diff options
Diffstat (limited to 'contrib/perl5/pod/perlsyn.pod')
| -rw-r--r-- | contrib/perl5/pod/perlsyn.pod | 30 |
1 files changed, 20 insertions, 10 deletions
diff --git a/contrib/perl5/pod/perlsyn.pod b/contrib/perl5/pod/perlsyn.pod index 724ba12..aad4efd 100644 --- a/contrib/perl5/pod/perlsyn.pod +++ b/contrib/perl5/pod/perlsyn.pod @@ -53,8 +53,8 @@ subroutine without defining it by saying C<sub name>, thus: sub myname; $me = myname $0 or die "can't get myname"; -Note that my() functions as a list operator, not as a unary operator; so -be careful to use C<or> instead of C<||> in this case. However, if +Note that myname() functions as a list operator, not as a unary operator; +so be careful to use C<or> instead of C<||> in this case. However, if you were to declare the subroutine as C<sub myname ($)>, then C<myname> would function as a unary operator, so either C<or> or C<||> would work. @@ -172,7 +172,7 @@ 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 C<use warnings> -praga or the B<-w> flag. +pragma or the B<-w> flag. Unlike a C<foreach> statement, a C<while> statement never implicitly localises any variables. @@ -263,7 +263,7 @@ available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. =head2 For Loops -Perl's C-style C<for> loop works exactly like the corresponding C<while> loop; +Perl's C-style C<for> loop works like the corresponding C<while> loop; that means that this: for ($i = 1; $i < 10; $i++) { @@ -279,8 +279,10 @@ is the same as this: $i++; } -(There is one minor difference: The first form implies a lexical scope -for variables declared with C<my> in the initialization expression.) +There is one minor difference: if variables are declared with C<my> +in the initialization section of the C<for>, the lexical scope of +those variables is exactly the C<for> loop (the body of the loop +and the control sections). Besides the normal array index looping, C<for> can lend itself to many other interesting applications. Here's one that avoids the @@ -309,9 +311,12 @@ The C<foreach> keyword is actually a synonym for the C<for> keyword, so you can use C<foreach> for readability or C<for> for brevity. (Or because the Bourne shell is more familiar to you than I<csh>, so writing C<for> comes more naturally.) If VAR is omitted, C<$_> 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<foreach> 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<foreach> 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<foreach> will get very confused if you add or remove elements within the loop body, for example with @@ -483,7 +488,7 @@ 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<HTTP_USER_AGENT> envariable. +C<HTTP_USER_AGENT> environment variable. #!/usr/bin/perl # pick out jargon file page based on browser @@ -598,6 +603,11 @@ C</^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/> 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: |
