Selecting the maximum “n” values

Question

If I have the following:

#include 
#include 
#include 
#include 

    struct Features{ int F1, F2,         


        

Answer 1

With a combination of std::transform, std::multiset, and an insert iterator you could.

vector<Features> v;
...fill it up
multiset<int> ms;
transform(v.begin(), v.end(), inserter(ms, ms.begin()), criterionFunction);

Then the three max values are the last three elements.

Answer 2

Here is an example using nth_element with a simpler feature object and criterion function (to reduce clutter):

#include <algorithm>
#include <vector>
#include <iterator>
#include <iostream>

typedef int Features;

int criterionFunction(Features features) {
  return features;
}

int main() {
  std::vector<Features> v { 0, 4, 2, 5, 4, 3, -2, 1 };
  std::nth_element(v.begin(), v.begin() + 3, v.end(),
                   [](Features a, Features b) {
                       return criterionFunction(a) > criterionFunction(b);
                   });
  std::copy(v.begin(), v.begin() + 3,
            std::ostream_iterator<Features>(std::cout, " "));
}

For your original Features object, it might be useful to cache/memoize the results of the criterionFunction to prevent duplicate calls.

Note that nth_element does not sort the elements in the two partitions; if you want the first three elements in sorted order, use partial_sort instead.

Answer 3

You can't. It's not what std::transform does.

transform applies a single function to every element in the sequence. It does not select specific elements.