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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 |
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11 | start with a C<null> or C<0> value until they are defined by some explicit |
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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 exception |
25 | to this is format declarations, for obvious reasons.) Text from a |
26 | C<"#"> character until the end of the line is a comment, and is |
27 | ignored. If you attempt to use C</* */> C-style comments, it will be |
28 | interpreted either as division or pattern matching, depending on the |
29 | context, and C++ C<//> comments just look like a null regular |
30 | expression, so don't do that. |
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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 C<my()>, you'll have to make sure |
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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 $0 or die "can't get myname"; |
46 | |
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47 | Note that my() functions as a list operator, not as a unary operator; so |
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48 | be careful to use C<or> instead of C<||> in this case. However, if |
49 | you were to declare the subroutine as C<sub myname ($)>, then |
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50 | C<myname> would function as a unary operator, so either C<or> or |
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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 |
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86 | iterator: For each value in EXPR, it aliases C<$_> to the value and |
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87 | executes the statement. The C<while> and C<until> modifiers have the |
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88 | usual "C<while> loop" semantics (conditional evaluated first), except |
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89 | when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE |
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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 | |
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98 | See L<perlfunc/do>. Note also that the loop control statements described |
99 | later will I<NOT> work in this construct, because modifiers don't take |
100 | loop labels. Sorry. You can always put another block inside of it |
101 | (for C<next>) or around it (for C<last>) to do that sort of thing. |
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102 | For C<next>, just double the braces: |
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103 | |
104 | do {{ |
105 | next if $x == $y; |
106 | # do something here |
107 | }} until $x++ > $z; |
108 | |
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109 | For C<last>, you have to be more elaborate: |
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110 | |
111 | LOOP: { |
112 | do { |
113 | last if $x = $y**2; |
114 | # do something here |
115 | } while $x++ <= $z; |
116 | } |
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117 | |
118 | =head2 Compound statements |
119 | |
120 | In Perl, a sequence of statements that defines a scope is called a block. |
121 | Sometimes a block is delimited by the file containing it (in the case |
122 | of a required file, or the program as a whole), and sometimes a block |
123 | is delimited by the extent of a string (in the case of an eval). |
124 | |
125 | But generally, a block is delimited by curly brackets, also known as braces. |
126 | We will call this syntactic construct a BLOCK. |
127 | |
128 | The following compound statements may be used to control flow: |
129 | |
130 | if (EXPR) BLOCK |
131 | if (EXPR) BLOCK else BLOCK |
132 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
133 | LABEL while (EXPR) BLOCK |
134 | LABEL while (EXPR) BLOCK continue BLOCK |
135 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
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136 | LABEL foreach VAR (LIST) BLOCK |
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137 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
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138 | LABEL BLOCK continue BLOCK |
139 | |
140 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, |
141 | not statements. This means that the curly brackets are I<required>--no |
142 | dangling statements allowed. If you want to write conditionals without |
143 | curly brackets there are several other ways to do it. The following |
144 | all do the same thing: |
145 | |
146 | if (!open(FOO)) { die "Can't open $FOO: $!"; } |
147 | die "Can't open $FOO: $!" unless open(FOO); |
148 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! |
149 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; |
150 | # a bit exotic, that last one |
151 | |
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152 | The C<if> statement is straightforward. Because BLOCKs are always |
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153 | bounded by curly brackets, there is never any ambiguity about which |
154 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, |
155 | the sense of the test is reversed. |
156 | |
157 | The C<while> statement executes the block as long as the expression is |
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158 | true (does not evaluate to the null string (C<"">) or C<0> or C<"0")>. The LABEL is |
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159 | optional, and if present, consists of an identifier followed by a colon. |
160 | The LABEL identifies the loop for the loop control statements C<next>, |
161 | C<last>, and C<redo>. If the LABEL is omitted, the loop control statement |
162 | refers to the innermost enclosing loop. This may include dynamically |
163 | looking back your call-stack at run time to find the LABEL. Such |
164 | desperate behavior triggers a warning if you use the B<-w> flag. |
165 | |
166 | If there is a C<continue> BLOCK, it is always executed just before the |
167 | conditional is about to be evaluated again, just like the third part of a |
168 | C<for> loop in C. Thus it can be used to increment a loop variable, even |
169 | when the loop has been continued via the C<next> statement (which is |
170 | similar to the C C<continue> statement). |
171 | |
172 | =head2 Loop Control |
173 | |
174 | The C<next> command is like the C<continue> statement in C; it starts |
175 | the next iteration of the loop: |
176 | |
177 | LINE: while (<STDIN>) { |
178 | next LINE if /^#/; # discard comments |
179 | ... |
180 | } |
181 | |
182 | The C<last> command is like the C<break> statement in C (as used in |
183 | loops); it immediately exits the loop in question. The |
184 | C<continue> block, if any, is not executed: |
185 | |
186 | LINE: while (<STDIN>) { |
187 | last LINE if /^$/; # exit when done with header |
188 | ... |
189 | } |
190 | |
191 | The C<redo> command restarts the loop block without evaluating the |
192 | conditional again. The C<continue> block, if any, is I<not> executed. |
193 | This command is normally used by programs that want to lie to themselves |
194 | about what was just input. |
195 | |
196 | For example, when processing a file like F</etc/termcap>. |
197 | If your input lines might end in backslashes to indicate continuation, you |
198 | want to skip ahead and get the next record. |
199 | |
200 | while (<>) { |
201 | chomp; |
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202 | if (s/\\$//) { |
203 | $_ .= <>; |
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204 | redo unless eof(); |
205 | } |
206 | # now process $_ |
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207 | } |
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208 | |
209 | which is Perl short-hand for the more explicitly written version: |
210 | |
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211 | LINE: while (defined($line = <ARGV>)) { |
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212 | chomp($line); |
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213 | if ($line =~ s/\\$//) { |
214 | $line .= <ARGV>; |
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215 | redo LINE unless eof(); # not eof(ARGV)! |
216 | } |
217 | # now process $line |
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218 | } |
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219 | |
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220 | Note that if there were a C<continue> block on the above code, it would get |
221 | executed even on discarded lines. This is often used to reset line counters |
222 | or C<?pat?> one-time matches. |
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223 | |
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224 | # inspired by :1,$g/fred/s//WILMA/ |
225 | while (<>) { |
226 | ?(fred)? && s//WILMA $1 WILMA/; |
227 | ?(barney)? && s//BETTY $1 BETTY/; |
228 | ?(homer)? && s//MARGE $1 MARGE/; |
229 | } continue { |
230 | print "$ARGV $.: $_"; |
231 | close ARGV if eof(); # reset $. |
232 | reset if eof(); # reset ?pat? |
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233 | } |
234 | |
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235 | If the word C<while> is replaced by the word C<until>, the sense of the |
236 | test is reversed, but the conditional is still tested before the first |
237 | iteration. |
238 | |
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239 | The loop control statements don't work in an C<if> or C<unless>, since |
240 | they aren't loops. You can double the braces to make them such, though. |
241 | |
242 | if (/pattern/) {{ |
243 | next if /fred/; |
244 | next if /barney/; |
245 | # so something here |
246 | }} |
247 | |
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248 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
249 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. |
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250 | |
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251 | =head2 For Loops |
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252 | |
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253 | Perl's C-style C<for> loop works exactly like the corresponding C<while> loop; |
254 | that means that this: |
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255 | |
256 | for ($i = 1; $i < 10; $i++) { |
257 | ... |
258 | } |
259 | |
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260 | is the same as this: |
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261 | |
262 | $i = 1; |
263 | while ($i < 10) { |
264 | ... |
265 | } continue { |
266 | $i++; |
267 | } |
268 | |
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269 | (There is one minor difference: The first form implies a lexical scope |
270 | for variables declared with C<my> in the initialization expression.) |
271 | |
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272 | Besides the normal array index looping, C<for> can lend itself |
273 | to many other interesting applications. Here's one that avoids the |
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274 | problem you get into if you explicitly test for end-of-file on |
275 | an interactive file descriptor causing your program to appear to |
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276 | hang. |
277 | |
278 | $on_a_tty = -t STDIN && -t STDOUT; |
279 | sub prompt { print "yes? " if $on_a_tty } |
280 | for ( prompt(); <STDIN>; prompt() ) { |
281 | # do something |
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282 | } |
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283 | |
284 | =head2 Foreach Loops |
285 | |
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286 | The C<foreach> loop iterates over a normal list value and sets the |
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287 | variable VAR to be each element of the list in turn. If the variable |
288 | is preceded with the keyword C<my>, then it is lexically scoped, and |
289 | is therefore visible only within the loop. Otherwise, the variable is |
290 | implicitly local to the loop and regains its former value upon exiting |
291 | the loop. If the variable was previously declared with C<my>, it uses |
292 | that variable instead of the global one, but it's still localized to |
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293 | the loop. |
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294 | |
295 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so |
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296 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
297 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> |
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298 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
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299 | If any element of LIST is an lvalue, you can modify it by modifying VAR |
300 | inside the loop. That's because the C<foreach> loop index variable is |
301 | an implicit alias for each item in the list that you're looping over. |
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302 | |
303 | If any part of LIST is an array, C<foreach> will get very confused if |
304 | you add or remove elements within the loop body, for example with |
305 | C<splice>. So don't do that. |
306 | |
307 | C<foreach> probably won't do what you expect if VAR is a tied or other |
308 | special variable. Don't do that either. |
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309 | |
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310 | Examples: |
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311 | |
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312 | for (@ary) { s/foo/bar/ } |
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313 | |
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314 | foreach my $elem (@elements) { |
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315 | $elem *= 2; |
316 | } |
317 | |
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318 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
319 | print $count, "\n"; sleep(1); |
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320 | } |
321 | |
322 | for (1..15) { print "Merry Christmas\n"; } |
323 | |
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324 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
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325 | print "Item: $item\n"; |
326 | } |
327 | |
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328 | Here's how a C programmer might code up a particular algorithm in Perl: |
329 | |
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330 | for (my $i = 0; $i < @ary1; $i++) { |
331 | for (my $j = 0; $j < @ary2; $j++) { |
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332 | if ($ary1[$i] > $ary2[$j]) { |
333 | last; # can't go to outer :-( |
334 | } |
335 | $ary1[$i] += $ary2[$j]; |
336 | } |
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337 | # this is where that last takes me |
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338 | } |
339 | |
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340 | Whereas here's how a Perl programmer more comfortable with the idiom might |
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341 | do it: |
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342 | |
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343 | OUTER: foreach my $wid (@ary1) { |
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344 | INNER: foreach my $jet (@ary2) { |
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345 | next OUTER if $wid > $jet; |
346 | $wid += $jet; |
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347 | } |
348 | } |
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349 | |
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350 | See how much easier this is? It's cleaner, safer, and faster. It's |
351 | cleaner because it's less noisy. It's safer because if code gets added |
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352 | between the inner and outer loops later on, the new code won't be |
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353 | accidentally executed. The C<next> explicitly iterates the other loop |
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354 | rather than merely terminating the inner one. And it's faster because |
355 | Perl executes a C<foreach> statement more rapidly than it would the |
356 | equivalent C<for> loop. |
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357 | |
358 | =head2 Basic BLOCKs and Switch Statements |
359 | |
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360 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
361 | loop that executes once. Thus you can use any of the loop control |
362 | statements in it to leave or restart the block. (Note that this is |
363 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief |
364 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> |
365 | block is optional. |
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366 | |
367 | The BLOCK construct is particularly nice for doing case |
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368 | structures. |
369 | |
370 | SWITCH: { |
371 | if (/^abc/) { $abc = 1; last SWITCH; } |
372 | if (/^def/) { $def = 1; last SWITCH; } |
373 | if (/^xyz/) { $xyz = 1; last SWITCH; } |
374 | $nothing = 1; |
375 | } |
376 | |
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377 | There is no official C<switch> statement in Perl, because there are |
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378 | already several ways to write the equivalent. In addition to the |
379 | above, you could write |
380 | |
381 | SWITCH: { |
382 | $abc = 1, last SWITCH if /^abc/; |
383 | $def = 1, last SWITCH if /^def/; |
384 | $xyz = 1, last SWITCH if /^xyz/; |
385 | $nothing = 1; |
386 | } |
387 | |
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388 | (That's actually not as strange as it looks once you realize that you can |
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389 | use loop control "operators" within an expression, That's just the normal |
390 | C comma operator.) |
391 | |
392 | or |
393 | |
394 | SWITCH: { |
395 | /^abc/ && do { $abc = 1; last SWITCH; }; |
396 | /^def/ && do { $def = 1; last SWITCH; }; |
397 | /^xyz/ && do { $xyz = 1; last SWITCH; }; |
398 | $nothing = 1; |
399 | } |
400 | |
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401 | or formatted so it stands out more as a "proper" C<switch> statement: |
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402 | |
403 | SWITCH: { |
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404 | /^abc/ && do { |
405 | $abc = 1; |
406 | last SWITCH; |
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407 | }; |
408 | |
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409 | /^def/ && do { |
410 | $def = 1; |
411 | last SWITCH; |
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412 | }; |
413 | |
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414 | /^xyz/ && do { |
415 | $xyz = 1; |
416 | last SWITCH; |
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417 | }; |
418 | $nothing = 1; |
419 | } |
420 | |
421 | or |
422 | |
423 | SWITCH: { |
424 | /^abc/ and $abc = 1, last SWITCH; |
425 | /^def/ and $def = 1, last SWITCH; |
426 | /^xyz/ and $xyz = 1, last SWITCH; |
427 | $nothing = 1; |
428 | } |
429 | |
430 | or even, horrors, |
431 | |
432 | if (/^abc/) |
433 | { $abc = 1 } |
434 | elsif (/^def/) |
435 | { $def = 1 } |
436 | elsif (/^xyz/) |
437 | { $xyz = 1 } |
438 | else |
439 | { $nothing = 1 } |
440 | |
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441 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make |
442 | a temporary assignment to C<$_> for convenient matching: |
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443 | |
444 | SWITCH: for ($where) { |
445 | /In Card Names/ && do { push @flags, '-e'; last; }; |
446 | /Anywhere/ && do { push @flags, '-h'; last; }; |
447 | /In Rulings/ && do { last; }; |
448 | die "unknown value for form variable where: `$where'"; |
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449 | } |
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450 | |
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451 | Another interesting approach to a switch statement is arrange |
452 | for a C<do> block to return the proper value: |
453 | |
454 | $amode = do { |
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455 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? |
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456 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
cb1a09d0 |
457 | elsif ($flag & O_RDWR) { |
458 | if ($flag & O_CREAT) { "w+" } |
c07a80fd |
459 | else { ($flag & O_APPEND) ? "a+" : "r+" } |
cb1a09d0 |
460 | } |
461 | }; |
462 | |
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463 | Or |
464 | |
465 | print do { |
466 | ($flags & O_WRONLY) ? "write-only" : |
467 | ($flags & O_RDWR) ? "read-write" : |
468 | "read-only"; |
469 | }; |
470 | |
471 | Or if you are certainly that all the C<&&> clauses are true, you can use |
472 | something like this, which "switches" on the value of the |
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473 | C<HTTP_USER_AGENT> envariable. |
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474 | |
475 | #!/usr/bin/perl |
476 | # pick out jargon file page based on browser |
477 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; |
478 | for ($ENV{HTTP_USER_AGENT}) { |
479 | $page = /Mac/ && 'm/Macintrash.html' |
480 | || /Win(dows )?NT/ && 'e/evilandrude.html' |
481 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' |
482 | || /Linux/ && 'l/Linux.html' |
483 | || /HP-UX/ && 'h/HP-SUX.html' |
484 | || /SunOS/ && 's/ScumOS.html' |
485 | || 'a/AppendixB.html'; |
486 | } |
487 | print "Location: $dir/$page\015\012\015\012"; |
488 | |
489 | That kind of switch statement only works when you know the C<&&> clauses |
490 | will be true. If you don't, the previous C<?:> example should be used. |
491 | |
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492 | You might also consider writing a hash of subroutine references |
493 | instead of synthesizing a C<switch> statement. |
5a964f20 |
494 | |
4633a7c4 |
495 | =head2 Goto |
496 | |
19799a22 |
497 | Although not for the faint of heart, Perl does support a C<goto> |
498 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and |
499 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for |
500 | a C<goto>; it's just the name of the loop. |
4633a7c4 |
501 | |
f86cebdf |
502 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 |
503 | execution there. It may not be used to go into any construct that |
f86cebdf |
504 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 |
505 | also can't be used to go into a construct that is optimized away. It |
506 | can be used to go almost anywhere else within the dynamic scope, |
507 | including out of subroutines, but it's usually better to use some other |
f86cebdf |
508 | construct such as C<last> or C<die>. The author of Perl has never felt the |
509 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
4633a7c4 |
510 | |
f86cebdf |
511 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
512 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't |
4633a7c4 |
513 | necessarily recommended if you're optimizing for maintainability: |
514 | |
515 | goto ("FOO", "BAR", "GLARCH")[$i]; |
516 | |
f86cebdf |
517 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 |
518 | named subroutine for the currently running subroutine. This is used by |
f86cebdf |
519 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 |
520 | pretend that the other subroutine had been called in the first place |
f86cebdf |
521 | (except that any modifications to C<@_> in the current subroutine are |
522 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> |
4633a7c4 |
523 | will be able to tell that this routine was called first. |
524 | |
c07a80fd |
525 | In almost all cases like this, it's usually a far, far better idea to use the |
526 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of |
4633a7c4 |
527 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
528 | C<eval{}> and die() for exception processing can also be a prudent approach. |
cb1a09d0 |
529 | |
530 | =head2 PODs: Embedded Documentation |
531 | |
532 | Perl has a mechanism for intermixing documentation with source code. |
c07a80fd |
533 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 |
534 | encounters a line that begins with an equal sign and a word, like this |
535 | |
536 | =head1 Here There Be Pods! |
537 | |
538 | Then that text and all remaining text up through and including a line |
539 | beginning with C<=cut> will be ignored. The format of the intervening |
54310121 |
540 | text is described in L<perlpod>. |
cb1a09d0 |
541 | |
542 | This allows you to intermix your source code |
543 | and your documentation text freely, as in |
544 | |
545 | =item snazzle($) |
546 | |
54310121 |
547 | The snazzle() function will behave in the most spectacular |
cb1a09d0 |
548 | form that you can possibly imagine, not even excepting |
549 | cybernetic pyrotechnics. |
550 | |
551 | =cut back to the compiler, nuff of this pod stuff! |
552 | |
553 | sub snazzle($) { |
554 | my $thingie = shift; |
555 | ......... |
54310121 |
556 | } |
cb1a09d0 |
557 | |
54310121 |
558 | Note that pod translators should look at only paragraphs beginning |
184e9718 |
559 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 |
560 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 |
561 | paragraph. This means that the following secret stuff will be |
562 | ignored by both the compiler and the translators. |
563 | |
564 | $a=3; |
565 | =secret stuff |
566 | warn "Neither POD nor CODE!?" |
567 | =cut back |
568 | print "got $a\n"; |
569 | |
f86cebdf |
570 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 |
571 | Not all pod translators are well-behaved in this regard, and perhaps |
572 | the compiler will become pickier. |
774d564b |
573 | |
574 | One may also use pod directives to quickly comment out a section |
575 | of code. |
576 | |
577 | =head2 Plain Old Comments (Not!) |
578 | |
5a964f20 |
579 | Much like the C preprocessor, Perl can process line directives. Using |
580 | this, one can control Perl's idea of filenames and line numbers in |
774d564b |
581 | error or warning messages (especially for strings that are processed |
f86cebdf |
582 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b |
583 | C preprocessors: it matches the regular expression |
4b094ceb |
584 | C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line |
774d564b |
585 | number for the next line, and C<$2> being the optional filename |
586 | (specified within quotes). |
587 | |
588 | Here are some examples that you should be able to type into your command |
589 | shell: |
590 | |
591 | % perl |
592 | # line 200 "bzzzt" |
593 | # the `#' on the previous line must be the first char on line |
594 | die 'foo'; |
595 | __END__ |
596 | foo at bzzzt line 201. |
54310121 |
597 | |
774d564b |
598 | % perl |
599 | # line 200 "bzzzt" |
600 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; |
601 | __END__ |
602 | foo at - line 2001. |
54310121 |
603 | |
774d564b |
604 | % perl |
605 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; |
606 | __END__ |
607 | foo at foo bar line 200. |
54310121 |
608 | |
774d564b |
609 | % perl |
610 | # line 345 "goop" |
611 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; |
612 | print $@; |
613 | __END__ |
614 | foo at goop line 345. |
615 | |
616 | =cut |