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