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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
cb1a09d0 | 3 | perldata - Perl data types |
a0d0e21e LW |
4 | |
5 | =head1 DESCRIPTION | |
6 | ||
7 | =head2 Variable names | |
d74e8afc | 8 | X<variable, name> X<variable name> X<data type> X<type> |
a0d0e21e | 9 | |
d55a8828 | 10 | Perl has three built-in data types: scalars, arrays of scalars, and |
692ef166 SF |
11 | associative arrays of scalars, known as "hashes". A scalar is a |
12 | single string (of any size, limited only by the available memory), | |
13 | number, or a reference to something (which will be discussed | |
14 | in L<perlref>). Normal arrays are ordered lists of scalars indexed | |
15 | by number, starting with 0. Hashes are unordered collections of scalar | |
16 | values indexed by their associated string key. | |
a0d0e21e | 17 | |
d55a8828 | 18 | Values are usually referred to by name, or through a named reference. |
b88cefa9 | 19 | The first character of the name tells you to what sort of data |
20 | structure it refers. The rest of the name tells you the particular | |
d55a8828 TC |
21 | value to which it refers. Usually this name is a single I<identifier>, |
22 | that is, a string beginning with a letter or underscore, and | |
23 | containing letters, underscores, and digits. In some cases, it may | |
24 | be a chain of identifiers, separated by C<::> (or by the slightly | |
25 | archaic C<'>); all but the last are interpreted as names of packages, | |
26 | to locate the namespace in which to look up the final identifier | |
32833930 BF |
27 | (see L<perlmod/Packages> for details). For a more in-depth discussion |
28 | on identifiers, see L<Identifier parsing>. It's possible to | |
29 | substitute for a simple identifier, an expression that produces a reference | |
d55a8828 TC |
30 | to the value at runtime. This is described in more detail below |
31 | and in L<perlref>. | |
d74e8afc | 32 | X<identifier> |
d55a8828 TC |
33 | |
34 | Perl also has its own built-in variables whose names don't follow | |
35 | these rules. They have strange names so they don't accidentally | |
36 | collide with one of your normal variables. Strings that match | |
37 | parenthesized parts of a regular expression are saved under names | |
38 | containing only digits after the C<$> (see L<perlop> and L<perlre>). | |
39 | In addition, several special variables that provide windows into | |
40 | the inner working of Perl have names containing punctuation characters | |
41 | and control characters. These are documented in L<perlvar>. | |
d74e8afc | 42 | X<variable, built-in> |
d55a8828 TC |
43 | |
44 | Scalar values are always named with '$', even when referring to a | |
45 | scalar that is part of an array or a hash. The '$' symbol works | |
46 | semantically like the English word "the" in that it indicates a | |
47 | single value is expected. | |
d74e8afc | 48 | X<scalar> |
a0d0e21e LW |
49 | |
50 | $days # the simple scalar value "days" | |
51 | $days[28] # the 29th element of array @days | |
52 | $days{'Feb'} # the 'Feb' value from hash %days | |
53 | $#days # the last index of array @days | |
54 | ||
d55a8828 | 55 | Entire arrays (and slices of arrays and hashes) are denoted by '@', |
3921068c | 56 | which works much as the word "these" or "those" does in English, |
d55a8828 | 57 | in that it indicates multiple values are expected. |
d74e8afc | 58 | X<array> |
a0d0e21e LW |
59 | |
60 | @days # ($days[0], $days[1],... $days[n]) | |
d55a8828 | 61 | @days[3,4,5] # same as ($days[3],$days[4],$days[5]) |
a0d0e21e LW |
62 | @days{'a','c'} # same as ($days{'a'},$days{'c'}) |
63 | ||
d55a8828 | 64 | Entire hashes are denoted by '%': |
d74e8afc | 65 | X<hash> |
a0d0e21e LW |
66 | |
67 | %days # (key1, val1, key2, val2 ...) | |
68 | ||
d55a8828 TC |
69 | In addition, subroutines are named with an initial '&', though this |
70 | is optional when unambiguous, just as the word "do" is often redundant | |
71 | in English. Symbol table entries can be named with an initial '*', | |
72 | but you don't really care about that yet (if ever :-). | |
73 | ||
74 | Every variable type has its own namespace, as do several | |
75 | non-variable identifiers. This means that you can, without fear | |
76 | of conflict, use the same name for a scalar variable, an array, or | |
77 | a hash--or, for that matter, for a filehandle, a directory handle, a | |
78 | subroutine name, a format name, or a label. This means that $foo | |
79 | and @foo are two different variables. It also means that C<$foo[1]> | |
80 | is a part of @foo, not a part of $foo. This may seem a bit weird, | |
81 | but that's okay, because it is weird. | |
d74e8afc | 82 | X<namespace> |
d55a8828 TC |
83 | |
84 | Because variable references always start with '$', '@', or '%', the | |
85 | "reserved" words aren't in fact reserved with respect to variable | |
86 | names. They I<are> reserved with respect to labels and filehandles, | |
87 | however, which don't have an initial special character. You can't | |
88 | have a filehandle named "log", for instance. Hint: you could say | |
89 | C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using | |
90 | uppercase filehandles also improves readability and protects you | |
91 | from conflict with future reserved words. Case I<is> significant--"FOO", | |
92 | "Foo", and "foo" are all different names. Names that start with a | |
93 | letter or underscore may also contain digits and underscores. | |
d74e8afc ITB |
94 | X<identifier, case sensitivity> |
95 | X<case> | |
a0d0e21e LW |
96 | |
97 | It is possible to replace such an alphanumeric name with an expression | |
d55a8828 | 98 | that returns a reference to the appropriate type. For a description |
a0d0e21e LW |
99 | of this, see L<perlref>. |
100 | ||
5f05dabc | 101 | Names that start with a digit may contain only more digits. Names |
9539f610 RGS |
102 | that do not start with a letter, underscore, digit or a caret (i.e. |
103 | a control character) are limited to one character, e.g., C<$%> or | |
104 | C<$$>. (Most of these one character names have a predefined | |
105 | significance to Perl. For instance, C<$$> is the current process | |
106 | id.) | |
a0d0e21e | 107 | |
32833930 BF |
108 | =head2 Identifier parsing |
109 | X<identifiers> | |
110 | ||
111 | Up until Perl 5.18, the actual rules of what a valid identifier | |
112 | was were a bit fuzzy. However, in general, anything defined here should | |
113 | work on previous versions of Perl, while the opposite -- edge cases | |
114 | that work in previous versions, but aren't defined here -- probably | |
115 | won't work on newer versions. | |
116 | As an important side note, please note that the following only applies | |
117 | to bareword identifiers as found in Perl source code, not identifiers | |
118 | introduced through symbolic references, which have much fewer | |
119 | restrictions. | |
120 | If working under the effect of the C<use utf8;> pragma, the following | |
121 | rules apply: | |
122 | ||
9c1129c7 KW |
123 | / (?[ ( \p{Word} & \p{XID_Start} ) + [_] ]) |
124 | (?[ ( \p{Word} & \p{XID_Continue} ) ]) * /x | |
125 | ||
126 | That is, a "start" character followed by any number of "continue" | |
127 | characters. Perl requires every character in an identifier to also | |
128 | match C<\w> (this prevents some problematic cases); and Perl | |
129 | additionally accepts identfier names beginning with an underscore. | |
32833930 BF |
130 | |
131 | If not under C<use utf8>, the source is treated as ASCII + 128 extra | |
132 | controls, and identifiers should match | |
133 | ||
134 | / (?aa) (?!\d) \w+ /x | |
135 | ||
136 | That is, any word character in the ASCII range, as long as the first | |
137 | character is not a digit. | |
138 | ||
139 | There are two package separators in Perl: A double colon (C<::>) and a single | |
140 | quote (C<'>). Normal identifiers can start or end with a double colon, and | |
141 | can contain several parts delimited by double colons. | |
142 | Single quotes have similar rules, but with the exception that they are not | |
143 | legal at the end of an identifier: That is, C<$'foo> and C<$foo'bar> are | |
1d268002 | 144 | legal, but C<$foo'bar'> is not. |
32833930 | 145 | |
1d268002 | 146 | Additionally, if the identifier is preceded by a sigil -- |
32833930 BF |
147 | that is, if the identifier is part of a variable name -- it |
148 | may optionally be enclosed in braces. | |
149 | ||
150 | While you can mix double colons with singles quotes, the quotes must come | |
151 | after the colons: C<$::::'foo> and C<$foo::'bar> are legal, but C<$::'::foo> | |
152 | and C<$foo'::bar> are not. | |
153 | ||
154 | Put together, a grammar to match a basic identifier becomes | |
155 | ||
156 | / | |
157 | (?(DEFINE) | |
158 | (?<variable> | |
159 | (?&sigil) | |
160 | (?: | |
161 | (?&normal_identifier) | |
162 | | \{ \s* (?&normal_identifier) \s* \} | |
163 | ) | |
164 | ) | |
165 | (?<normal_identifier> | |
166 | (?: :: )* '? | |
167 | (?&basic_identifier) | |
168 | (?: (?= (?: :: )+ '? | (?: :: )* ' ) (?&normal_identifier) )? | |
169 | (?: :: )* | |
170 | ) | |
171 | (?<basic_identifier> | |
172 | # is use utf8 on? | |
173 | (?(?{ (caller(0))[8] & $utf8::hint_bits }) | |
4c106081 | 174 | (?&Perl_XIDS) (?&Perl_XIDC)* |
32833930 BF |
175 | | (?aa) (?!\d) \w+ |
176 | ) | |
177 | ) | |
178 | (?<sigil> [&*\$\@\%]) | |
179 | (?<Perl_XIDS> (?[ ( \p{Word} & \p{XID_Start} ) + [_] ]) ) | |
4c106081 | 180 | (?<Perl_XIDC> (?[ \p{Word} & \p{XID_Continue} ]) ) |
32833930 BF |
181 | ) |
182 | /x | |
183 | ||
184 | Meanwhile, special identifiers don't follow the above rules; For the most | |
185 | part, all of the identifiers in this category have a special meaning given | |
186 | by Perl. Because they have special parsing rules, these generally can't be | |
187 | fully-qualified. They come in four forms: | |
188 | ||
189 | =over | |
190 | ||
191 | =item A sigil, followed solely by digits matching \p{POSIX_Digit}, like C<$0>, | |
192 | C<$1>, or C<$10000>. | |
193 | ||
194 | =item A sigil, followed by either a caret and a single POSIX uppercase letter, | |
195 | like C<$^V> or C<$^W>, or a sigil followed by a literal control character | |
b4e2e1dd FC |
196 | matching the C<\p{POSIX_Cntrl}> property. |
197 | Due to a historical oddity, if not | |
32833930 | 198 | running under C<use utf8>, the 128 extra controls in the C<[0x80-0xff]> range |
b50f77aa RS |
199 | may also be used in length one variables. The use of a literal control |
200 | character is deprecated. Support for this form will be removed in a future | |
201 | version of perl. | |
32833930 BF |
202 | |
203 | =item Similar to the above, a sigil, followed by bareword text in brackets, | |
204 | where the first character is either a caret followed by an uppercase letter, | |
b50f77aa RS |
205 | or a literal control, like C<${^GLOBAL_PHASE}> or C<${\7LOBAL_PHASE}>. The use |
206 | of a literal control character is deprecated. Support for this form will be | |
207 | removed in a future version of perl. | |
32833930 BF |
208 | |
209 | =item A sigil followed by a single character matching the C<\p{POSIX_Punct}> | |
210 | property, like C<$!> or C<%+>. | |
211 | ||
212 | =back | |
213 | ||
b29f65fc BF |
214 | Note that as of Perl 5.20, literal control characters in variable names |
215 | are deprecated. | |
216 | ||
a0d0e21e | 217 | =head2 Context |
d74e8afc | 218 | X<context> X<scalar context> X<list context> |
a0d0e21e LW |
219 | |
220 | The interpretation of operations and values in Perl sometimes depends | |
221 | on the requirements of the context around the operation or value. | |
d55a8828 | 222 | There are two major contexts: list and scalar. Certain operations |
a0d0e21e | 223 | return list values in contexts wanting a list, and scalar values |
d55a8828 TC |
224 | otherwise. If this is true of an operation it will be mentioned in |
225 | the documentation for that operation. In other words, Perl overloads | |
a0d0e21e | 226 | certain operations based on whether the expected return value is |
d55a8828 TC |
227 | singular or plural. Some words in English work this way, like "fish" |
228 | and "sheep". | |
a0d0e21e LW |
229 | |
230 | In a reciprocal fashion, an operation provides either a scalar or a | |
231 | list context to each of its arguments. For example, if you say | |
232 | ||
233 | int( <STDIN> ) | |
234 | ||
c47ff5f1 | 235 | the integer operation provides scalar context for the <> |
a0d0e21e LW |
236 | operator, which responds by reading one line from STDIN and passing it |
237 | back to the integer operation, which will then find the integer value | |
238 | of that line and return that. If, on the other hand, you say | |
239 | ||
240 | sort( <STDIN> ) | |
241 | ||
c47ff5f1 | 242 | then the sort operation provides list context for <>, which |
a0d0e21e LW |
243 | will proceed to read every line available up to the end of file, and |
244 | pass that list of lines back to the sort routine, which will then | |
245 | sort those lines and return them as a list to whatever the context | |
246 | of the sort was. | |
247 | ||
d55a8828 TC |
248 | Assignment is a little bit special in that it uses its left argument |
249 | to determine the context for the right argument. Assignment to a | |
250 | scalar evaluates the right-hand side in scalar context, while | |
251 | assignment to an array or hash evaluates the righthand side in list | |
252 | context. Assignment to a list (or slice, which is just a list | |
3921068c | 253 | anyway) also evaluates the right-hand side in list context. |
d55a8828 | 254 | |
9f1b1f2d GS |
255 | When you use the C<use warnings> pragma or Perl's B<-w> command-line |
256 | option, you may see warnings | |
d55a8828 TC |
257 | about useless uses of constants or functions in "void context". |
258 | Void context just means the value has been discarded, such as a | |
259 | statement containing only C<"fred";> or C<getpwuid(0);>. It still | |
260 | counts as scalar context for functions that care whether or not | |
261 | they're being called in list context. | |
262 | ||
263 | User-defined subroutines may choose to care whether they are being | |
264 | called in a void, scalar, or list context. Most subroutines do not | |
265 | need to bother, though. That's because both scalars and lists are | |
266 | automatically interpolated into lists. See L<perlfunc/wantarray> | |
267 | for how you would dynamically discern your function's calling | |
268 | context. | |
a0d0e21e LW |
269 | |
270 | =head2 Scalar values | |
d74e8afc | 271 | X<scalar> X<number> X<string> X<reference> |
a0d0e21e | 272 | |
d55a8828 TC |
273 | All data in Perl is a scalar, an array of scalars, or a hash of |
274 | scalars. A scalar may contain one single value in any of three | |
275 | different flavors: a number, a string, or a reference. In general, | |
276 | conversion from one form to another is transparent. Although a | |
277 | scalar may not directly hold multiple values, it may contain a | |
278 | reference to an array or hash which in turn contains multiple values. | |
279 | ||
280 | Scalars aren't necessarily one thing or another. There's no place | |
281 | to declare a scalar variable to be of type "string", type "number", | |
282 | type "reference", or anything else. Because of the automatic | |
283 | conversion of scalars, operations that return scalars don't need | |
284 | to care (and in fact, cannot care) whether their caller is looking | |
285 | for a string, a number, or a reference. Perl is a contextually | |
286 | polymorphic language whose scalars can be strings, numbers, or | |
287 | references (which includes objects). Although strings and numbers | |
288 | are considered pretty much the same thing for nearly all purposes, | |
289 | references are strongly-typed, uncastable pointers with builtin | |
290 | reference-counting and destructor invocation. | |
a0d0e21e | 291 | |
32860eee FC |
292 | A scalar value is interpreted as FALSE in the Boolean sense |
293 | if it is undefined, the null string or the number 0 (or its | |
294 | string equivalent, "0"), and TRUE if it is anything else. The | |
d55a8828 TC |
295 | Boolean context is just a special kind of scalar context where no |
296 | conversion to a string or a number is ever performed. | |
d74e8afc | 297 | X<boolean> X<bool> X<true> X<false> X<truth> |
d55a8828 TC |
298 | |
299 | There are actually two varieties of null strings (sometimes referred | |
300 | to as "empty" strings), a defined one and an undefined one. The | |
301 | defined version is just a string of length zero, such as C<"">. | |
302 | The undefined version is the value that indicates that there is | |
303 | no real value for something, such as when there was an error, or | |
304 | at end of file, or when you refer to an uninitialized variable or | |
305 | element of an array or hash. Although in early versions of Perl, | |
306 | an undefined scalar could become defined when first used in a | |
307 | place expecting a defined value, this no longer happens except for | |
308 | rare cases of autovivification as explained in L<perlref>. You can | |
309 | use the defined() operator to determine whether a scalar value is | |
310 | defined (this has no meaning on arrays or hashes), and the undef() | |
311 | operator to produce an undefined value. | |
d74e8afc | 312 | X<defined> X<undefined> X<undef> X<null> X<string, null> |
d55a8828 TC |
313 | |
314 | To find out whether a given string is a valid non-zero number, it's | |
315 | sometimes enough to test it against both numeric 0 and also lexical | |
692ef166 SF |
316 | "0" (although this will cause noises if warnings are on). That's |
317 | because strings that aren't numbers count as 0, just as they do in B<awk>: | |
4633a7c4 LW |
318 | |
319 | if ($str == 0 && $str ne "0") { | |
320 | warn "That doesn't look like a number"; | |
54310121 | 321 | } |
4633a7c4 | 322 | |
d55a8828 TC |
323 | That method may be best because otherwise you won't treat IEEE |
324 | notations like C<NaN> or C<Infinity> properly. At other times, you | |
325 | might prefer to determine whether string data can be used numerically | |
326 | by calling the POSIX::strtod() function or by inspecting your string | |
327 | with a regular expression (as documented in L<perlre>). | |
cb1a09d0 AD |
328 | |
329 | warn "has nondigits" if /\D/; | |
5a964f20 TC |
330 | warn "not a natural number" unless /^\d+$/; # rejects -3 |
331 | warn "not an integer" unless /^-?\d+$/; # rejects +3 | |
332 | warn "not an integer" unless /^[+-]?\d+$/; | |
333 | warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2 | |
334 | warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/; | |
54310121 | 335 | warn "not a C float" |
cb1a09d0 AD |
336 | unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/; |
337 | ||
d55a8828 | 338 | The length of an array is a scalar value. You may find the length |
fc518ee5 JA |
339 | of array @days by evaluating C<$#days>, as in B<csh>. However, this |
340 | isn't the length of the array; it's the subscript of the last element, | |
341 | which is a different value since there is ordinarily a 0th element. | |
d55a8828 TC |
342 | Assigning to C<$#days> actually changes the length of the array. |
343 | Shortening an array this way destroys intervening values. Lengthening | |
344 | an array that was previously shortened does not recover values | |
0568eccd | 345 | that were in those elements. |
d74e8afc | 346 | X<$#> X<array, length> |
d55a8828 | 347 | |
210b36aa | 348 | You can also gain some minuscule measure of efficiency by pre-extending |
d55a8828 TC |
349 | an array that is going to get big. You can also extend an array |
350 | by assigning to an element that is off the end of the array. You | |
19799a22 | 351 | can truncate an array down to nothing by assigning the null list |
d55a8828 | 352 | () to it. The following are equivalent: |
a0d0e21e | 353 | |
84f709e7 | 354 | @whatever = (); |
3e3baf6d | 355 | $#whatever = -1; |
a0d0e21e | 356 | |
d55a8828 TC |
357 | If you evaluate an array in scalar context, it returns the length |
358 | of the array. (Note that this is not true of lists, which return | |
359 | the last value, like the C comma operator, nor of built-in functions, | |
360 | which return whatever they feel like returning.) The following is | |
361 | always true: | |
d74e8afc | 362 | X<array, length> |
a0d0e21e | 363 | |
a0d0e21e LW |
364 | scalar(@whatever) == $#whatever + 1; |
365 | ||
d55a8828 TC |
366 | Some programmers choose to use an explicit conversion so as to |
367 | leave nothing to doubt: | |
4633a7c4 LW |
368 | |
369 | $element_count = scalar(@whatever); | |
370 | ||
d55a8828 TC |
371 | If you evaluate a hash in scalar context, it returns false if the |
372 | hash is empty. If there are any key/value pairs, it returns true; | |
373 | more precisely, the value returned is a string consisting of the | |
374 | number of used buckets and the number of allocated buckets, separated | |
375 | by a slash. This is pretty much useful only to find out whether | |
376 | Perl's internal hashing algorithm is performing poorly on your data | |
377 | set. For example, you stick 10,000 things in a hash, but evaluating | |
378 | %HASH in scalar context reveals C<"1/16">, which means only one out | |
379 | of sixteen buckets has been touched, and presumably contains all | |
126c71c8 | 380 | 10,000 of your items. This isn't supposed to happen. If a tied hash |
6e4c4703 DM |
381 | is evaluated in scalar context, the C<SCALAR> method is called (with a |
382 | fallback to C<FIRSTKEY>). | |
d74e8afc | 383 | X<hash, scalar context> X<hash, bucket> X<bucket> |
a0d0e21e | 384 | |
5a964f20 | 385 | You can preallocate space for a hash by assigning to the keys() function. |
65841adf | 386 | This rounds up the allocated buckets to the next power of two: |
5a964f20 TC |
387 | |
388 | keys(%users) = 1000; # allocate 1024 buckets | |
389 | ||
a0d0e21e | 390 | =head2 Scalar value constructors |
d74e8afc | 391 | X<scalar, literal> X<scalar, constant> |
a0d0e21e | 392 | |
d55a8828 | 393 | Numeric literals are specified in any of the following floating point or |
a0d0e21e LW |
394 | integer formats: |
395 | ||
a0d0e21e LW |
396 | 12345 |
397 | 12345.67 | |
d55a8828 | 398 | .23E-10 # a very small number |
928753ea | 399 | 3.14_15_92 # a very important number |
1d277562 | 400 | 4_294_967_296 # underscore for legibility |
d55a8828 | 401 | 0xff # hex |
928753ea | 402 | 0xdead_beef # more hex |
802a55ac | 403 | 0377 # octal (only numbers, begins with 0) |
d55a8828 | 404 | 0b011011 # binary |
a0d0e21e | 405 | |
d4ced10d | 406 | You are allowed to use underscores (underbars) in numeric literals |
d823de2e KW |
407 | between digits for legibility (but not multiple underscores in a row: |
408 | C<23__500> is not legal; C<23_500> is). | |
409 | You could, for example, group binary | |
d4ced10d JH |
410 | digits by threes (as for a Unix-style mode argument such as 0b110_100_100) |
411 | or by fours (to represent nibbles, as in 0b1010_0110) or in other groups. | |
d74e8afc | 412 | X<number, literal> |
1d277562 | 413 | |
55497cff | 414 | String literals are usually delimited by either single or double |
d55a8828 TC |
415 | quotes. They work much like quotes in the standard Unix shells: |
416 | double-quoted string literals are subject to backslash and variable | |
19799a22 GS |
417 | substitution; single-quoted strings are not (except for C<\'> and |
418 | C<\\>). The usual C-style backslash rules apply for making | |
d55a8828 | 419 | characters such as newline, tab, etc., as well as some more exotic |
4a4eefd0 | 420 | forms. See L<perlop/"Quote and Quote-like Operators"> for a list. |
d74e8afc | 421 | X<string, literal> |
d55a8828 TC |
422 | |
423 | Hexadecimal, octal, or binary, representations in string literals | |
424 | (e.g. '0xff') are not automatically converted to their integer | |
425 | representation. The hex() and oct() functions make these conversions | |
426 | for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details. | |
68dc0745 | 427 | |
5f05dabc | 428 | You can also embed newlines directly in your strings, i.e., they can end |
a0d0e21e LW |
429 | on a different line than they begin. This is nice, but if you forget |
430 | your trailing quote, the error will not be reported until Perl finds | |
431 | another line containing the quote character, which may be much further | |
432 | on in the script. Variable substitution inside strings is limited to | |
d55a8828 | 433 | scalar variables, arrays, and array or hash slices. (In other words, |
b88cefa9 | 434 | names beginning with $ or @, followed by an optional bracketed |
a0d0e21e | 435 | expression as a subscript.) The following code segment prints out "The |
184e9718 | 436 | price is $Z<>100." |
d74e8afc | 437 | X<interpolation> |
a0d0e21e | 438 | |
692ef166 SF |
439 | $Price = '$100'; # not interpolated |
440 | print "The price is $Price.\n"; # interpolated | |
441 | ||
442 | There is no double interpolation in Perl, so the C<$100> is left as is. | |
a0d0e21e | 443 | |
7e4353e9 RGS |
444 | By default floating point numbers substituted inside strings use the |
445 | dot (".") as the decimal separator. If C<use locale> is in effect, | |
446 | and POSIX::setlocale() has been called, the character used for the | |
447 | decimal separator is affected by the LC_NUMERIC locale. | |
448 | See L<perllocale> and L<POSIX>. | |
449 | ||
d55a8828 | 450 | As in some shells, you can enclose the variable name in braces to |
f1cbbd6e GS |
451 | disambiguate it from following alphanumerics (and underscores). |
452 | You must also do | |
d55a8828 TC |
453 | this when interpolating a variable into a string to separate the |
454 | variable name from a following double-colon or an apostrophe, since | |
455 | these would be otherwise treated as a package separator: | |
d74e8afc | 456 | X<interpolation> |
d55a8828 | 457 | |
84f709e7 | 458 | $who = "Larry"; |
d55a8828 TC |
459 | print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n"; |
460 | print "We use ${who}speak when ${who}'s here.\n"; | |
461 | ||
462 | Without the braces, Perl would have looked for a $whospeak, a | |
463 | C<$who::0>, and a C<$who's> variable. The last two would be the | |
464 | $0 and the $s variables in the (presumably) non-existent package | |
465 | C<who>. | |
466 | ||
34a2706e | 467 | In fact, a simple identifier within such curlies is forced to be |
b4e2e1dd | 468 | a string, and likewise within a hash subscript. Neither need |
d55a8828 TC |
469 | quoting. Our earlier example, C<$days{'Feb'}> can be written as |
470 | C<$days{Feb}> and the quotes will be assumed automatically. But | |
719b43e8 RGS |
471 | anything more complicated in the subscript will be interpreted as an |
472 | expression. This means for example that C<$version{2.0}++> is | |
473 | equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>. | |
d55a8828 | 474 | |
692ef166 | 475 | =head3 Version Strings |
d74e8afc | 476 | X<version string> X<vstring> X<v-string> |
692ef166 | 477 | |
191d61a7 | 478 | A literal of the form C<v1.20.300.4000> is parsed as a string composed |
6b2463a0 JH |
479 | of characters with the specified ordinals. This form, known as |
480 | v-strings, provides an alternative, more readable way to construct | |
481 | strings, rather than use the somewhat less readable interpolation form | |
482 | C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing | |
483 | Unicode strings, and for comparing version "numbers" using the string | |
484 | comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or | |
485 | more dots in the literal, the leading C<v> may be omitted. | |
b9c62f5b | 486 | |
2575c402 | 487 | print v9786; # prints SMILEY, "\x{263a}" |
b9c62f5b GS |
488 | print v102.111.111; # prints "foo" |
489 | print 102.111.111; # same | |
490 | ||
491 | Such literals are accepted by both C<require> and C<use> for | |
a32521b7 JD |
492 | doing a version check. Note that using the v-strings for IPv4 |
493 | addresses is not portable unless you also use the | |
494 | inet_aton()/inet_ntoa() routines of the Socket package. | |
191d61a7 | 495 | |
d32a65d2 | 496 | Note that since Perl 5.8.1 the single-number v-strings (like C<v65>) |
8fa72689 | 497 | are not v-strings before the C<< => >> operator (which is usually used |
3921068c | 498 | to separate a hash key from a hash value); instead they are interpreted |
15ecd4ae JH |
499 | as literal strings ('v65'). They were v-strings from Perl 5.6.0 to |
500 | Perl 5.8.0, but that caused more confusion and breakage than good. | |
501 | Multi-number v-strings like C<v65.66> and C<65.66.67> continue to | |
502 | be v-strings always. | |
d32a65d2 | 503 | |
692ef166 | 504 | =head3 Special Literals |
d74e8afc ITB |
505 | X<special literal> X<__END__> X<__DATA__> X<END> X<DATA> |
506 | X<end> X<data> X<^D> X<^Z> | |
692ef166 | 507 | |
d55a8828 | 508 | The special literals __FILE__, __LINE__, and __PACKAGE__ |
68dc0745 | 509 | represent the current filename, line number, and package name at that |
84ed0108 FC |
510 | point in your program. __SUB__ gives a reference to the current |
511 | subroutine. They may be used only as separate tokens; they | |
68dc0745 | 512 | will not be interpolated into strings. If there is no current package |
3e92a254 | 513 | (due to an empty C<package;> directive), __PACKAGE__ is the undefined |
8fdd8881 | 514 | value. (But the empty C<package;> is no longer supported, as of version |
84ed0108 FC |
515 | 5.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__ |
516 | is only available in 5.16 or higher, and only with a C<use v5.16> or | |
517 | C<use feature "current_sub"> declaration. | |
518 | X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__> | |
519 | X<line> X<file> X<package> | |
3e92a254 GS |
520 | |
521 | The two control characters ^D and ^Z, and the tokens __END__ and __DATA__ | |
522 | may be used to indicate the logical end of the script before the actual | |
523 | end of file. Any following text is ignored. | |
524 | ||
1bab44f9 | 525 | Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>, |
3e92a254 GS |
526 | where C<PACKNAME> is the package that was current when the __DATA__ |
527 | token was encountered. The filehandle is left open pointing to the | |
4d383607 | 528 | line after __DATA__. The program should C<close DATA> when it is done |
9c205800 FC |
529 | reading from it. (Leaving it open leaks filehandles if the module is |
530 | reloaded for any reason, so it's a safer practice to close it.) For | |
4d383607 JK |
531 | compatibility with older scripts written before __DATA__ was |
532 | introduced, __END__ behaves like __DATA__ in the top level script (but | |
533 | not in files loaded with C<require> or C<do>) and leaves the remaining | |
534 | contents of the file accessible via C<main::DATA>. | |
3e92a254 GS |
535 | |
536 | See L<SelfLoader> for more description of __DATA__, and | |
d55a8828 TC |
537 | an example of its use. Note that you cannot read from the DATA |
538 | filehandle in a BEGIN block: the BEGIN block is executed as soon | |
539 | as it is seen (during compilation), at which point the corresponding | |
a00c1fe5 | 540 | __DATA__ (or __END__) token has not yet been seen. |
a0d0e21e | 541 | |
692ef166 | 542 | =head3 Barewords |
d74e8afc | 543 | X<bareword> |
692ef166 | 544 | |
748a9306 | 545 | A word that has no other interpretation in the grammar will |
a0d0e21e LW |
546 | be treated as if it were a quoted string. These are known as |
547 | "barewords". As with filehandles and labels, a bareword that consists | |
548 | entirely of lowercase letters risks conflict with future reserved | |
9f1b1f2d | 549 | words, and if you use the C<use warnings> pragma or the B<-w> switch, |
05b4f1ec FW |
550 | Perl will warn you about any such words. Perl limits barewords (like |
551 | identifiers) to about 250 characters. Future versions of Perl are likely | |
552 | to eliminate these arbitrary limitations. | |
553 | ||
554 | Some people may wish to outlaw barewords entirely. If you | |
a0d0e21e LW |
555 | say |
556 | ||
557 | use strict 'subs'; | |
558 | ||
559 | then any bareword that would NOT be interpreted as a subroutine call | |
560 | produces a compile-time error instead. The restriction lasts to the | |
54310121 | 561 | end of the enclosing block. An inner block may countermand this |
a0d0e21e LW |
562 | by saying C<no strict 'subs'>. |
563 | ||
e2b457c0 | 564 | =head3 Array Interpolation |
d74e8afc | 565 | X<array, interpolation> X<interpolation, array> X<$"> |
692ef166 | 566 | |
d55a8828 TC |
567 | Arrays and slices are interpolated into double-quoted strings |
568 | by joining the elements with the delimiter specified in the C<$"> | |
692ef166 SF |
569 | variable (C<$LIST_SEPARATOR> if "use English;" is specified), |
570 | space by default. The following are equivalent: | |
a0d0e21e | 571 | |
84f709e7 | 572 | $temp = join($", @ARGV); |
a0d0e21e LW |
573 | system "echo $temp"; |
574 | ||
575 | system "echo @ARGV"; | |
576 | ||
577 | Within search patterns (which also undergo double-quotish substitution) | |
d55a8828 | 578 | there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as |
a0d0e21e LW |
579 | C</${foo}[bar]/> (where C<[bar]> is a character class for the regular |
580 | expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array | |
581 | @foo)? If @foo doesn't otherwise exist, then it's obviously a | |
582 | character class. If @foo exists, Perl takes a good guess about C<[bar]>, | |
583 | and is almost always right. If it does guess wrong, or if you're just | |
584 | plain paranoid, you can force the correct interpretation with curly | |
d55a8828 | 585 | braces as above. |
a0d0e21e | 586 | |
7e3b091d | 587 | If you're looking for the information on how to use here-documents, |
210b36aa AMS |
588 | which used to be here, that's been moved to |
589 | L<perlop/Quote and Quote-like Operators>. | |
be16fac9 | 590 | |
a0d0e21e | 591 | =head2 List value constructors |
d74e8afc | 592 | X<list> |
a0d0e21e LW |
593 | |
594 | List values are denoted by separating individual values by commas | |
595 | (and enclosing the list in parentheses where precedence requires it): | |
596 | ||
597 | (LIST) | |
598 | ||
d55a8828 TC |
599 | In a context not requiring a list value, the value of what appears |
600 | to be a list literal is simply the value of the final element, as | |
601 | with the C comma operator. For example, | |
a0d0e21e | 602 | |
84f709e7 | 603 | @foo = ('cc', '-E', $bar); |
a0d0e21e | 604 | |
d55a8828 | 605 | assigns the entire list value to array @foo, but |
a0d0e21e | 606 | |
84f709e7 | 607 | $foo = ('cc', '-E', $bar); |
a0d0e21e | 608 | |
d55a8828 TC |
609 | assigns the value of variable $bar to the scalar variable $foo. |
610 | Note that the value of an actual array in scalar context is the | |
611 | length of the array; the following assigns the value 3 to $foo: | |
a0d0e21e | 612 | |
84f709e7 | 613 | @foo = ('cc', '-E', $bar); |
7e3b091d | 614 | $foo = @foo; # $foo gets 3 |
a0d0e21e | 615 | |
54310121 | 616 | You may have an optional comma before the closing parenthesis of a |
a0d0e21e LW |
617 | list literal, so that you can say: |
618 | ||
84f709e7 | 619 | @foo = ( |
7e3b091d DA |
620 | 1, |
621 | 2, | |
622 | 3, | |
a0d0e21e LW |
623 | ); |
624 | ||
d55a8828 TC |
625 | To use a here-document to assign an array, one line per element, |
626 | you might use an approach like this: | |
627 | ||
84f709e7 | 628 | @sauces = <<End_Lines =~ m/(\S.*\S)/g; |
7e3b091d DA |
629 | normal tomato |
630 | spicy tomato | |
631 | green chile | |
632 | pesto | |
633 | white wine | |
d55a8828 TC |
634 | End_Lines |
635 | ||
a0d0e21e | 636 | LISTs do automatic interpolation of sublists. That is, when a LIST is |
d55a8828 | 637 | evaluated, each element of the list is evaluated in list context, and |
a0d0e21e | 638 | the resulting list value is interpolated into LIST just as if each |
5a964f20 | 639 | individual element were a member of LIST. Thus arrays and hashes lose their |
a0d0e21e LW |
640 | identity in a LIST--the list |
641 | ||
5a964f20 | 642 | (@foo,@bar,&SomeSub,%glarch) |
a0d0e21e LW |
643 | |
644 | contains all the elements of @foo followed by all the elements of @bar, | |
5a964f20 | 645 | followed by all the elements returned by the subroutine named SomeSub |
d55a8828 | 646 | called in list context, followed by the key/value pairs of %glarch. |
a0d0e21e LW |
647 | To make a list reference that does I<NOT> interpolate, see L<perlref>. |
648 | ||
19799a22 | 649 | The null list is represented by (). Interpolating it in a list |
a0d0e21e LW |
650 | has no effect. Thus ((),(),()) is equivalent to (). Similarly, |
651 | interpolating an array with no elements is the same as if no | |
652 | array had been interpolated at that point. | |
653 | ||
c2689353 | 654 | This interpolation combines with the facts that the opening |
ab1f959b | 655 | and closing parentheses are optional (except when necessary for |
c2689353 | 656 | precedence) and lists may end with an optional comma to mean that |
8fdd8881 | 657 | multiple commas within lists are legal syntax. The list C<1,,3> is a |
c2689353 NC |
658 | concatenation of two lists, C<1,> and C<3>, the first of which ends |
659 | with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And | |
660 | similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that | |
661 | we'd advise you to use this obfuscation. | |
662 | ||
a0d0e21e | 663 | A list value may also be subscripted like a normal array. You must |
54310121 | 664 | put the list in parentheses to avoid ambiguity. For example: |
a0d0e21e LW |
665 | |
666 | # Stat returns list value. | |
84f709e7 | 667 | $time = (stat($file))[8]; |
a0d0e21e | 668 | |
4633a7c4 | 669 | # SYNTAX ERROR HERE. |
84f709e7 | 670 | $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES |
4633a7c4 | 671 | |
a0d0e21e | 672 | # Find a hex digit. |
84f709e7 | 673 | $hexdigit = ('a','b','c','d','e','f')[$digit-10]; |
a0d0e21e LW |
674 | |
675 | # A "reverse comma operator". | |
676 | return (pop(@foo),pop(@foo))[0]; | |
677 | ||
d55a8828 TC |
678 | Lists may be assigned to only when each element of the list |
679 | is itself legal to assign to: | |
a0d0e21e | 680 | |
84f709e7 | 681 | ($a, $b, $c) = (1, 2, 3); |
a0d0e21e | 682 | |
84f709e7 | 683 | ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00); |
a0d0e21e | 684 | |
d55a8828 TC |
685 | An exception to this is that you may assign to C<undef> in a list. |
686 | This is useful for throwing away some of the return values of a | |
687 | function: | |
688 | ||
84f709e7 | 689 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file); |
d55a8828 TC |
690 | |
691 | List assignment in scalar context returns the number of elements | |
4633a7c4 LW |
692 | produced by the expression on the right side of the assignment: |
693 | ||
7e3b091d DA |
694 | $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2 |
695 | $x = (($foo,$bar) = f()); # set $x to f()'s return count | |
4633a7c4 | 696 | |
d55a8828 | 697 | This is handy when you want to do a list assignment in a Boolean |
19799a22 | 698 | context, because most list functions return a null list when finished, |
4633a7c4 LW |
699 | which when assigned produces a 0, which is interpreted as FALSE. |
700 | ||
ab1f959b PN |
701 | It's also the source of a useful idiom for executing a function or |
702 | performing an operation in list context and then counting the number of | |
703 | return values, by assigning to an empty list and then using that | |
8fdd8881 | 704 | assignment in scalar context. For example, this code: |
ab1f959b | 705 | |
84f709e7 | 706 | $count = () = $string =~ /\d+/g; |
ab1f959b PN |
707 | |
708 | will place into $count the number of digit groups found in $string. | |
709 | This happens because the pattern match is in list context (since it | |
710 | is being assigned to the empty list), and will therefore return a list | |
8fdd8881 | 711 | of all matching parts of the string. The list assignment in scalar |
ab1f959b | 712 | context will translate that into the number of elements (here, the |
8fdd8881 | 713 | number of times the pattern matched) and assign that to $count. Note |
ab1f959b PN |
714 | that simply using |
715 | ||
84f709e7 | 716 | $count = $string =~ /\d+/g; |
ab1f959b PN |
717 | |
718 | would not have worked, since a pattern match in scalar context will | |
719 | only return true or false, rather than a count of matches. | |
720 | ||
721 | The final element of a list assignment may be an array or a hash: | |
a0d0e21e | 722 | |
84f709e7 | 723 | ($a, $b, @rest) = split; |
5a964f20 | 724 | my($a, $b, %rest) = @_; |
a0d0e21e | 725 | |
4633a7c4 | 726 | You can actually put an array or hash anywhere in the list, but the first one |
d55a8828 TC |
727 | in the list will soak up all the values, and anything after it will become |
728 | undefined. This may be useful in a my() or local(). | |
a0d0e21e | 729 | |
d55a8828 TC |
730 | A hash can be initialized using a literal list holding pairs of |
731 | items to be interpreted as a key and a value: | |
a0d0e21e LW |
732 | |
733 | # same as map assignment above | |
84f709e7 | 734 | %map = ('red',0x00f,'blue',0x0f0,'green',0xf00); |
a0d0e21e | 735 | |
d55a8828 | 736 | While literal lists and named arrays are often interchangeable, that's |
4633a7c4 LW |
737 | not the case for hashes. Just because you can subscript a list value like |
738 | a normal array does not mean that you can subscript a list value as a | |
739 | hash. Likewise, hashes included as parts of other lists (including | |
740 | parameters lists and return lists from functions) always flatten out into | |
741 | key/value pairs. That's why it's good to use references sometimes. | |
a0d0e21e | 742 | |
c47ff5f1 GS |
743 | It is often more readable to use the C<< => >> operator between key/value |
744 | pairs. The C<< => >> operator is mostly just a more visually distinctive | |
b88cefa9 | 745 | synonym for a comma, but it also arranges for its left-hand operand to be |
ac036724 | 746 | interpreted as a string if it's a bareword that would be a legal simple |
8fdd8881 FC |
747 | identifier. C<< => >> doesn't quote compound identifiers, that contain |
748 | double colons. This makes it nice for initializing hashes: | |
a0d0e21e | 749 | |
84f709e7 | 750 | %map = ( |
7e3b091d DA |
751 | red => 0x00f, |
752 | blue => 0x0f0, | |
753 | green => 0xf00, | |
4633a7c4 LW |
754 | ); |
755 | ||
756 | or for initializing hash references to be used as records: | |
757 | ||
84f709e7 | 758 | $rec = { |
7e3b091d DA |
759 | witch => 'Mable the Merciless', |
760 | cat => 'Fluffy the Ferocious', | |
761 | date => '10/31/1776', | |
4633a7c4 LW |
762 | }; |
763 | ||
764 | or for using call-by-named-parameter to complicated functions: | |
765 | ||
84f709e7 | 766 | $field = $query->radio_group( |
7e3b091d | 767 | name => 'group_name', |
4633a7c4 LW |
768 | values => ['eenie','meenie','minie'], |
769 | default => 'meenie', | |
770 | linebreak => 'true', | |
84f709e7 | 771 | labels => \%labels |
4633a7c4 | 772 | ); |
cb1a09d0 AD |
773 | |
774 | Note that just because a hash is initialized in that order doesn't | |
775 | mean that it comes out in that order. See L<perlfunc/sort> for examples | |
776 | of how to arrange for an output ordering. | |
777 | ||
c9e3649f LM |
778 | If a key appears more than once in the initializer list of a hash, the last |
779 | occurrence wins: | |
780 | ||
781 | %circle = ( | |
782 | center => [5, 10], | |
783 | center => [27, 9], | |
784 | radius => 100, | |
785 | color => [0xDF, 0xFF, 0x00], | |
786 | radius => 54, | |
787 | ); | |
788 | ||
789 | # same as | |
790 | %circle = ( | |
791 | center => [27, 9], | |
792 | color => [0xDF, 0xFF, 0x00], | |
793 | radius => 54, | |
794 | ); | |
795 | ||
796 | This can be used to provide overridable configuration defaults: | |
797 | ||
798 | # values in %args take priority over %config_defaults | |
799 | %config = (%config_defaults, %args); | |
800 | ||
692ef166 SF |
801 | =head2 Subscripts |
802 | ||
aa80e1dc FC |
803 | An array can be accessed one scalar at a |
804 | time by specifying a dollar sign (C<$>), then the | |
692ef166 SF |
805 | name of the array (without the leading C<@>), then the subscript inside |
806 | square brackets. For example: | |
807 | ||
808 | @myarray = (5, 50, 500, 5000); | |
2adc35dd | 809 | print "The Third Element is", $myarray[2], "\n"; |
692ef166 | 810 | |
8fdd8881 | 811 | The array indices start with 0. A negative subscript retrieves its |
692ef166 SF |
812 | value from the end. In our example, C<$myarray[-1]> would have been |
813 | 5000, and C<$myarray[-2]> would have been 500. | |
814 | ||
815 | Hash subscripts are similar, only instead of square brackets curly brackets | |
8fdd8881 | 816 | are used. For example: |
692ef166 SF |
817 | |
818 | %scientists = | |
819 | ( | |
820 | "Newton" => "Isaac", | |
821 | "Einstein" => "Albert", | |
822 | "Darwin" => "Charles", | |
823 | "Feynman" => "Richard", | |
824 | ); | |
825 | ||
826 | print "Darwin's First Name is ", $scientists{"Darwin"}, "\n"; | |
827 | ||
aa80e1dc | 828 | You can also subscript a list to get a single element from it: |
d55a8828 | 829 | |
aa80e1dc | 830 | $dir = (getpwnam("daemon"))[7]; |
d55a8828 | 831 | |
9ed2a148 IG |
832 | =head2 Multi-dimensional array emulation |
833 | ||
834 | Multidimensional arrays may be emulated by subscripting a hash with a | |
8fdd8881 | 835 | list. The elements of the list are joined with the subscript separator |
b8db74f2 | 836 | (see L<perlvar/$;>). |
9ed2a148 IG |
837 | |
838 | $foo{$a,$b,$c} | |
839 | ||
840 | is equivalent to | |
841 | ||
842 | $foo{join($;, $a, $b, $c)} | |
843 | ||
844 | The default subscript separator is "\034", the same as SUBSEP in B<awk>. | |
845 | ||
aa80e1dc FC |
846 | =head2 Slices |
847 | X<slice> X<array, slice> X<hash, slice> | |
d55a8828 TC |
848 | |
849 | A slice accesses several elements of a list, an array, or a hash | |
56d7751a GS |
850 | simultaneously using a list of subscripts. It's more convenient |
851 | than writing out the individual elements as a list of separate | |
d55a8828 TC |
852 | scalar values. |
853 | ||
7e3b091d DA |
854 | ($him, $her) = @folks[0,-1]; # array slice |
855 | @them = @folks[0 .. 3]; # array slice | |
856 | ($who, $home) = @ENV{"USER", "HOME"}; # hash slice | |
857 | ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice | |
d55a8828 TC |
858 | |
859 | Since you can assign to a list of variables, you can also assign to | |
860 | an array or hash slice. | |
861 | ||
84f709e7 | 862 | @days[3..5] = qw/Wed Thu Fri/; |
d55a8828 | 863 | @colors{'red','blue','green'} |
7e3b091d | 864 | = (0xff0000, 0x0000ff, 0x00ff00); |
d55a8828 TC |
865 | @folks[0, -1] = @folks[-1, 0]; |
866 | ||
867 | The previous assignments are exactly equivalent to | |
868 | ||
84f709e7 JH |
869 | ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/; |
870 | ($colors{'red'}, $colors{'blue'}, $colors{'green'}) | |
7e3b091d | 871 | = (0xff0000, 0x0000ff, 0x00ff00); |
88fd19e3 | 872 | ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]); |
d55a8828 TC |
873 | |
874 | Since changing a slice changes the original array or hash that it's | |
56d7751a GS |
875 | slicing, a C<foreach> construct will alter some--or even all--of the |
876 | values of the array or hash. | |
d55a8828 TC |
877 | |
878 | foreach (@array[ 4 .. 10 ]) { s/peter/paul/ } | |
879 | ||
00cb5da1 | 880 | foreach (@hash{qw[key1 key2]}) { |
7e3b091d DA |
881 | s/^\s+//; # trim leading whitespace |
882 | s/\s+$//; # trim trailing whitespace | |
883 | s/(\w+)/\u\L$1/g; # "titlecase" words | |
d55a8828 TC |
884 | } |
885 | ||
08cd8952 GS |
886 | A slice of an empty list is still an empty list. Thus: |
887 | ||
84f709e7 JH |
888 | @a = ()[1,0]; # @a has no elements |
889 | @b = (@a)[0,1]; # @b has no elements | |
56d7751a GS |
890 | |
891 | But: | |
892 | ||
84f709e7 JH |
893 | @a = (1)[1,0]; # @a has two elements |
894 | @b = (1,undef)[1,0,2]; # @b has three elements | |
08cd8952 | 895 | |
f51152ef AP |
896 | More generally, a slice yields the empty list if it indexes only |
897 | beyond the end of a list: | |
898 | ||
899 | @a = (1)[ 1,2]; # @a has no elements | |
900 | @b = (1)[0,1,2]; # @b has three elements | |
901 | ||
19799a22 GS |
902 | This makes it easy to write loops that terminate when a null list |
903 | is returned: | |
d55a8828 | 904 | |
84f709e7 | 905 | while ( ($home, $user) = (getpwent)[7,0]) { |
7e3b091d | 906 | printf "%-8s %s\n", $user, $home; |
d55a8828 TC |
907 | } |
908 | ||
909 | As noted earlier in this document, the scalar sense of list assignment | |
910 | is the number of elements on the right-hand side of the assignment. | |
19799a22 | 911 | The null list contains no elements, so when the password file is |
d55a8828 TC |
912 | exhausted, the result is 0, not 2. |
913 | ||
ad1de9c6 ML |
914 | Slices in scalar context return the last item of the slice. |
915 | ||
916 | @a = qw/first second third/; | |
917 | %h = (first => 'A', second => 'B'); | |
918 | $t = @a[0, 1]; # $t is now 'second' | |
0de10106 | 919 | $u = @h{'first', 'second'}; # $u is now 'B' |
ad1de9c6 | 920 | |
d55a8828 TC |
921 | If you're confused about why you use an '@' there on a hash slice |
922 | instead of a '%', think of it like this. The type of bracket (square | |
923 | or curly) governs whether it's an array or a hash being looked at. | |
924 | On the other hand, the leading symbol ('$' or '@') on the array or | |
925 | hash indicates whether you are getting back a singular value (a | |
926 | scalar) or a plural one (a list). | |
927 | ||
8a7ab7dc | 928 | =head3 Key/Value Hash Slices |
23a22365 | 929 | |
c44d7536 FC |
930 | Starting in Perl 5.20, a hash slice operation |
931 | with the % symbol is a variant of slice operation | |
190c3990 | 932 | returning a list of key/value pairs rather than just values: |
23a22365 | 933 | |
190c3990 FC |
934 | %h = (blonk => 2, foo => 3, squink => 5, bar => 8); |
935 | %subset = %h{'foo', 'bar'}; # key/value hash slice | |
936 | # %subset is now (foo => 3, bar => 8) | |
23a22365 | 937 | |
190c3990 FC |
938 | However, the result of such a slice cannot be localized, deleted or used |
939 | in assignment. These are otherwise very much consistent with hash slices | |
940 | using the @ symbol. | |
23a22365 | 941 | |
8a7ab7dc | 942 | =head3 Index/Value Array Slices |
23a22365 | 943 | |
c44d7536 FC |
944 | Similar to key/value hash slices (and also introduced |
945 | in Perl 5.20), the % array slice syntax returns a list | |
190c3990 | 946 | of index/value pairs: |
23a22365 | 947 | |
190c3990 FC |
948 | @a = "a".."z"; |
949 | @list = %a[3,4,6]; | |
950 | # @list is now (3, "d", 4, "e", 6, "g") | |
23a22365 | 951 | |
5f05dabc | 952 | =head2 Typeglobs and Filehandles |
d74e8afc | 953 | X<typeglob> X<filehandle> X<*> |
cb1a09d0 AD |
954 | |
955 | Perl uses an internal type called a I<typeglob> to hold an entire | |
956 | symbol table entry. The type prefix of a typeglob is a C<*>, because | |
54310121 | 957 | it represents all types. This used to be the preferred way to |
cb1a09d0 | 958 | pass arrays and hashes by reference into a function, but now that |
5a964f20 TC |
959 | we have real references, this is seldom needed. |
960 | ||
961 | The main use of typeglobs in modern Perl is create symbol table aliases. | |
962 | This assignment: | |
963 | ||
964 | *this = *that; | |
965 | ||
966 | makes $this an alias for $that, @this an alias for @that, %this an alias | |
967 | for %that, &this an alias for &that, etc. Much safer is to use a reference. | |
968 | This: | |
5f05dabc | 969 | |
5a964f20 TC |
970 | local *Here::blue = \$There::green; |
971 | ||
972 | temporarily makes $Here::blue an alias for $There::green, but doesn't | |
973 | make @Here::blue an alias for @There::green, or %Here::blue an alias for | |
974 | %There::green, etc. See L<perlmod/"Symbol Tables"> for more examples | |
975 | of this. Strange though this may seem, this is the basis for the whole | |
84f709e7 | 976 | module import/export system. |
5a964f20 | 977 | |
d55a8828 | 978 | Another use for typeglobs is to pass filehandles into a function or |
5a964f20 TC |
979 | to create new filehandles. If you need to use a typeglob to save away |
980 | a filehandle, do it this way: | |
5f05dabc | 981 | |
84f709e7 | 982 | $fh = *STDOUT; |
5f05dabc | 983 | |
984 | or perhaps as a real reference, like this: | |
985 | ||
84f709e7 | 986 | $fh = \*STDOUT; |
5f05dabc | 987 | |
5a964f20 TC |
988 | See L<perlsub> for examples of using these as indirect filehandles |
989 | in functions. | |
990 | ||
991 | Typeglobs are also a way to create a local filehandle using the local() | |
992 | operator. These last until their block is exited, but may be passed back. | |
993 | For example: | |
5f05dabc | 994 | |
995 | sub newopen { | |
7e3b091d DA |
996 | my $path = shift; |
997 | local *FH; # not my! | |
998 | open (FH, $path) or return undef; | |
999 | return *FH; | |
5f05dabc | 1000 | } |
84f709e7 | 1001 | $fh = newopen('/etc/passwd'); |
5f05dabc | 1002 | |
d55a8828 | 1003 | Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much |
5a964f20 | 1004 | for filehandle manipulations, although they're still needed to pass brand |
8fdd8881 | 1005 | new file and directory handles into or out of functions. That's because |
d55a8828 TC |
1006 | C<*HANDLE{IO}> only works if HANDLE has already been used as a handle. |
1007 | In other words, C<*FH> must be used to create new symbol table entries; | |
1008 | C<*foo{THING}> cannot. When in doubt, use C<*FH>. | |
1009 | ||
36392fcf GS |
1010 | All functions that are capable of creating filehandles (open(), |
1011 | opendir(), pipe(), socketpair(), sysopen(), socket(), and accept()) | |
1012 | automatically create an anonymous filehandle if the handle passed to | |
8fdd8881 | 1013 | them is an uninitialized scalar variable. This allows the constructs |
36392fcf GS |
1014 | such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to |
1015 | create filehandles that will conveniently be closed automatically when | |
8fdd8881 | 1016 | the scope ends, provided there are no other references to them. This |
36392fcf GS |
1017 | largely eliminates the need for typeglobs when opening filehandles |
1018 | that must be passed around, as in the following example: | |
1019 | ||
1020 | sub myopen { | |
84f709e7 | 1021 | open my $fh, "@_" |
7e3b091d DA |
1022 | or die "Can't open '@_': $!"; |
1023 | return $fh; | |
36392fcf GS |
1024 | } |
1025 | ||
1026 | { | |
1027 | my $f = myopen("</etc/motd"); | |
7e3b091d DA |
1028 | print <$f>; |
1029 | # $f implicitly closed here | |
36392fcf GS |
1030 | } |
1031 | ||
b92795fe AMS |
1032 | Note that if an initialized scalar variable is used instead the |
1033 | result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent | |
1034 | to C<open( *{'zzz'}, ...)>. | |
d83fe814 AT |
1035 | C<use strict 'refs'> forbids such practice. |
1036 | ||
d55a8828 TC |
1037 | Another way to create anonymous filehandles is with the Symbol |
1038 | module or with the IO::Handle module and its ilk. These modules | |
1039 | have the advantage of not hiding different types of the same name | |
66b6e4ad KW |
1040 | during the local(). See the bottom of L<perlfunc/open> for an |
1041 | example. | |
d55a8828 TC |
1042 | |
1043 | =head1 SEE ALSO | |
1044 | ||
1045 | See L<perlvar> for a description of Perl's built-in variables and | |
1046 | a discussion of legal variable names. See L<perlref>, L<perlsub>, | |
1047 | and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and | |
1048 | the C<*foo{THING}> syntax. |