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| 128 | .rm #[ #] #H #V #F C |
| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "PERLSYN 1" |
| 132 | .TH PERLSYN 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | perlsyn \- Perl syntax |
| 135 | .SH "DESCRIPTION" |
| 136 | .IX Header "DESCRIPTION" |
| 137 | A Perl script consists of a sequence of declarations and statements. |
| 138 | The sequence of statements is executed just once, unlike in \fBsed\fR |
| 139 | and \fBawk\fR scripts, where the sequence of statements is executed |
| 140 | for each input line. While this means that you must explicitly |
| 141 | loop over the lines of your input file (or files), it also means |
| 142 | you have much more control over which files and which lines you look at. |
| 143 | (Actually, I'm lying\*(--it is possible to do an implicit loop with |
| 144 | either the \fB\-n\fR or \fB\-p\fR switch. It's just not the mandatory |
| 145 | default like it is in \fBsed\fR and \fBawk\fR.) |
| 146 | .PP |
| 147 | Perl is, for the most part, a free-form language. (The only exception |
| 148 | to this is format declarations, for obvious reasons.) Text from a |
| 149 | \&\f(CW"#"\fR character until the end of the line is a comment, and is |
| 150 | ignored. If you attempt to use \f(CW\*(C`/* */\*(C'\fR C\-style comments, it will be |
| 151 | interpreted either as division or pattern matching, depending on the |
| 152 | context, and \*(C+ \f(CW\*(C`//\*(C'\fR comments just look like a null regular |
| 153 | expression, so don't do that. |
| 154 | .Sh "Declarations" |
| 155 | .IX Subsection "Declarations" |
| 156 | The only things you need to declare in Perl are report formats |
| 157 | and subroutines\*(--and even undefined subroutines can be handled |
| 158 | through \s-1AUTOLOAD\s0. A variable holds the undefined value (\f(CW\*(C`undef\*(C'\fR) |
| 159 | until it has been assigned a defined value, which is anything |
| 160 | other than \f(CW\*(C`undef\*(C'\fR. When used as a number, \f(CW\*(C`undef\*(C'\fR is treated |
| 161 | as \f(CW0\fR; when used as a string, it is treated the empty string, |
| 162 | \&\f(CW""\fR; and when used as a reference that isn't being assigned |
| 163 | to, it is treated as an error. If you enable warnings, you'll |
| 164 | be notified of an uninitialized value whenever you treat \f(CW\*(C`undef\*(C'\fR |
| 165 | as a string or a number. Well, usually. Boolean contexts, such as: |
| 166 | .PP |
| 167 | .Vb 2 |
| 168 | \& my $a; |
| 169 | \& if ($a) {} |
| 170 | .Ve |
| 171 | .PP |
| 172 | are exempt from warnings (because they care about truth rather than |
| 173 | definedness). Operators such as \f(CW\*(C`++\*(C'\fR, \f(CW\*(C`\-\-\*(C'\fR, \f(CW\*(C`+=\*(C'\fR, |
| 174 | \&\f(CW\*(C`\-=\*(C'\fR, and \f(CW\*(C`.=\*(C'\fR, that operate on undefined left values such as: |
| 175 | .PP |
| 176 | .Vb 2 |
| 177 | \& my $a; |
| 178 | \& $a++; |
| 179 | .Ve |
| 180 | .PP |
| 181 | are also always exempt from such warnings. |
| 182 | .PP |
| 183 | A declaration can be put anywhere a statement can, but has no effect on |
| 184 | the execution of the primary sequence of statements\*(--declarations all |
| 185 | take effect at compile time. Typically all the declarations are put at |
| 186 | the beginning or the end of the script. However, if you're using |
| 187 | lexically-scoped private variables created with \f(CW\*(C`my()\*(C'\fR, you'll |
| 188 | have to make sure |
| 189 | your format or subroutine definition is within the same block scope |
| 190 | as the my if you expect to be able to access those private variables. |
| 191 | .PP |
| 192 | Declaring a subroutine allows a subroutine name to be used as if it were a |
| 193 | list operator from that point forward in the program. You can declare a |
| 194 | subroutine without defining it by saying \f(CW\*(C`sub name\*(C'\fR, thus: |
| 195 | .PP |
| 196 | .Vb 2 |
| 197 | \& sub myname; |
| 198 | \& $me = myname $0 or die "can't get myname"; |
| 199 | .Ve |
| 200 | .PP |
| 201 | Note that \fImyname()\fR functions as a list operator, not as a unary operator; |
| 202 | so be careful to use \f(CW\*(C`or\*(C'\fR instead of \f(CW\*(C`||\*(C'\fR in this case. However, if |
| 203 | you were to declare the subroutine as \f(CW\*(C`sub myname ($)\*(C'\fR, then |
| 204 | \&\f(CW\*(C`myname\*(C'\fR would function as a unary operator, so either \f(CW\*(C`or\*(C'\fR or |
| 205 | \&\f(CW\*(C`||\*(C'\fR would work. |
| 206 | .PP |
| 207 | Subroutines declarations can also be loaded up with the \f(CW\*(C`require\*(C'\fR statement |
| 208 | or both loaded and imported into your namespace with a \f(CW\*(C`use\*(C'\fR statement. |
| 209 | See perlmod for details on this. |
| 210 | .PP |
| 211 | A statement sequence may contain declarations of lexically-scoped |
| 212 | variables, but apart from declaring a variable name, the declaration acts |
| 213 | like an ordinary statement, and is elaborated within the sequence of |
| 214 | statements as if it were an ordinary statement. That means it actually |
| 215 | has both compile-time and run-time effects. |
| 216 | .Sh "Simple statements" |
| 217 | .IX Subsection "Simple statements" |
| 218 | The only kind of simple statement is an expression evaluated for its |
| 219 | side effects. Every simple statement must be terminated with a |
| 220 | semicolon, unless it is the final statement in a block, in which case |
| 221 | the semicolon is optional. (A semicolon is still encouraged there if the |
| 222 | block takes up more than one line, because you may eventually add another line.) |
| 223 | Note that there are some operators like \f(CW\*(C`eval {}\*(C'\fR and \f(CW\*(C`do {}\*(C'\fR that look |
| 224 | like compound statements, but aren't (they're just TERMs in an expression), |
| 225 | and thus need an explicit termination if used as the last item in a statement. |
| 226 | .PP |
| 227 | Any simple statement may optionally be followed by a \fI\s-1SINGLE\s0\fR modifier, |
| 228 | just before the terminating semicolon (or block ending). The possible |
| 229 | modifiers are: |
| 230 | .PP |
| 231 | .Vb 5 |
| 232 | \& if EXPR |
| 233 | \& unless EXPR |
| 234 | \& while EXPR |
| 235 | \& until EXPR |
| 236 | \& foreach EXPR |
| 237 | .Ve |
| 238 | .PP |
| 239 | The \f(CW\*(C`if\*(C'\fR and \f(CW\*(C`unless\*(C'\fR modifiers have the expected semantics, |
| 240 | presuming you're a speaker of English. The \f(CW\*(C`foreach\*(C'\fR modifier is an |
| 241 | iterator: For each value in \s-1EXPR\s0, it aliases \f(CW$_\fR to the value and |
| 242 | executes the statement. The \f(CW\*(C`while\*(C'\fR and \f(CW\*(C`until\*(C'\fR modifiers have the |
| 243 | usual "\f(CW\*(C`while\*(C'\fR loop" semantics (conditional evaluated first), except |
| 244 | when applied to a \f(CW\*(C`do\*(C'\fR\-BLOCK (or to the deprecated \f(CW\*(C`do\*(C'\fR\-SUBROUTINE |
| 245 | statement), in which case the block executes once before the |
| 246 | conditional is evaluated. This is so that you can write loops like: |
| 247 | .PP |
| 248 | .Vb 4 |
| 249 | \& do { |
| 250 | \& $line = <STDIN>; |
| 251 | \& ... |
| 252 | \& } until $line eq ".\en"; |
| 253 | .Ve |
| 254 | .PP |
| 255 | See \*(L"do\*(R" in perlfunc. Note also that the loop control statements described |
| 256 | later will \fI\s-1NOT\s0\fR work in this construct, because modifiers don't take |
| 257 | loop labels. Sorry. You can always put another block inside of it |
| 258 | (for \f(CW\*(C`next\*(C'\fR) or around it (for \f(CW\*(C`last\*(C'\fR) to do that sort of thing. |
| 259 | For \f(CW\*(C`next\*(C'\fR, just double the braces: |
| 260 | .PP |
| 261 | .Vb 4 |
| 262 | \& do {{ |
| 263 | \& next if $x == $y; |
| 264 | \& # do something here |
| 265 | \& }} until $x++ > $z; |
| 266 | .Ve |
| 267 | .PP |
| 268 | For \f(CW\*(C`last\*(C'\fR, you have to be more elaborate: |
| 269 | .PP |
| 270 | .Vb 6 |
| 271 | \& LOOP: { |
| 272 | \& do { |
| 273 | \& last if $x = $y**2; |
| 274 | \& # do something here |
| 275 | \& } while $x++ <= $z; |
| 276 | \& } |
| 277 | .Ve |
| 278 | .Sh "Compound statements" |
| 279 | .IX Subsection "Compound statements" |
| 280 | In Perl, a sequence of statements that defines a scope is called a block. |
| 281 | Sometimes a block is delimited by the file containing it (in the case |
| 282 | of a required file, or the program as a whole), and sometimes a block |
| 283 | is delimited by the extent of a string (in the case of an eval). |
| 284 | .PP |
| 285 | But generally, a block is delimited by curly brackets, also known as braces. |
| 286 | We will call this syntactic construct a \s-1BLOCK\s0. |
| 287 | .PP |
| 288 | The following compound statements may be used to control flow: |
| 289 | .PP |
| 290 | .Vb 9 |
| 291 | \& if (EXPR) BLOCK |
| 292 | \& if (EXPR) BLOCK else BLOCK |
| 293 | \& if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
| 294 | \& LABEL while (EXPR) BLOCK |
| 295 | \& LABEL while (EXPR) BLOCK continue BLOCK |
| 296 | \& LABEL for (EXPR; EXPR; EXPR) BLOCK |
| 297 | \& LABEL foreach VAR (LIST) BLOCK |
| 298 | \& LABEL foreach VAR (LIST) BLOCK continue BLOCK |
| 299 | \& LABEL BLOCK continue BLOCK |
| 300 | .Ve |
| 301 | .PP |
| 302 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, |
| 303 | not statements. This means that the curly brackets are \fIrequired\fR\-\-no |
| 304 | dangling statements allowed. If you want to write conditionals without |
| 305 | curly brackets there are several other ways to do it. The following |
| 306 | all do the same thing: |
| 307 | .PP |
| 308 | .Vb 5 |
| 309 | \& if (!open(FOO)) { die "Can't open $FOO: $!"; } |
| 310 | \& die "Can't open $FOO: $!" unless open(FOO); |
| 311 | \& open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! |
| 312 | \& open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; |
| 313 | \& # a bit exotic, that last one |
| 314 | .Ve |
| 315 | .PP |
| 316 | The \f(CW\*(C`if\*(C'\fR statement is straightforward. Because BLOCKs are always |
| 317 | bounded by curly brackets, there is never any ambiguity about which |
| 318 | \&\f(CW\*(C`if\*(C'\fR an \f(CW\*(C`else\*(C'\fR goes with. If you use \f(CW\*(C`unless\*(C'\fR in place of \f(CW\*(C`if\*(C'\fR, |
| 319 | the sense of the test is reversed. |
| 320 | .PP |
| 321 | The \f(CW\*(C`while\*(C'\fR statement executes the block as long as the expression is |
| 322 | true (does not evaluate to the null string \f(CW""\fR or \f(CW0\fR or \f(CW"0"\fR). |
| 323 | The \s-1LABEL\s0 is optional, and if present, consists of an identifier followed |
| 324 | by a colon. The \s-1LABEL\s0 identifies the loop for the loop control |
| 325 | statements \f(CW\*(C`next\*(C'\fR, \f(CW\*(C`last\*(C'\fR, and \f(CW\*(C`redo\*(C'\fR. |
| 326 | If the \s-1LABEL\s0 is omitted, the loop control statement |
| 327 | refers to the innermost enclosing loop. This may include dynamically |
| 328 | looking back your call-stack at run time to find the \s-1LABEL\s0. Such |
| 329 | desperate behavior triggers a warning if you use the \f(CW\*(C`use warnings\*(C'\fR |
| 330 | pragma or the \fB\-w\fR flag. |
| 331 | Unlike a \f(CW\*(C`foreach\*(C'\fR statement, a \f(CW\*(C`while\*(C'\fR statement never implicitly |
| 332 | localises any variables. |
| 333 | .PP |
| 334 | If there is a \f(CW\*(C`continue\*(C'\fR \s-1BLOCK\s0, it is always executed just before the |
| 335 | conditional is about to be evaluated again, just like the third part of a |
| 336 | \&\f(CW\*(C`for\*(C'\fR loop in C. Thus it can be used to increment a loop variable, even |
| 337 | when the loop has been continued via the \f(CW\*(C`next\*(C'\fR statement (which is |
| 338 | similar to the C \f(CW\*(C`continue\*(C'\fR statement). |
| 339 | .Sh "Loop Control" |
| 340 | .IX Subsection "Loop Control" |
| 341 | The \f(CW\*(C`next\*(C'\fR command is like the \f(CW\*(C`continue\*(C'\fR statement in C; it starts |
| 342 | the next iteration of the loop: |
| 343 | .PP |
| 344 | .Vb 4 |
| 345 | \& LINE: while (<STDIN>) { |
| 346 | \& next LINE if /^#/; # discard comments |
| 347 | \& ... |
| 348 | \& } |
| 349 | .Ve |
| 350 | .PP |
| 351 | The \f(CW\*(C`last\*(C'\fR command is like the \f(CW\*(C`break\*(C'\fR statement in C (as used in |
| 352 | loops); it immediately exits the loop in question. The |
| 353 | \&\f(CW\*(C`continue\*(C'\fR block, if any, is not executed: |
| 354 | .PP |
| 355 | .Vb 4 |
| 356 | \& LINE: while (<STDIN>) { |
| 357 | \& last LINE if /^$/; # exit when done with header |
| 358 | \& ... |
| 359 | \& } |
| 360 | .Ve |
| 361 | .PP |
| 362 | The \f(CW\*(C`redo\*(C'\fR command restarts the loop block without evaluating the |
| 363 | conditional again. The \f(CW\*(C`continue\*(C'\fR block, if any, is \fInot\fR executed. |
| 364 | This command is normally used by programs that want to lie to themselves |
| 365 | about what was just input. |
| 366 | .PP |
| 367 | For example, when processing a file like \fI/etc/termcap\fR. |
| 368 | If your input lines might end in backslashes to indicate continuation, you |
| 369 | want to skip ahead and get the next record. |
| 370 | .PP |
| 371 | .Vb 8 |
| 372 | \& while (<>) { |
| 373 | \& chomp; |
| 374 | \& if (s/\e\e$//) { |
| 375 | \& $_ .= <>; |
| 376 | \& redo unless eof(); |
| 377 | \& } |
| 378 | \& # now process $_ |
| 379 | \& } |
| 380 | .Ve |
| 381 | .PP |
| 382 | which is Perl short-hand for the more explicitly written version: |
| 383 | .PP |
| 384 | .Vb 8 |
| 385 | \& LINE: while (defined($line = <ARGV>)) { |
| 386 | \& chomp($line); |
| 387 | \& if ($line =~ s/\e\e$//) { |
| 388 | \& $line .= <ARGV>; |
| 389 | \& redo LINE unless eof(); # not eof(ARGV)! |
| 390 | \& } |
| 391 | \& # now process $line |
| 392 | \& } |
| 393 | .Ve |
| 394 | .PP |
| 395 | Note that if there were a \f(CW\*(C`continue\*(C'\fR block on the above code, it would |
| 396 | get executed only on lines discarded by the regex (since redo skips the |
| 397 | continue block). A continue block is often used to reset line counters |
| 398 | or \f(CW\*(C`?pat?\*(C'\fR one-time matches: |
| 399 | .PP |
| 400 | .Vb 10 |
| 401 | \& # inspired by :1,$g/fred/s//WILMA/ |
| 402 | \& while (<>) { |
| 403 | \& ?(fred)? && s//WILMA $1 WILMA/; |
| 404 | \& ?(barney)? && s//BETTY $1 BETTY/; |
| 405 | \& ?(homer)? && s//MARGE $1 MARGE/; |
| 406 | \& } continue { |
| 407 | \& print "$ARGV $.: $_"; |
| 408 | \& close ARGV if eof(); # reset $. |
| 409 | \& reset if eof(); # reset ?pat? |
| 410 | \& } |
| 411 | .Ve |
| 412 | .PP |
| 413 | If the word \f(CW\*(C`while\*(C'\fR is replaced by the word \f(CW\*(C`until\*(C'\fR, the sense of the |
| 414 | test is reversed, but the conditional is still tested before the first |
| 415 | iteration. |
| 416 | .PP |
| 417 | The loop control statements don't work in an \f(CW\*(C`if\*(C'\fR or \f(CW\*(C`unless\*(C'\fR, since |
| 418 | they aren't loops. You can double the braces to make them such, though. |
| 419 | .PP |
| 420 | .Vb 5 |
| 421 | \& if (/pattern/) {{ |
| 422 | \& last if /fred/; |
| 423 | \& next if /barney/; # same effect as "last", but doesn't document as well |
| 424 | \& # do something here |
| 425 | \& }} |
| 426 | .Ve |
| 427 | .PP |
| 428 | This is caused by the fact that a block by itself acts as a loop that |
| 429 | executes once, see \*(L"Basic BLOCKs and Switch Statements\*(R". |
| 430 | .PP |
| 431 | The form \f(CW\*(C`while/if BLOCK BLOCK\*(C'\fR, available in Perl 4, is no longer |
| 432 | available. Replace any occurrence of \f(CW\*(C`if BLOCK\*(C'\fR by \f(CW\*(C`if (do BLOCK)\*(C'\fR. |
| 433 | .Sh "For Loops" |
| 434 | .IX Subsection "For Loops" |
| 435 | Perl's C\-style \f(CW\*(C`for\*(C'\fR loop works like the corresponding \f(CW\*(C`while\*(C'\fR loop; |
| 436 | that means that this: |
| 437 | .PP |
| 438 | .Vb 3 |
| 439 | \& for ($i = 1; $i < 10; $i++) { |
| 440 | \& ... |
| 441 | \& } |
| 442 | .Ve |
| 443 | .PP |
| 444 | is the same as this: |
| 445 | .PP |
| 446 | .Vb 6 |
| 447 | \& $i = 1; |
| 448 | \& while ($i < 10) { |
| 449 | \& ... |
| 450 | \& } continue { |
| 451 | \& $i++; |
| 452 | \& } |
| 453 | .Ve |
| 454 | .PP |
| 455 | There is one minor difference: if variables are declared with \f(CW\*(C`my\*(C'\fR |
| 456 | in the initialization section of the \f(CW\*(C`for\*(C'\fR, the lexical scope of |
| 457 | those variables is exactly the \f(CW\*(C`for\*(C'\fR loop (the body of the loop |
| 458 | and the control sections). |
| 459 | .PP |
| 460 | Besides the normal array index looping, \f(CW\*(C`for\*(C'\fR can lend itself |
| 461 | to many other interesting applications. Here's one that avoids the |
| 462 | problem you get into if you explicitly test for end-of-file on |
| 463 | an interactive file descriptor causing your program to appear to |
| 464 | hang. |
| 465 | .PP |
| 466 | .Vb 5 |
| 467 | \& $on_a_tty = -t STDIN && -t STDOUT; |
| 468 | \& sub prompt { print "yes? " if $on_a_tty } |
| 469 | \& for ( prompt(); <STDIN>; prompt() ) { |
| 470 | \& # do something |
| 471 | \& } |
| 472 | .Ve |
| 473 | .Sh "Foreach Loops" |
| 474 | .IX Subsection "Foreach Loops" |
| 475 | The \f(CW\*(C`foreach\*(C'\fR loop iterates over a normal list value and sets the |
| 476 | variable \s-1VAR\s0 to be each element of the list in turn. If the variable |
| 477 | is preceded with the keyword \f(CW\*(C`my\*(C'\fR, then it is lexically scoped, and |
| 478 | is therefore visible only within the loop. Otherwise, the variable is |
| 479 | implicitly local to the loop and regains its former value upon exiting |
| 480 | the loop. If the variable was previously declared with \f(CW\*(C`my\*(C'\fR, it uses |
| 481 | that variable instead of the global one, but it's still localized to |
| 482 | the loop. |
| 483 | .PP |
| 484 | The \f(CW\*(C`foreach\*(C'\fR keyword is actually a synonym for the \f(CW\*(C`for\*(C'\fR keyword, so |
| 485 | you can use \f(CW\*(C`foreach\*(C'\fR for readability or \f(CW\*(C`for\*(C'\fR for brevity. (Or because |
| 486 | the Bourne shell is more familiar to you than \fIcsh\fR, so writing \f(CW\*(C`for\*(C'\fR |
| 487 | comes more naturally.) If \s-1VAR\s0 is omitted, \f(CW$_\fR is set to each value. |
| 488 | .PP |
| 489 | If any element of \s-1LIST\s0 is an lvalue, you can modify it by modifying |
| 490 | \&\s-1VAR\s0 inside the loop. Conversely, if any element of \s-1LIST\s0 is \s-1NOT\s0 an |
| 491 | lvalue, any attempt to modify that element will fail. In other words, |
| 492 | the \f(CW\*(C`foreach\*(C'\fR loop index variable is an implicit alias for each item |
| 493 | in the list that you're looping over. |
| 494 | .PP |
| 495 | If any part of \s-1LIST\s0 is an array, \f(CW\*(C`foreach\*(C'\fR will get very confused if |
| 496 | you add or remove elements within the loop body, for example with |
| 497 | \&\f(CW\*(C`splice\*(C'\fR. So don't do that. |
| 498 | .PP |
| 499 | \&\f(CW\*(C`foreach\*(C'\fR probably won't do what you expect if \s-1VAR\s0 is a tied or other |
| 500 | special variable. Don't do that either. |
| 501 | .PP |
| 502 | Examples: |
| 503 | .PP |
| 504 | .Vb 1 |
| 505 | \& for (@ary) { s/foo/bar/ } |
| 506 | .Ve |
| 507 | .PP |
| 508 | .Vb 3 |
| 509 | \& for my $elem (@elements) { |
| 510 | \& $elem *= 2; |
| 511 | \& } |
| 512 | .Ve |
| 513 | .PP |
| 514 | .Vb 3 |
| 515 | \& for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
| 516 | \& print $count, "\en"; sleep(1); |
| 517 | \& } |
| 518 | .Ve |
| 519 | .PP |
| 520 | .Vb 1 |
| 521 | \& for (1..15) { print "Merry Christmas\en"; } |
| 522 | .Ve |
| 523 | .PP |
| 524 | .Vb 3 |
| 525 | \& foreach $item (split(/:[\e\e\en:]*/, $ENV{TERMCAP})) { |
| 526 | \& print "Item: $item\en"; |
| 527 | \& } |
| 528 | .Ve |
| 529 | .PP |
| 530 | Here's how a C programmer might code up a particular algorithm in Perl: |
| 531 | .PP |
| 532 | .Vb 9 |
| 533 | \& for (my $i = 0; $i < @ary1; $i++) { |
| 534 | \& for (my $j = 0; $j < @ary2; $j++) { |
| 535 | \& if ($ary1[$i] > $ary2[$j]) { |
| 536 | \& last; # can't go to outer :-( |
| 537 | \& } |
| 538 | \& $ary1[$i] += $ary2[$j]; |
| 539 | \& } |
| 540 | \& # this is where that last takes me |
| 541 | \& } |
| 542 | .Ve |
| 543 | .PP |
| 544 | Whereas here's how a Perl programmer more comfortable with the idiom might |
| 545 | do it: |
| 546 | .PP |
| 547 | .Vb 6 |
| 548 | \& OUTER: for my $wid (@ary1) { |
| 549 | \& INNER: for my $jet (@ary2) { |
| 550 | \& next OUTER if $wid > $jet; |
| 551 | \& $wid += $jet; |
| 552 | \& } |
| 553 | \& } |
| 554 | .Ve |
| 555 | .PP |
| 556 | See how much easier this is? It's cleaner, safer, and faster. It's |
| 557 | cleaner because it's less noisy. It's safer because if code gets added |
| 558 | between the inner and outer loops later on, the new code won't be |
| 559 | accidentally executed. The \f(CW\*(C`next\*(C'\fR explicitly iterates the other loop |
| 560 | rather than merely terminating the inner one. And it's faster because |
| 561 | Perl executes a \f(CW\*(C`foreach\*(C'\fR statement more rapidly than it would the |
| 562 | equivalent \f(CW\*(C`for\*(C'\fR loop. |
| 563 | .Sh "Basic BLOCKs and Switch Statements" |
| 564 | .IX Subsection "Basic BLOCKs and Switch Statements" |
| 565 | A \s-1BLOCK\s0 by itself (labeled or not) is semantically equivalent to a |
| 566 | loop that executes once. Thus you can use any of the loop control |
| 567 | statements in it to leave or restart the block. (Note that this is |
| 568 | \&\fI\s-1NOT\s0\fR true in \f(CW\*(C`eval{}\*(C'\fR, \f(CW\*(C`sub{}\*(C'\fR, or contrary to popular belief |
| 569 | \&\f(CW\*(C`do{}\*(C'\fR blocks, which do \fI\s-1NOT\s0\fR count as loops.) The \f(CW\*(C`continue\*(C'\fR |
| 570 | block is optional. |
| 571 | .PP |
| 572 | The \s-1BLOCK\s0 construct is particularly nice for doing case |
| 573 | structures. |
| 574 | .PP |
| 575 | .Vb 6 |
| 576 | \& SWITCH: { |
| 577 | \& if (/^abc/) { $abc = 1; last SWITCH; } |
| 578 | \& if (/^def/) { $def = 1; last SWITCH; } |
| 579 | \& if (/^xyz/) { $xyz = 1; last SWITCH; } |
| 580 | \& $nothing = 1; |
| 581 | \& } |
| 582 | .Ve |
| 583 | .PP |
| 584 | There is no official \f(CW\*(C`switch\*(C'\fR statement in Perl, because there are |
| 585 | already several ways to write the equivalent. |
| 586 | .PP |
| 587 | However, starting from Perl 5.8 to get switch and case one can use |
| 588 | the Switch extension and say: |
| 589 | .PP |
| 590 | .Vb 1 |
| 591 | \& use Switch; |
| 592 | .Ve |
| 593 | .PP |
| 594 | after which one has switch and case. It is not as fast as it could be |
| 595 | because it's not really part of the language (it's done using source |
| 596 | filters) but it is available, and it's very flexible. |
| 597 | .PP |
| 598 | In addition to the above \s-1BLOCK\s0 construct, you could write |
| 599 | .PP |
| 600 | .Vb 6 |
| 601 | \& SWITCH: { |
| 602 | \& $abc = 1, last SWITCH if /^abc/; |
| 603 | \& $def = 1, last SWITCH if /^def/; |
| 604 | \& $xyz = 1, last SWITCH if /^xyz/; |
| 605 | \& $nothing = 1; |
| 606 | \& } |
| 607 | .Ve |
| 608 | .PP |
| 609 | (That's actually not as strange as it looks once you realize that you can |
| 610 | use loop control \*(L"operators\*(R" within an expression, That's just the normal |
| 611 | C comma operator.) |
| 612 | .PP |
| 613 | or |
| 614 | .PP |
| 615 | .Vb 6 |
| 616 | \& SWITCH: { |
| 617 | \& /^abc/ && do { $abc = 1; last SWITCH; }; |
| 618 | \& /^def/ && do { $def = 1; last SWITCH; }; |
| 619 | \& /^xyz/ && do { $xyz = 1; last SWITCH; }; |
| 620 | \& $nothing = 1; |
| 621 | \& } |
| 622 | .Ve |
| 623 | .PP |
| 624 | or formatted so it stands out more as a \*(L"proper\*(R" \f(CW\*(C`switch\*(C'\fR statement: |
| 625 | .PP |
| 626 | .Vb 5 |
| 627 | \& SWITCH: { |
| 628 | \& /^abc/ && do { |
| 629 | \& $abc = 1; |
| 630 | \& last SWITCH; |
| 631 | \& }; |
| 632 | .Ve |
| 633 | .PP |
| 634 | .Vb 4 |
| 635 | \& /^def/ && do { |
| 636 | \& $def = 1; |
| 637 | \& last SWITCH; |
| 638 | \& }; |
| 639 | .Ve |
| 640 | .PP |
| 641 | .Vb 6 |
| 642 | \& /^xyz/ && do { |
| 643 | \& $xyz = 1; |
| 644 | \& last SWITCH; |
| 645 | \& }; |
| 646 | \& $nothing = 1; |
| 647 | \& } |
| 648 | .Ve |
| 649 | .PP |
| 650 | or |
| 651 | .PP |
| 652 | .Vb 6 |
| 653 | \& SWITCH: { |
| 654 | \& /^abc/ and $abc = 1, last SWITCH; |
| 655 | \& /^def/ and $def = 1, last SWITCH; |
| 656 | \& /^xyz/ and $xyz = 1, last SWITCH; |
| 657 | \& $nothing = 1; |
| 658 | \& } |
| 659 | .Ve |
| 660 | .PP |
| 661 | or even, horrors, |
| 662 | .PP |
| 663 | .Vb 8 |
| 664 | \& if (/^abc/) |
| 665 | \& { $abc = 1 } |
| 666 | \& elsif (/^def/) |
| 667 | \& { $def = 1 } |
| 668 | \& elsif (/^xyz/) |
| 669 | \& { $xyz = 1 } |
| 670 | \& else |
| 671 | \& { $nothing = 1 } |
| 672 | .Ve |
| 673 | .PP |
| 674 | A common idiom for a \f(CW\*(C`switch\*(C'\fR statement is to use \f(CW\*(C`foreach\*(C'\fR's aliasing to make |
| 675 | a temporary assignment to \f(CW$_\fR for convenient matching: |
| 676 | .PP |
| 677 | .Vb 6 |
| 678 | \& SWITCH: for ($where) { |
| 679 | \& /In Card Names/ && do { push @flags, '-e'; last; }; |
| 680 | \& /Anywhere/ && do { push @flags, '-h'; last; }; |
| 681 | \& /In Rulings/ && do { last; }; |
| 682 | \& die "unknown value for form variable where: `$where'"; |
| 683 | \& } |
| 684 | .Ve |
| 685 | .PP |
| 686 | Another interesting approach to a switch statement is arrange |
| 687 | for a \f(CW\*(C`do\*(C'\fR block to return the proper value: |
| 688 | .PP |
| 689 | .Vb 8 |
| 690 | \& $amode = do { |
| 691 | \& if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? |
| 692 | \& elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
| 693 | \& elsif ($flag & O_RDWR) { |
| 694 | \& if ($flag & O_CREAT) { "w+" } |
| 695 | \& else { ($flag & O_APPEND) ? "a+" : "r+" } |
| 696 | \& } |
| 697 | \& }; |
| 698 | .Ve |
| 699 | .PP |
| 700 | Or |
| 701 | .PP |
| 702 | .Vb 5 |
| 703 | \& print do { |
| 704 | \& ($flags & O_WRONLY) ? "write-only" : |
| 705 | \& ($flags & O_RDWR) ? "read-write" : |
| 706 | \& "read-only"; |
| 707 | \& }; |
| 708 | .Ve |
| 709 | .PP |
| 710 | Or if you are certain that all the \f(CW\*(C`&&\*(C'\fR clauses are true, you can use |
| 711 | something like this, which \*(L"switches\*(R" on the value of the |
| 712 | \&\f(CW\*(C`HTTP_USER_AGENT\*(C'\fR environment variable. |
| 713 | .PP |
| 714 | .Vb 13 |
| 715 | \& #!/usr/bin/perl |
| 716 | \& # pick out jargon file page based on browser |
| 717 | \& $dir = 'http://www.wins.uva.nl/~mes/jargon'; |
| 718 | \& for ($ENV{HTTP_USER_AGENT}) { |
| 719 | \& $page = /Mac/ && 'm/Macintrash.html' |
| 720 | \& || /Win(dows )?NT/ && 'e/evilandrude.html' |
| 721 | \& || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' |
| 722 | \& || /Linux/ && 'l/Linux.html' |
| 723 | \& || /HP-UX/ && 'h/HP-SUX.html' |
| 724 | \& || /SunOS/ && 's/ScumOS.html' |
| 725 | \& || 'a/AppendixB.html'; |
| 726 | \& } |
| 727 | \& print "Location: $dir/$page\e015\e012\e015\e012"; |
| 728 | .Ve |
| 729 | .PP |
| 730 | That kind of switch statement only works when you know the \f(CW\*(C`&&\*(C'\fR clauses |
| 731 | will be true. If you don't, the previous \f(CW\*(C`?:\*(C'\fR example should be used. |
| 732 | .PP |
| 733 | You might also consider writing a hash of subroutine references |
| 734 | instead of synthesizing a \f(CW\*(C`switch\*(C'\fR statement. |
| 735 | .Sh "Goto" |
| 736 | .IX Subsection "Goto" |
| 737 | Although not for the faint of heart, Perl does support a \f(CW\*(C`goto\*(C'\fR |
| 738 | statement. There are three forms: \f(CW\*(C`goto\*(C'\fR\-LABEL, \f(CW\*(C`goto\*(C'\fR\-EXPR, and |
| 739 | \&\f(CW\*(C`goto\*(C'\fR\-&NAME. A loop's \s-1LABEL\s0 is not actually a valid target for |
| 740 | a \f(CW\*(C`goto\*(C'\fR; it's just the name of the loop. |
| 741 | .PP |
| 742 | The \f(CW\*(C`goto\*(C'\fR\-LABEL form finds the statement labeled with \s-1LABEL\s0 and resumes |
| 743 | execution there. It may not be used to go into any construct that |
| 744 | requires initialization, such as a subroutine or a \f(CW\*(C`foreach\*(C'\fR loop. It |
| 745 | also can't be used to go into a construct that is optimized away. It |
| 746 | can be used to go almost anywhere else within the dynamic scope, |
| 747 | including out of subroutines, but it's usually better to use some other |
| 748 | construct such as \f(CW\*(C`last\*(C'\fR or \f(CW\*(C`die\*(C'\fR. The author of Perl has never felt the |
| 749 | need to use this form of \f(CW\*(C`goto\*(C'\fR (in Perl, that is\*(--C is another matter). |
| 750 | .PP |
| 751 | The \f(CW\*(C`goto\*(C'\fR\-EXPR form expects a label name, whose scope will be resolved |
| 752 | dynamically. This allows for computed \f(CW\*(C`goto\*(C'\fRs per \s-1FORTRAN\s0, but isn't |
| 753 | necessarily recommended if you're optimizing for maintainability: |
| 754 | .PP |
| 755 | .Vb 1 |
| 756 | \& goto(("FOO", "BAR", "GLARCH")[$i]); |
| 757 | .Ve |
| 758 | .PP |
| 759 | The \f(CW\*(C`goto\*(C'\fR\-&NAME form is highly magical, and substitutes a call to the |
| 760 | named subroutine for the currently running subroutine. This is used by |
| 761 | \&\f(CW\*(C`AUTOLOAD()\*(C'\fR subroutines that wish to load another subroutine and then |
| 762 | pretend that the other subroutine had been called in the first place |
| 763 | (except that any modifications to \f(CW@_\fR in the current subroutine are |
| 764 | propagated to the other subroutine.) After the \f(CW\*(C`goto\*(C'\fR, not even \f(CW\*(C`caller()\*(C'\fR |
| 765 | will be able to tell that this routine was called first. |
| 766 | .PP |
| 767 | In almost all cases like this, it's usually a far, far better idea to use the |
| 768 | structured control flow mechanisms of \f(CW\*(C`next\*(C'\fR, \f(CW\*(C`last\*(C'\fR, or \f(CW\*(C`redo\*(C'\fR instead of |
| 769 | resorting to a \f(CW\*(C`goto\*(C'\fR. For certain applications, the catch and throw pair of |
| 770 | \&\f(CW\*(C`eval{}\*(C'\fR and \fIdie()\fR for exception processing can also be a prudent approach. |
| 771 | .Sh "PODs: Embedded Documentation" |
| 772 | .IX Subsection "PODs: Embedded Documentation" |
| 773 | Perl has a mechanism for intermixing documentation with source code. |
| 774 | While it's expecting the beginning of a new statement, if the compiler |
| 775 | encounters a line that begins with an equal sign and a word, like this |
| 776 | .PP |
| 777 | .Vb 1 |
| 778 | \& =head1 Here There Be Pods! |
| 779 | .Ve |
| 780 | .PP |
| 781 | Then that text and all remaining text up through and including a line |
| 782 | beginning with \f(CW\*(C`=cut\*(C'\fR will be ignored. The format of the intervening |
| 783 | text is described in perlpod. |
| 784 | .PP |
| 785 | This allows you to intermix your source code |
| 786 | and your documentation text freely, as in |
| 787 | .PP |
| 788 | .Vb 1 |
| 789 | \& =item snazzle($) |
| 790 | .Ve |
| 791 | .PP |
| 792 | .Vb 3 |
| 793 | \& The snazzle() function will behave in the most spectacular |
| 794 | \& form that you can possibly imagine, not even excepting |
| 795 | \& cybernetic pyrotechnics. |
| 796 | .Ve |
| 797 | .PP |
| 798 | .Vb 1 |
| 799 | \& =cut back to the compiler, nuff of this pod stuff! |
| 800 | .Ve |
| 801 | .PP |
| 802 | .Vb 4 |
| 803 | \& sub snazzle($) { |
| 804 | \& my $thingie = shift; |
| 805 | \& ......... |
| 806 | \& } |
| 807 | .Ve |
| 808 | .PP |
| 809 | Note that pod translators should look at only paragraphs beginning |
| 810 | with a pod directive (it makes parsing easier), whereas the compiler |
| 811 | actually knows to look for pod escapes even in the middle of a |
| 812 | paragraph. This means that the following secret stuff will be |
| 813 | ignored by both the compiler and the translators. |
| 814 | .PP |
| 815 | .Vb 5 |
| 816 | \& $a=3; |
| 817 | \& =secret stuff |
| 818 | \& warn "Neither POD nor CODE!?" |
| 819 | \& =cut back |
| 820 | \& print "got $a\en"; |
| 821 | .Ve |
| 822 | .PP |
| 823 | You probably shouldn't rely upon the \f(CW\*(C`warn()\*(C'\fR being podded out forever. |
| 824 | Not all pod translators are well-behaved in this regard, and perhaps |
| 825 | the compiler will become pickier. |
| 826 | .PP |
| 827 | One may also use pod directives to quickly comment out a section |
| 828 | of code. |
| 829 | .Sh "Plain Old Comments (Not!)" |
| 830 | .IX Subsection "Plain Old Comments (Not!)" |
| 831 | Much like the C preprocessor, Perl can process line directives. Using |
| 832 | this, one can control Perl's idea of filenames and line numbers in |
| 833 | error or warning messages (especially for strings that are processed |
| 834 | with \f(CW\*(C`eval()\*(C'\fR). The syntax for this mechanism is the same as for most |
| 835 | C preprocessors: it matches the regular expression |
| 836 | \&\f(CW\*(C`/^#\es*line\es+(\ed+)\es*(?:\es"([^"]+)")?\es*$/\*(C'\fR with \f(CW$1\fR being the line |
| 837 | number for the next line, and \f(CW$2\fR being the optional filename |
| 838 | (specified within quotes). |
| 839 | .PP |
| 840 | There is a fairly obvious gotcha included with the line directive: |
| 841 | Debuggers and profilers will only show the last source line to appear |
| 842 | at a particular line number in a given file. Care should be taken not |
| 843 | to cause line number collisions in code you'd like to debug later. |
| 844 | .PP |
| 845 | Here are some examples that you should be able to type into your command |
| 846 | shell: |
| 847 | .PP |
| 848 | .Vb 6 |
| 849 | \& % perl |
| 850 | \& # line 200 "bzzzt" |
| 851 | \& # the `#' on the previous line must be the first char on line |
| 852 | \& die 'foo'; |
| 853 | \& __END__ |
| 854 | \& foo at bzzzt line 201. |
| 855 | .Ve |
| 856 | .PP |
| 857 | .Vb 5 |
| 858 | \& % perl |
| 859 | \& # line 200 "bzzzt" |
| 860 | \& eval qq[\en#line 2001 ""\endie 'foo']; print $@; |
| 861 | \& __END__ |
| 862 | \& foo at - line 2001. |
| 863 | .Ve |
| 864 | .PP |
| 865 | .Vb 4 |
| 866 | \& % perl |
| 867 | \& eval qq[\en#line 200 "foo bar"\endie 'foo']; print $@; |
| 868 | \& __END__ |
| 869 | \& foo at foo bar line 200. |
| 870 | .Ve |
| 871 | .PP |
| 872 | .Vb 6 |
| 873 | \& % perl |
| 874 | \& # line 345 "goop" |
| 875 | \& eval "\en#line " . __LINE__ . ' "' . __FILE__ ."\e"\endie 'foo'"; |
| 876 | \& print $@; |
| 877 | \& __END__ |
| 878 | \& foo at goop line 345. |
| 879 | .Ve |