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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. |
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8 | The sequence of statements is executed just once, unlike in B<sed> |
9 | and B<awk> scripts, where the sequence of statements is executed |
10 | for each input line. While this means that you must explicitly |
11 | loop over the lines of your input file (or files), it also means |
12 | you have much more control over which files and which lines you look at. |
13 | (Actually, I'm lying--it is possible to do an implicit loop with |
14 | either the B<-n> or B<-p> switch. It's just not the mandatory |
15 | default like it is in B<sed> and B<awk>.) |
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16 | |
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17 | Perl is, for the most part, a free-form language. (The only exception |
18 | to this is format declarations, for obvious reasons.) Text from a |
19 | C<"#"> character until the end of the line is a comment, and is |
20 | ignored. If you attempt to use C</* */> C-style comments, it will be |
21 | interpreted either as division or pattern matching, depending on the |
22 | context, and C++ C<//> comments just look like a null regular |
23 | expression, so don't do that. |
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24 | |
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25 | =head2 Declarations |
26 | |
27 | The only things you need to declare in Perl are report formats |
28 | and subroutines--and even undefined subroutines can be handled |
29 | through AUTOLOAD. A variable holds the undefined value (C<undef>) |
30 | until it has been assigned a defined value, which is anything |
31 | other than C<undef>. When used as a number, C<undef> is treated |
32 | as C<0>; when used as a string, it is treated the empty string, |
33 | C<"">; and when used as a reference that isn't being assigned |
34 | to, it is treated as an error. If you enable warnings, you'll |
35 | be notified of an uninitialized value whenever you treat C<undef> |
36 | as a string or a number. Well, usually. Boolean ("don't-care") |
37 | contexts and operators such as C<++>, C<-->, C<+=>, C<-=>, and |
38 | C<.=> are always exempt from such warnings. |
39 | |
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40 | A declaration can be put anywhere a statement can, but has no effect on |
41 | the execution of the primary sequence of statements--declarations all |
42 | take effect at compile time. Typically all the declarations are put at |
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43 | the beginning or the end of the script. However, if you're using |
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44 | lexically-scoped private variables created with C<my()>, you'll |
45 | have to make sure |
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46 | your format or subroutine definition is within the same block scope |
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47 | as the my if you expect to be able to access those private variables. |
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48 | |
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49 | Declaring a subroutine allows a subroutine name to be used as if it were a |
50 | list operator from that point forward in the program. You can declare a |
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51 | subroutine without defining it by saying C<sub name>, thus: |
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52 | |
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53 | sub myname; |
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54 | $me = myname $0 or die "can't get myname"; |
55 | |
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56 | Note that myname() functions as a list operator, not as a unary operator; |
57 | so be careful to use C<or> instead of C<||> in this case. However, if |
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58 | you were to declare the subroutine as C<sub myname ($)>, then |
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59 | C<myname> would function as a unary operator, so either C<or> or |
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60 | C<||> would work. |
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61 | |
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62 | Subroutines declarations can also be loaded up with the C<require> statement |
63 | or both loaded and imported into your namespace with a C<use> statement. |
64 | See L<perlmod> for details on this. |
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65 | |
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66 | A statement sequence may contain declarations of lexically-scoped |
67 | variables, but apart from declaring a variable name, the declaration acts |
68 | like an ordinary statement, and is elaborated within the sequence of |
69 | statements as if it were an ordinary statement. That means it actually |
70 | has both compile-time and run-time effects. |
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71 | |
72 | =head2 Simple statements |
73 | |
74 | The only kind of simple statement is an expression evaluated for its |
75 | side effects. Every simple statement must be terminated with a |
76 | semicolon, unless it is the final statement in a block, in which case |
77 | the semicolon is optional. (A semicolon is still encouraged there if the |
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78 | block takes up more than one line, because you may eventually add another line.) |
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79 | Note that there are some operators like C<eval {}> and C<do {}> that look |
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80 | like compound statements, but aren't (they're just TERMs in an expression), |
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81 | and thus need an explicit termination if used as the last item in a statement. |
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82 | |
83 | Any simple statement may optionally be followed by a I<SINGLE> modifier, |
84 | just before the terminating semicolon (or block ending). The possible |
85 | modifiers are: |
86 | |
87 | if EXPR |
88 | unless EXPR |
89 | while EXPR |
90 | until EXPR |
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91 | foreach EXPR |
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92 | |
93 | The C<if> and C<unless> modifiers have the expected semantics, |
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94 | presuming you're a speaker of English. The C<foreach> modifier is an |
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95 | iterator: For each value in EXPR, it aliases C<$_> to the value and |
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96 | executes the statement. The C<while> and C<until> modifiers have the |
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97 | usual "C<while> loop" semantics (conditional evaluated first), except |
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98 | when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE |
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99 | statement), in which case the block executes once before the |
100 | conditional is evaluated. This is so that you can write loops like: |
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101 | |
102 | do { |
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103 | $line = <STDIN>; |
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104 | ... |
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105 | } until $line eq ".\n"; |
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106 | |
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107 | See L<perlfunc/do>. Note also that the loop control statements described |
108 | later will I<NOT> work in this construct, because modifiers don't take |
109 | loop labels. Sorry. You can always put another block inside of it |
110 | (for C<next>) or around it (for C<last>) to do that sort of thing. |
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111 | For C<next>, just double the braces: |
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112 | |
113 | do {{ |
114 | next if $x == $y; |
115 | # do something here |
116 | }} until $x++ > $z; |
117 | |
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118 | For C<last>, you have to be more elaborate: |
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119 | |
120 | LOOP: { |
121 | do { |
122 | last if $x = $y**2; |
123 | # do something here |
124 | } while $x++ <= $z; |
125 | } |
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126 | |
127 | =head2 Compound statements |
128 | |
129 | In Perl, a sequence of statements that defines a scope is called a block. |
130 | Sometimes a block is delimited by the file containing it (in the case |
131 | of a required file, or the program as a whole), and sometimes a block |
132 | is delimited by the extent of a string (in the case of an eval). |
133 | |
134 | But generally, a block is delimited by curly brackets, also known as braces. |
135 | We will call this syntactic construct a BLOCK. |
136 | |
137 | The following compound statements may be used to control flow: |
138 | |
139 | if (EXPR) BLOCK |
140 | if (EXPR) BLOCK else BLOCK |
141 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
142 | LABEL while (EXPR) BLOCK |
143 | LABEL while (EXPR) BLOCK continue BLOCK |
144 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
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145 | LABEL foreach VAR (LIST) BLOCK |
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146 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
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147 | LABEL BLOCK continue BLOCK |
148 | |
149 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, |
150 | not statements. This means that the curly brackets are I<required>--no |
151 | dangling statements allowed. If you want to write conditionals without |
152 | curly brackets there are several other ways to do it. The following |
153 | all do the same thing: |
154 | |
155 | if (!open(FOO)) { die "Can't open $FOO: $!"; } |
156 | die "Can't open $FOO: $!" unless open(FOO); |
157 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! |
158 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; |
159 | # a bit exotic, that last one |
160 | |
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161 | The C<if> statement is straightforward. Because BLOCKs are always |
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162 | bounded by curly brackets, there is never any ambiguity about which |
163 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, |
164 | the sense of the test is reversed. |
165 | |
166 | The C<while> statement executes the block as long as the expression is |
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167 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). |
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168 | The LABEL is optional, and if present, consists of an identifier followed |
169 | by a colon. The LABEL identifies the loop for the loop control |
170 | statements C<next>, C<last>, and C<redo>. |
171 | If the LABEL is omitted, the loop control statement |
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172 | refers to the innermost enclosing loop. This may include dynamically |
173 | looking back your call-stack at run time to find the LABEL. Such |
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174 | desperate behavior triggers a warning if you use the C<use warnings> |
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175 | pragma or the B<-w> flag. |
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176 | Unlike a C<foreach> statement, a C<while> statement never implicitly |
177 | localises any variables. |
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178 | |
179 | If there is a C<continue> BLOCK, it is always executed just before the |
180 | conditional is about to be evaluated again, just like the third part of a |
181 | C<for> loop in C. Thus it can be used to increment a loop variable, even |
182 | when the loop has been continued via the C<next> statement (which is |
183 | similar to the C C<continue> statement). |
184 | |
185 | =head2 Loop Control |
186 | |
187 | The C<next> command is like the C<continue> statement in C; it starts |
188 | the next iteration of the loop: |
189 | |
190 | LINE: while (<STDIN>) { |
191 | next LINE if /^#/; # discard comments |
192 | ... |
193 | } |
194 | |
195 | The C<last> command is like the C<break> statement in C (as used in |
196 | loops); it immediately exits the loop in question. The |
197 | C<continue> block, if any, is not executed: |
198 | |
199 | LINE: while (<STDIN>) { |
200 | last LINE if /^$/; # exit when done with header |
201 | ... |
202 | } |
203 | |
204 | The C<redo> command restarts the loop block without evaluating the |
205 | conditional again. The C<continue> block, if any, is I<not> executed. |
206 | This command is normally used by programs that want to lie to themselves |
207 | about what was just input. |
208 | |
209 | For example, when processing a file like F</etc/termcap>. |
210 | If your input lines might end in backslashes to indicate continuation, you |
211 | want to skip ahead and get the next record. |
212 | |
213 | while (<>) { |
214 | chomp; |
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215 | if (s/\\$//) { |
216 | $_ .= <>; |
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217 | redo unless eof(); |
218 | } |
219 | # now process $_ |
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220 | } |
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221 | |
222 | which is Perl short-hand for the more explicitly written version: |
223 | |
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224 | LINE: while (defined($line = <ARGV>)) { |
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225 | chomp($line); |
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226 | if ($line =~ s/\\$//) { |
227 | $line .= <ARGV>; |
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228 | redo LINE unless eof(); # not eof(ARGV)! |
229 | } |
230 | # now process $line |
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231 | } |
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232 | |
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233 | Note that if there were a C<continue> block on the above code, it would get |
234 | executed even on discarded lines. This is often used to reset line counters |
235 | or C<?pat?> one-time matches. |
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236 | |
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237 | # inspired by :1,$g/fred/s//WILMA/ |
238 | while (<>) { |
239 | ?(fred)? && s//WILMA $1 WILMA/; |
240 | ?(barney)? && s//BETTY $1 BETTY/; |
241 | ?(homer)? && s//MARGE $1 MARGE/; |
242 | } continue { |
243 | print "$ARGV $.: $_"; |
244 | close ARGV if eof(); # reset $. |
245 | reset if eof(); # reset ?pat? |
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246 | } |
247 | |
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248 | If the word C<while> is replaced by the word C<until>, the sense of the |
249 | test is reversed, but the conditional is still tested before the first |
250 | iteration. |
251 | |
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252 | The loop control statements don't work in an C<if> or C<unless>, since |
253 | they aren't loops. You can double the braces to make them such, though. |
254 | |
255 | if (/pattern/) {{ |
256 | next if /fred/; |
257 | next if /barney/; |
258 | # so something here |
259 | }} |
260 | |
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261 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
262 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. |
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263 | |
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264 | =head2 For Loops |
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265 | |
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266 | Perl's C-style C<for> loop works exactly like the corresponding C<while> loop; |
267 | that means that this: |
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268 | |
269 | for ($i = 1; $i < 10; $i++) { |
270 | ... |
271 | } |
272 | |
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273 | is the same as this: |
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274 | |
275 | $i = 1; |
276 | while ($i < 10) { |
277 | ... |
278 | } continue { |
279 | $i++; |
280 | } |
281 | |
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282 | (There is one minor difference: The first form implies a lexical scope |
283 | for variables declared with C<my> in the initialization expression.) |
284 | |
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285 | Besides the normal array index looping, C<for> can lend itself |
286 | to many other interesting applications. Here's one that avoids the |
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287 | problem you get into if you explicitly test for end-of-file on |
288 | an interactive file descriptor causing your program to appear to |
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289 | hang. |
290 | |
291 | $on_a_tty = -t STDIN && -t STDOUT; |
292 | sub prompt { print "yes? " if $on_a_tty } |
293 | for ( prompt(); <STDIN>; prompt() ) { |
294 | # do something |
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295 | } |
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296 | |
297 | =head2 Foreach Loops |
298 | |
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299 | The C<foreach> loop iterates over a normal list value and sets the |
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300 | variable VAR to be each element of the list in turn. If the variable |
301 | is preceded with the keyword C<my>, then it is lexically scoped, and |
302 | is therefore visible only within the loop. Otherwise, the variable is |
303 | implicitly local to the loop and regains its former value upon exiting |
304 | the loop. If the variable was previously declared with C<my>, it uses |
305 | that variable instead of the global one, but it's still localized to |
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306 | the loop. |
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307 | |
308 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so |
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309 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
310 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> |
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311 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
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312 | |
313 | If any element of LIST is an lvalue, you can modify it by modifying |
314 | VAR inside the loop. Conversely, if any element of LIST is NOT an |
315 | lvalue, any attempt to modify that element will fail. In other words, |
316 | the C<foreach> loop index variable is an implicit alias for each item |
317 | in the list that you're looping over. |
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318 | |
319 | If any part of LIST is an array, C<foreach> will get very confused if |
320 | you add or remove elements within the loop body, for example with |
321 | C<splice>. So don't do that. |
322 | |
323 | C<foreach> probably won't do what you expect if VAR is a tied or other |
324 | special variable. Don't do that either. |
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325 | |
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326 | Examples: |
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327 | |
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328 | for (@ary) { s/foo/bar/ } |
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329 | |
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330 | for my $elem (@elements) { |
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331 | $elem *= 2; |
332 | } |
333 | |
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334 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
335 | print $count, "\n"; sleep(1); |
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336 | } |
337 | |
338 | for (1..15) { print "Merry Christmas\n"; } |
339 | |
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340 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
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341 | print "Item: $item\n"; |
342 | } |
343 | |
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344 | Here's how a C programmer might code up a particular algorithm in Perl: |
345 | |
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346 | for (my $i = 0; $i < @ary1; $i++) { |
347 | for (my $j = 0; $j < @ary2; $j++) { |
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348 | if ($ary1[$i] > $ary2[$j]) { |
349 | last; # can't go to outer :-( |
350 | } |
351 | $ary1[$i] += $ary2[$j]; |
352 | } |
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353 | # this is where that last takes me |
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354 | } |
355 | |
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356 | Whereas here's how a Perl programmer more comfortable with the idiom might |
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357 | do it: |
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358 | |
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359 | OUTER: for my $wid (@ary1) { |
360 | INNER: for my $jet (@ary2) { |
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361 | next OUTER if $wid > $jet; |
362 | $wid += $jet; |
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363 | } |
364 | } |
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365 | |
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366 | See how much easier this is? It's cleaner, safer, and faster. It's |
367 | cleaner because it's less noisy. It's safer because if code gets added |
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368 | between the inner and outer loops later on, the new code won't be |
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369 | accidentally executed. The C<next> explicitly iterates the other loop |
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370 | rather than merely terminating the inner one. And it's faster because |
371 | Perl executes a C<foreach> statement more rapidly than it would the |
372 | equivalent C<for> loop. |
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373 | |
374 | =head2 Basic BLOCKs and Switch Statements |
375 | |
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376 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
377 | loop that executes once. Thus you can use any of the loop control |
378 | statements in it to leave or restart the block. (Note that this is |
379 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief |
380 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> |
381 | block is optional. |
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382 | |
383 | The BLOCK construct is particularly nice for doing case |
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384 | structures. |
385 | |
386 | SWITCH: { |
387 | if (/^abc/) { $abc = 1; last SWITCH; } |
388 | if (/^def/) { $def = 1; last SWITCH; } |
389 | if (/^xyz/) { $xyz = 1; last SWITCH; } |
390 | $nothing = 1; |
391 | } |
392 | |
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393 | There is no official C<switch> statement in Perl, because there are |
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394 | already several ways to write the equivalent. In addition to the |
395 | above, you could write |
396 | |
397 | SWITCH: { |
398 | $abc = 1, last SWITCH if /^abc/; |
399 | $def = 1, last SWITCH if /^def/; |
400 | $xyz = 1, last SWITCH if /^xyz/; |
401 | $nothing = 1; |
402 | } |
403 | |
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404 | (That's actually not as strange as it looks once you realize that you can |
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405 | use loop control "operators" within an expression, That's just the normal |
406 | C comma operator.) |
407 | |
408 | or |
409 | |
410 | SWITCH: { |
411 | /^abc/ && do { $abc = 1; last SWITCH; }; |
412 | /^def/ && do { $def = 1; last SWITCH; }; |
413 | /^xyz/ && do { $xyz = 1; last SWITCH; }; |
414 | $nothing = 1; |
415 | } |
416 | |
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417 | or formatted so it stands out more as a "proper" C<switch> statement: |
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418 | |
419 | SWITCH: { |
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420 | /^abc/ && do { |
421 | $abc = 1; |
422 | last SWITCH; |
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423 | }; |
424 | |
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425 | /^def/ && do { |
426 | $def = 1; |
427 | last SWITCH; |
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428 | }; |
429 | |
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430 | /^xyz/ && do { |
431 | $xyz = 1; |
432 | last SWITCH; |
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433 | }; |
434 | $nothing = 1; |
435 | } |
436 | |
437 | or |
438 | |
439 | SWITCH: { |
440 | /^abc/ and $abc = 1, last SWITCH; |
441 | /^def/ and $def = 1, last SWITCH; |
442 | /^xyz/ and $xyz = 1, last SWITCH; |
443 | $nothing = 1; |
444 | } |
445 | |
446 | or even, horrors, |
447 | |
448 | if (/^abc/) |
449 | { $abc = 1 } |
450 | elsif (/^def/) |
451 | { $def = 1 } |
452 | elsif (/^xyz/) |
453 | { $xyz = 1 } |
454 | else |
455 | { $nothing = 1 } |
456 | |
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457 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make |
458 | a temporary assignment to C<$_> for convenient matching: |
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459 | |
460 | SWITCH: for ($where) { |
461 | /In Card Names/ && do { push @flags, '-e'; last; }; |
462 | /Anywhere/ && do { push @flags, '-h'; last; }; |
463 | /In Rulings/ && do { last; }; |
464 | die "unknown value for form variable where: `$where'"; |
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465 | } |
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466 | |
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467 | Another interesting approach to a switch statement is arrange |
468 | for a C<do> block to return the proper value: |
469 | |
470 | $amode = do { |
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471 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? |
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472 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
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473 | elsif ($flag & O_RDWR) { |
474 | if ($flag & O_CREAT) { "w+" } |
c07a80fd |
475 | else { ($flag & O_APPEND) ? "a+" : "r+" } |
cb1a09d0 |
476 | } |
477 | }; |
478 | |
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479 | Or |
480 | |
481 | print do { |
482 | ($flags & O_WRONLY) ? "write-only" : |
483 | ($flags & O_RDWR) ? "read-write" : |
484 | "read-only"; |
485 | }; |
486 | |
487 | Or if you are certainly that all the C<&&> clauses are true, you can use |
488 | something like this, which "switches" on the value of the |
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489 | C<HTTP_USER_AGENT> environment variable. |
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490 | |
491 | #!/usr/bin/perl |
492 | # pick out jargon file page based on browser |
493 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; |
494 | for ($ENV{HTTP_USER_AGENT}) { |
495 | $page = /Mac/ && 'm/Macintrash.html' |
496 | || /Win(dows )?NT/ && 'e/evilandrude.html' |
497 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' |
498 | || /Linux/ && 'l/Linux.html' |
499 | || /HP-UX/ && 'h/HP-SUX.html' |
500 | || /SunOS/ && 's/ScumOS.html' |
501 | || 'a/AppendixB.html'; |
502 | } |
503 | print "Location: $dir/$page\015\012\015\012"; |
504 | |
505 | That kind of switch statement only works when you know the C<&&> clauses |
506 | will be true. If you don't, the previous C<?:> example should be used. |
507 | |
19799a22 |
508 | You might also consider writing a hash of subroutine references |
509 | instead of synthesizing a C<switch> statement. |
5a964f20 |
510 | |
4633a7c4 |
511 | =head2 Goto |
512 | |
19799a22 |
513 | Although not for the faint of heart, Perl does support a C<goto> |
514 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and |
515 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for |
516 | a C<goto>; it's just the name of the loop. |
4633a7c4 |
517 | |
f86cebdf |
518 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 |
519 | execution there. It may not be used to go into any construct that |
f86cebdf |
520 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 |
521 | also can't be used to go into a construct that is optimized away. It |
522 | can be used to go almost anywhere else within the dynamic scope, |
523 | including out of subroutines, but it's usually better to use some other |
f86cebdf |
524 | construct such as C<last> or C<die>. The author of Perl has never felt the |
525 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
4633a7c4 |
526 | |
f86cebdf |
527 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
528 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't |
4633a7c4 |
529 | necessarily recommended if you're optimizing for maintainability: |
530 | |
96f2dc66 |
531 | goto(("FOO", "BAR", "GLARCH")[$i]); |
4633a7c4 |
532 | |
f86cebdf |
533 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 |
534 | named subroutine for the currently running subroutine. This is used by |
f86cebdf |
535 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 |
536 | pretend that the other subroutine had been called in the first place |
f86cebdf |
537 | (except that any modifications to C<@_> in the current subroutine are |
538 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> |
4633a7c4 |
539 | will be able to tell that this routine was called first. |
540 | |
c07a80fd |
541 | In almost all cases like this, it's usually a far, far better idea to use the |
542 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of |
4633a7c4 |
543 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
544 | C<eval{}> and die() for exception processing can also be a prudent approach. |
cb1a09d0 |
545 | |
546 | =head2 PODs: Embedded Documentation |
547 | |
548 | Perl has a mechanism for intermixing documentation with source code. |
c07a80fd |
549 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 |
550 | encounters a line that begins with an equal sign and a word, like this |
551 | |
552 | =head1 Here There Be Pods! |
553 | |
554 | Then that text and all remaining text up through and including a line |
555 | beginning with C<=cut> will be ignored. The format of the intervening |
54310121 |
556 | text is described in L<perlpod>. |
cb1a09d0 |
557 | |
558 | This allows you to intermix your source code |
559 | and your documentation text freely, as in |
560 | |
561 | =item snazzle($) |
562 | |
54310121 |
563 | The snazzle() function will behave in the most spectacular |
cb1a09d0 |
564 | form that you can possibly imagine, not even excepting |
565 | cybernetic pyrotechnics. |
566 | |
567 | =cut back to the compiler, nuff of this pod stuff! |
568 | |
569 | sub snazzle($) { |
570 | my $thingie = shift; |
571 | ......... |
54310121 |
572 | } |
cb1a09d0 |
573 | |
54310121 |
574 | Note that pod translators should look at only paragraphs beginning |
184e9718 |
575 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 |
576 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 |
577 | paragraph. This means that the following secret stuff will be |
578 | ignored by both the compiler and the translators. |
579 | |
580 | $a=3; |
581 | =secret stuff |
582 | warn "Neither POD nor CODE!?" |
583 | =cut back |
584 | print "got $a\n"; |
585 | |
f86cebdf |
586 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 |
587 | Not all pod translators are well-behaved in this regard, and perhaps |
588 | the compiler will become pickier. |
774d564b |
589 | |
590 | One may also use pod directives to quickly comment out a section |
591 | of code. |
592 | |
593 | =head2 Plain Old Comments (Not!) |
594 | |
5a964f20 |
595 | Much like the C preprocessor, Perl can process line directives. Using |
596 | this, one can control Perl's idea of filenames and line numbers in |
774d564b |
597 | error or warning messages (especially for strings that are processed |
f86cebdf |
598 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b |
599 | C preprocessors: it matches the regular expression |
73659bf1 |
600 | C</^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/> with C<$1> being the line |
774d564b |
601 | number for the next line, and C<$2> being the optional filename |
602 | (specified within quotes). |
603 | |
003183f2 |
604 | There is a fairly obvious gotcha included with the line directive: |
605 | Debuggers and profilers will only show the last source line to appear |
606 | at a particular line number in a given file. Care should be taken not |
607 | to cause line number collisions in code you'd like to debug later. |
608 | |
774d564b |
609 | Here are some examples that you should be able to type into your command |
610 | shell: |
611 | |
612 | % perl |
613 | # line 200 "bzzzt" |
614 | # the `#' on the previous line must be the first char on line |
615 | die 'foo'; |
616 | __END__ |
617 | foo at bzzzt line 201. |
54310121 |
618 | |
774d564b |
619 | % perl |
620 | # line 200 "bzzzt" |
621 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; |
622 | __END__ |
623 | foo at - line 2001. |
54310121 |
624 | |
774d564b |
625 | % perl |
626 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; |
627 | __END__ |
628 | foo at foo bar line 200. |
54310121 |
629 | |
774d564b |
630 | % perl |
631 | # line 345 "goop" |
632 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; |
633 | print $@; |
634 | __END__ |
635 | foo at goop line 345. |
636 | |
637 | =cut |