Apologies for the late reply, I have been away for a while.

To answer your question: keys iterates through the keys of a hash while values iterates through the associated values in the same order. So if you find you are writing code like:

foreach my $key ( keys %hash ) {
    my $val = $hash{$key};
    # do something with $val
    ...
}

[download]

and not use $key otherwise you can write directly

foreach my $val ( values %hash ) {
    # do something with $val
    ...
}

[download]

If $val is a reference to a hash as in %site_length_catch, then %$val is a hash and the game can start again for the inner loop. The final line $count = keys %$count; takes the hash reference $count, counts its keys and overwrites the hash reference with the number of keys, in this case the number of patients.

Hope this is helpful even if you have worked it out yourself already...

In reply to Re^5: Memory issue with large cancer gene data structure by hdb
in thread Memory issue with large cancer gene data structure by ZWcarp

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