I'm getting numbers like 9 and 15 also prime by using this code . pls find the error in the code and if possible Pls edit my code to find the error Here is the code--

`#include<iostream>
    using namespace std;
    int main()
    {
        int n;
        cout<<"Enter the number : ";
        cin>>n;
        int flag=0;
        //to check prime check from 2 to n
        for(int i=2;i<=n;i  )
        {
            if(n%2==0)
            {
                cout<<n<<" is a non-prime number";
                flag  ;
                break;
            }
            else
            {
                if(flag==0)
                {
                    cout<<n<<" is a prime number";
                    break;
                }
            }
        }
        return 0;
    }

CodePudding user response：

Your check logic is flawed as you are checking against %2, you should check against %i. I've also added a little optimization as you only need to check up until n/2.

To check if a number is prime:

    #include<iostream>
    using namespace std;
    int main()
    {
        int n;
        cout<<"Enter the number : ";
        cin>>n;
        bool prime=true;
        //to check prime check from 2 to n
        for(int i=2;i<=n/2;i  )
        {
            if(n%i==0)
            {
                prime = false;
                break;
            }
        }
        if (prime)
            cout<<n<<" is a prime number";
        else
            cout<<n<<" is a non-prime number";
        return 0;
    }

CodePudding user response：

Your code is not checking if the number is prime or not instead it just check if all the number from 2 to n are even or odd and as the value of i become an odd number (which is 3, just after its first iteration) the code executes these lines

cout<<n<<" is a prime number";
flag  ;

Also what is the point of

flag = 0 

if you are using break for terminating the loop.

You must be trying to write something like this

#include<iostream>
using namespace std;
int main()
{
    int n;
    cout<<"Enter the number : ";
    cin>>n;
    int flag=0;
    for(int i=2;i<=n;i  )
    {
        if(n%i==0)
        {
            cout<<n<<" is a non-prime number";
            flag  ;
            break;
        }
    }
    if(flag==0)
    {
        cout<<n<<" is a prime number";
        break;
    }
    return 0;
}

Also to make this code better write

for(int i=2;i<=sqrt(n);i  )

instead