Question

The C++ program below should return a stricly positive value. However, it returns 0.

What happens ? I suspect an int-double conversion, but I can’t figure out why and how.

#include <iostream>
#include <vector>
#include <numeric>
using namespace std;
int main()
{

    vector<double> coordinates;
    coordinates.push_back(0.5);
    coordinates.push_back(0.5);
    coordinates.push_back(0.5);

     cout<<inner_product(coordinates.begin(), coordinates.end(), coordinates.begin(), 0)<<endl;

    return 0;
}

Answer 1

Because you’ve supplied an initial value of 0, an int. Your code is internally equivalent to:

int result = 0;

result = result + 0.5 * 0.5; // first iteration
result = result + 0.5 * 0.5; // second iteration
result = result + 0.5 * 0.5; // third iteration

return result;

While result + 0.5 * 0.5 produces the correct value (result is promoted to double in this expression), when that value is assigned back into result, it’s truncated (that expression is cast to int). You never get above 1, so you see 0.

Give it an initial value of 0.0 instead.