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103 lines
3.9 KiB
HTML
103 lines
3.9 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>log2 (LIBM)</title>
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<meta name="description" content="log2 (LIBM)">
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<meta name="keywords" content="log2 (LIBM)">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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="logb.html#logb" rel="next" title="logb">
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<link href="log1p.html#log1p" rel="prev" title="log1p">
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</head>
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<body lang="en">
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<a name="log2"></a>
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<div class="header">
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<p>
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Next: <a href="logb.html#logb" accesskey="n" rel="next">logb</a>, Previous: <a href="log1p.html#log1p" accesskey="p" rel="prev">log1p</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="log2_002c-log2f_002d_002d_002dbase-2-logarithm"></a>
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<h3 class="section">1.37 <code>log2</code>, <code>log2f</code>—base 2 logarithm</h3>
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<a name="index-log2"></a>
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<a name="index-log2f"></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 log2(double <var>x</var>);
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float log2f(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>log2</code> functions compute the base-2 logarithm of <var>x</var>. A domain error
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occurs if the argument is less than zero. A range error occurs if the
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argument is zero.
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</p>
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<p>The Newlib implementations are not full, intrinisic calculations, but
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rather are derivatives based on <code>log</code>. (Accuracy might be slightly off from
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a direct calculation.) In addition to functions, they are also implemented as
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macros defined in math.h:
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</p><div class="smallexample">
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<pre class="smallexample"> #define log2(x) (log (x) / _M_LN2)
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#define log2f(x) (logf (x) / (float) _M_LN2)
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</pre></div>
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<p>To use the functions instead, just undefine the macros first.
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</p>
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<br>
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<p><strong>Returns</strong><br>
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The <code>log2</code> functions return
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<code>log base-2(<var>x</var>)</code>
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on success.
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When <var>x</var> is zero, the
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returned value is <code>-HUGE_VAL</code> and <code>errno</code> is set to <code>ERANGE</code>.
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When <var>x</var> is negative, the returned value is NaN (not a number) and
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<code>errno</code> is set to <code>EDOM</code>.
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</p>
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<br>
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<p><strong>Portability</strong><br>
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C99, POSIX, System V Interface Definition (Issue 6).
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</p>
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<br>
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</body>
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</html>
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