avg

#include <stdio.h>

double avg(double * a, int N, int (*f)(double *)){
    int i, count = 0;
    double sum = 0;
    for(i = 0; i < N; i++)
        if ((*f)(&a[i])){
            sum += a[i];
            count++;
        }

    if (count == 0)
        return 0;
    else
        return sum/count;
}

int f(double * d){
    if (d >= 0)
        return 0;
    else
        return 1;
}

int main(){
    double a[] = {1,2,3,4,5,6,7};
    int N = sizeof(a)/sizeof(a[0]);

    double r = avg(a, N, &f);
    printf("avg = %f", r);
}