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<title>Logging floating point type numbers</title>
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</a><b>Logging floating point type numbers</b>
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<td><div class="spirit-nav">
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<a href="BOOST_TEST_PASSPOINT.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a href="log-hr-format.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="titlepage"><div><div><h6 class="title">
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<a name="utf.user-guide.test-output.log.FPT"></a>Logging floating point type numbers</h6></div></div></div>
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<p class="first-line-indented">
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It may appear that floating-point numbers are displayed by the <acronym class="acronym">UTF</acronym> with an excessive number of decimal digits.
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However the number of digits shown is chosen to avoid apparently nonsensical displays like <code class="computeroutput">[1.00000 != 1.00000]</code>
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when comparing exactly unity against a value which is increased by just one least significant binary digit using
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the default precision for float of just 6 decimal digits, given by
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<code class="computeroutput">std::numeric_limits</code><float>::digits10. The function used for the number of decimal
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digits displayed is that proposed for a future C++ Standard,
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<a href="http://www2.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1822.pdf" target="_top">A Proposal to add a max
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significant decimal digits value</a>, to be called <code class="computeroutput">std::numeric_limits</code>::max_digits10();.
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For 32-bit floats, 9 decimal digits are needed to ensure a single bit change produces a different decimal digit
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string.
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</p>
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<p class="first-line-indented">
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So a much more helpful display using 9 decimal digits is thus:
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<code class="computeroutput">[1.00000000 != 1.00000012]</code> showing that the two values are in fact different.
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</p>
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<p class="first-line-indented">
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For <acronym class="acronym">IEEE754</acronym> 32-bit float values - 9 decimal digits are shown. For 64-bit <acronym class="acronym">IEEE754</acronym> double - 17 decimal digits. For
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<acronym class="acronym">IEEE754</acronym> extended long double using 80-bit - 21 decimal digits. For <acronym class="acronym">IEEE754</acronym> quadruple long double 128-bit, and Sparc
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extended long double 128-bit - 36 decimal digits. For floating-point types, a convenient formula to calculate
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max_digits10 is: 2 + <code class="computeroutput">std::numeric_limits</code><FPT>::digits * 3010/10000;
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<div class="note"><table border="0" summary="Note">
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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Note that a user defined floating point type UDFPT must define
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<code class="computeroutput">std::numeric_limits</code><UDFPT>::is_specialized = true and provide an appropriate value
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for <code class="computeroutput">std::numeric_limits</code><UDFPT>::digits, the number of bits used for the significand
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or mantissa. For example, for the Sparc extended long double 128, 113 bits are used for the significand (one of
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which is implicit).
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