Adding Decimal Numbers

Adding decimal numbers were my exam question.
Our teacher said the right code is this (Look below), but it doesn’t work what’s the problem

This is the question: 9/10-11/12+13/14-15/16+….49/50 Write a c++ program to calculate this question

Here is the code:

#include <iostream>
using namespace std;
int main()
{
    double x;
    int p = 1;
    double s = 0;
    for (x = 9.0; x < 49.0; x + 2.0)
    {
        s = s + x / (x + 1) * p;
    }
    p = -p;
    cout << "Total: " << s << endl;
}

>Solution :

Well, this may be what you want:

#include <iostream>

int main( )
{
    int sign { 1 };
    double sum { };

    for ( std::size_t x = 9; x <= 49; x += 2 )
    {
        std::clog << "sum == " << sum << '\n';
        // here you have to cast both x and x+1 to double
        sum += static_cast<double>( x ) / static_cast<double>( x + 1 ) * sign;
        sign = -sign;
    }

    std::cout << "\nTotal: " << sum << '\n';
}

Also, don’t use anything except integral types for the initialization variable of a for-loop. For example, use std::size_t or int.

The result:

sum == 0
sum == 0.9
sum == -0.0166667
sum == 0.911905
sum == -0.0255952
sum == 0.918849
sum == -0.0311508
sum == 0.923395
sum == -0.0349387
sum == 0.9266
sum == -0.0376859
sum == 0.928981
sum == -0.0397693
sum == 0.930819
sum == -0.0414032
sum == 0.932281
sum == -0.042719
sum == 0.933471
sum == -0.0438013
sum == 0.93446
sum == -0.0447071

Total: 0.935293

As you can see, in this case after 20 iterations (because (49-9)/2 == 20), the total/sum will be 0.935293 so I hope this is what your teacher had in his/her mind.