Question

I need to create a program that takes ‘N’ as Input and show the first N prime numbers and finally sum them…
Here’s my code, but it doesn’t work. (It loops at: “if ((num == 2) || (num == 3))”)

Can you help me?
Thanks in advance.

I can’t use functions and arrays.

Code:

#include "stdafx.h"
#include <iostream>

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    int n, num = 1, rest, cont = 0, sum = 0;
    bool exit = false;
    int e = 1;

    cout<<"How many prime number do you want to show? ";
    cin>>n;

    while (cont <= n)
    {
        num++;
        if ((num == 2) || (num == 3))
        {
            cout<<num<<endl;
            sum += num;
            cont++;
        }
        else if (num%2 == 0)
        {
        }
        else
        {
            bool exit = false;
            int div = 3;
            while (!exit)
            {
                rest = num%div;

                if (num == div)
                {
                    exit = true;
                    cout<<num<<endl;
                    sum += num;
                    cont++;
                }

                if (rest == 0)
                    exit = true;                    
                div++;
            }
        }       
        num++;
    }    
    cout<<"Sum: "<<sum<<endl;

    return 0;
}

Answer 1

To begin with, you are using a variable name resto instead of rest, which is the name with which you declared it (if (resto == 0)).

Second, you increase num twice: at the beginning and at the end of your loop. Remove the last num++ and it should work better.

Third, initialize cont to 1 and not to 0, or it will compute sum of the first N+1 primes instead of the first N primes.

Here is the fixed version of your code:

#include <iostream>

using namespace std;

int main()
{
    int n, num = 1, rest, cont = 1, sum = 0;
    bool exit = false;
    int e = 1;

    cout<<"How many prime number do you want to show? ";
    cin>>n;

    while (cont <= n)
    {
        num++;
        if ((num == 2) || (num == 3))
        {
            cout<<num<<endl;
            sum += num;
            cont++;
        }
        else if (num%2 == 0)
        {
        }
        else
        {
            bool exit = false;
            int div = 3;
            while (!exit)
            {
                rest = num%div;

                if (num == div)
                {
                    exit = true;
                    cout<<num<<endl;
                    sum += num;
                    cont++;
                }

                if (rest == 0)
                    exit = true;

                div++;
            }
        }
    }
    cout<<"Sum: "<<sum<<endl;

    return 0;
}