nth Fibonacci term prime check

The Fibonacci series is 0, 1, 1, 2, 3, 5, 8, 13, 21, ... with each term the sum of the previous two. The function fib(n) returns the nth term (term 1 = 0, term 2 = 1). The function isprime() returns 1 when a number is prime. main() reads n, calls both functions and displays the result.

#include <stdio.h>
long fib(int n)
{
    long a = 0, b = 1, t;
    int i;
    if (n == 1) return 0;
    if (n == 2) return 1;
    for (i = 3; i <= n; i++)
    {
        t = a + b;
        a = b;
        b = t;
    }
    return b;
}
int isprime(long x)
{
    long i;
    if (x < 2) return 0;
    for (i = 2; i * i <= x; i++)
        if (x % i == 0)
            return 0;
    return 1;
}
int main(void)
{
    int n;
    long f;
    printf("Enter n: ");
    scanf("%d", &n);
    f = fib(n);
    printf("Term %d of the Fibonacci series = %ld\n", n, f);
    if (isprime(f))
        printf("%ld is a prime number\n", f);
    else
        printf("%ld is not a prime number\n", f);
    return 0;
}

Output for n = 7:

Enter n: Term 7 of the Fibonacci series = 8
8 is not a prime number

Output for n = 8: Term 8 of the Fibonacci series = 13 and 13 is a prime number.

Explanation

• fib() keeps the last two terms in a and b and moves forward with t = a + b.
• isprime() rejects numbers below 2 and tests divisors from 2 up to √x.