Write a function to calculate if a number is prime. Return 1 if it is prime and 0 if it is not a prime.

// to compile: gcc -Wall prime.c -lm -o prime

#include <math.h>    // for the square root function sqrt()	
#include <stdio.h>

int is_prime(int n);

int main() 
{
  printf("Write an integer: ");
  int var;
  scanf("%d", &var);
  if (is_prime(var)==1) {
    printf("A prime\n");
  } else {
    printf("Not a prime\n");
  }
  return 1;
}

int is_prime(int n)
{
  int x;
  int sq= sqrt(n)+1;
 
  // Checking the trivial cases first
  if ( n < 2 )
    return 0;
  if ( n == 2 || n == 3 )
    return 1;
 
  // Checking if n is divisible by 2 or odd numbers between 3 and the
  // square root of n.
  if ( n % 2 == 0 )
    return 0;
  for (x= 3; x <= sq; x += 2)
    {
      if ( n % x == 0 )
	return 0;
    }
 
  return 1; 
}

Another better solution that doesn't need to include math.h and faster than the one above.

#include<stdio.h>

int isPrime(const unsigned long long int);

int main(void) {
  unsigned long long n;
  scanf("%llu",&n);
  printf("%d\n",isPrime(n));
  while((n=getchar())!=10);
  getchar();
  return 0;
}

int isPrime(const unsigned long long int x) {
  if(x<4)
    return x>1;
  if(x%2==0||x%3==0)
    return 0;
  for(unsigned long int i=5;i*i<=x;i+=6)
    if(x%i==0||x%(i+2)==0)
      return 0;
  return 1;
}

References

Wikibooks contributors. "C Programming/Exercise solutions". Wikibooks. Wikibooks. Retrieved 13 May 2024.