要求是这样的：
create a text-based, menu-driven program that allows the user to choose whether to calculate the circumference of a circle, the area of a circle or the volume of a sphere. The program should then input a radius from the user, perform the appropriate calculation and display the result. Use an array of function pointers in which each pointer represents a function that returns void and receives a double parameter. The corresponding functions should each display messages indicating which calculation was performed, the value of the radius and the result of the calculation.

以下是我写的程序：
#include<stdio.h>
#define P 3.14

void main()
{
    int choice, r;
    float C,A,V;
    printf("Press 1, calculate the circumference of a circle.\n");
    printf("Press 2, calculate the area of a circle.\n");
    printf("Press 3, calculate the volume of a sphere.\n");
    printf("Enter your choice: ");
    scanf ("%d", &choice);
    switch(choice)
    {
    case 1:
        printf("Please enter the radius: ");
        scanf ("%d",&r);
        C = 2*P*r;
        printf("\n The circumference of circle is %.2f",C);
        break;
    case 2:
        printf("Please enter the radius: ");
        scanf ("%d",&r);
        A = P*r*r;
        printf("The area of a circle is %.2f",A);
        break;
    case 3:
        printf("Please enter the radius: ");
        scanf ("%d",&r);
        V = 4/3*P*r*r*r;
        printf("The volume of a sphere is %.2f",V);
        break;
    }
}
我知道不能完全符合要求，但是又没什么思路，哪位大侠给指条明路，多谢啊！！