output problem with if number is prime or not

[heights]

Hi

I was trying to write a code to know if the entered number is prime or not. It works fine other that it gives several outputs saying numbers is prime or not prime. How do I fix it so that it shows only once if the number is prime or not? Please help me. Thanks

#include <iostream>
#include <cstdlib>

using namespace std;

int main()

{
    int n, i;
    bool prime, notprime;

    cout << "Enter the number: ";
    cin >> n;

    for (i=2; i<=(n/2);i++)

    {
        if (n&#37;i != 0)
        cout << "prime" << endl;

        else
        cout << "notprime" << endl;
    }

    system("pause");
}

Output:

Enter the number: 13
prime
prime
prime
prime
prime
Press any key to continue . . .

[Eaglei022]

Try:

#include <iostream>
#include <cstdlib>

using namespace std;

int main()

{
int n, i;
bool prime=true;

cout << "Enter the number: ";
cin >> n;

for (i=2; i<=(n/2);i++)

{
if !(n&#37;i != 0)
prime = false;

}

if (prime)
cout << "prime" << endl;


else
cout << "notprime" << endl;

system("pause");
}

[D_Drmmr]

I was trying to write a code to know if the entered number is prime or not. It works fine other that it gives several outputs saying numbers is prime or not prime. How do I fix it so that it shows only once if the number is prime or not? Please help me. Thanks

The logic in your program is flawed. If a number is dividable by any number other than 1 or itself, it is not a prime. If it is only dividable by 1 and itself, it is a prime.

[D_Drmmr]

Try:

Merely providing the solution doesn't help people to learn from their mistakes.
And you should use code tags when posting code.

[henryswanson]

Merely providing the solution doesn't help people to learn from their mistakes.
And you should use code tags when posting code.

Yeah, just giving a solution doesn't help. If that solution is explained, that'd be a lot better. On the other hand, you just have to hope they'll read it. :P So, here goes.
The for loop is what checks the divisibility. It starts at 2, since 1 will divide anything. The expression in the if block should be "n % i == 0", as that is more readable. If the remainder is 0, n cannot be prime, so it sets the 'prime' flag to false. And it would be better to have it break out of the for loop after setting prime to false, since no more tests are needed. See if you can modify this to just check 2 and odd numbers.

[MrViggy]

And, you can stop your loop sooner then you are currently...

Viggy

[heights]

Thanks a lot, everyone. I will ask some follow-on questions tomorrow.

Best regards
H