老师给的题目，关于函数我在百度上搜了，可是是作用在0~9，而且是字符形式没法进行运算。
求帮助
这是原来的代码，要求在这上面加题目上的功能



#include <stdio.h>
float add(float x,float y)
{
    return x+y;
}
float minus(float x,float y)
{
    return x-y;
}
float multiply(float x,float y)
{
    return x*y;
}
float division(float x,float y)
{
    return x/y;
}

int main()
{
    //定义变量
    float a,b,result;
    int choose;
    do
    {
        //输入数据
   
        
        printf("1.  加法     2. 减法  \n");
        printf("3.  乘法     4. 除法  \n");
        printf("0.  退出程序\n") ;
        printf("请选择： \n");
        scanf("%d",&choose);
        if(choose!=0)
        {
            printf("请输入两个数：  ");
            scanf("%f %f",&a,&b);
        }
        
        switch(choose)
        {
            case 1:
                result=add(a,b) ;
                printf("result=%.2f\n",result);
                break;
            case 2:
                result=minus(a,b);
                printf("result=%.2f\n",result);
                break;
            case 3:
                result=multiply(a,b);
                printf("result=%.2f\n",result);
                break;
            case 4:
                {
                        if(b==0)
                    {
                        printf("结果错误\n");
                    }
                    else
                    {
                        result=division(a,b);
                        printf("result=%.2f\n",result);
                    }
                }
               
                break;
            case 0:
                printf("欢迎再次使用本软件，再见！\n");
                break;
            default:
                printf("输入错误\n");
        }
    }while(choose!=0);
    return 9;
}