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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
8
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9Perl has three built-in data types: scalars, arrays of scalars, and
10associative arrays of scalars, known as "hashes". Normal arrays
19799a22 11are ordered lists of scalars indexed by number, starting with 0 and with
d55a8828 12negative subscripts counting from the end. Hashes are unordered
19799a22 13collections of scalar values indexed by their associated string key.
a0d0e21e 14
d55a8828 15Values are usually referred to by name, or through a named reference.
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16The first character of the name tells you to what sort of data
17structure it refers. The rest of the name tells you the particular
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18value to which it refers. Usually this name is a single I<identifier>,
19that is, a string beginning with a letter or underscore, and
20containing letters, underscores, and digits. In some cases, it may
21be a chain of identifiers, separated by C<::> (or by the slightly
22archaic C<'>); all but the last are interpreted as names of packages,
23to locate the namespace in which to look up the final identifier
24(see L<perlmod/Packages> for details). It's possible to substitute
25for a simple identifier, an expression that produces a reference
26to the value at runtime. This is described in more detail below
27and in L<perlref>.
28
29Perl also has its own built-in variables whose names don't follow
30these rules. They have strange names so they don't accidentally
31collide with one of your normal variables. Strings that match
32parenthesized parts of a regular expression are saved under names
33containing only digits after the C<$> (see L<perlop> and L<perlre>).
34In addition, several special variables that provide windows into
35the inner working of Perl have names containing punctuation characters
36and control characters. These are documented in L<perlvar>.
37
38Scalar values are always named with '$', even when referring to a
39scalar that is part of an array or a hash. The '$' symbol works
40semantically like the English word "the" in that it indicates a
41single value is expected.
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42
43 $days # the simple scalar value "days"
44 $days[28] # the 29th element of array @days
45 $days{'Feb'} # the 'Feb' value from hash %days
46 $#days # the last index of array @days
47
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48Entire arrays (and slices of arrays and hashes) are denoted by '@',
49which works much like the word "these" or "those" does in English,
50in that it indicates multiple values are expected.
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51
52 @days # ($days[0], $days[1],... $days[n])
d55a8828 53 @days[3,4,5] # same as ($days[3],$days[4],$days[5])
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54 @days{'a','c'} # same as ($days{'a'},$days{'c'})
55
d55a8828 56Entire hashes are denoted by '%':
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57
58 %days # (key1, val1, key2, val2 ...)
59
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60In addition, subroutines are named with an initial '&', though this
61is optional when unambiguous, just as the word "do" is often redundant
62in English. Symbol table entries can be named with an initial '*',
63but you don't really care about that yet (if ever :-).
64
65Every variable type has its own namespace, as do several
66non-variable identifiers. This means that you can, without fear
67of conflict, use the same name for a scalar variable, an array, or
68a hash--or, for that matter, for a filehandle, a directory handle, a
69subroutine name, a format name, or a label. This means that $foo
70and @foo are two different variables. It also means that C<$foo[1]>
71is a part of @foo, not a part of $foo. This may seem a bit weird,
72but that's okay, because it is weird.
73
74Because variable references always start with '$', '@', or '%', the
75"reserved" words aren't in fact reserved with respect to variable
76names. They I<are> reserved with respect to labels and filehandles,
77however, which don't have an initial special character. You can't
78have a filehandle named "log", for instance. Hint: you could say
79C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using
80uppercase filehandles also improves readability and protects you
81from conflict with future reserved words. Case I<is> significant--"FOO",
82"Foo", and "foo" are all different names. Names that start with a
83letter or underscore may also contain digits and underscores.
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84
85It is possible to replace such an alphanumeric name with an expression
d55a8828 86that returns a reference to the appropriate type. For a description
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87of this, see L<perlref>.
88
5f05dabc 89Names that start with a digit may contain only more digits. Names
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90that do not start with a letter, underscore, digit or a caret (i.e.
91a control character) are limited to one character, e.g., C<$%> or
92C<$$>. (Most of these one character names have a predefined
93significance to Perl. For instance, C<$$> is the current process
94id.)
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95
96=head2 Context
97
98The interpretation of operations and values in Perl sometimes depends
99on the requirements of the context around the operation or value.
d55a8828 100There are two major contexts: list and scalar. Certain operations
a0d0e21e 101return list values in contexts wanting a list, and scalar values
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102otherwise. If this is true of an operation it will be mentioned in
103the documentation for that operation. In other words, Perl overloads
a0d0e21e 104certain operations based on whether the expected return value is
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105singular or plural. Some words in English work this way, like "fish"
106and "sheep".
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107
108In a reciprocal fashion, an operation provides either a scalar or a
109list context to each of its arguments. For example, if you say
110
111 int( <STDIN> )
112
c47ff5f1 113the integer operation provides scalar context for the <>
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114operator, which responds by reading one line from STDIN and passing it
115back to the integer operation, which will then find the integer value
116of that line and return that. If, on the other hand, you say
117
118 sort( <STDIN> )
119
c47ff5f1 120then the sort operation provides list context for <>, which
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121will proceed to read every line available up to the end of file, and
122pass that list of lines back to the sort routine, which will then
123sort those lines and return them as a list to whatever the context
124of the sort was.
125
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126Assignment is a little bit special in that it uses its left argument
127to determine the context for the right argument. Assignment to a
128scalar evaluates the right-hand side in scalar context, while
129assignment to an array or hash evaluates the righthand side in list
130context. Assignment to a list (or slice, which is just a list
131anyway) also evaluates the righthand side in list context.
132
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133When you use the C<use warnings> pragma or Perl's B<-w> command-line
134option, you may see warnings
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135about useless uses of constants or functions in "void context".
136Void context just means the value has been discarded, such as a
137statement containing only C<"fred";> or C<getpwuid(0);>. It still
138counts as scalar context for functions that care whether or not
139they're being called in list context.
140
141User-defined subroutines may choose to care whether they are being
142called in a void, scalar, or list context. Most subroutines do not
143need to bother, though. That's because both scalars and lists are
144automatically interpolated into lists. See L<perlfunc/wantarray>
145for how you would dynamically discern your function's calling
146context.
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147
148=head2 Scalar values
149
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150All data in Perl is a scalar, an array of scalars, or a hash of
151scalars. A scalar may contain one single value in any of three
152different flavors: a number, a string, or a reference. In general,
153conversion from one form to another is transparent. Although a
154scalar may not directly hold multiple values, it may contain a
155reference to an array or hash which in turn contains multiple values.
156
157Scalars aren't necessarily one thing or another. There's no place
158to declare a scalar variable to be of type "string", type "number",
159type "reference", or anything else. Because of the automatic
160conversion of scalars, operations that return scalars don't need
161to care (and in fact, cannot care) whether their caller is looking
162for a string, a number, or a reference. Perl is a contextually
163polymorphic language whose scalars can be strings, numbers, or
164references (which includes objects). Although strings and numbers
165are considered pretty much the same thing for nearly all purposes,
166references are strongly-typed, uncastable pointers with builtin
167reference-counting and destructor invocation.
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168
169A scalar value is interpreted as TRUE in the Boolean sense if it is not
19799a22 170the null string or the number 0 (or its string equivalent, "0"). The
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171Boolean context is just a special kind of scalar context where no
172conversion to a string or a number is ever performed.
173
174There are actually two varieties of null strings (sometimes referred
175to as "empty" strings), a defined one and an undefined one. The
176defined version is just a string of length zero, such as C<"">.
177The undefined version is the value that indicates that there is
178no real value for something, such as when there was an error, or
179at end of file, or when you refer to an uninitialized variable or
180element of an array or hash. Although in early versions of Perl,
181an undefined scalar could become defined when first used in a
182place expecting a defined value, this no longer happens except for
183rare cases of autovivification as explained in L<perlref>. You can
184use the defined() operator to determine whether a scalar value is
185defined (this has no meaning on arrays or hashes), and the undef()
186operator to produce an undefined value.
187
188To find out whether a given string is a valid non-zero number, it's
189sometimes enough to test it against both numeric 0 and also lexical
190"0" (although this will cause B<-w> noises). That's because strings
191that aren't numbers count as 0, just as they do in B<awk>:
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192
193 if ($str == 0 && $str ne "0") {
194 warn "That doesn't look like a number";
54310121 195 }
4633a7c4 196
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197That method may be best because otherwise you won't treat IEEE
198notations like C<NaN> or C<Infinity> properly. At other times, you
199might prefer to determine whether string data can be used numerically
200by calling the POSIX::strtod() function or by inspecting your string
201with a regular expression (as documented in L<perlre>).
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202
203 warn "has nondigits" if /\D/;
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204 warn "not a natural number" unless /^\d+$/; # rejects -3
205 warn "not an integer" unless /^-?\d+$/; # rejects +3
206 warn "not an integer" unless /^[+-]?\d+$/;
207 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
208 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
54310121 209 warn "not a C float"
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210 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
211
d55a8828 212The length of an array is a scalar value. You may find the length
fc518ee5 213of array @days by evaluating C<$#days>, as in B<csh>. However, this
214isn't the length of the array; it's the subscript of the last element,
215which is a different value since there is ordinarily a 0th element.
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216Assigning to C<$#days> actually changes the length of the array.
217Shortening an array this way destroys intervening values. Lengthening
218an array that was previously shortened does not recover values
219that were in those elements. (It used to do so in Perl 4, but we
220had to break this to make sure destructors were called when expected.)
221
222You can also gain some miniscule measure of efficiency by pre-extending
223an array that is going to get big. You can also extend an array
224by assigning to an element that is off the end of the array. You
19799a22 225can truncate an array down to nothing by assigning the null list
d55a8828 226() to it. The following are equivalent:
a0d0e21e 227
84f709e7 228 @whatever = ();
3e3baf6d 229 $#whatever = -1;
a0d0e21e 230
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231If you evaluate an array in scalar context, it returns the length
232of the array. (Note that this is not true of lists, which return
233the last value, like the C comma operator, nor of built-in functions,
234which return whatever they feel like returning.) The following is
235always true:
a0d0e21e 236
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237 scalar(@whatever) == $#whatever - $[ + 1;
238
239Version 5 of Perl changed the semantics of C<$[>: files that don't set
240the value of C<$[> no longer need to worry about whether another
241file changed its value. (In other words, use of C<$[> is deprecated.)
242So in general you can assume that
243
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244 scalar(@whatever) == $#whatever + 1;
245
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246Some programmers choose to use an explicit conversion so as to
247leave nothing to doubt:
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248
249 $element_count = scalar(@whatever);
250
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251If you evaluate a hash in scalar context, it returns false if the
252hash is empty. If there are any key/value pairs, it returns true;
253more precisely, the value returned is a string consisting of the
254number of used buckets and the number of allocated buckets, separated
255by a slash. This is pretty much useful only to find out whether
256Perl's internal hashing algorithm is performing poorly on your data
257set. For example, you stick 10,000 things in a hash, but evaluating
258%HASH in scalar context reveals C<"1/16">, which means only one out
259of sixteen buckets has been touched, and presumably contains all
26010,000 of your items. This isn't supposed to happen.
a0d0e21e 261
5a964f20 262You can preallocate space for a hash by assigning to the keys() function.
65841adf 263This rounds up the allocated buckets to the next power of two:
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264
265 keys(%users) = 1000; # allocate 1024 buckets
266
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267=head2 Scalar value constructors
268
d55a8828 269Numeric literals are specified in any of the following floating point or
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270integer formats:
271
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272 12345
273 12345.67
d55a8828 274 .23E-10 # a very small number
928753ea 275 3.14_15_92 # a very important number
1d277562 276 4_294_967_296 # underscore for legibility
d55a8828 277 0xff # hex
928753ea 278 0xdead_beef # more hex
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279 0377 # octal
280 0b011011 # binary
a0d0e21e 281
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282You are allowed to use underscores (underbars) in numeric literals
283between digits for legibility. You could, for example, group binary
284digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
285or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
1d277562 286
55497cff 287String literals are usually delimited by either single or double
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288quotes. They work much like quotes in the standard Unix shells:
289double-quoted string literals are subject to backslash and variable
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290substitution; single-quoted strings are not (except for C<\'> and
291C<\\>). The usual C-style backslash rules apply for making
d55a8828 292characters such as newline, tab, etc., as well as some more exotic
4a4eefd0 293forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
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294
295Hexadecimal, octal, or binary, representations in string literals
296(e.g. '0xff') are not automatically converted to their integer
297representation. The hex() and oct() functions make these conversions
298for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
68dc0745 299
5f05dabc 300You can also embed newlines directly in your strings, i.e., they can end
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301on a different line than they begin. This is nice, but if you forget
302your trailing quote, the error will not be reported until Perl finds
303another line containing the quote character, which may be much further
304on in the script. Variable substitution inside strings is limited to
d55a8828 305scalar variables, arrays, and array or hash slices. (In other words,
b88cefa9 306names beginning with $ or @, followed by an optional bracketed
a0d0e21e 307expression as a subscript.) The following code segment prints out "The
184e9718 308price is $Z<>100."
a0d0e21e 309
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310 $Price = '$100'; # not interpreted
311 print "The price is $Price.\n"; # interpreted
a0d0e21e 312
d55a8828 313As in some shells, you can enclose the variable name in braces to
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314disambiguate it from following alphanumerics (and underscores).
315You must also do
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316this when interpolating a variable into a string to separate the
317variable name from a following double-colon or an apostrophe, since
318these would be otherwise treated as a package separator:
319
84f709e7 320 $who = "Larry";
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321 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
322 print "We use ${who}speak when ${who}'s here.\n";
323
324Without the braces, Perl would have looked for a $whospeak, a
325C<$who::0>, and a C<$who's> variable. The last two would be the
326$0 and the $s variables in the (presumably) non-existent package
327C<who>.
328
329In fact, an identifier within such curlies is forced to be a string,
330as is any simple identifier within a hash subscript. Neither need
331quoting. Our earlier example, C<$days{'Feb'}> can be written as
332C<$days{Feb}> and the quotes will be assumed automatically. But
333anything more complicated in the subscript will be interpreted as
334an expression.
335
191d61a7 336A literal of the form C<v1.20.300.4000> is parsed as a string composed
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337of characters with the specified ordinals. This form, known as
338v-strings, provides an alternative, more readable way to construct
339strings, rather than use the somewhat less readable interpolation form
340C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
341Unicode strings, and for comparing version "numbers" using the string
342comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
343more dots in the literal, the leading C<v> may be omitted.
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344
345 print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
346 print v102.111.111; # prints "foo"
347 print 102.111.111; # same
348
349Such literals are accepted by both C<require> and C<use> for
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350doing a version check. The C<$^V> special variable also contains the
351running Perl interpreter's version in this form. See L<perlvar/$^V>.
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352Note that using the v-strings for IPv4 addresses is not portable unless
353you also use the inet_aton()/inet_ntoa() routines of the Socket package.
191d61a7 354
d55a8828 355The special literals __FILE__, __LINE__, and __PACKAGE__
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356represent the current filename, line number, and package name at that
357point in your program. They may be used only as separate tokens; they
358will not be interpolated into strings. If there is no current package
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359(due to an empty C<package;> directive), __PACKAGE__ is the undefined
360value.
361
362The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
363may be used to indicate the logical end of the script before the actual
364end of file. Any following text is ignored.
365
366Text after __DATA__ but may be read via the filehandle C<PACKNAME::DATA>,
367where C<PACKNAME> is the package that was current when the __DATA__
368token was encountered. The filehandle is left open pointing to the
369contents after __DATA__. It is the program's responsibility to
370C<close DATA> when it is done reading from it. For compatibility with
371older scripts written before __DATA__ was introduced, __END__ behaves
372like __DATA__ in the toplevel script (but not in files loaded with
373C<require> or C<do>) and leaves the remaining contents of the
374file accessible via C<main::DATA>.
375
376See L<SelfLoader> for more description of __DATA__, and
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377an example of its use. Note that you cannot read from the DATA
378filehandle in a BEGIN block: the BEGIN block is executed as soon
379as it is seen (during compilation), at which point the corresponding
a00c1fe5 380__DATA__ (or __END__) token has not yet been seen.
a0d0e21e 381
748a9306 382A word that has no other interpretation in the grammar will
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383be treated as if it were a quoted string. These are known as
384"barewords". As with filehandles and labels, a bareword that consists
385entirely of lowercase letters risks conflict with future reserved
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386words, and if you use the C<use warnings> pragma or the B<-w> switch,
387Perl will warn you about any
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388such words. Some people may wish to outlaw barewords entirely. If you
389say
390
391 use strict 'subs';
392
393then any bareword that would NOT be interpreted as a subroutine call
394produces a compile-time error instead. The restriction lasts to the
54310121 395end of the enclosing block. An inner block may countermand this
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396by saying C<no strict 'subs'>.
397
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398Arrays and slices are interpolated into double-quoted strings
399by joining the elements with the delimiter specified in the C<$">
400variable (C<$LIST_SEPARATOR> in English), space by default. The
401following are equivalent:
a0d0e21e 402
84f709e7 403 $temp = join($", @ARGV);
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404 system "echo $temp";
405
406 system "echo @ARGV";
407
408Within search patterns (which also undergo double-quotish substitution)
d55a8828 409there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
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410C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
411expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
412@foo)? If @foo doesn't otherwise exist, then it's obviously a
413character class. If @foo exists, Perl takes a good guess about C<[bar]>,
414and is almost always right. If it does guess wrong, or if you're just
415plain paranoid, you can force the correct interpretation with curly
d55a8828 416braces as above.
a0d0e21e 417
d55a8828 418A line-oriented form of quoting is based on the shell "here-document"
c47ff5f1 419syntax. Following a C<< << >> you specify a string to terminate
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420the quoted material, and all lines following the current line down to
421the terminating string are the value of the item. The terminating
422string may be either an identifier (a word), or some quoted text. If
423quoted, the type of quotes you use determines the treatment of the
424text, just as in regular quoting. An unquoted identifier works like
c47ff5f1 425double quotes. There must be no space between the C<< << >> and
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426the identifier, unless the identifier is quoted. (If you put a space it
427will be treated as a null identifier, which is valid, and matches the first
428empty line.) The terminating string must appear by itself (unquoted and
429with no surrounding whitespace) on the terminating line.
a0d0e21e 430
54310121 431 print <<EOF;
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432 The price is $Price.
433 EOF
434
be16fac9 435 print << "EOF"; # same as above
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436 The price is $Price.
437 EOF
438
be16fac9 439 print << `EOC`; # execute commands
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440 echo hi there
441 echo lo there
442 EOC
443
444 print <<"foo", <<"bar"; # you can stack them
445 I said foo.
446 foo
447 I said bar.
448 bar
449
be16fac9 450 myfunc(<< "THIS", 23, <<'THAT');
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451 Here's a line
452 or two.
453 THIS
54310121 454 and here's another.
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455 THAT
456
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457Just don't forget that you have to put a semicolon on the end
458to finish the statement, as Perl doesn't know you're not going to
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459try to do this:
460
461 print <<ABC
462 179231
463 ABC
464 + 20;
465
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466If you want your here-docs to be indented with the
467rest of the code, you'll need to remove leading whitespace
468from each line manually:
469
84f709e7 470 ($quote = <<'FINIS') =~ s/^\s+//gm;
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471 The Road goes ever on and on,
472 down from the door where it began.
473 FINIS
a0d0e21e 474
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475If you use a here-doc within a delimited construct, such as in C<s///eg>,
476the quoted material must come on the lines following the final delimiter.
477So instead of
478
479 s/this/<<E . 'that'
480 the other
481 E
482 . 'more '/eg;
483
484you have to write
485
486 s/this/<<E . 'that'
487 . 'more '/eg;
488 the other
489 E
490
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491If the terminating identifier is on the last line of the program, you
492must be sure there is a newline after it; otherwise, Perl will give the
493warning B<Can't find string terminator "END" anywhere before EOF...>.
494
495Additionally, the quoting rules for the identifier are not related to
496Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
497in place of C<''> and C<"">, and the only interpolation is for backslashing
498the quoting character:
499
500 print << "abc\"def";
501 testing...
502 abc"def
503
504Finally, quoted strings cannot span multiple lines. The general rule is
505that the identifier must be a string literal. Stick with that, and you
506should be safe.
507
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508=head2 List value constructors
509
510List values are denoted by separating individual values by commas
511(and enclosing the list in parentheses where precedence requires it):
512
513 (LIST)
514
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515In a context not requiring a list value, the value of what appears
516to be a list literal is simply the value of the final element, as
517with the C comma operator. For example,
a0d0e21e 518
84f709e7 519 @foo = ('cc', '-E', $bar);
a0d0e21e 520
d55a8828 521assigns the entire list value to array @foo, but
a0d0e21e 522
84f709e7 523 $foo = ('cc', '-E', $bar);
a0d0e21e 524
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525assigns the value of variable $bar to the scalar variable $foo.
526Note that the value of an actual array in scalar context is the
527length of the array; the following assigns the value 3 to $foo:
a0d0e21e 528
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529 @foo = ('cc', '-E', $bar);
530 $foo = @foo; # $foo gets 3
a0d0e21e 531
54310121 532You may have an optional comma before the closing parenthesis of a
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533list literal, so that you can say:
534
84f709e7 535 @foo = (
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536 1,
537 2,
538 3,
539 );
540
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541To use a here-document to assign an array, one line per element,
542you might use an approach like this:
543
84f709e7 544 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
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545 normal tomato
546 spicy tomato
547 green chile
548 pesto
549 white wine
550 End_Lines
551
a0d0e21e 552LISTs do automatic interpolation of sublists. That is, when a LIST is
d55a8828 553evaluated, each element of the list is evaluated in list context, and
a0d0e21e 554the resulting list value is interpolated into LIST just as if each
5a964f20 555individual element were a member of LIST. Thus arrays and hashes lose their
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556identity in a LIST--the list
557
5a964f20 558 (@foo,@bar,&SomeSub,%glarch)
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559
560contains all the elements of @foo followed by all the elements of @bar,
5a964f20 561followed by all the elements returned by the subroutine named SomeSub
d55a8828 562called in list context, followed by the key/value pairs of %glarch.
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563To make a list reference that does I<NOT> interpolate, see L<perlref>.
564
19799a22 565The null list is represented by (). Interpolating it in a list
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566has no effect. Thus ((),(),()) is equivalent to (). Similarly,
567interpolating an array with no elements is the same as if no
568array had been interpolated at that point.
569
c2689353 570This interpolation combines with the facts that the opening
ab1f959b 571and closing parentheses are optional (except when necessary for
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572precedence) and lists may end with an optional comma to mean that
573multiple commas within lists are legal syntax. The list C<1,,3> is a
574concatenation of two lists, C<1,> and C<3>, the first of which ends
575with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
576similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
577we'd advise you to use this obfuscation.
578
a0d0e21e 579A list value may also be subscripted like a normal array. You must
54310121 580put the list in parentheses to avoid ambiguity. For example:
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581
582 # Stat returns list value.
84f709e7 583 $time = (stat($file))[8];
a0d0e21e 584
4633a7c4 585 # SYNTAX ERROR HERE.
84f709e7 586 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
4633a7c4 587
a0d0e21e 588 # Find a hex digit.
84f709e7 589 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
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590
591 # A "reverse comma operator".
592 return (pop(@foo),pop(@foo))[0];
593
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594Lists may be assigned to only when each element of the list
595is itself legal to assign to:
a0d0e21e 596
84f709e7 597 ($a, $b, $c) = (1, 2, 3);
a0d0e21e 598
84f709e7 599 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
a0d0e21e 600
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601An exception to this is that you may assign to C<undef> in a list.
602This is useful for throwing away some of the return values of a
603function:
604
84f709e7 605 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
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606
607List assignment in scalar context returns the number of elements
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608produced by the expression on the right side of the assignment:
609
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610 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
611 $x = (($foo,$bar) = f()); # set $x to f()'s return count
4633a7c4 612
d55a8828 613This is handy when you want to do a list assignment in a Boolean
19799a22 614context, because most list functions return a null list when finished,
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615which when assigned produces a 0, which is interpreted as FALSE.
616
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617It's also the source of a useful idiom for executing a function or
618performing an operation in list context and then counting the number of
619return values, by assigning to an empty list and then using that
620assignment in scalar context. For example, this code:
621
84f709e7 622 $count = () = $string =~ /\d+/g;
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623
624will place into $count the number of digit groups found in $string.
625This happens because the pattern match is in list context (since it
626is being assigned to the empty list), and will therefore return a list
627of all matching parts of the string. The list assignment in scalar
628context will translate that into the number of elements (here, the
629number of times the pattern matched) and assign that to $count. Note
630that simply using
631
84f709e7 632 $count = $string =~ /\d+/g;
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633
634would not have worked, since a pattern match in scalar context will
635only return true or false, rather than a count of matches.
636
637The final element of a list assignment may be an array or a hash:
a0d0e21e 638
84f709e7 639 ($a, $b, @rest) = split;
5a964f20 640 my($a, $b, %rest) = @_;
a0d0e21e 641
4633a7c4 642You can actually put an array or hash anywhere in the list, but the first one
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643in the list will soak up all the values, and anything after it will become
644undefined. This may be useful in a my() or local().
a0d0e21e 645
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646A hash can be initialized using a literal list holding pairs of
647items to be interpreted as a key and a value:
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648
649 # same as map assignment above
84f709e7 650 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
a0d0e21e 651
d55a8828 652While literal lists and named arrays are often interchangeable, that's
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653not the case for hashes. Just because you can subscript a list value like
654a normal array does not mean that you can subscript a list value as a
655hash. Likewise, hashes included as parts of other lists (including
656parameters lists and return lists from functions) always flatten out into
657key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 658
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659It is often more readable to use the C<< => >> operator between key/value
660pairs. The C<< => >> operator is mostly just a more visually distinctive
b88cefa9 661synonym for a comma, but it also arranges for its left-hand operand to be
5a964f20 662interpreted as a string--if it's a bareword that would be a legal identifier.
b88cefa9 663This makes it nice for initializing hashes:
a0d0e21e 664
84f709e7 665 %map = (
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666 red => 0x00f,
667 blue => 0x0f0,
668 green => 0xf00,
669 );
670
671or for initializing hash references to be used as records:
672
84f709e7 673 $rec = {
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674 witch => 'Mable the Merciless',
675 cat => 'Fluffy the Ferocious',
676 date => '10/31/1776',
677 };
678
679or for using call-by-named-parameter to complicated functions:
680
84f709e7 681 $field = $query->radio_group(
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682 name => 'group_name',
683 values => ['eenie','meenie','minie'],
684 default => 'meenie',
685 linebreak => 'true',
84f709e7 686 labels => \%labels
4633a7c4 687 );
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688
689Note that just because a hash is initialized in that order doesn't
690mean that it comes out in that order. See L<perlfunc/sort> for examples
691of how to arrange for an output ordering.
692
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693=head2 Slices
694
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695A common way to access an array or a hash is one scalar element at a
696time. You can also subscript a list to get a single element from it.
d55a8828 697
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698 $whoami = $ENV{"USER"}; # one element from the hash
699 $parent = $ISA[0]; # one element from the array
700 $dir = (getpwnam("daemon"))[7]; # likewise, but with list
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701
702A slice accesses several elements of a list, an array, or a hash
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703simultaneously using a list of subscripts. It's more convenient
704than writing out the individual elements as a list of separate
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705scalar values.
706
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707 ($him, $her) = @folks[0,-1]; # array slice
708 @them = @folks[0 .. 3]; # array slice
709 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
710 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
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711
712Since you can assign to a list of variables, you can also assign to
713an array or hash slice.
714
84f709e7 715 @days[3..5] = qw/Wed Thu Fri/;
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716 @colors{'red','blue','green'}
717 = (0xff0000, 0x0000ff, 0x00ff00);
718 @folks[0, -1] = @folks[-1, 0];
719
720The previous assignments are exactly equivalent to
721
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722 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
723 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
d55a8828 724 = (0xff0000, 0x0000ff, 0x00ff00);
84f709e7 725 ($folks[0], $folks[-1]) = ($folks[0], $folks[-1]);
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726
727Since changing a slice changes the original array or hash that it's
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728slicing, a C<foreach> construct will alter some--or even all--of the
729values of the array or hash.
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730
731 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
732
733 foreach (@hash{keys %hash}) {
734 s/^\s+//; # trim leading whitespace
735 s/\s+$//; # trim trailing whitespace
736 s/(\w+)/\u\L$1/g; # "titlecase" words
737 }
738
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739A slice of an empty list is still an empty list. Thus:
740
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741 @a = ()[1,0]; # @a has no elements
742 @b = (@a)[0,1]; # @b has no elements
743 @c = (0,1)[2,3]; # @c has no elements
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744
745But:
746
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747 @a = (1)[1,0]; # @a has two elements
748 @b = (1,undef)[1,0,2]; # @b has three elements
08cd8952 749
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750This makes it easy to write loops that terminate when a null list
751is returned:
d55a8828 752
84f709e7 753 while ( ($home, $user) = (getpwent)[7,0]) {
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754 printf "%-8s %s\n", $user, $home;
755 }
756
757As noted earlier in this document, the scalar sense of list assignment
758is the number of elements on the right-hand side of the assignment.
19799a22 759The null list contains no elements, so when the password file is
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760exhausted, the result is 0, not 2.
761
762If you're confused about why you use an '@' there on a hash slice
763instead of a '%', think of it like this. The type of bracket (square
764or curly) governs whether it's an array or a hash being looked at.
765On the other hand, the leading symbol ('$' or '@') on the array or
766hash indicates whether you are getting back a singular value (a
767scalar) or a plural one (a list).
768
5f05dabc 769=head2 Typeglobs and Filehandles
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770
771Perl uses an internal type called a I<typeglob> to hold an entire
772symbol table entry. The type prefix of a typeglob is a C<*>, because
54310121 773it represents all types. This used to be the preferred way to
cb1a09d0 774pass arrays and hashes by reference into a function, but now that
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775we have real references, this is seldom needed.
776
777The main use of typeglobs in modern Perl is create symbol table aliases.
778This assignment:
779
780 *this = *that;
781
782makes $this an alias for $that, @this an alias for @that, %this an alias
783for %that, &this an alias for &that, etc. Much safer is to use a reference.
784This:
5f05dabc 785
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786 local *Here::blue = \$There::green;
787
788temporarily makes $Here::blue an alias for $There::green, but doesn't
789make @Here::blue an alias for @There::green, or %Here::blue an alias for
790%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
791of this. Strange though this may seem, this is the basis for the whole
84f709e7 792module import/export system.
5a964f20 793
d55a8828 794Another use for typeglobs is to pass filehandles into a function or
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795to create new filehandles. If you need to use a typeglob to save away
796a filehandle, do it this way:
5f05dabc 797
84f709e7 798 $fh = *STDOUT;
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799
800or perhaps as a real reference, like this:
801
84f709e7 802 $fh = \*STDOUT;
5f05dabc 803
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804See L<perlsub> for examples of using these as indirect filehandles
805in functions.
806
807Typeglobs are also a way to create a local filehandle using the local()
808operator. These last until their block is exited, but may be passed back.
809For example:
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810
811 sub newopen {
812 my $path = shift;
d55a8828 813 local *FH; # not my!
5a964f20 814 open (FH, $path) or return undef;
e05a3a1e 815 return *FH;
5f05dabc 816 }
84f709e7 817 $fh = newopen('/etc/passwd');
5f05dabc 818
d55a8828 819Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
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820for filehandle manipulations, although they're still needed to pass brand
821new file and directory handles into or out of functions. That's because
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822C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
823In other words, C<*FH> must be used to create new symbol table entries;
824C<*foo{THING}> cannot. When in doubt, use C<*FH>.
825
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826All functions that are capable of creating filehandles (open(),
827opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
828automatically create an anonymous filehandle if the handle passed to
829them is an uninitialized scalar variable. This allows the constructs
830such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
831create filehandles that will conveniently be closed automatically when
832the scope ends, provided there are no other references to them. This
833largely eliminates the need for typeglobs when opening filehandles
834that must be passed around, as in the following example:
835
836 sub myopen {
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837 open my $fh, "@_"
838 or die "Can't open '@_': $!";
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839 return $fh;
840 }
841
842 {
843 my $f = myopen("</etc/motd");
844 print <$f>;
845 # $f implicitly closed here
846 }
847
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848Another way to create anonymous filehandles is with the Symbol
849module or with the IO::Handle module and its ilk. These modules
850have the advantage of not hiding different types of the same name
851during the local(). See the bottom of L<perlfunc/open()> for an
852example.
853
854=head1 SEE ALSO
855
856See L<perlvar> for a description of Perl's built-in variables and
857a discussion of legal variable names. See L<perlref>, L<perlsub>,
858and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
859the C<*foo{THING}> syntax.