＞第 16 章 运算符详解＞位运算＞掩码＞习题

int countbit(unsigned int x) {
	int i;
	int cnt = 0;
	int mask = 0x00000001;

	for (i = 0; i < sizeof(unsigned int) << 3; i++) {
		if (x & mask)
			cnt++;
		x >>= 1;
	}

	return cnt;
}

unsigned int multiply(unsigned int x, unsigned int y) {
	int i;
	int sum = 0;
	int mask = 1;

	for (i = 0; i < 32; i++) {
		if (y & mask) 
			sum += x << i;

		y >>= 1;
	}

	return sum;
}

unsigned int rotate_right(unsigned int x, int n) {
	int i;
	int mask = 0x80000000;

	for (i = 0; i < n; i++) {
		if (x & 1) {
			x >>= 1;
			x |= mask;
		} else
			x >>= 1;
	}

	return x;
}

1. countbit
#include <stdio.h>
int countbit(unsigned int x){
	int count = 0;
	while (x != 0) {
		count += x & 1;
		x >>= 1;
	}
	return count;
}

int main(void){
	int n;
	scanf("%d", &n);
	printf("%d", countbit(n));
}

2. multiply
#include <stdio.h>
unsigned int multiply(unsigned int x, unsigned int y){
	unsigned int res = 0;
	while (y > 0) {
		int last = y & 1;
		if (last) {
			res += x;
		}
		x <<= 1;
		y >>= 1;
	}
	return res;
}
int main(void){
	unsigned int x, y;
	scanf("%d", &x);
	scanf("%d", &y);
	printf("%d", multiply(x, y));
}

3. rotate_right
#include <stdio.h>
unsigned int rotate_right(unsigned int x, int rol_num){
	unsigned int y = x;
	printf("x: %x\n", x);
	y <<= 32 - rol_num;
	x >>= rol_num;
	unsigned int res = x | y;
	printf("res: %x\n", res);
}

int main(void){
	unsigned int n;
	scanf("%d", &n);
	printf("n: %x\n", n);
	rotate_right(n, 16);
}

int countbit(unsigned int x)
{
    int cnt = 0;
    while(x != 0)
    {
        x &= (x - 1);
        cnt++;
    }
    return cnt;
}

自己做的第三题答案：
#include<stdio.h>

unsigned int roat_right(unsigned int x, int n)
{
	unsigned int mask = 0x00000001, i, y, N = 4 * n;
	for(i = 1; i <= N; i++){
		y = x & mask;
		x >>= 1;
		if(y == 1)
			x += y << 31;		
	}
	return x;
}

int main(void)
{
	unsigned int x = 0xdeadbeef;
	printf("%x\n", roat_right(x, 4));
	return 0;
}

如果您有建设性意见，哪怕只是纠正一个错别字，也请不吝赐教，您留下的姓名和email将会出现在本书前言的致谢中。再次感谢您的宝贵意见！