3 perldata - Perl data types
8 X<variable, name> X<variable name> X<data type> X<type>
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.
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). For a more in-depth discussion
28 on identifiers, see L<Identifier parsing>. It's possible to
29 substitute for a simple identifier, an expression that produces a reference
30 to the value at runtime. This is described in more detail below
34 Perl also has its own built-in variables whose names don't follow
35 these rules. They have strange names so they don't accidentally
36 collide with one of your normal variables. Strings that match
37 parenthesized parts of a regular expression are saved under names
38 containing only digits after the C<$> (see L<perlop> and L<perlre>).
39 In addition, several special variables that provide windows into
40 the inner working of Perl have names containing punctuation characters
41 and control characters. These are documented in L<perlvar>.
44 Scalar values are always named with '$', even when referring to a
45 scalar that is part of an array or a hash. The '$' symbol works
46 semantically like the English word "the" in that it indicates a
47 single value is expected.
50 $days # the simple scalar value "days"
51 $days[28] # the 29th element of array @days
52 $days{'Feb'} # the 'Feb' value from hash %days
53 $#days # the last index of array @days
55 Entire arrays (and slices of arrays and hashes) are denoted by '@',
56 which works much as the word "these" or "those" does in English,
57 in that it indicates multiple values are expected.
60 @days # ($days[0], $days[1],... $days[n])
61 @days[3,4,5] # same as ($days[3],$days[4],$days[5])
62 @days{'a','c'} # same as ($days{'a'},$days{'c'})
64 Entire hashes are denoted by '%':
67 %days # (key1, val1, key2, val2 ...)
69 In addition, subroutines are named with an initial '&', though this
70 is optional when unambiguous, just as the word "do" is often redundant
71 in English. Symbol table entries can be named with an initial '*',
72 but you don't really care about that yet (if ever :-).
74 Every variable type has its own namespace, as do several
75 non-variable identifiers. This means that you can, without fear
76 of conflict, use the same name for a scalar variable, an array, or
77 a hash--or, for that matter, for a filehandle, a directory handle, a
78 subroutine name, a format name, or a label. This means that $foo
79 and @foo are two different variables. It also means that C<$foo[1]>
80 is a part of @foo, not a part of $foo. This may seem a bit weird,
81 but that's okay, because it is weird.
84 Because variable references always start with '$', '@', or '%', the
85 "reserved" words aren't in fact reserved with respect to variable
86 names. They I<are> reserved with respect to labels and filehandles,
87 however, which don't have an initial special character. You can't
88 have a filehandle named "log", for instance. Hint: you could say
89 C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using
90 uppercase filehandles also improves readability and protects you
91 from conflict with future reserved words. Case I<is> significant--"FOO",
92 "Foo", and "foo" are all different names. Names that start with a
93 letter or underscore may also contain digits and underscores.
94 X<identifier, case sensitivity>
97 It is possible to replace such an alphanumeric name with an expression
98 that returns a reference to the appropriate type. For a description
99 of this, see L<perlref>.
101 Names that start with a digit may contain only more digits. Names
102 that do not start with a letter, underscore, digit or a caret (i.e.
103 a control character) are limited to one character, e.g., C<$%> or
104 C<$$>. (Most of these one character names have a predefined
105 significance to Perl. For instance, C<$$> is the current process
108 =head2 Identifier parsing
111 Up until Perl 5.18, the actual rules of what a valid identifier
112 was were a bit fuzzy. However, in general, anything defined here should
113 work on previous versions of Perl, while the opposite -- edge cases
114 that work in previous versions, but aren't defined here -- probably
115 won't work on newer versions.
116 As an important side note, please note that the following only applies
117 to bareword identifiers as found in Perl source code, not identifiers
118 introduced through symbolic references, which have much fewer
120 If working under the effect of the C<use utf8;> pragma, the following
123 / (?[ ( \p{Word} & \p{XID_Start} ) + [_] ])
124 (?[ ( \p{Word} & \p{XID_Continue} ) ]) * /x
126 That is, a "start" character followed by any number of "continue"
127 characters. Perl requires every character in an identifier to also
128 match C<\w> (this prevents some problematic cases); and Perl
129 additionally accepts identfier names beginning with an underscore.
131 If not under C<use utf8>, the source is treated as ASCII + 128 extra
132 controls, and identifiers should match
134 / (?aa) (?!\d) \w+ /x
136 That is, any word character in the ASCII range, as long as the first
137 character is not a digit.
139 There are two package separators in Perl: A double colon (C<::>) and a single
140 quote (C<'>). Normal identifiers can start or end with a double colon, and
141 can contain several parts delimited by double colons.
142 Single quotes have similar rules, but with the exception that they are not
143 legal at the end of an identifier: That is, C<$'foo> and C<$foo'bar> are
144 legal, but C<$foo'bar'> is not.
146 Additionally, if the identifier is preceded by a sigil --
147 that is, if the identifier is part of a variable name -- it
148 may optionally be enclosed in braces.
150 While you can mix double colons with singles quotes, the quotes must come
151 after the colons: C<$::::'foo> and C<$foo::'bar> are legal, but C<$::'::foo>
152 and C<$foo'::bar> are not.
154 Put together, a grammar to match a basic identifier becomes
161 (?&normal_identifier)
162 | \{ \s* (?&normal_identifier) \s* \}
165 (?<normal_identifier>
168 (?: (?= (?: :: )+ '? | (?: :: )* ' ) (?&normal_identifier) )?
173 (?(?{ (caller(0))[8] & $utf8::hint_bits })
174 (?&Perl_XIDS) (?&Perl_XIDC)*
178 (?<sigil> [&*\$\@\%])
179 (?<Perl_XIDS> (?[ ( \p{Word} & \p{XID_Start} ) + [_] ]) )
180 (?<Perl_XIDC> (?[ \p{Word} & \p{XID_Continue} ]) )
184 Meanwhile, special identifiers don't follow the above rules; For the most
185 part, all of the identifiers in this category have a special meaning given
186 by Perl. Because they have special parsing rules, these generally can't be
187 fully-qualified. They come in four forms:
191 =item A sigil, followed solely by digits matching \p{POSIX_Digit}, like C<$0>,
194 =item A sigil, followed by either a caret and a single POSIX uppercase letter,
195 like C<$^V> or C<$^W>, or a sigil followed by a literal control character
196 matching the C<\p{POSIX_Cntrl}> property. Due to a historical oddity, if not
197 running under C<use utf8>, the 128 extra controls in the C<[0x80-0xff]> range
198 may also be used in length one variables.
200 =item Similar to the above, a sigil, followed by bareword text in brackets,
201 where the first character is either a caret followed by an uppercase letter,
202 or a literal control, like C<${^GLOBAL_PHASE}> or C<${\7LOBAL_PHASE}>.
204 =item A sigil followed by a single character matching the C<\p{POSIX_Punct}>
205 property, like C<$!> or C<%+>.
210 X<context> X<scalar context> X<list context>
212 The interpretation of operations and values in Perl sometimes depends
213 on the requirements of the context around the operation or value.
214 There are two major contexts: list and scalar. Certain operations
215 return list values in contexts wanting a list, and scalar values
216 otherwise. If this is true of an operation it will be mentioned in
217 the documentation for that operation. In other words, Perl overloads
218 certain operations based on whether the expected return value is
219 singular or plural. Some words in English work this way, like "fish"
222 In a reciprocal fashion, an operation provides either a scalar or a
223 list context to each of its arguments. For example, if you say
227 the integer operation provides scalar context for the <>
228 operator, which responds by reading one line from STDIN and passing it
229 back to the integer operation, which will then find the integer value
230 of that line and return that. If, on the other hand, you say
234 then the sort operation provides list context for <>, which
235 will proceed to read every line available up to the end of file, and
236 pass that list of lines back to the sort routine, which will then
237 sort those lines and return them as a list to whatever the context
240 Assignment is a little bit special in that it uses its left argument
241 to determine the context for the right argument. Assignment to a
242 scalar evaluates the right-hand side in scalar context, while
243 assignment to an array or hash evaluates the righthand side in list
244 context. Assignment to a list (or slice, which is just a list
245 anyway) also evaluates the right-hand side in list context.
247 When you use the C<use warnings> pragma or Perl's B<-w> command-line
248 option, you may see warnings
249 about useless uses of constants or functions in "void context".
250 Void context just means the value has been discarded, such as a
251 statement containing only C<"fred";> or C<getpwuid(0);>. It still
252 counts as scalar context for functions that care whether or not
253 they're being called in list context.
255 User-defined subroutines may choose to care whether they are being
256 called in a void, scalar, or list context. Most subroutines do not
257 need to bother, though. That's because both scalars and lists are
258 automatically interpolated into lists. See L<perlfunc/wantarray>
259 for how you would dynamically discern your function's calling
263 X<scalar> X<number> X<string> X<reference>
265 All data in Perl is a scalar, an array of scalars, or a hash of
266 scalars. A scalar may contain one single value in any of three
267 different flavors: a number, a string, or a reference. In general,
268 conversion from one form to another is transparent. Although a
269 scalar may not directly hold multiple values, it may contain a
270 reference to an array or hash which in turn contains multiple values.
272 Scalars aren't necessarily one thing or another. There's no place
273 to declare a scalar variable to be of type "string", type "number",
274 type "reference", or anything else. Because of the automatic
275 conversion of scalars, operations that return scalars don't need
276 to care (and in fact, cannot care) whether their caller is looking
277 for a string, a number, or a reference. Perl is a contextually
278 polymorphic language whose scalars can be strings, numbers, or
279 references (which includes objects). Although strings and numbers
280 are considered pretty much the same thing for nearly all purposes,
281 references are strongly-typed, uncastable pointers with builtin
282 reference-counting and destructor invocation.
284 A scalar value is interpreted as FALSE in the Boolean sense
285 if it is undefined, the null string or the number 0 (or its
286 string equivalent, "0"), and TRUE if it is anything else. The
287 Boolean context is just a special kind of scalar context where no
288 conversion to a string or a number is ever performed.
289 X<boolean> X<bool> X<true> X<false> X<truth>
291 There are actually two varieties of null strings (sometimes referred
292 to as "empty" strings), a defined one and an undefined one. The
293 defined version is just a string of length zero, such as C<"">.
294 The undefined version is the value that indicates that there is
295 no real value for something, such as when there was an error, or
296 at end of file, or when you refer to an uninitialized variable or
297 element of an array or hash. Although in early versions of Perl,
298 an undefined scalar could become defined when first used in a
299 place expecting a defined value, this no longer happens except for
300 rare cases of autovivification as explained in L<perlref>. You can
301 use the defined() operator to determine whether a scalar value is
302 defined (this has no meaning on arrays or hashes), and the undef()
303 operator to produce an undefined value.
304 X<defined> X<undefined> X<undef> X<null> X<string, null>
306 To find out whether a given string is a valid non-zero number, it's
307 sometimes enough to test it against both numeric 0 and also lexical
308 "0" (although this will cause noises if warnings are on). That's
309 because strings that aren't numbers count as 0, just as they do in B<awk>:
311 if ($str == 0 && $str ne "0") {
312 warn "That doesn't look like a number";
315 That method may be best because otherwise you won't treat IEEE
316 notations like C<NaN> or C<Infinity> properly. At other times, you
317 might prefer to determine whether string data can be used numerically
318 by calling the POSIX::strtod() function or by inspecting your string
319 with a regular expression (as documented in L<perlre>).
321 warn "has nondigits" if /\D/;
322 warn "not a natural number" unless /^\d+$/; # rejects -3
323 warn "not an integer" unless /^-?\d+$/; # rejects +3
324 warn "not an integer" unless /^[+-]?\d+$/;
325 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
326 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
328 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
330 The length of an array is a scalar value. You may find the length
331 of array @days by evaluating C<$#days>, as in B<csh>. However, this
332 isn't the length of the array; it's the subscript of the last element,
333 which is a different value since there is ordinarily a 0th element.
334 Assigning to C<$#days> actually changes the length of the array.
335 Shortening an array this way destroys intervening values. Lengthening
336 an array that was previously shortened does not recover values
337 that were in those elements.
338 X<$#> X<array, length>
340 You can also gain some minuscule measure of efficiency by pre-extending
341 an array that is going to get big. You can also extend an array
342 by assigning to an element that is off the end of the array. You
343 can truncate an array down to nothing by assigning the null list
344 () to it. The following are equivalent:
349 If you evaluate an array in scalar context, it returns the length
350 of the array. (Note that this is not true of lists, which return
351 the last value, like the C comma operator, nor of built-in functions,
352 which return whatever they feel like returning.) The following is
356 scalar(@whatever) == $#whatever + 1;
358 Some programmers choose to use an explicit conversion so as to
359 leave nothing to doubt:
361 $element_count = scalar(@whatever);
363 If you evaluate a hash in scalar context, it returns false if the
364 hash is empty. If there are any key/value pairs, it returns true;
365 more precisely, the value returned is a string consisting of the
366 number of used buckets and the number of allocated buckets, separated
367 by a slash. This is pretty much useful only to find out whether
368 Perl's internal hashing algorithm is performing poorly on your data
369 set. For example, you stick 10,000 things in a hash, but evaluating
370 %HASH in scalar context reveals C<"1/16">, which means only one out
371 of sixteen buckets has been touched, and presumably contains all
372 10,000 of your items. This isn't supposed to happen. If a tied hash
373 is evaluated in scalar context, the C<SCALAR> method is called (with a
374 fallback to C<FIRSTKEY>).
375 X<hash, scalar context> X<hash, bucket> X<bucket>
377 You can preallocate space for a hash by assigning to the keys() function.
378 This rounds up the allocated buckets to the next power of two:
380 keys(%users) = 1000; # allocate 1024 buckets
382 =head2 Scalar value constructors
383 X<scalar, literal> X<scalar, constant>
385 Numeric literals are specified in any of the following floating point or
390 .23E-10 # a very small number
391 3.14_15_92 # a very important number
392 4_294_967_296 # underscore for legibility
394 0xdead_beef # more hex
395 0377 # octal (only numbers, begins with 0)
398 You are allowed to use underscores (underbars) in numeric literals
399 between digits for legibility (but not multiple underscores in a row:
400 C<23__500> is not legal; C<23_500> is).
401 You could, for example, group binary
402 digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
403 or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
406 String literals are usually delimited by either single or double
407 quotes. They work much like quotes in the standard Unix shells:
408 double-quoted string literals are subject to backslash and variable
409 substitution; single-quoted strings are not (except for C<\'> and
410 C<\\>). The usual C-style backslash rules apply for making
411 characters such as newline, tab, etc., as well as some more exotic
412 forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
415 Hexadecimal, octal, or binary, representations in string literals
416 (e.g. '0xff') are not automatically converted to their integer
417 representation. The hex() and oct() functions make these conversions
418 for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
420 You can also embed newlines directly in your strings, i.e., they can end
421 on a different line than they begin. This is nice, but if you forget
422 your trailing quote, the error will not be reported until Perl finds
423 another line containing the quote character, which may be much further
424 on in the script. Variable substitution inside strings is limited to
425 scalar variables, arrays, and array or hash slices. (In other words,
426 names beginning with $ or @, followed by an optional bracketed
427 expression as a subscript.) The following code segment prints out "The
431 $Price = '$100'; # not interpolated
432 print "The price is $Price.\n"; # interpolated
434 There is no double interpolation in Perl, so the C<$100> is left as is.
436 By default floating point numbers substituted inside strings use the
437 dot (".") as the decimal separator. If C<use locale> is in effect,
438 and POSIX::setlocale() has been called, the character used for the
439 decimal separator is affected by the LC_NUMERIC locale.
440 See L<perllocale> and L<POSIX>.
442 As in some shells, you can enclose the variable name in braces to
443 disambiguate it from following alphanumerics (and underscores).
445 this when interpolating a variable into a string to separate the
446 variable name from a following double-colon or an apostrophe, since
447 these would be otherwise treated as a package separator:
451 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
452 print "We use ${who}speak when ${who}'s here.\n";
454 Without the braces, Perl would have looked for a $whospeak, a
455 C<$who::0>, and a C<$who's> variable. The last two would be the
456 $0 and the $s variables in the (presumably) non-existent package
459 In fact, a simple identifier within such curlies is forced to be
460 a string, and likewise within a hash subscript. Neither need
461 quoting. Our earlier example, C<$days{'Feb'}> can be written as
462 C<$days{Feb}> and the quotes will be assumed automatically. But
463 anything more complicated in the subscript will be interpreted as an
464 expression. This means for example that C<$version{2.0}++> is
465 equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
467 =head3 Version Strings
468 X<version string> X<vstring> X<v-string>
470 A literal of the form C<v1.20.300.4000> is parsed as a string composed
471 of characters with the specified ordinals. This form, known as
472 v-strings, provides an alternative, more readable way to construct
473 strings, rather than use the somewhat less readable interpolation form
474 C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
475 Unicode strings, and for comparing version "numbers" using the string
476 comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
477 more dots in the literal, the leading C<v> may be omitted.
479 print v9786; # prints SMILEY, "\x{263a}"
480 print v102.111.111; # prints "foo"
481 print 102.111.111; # same
483 Such literals are accepted by both C<require> and C<use> for
484 doing a version check. Note that using the v-strings for IPv4
485 addresses is not portable unless you also use the
486 inet_aton()/inet_ntoa() routines of the Socket package.
488 Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
489 are not v-strings before the C<< => >> operator (which is usually used
490 to separate a hash key from a hash value); instead they are interpreted
491 as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
492 Perl 5.8.0, but that caused more confusion and breakage than good.
493 Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
496 =head3 Special Literals
497 X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
498 X<end> X<data> X<^D> X<^Z>
500 The special literals __FILE__, __LINE__, and __PACKAGE__
501 represent the current filename, line number, and package name at that
502 point in your program. __SUB__ gives a reference to the current
503 subroutine. They may be used only as separate tokens; they
504 will not be interpolated into strings. If there is no current package
505 (due to an empty C<package;> directive), __PACKAGE__ is the undefined
506 value. (But the empty C<package;> is no longer supported, as of version
507 5.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
508 is only available in 5.16 or higher, and only with a C<use v5.16> or
509 C<use feature "current_sub"> declaration.
510 X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
511 X<line> X<file> X<package>
513 The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
514 may be used to indicate the logical end of the script before the actual
515 end of file. Any following text is ignored.
517 Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
518 where C<PACKNAME> is the package that was current when the __DATA__
519 token was encountered. The filehandle is left open pointing to the
520 line after __DATA__. The program should C<close DATA> when it is done
521 reading from it. (Leaving it open leaks filehandles if the module is
522 reloaded for any reason, so it's a safer practice to close it.) For
523 compatibility with older scripts written before __DATA__ was
524 introduced, __END__ behaves like __DATA__ in the top level script (but
525 not in files loaded with C<require> or C<do>) and leaves the remaining
526 contents of the file accessible via C<main::DATA>.
528 See L<SelfLoader> for more description of __DATA__, and
529 an example of its use. Note that you cannot read from the DATA
530 filehandle in a BEGIN block: the BEGIN block is executed as soon
531 as it is seen (during compilation), at which point the corresponding
532 __DATA__ (or __END__) token has not yet been seen.
537 A word that has no other interpretation in the grammar will
538 be treated as if it were a quoted string. These are known as
539 "barewords". As with filehandles and labels, a bareword that consists
540 entirely of lowercase letters risks conflict with future reserved
541 words, and if you use the C<use warnings> pragma or the B<-w> switch,
542 Perl will warn you about any such words. Perl limits barewords (like
543 identifiers) to about 250 characters. Future versions of Perl are likely
544 to eliminate these arbitrary limitations.
546 Some people may wish to outlaw barewords entirely. If you
551 then any bareword that would NOT be interpreted as a subroutine call
552 produces a compile-time error instead. The restriction lasts to the
553 end of the enclosing block. An inner block may countermand this
554 by saying C<no strict 'subs'>.
556 =head3 Array Interpolation
557 X<array, interpolation> X<interpolation, array> X<$">
559 Arrays and slices are interpolated into double-quoted strings
560 by joining the elements with the delimiter specified in the C<$">
561 variable (C<$LIST_SEPARATOR> if "use English;" is specified),
562 space by default. The following are equivalent:
564 $temp = join($", @ARGV);
569 Within search patterns (which also undergo double-quotish substitution)
570 there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
571 C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
572 expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
573 @foo)? If @foo doesn't otherwise exist, then it's obviously a
574 character class. If @foo exists, Perl takes a good guess about C<[bar]>,
575 and is almost always right. If it does guess wrong, or if you're just
576 plain paranoid, you can force the correct interpretation with curly
579 If you're looking for the information on how to use here-documents,
580 which used to be here, that's been moved to
581 L<perlop/Quote and Quote-like Operators>.
583 =head2 List value constructors
586 List values are denoted by separating individual values by commas
587 (and enclosing the list in parentheses where precedence requires it):
591 In a context not requiring a list value, the value of what appears
592 to be a list literal is simply the value of the final element, as
593 with the C comma operator. For example,
595 @foo = ('cc', '-E', $bar);
597 assigns the entire list value to array @foo, but
599 $foo = ('cc', '-E', $bar);
601 assigns the value of variable $bar to the scalar variable $foo.
602 Note that the value of an actual array in scalar context is the
603 length of the array; the following assigns the value 3 to $foo:
605 @foo = ('cc', '-E', $bar);
606 $foo = @foo; # $foo gets 3
608 You may have an optional comma before the closing parenthesis of a
609 list literal, so that you can say:
617 To use a here-document to assign an array, one line per element,
618 you might use an approach like this:
620 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
628 LISTs do automatic interpolation of sublists. That is, when a LIST is
629 evaluated, each element of the list is evaluated in list context, and
630 the resulting list value is interpolated into LIST just as if each
631 individual element were a member of LIST. Thus arrays and hashes lose their
632 identity in a LIST--the list
634 (@foo,@bar,&SomeSub,%glarch)
636 contains all the elements of @foo followed by all the elements of @bar,
637 followed by all the elements returned by the subroutine named SomeSub
638 called in list context, followed by the key/value pairs of %glarch.
639 To make a list reference that does I<NOT> interpolate, see L<perlref>.
641 The null list is represented by (). Interpolating it in a list
642 has no effect. Thus ((),(),()) is equivalent to (). Similarly,
643 interpolating an array with no elements is the same as if no
644 array had been interpolated at that point.
646 This interpolation combines with the facts that the opening
647 and closing parentheses are optional (except when necessary for
648 precedence) and lists may end with an optional comma to mean that
649 multiple commas within lists are legal syntax. The list C<1,,3> is a
650 concatenation of two lists, C<1,> and C<3>, the first of which ends
651 with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
652 similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
653 we'd advise you to use this obfuscation.
655 A list value may also be subscripted like a normal array. You must
656 put the list in parentheses to avoid ambiguity. For example:
658 # Stat returns list value.
659 $time = (stat($file))[8];
662 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
665 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
667 # A "reverse comma operator".
668 return (pop(@foo),pop(@foo))[0];
670 Lists may be assigned to only when each element of the list
671 is itself legal to assign to:
673 ($a, $b, $c) = (1, 2, 3);
675 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
677 An exception to this is that you may assign to C<undef> in a list.
678 This is useful for throwing away some of the return values of a
681 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
683 List assignment in scalar context returns the number of elements
684 produced by the expression on the right side of the assignment:
686 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
687 $x = (($foo,$bar) = f()); # set $x to f()'s return count
689 This is handy when you want to do a list assignment in a Boolean
690 context, because most list functions return a null list when finished,
691 which when assigned produces a 0, which is interpreted as FALSE.
693 It's also the source of a useful idiom for executing a function or
694 performing an operation in list context and then counting the number of
695 return values, by assigning to an empty list and then using that
696 assignment in scalar context. For example, this code:
698 $count = () = $string =~ /\d+/g;
700 will place into $count the number of digit groups found in $string.
701 This happens because the pattern match is in list context (since it
702 is being assigned to the empty list), and will therefore return a list
703 of all matching parts of the string. The list assignment in scalar
704 context will translate that into the number of elements (here, the
705 number of times the pattern matched) and assign that to $count. Note
708 $count = $string =~ /\d+/g;
710 would not have worked, since a pattern match in scalar context will
711 only return true or false, rather than a count of matches.
713 The final element of a list assignment may be an array or a hash:
715 ($a, $b, @rest) = split;
716 my($a, $b, %rest) = @_;
718 You can actually put an array or hash anywhere in the list, but the first one
719 in the list will soak up all the values, and anything after it will become
720 undefined. This may be useful in a my() or local().
722 A hash can be initialized using a literal list holding pairs of
723 items to be interpreted as a key and a value:
725 # same as map assignment above
726 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
728 While literal lists and named arrays are often interchangeable, that's
729 not the case for hashes. Just because you can subscript a list value like
730 a normal array does not mean that you can subscript a list value as a
731 hash. Likewise, hashes included as parts of other lists (including
732 parameters lists and return lists from functions) always flatten out into
733 key/value pairs. That's why it's good to use references sometimes.
735 It is often more readable to use the C<< => >> operator between key/value
736 pairs. The C<< => >> operator is mostly just a more visually distinctive
737 synonym for a comma, but it also arranges for its left-hand operand to be
738 interpreted as a string if it's a bareword that would be a legal simple
739 identifier. C<< => >> doesn't quote compound identifiers, that contain
740 double colons. This makes it nice for initializing hashes:
748 or for initializing hash references to be used as records:
751 witch => 'Mable the Merciless',
752 cat => 'Fluffy the Ferocious',
753 date => '10/31/1776',
756 or for using call-by-named-parameter to complicated functions:
758 $field = $query->radio_group(
759 name => 'group_name',
760 values => ['eenie','meenie','minie'],
766 Note that just because a hash is initialized in that order doesn't
767 mean that it comes out in that order. See L<perlfunc/sort> for examples
768 of how to arrange for an output ordering.
770 If a key appears more than once in the initializer list of a hash, the last
777 color => [0xDF, 0xFF, 0x00],
784 color => [0xDF, 0xFF, 0x00],
788 This can be used to provide overridable configuration defaults:
790 # values in %args take priority over %config_defaults
791 %config = (%config_defaults, %args);
795 An array can be accessed one scalar at a
796 time by specifying a dollar sign (C<$>), then the
797 name of the array (without the leading C<@>), then the subscript inside
798 square brackets. For example:
800 @myarray = (5, 50, 500, 5000);
801 print "The Third Element is", $myarray[2], "\n";
803 The array indices start with 0. A negative subscript retrieves its
804 value from the end. In our example, C<$myarray[-1]> would have been
805 5000, and C<$myarray[-2]> would have been 500.
807 Hash subscripts are similar, only instead of square brackets curly brackets
808 are used. For example:
813 "Einstein" => "Albert",
814 "Darwin" => "Charles",
815 "Feynman" => "Richard",
818 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
820 You can also subscript a list to get a single element from it:
822 $dir = (getpwnam("daemon"))[7];
824 =head2 Multi-dimensional array emulation
826 Multidimensional arrays may be emulated by subscripting a hash with a
827 list. The elements of the list are joined with the subscript separator
834 $foo{join($;, $a, $b, $c)}
836 The default subscript separator is "\034", the same as SUBSEP in B<awk>.
839 X<slice> X<array, slice> X<hash, slice>
841 A slice accesses several elements of a list, an array, or a hash
842 simultaneously using a list of subscripts. It's more convenient
843 than writing out the individual elements as a list of separate
846 ($him, $her) = @folks[0,-1]; # array slice
847 @them = @folks[0 .. 3]; # array slice
848 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
849 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
851 Since you can assign to a list of variables, you can also assign to
852 an array or hash slice.
854 @days[3..5] = qw/Wed Thu Fri/;
855 @colors{'red','blue','green'}
856 = (0xff0000, 0x0000ff, 0x00ff00);
857 @folks[0, -1] = @folks[-1, 0];
859 The previous assignments are exactly equivalent to
861 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
862 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
863 = (0xff0000, 0x0000ff, 0x00ff00);
864 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
866 Since changing a slice changes the original array or hash that it's
867 slicing, a C<foreach> construct will alter some--or even all--of the
868 values of the array or hash.
870 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
872 foreach (@hash{qw[key1 key2]}) {
873 s/^\s+//; # trim leading whitespace
874 s/\s+$//; # trim trailing whitespace
875 s/(\w+)/\u\L$1/g; # "titlecase" words
878 A slice of an empty list is still an empty list. Thus:
880 @a = ()[1,0]; # @a has no elements
881 @b = (@a)[0,1]; # @b has no elements
885 @a = (1)[1,0]; # @a has two elements
886 @b = (1,undef)[1,0,2]; # @b has three elements
888 More generally, a slice yields the empty list if it indexes only
889 beyond the end of a list:
891 @a = (1)[ 1,2]; # @a has no elements
892 @b = (1)[0,1,2]; # @b has three elements
894 This makes it easy to write loops that terminate when a null list
897 while ( ($home, $user) = (getpwent)[7,0]) {
898 printf "%-8s %s\n", $user, $home;
901 As noted earlier in this document, the scalar sense of list assignment
902 is the number of elements on the right-hand side of the assignment.
903 The null list contains no elements, so when the password file is
904 exhausted, the result is 0, not 2.
906 Slices in scalar context return the last item of the slice.
908 @a = qw/first second third/;
909 %h = (first => 'A', second => 'B');
910 $t = @a[0, 1]; # $t is now 'second'
911 $u = @h{'first', 'second'}; # $u is now 'B'
913 If you're confused about why you use an '@' there on a hash slice
914 instead of a '%', think of it like this. The type of bracket (square
915 or curly) governs whether it's an array or a hash being looked at.
916 On the other hand, the leading symbol ('$' or '@') on the array or
917 hash indicates whether you are getting back a singular value (a
918 scalar) or a plural one (a list).
920 =head2 Typeglobs and Filehandles
921 X<typeglob> X<filehandle> X<*>
923 Perl uses an internal type called a I<typeglob> to hold an entire
924 symbol table entry. The type prefix of a typeglob is a C<*>, because
925 it represents all types. This used to be the preferred way to
926 pass arrays and hashes by reference into a function, but now that
927 we have real references, this is seldom needed.
929 The main use of typeglobs in modern Perl is create symbol table aliases.
934 makes $this an alias for $that, @this an alias for @that, %this an alias
935 for %that, &this an alias for &that, etc. Much safer is to use a reference.
938 local *Here::blue = \$There::green;
940 temporarily makes $Here::blue an alias for $There::green, but doesn't
941 make @Here::blue an alias for @There::green, or %Here::blue an alias for
942 %There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
943 of this. Strange though this may seem, this is the basis for the whole
944 module import/export system.
946 Another use for typeglobs is to pass filehandles into a function or
947 to create new filehandles. If you need to use a typeglob to save away
948 a filehandle, do it this way:
952 or perhaps as a real reference, like this:
956 See L<perlsub> for examples of using these as indirect filehandles
959 Typeglobs are also a way to create a local filehandle using the local()
960 operator. These last until their block is exited, but may be passed back.
966 open (FH, $path) or return undef;
969 $fh = newopen('/etc/passwd');
971 Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
972 for filehandle manipulations, although they're still needed to pass brand
973 new file and directory handles into or out of functions. That's because
974 C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
975 In other words, C<*FH> must be used to create new symbol table entries;
976 C<*foo{THING}> cannot. When in doubt, use C<*FH>.
978 All functions that are capable of creating filehandles (open(),
979 opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
980 automatically create an anonymous filehandle if the handle passed to
981 them is an uninitialized scalar variable. This allows the constructs
982 such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
983 create filehandles that will conveniently be closed automatically when
984 the scope ends, provided there are no other references to them. This
985 largely eliminates the need for typeglobs when opening filehandles
986 that must be passed around, as in the following example:
990 or die "Can't open '@_': $!";
995 my $f = myopen("</etc/motd");
997 # $f implicitly closed here
1000 Note that if an initialized scalar variable is used instead the
1001 result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
1002 to C<open( *{'zzz'}, ...)>.
1003 C<use strict 'refs'> forbids such practice.
1005 Another way to create anonymous filehandles is with the Symbol
1006 module or with the IO::Handle module and its ilk. These modules
1007 have the advantage of not hiding different types of the same name
1008 during the local(). See the bottom of L<perlfunc/open> for an
1013 See L<perlvar> for a description of Perl's built-in variables and
1014 a discussion of legal variable names. See L<perlref>, L<perlsub>,
1015 and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
1016 the C<*foo{THING}> syntax.