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$RANDOM not random in 2.05.0(1)-release


From: David Forrest
Subject: $RANDOM not random in 2.05.0(1)-release
Date: Tue, 26 Mar 2002 16:16:54 -0500 (EST)

    while true; do echo -n " $(( ( RANDOM >> 5) % 8 ))"; done

Shows a repeat interval of 256 (look for 6 6 0 0 0 1 1)

    while true; do
       R=$RANDOM
       R=$(( ( RANDOM >> 5) % 8 ))
       echo -n " $R"
    done

Shows the same

  Although the basic PRNG is fairly good (period 2^31-1), and BASH strips
off the low order bits already, the net result that appears in $RANDOM is
horrid for some reason.

### from bash-2.05a/variables.c ######################################
/* A linear congruential random number generator based on the example
   on in the ANSI C standard.  This one isn't very good, but a more
   complicated one is overkill. */

/* Returns a pseudo-random number between 0 and 32767. */
static int
brand ()
{
  rseed = rseed * 1103515245 + 12345;
  return ((unsigned int)((rseed >> 16) & 32767));       /* was % 32768 */
}
#######################################################################

#bash shell script equivalent:
RS=$(( ( RS * 1103515245 +12345  ) )) ; R=$(( (RS >>16 ) & 32767 ))

# A well tested alternate: The Park & Miller Minimum Standard RNG
RS=$(( ( RS * 16807 % 2147483647 ) )) ; R=$(( (RS >>16 ) & 32767 ))


RS=$RANDOM;
while true; do
  RS=$(( ( RS * 1103515245 +12345 ) ))
  R=$((  ( RS >> 16 ) & 32767 ))
  echo -n " $(( (R >>5) % 8))"
done

 This bash code should match what is in the current bash source for the
test above, but is much more reliable for some reason.  Why is the $RANDOM
variable so bad?

Dave.
-- 
 Dave Forrest                                   address@hidden
 (434)296-7283h 924-3954w      http://mug.sys.virginia.edu/~drf5n/




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