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<a name="boost.mpi.scan"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Function scan</span></h2>
<p>boost::mpi::scan — Compute a prefix reduction of values from all processes in the communicator. </p>
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<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../../mpi/reference.html#header.boost.mpi.collectives_hpp" title="Header <boost/mpi/collectives.hpp>">boost/mpi/collectives.hpp</a>>
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<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">></span>
<span class="keyword">void</span> <span class="identifier">scan</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> <span class="special">&</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span> in_value<span class="special">,</span> <span class="identifier">T</span> <span class="special">&</span> out_value<span class="special">,</span>
<span class="identifier">Op</span> op<span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">></span>
<span class="identifier">T</span> <span class="identifier">scan</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> <span class="special">&</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span> in_value<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">></span>
<span class="keyword">void</span> <span class="identifier">scan</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> <span class="special">&</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">*</span> in_values<span class="special">,</span> <span class="keyword">int</span> n<span class="special">,</span>
<span class="identifier">T</span> <span class="special">*</span> out_values<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">)</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="idm45555223575024"></a><h2>Description</h2>
<p><code class="computeroutput">scan</code> is a collective algorithm that combines the values stored by each process with the values of all processes with a smaller rank. The values can be arbitrarily combined, specified via a function object <code class="computeroutput">op</code>. The type <code class="computeroutput">T</code> of the values may be any type that is serializable or has an associated MPI data type. One can think of this operation as a <code class="computeroutput">gather</code> to some process, followed by an <code class="computeroutput">std::prefix_sum()</code> over the gathered values using the operation <code class="computeroutput">op</code>. The ith process returns the ith value emitted by <code class="computeroutput">std::prefix_sum()</code>.</p>
<p>When the type <code class="computeroutput">T</code> has an associated MPI data type, this routine invokes <code class="computeroutput">MPI_Scan</code> to perform the reduction. If possible, built-in MPI operations will be used; otherwise, <code class="computeroutput">scan()</code> will create a custom <code class="computeroutput">MPI_Op</code> for the call to MPI_Scan.</p>
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<td><p><span class="term">Parameters:</span></p></td>
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<td><p><span class="term"><code class="computeroutput">comm</code></span></p></td>
<td><p>The communicator over which the prefix reduction will occur.</p></td>
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<td><p><span class="term"><code class="computeroutput">in_value</code></span></p></td>
<td><p>The local value to be combined with the local values of other processes. For the array variant, the <code class="computeroutput">in_values</code> parameter points to the <code class="computeroutput">n</code> local values that will be combined.</p></td>
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<td><p><span class="term"><code class="computeroutput">op</code></span></p></td>
<td><p>The binary operation that combines two values of type <code class="computeroutput">T</code> into a third value of type <code class="computeroutput">T</code>. For types <code class="computeroutput">T</code> that has ssociated MPI data types, <code class="computeroutput">op</code> will either be translated into an <code class="computeroutput">MPI_Op</code> (via <code class="computeroutput">MPI_Op_create</code>) or, if possible, mapped directly to a built-in MPI operation. See <code class="computeroutput"><code class="computeroutput"><a class="link" href="is_mpi_op.html" title="Struct template is_mpi_op">is_mpi_op</a></code></code> in the <code class="computeroutput">operations.hpp</code> header for more details on this mapping. For any non-built-in operation, commutativity will be determined by the <code class="computeroutput">is_commmutative</code> trait (also in <code class="computeroutput">operations.hpp</code>).</p></td>
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<td><p><span class="term"><code class="computeroutput">out_value</code></span></p></td>
<td><p>If provided, the ith process will receive the value <code class="computeroutput">op</code>(in_value[0], op(in_value[1], op(..., in_value[i]) ... )). For the array variant, <code class="computeroutput">out_values</code> contains a pointer to storage for the <code class="computeroutput">n</code> output values. The prefix reduction occurs independently for each of the <code class="computeroutput">n</code> values referenced by <code class="computeroutput">in_values</code>, e.g., calling scan on an array of <code class="computeroutput">n</code> values is like calling <code class="computeroutput">scan</code> <code class="computeroutput">n</code> separate times, one for each location in <code class="computeroutput">in_values</code> and <code class="computeroutput">out_values</code>.</p></td>
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<td><p><span class="term">Returns:</span></p></td>
<td><p>If no <code class="computeroutput">out_value</code> parameter is provided, returns the result of prefix reduction. </p></td>
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<td align="right"><div class="copyright-footer">Copyright © 2005-2007 Douglas Gregor,
Matthias Troyer, Trustees of Indiana University<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">
http://www.boost.org/LICENSE_1_0.txt </a>)
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