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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
5a964f20 90that do not start with a letter, underscore, or digit are limited to
5f05dabc 91one character, e.g., C<$%> or C<$$>. (Most of these one character names
cb1a09d0 92have a predefined significance to Perl. For instance, C<$$> is the
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93current process id.)
94
95=head2 Context
96
97The interpretation of operations and values in Perl sometimes depends
98on the requirements of the context around the operation or value.
d55a8828 99There are two major contexts: list and scalar. Certain operations
a0d0e21e 100return list values in contexts wanting a list, and scalar values
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101otherwise. If this is true of an operation it will be mentioned in
102the documentation for that operation. In other words, Perl overloads
a0d0e21e 103certain operations based on whether the expected return value is
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104singular or plural. Some words in English work this way, like "fish"
105and "sheep".
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106
107In a reciprocal fashion, an operation provides either a scalar or a
108list context to each of its arguments. For example, if you say
109
110 int( <STDIN> )
111
c47ff5f1 112the integer operation provides scalar context for the <>
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113operator, which responds by reading one line from STDIN and passing it
114back to the integer operation, which will then find the integer value
115of that line and return that. If, on the other hand, you say
116
117 sort( <STDIN> )
118
c47ff5f1 119then the sort operation provides list context for <>, which
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120will proceed to read every line available up to the end of file, and
121pass that list of lines back to the sort routine, which will then
122sort those lines and return them as a list to whatever the context
123of the sort was.
124
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125Assignment is a little bit special in that it uses its left argument
126to determine the context for the right argument. Assignment to a
127scalar evaluates the right-hand side in scalar context, while
128assignment to an array or hash evaluates the righthand side in list
129context. Assignment to a list (or slice, which is just a list
130anyway) also evaluates the righthand side in list context.
131
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132When you use the C<use warnings> pragma or Perl's B<-w> command-line
133option, you may see warnings
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134about useless uses of constants or functions in "void context".
135Void context just means the value has been discarded, such as a
136statement containing only C<"fred";> or C<getpwuid(0);>. It still
137counts as scalar context for functions that care whether or not
138they're being called in list context.
139
140User-defined subroutines may choose to care whether they are being
141called in a void, scalar, or list context. Most subroutines do not
142need to bother, though. That's because both scalars and lists are
143automatically interpolated into lists. See L<perlfunc/wantarray>
144for how you would dynamically discern your function's calling
145context.
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146
147=head2 Scalar values
148
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149All data in Perl is a scalar, an array of scalars, or a hash of
150scalars. A scalar may contain one single value in any of three
151different flavors: a number, a string, or a reference. In general,
152conversion from one form to another is transparent. Although a
153scalar may not directly hold multiple values, it may contain a
154reference to an array or hash which in turn contains multiple values.
155
156Scalars aren't necessarily one thing or another. There's no place
157to declare a scalar variable to be of type "string", type "number",
158type "reference", or anything else. Because of the automatic
159conversion of scalars, operations that return scalars don't need
160to care (and in fact, cannot care) whether their caller is looking
161for a string, a number, or a reference. Perl is a contextually
162polymorphic language whose scalars can be strings, numbers, or
163references (which includes objects). Although strings and numbers
164are considered pretty much the same thing for nearly all purposes,
165references are strongly-typed, uncastable pointers with builtin
166reference-counting and destructor invocation.
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167
168A scalar value is interpreted as TRUE in the Boolean sense if it is not
19799a22 169the null string or the number 0 (or its string equivalent, "0"). The
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170Boolean context is just a special kind of scalar context where no
171conversion to a string or a number is ever performed.
172
173There are actually two varieties of null strings (sometimes referred
174to as "empty" strings), a defined one and an undefined one. The
175defined version is just a string of length zero, such as C<"">.
176The undefined version is the value that indicates that there is
177no real value for something, such as when there was an error, or
178at end of file, or when you refer to an uninitialized variable or
179element of an array or hash. Although in early versions of Perl,
180an undefined scalar could become defined when first used in a
181place expecting a defined value, this no longer happens except for
182rare cases of autovivification as explained in L<perlref>. You can
183use the defined() operator to determine whether a scalar value is
184defined (this has no meaning on arrays or hashes), and the undef()
185operator to produce an undefined value.
186
187To find out whether a given string is a valid non-zero number, it's
188sometimes enough to test it against both numeric 0 and also lexical
189"0" (although this will cause B<-w> noises). That's because strings
190that aren't numbers count as 0, just as they do in B<awk>:
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191
192 if ($str == 0 && $str ne "0") {
193 warn "That doesn't look like a number";
54310121 194 }
4633a7c4 195
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196That method may be best because otherwise you won't treat IEEE
197notations like C<NaN> or C<Infinity> properly. At other times, you
198might prefer to determine whether string data can be used numerically
199by calling the POSIX::strtod() function or by inspecting your string
200with a regular expression (as documented in L<perlre>).
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201
202 warn "has nondigits" if /\D/;
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203 warn "not a natural number" unless /^\d+$/; # rejects -3
204 warn "not an integer" unless /^-?\d+$/; # rejects +3
205 warn "not an integer" unless /^[+-]?\d+$/;
206 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
207 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
54310121 208 warn "not a C float"
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209 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
210
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211The length of an array is a scalar value. You may find the length
212of array @days by evaluating C<$#days>, as in B<csh>. Technically
213speaking, this isn't the length of the array; it's the subscript
214of the last element, since there is ordinarily a 0th element.
215Assigning to C<$#days> actually changes the length of the array.
216Shortening an array this way destroys intervening values. Lengthening
217an array that was previously shortened does not recover values
218that were in those elements. (It used to do so in Perl 4, but we
219had to break this to make sure destructors were called when expected.)
220
221You can also gain some miniscule measure of efficiency by pre-extending
222an array that is going to get big. You can also extend an array
223by assigning to an element that is off the end of the array. You
19799a22 224can truncate an array down to nothing by assigning the null list
d55a8828 225() to it. The following are equivalent:
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226
227 @whatever = ();
3e3baf6d 228 $#whatever = -1;
a0d0e21e 229
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230If you evaluate an array in scalar context, it returns the length
231of the array. (Note that this is not true of lists, which return
232the last value, like the C comma operator, nor of built-in functions,
233which return whatever they feel like returning.) The following is
234always true:
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235
236 scalar(@whatever) == $#whatever - $[ + 1;
237
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238Version 5 of Perl changed the semantics of C<$[>: files that don't set
239the value of C<$[> no longer need to worry about whether another
240file changed its value. (In other words, use of C<$[> is deprecated.)
5f05dabc 241So in general you can assume that
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242
243 scalar(@whatever) == $#whatever + 1;
244
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245Some programmers choose to use an explicit conversion so as to
246leave nothing to doubt:
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247
248 $element_count = scalar(@whatever);
249
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250If you evaluate a hash in scalar context, it returns false if the
251hash is empty. If there are any key/value pairs, it returns true;
252more precisely, the value returned is a string consisting of the
253number of used buckets and the number of allocated buckets, separated
254by a slash. This is pretty much useful only to find out whether
255Perl's internal hashing algorithm is performing poorly on your data
256set. For example, you stick 10,000 things in a hash, but evaluating
257%HASH in scalar context reveals C<"1/16">, which means only one out
258of sixteen buckets has been touched, and presumably contains all
25910,000 of your items. This isn't supposed to happen.
a0d0e21e 260
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261You can preallocate space for a hash by assigning to the keys() function.
262This rounds up the allocated bucked to the next power of two:
263
264 keys(%users) = 1000; # allocate 1024 buckets
265
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266=head2 Scalar value constructors
267
d55a8828 268Numeric literals are specified in any of the following floating point or
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269integer formats:
270
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271 12345
272 12345.67
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273 .23E-10 # a very small number
274 4_294_967_296 # underline for legibility
275 0xff # hex
276 0377 # octal
277 0b011011 # binary
a0d0e21e 278
55497cff 279String literals are usually delimited by either single or double
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280quotes. They work much like quotes in the standard Unix shells:
281double-quoted string literals are subject to backslash and variable
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282substitution; single-quoted strings are not (except for C<\'> and
283C<\\>). The usual C-style backslash rules apply for making
d55a8828 284characters such as newline, tab, etc., as well as some more exotic
4a4eefd0 285forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
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286
287Hexadecimal, octal, or binary, representations in string literals
288(e.g. '0xff') are not automatically converted to their integer
289representation. The hex() and oct() functions make these conversions
290for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
68dc0745 291
5f05dabc 292You can also embed newlines directly in your strings, i.e., they can end
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293on a different line than they begin. This is nice, but if you forget
294your trailing quote, the error will not be reported until Perl finds
295another line containing the quote character, which may be much further
296on in the script. Variable substitution inside strings is limited to
d55a8828 297scalar variables, arrays, and array or hash slices. (In other words,
b88cefa9 298names beginning with $ or @, followed by an optional bracketed
a0d0e21e 299expression as a subscript.) The following code segment prints out "The
184e9718 300price is $Z<>100."
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301
302 $Price = '$100'; # not interpreted
303 print "The price is $Price.\n"; # interpreted
304
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305As in some shells, you can enclose the variable name in braces to
306disambiguate it from following alphanumerics. You must also do
307this when interpolating a variable into a string to separate the
308variable name from a following double-colon or an apostrophe, since
309these would be otherwise treated as a package separator:
310
311 $who = "Larry";
312 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
313 print "We use ${who}speak when ${who}'s here.\n";
314
315Without the braces, Perl would have looked for a $whospeak, a
316C<$who::0>, and a C<$who's> variable. The last two would be the
317$0 and the $s variables in the (presumably) non-existent package
318C<who>.
319
320In fact, an identifier within such curlies is forced to be a string,
321as is any simple identifier within a hash subscript. Neither need
322quoting. Our earlier example, C<$days{'Feb'}> can be written as
323C<$days{Feb}> and the quotes will be assumed automatically. But
324anything more complicated in the subscript will be interpreted as
325an expression.
326
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327A literal of the form C<v1.20.300.4000> is parsed as a string composed
328of characters with the specified ordinals. This provides an alternative,
329more readable way to construct strings, rather than use the somewhat less
330readable interpolation form C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful
331for representing Unicode strings, and for comparing version "numbers"
332using the string comparison operators, C<cmp>, C<gt>, C<lt> etc.
333If there are two or more dots in the literal, the leading C<v> may be
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334omitted.
335
336 print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
337 print v102.111.111; # prints "foo"
338 print 102.111.111; # same
339
340Such literals are accepted by both C<require> and C<use> for
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341doing a version check. The C<$^V> special variable also contains the
342running Perl interpreter's version in this form. See L<perlvar/$^V>.
343
d55a8828 344The special literals __FILE__, __LINE__, and __PACKAGE__
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345represent the current filename, line number, and package name at that
346point in your program. They may be used only as separate tokens; they
347will not be interpolated into strings. If there is no current package
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348(due to an empty C<package;> directive), __PACKAGE__ is the undefined
349value.
350
351The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
352may be used to indicate the logical end of the script before the actual
353end of file. Any following text is ignored.
354
355Text after __DATA__ but may be read via the filehandle C<PACKNAME::DATA>,
356where C<PACKNAME> is the package that was current when the __DATA__
357token was encountered. The filehandle is left open pointing to the
358contents after __DATA__. It is the program's responsibility to
359C<close DATA> when it is done reading from it. For compatibility with
360older scripts written before __DATA__ was introduced, __END__ behaves
361like __DATA__ in the toplevel script (but not in files loaded with
362C<require> or C<do>) and leaves the remaining contents of the
363file accessible via C<main::DATA>.
364
365See L<SelfLoader> for more description of __DATA__, and
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366an example of its use. Note that you cannot read from the DATA
367filehandle in a BEGIN block: the BEGIN block is executed as soon
368as it is seen (during compilation), at which point the corresponding
a00c1fe5 369__DATA__ (or __END__) token has not yet been seen.
a0d0e21e 370
748a9306 371A word that has no other interpretation in the grammar will
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372be treated as if it were a quoted string. These are known as
373"barewords". As with filehandles and labels, a bareword that consists
374entirely of lowercase letters risks conflict with future reserved
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375words, and if you use the C<use warnings> pragma or the B<-w> switch,
376Perl will warn you about any
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377such words. Some people may wish to outlaw barewords entirely. If you
378say
379
380 use strict 'subs';
381
382then any bareword that would NOT be interpreted as a subroutine call
383produces a compile-time error instead. The restriction lasts to the
54310121 384end of the enclosing block. An inner block may countermand this
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385by saying C<no strict 'subs'>.
386
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387Arrays and slices are interpolated into double-quoted strings
388by joining the elements with the delimiter specified in the C<$">
389variable (C<$LIST_SEPARATOR> in English), space by default. The
390following are equivalent:
a0d0e21e 391
d55a8828 392 $temp = join($", @ARGV);
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393 system "echo $temp";
394
395 system "echo @ARGV";
396
397Within search patterns (which also undergo double-quotish substitution)
d55a8828 398there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
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399C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
400expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
401@foo)? If @foo doesn't otherwise exist, then it's obviously a
402character class. If @foo exists, Perl takes a good guess about C<[bar]>,
403and is almost always right. If it does guess wrong, or if you're just
404plain paranoid, you can force the correct interpretation with curly
d55a8828 405braces as above.
a0d0e21e 406
d55a8828 407A line-oriented form of quoting is based on the shell "here-document"
c47ff5f1 408syntax. Following a C<< << >> you specify a string to terminate
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409the quoted material, and all lines following the current line down to
410the terminating string are the value of the item. The terminating
411string may be either an identifier (a word), or some quoted text. If
412quoted, the type of quotes you use determines the treatment of the
413text, just as in regular quoting. An unquoted identifier works like
c47ff5f1 414double quotes. There must be no space between the C<< << >> and
55497cff 415the identifier. (If you put a space it will be treated as a null
3fe9a6f1 416identifier, which is valid, and matches the first empty line.) The
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417terminating string must appear by itself (unquoted and with no
418surrounding whitespace) on the terminating line.
a0d0e21e 419
54310121 420 print <<EOF;
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421 The price is $Price.
422 EOF
423
424 print <<"EOF"; # same as above
425 The price is $Price.
426 EOF
427
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428 print <<`EOC`; # execute commands
429 echo hi there
430 echo lo there
431 EOC
432
433 print <<"foo", <<"bar"; # you can stack them
434 I said foo.
435 foo
436 I said bar.
437 bar
438
d28ebecd 439 myfunc(<<"THIS", 23, <<'THAT');
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440 Here's a line
441 or two.
442 THIS
54310121 443 and here's another.
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444 THAT
445
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446Just don't forget that you have to put a semicolon on the end
447to finish the statement, as Perl doesn't know you're not going to
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448try to do this:
449
450 print <<ABC
451 179231
452 ABC
453 + 20;
454
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455If you want your here-docs to be indented with the
456rest of the code, you'll need to remove leading whitespace
457from each line manually:
458
459 ($quote = <<'FINIS') =~ s/^\s+//gm;
460 The Road goes ever on and on,
461 down from the door where it began.
462 FINIS
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463
464=head2 List value constructors
465
466List values are denoted by separating individual values by commas
467(and enclosing the list in parentheses where precedence requires it):
468
469 (LIST)
470
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471In a context not requiring a list value, the value of what appears
472to be a list literal is simply the value of the final element, as
473with the C comma operator. For example,
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474
475 @foo = ('cc', '-E', $bar);
476
d55a8828 477assigns the entire list value to array @foo, but
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478
479 $foo = ('cc', '-E', $bar);
480
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481assigns the value of variable $bar to the scalar variable $foo.
482Note that the value of an actual array in scalar context is the
483length of the array; the following assigns the value 3 to $foo:
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484
485 @foo = ('cc', '-E', $bar);
486 $foo = @foo; # $foo gets 3
487
54310121 488You may have an optional comma before the closing parenthesis of a
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489list literal, so that you can say:
490
491 @foo = (
492 1,
493 2,
494 3,
495 );
496
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497To use a here-document to assign an array, one line per element,
498you might use an approach like this:
499
500 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
501 normal tomato
502 spicy tomato
503 green chile
504 pesto
505 white wine
506 End_Lines
507
a0d0e21e 508LISTs do automatic interpolation of sublists. That is, when a LIST is
d55a8828 509evaluated, each element of the list is evaluated in list context, and
a0d0e21e 510the resulting list value is interpolated into LIST just as if each
5a964f20 511individual element were a member of LIST. Thus arrays and hashes lose their
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512identity in a LIST--the list
513
5a964f20 514 (@foo,@bar,&SomeSub,%glarch)
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515
516contains all the elements of @foo followed by all the elements of @bar,
5a964f20 517followed by all the elements returned by the subroutine named SomeSub
d55a8828 518called in list context, followed by the key/value pairs of %glarch.
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519To make a list reference that does I<NOT> interpolate, see L<perlref>.
520
19799a22 521The null list is represented by (). Interpolating it in a list
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522has no effect. Thus ((),(),()) is equivalent to (). Similarly,
523interpolating an array with no elements is the same as if no
524array had been interpolated at that point.
525
526A list value may also be subscripted like a normal array. You must
54310121 527put the list in parentheses to avoid ambiguity. For example:
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528
529 # Stat returns list value.
530 $time = (stat($file))[8];
531
4633a7c4 532 # SYNTAX ERROR HERE.
5f05dabc 533 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
4633a7c4 534
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535 # Find a hex digit.
536 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
537
538 # A "reverse comma operator".
539 return (pop(@foo),pop(@foo))[0];
540
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541Lists may be assigned to only when each element of the list
542is itself legal to assign to:
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543
544 ($a, $b, $c) = (1, 2, 3);
545
546 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
547
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548An exception to this is that you may assign to C<undef> in a list.
549This is useful for throwing away some of the return values of a
550function:
551
552 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
553
554List assignment in scalar context returns the number of elements
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555produced by the expression on the right side of the assignment:
556
557 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
558 $x = (($foo,$bar) = f()); # set $x to f()'s return count
559
d55a8828 560This is handy when you want to do a list assignment in a Boolean
19799a22 561context, because most list functions return a null list when finished,
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562which when assigned produces a 0, which is interpreted as FALSE.
563
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564The final element may be an array or a hash:
565
566 ($a, $b, @rest) = split;
5a964f20 567 my($a, $b, %rest) = @_;
a0d0e21e 568
4633a7c4 569You can actually put an array or hash anywhere in the list, but the first one
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570in the list will soak up all the values, and anything after it will become
571undefined. This may be useful in a my() or local().
a0d0e21e 572
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573A hash can be initialized using a literal list holding pairs of
574items to be interpreted as a key and a value:
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575
576 # same as map assignment above
577 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
578
d55a8828 579While literal lists and named arrays are often interchangeable, that's
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580not the case for hashes. Just because you can subscript a list value like
581a normal array does not mean that you can subscript a list value as a
582hash. Likewise, hashes included as parts of other lists (including
583parameters lists and return lists from functions) always flatten out into
584key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 585
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586It is often more readable to use the C<< => >> operator between key/value
587pairs. The C<< => >> operator is mostly just a more visually distinctive
b88cefa9 588synonym for a comma, but it also arranges for its left-hand operand to be
5a964f20 589interpreted as a string--if it's a bareword that would be a legal identifier.
b88cefa9 590This makes it nice for initializing hashes:
a0d0e21e 591
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592 %map = (
593 red => 0x00f,
594 blue => 0x0f0,
595 green => 0xf00,
596 );
597
598or for initializing hash references to be used as records:
599
600 $rec = {
601 witch => 'Mable the Merciless',
602 cat => 'Fluffy the Ferocious',
603 date => '10/31/1776',
604 };
605
606or for using call-by-named-parameter to complicated functions:
607
54310121 608 $field = $query->radio_group(
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609 name => 'group_name',
610 values => ['eenie','meenie','minie'],
611 default => 'meenie',
612 linebreak => 'true',
613 labels => \%labels
614 );
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615
616Note that just because a hash is initialized in that order doesn't
617mean that it comes out in that order. See L<perlfunc/sort> for examples
618of how to arrange for an output ordering.
619
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620=head2 Slices
621
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622A common way to access an array or a hash is one scalar element at a
623time. You can also subscript a list to get a single element from it.
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624
625 $whoami = $ENV{"USER"}; # one element from the hash
626 $parent = $ISA[0]; # one element from the array
627 $dir = (getpwnam("daemon"))[7]; # likewise, but with list
628
629A slice accesses several elements of a list, an array, or a hash
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630simultaneously using a list of subscripts. It's more convenient
631than writing out the individual elements as a list of separate
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632scalar values.
633
634 ($him, $her) = @folks[0,-1]; # array slice
635 @them = @folks[0 .. 3]; # array slice
636 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
637 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
638
639Since you can assign to a list of variables, you can also assign to
640an array or hash slice.
641
642 @days[3..5] = qw/Wed Thu Fri/;
643 @colors{'red','blue','green'}
644 = (0xff0000, 0x0000ff, 0x00ff00);
645 @folks[0, -1] = @folks[-1, 0];
646
647The previous assignments are exactly equivalent to
648
649 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
650 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
651 = (0xff0000, 0x0000ff, 0x00ff00);
652 ($folks[0], $folks[-1]) = ($folks[0], $folks[-1]);
653
654Since changing a slice changes the original array or hash that it's
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655slicing, a C<foreach> construct will alter some--or even all--of the
656values of the array or hash.
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657
658 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
659
660 foreach (@hash{keys %hash}) {
661 s/^\s+//; # trim leading whitespace
662 s/\s+$//; # trim trailing whitespace
663 s/(\w+)/\u\L$1/g; # "titlecase" words
664 }
665
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666A slice of an empty list is still an empty list. Thus:
667
668 @a = ()[1,0]; # @a has no elements
669 @b = (@a)[0,1]; # @b has no elements
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670 @c = (0,1)[2,3]; # @c has no elements
671
672But:
673
674 @a = (1)[1,0]; # @a has two elements
675 @b = (1,undef)[1,0,2]; # @b has three elements
08cd8952 676
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677This makes it easy to write loops that terminate when a null list
678is returned:
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679
680 while ( ($home, $user) = (getpwent)[7,0]) {
681 printf "%-8s %s\n", $user, $home;
682 }
683
684As noted earlier in this document, the scalar sense of list assignment
685is the number of elements on the right-hand side of the assignment.
19799a22 686The null list contains no elements, so when the password file is
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687exhausted, the result is 0, not 2.
688
689If you're confused about why you use an '@' there on a hash slice
690instead of a '%', think of it like this. The type of bracket (square
691or curly) governs whether it's an array or a hash being looked at.
692On the other hand, the leading symbol ('$' or '@') on the array or
693hash indicates whether you are getting back a singular value (a
694scalar) or a plural one (a list).
695
5f05dabc 696=head2 Typeglobs and Filehandles
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697
698Perl uses an internal type called a I<typeglob> to hold an entire
699symbol table entry. The type prefix of a typeglob is a C<*>, because
54310121 700it represents all types. This used to be the preferred way to
cb1a09d0 701pass arrays and hashes by reference into a function, but now that
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702we have real references, this is seldom needed.
703
704The main use of typeglobs in modern Perl is create symbol table aliases.
705This assignment:
706
707 *this = *that;
708
709makes $this an alias for $that, @this an alias for @that, %this an alias
710for %that, &this an alias for &that, etc. Much safer is to use a reference.
711This:
5f05dabc 712
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713 local *Here::blue = \$There::green;
714
715temporarily makes $Here::blue an alias for $There::green, but doesn't
716make @Here::blue an alias for @There::green, or %Here::blue an alias for
717%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
718of this. Strange though this may seem, this is the basis for the whole
719module import/export system.
720
d55a8828 721Another use for typeglobs is to pass filehandles into a function or
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722to create new filehandles. If you need to use a typeglob to save away
723a filehandle, do it this way:
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724
725 $fh = *STDOUT;
726
727or perhaps as a real reference, like this:
728
729 $fh = \*STDOUT;
730
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731See L<perlsub> for examples of using these as indirect filehandles
732in functions.
733
734Typeglobs are also a way to create a local filehandle using the local()
735operator. These last until their block is exited, but may be passed back.
736For example:
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737
738 sub newopen {
739 my $path = shift;
d55a8828 740 local *FH; # not my!
5a964f20 741 open (FH, $path) or return undef;
e05a3a1e 742 return *FH;
5f05dabc
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743 }
744 $fh = newopen('/etc/passwd');
745
d55a8828 746Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
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747for filehandle manipulations, although they're still needed to pass brand
748new file and directory handles into or out of functions. That's because
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749C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
750In other words, C<*FH> must be used to create new symbol table entries;
751C<*foo{THING}> cannot. When in doubt, use C<*FH>.
752
753Another way to create anonymous filehandles is with the Symbol
754module or with the IO::Handle module and its ilk. These modules
755have the advantage of not hiding different types of the same name
756during the local(). See the bottom of L<perlfunc/open()> for an
757example.
758
759=head1 SEE ALSO
760
761See L<perlvar> for a description of Perl's built-in variables and
762a discussion of legal variable names. See L<perlref>, L<perlsub>,
763and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
764the C<*foo{THING}> syntax.