drand48(3)
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drand48, erand48, lrand48, nrand48, mrand48, Subroutine
jrand48, srand48, seed48, lcong48
generate uniformly distributed pseudo-random numbers
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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];
DESCRIPTION
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, 231).
Functions mrand48 and jrand48 return signed long integers
uniformly distributed over the interval [-231, 231).
Functions srand48, seed48 and lcong48 are initialization entry
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drand48(3)
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.
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.
The parameter m=248; hence 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, and 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 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
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drand48(3)
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[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.
NOTES
The source code for the portable version can be used on computers
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].
SEE ALSO
rand(3C).
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