how to find the diameter of a circle with the circumference code example

Example 1: area and circumference of a circle

#include <stdio.h>
#define PI 3.142
int main(){
    float r, C, A;
    printf("What is the radius: \n");
    scanf("%f", &r);
    C = 2 * PI * r;
    A = PI * r * r;
    printf("\nCincumference is and %f Area is %f\n", C, A);
}

Example 2: circumference of circle

C = Pi * r * 2 (r stands for radius)

Or you could do this:
C = Pi * d (d stands for diameter)