C++ sort on vector using function object

Question

I'm trying to sort a vector v1 using another vector v2. I can't wrap my head around this error:

terminate called after throwing an instance of 'std::out_of_range'
what(): vector::_M_range_check
Abort trap

while running this code:

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

class Comp
{
    public:
        Comp(vector<double>& inVec): _V(inVec) {}
        bool operator()(int i, int j) {return (_V.at(i)<_V.at(j));}
    private:
        vector<double> _V;
};

int main(int argc, char** argv)
{
    double x1[] = {90.0, 100.0, 80.0};
    double x2[] = {9.0, 3.0, 1.0};
    vector<double> v1(x1,x1+3);
    vector<double> v2(x2,x2+3);

    sort(v1.begin(), v1.end(), Comp(v2));  // sort v1 according to v2

    for(unsigned int i=0; i<v1.size(); i++)
    {
        cout << v1.at(i) << " " << v2.at(i) << endl;
    }

    return 0;
}

v1 and v2 are of the same size. Why the out_of_range error?

Thanks in advance for any pointers.

Answer 1

I believe that your problem is in this line:

bool operator()(int i, int j) {return (_V.at(i)<_V.at(j));}

The problem is that when the std::sort algorithm uses a custom callback, it passes in the actual values stored in the vector at particular locations, not the indices of those locations within the vector. As a result, when you call

sort(v1.begin(), v1.end(), Comp(v2));  // sort v1 according to v2

The Comp comparator you've written will be getting passed as parameters the values stored in the v1 vector and will then try indexing at those positions into the v2 vector. Since the values in v1 are larger than the size of v2, the call to _V.at(i) will cause an out_of_range exception to be thrown.

If you want to sort the two ranges with respect to one another, you'll need to adopt a different approach. I'm not aware of a straightforward way of doing this, but I'll let you know if I think of one.