DRAND48(S) XENIX System V DRAND48(S)
Name
drand48, erand48, lrand48, nrand48, mrand48, jrand48,
srand48, seed48, lcong48 - Generates 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];
See Also
rand(S)
Description
This family of functions generates pseudo-random numbers
using the well-known linear congruential algorithm and 48-
bit integer arithmetic.
The 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 long31
integers uniformly distributed over the interval [0, 2 ].
Functions mrand48 and jrand48 return signed31ong31ntegers
uniformly distributed over the interval [-2 , 2 ].
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
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DRAND48(S) XENIX System V DRAND48(S)
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
require an initialization entry point to be called first.
All the routines work by generating a sequence of 48-bit
integer values, $Xi, according to the linear congruential
formula
Xn+1 = (aXn+c)mod m n_0.
48;
The parameter is m=2 thus, 48-bit integer arithmetic is
performed. Unless lcong48 has been invoked, the multiplier
value a and the addend value c are given by:
a = 5DEECE66D16 = 2736731631558
c = B16 = 138.
The value returned by any of the functions drand48, erand48,
lrand48, nrand48, mrand48 or jrand48 is computed by first
generating the next 48-bit Xi 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 Xi and transformed into the returned
value.
The functions drand48, lrand48 and mrand48 store the last
48-bit Xi 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 Xi 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 Xi 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 Xi to the 32 bits contained in its argument. The low-
order 16 bits of Xi are set to the arbitrary value 330E16.
The initializer function seed48 sets the value of Xi to the
48-bit value specified in the argument array. In addition,
the previous value of Xi 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
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DRAND48(S) XENIX System V DRAND48(S)
to be restarted from a given point at some future time - use
the pointer to get at and store the last Xi 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 Xi, the multiplier value $a, and the
addend value c. Argument array elements param[0-2] specify
Xi, 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.
See Also
rand(S)
Notes
These routines are coded in portable C. The source code for
the portable version can even be used on computers which do
not support floating-point arithmetic. In such a situation,
functions drand48 and erand48 do not exist; instead, they
are replaced by two new functions shown 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].
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