DRAND48(3C) INTERACTIVE UNIX System DRAND48(3C)
NAME
drand48, erand48, lrand48, nrand48, mrand48, jrand48,
srand48, seed48, lcong48 - generate uniformly distributed
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
delim $$ 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 $[0,~2 sup
31 ).$
Functions mrand48 and jrand48 return signed long integers
uniformly distributed over the interval $[-2 sup 31 ,~2 sup
31 ).$
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
Rev. C Software Development Set Page 1
DRAND48(3C) INTERACTIVE UNIX System DRAND48(3C)
entry point.) Functions erand48, nrand48, and jrand48 do
not require an initialization entry point to be called
first.
All the routines work by generating a sequence of 48-bit
integer values, $X sub i ,$ according to the linear
congruential formula
X sub{n+1}~=~(aX sub n^+^c) sub{roman mod~m}~~~~~~~~n>=0.
Rev. C Software Development Set Page 2
DRAND48(3C) INTERACTIVE UNIX System DRAND48(3C)
The parameter $m^=^2 sup 48$; hence 48-bit integer arith-
metic is performed. Unless lcong48 has been invoked, the
multiplier value $a$ and the addend value $c$ are given by
a~mark =~roman 5DEECE66D^sub 16~=~roman
273673163155^sub 8
c~lineup =~roman B^sub 16~=~roman 13^sub 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 $X sub i$ 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 $X sub i$ and transformed into the
returned value.
The functions drand48, lrand48, and mrand48 store the last
48-bit $X sub i$ generated in an internal buffer, and must
be initialized prior to being invoked. The functions
erand48, nrand48, and jrand48 require the calling program to
provide storage for the successive $X sub i$ values in the
array specified as an argument when the functions are
invoked. These routines do not have to be initialized; the
calling program must place the desired initial value of $X
sub i$ 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 $X sub i$ to the 32 bits contained in its argument. The
low-order 16 bits of $X sub i$ are set to the arbitrary
value $roman 330E sub 16 .$
The initializer function seed48 sets the value of $X sub i$
to the 48-bit value specified in the argument array. In
addition, the previous value of $X sub i$ 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
$X sub i$ value, and then use this value to reinitialize via
seed48 when the program is restarted.
The initialization function lcong48 allows the user to
specify the initial $X sub i ,$ the multiplier value $a,$
and the addend value $c.$ Argument array elements param[0-2]
specify $X sub i ,$ param[3-5] specify the multiplier $a,$
and param[6] specifies the 16-bit addend $c.$ After lcong48
Rev. C Software Development Set Page 3
DRAND48(3C) INTERACTIVE UNIX System DRAND48(3C)
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.
SEE ALSO
rand(3C).
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 $[0,~m-1
]$. delim off
Rev. C Software Development Set Page 4