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1 | =head1 NAME |
2 | |
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3 | perldata - Perl data types |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | =head2 Variable names |
8 | |
9 | Perl has three data structures: scalars, arrays of scalars, and |
10 | associative arrays of scalars, known as "hashes". Normal arrays are |
11 | indexed by number, starting with 0. (Negative subscripts count from |
12 | the end.) Hash arrays are indexed by string. |
13 | |
14 | Scalar values are always named with '$', even when referring to a scalar |
15 | that is part of an array. It works like the English word "the". Thus |
16 | we 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 | $#days # the last index of array @days |
22 | |
23 | but entire arrays or array slices are denoted by '@', which works much like |
24 | the word "these" or "those": |
25 | |
26 | @days # ($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 | |
30 | and entire hashes are denoted by '%': |
31 | |
32 | %days # (key1, val1, key2, val2 ...) |
33 | |
34 | In addition, subroutines are named with an initial '&', though this is |
35 | optional when it's otherwise unambiguous (just as "do" is often |
36 | redundant in English). Symbol table entries can be named with an |
37 | initial '*', but you don't really care about that yet. |
38 | |
39 | Every variable type has its own namespace. You can, without fear of |
40 | conflict, 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). |
42 | This means that $foo and @foo are two different variables. It also |
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43 | means that C<$foo[1]> is a part of @foo, not a part of $foo. This may |
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44 | seem a bit weird, but that's okay, because it is weird. |
45 | |
46 | Since variable and array references always start with '$', '@', or '%', |
47 | the "reserved" words aren't in fact reserved with respect to variable |
48 | names. (They ARE reserved with respect to labels and filehandles, |
49 | however, which don't have an initial special character. You can't have |
50 | a filehandle named "log", for instance. Hint: you could say |
51 | C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase |
52 | filehandles also improves readability and protects you from conflict |
53 | with future reserved words.) Case I<IS> significant--"FOO", "Foo" and |
54 | "foo" are all different names. Names that start with a letter or |
55 | underscore may also contain digits and underscores. |
56 | |
57 | It is possible to replace such an alphanumeric name with an expression |
58 | that returns a reference to an object of that type. For a description |
59 | of this, see L<perlref>. |
60 | |
61 | Names that start with a digit may only contain more digits. Names |
62 | which do not start with a letter, underscore, or digit are limited to |
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63 | one character, e.g. C<$%> or C<$$>. (Most of these one character names |
64 | have a predefined significance to Perl. For instance, C<$$> is the |
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65 | current process id.) |
66 | |
67 | =head2 Context |
68 | |
69 | The interpretation of operations and values in Perl sometimes depends |
70 | on the requirements of the context around the operation or value. |
71 | There are two major contexts: scalar and list. Certain operations |
72 | return list values in contexts wanting a list, and scalar values |
73 | otherwise. (If this is true of an operation it will be mentioned in |
74 | the documentation for that operation.) In other words, Perl overloads |
75 | certain operations based on whether the expected return value is |
76 | singular or plural. (Some words in English work this way, like "fish" |
77 | and "sheep".) |
78 | |
79 | In a reciprocal fashion, an operation provides either a scalar or a |
80 | list context to each of its arguments. For example, if you say |
81 | |
82 | int( <STDIN> ) |
83 | |
84 | the integer operation provides a scalar context for the <STDIN> |
85 | operator, which responds by reading one line from STDIN and passing it |
86 | back to the integer operation, which will then find the integer value |
87 | of that line and return that. If, on the other hand, you say |
88 | |
89 | sort( <STDIN> ) |
90 | |
91 | then the sort operation provides a list context for <STDIN>, which |
92 | will proceed to read every line available up to the end of file, and |
93 | pass that list of lines back to the sort routine, which will then |
94 | sort those lines and return them as a list to whatever the context |
95 | of the sort was. |
96 | |
97 | Assignment is a little bit special in that it uses its left argument to |
98 | determine the context for the right argument. Assignment to a scalar |
99 | evaluates the righthand side in a scalar context, while assignment to |
100 | an array or array slice evaluates the righthand side in a list |
101 | context. Assignment to a list also evaluates the righthand side in a |
102 | list context. |
103 | |
104 | User defined subroutines may choose to care whether they are being |
105 | called in a scalar or list context, but most subroutines do not |
106 | need to care, because scalars are automatically interpolated into |
107 | lists. See L<perlfunc/wantarray>. |
108 | |
109 | =head2 Scalar values |
110 | |
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111 | All data in Perl is a scalar or an array of scalars or a hash of scalars. |
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112 | Scalar variables may contain various kinds of singular data, such as |
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113 | numbers, strings, and references. In general, conversion from one form to |
114 | another is transparent. (A scalar may not contain multiple values, but |
115 | may contain a reference to an array or hash containing multiple values.) |
116 | Because of the automatic conversion of scalars, operations and functions |
117 | that return scalars don't need to care (and, in fact, can't care) whether |
118 | the context is looking for a string or a number. |
119 | |
120 | Scalars aren't necessarily one thing or another. There's no place to |
121 | declare a scalar variable to be of type "string", or of type "number", or |
122 | type "filehandle", or anything else. Perl is a contextually polymorphic |
123 | language whose scalars can be strings, numbers, or references (which |
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124 | includes objects). While strings and numbers are considered pretty |
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125 | much same thing for nearly all purposes, references are strongly-typed |
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126 | uncastable pointers with built-in reference-counting and destructor |
127 | invocation. |
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128 | |
129 | A scalar value is interpreted as TRUE in the Boolean sense if it is not |
130 | the null string or the number 0 (or its string equivalent, "0"). The |
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131 | Boolean context is just a special kind of scalar context. |
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132 | |
133 | There are actually two varieties of null scalars: defined and |
134 | undefined. Undefined null scalars are returned when there is no real |
135 | value for something, such as when there was an error, or at end of |
136 | file, or when you refer to an uninitialized variable or element of an |
137 | array. An undefined null scalar may become defined the first time you |
138 | use it as if it were defined, but prior to that you can use the |
139 | defined() operator to determine whether the value is defined or not. |
140 | |
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141 | To find out whether a given string is a valid non-zero number, it's usually |
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142 | enough to test it against both numeric 0 and also lexical "0" (although |
143 | this will cause B<-w> noises). That's because strings that aren't |
144 | numbers count as 0, just as the do in I<awk>: |
145 | |
146 | if ($str == 0 && $str ne "0") { |
147 | warn "That doesn't look like a number"; |
148 | } |
149 | |
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150 | That's usually preferable because otherwise you won't treat IEEE notations |
151 | like C<NaN> or C<Infinity> properly. At other times you might prefer to |
152 | use a regular expression to check whether data is numeric. See L<perlre> |
153 | for details on regular expressions. |
154 | |
155 | warn "has nondigits" if /\D/; |
156 | warn "not a whole number" unless /^\d+$/; |
157 | warn "not an integer" unless /^[+-]?\d+$/ |
158 | warn "not a decimal number" unless /^[+-]?\d+\.?\d*$/ |
159 | warn "not a C float" |
160 | unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/; |
161 | |
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162 | The length of an array is a scalar value. You may find the length of |
163 | array @days by evaluating C<$#days>, as in B<csh>. (Actually, it's not |
164 | the length of the array, it's the subscript of the last element, since |
165 | there is (ordinarily) a 0th element.) Assigning to C<$#days> changes the |
166 | length of the array. Shortening an array by this method destroys |
167 | intervening values. Lengthening an array that was previously shortened |
168 | I<NO LONGER> recovers the values that were in those elements. (It used to |
169 | in Perl 4, but we had to break this make to make sure destructors were |
170 | called when expected.) You can also gain some measure of efficiency by |
171 | preextending an array that is going to get big. (You can also extend |
172 | an array by assigning to an element that is off the end of the array.) |
173 | You can truncate an array down to nothing by assigning the null list () |
174 | to it. The following are equivalent: |
175 | |
176 | @whatever = (); |
177 | $#whatever = $[ - 1; |
178 | |
179 | If you evaluate a named array in a scalar context, it returns the length of |
180 | the array. (Note that this is not true of lists, which return the |
181 | last value, like the C comma operator.) The following is always true: |
182 | |
183 | scalar(@whatever) == $#whatever - $[ + 1; |
184 | |
185 | Version 5 of Perl changed the semantics of $[: files that don't set |
186 | the value of $[ no longer need to worry about whether another |
187 | file changed its value. (In other words, use of $[ is deprecated.) |
188 | So in general you can just assume that |
189 | |
190 | scalar(@whatever) == $#whatever + 1; |
191 | |
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192 | Some programmers choose to use an explicit conversion so nothing's |
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193 | left to doubt: |
194 | |
195 | $element_count = scalar(@whatever); |
196 | |
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197 | If you evaluate a hash in a scalar context, it returns a value which is |
198 | true if and only if the hash contains any key/value pairs. (If there |
199 | are any key/value pairs, the value returned is a string consisting of |
200 | the number of used buckets and the number of allocated buckets, separated |
201 | by a slash. This is pretty much only useful to find out whether Perl's |
202 | (compiled in) hashing algorithm is performing poorly on your data set. |
203 | For example, you stick 10,000 things in a hash, but evaluating %HASH in |
204 | scalar context reveals "1/16", which means only one out of sixteen buckets |
205 | has been touched, and presumably contains all 10,000 of your items. This |
206 | isn't supposed to happen.) |
207 | |
208 | =head2 Scalar value constructors |
209 | |
210 | Numeric literals are specified in any of the customary floating point or |
211 | integer formats: |
212 | |
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213 | 12345 |
214 | 12345.67 |
215 | .23E-10 |
216 | 0xffff # hex |
217 | 0377 # octal |
218 | 4_294_967_296 # underline for legibility |
219 | |
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220 | String literals are usually delimited by either single or double quotes. They |
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221 | work much like shell quotes: double-quoted string literals are subject |
222 | to backslash and variable substitution; single-quoted strings are not |
223 | (except for "C<\'>" and "C<\\>"). The usual Unix backslash rules apply for making |
224 | characters such as newline, tab, etc., as well as some more exotic |
225 | forms. See L<perlop/qq> for a list. |
226 | |
227 | You can also embed newlines directly in your strings, i.e. they can end |
228 | on a different line than they begin. This is nice, but if you forget |
229 | your trailing quote, the error will not be reported until Perl finds |
230 | another line containing the quote character, which may be much further |
231 | on in the script. Variable substitution inside strings is limited to |
232 | scalar variables, arrays, and array slices. (In other words, |
233 | identifiers beginning with $ or @, followed by an optional bracketed |
234 | expression as a subscript.) The following code segment prints out "The |
235 | price is $100." |
236 | |
237 | $Price = '$100'; # not interpreted |
238 | print "The price is $Price.\n"; # interpreted |
239 | |
240 | As in some shells, you can put curly brackets around the identifier to |
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241 | delimit it from following alphanumerics. In fact, an identifier |
242 | within such curlies is forced to be a string, as is any single |
243 | identifier within a hash subscript. Our earlier example, |
244 | |
245 | $days{'Feb'} |
246 | |
247 | can be written as |
248 | |
249 | $days{Feb} |
250 | |
251 | and the quotes will be assumed automatically. But anything more complicated |
252 | in the subscript will be interpreted as an expression. |
253 | |
254 | Note that a |
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255 | single-quoted string must be separated from a preceding word by a |
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256 | space, since single quote is a valid (though deprecated) character in |
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257 | an identifier (see L<perlmod/Packages>). |
258 | |
259 | Two special literals are __LINE__ and __FILE__, which represent the |
260 | current line number and filename at that point in your program. They |
261 | may only be used as separate tokens; they will not be interpolated into |
262 | strings. In addition, the token __END__ may be used to indicate the |
263 | logical end of the script before the actual end of file. Any following |
264 | text is ignored, but may be read via the DATA filehandle. (The DATA |
265 | filehandle may read data only from the main script, but not from any |
266 | required file or evaluated string.) The two control characters ^D and |
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267 | ^Z are synonyms for __END__ (or __DATA__ in a module; see L<SelfLoader> for |
268 | details on __DATA__). |
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269 | |
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270 | A word that has no other interpretation in the grammar will |
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271 | be treated as if it were a quoted string. These are known as |
272 | "barewords". As with filehandles and labels, a bareword that consists |
273 | entirely of lowercase letters risks conflict with future reserved |
274 | words, and if you use the B<-w> switch, Perl will warn you about any |
275 | such words. Some people may wish to outlaw barewords entirely. If you |
276 | say |
277 | |
278 | use strict 'subs'; |
279 | |
280 | then any bareword that would NOT be interpreted as a subroutine call |
281 | produces a compile-time error instead. The restriction lasts to the |
282 | end of the enclosing block. An inner block may countermand this |
283 | by saying C<no strict 'subs'>. |
284 | |
285 | Array variables are interpolated into double-quoted strings by joining all |
286 | the elements of the array with the delimiter specified in the C<$"> |
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287 | variable ($LIST_SEPARATOR in English), space by default. The following |
288 | are equivalent: |
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289 | |
290 | $temp = join($",@ARGV); |
291 | system "echo $temp"; |
292 | |
293 | system "echo @ARGV"; |
294 | |
295 | Within search patterns (which also undergo double-quotish substitution) |
296 | there is a bad ambiguity: Is C</$foo[bar]/> to be interpreted as |
297 | C</${foo}[bar]/> (where C<[bar]> is a character class for the regular |
298 | expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array |
299 | @foo)? If @foo doesn't otherwise exist, then it's obviously a |
300 | character class. If @foo exists, Perl takes a good guess about C<[bar]>, |
301 | and is almost always right. If it does guess wrong, or if you're just |
302 | plain paranoid, you can force the correct interpretation with curly |
303 | brackets as above. |
304 | |
305 | A line-oriented form of quoting is based on the shell "here-doc" syntax. |
306 | Following a C<E<lt>E<lt>> you specify a string to terminate the quoted material, |
307 | and all lines following the current line down to the terminating string |
308 | are the value of the item. The terminating string may be either an |
309 | identifier (a word), or some quoted text. If quoted, the type of |
310 | quotes you use determines the treatment of the text, just as in regular |
311 | quoting. An unquoted identifier works like double quotes. There must |
312 | be no space between the C<E<lt>E<lt>> and the identifier. (If you put a space it |
313 | will be treated as a null identifier, which is valid, and matches the |
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314 | first blank line.) The terminating string must appear by itself |
315 | (unquoted and with no surrounding whitespace) on the terminating line. |
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316 | |
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317 | print <<EOF; |
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318 | The price is $Price. |
319 | EOF |
320 | |
321 | print <<"EOF"; # same as above |
322 | The price is $Price. |
323 | EOF |
324 | |
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325 | print <<`EOC`; # execute commands |
326 | echo hi there |
327 | echo lo there |
328 | EOC |
329 | |
330 | print <<"foo", <<"bar"; # you can stack them |
331 | I said foo. |
332 | foo |
333 | I said bar. |
334 | bar |
335 | |
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336 | myfunc(<<"THIS", 23, <<'THAT'); |
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337 | Here's a line |
338 | or two. |
339 | THIS |
340 | and here another. |
341 | THAT |
342 | |
343 | Just don't forget that you have to put a semicolon on the end |
344 | to finish the statement, as Perl doesn't know you're not going to |
345 | try to do this: |
346 | |
347 | print <<ABC |
348 | 179231 |
349 | ABC |
350 | + 20; |
351 | |
352 | |
353 | =head2 List value constructors |
354 | |
355 | List values are denoted by separating individual values by commas |
356 | (and enclosing the list in parentheses where precedence requires it): |
357 | |
358 | (LIST) |
359 | |
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360 | In a context not requiring a list value, the value of the list |
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361 | literal is the value of the final element, as with the C comma operator. |
362 | For example, |
363 | |
364 | @foo = ('cc', '-E', $bar); |
365 | |
366 | assigns the entire list value to array foo, but |
367 | |
368 | $foo = ('cc', '-E', $bar); |
369 | |
370 | assigns the value of variable bar to variable foo. Note that the value |
371 | of an actual array in a scalar context is the length of the array; the |
372 | following assigns to $foo the value 3: |
373 | |
374 | @foo = ('cc', '-E', $bar); |
375 | $foo = @foo; # $foo gets 3 |
376 | |
377 | You may have an optional comma before the closing parenthesis of an |
378 | list literal, so that you can say: |
379 | |
380 | @foo = ( |
381 | 1, |
382 | 2, |
383 | 3, |
384 | ); |
385 | |
386 | LISTs do automatic interpolation of sublists. That is, when a LIST is |
387 | evaluated, each element of the list is evaluated in a list context, and |
388 | the resulting list value is interpolated into LIST just as if each |
389 | individual element were a member of LIST. Thus arrays lose their |
390 | identity in a LIST--the list |
391 | |
392 | (@foo,@bar,&SomeSub) |
393 | |
394 | contains all the elements of @foo followed by all the elements of @bar, |
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395 | followed by all the elements returned by the subroutine named SomeSub when |
396 | it's called in a list context. |
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397 | To make a list reference that does I<NOT> interpolate, see L<perlref>. |
398 | |
399 | The null list is represented by (). Interpolating it in a list |
400 | has no effect. Thus ((),(),()) is equivalent to (). Similarly, |
401 | interpolating an array with no elements is the same as if no |
402 | array had been interpolated at that point. |
403 | |
404 | A list value may also be subscripted like a normal array. You must |
405 | put the list in parentheses to avoid ambiguity. Examples: |
406 | |
407 | # Stat returns list value. |
408 | $time = (stat($file))[8]; |
409 | |
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410 | # SYNTAX ERROR HERE. |
411 | $time = stat($file)[8]; # OOPS, FORGOT PARENS |
412 | |
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413 | # Find a hex digit. |
414 | $hexdigit = ('a','b','c','d','e','f')[$digit-10]; |
415 | |
416 | # A "reverse comma operator". |
417 | return (pop(@foo),pop(@foo))[0]; |
418 | |
419 | Lists may be assigned to if and only if each element of the list |
420 | is legal to assign to: |
421 | |
422 | ($a, $b, $c) = (1, 2, 3); |
423 | |
424 | ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00); |
425 | |
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426 | Array assignment in a scalar context returns the number of elements |
427 | produced by the expression on the right side of the assignment: |
428 | |
429 | $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2 |
430 | $x = (($foo,$bar) = f()); # set $x to f()'s return count |
431 | |
432 | This is very handy when you want to do a list assignment in a Boolean |
433 | context, since most list functions return a null list when finished, |
434 | which when assigned produces a 0, which is interpreted as FALSE. |
435 | |
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436 | The final element may be an array or a hash: |
437 | |
438 | ($a, $b, @rest) = split; |
439 | local($a, $b, %rest) = @_; |
440 | |
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441 | You can actually put an array or hash anywhere in the list, but the first one |
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442 | in the list will soak up all the values, and anything after it will get |
443 | a null value. This may be useful in a local() or my(). |
444 | |
445 | A hash literal contains pairs of values to be interpreted |
446 | as a key and a value: |
447 | |
448 | # same as map assignment above |
449 | %map = ('red',0x00f,'blue',0x0f0,'green',0xf00); |
450 | |
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451 | While literal lists and named arrays are usually interchangeable, that's |
452 | not the case for hashes. Just because you can subscript a list value like |
453 | a normal array does not mean that you can subscript a list value as a |
454 | hash. Likewise, hashes included as parts of other lists (including |
455 | parameters lists and return lists from functions) always flatten out into |
456 | key/value pairs. That's why it's good to use references sometimes. |
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457 | |
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458 | It is often more readable to use the C<=E<gt>> operator between key/value |
459 | pairs. The C<=E<gt>> operator is mostly just a more visually distinctive |
460 | synonym for a comma, but it also quotes its left-hand operand, which makes |
461 | it nice for initializing hashes: |
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462 | |
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463 | %map = ( |
464 | red => 0x00f, |
465 | blue => 0x0f0, |
466 | green => 0xf00, |
467 | ); |
468 | |
469 | or for initializing hash references to be used as records: |
470 | |
471 | $rec = { |
472 | witch => 'Mable the Merciless', |
473 | cat => 'Fluffy the Ferocious', |
474 | date => '10/31/1776', |
475 | }; |
476 | |
477 | or for using call-by-named-parameter to complicated functions: |
478 | |
479 | $field = $query->radio_group( |
480 | name => 'group_name', |
481 | values => ['eenie','meenie','minie'], |
482 | default => 'meenie', |
483 | linebreak => 'true', |
484 | labels => \%labels |
485 | ); |
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486 | |
487 | Note that just because a hash is initialized in that order doesn't |
488 | mean that it comes out in that order. See L<perlfunc/sort> for examples |
489 | of how to arrange for an output ordering. |
490 | |
491 | =head2 Typeglobs and FileHandles |
492 | |
493 | Perl uses an internal type called a I<typeglob> to hold an entire |
494 | symbol table entry. The type prefix of a typeglob is a C<*>, because |
495 | it represents all types. This used to be the preferred way to |
496 | pass arrays and hashes by reference into a function, but now that |
497 | we have real references, this is seldom needed. |
498 | |
499 | One place where you still use typeglobs (or references thereto) |
500 | is for passing or storing filehandles. If you want to save away |
501 | a filehandle, do it this way: |
502 | |
503 | $fh = *STDOUT; |
504 | |
505 | or perhaps as a real reference, like this: |
506 | |
507 | $fh = \*STDOUT; |
508 | |
509 | This is also the way to create a local filehandle. For example: |
510 | |
511 | sub newopen { |
512 | my $path = shift; |
513 | local *FH; # not my! |
514 | open (FH, $path) || return undef; |
515 | return \*FH; |
516 | } |
517 | $fh = newopen('/etc/passwd'); |
518 | |
519 | See L<perlref>, L<perlsub>, and L<perlmod/"Symbols Tables"> for more |
520 | discussion on typeglobs. See L<perlfunc/open> for other ways of |
521 | generating filehandles. |