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
}
}
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
}
}
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. | [reply]
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