How can I sort two vectors in the same way, with criteria that uses only one of the vectors?
For example, suppose I have two vectors of the same size:
vector<MyObject> vectorA;
vector<int> vectorB;
I then sort vectorA using some comparison function. That sorting reordered vectorA. How can I have the same reordering applied to vectorB?
One option is to create a struct:
struct ExampleStruct {
MyObject mo;
int i;
};
and then sort a vector that contains the contents of vectorA and vectorB zipped up into a single vector:
// vectorC[i] is vectorA[i] and vectorB[i] combined
vector<ExampleStruct> vectorC;
This doesn't seem like an ideal solution. Are there other ways of doing this?
Given a std::vector<T> and a comparison for T's, we want to be able to find the permutation you would use if you were to sort the vector using this comparison.
template <typename T, typename Compare>
std::vector<std::size_t> sort_permutation(
const std::vector<T>& vec,
Compare& compare)
{
std::vector<std::size_t> p(vec.size());
std::iota(p.begin(), p.end(), 0);
std::sort(p.begin(), p.end(),
[&](std::size_t i, std::size_t j){ return compare(vec[i], vec[j]); });
return p;
}
Given a std::vector<T> and a permutation, we want to be able to build a new std::vector<T> that is reordered according to the permutation.
template <typename T>
std::vector<T> apply_permutation(
const std::vector<T>& vec,
const std::vector<std::size_t>& p)
{
std::vector<T> sorted_vec(vec.size());
std::transform(p.begin(), p.end(), sorted_vec.begin(),
[&](std::size_t i){ return vec[i]; });
return sorted_vec;
}
You could of course modify apply_permutation to mutate the vector you give it rather than returning a new sorted copy. This approach is still linear time complexity and uses one bit per item in your vector. Theoretically, it's still linear space complexity; but, in practice, when sizeof(T) is large the reduction in memory usage can be dramatic. (See details)
template <typename T>
void apply_permutation_in_place(
std::vector<T>& vec,
const std::vector<std::size_t>& p)
{
std::vector<bool> done(vec.size());
for (std::size_t i = 0; i < vec.size(); ++i)
{
if (done[i])
{
continue;
}
done[i] = true;
std::size_t prev_j = i;
std::size_t j = p[i];
while (i != j)
{
std::swap(vec[prev_j], vec[j]);
done[j] = true;
prev_j = j;
j = p[j];
}
}
}
vector<MyObject> vectorA;
vector<int> vectorB;
auto p = sort_permutation(vectorA,
[](T const& a, T const& b){ /*some comparison*/ });
vectorA = apply_permutation(vectorA, p);
vectorB = apply_permutation(vectorB, p);