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1=head1 NAME
2
3perldata - Perl data structures
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
14Scalar values are always named with '$', even when referring to a scalar
15that is part of an array. It works like the English word "the". Thus
16we have:
17
18 $days # the simple scalar value "days"
19 $days[28] # the 29th element of array @days
20 $days{'Feb'} # the 'Feb' value from hash %days
21 $#days # the last index of array @days
22
23but entire arrays or array slices are denoted by '@', which works much like
24the word "these" or "those":
25
26 @days # ($days[0], $days[1],... $days[n])
27 @days[3,4,5] # same as @days[3..5]
28 @days{'a','c'} # same as ($days{'a'},$days{'c'})
29
30and entire hashes are denoted by '%':
31
32 %days # (key1, val1, key2, val2 ...)
33
34In addition, subroutines are named with an initial '&', though this is
35optional when it's otherwise unambiguous (just as "do" is often
36redundant in English). Symbol table entries can be named with an
37initial '*', but you don't really care about that yet.
38
39Every variable type has its own namespace. You can, without fear of
40conflict, use the same name for a scalar variable, an array, or a hash
41(or, for that matter, a filehandle, a subroutine name, or a label).
42This means that $foo and @foo are two different variables. It also
748a9306 43means that C<$foo[1]> is a part of @foo, not a part of $foo. This may
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44seem a bit weird, but that's okay, because it is weird.
45
46Since variable and array references always start with '$', '@', or '%',
47the "reserved" words aren't in fact reserved with respect to variable
48names. (They ARE reserved with respect to labels and filehandles,
49however, which don't have an initial special character. You can't have
50a filehandle named "log", for instance. Hint: you could say
51C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase
52filehandles also improves readability and protects you from conflict
53with future reserved words.) Case I<IS> significant--"FOO", "Foo" and
54"foo" are all different names. Names that start with a letter or
55underscore may also contain digits and underscores.
56
57It is possible to replace such an alphanumeric name with an expression
58that returns a reference to an object of that type. For a description
59of this, see L<perlref>.
60
61Names that start with a digit may only contain more digits. Names
62which do not start with a letter, underscore, or digit are limited to
63one character, e.g. "$%" or "$$". (Most of these one character names
64have a predefined significance to Perl. For instance, $$ is the
65current process id.)
66
67=head2 Context
68
69The interpretation of operations and values in Perl sometimes depends
70on the requirements of the context around the operation or value.
71There are two major contexts: scalar and list. Certain operations
72return list values in contexts wanting a list, and scalar values
73otherwise. (If this is true of an operation it will be mentioned in
74the documentation for that operation.) In other words, Perl overloads
75certain operations based on whether the expected return value is
76singular or plural. (Some words in English work this way, like "fish"
77and "sheep".)
78
79In a reciprocal fashion, an operation provides either a scalar or a
80list context to each of its arguments. For example, if you say
81
82 int( <STDIN> )
83
84the integer operation provides a scalar context for the <STDIN>
85operator, which responds by reading one line from STDIN and passing it
86back to the integer operation, which will then find the integer value
87of that line and return that. If, on the other hand, you say
88
89 sort( <STDIN> )
90
91then the sort operation provides a list context for <STDIN>, which
92will proceed to read every line available up to the end of file, and
93pass that list of lines back to the sort routine, which will then
94sort those lines and return them as a list to whatever the context
95of the sort was.
96
97Assignment is a little bit special in that it uses its left argument to
98determine the context for the right argument. Assignment to a scalar
99evaluates the righthand side in a scalar context, while assignment to
100an array or array slice evaluates the righthand side in a list
101context. Assignment to a list also evaluates the righthand side in a
102list context.
103
104User defined subroutines may choose to care whether they are being
105called in a scalar or list context, but most subroutines do not
106need to care, because scalars are automatically interpolated into
107lists. See L<perlfunc/wantarray>.
108
109=head2 Scalar values
110
4633a7c4 111All data in Perl is a scalar or an array of scalars or a hash of scalars.
a0d0e21e 112Scalar variables may contain various kinds of singular data, such as
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113numbers, strings, and references. In general, conversion from one form to
114another is transparent. (A scalar may not contain multiple values, but
115may contain a reference to an array or hash containing multiple values.)
116Because of the automatic conversion of scalars, operations and functions
117that return scalars don't need to care (and, in fact, can't care) whether
118the context is looking for a string or a number.
119
120Scalars aren't necessarily one thing or another. There's no place to
121declare a scalar variable to be of type "string", or of type "number", or
122type "filehandle", or anything else. Perl is a contextually polymorphic
123language whose scalars can be strings, numbers, or references (which
124includes objects). While strings and numbers are considered the pretty
125much same thing for nearly all purposes, but references are strongly-typed
126uncastable pointers with built-in reference-counting and destructor
127invocation.
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128
129A scalar value is interpreted as TRUE in the Boolean sense if it is not
130the null string or the number 0 (or its string equivalent, "0"). The
4633a7c4 131Boolean context is just a special kind of scalar context.
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132
133There are actually two varieties of null scalars: defined and
134undefined. Undefined null scalars are returned when there is no real
135value for something, such as when there was an error, or at end of
136file, or when you refer to an uninitialized variable or element of an
137array. An undefined null scalar may become defined the first time you
138use it as if it were defined, but prior to that you can use the
139defined() operator to determine whether the value is defined or not.
140
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141To find out whether a given string is a valid non-zero number, it's usally
142enough to test it against both numeric 0 and also lexical "0" (although
143this will cause B<-w> noises). That's because strings that aren't
144numbers count as 0, just as the do in I<awk>:
145
146 if ($str == 0 && $str ne "0") {
147 warn "That doesn't look like a number";
148 }
149
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150The length of an array is a scalar value. You may find the length of
151array @days by evaluating C<$#days>, as in B<csh>. (Actually, it's not
152the length of the array, it's the subscript of the last element, since
153there is (ordinarily) a 0th element.) Assigning to C<$#days> changes the
154length of the array. Shortening an array by this method destroys
155intervening values. Lengthening an array that was previously shortened
156I<NO LONGER> recovers the values that were in those elements. (It used to
157in Perl 4, but we had to break this make to make sure destructors were
158called when expected.) You can also gain some measure of efficiency by
159preextending an array that is going to get big. (You can also extend
160an array by assigning to an element that is off the end of the array.)
161You can truncate an array down to nothing by assigning the null list ()
162to it. The following are equivalent:
163
164 @whatever = ();
165 $#whatever = $[ - 1;
166
167If you evaluate a named array in a scalar context, it returns the length of
168the array. (Note that this is not true of lists, which return the
169last value, like the C comma operator.) The following is always true:
170
171 scalar(@whatever) == $#whatever - $[ + 1;
172
173Version 5 of Perl changed the semantics of $[: files that don't set
174the value of $[ no longer need to worry about whether another
175file changed its value. (In other words, use of $[ is deprecated.)
176So in general you can just assume that
177
178 scalar(@whatever) == $#whatever + 1;
179
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180Some programmer choose to use an explcit conversion so nothing's
181left to doubt:
182
183 $element_count = scalar(@whatever);
184
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185If you evaluate a hash in a scalar context, it returns a value which is
186true if and only if the hash contains any key/value pairs. (If there
187are any key/value pairs, the value returned is a string consisting of
188the number of used buckets and the number of allocated buckets, separated
189by a slash. This is pretty much only useful to find out whether Perl's
190(compiled in) hashing algorithm is performing poorly on your data set.
191For example, you stick 10,000 things in a hash, but evaluating %HASH in
192scalar context reveals "1/16", which means only one out of sixteen buckets
193has been touched, and presumably contains all 10,000 of your items. This
194isn't supposed to happen.)
195
196=head2 Scalar value constructors
197
198Numeric literals are specified in any of the customary floating point or
199integer formats:
200
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201 12345
202 12345.67
203 .23E-10
204 0xffff # hex
205 0377 # octal
206 4_294_967_296 # underline for legibility
207
4633a7c4 208String literals are usually delimited by either single or double quotes. They
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209work much like shell quotes: double-quoted string literals are subject
210to backslash and variable substitution; single-quoted strings are not
211(except for "C<\'>" and "C<\\>"). The usual Unix backslash rules apply for making
212characters such as newline, tab, etc., as well as some more exotic
213forms. See L<perlop/qq> for a list.
214
215You can also embed newlines directly in your strings, i.e. they can end
216on a different line than they begin. This is nice, but if you forget
217your trailing quote, the error will not be reported until Perl finds
218another line containing the quote character, which may be much further
219on in the script. Variable substitution inside strings is limited to
220scalar variables, arrays, and array slices. (In other words,
221identifiers beginning with $ or @, followed by an optional bracketed
222expression as a subscript.) The following code segment prints out "The
223price is $100."
224
225 $Price = '$100'; # not interpreted
226 print "The price is $Price.\n"; # interpreted
227
228As in some shells, you can put curly brackets around the identifier to
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229delimit it from following alphanumerics. In fact, an identifier
230within such curlies is forced to be a string, as is any single
231identifier within a hash subscript. Our earlier example,
232
233 $days{'Feb'}
234
235can be written as
236
237 $days{Feb}
238
239and the quotes will be assumed automatically. But anything more complicated
240in the subscript will be interpreted as an expression.
241
242Note that a
a0d0e21e 243single-quoted string must be separated from a preceding word by a
748a9306 244space, since single quote is a valid (though deprecated) character in
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245an identifier (see L<perlmod/Packages>).
246
247Two special literals are __LINE__ and __FILE__, which represent the
248current line number and filename at that point in your program. They
249may only be used as separate tokens; they will not be interpolated into
250strings. In addition, the token __END__ may be used to indicate the
251logical end of the script before the actual end of file. Any following
252text is ignored, but may be read via the DATA filehandle. (The DATA
253filehandle may read data only from the main script, but not from any
254required file or evaluated string.) The two control characters ^D and
4633a7c4 255^Z are synonyms for __END__ (or __DATA__ in a module).
a0d0e21e 256
748a9306 257A word that has no other interpretation in the grammar will
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258be treated as if it were a quoted string. These are known as
259"barewords". As with filehandles and labels, a bareword that consists
260entirely of lowercase letters risks conflict with future reserved
261words, and if you use the B<-w> switch, Perl will warn you about any
262such words. Some people may wish to outlaw barewords entirely. If you
263say
264
265 use strict 'subs';
266
267then any bareword that would NOT be interpreted as a subroutine call
268produces a compile-time error instead. The restriction lasts to the
269end of the enclosing block. An inner block may countermand this
270by saying C<no strict 'subs'>.
271
272Array variables are interpolated into double-quoted strings by joining all
273the elements of the array with the delimiter specified in the C<$">
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274variable ($LIST_SEPARATOR in English), space by default. The following
275are equivalent:
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276
277 $temp = join($",@ARGV);
278 system "echo $temp";
279
280 system "echo @ARGV";
281
282Within search patterns (which also undergo double-quotish substitution)
283there is a bad ambiguity: Is C</$foo[bar]/> to be interpreted as
284C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
285expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
286@foo)? If @foo doesn't otherwise exist, then it's obviously a
287character class. If @foo exists, Perl takes a good guess about C<[bar]>,
288and is almost always right. If it does guess wrong, or if you're just
289plain paranoid, you can force the correct interpretation with curly
290brackets as above.
291
292A line-oriented form of quoting is based on the shell "here-doc" syntax.
293Following a C<E<lt>E<lt>> you specify a string to terminate the quoted material,
294and all lines following the current line down to the terminating string
295are the value of the item. The terminating string may be either an
296identifier (a word), or some quoted text. If quoted, the type of
297quotes you use determines the treatment of the text, just as in regular
298quoting. An unquoted identifier works like double quotes. There must
299be no space between the C<E<lt>E<lt>> and the identifier. (If you put a space it
300will be treated as a null identifier, which is valid, and matches the
301first blank line--see the Merry Christmas example below.) The terminating
302string must appear by itself (unquoted and with no surrounding
303whitespace) on the terminating line.
304
305 print <<EOF; # same as above
306 The price is $Price.
307 EOF
308
309 print <<"EOF"; # same as above
310 The price is $Price.
311 EOF
312
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313 print <<`EOC`; # execute commands
314 echo hi there
315 echo lo there
316 EOC
317
318 print <<"foo", <<"bar"; # you can stack them
319 I said foo.
320 foo
321 I said bar.
322 bar
323
324 myfunc(<<"THIS", 23, <<'THAT'');
325 Here's a line
326 or two.
327 THIS
328 and here another.
329 THAT
330
331Just don't forget that you have to put a semicolon on the end
332to finish the statement, as Perl doesn't know you're not going to
333try to do this:
334
335 print <<ABC
336 179231
337 ABC
338 + 20;
339
340
341=head2 List value constructors
342
343List values are denoted by separating individual values by commas
344(and enclosing the list in parentheses where precedence requires it):
345
346 (LIST)
347
748a9306 348In a context not requiring a list value, the value of the list
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349literal is the value of the final element, as with the C comma operator.
350For example,
351
352 @foo = ('cc', '-E', $bar);
353
354assigns the entire list value to array foo, but
355
356 $foo = ('cc', '-E', $bar);
357
358assigns the value of variable bar to variable foo. Note that the value
359of an actual array in a scalar context is the length of the array; the
360following assigns to $foo the value 3:
361
362 @foo = ('cc', '-E', $bar);
363 $foo = @foo; # $foo gets 3
364
365You may have an optional comma before the closing parenthesis of an
366list literal, so that you can say:
367
368 @foo = (
369 1,
370 2,
371 3,
372 );
373
374LISTs do automatic interpolation of sublists. That is, when a LIST is
375evaluated, each element of the list is evaluated in a list context, and
376the resulting list value is interpolated into LIST just as if each
377individual element were a member of LIST. Thus arrays lose their
378identity in a LIST--the list
379
380 (@foo,@bar,&SomeSub)
381
382contains all the elements of @foo followed by all the elements of @bar,
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383followed by all the elements returned by the subroutine named SomeSub when
384it's called in a list context.
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385To make a list reference that does I<NOT> interpolate, see L<perlref>.
386
387The null list is represented by (). Interpolating it in a list
388has no effect. Thus ((),(),()) is equivalent to (). Similarly,
389interpolating an array with no elements is the same as if no
390array had been interpolated at that point.
391
392A list value may also be subscripted like a normal array. You must
393put the list in parentheses to avoid ambiguity. Examples:
394
395 # Stat returns list value.
396 $time = (stat($file))[8];
397
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398 # SYNTAX ERROR HERE.
399 $time = stat($file)[8]; # OOPS, FORGOT PARENS
400
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401 # Find a hex digit.
402 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
403
404 # A "reverse comma operator".
405 return (pop(@foo),pop(@foo))[0];
406
407Lists may be assigned to if and only if each element of the list
408is legal to assign to:
409
410 ($a, $b, $c) = (1, 2, 3);
411
412 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
413
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414Array assignment in a scalar context returns the number of elements
415produced by the expression on the right side of the assignment:
416
417 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
418 $x = (($foo,$bar) = f()); # set $x to f()'s return count
419
420This is very handy when you want to do a list assignment in a Boolean
421context, since most list functions return a null list when finished,
422which when assigned produces a 0, which is interpreted as FALSE.
423
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424The final element may be an array or a hash:
425
426 ($a, $b, @rest) = split;
427 local($a, $b, %rest) = @_;
428
4633a7c4 429You can actually put an array or hash anywhere in the list, but the first one
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430in the list will soak up all the values, and anything after it will get
431a null value. This may be useful in a local() or my().
432
433A hash literal contains pairs of values to be interpreted
434as a key and a value:
435
436 # same as map assignment above
437 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
438
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439While literal lists and named arrays are usually interchangeable, that's
440not the case for hashes. Just because you can subscript a list value like
441a normal array does not mean that you can subscript a list value as a
442hash. Likewise, hashes included as parts of other lists (including
443parameters lists and return lists from functions) always flatten out into
444key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 445
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446It is often more readable to use the C<=E<gt>> operator between key/value
447pairs. The C<=E<gt>> operator is mostly just a more visually distinctive
448synonym for a comma, but it also quotes its left-hand operand, which makes
449it nice for initializing hashes:
a0d0e21e 450
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451 %map = (
452 red => 0x00f,
453 blue => 0x0f0,
454 green => 0xf00,
455 );
456
457or for initializing hash references to be used as records:
458
459 $rec = {
460 witch => 'Mable the Merciless',
461 cat => 'Fluffy the Ferocious',
462 date => '10/31/1776',
463 };
464
465or for using call-by-named-parameter to complicated functions:
466
467 $field = $query->radio_group(
468 name => 'group_name',
469 values => ['eenie','meenie','minie'],
470 default => 'meenie',
471 linebreak => 'true',
472 labels => \%labels
473 );