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| <div class="titlepage"><div><div><h5 class="title"> |
| <a name="boost_multiprecision.tut.floats.fp_eg.aos"></a><a class="link" href="aos.html" title="Area of Circle">Area of |
| Circle</a> |
| </h5></div></div></div> |
| <p> |
| Generic numeric programming employs templates to use the same code for |
| different floating-point types and functions. Consider the area of a |
| circle a of radius r, given by |
| </p> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <span class="emphasis"><em>a = π * r<sup>2</sup></em></span> |
| </p></blockquote></div> |
| <p> |
| The area of a circle can be computed in generic programming using Boost.Math |
| for the constant π as shown below: |
| </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">constants</span><span class="special">/</span><span class="identifier">constants</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">area_of_a_circle</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">r</span><span class="special">)</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">constants</span><span class="special">::</span><span class="identifier">pi</span><span class="special">;</span> |
| <span class="keyword">return</span> <span class="identifier">pi</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span> <span class="special">*</span> <span class="identifier">r</span> <span class="special">*</span> <span class="identifier">r</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| It is possible to use <code class="computeroutput"><span class="identifier">area_of_a_circle</span><span class="special">()</span></code> with built-in floating-point types |
| as well as floating-point types from Boost.Multiprecision. In particular, |
| consider a system with 4-byte single-precision float, 8-byte double-precision |
| double and also the <code class="computeroutput"><span class="identifier">cpp_dec_float_50</span></code> |
| data type from Boost.Multiprecision with 50 decimal digits of precision. |
| </p> |
| <p> |
| We can compute and print the approximate area of a circle with radius |
| 123/100 for <code class="computeroutput"><span class="keyword">float</span></code>, <code class="computeroutput"><span class="keyword">double</span></code> and <code class="computeroutput"><span class="identifier">cpp_dec_float_50</span></code> |
| with the program below. |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iomanip</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">multiprecision</span><span class="special">/</span><span class="identifier">cpp_dec_float</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">multiprecision</span><span class="special">::</span><span class="identifier">cpp_dec_float_50</span><span class="special">;</span> |
| |
| <span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">char</span><span class="special">**)</span> |
| <span class="special">{</span> |
| <span class="keyword">const</span> <span class="keyword">float</span> <span class="identifier">r_f</span><span class="special">(</span><span class="keyword">float</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">/</span> <span class="number">100</span><span class="special">);</span> |
| <span class="keyword">const</span> <span class="keyword">float</span> <span class="identifier">a_f</span> <span class="special">=</span> <span class="identifier">area_of_a_circle</span><span class="special">(</span><span class="identifier">r_f</span><span class="special">);</span> |
| |
| <span class="keyword">const</span> <span class="keyword">double</span> <span class="identifier">r_d</span><span class="special">(</span><span class="keyword">double</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">/</span> <span class="number">100</span><span class="special">);</span> |
| <span class="keyword">const</span> <span class="keyword">double</span> <span class="identifier">a_d</span> <span class="special">=</span> <span class="identifier">area_of_a_circle</span><span class="special">(</span><span class="identifier">r_d</span><span class="special">);</span> |
| |
| <span class="keyword">const</span> <span class="identifier">cpp_dec_float_50</span> <span class="identifier">r_mp</span><span class="special">(</span><span class="identifier">cpp_dec_float_50</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">/</span> <span class="number">100</span><span class="special">);</span> |
| <span class="keyword">const</span> <span class="identifier">cpp_dec_float_50</span> <span class="identifier">a_mp</span> <span class="special">=</span> <span class="identifier">area_of_a_circle</span><span class="special">(</span><span class="identifier">r_mp</span><span class="special">);</span> |
| |
| <span class="comment">// 4.75292</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> |
| <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">setprecision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">float</span><span class="special">>::</span><span class="identifier">digits10</span><span class="special">)</span> |
| <span class="special"><<</span> <span class="identifier">a_f</span> |
| <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> |
| |
| <span class="comment">// 4.752915525616</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> |
| <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">setprecision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">digits10</span><span class="special">)</span> |
| <span class="special"><<</span> <span class="identifier">a_d</span> |
| <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> |
| |
| <span class="comment">// 4.7529155256159981904701331745635599135018975843146</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> |
| <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">setprecision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">cpp_dec_float_50</span><span class="special">>::</span><span class="identifier">digits10</span><span class="special">)</span> |
| <span class="special"><<</span> <span class="identifier">a_mp</span> |
| <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| In the next example we'll look at calling both standard library and Boost.Math |
| functions from within generic code. We'll also show how to cope with |
| template arguments which are expression-templates rather than number |
| types. |
| </p> |
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