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### 2339. Wording issue in `nth_element`

Section: 25.8.3 [alg.nth.element] Status: C++14 Submitter: Christopher Jefferson Opened: 2013-10-19 Last modified: 2017-07-05

Priority: 0

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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).

Is it intended to accept `nth == last`? If so, then I would suggest adding this to the wording explicitly, say:

> 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 25.8.3 [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`.