Design an algorithm (and provide the c code) to accept a number from the user and check if it is a prime number or not?

Question

Write a C program to check if the given string is repeated substring or not.
ex
1: abcabcabc - yes (as abc is repeated.)
2: abcdabababababab - no

Answer 1

A number if prime if it is not divisible by other then 1 and self. So it means if given number is N and if we check from 2 to n-1 if number if not divisible by anyone then its prime. In practice if we do the same from 2 to n/2 then the above logic is suffice.

See the sample code -

#include <stdio.h>
#include <stdlib.h>

void main()
{
    int n, i, isprime;

    printf("Enter a number \n");
    scanf("%d", &n);

    if (n <= 1)
    {
        printf("%d is not a prime numbers \n", n);
        exit(1);
    }
    isprime = 1;
    for (i = 2; i <= n / 2; i++)
    {
        if ((n % i) == 0)
        {
            isprime = 0;
            break;
        }
    }

   if (isprime == 1)
        printf("%d is a prime number \n", n);
     else
        printf("%d is not a prime number \n", n);
}