drand48(3C) drand48(3C)
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
lcong48  generate uniformly distributed pseudorandom numbers
#include <stdlib.h>
double drand48 (void);
double erand48 (unsigned short xsubi[3]);
long lrand48 (void);
long nrand48 (unsigned short xsubi[3]);
long mrand48 (void);
long jrand48 (unsigned short xsubi[3]);
void srand48 (long seedval);
unsigned short *seed48 (unsigned short seed16v[3]);
void lcong48 (unsigned short param[7]);
This family of functions generates pseudorandom numbers using the wellknown
linear congruential algorithm and 48bit integer arithmetic.
Functions drand48 and erand48 return nonnegative doubleprecision
floatingpoint values uniformly distributed over the interval [0.0,~1.0)
.
Functions lrand48 and nrand48 return nonnegative long integers uniformly
distributed over the
31
interval [0,~2 ).
Functions mrand48 and jrand48 return signed long integers uniformly
distributed over the interval
31 31
[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 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.
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drand48(3C) drand48(3C)
All the routines work by generating a sequence of 48bit integer values,
Xi, according to the linear congruential formula
Xn+1 = (aXn+c)mod m n>0.
48
The parameter m=2 ; hence 48bit 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
48bit Xi in the sequence. Then the appropriate number of bits,
according to the type of
data item to be returned, are copied from the highorder (leftmost) bits
of Xi and transformed into the returned value.
The functions drand48, lrand48, and mrand48 store the last 48bit Xi
generated in an internal buffer. Xi 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. These routines
do not have to be initialized; the calling program must 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 pseudorandom 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 highorder 32 bits of Xi to the
32 bits contained in its argument. The loworder 16 bits of Xi are set
to the arbitrary value 330E16.
The initializer function seed48 sets the value of Xi to the 48bit value
specified in the argument array. In addition, the previous value of Xi
is copied into a 48bit 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 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[02] specify Xi, param[35] specify the multiplier
a, and param[6] specifies the 16bit 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.
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drand48(3C) drand48(3C)
rand(3C), random(3B)
PPPPaaaaggggeeee 3333 [ Back ]
