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<title>Issue 2163: nth_element requires inconsistent post-conditions</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="2163"><a href="lwg-defects.html#2163">2163</a>. <code>nth_element</code> requires inconsistent post-conditions</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> Peter Sommerlad <b>Opened:</b> 2012-07-06 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</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 specification of nth_element refers to <code>operator&lt;</code> whereas all sorting without a compare function is based on 
<code>operator&lt;</code>. While it is true that for all regular types both operators should be defined accordingly, all other 
sorting algorithms only rely on existence of <code>operator&lt;</code>. So I guess the paragraph p1
</p>
<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 any iterator <code>i</code> in the range <code>[first,nth)</code> and any
iterator <code>j</code> in the range <code>[nth,last)</code> it holds that: <code>!(*i &gt; *j)</code> or <code>comp(*j, *i) == false</code>.
</p></blockquote>
<p>
should read
</p>
<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 any iterator <code>i</code> in the range <code>[first,nth)</code> and any
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>
Note only "<code>!(*i &gt; *j)</code>" was changed to "<code>!(*j &lt; *i)</code>" and it would be more symmetric with 
<code>comp(*j, *i)</code> as well.
<p/>
In theory this might be a semantic change to the spec, but I believe the mistake is unintended.
</p>

<p><i>[
2012-10 Portland: Move to Ready 
]</i></p>


<p>
This is clearly correct by inspection, moved to Ready by unanimous consent.
</p>

<p><i>[2013-04-20 Bristol]</i></p>




<p id="res-2163"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3376.</p>

<ol>
<li><p>Change 26.8.3 <a href="https://wg21.link/alg.nth.element">[alg.nth.element]</a> p1 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. Also for any iterator <code>i</code> in the range <code>[first,nth)</code> and any
iterator <code>j</code> in the range <code>[nth,last)</code> it holds that: <code><del>!(*i &gt; *j)</del><ins>!(*j &lt; *i)</ins></code> 
or <code>comp(*j, *i) == false</code>.
</p>
</blockquote></blockquote>
</li>
</ol>






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