Question

I wrote the following code to sum the series (-1)^i*(i/(i+1)). But when I run it I get -1 for any value of n.

Can some one please point out what I am doing wrong? Thank you in advance!

#include <iostream>
using namespace std;

int main()
{
    int sum = 0;
    int i = 1.0;
    int n = 5.0;

    for(i=1;i<=n;i++)
        sum = (-1)^i*(i/(i+1));

    cout << "Sum" <<" = "<< sum << endl;
    return 0;
}

Answer 1

Problem #1: The C++ ^ operator isn’t the math power operator. It’s a bitwise XOR.

You should use pow() instead.

Problem #2:
You are storing floating-point types into an integer type. So the following will result in integer division (truncated division):

i/(i+1)

Problem #3:
You are not actually summing anything up:

sum = ...

should be:

sum += ...

---

A corrected version of the code is as follows:

double sum = 0;
int i = 1;
int n = 5;

for(i = 1; i <= n; i++)
    sum += pow(-1.,(double)i) * ((double)i / (i + 1));

---

Although you really don’t need to use pow in this case. A simple test for odd/even will do.

double sum = 0;
int i = 1;
int n = 5;

for(i = 1; i <= n; i++){
    double val = (double)i / (i + 1);
    if (i % 2 != 0){
        val *= -1.;
    }
    sum += val;
}