nan when executing printf

Question

When I execute program

#include <stdio.h>
#include <math.h>
#include <unistd.h>

double exponential(double u);

double exponential(double u)
{
    double a = (double)rand();
    return (-u * log(1.0 - a));
}

int main(void)
{
    printf("%e\n",exponential(2.3));
    return 0;
}

I obtain:

nan

Why?

Answer 1

Because rand() returns an integer (between 0 and RAND_MAX ), most often this integer is larger than 1 and your log expression will be negative. log returns NaN (not a number) for negative inputs.