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
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.
} 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.
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.
-Unlike a C<foreach> statement, a C<while> statement never implicitly
-localises any variables.
+desperate behavior triggers a warning if you use the C<use warnings>
+pragma or 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
# 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++) {
$i++;
}
-(There is one minor difference: The first form implies a lexical scope
-for variables declared with C<my> in the initialization expression.)
+There is one minor difference: if variables are declared with C<my>
+in the initialization section of the C<for>, the lexical scope of
+those variables is exactly the C<for> loop (the body of the loop
+and the control sections).
Besides the normal array index looping, C<for> can lend itself
to many other interesting applications. Here's one that avoids the
implicitly local to the loop and regains its former value upon exiting
the loop. If the variable was previously declared with C<my>, it uses
that variable instead of the global one, but it's still localized to
-the loop.
+the loop. This implicit localisation occurs I<only> in a C<foreach>
+loop.
The C<foreach> keyword is actually a synonym for the C<for> keyword, so
you can use C<foreach> for readability or C<for> for brevity. (Or because
the Bourne shell is more familiar to you than I<csh>, so writing C<for>
comes more naturally.) If VAR is omitted, C<$_> 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
for (@ary) { s/foo/bar/ }
- foreach my $elem (@elements) {
+ for my $elem (@elements) {
$elem *= 2;
}
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;
}
}
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: {
$abc = 1, last SWITCH if /^abc/;
"read-only";
};
-Or if you are certainly that all the C<&&> clauses are true, you can use
+Or if you are certain that all the C<&&> clauses are true, you can use
something like this, which "switches" on the value of the
-C<HTTP_USER_AGENT> envariable.
+C<HTTP_USER_AGENT> environment variable.
#!/usr/bin/perl
# pick out jargon file page based on browser
dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:
- goto ("FOO", "BAR", "GLARCH")[$i];
+ goto(("FOO", "BAR", "GLARCH")[$i]);
The C<goto>-&NAME form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine. This is used by
error or warning messages (especially for strings that are processed
with C<eval()>). The syntax for this mechanism is the same as for most
C preprocessors: it matches the regular expression
-C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
+C</^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/> 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:
projects / p5sagit/p5-mst-13.2.git / blobdiff