Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<syntax> |
a0d0e21e LW |
3 | |
4 | perlsyn - Perl syntax | |
5 | ||
6 | =head1 DESCRIPTION | |
7 | ||
6014d0cb MS |
8 | A Perl program consists of a sequence of declarations and statements |
9 | which run from the top to the bottom. Loops, subroutines and other | |
10 | control structures allow you to jump around within the code. | |
11 | ||
12 | Perl is a B<free-form> language, you can format and indent it however | |
13 | you like. Whitespace mostly serves to separate tokens, unlike | |
14 | languages like Python where it is an important part of the syntax. | |
15 | ||
16 | Many of Perl's syntactic elements are B<optional>. Rather than | |
110b9c83 | 17 | requiring you to put parentheses around every function call and |
6014d0cb MS |
18 | declare every variable, you can often leave such explicit elements off |
19 | and Perl will figure out what you meant. This is known as B<Do What I | |
20 | Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to | |
110b9c83 | 21 | code in a style with which they are comfortable. |
6014d0cb MS |
22 | |
23 | Perl B<borrows syntax> and concepts from many languages: awk, sed, C, | |
24 | Bourne Shell, Smalltalk, Lisp and even English. Other | |
25 | languages have borrowed syntax from Perl, particularly its regular | |
26 | expression extensions. So if you have programmed in another language | |
27 | you will see familiar pieces in Perl. They often work the same, but | |
28 | see L<perltrap> for information about how they differ. | |
a0d0e21e | 29 | |
0b8d69e9 | 30 | =head2 Declarations |
d74e8afc | 31 | X<declaration> X<undef> X<undefined> X<uninitialized> |
0b8d69e9 | 32 | |
cf48932e SF |
33 | The only things you need to declare in Perl are report formats and |
34 | subroutines (and sometimes not even subroutines). A variable holds | |
35 | the undefined value (C<undef>) until it has been assigned a defined | |
36 | value, which is anything other than C<undef>. When used as a number, | |
37 | C<undef> is treated as C<0>; when used as a string, it is treated as | |
38 | the empty string, C<"">; and when used as a reference that isn't being | |
39 | assigned to, it is treated as an error. If you enable warnings, | |
40 | you'll be notified of an uninitialized value whenever you treat | |
41 | C<undef> as a string or a number. Well, usually. Boolean contexts, | |
42 | such as: | |
7bd1983c EM |
43 | |
44 | my $a; | |
45 | if ($a) {} | |
46 | ||
a6b1f6d8 RGS |
47 | are exempt from warnings (because they care about truth rather than |
48 | definedness). Operators such as C<++>, C<-->, C<+=>, | |
7bd1983c EM |
49 | C<-=>, and C<.=>, that operate on undefined left values such as: |
50 | ||
51 | my $a; | |
52 | $a++; | |
53 | ||
54 | are also always exempt from such warnings. | |
0b8d69e9 | 55 | |
a0d0e21e LW |
56 | A declaration can be put anywhere a statement can, but has no effect on |
57 | the execution of the primary sequence of statements--declarations all | |
58 | take effect at compile time. Typically all the declarations are put at | |
54310121 | 59 | the beginning or the end of the script. However, if you're using |
0b8d69e9 GS |
60 | lexically-scoped private variables created with C<my()>, you'll |
61 | have to make sure | |
4633a7c4 | 62 | your format or subroutine definition is within the same block scope |
5f05dabc | 63 | as the my if you expect to be able to access those private variables. |
a0d0e21e | 64 | |
4633a7c4 LW |
65 | Declaring a subroutine allows a subroutine name to be used as if it were a |
66 | list operator from that point forward in the program. You can declare a | |
54310121 | 67 | subroutine without defining it by saying C<sub name>, thus: |
d74e8afc | 68 | X<subroutine, declaration> |
a0d0e21e | 69 | |
54310121 | 70 | sub myname; |
a0d0e21e LW |
71 | $me = myname $0 or die "can't get myname"; |
72 | ||
1f950eb4 JB |
73 | Note that myname() functions as a list operator, not as a unary operator; |
74 | so be careful to use C<or> instead of C<||> in this case. However, if | |
54310121 | 75 | you were to declare the subroutine as C<sub myname ($)>, then |
02c45c47 | 76 | C<myname> would function as a unary operator, so either C<or> or |
54310121 | 77 | C<||> would work. |
a0d0e21e | 78 | |
4633a7c4 LW |
79 | Subroutines declarations can also be loaded up with the C<require> statement |
80 | or both loaded and imported into your namespace with a C<use> statement. | |
81 | See L<perlmod> for details on this. | |
a0d0e21e | 82 | |
4633a7c4 LW |
83 | A statement sequence may contain declarations of lexically-scoped |
84 | variables, but apart from declaring a variable name, the declaration acts | |
85 | like an ordinary statement, and is elaborated within the sequence of | |
86 | statements as if it were an ordinary statement. That means it actually | |
87 | has both compile-time and run-time effects. | |
a0d0e21e | 88 | |
6014d0cb | 89 | =head2 Comments |
d74e8afc | 90 | X<comment> X<#> |
6014d0cb MS |
91 | |
92 | Text from a C<"#"> character until the end of the line is a comment, | |
93 | and is ignored. Exceptions include C<"#"> inside a string or regular | |
94 | expression. | |
95 | ||
6ec4bd10 | 96 | =head2 Simple Statements |
d74e8afc | 97 | X<statement> X<semicolon> X<expression> X<;> |
a0d0e21e LW |
98 | |
99 | The only kind of simple statement is an expression evaluated for its | |
100 | side effects. Every simple statement must be terminated with a | |
101 | semicolon, unless it is the final statement in a block, in which case | |
f386e492 AMS |
102 | the semicolon is optional. (A semicolon is still encouraged if the |
103 | block takes up more than one line, because you may eventually add | |
cf48932e SF |
104 | another line.) Note that there are some operators like C<eval {}> and |
105 | C<do {}> that look like compound statements, but aren't (they're just | |
106 | TERMs in an expression), and thus need an explicit termination if used | |
107 | as the last item in a statement. | |
108 | ||
109 | =head2 Truth and Falsehood | |
d74e8afc | 110 | X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0> |
cf48932e | 111 | |
f92061c1 AMS |
112 | The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and |
113 | C<undef> are all false in a boolean context. All other values are true. | |
52ea55c9 SP |
114 | Negation of a true value by C<!> or C<not> returns a special false value. |
115 | When evaluated as a string it is treated as C<''>, but as a number, it | |
116 | is treated as 0. | |
cf48932e | 117 | |
cf48932e | 118 | =head2 Statement Modifiers |
d74e8afc | 119 | X<statement modifier> X<modifier> X<if> X<unless> X<while> |
4f8ea571 | 120 | X<until> X<when> X<foreach> X<for> |
a0d0e21e LW |
121 | |
122 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
123 | just before the terminating semicolon (or block ending). The possible | |
124 | modifiers are: | |
125 | ||
126 | if EXPR | |
127 | unless EXPR | |
128 | while EXPR | |
129 | until EXPR | |
4f8ea571 VP |
130 | when EXPR |
131 | for LIST | |
cf48932e SF |
132 | foreach LIST |
133 | ||
134 | The C<EXPR> following the modifier is referred to as the "condition". | |
135 | Its truth or falsehood determines how the modifier will behave. | |
136 | ||
137 | C<if> executes the statement once I<if> and only if the condition is | |
138 | true. C<unless> is the opposite, it executes the statement I<unless> | |
139 | the condition is true (i.e., if the condition is false). | |
140 | ||
141 | print "Basset hounds got long ears" if length $ear >= 10; | |
142 | go_outside() and play() unless $is_raining; | |
143 | ||
4f8ea571 VP |
144 | C<when> executes the statement I<when> C<$_> smart matches C<EXPR>, and |
145 | then either C<break>s out if it's enclosed in a C<given> scope or skips | |
146 | to the C<next> element when it lies directly inside a C<for> loop. | |
147 | See also L</"Switch statements">. | |
148 | ||
149 | given ($something) { | |
150 | $abc = 1 when /^abc/; | |
151 | $just_a = 1 when /^a/; | |
152 | $other = 1; | |
153 | } | |
154 | ||
155 | for (@names) { | |
156 | admin($_) when [ qw/Alice Bob/ ]; | |
157 | regular($_) when [ qw/Chris David Ellen/ ]; | |
158 | } | |
159 | ||
cf48932e SF |
160 | The C<foreach> modifier is an iterator: it executes the statement once |
161 | for each item in the LIST (with C<$_> aliased to each item in turn). | |
162 | ||
163 | print "Hello $_!\n" foreach qw(world Dolly nurse); | |
164 | ||
165 | C<while> repeats the statement I<while> the condition is true. | |
166 | C<until> does the opposite, it repeats the statement I<until> the | |
167 | condition is true (or while the condition is false): | |
168 | ||
169 | # Both of these count from 0 to 10. | |
170 | print $i++ while $i <= 10; | |
171 | print $j++ until $j > 10; | |
172 | ||
173 | The C<while> and C<until> modifiers have the usual "C<while> loop" | |
174 | semantics (conditional evaluated first), except when applied to a | |
175 | C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in | |
176 | which case the block executes once before the conditional is | |
177 | evaluated. This is so that you can write loops like: | |
a0d0e21e LW |
178 | |
179 | do { | |
4633a7c4 | 180 | $line = <STDIN>; |
a0d0e21e | 181 | ... |
4633a7c4 | 182 | } until $line eq ".\n"; |
a0d0e21e | 183 | |
5a964f20 TC |
184 | See L<perlfunc/do>. Note also that the loop control statements described |
185 | later will I<NOT> work in this construct, because modifiers don't take | |
186 | loop labels. Sorry. You can always put another block inside of it | |
187 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
f86cebdf | 188 | For C<next>, just double the braces: |
d74e8afc | 189 | X<next> X<last> X<redo> |
5a964f20 TC |
190 | |
191 | do {{ | |
192 | next if $x == $y; | |
193 | # do something here | |
194 | }} until $x++ > $z; | |
195 | ||
f86cebdf | 196 | For C<last>, you have to be more elaborate: |
d74e8afc | 197 | X<last> |
5a964f20 TC |
198 | |
199 | LOOP: { | |
200 | do { | |
201 | last if $x = $y**2; | |
202 | # do something here | |
203 | } while $x++ <= $z; | |
204 | } | |
a0d0e21e | 205 | |
457b36cb MV |
206 | B<NOTE:> The behaviour of a C<my> statement modified with a statement |
207 | modifier conditional or loop construct (e.g. C<my $x if ...>) is | |
208 | B<undefined>. The value of the C<my> variable may be C<undef>, any | |
209 | previously assigned value, or possibly anything else. Don't rely on | |
210 | it. Future versions of perl might do something different from the | |
211 | version of perl you try it out on. Here be dragons. | |
d74e8afc | 212 | X<my> |
457b36cb | 213 | |
6ec4bd10 | 214 | =head2 Compound Statements |
d74e8afc ITB |
215 | X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace> |
216 | X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue> | |
a0d0e21e LW |
217 | |
218 | In Perl, a sequence of statements that defines a scope is called a block. | |
219 | Sometimes a block is delimited by the file containing it (in the case | |
220 | of a required file, or the program as a whole), and sometimes a block | |
221 | is delimited by the extent of a string (in the case of an eval). | |
222 | ||
223 | But generally, a block is delimited by curly brackets, also known as braces. | |
224 | We will call this syntactic construct a BLOCK. | |
225 | ||
226 | The following compound statements may be used to control flow: | |
227 | ||
228 | if (EXPR) BLOCK | |
229 | if (EXPR) BLOCK else BLOCK | |
230 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK | |
231 | LABEL while (EXPR) BLOCK | |
232 | LABEL while (EXPR) BLOCK continue BLOCK | |
5ec6d87f EA |
233 | LABEL until (EXPR) BLOCK |
234 | LABEL until (EXPR) BLOCK continue BLOCK | |
a0d0e21e | 235 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
748a9306 | 236 | LABEL foreach VAR (LIST) BLOCK |
b303ae78 | 237 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
a0d0e21e LW |
238 | LABEL BLOCK continue BLOCK |
239 | ||
240 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, | |
241 | not statements. This means that the curly brackets are I<required>--no | |
242 | dangling statements allowed. If you want to write conditionals without | |
243 | curly brackets there are several other ways to do it. The following | |
244 | all do the same thing: | |
245 | ||
246 | if (!open(FOO)) { die "Can't open $FOO: $!"; } | |
247 | die "Can't open $FOO: $!" unless open(FOO); | |
248 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! | |
249 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; | |
250 | # a bit exotic, that last one | |
251 | ||
5f05dabc | 252 | The C<if> statement is straightforward. Because BLOCKs are always |
a0d0e21e LW |
253 | bounded by curly brackets, there is never any ambiguity about which |
254 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
255 | the sense of the test is reversed. | |
256 | ||
257 | The C<while> statement executes the block as long as the expression is | |
e17b7802 | 258 | L<true|/"Truth and Falsehood">. |
1d5653dd RGS |
259 | The C<until> statement executes the block as long as the expression is |
260 | false. | |
b78218b7 GS |
261 | The LABEL is optional, and if present, consists of an identifier followed |
262 | by a colon. The LABEL identifies the loop for the loop control | |
263 | statements C<next>, C<last>, and C<redo>. | |
264 | If the LABEL is omitted, the loop control statement | |
4633a7c4 LW |
265 | refers to the innermost enclosing loop. This may include dynamically |
266 | looking back your call-stack at run time to find the LABEL. Such | |
9f1b1f2d | 267 | desperate behavior triggers a warning if you use the C<use warnings> |
a2293a43 | 268 | pragma or the B<-w> flag. |
4633a7c4 LW |
269 | |
270 | If there is a C<continue> BLOCK, it is always executed just before the | |
6ec4bd10 MS |
271 | conditional is about to be evaluated again. Thus it can be used to |
272 | increment a loop variable, even when the loop has been continued via | |
273 | the C<next> statement. | |
4633a7c4 LW |
274 | |
275 | =head2 Loop Control | |
d74e8afc | 276 | X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue> |
4633a7c4 | 277 | |
6ec4bd10 | 278 | The C<next> command starts the next iteration of the loop: |
4633a7c4 LW |
279 | |
280 | LINE: while (<STDIN>) { | |
281 | next LINE if /^#/; # discard comments | |
282 | ... | |
283 | } | |
284 | ||
6ec4bd10 | 285 | The C<last> command immediately exits the loop in question. The |
4633a7c4 LW |
286 | C<continue> block, if any, is not executed: |
287 | ||
288 | LINE: while (<STDIN>) { | |
289 | last LINE if /^$/; # exit when done with header | |
290 | ... | |
291 | } | |
292 | ||
293 | The C<redo> command restarts the loop block without evaluating the | |
294 | conditional again. The C<continue> block, if any, is I<not> executed. | |
295 | This command is normally used by programs that want to lie to themselves | |
296 | about what was just input. | |
297 | ||
298 | For example, when processing a file like F</etc/termcap>. | |
299 | If your input lines might end in backslashes to indicate continuation, you | |
300 | want to skip ahead and get the next record. | |
301 | ||
302 | while (<>) { | |
303 | chomp; | |
54310121 | 304 | if (s/\\$//) { |
305 | $_ .= <>; | |
4633a7c4 LW |
306 | redo unless eof(); |
307 | } | |
308 | # now process $_ | |
54310121 | 309 | } |
4633a7c4 LW |
310 | |
311 | which is Perl short-hand for the more explicitly written version: | |
312 | ||
54310121 | 313 | LINE: while (defined($line = <ARGV>)) { |
4633a7c4 | 314 | chomp($line); |
54310121 | 315 | if ($line =~ s/\\$//) { |
316 | $line .= <ARGV>; | |
4633a7c4 LW |
317 | redo LINE unless eof(); # not eof(ARGV)! |
318 | } | |
319 | # now process $line | |
54310121 | 320 | } |
4633a7c4 | 321 | |
36e7a065 AMS |
322 | Note that if there were a C<continue> block on the above code, it would |
323 | get executed only on lines discarded by the regex (since redo skips the | |
324 | continue block). A continue block is often used to reset line counters | |
325 | or C<?pat?> one-time matches: | |
4633a7c4 | 326 | |
5a964f20 TC |
327 | # inspired by :1,$g/fred/s//WILMA/ |
328 | while (<>) { | |
329 | ?(fred)? && s//WILMA $1 WILMA/; | |
330 | ?(barney)? && s//BETTY $1 BETTY/; | |
331 | ?(homer)? && s//MARGE $1 MARGE/; | |
332 | } continue { | |
333 | print "$ARGV $.: $_"; | |
334 | close ARGV if eof(); # reset $. | |
335 | reset if eof(); # reset ?pat? | |
4633a7c4 LW |
336 | } |
337 | ||
a0d0e21e LW |
338 | If the word C<while> is replaced by the word C<until>, the sense of the |
339 | test is reversed, but the conditional is still tested before the first | |
340 | iteration. | |
341 | ||
5a964f20 TC |
342 | The loop control statements don't work in an C<if> or C<unless>, since |
343 | they aren't loops. You can double the braces to make them such, though. | |
344 | ||
345 | if (/pattern/) {{ | |
7bd1983c EM |
346 | last if /fred/; |
347 | next if /barney/; # same effect as "last", but doesn't document as well | |
348 | # do something here | |
5a964f20 TC |
349 | }} |
350 | ||
7bd1983c | 351 | This is caused by the fact that a block by itself acts as a loop that |
27cec4bd | 352 | executes once, see L<"Basic BLOCKs">. |
7bd1983c | 353 | |
5b23ba8b MG |
354 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
355 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
4633a7c4 | 356 | |
cb1a09d0 | 357 | =head2 For Loops |
d74e8afc | 358 | X<for> X<foreach> |
a0d0e21e | 359 | |
b78df5de | 360 | Perl's C-style C<for> loop works like the corresponding C<while> loop; |
cb1a09d0 | 361 | that means that this: |
a0d0e21e LW |
362 | |
363 | for ($i = 1; $i < 10; $i++) { | |
364 | ... | |
365 | } | |
366 | ||
cb1a09d0 | 367 | is the same as this: |
a0d0e21e LW |
368 | |
369 | $i = 1; | |
370 | while ($i < 10) { | |
371 | ... | |
372 | } continue { | |
373 | $i++; | |
374 | } | |
375 | ||
b78df5de JA |
376 | There is one minor difference: if variables are declared with C<my> |
377 | in the initialization section of the C<for>, the lexical scope of | |
378 | those variables is exactly the C<for> loop (the body of the loop | |
379 | and the control sections). | |
d74e8afc | 380 | X<my> |
55497cff | 381 | |
cb1a09d0 AD |
382 | Besides the normal array index looping, C<for> can lend itself |
383 | to many other interesting applications. Here's one that avoids the | |
54310121 | 384 | problem you get into if you explicitly test for end-of-file on |
385 | an interactive file descriptor causing your program to appear to | |
cb1a09d0 | 386 | hang. |
d74e8afc | 387 | X<eof> X<end-of-file> X<end of file> |
cb1a09d0 AD |
388 | |
389 | $on_a_tty = -t STDIN && -t STDOUT; | |
390 | sub prompt { print "yes? " if $on_a_tty } | |
391 | for ( prompt(); <STDIN>; prompt() ) { | |
392 | # do something | |
54310121 | 393 | } |
cb1a09d0 | 394 | |
00cb5da1 CW |
395 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the |
396 | conditional of a C<for> loop is shorthand for the following. This | |
397 | behaviour is the same as a C<while> loop conditional. | |
d74e8afc | 398 | X<readline> X<< <> >> |
00cb5da1 CW |
399 | |
400 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { | |
401 | # do something | |
402 | } | |
403 | ||
cb1a09d0 | 404 | =head2 Foreach Loops |
d74e8afc | 405 | X<for> X<foreach> |
cb1a09d0 | 406 | |
4633a7c4 | 407 | The C<foreach> loop iterates over a normal list value and sets the |
55497cff | 408 | variable VAR to be each element of the list in turn. If the variable |
409 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
410 | is therefore visible only within the loop. Otherwise, the variable is | |
411 | implicitly local to the loop and regains its former value upon exiting | |
412 | the loop. If the variable was previously declared with C<my>, it uses | |
413 | that variable instead of the global one, but it's still localized to | |
5c502d37 MV |
414 | the loop. This implicit localisation occurs I<only> in a C<foreach> |
415 | loop. | |
d74e8afc | 416 | X<my> X<local> |
4633a7c4 LW |
417 | |
418 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
5a964f20 TC |
419 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
420 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> | |
f86cebdf | 421 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
d74e8afc | 422 | X<$_> |
c5674021 |
423 | |
424 | If any element of LIST is an lvalue, you can modify it by modifying | |
425 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
426 | lvalue, any attempt to modify that element will fail. In other words, | |
427 | the C<foreach> loop index variable is an implicit alias for each item | |
428 | in the list that you're looping over. | |
d74e8afc | 429 | X<alias> |
302617ea MG |
430 | |
431 | If any part of LIST is an array, C<foreach> will get very confused if | |
432 | you add or remove elements within the loop body, for example with | |
433 | C<splice>. So don't do that. | |
d74e8afc | 434 | X<splice> |
302617ea MG |
435 | |
436 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
437 | special variable. Don't do that either. | |
4633a7c4 | 438 | |
748a9306 | 439 | Examples: |
a0d0e21e | 440 | |
4633a7c4 | 441 | for (@ary) { s/foo/bar/ } |
a0d0e21e | 442 | |
96f2dc66 | 443 | for my $elem (@elements) { |
a0d0e21e LW |
444 | $elem *= 2; |
445 | } | |
446 | ||
4633a7c4 LW |
447 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
448 | print $count, "\n"; sleep(1); | |
a0d0e21e LW |
449 | } |
450 | ||
451 | for (1..15) { print "Merry Christmas\n"; } | |
452 | ||
4633a7c4 | 453 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
a0d0e21e LW |
454 | print "Item: $item\n"; |
455 | } | |
456 | ||
4633a7c4 LW |
457 | Here's how a C programmer might code up a particular algorithm in Perl: |
458 | ||
55497cff | 459 | for (my $i = 0; $i < @ary1; $i++) { |
460 | for (my $j = 0; $j < @ary2; $j++) { | |
4633a7c4 LW |
461 | if ($ary1[$i] > $ary2[$j]) { |
462 | last; # can't go to outer :-( | |
463 | } | |
464 | $ary1[$i] += $ary2[$j]; | |
465 | } | |
cb1a09d0 | 466 | # this is where that last takes me |
4633a7c4 LW |
467 | } |
468 | ||
184e9718 | 469 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 | 470 | do it: |
4633a7c4 | 471 | |
96f2dc66 GS |
472 | OUTER: for my $wid (@ary1) { |
473 | INNER: for my $jet (@ary2) { | |
cb1a09d0 AD |
474 | next OUTER if $wid > $jet; |
475 | $wid += $jet; | |
54310121 | 476 | } |
477 | } | |
4633a7c4 | 478 | |
cb1a09d0 AD |
479 | See how much easier this is? It's cleaner, safer, and faster. It's |
480 | cleaner because it's less noisy. It's safer because if code gets added | |
c07a80fd | 481 | between the inner and outer loops later on, the new code won't be |
5f05dabc | 482 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd | 483 | rather than merely terminating the inner one. And it's faster because |
484 | Perl executes a C<foreach> statement more rapidly than it would the | |
485 | equivalent C<for> loop. | |
4633a7c4 | 486 | |
0d863452 RH |
487 | =head2 Basic BLOCKs |
488 | X<block> | |
4633a7c4 | 489 | |
55497cff | 490 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
491 | loop that executes once. Thus you can use any of the loop control | |
492 | statements in it to leave or restart the block. (Note that this is | |
493 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
494 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
495 | block is optional. | |
4633a7c4 | 496 | |
27cec4bd | 497 | The BLOCK construct can be used to emulate case structures. |
a0d0e21e LW |
498 | |
499 | SWITCH: { | |
500 | if (/^abc/) { $abc = 1; last SWITCH; } | |
501 | if (/^def/) { $def = 1; last SWITCH; } | |
502 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
503 | $nothing = 1; | |
504 | } | |
505 | ||
0d863452 RH |
506 | Such constructs are quite frequently used, because older versions |
507 | of Perl had no official C<switch> statement. | |
83df6a1d | 508 | |
0d863452 RH |
509 | =head2 Switch statements |
510 | X<switch> X<case> X<given> X<when> X<default> | |
83df6a1d | 511 | |
27cec4bd | 512 | Starting from Perl 5.10, you can say |
83df6a1d | 513 | |
27cec4bd | 514 | use feature "switch"; |
a0d0e21e | 515 | |
0d863452 RH |
516 | which enables a switch feature that is closely based on the |
517 | Perl 6 proposal. | |
518 | ||
519 | The keywords C<given> and C<when> are analogous | |
520 | to C<switch> and C<case> in other languages, so the code | |
521 | above could be written as | |
522 | ||
27cec4bd RGS |
523 | given($_) { |
524 | when (/^abc/) { $abc = 1; } | |
525 | when (/^def/) { $def = 1; } | |
526 | when (/^xyz/) { $xyz = 1; } | |
527 | default { $nothing = 1; } | |
a0d0e21e LW |
528 | } |
529 | ||
0d863452 | 530 | This construct is very flexible and powerful. For example: |
a0d0e21e | 531 | |
4b7b0ae4 RH |
532 | use feature ":5.10"; |
533 | given($foo) { | |
534 | when (undef) { | |
535 | say '$foo is undefined'; | |
536 | } | |
4b7b0ae4 RH |
537 | when ("foo") { |
538 | say '$foo is the string "foo"'; | |
539 | } | |
4b7b0ae4 RH |
540 | when ([1,3,5,7,9]) { |
541 | say '$foo is an odd digit'; | |
542 | continue; # Fall through | |
9f435386 | 543 | } |
4b7b0ae4 RH |
544 | when ($_ < 100) { |
545 | say '$foo is numerically less than 100'; | |
546 | } | |
4b7b0ae4 | 547 | when (\&complicated_check) { |
f92e1a16 | 548 | say 'a complicated check for $foo is true'; |
4b7b0ae4 | 549 | } |
4b7b0ae4 RH |
550 | default { |
551 | die q(I don't know what to do with $foo); | |
552 | } | |
553 | } | |
554 | ||
555 | C<given(EXPR)> will assign the value of EXPR to C<$_> | |
556 | within the lexical scope of the block, so it's similar to | |
557 | ||
558 | do { my $_ = EXPR; ... } | |
559 | ||
560 | except that the block is automatically broken out of by a | |
561 | successful C<when> or an explicit C<break>. | |
562 | ||
563 | Most of the power comes from implicit smart matching: | |
a0d0e21e | 564 | |
4b7b0ae4 | 565 | when($foo) |
a0d0e21e | 566 | |
0d863452 | 567 | is exactly equivalent to |
a0d0e21e | 568 | |
4b7b0ae4 | 569 | when($_ ~~ $foo) |
a0d0e21e | 570 | |
b3ed409d CS |
571 | Most of the time, C<when(EXPR)> is treated as an implicit smart match of |
572 | C<$_>, i.e. C<$_ ~~ EXPR>. (See L</"Smart matching in detail"> for more | |
573 | information on smart matching.) But when EXPR is one of the below | |
574 | exceptional cases, it is used directly as a boolean: | |
0d863452 RH |
575 | |
576 | =over 4 | |
577 | ||
d991eed6 | 578 | =item * |
0d863452 RH |
579 | |
580 | a subroutine or method call | |
581 | ||
d991eed6 | 582 | =item * |
0d863452 RH |
583 | |
584 | a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>, | |
f92e1a16 | 585 | or a negated regular expression match (C<!/REGEX/> or C<$foo !~ /REGEX/>). |
0d863452 | 586 | |
d991eed6 | 587 | =item * |
0d863452 | 588 | |
4b7b0ae4 RH |
589 | a comparison such as C<$_ E<lt> 10> or C<$x eq "abc"> |
590 | (or of course C<$_ ~~ $c>) | |
0d863452 | 591 | |
d991eed6 | 592 | =item * |
0d863452 RH |
593 | |
594 | C<defined(...)>, C<exists(...)>, or C<eof(...)> | |
595 | ||
d991eed6 | 596 | =item * |
4633a7c4 | 597 | |
f92e1a16 | 598 | a negated expression C<!(...)> or C<not (...)>, or a logical |
0d863452 | 599 | exclusive-or C<(...) xor (...)>. |
cb1a09d0 | 600 | |
516817b4 RGS |
601 | =item * |
602 | ||
603 | a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>, | |
604 | that return numerical values, not boolean ones. | |
605 | ||
202d7cbd RGS |
606 | =item * |
607 | ||
f118ea0d | 608 | the C<..> and C<...> flip-flop operators. |
202d7cbd | 609 | |
0d863452 RH |
610 | =back |
611 | ||
f92e1a16 RGS |
612 | In those cases the value of EXPR is used directly as a boolean. |
613 | ||
0d863452 RH |
614 | Furthermore: |
615 | ||
616 | =over 4 | |
617 | ||
f92e1a16 | 618 | =item * |
0d863452 RH |
619 | |
620 | If EXPR is C<... && ...> or C<... and ...>, the test | |
621 | is applied recursively to both arguments. If I<both> | |
622 | arguments pass the test, then the argument is treated | |
623 | as boolean. | |
624 | ||
f92e1a16 | 625 | =item * |
0d863452 | 626 | |
f92e1a16 | 627 | If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test |
0d863452 RH |
628 | is applied recursively to the first argument. |
629 | ||
630 | =back | |
631 | ||
632 | These rules look complicated, but usually they will do what | |
633 | you want. For example you could write: | |
634 | ||
f849b90f | 635 | when (/^\d+$/ && $_ < 75) { ... } |
0d863452 | 636 | |
4b7b0ae4 | 637 | Another useful shortcut is that, if you use a literal array |
107bd117 | 638 | or hash as the argument to C<given>, it is turned into a |
4b7b0ae4 RH |
639 | reference. So C<given(@foo)> is the same as C<given(\@foo)>, |
640 | for example. | |
641 | ||
0d863452 RH |
642 | C<default> behaves exactly like C<when(1 == 1)>, which is |
643 | to say that it always matches. | |
644 | ||
4b7b0ae4 RH |
645 | =head3 Breaking out |
646 | ||
647 | You can use the C<break> keyword to break out of the enclosing | |
648 | C<given> block. Every C<when> block is implicitly ended with | |
649 | a C<break>. | |
650 | ||
0d863452 RH |
651 | =head3 Fall-through |
652 | ||
653 | You can use the C<continue> keyword to fall through from one | |
654 | case to the next: | |
655 | ||
27cec4bd | 656 | given($foo) { |
4b7b0ae4 RH |
657 | when (/x/) { say '$foo contains an x'; continue } |
658 | when (/y/) { say '$foo contains a y' } | |
02e7afe2 | 659 | default { say '$foo does not contain a y' } |
27cec4bd | 660 | } |
0d863452 RH |
661 | |
662 | =head3 Switching in a loop | |
663 | ||
664 | Instead of using C<given()>, you can use a C<foreach()> loop. | |
665 | For example, here's one way to count how many times a particular | |
666 | string occurs in an array: | |
667 | ||
27cec4bd RGS |
668 | my $count = 0; |
669 | for (@array) { | |
670 | when ("foo") { ++$count } | |
5a964f20 | 671 | } |
27cec4bd | 672 | print "\@array contains $count copies of 'foo'\n"; |
0d863452 RH |
673 | |
674 | On exit from the C<when> block, there is an implicit C<next>. | |
675 | You can override that with an explicit C<last> if you're only | |
676 | interested in the first match. | |
677 | ||
678 | This doesn't work if you explicitly specify a loop variable, | |
679 | as in C<for $item (@array)>. You have to use the default | |
680 | variable C<$_>. (You can use C<for my $_ (@array)>.) | |
681 | ||
682 | =head3 Smart matching in detail | |
683 | ||
202d7cbd RGS |
684 | The behaviour of a smart match depends on what type of thing its arguments |
685 | are. The behaviour is determined by the following table: the first row | |
686 | that applies determines the match behaviour (which is thus mostly | |
687 | determined by the type of the right operand). Note that the smart match | |
d0b243e3 RGS |
688 | implicitly dereferences any non-blessed hash or array ref, so the "Hash" |
689 | and "Array" entries apply in those cases. (For blessed references, the | |
c6ebb512 | 690 | "Object" entries apply.) |
4b7b0ae4 | 691 | |
b3ed409d CS |
692 | Note that the "Matching Code" column is not always an exact rendition. For |
693 | example, the smart match operator short-circuits whenever possible, but | |
694 | C<grep> does not. | |
695 | ||
4b7b0ae4 RH |
696 | $a $b Type of Match Implied Matching Code |
697 | ====== ===== ===================== ============= | |
202d7cbd RGS |
698 | Any undef undefined !defined $a |
699 | ||
c6ebb512 | 700 | Any Object invokes ~~ overloading on $object, or dies |
4b7b0ae4 | 701 | |
168ff818 RGS |
702 | Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a |
703 | Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a | |
704 | Any CodeRef scalar sub truth $b->($a) | |
4b7b0ae4 | 705 | |
6f76d139 | 706 | Hash Hash hash keys identical (every key is found in both hashes) |
c3886e8b | 707 | Array Hash hash slice existence grep { exists $b->{$_} } @$a |
07edf497 | 708 | Regex Hash hash key grep grep /$a/, keys %$b |
202d7cbd RGS |
709 | undef Hash always false (undef can't be a key) |
710 | Any Hash hash entry existence exists $b->{$a} | |
711 | ||
c3886e8b | 712 | Hash Array hash slice existence grep { exists $a->{$_} } @$b |
168ff818 | 713 | Array Array arrays are comparable[2] |
c3886e8b RGS |
714 | Regex Array array grep grep /$a/, @$b |
715 | undef Array array contains undef grep !defined, @$b | |
168ff818 | 716 | Any Array match against an array element[3] |
c3886e8b | 717 | grep $a ~~ $_, @$b |
4b7b0ae4 | 718 | |
202d7cbd | 719 | Hash Regex hash key grep grep /$b/, keys %$a |
4b7b0ae4 | 720 | Array Regex array grep grep /$b/, @$a |
4b7b0ae4 | 721 | Any Regex pattern match $a =~ /$b/ |
202d7cbd | 722 | |
2c9d2554 | 723 | Object Any invokes ~~ overloading on $object, or falls back: |
4b7b0ae4 | 724 | Any Num numeric equality $a == $b |
f118ea0d | 725 | Num numish[4] numeric equality $a == $b |
fb51372e | 726 | undef Any undefined !defined($b) |
4b7b0ae4 RH |
727 | Any Any string equality $a eq $b |
728 | ||
07edf497 | 729 | 1 - empty hashes or arrays will match. |
329802ba RGS |
730 | 2 - that is, each element smart-matches the element of same index in the |
731 | other array. [3] | |
168ff818 | 732 | 3 - If a circular reference is found, we fall back to referential equality. |
f118ea0d | 733 | 4 - either a real number, or a string that looks like a number |
0d863452 | 734 | |
0d863452 | 735 | =head3 Custom matching via overloading |
5a964f20 | 736 | |
0d863452 | 737 | You can change the way that an object is matched by overloading |
0de1c906 | 738 | the C<~~> operator. This may alter the usual smart match semantics. |
5a964f20 | 739 | |
202d7cbd RGS |
740 | It should be noted that C<~~> will refuse to work on objects that |
741 | don't overload it (in order to avoid relying on the object's | |
2da5311b | 742 | underlying structure). |
202d7cbd | 743 | |
0de1c906 DM |
744 | Note also that smart match's matching rules take precedence over |
745 | overloading, so if C<$obj> has smart match overloading, then | |
746 | ||
747 | $obj ~~ X | |
748 | ||
749 | will not automatically invoke the overload method with X as an argument; | |
750 | instead the table above is consulted as normal, and based in the type of X, | |
751 | overloading may or may not be invoked. | |
752 | ||
753 | See L<overload>. | |
754 | ||
54a85b95 RH |
755 | =head3 Differences from Perl 6 |
756 | ||
757 | The Perl 5 smart match and C<given>/C<when> constructs are not | |
758 | absolutely identical to their Perl 6 analogues. The most visible | |
759 | difference is that, in Perl 5, parentheses are required around | |
4f8ea571 VP |
760 | the argument to C<given()> and C<when()> (except when this last |
761 | one is used as a statement modifier). Parentheses in Perl 6 | |
54a85b95 RH |
762 | are always optional in a control construct such as C<if()>, |
763 | C<while()>, or C<when()>; they can't be made optional in Perl | |
764 | 5 without a great deal of potential confusion, because Perl 5 | |
765 | would parse the expression | |
766 | ||
767 | given $foo { | |
768 | ... | |
769 | } | |
770 | ||
771 | as though the argument to C<given> were an element of the hash | |
772 | C<%foo>, interpreting the braces as hash-element syntax. | |
773 | ||
ccc668fa RGS |
774 | The table of smart matches is not identical to that proposed by the |
775 | Perl 6 specification, mainly due to the differences between Perl 6's | |
776 | and Perl 5's data models. | |
54a85b95 RH |
777 | |
778 | In Perl 6, C<when()> will always do an implicit smart match | |
779 | with its argument, whilst it is convenient in Perl 5 to | |
780 | suppress this implicit smart match in certain situations, | |
781 | as documented above. (The difference is largely because Perl 5 | |
782 | does not, even internally, have a boolean type.) | |
783 | ||
4633a7c4 | 784 | =head2 Goto |
d74e8afc | 785 | X<goto> |
4633a7c4 | 786 | |
19799a22 GS |
787 | Although not for the faint of heart, Perl does support a C<goto> |
788 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
789 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
790 | a C<goto>; it's just the name of the loop. | |
4633a7c4 | 791 | |
f86cebdf | 792 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 | 793 | execution there. It may not be used to go into any construct that |
f86cebdf | 794 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 LW |
795 | also can't be used to go into a construct that is optimized away. It |
796 | can be used to go almost anywhere else within the dynamic scope, | |
797 | including out of subroutines, but it's usually better to use some other | |
f86cebdf GS |
798 | construct such as C<last> or C<die>. The author of Perl has never felt the |
799 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
4633a7c4 | 800 | |
f86cebdf GS |
801 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
802 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
4633a7c4 LW |
803 | necessarily recommended if you're optimizing for maintainability: |
804 | ||
96f2dc66 | 805 | goto(("FOO", "BAR", "GLARCH")[$i]); |
4633a7c4 | 806 | |
f86cebdf | 807 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 | 808 | named subroutine for the currently running subroutine. This is used by |
f86cebdf | 809 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 | 810 | pretend that the other subroutine had been called in the first place |
f86cebdf GS |
811 | (except that any modifications to C<@_> in the current subroutine are |
812 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
4633a7c4 LW |
813 | will be able to tell that this routine was called first. |
814 | ||
c07a80fd | 815 | In almost all cases like this, it's usually a far, far better idea to use the |
816 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
4633a7c4 LW |
817 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
818 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
cb1a09d0 AD |
819 | |
820 | =head2 PODs: Embedded Documentation | |
d74e8afc | 821 | X<POD> X<documentation> |
cb1a09d0 AD |
822 | |
823 | Perl has a mechanism for intermixing documentation with source code. | |
c07a80fd | 824 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 AD |
825 | encounters a line that begins with an equal sign and a word, like this |
826 | ||
827 | =head1 Here There Be Pods! | |
828 | ||
829 | Then that text and all remaining text up through and including a line | |
830 | beginning with C<=cut> will be ignored. The format of the intervening | |
54310121 | 831 | text is described in L<perlpod>. |
cb1a09d0 AD |
832 | |
833 | This allows you to intermix your source code | |
834 | and your documentation text freely, as in | |
835 | ||
836 | =item snazzle($) | |
837 | ||
54310121 | 838 | The snazzle() function will behave in the most spectacular |
cb1a09d0 AD |
839 | form that you can possibly imagine, not even excepting |
840 | cybernetic pyrotechnics. | |
841 | ||
842 | =cut back to the compiler, nuff of this pod stuff! | |
843 | ||
844 | sub snazzle($) { | |
845 | my $thingie = shift; | |
846 | ......... | |
54310121 | 847 | } |
cb1a09d0 | 848 | |
54310121 | 849 | Note that pod translators should look at only paragraphs beginning |
184e9718 | 850 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 | 851 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 AD |
852 | paragraph. This means that the following secret stuff will be |
853 | ignored by both the compiler and the translators. | |
854 | ||
855 | $a=3; | |
856 | =secret stuff | |
857 | warn "Neither POD nor CODE!?" | |
858 | =cut back | |
859 | print "got $a\n"; | |
860 | ||
f86cebdf | 861 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 AD |
862 | Not all pod translators are well-behaved in this regard, and perhaps |
863 | the compiler will become pickier. | |
774d564b | 864 | |
865 | One may also use pod directives to quickly comment out a section | |
866 | of code. | |
867 | ||
868 | =head2 Plain Old Comments (Not!) | |
d74e8afc | 869 | X<comment> X<line> X<#> X<preprocessor> X<eval> |
774d564b | 870 | |
6ec4bd10 | 871 | Perl can process line directives, much like the C preprocessor. Using |
5a964f20 | 872 | this, one can control Perl's idea of filenames and line numbers in |
774d564b | 873 | error or warning messages (especially for strings that are processed |
f86cebdf | 874 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b | 875 | C preprocessors: it matches the regular expression |
6ec4bd10 MS |
876 | |
877 | # example: '# line 42 "new_filename.plx"' | |
82d4537c | 878 | /^\# \s* |
6ec4bd10 | 879 | line \s+ (\d+) \s* |
7b6e93a8 | 880 | (?:\s("?)([^"]+)\2)? \s* |
6ec4bd10 MS |
881 | $/x |
882 | ||
7b6e93a8 CW |
883 | with C<$1> being the line number for the next line, and C<$3> being |
884 | the optional filename (specified with or without quotes). | |
774d564b | 885 | |
003183f2 GS |
886 | There is a fairly obvious gotcha included with the line directive: |
887 | Debuggers and profilers will only show the last source line to appear | |
888 | at a particular line number in a given file. Care should be taken not | |
889 | to cause line number collisions in code you'd like to debug later. | |
890 | ||
774d564b | 891 | Here are some examples that you should be able to type into your command |
892 | shell: | |
893 | ||
894 | % perl | |
895 | # line 200 "bzzzt" | |
896 | # the `#' on the previous line must be the first char on line | |
897 | die 'foo'; | |
898 | __END__ | |
899 | foo at bzzzt line 201. | |
54310121 | 900 | |
774d564b | 901 | % perl |
902 | # line 200 "bzzzt" | |
903 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
904 | __END__ | |
905 | foo at - line 2001. | |
54310121 | 906 | |
774d564b | 907 | % perl |
908 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
909 | __END__ | |
910 | foo at foo bar line 200. | |
54310121 | 911 | |
774d564b | 912 | % perl |
913 | # line 345 "goop" | |
914 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
915 | print $@; | |
916 | __END__ | |
917 | foo at goop line 345. | |
918 | ||
919 | =cut |