Function to compute sum of all even integers from 1 to num (inclusive)

[ComradeCookie]

I'm having a bit of a hard time creating a function, using iteration, to find the sum of all even integers between 1 and the number the user inputs. The program guidelines require a function to solve this three ways; a formula, iteration, and recursion. This is what I have so far.

#include <iostream>
#include <iomanip>
#include <cstdlib>

using namespace std;


void formulaEvenSum(int num, int& evenSum)
{

    evenSum = num / 2 * (num / 2 + 1);

    return;
}

void loopEvenSum(int num, int& evenSum2)
{
   
}


int main()
{
    int num, evenSum, evenSum2;
  
       cout << "Program to compute sum of even integers from 1 to num.";
       cout << endl << endl;

       cout << "Enter a positive integer (or 0 to exit): ";
       cin >> num;

       formulaEvenSum(num, evenSum);
       loopEvenSum(num, evenSum2);
    
       cout << "Formula result = " << evenSum << endl;
       cout << "Iterative result = " << evenSum2 << endl;
       
       system("PAUSE");
       return 0;

}

Any and all help would be appreciated. :]

[ryancb06]

To do this by reference like you are attempting to do.

void loopEvenSum(int num, int& evenSum2)
   {
    evenSum2 = 0;
    for(int i = 1; int <= num; i++)
       {
       evenSum2 += i;
       }
   }

or what I prefer more:

int loopEvenSum(int num)
   {
    int sum = 0;
    for(int i = 1; int <= num; i++)
       {
       sum =+ i;
       } 
   return sum;
   }

I prefer the latter but both should work!

[ComradeCookie]

Thanks. :)

I need to have it pass via reference, so I went with the first one. I"m at work, and am using compilr.com, but the function isn't working?

Could you give a recursive example too?

[3qX21hU5]

never trust online compilers is my experiance usually they get it wrong.

[vlad from moscow]

It is a bad idea to declare a function such a way when the function has return type void and at the same time has one "output" parameter. It is better to declare it returning a value. So instead of

void formulaEvenSum(int num, int& evenSum)
{

    evenSum = num / 2 * (num / 2 + 1);

    return;
}

I would write

unsigned int formulaEvenSum( unsigned int num )
{

    unsigned int evenSum = num / 2 * (num / 2 + 1);

    return ( evenSum );
}

Take into account that your original function does not work correctly if an argument is negative. So it is better to declare it having a parameter of type unsigned int.

As for the second function then it is very simple

unsigned int loopEvenSum( unsigned int num )
{

    unsigned int evenSum = 0;

    for ( unsigned int i = 2; i <= num; i += 2 ) evenSum += i;

    return ( evenSum );
}