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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
32833930 27(see L<perlmod/Packages> for details). For a more in-depth discussion
5a0de581 28on identifiers, see L</Identifier parsing>. It's possible to
32833930 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<$> (see L<perlop> and L<perlre>).
39In addition, several special variables that provide windows into
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40the inner working of Perl have names containing punctuation characters.
41These are documented in L<perlvar>.
d74e8afc 42X<variable, built-in>
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43
44Scalar values are always named with '$', 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 are
103limited to one character, e.g., C<$%> or
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104C<$$>. (Most of these one character names have a predefined
105significance to Perl. For instance, C<$$> is the current process
ce4793f1 106id. And all such names are reserved for Perl's possible use.)
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
ce4793f1 132generic characters, and identifiers should match
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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
ce4793f1 187fully-qualified. They come in six forms (but don't use forms 5 and 6):
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188
189=over
190
ce4793f1 191=item 1.
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192
193A sigil, followed solely by digits matching C<\p{POSIX_Digit}>, like
194C<$0>, C<$1>, or C<$10000>.
195
ce4793f1 196=item 2.
42327f06 197
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198A sigil followed by a single character matching the C<\p{POSIX_Punct}>
199property, like C<$!> or C<%+>, except the character C<"{"> doesn't work.
42327f06 200
ce4793f1 201=item 3.
42327f06 202
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203A sigil, followed by a caret and any one of the characters
204C<[][A-Z^_?\]>, like C<$^V> or C<$^]>.
42327f06 205
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206=item 4.
207
208Similar to the above, a sigil, followed by bareword text in braces,
209where the first character is a caret. The next character is any one of
210the characters C<[][A-Z^_?\]>, followed by ASCII word characters. An
211example is C<${^GLOBAL_PHASE}>.
212
213=item 5.
214
5741d7e6 215A sigil, followed by any single character in the range C<[\xA1-\xAC\xAE-\xFF]>
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216when not under C<S<"use utf8">>. (Under C<S<"use utf8">>, the normal
217identifier rules given earlier in this section apply.) Use of
218non-graphic characters (the C1 controls, the NO-BREAK SPACE, and the
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219SOFT HYPHEN) has been disallowed since v5.26.0.
220The use of the other characters is unwise, as these are all
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221reserved to have special meaning to Perl, and none of them currently
222do have special meaning, though this could change without notice.
223
224Note that an implication of this form is that there are identifiers only
225legal under C<S<"use utf8">>, and vice-versa, for example the identifier
226C<$E<233>tat> is legal under C<S<"use utf8">>, but is otherwise
227considered to be the single character variable C<$E<233>> followed by
228the bareword C<"tat">, the combination of which is a syntax error.
229
230=item 6.
231
232This is a combination of the previous two forms. It is valid only when
233not under S<C<"use utf8">> (normal identifier rules apply when under
234S<C<"use utf8">>). The form is a sigil, followed by text in braces,
235where the first character is any one of the characters in the range
236C<[\x80-\xFF]> followed by ASCII word characters up to the trailing
237brace.
238
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239The same caveats as the previous form apply: The non-graphic
240characters are no longer allowed with S<"use utf8">, it is unwise
241to use this form at all, and utf8ness makes a big difference.
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242
243=back
244
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245Prior to Perl v5.24, non-graphical ASCII control characters were also
246allowed in some situations; this had been deprecated since v5.20.
b29f65fc 247
a0d0e21e 248=head2 Context
d74e8afc 249X<context> X<scalar context> X<list context>
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250
251The interpretation of operations and values in Perl sometimes depends
252on the requirements of the context around the operation or value.
d55a8828 253There are two major contexts: list and scalar. Certain operations
a0d0e21e 254return list values in contexts wanting a list, and scalar values
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255otherwise. If this is true of an operation it will be mentioned in
256the documentation for that operation. In other words, Perl overloads
a0d0e21e 257certain operations based on whether the expected return value is
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258singular or plural. Some words in English work this way, like "fish"
259and "sheep".
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260
261In a reciprocal fashion, an operation provides either a scalar or a
262list context to each of its arguments. For example, if you say
263
264 int( <STDIN> )
265
c47ff5f1 266the integer operation provides scalar context for the <>
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267operator, which responds by reading one line from STDIN and passing it
268back to the integer operation, which will then find the integer value
269of that line and return that. If, on the other hand, you say
270
271 sort( <STDIN> )
272
c47ff5f1 273then the sort operation provides list context for <>, which
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274will proceed to read every line available up to the end of file, and
275pass that list of lines back to the sort routine, which will then
276sort those lines and return them as a list to whatever the context
277of the sort was.
278
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279Assignment is a little bit special in that it uses its left argument
280to determine the context for the right argument. Assignment to a
281scalar evaluates the right-hand side in scalar context, while
282assignment to an array or hash evaluates the righthand side in list
283context. Assignment to a list (or slice, which is just a list
3921068c 284anyway) also evaluates the right-hand side in list context.
d55a8828 285
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286When you use the C<use warnings> pragma or Perl's B<-w> command-line
287option, you may see warnings
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288about useless uses of constants or functions in "void context".
289Void context just means the value has been discarded, such as a
290statement containing only C<"fred";> or C<getpwuid(0);>. It still
291counts as scalar context for functions that care whether or not
292they're being called in list context.
293
294User-defined subroutines may choose to care whether they are being
295called in a void, scalar, or list context. Most subroutines do not
296need to bother, though. That's because both scalars and lists are
297automatically interpolated into lists. See L<perlfunc/wantarray>
298for how you would dynamically discern your function's calling
299context.
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300
301=head2 Scalar values
d74e8afc 302X<scalar> X<number> X<string> X<reference>
a0d0e21e 303
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304All data in Perl is a scalar, an array of scalars, or a hash of
305scalars. A scalar may contain one single value in any of three
306different flavors: a number, a string, or a reference. In general,
307conversion from one form to another is transparent. Although a
308scalar may not directly hold multiple values, it may contain a
309reference to an array or hash which in turn contains multiple values.
310
311Scalars aren't necessarily one thing or another. There's no place
312to declare a scalar variable to be of type "string", type "number",
313type "reference", or anything else. Because of the automatic
314conversion of scalars, operations that return scalars don't need
315to care (and in fact, cannot care) whether their caller is looking
316for a string, a number, or a reference. Perl is a contextually
317polymorphic language whose scalars can be strings, numbers, or
318references (which includes objects). Although strings and numbers
319are considered pretty much the same thing for nearly all purposes,
320references are strongly-typed, uncastable pointers with builtin
321reference-counting and destructor invocation.
a0d0e21e 322
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323A scalar value is interpreted as FALSE in the Boolean sense
324if it is undefined, the null string or the number 0 (or its
325string equivalent, "0"), and TRUE if it is anything else. The
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326Boolean context is just a special kind of scalar context where no
327conversion to a string or a number is ever performed.
d74e8afc 328X<boolean> X<bool> X<true> X<false> X<truth>
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329
330There are actually two varieties of null strings (sometimes referred
331to as "empty" strings), a defined one and an undefined one. The
332defined version is just a string of length zero, such as C<"">.
333The undefined version is the value that indicates that there is
334no real value for something, such as when there was an error, or
335at end of file, or when you refer to an uninitialized variable or
336element of an array or hash. Although in early versions of Perl,
337an undefined scalar could become defined when first used in a
338place expecting a defined value, this no longer happens except for
339rare cases of autovivification as explained in L<perlref>. You can
340use the defined() operator to determine whether a scalar value is
341defined (this has no meaning on arrays or hashes), and the undef()
342operator to produce an undefined value.
d74e8afc 343X<defined> X<undefined> X<undef> X<null> X<string, null>
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344
345To find out whether a given string is a valid non-zero number, it's
346sometimes enough to test it against both numeric 0 and also lexical
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347"0" (although this will cause noises if warnings are on). That's
348because strings that aren't numbers count as 0, just as they do in B<awk>:
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349
350 if ($str == 0 && $str ne "0") {
351 warn "That doesn't look like a number";
54310121 352 }
4633a7c4 353
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354That method may be best because otherwise you won't treat IEEE
355notations like C<NaN> or C<Infinity> properly. At other times, you
356might prefer to determine whether string data can be used numerically
357by calling the POSIX::strtod() function or by inspecting your string
358with a regular expression (as documented in L<perlre>).
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359
360 warn "has nondigits" if /\D/;
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361 warn "not a natural number" unless /^\d+$/; # rejects -3
362 warn "not an integer" unless /^-?\d+$/; # rejects +3
363 warn "not an integer" unless /^[+-]?\d+$/;
364 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
365 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
54310121 366 warn "not a C float"
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367 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
368
d55a8828 369The length of an array is a scalar value. You may find the length
fc518ee5 370of array @days by evaluating C<$#days>, as in B<csh>. However, this
371isn't the length of the array; it's the subscript of the last element,
372which is a different value since there is ordinarily a 0th element.
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373Assigning to C<$#days> actually changes the length of the array.
374Shortening an array this way destroys intervening values. Lengthening
375an array that was previously shortened does not recover values
0568eccd 376that were in those elements.
d74e8afc 377X<$#> X<array, length>
d55a8828 378
210b36aa 379You can also gain some minuscule measure of efficiency by pre-extending
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380an array that is going to get big. You can also extend an array
381by assigning to an element that is off the end of the array. You
19799a22 382can truncate an array down to nothing by assigning the null list
d55a8828 383() to it. The following are equivalent:
a0d0e21e 384
84f709e7 385 @whatever = ();
3e3baf6d 386 $#whatever = -1;
a0d0e21e 387
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388If you evaluate an array in scalar context, it returns the length
389of the array. (Note that this is not true of lists, which return
390the last value, like the C comma operator, nor of built-in functions,
391which return whatever they feel like returning.) The following is
392always true:
d74e8afc 393X<array, length>
a0d0e21e 394
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395 scalar(@whatever) == $#whatever + 1;
396
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397Some programmers choose to use an explicit conversion so as to
398leave nothing to doubt:
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399
400 $element_count = scalar(@whatever);
401
d55a8828 402If you evaluate a hash in scalar context, it returns false if the
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403hash is empty. If there are any key/value pairs, it returns true.
404A more precise definition is version dependent.
405
406Prior to Perl 5.25 the value returned was a string consisting of the
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407number of used buckets and the number of allocated buckets, separated
408by a slash. This is pretty much useful only to find out whether
409Perl's internal hashing algorithm is performing poorly on your data
410set. For example, you stick 10,000 things in a hash, but evaluating
411%HASH in scalar context reveals C<"1/16">, which means only one out
412of sixteen buckets has been touched, and presumably contains all
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41310,000 of your items. This isn't supposed to happen.
414
415As of Perl 5.25 the return was changed to be the count of keys in the
416hash. If you need access to the old behavior you can use
417C<Hash::Util::bucket_ratio()> instead.
418
419If a tied hash is evaluated in scalar context, the C<SCALAR> method is
420called (with a fallback to C<FIRSTKEY>).
d74e8afc 421X<hash, scalar context> X<hash, bucket> X<bucket>
a0d0e21e 422
5a964f20 423You can preallocate space for a hash by assigning to the keys() function.
65841adf 424This rounds up the allocated buckets to the next power of two:
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425
426 keys(%users) = 1000; # allocate 1024 buckets
427
a0d0e21e 428=head2 Scalar value constructors
d74e8afc 429X<scalar, literal> X<scalar, constant>
a0d0e21e 430
d55a8828 431Numeric literals are specified in any of the following floating point or
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432integer formats:
433
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434 12345
435 12345.67
436 .23E-10 # a very small number
437 3.14_15_92 # a very important number
438 4_294_967_296 # underscore for legibility
439 0xff # hex
440 0xdead_beef # more hex
441 0377 # octal (only numbers, begins with 0)
442 0b011011 # binary
443 0x1.999ap-4 # hexadecimal floating point (the 'p' is required)
a0d0e21e 444
d4ced10d 445You are allowed to use underscores (underbars) in numeric literals
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446between digits for legibility (but not multiple underscores in a row:
447C<23__500> is not legal; C<23_500> is).
448You could, for example, group binary
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449digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
450or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
d74e8afc 451X<number, literal>
1d277562 452
55497cff 453String literals are usually delimited by either single or double
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454quotes. They work much like quotes in the standard Unix shells:
455double-quoted string literals are subject to backslash and variable
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456substitution; single-quoted strings are not (except for C<\'> and
457C<\\>). The usual C-style backslash rules apply for making
d55a8828 458characters such as newline, tab, etc., as well as some more exotic
4a4eefd0 459forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
d74e8afc 460X<string, literal>
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461
462Hexadecimal, octal, or binary, representations in string literals
463(e.g. '0xff') are not automatically converted to their integer
464representation. The hex() and oct() functions make these conversions
465for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
68dc0745 466
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467Hexadecimal floating point can start just like a hexadecimal literal,
468and it can be followed by an optional fractional hexadecimal part,
469but it must be followed by C<p>, an optional sign, and a power of two.
470The format is useful for accurately presenting floating point values,
471avoiding conversions to or from decimal floating point, and therefore
472avoiding possible loss in precision. Notice that while most current
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473platforms use the 64-bit IEEE 754 floating point, not all do. Another
474potential source of (low-order) differences are the floating point
475rounding modes, which can differ between CPUs, operating systems,
476and compilers, and which Perl doesn't control.
61e61fbc 477
5f05dabc 478You can also embed newlines directly in your strings, i.e., they can end
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479on a different line than they begin. This is nice, but if you forget
480your trailing quote, the error will not be reported until Perl finds
481another line containing the quote character, which may be much further
482on in the script. Variable substitution inside strings is limited to
d55a8828 483scalar variables, arrays, and array or hash slices. (In other words,
b88cefa9 484names beginning with $ or @, followed by an optional bracketed
a0d0e21e 485expression as a subscript.) The following code segment prints out "The
184e9718 486price is $Z<>100."
d74e8afc 487X<interpolation>
a0d0e21e 488
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489 $Price = '$100'; # not interpolated
490 print "The price is $Price.\n"; # interpolated
491
492There is no double interpolation in Perl, so the C<$100> is left as is.
a0d0e21e 493
7e4353e9
RGS
494By default floating point numbers substituted inside strings use the
495dot (".") as the decimal separator. If C<use locale> is in effect,
496and POSIX::setlocale() has been called, the character used for the
497decimal separator is affected by the LC_NUMERIC locale.
498See L<perllocale> and L<POSIX>.
499
d55a8828 500As in some shells, you can enclose the variable name in braces to
f1cbbd6e
GS
501disambiguate it from following alphanumerics (and underscores).
502You must also do
d55a8828
TC
503this when interpolating a variable into a string to separate the
504variable name from a following double-colon or an apostrophe, since
505these would be otherwise treated as a package separator:
d74e8afc 506X<interpolation>
d55a8828 507
84f709e7 508 $who = "Larry";
d55a8828
TC
509 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
510 print "We use ${who}speak when ${who}'s here.\n";
511
512Without the braces, Perl would have looked for a $whospeak, a
513C<$who::0>, and a C<$who's> variable. The last two would be the
514$0 and the $s variables in the (presumably) non-existent package
515C<who>.
516
34a2706e 517In fact, a simple identifier within such curlies is forced to be
b4e2e1dd 518a string, and likewise within a hash subscript. Neither need
d55a8828
TC
519quoting. Our earlier example, C<$days{'Feb'}> can be written as
520C<$days{Feb}> and the quotes will be assumed automatically. But
719b43e8
RGS
521anything more complicated in the subscript will be interpreted as an
522expression. This means for example that C<$version{2.0}++> is
523equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
d55a8828 524
f17ecf24
JH
525=head3 Special floating point: infinity (Inf) and not-a-number (NaN)
526
527Floating point values include the special values C<Inf> and C<NaN>,
528for infinity and not-a-number. The infinity can be also negative.
529
530The infinity is the result of certain math operations that overflow
531the floating point range, like 9**9**9. The not-a-number is the
532result when the result is undefined or unrepresentable. Though note
533that you cannot get C<NaN> from some common "undefined" or
534"out-of-range" operations like dividing by zero, or square root of
535a negative number, since Perl generates fatal errors for those.
536
537The infinity and not-a-number have their own special arithmetic rules.
538The general rule is that they are "contagious": C<Inf> plus one is
539C<Inf>, and C<NaN> plus one is C<NaN>. Where things get interesting
540is when you combine infinities and not-a-numbers: C<Inf> minus C<Inf>
f38a07a3 541and C<Inf> divided by C<Inf> are C<NaN> (while C<Inf> plus C<Inf> is
f17ecf24
JH
542C<Inf> and C<Inf> times C<Inf> is C<Inf>). C<NaN> is also curious
543in that it does not equal any number, I<including> itself:
544C<NaN> != C<NaN>.
545
546Perl doesn't understand C<Inf> and C<NaN> as numeric literals, but
547you can have them as strings, and Perl will convert them as needed:
548"Inf" + 1. (You can, however, import them from the POSIX extension;
549C<use POSIX qw(Inf NaN);> and then use them as literals.)
550
551Note that on input (string to number) Perl accepts C<Inf> and C<NaN>
552in many forms. Case is ignored, and the Win32-specific forms like
553C<1.#INF> are understood, but on output the values are normalized to
554C<Inf> and C<NaN>.
555
692ef166 556=head3 Version Strings
d74e8afc 557X<version string> X<vstring> X<v-string>
692ef166 558
191d61a7 559A literal of the form C<v1.20.300.4000> is parsed as a string composed
6b2463a0
JH
560of characters with the specified ordinals. This form, known as
561v-strings, provides an alternative, more readable way to construct
562strings, rather than use the somewhat less readable interpolation form
563C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
564Unicode strings, and for comparing version "numbers" using the string
565comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
566more dots in the literal, the leading C<v> may be omitted.
b9c62f5b 567
2575c402 568 print v9786; # prints SMILEY, "\x{263a}"
b9c62f5b
GS
569 print v102.111.111; # prints "foo"
570 print 102.111.111; # same
571
572Such literals are accepted by both C<require> and C<use> for
a32521b7
JD
573doing a version check. Note that using the v-strings for IPv4
574addresses is not portable unless you also use the
575inet_aton()/inet_ntoa() routines of the Socket package.
191d61a7 576
d32a65d2 577Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
8fa72689 578are not v-strings before the C<< => >> operator (which is usually used
3921068c 579to separate a hash key from a hash value); instead they are interpreted
15ecd4ae
JH
580as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
581Perl 5.8.0, but that caused more confusion and breakage than good.
582Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
583be v-strings always.
d32a65d2 584
692ef166 585=head3 Special Literals
d74e8afc
ITB
586X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
587X<end> X<data> X<^D> X<^Z>
692ef166 588
d55a8828 589The special literals __FILE__, __LINE__, and __PACKAGE__
68dc0745 590represent the current filename, line number, and package name at that
84ed0108
FC
591point in your program. __SUB__ gives a reference to the current
592subroutine. They may be used only as separate tokens; they
68dc0745 593will not be interpolated into strings. If there is no current package
3e92a254 594(due to an empty C<package;> directive), __PACKAGE__ is the undefined
8fdd8881 595value. (But the empty C<package;> is no longer supported, as of version
84ed0108
FC
5965.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
597is only available in 5.16 or higher, and only with a C<use v5.16> or
598C<use feature "current_sub"> declaration.
599X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
600X<line> X<file> X<package>
3e92a254
GS
601
602The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
603may be used to indicate the logical end of the script before the actual
604end of file. Any following text is ignored.
605
1bab44f9 606Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
3e92a254
GS
607where C<PACKNAME> is the package that was current when the __DATA__
608token was encountered. The filehandle is left open pointing to the
4d383607 609line after __DATA__. The program should C<close DATA> when it is done
9c205800
FC
610reading from it. (Leaving it open leaks filehandles if the module is
611reloaded for any reason, so it's a safer practice to close it.) For
4d383607
JK
612compatibility with older scripts written before __DATA__ was
613introduced, __END__ behaves like __DATA__ in the top level script (but
614not in files loaded with C<require> or C<do>) and leaves the remaining
615contents of the file accessible via C<main::DATA>.
3e92a254
GS
616
617See L<SelfLoader> for more description of __DATA__, and
d55a8828
TC
618an example of its use. Note that you cannot read from the DATA
619filehandle in a BEGIN block: the BEGIN block is executed as soon
620as it is seen (during compilation), at which point the corresponding
a00c1fe5 621__DATA__ (or __END__) token has not yet been seen.
a0d0e21e 622
692ef166 623=head3 Barewords
d74e8afc 624X<bareword>
692ef166 625
748a9306 626A word that has no other interpretation in the grammar will
a0d0e21e
LW
627be treated as if it were a quoted string. These are known as
628"barewords". As with filehandles and labels, a bareword that consists
629entirely of lowercase letters risks conflict with future reserved
9f1b1f2d 630words, and if you use the C<use warnings> pragma or the B<-w> switch,
05b4f1ec
FW
631Perl will warn you about any such words. Perl limits barewords (like
632identifiers) to about 250 characters. Future versions of Perl are likely
633to eliminate these arbitrary limitations.
634
635Some people may wish to outlaw barewords entirely. If you
a0d0e21e
LW
636say
637
638 use strict 'subs';
639
640then any bareword that would NOT be interpreted as a subroutine call
641produces a compile-time error instead. The restriction lasts to the
54310121 642end of the enclosing block. An inner block may countermand this
a0d0e21e
LW
643by saying C<no strict 'subs'>.
644
e2b457c0 645=head3 Array Interpolation
d74e8afc 646X<array, interpolation> X<interpolation, array> X<$">
692ef166 647
d55a8828
TC
648Arrays and slices are interpolated into double-quoted strings
649by joining the elements with the delimiter specified in the C<$">
692ef166
SF
650variable (C<$LIST_SEPARATOR> if "use English;" is specified),
651space by default. The following are equivalent:
a0d0e21e 652
84f709e7 653 $temp = join($", @ARGV);
a0d0e21e
LW
654 system "echo $temp";
655
656 system "echo @ARGV";
657
658Within search patterns (which also undergo double-quotish substitution)
d55a8828 659there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
a0d0e21e
LW
660C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
661expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
662@foo)? If @foo doesn't otherwise exist, then it's obviously a
663character class. If @foo exists, Perl takes a good guess about C<[bar]>,
664and is almost always right. If it does guess wrong, or if you're just
665plain paranoid, you can force the correct interpretation with curly
d55a8828 666braces as above.
a0d0e21e 667
7e3b091d 668If you're looking for the information on how to use here-documents,
210b36aa
AMS
669which used to be here, that's been moved to
670L<perlop/Quote and Quote-like Operators>.
be16fac9 671
a0d0e21e 672=head2 List value constructors
d74e8afc 673X<list>
a0d0e21e
LW
674
675List values are denoted by separating individual values by commas
676(and enclosing the list in parentheses where precedence requires it):
677
678 (LIST)
679
d55a8828
TC
680In a context not requiring a list value, the value of what appears
681to be a list literal is simply the value of the final element, as
682with the C comma operator. For example,
a0d0e21e 683
84f709e7 684 @foo = ('cc', '-E', $bar);
a0d0e21e 685
d55a8828 686assigns the entire list value to array @foo, but
a0d0e21e 687
84f709e7 688 $foo = ('cc', '-E', $bar);
a0d0e21e 689
d55a8828
TC
690assigns the value of variable $bar to the scalar variable $foo.
691Note that the value of an actual array in scalar context is the
692length of the array; the following assigns the value 3 to $foo:
a0d0e21e 693
84f709e7 694 @foo = ('cc', '-E', $bar);
7e3b091d 695 $foo = @foo; # $foo gets 3
a0d0e21e 696
54310121 697You may have an optional comma before the closing parenthesis of a
a0d0e21e
LW
698list literal, so that you can say:
699
84f709e7 700 @foo = (
7e3b091d
DA
701 1,
702 2,
703 3,
a0d0e21e
LW
704 );
705
d55a8828
TC
706To use a here-document to assign an array, one line per element,
707you might use an approach like this:
708
84f709e7 709 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
7e3b091d
DA
710 normal tomato
711 spicy tomato
712 green chile
713 pesto
714 white wine
d55a8828
TC
715 End_Lines
716
a0d0e21e 717LISTs do automatic interpolation of sublists. That is, when a LIST is
d55a8828 718evaluated, each element of the list is evaluated in list context, and
a0d0e21e 719the resulting list value is interpolated into LIST just as if each
5a964f20 720individual element were a member of LIST. Thus arrays and hashes lose their
a0d0e21e
LW
721identity in a LIST--the list
722
5a964f20 723 (@foo,@bar,&SomeSub,%glarch)
a0d0e21e
LW
724
725contains all the elements of @foo followed by all the elements of @bar,
5a964f20 726followed by all the elements returned by the subroutine named SomeSub
d55a8828 727called in list context, followed by the key/value pairs of %glarch.
a0d0e21e
LW
728To make a list reference that does I<NOT> interpolate, see L<perlref>.
729
19799a22 730The null list is represented by (). Interpolating it in a list
a0d0e21e
LW
731has no effect. Thus ((),(),()) is equivalent to (). Similarly,
732interpolating an array with no elements is the same as if no
733array had been interpolated at that point.
734
c2689353 735This interpolation combines with the facts that the opening
ab1f959b 736and closing parentheses are optional (except when necessary for
c2689353 737precedence) and lists may end with an optional comma to mean that
8fdd8881 738multiple commas within lists are legal syntax. The list C<1,,3> is a
c2689353
NC
739concatenation of two lists, C<1,> and C<3>, the first of which ends
740with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
741similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
742we'd advise you to use this obfuscation.
743
a0d0e21e 744A list value may also be subscripted like a normal array. You must
54310121 745put the list in parentheses to avoid ambiguity. For example:
a0d0e21e
LW
746
747 # Stat returns list value.
84f709e7 748 $time = (stat($file))[8];
a0d0e21e 749
4633a7c4 750 # SYNTAX ERROR HERE.
84f709e7 751 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
4633a7c4 752
a0d0e21e 753 # Find a hex digit.
84f709e7 754 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
a0d0e21e
LW
755
756 # A "reverse comma operator".
757 return (pop(@foo),pop(@foo))[0];
758
d55a8828
TC
759Lists may be assigned to only when each element of the list
760is itself legal to assign to:
a0d0e21e 761
84f709e7 762 ($a, $b, $c) = (1, 2, 3);
a0d0e21e 763
84f709e7 764 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
a0d0e21e 765
d55a8828
TC
766An exception to this is that you may assign to C<undef> in a list.
767This is useful for throwing away some of the return values of a
768function:
769
84f709e7 770 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
d55a8828 771
e1817ab9
FC
772As of Perl 5.22, you can also use C<(undef)x2> instead of C<undef, undef>.
773(You can also do C<($x) x 2>, which is less useful, because it assigns to
774the same variable twice, clobbering the first value assigned.)
775
d55a8828 776List assignment in scalar context returns the number of elements
4633a7c4
LW
777produced by the expression on the right side of the assignment:
778
7e3b091d
DA
779 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
780 $x = (($foo,$bar) = f()); # set $x to f()'s return count
4633a7c4 781
d55a8828 782This is handy when you want to do a list assignment in a Boolean
19799a22 783context, because most list functions return a null list when finished,
4633a7c4
LW
784which when assigned produces a 0, which is interpreted as FALSE.
785
ab1f959b
PN
786It's also the source of a useful idiom for executing a function or
787performing an operation in list context and then counting the number of
788return values, by assigning to an empty list and then using that
8fdd8881 789assignment in scalar context. For example, this code:
ab1f959b 790
84f709e7 791 $count = () = $string =~ /\d+/g;
ab1f959b
PN
792
793will place into $count the number of digit groups found in $string.
794This happens because the pattern match is in list context (since it
795is being assigned to the empty list), and will therefore return a list
8fdd8881 796of all matching parts of the string. The list assignment in scalar
ab1f959b 797context will translate that into the number of elements (here, the
8fdd8881 798number of times the pattern matched) and assign that to $count. Note
ab1f959b
PN
799that simply using
800
84f709e7 801 $count = $string =~ /\d+/g;
ab1f959b
PN
802
803would not have worked, since a pattern match in scalar context will
804only return true or false, rather than a count of matches.
805
806The final element of a list assignment may be an array or a hash:
a0d0e21e 807
84f709e7 808 ($a, $b, @rest) = split;
5a964f20 809 my($a, $b, %rest) = @_;
a0d0e21e 810
4633a7c4 811You can actually put an array or hash anywhere in the list, but the first one
d55a8828
TC
812in the list will soak up all the values, and anything after it will become
813undefined. This may be useful in a my() or local().
a0d0e21e 814
d55a8828
TC
815A hash can be initialized using a literal list holding pairs of
816items to be interpreted as a key and a value:
a0d0e21e
LW
817
818 # same as map assignment above
84f709e7 819 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
a0d0e21e 820
d55a8828 821While literal lists and named arrays are often interchangeable, that's
4633a7c4
LW
822not the case for hashes. Just because you can subscript a list value like
823a normal array does not mean that you can subscript a list value as a
824hash. Likewise, hashes included as parts of other lists (including
825parameters lists and return lists from functions) always flatten out into
826key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 827
c47ff5f1
GS
828It is often more readable to use the C<< => >> operator between key/value
829pairs. The C<< => >> operator is mostly just a more visually distinctive
b88cefa9 830synonym for a comma, but it also arranges for its left-hand operand to be
ac036724 831interpreted as a string if it's a bareword that would be a legal simple
8fdd8881
FC
832identifier. C<< => >> doesn't quote compound identifiers, that contain
833double colons. This makes it nice for initializing hashes:
a0d0e21e 834
84f709e7 835 %map = (
7e3b091d
DA
836 red => 0x00f,
837 blue => 0x0f0,
838 green => 0xf00,
4633a7c4
LW
839 );
840
841or for initializing hash references to be used as records:
842
84f709e7 843 $rec = {
7e3b091d
DA
844 witch => 'Mable the Merciless',
845 cat => 'Fluffy the Ferocious',
846 date => '10/31/1776',
4633a7c4
LW
847 };
848
849or for using call-by-named-parameter to complicated functions:
850
84f709e7 851 $field = $query->radio_group(
7e3b091d 852 name => 'group_name',
4633a7c4
LW
853 values => ['eenie','meenie','minie'],
854 default => 'meenie',
855 linebreak => 'true',
84f709e7 856 labels => \%labels
4633a7c4 857 );
cb1a09d0
AD
858
859Note that just because a hash is initialized in that order doesn't
860mean that it comes out in that order. See L<perlfunc/sort> for examples
861of how to arrange for an output ordering.
862
c9e3649f
LM
863If a key appears more than once in the initializer list of a hash, the last
864occurrence wins:
865
866 %circle = (
867 center => [5, 10],
868 center => [27, 9],
869 radius => 100,
870 color => [0xDF, 0xFF, 0x00],
871 radius => 54,
872 );
873
874 # same as
875 %circle = (
876 center => [27, 9],
877 color => [0xDF, 0xFF, 0x00],
878 radius => 54,
879 );
880
881This can be used to provide overridable configuration defaults:
882
883 # values in %args take priority over %config_defaults
884 %config = (%config_defaults, %args);
885
692ef166
SF
886=head2 Subscripts
887
aa80e1dc
FC
888An array can be accessed one scalar at a
889time by specifying a dollar sign (C<$>), then the
692ef166
SF
890name of the array (without the leading C<@>), then the subscript inside
891square brackets. For example:
892
893 @myarray = (5, 50, 500, 5000);
2adc35dd 894 print "The Third Element is", $myarray[2], "\n";
692ef166 895
8fdd8881 896The array indices start with 0. A negative subscript retrieves its
692ef166
SF
897value from the end. In our example, C<$myarray[-1]> would have been
8985000, and C<$myarray[-2]> would have been 500.
899
900Hash subscripts are similar, only instead of square brackets curly brackets
8fdd8881 901are used. For example:
692ef166
SF
902
903 %scientists =
904 (
905 "Newton" => "Isaac",
906 "Einstein" => "Albert",
907 "Darwin" => "Charles",
908 "Feynman" => "Richard",
909 );
910
911 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
912
aa80e1dc 913You can also subscript a list to get a single element from it:
d55a8828 914
aa80e1dc 915 $dir = (getpwnam("daemon"))[7];
d55a8828 916
9ed2a148
IG
917=head2 Multi-dimensional array emulation
918
919Multidimensional arrays may be emulated by subscripting a hash with a
8fdd8881 920list. The elements of the list are joined with the subscript separator
b8db74f2 921(see L<perlvar/$;>).
9ed2a148
IG
922
923 $foo{$a,$b,$c}
924
925is equivalent to
926
927 $foo{join($;, $a, $b, $c)}
928
929The default subscript separator is "\034", the same as SUBSEP in B<awk>.
930
aa80e1dc
FC
931=head2 Slices
932X<slice> X<array, slice> X<hash, slice>
d55a8828
TC
933
934A slice accesses several elements of a list, an array, or a hash
56d7751a
GS
935simultaneously using a list of subscripts. It's more convenient
936than writing out the individual elements as a list of separate
d55a8828
TC
937scalar values.
938
7e3b091d
DA
939 ($him, $her) = @folks[0,-1]; # array slice
940 @them = @folks[0 .. 3]; # array slice
941 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
942 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
d55a8828
TC
943
944Since you can assign to a list of variables, you can also assign to
945an array or hash slice.
946
84f709e7 947 @days[3..5] = qw/Wed Thu Fri/;
d55a8828 948 @colors{'red','blue','green'}
7e3b091d 949 = (0xff0000, 0x0000ff, 0x00ff00);
d55a8828
TC
950 @folks[0, -1] = @folks[-1, 0];
951
952The previous assignments are exactly equivalent to
953
84f709e7
JH
954 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
955 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
7e3b091d 956 = (0xff0000, 0x0000ff, 0x00ff00);
88fd19e3 957 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
d55a8828
TC
958
959Since changing a slice changes the original array or hash that it's
56d7751a
GS
960slicing, a C<foreach> construct will alter some--or even all--of the
961values of the array or hash.
d55a8828
TC
962
963 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
964
00cb5da1 965 foreach (@hash{qw[key1 key2]}) {
7e3b091d
DA
966 s/^\s+//; # trim leading whitespace
967 s/\s+$//; # trim trailing whitespace
968 s/(\w+)/\u\L$1/g; # "titlecase" words
d55a8828
TC
969 }
970
e2ec1b05
AP
971As a special exception, when you slice a list (but not an array or a hash),
972if the list evaluates to empty, then taking a slice of that empty list will
973always yield the empty list in turn. Thus:
08cd8952 974
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AP
975 @a = ()[0,1]; # @a has no elements
976 @b = (@a)[0,1]; # @b has no elements
977 @c = (sub{}->())[0,1]; # @c has no elements
978 @d = ('a','b')[0,1]; # @d has two elements
979 @e = (@d)[0,1,8,9]; # @e has four elements
980 @f = (@d)[8,9]; # @f has two elements
f51152ef 981
19799a22
GS
982This makes it easy to write loops that terminate when a null list
983is returned:
d55a8828 984
e2ec1b05 985 while ( ($home, $user) = (getpwent)[7,0] ) {
7e3b091d 986 printf "%-8s %s\n", $user, $home;
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TC
987 }
988
989As noted earlier in this document, the scalar sense of list assignment
990is the number of elements on the right-hand side of the assignment.
19799a22 991The null list contains no elements, so when the password file is
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TC
992exhausted, the result is 0, not 2.
993
ad1de9c6
ML
994Slices in scalar context return the last item of the slice.
995
996 @a = qw/first second third/;
997 %h = (first => 'A', second => 'B');
998 $t = @a[0, 1]; # $t is now 'second'
0de10106 999 $u = @h{'first', 'second'}; # $u is now 'B'
ad1de9c6 1000
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1001If you're confused about why you use an '@' there on a hash slice
1002instead of a '%', think of it like this. The type of bracket (square
1003or curly) governs whether it's an array or a hash being looked at.
1004On the other hand, the leading symbol ('$' or '@') on the array or
1005hash indicates whether you are getting back a singular value (a
1006scalar) or a plural one (a list).
1007
8a7ab7dc 1008=head3 Key/Value Hash Slices
23a22365 1009
c44d7536
FC
1010Starting in Perl 5.20, a hash slice operation
1011with the % symbol is a variant of slice operation
190c3990 1012returning a list of key/value pairs rather than just values:
23a22365 1013
190c3990
FC
1014 %h = (blonk => 2, foo => 3, squink => 5, bar => 8);
1015 %subset = %h{'foo', 'bar'}; # key/value hash slice
1016 # %subset is now (foo => 3, bar => 8)
23a22365 1017
190c3990
FC
1018However, the result of such a slice cannot be localized, deleted or used
1019in assignment. These are otherwise very much consistent with hash slices
1020using the @ symbol.
23a22365 1021
8a7ab7dc 1022=head3 Index/Value Array Slices
23a22365 1023
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1024Similar to key/value hash slices (and also introduced
1025in Perl 5.20), the % array slice syntax returns a list
190c3990 1026of index/value pairs:
23a22365 1027
190c3990
FC
1028 @a = "a".."z";
1029 @list = %a[3,4,6];
1030 # @list is now (3, "d", 4, "e", 6, "g")
23a22365 1031
5f05dabc 1032=head2 Typeglobs and Filehandles
d74e8afc 1033X<typeglob> X<filehandle> X<*>
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AD
1034
1035Perl uses an internal type called a I<typeglob> to hold an entire
1036symbol table entry. The type prefix of a typeglob is a C<*>, because
54310121 1037it represents all types. This used to be the preferred way to
cb1a09d0 1038pass arrays and hashes by reference into a function, but now that
5a964f20
TC
1039we have real references, this is seldom needed.
1040
1041The main use of typeglobs in modern Perl is create symbol table aliases.
1042This assignment:
1043
1044 *this = *that;
1045
1046makes $this an alias for $that, @this an alias for @that, %this an alias
1047for %that, &this an alias for &that, etc. Much safer is to use a reference.
1048This:
5f05dabc 1049
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1050 local *Here::blue = \$There::green;
1051
1052temporarily makes $Here::blue an alias for $There::green, but doesn't
1053make @Here::blue an alias for @There::green, or %Here::blue an alias for
1054%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
1055of this. Strange though this may seem, this is the basis for the whole
84f709e7 1056module import/export system.
5a964f20 1057
d55a8828 1058Another use for typeglobs is to pass filehandles into a function or
5a964f20
TC
1059to create new filehandles. If you need to use a typeglob to save away
1060a filehandle, do it this way:
5f05dabc 1061
84f709e7 1062 $fh = *STDOUT;
5f05dabc
PP
1063
1064or perhaps as a real reference, like this:
1065
84f709e7 1066 $fh = \*STDOUT;
5f05dabc 1067
5a964f20
TC
1068See L<perlsub> for examples of using these as indirect filehandles
1069in functions.
1070
1071Typeglobs are also a way to create a local filehandle using the local()
1072operator. These last until their block is exited, but may be passed back.
1073For example:
5f05dabc
PP
1074
1075 sub newopen {
7e3b091d
DA
1076 my $path = shift;
1077 local *FH; # not my!
1078 open (FH, $path) or return undef;
1079 return *FH;
5f05dabc 1080 }
84f709e7 1081 $fh = newopen('/etc/passwd');
5f05dabc 1082
d55a8828 1083Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
5a964f20 1084for filehandle manipulations, although they're still needed to pass brand
8fdd8881 1085new file and directory handles into or out of functions. That's because
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1086C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
1087In other words, C<*FH> must be used to create new symbol table entries;
1088C<*foo{THING}> cannot. When in doubt, use C<*FH>.
1089
36392fcf
GS
1090All functions that are capable of creating filehandles (open(),
1091opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
1092automatically create an anonymous filehandle if the handle passed to
8fdd8881 1093them is an uninitialized scalar variable. This allows the constructs
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GS
1094such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
1095create filehandles that will conveniently be closed automatically when
8fdd8881 1096the scope ends, provided there are no other references to them. This
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GS
1097largely eliminates the need for typeglobs when opening filehandles
1098that must be passed around, as in the following example:
1099
1100 sub myopen {
84f709e7 1101 open my $fh, "@_"
7e3b091d
DA
1102 or die "Can't open '@_': $!";
1103 return $fh;
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GS
1104 }
1105
1106 {
1107 my $f = myopen("</etc/motd");
7e3b091d
DA
1108 print <$f>;
1109 # $f implicitly closed here
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GS
1110 }
1111
b92795fe
AMS
1112Note that if an initialized scalar variable is used instead the
1113result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
1114to C<open( *{'zzz'}, ...)>.
d83fe814
AT
1115C<use strict 'refs'> forbids such practice.
1116
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1117Another way to create anonymous filehandles is with the Symbol
1118module or with the IO::Handle module and its ilk. These modules
1119have the advantage of not hiding different types of the same name
66b6e4ad
KW
1120during the local(). See the bottom of L<perlfunc/open> for an
1121example.
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1122
1123=head1 SEE ALSO
1124
1125See L<perlvar> for a description of Perl's built-in variables and
1126a discussion of legal variable names. See L<perlref>, L<perlsub>,
1127and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
1128the C<*foo{THING}> syntax.