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**Section:** 25.7.2 [alg.nth.element] **Status:** C++14
**Submitter:** Christopher Jefferson **Opened:** 2013-10-19 **Last modified:** 2017-07-06

**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_elementthe element in the position pointed to bynthis the element that would be in that position if the whole range were sorted. Also for every iteratoriin the range[first,nth)and every iteratorjin the range[nth,last)it holds that:!(*j < *i)orcomp(*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_elementthe element in the position pointed to bynth, if any, is the element that would be in that position if the whole range were sorted. Also for every iteratoriin the range[first,nth)and every iteratorjin the range[nth,last)it holds that:!(*j < *i)orcomp(*j, *i) == false.

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

**Proposed resolution:**

This wording is relative to N3797.

Modify 25.7.2 [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`.