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| <div class="titlepage"><div><div><h3 class="title"> |
| <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 |
| Promotion Policies</a> |
| </h3></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> |
| </tr> |
| </tbody> |
| </table></div> |
| <h5> |
| <a name="math_toolkit.pol_ref.internal_promotion.h0"></a> |
| <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> |
| </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> |
| <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> |
| 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> |
| <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> |
| </div> |
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