4 – Best C program to check prime number

#include <stdio.h>
int main()
{
    int num,i,flag=0;
    printf("Enter any number : ");
    scanf("%d", &num);

    if(num==1) 
    {
        printf("1 is not a prime or composite number");
    }
    else 
    {
        for (i=2; i<=num/2; i++)
        {
            if (num%i == 0) 
            {
                flag = 1;
                break;
            }
        }
        if(flag==0)
        {
            printf("%d is a prime number", num);
        }
        else
        {
            printf("%d is not a prime number", num);
        }
    }
    return 0;
}