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<title>Internal Floating-point Promotion Policies</title>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="math_toolkit.pol_ref.internal_promotion"></a><a class="link" href="internal_promotion.html" title="Internal Floating-point Promotion Policies">Internal Floating-point
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Promotion Policies</a>
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</h3></div></div></div>
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<p>
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Normally when evaluating a function at say <code class="computeroutput"><span class="keyword">float</span></code>
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precision, maximal accuracy is assured by conducting the calculation at
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<code class="computeroutput"><span class="keyword">double</span></code> precision internally,
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and then rounding the result. There are two policies that control whether
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internal promotion to a higher precision floating-point type takes place,
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or not:
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</p>
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<div class="informaltable"><table class="table">
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<colgroup>
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<thead><tr>
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<p>
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Policy
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</p>
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</th>
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<th>
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<p>
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Meaning
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">promote_float</span><span class="special"><</span><span class="identifier">B</span><span class="special">></span></code>
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</p>
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</td>
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<td>
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<p>
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Indicates whether <code class="computeroutput"><span class="keyword">float</span></code>
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arguments should be promoted to <code class="computeroutput"><span class="keyword">double</span></code>
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precision internally: defaults to <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">promote_float</span><span class="special"><</span><span class="keyword">true</span><span class="special">></span></code>
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">promote_double</span><span class="special"><</span><span class="identifier">B</span><span class="special">></span></code>
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</p>
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</td>
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<td>
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<p>
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Indicates whether <code class="computeroutput"><span class="keyword">double</span></code>
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arguments should be promoted to <code class="computeroutput"><span class="keyword">long</span>
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<span class="keyword">double</span></code> precision internally:
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defaults to <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">promote_double</span><span class="special"><</span><span class="keyword">true</span><span class="special">></span></code>
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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<h5>
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<a name="math_toolkit.pol_ref.internal_promotion.h0"></a>
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<span class="phrase"><a name="math_toolkit.pol_ref.internal_promotion.examples"></a></span><a class="link" href="internal_promotion.html#math_toolkit.pol_ref.internal_promotion.examples">Examples</a>
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</h5>
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<p>
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Suppose we want <code class="computeroutput"><span class="identifier">tgamma</span></code> to
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be evaluated without internal promotion to <code class="computeroutput"><span class="keyword">long</span>
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<span class="keyword">double</span></code>, then we could use:
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">special_functions</span><span class="special">/</span><span class="identifier">gamma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">;</span>
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<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">tgamma</span><span class="special">;</span>
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<span class="comment">// Define a new policy *not* internally promoting RealType to double:</span>
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<span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span>
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<span class="identifier">promote_double</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span>
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<span class="special">></span> <span class="identifier">my_policy</span><span class="special">;</span>
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<span class="comment">// Call the function, applying the new policy:</span>
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<span class="keyword">double</span> <span class="identifier">t1</span> <span class="special">=</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">some_value</span><span class="special">,</span> <span class="identifier">my_policy</span><span class="special">());</span>
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<span class="comment">// Alternatively we could use helper function make_policy,</span>
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<span class="comment">// and concisely define everything at the call site:</span>
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<span class="keyword">double</span> <span class="identifier">t2</span> <span class="special">=</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">some_value</span><span class="special">,</span> <span class="identifier">make_policy</span><span class="special">(</span><span class="identifier">promote_double</span><span class="special"><</span><span class="keyword">false</span><span class="special">>()));</span>
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</pre>
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<p>
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Alternatively, suppose we want a distribution to perform calculations without
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promoting <code class="computeroutput"><span class="keyword">float</span></code> to <code class="computeroutput"><span class="keyword">double</span></code>, then we could use:
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">distributions</span><span class="special">/</span><span class="identifier">normal</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">normal_distribution</span><span class="special">;</span>
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<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">;</span>
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<span class="comment">// Define a policy:</span>
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<span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span>
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<span class="identifier">promote_float</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span>
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<span class="special">></span> <span class="identifier">my_policy</span><span class="special">;</span>
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<span class="comment">// Define the new normal distribution using my_policy:</span>
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<span class="keyword">typedef</span> <span class="identifier">normal_distribution</span><span class="special"><</span><span class="keyword">float</span><span class="special">,</span> <span class="identifier">my_policy</span><span class="special">></span> <span class="identifier">my_norm</span><span class="special">;</span>
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<span class="comment">// Get a quantile:</span>
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<span class="keyword">float</span> <span class="identifier">q</span> <span class="special">=</span> <span class="identifier">quantile</span><span class="special">(</span><span class="identifier">my_norm</span><span class="special">(),</span> <span class="number">0.05f</span><span class="special">);</span>
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</pre>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2006-2019 Nikhar
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Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
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Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
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Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
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Daryle Walker and Xiaogang Zhang<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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