DRAND48(3C-SVR3)    RISC/os Reference Manual     DRAND48(3C-SVR3)



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

SYNOPSIS
     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-SVR3)    RISC/os Reference Manual     DRAND48(3C-SVR3)



     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 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 by first
     generating the next 48-bit X  in the sequence.  Then the
     appropriate number of bits, a
iccording to the type of data
     item to be returned, are copied from the high-order (left-
     most) 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, and must be ini-
     tializediprior 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 funci
tions are invoked.  These rou-
     tines do not have to be initialized; the calling program
     must place the desired initial value of X  into the array
     and pass 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
     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 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 i
when the program
     is restarted.



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DRAND48(3C-SVR3)    RISC/os Reference Manual     DRAND48(3C-SVR3)



     The initialization function lcong48 allows the user to
     specify the initial X , the multiplier value a, and the
     addend value c.  Argui
ment 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, speci-
     fied on the previous page.

NOTES
     The source code for the portable version can be used on com-
     puters which do not have floating-point arithmetic.  In such
     a situation, functions drand48 and erand48 are replaced by
     the two new functions below.

     long irand48 (m)
     unsigned short m;

     long krand48 (xsubi, m)
     unsigned short xsubi[3], m;

     Functions irand48 and krand48 return non-negative long
     integers uniformly distributed over the interval (0, m-1).

SEE ALSO
     rand(3C).





























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