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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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323X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0>
324X<boolean> X<bool>
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325A scalar value is interpreted as FALSE in the Boolean sense
326if it is undefined, the null string or the number 0 (or its
327string equivalent, "0"), and TRUE if it is anything else. The
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328Boolean context is just a special kind of scalar context where no
329conversion to a string or a number is ever performed.
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330Negation of a true value by C<!> or C<not> returns a special false value.
331When evaluated as a string it is treated as C<"">, but as a number, it
332is treated as 0. Most Perl operators
333that return true or false behave this way.
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334
335There are actually two varieties of null strings (sometimes referred
336to as "empty" strings), a defined one and an undefined one. The
337defined version is just a string of length zero, such as C<"">.
338The undefined version is the value that indicates that there is
339no real value for something, such as when there was an error, or
340at end of file, or when you refer to an uninitialized variable or
341element of an array or hash. Although in early versions of Perl,
342an undefined scalar could become defined when first used in a
343place expecting a defined value, this no longer happens except for
344rare cases of autovivification as explained in L<perlref>. You can
345use the defined() operator to determine whether a scalar value is
346defined (this has no meaning on arrays or hashes), and the undef()
347operator to produce an undefined value.
d74e8afc 348X<defined> X<undefined> X<undef> X<null> X<string, null>
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349
350To find out whether a given string is a valid non-zero number, it's
351sometimes enough to test it against both numeric 0 and also lexical
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352"0" (although this will cause noises if warnings are on). That's
353because strings that aren't numbers count as 0, just as they do in B<awk>:
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354
355 if ($str == 0 && $str ne "0") {
356 warn "That doesn't look like a number";
54310121 357 }
4633a7c4 358
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359That method may be best because otherwise you won't treat IEEE
360notations like C<NaN> or C<Infinity> properly. At other times, you
361might prefer to determine whether string data can be used numerically
362by calling the POSIX::strtod() function or by inspecting your string
363with a regular expression (as documented in L<perlre>).
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364
365 warn "has nondigits" if /\D/;
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366 warn "not a natural number" unless /^\d+$/; # rejects -3
367 warn "not an integer" unless /^-?\d+$/; # rejects +3
368 warn "not an integer" unless /^[+-]?\d+$/;
369 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
370 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
54310121 371 warn "not a C float"
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372 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
373
d55a8828 374The length of an array is a scalar value. You may find the length
fc518ee5 375of array @days by evaluating C<$#days>, as in B<csh>. However, this
376isn't the length of the array; it's the subscript of the last element,
377which is a different value since there is ordinarily a 0th element.
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378Assigning to C<$#days> actually changes the length of the array.
379Shortening an array this way destroys intervening values. Lengthening
380an array that was previously shortened does not recover values
0568eccd 381that were in those elements.
d74e8afc 382X<$#> X<array, length>
d55a8828 383
210b36aa 384You can also gain some minuscule measure of efficiency by pre-extending
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385an array that is going to get big. You can also extend an array
386by assigning to an element that is off the end of the array. You
19799a22 387can truncate an array down to nothing by assigning the null list
d55a8828 388() to it. The following are equivalent:
a0d0e21e 389
84f709e7 390 @whatever = ();
3e3baf6d 391 $#whatever = -1;
a0d0e21e 392
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393If you evaluate an array in scalar context, it returns the length
394of the array. (Note that this is not true of lists, which return
395the last value, like the C comma operator, nor of built-in functions,
396which return whatever they feel like returning.) The following is
397always true:
d74e8afc 398X<array, length>
a0d0e21e 399
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400 scalar(@whatever) == $#whatever + 1;
401
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402Some programmers choose to use an explicit conversion so as to
403leave nothing to doubt:
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404
405 $element_count = scalar(@whatever);
406
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407If you evaluate a hash in scalar context, it returns a false value if
408the hash is empty. If there are any key/value pairs, it returns a
409true value. A more precise definition is version dependent.
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410
411Prior to Perl 5.25 the value returned was a string consisting of the
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412number of used buckets and the number of allocated buckets, separated
413by a slash. This is pretty much useful only to find out whether
414Perl's internal hashing algorithm is performing poorly on your data
415set. For example, you stick 10,000 things in a hash, but evaluating
416%HASH in scalar context reveals C<"1/16">, which means only one out
417of sixteen buckets has been touched, and presumably contains all
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41810,000 of your items. This isn't supposed to happen.
419
420As of Perl 5.25 the return was changed to be the count of keys in the
421hash. If you need access to the old behavior you can use
422C<Hash::Util::bucket_ratio()> instead.
423
424If a tied hash is evaluated in scalar context, the C<SCALAR> method is
425called (with a fallback to C<FIRSTKEY>).
d74e8afc 426X<hash, scalar context> X<hash, bucket> X<bucket>
a0d0e21e 427
5a964f20 428You can preallocate space for a hash by assigning to the keys() function.
65841adf 429This rounds up the allocated buckets to the next power of two:
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430
431 keys(%users) = 1000; # allocate 1024 buckets
432
a0d0e21e 433=head2 Scalar value constructors
d74e8afc 434X<scalar, literal> X<scalar, constant>
a0d0e21e 435
d55a8828 436Numeric literals are specified in any of the following floating point or
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437integer formats:
438
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439 12345
440 12345.67
441 .23E-10 # a very small number
442 3.14_15_92 # a very important number
443 4_294_967_296 # underscore for legibility
444 0xff # hex
445 0xdead_beef # more hex
446 0377 # octal (only numbers, begins with 0)
447 0b011011 # binary
448 0x1.999ap-4 # hexadecimal floating point (the 'p' is required)
a0d0e21e 449
d4ced10d 450You are allowed to use underscores (underbars) in numeric literals
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451between digits for legibility (but not multiple underscores in a row:
452C<23__500> is not legal; C<23_500> is).
453You could, for example, group binary
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454digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
455or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
d74e8afc 456X<number, literal>
1d277562 457
55497cff 458String literals are usually delimited by either single or double
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459quotes. They work much like quotes in the standard Unix shells:
460double-quoted string literals are subject to backslash and variable
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461substitution; single-quoted strings are not (except for C<\'> and
462C<\\>). The usual C-style backslash rules apply for making
d55a8828 463characters such as newline, tab, etc., as well as some more exotic
4a4eefd0 464forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
d74e8afc 465X<string, literal>
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466
467Hexadecimal, octal, or binary, representations in string literals
468(e.g. '0xff') are not automatically converted to their integer
469representation. The hex() and oct() functions make these conversions
470for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
68dc0745 471
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472Hexadecimal floating point can start just like a hexadecimal literal,
473and it can be followed by an optional fractional hexadecimal part,
474but it must be followed by C<p>, an optional sign, and a power of two.
475The format is useful for accurately presenting floating point values,
476avoiding conversions to or from decimal floating point, and therefore
477avoiding possible loss in precision. Notice that while most current
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478platforms use the 64-bit IEEE 754 floating point, not all do. Another
479potential source of (low-order) differences are the floating point
480rounding modes, which can differ between CPUs, operating systems,
481and compilers, and which Perl doesn't control.
61e61fbc 482
5f05dabc 483You can also embed newlines directly in your strings, i.e., they can end
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484on a different line than they begin. This is nice, but if you forget
485your trailing quote, the error will not be reported until Perl finds
486another line containing the quote character, which may be much further
487on in the script. Variable substitution inside strings is limited to
d55a8828 488scalar variables, arrays, and array or hash slices. (In other words,
b88cefa9 489names beginning with $ or @, followed by an optional bracketed
a0d0e21e 490expression as a subscript.) The following code segment prints out "The
184e9718 491price is $Z<>100."
d74e8afc 492X<interpolation>
a0d0e21e 493
692ef166
SF
494 $Price = '$100'; # not interpolated
495 print "The price is $Price.\n"; # interpolated
496
497There is no double interpolation in Perl, so the C<$100> is left as is.
a0d0e21e 498
7e4353e9
RGS
499By default floating point numbers substituted inside strings use the
500dot (".") as the decimal separator. If C<use locale> is in effect,
501and POSIX::setlocale() has been called, the character used for the
502decimal separator is affected by the LC_NUMERIC locale.
503See L<perllocale> and L<POSIX>.
504
d55a8828 505As in some shells, you can enclose the variable name in braces to
f1cbbd6e
GS
506disambiguate it from following alphanumerics (and underscores).
507You must also do
d55a8828
TC
508this when interpolating a variable into a string to separate the
509variable name from a following double-colon or an apostrophe, since
510these would be otherwise treated as a package separator:
d74e8afc 511X<interpolation>
d55a8828 512
84f709e7 513 $who = "Larry";
d55a8828
TC
514 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
515 print "We use ${who}speak when ${who}'s here.\n";
516
517Without the braces, Perl would have looked for a $whospeak, a
518C<$who::0>, and a C<$who's> variable. The last two would be the
519$0 and the $s variables in the (presumably) non-existent package
520C<who>.
521
34a2706e 522In fact, a simple identifier within such curlies is forced to be
b4e2e1dd 523a string, and likewise within a hash subscript. Neither need
d55a8828
TC
524quoting. Our earlier example, C<$days{'Feb'}> can be written as
525C<$days{Feb}> and the quotes will be assumed automatically. But
719b43e8
RGS
526anything more complicated in the subscript will be interpreted as an
527expression. This means for example that C<$version{2.0}++> is
528equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
d55a8828 529
f17ecf24
JH
530=head3 Special floating point: infinity (Inf) and not-a-number (NaN)
531
532Floating point values include the special values C<Inf> and C<NaN>,
533for infinity and not-a-number. The infinity can be also negative.
534
535The infinity is the result of certain math operations that overflow
536the floating point range, like 9**9**9. The not-a-number is the
537result when the result is undefined or unrepresentable. Though note
538that you cannot get C<NaN> from some common "undefined" or
539"out-of-range" operations like dividing by zero, or square root of
540a negative number, since Perl generates fatal errors for those.
541
542The infinity and not-a-number have their own special arithmetic rules.
543The general rule is that they are "contagious": C<Inf> plus one is
544C<Inf>, and C<NaN> plus one is C<NaN>. Where things get interesting
545is when you combine infinities and not-a-numbers: C<Inf> minus C<Inf>
f38a07a3 546and C<Inf> divided by C<Inf> are C<NaN> (while C<Inf> plus C<Inf> is
f17ecf24
JH
547C<Inf> and C<Inf> times C<Inf> is C<Inf>). C<NaN> is also curious
548in that it does not equal any number, I<including> itself:
549C<NaN> != C<NaN>.
550
551Perl doesn't understand C<Inf> and C<NaN> as numeric literals, but
552you can have them as strings, and Perl will convert them as needed:
553"Inf" + 1. (You can, however, import them from the POSIX extension;
554C<use POSIX qw(Inf NaN);> and then use them as literals.)
555
556Note that on input (string to number) Perl accepts C<Inf> and C<NaN>
557in many forms. Case is ignored, and the Win32-specific forms like
558C<1.#INF> are understood, but on output the values are normalized to
559C<Inf> and C<NaN>.
560
692ef166 561=head3 Version Strings
d74e8afc 562X<version string> X<vstring> X<v-string>
692ef166 563
191d61a7 564A literal of the form C<v1.20.300.4000> is parsed as a string composed
6b2463a0
JH
565of characters with the specified ordinals. This form, known as
566v-strings, provides an alternative, more readable way to construct
567strings, rather than use the somewhat less readable interpolation form
568C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
569Unicode strings, and for comparing version "numbers" using the string
570comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
571more dots in the literal, the leading C<v> may be omitted.
b9c62f5b 572
2575c402 573 print v9786; # prints SMILEY, "\x{263a}"
b9c62f5b
GS
574 print v102.111.111; # prints "foo"
575 print 102.111.111; # same
576
577Such literals are accepted by both C<require> and C<use> for
a32521b7
JD
578doing a version check. Note that using the v-strings for IPv4
579addresses is not portable unless you also use the
580inet_aton()/inet_ntoa() routines of the Socket package.
191d61a7 581
d32a65d2 582Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
8fa72689 583are not v-strings before the C<< => >> operator (which is usually used
3921068c 584to separate a hash key from a hash value); instead they are interpreted
15ecd4ae
JH
585as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
586Perl 5.8.0, but that caused more confusion and breakage than good.
587Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
588be v-strings always.
d32a65d2 589
692ef166 590=head3 Special Literals
d74e8afc
ITB
591X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
592X<end> X<data> X<^D> X<^Z>
692ef166 593
d55a8828 594The special literals __FILE__, __LINE__, and __PACKAGE__
68dc0745 595represent the current filename, line number, and package name at that
84ed0108
FC
596point in your program. __SUB__ gives a reference to the current
597subroutine. They may be used only as separate tokens; they
68dc0745 598will not be interpolated into strings. If there is no current package
3e92a254 599(due to an empty C<package;> directive), __PACKAGE__ is the undefined
8fdd8881 600value. (But the empty C<package;> is no longer supported, as of version
84ed0108
FC
6015.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
602is only available in 5.16 or higher, and only with a C<use v5.16> or
603C<use feature "current_sub"> declaration.
604X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
605X<line> X<file> X<package>
3e92a254
GS
606
607The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
608may be used to indicate the logical end of the script before the actual
609end of file. Any following text is ignored.
610
1bab44f9 611Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
3e92a254
GS
612where C<PACKNAME> is the package that was current when the __DATA__
613token was encountered. The filehandle is left open pointing to the
4d383607 614line after __DATA__. The program should C<close DATA> when it is done
9c205800
FC
615reading from it. (Leaving it open leaks filehandles if the module is
616reloaded for any reason, so it's a safer practice to close it.) For
4d383607
JK
617compatibility with older scripts written before __DATA__ was
618introduced, __END__ behaves like __DATA__ in the top level script (but
619not in files loaded with C<require> or C<do>) and leaves the remaining
620contents of the file accessible via C<main::DATA>.
3e92a254 621
e7e8ce85
Z
622The C<DATA> file handle by default has whatever PerlIO layers were
623in place when Perl read the file to parse the source. Normally that
624means that the file is being read bytewise, as if it were encoded in
625Latin-1, but there are two major ways for it to be otherwise. Firstly,
626if the C<__END__>/C<__DATA__> token is in the scope of a C<use utf8>
627pragma then the C<DATA> handle will be in UTF-8 mode. And secondly,
628if the source is being read from perl's standard input then the C<DATA>
629file handle is actually aliased to the C<STDIN> file handle, and may
630be in UTF-8 mode because of the C<PERL_UNICODE> environment variable or
631perl's command-line switches.
632
3e92a254 633See L<SelfLoader> for more description of __DATA__, and
d55a8828
TC
634an example of its use. Note that you cannot read from the DATA
635filehandle in a BEGIN block: the BEGIN block is executed as soon
636as it is seen (during compilation), at which point the corresponding
a00c1fe5 637__DATA__ (or __END__) token has not yet been seen.
a0d0e21e 638
692ef166 639=head3 Barewords
d74e8afc 640X<bareword>
692ef166 641
748a9306 642A word that has no other interpretation in the grammar will
a0d0e21e
LW
643be treated as if it were a quoted string. These are known as
644"barewords". As with filehandles and labels, a bareword that consists
645entirely of lowercase letters risks conflict with future reserved
9f1b1f2d 646words, and if you use the C<use warnings> pragma or the B<-w> switch,
05b4f1ec
FW
647Perl will warn you about any such words. Perl limits barewords (like
648identifiers) to about 250 characters. Future versions of Perl are likely
649to eliminate these arbitrary limitations.
650
651Some people may wish to outlaw barewords entirely. If you
a0d0e21e
LW
652say
653
654 use strict 'subs';
655
656then any bareword that would NOT be interpreted as a subroutine call
657produces a compile-time error instead. The restriction lasts to the
54310121 658end of the enclosing block. An inner block may countermand this
a0d0e21e
LW
659by saying C<no strict 'subs'>.
660
e2b457c0 661=head3 Array Interpolation
d74e8afc 662X<array, interpolation> X<interpolation, array> X<$">
692ef166 663
d55a8828
TC
664Arrays and slices are interpolated into double-quoted strings
665by joining the elements with the delimiter specified in the C<$">
692ef166
SF
666variable (C<$LIST_SEPARATOR> if "use English;" is specified),
667space by default. The following are equivalent:
a0d0e21e 668
84f709e7 669 $temp = join($", @ARGV);
a0d0e21e
LW
670 system "echo $temp";
671
672 system "echo @ARGV";
673
674Within search patterns (which also undergo double-quotish substitution)
d55a8828 675there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
a0d0e21e
LW
676C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
677expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
678@foo)? If @foo doesn't otherwise exist, then it's obviously a
679character class. If @foo exists, Perl takes a good guess about C<[bar]>,
680and is almost always right. If it does guess wrong, or if you're just
681plain paranoid, you can force the correct interpretation with curly
d55a8828 682braces as above.
a0d0e21e 683
7e3b091d 684If you're looking for the information on how to use here-documents,
210b36aa
AMS
685which used to be here, that's been moved to
686L<perlop/Quote and Quote-like Operators>.
be16fac9 687
a0d0e21e 688=head2 List value constructors
d74e8afc 689X<list>
a0d0e21e
LW
690
691List values are denoted by separating individual values by commas
692(and enclosing the list in parentheses where precedence requires it):
693
694 (LIST)
695
d55a8828
TC
696In a context not requiring a list value, the value of what appears
697to be a list literal is simply the value of the final element, as
698with the C comma operator. For example,
a0d0e21e 699
84f709e7 700 @foo = ('cc', '-E', $bar);
a0d0e21e 701
d55a8828 702assigns the entire list value to array @foo, but
a0d0e21e 703
84f709e7 704 $foo = ('cc', '-E', $bar);
a0d0e21e 705
d55a8828
TC
706assigns the value of variable $bar to the scalar variable $foo.
707Note that the value of an actual array in scalar context is the
708length of the array; the following assigns the value 3 to $foo:
a0d0e21e 709
84f709e7 710 @foo = ('cc', '-E', $bar);
7e3b091d 711 $foo = @foo; # $foo gets 3
a0d0e21e 712
54310121 713You may have an optional comma before the closing parenthesis of a
a0d0e21e
LW
714list literal, so that you can say:
715
84f709e7 716 @foo = (
7e3b091d
DA
717 1,
718 2,
719 3,
a0d0e21e
LW
720 );
721
d55a8828
TC
722To use a here-document to assign an array, one line per element,
723you might use an approach like this:
724
84f709e7 725 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
7e3b091d
DA
726 normal tomato
727 spicy tomato
728 green chile
729 pesto
730 white wine
d55a8828
TC
731 End_Lines
732
a0d0e21e 733LISTs do automatic interpolation of sublists. That is, when a LIST is
d55a8828 734evaluated, each element of the list is evaluated in list context, and
a0d0e21e 735the resulting list value is interpolated into LIST just as if each
5a964f20 736individual element were a member of LIST. Thus arrays and hashes lose their
a0d0e21e
LW
737identity in a LIST--the list
738
5a964f20 739 (@foo,@bar,&SomeSub,%glarch)
a0d0e21e
LW
740
741contains all the elements of @foo followed by all the elements of @bar,
5a964f20 742followed by all the elements returned by the subroutine named SomeSub
d55a8828 743called in list context, followed by the key/value pairs of %glarch.
a0d0e21e
LW
744To make a list reference that does I<NOT> interpolate, see L<perlref>.
745
19799a22 746The null list is represented by (). Interpolating it in a list
a0d0e21e
LW
747has no effect. Thus ((),(),()) is equivalent to (). Similarly,
748interpolating an array with no elements is the same as if no
749array had been interpolated at that point.
750
c2689353 751This interpolation combines with the facts that the opening
ab1f959b 752and closing parentheses are optional (except when necessary for
c2689353 753precedence) and lists may end with an optional comma to mean that
8fdd8881 754multiple commas within lists are legal syntax. The list C<1,,3> is a
c2689353
NC
755concatenation of two lists, C<1,> and C<3>, the first of which ends
756with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
757similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
758we'd advise you to use this obfuscation.
759
a0d0e21e 760A list value may also be subscripted like a normal array. You must
54310121 761put the list in parentheses to avoid ambiguity. For example:
a0d0e21e
LW
762
763 # Stat returns list value.
84f709e7 764 $time = (stat($file))[8];
a0d0e21e 765
4633a7c4 766 # SYNTAX ERROR HERE.
84f709e7 767 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
4633a7c4 768
a0d0e21e 769 # Find a hex digit.
84f709e7 770 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
a0d0e21e
LW
771
772 # A "reverse comma operator".
773 return (pop(@foo),pop(@foo))[0];
774
d55a8828
TC
775Lists may be assigned to only when each element of the list
776is itself legal to assign to:
a0d0e21e 777
84f709e7 778 ($a, $b, $c) = (1, 2, 3);
a0d0e21e 779
84f709e7 780 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
a0d0e21e 781
d55a8828
TC
782An exception to this is that you may assign to C<undef> in a list.
783This is useful for throwing away some of the return values of a
784function:
785
84f709e7 786 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
d55a8828 787
e1817ab9
FC
788As of Perl 5.22, you can also use C<(undef)x2> instead of C<undef, undef>.
789(You can also do C<($x) x 2>, which is less useful, because it assigns to
790the same variable twice, clobbering the first value assigned.)
791
436908e5
JK
792When you assign a list of scalars to an array, all previous values in that
793array are wiped out and the number of elements in the array will now be equal to
794the number of elements in the right-hand list -- the list from which
795assignment was made. The array will automatically resize itself to precisely
796accommodate each element in the right-hand list.
797
798 use warnings;
799 my (@xyz, $x, $y, $z);
800
801 @xyz = (1, 2, 3);
802 print "@xyz\n"; # 1 2 3
803
804 @xyz = ('al', 'be', 'ga', 'de');
805 print "@xyz\n"; # al be ga de
806
807 @xyz = (101, 102);
808 print "@xyz\n"; # 101 102
809
810When, however, you assign a list of scalars to another list of scalars, the
811results differ according to whether the left-hand list -- the list being
812assigned to -- has the same, more or fewer elements than the right-hand list.
813
814 ($x, $y, $z) = (1, 2, 3);
815 print "$x $y $z\n"; # 1 2 3
816
817 ($x, $y, $z) = ('al', 'be', 'ga', 'de');
818 print "$x $y $z\n"; # al be ga
819
820 ($x, $y, $z) = (101, 102);
821 print "$x $y $z\n"; # 101 102
822 # Use of uninitialized value $z in concatenation (.)
823 # or string at [program] line [line number].
824
825If the number of scalars in the left-hand list is less than that in the
826right-hand list, the "extra" scalars in the right-hand list will simply not be
827assigned.
828
829If the number of scalars in the left-hand list is greater than that in the
830left-hand list, the "missing" scalars will become undefined.
831
832 ($x, $y, $z) = (101, 102);
833 for my $el ($x, $y, $z) {
834 (defined $el) ? print "$el " : print "<undef>";
835 }
836 print "\n";
837 # 101 102 <undef>
838
d55a8828 839List assignment in scalar context returns the number of elements
4633a7c4
LW
840produced by the expression on the right side of the assignment:
841
7e3b091d
DA
842 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
843 $x = (($foo,$bar) = f()); # set $x to f()'s return count
4633a7c4 844
d55a8828 845This is handy when you want to do a list assignment in a Boolean
19799a22 846context, because most list functions return a null list when finished,
4633a7c4
LW
847which when assigned produces a 0, which is interpreted as FALSE.
848
ab1f959b
PN
849It's also the source of a useful idiom for executing a function or
850performing an operation in list context and then counting the number of
851return values, by assigning to an empty list and then using that
8fdd8881 852assignment in scalar context. For example, this code:
ab1f959b 853
84f709e7 854 $count = () = $string =~ /\d+/g;
ab1f959b
PN
855
856will place into $count the number of digit groups found in $string.
857This happens because the pattern match is in list context (since it
858is being assigned to the empty list), and will therefore return a list
8fdd8881 859of all matching parts of the string. The list assignment in scalar
ab1f959b 860context will translate that into the number of elements (here, the
8fdd8881 861number of times the pattern matched) and assign that to $count. Note
ab1f959b
PN
862that simply using
863
84f709e7 864 $count = $string =~ /\d+/g;
ab1f959b
PN
865
866would not have worked, since a pattern match in scalar context will
867only return true or false, rather than a count of matches.
868
869The final element of a list assignment may be an array or a hash:
a0d0e21e 870
84f709e7 871 ($a, $b, @rest) = split;
5a964f20 872 my($a, $b, %rest) = @_;
a0d0e21e 873
4633a7c4 874You can actually put an array or hash anywhere in the list, but the first one
d55a8828
TC
875in the list will soak up all the values, and anything after it will become
876undefined. This may be useful in a my() or local().
a0d0e21e 877
d55a8828
TC
878A hash can be initialized using a literal list holding pairs of
879items to be interpreted as a key and a value:
a0d0e21e
LW
880
881 # same as map assignment above
84f709e7 882 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
a0d0e21e 883
d55a8828 884While literal lists and named arrays are often interchangeable, that's
4633a7c4
LW
885not the case for hashes. Just because you can subscript a list value like
886a normal array does not mean that you can subscript a list value as a
887hash. Likewise, hashes included as parts of other lists (including
888parameters lists and return lists from functions) always flatten out into
889key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 890
c47ff5f1
GS
891It is often more readable to use the C<< => >> operator between key/value
892pairs. The C<< => >> operator is mostly just a more visually distinctive
b88cefa9 893synonym for a comma, but it also arranges for its left-hand operand to be
ac036724 894interpreted as a string if it's a bareword that would be a legal simple
8fdd8881
FC
895identifier. C<< => >> doesn't quote compound identifiers, that contain
896double colons. This makes it nice for initializing hashes:
a0d0e21e 897
84f709e7 898 %map = (
7e3b091d
DA
899 red => 0x00f,
900 blue => 0x0f0,
901 green => 0xf00,
4633a7c4
LW
902 );
903
904or for initializing hash references to be used as records:
905
84f709e7 906 $rec = {
7e3b091d
DA
907 witch => 'Mable the Merciless',
908 cat => 'Fluffy the Ferocious',
909 date => '10/31/1776',
4633a7c4
LW
910 };
911
912or for using call-by-named-parameter to complicated functions:
913
84f709e7 914 $field = $query->radio_group(
7e3b091d 915 name => 'group_name',
4633a7c4
LW
916 values => ['eenie','meenie','minie'],
917 default => 'meenie',
918 linebreak => 'true',
84f709e7 919 labels => \%labels
4633a7c4 920 );
cb1a09d0
AD
921
922Note that just because a hash is initialized in that order doesn't
923mean that it comes out in that order. See L<perlfunc/sort> for examples
924of how to arrange for an output ordering.
925
c9e3649f
LM
926If a key appears more than once in the initializer list of a hash, the last
927occurrence wins:
928
929 %circle = (
930 center => [5, 10],
931 center => [27, 9],
932 radius => 100,
933 color => [0xDF, 0xFF, 0x00],
934 radius => 54,
935 );
936
937 # same as
938 %circle = (
939 center => [27, 9],
940 color => [0xDF, 0xFF, 0x00],
941 radius => 54,
942 );
943
944This can be used to provide overridable configuration defaults:
945
946 # values in %args take priority over %config_defaults
947 %config = (%config_defaults, %args);
948
692ef166
SF
949=head2 Subscripts
950
aa80e1dc
FC
951An array can be accessed one scalar at a
952time by specifying a dollar sign (C<$>), then the
692ef166
SF
953name of the array (without the leading C<@>), then the subscript inside
954square brackets. For example:
955
956 @myarray = (5, 50, 500, 5000);
2adc35dd 957 print "The Third Element is", $myarray[2], "\n";
692ef166 958
8fdd8881 959The array indices start with 0. A negative subscript retrieves its
692ef166
SF
960value from the end. In our example, C<$myarray[-1]> would have been
9615000, and C<$myarray[-2]> would have been 500.
962
963Hash subscripts are similar, only instead of square brackets curly brackets
8fdd8881 964are used. For example:
692ef166
SF
965
966 %scientists =
967 (
968 "Newton" => "Isaac",
969 "Einstein" => "Albert",
970 "Darwin" => "Charles",
971 "Feynman" => "Richard",
972 );
973
974 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
975
aa80e1dc 976You can also subscript a list to get a single element from it:
d55a8828 977
aa80e1dc 978 $dir = (getpwnam("daemon"))[7];
d55a8828 979
9ed2a148
IG
980=head2 Multi-dimensional array emulation
981
982Multidimensional arrays may be emulated by subscripting a hash with a
8fdd8881 983list. The elements of the list are joined with the subscript separator
b8db74f2 984(see L<perlvar/$;>).
9ed2a148
IG
985
986 $foo{$a,$b,$c}
987
988is equivalent to
989
990 $foo{join($;, $a, $b, $c)}
991
992The default subscript separator is "\034", the same as SUBSEP in B<awk>.
993
aa80e1dc
FC
994=head2 Slices
995X<slice> X<array, slice> X<hash, slice>
d55a8828
TC
996
997A slice accesses several elements of a list, an array, or a hash
56d7751a
GS
998simultaneously using a list of subscripts. It's more convenient
999than writing out the individual elements as a list of separate
d55a8828
TC
1000scalar values.
1001
7e3b091d
DA
1002 ($him, $her) = @folks[0,-1]; # array slice
1003 @them = @folks[0 .. 3]; # array slice
1004 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
1005 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
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TC
1006
1007Since you can assign to a list of variables, you can also assign to
1008an array or hash slice.
1009
84f709e7 1010 @days[3..5] = qw/Wed Thu Fri/;
d55a8828 1011 @colors{'red','blue','green'}
7e3b091d 1012 = (0xff0000, 0x0000ff, 0x00ff00);
d55a8828
TC
1013 @folks[0, -1] = @folks[-1, 0];
1014
1015The previous assignments are exactly equivalent to
1016
84f709e7
JH
1017 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
1018 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
7e3b091d 1019 = (0xff0000, 0x0000ff, 0x00ff00);
88fd19e3 1020 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
d55a8828
TC
1021
1022Since changing a slice changes the original array or hash that it's
56d7751a
GS
1023slicing, a C<foreach> construct will alter some--or even all--of the
1024values of the array or hash.
d55a8828
TC
1025
1026 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
1027
00cb5da1 1028 foreach (@hash{qw[key1 key2]}) {
7e3b091d
DA
1029 s/^\s+//; # trim leading whitespace
1030 s/\s+$//; # trim trailing whitespace
1031 s/(\w+)/\u\L$1/g; # "titlecase" words
d55a8828
TC
1032 }
1033
e2ec1b05
AP
1034As a special exception, when you slice a list (but not an array or a hash),
1035if the list evaluates to empty, then taking a slice of that empty list will
1036always yield the empty list in turn. Thus:
08cd8952 1037
e2ec1b05
AP
1038 @a = ()[0,1]; # @a has no elements
1039 @b = (@a)[0,1]; # @b has no elements
1040 @c = (sub{}->())[0,1]; # @c has no elements
1041 @d = ('a','b')[0,1]; # @d has two elements
1042 @e = (@d)[0,1,8,9]; # @e has four elements
1043 @f = (@d)[8,9]; # @f has two elements
f51152ef 1044
19799a22
GS
1045This makes it easy to write loops that terminate when a null list
1046is returned:
d55a8828 1047
e2ec1b05 1048 while ( ($home, $user) = (getpwent)[7,0] ) {
7e3b091d 1049 printf "%-8s %s\n", $user, $home;
d55a8828
TC
1050 }
1051
1052As noted earlier in this document, the scalar sense of list assignment
1053is the number of elements on the right-hand side of the assignment.
19799a22 1054The null list contains no elements, so when the password file is
d55a8828
TC
1055exhausted, the result is 0, not 2.
1056
ad1de9c6
ML
1057Slices in scalar context return the last item of the slice.
1058
1059 @a = qw/first second third/;
1060 %h = (first => 'A', second => 'B');
1061 $t = @a[0, 1]; # $t is now 'second'
0de10106 1062 $u = @h{'first', 'second'}; # $u is now 'B'
ad1de9c6 1063
d55a8828
TC
1064If you're confused about why you use an '@' there on a hash slice
1065instead of a '%', think of it like this. The type of bracket (square
1066or curly) governs whether it's an array or a hash being looked at.
1067On the other hand, the leading symbol ('$' or '@') on the array or
1068hash indicates whether you are getting back a singular value (a
1069scalar) or a plural one (a list).
1070
8a7ab7dc 1071=head3 Key/Value Hash Slices
23a22365 1072
c44d7536
FC
1073Starting in Perl 5.20, a hash slice operation
1074with the % symbol is a variant of slice operation
190c3990 1075returning a list of key/value pairs rather than just values:
23a22365 1076
190c3990
FC
1077 %h = (blonk => 2, foo => 3, squink => 5, bar => 8);
1078 %subset = %h{'foo', 'bar'}; # key/value hash slice
1079 # %subset is now (foo => 3, bar => 8)
cc0776d6
DIM
1080 %removed = delete %h{'foo', 'bar'};
1081 # %removed is now (foo => 3, bar => 8)
1082 # %h is now (blonk => 2, squink => 5)
23a22365 1083
cc0776d6 1084However, the result of such a slice cannot be localized or used
190c3990
FC
1085in assignment. These are otherwise very much consistent with hash slices
1086using the @ symbol.
23a22365 1087
8a7ab7dc 1088=head3 Index/Value Array Slices
23a22365 1089
c44d7536
FC
1090Similar to key/value hash slices (and also introduced
1091in Perl 5.20), the % array slice syntax returns a list
190c3990 1092of index/value pairs:
23a22365 1093
190c3990
FC
1094 @a = "a".."z";
1095 @list = %a[3,4,6];
1096 # @list is now (3, "d", 4, "e", 6, "g")
cc0776d6
DIM
1097 @removed = delete %a[3,4,6]
1098 # @removed is now (3, "d", 4, "e", 6, "g")
1099 # @list[3,4,6] are now undef
1100
1101Note that calling L<C<delete>|perlfunc/delete EXPR> on array values is
1102strongly discouraged.
23a22365 1103
5f05dabc 1104=head2 Typeglobs and Filehandles
d74e8afc 1105X<typeglob> X<filehandle> X<*>
cb1a09d0
AD
1106
1107Perl uses an internal type called a I<typeglob> to hold an entire
1108symbol table entry. The type prefix of a typeglob is a C<*>, because
54310121 1109it represents all types. This used to be the preferred way to
cb1a09d0 1110pass arrays and hashes by reference into a function, but now that
5a964f20
TC
1111we have real references, this is seldom needed.
1112
1113The main use of typeglobs in modern Perl is create symbol table aliases.
1114This assignment:
1115
1116 *this = *that;
1117
1118makes $this an alias for $that, @this an alias for @that, %this an alias
1119for %that, &this an alias for &that, etc. Much safer is to use a reference.
1120This:
5f05dabc 1121
5a964f20
TC
1122 local *Here::blue = \$There::green;
1123
1124temporarily makes $Here::blue an alias for $There::green, but doesn't
1125make @Here::blue an alias for @There::green, or %Here::blue an alias for
1126%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
1127of this. Strange though this may seem, this is the basis for the whole
84f709e7 1128module import/export system.
5a964f20 1129
d55a8828 1130Another use for typeglobs is to pass filehandles into a function or
5a964f20
TC
1131to create new filehandles. If you need to use a typeglob to save away
1132a filehandle, do it this way:
5f05dabc 1133
84f709e7 1134 $fh = *STDOUT;
5f05dabc
PP
1135
1136or perhaps as a real reference, like this:
1137
84f709e7 1138 $fh = \*STDOUT;
5f05dabc 1139
5a964f20
TC
1140See L<perlsub> for examples of using these as indirect filehandles
1141in functions.
1142
1143Typeglobs are also a way to create a local filehandle using the local()
1144operator. These last until their block is exited, but may be passed back.
1145For example:
5f05dabc
PP
1146
1147 sub newopen {
7e3b091d
DA
1148 my $path = shift;
1149 local *FH; # not my!
1150 open (FH, $path) or return undef;
1151 return *FH;
5f05dabc 1152 }
84f709e7 1153 $fh = newopen('/etc/passwd');
5f05dabc 1154
d55a8828 1155Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
5a964f20 1156for filehandle manipulations, although they're still needed to pass brand
8fdd8881 1157new file and directory handles into or out of functions. That's because
d55a8828
TC
1158C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
1159In other words, C<*FH> must be used to create new symbol table entries;
1160C<*foo{THING}> cannot. When in doubt, use C<*FH>.
1161
36392fcf
GS
1162All functions that are capable of creating filehandles (open(),
1163opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
1164automatically create an anonymous filehandle if the handle passed to
8fdd8881 1165them is an uninitialized scalar variable. This allows the constructs
36392fcf
GS
1166such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
1167create filehandles that will conveniently be closed automatically when
8fdd8881 1168the scope ends, provided there are no other references to them. This
36392fcf
GS
1169largely eliminates the need for typeglobs when opening filehandles
1170that must be passed around, as in the following example:
1171
1172 sub myopen {
84f709e7 1173 open my $fh, "@_"
7e3b091d
DA
1174 or die "Can't open '@_': $!";
1175 return $fh;
36392fcf
GS
1176 }
1177
1178 {
1179 my $f = myopen("</etc/motd");
7e3b091d
DA
1180 print <$f>;
1181 # $f implicitly closed here
36392fcf
GS
1182 }
1183
b92795fe
AMS
1184Note that if an initialized scalar variable is used instead the
1185result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
1186to C<open( *{'zzz'}, ...)>.
d83fe814
AT
1187C<use strict 'refs'> forbids such practice.
1188
d55a8828
TC
1189Another way to create anonymous filehandles is with the Symbol
1190module or with the IO::Handle module and its ilk. These modules
1191have the advantage of not hiding different types of the same name
66b6e4ad
KW
1192during the local(). See the bottom of L<perlfunc/open> for an
1193example.
d55a8828
TC
1194
1195=head1 SEE ALSO
1196
1197See L<perlvar> for a description of Perl's built-in variables and
1198a discussion of legal variable names. See L<perlref>, L<perlsub>,
1199and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
1200the C<*foo{THING}> syntax.