Re: Reset while loop to beginning of hash when using 'each'

[davido]

According to the perldoc for each you can reset the each iterator function in one of the following ways:

• Iterate over the entire hash. Upon final iteration, zero or null is returned depending on context, and the next call to each will start over again. You might find this to be slightly inconvenient if the point is to break out of a loop before iterating over all elements.
• evaluate keys %hash.
• evaluate values %hash.

You might think that evaluating keys for the hash is an inefficient way of resetting the each iterator. It might be, if you were to evaluate keys in list context. Don't. Just evaluate keys in scalar context, which efficiently returns the number of keys, rather than the list of keys. After that, you'll find each has been reset as desired.

Here's a sample illustration:

use strict;
use warnings;
my %hash = ( Who   => 'Me',
             What  => 'Person',
             How   => 'Conception',
             When  => '35 years ago',
             Where => 'I dont want to know',
             Why   => 'Irrelevant' );
while ( my ( $key, $value ) = each %hash ) {
    print "$key => $value\n";
    scalar keys %hash;
}

If you run that you'll now find yourself in a neverending loop, since each gets reset each time through the while loop. In this illustration, the use of the pseudo-function scalar is unnecessary. As long as keys isn't being evaluated in list context, Perl is smart enough to not go to the work of creating a list of keys that just gets thrown away.

Hope this helps...

Dave

"If I had my life to do over again, I'd be a plumber." -- Albert Einstein

[seaver]

Fantastic, it hadnt occured to me that the iteration was in the 'each' function. Great answer too, so easy to read!

Cheers
Sam