How use bitmask operator

Question

I'm learning bit mask. And found and example but couldn't make it work.

I'm trying to calculate all sum combination from one array. The result should be

0 - 1 - 2 - 3 - 3 - 4 - 5 - 6

My problem is with (i & mask) should only result in {0,1} and isn't. Instead is producing.

0 - 1 - 4 - 5 - 12 - 13 - 16 - 17

        int[] elem = new int[] { 1, 2, 3 };

        double maxElem = Math.Pow(2, elem.Length);

        for (int i = 0; i < maxElem; first++)
        {
            int mask = 1, sum = 0;
            for (int run = 0; run < elem.Length; run++)
            {
                sum += elem[run] * (i & mask);
                mask <<= 1;
            }
            Debug.Write(sum + " - ");
        }

`i & mask` will produce either 0 or `mask`, not 0 or 1: Eg. `0110b & 0100b == 0100b`, and `1010b & 0100b == 0000b` — Blorgbeard, May 21 '15 at 01:52
In your case `mask` is 2^`run` so that means only one bit is set - not that it's 1 or 0 — BrokenGlass, May 21 '15 at 01:53
@Blorgbeard I want sum {0,1}elem[0] + {0,1}elem[1] + {0,1}elem[2] for all combination. The {0,1} changes I think can be solve by a bitmask sequence. — Juan Carlos Oropeza, May 21 '15 at 01:55

score 1 · Answer 1 · answered May 21 '15 at 01:58

(i & mask) should only result in {0,1} and isn't

(i & mask) should return a result in {0,1} only when mask is 1 - that is, on the initial iteration. However, as soon as mask gets shifted by mask <<= 1 operation, the result of the next operation will be in {0,2}. As the mask gets shifted, possible results will become {0,4}, {0,8}, {0,16} and so on, because the only bit set to 1 in the mask would be moving to the left.

The reason why << operator doubles the number is the same as the reason why writing a zero after a decimal number has the effect of multiplying the number by ten: appending a zero to a number of any base is the same as multiplying that number by the value of base.

Juan Carlos Oropeza · Accepted Answer · 2015-05-21T02:46:32.430

1

Ok, I solve it creating an IF.

int[] elem = new int[] { 1, 2, 3 };
double maxElem = Math.Pow(2, elem.Length);

for (int i = 0; i < maxElem; first++)
{
    for (int run = 0; run < elem.Length; run++)
    {
        int mask = 1, sum = 0;
        if ((i & mask) > 0) // ADD THIS LINE
        {
            sum += elem[run];                    
        }
        mask <<= 1;
    }
}

edited May 21 '15 at 02:46

answered May 21 '15 at 02:07

Juan Carlos Oropeza

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How use bitmask operator

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