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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
9Perl has three data structures: scalars, arrays of scalars, and
10associative arrays of scalars, known as "hashes". Normal arrays are
11indexed by number, starting with 0. (Negative subscripts count from
12the end.) Hash arrays are indexed by string.
13
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14Values are usually referred to by name (or through a named reference).
15The first character of the name tells you to what sort of data
16structure it refers. The rest of the name tells you the particular
17value to which it refers. Most often, it consists of a single
18I<identifier>, that is, a string beginning with a letter or underscore,
19and containing letters, underscores, and digits. In some cases, it
20may be a chain of identifiers, separated by C<::> (or by C<'>, but
21that's deprecated); all but the last are interpreted as names of
5f05dabc 22packages, to locate the namespace in which to look
b88cefa9 23up the final identifier (see L<perlmod/Packages> for details).
184e9718 24It's possible to substitute for a simple identifier an expression
5a964f20 25that produces a reference to the value at runtime; this is
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26described in more detail below, and in L<perlref>.
27
28There are also special variables whose names don't follow these
29rules, so that they don't accidentally collide with one of your
5a964f20 30normal variables. Strings that match parenthesized parts of a
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31regular expression are saved under names containing only digits after
32the C<$> (see L<perlop> and L<perlre>). In addition, several special
5a964f20 33variables that provide windows into the inner working of Perl have names
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34containing punctuation characters (see L<perlvar>).
35
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36Scalar values are always named with '$', even when referring to a scalar
37that is part of an array. It works like the English word "the". Thus
38we have:
39
40 $days # the simple scalar value "days"
41 $days[28] # the 29th element of array @days
42 $days{'Feb'} # the 'Feb' value from hash %days
43 $#days # the last index of array @days
44
45but entire arrays or array slices are denoted by '@', which works much like
46the word "these" or "those":
47
48 @days # ($days[0], $days[1],... $days[n])
49 @days[3,4,5] # same as @days[3..5]
50 @days{'a','c'} # same as ($days{'a'},$days{'c'})
51
52and entire hashes are denoted by '%':
53
54 %days # (key1, val1, key2, val2 ...)
55
56In addition, subroutines are named with an initial '&', though this is
57optional when it's otherwise unambiguous (just as "do" is often
58redundant in English). Symbol table entries can be named with an
59initial '*', but you don't really care about that yet.
60
61Every variable type has its own namespace. You can, without fear of
62conflict, use the same name for a scalar variable, an array, or a hash
63(or, for that matter, a filehandle, a subroutine name, or a label).
64This means that $foo and @foo are two different variables. It also
748a9306 65means that C<$foo[1]> is a part of @foo, not a part of $foo. This may
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66seem a bit weird, but that's okay, because it is weird.
67
5f05dabc 68Because variable and array references always start with '$', '@', or '%',
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69the "reserved" words aren't in fact reserved with respect to variable
70names. (They ARE reserved with respect to labels and filehandles,
71however, which don't have an initial special character. You can't have
72a filehandle named "log", for instance. Hint: you could say
73C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase
74filehandles also improves readability and protects you from conflict
5f05dabc 75with future reserved words.) Case I<IS> significant--"FOO", "Foo", and
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76"foo" are all different names. Names that start with a letter or
77underscore may also contain digits and underscores.
78
79It is possible to replace such an alphanumeric name with an expression
80that returns a reference to an object of that type. For a description
81of this, see L<perlref>.
82
5f05dabc 83Names that start with a digit may contain only more digits. Names
5a964f20 84that do not start with a letter, underscore, or digit are limited to
5f05dabc 85one character, e.g., C<$%> or C<$$>. (Most of these one character names
cb1a09d0 86have a predefined significance to Perl. For instance, C<$$> is the
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87current process id.)
88
89=head2 Context
90
91The interpretation of operations and values in Perl sometimes depends
92on the requirements of the context around the operation or value.
93There are two major contexts: scalar and list. Certain operations
94return list values in contexts wanting a list, and scalar values
95otherwise. (If this is true of an operation it will be mentioned in
96the documentation for that operation.) In other words, Perl overloads
97certain operations based on whether the expected return value is
98singular or plural. (Some words in English work this way, like "fish"
99and "sheep".)
100
101In a reciprocal fashion, an operation provides either a scalar or a
102list context to each of its arguments. For example, if you say
103
104 int( <STDIN> )
105
184e9718 106the integer operation provides a scalar context for the E<lt>STDINE<gt>
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107operator, which responds by reading one line from STDIN and passing it
108back to the integer operation, which will then find the integer value
109of that line and return that. If, on the other hand, you say
110
111 sort( <STDIN> )
112
184e9718 113then the sort operation provides a list context for E<lt>STDINE<gt>, which
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114will proceed to read every line available up to the end of file, and
115pass that list of lines back to the sort routine, which will then
116sort those lines and return them as a list to whatever the context
117of the sort was.
118
119Assignment is a little bit special in that it uses its left argument to
120determine the context for the right argument. Assignment to a scalar
121evaluates the righthand side in a scalar context, while assignment to
122an array or array slice evaluates the righthand side in a list
123context. Assignment to a list also evaluates the righthand side in a
124list context.
125
126User defined subroutines may choose to care whether they are being
127called in a scalar or list context, but most subroutines do not
128need to care, because scalars are automatically interpolated into
129lists. See L<perlfunc/wantarray>.
130
131=head2 Scalar values
132
4633a7c4 133All data in Perl is a scalar or an array of scalars or a hash of scalars.
a0d0e21e 134Scalar variables may contain various kinds of singular data, such as
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135numbers, strings, and references. In general, conversion from one form to
136another is transparent. (A scalar may not contain multiple values, but
137may contain a reference to an array or hash containing multiple values.)
5f05dabc 138Because of the automatic conversion of scalars, operations, and functions
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139that return scalars don't need to care (and, in fact, can't care) whether
140the context is looking for a string or a number.
141
142Scalars aren't necessarily one thing or another. There's no place to
143declare a scalar variable to be of type "string", or of type "number", or
144type "filehandle", or anything else. Perl is a contextually polymorphic
145language whose scalars can be strings, numbers, or references (which
d28ebecd 146includes objects). While strings and numbers are considered pretty
b88cefa9 147much the same thing for nearly all purposes, references are strongly-typed
54310121 148uncastable pointers with builtin reference-counting and destructor
4633a7c4 149invocation.
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150
151A scalar value is interpreted as TRUE in the Boolean sense if it is not
152the null string or the number 0 (or its string equivalent, "0"). The
54310121 153Boolean context is just a special kind of scalar context.
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154
155There are actually two varieties of null scalars: defined and
156undefined. Undefined null scalars are returned when there is no real
157value for something, such as when there was an error, or at end of
158file, or when you refer to an uninitialized variable or element of an
159array. An undefined null scalar may become defined the first time you
160use it as if it were defined, but prior to that you can use the
161defined() operator to determine whether the value is defined or not.
162
54310121 163To find out whether a given string is a valid nonzero number, it's usually
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164enough to test it against both numeric 0 and also lexical "0" (although
165this will cause B<-w> noises). That's because strings that aren't
184e9718 166numbers count as 0, just as they do in B<awk>:
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167
168 if ($str == 0 && $str ne "0") {
169 warn "That doesn't look like a number";
54310121 170 }
4633a7c4 171
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172That's usually preferable because otherwise you won't treat IEEE notations
173like C<NaN> or C<Infinity> properly. At other times you might prefer to
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174use the POSIX::strtod function or a regular expression to check whether
175data is numeric. See L<perlre> for details on regular expressions.
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176
177 warn "has nondigits" if /\D/;
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178 warn "not a natural number" unless /^\d+$/; # rejects -3
179 warn "not an integer" unless /^-?\d+$/; # rejects +3
180 warn "not an integer" unless /^[+-]?\d+$/;
181 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
182 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
54310121 183 warn "not a C float"
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184 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
185
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186The length of an array is a scalar value. You may find the length of
187array @days by evaluating C<$#days>, as in B<csh>. (Actually, it's not
5f05dabc 188the length of the array, it's the subscript of the last element, because
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189there is (ordinarily) a 0th element.) Assigning to C<$#days> changes the
190length of the array. Shortening an array by this method destroys
191intervening values. Lengthening an array that was previously shortened
192I<NO LONGER> recovers the values that were in those elements. (It used to
b88cefa9 193in Perl 4, but we had to break this to make sure destructors were
5a964f20 194called when expected.) You can also gain some miniscule measure of efficiency by
4a6725af 195pre-extending an array that is going to get big. (You can also extend
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196an array by assigning to an element that is off the end of the array.)
197You can truncate an array down to nothing by assigning the null list ()
198to it. The following are equivalent:
199
200 @whatever = ();
3e3baf6d 201 $#whatever = -1;
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202
203If you evaluate a named array in a scalar context, it returns the length of
204the array. (Note that this is not true of lists, which return the
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205last value, like the C comma operator, nor of built-in functions, which return
206whatever they feel like returning.) The following is always true:
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207
208 scalar(@whatever) == $#whatever - $[ + 1;
209
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210Version 5 of Perl changed the semantics of C<$[>: files that don't set
211the value of C<$[> no longer need to worry about whether another
212file changed its value. (In other words, use of C<$[> is deprecated.)
5f05dabc 213So in general you can assume that
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214
215 scalar(@whatever) == $#whatever + 1;
216
d28ebecd 217Some programmers choose to use an explicit conversion so nothing's
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218left to doubt:
219
220 $element_count = scalar(@whatever);
221
5a964f20 222If you evaluate a hash in a scalar context, it returns a value that is
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223true if and only if the hash contains any key/value pairs. (If there
224are any key/value pairs, the value returned is a string consisting of
225the number of used buckets and the number of allocated buckets, separated
5f05dabc 226by a slash. This is pretty much useful only to find out whether Perl's
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227(compiled in) hashing algorithm is performing poorly on your data set.
228For example, you stick 10,000 things in a hash, but evaluating %HASH in
229scalar context reveals "1/16", which means only one out of sixteen buckets
230has been touched, and presumably contains all 10,000 of your items. This
231isn't supposed to happen.)
232
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233You can preallocate space for a hash by assigning to the keys() function.
234This rounds up the allocated bucked to the next power of two:
235
236 keys(%users) = 1000; # allocate 1024 buckets
237
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238=head2 Scalar value constructors
239
240Numeric literals are specified in any of the customary floating point or
241integer formats:
242
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243 12345
244 12345.67
245 .23E-10
246 0xffff # hex
247 0377 # octal
4f19785b 248 0b111000 # binary
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249 4_294_967_296 # underline for legibility
250
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251String literals are usually delimited by either single or double
252quotes. They work much like shell quotes: double-quoted string
253literals are subject to backslash and variable substitution;
254single-quoted strings are not (except for "C<\'>" and "C<\\>").
255The usual Unix backslash rules apply for making characters such as
256newline, tab, etc., as well as some more exotic forms. See
b687b08b 257L<perlop/"Quote and Quotelike Operators"> for a list.
a0d0e21e 258
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259Octal or hex representations in string literals (e.g. '0xffff') are not
260automatically converted to their integer representation. The hex() and
261oct() functions make these conversions for you. See L<perlfunc/hex> and
262L<perlfunc/oct> for more details.
263
5f05dabc 264You can also embed newlines directly in your strings, i.e., they can end
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265on a different line than they begin. This is nice, but if you forget
266your trailing quote, the error will not be reported until Perl finds
267another line containing the quote character, which may be much further
268on in the script. Variable substitution inside strings is limited to
269scalar variables, arrays, and array slices. (In other words,
b88cefa9 270names beginning with $ or @, followed by an optional bracketed
a0d0e21e 271expression as a subscript.) The following code segment prints out "The
184e9718 272price is $Z<>100."
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273
274 $Price = '$100'; # not interpreted
275 print "The price is $Price.\n"; # interpreted
276
b88cefa9 277As in some shells, you can put curly brackets around the name to
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278delimit it from following alphanumerics. In fact, an identifier
279within such curlies is forced to be a string, as is any single
280identifier within a hash subscript. Our earlier example,
281
282 $days{'Feb'}
283
284can be written as
285
286 $days{Feb}
287
288and the quotes will be assumed automatically. But anything more complicated
289in the subscript will be interpreted as an expression.
290
291Note that a
a0d0e21e 292single-quoted string must be separated from a preceding word by a
5f05dabc 293space, because single quote is a valid (though deprecated) character in
b88cefa9 294a variable name (see L<perlmod/Packages>).
a0d0e21e 295
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296Three special literals are __FILE__, __LINE__, and __PACKAGE__, which
297represent the current filename, line number, and package name at that
298point in your program. They may be used only as separate tokens; they
299will not be interpolated into strings. If there is no current package
5a964f20 300(due to an empty C<package;> directive), __PACKAGE__ is the undefined value.
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301
302The tokens __END__ and __DATA__ may be used to indicate the logical end
303of the script before the actual end of file. Any following text is
304ignored, but may be read via a DATA filehandle: main::DATA for __END__,
305or PACKNAME::DATA (where PACKNAME is the current package) for __DATA__.
306The two control characters ^D and ^Z are synonyms for __END__ (or
307__DATA__ in a module). See L<SelfLoader> for more description of
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308__DATA__, and an example of its use. Note that you cannot read from the
309DATA filehandle in a BEGIN block: the BEGIN block is executed as soon as
310it is seen (during compilation), at which point the corresponding
311__DATA__ (or __END__) token has not yet been seen.
a0d0e21e 312
748a9306 313A word that has no other interpretation in the grammar will
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314be treated as if it were a quoted string. These are known as
315"barewords". As with filehandles and labels, a bareword that consists
316entirely of lowercase letters risks conflict with future reserved
317words, and if you use the B<-w> switch, Perl will warn you about any
318such words. Some people may wish to outlaw barewords entirely. If you
319say
320
321 use strict 'subs';
322
323then any bareword that would NOT be interpreted as a subroutine call
324produces a compile-time error instead. The restriction lasts to the
54310121 325end of the enclosing block. An inner block may countermand this
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326by saying C<no strict 'subs'>.
327
328Array variables are interpolated into double-quoted strings by joining all
329the elements of the array with the delimiter specified in the C<$">
184e9718 330variable (C<$LIST_SEPARATOR> in English), space by default. The following
4633a7c4 331are equivalent:
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332
333 $temp = join($",@ARGV);
334 system "echo $temp";
335
336 system "echo @ARGV";
337
338Within search patterns (which also undergo double-quotish substitution)
339there is a bad ambiguity: Is C</$foo[bar]/> to be interpreted as
340C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
341expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
342@foo)? If @foo doesn't otherwise exist, then it's obviously a
343character class. If @foo exists, Perl takes a good guess about C<[bar]>,
344and is almost always right. If it does guess wrong, or if you're just
345plain paranoid, you can force the correct interpretation with curly
346brackets as above.
347
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348A line-oriented form of quoting is based on the shell "here-doc"
349syntax. Following a C<E<lt>E<lt>> you specify a string to terminate
350the quoted material, and all lines following the current line down to
351the terminating string are the value of the item. The terminating
352string may be either an identifier (a word), or some quoted text. If
353quoted, the type of quotes you use determines the treatment of the
354text, just as in regular quoting. An unquoted identifier works like
355double quotes. There must be no space between the C<E<lt>E<lt>> and
356the identifier. (If you put a space it will be treated as a null
3fe9a6f1 357identifier, which is valid, and matches the first empty line.) The
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358terminating string must appear by itself (unquoted and with no
359surrounding whitespace) on the terminating line.
a0d0e21e 360
54310121 361 print <<EOF;
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362 The price is $Price.
363 EOF
364
365 print <<"EOF"; # same as above
366 The price is $Price.
367 EOF
368
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369 print <<`EOC`; # execute commands
370 echo hi there
371 echo lo there
372 EOC
373
374 print <<"foo", <<"bar"; # you can stack them
375 I said foo.
376 foo
377 I said bar.
378 bar
379
d28ebecd 380 myfunc(<<"THIS", 23, <<'THAT');
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381 Here's a line
382 or two.
383 THIS
54310121 384 and here's another.
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385 THAT
386
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387Just don't forget that you have to put a semicolon on the end
388to finish the statement, as Perl doesn't know you're not going to
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389try to do this:
390
391 print <<ABC
392 179231
393 ABC
394 + 20;
395
396
397=head2 List value constructors
398
399List values are denoted by separating individual values by commas
400(and enclosing the list in parentheses where precedence requires it):
401
402 (LIST)
403
748a9306 404In a context not requiring a list value, the value of the list
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405literal is the value of the final element, as with the C comma operator.
406For example,
407
408 @foo = ('cc', '-E', $bar);
409
410assigns the entire list value to array foo, but
411
412 $foo = ('cc', '-E', $bar);
413
414assigns the value of variable bar to variable foo. Note that the value
415of an actual array in a scalar context is the length of the array; the
54310121 416following assigns the value 3 to $foo:
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417
418 @foo = ('cc', '-E', $bar);
419 $foo = @foo; # $foo gets 3
420
54310121 421You may have an optional comma before the closing parenthesis of a
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422list literal, so that you can say:
423
424 @foo = (
425 1,
426 2,
427 3,
428 );
429
430LISTs do automatic interpolation of sublists. That is, when a LIST is
431evaluated, each element of the list is evaluated in a list context, and
432the resulting list value is interpolated into LIST just as if each
5a964f20 433individual element were a member of LIST. Thus arrays and hashes lose their
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434identity in a LIST--the list
435
5a964f20 436 (@foo,@bar,&SomeSub,%glarch)
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437
438contains all the elements of @foo followed by all the elements of @bar,
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439followed by all the elements returned by the subroutine named SomeSub
440called in a list context, followed by the key/value pairs of %glarch.
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441To make a list reference that does I<NOT> interpolate, see L<perlref>.
442
443The null list is represented by (). Interpolating it in a list
444has no effect. Thus ((),(),()) is equivalent to (). Similarly,
445interpolating an array with no elements is the same as if no
446array had been interpolated at that point.
447
448A list value may also be subscripted like a normal array. You must
54310121 449put the list in parentheses to avoid ambiguity. For example:
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450
451 # Stat returns list value.
452 $time = (stat($file))[8];
453
4633a7c4 454 # SYNTAX ERROR HERE.
5f05dabc 455 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
4633a7c4 456
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457 # Find a hex digit.
458 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
459
460 # A "reverse comma operator".
461 return (pop(@foo),pop(@foo))[0];
462
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463You may assign to C<undef> in a list. This is useful for throwing
464away some of the return values of a function:
465
466 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
467
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468Lists may be assigned to if and only if each element of the list
469is legal to assign to:
470
471 ($a, $b, $c) = (1, 2, 3);
472
473 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
474
feedc0d7 475List assignment in a scalar context returns the number of elements
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476produced by the expression on the right side of the assignment:
477
478 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
479 $x = (($foo,$bar) = f()); # set $x to f()'s return count
480
481This is very handy when you want to do a list assignment in a Boolean
5f05dabc 482context, because most list functions return a null list when finished,
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483which when assigned produces a 0, which is interpreted as FALSE.
484
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485The final element may be an array or a hash:
486
487 ($a, $b, @rest) = split;
5a964f20 488 my($a, $b, %rest) = @_;
a0d0e21e 489
4633a7c4 490You can actually put an array or hash anywhere in the list, but the first one
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491in the list will soak up all the values, and anything after it will get
492a null value. This may be useful in a local() or my().
493
494A hash literal contains pairs of values to be interpreted
495as a key and a value:
496
497 # same as map assignment above
498 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
499
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500While literal lists and named arrays are usually interchangeable, that's
501not the case for hashes. Just because you can subscript a list value like
502a normal array does not mean that you can subscript a list value as a
503hash. Likewise, hashes included as parts of other lists (including
504parameters lists and return lists from functions) always flatten out into
505key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 506
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507It is often more readable to use the C<=E<gt>> operator between key/value
508pairs. The C<=E<gt>> operator is mostly just a more visually distinctive
b88cefa9 509synonym for a comma, but it also arranges for its left-hand operand to be
5a964f20 510interpreted as a string--if it's a bareword that would be a legal identifier.
b88cefa9 511This makes it nice for initializing hashes:
a0d0e21e 512
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513 %map = (
514 red => 0x00f,
515 blue => 0x0f0,
516 green => 0xf00,
517 );
518
519or for initializing hash references to be used as records:
520
521 $rec = {
522 witch => 'Mable the Merciless',
523 cat => 'Fluffy the Ferocious',
524 date => '10/31/1776',
525 };
526
527or for using call-by-named-parameter to complicated functions:
528
54310121 529 $field = $query->radio_group(
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530 name => 'group_name',
531 values => ['eenie','meenie','minie'],
532 default => 'meenie',
533 linebreak => 'true',
534 labels => \%labels
535 );
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536
537Note that just because a hash is initialized in that order doesn't
538mean that it comes out in that order. See L<perlfunc/sort> for examples
539of how to arrange for an output ordering.
540
5f05dabc 541=head2 Typeglobs and Filehandles
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542
543Perl uses an internal type called a I<typeglob> to hold an entire
544symbol table entry. The type prefix of a typeglob is a C<*>, because
54310121 545it represents all types. This used to be the preferred way to
cb1a09d0 546pass arrays and hashes by reference into a function, but now that
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547we have real references, this is seldom needed.
548
549The main use of typeglobs in modern Perl is create symbol table aliases.
550This assignment:
551
552 *this = *that;
553
554makes $this an alias for $that, @this an alias for @that, %this an alias
555for %that, &this an alias for &that, etc. Much safer is to use a reference.
556This:
5f05dabc 557
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558 local *Here::blue = \$There::green;
559
560temporarily makes $Here::blue an alias for $There::green, but doesn't
561make @Here::blue an alias for @There::green, or %Here::blue an alias for
562%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
563of this. Strange though this may seem, this is the basis for the whole
564module import/export system.
565
566Another use for typeglobs is to to pass filehandles into a function or
567to create new filehandles. If you need to use a typeglob to save away
568a filehandle, do it this way:
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569
570 $fh = *STDOUT;
571
572or perhaps as a real reference, like this:
573
574 $fh = \*STDOUT;
575
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576See L<perlsub> for examples of using these as indirect filehandles
577in functions.
578
579Typeglobs are also a way to create a local filehandle using the local()
580operator. These last until their block is exited, but may be passed back.
581For example:
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582
583 sub newopen {
584 my $path = shift;
585 local *FH; # not my!
5a964f20 586 open (FH, $path) or return undef;
e05a3a1e 587 return *FH;
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588 }
589 $fh = newopen('/etc/passwd');
590
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591Now that we have the *foo{THING} notation, typeglobs aren't used as much
592for filehandle manipulations, although they're still needed to pass brand
593new file and directory handles into or out of functions. That's because
594*HANDLE{IO} only works if HANDLE has already been used as a handle.
595In other words, *FH can be used to create new symbol table entries,
596but *foo{THING} cannot.
597
598Another way to create anonymous filehandles is with the IO::Handle
599module and its ilk. These modules have the advantage of not hiding
600different types of the same name during the local(). See the bottom of
601L<perlfunc/open()> for an example.
cb1a09d0 602
55497cff 603See L<perlref>, L<perlsub>, and L<perlmod/"Symbol Tables"> for more
5a964f20 604discussion on typeglobs and the *foo{THING} syntax.