switch 与 if/else

效率问题：见e文。
  For just a few items, the difference is small. If you have many items you should definitely use a switch.
If a switch contains more than five items, it's implemented using a lookup table or a hash list. This means that all items get the same access time, compared to a list of if:s where the last item takes much more time to reach as it has to evaluate every previous condition first.
 简单的说：switch在分支多的时候效率高点，其他差不多。

switch的语法说明一下：根据输入的值，决定跳到哪个case语句或default，然后再一直往下执行(除非碰到break)。  

#include <stdio.h>
int main()
{
	int num[6] = {3, 2, 1, 0, 2, 1};
	int v0 = 0, v1 = 0, v2 = 0;
	int i;
	for (i = 0; i < 1; i++)
	{
		switch (num[i])
		{
		default://2
			v2++;
		case 2://4
			v0++;
		case 0://6
			v1++;
		case 1://
			v2++;
		}
	}
	printf("v0 = %d, v1 = %d, v2 = %d\r\n", v0, v1, v2);
	getchar();
	return 0;
}

如果我这样写，把default放到最前面，输入3，除了default外，下面的所有case都会执行的。(没break哟)