


      DRAND48(3C)		  (UNIX	5.0)		    DRAND48(3C)



      NAME
	   drand48, erand48, lrand48, nrand48, mrand48,	jrand48,
	   srand48, seed48, lcong48 - generate uniformly distributed
	   pseudo-random numbers

      SYNTAX
	   double drand48 ( )

	   double erand48 (xsubi)
	   unsigned short xsubi[3];

	   long	lrand48	( )

	   long	nrand48	(xsubi)
	   unsigned short xsubi[3];

	   long	mrand48	( )

	   long	jrand48	(xsubi)
	   unsigned short xsubi[3];

	   void	srand48	(seedval)
	   long	seedval;

	   unsigned short *seed48 (seed16v)
	   unsigned short seed16v[3];

	   void	lcong48	(param)
	   unsigned short param[7];

      DESCRIPTION
	   This	family of functions generates pseudo-random numbers
	   using the well-known	linear congruential algorithm and 48-
	   bit integer arithmetic.

	   Functions drand48 and erand48 return	non-negative double-
	   precision floating-point values uniformly distributed over
	   the interval	[0.0, 1.0).

	   Functions lrand48 and nrand48 return	non-negative long
	   integers uniformly distributed over the interval [0,	231).

	   Functions mrand48 and jrand48 return	signed long integers
	   uniformly distributed over the interval [-231, 231).

	   Functions srand48, seed48 and lcong48 are initialization
	   entry points, one of	which should be	invoked	before either
	   drand48, lrand48 or mrand48 is called.  (Although it	is not
	   recommended practice, constant default initializer values
	   will	be supplied automatically if drand48, lrand48 or
	   mrand48 is called without a prior call to an	initialization
	   entry point.) Functions erand48, nrand48 and	jrand48	do not



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      DRAND48(3C)		  (UNIX	5.0)		    DRAND48(3C)



	   require an initialization entry point to be called first.

	   All the routines work by generating a sequence of 48-bit
	   integer values, X , according to the	linear congruential
	   formula	    i

			 X    =	(aX +c)		    n>0.
			  n+1	   n   mod m
	   The parameter m=248;	hence 48-bit integer arithmetic	is
	   performed.  Unless lcong48 has been invoked,	the multiplier
	   value a and the addend value	c are given by

		 a = 5DEECE66D	 = 273673163155
		 c = B	 = 13 .6	       8
		      16     8
	   The value returned by any of	the functions drand48, erand48,
	   lrand48, nrand48, mrand48 or	jrand48	is computed by first
	   generating the next 48-bit X	 in the	sequence.  Then	the
	   appropriate number of bits, according to the	type of	data
	   item	to be returned,	are copied from	the high-order
	   (leftmost) bits of X	 and transformed into the returned
	   value.	       i

	   The functions drand48, lrand48 and mrand48 store the	last
	   48-bit X  generated in an internal buffer; that is why they
	   must	be initialized prior to	being invoked.	The functions
	   erand48, nrand48 and	jrand48	require	the calling program to
	   provide storage for the successive X	 values	in the array
	   specified as	an argument when the functions are invoked.
	   That	is why these routines do not have to be	initialized;
	   the calling program merely has to place the desired initial
	   value of X  into the	array and pass it as an	argument.  By
	   using different arguments, functions	erand48, nrand48 and
	   jrand48 allow separate modules of a large program to
	   generate several independent	streams	of pseudo-random
	   numbers, i.e., the sequence of numbers in each stream will
	   not depend upon how many times the routines have been called
	   to generate numbers for the other streams.

	   The initializer function srand48 sets the high-order	32 bits
	   of X	 to the	32 bits	contained in its argument.  The	low-
	   order 16 bits of X  are set to the arbitrary	value 330E  .
			     i					  16
	   The initializer function seed48 sets	the value of X	to the
	   48-bit value	specified in the argument array.  In addition,
	   the previous	value of X  is copied into a 48-bit internal
	   buffer, used	only by	seed48,	and a pointer to this buffer is
	   the value returned by seed48. This returned pointer,	which
	   can just be ignored if not needed, is useful	if a program is
	   to be restarted from	a given	point at some future time - use
	   the pointer to get at and store the last X  value, and then
	   use this value to reinitialize via seed48 when the program



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      DRAND48(3C)		  (UNIX	5.0)		    DRAND48(3C)



	   is restarted.

	   The initialization function lcong48 allows the user to
	   specify the initial X , the multiplier value	a, and the
	   addend value	c. Argument array elements param[0-2] specify
	   X , param[3-5] specify the multiplier a, and	param[6]
	   specifies the 16-bit	addend c. After	lcong48	has been
	   called, a subsequent	call to	either srand48 or seed48 will
	   restore the ``standard'' multiplier and addend values, a and
	   c, specified	on the previous	page.

      NOTES
	   The versions	of these routines for the Bull-SPS9 are	coded
	   in portable C.

      SEE ALSO
	   rand(3C).






































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