If you're confused about why you use an '@' there on a hash slice 767 instead of a '%', think of it like this. The type of bracket (square 768 or curly) governs whether it's an array or a hash being looked at. 769 On the other hand, the leading symbol ('$' or '@') on the array or 770 hash indicates whether you are getting back a singular value (a 771 scalar) or a plural one (a list). 772 773 =head2 Typeglobs and Filehandles 774 X<typeglob> X<filehandle> X<*> 775 776 Perl uses an internal type called a I<typeglob> to hold an entire 777 symbol table entry. The type prefix of a typeglob is a C<*>, because 778 it represents all types. This used to be the preferred way to 779 pass arrays and hashes by reference into a function, but now that 780 we have real references, this is seldom needed. 781 782 The main use of typeglobs in modern Perl is create symbol table aliases. 783 This assignment: 784 785 *this = *that; 786 787 makes $this an alias for $that, @this an alias for @that, %this an alias 788 for %that, &this an alias for &that, etc. Much safer is to use a reference. 789 This: 790 791 local *Here::blue = \$There::green; 792 793 temporarily makes $Here::blue an alias for $There::green, but doesn't 794 make @Here::blue an alias for @There::green, or %Here::blue an alias for 795 %There::green, etc. See L<perlmod/"Symbol Tables"> for more examples 796 of this. Strange though this may seem, this is the basis for the whole 797 module import/export system. 798 799 Another use for typeglobs is to pass filehandles into a function or 800 to create new filehandles. If you need to use a typeglob to save away 801 a filehandle, do it this way: 802 803 $fh = *STDOUT; 804 805 or perhaps as a real reference, like this: 806 807 $fh = \*STDOUT; 808 809 See L<perlsub> for examples of using these as indirect filehandles 810 in functions. 811 812 Typeglobs are also a way to create a local filehandle using the local() 813 operator. These last until their block is exited, but may be passed back. 814 For example: 815 816 sub newopen { 817 my $path = shift; 818 local *FH; # not my! 819 open (FH, $path) or return undef; 820 return *FH; 821 } 822 $fh = newopen('/etc/passwd'); 823 824 Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much 825 for filehandle manipulations, although they're still needed to pass brand 826 new file and directory handles into or out of functions. That's because 827 C<*HANDLE{IO}> only works if HANDLE has already been used as a handle. 828 In other words, C<*FH> must be used to create new symbol table entries; 829 C<*foo{THING}> cannot. When in doubt, use C<*FH>. 830 831 All functions that are capable of creating filehandles (open(), 832 opendir(), pipe(), socketpair(), sysopen(), socket(), and accept()) 833 automatically create an anonymous filehandle if the handle passed to 834 them is an uninitialized scalar variable. This allows the constructs 835 such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to 836 create filehandles that will conveniently be closed automatically when 837 the scope ends, provided there are no other references to them. This 838 largely eliminates the need for typeglobs when opening filehandles 839 that must be passed around, as in the following example: 840 841 sub myopen { 842 open my $fh, "@_" 843 or die "Can't open '@_': $!"; 844 return $fh; 845 } 846 847 { 848 my $f = myopen("</etc/motd"); 849 print <$f>; 850 # $f implicitly closed here 851 } 852 853 Note that if an initialized scalar variable is used instead the 854 result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent 855 to C<open( *{'zzz'}, ...)>. 856 C<use strict 'refs'> forbids such practice. 857 858 Another way to create anonymous filehandles is with the Symbol 859 module or with the IO::Handle module and its ilk. These modules 860 have the advantage of not hiding different types of the same name 861 during the local(). See the bottom of L<perlfunc/open()> for an 862 example. 863 864 =head1 SEE ALSO 865 866 See L<perlvar> for a description of Perl's built-in variables and 867 a discussion of legal variable names. See L<perlref>, L<perlsub>, 868 and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and 869 the C<*foo{THING}> syntax.
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 |