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193 lines
9.3 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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<!-- This manual describes how to install and use the GNU multiple precision
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arithmetic library, version 6.1.0.
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Copyright 1991, 1993-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document under
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the terms of the GNU Free Documentation License, Version 1.3 or any later
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version published by the Free Software Foundation; with no Invariant Sections,
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with the Front-Cover Texts being "A GNU Manual", and with the Back-Cover
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Texts being "You have freedom to copy and modify this GNU Manual, like GNU
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software". A copy of the license is included in
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GNU Free Documentation License. -->
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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>Initializing Floats (GNU MP 6.1.0)</title>
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<meta name="description" content="How to install and use the GNU multiple precision arithmetic library, version 6.1.0.">
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<meta name="keywords" content="Initializing Floats (GNU MP 6.1.0)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="Floating_002dpoint-Functions.html#Floating_002dpoint-Functions" rel="up" title="Floating-point Functions">
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<link href="Assigning-Floats.html#Assigning-Floats" rel="next" title="Assigning Floats">
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<link href="Floating_002dpoint-Functions.html#Floating_002dpoint-Functions" rel="prev" title="Floating-point Functions">
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<style type="text/css">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Initializing-Floats"></a>
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<div class="header">
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<p>
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Next: <a href="Assigning-Floats.html#Assigning-Floats" accesskey="n" rel="next">Assigning Floats</a>, Previous: <a href="Floating_002dpoint-Functions.html#Floating_002dpoint-Functions" accesskey="p" rel="prev">Floating-point Functions</a>, Up: <a href="Floating_002dpoint-Functions.html#Floating_002dpoint-Functions" accesskey="u" rel="up">Floating-point Functions</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Initialization-Functions-1"></a>
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<h3 class="section">7.1 Initialization Functions</h3>
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<a name="index-Float-initialization-functions"></a>
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<a name="index-Initialization-functions-3"></a>
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<dl>
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<dt><a name="index-mpf_005fset_005fdefault_005fprec"></a>Function: <em>void</em> <strong>mpf_set_default_prec</strong> <em>(mp_bitcnt_t <var>prec</var>)</em></dt>
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<dd><p>Set the default precision to be <strong>at least</strong> <var>prec</var> bits. All
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subsequent calls to <code>mpf_init</code> will use this precision, but previously
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initialized variables are unaffected.
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</p></dd></dl>
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<dl>
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<dt><a name="index-mpf_005fget_005fdefault_005fprec"></a>Function: <em>mp_bitcnt_t</em> <strong>mpf_get_default_prec</strong> <em>(void)</em></dt>
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<dd><p>Return the default precision actually used.
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</p></dd></dl>
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<p>An <code>mpf_t</code> object must be initialized before storing the first value in
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it. The functions <code>mpf_init</code> and <code>mpf_init2</code> are used for that
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purpose.
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</p>
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<dl>
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<dt><a name="index-mpf_005finit"></a>Function: <em>void</em> <strong>mpf_init</strong> <em>(mpf_t <var>x</var>)</em></dt>
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<dd><p>Initialize <var>x</var> to 0. Normally, a variable should be initialized once only
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or at least be cleared, using <code>mpf_clear</code>, between initializations. The
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precision of <var>x</var> is undefined unless a default precision has already been
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established by a call to <code>mpf_set_default_prec</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-mpf_005finit2"></a>Function: <em>void</em> <strong>mpf_init2</strong> <em>(mpf_t <var>x</var>, mp_bitcnt_t <var>prec</var>)</em></dt>
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<dd><p>Initialize <var>x</var> to 0 and set its precision to be <strong>at least</strong>
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<var>prec</var> bits. Normally, a variable should be initialized once only or at
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least be cleared, using <code>mpf_clear</code>, between initializations.
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</p></dd></dl>
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<dl>
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<dt><a name="index-mpf_005finits"></a>Function: <em>void</em> <strong>mpf_inits</strong> <em>(mpf_t <var>x</var>, ...)</em></dt>
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<dd><p>Initialize a NULL-terminated list of <code>mpf_t</code> variables, and set their
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values to 0. The precision of the initialized variables is undefined unless a
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default precision has already been established by a call to
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<code>mpf_set_default_prec</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-mpf_005fclear"></a>Function: <em>void</em> <strong>mpf_clear</strong> <em>(mpf_t <var>x</var>)</em></dt>
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<dd><p>Free the space occupied by <var>x</var>. Make sure to call this function for all
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<code>mpf_t</code> variables when you are done with them.
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</p></dd></dl>
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<dl>
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<dt><a name="index-mpf_005fclears"></a>Function: <em>void</em> <strong>mpf_clears</strong> <em>(mpf_t <var>x</var>, ...)</em></dt>
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<dd><p>Free the space occupied by a NULL-terminated list of <code>mpf_t</code> variables.
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</p></dd></dl>
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<p>Here is an example on how to initialize floating-point variables:
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</p><div class="example">
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<pre class="example">{
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mpf_t x, y;
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mpf_init (x); /* use default precision */
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mpf_init2 (y, 256); /* precision <em>at least</em> 256 bits */
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…
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/* Unless the program is about to exit, do ... */
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mpf_clear (x);
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mpf_clear (y);
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}
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</pre></div>
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<p>The following three functions are useful for changing the precision during a
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calculation. A typical use would be for adjusting the precision gradually in
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iterative algorithms like Newton-Raphson, making the computation precision
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closely match the actual accurate part of the numbers.
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</p>
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<dl>
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<dt><a name="index-mpf_005fget_005fprec"></a>Function: <em>mp_bitcnt_t</em> <strong>mpf_get_prec</strong> <em>(const mpf_t <var>op</var>)</em></dt>
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<dd><p>Return the current precision of <var>op</var>, in bits.
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</p></dd></dl>
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<dl>
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<dt><a name="index-mpf_005fset_005fprec"></a>Function: <em>void</em> <strong>mpf_set_prec</strong> <em>(mpf_t <var>rop</var>, mp_bitcnt_t <var>prec</var>)</em></dt>
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<dd><p>Set the precision of <var>rop</var> to be <strong>at least</strong> <var>prec</var> bits. The
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value in <var>rop</var> will be truncated to the new precision.
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</p>
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<p>This function requires a call to <code>realloc</code>, and so should not be used in
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a tight loop.
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</p></dd></dl>
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<dl>
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<dt><a name="index-mpf_005fset_005fprec_005fraw"></a>Function: <em>void</em> <strong>mpf_set_prec_raw</strong> <em>(mpf_t <var>rop</var>, mp_bitcnt_t <var>prec</var>)</em></dt>
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<dd><p>Set the precision of <var>rop</var> to be <strong>at least</strong> <var>prec</var> bits,
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without changing the memory allocated.
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</p>
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<p><var>prec</var> must be no more than the allocated precision for <var>rop</var>, that
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being the precision when <var>rop</var> was initialized, or in the most recent
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<code>mpf_set_prec</code>.
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</p>
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<p>The value in <var>rop</var> is unchanged, and in particular if it had a higher
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precision than <var>prec</var> it will retain that higher precision. New values
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written to <var>rop</var> will use the new <var>prec</var>.
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</p>
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<p>Before calling <code>mpf_clear</code> or the full <code>mpf_set_prec</code>, another
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<code>mpf_set_prec_raw</code> call must be made to restore <var>rop</var> to its original
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allocated precision. Failing to do so will have unpredictable results.
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</p>
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<p><code>mpf_get_prec</code> can be used before <code>mpf_set_prec_raw</code> to get the
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original allocated precision. After <code>mpf_set_prec_raw</code> it reflects the
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<var>prec</var> value set.
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</p>
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<p><code>mpf_set_prec_raw</code> is an efficient way to use an <code>mpf_t</code> variable at
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different precisions during a calculation, perhaps to gradually increase
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precision in an iteration, or just to use various different precisions for
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different purposes during a calculation.
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</p></dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Assigning-Floats.html#Assigning-Floats" accesskey="n" rel="next">Assigning Floats</a>, Previous: <a href="Floating_002dpoint-Functions.html#Floating_002dpoint-Functions" accesskey="p" rel="prev">Floating-point Functions</a>, Up: <a href="Floating_002dpoint-Functions.html#Floating_002dpoint-Functions" accesskey="u" rel="up">Floating-point Functions</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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