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