How do you write a C program to check whether a number is prime or not and to find the nth prime number?

Answer 1

int number;

int i=2;

while (i<number)

{

if(number%i==0)

{

printf("Not a prime no.");

break;

}

else

printf("number entered is prime");

getch();

}

Answer 2

#include
#include

bool is_prime(unsigned num) {

unsigned max, factor;
if (num<2) return false;
if (!(num%2)) return num==2;
max = (unsigned) sqrt((double)num) + 1.0;
for (factor=3; factorif (!(num%factor)) return false;
return true;
}

unsigned next_prime(unsigned num) {

do { ++num; } while (!isprime(num));

return num;

}

unsigned nth_prime (unsigned n) {

// Returns the nth prime. Returns 0 when n is zero.

// Note: does not guard against underflow!

unsigned prime;

if (!n) return 0;

prime=2;

while (--n) {prime = next_prime(prime);}

return prime;

}

int main() {
printf ("The 100th prime is %u\n", nth_prime(100));
return 0;
}

Answer 3

234