Question

Given two vectors foo and bar, I want to output a vector of length foo.size() containing the index to the “closest” element of bar. I don’t like reinventing the wheel – are there any STL algorithms or otherwise to do this concisely?

#include <vector>
#include <cmath>
#include <float.h>

int main() {
    vector<double> foo;
    vector<double> bar;

    // example data setup
    double array_foo[] = {0.0, 1.0, 2.0, 3.0, 4.0,
                          5.0, 6.0, 7.0, 8.0, 9.0};
    double array_bar[] = {4.8, 1.5, 12.0};
    foo.assign(array_foo, array_foo + 10);
    bar.assign(array_bar, array_bar + 3);

    // output array
    vector<int> indices;
    indices.resize(foo.size());

    for(int i = 0; i < foo.size(); i++) {
        double dist = DBL_MAX;
        int idx = 0;
        // find index of closest element in foo
        for(int j = 0; j < bar.size(); j++) {
            if(abs(foo[i] - bar[j]) < dist) {
                dist = abs(foo[i] - bar[j]);
                idx = j;
            }
        }
        indices[i] = idx;
    }
    // expected result: indices = [1,1,1,1,0,0,0,0,0,2]
    return 0;
}

Answer 1

This exact algorithm doesn’t exist, but you could implement it in an idiomatic STL way by using std::min_element and a custom functor:

template <typename T>
T norm(const T& a, const T& b)
{
    return abs(b - a);
}

template <typename T>
struct closer_compare
{
    closer_compare(const T& to) : to(to) {}
    bool operator()(const T& a, const T& b) const
    {
        return norm(a, to) < norm(b, to);
    }
    const T& to;
};

template <typename It1, typename It2, typename OutIt>
void find_nearest_indices(It1 in1_begin, It1 in1_end, It2 in2_begin, It2 in2_end, OutIt out)
{
    typedef typename std::iterator_traits<It1>::value_type value;
    for (It1 it = in1_begin; it != in1_end; ++it)
    {
        It2 closest = std::min_element(in2_begin, in2_end, closer_compare<value>(*it));
        *out++ = std::distance(in2_begin, closest);
    }
}

Your algorithm would then be replaced with:

find_nearest_indices(foo.begin(), foo.end(), bar.begin(), bar.end(), indices.begin());

I tested with your input and got your expected results.