# Prime number program in C

Prime number program in C language to check whether a number is prime or composite, to print prime numbers. A number is prime if it's divisible only by one and itself.

Two is the only even and the smallest prime number. First few prime numbers are 2, 3, 5, 7, 11, 13, 17, .... Prime numbers have many applications in computer science and mathematics. A number greater than one can be factorized into prime numbers, for example, 120 = 23*31*51 (8*3*5).

## Prime number in C language

#include <stdio.h>

int main()
{
int n, i = 3, count, c;

printf("Enter the number of prime numbers to print\n");
scanf("%d", &n);

if (n >= 1) {
printf("First %d prime numbers are:\n",n);
printf("2\n");
}

for (count = 2; count <= n;)
{
for (c = 2; c <= i - 1; c++)
{
if (i%c == 0)
break;
}
if (c == i)
{
printf("%d\n", i);
count++;
}
i++;
}

return 0;
}

Output of program:

## C program for prime number or not

#include<stdio.h>

int main()
{
int n, c;

printf("Enter a number to check if it's prime\n");
scanf("%d", &n);

for (c = 2; c <= n/2; c++)
{
if (n%c == 0)
{
printf("%d is a composite number.\n", n);
break;
}
}

if (c == n/2 + 1)
printf("%d is prime.\n", n);

return 0;
}

## C program for prime number using function

#include<stdio.h>

int check_prime(int);

int main()
{
int n, result;

printf("Enter an integer to check whether it's prime or not.\n");
scanf("%d",&n);

result = check_prime(n);

if (result == 1)
printf("%d is prime.\n", n);
else
printf("%d isn't prime.\n", n);

return 0;
}

int check_prime(int a)
{
int c;

for (c = 2; c <= a - 1; c++)
{
if (a%c == 0)
return 0;
}
if (c == a)
return 1;
}

There are many dynamic logics than this to check primality, one method given below.

`for (c = 2; c <= (int)sqrt(n); c++)`

Only checking from 2 to square root of the number is sufficient.