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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 | |
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31 | The only things you need to declare in Perl are report formats and |
32 | subroutines (and sometimes not even subroutines). A variable holds |
33 | the undefined value (C<undef>) until it has been assigned a defined |
34 | value, which is anything other than C<undef>. When used as a number, |
35 | C<undef> is treated as C<0>; when used as a string, it is treated as |
36 | the empty string, C<"">; and when used as a reference that isn't being |
37 | assigned to, it is treated as an error. If you enable warnings, |
38 | you'll be notified of an uninitialized value whenever you treat |
39 | C<undef> as a string or a number. Well, usually. Boolean contexts, |
40 | 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 |
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97 | the semicolon is optional. (A semicolon is still encouraged if the |
98 | block takes up more than one line, because you may eventually add |
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99 | another line.) Note that there are some operators like C<eval {}> and |
100 | C<do {}> that look like compound statements, but aren't (they're just |
101 | TERMs in an expression), and thus need an explicit termination if used |
102 | as the last item in a statement. |
103 | |
104 | =head2 Truth and Falsehood |
105 | |
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106 | The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and |
107 | C<undef> are all false in a boolean context. All other values are true. |
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108 | Negation of a true value by C<!> or C<not> returns a special false value. |
109 | When evaluated as a string it is treated as C<''>, but as a number, it |
110 | is treated as 0. |
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111 | |
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112 | =head2 Statement Modifiers |
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113 | |
114 | Any simple statement may optionally be followed by a I<SINGLE> modifier, |
115 | just before the terminating semicolon (or block ending). The possible |
116 | modifiers are: |
117 | |
118 | if EXPR |
119 | unless EXPR |
120 | while EXPR |
121 | until EXPR |
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122 | foreach LIST |
123 | |
124 | The C<EXPR> following the modifier is referred to as the "condition". |
125 | Its truth or falsehood determines how the modifier will behave. |
126 | |
127 | C<if> executes the statement once I<if> and only if the condition is |
128 | true. C<unless> is the opposite, it executes the statement I<unless> |
129 | the condition is true (i.e., if the condition is false). |
130 | |
131 | print "Basset hounds got long ears" if length $ear >= 10; |
132 | go_outside() and play() unless $is_raining; |
133 | |
134 | The C<foreach> modifier is an iterator: it executes the statement once |
135 | for each item in the LIST (with C<$_> aliased to each item in turn). |
136 | |
137 | print "Hello $_!\n" foreach qw(world Dolly nurse); |
138 | |
139 | C<while> repeats the statement I<while> the condition is true. |
140 | C<until> does the opposite, it repeats the statement I<until> the |
141 | condition is true (or while the condition is false): |
142 | |
143 | # Both of these count from 0 to 10. |
144 | print $i++ while $i <= 10; |
145 | print $j++ until $j > 10; |
146 | |
147 | The C<while> and C<until> modifiers have the usual "C<while> loop" |
148 | semantics (conditional evaluated first), except when applied to a |
149 | C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in |
150 | which case the block executes once before the conditional is |
151 | evaluated. This is so that you can write loops like: |
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152 | |
153 | do { |
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154 | $line = <STDIN>; |
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155 | ... |
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156 | } until $line eq ".\n"; |
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157 | |
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158 | See L<perlfunc/do>. Note also that the loop control statements described |
159 | later will I<NOT> work in this construct, because modifiers don't take |
160 | loop labels. Sorry. You can always put another block inside of it |
161 | (for C<next>) or around it (for C<last>) to do that sort of thing. |
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162 | For C<next>, just double the braces: |
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163 | |
164 | do {{ |
165 | next if $x == $y; |
166 | # do something here |
167 | }} until $x++ > $z; |
168 | |
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169 | For C<last>, you have to be more elaborate: |
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170 | |
171 | LOOP: { |
172 | do { |
173 | last if $x = $y**2; |
174 | # do something here |
175 | } while $x++ <= $z; |
176 | } |
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177 | |
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178 | B<NOTE:> The behaviour of a C<my> statement modified with a statement |
179 | modifier conditional or loop construct (e.g. C<my $x if ...>) is |
180 | B<undefined>. The value of the C<my> variable may be C<undef>, any |
181 | previously assigned value, or possibly anything else. Don't rely on |
182 | it. Future versions of perl might do something different from the |
183 | version of perl you try it out on. Here be dragons. |
184 | |
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185 | =head2 Compound Statements |
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186 | |
187 | In Perl, a sequence of statements that defines a scope is called a block. |
188 | Sometimes a block is delimited by the file containing it (in the case |
189 | of a required file, or the program as a whole), and sometimes a block |
190 | is delimited by the extent of a string (in the case of an eval). |
191 | |
192 | But generally, a block is delimited by curly brackets, also known as braces. |
193 | We will call this syntactic construct a BLOCK. |
194 | |
195 | The following compound statements may be used to control flow: |
196 | |
197 | if (EXPR) BLOCK |
198 | if (EXPR) BLOCK else BLOCK |
199 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
200 | LABEL while (EXPR) BLOCK |
201 | LABEL while (EXPR) BLOCK continue BLOCK |
202 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
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203 | LABEL foreach VAR (LIST) BLOCK |
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204 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
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205 | LABEL BLOCK continue BLOCK |
206 | |
207 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, |
208 | not statements. This means that the curly brackets are I<required>--no |
209 | dangling statements allowed. If you want to write conditionals without |
210 | curly brackets there are several other ways to do it. The following |
211 | all do the same thing: |
212 | |
213 | if (!open(FOO)) { die "Can't open $FOO: $!"; } |
214 | die "Can't open $FOO: $!" unless open(FOO); |
215 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! |
216 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; |
217 | # a bit exotic, that last one |
218 | |
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219 | The C<if> statement is straightforward. Because BLOCKs are always |
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220 | bounded by curly brackets, there is never any ambiguity about which |
221 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, |
222 | the sense of the test is reversed. |
223 | |
224 | The C<while> statement executes the block as long as the expression is |
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225 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). |
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226 | The LABEL is optional, and if present, consists of an identifier followed |
227 | by a colon. The LABEL identifies the loop for the loop control |
228 | statements C<next>, C<last>, and C<redo>. |
229 | If the LABEL is omitted, the loop control statement |
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230 | refers to the innermost enclosing loop. This may include dynamically |
231 | looking back your call-stack at run time to find the LABEL. Such |
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232 | desperate behavior triggers a warning if you use the C<use warnings> |
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233 | pragma or the B<-w> flag. |
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234 | |
235 | If there is a C<continue> BLOCK, it is always executed just before the |
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236 | conditional is about to be evaluated again. Thus it can be used to |
237 | increment a loop variable, even when the loop has been continued via |
238 | the C<next> statement. |
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239 | |
240 | =head2 Loop Control |
241 | |
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242 | The C<next> command starts the next iteration of the loop: |
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243 | |
244 | LINE: while (<STDIN>) { |
245 | next LINE if /^#/; # discard comments |
246 | ... |
247 | } |
248 | |
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249 | The C<last> command immediately exits the loop in question. The |
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250 | C<continue> block, if any, is not executed: |
251 | |
252 | LINE: while (<STDIN>) { |
253 | last LINE if /^$/; # exit when done with header |
254 | ... |
255 | } |
256 | |
257 | The C<redo> command restarts the loop block without evaluating the |
258 | conditional again. The C<continue> block, if any, is I<not> executed. |
259 | This command is normally used by programs that want to lie to themselves |
260 | about what was just input. |
261 | |
262 | For example, when processing a file like F</etc/termcap>. |
263 | If your input lines might end in backslashes to indicate continuation, you |
264 | want to skip ahead and get the next record. |
265 | |
266 | while (<>) { |
267 | chomp; |
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268 | if (s/\\$//) { |
269 | $_ .= <>; |
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270 | redo unless eof(); |
271 | } |
272 | # now process $_ |
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273 | } |
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274 | |
275 | which is Perl short-hand for the more explicitly written version: |
276 | |
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277 | LINE: while (defined($line = <ARGV>)) { |
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278 | chomp($line); |
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279 | if ($line =~ s/\\$//) { |
280 | $line .= <ARGV>; |
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281 | redo LINE unless eof(); # not eof(ARGV)! |
282 | } |
283 | # now process $line |
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284 | } |
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285 | |
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286 | Note that if there were a C<continue> block on the above code, it would |
287 | get executed only on lines discarded by the regex (since redo skips the |
288 | continue block). A continue block is often used to reset line counters |
289 | or C<?pat?> one-time matches: |
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290 | |
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291 | # inspired by :1,$g/fred/s//WILMA/ |
292 | while (<>) { |
293 | ?(fred)? && s//WILMA $1 WILMA/; |
294 | ?(barney)? && s//BETTY $1 BETTY/; |
295 | ?(homer)? && s//MARGE $1 MARGE/; |
296 | } continue { |
297 | print "$ARGV $.: $_"; |
298 | close ARGV if eof(); # reset $. |
299 | reset if eof(); # reset ?pat? |
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300 | } |
301 | |
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302 | If the word C<while> is replaced by the word C<until>, the sense of the |
303 | test is reversed, but the conditional is still tested before the first |
304 | iteration. |
305 | |
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306 | The loop control statements don't work in an C<if> or C<unless>, since |
307 | they aren't loops. You can double the braces to make them such, though. |
308 | |
309 | if (/pattern/) {{ |
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310 | last if /fred/; |
311 | next if /barney/; # same effect as "last", but doesn't document as well |
312 | # do something here |
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313 | }} |
314 | |
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315 | This is caused by the fact that a block by itself acts as a loop that |
316 | executes once, see L<"Basic BLOCKs and Switch Statements">. |
317 | |
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318 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
319 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. |
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320 | |
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321 | =head2 For Loops |
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322 | |
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323 | Perl's C-style C<for> loop works like the corresponding C<while> loop; |
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324 | that means that this: |
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325 | |
326 | for ($i = 1; $i < 10; $i++) { |
327 | ... |
328 | } |
329 | |
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330 | is the same as this: |
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331 | |
332 | $i = 1; |
333 | while ($i < 10) { |
334 | ... |
335 | } continue { |
336 | $i++; |
337 | } |
338 | |
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339 | There is one minor difference: if variables are declared with C<my> |
340 | in the initialization section of the C<for>, the lexical scope of |
341 | those variables is exactly the C<for> loop (the body of the loop |
342 | and the control sections). |
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343 | |
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344 | Besides the normal array index looping, C<for> can lend itself |
345 | to many other interesting applications. Here's one that avoids the |
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346 | problem you get into if you explicitly test for end-of-file on |
347 | an interactive file descriptor causing your program to appear to |
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348 | hang. |
349 | |
350 | $on_a_tty = -t STDIN && -t STDOUT; |
351 | sub prompt { print "yes? " if $on_a_tty } |
352 | for ( prompt(); <STDIN>; prompt() ) { |
353 | # do something |
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354 | } |
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355 | |
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356 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the |
357 | conditional of a C<for> loop is shorthand for the following. This |
358 | behaviour is the same as a C<while> loop conditional. |
359 | |
360 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { |
361 | # do something |
362 | } |
363 | |
cb1a09d0 |
364 | =head2 Foreach Loops |
365 | |
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366 | The C<foreach> loop iterates over a normal list value and sets the |
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367 | variable VAR to be each element of the list in turn. If the variable |
368 | is preceded with the keyword C<my>, then it is lexically scoped, and |
369 | is therefore visible only within the loop. Otherwise, the variable is |
370 | implicitly local to the loop and regains its former value upon exiting |
371 | the loop. If the variable was previously declared with C<my>, it uses |
372 | that variable instead of the global one, but it's still localized to |
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373 | the loop. This implicit localisation occurs I<only> in a C<foreach> |
374 | loop. |
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375 | |
376 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so |
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377 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
378 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> |
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379 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
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380 | |
381 | If any element of LIST is an lvalue, you can modify it by modifying |
382 | VAR inside the loop. Conversely, if any element of LIST is NOT an |
383 | lvalue, any attempt to modify that element will fail. In other words, |
384 | the C<foreach> loop index variable is an implicit alias for each item |
385 | in the list that you're looping over. |
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386 | |
387 | If any part of LIST is an array, C<foreach> will get very confused if |
388 | you add or remove elements within the loop body, for example with |
389 | C<splice>. So don't do that. |
390 | |
391 | C<foreach> probably won't do what you expect if VAR is a tied or other |
392 | special variable. Don't do that either. |
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393 | |
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394 | Examples: |
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395 | |
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396 | for (@ary) { s/foo/bar/ } |
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397 | |
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398 | for my $elem (@elements) { |
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399 | $elem *= 2; |
400 | } |
401 | |
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402 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
403 | print $count, "\n"; sleep(1); |
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404 | } |
405 | |
406 | for (1..15) { print "Merry Christmas\n"; } |
407 | |
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408 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
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409 | print "Item: $item\n"; |
410 | } |
411 | |
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412 | Here's how a C programmer might code up a particular algorithm in Perl: |
413 | |
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414 | for (my $i = 0; $i < @ary1; $i++) { |
415 | for (my $j = 0; $j < @ary2; $j++) { |
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416 | if ($ary1[$i] > $ary2[$j]) { |
417 | last; # can't go to outer :-( |
418 | } |
419 | $ary1[$i] += $ary2[$j]; |
420 | } |
cb1a09d0 |
421 | # this is where that last takes me |
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422 | } |
423 | |
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424 | Whereas here's how a Perl programmer more comfortable with the idiom might |
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425 | do it: |
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426 | |
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427 | OUTER: for my $wid (@ary1) { |
428 | INNER: for my $jet (@ary2) { |
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429 | next OUTER if $wid > $jet; |
430 | $wid += $jet; |
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431 | } |
432 | } |
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433 | |
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434 | See how much easier this is? It's cleaner, safer, and faster. It's |
435 | cleaner because it's less noisy. It's safer because if code gets added |
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436 | between the inner and outer loops later on, the new code won't be |
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437 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd |
438 | rather than merely terminating the inner one. And it's faster because |
439 | Perl executes a C<foreach> statement more rapidly than it would the |
440 | equivalent C<for> loop. |
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441 | |
442 | =head2 Basic BLOCKs and Switch Statements |
443 | |
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444 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
445 | loop that executes once. Thus you can use any of the loop control |
446 | statements in it to leave or restart the block. (Note that this is |
447 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief |
448 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> |
449 | block is optional. |
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450 | |
451 | The BLOCK construct is particularly nice for doing case |
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452 | structures. |
453 | |
454 | SWITCH: { |
455 | if (/^abc/) { $abc = 1; last SWITCH; } |
456 | if (/^def/) { $def = 1; last SWITCH; } |
457 | if (/^xyz/) { $xyz = 1; last SWITCH; } |
458 | $nothing = 1; |
459 | } |
460 | |
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461 | There is no official C<switch> statement in Perl, because there are |
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462 | already several ways to write the equivalent. |
463 | |
464 | However, starting from Perl 5.8 to get switch and case one can use |
465 | the Switch extension and say: |
466 | |
467 | use Switch; |
468 | |
469 | after which one has switch and case. It is not as fast as it could be |
470 | because it's not really part of the language (it's done using source |
471 | filters) but it is available, and it's very flexible. |
472 | |
473 | In addition to the above BLOCK construct, you could write |
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474 | |
475 | SWITCH: { |
476 | $abc = 1, last SWITCH if /^abc/; |
477 | $def = 1, last SWITCH if /^def/; |
478 | $xyz = 1, last SWITCH if /^xyz/; |
479 | $nothing = 1; |
480 | } |
481 | |
cb1a09d0 |
482 | (That's actually not as strange as it looks once you realize that you can |
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483 | use loop control "operators" within an expression. That's just the binary |
484 | comma operator in scalar context. See L<perlop/"Comma Operator">.) |
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485 | |
486 | or |
487 | |
488 | SWITCH: { |
489 | /^abc/ && do { $abc = 1; last SWITCH; }; |
490 | /^def/ && do { $def = 1; last SWITCH; }; |
491 | /^xyz/ && do { $xyz = 1; last SWITCH; }; |
492 | $nothing = 1; |
493 | } |
494 | |
f86cebdf |
495 | or formatted so it stands out more as a "proper" C<switch> statement: |
a0d0e21e |
496 | |
497 | SWITCH: { |
54310121 |
498 | /^abc/ && do { |
499 | $abc = 1; |
500 | last SWITCH; |
a0d0e21e |
501 | }; |
502 | |
54310121 |
503 | /^def/ && do { |
504 | $def = 1; |
505 | last SWITCH; |
a0d0e21e |
506 | }; |
507 | |
54310121 |
508 | /^xyz/ && do { |
509 | $xyz = 1; |
510 | last SWITCH; |
a0d0e21e |
511 | }; |
512 | $nothing = 1; |
513 | } |
514 | |
515 | or |
516 | |
517 | SWITCH: { |
518 | /^abc/ and $abc = 1, last SWITCH; |
519 | /^def/ and $def = 1, last SWITCH; |
520 | /^xyz/ and $xyz = 1, last SWITCH; |
521 | $nothing = 1; |
522 | } |
523 | |
524 | or even, horrors, |
525 | |
526 | if (/^abc/) |
527 | { $abc = 1 } |
528 | elsif (/^def/) |
529 | { $def = 1 } |
530 | elsif (/^xyz/) |
531 | { $xyz = 1 } |
532 | else |
533 | { $nothing = 1 } |
534 | |
f86cebdf |
535 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make |
536 | a temporary assignment to C<$_> for convenient matching: |
4633a7c4 |
537 | |
538 | SWITCH: for ($where) { |
539 | /In Card Names/ && do { push @flags, '-e'; last; }; |
540 | /Anywhere/ && do { push @flags, '-h'; last; }; |
541 | /In Rulings/ && do { last; }; |
542 | die "unknown value for form variable where: `$where'"; |
54310121 |
543 | } |
4633a7c4 |
544 | |
cb1a09d0 |
545 | Another interesting approach to a switch statement is arrange |
546 | for a C<do> block to return the proper value: |
547 | |
548 | $amode = do { |
5a964f20 |
549 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? |
54310121 |
550 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
cb1a09d0 |
551 | elsif ($flag & O_RDWR) { |
552 | if ($flag & O_CREAT) { "w+" } |
c07a80fd |
553 | else { ($flag & O_APPEND) ? "a+" : "r+" } |
cb1a09d0 |
554 | } |
555 | }; |
556 | |
5a964f20 |
557 | Or |
558 | |
559 | print do { |
560 | ($flags & O_WRONLY) ? "write-only" : |
561 | ($flags & O_RDWR) ? "read-write" : |
562 | "read-only"; |
563 | }; |
564 | |
a031eab2 |
565 | Or if you are certain that all the C<&&> clauses are true, you can use |
5a964f20 |
566 | something like this, which "switches" on the value of the |
a2293a43 |
567 | C<HTTP_USER_AGENT> environment variable. |
5a964f20 |
568 | |
569 | #!/usr/bin/perl |
570 | # pick out jargon file page based on browser |
571 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; |
572 | for ($ENV{HTTP_USER_AGENT}) { |
573 | $page = /Mac/ && 'm/Macintrash.html' |
574 | || /Win(dows )?NT/ && 'e/evilandrude.html' |
575 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' |
576 | || /Linux/ && 'l/Linux.html' |
577 | || /HP-UX/ && 'h/HP-SUX.html' |
578 | || /SunOS/ && 's/ScumOS.html' |
579 | || 'a/AppendixB.html'; |
580 | } |
581 | print "Location: $dir/$page\015\012\015\012"; |
582 | |
583 | That kind of switch statement only works when you know the C<&&> clauses |
584 | will be true. If you don't, the previous C<?:> example should be used. |
585 | |
19799a22 |
586 | You might also consider writing a hash of subroutine references |
587 | instead of synthesizing a C<switch> statement. |
5a964f20 |
588 | |
4633a7c4 |
589 | =head2 Goto |
590 | |
19799a22 |
591 | Although not for the faint of heart, Perl does support a C<goto> |
592 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and |
593 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for |
594 | a C<goto>; it's just the name of the loop. |
4633a7c4 |
595 | |
f86cebdf |
596 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 |
597 | execution there. It may not be used to go into any construct that |
f86cebdf |
598 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 |
599 | also can't be used to go into a construct that is optimized away. It |
600 | can be used to go almost anywhere else within the dynamic scope, |
601 | including out of subroutines, but it's usually better to use some other |
f86cebdf |
602 | construct such as C<last> or C<die>. The author of Perl has never felt the |
603 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
4633a7c4 |
604 | |
f86cebdf |
605 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
606 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't |
4633a7c4 |
607 | necessarily recommended if you're optimizing for maintainability: |
608 | |
96f2dc66 |
609 | goto(("FOO", "BAR", "GLARCH")[$i]); |
4633a7c4 |
610 | |
f86cebdf |
611 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 |
612 | named subroutine for the currently running subroutine. This is used by |
f86cebdf |
613 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 |
614 | pretend that the other subroutine had been called in the first place |
f86cebdf |
615 | (except that any modifications to C<@_> in the current subroutine are |
616 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> |
4633a7c4 |
617 | will be able to tell that this routine was called first. |
618 | |
c07a80fd |
619 | In almost all cases like this, it's usually a far, far better idea to use the |
620 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of |
4633a7c4 |
621 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
622 | C<eval{}> and die() for exception processing can also be a prudent approach. |
cb1a09d0 |
623 | |
624 | =head2 PODs: Embedded Documentation |
625 | |
626 | Perl has a mechanism for intermixing documentation with source code. |
c07a80fd |
627 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 |
628 | encounters a line that begins with an equal sign and a word, like this |
629 | |
630 | =head1 Here There Be Pods! |
631 | |
632 | Then that text and all remaining text up through and including a line |
633 | beginning with C<=cut> will be ignored. The format of the intervening |
54310121 |
634 | text is described in L<perlpod>. |
cb1a09d0 |
635 | |
636 | This allows you to intermix your source code |
637 | and your documentation text freely, as in |
638 | |
639 | =item snazzle($) |
640 | |
54310121 |
641 | The snazzle() function will behave in the most spectacular |
cb1a09d0 |
642 | form that you can possibly imagine, not even excepting |
643 | cybernetic pyrotechnics. |
644 | |
645 | =cut back to the compiler, nuff of this pod stuff! |
646 | |
647 | sub snazzle($) { |
648 | my $thingie = shift; |
649 | ......... |
54310121 |
650 | } |
cb1a09d0 |
651 | |
54310121 |
652 | Note that pod translators should look at only paragraphs beginning |
184e9718 |
653 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 |
654 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 |
655 | paragraph. This means that the following secret stuff will be |
656 | ignored by both the compiler and the translators. |
657 | |
658 | $a=3; |
659 | =secret stuff |
660 | warn "Neither POD nor CODE!?" |
661 | =cut back |
662 | print "got $a\n"; |
663 | |
f86cebdf |
664 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 |
665 | Not all pod translators are well-behaved in this regard, and perhaps |
666 | the compiler will become pickier. |
774d564b |
667 | |
668 | One may also use pod directives to quickly comment out a section |
669 | of code. |
670 | |
671 | =head2 Plain Old Comments (Not!) |
672 | |
6ec4bd10 |
673 | Perl can process line directives, much like the C preprocessor. Using |
5a964f20 |
674 | this, one can control Perl's idea of filenames and line numbers in |
774d564b |
675 | error or warning messages (especially for strings that are processed |
f86cebdf |
676 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b |
677 | C preprocessors: it matches the regular expression |
6ec4bd10 |
678 | |
679 | # example: '# line 42 "new_filename.plx"' |
82d4537c |
680 | /^\# \s* |
6ec4bd10 |
681 | line \s+ (\d+) \s* |
7b6e93a8 |
682 | (?:\s("?)([^"]+)\2)? \s* |
6ec4bd10 |
683 | $/x |
684 | |
7b6e93a8 |
685 | with C<$1> being the line number for the next line, and C<$3> being |
686 | the optional filename (specified with or without quotes). |
774d564b |
687 | |
003183f2 |
688 | There is a fairly obvious gotcha included with the line directive: |
689 | Debuggers and profilers will only show the last source line to appear |
690 | at a particular line number in a given file. Care should be taken not |
691 | to cause line number collisions in code you'd like to debug later. |
692 | |
774d564b |
693 | Here are some examples that you should be able to type into your command |
694 | shell: |
695 | |
696 | % perl |
697 | # line 200 "bzzzt" |
698 | # the `#' on the previous line must be the first char on line |
699 | die 'foo'; |
700 | __END__ |
701 | foo at bzzzt line 201. |
54310121 |
702 | |
774d564b |
703 | % perl |
704 | # line 200 "bzzzt" |
705 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; |
706 | __END__ |
707 | foo at - line 2001. |
54310121 |
708 | |
774d564b |
709 | % perl |
710 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; |
711 | __END__ |
712 | foo at foo bar line 200. |
54310121 |
713 | |
774d564b |
714 | % perl |
715 | # line 345 "goop" |
716 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; |
717 | print $@; |
718 | __END__ |
719 | foo at goop line 345. |
720 | |
721 | =cut |