c语言初学者，急求高手解答疑惑！

#include <stdio.h>
int main (void)
{
float value1,value2;
char operator;
printf("type in your expression.\n");
scanf("%f %c %f",&value1,&operator,&value2);
if(operator=='+')
printf("%.2f\n",value1+value2);
else if (operator=='-')
printf("%.2f\n",value1-value2);
else if (operator=='*')
printf("%.2f\n",value1*value2);
else if (operator=='/')
if(value2=0)
printf("division by zero.\n");
else
printf("%.2f\n",value1/value2);
else
printf("unknow operator.\n");
return 0;
}
这个程序是对简单的表达式求值，是照着参考书敲得，为啥编译的时候就有一个错误，检查了多遍就是没检查出来哪里出错啦，求高手帮忙呀，谢谢！