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 OPENSOLARIS

srand

rand(3UCB)					     SunOS/BSD Compatibility Library Functions						rand(3UCB)

NAME

       rand, srand - simple random number generator

SYNOPSIS

       /usr/ucb/cc [ flag ... ] file ...

       int rand();

       int srand(seed)
       unsigned seed;

DESCRIPTION

       rand()  uses a multiplicative congruential random number generator with period 2^32 to return successive pseudo-random numbers in the range
       from 0 to 2^31 -1.

       srand() can be called at any time to reset the random-number generator to a random starting point. The generator is initially seeded with a
       value of 1.

SEE ALSO

       cc(1B), drand48(3C), rand(3C), random(3C)

NOTES

       Use of these interfaces should be restricted to only applications written on BSD platforms.  Use of these interfaces with any of the system
       libraries or in multi-thread applications is unsupported.

       The spectral properties of rand() leave a great deal to be desired. drand48(3C) and random(3C) provide much better, though more	elaborate,
       random-number generators.

       The low bits of the numbers generated are not very random; use the middle bits. In particular the lowest bit alternates between 0 and 1.

SunOS 5.11							    30 Oct 2007 							rand(3UCB)