indices of the k largest elements in an unsorted length n array
arrays, c++, indices, max
Solution
This should be an improved version of @hazelnusse which is executed in `O(nlogk)` instead of `O(nlogn)`
#include <queue>
#include <iostream>
#include <vector>
// maxindices.cc
// compile with:
// g++ -std=c++11 maxindices.cc -o maxindices
int main()
{
std::vector<double> test = {2, 8, 7, 5, 9, 3, 6, 1, 10, 4};
std::priority_queue< std::pair<double, int>, std::vector< std::pair<double, int> >, std::greater <std::pair<double, int> > > q;
int k = 5; // number of indices we need
for (int i = 0; i < test.size(); ++i) {
if(q.size()<k)
q.push(std::pair<double, int>(test[i], i));
else if(q.top().first < test[i]){
q.pop();
q.push(std::pair<double, int>(test[i], i));
}
}
k = q.size();
std::vector<int> res(k);
for (int i = 0; i < k; ++i) {
res[k - i - 1] = q.top().second;
q.pop();
}
for (int i = 0; i < k; ++i) {
std::cout<< res[i] <<std::endl;
}
}
8 4 1 2 6
Problem
I need to find the indices of the k largest elements of an unsorted, length n, array/vector in C++, with k < n. I have seen how to use nth_element() to find the k-th statistic, but I'm not sure if using this is the right choice for my problem as it seems like I would need to make k calls to nth_statistic, which I guess it would have complexity O(kn), which may be as good as it can get? Or is there a way to do this just in O(n)? Implementing it without nth_element() seems like I will have to iterate over the whole array once, populating a list of indices of the largest elements at each step. Is there anything in the standard C++ library that makes this a one-liner or any clever way to implement this myself in just a couple lines? In my particular case, k = 3, and n = 6, so efficiency isn't a huge concern, but it would be nice to find a clean and efficient way to do this for arbitrary k and n. It looks like Mark the top N elements of an unsorted array is probably the closest posting I can find on SO, the postings there are in Python and PHP.