C Program to Perform Fermat Primality Test

This is a C Program to test whether a number is prime or not. The Fermat primality test is a probabilistic test to determine if a number is probable prime.

Here is source code of the C Program to Perform Fermat Primality Test. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

```
#include <stdio.h>
```
```
#include <stdlib.h>
```
```
 
```
```
#define ll long long
```
```
/*
```
```
 * modular exponentiation
```

 */ll modulo(ll base, ll exponent, ll mod) {

```
    ll x = 1;
```
```
    ll y = base;
```
```
    while (exponent > 0) {
```
```
        if (exponent % 2 == 1)
```
```
            x = (x * y) % mod;
```
```
        y = (y * y) % mod;
```
```
        exponent = exponent / 2;
```
```
    }
```
```
    return x % mod;
```
```
}
```
```
 
```
```
/*
```

 * Fermat's test for checking primality

```
 */
```
```
int Fermat(ll p, int iterations) {
```
```
    int i;
```
```
    if (p == 1) {
```
```
        return 0;
```
```
    }
```
```
    for (i = 0; i < iterations; i++) {
```
```
        ll a = rand() % (p - 1) + 1;
```
```
        if (modulo(a, p - 1, p) != 1) {
```
```
            return 0;
```
```
        }
```
```
    }
```
```
    return 1;
```
```
}
```
```
/*
```
```
 * Main
```
```
 */
```
```
int main() {
```
```
    int iteration = 50;
```
```
    ll num;
```

    printf("Enter integer to test primality: ");

```
    scanf("%lld", &num);
```
```
    if (Fermat(num, iteration) == 1)
```
```
        printf("%lld is prime ", num);
```
```
    else
```

        printf("%lld is not prime ", num);

```
    return 0;
```
```
}
```

Output:

$ gcc FermatPrimeTest.c
$ ./a.out
 
Enter integer to test primality: 45
45 is not prime 
 
Enter integer to test primality: 97
97 is prime

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