| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Input Output</title> |
| <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.77.1"> |
| <link rel="home" href="../../index.html" title="Chapter 1. Boost.Multiprecision"> |
| <link rel="up" href="../tut.html" title="Tutorial"> |
| <link rel="prev" href="limits/how_to_tell.html" title="How to Determine the Kind of a Number From std::numeric_limits"> |
| <link rel="next" href="../ref.html" title="Reference"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> |
| <td align="center"><a href="../../../../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="limits/how_to_tell.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tut.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../ref.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_multiprecision.tut.input_output"></a><a class="link" href="input_output.html" title="Input Output">Input Output</a> |
| </h3></div></div></div> |
| <h5> |
| <a name="boost_multiprecision.tut.input_output.h0"></a> |
| <span class="phrase"><a name="boost_multiprecision.tut.input_output.loopback_testing"></a></span><a class="link" href="input_output.html#boost_multiprecision.tut.input_output.loopback_testing">Loopback |
| testing</a> |
| </h5> |
| <p> |
| <span class="emphasis"><em>Loopback</em></span> or <span class="emphasis"><em>round-tripping</em></span> refers |
| to writing out a value as a decimal digit string using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">iostream</span></code>, |
| usually to a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span></code>, and then reading the string |
| back in to another value, and confirming that the two values are identical. |
| A trivial example using <code class="computeroutput"><span class="keyword">float</span></code> |
| is: |
| </p> |
| <pre class="programlisting"><span class="keyword">float</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Value to round-trip.</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">ss</span><span class="special">;</span> <span class="comment">// Read and write std::stringstream.</span> |
| <span class="identifier">ss</span><span class="special">.</span><span class="identifier">precision</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">T</span><span class="special">>::</span><span class="identifier">max_digits10</span><span class="special">);</span> <span class="comment">// Ensure all potentially significant bits are output.</span> |
| <span class="identifier">ss</span><span class="special">.</span><span class="identifier">flags</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">fmtflags</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">scientific</span><span class="special">));</span> <span class="comment">// Use scientific format.</span> |
| <span class="identifier">ss</span> <span class="special"><<</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Output to string.</span> |
| <span class="keyword">float</span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Expected.</span> |
| <span class="identifier">ss</span> <span class="special">>></span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Read decimal digits string from stringstream.</span> |
| <span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">write</span><span class="special">,</span> <span class="identifier">read</span><span class="special">);</span> <span class="comment">// Should be the same.</span> |
| </pre> |
| <p> |
| and this can be run in a loop for all possible values of a 32-bit float. |
| For other floating-point types <code class="computeroutput"><span class="identifier">T</span></code>, |
| including built-in <code class="computeroutput"><span class="keyword">double</span></code>, it |
| takes far too long to test all values, so a reasonable test strategy is to |
| use a large number of random values. |
| </p> |
| <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">write</span><span class="special">;</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">ss</span><span class="special">;</span> |
| <span class="identifier">ss</span><span class="special">.</span><span class="identifier">precision</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">T</span><span class="special">>::</span><span class="identifier">max_digits10</span><span class="special">);</span> <span class="comment">// Ensure all potentially significant bits are output.</span> |
| <span class="identifier">ss</span><span class="special">.</span><span class="identifier">flags</span><span class="special">(</span><span class="identifier">f</span><span class="special">);</span> <span class="comment">// Changed from default iostream format flags if desired.</span> |
| <span class="identifier">ss</span> <span class="special"><<</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Output to stringstream.</span> |
| |
| <span class="identifier">T</span> <span class="identifier">read</span><span class="special">;</span> |
| <span class="identifier">ss</span> <span class="special">>></span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Get read using operator>> from stringstream.</span> |
| <span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">read</span><span class="special">,</span> <span class="identifier">write</span><span class="special">);</span> |
| |
| <span class="identifier">read</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">ss</span><span class="special">.</span><span class="identifier">str</span><span class="special">());</span> <span class="comment">// Get read by converting from decimal digits string representation of write.</span> |
| <span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">read</span><span class="special">,</span> <span class="identifier">write</span><span class="special">);</span> |
| |
| <span class="identifier">read</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">write</span><span class="special">.</span><span class="identifier">str</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="identifier">f</span><span class="special">));</span> <span class="comment">// Get read using format specified when written.</span> |
| <span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">read</span><span class="special">,</span> <span class="identifier">write</span><span class="special">);</span> |
| </pre> |
| <p> |
| The test at <a href="../../../../test/test_cpp_bin_float_io.cpp" target="_top">test_cpp_bin_float_io.cpp</a> |
| allows any floating-point type to be <span class="emphasis"><em>round_tripped</em></span> using |
| a wide range of fairly random values. It also includes tests compared a collection |
| of <a href="../../../../test/string_data.ipp" target="_top">stringdata</a> test cases |
| in a file. |
| </p> |
| <h5> |
| <a name="boost_multiprecision.tut.input_output.h1"></a> |
| <span class="phrase"><a name="boost_multiprecision.tut.input_output.comparing_with_output_using_buil"></a></span><a class="link" href="input_output.html#boost_multiprecision.tut.input_output.comparing_with_output_using_buil">Comparing |
| with output using Built-in types</a> |
| </h5> |
| <p> |
| One can make some comparisons with the output of |
| </p> |
| <pre class="programlisting"><span class="special"><</span><span class="identifier">number</span><span class="special"><</span><span class="identifier">cpp_bin_float</span><span class="special"><</span><span class="number">53</span><span class="special">,</span> <span class="identifier">digit_count_2</span><span class="special">></span> <span class="special">></span> |
| </pre> |
| <p> |
| which has the same number of significant bits (53) as 64-bit double precision |
| floating-point. |
| </p> |
| <p> |
| However, although most outputs are identical, there are differences on some |
| platforms caused by the implementation-dependent behaviours allowed by the |
| C99 specification <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf" target="_top">C99 |
| ISO/IEC 9899:TC2</a>, incorporated by C++. |
| </p> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <span class="emphasis"><em>"For e, E, f, F, g, and G conversions, if the number of |
| significant decimal digits is at most DECIMAL_DIG, then the result should |
| be correctly rounded. If the number of significant decimal digits is more |
| than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG |
| digits, then the result should be an exact representation with trailing |
| zeros. Otherwise, the source value is bounded by two adjacent decimal strings |
| L < U, both having DECIMAL_DIG significant digits; the value of the |
| resultant decimal string D should satisfy L<= D <= U, with the extra |
| stipulation that the error should have a correct sign for the current rounding |
| direction."</em></span> |
| </p></blockquote></div> |
| <p> |
| So not only is correct rounding for the full number of digits not required, |
| but even if the <span class="bold"><strong>optional</strong></span> recommended practice |
| is followed, then the value of these last few digits is unspecified as long |
| as the value is within certain bounds. |
| </p> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Do not expect the output from different platforms to be <span class="bold"><strong>identical</strong></span>, |
| but <code class="computeroutput"><span class="identifier">cpp_dec_float</span></code>, <code class="computeroutput"><span class="identifier">cpp_bin_float</span></code> (and other backends) outputs |
| should be correctly rounded to the number of digits requested by the set |
| precision and format. |
| </p></td></tr> |
| </table></div> |
| <h5> |
| <a name="boost_multiprecision.tut.input_output.h2"></a> |
| <span class="phrase"><a name="boost_multiprecision.tut.input_output.macro_boost_mp_min_exponent_digi"></a></span><a class="link" href="input_output.html#boost_multiprecision.tut.input_output.macro_boost_mp_min_exponent_digi">Macro |
| BOOST_MP_MIN_EXPONENT_DIGITS</a> |
| </h5> |
| <p> |
| <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf" target="_top">C99 |
| Standard</a> for format specifiers, 7.19.6 Formatted input/output functions |
| requires: |
| </p> |
| <p> |
| "The exponent always contains at least two digits, and only as many |
| more digits as necessary to represent the exponent." |
| </p> |
| <p> |
| So to conform to the C99 standard (incorporated by C++) |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">2</span> |
| </pre> |
| <p> |
| Confusingly, Microsoft (and MinGW) do not conform to this standard and provide |
| <span class="bold"><strong>at least three digits</strong></span>, for example <code class="computeroutput"><span class="number">1e+001</span></code>. So if you want the output to match |
| that from built-in floating-point types on compilers that use Microsofts |
| runtime then use: |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">3</span> |
| </pre> |
| <p> |
| Also useful to get the minimum exponent field width is |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">1</span> |
| </pre> |
| <p> |
| producing a compact output like <code class="computeroutput"><span class="number">2e+4</span></code>, |
| useful when conserving space is important. |
| </p> |
| <p> |
| Larger values are also supported, for example, value 4 for <code class="computeroutput"><span class="number">2e+0004</span></code> which may be useful to ensure that |
| columns line up. |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2002-2013 John Maddock and Christopher Kormanyos<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>) |
| </p> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="limits/how_to_tell.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tut.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../ref.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
