C<"">; and when used as a reference that isn't being assigned
to, it is treated as an error. If you enable warnings, you'll
be notified of an uninitialized value whenever you treat C<undef>
-as a string or a number. Well, usually. Boolean ("don't-care")
-contexts and operators such as C<++>, C<-->, C<+=>, C<-=>, and
-C<.=> are always exempt from such warnings.
+as a string or a number. Well, usually. Boolean contexts, such as:
+
+ my $a;
+ if ($a) {}
+
+are exempt from warnings (because they care about truth rather than
+definedness). Operators such as C<++>, C<-->, C<+=>,
+C<-=>, and C<.=>, that operate on undefined left values such as:
+
+ my $a;
+ $a++;
+
+are also always exempt from such warnings.
A declaration can be put anywhere a statement can, but has no effect on
the execution of the primary sequence of statements--declarations all
} while $x++ <= $z;
}
+B<NOTE:> The behaviour of a C<my> statement modified with a statement
+modifier conditional or loop construct (e.g. C<my $x if ...>) is
+B<undefined>. The value of the C<my> variable may be C<undef>, any
+previously assigned value, or possibly anything else. Don't rely on
+it. Future versions of perl might do something different from the
+version of perl you try it out on. Here be dragons.
+
=head2 Compound statements
In Perl, a sequence of statements that defines a scope is called a block.
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>
pragma or the B<-w> flag.
-Unlike a C<foreach> statement, a C<while> statement never implicitly
-localises any variables.
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
# now process $line
}
-Note that if there were a C<continue> block on the above code, it would get
-executed even on discarded lines. This is often used to reset line counters
-or C<?pat?> one-time matches.
+Note that if there were a C<continue> block on the above code, it would
+get executed only on lines discarded by the regex (since redo skips the
+continue block). A continue block is often used to reset line counters
+or C<?pat?> one-time matches:
# inspired by :1,$g/fred/s//WILMA/
while (<>) {
they aren't loops. You can double the braces to make them such, though.
if (/pattern/) {{
- next if /fred/;
- next if /barney/;
- # so something here
+ last if /fred/;
+ next if /barney/; # same effect as "last", but doesn't document as well
+ # do something here
}}
+This is caused by the fact that a block by itself acts as a loop that
+executes once, see L<"Basic BLOCKs and Switch Statements">.
+
The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
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 loop. This implicit localisation occurs I<only> in a C<foreach>
+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. (Or because
"read-only";
};
-Or if you are certainly that all the C<&&> clauses are true, you can use
+Or if you are certain that all the C<&&> clauses are true, you can use
something like this, which "switches" on the value of the
C<HTTP_USER_AGENT> environment variable.
projects / p5sagit/p5-mst-13.2.git / blobdiff