DRAND48(3C)              DOMAIN/IX SYS5               DRAND48(3C)



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

USAGE
     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 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.  You should invoke one of these before your
     program calls either drand48, lrand48, or mrand48.  (These
     functions do supply default initialization values



Printed 12/4/86                                         DRAND48-1

DRAND48(3C)              DOMAIN/IX SYS5               DRAND48(3C)



     automatically if your program calls drand48, lrand48, or
     mrand48 without calling an initialization entry point, but
     we recommend that you specify one explicitly.) Functions
     erand48, nrand48, and jrand48 do not require an initializa-
     tion entry point.

     All of these 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 per-
     formed.  Unless lcong48 has been invoked, the multiplier
     value a and the addend value c are given by

           a = 5DEECE66D   = 273673163155
           c = B   = 13 .
16               8
                16     8
     The value returned by any of the functions drand48, erand48,
     lrand48, nrand48, mrand48 or jrand48 is computed in the fol-
     lowing fashion.  First, the function generates the next 48-
     bit X  in the sequence.  Then it copies the appropriate
     numbei
r of bits, according to the type of data item to be
     returned, from the high-order (leftmost) bits of X  and
     transforms them 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 your
     program i
must initialize them before invoking them.  The
     functions erand48, nrand48, and jrand48 require the calling
     program to provide storage for the successive X  values in
     the array that is specified as an argument, at i
the time that
     the functions are invoked.  That is why these routines do
     not have to be initialized; the calling program merely
     places the initial value of X  into the array and passes it
     as an argument.  By using diff
ierent 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-
     ordei
r 16 bits of X  are set to the arbitrary value 330E  .
                       i                                    16






DRAND48-2                                         Printed 12/4/86

DRAND48(3C)              DOMAIN/IX SYS5               DRAND48(3C)



     The initializer function seed48 sets the value of X  to the
     48-bit value specified in the argument array.  In ad
idition,
     the previous value of X  is copied into a 48-bit internal
     buffer, used only by see
id48, 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 i
when the program
     is restarted.

     The initialization function lcong48 allows the user to
     specify the initial X , the multiplier value a, and the
     addend value c. Argume
int array elements param[0-2] specify
     X , param[3-5] specify the multiplier a, and param[6] speci-
     fies 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.

RELATED INFORMATION
     rand(3C).































Printed 12/4/86                                         DRAND48-3