刚学c语言，求解，这个计算器为什么没有办法算出结果呀

Question

问题遇到的现象和发生背景

问题相关代码，请勿粘贴截图

#include<stdio.h>
int main()
{
double a=0, b=0;
int c;
char i;
printf("Input expression:");
scanf("%lf%c%lf",&a,&i,&b);
switch (i)
{
case '+':c=a+b; printf("%lf+%lf=%lf",a,b,c); break;
case '-':c=a-b; printf("%lf-%lf=%lf",a,b,c); break;
case '':c+ab; printf("%lf*%lf=%lf",a,b,c); break;
case '/':if (b==0)
{printf("Can not divid 0!\nExpression error!");
}
else c=a/b; printf("%lf/%lf=%lf",a,b,c); break;
default:printf("Expression error!"); break;
}
return 0;
}

运行结果及报错内容

我的解答思路和尝试过的方法

我想要达到的结果

Answer 1

int c;改成double c;
case '':c+ab; printf("%lf*%lf=%lf",a,b,c); break;
这句也不对，改成这样：
case '*':c = a * b; printf("%lf*%lf=%lf", a, b, c); break;
完整代码：

#include<stdio.h>
int main()
{
    double a = 0, b = 0;
    double c;
    char i;
    printf("Input expression:");
    scanf("%lf%c%lf", &a, &i, &b);
    switch (i)
    {
    case '+':c = a + b; printf("%lf+%lf=%lf", a, b, c); break;
    case '-':c = a - b; printf("%lf-%lf=%lf", a, b, c); break;
    case '*':c = a * b; printf("%lf*%lf=%lf", a, b, c); break;
    case '/':if (b == 0)
    {
        printf("Can not divid 0!\nExpression error!");
    }
             else c = a / b; printf("%lf/%lf=%lf", a, b, c); break;
    default:printf("Expression error!"); break;
    }
    return 0;
}