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<title>Issue 2339: Wording issue in nth_element</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++14">C++14</a> status.</em></p>
<h3 id="2339"><a href="lwg-defects.html#2339">2339</a>. Wording issue in <code>nth_element</code></h3>
<p><b>Section:</b> 26.8.3 <a href="https://wg21.link/alg.nth.element">[alg.nth.element]</a> <b>Status:</b> <a href="lwg-active.html#C++14">C++14</a>
 <b>Submitter:</b> Christopher Jefferson <b>Opened:</b> 2013-10-19 <b>Last modified:</b> 2017-07-05</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.nth.element">issues</a> in [alg.nth.element].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The wording of <code>nth_element</code> says:
</p>
<blockquote><pre>
template&lt;class RandomAccessIterator&gt;
  void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
                   RandomAccessIterator last);
</pre><blockquote>
<p>
After <code>nth_element</code> the element in the position pointed to by <code>nth</code> is the element that would be in that
position if the whole range were sorted. Also for every iterator <code>i</code> in the range <code>[first,nth)</code> and every
iterator <code>j</code> in the range <code>[nth,last)</code> it holds that: <code>!(*j &lt; *i)</code> or <code>comp(*j, *i) == false</code>.
</p>
</blockquote></blockquote>
<p>
That wording, to me, implies that there must be an element at '<code>nth</code>'.
However, gcc at least accepts <code>nth == last</code>, and returns without effect
(which seems like the sensible option).
<p/>
Is it intended to accept <code>nth == last</code>? If so, then I would suggest adding
this to the wording explicitly, say:
</p>
<blockquote>
<p>
After <code>nth_element</code> the element in the position pointed to by <code>nth</code><ins>, if any,</ins> is the element that
would be in that position if the whole range were sorted. Also for every iterator <code>i</code> in the range <code>[first,nth)</code>
and every iterator <code>j</code> in the range <code>[nth,last)</code> it holds that: <code>!(*j &lt; *i)</code> or <code>comp(*j, *i) == false</code>.
</p>
</blockquote>

<p><i>[Issaquah 2014-02-11: Move to Immediate]</i></p>




<p id="res-2339"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3797.</p>

<ol>
<li><p>Modify 26.8.3 <a href="https://wg21.link/alg.nth.element">[alg.nth.element]</a>/1 as indicated:</p>

<blockquote><pre>
template&lt;class RandomAccessIterator&gt;
  void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
                   RandomAccessIterator last);
template&lt;class RandomAccessIterator, class Compare&gt;
  void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
                   RandomAccessIterator last, Compare comp);
</pre><blockquote>
<p>
-1- After <code>nth_element</code> the element in the position pointed to by <code>nth</code> is the element that would be in that
position if the whole range were sorted<ins>, unless <code>nth == last</code></ins>. Also for every iterator <code>i</code> in the 
range <code>[first,nth)</code> and every iterator <code>j</code> in the range <code>[nth,last)</code> it holds that: <code>!(*j &lt; *i)</code> 
or <code>comp(*j, *i) == false</code>.
</p>
</blockquote></blockquote>
</li>

</ol>





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