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<title>Internal Floating-point Promotion Policies</title>
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<div class="section math_toolkit_policy_pol_ref_internal_promotion">
<div class="titlepage"><div><div><h4 class="title">
<a name="math_toolkit.policy.pol_ref.internal_promotion"></a><a class="link" href="internal_promotion.html" title="Internal Floating-point Promotion Policies">Internal
Floating-point Promotion Policies</a>
</h4></div></div></div>
<p>
Normally when evaluating a function at say <code class="computeroutput"><span class="keyword">float</span></code>
precision, maximal accuracy is assured by conducting the calculation at
<code class="computeroutput"><span class="keyword">double</span></code> precision internally,
and then rounding the result. There are two policies that control whether
internal promotion to a higher precision floating-point type takes place,
or not:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Policy
</p>
</th>
<th>
<p>
Meaning
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<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>
</p>
</td>
<td>
<p>
Indicates whether <code class="computeroutput"><span class="keyword">float</span></code>
arguments should be promoted to <code class="computeroutput"><span class="keyword">double</span></code>
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>
</p>
</td>
</tr>
<tr>
<td>
<p>
<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>
</p>
</td>
<td>
<p>
Indicates whether <code class="computeroutput"><span class="keyword">double</span></code>
arguments should be promoted to <code class="computeroutput"><span class="keyword">long</span>
<span class="keyword">double</span></code> 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_double</span><span class="special"><</span><span class="keyword">true</span><span class="special">></span></code>
</p>
</td>
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</tbody>
</table></div>
<h5>
<a name="math_toolkit.policy.pol_ref.internal_promotion.h0"></a>
<span><a name="math_toolkit.policy.pol_ref.internal_promotion.examples"></a></span><a class="link" href="internal_promotion.html#math_toolkit.policy.pol_ref.internal_promotion.examples">Examples</a>
</h5>
<p>
Suppose we want <code class="computeroutput"><span class="identifier">tgamma</span></code>
to be evaluated without internal promotion to <code class="computeroutput"><span class="keyword">long</span>
<span class="keyword">double</span></code>, then we could use:
</p>
<p>
</p>
<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>
<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>
<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>
<span class="comment">// Define a new policy *not* internally promoting RealType to double:</span>
<span class="keyword">typedef</span> <span class="identifier">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>
<span class="special">></span> <span class="identifier">my_policy</span><span class="special">;</span>
<span class="comment">// Call the function, applying the new policy:</span>
<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>
<span class="comment">// Alternatively we could use helper function make_policy,</span>
<span class="comment">// and concisely define everything at the call site:</span>
<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>
</pre>
<p>
</p>
<p>
Alternatively, suppose we want a distribution to perform calculations without
promoting <code class="computeroutput"><span class="keyword">float</span></code> to <code class="computeroutput"><span class="keyword">double</span></code>, then we could use:
</p>
<p>
</p>
<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>
<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>
<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>
<span class="comment">// Define a policy:</span>
<span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span>
<span class="identifier">promote_float</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span>
<span class="special">></span> <span class="identifier">my_policy</span><span class="special">;</span>
<span class="comment">// Define the new normal distribution using my_policy:</span>
<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>
<span class="comment">// Get a quantile:</span>
<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>
</pre>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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