Replace for loop with formula

Posted by hamax on Stack Overflow See other posts from Stack Overflow or by hamax
Published on 2010-04-09T18:26:56Z Indexed on 2010/04/09 18:53 UTC
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I have this loop that runs in O(end - start) and I would like to replace it with something O(1).
If "width" wouldn't be decreasing, it would be pretty simple.

for (int i = start; i <= end; i++, width--)
    if (i % 3 > 0) // 1 or 2, but not 0
        z += width;

start, end and width have positive values

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