Question

I am writing a function in C++ which will take in 2 vectors of doubles called xvalues and yvalues. My aim is to create an interpolation with these inputs. However, it would be really convenient if the (x,y) pairs were sorted so that the x-values were in increasing order and the y-values still corresponded to the correct x-value.

Does anyone know how I can do this efficiently?

Answer 1

Or you use a vector of pairs, which you then can sort easily with the stl sort algorithm, or you write your own sort method. Therefore you’ve several options.
Within your own sorting algorithm you can then take care of not only sorting your x-vector but also the y-vector respectively.

Here as an example using bubble sort for your two vectors (vec1 and vec2).

bool bDone = false;
while (!done) {
    done = true;
    for(unsigned int i=0; i<=vec1.size()-1; ++i) {
        if ( vec1.at(i) > vec1.at(i+1) ) {
            double tmp   = vec1.at(i);
            vec1.at(i)   = vec1.at(i+1);
            vec1.at(i+1) = tmp;
            tmp          = vec2.at(i);
            vec2.at(i)   = vec2.at(i+1);
            vec2.at(i+1) = tmp;
            done = false;
        }
    }
}

But again, as others pointed out here, you should defenitely use std::vector< std::pair<double, double> > and the just sort it.