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<title>Issue 618: valarray::cshift() effects on empty array</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#CD1">CD1</a> status.</em></p>
<h3 id="618"><a href="lwg-defects.html#618">618</a>. valarray::cshift() effects on empty array</h3>
<p><b>Section:</b> 29.6.2.8 <a href="https://wg21.link/valarray.members">[valarray.members]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Gabriel Dos Reis <b>Opened:</b> 2007-01-10 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
I would respectfully request an issue be opened with the intention to
clarify the wording for <code>size() == 0</code> for <code>cshift</code>.
</p>


<p id="res-618"><b>Proposed resolution:</b></p>
<p>
Change 29.6.2.8 <a href="https://wg21.link/valarray.members">[valarray.members]</a>, paragraph 10:
</p>

<blockquote>

<pre>
valarray&lt;T&gt; cshift(int <i>n</i>) const;
</pre>

<blockquote>
<p>
This function returns an object of class <code>valarray&lt;T&gt;</code>, of
length <code>size()</code>, <del>each of whose elements <code>I</code> is
<code>(*this)[(I + n ) % size()]</code>. Thus, if element zero is taken as
the leftmost element, a positive value of <i>n</i> shifts the elements
circularly left <i>n</i> places.</del> <ins>that is a circular shift of <code>*this</code>. If
element zero is taken as the leftmost element, a non-negative value of
<i>n</i> shifts the elements circularly left <i>n</i> places and a
negative value of <i>n</i> shifts the elements circularly right
-<i>n</i> places.</ins>
</p>
</blockquote>
</blockquote>



<p><b>Rationale:</b></p>
<p>
We do not believe that there is any real ambiguity about what happens
when <code>size() == 0</code>, but we do believe that spelling this out as a C++
expression causes more trouble that it solves. The expression is
certainly wrong when <code>n &lt; 0</code>, since the sign of % with negative arguments
is implementation defined.
</p>


<p><i>[
Kona (2007) Changed proposed wording, added rationale and set to Review.
]</i></p>





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