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Showing content from https://cplusplus.github.io/LWG/issue2339 below:

Issue 2339: Wording issue in nth_element

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Section: 26.8.3 [alg.nth.element] Status: C++14 Submitter: Christopher Jefferson Opened: 2013-10-19 Last modified: 2017-07-05

Priority: 0

View all other issues in [alg.nth.element].

View all issues with C++14 status.

Discussion:

The wording of nth_element says:

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

After nth_element the element in the position pointed to by nth is the element that would be in that position if the whole range were sorted. Also for every iterator i in the range [first,nth) and every iterator j in the range [nth,last) it holds that: !(*j < *i) or comp(*j, *i) == false.

That wording, to me, implies that there must be an element at 'nth'. However, gcc at least accepts nth == last, and returns without effect (which seems like the sensible option).

After nth_element the element in the position pointed to by nth , if any, is the element that would be in that position if the whole range were sorted. Also for every iterator i in the range [first,nth) and every iterator j in the range [nth,last) it holds that: !(*j < *i) or comp(*j, *i) == false.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 26.8.3 [alg.nth.element]/1 as indicated:

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

   -1- After nth_element the element in the position pointed to by nth is the element that would be in that position if the whole range were sorted , unless nth == last. Also for every iterator i in the range [first,nth) and every iterator j in the range [nth,last) it holds that: !(*j < *i) or comp(*j, *i) == false.

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