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105 lines
4.2 KiB
HTML
105 lines
4.2 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>logb (LIBM)</title>
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<meta name="description" content="logb (LIBM)">
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<meta name="keywords" content="logb (LIBM)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Document-Index.html#Document-Index" rel="index" title="Document Index">
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<link href="Document-Index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Math.html#Math" rel="up" title="Math">
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<link href="lrint.html#lrint" rel="next" title="lrint">
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<link href="log2.html#log2" rel="prev" title="log2">
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</head>
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<body lang="en">
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<a name="logb"></a>
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<div class="header">
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<p>
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Next: <a href="lrint.html#lrint" accesskey="n" rel="next">lrint</a>, Previous: <a href="log2.html#log2" accesskey="p" rel="prev">log2</a>, Up: <a href="Math.html#Math" accesskey="u" rel="up">Math</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="logb_002c-logbf_002d_002d_002dget-exponent-of-floating_002dpoint-number"></a>
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<h3 class="section">1.38 <code>logb</code>, <code>logbf</code>—get exponent of floating-point number</h3>
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<a name="index-logb"></a>
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<a name="index-logbf"></a>
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<p><strong>Synopsis</strong>
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</p><div class="example">
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<pre class="example">#include <math.h>
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double logb(double <var>x</var>);
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float logbf(float <var>x</var>);
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</pre></div>
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<p><strong>Description</strong><br>
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The <code>logb</code> functions extract the exponent of <var>x</var>, as a signed integer value
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in floating-point format. If <var>x</var> is subnormal it is treated as though it were
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normalized; thus, for positive finite <var>x</var>,
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1 <= (<var>x</var> * FLT_RADIX to the power (-logb(<var>x</var>))) < FLT_RADIX.
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A domain error may occur if the argument is zero.
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In this floating-point implementation, FLT_RADIX is 2. Which also means
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that for finite <var>x</var>, <code>logb</code>(<var>x</var>) = <code>floor</code>(<code>log2</code>(<code>fabs</code>(<var>x</var>))).
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</p>
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<p>All nonzero, normal numbers can be described as
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<var>m</var> * 2**<var>p</var>, where 1.0 <= <var>m</var> < 2.0.
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The <code>logb</code> functions examine the argument <var>x</var>, and return <var>p</var>.
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The <code>frexp</code> functions are similar to the <code>logb</code> functions, but
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returning <var>m</var> adjusted to the interval [.5, 1) or 0, and <var>p</var>+1.
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</p>
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<br>
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<p><strong>Returns</strong><br>
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When <var>x</var> is:<br>
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+inf or -inf, +inf is returned;<br>
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NaN, NaN is returned;<br>
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0, -inf is returned, and the divide-by-zero exception is raised;<br>
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otherwise, the <code>logb</code> functions return the signed exponent of <var>x</var>.
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</p>
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<br>
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<p><strong>Portability</strong><br>
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ANSI C, POSIX
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</p>
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<br>
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<p><strong>See Also</strong><br>
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frexp, ilogb
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</p>
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<br>
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</body>
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</html>
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