mod on %RANDOM Post: 302272025

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Top Forums Shell Programming and Scripting mod on %RANDOM Post 302272025 by Perderabo on Monday 29th of December 2008 10:29:11 AM

12-29-2008

Administrator Emeritus

When you use a modulus operation you are selecting information from the low order bits of a number and discarding information from the high order bits.

"these bits should be extracted from the most significant (left-hand) part
of the computer word, since the least significant bits produced by many random number generators are not sufficiently random."

and

"The least significant (right-hand) digits of X are not very random, so decisions based on the number X should always be influenced primarily by the most significant digits. It is generally best to think of X as a random fraction X/m between 0 and 1, that is, to visualize X with a decimal point at its left, rather than to regard X as a random integer between 0 and m - 1. To compute a random integer between 0 and k - 1, one should multiply by k and truncate the result."
both from The Art of Computer Programming, Volume 2 Seminumerical Algorithms by Donald Knuth

While neither is perfect, assuming that 0 <= RANDOM <= 32767,
((myrandom = RANDOM * 50 / 32768))
will behave better than
((myrandom = RANDOM % 50))

Perderabo

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LEARN ABOUT REDHAT

srandom

RANDOM(3)						     Linux Programmer's Manual							 RANDOM(3)

NAME

       random, srandom, initstate, setstate - random number generator.

SYNOPSIS

       #include <stdlib.h>

       long int random(void);
       void srandom(unsigned int seed);
       char *initstate(unsigned int seed, char *state, size_t n);
       char *setstate(char *state);

DESCRIPTION

       The  random()  function	uses  a non-linear additive feedback random number generator employing a default table of size 31 long integers to
       return successive pseudo-random numbers in the range from 0 to RAND_MAX.  The period of this random number generator is very large, approx-
       imately 16*((2**31)-1).

       The  srandom()  function  sets  its  argument  as  the seed for a new sequence of pseudo-random integers to be returned by random().  These
       sequences are repeatable by calling srandom() with the same seed value.	If no seed value is provided, the random() function  is  automati-
       cally seeded with a value of 1.

       The  initstate() function allows a state array state to be initialized for use by random().  The size of the state array n is used by init-
       state() to decide how sophisticated a random number generator it should use -- the larger the state array, the better  the  random  numbers
       will  be.   seed  is  the  seed	for  the initialization, which specifies a starting point for the random number sequence, and provides for
       restarting at the same point.

       The setstate() function changes the state array used by the random() function.  The state array state is used for random number	generation
       until  the next call to initstate() or setstate().  state must first have been initialized using initstate() or be the result of a previous
       call of setstate().

RETURN VALUE

       The random() function returns a value between 0 and RAND_MAX.  The srandom() function returns no value.	 The  initstate()  and	setstate()
       functions return a pointer to the previous state array, or NULL on error.

ERRORS

       EINVAL A state array of less than 8 bytes was specified to initstate().

NOTES

       Current	"optimal"  values  for	the size of the state array n are 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to the
       nearest known amount.  Using less than 8 bytes will cause an error.

CONFORMING TO

       BSD 4.3

SEE ALSO

       rand(3), srand(3)

GNU
								    2000-08-20								 RANDOM(3)