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rand48
The drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48,
seed48, and lcong48 functions generate 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];
Parameters
xsubi A pointer to a 3-word (48-bit) unsigned short int array
used by the random number generator to store successive
values of X.
seedval A 32-bit seed value used to initialize the high-order bits
of seed value to the random number generator
seed16v A pointer to a 3-word (48-bit) unsigned short int array
used internally by the random number generator to hold the
previous value of the seed.
param A pointer to a 7-word unsigned short int array arranged as
follows:
param[0-2] The 48-bit seed value.
param[3-5] The multiplier A used to expand the random
number from the 0 to 1 range to the desire
range.
param[6] The addend C used to shift the random number
from the 0 to 1 range to the desired range.
Return Values
x Random numbers appropriate to the type and function called
(except seed, which returns a pointer to the internal
buffer where X is stored).
Description
This family of functions generates uniform pseudo-random numbers using
the linear congruential algorithm and 48-bit integer arithmetic.
The drand48 and erand48 functions return non-negative double-precision
floating-point values uniformly distributed over the interval of 0.0
(inclusive) to 1.0 (non-inclusive) or, in mathematical nomenclature,
(0.0,1.0).
The lrand48 and nrand48 functions return non-negative long integers
uniformly distributed over the interval of 0 (inclusive) to 231
(non-inclusive), or (0, 231).
The mrand48 and jrand48 functions return signed long integers uniformly
distributed over the interval of -231 (inclusive) to 231 (non-inclusive),
or (-231, 231).
The srand48, seed48 and lcong48 functions 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 are supplied automatically if drand48, lrand48
or mrand48 is called without a prior call to an initialization entry
point. The erand48, nrand48, and jrand48 functions do not require an
initialization entry point to be called first.
All the functions 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.
The parameter m = 248; therefore 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 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 drand48, lrand48 and mrand48 functions store the last 48-bit Xi
generated in an internal buffer, which is why they must be initialized
prior to being invoked. The erand48, nrand48 and jrand48 functions
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 functions 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, the
erand48, nrand48 and jrand48 functions allow separate modules of a large
program to generate several independent streams of pseudo-random numbers.
For example, the sequence of numbers in each stream does not depend upon
how many times the functions 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 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 Xi value, and then use this value to
reinitialize using 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 is called,
a subsequent call to either srand48 or seed48 restores the standard
multiplier and addend values a and c, as specified previously.
See Also
rand()