学步园

以下几个语句都能交换a,b的值

1）

a^=b^=a^=b;

2）

temp = a;
a = b;
b = temp;

3）

a = a^b;
b = a^b;
a = a^b;

效率为2） > 3） > 1）

验证代码如下：（linux下GCC和win下codeblocks通过编译）

#include <stdio.h>
#include <sys/time.h>
#include <unistd.h>

void speed_1()
{
	long i = 100000000;
	while(i){
		int a = 10;
		int b = 20;
		a^=b^=a^=b; 
		i--;
	}
}

void speed_2()
{
	long i = 100000000;
	int temp = 0;
	while(i){
		int a = 10;
		int b = 20;
		temp = a;
		a = b;
		b =  temp;
		i--;
	}
}

void speed_3()
{
	long i = 100000000;
	while(i) {
		int a = 10;
		int b = 20;
		a = a^b;
		b = a^b;
		a = a^b;
		i--;
	}
}

int main(void)
{
	struct timeval start;
	struct timeval end;
	
	unsigned long diff_1;             //此处的1，2，3数字代表的代码与上面所列一致
	unsigned long diff_2;
	unsigned long diff_3;
	   
	gettimeofday(&start, NULL);       //gettimeofday()是C语言获得精确时间函数
	speed_1();
	gettimeofday(&end, NULL);
	diff_1 = 1000000 * (end.tv_sec - start.tv_sec) + end.tv_usec - start.tv_usec;

	gettimeofday(&start, NULL);
	speed_2();
	gettimeofday(&end, NULL);
	diff_2 = 1000000 * (end.tv_sec - start.tv_sec) + end.tv_usec - start.tv_usec;

	gettimeofday(&start, NULL);
	speed_3();
	gettimeofday(&end, NULL);
	diff_3 = 1000000 * (end.tv_sec - start.tv_sec) + end.tv_usec - start.tv_usec;


	printf("time_1 = %ld\n", diff_1);
	printf("time_2 = %ld\n", diff_2);
	printf("time_3 = %ld\n", diff_3);

	return 0;
}

记得以前有人深入的分析过，我也不会，就简单的控制变量看下表面现象，看谁的效率更高些。

等以后在尝试从代码角度进行分析吧。