16 replies to this topic

[jackson6612]

Hi :)

I was trying to write a C++ code to find if the entered number is prime or not. There is a big logic problem in the code which I have failed to fix. At least you can see I tried. Now please help me with it. It would be kind of you.

#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%i != 0)

        prime = true;



        else if (n%i = 0)

        prime = false;


        {

            if (prime = false)

            break;

        }

    }


    if (prime = true)

    cout << "the number is prime" << endl;

    

    else

    cout << "not prime" << endl;


    system("pause");

}

[Flying Dutchman]

= is assignment operator and == is comparison operator.

[jackson6612]

Hi Dutchman

I don't get it. I replaced many of the "=" with "==" but it didn't work. Could you please help me to fix it?

[Flying Dutchman]

In if statements you want to use comparison rather than assignment.

[dargueta]

Any comparisons, actually, e.g. in while loops, do-while and some for loops as well.

[jackson6612]

Thanks a lot, Dutchman, dargueta. It was very helpful. Do you find any syntax or logical errors now? Any suggestions?

#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%i != 0)

        prime = true;



        else if (n%i == 0)

        prime = false;


        {

            if (prime == false)

            break;

        }

    }


    if (prime == true)

    cout << "the number is prime" << endl;


    else

    cout << "not prime" << endl;


    system("pause");

}

[Flying Dutchman]

One logical error I see is if user enter 1 it will say that it's a prime number. Number is not a prime unless proven so.

[jackson6612]

Flying Dutchman said:

One logical error I see is if user enter 1 it will say that it's a prime number. Number is not a prime unless proven so.

Thanks a lot, Dutchman. What about this new code? There is an obvious problem. Although I used the exit function call, it still runs the loop. How do I jump to the end of the program without running the code? Please help me. Thank you.:)

#include <iostream>

#include <cstdlib>


using namespace std;


int main()


{

    int n, i;

    bool prime = true;


    cout << "Enter the number: ";

    cin >> n;


    [B][COLOR="darkorange"]if (n == 1)

    cout << "1 is neither considered a prime nor non-prime" << endl;

    exit;[/COLOR][/B]


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


    {

        if (n%i != 0)

        prime = true;



        else if (n%i == 0)

        prime = false;


        {

            if (prime == false)

            break;

        }

    }


    if (prime == true)

    cout << "the number is prime" << endl;


    else

    cout << "not prime" << endl;


    system("pause");

}

Output:

Enter the number: 1

1 is neither considered a prime nor non-prime

[COLOR="red"]the number is prime[/COLOR]

Press any key to continue . . .

[dargueta]

if (n == 1)

    cout << "1 is neither considered a prime nor non-prime" << endl;

    exit;

Change to

if (n == 1)

[B][COLOR="RED"]{[/COLOR][/B]

    cout << "1 is neither considered a prime nor non-prime" << endl;

    exit;

[B][COLOR="RED"]}[/COLOR][/B]

Otherwise your program will always exit regardless of the input.

[jackson6612]

dargueta said:

if (n == 1)

    cout << "1 is neither considered a prime nor non-prime" << endl;

    exit;

Change to

if (n == 1)

[B][COLOR="RED"]{[/COLOR][/B]

    cout << "1 is neither considered a prime nor non-prime" << endl;

    exit;

[B][COLOR="RED"]}[/COLOR][/B]

Otherwise your program will always exit regardless of the input.

Thanks a lot, dargueta. :)

I followed your guideline and used the parentheses. Unfortunately the loop still runs. Perhaps I need to place the part:

if (n == 1)

    {

    cout << "1 is neither considered a prime nor non-prime" << endl;

    exit;

    }

at some place else in the main code. Please guide me. Thanks a lot for your help and time.

Best regards
Jackson

#include <iostream>

#include <cstdlib>


using namespace std;


int main()


{

    int n, i;

    bool prime = true;


    cout << "Enter the number: ";

    cin >> n;


    if (n == 1)

    [COLOR="red"]{[/COLOR]

    cout << "1 is neither considered a prime nor non-prime" << endl;

    exit;

   [COLOR="red"] }[/COLOR]


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


    {

        if (n%i != 0)

        prime = true;



        else if (n%i == 0)

        prime = false;


        {

            if (prime == false)

            break;

        }

    }


    if (prime == true)

    cout << "the number is prime" << endl;


    else

    cout << "not prime" << endl;


    system("pause");

}

[dargueta]

I didn't really change your code all that much, but this seems to work.

#include <iostream>

#include <cstdlib>


using namespace std;


int main()

{

    int n, i;

    bool prime = true;


    cout << "Enter the number: ";

    cin >> n;


    if (n == 1)

    {

        cout << "1 is neither considered a prime nor non-prime" << endl;

        return 0;

    }


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

    {

        prime = (n % i) != 0;

        if( !prime )

            break;

    }


    if( prime )

        cout << "the number is prime" << endl;

    else

        cout << "not prime" << endl;


    return 0;

}

[Flying Dutchman]

@jackson6612,

exit;

This is not a function call, you're missing a parenthesis.

exit(1);