Steve Bertrand <[email protected]> wrote:
> Roman Makurin wrote:
> > On Wed, Jun 24, 2009 at 03:25:57PM +0200, Jenda Krynicky wrote:
> >> From: Roman Makurin <[email protected]>
> >>> here is complite perl script which produces such results without
> >>> any warning:
> >>>
> >>> #!/usr/bin/perl
> >>>
> >>> use strict;
> >>> use warnings;
> >>>
> >>> use constant {
> >>> A => 0,
> >>> B => 1,
> >>> C => 2 };
> >>>
> >>> my @a = (A, B, C);
> >>> my @b = (1, 2, 3);
> >>>
> >>> while(my $i = shift @a) {
> >>> print $i, $/
> >>> }
> >> But of course this does not print anything. The shift(@a) returns the
> >> first element of @a which is zero, assigns that to $i and then checks
> >> whether it's true. And of course it's not. So it skips the body and
> >> leaves the loop. Keep in mind that the value of
> >>
> >> my $i = shift @a
> >>
> >> is NOT a true/false whether there was something shifted from the
> >> array. It's the value that was removed from the array and assigned to
> >> the $i. And if that value it false (undef, 0, 0.0, "0", "0.0", "" -
> >> if I remember rigth) then the whole expression evaluates to false in
> >> boolean context.
>
> If I understand correctly, what you are saying is that while() is
> evaluating the left side of the '=' as it's condition, culminating into:
>
> while($i)
>
> Which eventually equates into:
>
> while(0)
>
> ...on the very first pass.
Well, you could understand it like that in this case. The problem
comes as soon as the assignment is not a scalar, but rather a list
one. Which doesn't mean just this
my @foo = whatever();
but also
my ($x, $y) = whatever();
and even
my ($x) = whatever();
Because the scalar value of such an assignment is the number of
assigned values, not the first or last such value. See
print scalar(my $x = 'hello'),"\n";
print scalar(my ($x) = 'hello'),"\n";
print scalar(my ($x,$y) = 'hello'),"\n";
If you want to loop over a list and "consume" it by the loop you
should use this:
while (@array) {
my $x = shift(@array);
...
}
Jenda
===== [email protected] === http://Jenda.Krynicky.cz =====
When it comes to wine, women and song, wizards are allowed
to get drunk and croon as much as they like.
-- Terry Pratchett in Sourcery
--
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
http://learn.perl.org/
