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<div class="refentry" title="Function reduce">
<a name="boost.mpi.reduce"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Function reduce</span></h2>
<p>boost::mpi::reduce — Combine the values stored by each process into a single value at the root. </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="emphasis"><em>// 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>>
</em></span>
<span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> T, <span class="bold"><strong>typename</strong></span> Op>
<span class="type"><span class="bold"><strong>void</strong></span></span> reduce(<span class="bold"><strong>const</strong></span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> & comm, <span class="bold"><strong>const</strong></span> T & in_value, T & out_value,
Op op, <span class="bold"><strong>int</strong></span> root);
<span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> T, <span class="bold"><strong>typename</strong></span> Op>
<span class="type"><span class="bold"><strong>void</strong></span></span> reduce(<span class="bold"><strong>const</strong></span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> & comm, <span class="bold"><strong>const</strong></span> T & in_value, Op op, <span class="bold"><strong>int</strong></span> root);
<span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> T, <span class="bold"><strong>typename</strong></span> Op>
<span class="type"><span class="bold"><strong>void</strong></span></span> reduce(<span class="bold"><strong>const</strong></span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> & comm, <span class="bold"><strong>const</strong></span> T * in_values, <span class="bold"><strong>int</strong></span> n,
T * out_values, Op op, <span class="bold"><strong>int</strong></span> root);
<span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> T, <span class="bold"><strong>typename</strong></span> Op>
<span class="type"><span class="bold"><strong>void</strong></span></span> reduce(<span class="bold"><strong>const</strong></span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> & comm, <span class="bold"><strong>const</strong></span> T * in_values, <span class="bold"><strong>int</strong></span> n, Op op,
<span class="bold"><strong>int</strong></span> root);</pre></div>
<div class="refsect1" title="Description">
<a name="id1392233"></a><h2>Description</h2>
<p><code class="computeroutput">reduce</code> is a collective algorithm that combines the values stored by each process into a single value at the <code class="computeroutput">root</code>. The values can be combined arbitrarily, specified via a function object. 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 the <code class="computeroutput">root</code>, followed by an <code class="computeroutput">std::accumulate()</code> over the gathered values and using the operation <code class="computeroutput">op</code>.</p>
<p>When the type <code class="computeroutput">T</code> has an associated MPI data type, this routine invokes <code class="computeroutput">MPI_Reduce</code> to perform the reduction. If possible, built-in MPI operations will be used; otherwise, <code class="computeroutput">reduce()</code> will create a custom MPI_Op for the call to MPI_Reduce.</p>
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</p>
<div class="variablelist"><table border="0">
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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 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 every other process. For reducing arrays, <code class="computeroutput">in_values</code> contains a pointer to the local values. In this case, <code class="computeroutput">n</code> is the number of values that will be reduced. Reduction occurs independently for each of the <code class="computeroutput">n</code> values referenced by <code class="computeroutput">in_values</code>, e.g., calling reduce on an array of <code class="computeroutput">n</code> values is like calling <code class="computeroutput">reduce</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"><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">is_mpi_op</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>): users are encouraged to mark commutative operations as such, because it gives the implementation additional lattitude to optimize the reduction operation.</p></td>
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<td><p><span class="term"><code class="computeroutput">out_value</code></span></p></td>
<td><p>Will receive the result of the reduction operation, but only for the <code class="computeroutput">root</code> process. Non-root processes may omit if parameter; if they choose to supply the parameter, it will be unchanged. For reducing arrays, <code class="computeroutput">out_values</code> contains a pointer to the storage for the output values.</p></td>
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<td><p><span class="term"><code class="computeroutput">root</code></span></p></td>
<td><p>The process ID number that will receive the final, combined value. This value must be the same on all processes. </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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