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<title>Initializing Floats (GNU MP 6.1.0)</title>
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<a name="Initializing-Floats"></a>
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<p>
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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<hr>
<a name="Initialization-Functions-1"></a>
<h3 class="section">7.1 Initialization Functions</h3>
<a name="index-Float-initialization-functions"></a>
<a name="index-Initialization-functions-3"></a>
<dl>
<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>
<dd><p>Set the default precision to be <strong>at least</strong> <var>prec</var> bits. All
subsequent calls to <code>mpf_init</code> will use this precision, but previously
initialized variables are unaffected.
</p></dd></dl>
<dl>
<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>
<dd><p>Return the default precision actually used.
</p></dd></dl>
<p>An <code>mpf_t</code> object must be initialized before storing the first value in
it. The functions <code>mpf_init</code> and <code>mpf_init2</code> are used for that
purpose.
</p>
<dl>
<dt><a name="index-mpf_005finit"></a>Function: <em>void</em> <strong>mpf_init</strong> <em>(mpf_t <var>x</var>)</em></dt>
<dd><p>Initialize <var>x</var> to 0. Normally, a variable should be initialized once only
or at least be cleared, using <code>mpf_clear</code>, between initializations. The
precision of <var>x</var> is undefined unless a default precision has already been
established by a call to <code>mpf_set_default_prec</code>.
</p></dd></dl>
<dl>
<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>
<dd><p>Initialize <var>x</var> to 0 and set its precision to be <strong>at least</strong>
<var>prec</var> bits. Normally, a variable should be initialized once only or at
least be cleared, using <code>mpf_clear</code>, between initializations.
</p></dd></dl>
<dl>
<dt><a name="index-mpf_005finits"></a>Function: <em>void</em> <strong>mpf_inits</strong> <em>(mpf_t <var>x</var>, ...)</em></dt>
<dd><p>Initialize a NULL-terminated list of <code>mpf_t</code> variables, and set their
values to 0. The precision of the initialized variables is undefined unless a
default precision has already been established by a call to
<code>mpf_set_default_prec</code>.
</p></dd></dl>
<dl>
<dt><a name="index-mpf_005fclear"></a>Function: <em>void</em> <strong>mpf_clear</strong> <em>(mpf_t <var>x</var>)</em></dt>
<dd><p>Free the space occupied by <var>x</var>. Make sure to call this function for all
<code>mpf_t</code> variables when you are done with them.
</p></dd></dl>
<dl>
<dt><a name="index-mpf_005fclears"></a>Function: <em>void</em> <strong>mpf_clears</strong> <em>(mpf_t <var>x</var>, ...)</em></dt>
<dd><p>Free the space occupied by a NULL-terminated list of <code>mpf_t</code> variables.
</p></dd></dl>
<p>Here is an example on how to initialize floating-point variables:
</p><div class="example">
<pre class="example">{
mpf_t x, y;
mpf_init (x); /* use default precision */
mpf_init2 (y, 256); /* precision <em>at least</em> 256 bits */
…
/* Unless the program is about to exit, do ... */
mpf_clear (x);
mpf_clear (y);
}
</pre></div>
<p>The following three functions are useful for changing the precision during a
calculation. A typical use would be for adjusting the precision gradually in
iterative algorithms like Newton-Raphson, making the computation precision
closely match the actual accurate part of the numbers.
</p>
<dl>
<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>
<dd><p>Return the current precision of <var>op</var>, in bits.
</p></dd></dl>
<dl>
<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>
<dd><p>Set the precision of <var>rop</var> to be <strong>at least</strong> <var>prec</var> bits. The
value in <var>rop</var> will be truncated to the new precision.
</p>
<p>This function requires a call to <code>realloc</code>, and so should not be used in
a tight loop.
</p></dd></dl>
<dl>
<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>
<dd><p>Set the precision of <var>rop</var> to be <strong>at least</strong> <var>prec</var> bits,
without changing the memory allocated.
</p>
<p><var>prec</var> must be no more than the allocated precision for <var>rop</var>, that
being the precision when <var>rop</var> was initialized, or in the most recent
<code>mpf_set_prec</code>.
</p>
<p>The value in <var>rop</var> is unchanged, and in particular if it had a higher
precision than <var>prec</var> it will retain that higher precision. New values
written to <var>rop</var> will use the new <var>prec</var>.
</p>
<p>Before calling <code>mpf_clear</code> or the full <code>mpf_set_prec</code>, another
<code>mpf_set_prec_raw</code> call must be made to restore <var>rop</var> to its original
allocated precision. Failing to do so will have unpredictable results.
</p>
<p><code>mpf_get_prec</code> can be used before <code>mpf_set_prec_raw</code> to get the
original allocated precision. After <code>mpf_set_prec_raw</code> it reflects the
<var>prec</var> value set.
</p>
<p><code>mpf_set_prec_raw</code> is an efficient way to use an <code>mpf_t</code> variable at
different precisions during a calculation, perhaps to gradually increase
precision in an iteration, or just to use various different precisions for
different purposes during a calculation.
</p></dd></dl>
<hr>
<div class="header">
<p>
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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