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Diffstat (limited to 'pod/perlsyn.pod')
| -rw-r--r-- | pod/perlsyn.pod | 250 |
1 files changed, 208 insertions, 42 deletions
diff --git a/pod/perlsyn.pod b/pod/perlsyn.pod index 252e679b72..4b1d607e7e 100644 --- a/pod/perlsyn.pod +++ b/pod/perlsyn.pod @@ -19,35 +19,43 @@ which lines you look at. (Actually, I'm lying--it is possible to do an implicit loop with either the B<-n> or B<-p> switch. It's just not the mandatory default like it is in B<sed> and B<awk>.) +=head2 Declarations + Perl is, for the most part, a free-form language. (The only exception to this is format declarations, for obvious reasons.) Comments are indicated by the "#" character, and extend to the end of the line. If you attempt to use C</* */> C-style comments, it will be interpreted either as division or pattern matching, depending on the context, and C++ -C<//> comments just look like a null regular expression, So don't do +C<//> comments just look like a null regular expression, so don't do that. A declaration can be put anywhere a statement can, but has no effect on the execution of the primary sequence of statements--declarations all take effect at compile time. Typically all the declarations are put at -the beginning or the end of the script. +the beginning or the end of the script. However, if you're using +lexically-scoped private variables created with my(), you'll have to make sure +your format or subroutine definition is within the same block scope +as the my if you expect to to be able to access those private variables. -As of Perl 5, declaring a subroutine allows a subroutine name to be used -as if it were a list operator from that point forward in the program. You -can declare a subroutine without defining it by saying just +Declaring a subroutine allows a subroutine name to be used as if it were a +list operator from that point forward in the program. You can declare a +subroutine without defining it by saying just sub myname; $me = myname $0 or die "can't get myname"; -Note that it functions as a list operator though, not a unary +Note that it functions as a list operator though, not as a unary operator, so be careful to use C<or> instead of C<||> there. -Subroutines declarations can also be imported by a C<use> statement. +Subroutines declarations can also be loaded up with the C<require> statement +or both loaded and imported into your namespace with a C<use> statement. +See L<perlmod> for details on this. -Also as of Perl 5, a statement sequence may contain declarations of -lexically scoped variables, but apart from declaring a variable name, -the declaration acts like an ordinary statement, and is elaborated within -the sequence of statements as if it were an ordinary statement. +A statement sequence may contain declarations of lexically-scoped +variables, but apart from declaring a variable name, the declaration acts +like an ordinary statement, and is elaborated within the sequence of +statements as if it were an ordinary statement. That means it actually +has both compile-time and run-time effects. =head2 Simple statements @@ -58,8 +66,7 @@ the semicolon is optional. (A semicolon is still encouraged there if the block takes up more than one line, since you may eventually add another line.) Note that there are some operators like C<eval {}> and C<do {}> that look like compound statements, but aren't (they're just TERMs in an expression), -and thus need an explicit termination -if used as the last item in a statement. +and thus need an explicit termination if used as the last item in a statement. Any simple statement may optionally be followed by a I<SINGLE> modifier, just before the terminating semicolon (or block ending). The possible @@ -79,14 +86,14 @@ executes once before the conditional is evaluated. This is so that you can write loops like: do { - $_ = <STDIN>; + $line = <STDIN>; ... - } until $_ eq ".\n"; + } until $line eq ".\n"; See L<perlfunc/do>. Note also that the loop control statements described later will I<NOT> work in this construct, since modifiers don't take loop labels. Sorry. You can always wrap -another block around it to do that sort of thing.) +another block around it to do that sort of thing. =head2 Compound statements @@ -128,14 +135,86 @@ the sense of the test is reversed. The C<while> statement executes the block as long as the expression is true (does not evaluate to the null string or 0 or "0"). The LABEL is -optional, and if present, consists of an identifier followed by a -colon. The LABEL identifies the loop for the loop control statements -C<next>, C<last>, and C<redo> (see below). If there is a C<continue> -BLOCK, it is always executed just before the conditional is about to be -evaluated again, just like the third part of a C<for> loop in C. -Thus it can be used to increment a loop variable, even when the loop -has been continued via the C<next> statement (which is similar to the C -C<continue> statement). +optional, and if present, consists of an identifier followed by a colon. +The LABEL identifies the loop for the loop control statements C<next>, +C<last>, and C<redo>. If the LABEL is omitted, the loop control statement +refers to the innermost enclosing loop. This may include dynamically +looking back your call-stack at run time to find the LABEL. Such +desperate behavior triggers a warning if you use the B<-w> flag. + +If there is a C<continue> BLOCK, it is always executed just before the +conditional is about to be evaluated again, just like the third part of a +C<for> loop in C. Thus it can be used to increment a loop variable, even +when the loop has been continued via the C<next> statement (which is +similar to the C C<continue> statement). + +=head2 Loop Control + +The C<next> command is like the C<continue> statement in C; it starts +the next iteration of the loop: + + LINE: while (<STDIN>) { + next LINE if /^#/; # discard comments + ... + } + +The C<last> command is like the C<break> statement in C (as used in +loops); it immediately exits the loop in question. The +C<continue> block, if any, is not executed: + + LINE: while (<STDIN>) { + last LINE if /^$/; # exit when done with header + ... + } + +The C<redo> command restarts the loop block without evaluating the +conditional again. The C<continue> block, if any, is I<not> executed. +This command is normally used by programs that want to lie to themselves +about what was just input. + +For example, when processing a file like F</etc/termcap>. +If your input lines might end in backslashes to indicate continuation, you +want to skip ahead and get the next record. + + while (<>) { + chomp; + if (s/\\$//) { + $_ .= <>; + redo unless eof(); + } + # now process $_ + } + +which is Perl short-hand for the more explicitly written version: + + LINE: while ($line = <ARGV>) { + chomp($line); + if ($line =~ s/\\$//) { + $line .= <ARGV>; + redo LINE unless eof(); # not eof(ARGV)! + } + # now process $line + } + +Or here's a a simpleminded Pascal comment stripper (warning: assumes no { or } in strings) + + LINE: while (<STDIN>) { + while (s|({.*}.*){.*}|$1 |) {} + s|{.*}| |; + if (s|{.*| |) { + $front = $_; + while (<STDIN>) { + if (/}/) { # end of comment? + s|^|$front{|; + redo LINE; + } + } + } + print; + } + +Note that if there were a C<continue> block on the above code, it would get +executed even on discarded lines. If the word C<while> is replaced by the word C<until>, the sense of the test is reversed, but the conditional is still tested before the first @@ -143,11 +222,13 @@ iteration. In either the C<if> or the C<while> statement, you may replace "(EXPR)" with a BLOCK, and the conditional is true if the value of the last -statement in that block is true. (This feature continues to work in Perl -5 but is deprecated. Please change any occurrences of "if BLOCK" to -"if (do BLOCK)".) +statement in that block is true. While this "feature" continues to work in +version 5, it has been deprecated, so please change any occurrences of "if BLOCK" to +"if (do BLOCK)". + +=head2 For and Foreach -The C-style C<for> loop works exactly like the corresponding C<while> loop: +Perl's C-style C<for> loop works exactly like the corresponding C<while> loop: for ($i = 1; $i < 10; $i++) { ... @@ -162,38 +243,78 @@ is the same as $i++; } -The foreach loop iterates over a normal list value and sets the +The C<foreach> loop iterates over a normal list value and sets the variable VAR to be each element of the list in turn. The variable is -implicitly local to the loop and regains its former value upon exiting -the loop. (If the variable was previously declared with C<my>, it uses -that variable instead of the global one, but it's still localized to -the loop.) The C<foreach> keyword is actually a synonym for the C<for> -keyword, so you can use C<foreach> for readability or C<for> for -brevity. If VAR is omitted, $_ is set to each value. If LIST is an -actual array (as opposed to an expression returning a list value), you -can modify each element of the array by modifying VAR inside the loop. +implicitly local to the loop and regains its former value upon exiting the +loop. If the variable was previously declared with C<my>, it uses that +variable instead of the global one, but it's still localized to the loop. +This can cause problems if you have subroutine or format declarations +within that block's scope. + +The C<foreach> keyword is actually a synonym for the C<for> keyword, so +you can use C<foreach> for readability or C<for> for brevity. If VAR is +omitted, $_ is set to each value. If LIST is an actual array (as opposed +to an expression returning a list value), you can modify each element of +the array by modifying VAR inside the loop. That's because the C<foreach> +loop index variable is an implicit alias for each item in the list that +you're looping over. + Examples: - for (@ary) { s/foo/bar/; } + for (@ary) { s/foo/bar/ } foreach $elem (@elements) { $elem *= 2; } - for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) { - print $_, "\n"; sleep(1); + for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { + print $count, "\n"; sleep(1); } for (1..15) { print "Merry Christmas\n"; } - foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) { + foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { print "Item: $item\n"; } +Here's how a C programmer might code up a particular algorithm in Perl: + + for ($i = 0; $i < @ary1; $i++) { + for ($j = 0; $j < @ary2; $j++) { + if ($ary1[$i] > $ary2[$j]) { + last; # can't go to outer :-( + } + $ary1[$i] += $ary2[$j]; + } + } + +Whereas here's how a Perl programmer more confortable with the idiom might +do it this way: + + OUTER: foreach $i (@ary1) { + INNER: foreach $j (@ary2) { + next OUTER if $i > $j; + $i += $j; + } + } + +See how much easier this is? It's cleaner, safer, and faster. +It's cleaner because it's less noisy. +It's safer because if code gets added +between the inner and outer loops later, you won't accidentally excecute +it because you've explicitly asked to iterate the other loop rather than +merely terminating the inner one. +And it's faster because Perl exececute C<foreach> statement more +rapidly than it would the equivalent C<for> loop. + +=head2 Basic BLOCKs and Switch Statements + A BLOCK by itself (labeled or not) is semantically equivalent to a loop that executes once. Thus you can use any of the loop control statements in it to leave or restart the block. The C<continue> block -is optional. This construct is particularly nice for doing case +is optional. + +The BLOCK construct is particularly nice for doing case structures. SWITCH: { @@ -267,3 +388,48 @@ or even, horrors, else { $nothing = 1 } + +A common idiom for a switch statement is to use C<foreach>'s aliasing to make +a temporary assignment to $_ for convenient matching: + + SWITCH: for ($where) { + /In Card Names/ && do { push @flags, '-e'; last; }; + /Anywhere/ && do { push @flags, '-h'; last; }; + /In Rulings/ && do { last; }; + die "unknown value for form variable where: `$where'"; + } + +=head2 Goto + +Although not for the faint of heart, Perl does support a C<goto> statement. +A loop's LABEL is not actually a valid target for a C<goto>; +it's just the name of the loop. There are three forms: goto-LABEL, +goto-EXPR, and goto-&NAME. + +The goto-LABEL form finds the statement labeled with LABEL and resumes +execution there. It may not be used to go into any construct that +requires initialization, such as a subroutine or a foreach loop. It +also can't be used to go into a construct that is optimized away. It +can be used to go almost anywhere else within the dynamic scope, +including out of subroutines, but it's usually better to use some other +construct such as last or die. The author of Perl has never felt the +need to use this form of goto (in Perl, that is--C is another matter). + +The goto-EXPR form expects a label name, whose scope will be resolved +dynamically. This allows for computed gotos per FORTRAN, but isn't +necessarily recommended if you're optimizing for maintainability: + + goto ("FOO", "BAR", "GLARCH")[$i]; + +The goto-&NAME form is highly magical, and substitutes a call to the +named subroutine for the currently running subroutine. This is used by +AUTOLOAD() subroutines that wish to load another subroutine and then +pretend that the other subroutine had been called in the first place +(except that any modifications to @_ in the current subroutine are +propagated to the other subroutine.) After the C<goto>, not even caller() +will be able to tell that this routine was called first. + +In almost cases like this, it's usually a far, far better idea to use the +structured control flow mechanisms of C<next>, C<last>, or C<redo> insetad +resorting to a C<goto>. For certain applications, the catch and throw pair of +C<eval{}> and die() for exception processing can also be a prudent approach. |