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
# 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)>.
=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++) {
$i++;
}
-(There is one minor difference: The first form implies a lexical scope
-for variables declared with C<my> in the initialization expression.)
+There is one minor difference: if variables are declared with C<my>
+in the initialization section of the C<for>, the lexical scope of
+those variables is exactly the C<for> loop (the body of the loop
+and the control sections).
Besides the normal array index looping, C<for> can lend itself
to many other interesting applications. Here's one that avoids the
you can use C<foreach> for readability or C<for> for brevity. (Or because
the Bourne shell is more familiar to you than I<csh>, so writing C<for>
comes more naturally.) If VAR is omitted, C<$_> 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
}
There is no official C<switch> 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/;
"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.