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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlsyn - Perl syntax | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
6014d0cb MS |
7 | A Perl program consists of a sequence of declarations and statements |
8 | which run from the top to the bottom. Loops, subroutines and other | |
9 | control structures allow you to jump around within the code. | |
10 | ||
11 | Perl is a B<free-form> language, you can format and indent it however | |
12 | you like. Whitespace mostly serves to separate tokens, unlike | |
13 | languages like Python where it is an important part of the syntax. | |
14 | ||
15 | Many of Perl's syntactic elements are B<optional>. Rather than | |
110b9c83 | 16 | requiring you to put parentheses around every function call and |
6014d0cb MS |
17 | declare every variable, you can often leave such explicit elements off |
18 | and Perl will figure out what you meant. This is known as B<Do What I | |
19 | Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to | |
110b9c83 | 20 | code in a style with which they are comfortable. |
6014d0cb MS |
21 | |
22 | Perl B<borrows syntax> and concepts from many languages: awk, sed, C, | |
23 | Bourne Shell, Smalltalk, Lisp and even English. Other | |
24 | languages have borrowed syntax from Perl, particularly its regular | |
25 | expression extensions. So if you have programmed in another language | |
26 | you will see familiar pieces in Perl. They often work the same, but | |
27 | see L<perltrap> for information about how they differ. | |
a0d0e21e | 28 | |
0b8d69e9 GS |
29 | =head2 Declarations |
30 | ||
cf48932e SF |
31 | The only things you need to declare in Perl are report formats and |
32 | subroutines (and sometimes not even subroutines). A variable holds | |
33 | the undefined value (C<undef>) until it has been assigned a defined | |
34 | value, which is anything other than C<undef>. When used as a number, | |
35 | C<undef> is treated as C<0>; when used as a string, it is treated as | |
36 | the empty string, C<"">; and when used as a reference that isn't being | |
37 | assigned to, it is treated as an error. If you enable warnings, | |
38 | you'll be notified of an uninitialized value whenever you treat | |
39 | C<undef> as a string or a number. Well, usually. Boolean contexts, | |
40 | such as: | |
7bd1983c EM |
41 | |
42 | my $a; | |
43 | if ($a) {} | |
44 | ||
a6b1f6d8 RGS |
45 | are exempt from warnings (because they care about truth rather than |
46 | definedness). Operators such as C<++>, C<-->, C<+=>, | |
7bd1983c EM |
47 | C<-=>, and C<.=>, that operate on undefined left values such as: |
48 | ||
49 | my $a; | |
50 | $a++; | |
51 | ||
52 | are also always exempt from such warnings. | |
0b8d69e9 | 53 | |
a0d0e21e LW |
54 | A declaration can be put anywhere a statement can, but has no effect on |
55 | the execution of the primary sequence of statements--declarations all | |
56 | take effect at compile time. Typically all the declarations are put at | |
54310121 | 57 | the beginning or the end of the script. However, if you're using |
0b8d69e9 GS |
58 | lexically-scoped private variables created with C<my()>, you'll |
59 | have to make sure | |
4633a7c4 | 60 | your format or subroutine definition is within the same block scope |
5f05dabc | 61 | as the my if you expect to be able to access those private variables. |
a0d0e21e | 62 | |
4633a7c4 LW |
63 | Declaring a subroutine allows a subroutine name to be used as if it were a |
64 | list operator from that point forward in the program. You can declare a | |
54310121 | 65 | subroutine without defining it by saying C<sub name>, thus: |
a0d0e21e | 66 | |
54310121 | 67 | sub myname; |
a0d0e21e LW |
68 | $me = myname $0 or die "can't get myname"; |
69 | ||
1f950eb4 JB |
70 | Note that myname() functions as a list operator, not as a unary operator; |
71 | so be careful to use C<or> instead of C<||> in this case. However, if | |
54310121 | 72 | you were to declare the subroutine as C<sub myname ($)>, then |
02c45c47 | 73 | C<myname> would function as a unary operator, so either C<or> or |
54310121 | 74 | C<||> would work. |
a0d0e21e | 75 | |
4633a7c4 LW |
76 | Subroutines declarations can also be loaded up with the C<require> statement |
77 | or both loaded and imported into your namespace with a C<use> statement. | |
78 | See L<perlmod> for details on this. | |
a0d0e21e | 79 | |
4633a7c4 LW |
80 | A statement sequence may contain declarations of lexically-scoped |
81 | variables, but apart from declaring a variable name, the declaration acts | |
82 | like an ordinary statement, and is elaborated within the sequence of | |
83 | statements as if it were an ordinary statement. That means it actually | |
84 | has both compile-time and run-time effects. | |
a0d0e21e | 85 | |
6014d0cb MS |
86 | =head2 Comments |
87 | ||
88 | Text from a C<"#"> character until the end of the line is a comment, | |
89 | and is ignored. Exceptions include C<"#"> inside a string or regular | |
90 | expression. | |
91 | ||
6ec4bd10 | 92 | =head2 Simple Statements |
a0d0e21e LW |
93 | |
94 | The only kind of simple statement is an expression evaluated for its | |
95 | side effects. Every simple statement must be terminated with a | |
96 | semicolon, unless it is the final statement in a block, in which case | |
f386e492 AMS |
97 | the semicolon is optional. (A semicolon is still encouraged if the |
98 | block takes up more than one line, because you may eventually add | |
cf48932e SF |
99 | another line.) Note that there are some operators like C<eval {}> and |
100 | C<do {}> that look like compound statements, but aren't (they're just | |
101 | TERMs in an expression), and thus need an explicit termination if used | |
102 | as the last item in a statement. | |
103 | ||
104 | =head2 Truth and Falsehood | |
105 | ||
f92061c1 AMS |
106 | The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and |
107 | C<undef> are all false in a boolean context. All other values are true. | |
52ea55c9 SP |
108 | Negation of a true value by C<!> or C<not> returns a special false value. |
109 | When evaluated as a string it is treated as C<''>, but as a number, it | |
110 | is treated as 0. | |
cf48932e | 111 | |
cf48932e | 112 | =head2 Statement Modifiers |
a0d0e21e LW |
113 | |
114 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
115 | just before the terminating semicolon (or block ending). The possible | |
116 | modifiers are: | |
117 | ||
118 | if EXPR | |
119 | unless EXPR | |
120 | while EXPR | |
121 | until EXPR | |
cf48932e SF |
122 | foreach LIST |
123 | ||
124 | The C<EXPR> following the modifier is referred to as the "condition". | |
125 | Its truth or falsehood determines how the modifier will behave. | |
126 | ||
127 | C<if> executes the statement once I<if> and only if the condition is | |
128 | true. C<unless> is the opposite, it executes the statement I<unless> | |
129 | the condition is true (i.e., if the condition is false). | |
130 | ||
131 | print "Basset hounds got long ears" if length $ear >= 10; | |
132 | go_outside() and play() unless $is_raining; | |
133 | ||
134 | The C<foreach> modifier is an iterator: it executes the statement once | |
135 | for each item in the LIST (with C<$_> aliased to each item in turn). | |
136 | ||
137 | print "Hello $_!\n" foreach qw(world Dolly nurse); | |
138 | ||
139 | C<while> repeats the statement I<while> the condition is true. | |
140 | C<until> does the opposite, it repeats the statement I<until> the | |
141 | condition is true (or while the condition is false): | |
142 | ||
143 | # Both of these count from 0 to 10. | |
144 | print $i++ while $i <= 10; | |
145 | print $j++ until $j > 10; | |
146 | ||
147 | The C<while> and C<until> modifiers have the usual "C<while> loop" | |
148 | semantics (conditional evaluated first), except when applied to a | |
149 | C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in | |
150 | which case the block executes once before the conditional is | |
151 | evaluated. This is so that you can write loops like: | |
a0d0e21e LW |
152 | |
153 | do { | |
4633a7c4 | 154 | $line = <STDIN>; |
a0d0e21e | 155 | ... |
4633a7c4 | 156 | } until $line eq ".\n"; |
a0d0e21e | 157 | |
5a964f20 TC |
158 | See L<perlfunc/do>. Note also that the loop control statements described |
159 | later will I<NOT> work in this construct, because modifiers don't take | |
160 | loop labels. Sorry. You can always put another block inside of it | |
161 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
f86cebdf | 162 | For C<next>, just double the braces: |
5a964f20 TC |
163 | |
164 | do {{ | |
165 | next if $x == $y; | |
166 | # do something here | |
167 | }} until $x++ > $z; | |
168 | ||
f86cebdf | 169 | For C<last>, you have to be more elaborate: |
5a964f20 TC |
170 | |
171 | LOOP: { | |
172 | do { | |
173 | last if $x = $y**2; | |
174 | # do something here | |
175 | } while $x++ <= $z; | |
176 | } | |
a0d0e21e | 177 | |
457b36cb MV |
178 | B<NOTE:> The behaviour of a C<my> statement modified with a statement |
179 | modifier conditional or loop construct (e.g. C<my $x if ...>) is | |
180 | B<undefined>. The value of the C<my> variable may be C<undef>, any | |
181 | previously assigned value, or possibly anything else. Don't rely on | |
182 | it. Future versions of perl might do something different from the | |
183 | version of perl you try it out on. Here be dragons. | |
184 | ||
6ec4bd10 | 185 | =head2 Compound Statements |
a0d0e21e LW |
186 | |
187 | In Perl, a sequence of statements that defines a scope is called a block. | |
188 | Sometimes a block is delimited by the file containing it (in the case | |
189 | of a required file, or the program as a whole), and sometimes a block | |
190 | is delimited by the extent of a string (in the case of an eval). | |
191 | ||
192 | But generally, a block is delimited by curly brackets, also known as braces. | |
193 | We will call this syntactic construct a BLOCK. | |
194 | ||
195 | The following compound statements may be used to control flow: | |
196 | ||
197 | if (EXPR) BLOCK | |
198 | if (EXPR) BLOCK else BLOCK | |
199 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK | |
200 | LABEL while (EXPR) BLOCK | |
201 | LABEL while (EXPR) BLOCK continue BLOCK | |
5ec6d87f EA |
202 | LABEL until (EXPR) BLOCK |
203 | LABEL until (EXPR) BLOCK continue BLOCK | |
a0d0e21e | 204 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
748a9306 | 205 | LABEL foreach VAR (LIST) BLOCK |
b303ae78 | 206 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
a0d0e21e LW |
207 | LABEL BLOCK continue BLOCK |
208 | ||
209 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, | |
210 | not statements. This means that the curly brackets are I<required>--no | |
211 | dangling statements allowed. If you want to write conditionals without | |
212 | curly brackets there are several other ways to do it. The following | |
213 | all do the same thing: | |
214 | ||
215 | if (!open(FOO)) { die "Can't open $FOO: $!"; } | |
216 | die "Can't open $FOO: $!" unless open(FOO); | |
217 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! | |
218 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; | |
219 | # a bit exotic, that last one | |
220 | ||
5f05dabc | 221 | The C<if> statement is straightforward. Because BLOCKs are always |
a0d0e21e LW |
222 | bounded by curly brackets, there is never any ambiguity about which |
223 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
224 | the sense of the test is reversed. | |
225 | ||
226 | The C<while> statement executes the block as long as the expression is | |
0eb389d5 | 227 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). |
b78218b7 GS |
228 | The LABEL is optional, and if present, consists of an identifier followed |
229 | by a colon. The LABEL identifies the loop for the loop control | |
230 | statements C<next>, C<last>, and C<redo>. | |
231 | If the LABEL is omitted, the loop control statement | |
4633a7c4 LW |
232 | refers to the innermost enclosing loop. This may include dynamically |
233 | looking back your call-stack at run time to find the LABEL. Such | |
9f1b1f2d | 234 | desperate behavior triggers a warning if you use the C<use warnings> |
a2293a43 | 235 | pragma or the B<-w> flag. |
4633a7c4 LW |
236 | |
237 | If there is a C<continue> BLOCK, it is always executed just before the | |
6ec4bd10 MS |
238 | conditional is about to be evaluated again. Thus it can be used to |
239 | increment a loop variable, even when the loop has been continued via | |
240 | the C<next> statement. | |
4633a7c4 LW |
241 | |
242 | =head2 Loop Control | |
243 | ||
6ec4bd10 | 244 | The C<next> command starts the next iteration of the loop: |
4633a7c4 LW |
245 | |
246 | LINE: while (<STDIN>) { | |
247 | next LINE if /^#/; # discard comments | |
248 | ... | |
249 | } | |
250 | ||
6ec4bd10 | 251 | The C<last> command immediately exits the loop in question. The |
4633a7c4 LW |
252 | C<continue> block, if any, is not executed: |
253 | ||
254 | LINE: while (<STDIN>) { | |
255 | last LINE if /^$/; # exit when done with header | |
256 | ... | |
257 | } | |
258 | ||
259 | The C<redo> command restarts the loop block without evaluating the | |
260 | conditional again. The C<continue> block, if any, is I<not> executed. | |
261 | This command is normally used by programs that want to lie to themselves | |
262 | about what was just input. | |
263 | ||
264 | For example, when processing a file like F</etc/termcap>. | |
265 | If your input lines might end in backslashes to indicate continuation, you | |
266 | want to skip ahead and get the next record. | |
267 | ||
268 | while (<>) { | |
269 | chomp; | |
54310121 | 270 | if (s/\\$//) { |
271 | $_ .= <>; | |
4633a7c4 LW |
272 | redo unless eof(); |
273 | } | |
274 | # now process $_ | |
54310121 | 275 | } |
4633a7c4 LW |
276 | |
277 | which is Perl short-hand for the more explicitly written version: | |
278 | ||
54310121 | 279 | LINE: while (defined($line = <ARGV>)) { |
4633a7c4 | 280 | chomp($line); |
54310121 | 281 | if ($line =~ s/\\$//) { |
282 | $line .= <ARGV>; | |
4633a7c4 LW |
283 | redo LINE unless eof(); # not eof(ARGV)! |
284 | } | |
285 | # now process $line | |
54310121 | 286 | } |
4633a7c4 | 287 | |
36e7a065 AMS |
288 | Note that if there were a C<continue> block on the above code, it would |
289 | get executed only on lines discarded by the regex (since redo skips the | |
290 | continue block). A continue block is often used to reset line counters | |
291 | or C<?pat?> one-time matches: | |
4633a7c4 | 292 | |
5a964f20 TC |
293 | # inspired by :1,$g/fred/s//WILMA/ |
294 | while (<>) { | |
295 | ?(fred)? && s//WILMA $1 WILMA/; | |
296 | ?(barney)? && s//BETTY $1 BETTY/; | |
297 | ?(homer)? && s//MARGE $1 MARGE/; | |
298 | } continue { | |
299 | print "$ARGV $.: $_"; | |
300 | close ARGV if eof(); # reset $. | |
301 | reset if eof(); # reset ?pat? | |
4633a7c4 LW |
302 | } |
303 | ||
a0d0e21e LW |
304 | If the word C<while> is replaced by the word C<until>, the sense of the |
305 | test is reversed, but the conditional is still tested before the first | |
306 | iteration. | |
307 | ||
5a964f20 TC |
308 | The loop control statements don't work in an C<if> or C<unless>, since |
309 | they aren't loops. You can double the braces to make them such, though. | |
310 | ||
311 | if (/pattern/) {{ | |
7bd1983c EM |
312 | last if /fred/; |
313 | next if /barney/; # same effect as "last", but doesn't document as well | |
314 | # do something here | |
5a964f20 TC |
315 | }} |
316 | ||
7bd1983c EM |
317 | This is caused by the fact that a block by itself acts as a loop that |
318 | executes once, see L<"Basic BLOCKs and Switch Statements">. | |
319 | ||
5b23ba8b MG |
320 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
321 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
4633a7c4 | 322 | |
cb1a09d0 | 323 | =head2 For Loops |
a0d0e21e | 324 | |
b78df5de | 325 | Perl's C-style C<for> loop works like the corresponding C<while> loop; |
cb1a09d0 | 326 | that means that this: |
a0d0e21e LW |
327 | |
328 | for ($i = 1; $i < 10; $i++) { | |
329 | ... | |
330 | } | |
331 | ||
cb1a09d0 | 332 | is the same as this: |
a0d0e21e LW |
333 | |
334 | $i = 1; | |
335 | while ($i < 10) { | |
336 | ... | |
337 | } continue { | |
338 | $i++; | |
339 | } | |
340 | ||
b78df5de JA |
341 | There is one minor difference: if variables are declared with C<my> |
342 | in the initialization section of the C<for>, the lexical scope of | |
343 | those variables is exactly the C<for> loop (the body of the loop | |
344 | and the control sections). | |
55497cff | 345 | |
cb1a09d0 AD |
346 | Besides the normal array index looping, C<for> can lend itself |
347 | to many other interesting applications. Here's one that avoids the | |
54310121 | 348 | problem you get into if you explicitly test for end-of-file on |
349 | an interactive file descriptor causing your program to appear to | |
cb1a09d0 AD |
350 | hang. |
351 | ||
352 | $on_a_tty = -t STDIN && -t STDOUT; | |
353 | sub prompt { print "yes? " if $on_a_tty } | |
354 | for ( prompt(); <STDIN>; prompt() ) { | |
355 | # do something | |
54310121 | 356 | } |
cb1a09d0 | 357 | |
00cb5da1 CW |
358 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the |
359 | conditional of a C<for> loop is shorthand for the following. This | |
360 | behaviour is the same as a C<while> loop conditional. | |
361 | ||
362 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { | |
363 | # do something | |
364 | } | |
365 | ||
cb1a09d0 AD |
366 | =head2 Foreach Loops |
367 | ||
4633a7c4 | 368 | The C<foreach> loop iterates over a normal list value and sets the |
55497cff | 369 | variable VAR to be each element of the list in turn. If the variable |
370 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
371 | is therefore visible only within the loop. Otherwise, the variable is | |
372 | implicitly local to the loop and regains its former value upon exiting | |
373 | the loop. If the variable was previously declared with C<my>, it uses | |
374 | that variable instead of the global one, but it's still localized to | |
5c502d37 MV |
375 | the loop. This implicit localisation occurs I<only> in a C<foreach> |
376 | loop. | |
4633a7c4 LW |
377 | |
378 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
5a964f20 TC |
379 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
380 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> | |
f86cebdf | 381 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
c5674021 |
382 | |
383 | If any element of LIST is an lvalue, you can modify it by modifying | |
384 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
385 | lvalue, any attempt to modify that element will fail. In other words, | |
386 | the C<foreach> loop index variable is an implicit alias for each item | |
387 | in the list that you're looping over. | |
302617ea MG |
388 | |
389 | If any part of LIST is an array, C<foreach> will get very confused if | |
390 | you add or remove elements within the loop body, for example with | |
391 | C<splice>. So don't do that. | |
392 | ||
393 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
394 | special variable. Don't do that either. | |
4633a7c4 | 395 | |
748a9306 | 396 | Examples: |
a0d0e21e | 397 | |
4633a7c4 | 398 | for (@ary) { s/foo/bar/ } |
a0d0e21e | 399 | |
96f2dc66 | 400 | for my $elem (@elements) { |
a0d0e21e LW |
401 | $elem *= 2; |
402 | } | |
403 | ||
4633a7c4 LW |
404 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
405 | print $count, "\n"; sleep(1); | |
a0d0e21e LW |
406 | } |
407 | ||
408 | for (1..15) { print "Merry Christmas\n"; } | |
409 | ||
4633a7c4 | 410 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
a0d0e21e LW |
411 | print "Item: $item\n"; |
412 | } | |
413 | ||
4633a7c4 LW |
414 | Here's how a C programmer might code up a particular algorithm in Perl: |
415 | ||
55497cff | 416 | for (my $i = 0; $i < @ary1; $i++) { |
417 | for (my $j = 0; $j < @ary2; $j++) { | |
4633a7c4 LW |
418 | if ($ary1[$i] > $ary2[$j]) { |
419 | last; # can't go to outer :-( | |
420 | } | |
421 | $ary1[$i] += $ary2[$j]; | |
422 | } | |
cb1a09d0 | 423 | # this is where that last takes me |
4633a7c4 LW |
424 | } |
425 | ||
184e9718 | 426 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 | 427 | do it: |
4633a7c4 | 428 | |
96f2dc66 GS |
429 | OUTER: for my $wid (@ary1) { |
430 | INNER: for my $jet (@ary2) { | |
cb1a09d0 AD |
431 | next OUTER if $wid > $jet; |
432 | $wid += $jet; | |
54310121 | 433 | } |
434 | } | |
4633a7c4 | 435 | |
cb1a09d0 AD |
436 | See how much easier this is? It's cleaner, safer, and faster. It's |
437 | cleaner because it's less noisy. It's safer because if code gets added | |
c07a80fd | 438 | between the inner and outer loops later on, the new code won't be |
5f05dabc | 439 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd | 440 | rather than merely terminating the inner one. And it's faster because |
441 | Perl executes a C<foreach> statement more rapidly than it would the | |
442 | equivalent C<for> loop. | |
4633a7c4 LW |
443 | |
444 | =head2 Basic BLOCKs and Switch Statements | |
445 | ||
55497cff | 446 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
447 | loop that executes once. Thus you can use any of the loop control | |
448 | statements in it to leave or restart the block. (Note that this is | |
449 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
450 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
451 | block is optional. | |
4633a7c4 LW |
452 | |
453 | The BLOCK construct is particularly nice for doing case | |
a0d0e21e LW |
454 | structures. |
455 | ||
456 | SWITCH: { | |
457 | if (/^abc/) { $abc = 1; last SWITCH; } | |
458 | if (/^def/) { $def = 1; last SWITCH; } | |
459 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
460 | $nothing = 1; | |
461 | } | |
462 | ||
f86cebdf | 463 | There is no official C<switch> statement in Perl, because there are |
83df6a1d JH |
464 | already several ways to write the equivalent. |
465 | ||
466 | However, starting from Perl 5.8 to get switch and case one can use | |
467 | the Switch extension and say: | |
468 | ||
469 | use Switch; | |
470 | ||
471 | after which one has switch and case. It is not as fast as it could be | |
472 | because it's not really part of the language (it's done using source | |
473 | filters) but it is available, and it's very flexible. | |
474 | ||
475 | In addition to the above BLOCK construct, you could write | |
a0d0e21e LW |
476 | |
477 | SWITCH: { | |
478 | $abc = 1, last SWITCH if /^abc/; | |
479 | $def = 1, last SWITCH if /^def/; | |
480 | $xyz = 1, last SWITCH if /^xyz/; | |
481 | $nothing = 1; | |
482 | } | |
483 | ||
cb1a09d0 | 484 | (That's actually not as strange as it looks once you realize that you can |
6ec4bd10 MS |
485 | use loop control "operators" within an expression. That's just the binary |
486 | comma operator in scalar context. See L<perlop/"Comma Operator">.) | |
a0d0e21e LW |
487 | |
488 | or | |
489 | ||
490 | SWITCH: { | |
491 | /^abc/ && do { $abc = 1; last SWITCH; }; | |
492 | /^def/ && do { $def = 1; last SWITCH; }; | |
493 | /^xyz/ && do { $xyz = 1; last SWITCH; }; | |
494 | $nothing = 1; | |
495 | } | |
496 | ||
f86cebdf | 497 | or formatted so it stands out more as a "proper" C<switch> statement: |
a0d0e21e LW |
498 | |
499 | SWITCH: { | |
54310121 | 500 | /^abc/ && do { |
501 | $abc = 1; | |
502 | last SWITCH; | |
a0d0e21e LW |
503 | }; |
504 | ||
54310121 | 505 | /^def/ && do { |
506 | $def = 1; | |
507 | last SWITCH; | |
a0d0e21e LW |
508 | }; |
509 | ||
54310121 | 510 | /^xyz/ && do { |
511 | $xyz = 1; | |
512 | last SWITCH; | |
a0d0e21e LW |
513 | }; |
514 | $nothing = 1; | |
515 | } | |
516 | ||
517 | or | |
518 | ||
519 | SWITCH: { | |
520 | /^abc/ and $abc = 1, last SWITCH; | |
521 | /^def/ and $def = 1, last SWITCH; | |
522 | /^xyz/ and $xyz = 1, last SWITCH; | |
523 | $nothing = 1; | |
524 | } | |
525 | ||
526 | or even, horrors, | |
527 | ||
528 | if (/^abc/) | |
529 | { $abc = 1 } | |
530 | elsif (/^def/) | |
531 | { $def = 1 } | |
532 | elsif (/^xyz/) | |
533 | { $xyz = 1 } | |
534 | else | |
535 | { $nothing = 1 } | |
536 | ||
f86cebdf GS |
537 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make |
538 | a temporary assignment to C<$_> for convenient matching: | |
4633a7c4 LW |
539 | |
540 | SWITCH: for ($where) { | |
541 | /In Card Names/ && do { push @flags, '-e'; last; }; | |
542 | /Anywhere/ && do { push @flags, '-h'; last; }; | |
543 | /In Rulings/ && do { last; }; | |
544 | die "unknown value for form variable where: `$where'"; | |
54310121 | 545 | } |
4633a7c4 | 546 | |
cb1a09d0 AD |
547 | Another interesting approach to a switch statement is arrange |
548 | for a C<do> block to return the proper value: | |
549 | ||
550 | $amode = do { | |
5a964f20 | 551 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? |
54310121 | 552 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
cb1a09d0 AD |
553 | elsif ($flag & O_RDWR) { |
554 | if ($flag & O_CREAT) { "w+" } | |
c07a80fd | 555 | else { ($flag & O_APPEND) ? "a+" : "r+" } |
cb1a09d0 AD |
556 | } |
557 | }; | |
558 | ||
5a964f20 TC |
559 | Or |
560 | ||
561 | print do { | |
562 | ($flags & O_WRONLY) ? "write-only" : | |
563 | ($flags & O_RDWR) ? "read-write" : | |
564 | "read-only"; | |
565 | }; | |
566 | ||
a031eab2 | 567 | Or if you are certain that all the C<&&> clauses are true, you can use |
5a964f20 | 568 | something like this, which "switches" on the value of the |
a2293a43 | 569 | C<HTTP_USER_AGENT> environment variable. |
5a964f20 TC |
570 | |
571 | #!/usr/bin/perl | |
572 | # pick out jargon file page based on browser | |
573 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; | |
574 | for ($ENV{HTTP_USER_AGENT}) { | |
575 | $page = /Mac/ && 'm/Macintrash.html' | |
576 | || /Win(dows )?NT/ && 'e/evilandrude.html' | |
577 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' | |
578 | || /Linux/ && 'l/Linux.html' | |
579 | || /HP-UX/ && 'h/HP-SUX.html' | |
580 | || /SunOS/ && 's/ScumOS.html' | |
581 | || 'a/AppendixB.html'; | |
582 | } | |
583 | print "Location: $dir/$page\015\012\015\012"; | |
584 | ||
585 | That kind of switch statement only works when you know the C<&&> clauses | |
586 | will be true. If you don't, the previous C<?:> example should be used. | |
587 | ||
19799a22 GS |
588 | You might also consider writing a hash of subroutine references |
589 | instead of synthesizing a C<switch> statement. | |
5a964f20 | 590 | |
4633a7c4 LW |
591 | =head2 Goto |
592 | ||
19799a22 GS |
593 | Although not for the faint of heart, Perl does support a C<goto> |
594 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
595 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
596 | a C<goto>; it's just the name of the loop. | |
4633a7c4 | 597 | |
f86cebdf | 598 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 | 599 | execution there. It may not be used to go into any construct that |
f86cebdf | 600 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 LW |
601 | also can't be used to go into a construct that is optimized away. It |
602 | can be used to go almost anywhere else within the dynamic scope, | |
603 | including out of subroutines, but it's usually better to use some other | |
f86cebdf GS |
604 | construct such as C<last> or C<die>. The author of Perl has never felt the |
605 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
4633a7c4 | 606 | |
f86cebdf GS |
607 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
608 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
4633a7c4 LW |
609 | necessarily recommended if you're optimizing for maintainability: |
610 | ||
96f2dc66 | 611 | goto(("FOO", "BAR", "GLARCH")[$i]); |
4633a7c4 | 612 | |
f86cebdf | 613 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 | 614 | named subroutine for the currently running subroutine. This is used by |
f86cebdf | 615 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 | 616 | pretend that the other subroutine had been called in the first place |
f86cebdf GS |
617 | (except that any modifications to C<@_> in the current subroutine are |
618 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
4633a7c4 LW |
619 | will be able to tell that this routine was called first. |
620 | ||
c07a80fd | 621 | In almost all cases like this, it's usually a far, far better idea to use the |
622 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
4633a7c4 LW |
623 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
624 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
cb1a09d0 AD |
625 | |
626 | =head2 PODs: Embedded Documentation | |
627 | ||
628 | Perl has a mechanism for intermixing documentation with source code. | |
c07a80fd | 629 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 AD |
630 | encounters a line that begins with an equal sign and a word, like this |
631 | ||
632 | =head1 Here There Be Pods! | |
633 | ||
634 | Then that text and all remaining text up through and including a line | |
635 | beginning with C<=cut> will be ignored. The format of the intervening | |
54310121 | 636 | text is described in L<perlpod>. |
cb1a09d0 AD |
637 | |
638 | This allows you to intermix your source code | |
639 | and your documentation text freely, as in | |
640 | ||
641 | =item snazzle($) | |
642 | ||
54310121 | 643 | The snazzle() function will behave in the most spectacular |
cb1a09d0 AD |
644 | form that you can possibly imagine, not even excepting |
645 | cybernetic pyrotechnics. | |
646 | ||
647 | =cut back to the compiler, nuff of this pod stuff! | |
648 | ||
649 | sub snazzle($) { | |
650 | my $thingie = shift; | |
651 | ......... | |
54310121 | 652 | } |
cb1a09d0 | 653 | |
54310121 | 654 | Note that pod translators should look at only paragraphs beginning |
184e9718 | 655 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 | 656 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 AD |
657 | paragraph. This means that the following secret stuff will be |
658 | ignored by both the compiler and the translators. | |
659 | ||
660 | $a=3; | |
661 | =secret stuff | |
662 | warn "Neither POD nor CODE!?" | |
663 | =cut back | |
664 | print "got $a\n"; | |
665 | ||
f86cebdf | 666 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 AD |
667 | Not all pod translators are well-behaved in this regard, and perhaps |
668 | the compiler will become pickier. | |
774d564b | 669 | |
670 | One may also use pod directives to quickly comment out a section | |
671 | of code. | |
672 | ||
673 | =head2 Plain Old Comments (Not!) | |
674 | ||
6ec4bd10 | 675 | Perl can process line directives, much like the C preprocessor. Using |
5a964f20 | 676 | this, one can control Perl's idea of filenames and line numbers in |
774d564b | 677 | error or warning messages (especially for strings that are processed |
f86cebdf | 678 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b | 679 | C preprocessors: it matches the regular expression |
6ec4bd10 MS |
680 | |
681 | # example: '# line 42 "new_filename.plx"' | |
82d4537c | 682 | /^\# \s* |
6ec4bd10 | 683 | line \s+ (\d+) \s* |
7b6e93a8 | 684 | (?:\s("?)([^"]+)\2)? \s* |
6ec4bd10 MS |
685 | $/x |
686 | ||
7b6e93a8 CW |
687 | with C<$1> being the line number for the next line, and C<$3> being |
688 | the optional filename (specified with or without quotes). | |
774d564b | 689 | |
003183f2 GS |
690 | There is a fairly obvious gotcha included with the line directive: |
691 | Debuggers and profilers will only show the last source line to appear | |
692 | at a particular line number in a given file. Care should be taken not | |
693 | to cause line number collisions in code you'd like to debug later. | |
694 | ||
774d564b | 695 | Here are some examples that you should be able to type into your command |
696 | shell: | |
697 | ||
698 | % perl | |
699 | # line 200 "bzzzt" | |
700 | # the `#' on the previous line must be the first char on line | |
701 | die 'foo'; | |
702 | __END__ | |
703 | foo at bzzzt line 201. | |
54310121 | 704 | |
774d564b | 705 | % perl |
706 | # line 200 "bzzzt" | |
707 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
708 | __END__ | |
709 | foo at - line 2001. | |
54310121 | 710 | |
774d564b | 711 | % perl |
712 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
713 | __END__ | |
714 | foo at foo bar line 200. | |
54310121 | 715 | |
774d564b | 716 | % perl |
717 | # line 345 "goop" | |
718 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
719 | print $@; | |
720 | __END__ | |
721 | foo at goop line 345. | |
722 | ||
723 | =cut |