Zero inner product when using std::inner_product

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

This is because you provided zero as an integer constant. The resultant operations are all in integers, so the final value (0.75) is truncated to an int as well.

Change zero to 0.0 to make it work:

cout << inner_product(coord.begin(), coord.end(),coord.begin(), 0.0) << endl;

This produces 0.75 on ideone.