Algorithm: bit manipulation

1. 一个数的从右起第p1位和第p2位swap n位

 1 unsigned int swapBits(unsigned int x, unsigned int p1, unsigned int p2, unsigned int n) {
 2     unsigned int set1 = (x >> p1) & ((1 << n) - 1);
 3     unsigned int set2 = (x >> p2) & ((1 << n) - 1);
 4     unsigned int xored = set1 ^ set2; //same is 0, different is 1
 5     xored = (xored << p1) | (xored << p2);
 6     return x ^ xored;  //anybit ^ 0 = anybit, anybit ^ 1 = !anybit, so same no need to change, different need to be changed
 7 }
 8 
 9 int main()
10 {
11     cout << swapBits(28, 0, 3, 2) << endl;
12     return 0;
13 }

2. Add two integer

1 int Add(int x, int y) {
2     while (y) {
3         int carry = x & y;
4         x = x ^ y;
5         y = carry << 1;
6        // cout << carry << " " << x << " " << y << endl;
7     }
8     return x;
9 }

3. min函数

1 int minnum(int x, int y) {
2     return y + ((x - y) & ((x - y) >> (sizeof(int) * 8 - 1)));
3 }

also can use division

4. count bit set in an integer

1 int countsetbits(int x) {
2     int ans = 0;
3     while(x) {
4         x &= (x-1);
5         ans++;
6     }
7     return ans;
8 }

posted @ 2014-02-25 10:13 ying_vincent 阅读(198) 评论(0) 收藏举报

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