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### 2163. `nth_element` requires inconsistent post-conditions

Section: 27.8.3 [alg.nth.element] Status: C++14 Submitter: Peter Sommerlad Opened: 2012-07-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The specification of nth_element refers to `operator<` whereas all sorting without a compare function is based on `operator<`. While it is true that for all regular types both operators should be defined accordingly, all other sorting algorithms only rely on existence of `operator<`. So I guess the paragraph p1

> 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 any iterator `i` in the range `[first,nth)` and any iterator `j` in the range `[nth,last)` it holds that: `!(*i > *j)` or `comp(*j, *i) == false`.

should read

> 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 any iterator `i` in the range `[first,nth)` and any iterator `j` in the range `[nth,last)` it holds that: `!(*j < *i)` or `comp(*j, *i) == false`.

Note only "`!(*i > *j)`" was changed to "`!(*j < *i)`" and it would be more symmetric with `comp(*j, *i)` as well.

In theory this might be a semantic change to the spec, but I believe the mistake is unintended.

[ 2012-10 Portland: Move to Ready ]

This is clearly correct by inspection, moved to Ready by unanimous consent.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change 27.8.3 [alg.nth.element] p1 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. Also for any iterator `i` in the range `[first,nth)` and any iterator `j` in the range `[nth,last)` it holds that: `!(*i > *j)!(*j < *i)` or `comp(*j, *i) == false`.