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Showing content from https://cplusplus.com/reference/algorithm/nth_element/ below:

function template

std::nth_element default (1)

template <class RandomAccessIterator>  void nth_element (RandomAccessIterator first, RandomAccessIterator nth,                    RandomAccessIterator last);

custom (2)

template <class RandomAccessIterator, class Compare>  void nth_element (RandomAccessIterator first, RandomAccessIterator nth,                    RandomAccessIterator last, Compare comp);

Sort element in range

Rearranges the elements in the range [first,last), in such a way that the element at the nth position is the element that would be in that position in a sorted sequence.

The other elements are left without any specific order, except that none of the elements preceding nth are greater than it, and none of the elements following it are less.

The elements are compared using operator< for the first version, and comp for the second.

Parameters

first, last

Random-access iterators to the initial and final positions of the sequence to be used. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
Notice that in this function, these are not consecutive parameters, but the first and the third.

nth

Random-access iterator

pointing to the location within the range

[first,last)

that will contain the sorted element.

Notice that the value of the element pointed by

nth

before the call is not relevant.

Random-access iterator

pointing to the location within the range

[first,last]

that will contain the sorted element.

If this points to

last

, the function call has no effect.

Notice that the value of the element pointed by

nth

before the call is not relevant.

comp

Binary function that accepts two elements in the range as arguments, and returns a value convertible to bool. The value returned indicates whether the element passed as first argument is considered to go before the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

Return value none

Example

// nth_element example
#include <iostream>     // std::cout
#include <algorithm>    // std::nth_element, std::random_shuffle
#include <vector>       // std::vector

bool myfunction (int i,int j) { return (i<j); }

int main () {
  std::vector<int> myvector;

  // set some values:
  for (int i=1; i<10; i++) myvector.push_back(i);   // 1 2 3 4 5 6 7 8 9

  std::random_shuffle (myvector.begin(), myvector.end());

  // using default comparison (operator <):
  std::nth_element (myvector.begin(), myvector.begin()+5, myvector.end());

  // using function as comp
  std::nth_element (myvector.begin(), myvector.begin()+5, myvector.end(),myfunction);

  // print out content:
  std::cout << "myvector contains:";
  for (std::vector<int>::iterator it=myvector.begin(); it!=myvector.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

Possible output:

myvector contains: 3 1 4 2 5 6 9 7 8

Complexity On average, linear in the distance between first and last: Compares elements, and possibly swaps (or moves) them, until the elements are properly rearranged.

Data races The objects in the range [first,last) are modified.

Exceptions Throws if any of the element comparisons, the element swaps (or moves) or the operations on iterators throws.
Note that invalid arguments cause undefined behavior.

See also

sort: Sort elements in range (function template)

partial_sort: Partially sort elements in range (function template)

partition: Partition range in two (function template)

find_if: Find element in range (function template)

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