Reorder vector using a vector of indices

Question

I\'d like to reorder the items in a vector, using another vector to specify the order:

char   A[]     = { \'a\', \'b\', \'c\' };
size_t ORDER[] = { 1, 0, 2 }         


        

Answer 1

It's an interesting intellectual exercise to do the reorder with O(1) space requirement but in 99.9% of the cases the simpler answer will perform to your needs:

void permute(vector& values, const vector& indices)  
{   
    vector out;
    out.reserve(indices.size());
    for(size_t index: indices)
    {
        assert(0 <= index && index < values.size());
        out.push_back(values[index]);
    }
    values = std::move(out);
}

Beyond memory requirements, the only way I can think of this being slower would be due to the memory of out being in a different cache page than that of values and indices.