X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlsyn.pod;h=9d0c9209eaa8d220486b666a7a84e3b9226c282f;hb=b851fbc1add6c3d9fa6158884279133c311a3efc;hp=f07bdfeabfbab28467ef83d416c29cee1268d9e1;hpb=0b8d69e96040ec811c067522a2d9770121123a35;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlsyn.pod b/pod/perlsyn.pod index f07bdfe..9d0c920 100644 --- a/pod/perlsyn.pod +++ b/pod/perlsyn.pod @@ -53,8 +53,8 @@ subroutine without defining it by saying C, thus: sub myname; $me = myname $0 or die "can't get myname"; -Note that my() functions as a list operator, not as a unary operator; so -be careful to use C instead of C<||> in this case. However, if +Note that myname() functions as a list operator, not as a unary operator; +so be careful to use C instead of C<||> in this case. However, if you were to declare the subroutine as C, then C would function as a unary operator, so either C or C<||> would work. @@ -171,7 +171,8 @@ statements C, C, and C. If the LABEL is omitted, the loop control statement refers to the innermost enclosing loop. This may include dynamically looking back your call-stack at run time to find the LABEL. Such -desperate behavior triggers a warning if you use the B<-w> flag. +desperate behavior triggers a warning if you use the C +pragma or the B<-w> flag. Unlike a C statement, a C statement never implicitly localises any variables. @@ -262,7 +263,7 @@ available. Replace any occurrence of C by C. =head2 For Loops -Perl's C-style C loop works exactly like the corresponding C loop; +Perl's C-style C loop works like the corresponding C loop; that means that this: for ($i = 1; $i < 10; $i++) { @@ -278,8 +279,10 @@ is the same as this: $i++; } -(There is one minor difference: The first form implies a lexical scope -for variables declared with C in the initialization expression.) +There is one minor difference: if variables are declared with C +in the initialization section of the C, the lexical scope of +those variables is exactly the C loop (the body of the loop +and the control sections). Besides the normal array index looping, C can lend itself to many other interesting applications. Here's one that avoids the @@ -308,9 +311,12 @@ The C keyword is actually a synonym for the C keyword, so you can use C for readability or C for brevity. (Or because the Bourne shell is more familiar to you than I, so writing C comes more naturally.) If VAR is omitted, C<$_> is set to each value. -If any element of LIST is an lvalue, you can modify it by modifying VAR -inside the loop. That's because the C loop index variable is -an implicit alias for each item in the list that you're looping over. + +If any element of LIST is an lvalue, you can modify it by modifying +VAR inside the loop. Conversely, if any element of LIST is NOT an +lvalue, any attempt to modify that element will fail. In other words, +the C loop index variable is an implicit alias for each item +in the list that you're looping over. If any part of LIST is an array, C will get very confused if you add or remove elements within the loop body, for example with @@ -323,7 +329,7 @@ Examples: for (@ary) { s/foo/bar/ } - foreach my $elem (@elements) { + for my $elem (@elements) { $elem *= 2; } @@ -352,8 +358,8 @@ Here's how a C programmer might code up a particular algorithm in Perl: Whereas here's how a Perl programmer more comfortable with the idiom might do it: - OUTER: foreach my $wid (@ary1) { - INNER: foreach my $jet (@ary2) { + OUTER: for my $wid (@ary1) { + INNER: for my $jet (@ary2) { next OUTER if $wid > $jet; $wid += $jet; } @@ -387,8 +393,18 @@ structures. } There is no official C statement in Perl, because there are -already several ways to write the equivalent. In addition to the -above, you could write +already several ways to write the equivalent. + +However, starting from Perl 5.8 to get switch and case one can use +the Switch extension and say: + + use Switch; + +after which one has switch and case. It is not as fast as it could be +because it's not really part of the language (it's done using source +filters) but it is available, and it's very flexible. + +In addition to the above BLOCK construct, you could write SWITCH: { $abc = 1, last SWITCH if /^abc/; @@ -482,7 +498,7 @@ Or Or if you are certainly that all the C<&&> clauses are true, you can use something like this, which "switches" on the value of the -C envariable. +C environment variable. #!/usr/bin/perl # pick out jargon file page based on browser @@ -524,7 +540,7 @@ The C-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computed Cs per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability: - goto ("FOO", "BAR", "GLARCH")[$i]; + goto(("FOO", "BAR", "GLARCH")[$i]); The C-&NAME form is highly magical, and substitutes a call to the named subroutine for the currently running subroutine. This is used by @@ -593,10 +609,15 @@ this, one can control Perl's idea of filenames and line numbers in error or warning messages (especially for strings that are processed with C). The syntax for this mechanism is the same as for most C preprocessors: it matches the regular expression -C with C<$1> being the line +C with C<$1> being the line number for the next line, and C<$2> being the optional filename (specified within quotes). +There is a fairly obvious gotcha included with the line directive: +Debuggers and profilers will only show the last source line to appear +at a particular line number in a given file. Care should be taken not +to cause line number collisions in code you'd like to debug later. + Here are some examples that you should be able to type into your command shell: