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174 lines
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174 lines
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4 years ago
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<title>rand48 (The Red Hat newlib C Library)</title>
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<meta name="description" content="rand48 (The Red Hat newlib C Library)">
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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="Stdlib.html#Stdlib" rel="up" title="Stdlib">
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<body lang="en">
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<a name="rand48"></a>
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<div class="header">
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<p>
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Next: <a href="rpmatch.html#rpmatch" accesskey="n" rel="next">rpmatch</a>, Previous: <a href="random.html#random" accesskey="p" rel="prev">random</a>, Up: <a href="Stdlib.html#Stdlib" accesskey="u" rel="up">Stdlib</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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<hr>
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<a name="rand48_002c-drand48_002c-erand48_002c-lrand48_002c-nrand48_002c-mrand48_002c-jrand48_002c-srand48_002c-seed48_002c-lcong48_002d_002d_002dpseudo_002drandom-number-generators-and-initialization-routines"></a>
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<h3 class="section">2.35 <code>rand48</code>, <code>drand48</code>, <code>erand48</code>, <code>lrand48</code>, <code>nrand48</code>, <code>mrand48</code>, <code>jrand48</code>, <code>srand48</code>, <code>seed48</code>, <code>lcong48</code>—pseudo-random number generators and initialization routines</h3>
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<a name="index-rand48"></a>
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<a name="index-drand48"></a>
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<a name="index-erand48"></a>
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<a name="index-lrand48"></a>
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<a name="index-nrand48"></a>
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<a name="index-mrand48"></a>
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<a name="index-jrand48"></a>
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<a name="index-srand48"></a>
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<a name="index-seed48"></a>
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<a name="index-lcong48"></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 <stdlib.h>
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double drand48(void);
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double erand48(unsigned short <var>xseed</var>[3]);
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long lrand48(void);
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long nrand48(unsigned short <var>xseed</var>[3]);
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long mrand48(void);
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long jrand48(unsigned short <var>xseed</var>[3]);
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void srand48(long <var>seed</var>);
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unsigned short *seed48(unsigned short <var>xseed</var>[3]);
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void lcong48(unsigned short <var>p</var>[7]);
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</pre></div>
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<p><strong>Description</strong><br>
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The <code>rand48</code> family of functions generates pseudo-random numbers
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using a linear congruential algorithm working on integers 48 bits in size.
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The particular formula employed is
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r(n+1) = (a * r(n) + c) mod m
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where the default values are
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for the multiplicand a = 0xfdeece66d = 25214903917 and
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the addend c = 0xb = 11. The modulo is always fixed at m = 2 ** 48.
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r(n) is called the seed of the random number generator.
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</p>
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<p>For all the six generator routines described next, the first
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computational step is to perform a single iteration of the algorithm.
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</p>
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<p><code>drand48</code> and <code>erand48</code>
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return values of type double. The full 48 bits of r(n+1) are
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loaded into the mantissa of the returned value, with the exponent set
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such that the values produced lie in the interval [0.0, 1.0].
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</p>
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<p><code>lrand48</code> and <code>nrand48</code>
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return values of type long in the range
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[0, 2**31-1]. The high-order (31) bits of
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r(n+1) are loaded into the lower bits of the returned value, with
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the topmost (sign) bit set to zero.
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</p>
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<p><code>mrand48</code> and <code>jrand48</code>
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return values of type long in the range
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[-2**31, 2**31-1]. The high-order (32) bits of
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r(n+1) are loaded into the returned value.
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</p>
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<p><code>drand48</code>, <code>lrand48</code>, and <code>mrand48</code>
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use an internal buffer to store r(n). For these functions
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the initial value of r(0) = 0x1234abcd330e = 20017429951246.
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</p>
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<p>On the other hand, <code>erand48</code>, <code>nrand48</code>, and <code>jrand48</code>
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use a user-supplied buffer to store the seed r(n),
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which consists of an array of 3 shorts, where the zeroth member
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holds the least significant bits.
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</p>
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<p>All functions share the same multiplicand and addend.
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</p>
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<p><code>srand48</code> is used to initialize the internal buffer r(n) of
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<code>drand48</code>, <code>lrand48</code>, and <code>mrand48</code>
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such that the 32 bits of the seed value are copied into the upper 32 bits
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of r(n), with the lower 16 bits of r(n) arbitrarily being set to 0x330e.
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Additionally, the constant multiplicand and addend of the algorithm are
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reset to the default values given above.
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</p>
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<p><code>seed48</code> also initializes the internal buffer r(n) of
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<code>drand48</code>, <code>lrand48</code>, and <code>mrand48</code>,
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but here all 48 bits of the seed can be specified in an array of 3 shorts,
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where the zeroth member specifies the lowest bits. Again,
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the constant multiplicand and addend of the algorithm are
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reset to the default values given above.
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<code>seed48</code> returns a pointer to an array of 3 shorts which contains
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the old seed.
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This array is statically allocated, thus its contents are lost after
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each new call to <code>seed48</code>.
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</p>
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<p>Finally, <code>lcong48</code> allows full control over the multiplicand and
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addend used in <code>drand48</code>, <code>erand48</code>, <code>lrand48</code>, <code>nrand48</code>,
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<code>mrand48</code>, and <code>jrand48</code>,
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and the seed used in <code>drand48</code>, <code>lrand48</code>, and <code>mrand48</code>.
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An array of 7 shorts is passed as parameter; the first three shorts are
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used to initialize the seed; the second three are used to initialize the
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multiplicand; and the last short is used to initialize the addend.
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It is thus not possible to use values greater than 0xffff as the addend.
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</p>
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<p>Note that all three methods of seeding the random number generator
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always also set the multiplicand and addend for any of the six
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generator calls.
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</p>
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<p>For a more powerful random number generator, see <code>random</code>.
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</p>
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<br>
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<p><strong>Portability</strong><br>
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SUS requires these functions.
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</p>
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<p>No supporting OS subroutines are required.
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</p>
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<br>
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<hr>
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<div class="header">
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<p>
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Next: <a href="rpmatch.html#rpmatch" accesskey="n" rel="next">rpmatch</a>, Previous: <a href="random.html#random" accesskey="p" rel="prev">random</a>, Up: <a href="Stdlib.html#Stdlib" accesskey="u" rel="up">Stdlib</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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