The problem still is that the OP said they sorted the array just to try to get the grep to be faster.

Yes, binary search is faster than a straight linear search.

No, it's not going to be as fast as eliminating 1000 comparisons altogether. 

for ( @array ) {
    if ( exists $hash{$_} ) {
        # do something
    } else {
        # do something else
    }
}

[download]
Here, you have one hash lookup for each array item. 
@array = sort @array;
foreach ( keys %hash ) {
    if ( is_in_array( $_ ) ) {
       # do something
    } else {
       # do something else
    }
}

[download]
Here, you're doing a sort (quicksort, if using Perl's sort(), which is O(n log(n)) and worst case O(n**2) ), then using binary search which is O(log2 n) for each key of the hash.

So your solution ends up being O( (n log(n)) + (m * log n) ) with a worst case of O(n**2 + m * log(n)). Just going through the loop and doing a hash lookup is predictably O(n).

In simplest terms, with an 800k element array you can do 800k comparisons the way GrandFather and I have been recommending. In order to do a quicksort then a binary search for each key of the hash, you're possibly talking about 800k * 800k + 1000 * log2(800k). log2(800,000) is roughly 20. So 640,000,020,000 comparisons in the worst case, even using fairly good sort and search algorithms.

The loop is simply inside out.

In reply to Re^3: performance issues with grepping large sorted arrays by mr_mischief
in thread performance issues with grepping large sorted arrays by HarshaHegde

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