error C2440: “=”: 无法从“const char [2]”转换为“char”

// 112,5.10.6.cpp : 定义控制台应用程序的入口点。
//
#include "stdafx.h"
#include <stdio.h>
int _tmain(int argc, _TCHAR* argv[])
{
char c1, c2;
int diff;
float num;
c1 = "S";
c2 = "O";
diff = c1 - c2;
num = diff;

printf("%c%c%c: %d %3.2f\n", c1, c2, c1, diff, num);
return 0;
}

c1 = "S"; - > c1 = 'S';
c2 = "O"; -> c2 = 'O';
这里的c1和c2是char，而"S"和"O"是字符串。