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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 | |
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14 | Values are usually referred to by name (or through a named reference). |
15 | The first character of the name tells you to what sort of data |
16 | structure it refers. The rest of the name tells you the particular |
17 | value to which it refers. Most often, it consists of a single |
18 | I<identifier>, that is, a string beginning with a letter or underscore, |
19 | and containing letters, underscores, and digits. In some cases, it |
20 | may be a chain of identifiers, separated by C<::> (or by C<'>, but |
21 | that's deprecated); all but the last are interpreted as names of |
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22 | packages, to locate the namespace in which to look |
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23 | up the final identifier (see L<perlmod/Packages> for details). |
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24 | It's possible to substitute for a simple identifier an expression |
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25 | that produces a reference to the value at runtime; this is |
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26 | described in more detail below, and in L<perlref>. |
27 | |
28 | There are also special variables whose names don't follow these |
29 | rules, so that they don't accidentally collide with one of your |
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30 | normal variables. Strings that match parenthesized parts of a |
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31 | regular expression are saved under names containing only digits after |
32 | the C<$> (see L<perlop> and L<perlre>). In addition, several special |
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33 | variables that provide windows into the inner working of Perl have names |
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34 | containing punctuation characters (see L<perlvar>). |
35 | |
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36 | Scalar values are always named with '$', even when referring to a scalar |
37 | that is part of an array. It works like the English word "the". Thus |
38 | we 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 | $#days # the last index of array @days |
44 | |
45 | but entire arrays or array slices are denoted by '@', which works much like |
46 | the word "these" or "those": |
47 | |
48 | @days # ($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 | |
52 | and entire hashes are denoted by '%': |
53 | |
54 | %days # (key1, val1, key2, val2 ...) |
55 | |
56 | In addition, subroutines are named with an initial '&', though this is |
57 | optional when it's otherwise unambiguous (just as "do" is often |
58 | redundant in English). Symbol table entries can be named with an |
59 | initial '*', but you don't really care about that yet. |
60 | |
61 | Every variable type has its own namespace. You can, without fear of |
62 | conflict, 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). |
64 | This means that $foo and @foo are two different variables. It also |
748a9306 |
65 | means that C<$foo[1]> is a part of @foo, not a part of $foo. This may |
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66 | seem a bit weird, but that's okay, because it is weird. |
67 | |
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68 | Because variable and array references always start with '$', '@', or '%', |
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69 | the "reserved" words aren't in fact reserved with respect to variable |
70 | names. (They ARE reserved with respect to labels and filehandles, |
71 | however, which don't have an initial special character. You can't have |
72 | a filehandle named "log", for instance. Hint: you could say |
73 | C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase |
74 | filehandles also improves readability and protects you from conflict |
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75 | with 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 |
77 | underscore may also contain digits and underscores. |
78 | |
79 | It is possible to replace such an alphanumeric name with an expression |
80 | that returns a reference to an object of that type. For a description |
81 | of this, see L<perlref>. |
82 | |
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83 | Names that start with a digit may contain only more digits. Names |
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84 | that do not start with a letter, underscore, or digit are limited to |
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85 | one character, e.g., C<$%> or C<$$>. (Most of these one character names |
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86 | have a predefined significance to Perl. For instance, C<$$> is the |
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87 | current process id.) |
88 | |
89 | =head2 Context |
90 | |
91 | The interpretation of operations and values in Perl sometimes depends |
92 | on the requirements of the context around the operation or value. |
93 | There are two major contexts: scalar and list. Certain operations |
94 | return list values in contexts wanting a list, and scalar values |
95 | otherwise. (If this is true of an operation it will be mentioned in |
96 | the documentation for that operation.) In other words, Perl overloads |
97 | certain operations based on whether the expected return value is |
98 | singular or plural. (Some words in English work this way, like "fish" |
99 | and "sheep".) |
100 | |
101 | In a reciprocal fashion, an operation provides either a scalar or a |
102 | list context to each of its arguments. For example, if you say |
103 | |
104 | int( <STDIN> ) |
105 | |
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106 | the integer operation provides a scalar context for the E<lt>STDINE<gt> |
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107 | operator, which responds by reading one line from STDIN and passing it |
108 | back to the integer operation, which will then find the integer value |
109 | of that line and return that. If, on the other hand, you say |
110 | |
111 | sort( <STDIN> ) |
112 | |
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113 | then the sort operation provides a list context for E<lt>STDINE<gt>, which |
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114 | will proceed to read every line available up to the end of file, and |
115 | pass that list of lines back to the sort routine, which will then |
116 | sort those lines and return them as a list to whatever the context |
117 | of the sort was. |
118 | |
119 | Assignment is a little bit special in that it uses its left argument to |
120 | determine the context for the right argument. Assignment to a scalar |
121 | evaluates the righthand side in a scalar context, while assignment to |
122 | an array or array slice evaluates the righthand side in a list |
123 | context. Assignment to a list also evaluates the righthand side in a |
124 | list context. |
125 | |
126 | User defined subroutines may choose to care whether they are being |
127 | called in a scalar or list context, but most subroutines do not |
128 | need to care, because scalars are automatically interpolated into |
129 | lists. See L<perlfunc/wantarray>. |
130 | |
131 | =head2 Scalar values |
132 | |
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133 | All data in Perl is a scalar or an array of scalars or a hash of scalars. |
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134 | Scalar variables may contain various kinds of singular data, such as |
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135 | numbers, strings, and references. In general, conversion from one form to |
136 | another is transparent. (A scalar may not contain multiple values, but |
137 | may contain a reference to an array or hash containing multiple values.) |
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138 | Because of the automatic conversion of scalars, operations, and functions |
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139 | that return scalars don't need to care (and, in fact, can't care) whether |
140 | the context is looking for a string or a number. |
141 | |
142 | Scalars aren't necessarily one thing or another. There's no place to |
143 | declare a scalar variable to be of type "string", or of type "number", or |
144 | type "filehandle", or anything else. Perl is a contextually polymorphic |
145 | language whose scalars can be strings, numbers, or references (which |
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146 | includes objects). While strings and numbers are considered pretty |
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147 | much the same thing for nearly all purposes, references are strongly-typed |
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148 | uncastable pointers with builtin reference-counting and destructor |
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149 | invocation. |
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150 | |
151 | A scalar value is interpreted as TRUE in the Boolean sense if it is not |
152 | the null string or the number 0 (or its string equivalent, "0"). The |
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153 | Boolean context is just a special kind of scalar context. |
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154 | |
155 | There are actually two varieties of null scalars: defined and |
156 | undefined. Undefined null scalars are returned when there is no real |
157 | value for something, such as when there was an error, or at end of |
158 | file, or when you refer to an uninitialized variable or element of an |
159 | array. An undefined null scalar may become defined the first time you |
160 | use it as if it were defined, but prior to that you can use the |
161 | defined() operator to determine whether the value is defined or not. |
162 | |
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163 | To find out whether a given string is a valid nonzero number, it's usually |
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164 | enough to test it against both numeric 0 and also lexical "0" (although |
165 | this will cause B<-w> noises). That's because strings that aren't |
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166 | numbers count as 0, just as they do in B<awk>: |
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167 | |
168 | if ($str == 0 && $str ne "0") { |
169 | warn "That doesn't look like a number"; |
54310121 |
170 | } |
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171 | |
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172 | That's usually preferable because otherwise you won't treat IEEE notations |
173 | like C<NaN> or C<Infinity> properly. At other times you might prefer to |
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174 | use the POSIX::strtod function or a regular expression to check whether |
175 | data is numeric. See L<perlre> for details on regular expressions. |
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176 | |
177 | warn "has nondigits" if /\D/; |
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178 | warn "not a natural number" unless /^\d+$/; # 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*$/; # rejects .2 |
182 | warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/; |
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183 | warn "not a C float" |
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184 | unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/; |
185 | |
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186 | The length of an array is a scalar value. You may find the length of |
187 | array @days by evaluating C<$#days>, as in B<csh>. (Actually, it's not |
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188 | the length of the array, it's the subscript of the last element, because |
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189 | there is (ordinarily) a 0th element.) Assigning to C<$#days> changes the |
190 | length of the array. Shortening an array by this method destroys |
191 | intervening values. Lengthening an array that was previously shortened |
192 | I<NO LONGER> recovers the values that were in those elements. (It used to |
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193 | in Perl 4, but we had to break this to make sure destructors were |
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194 | called when expected.) You can also gain some miniscule measure of efficiency by |
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195 | pre-extending an array that is going to get big. (You can also extend |
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196 | an array by assigning to an element that is off the end of the array.) |
197 | You can truncate an array down to nothing by assigning the null list () |
198 | to it. The following are equivalent: |
199 | |
200 | @whatever = (); |
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201 | $#whatever = -1; |
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202 | |
203 | If you evaluate a named array in a scalar context, it returns the length of |
204 | the array. (Note that this is not true of lists, which return the |
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205 | last value, like the C comma operator, nor of built-in functions, which return |
206 | whatever they feel like returning.) The following is always true: |
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207 | |
208 | scalar(@whatever) == $#whatever - $[ + 1; |
209 | |
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210 | Version 5 of Perl changed the semantics of C<$[>: files that don't set |
211 | the value of C<$[> no longer need to worry about whether another |
212 | file changed its value. (In other words, use of C<$[> is deprecated.) |
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213 | So in general you can assume that |
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214 | |
215 | scalar(@whatever) == $#whatever + 1; |
216 | |
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217 | Some programmers choose to use an explicit conversion so nothing's |
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218 | left to doubt: |
219 | |
220 | $element_count = scalar(@whatever); |
221 | |
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222 | If you evaluate a hash in a scalar context, it returns a value that is |
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223 | true if and only if the hash contains any key/value pairs. (If there |
224 | are any key/value pairs, the value returned is a string consisting of |
225 | the number of used buckets and the number of allocated buckets, separated |
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226 | by 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. |
228 | For example, you stick 10,000 things in a hash, but evaluating %HASH in |
229 | scalar context reveals "1/16", which means only one out of sixteen buckets |
230 | has been touched, and presumably contains all 10,000 of your items. This |
231 | isn't supposed to happen.) |
232 | |
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233 | You can preallocate space for a hash by assigning to the keys() function. |
234 | This 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 | |
240 | Numeric literals are specified in any of the customary floating point or |
241 | integer formats: |
242 | |
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243 | 12345 |
244 | 12345.67 |
245 | .23E-10 |
246 | 0xffff # hex |
247 | 0377 # octal |
248 | 4_294_967_296 # underline for legibility |
249 | |
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250 | String literals are usually delimited by either single or double |
251 | quotes. They work much like shell quotes: double-quoted string |
252 | literals are subject to backslash and variable substitution; |
253 | single-quoted strings are not (except for "C<\'>" and "C<\\>"). |
254 | The usual Unix backslash rules apply for making characters such as |
255 | newline, tab, etc., as well as some more exotic forms. See |
256 | L<perlop/Quote and Quotelike Operators> for a list. |
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257 | |
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258 | Octal or hex representations in string literals (e.g. '0xffff') are not |
259 | automatically converted to their integer representation. The hex() and |
260 | oct() functions make these conversions for you. See L<perlfunc/hex> and |
261 | L<perlfunc/oct> for more details. |
262 | |
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263 | You can also embed newlines directly in your strings, i.e., they can end |
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264 | on a different line than they begin. This is nice, but if you forget |
265 | your trailing quote, the error will not be reported until Perl finds |
266 | another line containing the quote character, which may be much further |
267 | on in the script. Variable substitution inside strings is limited to |
268 | scalar variables, arrays, and array slices. (In other words, |
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269 | names beginning with $ or @, followed by an optional bracketed |
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270 | expression as a subscript.) The following code segment prints out "The |
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271 | price is $Z<>100." |
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272 | |
273 | $Price = '$100'; # not interpreted |
274 | print "The price is $Price.\n"; # interpreted |
275 | |
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276 | As in some shells, you can put curly brackets around the name to |
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277 | delimit it from following alphanumerics. In fact, an identifier |
278 | within such curlies is forced to be a string, as is any single |
279 | identifier within a hash subscript. Our earlier example, |
280 | |
281 | $days{'Feb'} |
282 | |
283 | can be written as |
284 | |
285 | $days{Feb} |
286 | |
287 | and the quotes will be assumed automatically. But anything more complicated |
288 | in the subscript will be interpreted as an expression. |
289 | |
290 | Note that a |
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291 | single-quoted string must be separated from a preceding word by a |
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292 | space, because single quote is a valid (though deprecated) character in |
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293 | a variable name (see L<perlmod/Packages>). |
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294 | |
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295 | Three special literals are __FILE__, __LINE__, and __PACKAGE__, which |
296 | represent the current filename, line number, and package name at that |
297 | point in your program. They may be used only as separate tokens; they |
298 | will not be interpolated into strings. If there is no current package |
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299 | (due to an empty C<package;> directive), __PACKAGE__ is the undefined value. |
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300 | |
301 | The tokens __END__ and __DATA__ may be used to indicate the logical end |
302 | of the script before the actual end of file. Any following text is |
303 | ignored, but may be read via a DATA filehandle: main::DATA for __END__, |
304 | or PACKNAME::DATA (where PACKNAME is the current package) for __DATA__. |
305 | The two control characters ^D and ^Z are synonyms for __END__ (or |
306 | __DATA__ in a module). See L<SelfLoader> for more description of |
a00c1fe5 |
307 | __DATA__, and an example of its use. Note that you cannot read from the |
308 | DATA filehandle in a BEGIN block: the BEGIN block is executed as soon as |
309 | it is seen (during compilation), at which point the corresponding |
310 | __DATA__ (or __END__) token has not yet been seen. |
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311 | |
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312 | A word that has no other interpretation in the grammar will |
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313 | be treated as if it were a quoted string. These are known as |
314 | "barewords". As with filehandles and labels, a bareword that consists |
315 | entirely of lowercase letters risks conflict with future reserved |
316 | words, and if you use the B<-w> switch, Perl will warn you about any |
317 | such words. Some people may wish to outlaw barewords entirely. If you |
318 | say |
319 | |
320 | use strict 'subs'; |
321 | |
322 | then any bareword that would NOT be interpreted as a subroutine call |
323 | produces a compile-time error instead. The restriction lasts to the |
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324 | end of the enclosing block. An inner block may countermand this |
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325 | by saying C<no strict 'subs'>. |
326 | |
327 | Array variables are interpolated into double-quoted strings by joining all |
328 | the elements of the array with the delimiter specified in the C<$"> |
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329 | variable (C<$LIST_SEPARATOR> in English), space by default. The following |
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330 | are equivalent: |
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331 | |
332 | $temp = join($",@ARGV); |
333 | system "echo $temp"; |
334 | |
335 | system "echo @ARGV"; |
336 | |
337 | Within search patterns (which also undergo double-quotish substitution) |
338 | there is a bad ambiguity: Is C</$foo[bar]/> to be interpreted as |
339 | C</${foo}[bar]/> (where C<[bar]> is a character class for the regular |
340 | expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array |
341 | @foo)? If @foo doesn't otherwise exist, then it's obviously a |
342 | character class. If @foo exists, Perl takes a good guess about C<[bar]>, |
343 | and is almost always right. If it does guess wrong, or if you're just |
344 | plain paranoid, you can force the correct interpretation with curly |
345 | brackets as above. |
346 | |
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347 | A line-oriented form of quoting is based on the shell "here-doc" |
348 | syntax. Following a C<E<lt>E<lt>> you specify a string to terminate |
349 | the quoted material, and all lines following the current line down to |
350 | the terminating string are the value of the item. The terminating |
351 | string may be either an identifier (a word), or some quoted text. If |
352 | quoted, the type of quotes you use determines the treatment of the |
353 | text, just as in regular quoting. An unquoted identifier works like |
354 | double quotes. There must be no space between the C<E<lt>E<lt>> and |
355 | the identifier. (If you put a space it will be treated as a null |
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356 | identifier, which is valid, and matches the first empty line.) The |
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357 | terminating string must appear by itself (unquoted and with no |
358 | surrounding whitespace) on the terminating line. |
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359 | |
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360 | print <<EOF; |
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361 | The price is $Price. |
362 | EOF |
363 | |
364 | print <<"EOF"; # same as above |
365 | The price is $Price. |
366 | EOF |
367 | |
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368 | print <<`EOC`; # execute commands |
369 | echo hi there |
370 | echo lo there |
371 | EOC |
372 | |
373 | print <<"foo", <<"bar"; # you can stack them |
374 | I said foo. |
375 | foo |
376 | I said bar. |
377 | bar |
378 | |
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379 | myfunc(<<"THIS", 23, <<'THAT'); |
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380 | Here's a line |
381 | or two. |
382 | THIS |
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383 | and here's another. |
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384 | THAT |
385 | |
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386 | Just don't forget that you have to put a semicolon on the end |
387 | to finish the statement, as Perl doesn't know you're not going to |
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388 | try to do this: |
389 | |
390 | print <<ABC |
391 | 179231 |
392 | ABC |
393 | + 20; |
394 | |
395 | |
396 | =head2 List value constructors |
397 | |
398 | List values are denoted by separating individual values by commas |
399 | (and enclosing the list in parentheses where precedence requires it): |
400 | |
401 | (LIST) |
402 | |
748a9306 |
403 | In a context not requiring a list value, the value of the list |
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404 | literal is the value of the final element, as with the C comma operator. |
405 | For example, |
406 | |
407 | @foo = ('cc', '-E', $bar); |
408 | |
409 | assigns the entire list value to array foo, but |
410 | |
411 | $foo = ('cc', '-E', $bar); |
412 | |
413 | assigns the value of variable bar to variable foo. Note that the value |
414 | of an actual array in a scalar context is the length of the array; the |
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415 | following assigns the value 3 to $foo: |
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416 | |
417 | @foo = ('cc', '-E', $bar); |
418 | $foo = @foo; # $foo gets 3 |
419 | |
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420 | You may have an optional comma before the closing parenthesis of a |
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421 | list literal, so that you can say: |
422 | |
423 | @foo = ( |
424 | 1, |
425 | 2, |
426 | 3, |
427 | ); |
428 | |
429 | LISTs do automatic interpolation of sublists. That is, when a LIST is |
430 | evaluated, each element of the list is evaluated in a list context, and |
431 | the resulting list value is interpolated into LIST just as if each |
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432 | individual element were a member of LIST. Thus arrays and hashes lose their |
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433 | identity in a LIST--the list |
434 | |
5a964f20 |
435 | (@foo,@bar,&SomeSub,%glarch) |
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436 | |
437 | contains all the elements of @foo followed by all the elements of @bar, |
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438 | followed by all the elements returned by the subroutine named SomeSub |
439 | called in a list context, followed by the key/value pairs of %glarch. |
a0d0e21e |
440 | To make a list reference that does I<NOT> interpolate, see L<perlref>. |
441 | |
442 | The null list is represented by (). Interpolating it in a list |
443 | has no effect. Thus ((),(),()) is equivalent to (). Similarly, |
444 | interpolating an array with no elements is the same as if no |
445 | array had been interpolated at that point. |
446 | |
447 | A list value may also be subscripted like a normal array. You must |
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448 | put the list in parentheses to avoid ambiguity. For example: |
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449 | |
450 | # Stat returns list value. |
451 | $time = (stat($file))[8]; |
452 | |
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453 | # SYNTAX ERROR HERE. |
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454 | $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES |
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455 | |
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456 | # Find a hex digit. |
457 | $hexdigit = ('a','b','c','d','e','f')[$digit-10]; |
458 | |
459 | # A "reverse comma operator". |
460 | return (pop(@foo),pop(@foo))[0]; |
461 | |
68dc0745 |
462 | You may assign to C<undef> in a list. This is useful for throwing |
463 | away some of the return values of a function: |
464 | |
465 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file); |
466 | |
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467 | Lists may be assigned to if and only if each element of the list |
468 | is legal to assign to: |
469 | |
470 | ($a, $b, $c) = (1, 2, 3); |
471 | |
472 | ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00); |
473 | |
4633a7c4 |
474 | Array assignment in a scalar context returns the number of elements |
475 | produced by the expression on the right side of the assignment: |
476 | |
477 | $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2 |
478 | $x = (($foo,$bar) = f()); # set $x to f()'s return count |
479 | |
480 | This is very handy when you want to do a list assignment in a Boolean |
5f05dabc |
481 | context, because most list functions return a null list when finished, |
4633a7c4 |
482 | which when assigned produces a 0, which is interpreted as FALSE. |
483 | |
a0d0e21e |
484 | The final element may be an array or a hash: |
485 | |
486 | ($a, $b, @rest) = split; |
5a964f20 |
487 | my($a, $b, %rest) = @_; |
a0d0e21e |
488 | |
4633a7c4 |
489 | You can actually put an array or hash anywhere in the list, but the first one |
a0d0e21e |
490 | in the list will soak up all the values, and anything after it will get |
491 | a null value. This may be useful in a local() or my(). |
492 | |
493 | A hash literal contains pairs of values to be interpreted |
494 | as a key and a value: |
495 | |
496 | # same as map assignment above |
497 | %map = ('red',0x00f,'blue',0x0f0,'green',0xf00); |
498 | |
4633a7c4 |
499 | While literal lists and named arrays are usually interchangeable, that's |
500 | not the case for hashes. Just because you can subscript a list value like |
501 | a normal array does not mean that you can subscript a list value as a |
502 | hash. Likewise, hashes included as parts of other lists (including |
503 | parameters lists and return lists from functions) always flatten out into |
504 | key/value pairs. That's why it's good to use references sometimes. |
a0d0e21e |
505 | |
4633a7c4 |
506 | It is often more readable to use the C<=E<gt>> operator between key/value |
507 | pairs. The C<=E<gt>> operator is mostly just a more visually distinctive |
b88cefa9 |
508 | synonym for a comma, but it also arranges for its left-hand operand to be |
5a964f20 |
509 | interpreted as a string--if it's a bareword that would be a legal identifier. |
b88cefa9 |
510 | This makes it nice for initializing hashes: |
a0d0e21e |
511 | |
4633a7c4 |
512 | %map = ( |
513 | red => 0x00f, |
514 | blue => 0x0f0, |
515 | green => 0xf00, |
516 | ); |
517 | |
518 | or for initializing hash references to be used as records: |
519 | |
520 | $rec = { |
521 | witch => 'Mable the Merciless', |
522 | cat => 'Fluffy the Ferocious', |
523 | date => '10/31/1776', |
524 | }; |
525 | |
526 | or for using call-by-named-parameter to complicated functions: |
527 | |
54310121 |
528 | $field = $query->radio_group( |
4633a7c4 |
529 | name => 'group_name', |
530 | values => ['eenie','meenie','minie'], |
531 | default => 'meenie', |
532 | linebreak => 'true', |
533 | labels => \%labels |
534 | ); |
cb1a09d0 |
535 | |
536 | Note that just because a hash is initialized in that order doesn't |
537 | mean that it comes out in that order. See L<perlfunc/sort> for examples |
538 | of how to arrange for an output ordering. |
539 | |
5f05dabc |
540 | =head2 Typeglobs and Filehandles |
cb1a09d0 |
541 | |
542 | Perl uses an internal type called a I<typeglob> to hold an entire |
543 | symbol table entry. The type prefix of a typeglob is a C<*>, because |
54310121 |
544 | it represents all types. This used to be the preferred way to |
cb1a09d0 |
545 | pass arrays and hashes by reference into a function, but now that |
5a964f20 |
546 | we have real references, this is seldom needed. |
547 | |
548 | The main use of typeglobs in modern Perl is create symbol table aliases. |
549 | This assignment: |
550 | |
551 | *this = *that; |
552 | |
553 | makes $this an alias for $that, @this an alias for @that, %this an alias |
554 | for %that, &this an alias for &that, etc. Much safer is to use a reference. |
555 | This: |
5f05dabc |
556 | |
5a964f20 |
557 | local *Here::blue = \$There::green; |
558 | |
559 | temporarily makes $Here::blue an alias for $There::green, but doesn't |
560 | make @Here::blue an alias for @There::green, or %Here::blue an alias for |
561 | %There::green, etc. See L<perlmod/"Symbol Tables"> for more examples |
562 | of this. Strange though this may seem, this is the basis for the whole |
563 | module import/export system. |
564 | |
565 | Another use for typeglobs is to to pass filehandles into a function or |
566 | to create new filehandles. If you need to use a typeglob to save away |
567 | a filehandle, do it this way: |
5f05dabc |
568 | |
569 | $fh = *STDOUT; |
570 | |
571 | or perhaps as a real reference, like this: |
572 | |
573 | $fh = \*STDOUT; |
574 | |
5a964f20 |
575 | See L<perlsub> for examples of using these as indirect filehandles |
576 | in functions. |
577 | |
578 | Typeglobs are also a way to create a local filehandle using the local() |
579 | operator. These last until their block is exited, but may be passed back. |
580 | For example: |
5f05dabc |
581 | |
582 | sub newopen { |
583 | my $path = shift; |
584 | local *FH; # not my! |
5a964f20 |
585 | open (FH, $path) or return undef; |
e05a3a1e |
586 | return *FH; |
5f05dabc |
587 | } |
588 | $fh = newopen('/etc/passwd'); |
589 | |
5a964f20 |
590 | Now that we have the *foo{THING} notation, typeglobs aren't used as much |
591 | for filehandle manipulations, although they're still needed to pass brand |
592 | new file and directory handles into or out of functions. That's because |
593 | *HANDLE{IO} only works if HANDLE has already been used as a handle. |
594 | In other words, *FH can be used to create new symbol table entries, |
595 | but *foo{THING} cannot. |
596 | |
597 | Another way to create anonymous filehandles is with the IO::Handle |
598 | module and its ilk. These modules have the advantage of not hiding |
599 | different types of the same name during the local(). See the bottom of |
600 | L<perlfunc/open()> for an example. |
cb1a09d0 |
601 | |
55497cff |
602 | See L<perlref>, L<perlsub>, and L<perlmod/"Symbol Tables"> for more |
5a964f20 |
603 | discussion on typeglobs and the *foo{THING} syntax. |