SPUG: interesting while() behavior and hosting recs

[John W. Krahn]

Joel Grow wrote:
> Hey Perl gurus, I recently noticed this very strange (to me at least)
> while() behavior:
> 
>  my $count = 1;
>  while() {     # this evaluates to true(!)
>     print "hello world\n";
>     last if (++$count > 3);
>  }
> 
> Prints "hello world" 3 times.  How is () evaluated to true?!?
> 
> If I put "0", "undef", "", or a scalar set to any of those in the parens,
> it does what I expect--it skips the while loop.  I thought "Hmm, maybe
> this is a strange empty list issue?", but no, the code below still skips
> the while loop:
> 
>  my $count = 1;
> 
>  my @empty = ();
> 
>  while(@empty) {  # skips this while loop, as I'd expect
>     print "hello world\n";
>     last if (++$count > 3);
>  }
> 
> In desperation, I thought maybe while() was evaluating $_ by default,
> since you can of course do:
> 
>  while (<STDIN>) {
> 
> but printing out $_ just before the while() loop showed it is undefined.
> 
> What's going on?


That is working for the same reason that

for ( ;; ) { ... }

works in Perl as well as in C where Perl inherited it from.



John
-- 
use Perl;
program
fulfillment