Commit | Line | Data |
a0d0e21e |
1 | =head1 NAME |
2 | |
cb1a09d0 |
3 | perldata - Perl data types |
a0d0e21e |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | =head2 Variable names |
8 | |
d55a8828 |
9 | Perl has three built-in data types: scalars, arrays of scalars, and |
10 | associative arrays of scalars, known as "hashes". Normal arrays |
19799a22 |
11 | are ordered lists of scalars indexed by number, starting with 0 and with |
d55a8828 |
12 | negative subscripts counting from the end. Hashes are unordered |
19799a22 |
13 | collections of scalar values indexed by their associated string key. |
a0d0e21e |
14 | |
d55a8828 |
15 | Values are usually referred to by name, or through a named reference. |
b88cefa9 |
16 | The first character of the name tells you to what sort of data |
17 | structure it refers. The rest of the name tells you the particular |
d55a8828 |
18 | value to which it refers. Usually this name is a single I<identifier>, |
19 | that is, a string beginning with a letter or underscore, and |
20 | containing letters, underscores, and digits. In some cases, it may |
21 | be a chain of identifiers, separated by C<::> (or by the slightly |
22 | archaic C<'>); all but the last are interpreted as names of packages, |
23 | to locate the namespace in which to look up the final identifier |
24 | (see L<perlmod/Packages> for details). It's possible to substitute |
25 | for a simple identifier, an expression that produces a reference |
26 | to the value at runtime. This is described in more detail below |
27 | and in L<perlref>. |
28 | |
29 | Perl also has its own built-in variables whose names don't follow |
30 | these rules. They have strange names so they don't accidentally |
31 | collide with one of your normal variables. Strings that match |
32 | parenthesized parts of a regular expression are saved under names |
33 | containing only digits after the C<$> (see L<perlop> and L<perlre>). |
34 | In addition, several special variables that provide windows into |
35 | the inner working of Perl have names containing punctuation characters |
36 | and control characters. These are documented in L<perlvar>. |
37 | |
38 | Scalar values are always named with '$', even when referring to a |
39 | scalar that is part of an array or a hash. The '$' symbol works |
40 | semantically like the English word "the" in that it indicates a |
41 | single value is expected. |
a0d0e21e |
42 | |
43 | $days # the simple scalar value "days" |
44 | $days[28] # the 29th element of array @days |
45 | $days{'Feb'} # the 'Feb' value from hash %days |
46 | $#days # the last index of array @days |
47 | |
d55a8828 |
48 | Entire arrays (and slices of arrays and hashes) are denoted by '@', |
49 | which works much like the word "these" or "those" does in English, |
50 | in that it indicates multiple values are expected. |
a0d0e21e |
51 | |
52 | @days # ($days[0], $days[1],... $days[n]) |
d55a8828 |
53 | @days[3,4,5] # same as ($days[3],$days[4],$days[5]) |
a0d0e21e |
54 | @days{'a','c'} # same as ($days{'a'},$days{'c'}) |
55 | |
d55a8828 |
56 | Entire hashes are denoted by '%': |
a0d0e21e |
57 | |
58 | %days # (key1, val1, key2, val2 ...) |
59 | |
d55a8828 |
60 | In addition, subroutines are named with an initial '&', though this |
61 | is optional when unambiguous, just as the word "do" is often redundant |
62 | in English. Symbol table entries can be named with an initial '*', |
63 | but you don't really care about that yet (if ever :-). |
64 | |
65 | Every variable type has its own namespace, as do several |
66 | non-variable identifiers. This means that you can, without fear |
67 | of conflict, use the same name for a scalar variable, an array, or |
68 | a hash--or, for that matter, for a filehandle, a directory handle, a |
69 | subroutine name, a format name, or a label. This means that $foo |
70 | and @foo are two different variables. It also means that C<$foo[1]> |
71 | is a part of @foo, not a part of $foo. This may seem a bit weird, |
72 | but that's okay, because it is weird. |
73 | |
74 | Because variable references always start with '$', '@', or '%', the |
75 | "reserved" words aren't in fact reserved with respect to variable |
76 | names. They I<are> reserved with respect to labels and filehandles, |
77 | however, which don't have an initial special character. You can't |
78 | have a filehandle named "log", for instance. Hint: you could say |
79 | C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using |
80 | uppercase filehandles also improves readability and protects you |
81 | from conflict with future reserved words. Case I<is> significant--"FOO", |
82 | "Foo", and "foo" are all different names. Names that start with a |
83 | letter or underscore may also contain digits and underscores. |
a0d0e21e |
84 | |
85 | It is possible to replace such an alphanumeric name with an expression |
d55a8828 |
86 | that returns a reference to the appropriate type. For a description |
a0d0e21e |
87 | of this, see L<perlref>. |
88 | |
5f05dabc |
89 | Names that start with a digit may contain only more digits. Names |
5a964f20 |
90 | that do not start with a letter, underscore, or digit are limited to |
5f05dabc |
91 | one character, e.g., C<$%> or C<$$>. (Most of these one character names |
cb1a09d0 |
92 | have a predefined significance to Perl. For instance, C<$$> is the |
a0d0e21e |
93 | current process id.) |
94 | |
95 | =head2 Context |
96 | |
97 | The interpretation of operations and values in Perl sometimes depends |
98 | on the requirements of the context around the operation or value. |
d55a8828 |
99 | There are two major contexts: list and scalar. Certain operations |
a0d0e21e |
100 | return list values in contexts wanting a list, and scalar values |
d55a8828 |
101 | otherwise. If this is true of an operation it will be mentioned in |
102 | the documentation for that operation. In other words, Perl overloads |
a0d0e21e |
103 | certain operations based on whether the expected return value is |
d55a8828 |
104 | singular or plural. Some words in English work this way, like "fish" |
105 | and "sheep". |
a0d0e21e |
106 | |
107 | In a reciprocal fashion, an operation provides either a scalar or a |
108 | list context to each of its arguments. For example, if you say |
109 | |
110 | int( <STDIN> ) |
111 | |
d55a8828 |
112 | the integer operation provides scalar context for the E<lt><gt> |
a0d0e21e |
113 | operator, which responds by reading one line from STDIN and passing it |
114 | back to the integer operation, which will then find the integer value |
115 | of that line and return that. If, on the other hand, you say |
116 | |
117 | sort( <STDIN> ) |
118 | |
d55a8828 |
119 | then the sort operation provides list context for E<lt><gt>, which |
a0d0e21e |
120 | will proceed to read every line available up to the end of file, and |
121 | pass that list of lines back to the sort routine, which will then |
122 | sort those lines and return them as a list to whatever the context |
123 | of the sort was. |
124 | |
d55a8828 |
125 | Assignment is a little bit special in that it uses its left argument |
126 | to determine the context for the right argument. Assignment to a |
127 | scalar evaluates the right-hand side in scalar context, while |
128 | assignment to an array or hash evaluates the righthand side in list |
129 | context. Assignment to a list (or slice, which is just a list |
130 | anyway) also evaluates the righthand side in list context. |
131 | |
132 | When you use Perl's B<-w> command-line option, you may see warnings |
133 | about useless uses of constants or functions in "void context". |
134 | Void context just means the value has been discarded, such as a |
135 | statement containing only C<"fred";> or C<getpwuid(0);>. It still |
136 | counts as scalar context for functions that care whether or not |
137 | they're being called in list context. |
138 | |
139 | User-defined subroutines may choose to care whether they are being |
140 | called in a void, scalar, or list context. Most subroutines do not |
141 | need to bother, though. That's because both scalars and lists are |
142 | automatically interpolated into lists. See L<perlfunc/wantarray> |
143 | for how you would dynamically discern your function's calling |
144 | context. |
a0d0e21e |
145 | |
146 | =head2 Scalar values |
147 | |
d55a8828 |
148 | All data in Perl is a scalar, an array of scalars, or a hash of |
149 | scalars. A scalar may contain one single value in any of three |
150 | different flavors: a number, a string, or a reference. In general, |
151 | conversion from one form to another is transparent. Although a |
152 | scalar may not directly hold multiple values, it may contain a |
153 | reference to an array or hash which in turn contains multiple values. |
154 | |
155 | Scalars aren't necessarily one thing or another. There's no place |
156 | to declare a scalar variable to be of type "string", type "number", |
157 | type "reference", or anything else. Because of the automatic |
158 | conversion of scalars, operations that return scalars don't need |
159 | to care (and in fact, cannot care) whether their caller is looking |
160 | for a string, a number, or a reference. Perl is a contextually |
161 | polymorphic language whose scalars can be strings, numbers, or |
162 | references (which includes objects). Although strings and numbers |
163 | are considered pretty much the same thing for nearly all purposes, |
164 | references are strongly-typed, uncastable pointers with builtin |
165 | reference-counting and destructor invocation. |
a0d0e21e |
166 | |
167 | A scalar value is interpreted as TRUE in the Boolean sense if it is not |
19799a22 |
168 | the null string or the number 0 (or its string equivalent, "0"). The |
d55a8828 |
169 | Boolean context is just a special kind of scalar context where no |
170 | conversion to a string or a number is ever performed. |
171 | |
172 | There are actually two varieties of null strings (sometimes referred |
173 | to as "empty" strings), a defined one and an undefined one. The |
174 | defined version is just a string of length zero, such as C<"">. |
175 | The undefined version is the value that indicates that there is |
176 | no real value for something, such as when there was an error, or |
177 | at end of file, or when you refer to an uninitialized variable or |
178 | element of an array or hash. Although in early versions of Perl, |
179 | an undefined scalar could become defined when first used in a |
180 | place expecting a defined value, this no longer happens except for |
181 | rare cases of autovivification as explained in L<perlref>. You can |
182 | use the defined() operator to determine whether a scalar value is |
183 | defined (this has no meaning on arrays or hashes), and the undef() |
184 | operator to produce an undefined value. |
185 | |
186 | To find out whether a given string is a valid non-zero number, it's |
187 | sometimes enough to test it against both numeric 0 and also lexical |
188 | "0" (although this will cause B<-w> noises). That's because strings |
189 | that aren't numbers count as 0, just as they do in B<awk>: |
4633a7c4 |
190 | |
191 | if ($str == 0 && $str ne "0") { |
192 | warn "That doesn't look like a number"; |
54310121 |
193 | } |
4633a7c4 |
194 | |
d55a8828 |
195 | That method may be best because otherwise you won't treat IEEE |
196 | notations like C<NaN> or C<Infinity> properly. At other times, you |
197 | might prefer to determine whether string data can be used numerically |
198 | by calling the POSIX::strtod() function or by inspecting your string |
199 | with a regular expression (as documented in L<perlre>). |
cb1a09d0 |
200 | |
201 | warn "has nondigits" if /\D/; |
5a964f20 |
202 | warn "not a natural number" unless /^\d+$/; # rejects -3 |
203 | warn "not an integer" unless /^-?\d+$/; # rejects +3 |
204 | warn "not an integer" unless /^[+-]?\d+$/; |
205 | warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2 |
206 | warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/; |
54310121 |
207 | warn "not a C float" |
cb1a09d0 |
208 | unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/; |
209 | |
d55a8828 |
210 | The length of an array is a scalar value. You may find the length |
211 | of array @days by evaluating C<$#days>, as in B<csh>. Technically |
212 | speaking, this isn't the length of the array; it's the subscript |
213 | of the last element, since there is ordinarily a 0th element. |
214 | Assigning to C<$#days> actually changes the length of the array. |
215 | Shortening an array this way destroys intervening values. Lengthening |
216 | an array that was previously shortened does not recover values |
217 | that were in those elements. (It used to do so in Perl 4, but we |
218 | had to break this to make sure destructors were called when expected.) |
219 | |
220 | You can also gain some miniscule measure of efficiency by pre-extending |
221 | an array that is going to get big. You can also extend an array |
222 | by assigning to an element that is off the end of the array. You |
19799a22 |
223 | can truncate an array down to nothing by assigning the null list |
d55a8828 |
224 | () to it. The following are equivalent: |
a0d0e21e |
225 | |
226 | @whatever = (); |
3e3baf6d |
227 | $#whatever = -1; |
a0d0e21e |
228 | |
d55a8828 |
229 | If you evaluate an array in scalar context, it returns the length |
230 | of the array. (Note that this is not true of lists, which return |
231 | the last value, like the C comma operator, nor of built-in functions, |
232 | which return whatever they feel like returning.) The following is |
233 | always true: |
a0d0e21e |
234 | |
235 | scalar(@whatever) == $#whatever - $[ + 1; |
236 | |
184e9718 |
237 | Version 5 of Perl changed the semantics of C<$[>: files that don't set |
238 | the value of C<$[> no longer need to worry about whether another |
239 | file changed its value. (In other words, use of C<$[> is deprecated.) |
5f05dabc |
240 | So in general you can assume that |
a0d0e21e |
241 | |
242 | scalar(@whatever) == $#whatever + 1; |
243 | |
d55a8828 |
244 | Some programmers choose to use an explicit conversion so as to |
245 | leave nothing to doubt: |
4633a7c4 |
246 | |
247 | $element_count = scalar(@whatever); |
248 | |
d55a8828 |
249 | If you evaluate a hash in scalar context, it returns false if the |
250 | hash is empty. If there are any key/value pairs, it returns true; |
251 | more precisely, the value returned is a string consisting of the |
252 | number of used buckets and the number of allocated buckets, separated |
253 | by a slash. This is pretty much useful only to find out whether |
254 | Perl's internal hashing algorithm is performing poorly on your data |
255 | set. For example, you stick 10,000 things in a hash, but evaluating |
256 | %HASH in scalar context reveals C<"1/16">, which means only one out |
257 | of sixteen buckets has been touched, and presumably contains all |
258 | 10,000 of your items. This isn't supposed to happen. |
a0d0e21e |
259 | |
5a964f20 |
260 | You can preallocate space for a hash by assigning to the keys() function. |
261 | This rounds up the allocated bucked to the next power of two: |
262 | |
263 | keys(%users) = 1000; # allocate 1024 buckets |
264 | |
a0d0e21e |
265 | =head2 Scalar value constructors |
266 | |
d55a8828 |
267 | Numeric literals are specified in any of the following floating point or |
a0d0e21e |
268 | integer formats: |
269 | |
a0d0e21e |
270 | 12345 |
271 | 12345.67 |
d55a8828 |
272 | .23E-10 # a very small number |
273 | 4_294_967_296 # underline for legibility |
274 | 0xff # hex |
275 | 0377 # octal |
276 | 0b011011 # binary |
a0d0e21e |
277 | |
55497cff |
278 | String literals are usually delimited by either single or double |
d55a8828 |
279 | quotes. They work much like quotes in the standard Unix shells: |
280 | double-quoted string literals are subject to backslash and variable |
19799a22 |
281 | substitution; single-quoted strings are not (except for C<\'> and |
282 | C<\\>). The usual C-style backslash rules apply for making |
d55a8828 |
283 | characters such as newline, tab, etc., as well as some more exotic |
284 | forms. See L<perlop/"Quote and Quotelike Operators"> for a list. |
285 | |
286 | Hexadecimal, octal, or binary, representations in string literals |
287 | (e.g. '0xff') are not automatically converted to their integer |
288 | representation. The hex() and oct() functions make these conversions |
289 | for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details. |
68dc0745 |
290 | |
5f05dabc |
291 | You can also embed newlines directly in your strings, i.e., they can end |
a0d0e21e |
292 | on a different line than they begin. This is nice, but if you forget |
293 | your trailing quote, the error will not be reported until Perl finds |
294 | another line containing the quote character, which may be much further |
295 | on in the script. Variable substitution inside strings is limited to |
d55a8828 |
296 | scalar variables, arrays, and array or hash slices. (In other words, |
b88cefa9 |
297 | names beginning with $ or @, followed by an optional bracketed |
a0d0e21e |
298 | expression as a subscript.) The following code segment prints out "The |
184e9718 |
299 | price is $Z<>100." |
a0d0e21e |
300 | |
301 | $Price = '$100'; # not interpreted |
302 | print "The price is $Price.\n"; # interpreted |
303 | |
d55a8828 |
304 | As in some shells, you can enclose the variable name in braces to |
305 | disambiguate it from following alphanumerics. You must also do |
306 | this when interpolating a variable into a string to separate the |
307 | variable name from a following double-colon or an apostrophe, since |
308 | these would be otherwise treated as a package separator: |
309 | |
310 | $who = "Larry"; |
311 | print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n"; |
312 | print "We use ${who}speak when ${who}'s here.\n"; |
313 | |
314 | Without the braces, Perl would have looked for a $whospeak, a |
315 | C<$who::0>, and a C<$who's> variable. The last two would be the |
316 | $0 and the $s variables in the (presumably) non-existent package |
317 | C<who>. |
318 | |
319 | In fact, an identifier within such curlies is forced to be a string, |
320 | as is any simple identifier within a hash subscript. Neither need |
321 | quoting. Our earlier example, C<$days{'Feb'}> can be written as |
322 | C<$days{Feb}> and the quotes will be assumed automatically. But |
323 | anything more complicated in the subscript will be interpreted as |
324 | an expression. |
325 | |
326 | The special literals __FILE__, __LINE__, and __PACKAGE__ |
68dc0745 |
327 | represent the current filename, line number, and package name at that |
328 | point in your program. They may be used only as separate tokens; they |
329 | will not be interpolated into strings. If there is no current package |
5a964f20 |
330 | (due to an empty C<package;> directive), __PACKAGE__ is the undefined value. |
68dc0745 |
331 | |
d55a8828 |
332 | The tokens __END__ and __DATA__ may be used to indicate the logical |
333 | end of the script before the actual end of file. Any following |
334 | text is ignored, but may be read via a DATA filehandle: main::DATA |
335 | for __END__, or PACKNAME::DATA (where PACKNAME is the current |
336 | package) for __DATA__. The two control characters ^D and ^Z are |
337 | synonyms for __END__ in the main program, __DATA__ in a separate |
338 | module. See L<SelfLoader> for more description of __DATA__, and |
339 | an example of its use. Note that you cannot read from the DATA |
340 | filehandle in a BEGIN block: the BEGIN block is executed as soon |
341 | as it is seen (during compilation), at which point the corresponding |
a00c1fe5 |
342 | __DATA__ (or __END__) token has not yet been seen. |
a0d0e21e |
343 | |
748a9306 |
344 | A word that has no other interpretation in the grammar will |
a0d0e21e |
345 | be treated as if it were a quoted string. These are known as |
346 | "barewords". As with filehandles and labels, a bareword that consists |
347 | entirely of lowercase letters risks conflict with future reserved |
348 | words, and if you use the B<-w> switch, Perl will warn you about any |
349 | such words. Some people may wish to outlaw barewords entirely. If you |
350 | say |
351 | |
352 | use strict 'subs'; |
353 | |
354 | then any bareword that would NOT be interpreted as a subroutine call |
355 | produces a compile-time error instead. The restriction lasts to the |
54310121 |
356 | end of the enclosing block. An inner block may countermand this |
a0d0e21e |
357 | by saying C<no strict 'subs'>. |
358 | |
d55a8828 |
359 | Arrays and slices are interpolated into double-quoted strings |
360 | by joining the elements with the delimiter specified in the C<$"> |
361 | variable (C<$LIST_SEPARATOR> in English), space by default. The |
362 | following are equivalent: |
a0d0e21e |
363 | |
d55a8828 |
364 | $temp = join($", @ARGV); |
a0d0e21e |
365 | system "echo $temp"; |
366 | |
367 | system "echo @ARGV"; |
368 | |
369 | Within search patterns (which also undergo double-quotish substitution) |
d55a8828 |
370 | there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as |
a0d0e21e |
371 | C</${foo}[bar]/> (where C<[bar]> is a character class for the regular |
372 | expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array |
373 | @foo)? If @foo doesn't otherwise exist, then it's obviously a |
374 | character class. If @foo exists, Perl takes a good guess about C<[bar]>, |
375 | and is almost always right. If it does guess wrong, or if you're just |
376 | plain paranoid, you can force the correct interpretation with curly |
d55a8828 |
377 | braces as above. |
a0d0e21e |
378 | |
d55a8828 |
379 | A line-oriented form of quoting is based on the shell "here-document" |
55497cff |
380 | syntax. Following a C<E<lt>E<lt>> you specify a string to terminate |
381 | the quoted material, and all lines following the current line down to |
382 | the terminating string are the value of the item. The terminating |
383 | string may be either an identifier (a word), or some quoted text. If |
384 | quoted, the type of quotes you use determines the treatment of the |
385 | text, just as in regular quoting. An unquoted identifier works like |
386 | double quotes. There must be no space between the C<E<lt>E<lt>> and |
387 | the identifier. (If you put a space it will be treated as a null |
3fe9a6f1 |
388 | identifier, which is valid, and matches the first empty line.) The |
55497cff |
389 | terminating string must appear by itself (unquoted and with no |
390 | surrounding whitespace) on the terminating line. |
a0d0e21e |
391 | |
54310121 |
392 | print <<EOF; |
a0d0e21e |
393 | The price is $Price. |
394 | EOF |
395 | |
396 | print <<"EOF"; # same as above |
397 | The price is $Price. |
398 | EOF |
399 | |
a0d0e21e |
400 | print <<`EOC`; # execute commands |
401 | echo hi there |
402 | echo lo there |
403 | EOC |
404 | |
405 | print <<"foo", <<"bar"; # you can stack them |
406 | I said foo. |
407 | foo |
408 | I said bar. |
409 | bar |
410 | |
d28ebecd |
411 | myfunc(<<"THIS", 23, <<'THAT'); |
a0d0e21e |
412 | Here's a line |
413 | or two. |
414 | THIS |
54310121 |
415 | and here's another. |
a0d0e21e |
416 | THAT |
417 | |
54310121 |
418 | Just don't forget that you have to put a semicolon on the end |
419 | to finish the statement, as Perl doesn't know you're not going to |
a0d0e21e |
420 | try to do this: |
421 | |
422 | print <<ABC |
423 | 179231 |
424 | ABC |
425 | + 20; |
426 | |
d55a8828 |
427 | If you want your here-docs to be indented with the |
428 | rest of the code, you'll need to remove leading whitespace |
429 | from each line manually: |
430 | |
431 | ($quote = <<'FINIS') =~ s/^\s+//gm; |
432 | The Road goes ever on and on, |
433 | down from the door where it began. |
434 | FINIS |
a0d0e21e |
435 | |
436 | =head2 List value constructors |
437 | |
438 | List values are denoted by separating individual values by commas |
439 | (and enclosing the list in parentheses where precedence requires it): |
440 | |
441 | (LIST) |
442 | |
d55a8828 |
443 | In a context not requiring a list value, the value of what appears |
444 | to be a list literal is simply the value of the final element, as |
445 | with the C comma operator. For example, |
a0d0e21e |
446 | |
447 | @foo = ('cc', '-E', $bar); |
448 | |
d55a8828 |
449 | assigns the entire list value to array @foo, but |
a0d0e21e |
450 | |
451 | $foo = ('cc', '-E', $bar); |
452 | |
d55a8828 |
453 | assigns the value of variable $bar to the scalar variable $foo. |
454 | Note that the value of an actual array in scalar context is the |
455 | length of the array; the following assigns the value 3 to $foo: |
a0d0e21e |
456 | |
457 | @foo = ('cc', '-E', $bar); |
458 | $foo = @foo; # $foo gets 3 |
459 | |
54310121 |
460 | You may have an optional comma before the closing parenthesis of a |
a0d0e21e |
461 | list literal, so that you can say: |
462 | |
463 | @foo = ( |
464 | 1, |
465 | 2, |
466 | 3, |
467 | ); |
468 | |
d55a8828 |
469 | To use a here-document to assign an array, one line per element, |
470 | you might use an approach like this: |
471 | |
472 | @sauces = <<End_Lines =~ m/(\S.*\S)/g; |
473 | normal tomato |
474 | spicy tomato |
475 | green chile |
476 | pesto |
477 | white wine |
478 | End_Lines |
479 | |
a0d0e21e |
480 | LISTs do automatic interpolation of sublists. That is, when a LIST is |
d55a8828 |
481 | evaluated, each element of the list is evaluated in list context, and |
a0d0e21e |
482 | the resulting list value is interpolated into LIST just as if each |
5a964f20 |
483 | individual element were a member of LIST. Thus arrays and hashes lose their |
a0d0e21e |
484 | identity in a LIST--the list |
485 | |
5a964f20 |
486 | (@foo,@bar,&SomeSub,%glarch) |
a0d0e21e |
487 | |
488 | contains all the elements of @foo followed by all the elements of @bar, |
5a964f20 |
489 | followed by all the elements returned by the subroutine named SomeSub |
d55a8828 |
490 | called in list context, followed by the key/value pairs of %glarch. |
a0d0e21e |
491 | To make a list reference that does I<NOT> interpolate, see L<perlref>. |
492 | |
19799a22 |
493 | The null list is represented by (). Interpolating it in a list |
a0d0e21e |
494 | has no effect. Thus ((),(),()) is equivalent to (). Similarly, |
495 | interpolating an array with no elements is the same as if no |
496 | array had been interpolated at that point. |
497 | |
498 | A list value may also be subscripted like a normal array. You must |
54310121 |
499 | put the list in parentheses to avoid ambiguity. For example: |
a0d0e21e |
500 | |
501 | # Stat returns list value. |
502 | $time = (stat($file))[8]; |
503 | |
4633a7c4 |
504 | # SYNTAX ERROR HERE. |
5f05dabc |
505 | $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES |
4633a7c4 |
506 | |
a0d0e21e |
507 | # Find a hex digit. |
508 | $hexdigit = ('a','b','c','d','e','f')[$digit-10]; |
509 | |
510 | # A "reverse comma operator". |
511 | return (pop(@foo),pop(@foo))[0]; |
512 | |
d55a8828 |
513 | Lists may be assigned to only when each element of the list |
514 | is itself legal to assign to: |
a0d0e21e |
515 | |
516 | ($a, $b, $c) = (1, 2, 3); |
517 | |
518 | ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00); |
519 | |
d55a8828 |
520 | An exception to this is that you may assign to C<undef> in a list. |
521 | This is useful for throwing away some of the return values of a |
522 | function: |
523 | |
524 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file); |
525 | |
526 | List assignment in scalar context returns the number of elements |
4633a7c4 |
527 | produced by the expression on the right side of the assignment: |
528 | |
529 | $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2 |
530 | $x = (($foo,$bar) = f()); # set $x to f()'s return count |
531 | |
d55a8828 |
532 | This is handy when you want to do a list assignment in a Boolean |
19799a22 |
533 | context, because most list functions return a null list when finished, |
4633a7c4 |
534 | which when assigned produces a 0, which is interpreted as FALSE. |
535 | |
a0d0e21e |
536 | The final element may be an array or a hash: |
537 | |
538 | ($a, $b, @rest) = split; |
5a964f20 |
539 | my($a, $b, %rest) = @_; |
a0d0e21e |
540 | |
4633a7c4 |
541 | You can actually put an array or hash anywhere in the list, but the first one |
d55a8828 |
542 | in the list will soak up all the values, and anything after it will become |
543 | undefined. This may be useful in a my() or local(). |
a0d0e21e |
544 | |
d55a8828 |
545 | A hash can be initialized using a literal list holding pairs of |
546 | items to be interpreted as a key and a value: |
a0d0e21e |
547 | |
548 | # same as map assignment above |
549 | %map = ('red',0x00f,'blue',0x0f0,'green',0xf00); |
550 | |
d55a8828 |
551 | While literal lists and named arrays are often interchangeable, that's |
4633a7c4 |
552 | not the case for hashes. Just because you can subscript a list value like |
553 | a normal array does not mean that you can subscript a list value as a |
554 | hash. Likewise, hashes included as parts of other lists (including |
555 | parameters lists and return lists from functions) always flatten out into |
556 | key/value pairs. That's why it's good to use references sometimes. |
a0d0e21e |
557 | |
4633a7c4 |
558 | It is often more readable to use the C<=E<gt>> operator between key/value |
559 | pairs. The C<=E<gt>> operator is mostly just a more visually distinctive |
b88cefa9 |
560 | synonym for a comma, but it also arranges for its left-hand operand to be |
5a964f20 |
561 | interpreted as a string--if it's a bareword that would be a legal identifier. |
b88cefa9 |
562 | This makes it nice for initializing hashes: |
a0d0e21e |
563 | |
4633a7c4 |
564 | %map = ( |
565 | red => 0x00f, |
566 | blue => 0x0f0, |
567 | green => 0xf00, |
568 | ); |
569 | |
570 | or for initializing hash references to be used as records: |
571 | |
572 | $rec = { |
573 | witch => 'Mable the Merciless', |
574 | cat => 'Fluffy the Ferocious', |
575 | date => '10/31/1776', |
576 | }; |
577 | |
578 | or for using call-by-named-parameter to complicated functions: |
579 | |
54310121 |
580 | $field = $query->radio_group( |
4633a7c4 |
581 | name => 'group_name', |
582 | values => ['eenie','meenie','minie'], |
583 | default => 'meenie', |
584 | linebreak => 'true', |
585 | labels => \%labels |
586 | ); |
cb1a09d0 |
587 | |
588 | Note that just because a hash is initialized in that order doesn't |
589 | mean that it comes out in that order. See L<perlfunc/sort> for examples |
590 | of how to arrange for an output ordering. |
591 | |
d55a8828 |
592 | =head2 Slices |
593 | |
594 | A common way access an array or a hash is one scalar element at a time. |
595 | You can also subscript a list to get a single element from it. |
596 | |
597 | $whoami = $ENV{"USER"}; # one element from the hash |
598 | $parent = $ISA[0]; # one element from the array |
599 | $dir = (getpwnam("daemon"))[7]; # likewise, but with list |
600 | |
601 | A slice accesses several elements of a list, an array, or a hash |
602 | simultaneously using a list of subscripts. It's a more convenient |
603 | that writing out the individual elements as a list of separate |
604 | scalar values. |
605 | |
606 | ($him, $her) = @folks[0,-1]; # array slice |
607 | @them = @folks[0 .. 3]; # array slice |
608 | ($who, $home) = @ENV{"USER", "HOME"}; # hash slice |
609 | ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice |
610 | |
611 | Since you can assign to a list of variables, you can also assign to |
612 | an array or hash slice. |
613 | |
614 | @days[3..5] = qw/Wed Thu Fri/; |
615 | @colors{'red','blue','green'} |
616 | = (0xff0000, 0x0000ff, 0x00ff00); |
617 | @folks[0, -1] = @folks[-1, 0]; |
618 | |
619 | The previous assignments are exactly equivalent to |
620 | |
621 | ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/; |
622 | ($colors{'red'}, $colors{'blue'}, $colors{'green'}) |
623 | = (0xff0000, 0x0000ff, 0x00ff00); |
624 | ($folks[0], $folks[-1]) = ($folks[0], $folks[-1]); |
625 | |
626 | Since changing a slice changes the original array or hash that it's |
627 | slicing, a C<foreach> construct will alter through some--or even |
628 | all--of the values of the array or hash. |
629 | |
630 | foreach (@array[ 4 .. 10 ]) { s/peter/paul/ } |
631 | |
632 | foreach (@hash{keys %hash}) { |
633 | s/^\s+//; # trim leading whitespace |
634 | s/\s+$//; # trim trailing whitespace |
635 | s/(\w+)/\u\L$1/g; # "titlecase" words |
636 | } |
637 | |
638 | You couldn't just loop through C<values %hash> to do this because |
639 | that function produces a new list which is a copy of the values, |
640 | so changing them doesn't change the original. |
641 | |
19799a22 |
642 | As a special rule, if a list slice would produce a list consisting |
643 | entirely of undefined values, the null list is produced instead. |
644 | This makes it easy to write loops that terminate when a null list |
645 | is returned: |
d55a8828 |
646 | |
647 | while ( ($home, $user) = (getpwent)[7,0]) { |
648 | printf "%-8s %s\n", $user, $home; |
649 | } |
650 | |
651 | As noted earlier in this document, the scalar sense of list assignment |
652 | is the number of elements on the right-hand side of the assignment. |
19799a22 |
653 | The null list contains no elements, so when the password file is |
d55a8828 |
654 | exhausted, the result is 0, not 2. |
655 | |
656 | If you're confused about why you use an '@' there on a hash slice |
657 | instead of a '%', think of it like this. The type of bracket (square |
658 | or curly) governs whether it's an array or a hash being looked at. |
659 | On the other hand, the leading symbol ('$' or '@') on the array or |
660 | hash indicates whether you are getting back a singular value (a |
661 | scalar) or a plural one (a list). |
662 | |
5f05dabc |
663 | =head2 Typeglobs and Filehandles |
cb1a09d0 |
664 | |
665 | Perl uses an internal type called a I<typeglob> to hold an entire |
666 | symbol table entry. The type prefix of a typeglob is a C<*>, because |
54310121 |
667 | it represents all types. This used to be the preferred way to |
cb1a09d0 |
668 | pass arrays and hashes by reference into a function, but now that |
5a964f20 |
669 | we have real references, this is seldom needed. |
670 | |
671 | The main use of typeglobs in modern Perl is create symbol table aliases. |
672 | This assignment: |
673 | |
674 | *this = *that; |
675 | |
676 | makes $this an alias for $that, @this an alias for @that, %this an alias |
677 | for %that, &this an alias for &that, etc. Much safer is to use a reference. |
678 | This: |
5f05dabc |
679 | |
5a964f20 |
680 | local *Here::blue = \$There::green; |
681 | |
682 | temporarily makes $Here::blue an alias for $There::green, but doesn't |
683 | make @Here::blue an alias for @There::green, or %Here::blue an alias for |
684 | %There::green, etc. See L<perlmod/"Symbol Tables"> for more examples |
685 | of this. Strange though this may seem, this is the basis for the whole |
686 | module import/export system. |
687 | |
d55a8828 |
688 | Another use for typeglobs is to pass filehandles into a function or |
5a964f20 |
689 | to create new filehandles. If you need to use a typeglob to save away |
690 | a filehandle, do it this way: |
5f05dabc |
691 | |
692 | $fh = *STDOUT; |
693 | |
694 | or perhaps as a real reference, like this: |
695 | |
696 | $fh = \*STDOUT; |
697 | |
5a964f20 |
698 | See L<perlsub> for examples of using these as indirect filehandles |
699 | in functions. |
700 | |
701 | Typeglobs are also a way to create a local filehandle using the local() |
702 | operator. These last until their block is exited, but may be passed back. |
703 | For example: |
5f05dabc |
704 | |
705 | sub newopen { |
706 | my $path = shift; |
d55a8828 |
707 | local *FH; # not my! |
5a964f20 |
708 | open (FH, $path) or return undef; |
e05a3a1e |
709 | return *FH; |
5f05dabc |
710 | } |
711 | $fh = newopen('/etc/passwd'); |
712 | |
d55a8828 |
713 | Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much |
5a964f20 |
714 | for filehandle manipulations, although they're still needed to pass brand |
715 | new file and directory handles into or out of functions. That's because |
d55a8828 |
716 | C<*HANDLE{IO}> only works if HANDLE has already been used as a handle. |
717 | In other words, C<*FH> must be used to create new symbol table entries; |
718 | C<*foo{THING}> cannot. When in doubt, use C<*FH>. |
719 | |
720 | Another way to create anonymous filehandles is with the Symbol |
721 | module or with the IO::Handle module and its ilk. These modules |
722 | have the advantage of not hiding different types of the same name |
723 | during the local(). See the bottom of L<perlfunc/open()> for an |
724 | example. |
725 | |
726 | =head1 SEE ALSO |
727 | |
728 | See L<perlvar> for a description of Perl's built-in variables and |
729 | a discussion of legal variable names. See L<perlref>, L<perlsub>, |
730 | and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and |
731 | the C<*foo{THING}> syntax. |