# include<stdio.h>

int main(void)
{   
    float a;
    float b;
    char c;
    float xz;
    float d;
    float e;
    float f;
    float g;
    float h;
    float i;
    printf("                               欢迎进入计算器系统\n");
a:
    printf("                                    1：四则计算\n");
    printf("                                    2：万能计算\n");
    printf("                                    3:   退出\n");
    printf("                                        请选择：");
    scanf("%d",&xz);
    if(xz==1)
    {
        printf("请输入要计算的式子:  ");
        scanf("%f%c%f",&a,&c,&b);
        switch(c)
        {
        case'+':printf("=%f\n",a+b);break;
        case'-':printf("=%f\n",a-b);break;
        case'*':printf("=%f\n",a*b);break;
        case'/':printf("=%f\n",a/b);break;
        default:printf("输入错误\n");
        }
        goto a;
    }
    else if(xz==2)
    {
        printf("请输入要计算的数字（最多两个）： ");
        scanf("%f%f",&a,&b);
        float d=a+b;
        float e=a-b;
        float f=b-a;
        float g=a*b;
        float h=a/b;
        float i=b/a;
        printf("%f%f%f%f%f%f",d,e,f,g,h,i);
        goto a;
    }
     else if(xz==3)
        return 0;
}

d=a+b;
         e=a-b;
         f=b-a;
         g=a*b;
         h=a/b;
         i=b/a;
你已经定义了浮点型，不要再定义了，删除float，最后在返回一个return 0；