DRAND48(3C) DOMAIN/IX Reference Manual (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
31
integers uniformly distributed over the interval [0, 2 ).
Functions mrand48 and jrand48 return signed long integers
31 31
uniformly distributed over the interval [-2 , 2 ).
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 initializion values automati-
cally if your program calls drand48, lrand48 or mrand48
without calling an initialization entry point, but we recom-
mend that you specify one explicitly.) Functions erand48,
Printed 5/31/85 DRAND48-1
DRAND48(3C) DOMAIN/IX Reference Manual (SYS5) DRAND48(3C)
nrand48 and jrand48 do not require an initialization entry
point.
All of these routines work by generating a sequence of 48-
bit integer values, X , according to the linear congruential
-i
formula -
X = (aX +c) n>0.
-n+1 --n - mod m --
- - -
48
The parameter m=2 ; 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
- 16 8
c = B = 13 .
- 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
-i
numbe- of bits, according to the type of data item to be
returned, from the high-order (leftmost) bits of X and
-i
transforms them into the returned value. -
The functions drand48, lrand48 and mrand48 store the last
48-bit X generated in an internal buffer; that is why your
-i
program -ust initialize them before invoking them. The
functions erand48, nrand48 and jrand48 require the calling
program to provide storage for the successive X values in
-i
the array that is specified as an argument, at -he 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
-i
as an argument. By using dif-erent 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-
-i
orde- 16 bits of X are set to the arbitrary value 330E .
-i 16
-
The initializer function seed48 sets the value of X to the
-i
48-bit value specified in the argument array. In a-dition,
the previous value of X is copied into a 48-bit internal
-i
buffer, used only by se-d48, 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
-i
-
DRAND48-2 Printed 5/31/85
DRAND48(3C) DOMAIN/IX Reference Manual (SYS5) DRAND48(3C)
use this value to reinitialize via seed48 when the program
is restarted.
The initialization function lcong48 allows the user to
specify the initial X , the multiplier value a, and the
-i -
addend value c. Argum-nt array elements param[0-2] specify
-
X , param[3-5] specify the multiplier a, and param[6] speci-
-i -
f-es 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.
RELATED INFORMATION
rand(3C).
Printed 5/31/85 DRAND48-3