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1 | =head1 NAME |
2 | |
3 | perlsyn - Perl syntax |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | A Perl script consists of a sequence of declarations and statements. |
8 | The only things that need to be declared in Perl are report formats |
9 | and subroutines. See the sections below for more information on those |
10 | declarations. All uninitialized user-created objects are assumed to |
11 | start with a null or 0 value until they are defined by some explicit |
12 | operation such as assignment. (Though you can get warnings about the |
13 | use of undefined values if you like.) The sequence of statements is |
14 | executed just once, unlike in B<sed> and B<awk> scripts, where the |
15 | sequence of statements is executed for each input line. While this means |
16 | that you must explicitly loop over the lines of your input file (or |
17 | files), it also means you have much more control over which files and |
18 | which lines you look at. (Actually, I'm lying--it is possible to do an |
19 | implicit loop with either the B<-n> or B<-p> switch. It's just not the |
20 | mandatory default like it is in B<sed> and B<awk>.) |
21 | |
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22 | =head2 Declarations |
23 | |
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24 | Perl is, for the most part, a free-form language. (The only |
25 | exception to this is format declarations, for obvious reasons.) Comments |
26 | are indicated by the "#" character, and extend to the end of the line. If |
27 | you attempt to use C</* */> C-style comments, it will be interpreted |
28 | either as division or pattern matching, depending on the context, and C++ |
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29 | C<//> comments just look like a null regular expression, so don't do |
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30 | that. |
31 | |
32 | A declaration can be put anywhere a statement can, but has no effect on |
33 | the execution of the primary sequence of statements--declarations all |
34 | take effect at compile time. Typically all the declarations are put at |
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35 | the beginning or the end of the script. However, if you're using |
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36 | lexically-scoped private variables created with my(), you'll have to make sure |
37 | your format or subroutine definition is within the same block scope |
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38 | as the my if you expect to be able to access those private variables. |
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39 | |
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40 | Declaring a subroutine allows a subroutine name to be used as if it were a |
41 | list operator from that point forward in the program. You can declare a |
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42 | subroutine without defining it by saying C<sub name>, thus: |
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43 | |
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44 | sub myname; |
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45 | $me = myname $0 or die "can't get myname"; |
46 | |
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47 | Note that it functions as a list operator, not as a unary operator; so |
48 | be careful to use C<or> instead of C<||> in this case. However, if |
49 | you were to declare the subroutine as C<sub myname ($)>, then |
50 | C<myname> would functonion as a unary operator, so either C<or> or |
51 | C<||> would work. |
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52 | |
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53 | Subroutines declarations can also be loaded up with the C<require> statement |
54 | or both loaded and imported into your namespace with a C<use> statement. |
55 | See L<perlmod> for details on this. |
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56 | |
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57 | A statement sequence may contain declarations of lexically-scoped |
58 | variables, but apart from declaring a variable name, the declaration acts |
59 | like an ordinary statement, and is elaborated within the sequence of |
60 | statements as if it were an ordinary statement. That means it actually |
61 | has both compile-time and run-time effects. |
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62 | |
63 | =head2 Simple statements |
64 | |
65 | The only kind of simple statement is an expression evaluated for its |
66 | side effects. Every simple statement must be terminated with a |
67 | semicolon, unless it is the final statement in a block, in which case |
68 | the semicolon is optional. (A semicolon is still encouraged there if the |
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69 | block takes up more than one line, because you may eventually add another line.) |
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70 | Note that there are some operators like C<eval {}> and C<do {}> that look |
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71 | like compound statements, but aren't (they're just TERMs in an expression), |
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72 | and thus need an explicit termination if used as the last item in a statement. |
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73 | |
74 | Any simple statement may optionally be followed by a I<SINGLE> modifier, |
75 | just before the terminating semicolon (or block ending). The possible |
76 | modifiers are: |
77 | |
78 | if EXPR |
79 | unless EXPR |
80 | while EXPR |
81 | until EXPR |
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82 | foreach EXPR |
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83 | |
84 | The C<if> and C<unless> modifiers have the expected semantics, |
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85 | presuming you're a speaker of English. The C<foreach> modifier is an |
86 | iterator: For each value in EXPR, it aliases $_ to the value and |
87 | executes the statement. The C<while> and C<until> modifiers have the |
88 | usual "while loop" semantics (conditional evaluated first), except |
89 | when applied to a do-BLOCK (or to the now-deprecated do-SUBROUTINE |
90 | statement), in which case the block executes once before the |
91 | conditional is evaluated. This is so that you can write loops like: |
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92 | |
93 | do { |
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94 | $line = <STDIN>; |
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95 | ... |
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96 | } until $line eq ".\n"; |
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97 | |
98 | See L<perlfunc/do>. Note also that the loop control |
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99 | statements described later will I<NOT> work in this construct, because |
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100 | modifiers don't take loop labels. Sorry. You can always wrap |
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101 | another block around it to do that sort of thing. |
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102 | |
103 | =head2 Compound statements |
104 | |
105 | In Perl, a sequence of statements that defines a scope is called a block. |
106 | Sometimes a block is delimited by the file containing it (in the case |
107 | of a required file, or the program as a whole), and sometimes a block |
108 | is delimited by the extent of a string (in the case of an eval). |
109 | |
110 | But generally, a block is delimited by curly brackets, also known as braces. |
111 | We will call this syntactic construct a BLOCK. |
112 | |
113 | The following compound statements may be used to control flow: |
114 | |
115 | if (EXPR) BLOCK |
116 | if (EXPR) BLOCK else BLOCK |
117 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
118 | LABEL while (EXPR) BLOCK |
119 | LABEL while (EXPR) BLOCK continue BLOCK |
120 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
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121 | LABEL foreach VAR (LIST) BLOCK |
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122 | LABEL BLOCK continue BLOCK |
123 | |
124 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, |
125 | not statements. This means that the curly brackets are I<required>--no |
126 | dangling statements allowed. If you want to write conditionals without |
127 | curly brackets there are several other ways to do it. The following |
128 | all do the same thing: |
129 | |
130 | if (!open(FOO)) { die "Can't open $FOO: $!"; } |
131 | die "Can't open $FOO: $!" unless open(FOO); |
132 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! |
133 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; |
134 | # a bit exotic, that last one |
135 | |
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136 | The C<if> statement is straightforward. Because BLOCKs are always |
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137 | bounded by curly brackets, there is never any ambiguity about which |
138 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, |
139 | the sense of the test is reversed. |
140 | |
141 | The C<while> statement executes the block as long as the expression is |
142 | true (does not evaluate to the null string or 0 or "0"). The LABEL is |
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143 | optional, and if present, consists of an identifier followed by a colon. |
144 | The LABEL identifies the loop for the loop control statements C<next>, |
145 | C<last>, and C<redo>. If the LABEL is omitted, the loop control statement |
146 | refers to the innermost enclosing loop. This may include dynamically |
147 | looking back your call-stack at run time to find the LABEL. Such |
148 | desperate behavior triggers a warning if you use the B<-w> flag. |
149 | |
150 | If there is a C<continue> BLOCK, it is always executed just before the |
151 | conditional is about to be evaluated again, just like the third part of a |
152 | C<for> loop in C. Thus it can be used to increment a loop variable, even |
153 | when the loop has been continued via the C<next> statement (which is |
154 | similar to the C C<continue> statement). |
155 | |
156 | =head2 Loop Control |
157 | |
158 | The C<next> command is like the C<continue> statement in C; it starts |
159 | the next iteration of the loop: |
160 | |
161 | LINE: while (<STDIN>) { |
162 | next LINE if /^#/; # discard comments |
163 | ... |
164 | } |
165 | |
166 | The C<last> command is like the C<break> statement in C (as used in |
167 | loops); it immediately exits the loop in question. The |
168 | C<continue> block, if any, is not executed: |
169 | |
170 | LINE: while (<STDIN>) { |
171 | last LINE if /^$/; # exit when done with header |
172 | ... |
173 | } |
174 | |
175 | The C<redo> command restarts the loop block without evaluating the |
176 | conditional again. The C<continue> block, if any, is I<not> executed. |
177 | This command is normally used by programs that want to lie to themselves |
178 | about what was just input. |
179 | |
180 | For example, when processing a file like F</etc/termcap>. |
181 | If your input lines might end in backslashes to indicate continuation, you |
182 | want to skip ahead and get the next record. |
183 | |
184 | while (<>) { |
185 | chomp; |
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186 | if (s/\\$//) { |
187 | $_ .= <>; |
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188 | redo unless eof(); |
189 | } |
190 | # now process $_ |
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191 | } |
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192 | |
193 | which is Perl short-hand for the more explicitly written version: |
194 | |
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195 | LINE: while (defined($line = <ARGV>)) { |
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196 | chomp($line); |
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197 | if ($line =~ s/\\$//) { |
198 | $line .= <ARGV>; |
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199 | redo LINE unless eof(); # not eof(ARGV)! |
200 | } |
201 | # now process $line |
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202 | } |
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203 | |
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204 | Or here's a simpleminded Pascal comment stripper (warning: assumes no |
205 | { or } in strings). |
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206 | |
207 | LINE: while (<STDIN>) { |
208 | while (s|({.*}.*){.*}|$1 |) {} |
209 | s|{.*}| |; |
210 | if (s|{.*| |) { |
211 | $front = $_; |
212 | while (<STDIN>) { |
213 | if (/}/) { # end of comment? |
214 | s|^|$front{|; |
215 | redo LINE; |
216 | } |
217 | } |
218 | } |
219 | print; |
220 | } |
221 | |
222 | Note that if there were a C<continue> block on the above code, it would get |
223 | executed even on discarded lines. |
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224 | |
225 | If the word C<while> is replaced by the word C<until>, the sense of the |
226 | test is reversed, but the conditional is still tested before the first |
227 | iteration. |
228 | |
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229 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
230 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. |
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231 | |
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232 | =head2 For Loops |
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233 | |
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234 | Perl's C-style C<for> loop works exactly like the corresponding C<while> loop; |
235 | that means that this: |
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236 | |
237 | for ($i = 1; $i < 10; $i++) { |
238 | ... |
239 | } |
240 | |
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241 | is the same as this: |
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242 | |
243 | $i = 1; |
244 | while ($i < 10) { |
245 | ... |
246 | } continue { |
247 | $i++; |
248 | } |
249 | |
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250 | (There is one minor difference: The first form implies a lexical scope |
251 | for variables declared with C<my> in the initialization expression.) |
252 | |
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253 | Besides the normal array index looping, C<for> can lend itself |
254 | to many other interesting applications. Here's one that avoids the |
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255 | problem you get into if you explicitly test for end-of-file on |
256 | an interactive file descriptor causing your program to appear to |
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257 | hang. |
258 | |
259 | $on_a_tty = -t STDIN && -t STDOUT; |
260 | sub prompt { print "yes? " if $on_a_tty } |
261 | for ( prompt(); <STDIN>; prompt() ) { |
262 | # do something |
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263 | } |
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264 | |
265 | =head2 Foreach Loops |
266 | |
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267 | The C<foreach> loop iterates over a normal list value and sets the |
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268 | variable VAR to be each element of the list in turn. If the variable |
269 | is preceded with the keyword C<my>, then it is lexically scoped, and |
270 | is therefore visible only within the loop. Otherwise, the variable is |
271 | implicitly local to the loop and regains its former value upon exiting |
272 | the loop. If the variable was previously declared with C<my>, it uses |
273 | that variable instead of the global one, but it's still localized to |
274 | the loop. (Note that a lexically scoped variable can cause problems |
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275 | if you have subroutine or format declarations within the loop which |
276 | refer to it.) |
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277 | |
278 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so |
279 | you can use C<foreach> for readability or C<for> for brevity. If VAR is |
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280 | omitted, $_ is set to each value. If any element of LIST is an lvalue, |
281 | you can modify it by modifying VAR inside the loop. That's because |
282 | the C<foreach> loop index variable is an implicit alias for each item |
283 | in the list that you're looping over. |
284 | |
285 | If any part of LIST is an array, C<foreach> will get very confused if |
286 | you add or remove elements within the loop body, for example with |
287 | C<splice>. So don't do that. |
288 | |
289 | C<foreach> probably won't do what you expect if VAR is a tied or other |
290 | special variable. Don't do that either. |
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291 | |
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292 | Examples: |
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293 | |
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294 | for (@ary) { s/foo/bar/ } |
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295 | |
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296 | foreach my $elem (@elements) { |
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297 | $elem *= 2; |
298 | } |
299 | |
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300 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
301 | print $count, "\n"; sleep(1); |
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302 | } |
303 | |
304 | for (1..15) { print "Merry Christmas\n"; } |
305 | |
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306 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
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307 | print "Item: $item\n"; |
308 | } |
309 | |
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310 | Here's how a C programmer might code up a particular algorithm in Perl: |
311 | |
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312 | for (my $i = 0; $i < @ary1; $i++) { |
313 | for (my $j = 0; $j < @ary2; $j++) { |
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314 | if ($ary1[$i] > $ary2[$j]) { |
315 | last; # can't go to outer :-( |
316 | } |
317 | $ary1[$i] += $ary2[$j]; |
318 | } |
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319 | # this is where that last takes me |
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320 | } |
321 | |
184e9718 |
322 | Whereas here's how a Perl programmer more comfortable with the idiom might |
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323 | do it: |
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324 | |
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325 | OUTER: foreach my $wid (@ary1) { |
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326 | INNER: foreach my $jet (@ary2) { |
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327 | next OUTER if $wid > $jet; |
328 | $wid += $jet; |
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329 | } |
330 | } |
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331 | |
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332 | See how much easier this is? It's cleaner, safer, and faster. It's |
333 | cleaner because it's less noisy. It's safer because if code gets added |
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334 | between the inner and outer loops later on, the new code won't be |
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335 | accidentally executed. The C<next> explicitly iterates the other loop |
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336 | rather than merely terminating the inner one. And it's faster because |
337 | Perl executes a C<foreach> statement more rapidly than it would the |
338 | equivalent C<for> loop. |
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339 | |
340 | =head2 Basic BLOCKs and Switch Statements |
341 | |
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342 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
343 | loop that executes once. Thus you can use any of the loop control |
344 | statements in it to leave or restart the block. (Note that this is |
345 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief |
346 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> |
347 | block is optional. |
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348 | |
349 | The BLOCK construct is particularly nice for doing case |
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350 | structures. |
351 | |
352 | SWITCH: { |
353 | if (/^abc/) { $abc = 1; last SWITCH; } |
354 | if (/^def/) { $def = 1; last SWITCH; } |
355 | if (/^xyz/) { $xyz = 1; last SWITCH; } |
356 | $nothing = 1; |
357 | } |
358 | |
359 | There is no official switch statement in Perl, because there are |
360 | already several ways to write the equivalent. In addition to the |
361 | above, you could write |
362 | |
363 | SWITCH: { |
364 | $abc = 1, last SWITCH if /^abc/; |
365 | $def = 1, last SWITCH if /^def/; |
366 | $xyz = 1, last SWITCH if /^xyz/; |
367 | $nothing = 1; |
368 | } |
369 | |
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370 | (That's actually not as strange as it looks once you realize that you can |
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371 | use loop control "operators" within an expression, That's just the normal |
372 | C comma operator.) |
373 | |
374 | or |
375 | |
376 | SWITCH: { |
377 | /^abc/ && do { $abc = 1; last SWITCH; }; |
378 | /^def/ && do { $def = 1; last SWITCH; }; |
379 | /^xyz/ && do { $xyz = 1; last SWITCH; }; |
380 | $nothing = 1; |
381 | } |
382 | |
383 | or formatted so it stands out more as a "proper" switch statement: |
384 | |
385 | SWITCH: { |
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386 | /^abc/ && do { |
387 | $abc = 1; |
388 | last SWITCH; |
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389 | }; |
390 | |
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391 | /^def/ && do { |
392 | $def = 1; |
393 | last SWITCH; |
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394 | }; |
395 | |
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396 | /^xyz/ && do { |
397 | $xyz = 1; |
398 | last SWITCH; |
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399 | }; |
400 | $nothing = 1; |
401 | } |
402 | |
403 | or |
404 | |
405 | SWITCH: { |
406 | /^abc/ and $abc = 1, last SWITCH; |
407 | /^def/ and $def = 1, last SWITCH; |
408 | /^xyz/ and $xyz = 1, last SWITCH; |
409 | $nothing = 1; |
410 | } |
411 | |
412 | or even, horrors, |
413 | |
414 | if (/^abc/) |
415 | { $abc = 1 } |
416 | elsif (/^def/) |
417 | { $def = 1 } |
418 | elsif (/^xyz/) |
419 | { $xyz = 1 } |
420 | else |
421 | { $nothing = 1 } |
422 | |
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423 | |
424 | A common idiom for a switch statement is to use C<foreach>'s aliasing to make |
425 | a temporary assignment to $_ for convenient matching: |
426 | |
427 | SWITCH: for ($where) { |
428 | /In Card Names/ && do { push @flags, '-e'; last; }; |
429 | /Anywhere/ && do { push @flags, '-h'; last; }; |
430 | /In Rulings/ && do { last; }; |
431 | die "unknown value for form variable where: `$where'"; |
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432 | } |
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433 | |
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434 | Another interesting approach to a switch statement is arrange |
435 | for a C<do> block to return the proper value: |
436 | |
437 | $amode = do { |
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438 | if ($flag & O_RDONLY) { "r" } |
439 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
cb1a09d0 |
440 | elsif ($flag & O_RDWR) { |
441 | if ($flag & O_CREAT) { "w+" } |
c07a80fd |
442 | else { ($flag & O_APPEND) ? "a+" : "r+" } |
cb1a09d0 |
443 | } |
444 | }; |
445 | |
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446 | =head2 Goto |
447 | |
448 | Although not for the faint of heart, Perl does support a C<goto> statement. |
449 | A loop's LABEL is not actually a valid target for a C<goto>; |
450 | it's just the name of the loop. There are three forms: goto-LABEL, |
451 | goto-EXPR, and goto-&NAME. |
452 | |
453 | The goto-LABEL form finds the statement labeled with LABEL and resumes |
454 | execution there. It may not be used to go into any construct that |
455 | requires initialization, such as a subroutine or a foreach loop. It |
456 | also can't be used to go into a construct that is optimized away. It |
457 | can be used to go almost anywhere else within the dynamic scope, |
458 | including out of subroutines, but it's usually better to use some other |
459 | construct such as last or die. The author of Perl has never felt the |
460 | need to use this form of goto (in Perl, that is--C is another matter). |
461 | |
462 | The goto-EXPR form expects a label name, whose scope will be resolved |
463 | dynamically. This allows for computed gotos per FORTRAN, but isn't |
464 | necessarily recommended if you're optimizing for maintainability: |
465 | |
466 | goto ("FOO", "BAR", "GLARCH")[$i]; |
467 | |
468 | The goto-&NAME form is highly magical, and substitutes a call to the |
469 | named subroutine for the currently running subroutine. This is used by |
470 | AUTOLOAD() subroutines that wish to load another subroutine and then |
471 | pretend that the other subroutine had been called in the first place |
472 | (except that any modifications to @_ in the current subroutine are |
473 | propagated to the other subroutine.) After the C<goto>, not even caller() |
474 | will be able to tell that this routine was called first. |
475 | |
c07a80fd |
476 | In almost all cases like this, it's usually a far, far better idea to use the |
477 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of |
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478 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
479 | C<eval{}> and die() for exception processing can also be a prudent approach. |
cb1a09d0 |
480 | |
481 | =head2 PODs: Embedded Documentation |
482 | |
483 | Perl has a mechanism for intermixing documentation with source code. |
c07a80fd |
484 | While it's expecting the beginning of a new statement, if the compiler |
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485 | encounters a line that begins with an equal sign and a word, like this |
486 | |
487 | =head1 Here There Be Pods! |
488 | |
489 | Then that text and all remaining text up through and including a line |
490 | beginning with C<=cut> will be ignored. The format of the intervening |
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491 | text is described in L<perlpod>. |
cb1a09d0 |
492 | |
493 | This allows you to intermix your source code |
494 | and your documentation text freely, as in |
495 | |
496 | =item snazzle($) |
497 | |
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498 | The snazzle() function will behave in the most spectacular |
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499 | form that you can possibly imagine, not even excepting |
500 | cybernetic pyrotechnics. |
501 | |
502 | =cut back to the compiler, nuff of this pod stuff! |
503 | |
504 | sub snazzle($) { |
505 | my $thingie = shift; |
506 | ......... |
54310121 |
507 | } |
cb1a09d0 |
508 | |
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509 | Note that pod translators should look at only paragraphs beginning |
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510 | with a pod directive (it makes parsing easier), whereas the compiler |
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511 | actually knows to look for pod escapes even in the middle of a |
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512 | paragraph. This means that the following secret stuff will be |
513 | ignored by both the compiler and the translators. |
514 | |
515 | $a=3; |
516 | =secret stuff |
517 | warn "Neither POD nor CODE!?" |
518 | =cut back |
519 | print "got $a\n"; |
520 | |
521 | You probably shouldn't rely upon the warn() being podded out forever. |
522 | Not all pod translators are well-behaved in this regard, and perhaps |
523 | the compiler will become pickier. |
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524 | |
525 | One may also use pod directives to quickly comment out a section |
526 | of code. |
527 | |
528 | =head2 Plain Old Comments (Not!) |
529 | |
530 | Much like the C preprocessor, perl can process line directives. Using |
531 | this, one can control perl's idea of filenames and line numbers in |
532 | error or warning messages (especially for strings that are processed |
533 | with eval()). The syntax for this mechanism is the same as for most |
534 | C preprocessors: it matches the regular expression |
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535 | C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line |
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536 | number for the next line, and C<$2> being the optional filename |
537 | (specified within quotes). |
538 | |
539 | Here are some examples that you should be able to type into your command |
540 | shell: |
541 | |
542 | % perl |
543 | # line 200 "bzzzt" |
544 | # the `#' on the previous line must be the first char on line |
545 | die 'foo'; |
546 | __END__ |
547 | foo at bzzzt line 201. |
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548 | |
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549 | % perl |
550 | # line 200 "bzzzt" |
551 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; |
552 | __END__ |
553 | foo at - line 2001. |
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554 | |
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555 | % perl |
556 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; |
557 | __END__ |
558 | foo at foo bar line 200. |
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559 | |
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560 | % perl |
561 | # line 345 "goop" |
562 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; |
563 | print $@; |
564 | __END__ |
565 | foo at goop line 345. |
566 | |
567 | =cut |