C Program to Check whether the Given Number is a Prime

A prime number is a natural number that has only one and itself as factors.

Example:

2, 3, 5, 7, 11 and 13 are few prime numbers.

Above numbers can only be divided evenly by 1 or itself, so these numbers are prime numbers.

Prime Number Check Program in C

Program:

#include <stdio.h> 

main() {
  int n, i, c = 0;
  printf("Enter any number n:");
  scanf("%d", &n);

  //logic
  for (i = 1; i <= n; i++) {
      if (n % i == 0) {
         c++;
      }
  }

  if (c == 2) {
        printf("n is a Prime number");
  }
  else {
         printf("n is not a Prime number");
  }
  return 0;    
}

Program Output:

Enter any number n: 7

n is Prime

Explanation:

consider a number n=5
for(i=0;i<=n;i++) /* for loop is executed until the n value equals i */
i.e. for(i=0;i<=5;i++) /* here the for loop is executed until i is equal to n */

1st iteration: i = 1; i <= 5; i++
here i is incremented i.e. i value for next iteration is 2
now if(n%i==0) then c is incremented
i.e.if(5%1==0)then c is incremented, here 5%1=0 thus c is incremented.
now c=1;

2nd iteration: i = 2; i <= 5; i++
here i is incremented i.e. i value for next iteration is 3
now if(n%i==0) then c is incremented
i.e.if(5%2==0) then c is incremented, but 5%2!=0 and so c is not incremented, c remains 1
c=1;

3rd iteration: i = 3; i <= 5; i++
here i is incremented i.e. i value for next iteration is 4
now if(n%i==0) then c is incremented
i.e.if(5%3==0) then c ic incremented, but 5%3!=0 and so c is not incremented, c remains 1
c=1;

4th iteration: i = 4; i <= 5; i++
here i is incremented i.e. i value for next iteration is 5
now if(n%i==0) then c is incremented
i.e. if(5%4==0) then c is incremented, but 5%4!=0 and so c is not incremented, c remains 1
c=1;

5th iteration: i = 5; i <= 5; i++
here i is incremented i.e. i value for next iteration is 6
now if(n%i==0) then c is incremented
i.e. if(5%5==0) then c is incremented, 5%5=0 and so c is incremented.
i.e. c=2

6th iteration: i = 6; i <= 5; i++
here i value is 6 and 6<=5 is false thus the condition fails and control leaves the for loop.
now if(c==2) then n is a prime number
we have c=2 from the 5th iteration and thus n=5 is a Prime number.