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:
193 A sigil, followed solely by digits matching C<\p{POSIX_Digit}>, like
194 C<$0>, C<$1>, or C<$10000>.
198 A sigil, followed by either a caret and a single POSIX uppercase letter,
199 like C<$^V> or C<$^W>, or a sigil followed by a literal non-space,
200 non-C<NUL> control character matching the C<\p{POSIX_Cntrl}> property.
201 Due to a historical oddity, if not running under C<use utf8>, the 128
202 characters in the C<[0x80-0xff]> range are considered to be controls,
203 and may also be used in length-one variables. However, the use of
204 non-graphical characters is deprecated as of v5.22, and support for them
205 will be removed in a future version of perl. ASCII space characters and
206 C<NUL> already aren't allowed, so this means that a single-character
207 variable name with that name being any other C0 control C<[0x01-0x1F]>,
208 or C<DEL> will generate a deprecated warning. Already, under C<"use
209 utf8">, non-ASCII characters must match C<Perl_XIDS>. As of v5.22, when
210 not under C<"use utf8"> C1 controls C<[0x80-0x9F]>, NO BREAK SPACE, and
211 SOFT HYPHEN (C<SHY>)) generate a deprecated warning.
215 Similar to the above, a sigil, followed by bareword text in brackets,
216 where the first character is either a caret followed by an uppercase
217 letter, like C<${^GLOBAL_PHASE}> or a non-C<NUL>, non-space literal
218 control like C<${\7LOBAL_PHASE}>. Like the above, when not under
219 C<"use utf8">, the characters in C<[0x80-0xFF]> are considered controls, but as
220 of v5.22, the use of any that are non-graphical are deprecated, and as
221 of v5.20 the use of any ASCII-range literal control is deprecated.
222 Support for these will be removed in a future version of perl.
226 A sigil followed by a single character matching the C<\p{POSIX_Punct}>
227 property, like C<$!> or C<%+>, except the character C<"{"> doesn't work.
231 Note that as of Perl 5.20, literal control characters in variable names
232 are deprecated; and as of Perl 5.22, any other non-graphic characters
236 X<context> X<scalar context> X<list context>
238 The interpretation of operations and values in Perl sometimes depends
239 on the requirements of the context around the operation or value.
240 There are two major contexts: list and scalar. Certain operations
241 return list values in contexts wanting a list, and scalar values
242 otherwise. If this is true of an operation it will be mentioned in
243 the documentation for that operation. In other words, Perl overloads
244 certain operations based on whether the expected return value is
245 singular or plural. Some words in English work this way, like "fish"
248 In a reciprocal fashion, an operation provides either a scalar or a
249 list context to each of its arguments. For example, if you say
253 the integer operation provides scalar context for the <>
254 operator, which responds by reading one line from STDIN and passing it
255 back to the integer operation, which will then find the integer value
256 of that line and return that. If, on the other hand, you say
260 then the sort operation provides list context for <>, which
261 will proceed to read every line available up to the end of file, and
262 pass that list of lines back to the sort routine, which will then
263 sort those lines and return them as a list to whatever the context
266 Assignment is a little bit special in that it uses its left argument
267 to determine the context for the right argument. Assignment to a
268 scalar evaluates the right-hand side in scalar context, while
269 assignment to an array or hash evaluates the righthand side in list
270 context. Assignment to a list (or slice, which is just a list
271 anyway) also evaluates the right-hand side in list context.
273 When you use the C<use warnings> pragma or Perl's B<-w> command-line
274 option, you may see warnings
275 about useless uses of constants or functions in "void context".
276 Void context just means the value has been discarded, such as a
277 statement containing only C<"fred";> or C<getpwuid(0);>. It still
278 counts as scalar context for functions that care whether or not
279 they're being called in list context.
281 User-defined subroutines may choose to care whether they are being
282 called in a void, scalar, or list context. Most subroutines do not
283 need to bother, though. That's because both scalars and lists are
284 automatically interpolated into lists. See L<perlfunc/wantarray>
285 for how you would dynamically discern your function's calling
289 X<scalar> X<number> X<string> X<reference>
291 All data in Perl is a scalar, an array of scalars, or a hash of
292 scalars. A scalar may contain one single value in any of three
293 different flavors: a number, a string, or a reference. In general,
294 conversion from one form to another is transparent. Although a
295 scalar may not directly hold multiple values, it may contain a
296 reference to an array or hash which in turn contains multiple values.
298 Scalars aren't necessarily one thing or another. There's no place
299 to declare a scalar variable to be of type "string", type "number",
300 type "reference", or anything else. Because of the automatic
301 conversion of scalars, operations that return scalars don't need
302 to care (and in fact, cannot care) whether their caller is looking
303 for a string, a number, or a reference. Perl is a contextually
304 polymorphic language whose scalars can be strings, numbers, or
305 references (which includes objects). Although strings and numbers
306 are considered pretty much the same thing for nearly all purposes,
307 references are strongly-typed, uncastable pointers with builtin
308 reference-counting and destructor invocation.
310 A scalar value is interpreted as FALSE in the Boolean sense
311 if it is undefined, the null string or the number 0 (or its
312 string equivalent, "0"), and TRUE if it is anything else. The
313 Boolean context is just a special kind of scalar context where no
314 conversion to a string or a number is ever performed.
315 X<boolean> X<bool> X<true> X<false> X<truth>
317 There are actually two varieties of null strings (sometimes referred
318 to as "empty" strings), a defined one and an undefined one. The
319 defined version is just a string of length zero, such as C<"">.
320 The undefined version is the value that indicates that there is
321 no real value for something, such as when there was an error, or
322 at end of file, or when you refer to an uninitialized variable or
323 element of an array or hash. Although in early versions of Perl,
324 an undefined scalar could become defined when first used in a
325 place expecting a defined value, this no longer happens except for
326 rare cases of autovivification as explained in L<perlref>. You can
327 use the defined() operator to determine whether a scalar value is
328 defined (this has no meaning on arrays or hashes), and the undef()
329 operator to produce an undefined value.
330 X<defined> X<undefined> X<undef> X<null> X<string, null>
332 To find out whether a given string is a valid non-zero number, it's
333 sometimes enough to test it against both numeric 0 and also lexical
334 "0" (although this will cause noises if warnings are on). That's
335 because strings that aren't numbers count as 0, just as they do in B<awk>:
337 if ($str == 0 && $str ne "0") {
338 warn "That doesn't look like a number";
341 That method may be best because otherwise you won't treat IEEE
342 notations like C<NaN> or C<Infinity> properly. At other times, you
343 might prefer to determine whether string data can be used numerically
344 by calling the POSIX::strtod() function or by inspecting your string
345 with a regular expression (as documented in L<perlre>).
347 warn "has nondigits" if /\D/;
348 warn "not a natural number" unless /^\d+$/; # rejects -3
349 warn "not an integer" unless /^-?\d+$/; # rejects +3
350 warn "not an integer" unless /^[+-]?\d+$/;
351 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
352 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
354 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
356 The length of an array is a scalar value. You may find the length
357 of array @days by evaluating C<$#days>, as in B<csh>. However, this
358 isn't the length of the array; it's the subscript of the last element,
359 which is a different value since there is ordinarily a 0th element.
360 Assigning to C<$#days> actually changes the length of the array.
361 Shortening an array this way destroys intervening values. Lengthening
362 an array that was previously shortened does not recover values
363 that were in those elements.
364 X<$#> X<array, length>
366 You can also gain some minuscule measure of efficiency by pre-extending
367 an array that is going to get big. You can also extend an array
368 by assigning to an element that is off the end of the array. You
369 can truncate an array down to nothing by assigning the null list
370 () to it. The following are equivalent:
375 If you evaluate an array in scalar context, it returns the length
376 of the array. (Note that this is not true of lists, which return
377 the last value, like the C comma operator, nor of built-in functions,
378 which return whatever they feel like returning.) The following is
382 scalar(@whatever) == $#whatever + 1;
384 Some programmers choose to use an explicit conversion so as to
385 leave nothing to doubt:
387 $element_count = scalar(@whatever);
389 If you evaluate a hash in scalar context, it returns false if the
390 hash is empty. If there are any key/value pairs, it returns true;
391 more precisely, the value returned is a string consisting of the
392 number of used buckets and the number of allocated buckets, separated
393 by a slash. This is pretty much useful only to find out whether
394 Perl's internal hashing algorithm is performing poorly on your data
395 set. For example, you stick 10,000 things in a hash, but evaluating
396 %HASH in scalar context reveals C<"1/16">, which means only one out
397 of sixteen buckets has been touched, and presumably contains all
398 10,000 of your items. This isn't supposed to happen. If a tied hash
399 is evaluated in scalar context, the C<SCALAR> method is called (with a
400 fallback to C<FIRSTKEY>).
401 X<hash, scalar context> X<hash, bucket> X<bucket>
403 You can preallocate space for a hash by assigning to the keys() function.
404 This rounds up the allocated buckets to the next power of two:
406 keys(%users) = 1000; # allocate 1024 buckets
408 =head2 Scalar value constructors
409 X<scalar, literal> X<scalar, constant>
411 Numeric literals are specified in any of the following floating point or
416 .23E-10 # a very small number
417 3.14_15_92 # a very important number
418 4_294_967_296 # underscore for legibility
420 0xdead_beef # more hex
421 0377 # octal (only numbers, begins with 0)
423 0x1.999ap-4 # hexadecimal floating point (the 'p' is required)
425 You are allowed to use underscores (underbars) in numeric literals
426 between digits for legibility (but not multiple underscores in a row:
427 C<23__500> is not legal; C<23_500> is).
428 You could, for example, group binary
429 digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
430 or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
433 String literals are usually delimited by either single or double
434 quotes. They work much like quotes in the standard Unix shells:
435 double-quoted string literals are subject to backslash and variable
436 substitution; single-quoted strings are not (except for C<\'> and
437 C<\\>). The usual C-style backslash rules apply for making
438 characters such as newline, tab, etc., as well as some more exotic
439 forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
442 Hexadecimal, octal, or binary, representations in string literals
443 (e.g. '0xff') are not automatically converted to their integer
444 representation. The hex() and oct() functions make these conversions
445 for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
447 Hexadecimal floating point can start just like a hexadecimal literal,
448 and it can be followed by an optional fractional hexadecimal part,
449 but it must be followed by C<p>, an optional sign, and a power of two.
450 The format is useful for accurately presenting floating point values,
451 avoiding conversions to or from decimal floating point, and therefore
452 avoiding possible loss in precision. Notice that while most current
453 platforms use the 64-bit IEEE 754 floating point, not all do. Another
454 potential source of (low-order) differences are the floating point
455 rounding modes, which can differ between CPUs, operating systems,
456 and compilers, and which Perl doesn't control.
458 You can also embed newlines directly in your strings, i.e., they can end
459 on a different line than they begin. This is nice, but if you forget
460 your trailing quote, the error will not be reported until Perl finds
461 another line containing the quote character, which may be much further
462 on in the script. Variable substitution inside strings is limited to
463 scalar variables, arrays, and array or hash slices. (In other words,
464 names beginning with $ or @, followed by an optional bracketed
465 expression as a subscript.) The following code segment prints out "The
469 $Price = '$100'; # not interpolated
470 print "The price is $Price.\n"; # interpolated
472 There is no double interpolation in Perl, so the C<$100> is left as is.
474 By default floating point numbers substituted inside strings use the
475 dot (".") as the decimal separator. If C<use locale> is in effect,
476 and POSIX::setlocale() has been called, the character used for the
477 decimal separator is affected by the LC_NUMERIC locale.
478 See L<perllocale> and L<POSIX>.
480 As in some shells, you can enclose the variable name in braces to
481 disambiguate it from following alphanumerics (and underscores).
483 this when interpolating a variable into a string to separate the
484 variable name from a following double-colon or an apostrophe, since
485 these would be otherwise treated as a package separator:
489 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
490 print "We use ${who}speak when ${who}'s here.\n";
492 Without the braces, Perl would have looked for a $whospeak, a
493 C<$who::0>, and a C<$who's> variable. The last two would be the
494 $0 and the $s variables in the (presumably) non-existent package
497 In fact, a simple identifier within such curlies is forced to be
498 a string, and likewise within a hash subscript. Neither need
499 quoting. Our earlier example, C<$days{'Feb'}> can be written as
500 C<$days{Feb}> and the quotes will be assumed automatically. But
501 anything more complicated in the subscript will be interpreted as an
502 expression. This means for example that C<$version{2.0}++> is
503 equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
505 =head3 Version Strings
506 X<version string> X<vstring> X<v-string>
508 A literal of the form C<v1.20.300.4000> is parsed as a string composed
509 of characters with the specified ordinals. This form, known as
510 v-strings, provides an alternative, more readable way to construct
511 strings, rather than use the somewhat less readable interpolation form
512 C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
513 Unicode strings, and for comparing version "numbers" using the string
514 comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
515 more dots in the literal, the leading C<v> may be omitted.
517 print v9786; # prints SMILEY, "\x{263a}"
518 print v102.111.111; # prints "foo"
519 print 102.111.111; # same
521 Such literals are accepted by both C<require> and C<use> for
522 doing a version check. Note that using the v-strings for IPv4
523 addresses is not portable unless you also use the
524 inet_aton()/inet_ntoa() routines of the Socket package.
526 Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
527 are not v-strings before the C<< => >> operator (which is usually used
528 to separate a hash key from a hash value); instead they are interpreted
529 as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
530 Perl 5.8.0, but that caused more confusion and breakage than good.
531 Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
534 =head3 Special Literals
535 X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
536 X<end> X<data> X<^D> X<^Z>
538 The special literals __FILE__, __LINE__, and __PACKAGE__
539 represent the current filename, line number, and package name at that
540 point in your program. __SUB__ gives a reference to the current
541 subroutine. They may be used only as separate tokens; they
542 will not be interpolated into strings. If there is no current package
543 (due to an empty C<package;> directive), __PACKAGE__ is the undefined
544 value. (But the empty C<package;> is no longer supported, as of version
545 5.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
546 is only available in 5.16 or higher, and only with a C<use v5.16> or
547 C<use feature "current_sub"> declaration.
548 X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
549 X<line> X<file> X<package>
551 The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
552 may be used to indicate the logical end of the script before the actual
553 end of file. Any following text is ignored.
555 Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
556 where C<PACKNAME> is the package that was current when the __DATA__
557 token was encountered. The filehandle is left open pointing to the
558 line after __DATA__. The program should C<close DATA> when it is done
559 reading from it. (Leaving it open leaks filehandles if the module is
560 reloaded for any reason, so it's a safer practice to close it.) For
561 compatibility with older scripts written before __DATA__ was
562 introduced, __END__ behaves like __DATA__ in the top level script (but
563 not in files loaded with C<require> or C<do>) and leaves the remaining
564 contents of the file accessible via C<main::DATA>.
566 See L<SelfLoader> for more description of __DATA__, and
567 an example of its use. Note that you cannot read from the DATA
568 filehandle in a BEGIN block: the BEGIN block is executed as soon
569 as it is seen (during compilation), at which point the corresponding
570 __DATA__ (or __END__) token has not yet been seen.
575 A word that has no other interpretation in the grammar will
576 be treated as if it were a quoted string. These are known as
577 "barewords". As with filehandles and labels, a bareword that consists
578 entirely of lowercase letters risks conflict with future reserved
579 words, and if you use the C<use warnings> pragma or the B<-w> switch,
580 Perl will warn you about any such words. Perl limits barewords (like
581 identifiers) to about 250 characters. Future versions of Perl are likely
582 to eliminate these arbitrary limitations.
584 Some people may wish to outlaw barewords entirely. If you
589 then any bareword that would NOT be interpreted as a subroutine call
590 produces a compile-time error instead. The restriction lasts to the
591 end of the enclosing block. An inner block may countermand this
592 by saying C<no strict 'subs'>.
594 =head3 Array Interpolation
595 X<array, interpolation> X<interpolation, array> X<$">
597 Arrays and slices are interpolated into double-quoted strings
598 by joining the elements with the delimiter specified in the C<$">
599 variable (C<$LIST_SEPARATOR> if "use English;" is specified),
600 space by default. The following are equivalent:
602 $temp = join($", @ARGV);
607 Within search patterns (which also undergo double-quotish substitution)
608 there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
609 C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
610 expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
611 @foo)? If @foo doesn't otherwise exist, then it's obviously a
612 character class. If @foo exists, Perl takes a good guess about C<[bar]>,
613 and is almost always right. If it does guess wrong, or if you're just
614 plain paranoid, you can force the correct interpretation with curly
617 If you're looking for the information on how to use here-documents,
618 which used to be here, that's been moved to
619 L<perlop/Quote and Quote-like Operators>.
621 =head2 List value constructors
624 List values are denoted by separating individual values by commas
625 (and enclosing the list in parentheses where precedence requires it):
629 In a context not requiring a list value, the value of what appears
630 to be a list literal is simply the value of the final element, as
631 with the C comma operator. For example,
633 @foo = ('cc', '-E', $bar);
635 assigns the entire list value to array @foo, but
637 $foo = ('cc', '-E', $bar);
639 assigns the value of variable $bar to the scalar variable $foo.
640 Note that the value of an actual array in scalar context is the
641 length of the array; the following assigns the value 3 to $foo:
643 @foo = ('cc', '-E', $bar);
644 $foo = @foo; # $foo gets 3
646 You may have an optional comma before the closing parenthesis of a
647 list literal, so that you can say:
655 To use a here-document to assign an array, one line per element,
656 you might use an approach like this:
658 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
666 LISTs do automatic interpolation of sublists. That is, when a LIST is
667 evaluated, each element of the list is evaluated in list context, and
668 the resulting list value is interpolated into LIST just as if each
669 individual element were a member of LIST. Thus arrays and hashes lose their
670 identity in a LIST--the list
672 (@foo,@bar,&SomeSub,%glarch)
674 contains all the elements of @foo followed by all the elements of @bar,
675 followed by all the elements returned by the subroutine named SomeSub
676 called in list context, followed by the key/value pairs of %glarch.
677 To make a list reference that does I<NOT> interpolate, see L<perlref>.
679 The null list is represented by (). Interpolating it in a list
680 has no effect. Thus ((),(),()) is equivalent to (). Similarly,
681 interpolating an array with no elements is the same as if no
682 array had been interpolated at that point.
684 This interpolation combines with the facts that the opening
685 and closing parentheses are optional (except when necessary for
686 precedence) and lists may end with an optional comma to mean that
687 multiple commas within lists are legal syntax. The list C<1,,3> is a
688 concatenation of two lists, C<1,> and C<3>, the first of which ends
689 with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
690 similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
691 we'd advise you to use this obfuscation.
693 A list value may also be subscripted like a normal array. You must
694 put the list in parentheses to avoid ambiguity. For example:
696 # Stat returns list value.
697 $time = (stat($file))[8];
700 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
703 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
705 # A "reverse comma operator".
706 return (pop(@foo),pop(@foo))[0];
708 Lists may be assigned to only when each element of the list
709 is itself legal to assign to:
711 ($a, $b, $c) = (1, 2, 3);
713 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
715 An exception to this is that you may assign to C<undef> in a list.
716 This is useful for throwing away some of the return values of a
719 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
721 As of Perl 5.22, you can also use C<(undef)x2> instead of C<undef, undef>.
722 (You can also do C<($x) x 2>, which is less useful, because it assigns to
723 the same variable twice, clobbering the first value assigned.)
725 List assignment in scalar context returns the number of elements
726 produced by the expression on the right side of the assignment:
728 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
729 $x = (($foo,$bar) = f()); # set $x to f()'s return count
731 This is handy when you want to do a list assignment in a Boolean
732 context, because most list functions return a null list when finished,
733 which when assigned produces a 0, which is interpreted as FALSE.
735 It's also the source of a useful idiom for executing a function or
736 performing an operation in list context and then counting the number of
737 return values, by assigning to an empty list and then using that
738 assignment in scalar context. For example, this code:
740 $count = () = $string =~ /\d+/g;
742 will place into $count the number of digit groups found in $string.
743 This happens because the pattern match is in list context (since it
744 is being assigned to the empty list), and will therefore return a list
745 of all matching parts of the string. The list assignment in scalar
746 context will translate that into the number of elements (here, the
747 number of times the pattern matched) and assign that to $count. Note
750 $count = $string =~ /\d+/g;
752 would not have worked, since a pattern match in scalar context will
753 only return true or false, rather than a count of matches.
755 The final element of a list assignment may be an array or a hash:
757 ($a, $b, @rest) = split;
758 my($a, $b, %rest) = @_;
760 You can actually put an array or hash anywhere in the list, but the first one
761 in the list will soak up all the values, and anything after it will become
762 undefined. This may be useful in a my() or local().
764 A hash can be initialized using a literal list holding pairs of
765 items to be interpreted as a key and a value:
767 # same as map assignment above
768 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
770 While literal lists and named arrays are often interchangeable, that's
771 not the case for hashes. Just because you can subscript a list value like
772 a normal array does not mean that you can subscript a list value as a
773 hash. Likewise, hashes included as parts of other lists (including
774 parameters lists and return lists from functions) always flatten out into
775 key/value pairs. That's why it's good to use references sometimes.
777 It is often more readable to use the C<< => >> operator between key/value
778 pairs. The C<< => >> operator is mostly just a more visually distinctive
779 synonym for a comma, but it also arranges for its left-hand operand to be
780 interpreted as a string if it's a bareword that would be a legal simple
781 identifier. C<< => >> doesn't quote compound identifiers, that contain
782 double colons. This makes it nice for initializing hashes:
790 or for initializing hash references to be used as records:
793 witch => 'Mable the Merciless',
794 cat => 'Fluffy the Ferocious',
795 date => '10/31/1776',
798 or for using call-by-named-parameter to complicated functions:
800 $field = $query->radio_group(
801 name => 'group_name',
802 values => ['eenie','meenie','minie'],
808 Note that just because a hash is initialized in that order doesn't
809 mean that it comes out in that order. See L<perlfunc/sort> for examples
810 of how to arrange for an output ordering.
812 If a key appears more than once in the initializer list of a hash, the last
819 color => [0xDF, 0xFF, 0x00],
826 color => [0xDF, 0xFF, 0x00],
830 This can be used to provide overridable configuration defaults:
832 # values in %args take priority over %config_defaults
833 %config = (%config_defaults, %args);
837 An array can be accessed one scalar at a
838 time by specifying a dollar sign (C<$>), then the
839 name of the array (without the leading C<@>), then the subscript inside
840 square brackets. For example:
842 @myarray = (5, 50, 500, 5000);
843 print "The Third Element is", $myarray[2], "\n";
845 The array indices start with 0. A negative subscript retrieves its
846 value from the end. In our example, C<$myarray[-1]> would have been
847 5000, and C<$myarray[-2]> would have been 500.
849 Hash subscripts are similar, only instead of square brackets curly brackets
850 are used. For example:
855 "Einstein" => "Albert",
856 "Darwin" => "Charles",
857 "Feynman" => "Richard",
860 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
862 You can also subscript a list to get a single element from it:
864 $dir = (getpwnam("daemon"))[7];
866 =head2 Multi-dimensional array emulation
868 Multidimensional arrays may be emulated by subscripting a hash with a
869 list. The elements of the list are joined with the subscript separator
876 $foo{join($;, $a, $b, $c)}
878 The default subscript separator is "\034", the same as SUBSEP in B<awk>.
881 X<slice> X<array, slice> X<hash, slice>
883 A slice accesses several elements of a list, an array, or a hash
884 simultaneously using a list of subscripts. It's more convenient
885 than writing out the individual elements as a list of separate
888 ($him, $her) = @folks[0,-1]; # array slice
889 @them = @folks[0 .. 3]; # array slice
890 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
891 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
893 Since you can assign to a list of variables, you can also assign to
894 an array or hash slice.
896 @days[3..5] = qw/Wed Thu Fri/;
897 @colors{'red','blue','green'}
898 = (0xff0000, 0x0000ff, 0x00ff00);
899 @folks[0, -1] = @folks[-1, 0];
901 The previous assignments are exactly equivalent to
903 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
904 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
905 = (0xff0000, 0x0000ff, 0x00ff00);
906 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
908 Since changing a slice changes the original array or hash that it's
909 slicing, a C<foreach> construct will alter some--or even all--of the
910 values of the array or hash.
912 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
914 foreach (@hash{qw[key1 key2]}) {
915 s/^\s+//; # trim leading whitespace
916 s/\s+$//; # trim trailing whitespace
917 s/(\w+)/\u\L$1/g; # "titlecase" words
920 As a special exception, when you slice a list (but not an array or a hash),
921 if the list evaluates to empty, then taking a slice of that empty list will
922 always yield the empty list in turn. Thus:
924 @a = ()[0,1]; # @a has no elements
925 @b = (@a)[0,1]; # @b has no elements
926 @c = (sub{}->())[0,1]; # @c has no elements
927 @d = ('a','b')[0,1]; # @d has two elements
928 @e = (@d)[0,1,8,9]; # @e has four elements
929 @f = (@d)[8,9]; # @f has two elements
931 This makes it easy to write loops that terminate when a null list
934 while ( ($home, $user) = (getpwent)[7,0] ) {
935 printf "%-8s %s\n", $user, $home;
938 As noted earlier in this document, the scalar sense of list assignment
939 is the number of elements on the right-hand side of the assignment.
940 The null list contains no elements, so when the password file is
941 exhausted, the result is 0, not 2.
943 Slices in scalar context return the last item of the slice.
945 @a = qw/first second third/;
946 %h = (first => 'A', second => 'B');
947 $t = @a[0, 1]; # $t is now 'second'
948 $u = @h{'first', 'second'}; # $u is now 'B'
950 If you're confused about why you use an '@' there on a hash slice
951 instead of a '%', think of it like this. The type of bracket (square
952 or curly) governs whether it's an array or a hash being looked at.
953 On the other hand, the leading symbol ('$' or '@') on the array or
954 hash indicates whether you are getting back a singular value (a
955 scalar) or a plural one (a list).
957 =head3 Key/Value Hash Slices
959 Starting in Perl 5.20, a hash slice operation
960 with the % symbol is a variant of slice operation
961 returning a list of key/value pairs rather than just values:
963 %h = (blonk => 2, foo => 3, squink => 5, bar => 8);
964 %subset = %h{'foo', 'bar'}; # key/value hash slice
965 # %subset is now (foo => 3, bar => 8)
967 However, the result of such a slice cannot be localized, deleted or used
968 in assignment. These are otherwise very much consistent with hash slices
971 =head3 Index/Value Array Slices
973 Similar to key/value hash slices (and also introduced
974 in Perl 5.20), the % array slice syntax returns a list
975 of index/value pairs:
979 # @list is now (3, "d", 4, "e", 6, "g")
981 =head2 Typeglobs and Filehandles
982 X<typeglob> X<filehandle> X<*>
984 Perl uses an internal type called a I<typeglob> to hold an entire
985 symbol table entry. The type prefix of a typeglob is a C<*>, because
986 it represents all types. This used to be the preferred way to
987 pass arrays and hashes by reference into a function, but now that
988 we have real references, this is seldom needed.
990 The main use of typeglobs in modern Perl is create symbol table aliases.
995 makes $this an alias for $that, @this an alias for @that, %this an alias
996 for %that, &this an alias for &that, etc. Much safer is to use a reference.
999 local *Here::blue = \$There::green;
1001 temporarily makes $Here::blue an alias for $There::green, but doesn't
1002 make @Here::blue an alias for @There::green, or %Here::blue an alias for
1003 %There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
1004 of this. Strange though this may seem, this is the basis for the whole
1005 module import/export system.
1007 Another use for typeglobs is to pass filehandles into a function or
1008 to create new filehandles. If you need to use a typeglob to save away
1009 a filehandle, do it this way:
1013 or perhaps as a real reference, like this:
1017 See L<perlsub> for examples of using these as indirect filehandles
1020 Typeglobs are also a way to create a local filehandle using the local()
1021 operator. These last until their block is exited, but may be passed back.
1026 local *FH; # not my!
1027 open (FH, $path) or return undef;
1030 $fh = newopen('/etc/passwd');
1032 Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
1033 for filehandle manipulations, although they're still needed to pass brand
1034 new file and directory handles into or out of functions. That's because
1035 C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
1036 In other words, C<*FH> must be used to create new symbol table entries;
1037 C<*foo{THING}> cannot. When in doubt, use C<*FH>.
1039 All functions that are capable of creating filehandles (open(),
1040 opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
1041 automatically create an anonymous filehandle if the handle passed to
1042 them is an uninitialized scalar variable. This allows the constructs
1043 such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
1044 create filehandles that will conveniently be closed automatically when
1045 the scope ends, provided there are no other references to them. This
1046 largely eliminates the need for typeglobs when opening filehandles
1047 that must be passed around, as in the following example:
1051 or die "Can't open '@_': $!";
1056 my $f = myopen("</etc/motd");
1058 # $f implicitly closed here
1061 Note that if an initialized scalar variable is used instead the
1062 result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
1063 to C<open( *{'zzz'}, ...)>.
1064 C<use strict 'refs'> forbids such practice.
1066 Another way to create anonymous filehandles is with the Symbol
1067 module or with the IO::Handle module and its ilk. These modules
1068 have the advantage of not hiding different types of the same name
1069 during the local(). See the bottom of L<perlfunc/open> for an
1074 See L<perlvar> for a description of Perl's built-in variables and
1075 a discussion of legal variable names. See L<perlref>, L<perlsub>,
1076 and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
1077 the C<*foo{THING}> syntax.