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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
27(see L<perlmod/Packages> for details). It's possible to substitute
28for a simple identifier, an expression that produces a reference
29to the value at runtime. This is described in more detail below
30and in L<perlref>.
d74e8afc 31X<identifier>
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32
33Perl also has its own built-in variables whose names don't follow
34these rules. They have strange names so they don't accidentally
35collide with one of your normal variables. Strings that match
36parenthesized parts of a regular expression are saved under names
37containing only digits after the C<$> (see L<perlop> and L<perlre>).
38In addition, several special variables that provide windows into
39the inner working of Perl have names containing punctuation characters
40and control characters. These are documented in L<perlvar>.
d74e8afc 41X<variable, built-in>
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42
43Scalar values are always named with '$', even when referring to a
44scalar that is part of an array or a hash. The '$' symbol works
45semantically like the English word "the" in that it indicates a
46single value is expected.
d74e8afc 47X<scalar>
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48
49 $days # the simple scalar value "days"
50 $days[28] # the 29th element of array @days
51 $days{'Feb'} # the 'Feb' value from hash %days
52 $#days # the last index of array @days
53
d55a8828 54Entire arrays (and slices of arrays and hashes) are denoted by '@',
3921068c 55which works much as the word "these" or "those" does in English,
d55a8828 56in that it indicates multiple values are expected.
d74e8afc 57X<array>
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58
59 @days # ($days[0], $days[1],... $days[n])
d55a8828 60 @days[3,4,5] # same as ($days[3],$days[4],$days[5])
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61 @days{'a','c'} # same as ($days{'a'},$days{'c'})
62
d55a8828 63Entire hashes are denoted by '%':
d74e8afc 64X<hash>
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65
66 %days # (key1, val1, key2, val2 ...)
67
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68In addition, subroutines are named with an initial '&', though this
69is optional when unambiguous, just as the word "do" is often redundant
70in English. Symbol table entries can be named with an initial '*',
71but you don't really care about that yet (if ever :-).
72
73Every variable type has its own namespace, as do several
74non-variable identifiers. This means that you can, without fear
75of conflict, use the same name for a scalar variable, an array, or
76a hash--or, for that matter, for a filehandle, a directory handle, a
77subroutine name, a format name, or a label. This means that $foo
78and @foo are two different variables. It also means that C<$foo[1]>
79is a part of @foo, not a part of $foo. This may seem a bit weird,
80but that's okay, because it is weird.
d74e8afc 81X<namespace>
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82
83Because variable references always start with '$', '@', or '%', the
84"reserved" words aren't in fact reserved with respect to variable
85names. They I<are> reserved with respect to labels and filehandles,
86however, which don't have an initial special character. You can't
87have a filehandle named "log", for instance. Hint: you could say
88C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using
89uppercase filehandles also improves readability and protects you
90from conflict with future reserved words. Case I<is> significant--"FOO",
91"Foo", and "foo" are all different names. Names that start with a
92letter or underscore may also contain digits and underscores.
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93X<identifier, case sensitivity>
94X<case>
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95
96It is possible to replace such an alphanumeric name with an expression
d55a8828 97that returns a reference to the appropriate type. For a description
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98of this, see L<perlref>.
99
5f05dabc 100Names that start with a digit may contain only more digits. Names
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101that do not start with a letter, underscore, digit or a caret (i.e.
102a control character) are limited to one character, e.g., C<$%> or
103C<$$>. (Most of these one character names have a predefined
104significance to Perl. For instance, C<$$> is the current process
105id.)
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106
107=head2 Context
d74e8afc 108X<context> X<scalar context> X<list context>
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109
110The interpretation of operations and values in Perl sometimes depends
111on the requirements of the context around the operation or value.
d55a8828 112There are two major contexts: list and scalar. Certain operations
a0d0e21e 113return list values in contexts wanting a list, and scalar values
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114otherwise. If this is true of an operation it will be mentioned in
115the documentation for that operation. In other words, Perl overloads
a0d0e21e 116certain operations based on whether the expected return value is
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117singular or plural. Some words in English work this way, like "fish"
118and "sheep".
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119
120In a reciprocal fashion, an operation provides either a scalar or a
121list context to each of its arguments. For example, if you say
122
123 int( <STDIN> )
124
c47ff5f1 125the integer operation provides scalar context for the <>
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126operator, which responds by reading one line from STDIN and passing it
127back to the integer operation, which will then find the integer value
128of that line and return that. If, on the other hand, you say
129
130 sort( <STDIN> )
131
c47ff5f1 132then the sort operation provides list context for <>, which
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133will proceed to read every line available up to the end of file, and
134pass that list of lines back to the sort routine, which will then
135sort those lines and return them as a list to whatever the context
136of the sort was.
137
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138Assignment is a little bit special in that it uses its left argument
139to determine the context for the right argument. Assignment to a
140scalar evaluates the right-hand side in scalar context, while
141assignment to an array or hash evaluates the righthand side in list
142context. Assignment to a list (or slice, which is just a list
3921068c 143anyway) also evaluates the right-hand side in list context.
d55a8828 144
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145When you use the C<use warnings> pragma or Perl's B<-w> command-line
146option, you may see warnings
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147about useless uses of constants or functions in "void context".
148Void context just means the value has been discarded, such as a
149statement containing only C<"fred";> or C<getpwuid(0);>. It still
150counts as scalar context for functions that care whether or not
151they're being called in list context.
152
153User-defined subroutines may choose to care whether they are being
154called in a void, scalar, or list context. Most subroutines do not
155need to bother, though. That's because both scalars and lists are
156automatically interpolated into lists. See L<perlfunc/wantarray>
157for how you would dynamically discern your function's calling
158context.
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159
160=head2 Scalar values
d74e8afc 161X<scalar> X<number> X<string> X<reference>
a0d0e21e 162
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163All data in Perl is a scalar, an array of scalars, or a hash of
164scalars. A scalar may contain one single value in any of three
165different flavors: a number, a string, or a reference. In general,
166conversion from one form to another is transparent. Although a
167scalar may not directly hold multiple values, it may contain a
168reference to an array or hash which in turn contains multiple values.
169
170Scalars aren't necessarily one thing or another. There's no place
171to declare a scalar variable to be of type "string", type "number",
172type "reference", or anything else. Because of the automatic
173conversion of scalars, operations that return scalars don't need
174to care (and in fact, cannot care) whether their caller is looking
175for a string, a number, or a reference. Perl is a contextually
176polymorphic language whose scalars can be strings, numbers, or
177references (which includes objects). Although strings and numbers
178are considered pretty much the same thing for nearly all purposes,
179references are strongly-typed, uncastable pointers with builtin
180reference-counting and destructor invocation.
a0d0e21e 181
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182A scalar value is interpreted as FALSE in the Boolean sense
183if it is undefined, the null string or the number 0 (or its
184string equivalent, "0"), and TRUE if it is anything else. The
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185Boolean context is just a special kind of scalar context where no
186conversion to a string or a number is ever performed.
d74e8afc 187X<boolean> X<bool> X<true> X<false> X<truth>
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188
189There are actually two varieties of null strings (sometimes referred
190to as "empty" strings), a defined one and an undefined one. The
191defined version is just a string of length zero, such as C<"">.
192The undefined version is the value that indicates that there is
193no real value for something, such as when there was an error, or
194at end of file, or when you refer to an uninitialized variable or
195element of an array or hash. Although in early versions of Perl,
196an undefined scalar could become defined when first used in a
197place expecting a defined value, this no longer happens except for
198rare cases of autovivification as explained in L<perlref>. You can
199use the defined() operator to determine whether a scalar value is
200defined (this has no meaning on arrays or hashes), and the undef()
201operator to produce an undefined value.
d74e8afc 202X<defined> X<undefined> X<undef> X<null> X<string, null>
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203
204To find out whether a given string is a valid non-zero number, it's
205sometimes enough to test it against both numeric 0 and also lexical
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206"0" (although this will cause noises if warnings are on). That's
207because strings that aren't numbers count as 0, just as they do in B<awk>:
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208
209 if ($str == 0 && $str ne "0") {
210 warn "That doesn't look like a number";
54310121 211 }
4633a7c4 212
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213That method may be best because otherwise you won't treat IEEE
214notations like C<NaN> or C<Infinity> properly. At other times, you
215might prefer to determine whether string data can be used numerically
216by calling the POSIX::strtod() function or by inspecting your string
217with a regular expression (as documented in L<perlre>).
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218
219 warn "has nondigits" if /\D/;
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220 warn "not a natural number" unless /^\d+$/; # rejects -3
221 warn "not an integer" unless /^-?\d+$/; # rejects +3
222 warn "not an integer" unless /^[+-]?\d+$/;
223 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
224 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
54310121 225 warn "not a C float"
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226 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
227
d55a8828 228The length of an array is a scalar value. You may find the length
fc518ee5 229of array @days by evaluating C<$#days>, as in B<csh>. However, this
230isn't the length of the array; it's the subscript of the last element,
231which is a different value since there is ordinarily a 0th element.
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232Assigning to C<$#days> actually changes the length of the array.
233Shortening an array this way destroys intervening values. Lengthening
234an array that was previously shortened does not recover values
235that were in those elements. (It used to do so in Perl 4, but we
236had to break this to make sure destructors were called when expected.)
d74e8afc 237X<$#> X<array, length>
d55a8828 238
210b36aa 239You can also gain some minuscule measure of efficiency by pre-extending
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240an array that is going to get big. You can also extend an array
241by assigning to an element that is off the end of the array. You
19799a22 242can truncate an array down to nothing by assigning the null list
d55a8828 243() to it. The following are equivalent:
a0d0e21e 244
84f709e7 245 @whatever = ();
3e3baf6d 246 $#whatever = -1;
a0d0e21e 247
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248If you evaluate an array in scalar context, it returns the length
249of the array. (Note that this is not true of lists, which return
250the last value, like the C comma operator, nor of built-in functions,
251which return whatever they feel like returning.) The following is
252always true:
d74e8afc 253X<array, length>
a0d0e21e 254
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255 scalar(@whatever) == $#whatever + 1;
256
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257Some programmers choose to use an explicit conversion so as to
258leave nothing to doubt:
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259
260 $element_count = scalar(@whatever);
261
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262If you evaluate a hash in scalar context, it returns false if the
263hash is empty. If there are any key/value pairs, it returns true;
264more precisely, the value returned is a string consisting of the
265number of used buckets and the number of allocated buckets, separated
266by a slash. This is pretty much useful only to find out whether
267Perl's internal hashing algorithm is performing poorly on your data
268set. For example, you stick 10,000 things in a hash, but evaluating
269%HASH in scalar context reveals C<"1/16">, which means only one out
270of sixteen buckets has been touched, and presumably contains all
126c71c8 27110,000 of your items. This isn't supposed to happen. If a tied hash
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272is evaluated in scalar context, the C<SCALAR> method is called (with a
273fallback to C<FIRSTKEY>).
d74e8afc 274X<hash, scalar context> X<hash, bucket> X<bucket>
a0d0e21e 275
5a964f20 276You can preallocate space for a hash by assigning to the keys() function.
65841adf 277This rounds up the allocated buckets to the next power of two:
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278
279 keys(%users) = 1000; # allocate 1024 buckets
280
a0d0e21e 281=head2 Scalar value constructors
d74e8afc 282X<scalar, literal> X<scalar, constant>
a0d0e21e 283
d55a8828 284Numeric literals are specified in any of the following floating point or
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285integer formats:
286
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287 12345
288 12345.67
d55a8828 289 .23E-10 # a very small number
928753ea 290 3.14_15_92 # a very important number
1d277562 291 4_294_967_296 # underscore for legibility
d55a8828 292 0xff # hex
928753ea 293 0xdead_beef # more hex
802a55ac 294 0377 # octal (only numbers, begins with 0)
d55a8828 295 0b011011 # binary
a0d0e21e 296
d4ced10d 297You are allowed to use underscores (underbars) in numeric literals
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298between digits for legibility (but not multiple underscores in a row:
299C<23__500> is not legal; C<23_500> is).
300You could, for example, group binary
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301digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
302or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
d74e8afc 303X<number, literal>
1d277562 304
55497cff 305String literals are usually delimited by either single or double
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306quotes. They work much like quotes in the standard Unix shells:
307double-quoted string literals are subject to backslash and variable
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308substitution; single-quoted strings are not (except for C<\'> and
309C<\\>). The usual C-style backslash rules apply for making
d55a8828 310characters such as newline, tab, etc., as well as some more exotic
4a4eefd0 311forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
d74e8afc 312X<string, literal>
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313
314Hexadecimal, octal, or binary, representations in string literals
315(e.g. '0xff') are not automatically converted to their integer
316representation. The hex() and oct() functions make these conversions
317for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
68dc0745 318
5f05dabc 319You can also embed newlines directly in your strings, i.e., they can end
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320on a different line than they begin. This is nice, but if you forget
321your trailing quote, the error will not be reported until Perl finds
322another line containing the quote character, which may be much further
323on in the script. Variable substitution inside strings is limited to
d55a8828 324scalar variables, arrays, and array or hash slices. (In other words,
b88cefa9 325names beginning with $ or @, followed by an optional bracketed
a0d0e21e 326expression as a subscript.) The following code segment prints out "The
184e9718 327price is $Z<>100."
d74e8afc 328X<interpolation>
a0d0e21e 329
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330 $Price = '$100'; # not interpolated
331 print "The price is $Price.\n"; # interpolated
332
333There is no double interpolation in Perl, so the C<$100> is left as is.
a0d0e21e 334
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335By default floating point numbers substituted inside strings use the
336dot (".") as the decimal separator. If C<use locale> is in effect,
337and POSIX::setlocale() has been called, the character used for the
338decimal separator is affected by the LC_NUMERIC locale.
339See L<perllocale> and L<POSIX>.
340
d55a8828 341As in some shells, you can enclose the variable name in braces to
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342disambiguate it from following alphanumerics (and underscores).
343You must also do
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344this when interpolating a variable into a string to separate the
345variable name from a following double-colon or an apostrophe, since
346these would be otherwise treated as a package separator:
d74e8afc 347X<interpolation>
d55a8828 348
84f709e7 349 $who = "Larry";
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350 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
351 print "We use ${who}speak when ${who}'s here.\n";
352
353Without the braces, Perl would have looked for a $whospeak, a
354C<$who::0>, and a C<$who's> variable. The last two would be the
355$0 and the $s variables in the (presumably) non-existent package
356C<who>.
357
358In fact, an identifier within such curlies is forced to be a string,
359as is any simple identifier within a hash subscript. Neither need
360quoting. Our earlier example, C<$days{'Feb'}> can be written as
361C<$days{Feb}> and the quotes will be assumed automatically. But
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362anything more complicated in the subscript will be interpreted as an
363expression. This means for example that C<$version{2.0}++> is
364equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
d55a8828 365
692ef166 366=head3 Version Strings
d74e8afc 367X<version string> X<vstring> X<v-string>
692ef166 368
191d61a7 369A literal of the form C<v1.20.300.4000> is parsed as a string composed
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370of characters with the specified ordinals. This form, known as
371v-strings, provides an alternative, more readable way to construct
372strings, rather than use the somewhat less readable interpolation form
373C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
374Unicode strings, and for comparing version "numbers" using the string
375comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
376more dots in the literal, the leading C<v> may be omitted.
b9c62f5b 377
2575c402 378 print v9786; # prints SMILEY, "\x{263a}"
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379 print v102.111.111; # prints "foo"
380 print 102.111.111; # same
381
382Such literals are accepted by both C<require> and C<use> for
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383doing a version check. Note that using the v-strings for IPv4
384addresses is not portable unless you also use the
385inet_aton()/inet_ntoa() routines of the Socket package.
191d61a7 386
d32a65d2 387Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
8fa72689 388are not v-strings before the C<< => >> operator (which is usually used
3921068c 389to separate a hash key from a hash value); instead they are interpreted
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390as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
391Perl 5.8.0, but that caused more confusion and breakage than good.
392Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
393be v-strings always.
d32a65d2 394
692ef166 395=head3 Special Literals
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396X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
397X<end> X<data> X<^D> X<^Z>
692ef166 398
d55a8828 399The special literals __FILE__, __LINE__, and __PACKAGE__
68dc0745 400represent the current filename, line number, and package name at that
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401point in your program. __SUB__ gives a reference to the current
402subroutine. They may be used only as separate tokens; they
68dc0745 403will not be interpolated into strings. If there is no current package
3e92a254 404(due to an empty C<package;> directive), __PACKAGE__ is the undefined
8fdd8881 405value. (But the empty C<package;> is no longer supported, as of version
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4065.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
407is only available in 5.16 or higher, and only with a C<use v5.16> or
408C<use feature "current_sub"> declaration.
409X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
410X<line> X<file> X<package>
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411
412The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
413may be used to indicate the logical end of the script before the actual
414end of file. Any following text is ignored.
415
1bab44f9 416Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
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417where C<PACKNAME> is the package that was current when the __DATA__
418token was encountered. The filehandle is left open pointing to the
4d383607 419line after __DATA__. The program should C<close DATA> when it is done
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420reading from it. (Leaving it open leaks filehandles if the module is
421reloaded for any reason, so it's a safer practice to close it.) For
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422compatibility with older scripts written before __DATA__ was
423introduced, __END__ behaves like __DATA__ in the top level script (but
424not in files loaded with C<require> or C<do>) and leaves the remaining
425contents of the file accessible via C<main::DATA>.
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426
427See L<SelfLoader> for more description of __DATA__, and
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428an example of its use. Note that you cannot read from the DATA
429filehandle in a BEGIN block: the BEGIN block is executed as soon
430as it is seen (during compilation), at which point the corresponding
a00c1fe5 431__DATA__ (or __END__) token has not yet been seen.
a0d0e21e 432
692ef166 433=head3 Barewords
d74e8afc 434X<bareword>
692ef166 435
748a9306 436A word that has no other interpretation in the grammar will
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437be treated as if it were a quoted string. These are known as
438"barewords". As with filehandles and labels, a bareword that consists
439entirely of lowercase letters risks conflict with future reserved
9f1b1f2d 440words, and if you use the C<use warnings> pragma or the B<-w> switch,
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441Perl will warn you about any such words. Perl limits barewords (like
442identifiers) to about 250 characters. Future versions of Perl are likely
443to eliminate these arbitrary limitations.
444
445Some people may wish to outlaw barewords entirely. If you
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446say
447
448 use strict 'subs';
449
450then any bareword that would NOT be interpreted as a subroutine call
451produces a compile-time error instead. The restriction lasts to the
54310121 452end of the enclosing block. An inner block may countermand this
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453by saying C<no strict 'subs'>.
454
e2b457c0 455=head3 Array Interpolation
d74e8afc 456X<array, interpolation> X<interpolation, array> X<$">
692ef166 457
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458Arrays and slices are interpolated into double-quoted strings
459by joining the elements with the delimiter specified in the C<$">
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460variable (C<$LIST_SEPARATOR> if "use English;" is specified),
461space by default. The following are equivalent:
a0d0e21e 462
84f709e7 463 $temp = join($", @ARGV);
a0d0e21e
LW
464 system "echo $temp";
465
466 system "echo @ARGV";
467
468Within search patterns (which also undergo double-quotish substitution)
d55a8828 469there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
a0d0e21e
LW
470C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
471expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
472@foo)? If @foo doesn't otherwise exist, then it's obviously a
473character class. If @foo exists, Perl takes a good guess about C<[bar]>,
474and is almost always right. If it does guess wrong, or if you're just
475plain paranoid, you can force the correct interpretation with curly
d55a8828 476braces as above.
a0d0e21e 477
7e3b091d 478If you're looking for the information on how to use here-documents,
210b36aa
AMS
479which used to be here, that's been moved to
480L<perlop/Quote and Quote-like Operators>.
be16fac9 481
a0d0e21e 482=head2 List value constructors
d74e8afc 483X<list>
a0d0e21e
LW
484
485List values are denoted by separating individual values by commas
486(and enclosing the list in parentheses where precedence requires it):
487
488 (LIST)
489
d55a8828
TC
490In a context not requiring a list value, the value of what appears
491to be a list literal is simply the value of the final element, as
492with the C comma operator. For example,
a0d0e21e 493
84f709e7 494 @foo = ('cc', '-E', $bar);
a0d0e21e 495
d55a8828 496assigns the entire list value to array @foo, but
a0d0e21e 497
84f709e7 498 $foo = ('cc', '-E', $bar);
a0d0e21e 499
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TC
500assigns the value of variable $bar to the scalar variable $foo.
501Note that the value of an actual array in scalar context is the
502length of the array; the following assigns the value 3 to $foo:
a0d0e21e 503
84f709e7 504 @foo = ('cc', '-E', $bar);
7e3b091d 505 $foo = @foo; # $foo gets 3
a0d0e21e 506
54310121 507You may have an optional comma before the closing parenthesis of a
a0d0e21e
LW
508list literal, so that you can say:
509
84f709e7 510 @foo = (
7e3b091d
DA
511 1,
512 2,
513 3,
a0d0e21e
LW
514 );
515
d55a8828
TC
516To use a here-document to assign an array, one line per element,
517you might use an approach like this:
518
84f709e7 519 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
7e3b091d
DA
520 normal tomato
521 spicy tomato
522 green chile
523 pesto
524 white wine
d55a8828
TC
525 End_Lines
526
a0d0e21e 527LISTs do automatic interpolation of sublists. That is, when a LIST is
d55a8828 528evaluated, each element of the list is evaluated in list context, and
a0d0e21e 529the resulting list value is interpolated into LIST just as if each
5a964f20 530individual element were a member of LIST. Thus arrays and hashes lose their
a0d0e21e
LW
531identity in a LIST--the list
532
5a964f20 533 (@foo,@bar,&SomeSub,%glarch)
a0d0e21e
LW
534
535contains all the elements of @foo followed by all the elements of @bar,
5a964f20 536followed by all the elements returned by the subroutine named SomeSub
d55a8828 537called in list context, followed by the key/value pairs of %glarch.
a0d0e21e
LW
538To make a list reference that does I<NOT> interpolate, see L<perlref>.
539
19799a22 540The null list is represented by (). Interpolating it in a list
a0d0e21e
LW
541has no effect. Thus ((),(),()) is equivalent to (). Similarly,
542interpolating an array with no elements is the same as if no
543array had been interpolated at that point.
544
c2689353 545This interpolation combines with the facts that the opening
ab1f959b 546and closing parentheses are optional (except when necessary for
c2689353 547precedence) and lists may end with an optional comma to mean that
8fdd8881 548multiple commas within lists are legal syntax. The list C<1,,3> is a
c2689353
NC
549concatenation of two lists, C<1,> and C<3>, the first of which ends
550with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
551similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
552we'd advise you to use this obfuscation.
553
a0d0e21e 554A list value may also be subscripted like a normal array. You must
54310121 555put the list in parentheses to avoid ambiguity. For example:
a0d0e21e
LW
556
557 # Stat returns list value.
84f709e7 558 $time = (stat($file))[8];
a0d0e21e 559
4633a7c4 560 # SYNTAX ERROR HERE.
84f709e7 561 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
4633a7c4 562
a0d0e21e 563 # Find a hex digit.
84f709e7 564 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
a0d0e21e
LW
565
566 # A "reverse comma operator".
567 return (pop(@foo),pop(@foo))[0];
568
d55a8828
TC
569Lists may be assigned to only when each element of the list
570is itself legal to assign to:
a0d0e21e 571
84f709e7 572 ($a, $b, $c) = (1, 2, 3);
a0d0e21e 573
84f709e7 574 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
a0d0e21e 575
d55a8828
TC
576An exception to this is that you may assign to C<undef> in a list.
577This is useful for throwing away some of the return values of a
578function:
579
84f709e7 580 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
d55a8828
TC
581
582List assignment in scalar context returns the number of elements
4633a7c4
LW
583produced by the expression on the right side of the assignment:
584
7e3b091d
DA
585 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
586 $x = (($foo,$bar) = f()); # set $x to f()'s return count
4633a7c4 587
d55a8828 588This is handy when you want to do a list assignment in a Boolean
19799a22 589context, because most list functions return a null list when finished,
4633a7c4
LW
590which when assigned produces a 0, which is interpreted as FALSE.
591
ab1f959b
PN
592It's also the source of a useful idiom for executing a function or
593performing an operation in list context and then counting the number of
594return values, by assigning to an empty list and then using that
8fdd8881 595assignment in scalar context. For example, this code:
ab1f959b 596
84f709e7 597 $count = () = $string =~ /\d+/g;
ab1f959b
PN
598
599will place into $count the number of digit groups found in $string.
600This happens because the pattern match is in list context (since it
601is being assigned to the empty list), and will therefore return a list
8fdd8881 602of all matching parts of the string. The list assignment in scalar
ab1f959b 603context will translate that into the number of elements (here, the
8fdd8881 604number of times the pattern matched) and assign that to $count. Note
ab1f959b
PN
605that simply using
606
84f709e7 607 $count = $string =~ /\d+/g;
ab1f959b
PN
608
609would not have worked, since a pattern match in scalar context will
610only return true or false, rather than a count of matches.
611
612The final element of a list assignment may be an array or a hash:
a0d0e21e 613
84f709e7 614 ($a, $b, @rest) = split;
5a964f20 615 my($a, $b, %rest) = @_;
a0d0e21e 616
4633a7c4 617You can actually put an array or hash anywhere in the list, but the first one
d55a8828
TC
618in the list will soak up all the values, and anything after it will become
619undefined. This may be useful in a my() or local().
a0d0e21e 620
d55a8828
TC
621A hash can be initialized using a literal list holding pairs of
622items to be interpreted as a key and a value:
a0d0e21e
LW
623
624 # same as map assignment above
84f709e7 625 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
a0d0e21e 626
d55a8828 627While literal lists and named arrays are often interchangeable, that's
4633a7c4
LW
628not the case for hashes. Just because you can subscript a list value like
629a normal array does not mean that you can subscript a list value as a
630hash. Likewise, hashes included as parts of other lists (including
631parameters lists and return lists from functions) always flatten out into
632key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 633
c47ff5f1
GS
634It is often more readable to use the C<< => >> operator between key/value
635pairs. The C<< => >> operator is mostly just a more visually distinctive
b88cefa9 636synonym for a comma, but it also arranges for its left-hand operand to be
ac036724 637interpreted as a string if it's a bareword that would be a legal simple
8fdd8881
FC
638identifier. C<< => >> doesn't quote compound identifiers, that contain
639double colons. This makes it nice for initializing hashes:
a0d0e21e 640
84f709e7 641 %map = (
7e3b091d
DA
642 red => 0x00f,
643 blue => 0x0f0,
644 green => 0xf00,
4633a7c4
LW
645 );
646
647or for initializing hash references to be used as records:
648
84f709e7 649 $rec = {
7e3b091d
DA
650 witch => 'Mable the Merciless',
651 cat => 'Fluffy the Ferocious',
652 date => '10/31/1776',
4633a7c4
LW
653 };
654
655or for using call-by-named-parameter to complicated functions:
656
84f709e7 657 $field = $query->radio_group(
7e3b091d 658 name => 'group_name',
4633a7c4
LW
659 values => ['eenie','meenie','minie'],
660 default => 'meenie',
661 linebreak => 'true',
84f709e7 662 labels => \%labels
4633a7c4 663 );
cb1a09d0
AD
664
665Note that just because a hash is initialized in that order doesn't
666mean that it comes out in that order. See L<perlfunc/sort> for examples
667of how to arrange for an output ordering.
668
692ef166
SF
669=head2 Subscripts
670
aa80e1dc
FC
671An array can be accessed one scalar at a
672time by specifying a dollar sign (C<$>), then the
692ef166
SF
673name of the array (without the leading C<@>), then the subscript inside
674square brackets. For example:
675
676 @myarray = (5, 50, 500, 5000);
2adc35dd 677 print "The Third Element is", $myarray[2], "\n";
692ef166 678
8fdd8881 679The array indices start with 0. A negative subscript retrieves its
692ef166
SF
680value from the end. In our example, C<$myarray[-1]> would have been
6815000, and C<$myarray[-2]> would have been 500.
682
683Hash subscripts are similar, only instead of square brackets curly brackets
8fdd8881 684are used. For example:
692ef166
SF
685
686 %scientists =
687 (
688 "Newton" => "Isaac",
689 "Einstein" => "Albert",
690 "Darwin" => "Charles",
691 "Feynman" => "Richard",
692 );
693
694 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
695
aa80e1dc 696You can also subscript a list to get a single element from it:
d55a8828 697
aa80e1dc 698 $dir = (getpwnam("daemon"))[7];
d55a8828 699
9ed2a148
IG
700=head2 Multi-dimensional array emulation
701
702Multidimensional arrays may be emulated by subscripting a hash with a
8fdd8881 703list. The elements of the list are joined with the subscript separator
b8db74f2 704(see L<perlvar/$;>).
9ed2a148
IG
705
706 $foo{$a,$b,$c}
707
708is equivalent to
709
710 $foo{join($;, $a, $b, $c)}
711
712The default subscript separator is "\034", the same as SUBSEP in B<awk>.
713
aa80e1dc
FC
714=head2 Slices
715X<slice> X<array, slice> X<hash, slice>
d55a8828
TC
716
717A slice accesses several elements of a list, an array, or a hash
56d7751a
GS
718simultaneously using a list of subscripts. It's more convenient
719than writing out the individual elements as a list of separate
d55a8828
TC
720scalar values.
721
7e3b091d
DA
722 ($him, $her) = @folks[0,-1]; # array slice
723 @them = @folks[0 .. 3]; # array slice
724 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
725 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
d55a8828
TC
726
727Since you can assign to a list of variables, you can also assign to
728an array or hash slice.
729
84f709e7 730 @days[3..5] = qw/Wed Thu Fri/;
d55a8828 731 @colors{'red','blue','green'}
7e3b091d 732 = (0xff0000, 0x0000ff, 0x00ff00);
d55a8828
TC
733 @folks[0, -1] = @folks[-1, 0];
734
735The previous assignments are exactly equivalent to
736
84f709e7
JH
737 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
738 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
7e3b091d 739 = (0xff0000, 0x0000ff, 0x00ff00);
88fd19e3 740 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
d55a8828
TC
741
742Since changing a slice changes the original array or hash that it's
56d7751a
GS
743slicing, a C<foreach> construct will alter some--or even all--of the
744values of the array or hash.
d55a8828
TC
745
746 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
747
00cb5da1 748 foreach (@hash{qw[key1 key2]}) {
7e3b091d
DA
749 s/^\s+//; # trim leading whitespace
750 s/\s+$//; # trim trailing whitespace
751 s/(\w+)/\u\L$1/g; # "titlecase" words
d55a8828
TC
752 }
753
08cd8952
GS
754A slice of an empty list is still an empty list. Thus:
755
84f709e7
JH
756 @a = ()[1,0]; # @a has no elements
757 @b = (@a)[0,1]; # @b has no elements
758 @c = (0,1)[2,3]; # @c has no elements
56d7751a
GS
759
760But:
761
84f709e7
JH
762 @a = (1)[1,0]; # @a has two elements
763 @b = (1,undef)[1,0,2]; # @b has three elements
08cd8952 764
19799a22
GS
765This makes it easy to write loops that terminate when a null list
766is returned:
d55a8828 767
84f709e7 768 while ( ($home, $user) = (getpwent)[7,0]) {
7e3b091d 769 printf "%-8s %s\n", $user, $home;
d55a8828
TC
770 }
771
772As noted earlier in this document, the scalar sense of list assignment
773is the number of elements on the right-hand side of the assignment.
19799a22 774The null list contains no elements, so when the password file is
d55a8828
TC
775exhausted, the result is 0, not 2.
776
ad1de9c6
ML
777Slices in scalar context return the last item of the slice.
778
779 @a = qw/first second third/;
780 %h = (first => 'A', second => 'B');
781 $t = @a[0, 1]; # $t is now 'second'
0de10106 782 $u = @h{'first', 'second'}; # $u is now 'B'
ad1de9c6 783
d55a8828
TC
784If you're confused about why you use an '@' there on a hash slice
785instead of a '%', think of it like this. The type of bracket (square
786or curly) governs whether it's an array or a hash being looked at.
787On the other hand, the leading symbol ('$' or '@') on the array or
788hash indicates whether you are getting back a singular value (a
789scalar) or a plural one (a list).
790
5f05dabc 791=head2 Typeglobs and Filehandles
d74e8afc 792X<typeglob> X<filehandle> X<*>
cb1a09d0
AD
793
794Perl uses an internal type called a I<typeglob> to hold an entire
795symbol table entry. The type prefix of a typeglob is a C<*>, because
54310121 796it represents all types. This used to be the preferred way to
cb1a09d0 797pass arrays and hashes by reference into a function, but now that
5a964f20
TC
798we have real references, this is seldom needed.
799
800The main use of typeglobs in modern Perl is create symbol table aliases.
801This assignment:
802
803 *this = *that;
804
805makes $this an alias for $that, @this an alias for @that, %this an alias
806for %that, &this an alias for &that, etc. Much safer is to use a reference.
807This:
5f05dabc 808
5a964f20
TC
809 local *Here::blue = \$There::green;
810
811temporarily makes $Here::blue an alias for $There::green, but doesn't
812make @Here::blue an alias for @There::green, or %Here::blue an alias for
813%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
814of this. Strange though this may seem, this is the basis for the whole
84f709e7 815module import/export system.
5a964f20 816
d55a8828 817Another use for typeglobs is to pass filehandles into a function or
5a964f20
TC
818to create new filehandles. If you need to use a typeglob to save away
819a filehandle, do it this way:
5f05dabc 820
84f709e7 821 $fh = *STDOUT;
5f05dabc
PP
822
823or perhaps as a real reference, like this:
824
84f709e7 825 $fh = \*STDOUT;
5f05dabc 826
5a964f20
TC
827See L<perlsub> for examples of using these as indirect filehandles
828in functions.
829
830Typeglobs are also a way to create a local filehandle using the local()
831operator. These last until their block is exited, but may be passed back.
832For example:
5f05dabc
PP
833
834 sub newopen {
7e3b091d
DA
835 my $path = shift;
836 local *FH; # not my!
837 open (FH, $path) or return undef;
838 return *FH;
5f05dabc 839 }
84f709e7 840 $fh = newopen('/etc/passwd');
5f05dabc 841
d55a8828 842Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
5a964f20 843for filehandle manipulations, although they're still needed to pass brand
8fdd8881 844new file and directory handles into or out of functions. That's because
d55a8828
TC
845C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
846In other words, C<*FH> must be used to create new symbol table entries;
847C<*foo{THING}> cannot. When in doubt, use C<*FH>.
848
36392fcf
GS
849All functions that are capable of creating filehandles (open(),
850opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
851automatically create an anonymous filehandle if the handle passed to
8fdd8881 852them is an uninitialized scalar variable. This allows the constructs
36392fcf
GS
853such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
854create filehandles that will conveniently be closed automatically when
8fdd8881 855the scope ends, provided there are no other references to them. This
36392fcf
GS
856largely eliminates the need for typeglobs when opening filehandles
857that must be passed around, as in the following example:
858
859 sub myopen {
84f709e7 860 open my $fh, "@_"
7e3b091d
DA
861 or die "Can't open '@_': $!";
862 return $fh;
36392fcf
GS
863 }
864
865 {
866 my $f = myopen("</etc/motd");
7e3b091d
DA
867 print <$f>;
868 # $f implicitly closed here
36392fcf
GS
869 }
870
b92795fe
AMS
871Note that if an initialized scalar variable is used instead the
872result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
873to C<open( *{'zzz'}, ...)>.
d83fe814
AT
874C<use strict 'refs'> forbids such practice.
875
d55a8828
TC
876Another way to create anonymous filehandles is with the Symbol
877module or with the IO::Handle module and its ilk. These modules
878have the advantage of not hiding different types of the same name
66b6e4ad
KW
879during the local(). See the bottom of L<perlfunc/open> for an
880example.
d55a8828
TC
881
882=head1 SEE ALSO
883
884See L<perlvar> for a description of Perl's built-in variables and
885a discussion of legal variable names. See L<perlref>, L<perlsub>,
886and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
887the C<*foo{THING}> syntax.