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1=head1 NAME
2
cb1a09d0 3perldata - Perl data types
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4
5=head1 DESCRIPTION
6
7=head2 Variable names
d74e8afc 8X<variable, name> X<variable name> X<data type> X<type>
a0d0e21e 9
d55a8828 10Perl has three built-in data types: scalars, arrays of scalars, and
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11associative arrays of scalars, known as "hashes". A scalar is a
12single string (of any size, limited only by the available memory),
13number, or a reference to something (which will be discussed
14in L<perlref>). Normal arrays are ordered lists of scalars indexed
15by number, starting with 0. Hashes are unordered collections of scalar
16values indexed by their associated string key.
a0d0e21e 17
d55a8828 18Values are usually referred to by name, or through a named reference.
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19The first character of the name tells you to what sort of data
20structure it refers. The rest of the name tells you the particular
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21value to which it refers. Usually this name is a single I<identifier>,
22that is, a string beginning with a letter or underscore, and
23containing letters, underscores, and digits. In some cases, it may
24be a chain of identifiers, separated by C<::> (or by the slightly
25archaic C<'>); all but the last are interpreted as names of packages,
26to locate the namespace in which to look up the final identifier
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27(see L<perlmod/Packages> for details). For a more in-depth discussion
28on identifiers, see L<Identifier parsing>. It's possible to
29substitute for a simple identifier, an expression that produces a reference
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30to the value at runtime. This is described in more detail below
31and in L<perlref>.
d74e8afc 32X<identifier>
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33
34Perl also has its own built-in variables whose names don't follow
35these rules. They have strange names so they don't accidentally
36collide with one of your normal variables. Strings that match
37parenthesized parts of a regular expression are saved under names
38containing only digits after the C<$> (see L<perlop> and L<perlre>).
39In addition, several special variables that provide windows into
40the inner working of Perl have names containing punctuation characters
41and control characters. These are documented in L<perlvar>.
d74e8afc 42X<variable, built-in>
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43
44Scalar values are always named with '$', even when referring to a
45scalar that is part of an array or a hash. The '$' symbol works
46semantically like the English word "the" in that it indicates a
47single value is expected.
d74e8afc 48X<scalar>
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49
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 $#days # the last index of array @days
54
d55a8828 55Entire arrays (and slices of arrays and hashes) are denoted by '@',
3921068c 56which works much as the word "these" or "those" does in English,
d55a8828 57in that it indicates multiple values are expected.
d74e8afc 58X<array>
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59
60 @days # ($days[0], $days[1],... $days[n])
d55a8828 61 @days[3,4,5] # same as ($days[3],$days[4],$days[5])
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62 @days{'a','c'} # same as ($days{'a'},$days{'c'})
63
d55a8828 64Entire hashes are denoted by '%':
d74e8afc 65X<hash>
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66
67 %days # (key1, val1, key2, val2 ...)
68
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69In addition, subroutines are named with an initial '&', though this
70is optional when unambiguous, just as the word "do" is often redundant
71in English. Symbol table entries can be named with an initial '*',
72but you don't really care about that yet (if ever :-).
73
74Every variable type has its own namespace, as do several
75non-variable identifiers. This means that you can, without fear
76of conflict, use the same name for a scalar variable, an array, or
77a hash--or, for that matter, for a filehandle, a directory handle, a
78subroutine name, a format name, or a label. This means that $foo
79and @foo are two different variables. It also means that C<$foo[1]>
80is a part of @foo, not a part of $foo. This may seem a bit weird,
81but that's okay, because it is weird.
d74e8afc 82X<namespace>
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83
84Because variable references always start with '$', '@', or '%', the
85"reserved" words aren't in fact reserved with respect to variable
86names. They I<are> reserved with respect to labels and filehandles,
87however, which don't have an initial special character. You can't
88have a filehandle named "log", for instance. Hint: you could say
89C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using
90uppercase filehandles also improves readability and protects you
91from conflict with future reserved words. Case I<is> significant--"FOO",
92"Foo", and "foo" are all different names. Names that start with a
93letter or underscore may also contain digits and underscores.
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94X<identifier, case sensitivity>
95X<case>
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96
97It is possible to replace such an alphanumeric name with an expression
d55a8828 98that returns a reference to the appropriate type. For a description
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99of this, see L<perlref>.
100
5f05dabc 101Names that start with a digit may contain only more digits. Names
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102that do not start with a letter, underscore, digit or a caret (i.e.
103a control character) are limited to one character, e.g., C<$%> or
104C<$$>. (Most of these one character names have a predefined
105significance to Perl. For instance, C<$$> is the current process
106id.)
a0d0e21e 107
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108=head2 Identifier parsing
109X<identifiers>
110
111Up until Perl 5.18, the actual rules of what a valid identifier
112was were a bit fuzzy. However, in general, anything defined here should
113work on previous versions of Perl, while the opposite -- edge cases
114that work in previous versions, but aren't defined here -- probably
115won't work on newer versions.
116As an important side note, please note that the following only applies
117to bareword identifiers as found in Perl source code, not identifiers
118introduced through symbolic references, which have much fewer
119restrictions.
120If working under the effect of the C<use utf8;> pragma, the following
121rules apply:
122
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123 / (?[ ( \p{Word} & \p{XID_Start} ) + [_] ])
124 (?[ ( \p{Word} & \p{XID_Continue} ) ]) * /x
125
126That is, a "start" character followed by any number of "continue"
127characters. Perl requires every character in an identifier to also
128match C<\w> (this prevents some problematic cases); and Perl
129additionally accepts identfier names beginning with an underscore.
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130
131If not under C<use utf8>, the source is treated as ASCII + 128 extra
132controls, and identifiers should match
133
134 / (?aa) (?!\d) \w+ /x
135
136That is, any word character in the ASCII range, as long as the first
137character is not a digit.
138
139There are two package separators in Perl: A double colon (C<::>) and a single
140quote (C<'>). Normal identifiers can start or end with a double colon, and
141can contain several parts delimited by double colons.
142Single quotes have similar rules, but with the exception that they are not
143legal at the end of an identifier: That is, C<$'foo> and C<$foo'bar> are
1d268002 144legal, but C<$foo'bar'> is not.
32833930 145
1d268002 146Additionally, if the identifier is preceded by a sigil --
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147that is, if the identifier is part of a variable name -- it
148may optionally be enclosed in braces.
149
150While you can mix double colons with singles quotes, the quotes must come
151after the colons: C<$::::'foo> and C<$foo::'bar> are legal, but C<$::'::foo>
152and C<$foo'::bar> are not.
153
154Put together, a grammar to match a basic identifier becomes
155
156 /
157 (?(DEFINE)
158 (?<variable>
159 (?&sigil)
160 (?:
161 (?&normal_identifier)
162 | \{ \s* (?&normal_identifier) \s* \}
163 )
164 )
165 (?<normal_identifier>
166 (?: :: )* '?
167 (?&basic_identifier)
168 (?: (?= (?: :: )+ '? | (?: :: )* ' ) (?&normal_identifier) )?
169 (?: :: )*
170 )
171 (?<basic_identifier>
172 # is use utf8 on?
173 (?(?{ (caller(0))[8] & $utf8::hint_bits })
4c106081 174 (?&Perl_XIDS) (?&Perl_XIDC)*
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175 | (?aa) (?!\d) \w+
176 )
177 )
178 (?<sigil> [&*\$\@\%])
179 (?<Perl_XIDS> (?[ ( \p{Word} & \p{XID_Start} ) + [_] ]) )
4c106081 180 (?<Perl_XIDC> (?[ \p{Word} & \p{XID_Continue} ]) )
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181 )
182 /x
183
184Meanwhile, special identifiers don't follow the above rules; For the most
185part, all of the identifiers in this category have a special meaning given
186by Perl. Because they have special parsing rules, these generally can't be
187fully-qualified. They come in four forms:
188
189=over
190
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191=item *
192
193A sigil, followed solely by digits matching C<\p{POSIX_Digit}>, like
194C<$0>, C<$1>, or C<$10000>.
195
196=item *
197
198A sigil, followed by either a caret and a single POSIX uppercase letter,
199like C<$^V> or C<$^W>, or a sigil followed by a literal non-space,
200non-C<NUL> control character matching the C<\p{POSIX_Cntrl}> property.
201Due to a historical oddity, if not running under C<use utf8>, the 128
202characters in the C<[0x80-0xff]> range are considered to be controls,
203and may also be used in length-one variables. However, the use of
204non-graphical characters is deprecated as of v5.22, and support for them
205will be removed in a future version of perl. ASCII space characters and
206C<NUL> already aren't allowed, so this means that a single-character
207variable name with that name being any other C0 control C<[0x01-0x1F]>,
208or C<DEL> will generate a deprecated warning. Already, under C<"use
209utf8">, non-ASCII characters must match C<Perl_XIDS>. As of v5.22, when
210not under C<"use utf8"> C1 controls C<[0x80-0x9F]>, NO BREAK SPACE, and
211SOFT HYPHEN (C<SHY>)) generate a deprecated warning.
212
213=item *
214
215Similar to the above, a sigil, followed by bareword text in brackets,
216where the first character is either a caret followed by an uppercase
217letter, like C<${^GLOBAL_PHASE}> or a non-C<NUL>, non-space literal
218control like C<${\7LOBAL_PHASE}>. Like the above, when not under
219C<"use utf8">, the characters in C<[0x80-0xFF]> are considered controls, but as
220of v5.22, the use of any that are non-graphical are deprecated, and as
221of v5.20 the use of any ASCII-range literal control is deprecated.
222Support for these will be removed in a future version of perl.
223
224=item *
225
226A sigil followed by a single character matching the C<\p{POSIX_Punct}>
227property, like C<$!> or C<%+>, except the character C<"{"> doesn't work.
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228
229=back
230
b29f65fc 231Note that as of Perl 5.20, literal control characters in variable names
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232are deprecated; and as of Perl 5.22, any other non-graphic characters
233are also deprecated.
b29f65fc 234
a0d0e21e 235=head2 Context
d74e8afc 236X<context> X<scalar context> X<list context>
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237
238The interpretation of operations and values in Perl sometimes depends
239on the requirements of the context around the operation or value.
d55a8828 240There are two major contexts: list and scalar. Certain operations
a0d0e21e 241return list values in contexts wanting a list, and scalar values
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242otherwise. If this is true of an operation it will be mentioned in
243the documentation for that operation. In other words, Perl overloads
a0d0e21e 244certain operations based on whether the expected return value is
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245singular or plural. Some words in English work this way, like "fish"
246and "sheep".
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247
248In a reciprocal fashion, an operation provides either a scalar or a
249list context to each of its arguments. For example, if you say
250
251 int( <STDIN> )
252
c47ff5f1 253the integer operation provides scalar context for the <>
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254operator, which responds by reading one line from STDIN and passing it
255back to the integer operation, which will then find the integer value
256of that line and return that. If, on the other hand, you say
257
258 sort( <STDIN> )
259
c47ff5f1 260then the sort operation provides list context for <>, which
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261will proceed to read every line available up to the end of file, and
262pass that list of lines back to the sort routine, which will then
263sort those lines and return them as a list to whatever the context
264of the sort was.
265
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266Assignment is a little bit special in that it uses its left argument
267to determine the context for the right argument. Assignment to a
268scalar evaluates the right-hand side in scalar context, while
269assignment to an array or hash evaluates the righthand side in list
270context. Assignment to a list (or slice, which is just a list
3921068c 271anyway) also evaluates the right-hand side in list context.
d55a8828 272
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273When you use the C<use warnings> pragma or Perl's B<-w> command-line
274option, you may see warnings
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275about useless uses of constants or functions in "void context".
276Void context just means the value has been discarded, such as a
277statement containing only C<"fred";> or C<getpwuid(0);>. It still
278counts as scalar context for functions that care whether or not
279they're being called in list context.
280
281User-defined subroutines may choose to care whether they are being
282called in a void, scalar, or list context. Most subroutines do not
283need to bother, though. That's because both scalars and lists are
284automatically interpolated into lists. See L<perlfunc/wantarray>
285for how you would dynamically discern your function's calling
286context.
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287
288=head2 Scalar values
d74e8afc 289X<scalar> X<number> X<string> X<reference>
a0d0e21e 290
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291All data in Perl is a scalar, an array of scalars, or a hash of
292scalars. A scalar may contain one single value in any of three
293different flavors: a number, a string, or a reference. In general,
294conversion from one form to another is transparent. Although a
295scalar may not directly hold multiple values, it may contain a
296reference to an array or hash which in turn contains multiple values.
297
298Scalars aren't necessarily one thing or another. There's no place
299to declare a scalar variable to be of type "string", type "number",
300type "reference", or anything else. Because of the automatic
301conversion of scalars, operations that return scalars don't need
302to care (and in fact, cannot care) whether their caller is looking
303for a string, a number, or a reference. Perl is a contextually
304polymorphic language whose scalars can be strings, numbers, or
305references (which includes objects). Although strings and numbers
306are considered pretty much the same thing for nearly all purposes,
307references are strongly-typed, uncastable pointers with builtin
308reference-counting and destructor invocation.
a0d0e21e 309
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310A scalar value is interpreted as FALSE in the Boolean sense
311if it is undefined, the null string or the number 0 (or its
312string equivalent, "0"), and TRUE if it is anything else. The
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313Boolean context is just a special kind of scalar context where no
314conversion to a string or a number is ever performed.
d74e8afc 315X<boolean> X<bool> X<true> X<false> X<truth>
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316
317There are actually two varieties of null strings (sometimes referred
318to as "empty" strings), a defined one and an undefined one. The
319defined version is just a string of length zero, such as C<"">.
320The undefined version is the value that indicates that there is
321no real value for something, such as when there was an error, or
322at end of file, or when you refer to an uninitialized variable or
323element of an array or hash. Although in early versions of Perl,
324an undefined scalar could become defined when first used in a
325place expecting a defined value, this no longer happens except for
326rare cases of autovivification as explained in L<perlref>. You can
327use the defined() operator to determine whether a scalar value is
328defined (this has no meaning on arrays or hashes), and the undef()
329operator to produce an undefined value.
d74e8afc 330X<defined> X<undefined> X<undef> X<null> X<string, null>
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331
332To find out whether a given string is a valid non-zero number, it's
333sometimes enough to test it against both numeric 0 and also lexical
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334"0" (although this will cause noises if warnings are on). That's
335because strings that aren't numbers count as 0, just as they do in B<awk>:
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336
337 if ($str == 0 && $str ne "0") {
338 warn "That doesn't look like a number";
54310121 339 }
4633a7c4 340
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341That method may be best because otherwise you won't treat IEEE
342notations like C<NaN> or C<Infinity> properly. At other times, you
343might prefer to determine whether string data can be used numerically
344by calling the POSIX::strtod() function or by inspecting your string
345with a regular expression (as documented in L<perlre>).
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346
347 warn "has nondigits" if /\D/;
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348 warn "not a natural number" unless /^\d+$/; # 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*$/; # rejects .2
352 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
54310121 353 warn "not a C float"
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354 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
355
d55a8828 356The length of an array is a scalar value. You may find the length
fc518ee5 357of array @days by evaluating C<$#days>, as in B<csh>. However, this
358isn't the length of the array; it's the subscript of the last element,
359which is a different value since there is ordinarily a 0th element.
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360Assigning to C<$#days> actually changes the length of the array.
361Shortening an array this way destroys intervening values. Lengthening
362an array that was previously shortened does not recover values
0568eccd 363that were in those elements.
d74e8afc 364X<$#> X<array, length>
d55a8828 365
210b36aa 366You can also gain some minuscule measure of efficiency by pre-extending
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367an array that is going to get big. You can also extend an array
368by assigning to an element that is off the end of the array. You
19799a22 369can truncate an array down to nothing by assigning the null list
d55a8828 370() to it. The following are equivalent:
a0d0e21e 371
84f709e7 372 @whatever = ();
3e3baf6d 373 $#whatever = -1;
a0d0e21e 374
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375If you evaluate an array in scalar context, it returns the length
376of the array. (Note that this is not true of lists, which return
377the last value, like the C comma operator, nor of built-in functions,
378which return whatever they feel like returning.) The following is
379always true:
d74e8afc 380X<array, length>
a0d0e21e 381
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382 scalar(@whatever) == $#whatever + 1;
383
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384Some programmers choose to use an explicit conversion so as to
385leave nothing to doubt:
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386
387 $element_count = scalar(@whatever);
388
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389If you evaluate a hash in scalar context, it returns false if the
390hash is empty. If there are any key/value pairs, it returns true;
391more precisely, the value returned is a string consisting of the
392number of used buckets and the number of allocated buckets, separated
393by a slash. This is pretty much useful only to find out whether
394Perl's internal hashing algorithm is performing poorly on your data
395set. 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
397of sixteen buckets has been touched, and presumably contains all
126c71c8 39810,000 of your items. This isn't supposed to happen. If a tied hash
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399is evaluated in scalar context, the C<SCALAR> method is called (with a
400fallback to C<FIRSTKEY>).
d74e8afc 401X<hash, scalar context> X<hash, bucket> X<bucket>
a0d0e21e 402
5a964f20 403You can preallocate space for a hash by assigning to the keys() function.
65841adf 404This rounds up the allocated buckets to the next power of two:
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405
406 keys(%users) = 1000; # allocate 1024 buckets
407
a0d0e21e 408=head2 Scalar value constructors
d74e8afc 409X<scalar, literal> X<scalar, constant>
a0d0e21e 410
d55a8828 411Numeric literals are specified in any of the following floating point or
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412integer formats:
413
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414 12345
415 12345.67
416 .23E-10 # a very small number
417 3.14_15_92 # a very important number
418 4_294_967_296 # underscore for legibility
419 0xff # hex
420 0xdead_beef # more hex
421 0377 # octal (only numbers, begins with 0)
422 0b011011 # binary
423 0x1.999ap-4 # hexadecimal floating point (the 'p' is required)
a0d0e21e 424
d4ced10d 425You are allowed to use underscores (underbars) in numeric literals
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426between digits for legibility (but not multiple underscores in a row:
427C<23__500> is not legal; C<23_500> is).
428You could, for example, group binary
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429digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
430or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
d74e8afc 431X<number, literal>
1d277562 432
55497cff 433String literals are usually delimited by either single or double
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434quotes. They work much like quotes in the standard Unix shells:
435double-quoted string literals are subject to backslash and variable
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436substitution; single-quoted strings are not (except for C<\'> and
437C<\\>). The usual C-style backslash rules apply for making
d55a8828 438characters such as newline, tab, etc., as well as some more exotic
4a4eefd0 439forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
d74e8afc 440X<string, literal>
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441
442Hexadecimal, octal, or binary, representations in string literals
443(e.g. '0xff') are not automatically converted to their integer
444representation. The hex() and oct() functions make these conversions
445for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
68dc0745 446
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447Hexadecimal floating point can start just like a hexadecimal literal,
448and it can be followed by an optional fractional hexadecimal part,
449but it must be followed by C<p>, an optional sign, and a power of two.
450The format is useful for accurately presenting floating point values,
451avoiding conversions to or from decimal floating point, and therefore
452avoiding possible loss in precision. Notice that while most current
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453platforms use the 64-bit IEEE 754 floating point, not all do. Another
454potential source of (low-order) differences are the floating point
455rounding modes, which can differ between CPUs, operating systems,
456and compilers, and which Perl doesn't control.
61e61fbc 457
5f05dabc 458You can also embed newlines directly in your strings, i.e., they can end
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459on a different line than they begin. This is nice, but if you forget
460your trailing quote, the error will not be reported until Perl finds
461another line containing the quote character, which may be much further
462on in the script. Variable substitution inside strings is limited to
d55a8828 463scalar variables, arrays, and array or hash slices. (In other words,
b88cefa9 464names beginning with $ or @, followed by an optional bracketed
a0d0e21e 465expression as a subscript.) The following code segment prints out "The
184e9718 466price is $Z<>100."
d74e8afc 467X<interpolation>
a0d0e21e 468
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469 $Price = '$100'; # not interpolated
470 print "The price is $Price.\n"; # interpolated
471
472There is no double interpolation in Perl, so the C<$100> is left as is.
a0d0e21e 473
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474By default floating point numbers substituted inside strings use the
475dot (".") as the decimal separator. If C<use locale> is in effect,
476and POSIX::setlocale() has been called, the character used for the
477decimal separator is affected by the LC_NUMERIC locale.
478See L<perllocale> and L<POSIX>.
479
d55a8828 480As in some shells, you can enclose the variable name in braces to
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481disambiguate it from following alphanumerics (and underscores).
482You must also do
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483this when interpolating a variable into a string to separate the
484variable name from a following double-colon or an apostrophe, since
485these would be otherwise treated as a package separator:
d74e8afc 486X<interpolation>
d55a8828 487
84f709e7 488 $who = "Larry";
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489 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
490 print "We use ${who}speak when ${who}'s here.\n";
491
492Without the braces, Perl would have looked for a $whospeak, a
493C<$who::0>, and a C<$who's> variable. The last two would be the
494$0 and the $s variables in the (presumably) non-existent package
495C<who>.
496
34a2706e 497In fact, a simple identifier within such curlies is forced to be
b4e2e1dd 498a string, and likewise within a hash subscript. Neither need
d55a8828
TC
499quoting. Our earlier example, C<$days{'Feb'}> can be written as
500C<$days{Feb}> and the quotes will be assumed automatically. But
719b43e8
RGS
501anything more complicated in the subscript will be interpreted as an
502expression. This means for example that C<$version{2.0}++> is
503equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
d55a8828 504
692ef166 505=head3 Version Strings
d74e8afc 506X<version string> X<vstring> X<v-string>
692ef166 507
191d61a7 508A literal of the form C<v1.20.300.4000> is parsed as a string composed
6b2463a0
JH
509of characters with the specified ordinals. This form, known as
510v-strings, provides an alternative, more readable way to construct
511strings, rather than use the somewhat less readable interpolation form
512C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
513Unicode strings, and for comparing version "numbers" using the string
514comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
515more dots in the literal, the leading C<v> may be omitted.
b9c62f5b 516
2575c402 517 print v9786; # prints SMILEY, "\x{263a}"
b9c62f5b
GS
518 print v102.111.111; # prints "foo"
519 print 102.111.111; # same
520
521Such literals are accepted by both C<require> and C<use> for
a32521b7
JD
522doing a version check. Note that using the v-strings for IPv4
523addresses is not portable unless you also use the
524inet_aton()/inet_ntoa() routines of the Socket package.
191d61a7 525
d32a65d2 526Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
8fa72689 527are not v-strings before the C<< => >> operator (which is usually used
3921068c 528to separate a hash key from a hash value); instead they are interpreted
15ecd4ae
JH
529as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
530Perl 5.8.0, but that caused more confusion and breakage than good.
531Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
532be v-strings always.
d32a65d2 533
692ef166 534=head3 Special Literals
d74e8afc
ITB
535X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
536X<end> X<data> X<^D> X<^Z>
692ef166 537
d55a8828 538The special literals __FILE__, __LINE__, and __PACKAGE__
68dc0745 539represent the current filename, line number, and package name at that
84ed0108
FC
540point in your program. __SUB__ gives a reference to the current
541subroutine. They may be used only as separate tokens; they
68dc0745 542will not be interpolated into strings. If there is no current package
3e92a254 543(due to an empty C<package;> directive), __PACKAGE__ is the undefined
8fdd8881 544value. (But the empty C<package;> is no longer supported, as of version
84ed0108
FC
5455.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
546is only available in 5.16 or higher, and only with a C<use v5.16> or
547C<use feature "current_sub"> declaration.
548X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
549X<line> X<file> X<package>
3e92a254
GS
550
551The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
552may be used to indicate the logical end of the script before the actual
553end of file. Any following text is ignored.
554
1bab44f9 555Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
3e92a254
GS
556where C<PACKNAME> is the package that was current when the __DATA__
557token was encountered. The filehandle is left open pointing to the
4d383607 558line after __DATA__. The program should C<close DATA> when it is done
9c205800
FC
559reading from it. (Leaving it open leaks filehandles if the module is
560reloaded for any reason, so it's a safer practice to close it.) For
4d383607
JK
561compatibility with older scripts written before __DATA__ was
562introduced, __END__ behaves like __DATA__ in the top level script (but
563not in files loaded with C<require> or C<do>) and leaves the remaining
564contents of the file accessible via C<main::DATA>.
3e92a254
GS
565
566See L<SelfLoader> for more description of __DATA__, and
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TC
567an example of its use. Note that you cannot read from the DATA
568filehandle in a BEGIN block: the BEGIN block is executed as soon
569as it is seen (during compilation), at which point the corresponding
a00c1fe5 570__DATA__ (or __END__) token has not yet been seen.
a0d0e21e 571
692ef166 572=head3 Barewords
d74e8afc 573X<bareword>
692ef166 574
748a9306 575A word that has no other interpretation in the grammar will
a0d0e21e
LW
576be treated as if it were a quoted string. These are known as
577"barewords". As with filehandles and labels, a bareword that consists
578entirely of lowercase letters risks conflict with future reserved
9f1b1f2d 579words, and if you use the C<use warnings> pragma or the B<-w> switch,
05b4f1ec
FW
580Perl will warn you about any such words. Perl limits barewords (like
581identifiers) to about 250 characters. Future versions of Perl are likely
582to eliminate these arbitrary limitations.
583
584Some people may wish to outlaw barewords entirely. If you
a0d0e21e
LW
585say
586
587 use strict 'subs';
588
589then any bareword that would NOT be interpreted as a subroutine call
590produces a compile-time error instead. The restriction lasts to the
54310121 591end of the enclosing block. An inner block may countermand this
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592by saying C<no strict 'subs'>.
593
e2b457c0 594=head3 Array Interpolation
d74e8afc 595X<array, interpolation> X<interpolation, array> X<$">
692ef166 596
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597Arrays and slices are interpolated into double-quoted strings
598by joining the elements with the delimiter specified in the C<$">
692ef166
SF
599variable (C<$LIST_SEPARATOR> if "use English;" is specified),
600space by default. The following are equivalent:
a0d0e21e 601
84f709e7 602 $temp = join($", @ARGV);
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LW
603 system "echo $temp";
604
605 system "echo @ARGV";
606
607Within search patterns (which also undergo double-quotish substitution)
d55a8828 608there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
a0d0e21e
LW
609C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
610expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
611@foo)? If @foo doesn't otherwise exist, then it's obviously a
612character class. If @foo exists, Perl takes a good guess about C<[bar]>,
613and is almost always right. If it does guess wrong, or if you're just
614plain paranoid, you can force the correct interpretation with curly
d55a8828 615braces as above.
a0d0e21e 616
7e3b091d 617If you're looking for the information on how to use here-documents,
210b36aa
AMS
618which used to be here, that's been moved to
619L<perlop/Quote and Quote-like Operators>.
be16fac9 620
a0d0e21e 621=head2 List value constructors
d74e8afc 622X<list>
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LW
623
624List values are denoted by separating individual values by commas
625(and enclosing the list in parentheses where precedence requires it):
626
627 (LIST)
628
d55a8828
TC
629In a context not requiring a list value, the value of what appears
630to be a list literal is simply the value of the final element, as
631with the C comma operator. For example,
a0d0e21e 632
84f709e7 633 @foo = ('cc', '-E', $bar);
a0d0e21e 634
d55a8828 635assigns the entire list value to array @foo, but
a0d0e21e 636
84f709e7 637 $foo = ('cc', '-E', $bar);
a0d0e21e 638
d55a8828
TC
639assigns the value of variable $bar to the scalar variable $foo.
640Note that the value of an actual array in scalar context is the
641length of the array; the following assigns the value 3 to $foo:
a0d0e21e 642
84f709e7 643 @foo = ('cc', '-E', $bar);
7e3b091d 644 $foo = @foo; # $foo gets 3
a0d0e21e 645
54310121 646You may have an optional comma before the closing parenthesis of a
a0d0e21e
LW
647list literal, so that you can say:
648
84f709e7 649 @foo = (
7e3b091d
DA
650 1,
651 2,
652 3,
a0d0e21e
LW
653 );
654
d55a8828
TC
655To use a here-document to assign an array, one line per element,
656you might use an approach like this:
657
84f709e7 658 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
7e3b091d
DA
659 normal tomato
660 spicy tomato
661 green chile
662 pesto
663 white wine
d55a8828
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664 End_Lines
665
a0d0e21e 666LISTs do automatic interpolation of sublists. That is, when a LIST is
d55a8828 667evaluated, each element of the list is evaluated in list context, and
a0d0e21e 668the resulting list value is interpolated into LIST just as if each
5a964f20 669individual element were a member of LIST. Thus arrays and hashes lose their
a0d0e21e
LW
670identity in a LIST--the list
671
5a964f20 672 (@foo,@bar,&SomeSub,%glarch)
a0d0e21e
LW
673
674contains all the elements of @foo followed by all the elements of @bar,
5a964f20 675followed by all the elements returned by the subroutine named SomeSub
d55a8828 676called in list context, followed by the key/value pairs of %glarch.
a0d0e21e
LW
677To make a list reference that does I<NOT> interpolate, see L<perlref>.
678
19799a22 679The null list is represented by (). Interpolating it in a list
a0d0e21e
LW
680has no effect. Thus ((),(),()) is equivalent to (). Similarly,
681interpolating an array with no elements is the same as if no
682array had been interpolated at that point.
683
c2689353 684This interpolation combines with the facts that the opening
ab1f959b 685and closing parentheses are optional (except when necessary for
c2689353 686precedence) and lists may end with an optional comma to mean that
8fdd8881 687multiple commas within lists are legal syntax. The list C<1,,3> is a
c2689353
NC
688concatenation of two lists, C<1,> and C<3>, the first of which ends
689with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
690similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
691we'd advise you to use this obfuscation.
692
a0d0e21e 693A list value may also be subscripted like a normal array. You must
54310121 694put the list in parentheses to avoid ambiguity. For example:
a0d0e21e
LW
695
696 # Stat returns list value.
84f709e7 697 $time = (stat($file))[8];
a0d0e21e 698
4633a7c4 699 # SYNTAX ERROR HERE.
84f709e7 700 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
4633a7c4 701
a0d0e21e 702 # Find a hex digit.
84f709e7 703 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
a0d0e21e
LW
704
705 # A "reverse comma operator".
706 return (pop(@foo),pop(@foo))[0];
707
d55a8828
TC
708Lists may be assigned to only when each element of the list
709is itself legal to assign to:
a0d0e21e 710
84f709e7 711 ($a, $b, $c) = (1, 2, 3);
a0d0e21e 712
84f709e7 713 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
a0d0e21e 714
d55a8828
TC
715An exception to this is that you may assign to C<undef> in a list.
716This is useful for throwing away some of the return values of a
717function:
718
84f709e7 719 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
d55a8828 720
e1817ab9
FC
721As 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
723the same variable twice, clobbering the first value assigned.)
724
d55a8828 725List assignment in scalar context returns the number of elements
4633a7c4
LW
726produced by the expression on the right side of the assignment:
727
7e3b091d
DA
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
4633a7c4 730
d55a8828 731This is handy when you want to do a list assignment in a Boolean
19799a22 732context, because most list functions return a null list when finished,
4633a7c4
LW
733which when assigned produces a 0, which is interpreted as FALSE.
734
ab1f959b
PN
735It's also the source of a useful idiom for executing a function or
736performing an operation in list context and then counting the number of
737return values, by assigning to an empty list and then using that
8fdd8881 738assignment in scalar context. For example, this code:
ab1f959b 739
84f709e7 740 $count = () = $string =~ /\d+/g;
ab1f959b
PN
741
742will place into $count the number of digit groups found in $string.
743This happens because the pattern match is in list context (since it
744is being assigned to the empty list), and will therefore return a list
8fdd8881 745of all matching parts of the string. The list assignment in scalar
ab1f959b 746context will translate that into the number of elements (here, the
8fdd8881 747number of times the pattern matched) and assign that to $count. Note
ab1f959b
PN
748that simply using
749
84f709e7 750 $count = $string =~ /\d+/g;
ab1f959b
PN
751
752would not have worked, since a pattern match in scalar context will
753only return true or false, rather than a count of matches.
754
755The final element of a list assignment may be an array or a hash:
a0d0e21e 756
84f709e7 757 ($a, $b, @rest) = split;
5a964f20 758 my($a, $b, %rest) = @_;
a0d0e21e 759
4633a7c4 760You can actually put an array or hash anywhere in the list, but the first one
d55a8828
TC
761in the list will soak up all the values, and anything after it will become
762undefined. This may be useful in a my() or local().
a0d0e21e 763
d55a8828
TC
764A hash can be initialized using a literal list holding pairs of
765items to be interpreted as a key and a value:
a0d0e21e
LW
766
767 # same as map assignment above
84f709e7 768 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
a0d0e21e 769
d55a8828 770While literal lists and named arrays are often interchangeable, that's
4633a7c4
LW
771not the case for hashes. Just because you can subscript a list value like
772a normal array does not mean that you can subscript a list value as a
773hash. Likewise, hashes included as parts of other lists (including
774parameters lists and return lists from functions) always flatten out into
775key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 776
c47ff5f1
GS
777It is often more readable to use the C<< => >> operator between key/value
778pairs. The C<< => >> operator is mostly just a more visually distinctive
b88cefa9 779synonym for a comma, but it also arranges for its left-hand operand to be
ac036724 780interpreted as a string if it's a bareword that would be a legal simple
8fdd8881
FC
781identifier. C<< => >> doesn't quote compound identifiers, that contain
782double colons. This makes it nice for initializing hashes:
a0d0e21e 783
84f709e7 784 %map = (
7e3b091d
DA
785 red => 0x00f,
786 blue => 0x0f0,
787 green => 0xf00,
4633a7c4
LW
788 );
789
790or for initializing hash references to be used as records:
791
84f709e7 792 $rec = {
7e3b091d
DA
793 witch => 'Mable the Merciless',
794 cat => 'Fluffy the Ferocious',
795 date => '10/31/1776',
4633a7c4
LW
796 };
797
798or for using call-by-named-parameter to complicated functions:
799
84f709e7 800 $field = $query->radio_group(
7e3b091d 801 name => 'group_name',
4633a7c4
LW
802 values => ['eenie','meenie','minie'],
803 default => 'meenie',
804 linebreak => 'true',
84f709e7 805 labels => \%labels
4633a7c4 806 );
cb1a09d0
AD
807
808Note that just because a hash is initialized in that order doesn't
809mean that it comes out in that order. See L<perlfunc/sort> for examples
810of how to arrange for an output ordering.
811
c9e3649f
LM
812If a key appears more than once in the initializer list of a hash, the last
813occurrence wins:
814
815 %circle = (
816 center => [5, 10],
817 center => [27, 9],
818 radius => 100,
819 color => [0xDF, 0xFF, 0x00],
820 radius => 54,
821 );
822
823 # same as
824 %circle = (
825 center => [27, 9],
826 color => [0xDF, 0xFF, 0x00],
827 radius => 54,
828 );
829
830This can be used to provide overridable configuration defaults:
831
832 # values in %args take priority over %config_defaults
833 %config = (%config_defaults, %args);
834
692ef166
SF
835=head2 Subscripts
836
aa80e1dc
FC
837An array can be accessed one scalar at a
838time by specifying a dollar sign (C<$>), then the
692ef166
SF
839name of the array (without the leading C<@>), then the subscript inside
840square brackets. For example:
841
842 @myarray = (5, 50, 500, 5000);
2adc35dd 843 print "The Third Element is", $myarray[2], "\n";
692ef166 844
8fdd8881 845The array indices start with 0. A negative subscript retrieves its
692ef166
SF
846value from the end. In our example, C<$myarray[-1]> would have been
8475000, and C<$myarray[-2]> would have been 500.
848
849Hash subscripts are similar, only instead of square brackets curly brackets
8fdd8881 850are used. For example:
692ef166
SF
851
852 %scientists =
853 (
854 "Newton" => "Isaac",
855 "Einstein" => "Albert",
856 "Darwin" => "Charles",
857 "Feynman" => "Richard",
858 );
859
860 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
861
aa80e1dc 862You can also subscript a list to get a single element from it:
d55a8828 863
aa80e1dc 864 $dir = (getpwnam("daemon"))[7];
d55a8828 865
9ed2a148
IG
866=head2 Multi-dimensional array emulation
867
868Multidimensional arrays may be emulated by subscripting a hash with a
8fdd8881 869list. The elements of the list are joined with the subscript separator
b8db74f2 870(see L<perlvar/$;>).
9ed2a148
IG
871
872 $foo{$a,$b,$c}
873
874is equivalent to
875
876 $foo{join($;, $a, $b, $c)}
877
878The default subscript separator is "\034", the same as SUBSEP in B<awk>.
879
aa80e1dc
FC
880=head2 Slices
881X<slice> X<array, slice> X<hash, slice>
d55a8828
TC
882
883A slice accesses several elements of a list, an array, or a hash
56d7751a
GS
884simultaneously using a list of subscripts. It's more convenient
885than writing out the individual elements as a list of separate
d55a8828
TC
886scalar values.
887
7e3b091d
DA
888 ($him, $her) = @folks[0,-1]; # array slice
889 @them = @folks[0 .. 3]; # array slice
890 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
891 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
d55a8828
TC
892
893Since you can assign to a list of variables, you can also assign to
894an array or hash slice.
895
84f709e7 896 @days[3..5] = qw/Wed Thu Fri/;
d55a8828 897 @colors{'red','blue','green'}
7e3b091d 898 = (0xff0000, 0x0000ff, 0x00ff00);
d55a8828
TC
899 @folks[0, -1] = @folks[-1, 0];
900
901The previous assignments are exactly equivalent to
902
84f709e7
JH
903 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
904 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
7e3b091d 905 = (0xff0000, 0x0000ff, 0x00ff00);
88fd19e3 906 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
d55a8828
TC
907
908Since changing a slice changes the original array or hash that it's
56d7751a
GS
909slicing, a C<foreach> construct will alter some--or even all--of the
910values of the array or hash.
d55a8828
TC
911
912 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
913
00cb5da1 914 foreach (@hash{qw[key1 key2]}) {
7e3b091d
DA
915 s/^\s+//; # trim leading whitespace
916 s/\s+$//; # trim trailing whitespace
917 s/(\w+)/\u\L$1/g; # "titlecase" words
d55a8828
TC
918 }
919
e2ec1b05
AP
920As a special exception, when you slice a list (but not an array or a hash),
921if the list evaluates to empty, then taking a slice of that empty list will
922always yield the empty list in turn. Thus:
08cd8952 923
e2ec1b05
AP
924 @a = ()[0,1]; # @a has no elements
925 @b = (@a)[0,1]; # @b has no elements
926 @c = (sub{}->())[0,1]; # @c has no elements
927 @d = ('a','b')[0,1]; # @d has two elements
928 @e = (@d)[0,1,8,9]; # @e has four elements
929 @f = (@d)[8,9]; # @f has two elements
f51152ef 930
19799a22
GS
931This makes it easy to write loops that terminate when a null list
932is returned:
d55a8828 933
e2ec1b05 934 while ( ($home, $user) = (getpwent)[7,0] ) {
7e3b091d 935 printf "%-8s %s\n", $user, $home;
d55a8828
TC
936 }
937
938As noted earlier in this document, the scalar sense of list assignment
939is the number of elements on the right-hand side of the assignment.
19799a22 940The null list contains no elements, so when the password file is
d55a8828
TC
941exhausted, the result is 0, not 2.
942
ad1de9c6
ML
943Slices in scalar context return the last item of the slice.
944
945 @a = qw/first second third/;
946 %h = (first => 'A', second => 'B');
947 $t = @a[0, 1]; # $t is now 'second'
0de10106 948 $u = @h{'first', 'second'}; # $u is now 'B'
ad1de9c6 949
d55a8828
TC
950If you're confused about why you use an '@' there on a hash slice
951instead of a '%', think of it like this. The type of bracket (square
952or curly) governs whether it's an array or a hash being looked at.
953On the other hand, the leading symbol ('$' or '@') on the array or
954hash indicates whether you are getting back a singular value (a
955scalar) or a plural one (a list).
956
8a7ab7dc 957=head3 Key/Value Hash Slices
23a22365 958
c44d7536
FC
959Starting in Perl 5.20, a hash slice operation
960with the % symbol is a variant of slice operation
190c3990 961returning a list of key/value pairs rather than just values:
23a22365 962
190c3990
FC
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)
23a22365 966
190c3990
FC
967However, the result of such a slice cannot be localized, deleted or used
968in assignment. These are otherwise very much consistent with hash slices
969using the @ symbol.
23a22365 970
8a7ab7dc 971=head3 Index/Value Array Slices
23a22365 972
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973Similar to key/value hash slices (and also introduced
974in Perl 5.20), the % array slice syntax returns a list
190c3990 975of index/value pairs:
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977 @a = "a".."z";
978 @list = %a[3,4,6];
979 # @list is now (3, "d", 4, "e", 6, "g")
23a22365 980
5f05dabc 981=head2 Typeglobs and Filehandles
d74e8afc 982X<typeglob> X<filehandle> X<*>
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983
984Perl uses an internal type called a I<typeglob> to hold an entire
985symbol table entry. The type prefix of a typeglob is a C<*>, because
54310121 986it represents all types. This used to be the preferred way to
cb1a09d0 987pass arrays and hashes by reference into a function, but now that
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988we have real references, this is seldom needed.
989
990The main use of typeglobs in modern Perl is create symbol table aliases.
991This assignment:
992
993 *this = *that;
994
995makes $this an alias for $that, @this an alias for @that, %this an alias
996for %that, &this an alias for &that, etc. Much safer is to use a reference.
997This:
5f05dabc 998
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999 local *Here::blue = \$There::green;
1000
1001temporarily makes $Here::blue an alias for $There::green, but doesn't
1002make @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
1004of this. Strange though this may seem, this is the basis for the whole
84f709e7 1005module import/export system.
5a964f20 1006
d55a8828 1007Another use for typeglobs is to pass filehandles into a function or
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1008to create new filehandles. If you need to use a typeglob to save away
1009a filehandle, do it this way:
5f05dabc 1010
84f709e7 1011 $fh = *STDOUT;
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1012
1013or perhaps as a real reference, like this:
1014
84f709e7 1015 $fh = \*STDOUT;
5f05dabc 1016
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1017See L<perlsub> for examples of using these as indirect filehandles
1018in functions.
1019
1020Typeglobs are also a way to create a local filehandle using the local()
1021operator. These last until their block is exited, but may be passed back.
1022For example:
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1023
1024 sub newopen {
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1025 my $path = shift;
1026 local *FH; # not my!
1027 open (FH, $path) or return undef;
1028 return *FH;
5f05dabc 1029 }
84f709e7 1030 $fh = newopen('/etc/passwd');
5f05dabc 1031
d55a8828 1032Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
5a964f20 1033for filehandle manipulations, although they're still needed to pass brand
8fdd8881 1034new file and directory handles into or out of functions. That's because
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1035C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
1036In other words, C<*FH> must be used to create new symbol table entries;
1037C<*foo{THING}> cannot. When in doubt, use C<*FH>.
1038
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1039All functions that are capable of creating filehandles (open(),
1040opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
1041automatically create an anonymous filehandle if the handle passed to
8fdd8881 1042them is an uninitialized scalar variable. This allows the constructs
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1043such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
1044create filehandles that will conveniently be closed automatically when
8fdd8881 1045the scope ends, provided there are no other references to them. This
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1046largely eliminates the need for typeglobs when opening filehandles
1047that must be passed around, as in the following example:
1048
1049 sub myopen {
84f709e7 1050 open my $fh, "@_"
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1051 or die "Can't open '@_': $!";
1052 return $fh;
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1053 }
1054
1055 {
1056 my $f = myopen("</etc/motd");
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1057 print <$f>;
1058 # $f implicitly closed here
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1059 }
1060
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1061Note that if an initialized scalar variable is used instead the
1062result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
1063to C<open( *{'zzz'}, ...)>.
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1064C<use strict 'refs'> forbids such practice.
1065
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1066Another way to create anonymous filehandles is with the Symbol
1067module or with the IO::Handle module and its ilk. These modules
1068have the advantage of not hiding different types of the same name
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1069during the local(). See the bottom of L<perlfunc/open> for an
1070example.
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1071
1072=head1 SEE ALSO
1073
1074See L<perlvar> for a description of Perl's built-in variables and
1075a discussion of legal variable names. See L<perlref>, L<perlsub>,
1076and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
1077the C<*foo{THING}> syntax.