C Program to find Prime Numbers

Given a positive integer N. The task is to write a C program to check if the number is prime or not.

Definition – A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself. (Wikipedia)

Program to check Prime Number

In this program user can check the entered number N is prime or not, where the value of n is input by the user

#include <stdio.h>
int main() {
  int n, i, flag = 0;
  printf("Enter a positive integer: ");
  scanf("%d", &n);

for (i = 2; i <= n / 2; ++i) {
// condition for non-prime
if (n % i == 0) {
flag = 1;
break;
}
}

if (n == 1) {
printf(“1 is neither prime nor composite.”);
}
else {
if (flag == 0)
printf(“%d is a prime number.”, n);
else
printf(“%d is not a prime number.”, n);
}

return 0;
}

Program to find first N Prime Numbers

Below is a program to find first n prime numbers using nested for loops, where the value of n is input by the user.

#include<stdio.h>
int main()
{
    int n,i = 3, count, c;
    printf("nEnter the number of prime numbers required :  ");
    scanf("%d", &n);
    if(n >= 1)
    {
        printf("nnFirst %d prime numbers are :  ", n);
        printf("2 ");
    }
    // iteration for n prime numbers
    // i is the number to be checked in each iteration starting from 3
    for(count = 2; count <= n; i++)  
    {
        // iteration to check c is prime or not
        for(c = 2; c < i; c++)
        {
            if(i%c == 0)
                break;
        }
        if(c == i)  // c is prime
        {
            printf("%d ", i);
            count++;    // increment the count of prime numbers
        }
    }
    return 0;
}

A K Singh Rajpoot is enthusiastic programmer & founder of XNetKing. As well He love blogging and shares programming and tech tips to improve your tech knowledge and experience.

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