大伙们，c语言程序设计帮看看问题出在哪！

设计这个程序的时候输入数字得到结果都是0.000，请问哥们问题出在哪了？请指导下，感谢了

#include
#include
int main()

{
float operation;
int function;
float num1;
float num2;
printf("请输入一个数：");
scanf("%f",&num1);
printf("请输入一个数：");
scanf("%f",&num2);
printf("请输入1-4的一个数:");
scanf("%d",&function);
int a = num1+num2;
int b = num1-num2;
int c = num1num2;
int d = num1/num2;
switch(function)
{
case 1: printf("num1+num2=%f",a);break;
case 2: printf("num1-num2=%f",b);break;
case 3: printf("num1num2=%f",c);break;
case 4: printf("num1/num2=%f",d);break;
}
return 0;
}

把变量a,b,c,d的类型修改为float类型，然后四个printf语句修改下即可，修改如下：

#include<stdio.h>
#include<string.h>
int main()

{
    float operation;
    int function;
    float num1;
    float num2;
    printf("请输入一个数：");
    scanf("%f",&num1);
    printf("请输入一个数：");
    scanf("%f",&num2);
    printf("请输入1-4的一个数:");
    scanf("%d",&function);
    float a = num1+num2;
    float b = num1-num2;
    float c = num1*num2;
    float d = num1/num2;
    switch(function)
    {
    case 1: printf("%f+%f=%f",num1,num2,a);break;
    case 2: printf("%f-%f=%f",num1,num2,b);break;
    case 3: printf("%f*%f=%f",num1,num2,c);break;
    case 4: printf("%f/%f=%f",num1,num2,d);break;
    }
    return 0;
}

还有一个+-×÷那几行，num1×num2，乘号没写，用不用加？