Bit manipulation, permutate bits

Posted by triktae on Stack Overflow See other posts from Stack Overflow or by triktae
Published on 2010-05-19T12:23:21Z Indexed on 2010/05/19 14:10 UTC
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Hi.

I am trying to make a loop that loops through all different integers where exactly 10 of the last 40 bits are set high, the rest set low. The reason is that I have a map with 40 different values, and I want to sum all different ways ten of these values can be multiplied. (This is just out of curiosity, so it's really the "bitmanip"-loop that is of interest, not the sum as such.)

If I were to do this with e.g. 2 out of 4 bits, it would be easy to set all manually,

0011 = 7,
0101 = 5,
1001 = 9,
0110 = 6,
1010 = 10,
1100 = 12,

but with 10 out of 40 I can't seem to find a method to generate these effectively. I tried, starting with 1023 ( = 1111111111 in binary), finding a good way to manipulate this, but with no success. I've been trying to do this in C++, but it's really the general method(if any) that is of interest. I did some googling, but with little success, if anyone has a good link, that would of course be appreciated too. :)

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