Random algorithm

[Dave Cragg]

On 13 Nov 2008, at 19:38, Richard Gaskin wrote:

> So unless I'm missing something obvious (and it certainly wouldn't  
> be the first time), beginning with a fresh seed as Rev does and then  
> resetting it each time during the session seems a fair way to avoid  
> discernible reproducible patterns for most applications.


Sorry for going back to this, but I'd just like to repeat my concern  
for resetting the seed using a "random" value.  I may have  
misunderstood what "resetting it each time" means. In some posts  
people have talked about resetting it before "each run" and others  
have talked about resetting before each call to the random function.  
My concern was with a particular "each run" situation.

Say the task is to produce 5000 sets of 5 random numbers from 1 to  
1000. (perhaps for a gaming task) The straightforward way to do this  
is to use the default Rev seed and call random(1000) 25000 times,  
dividing the results into 5000 sets of 5.

An alternative would be to reset the randomSeed before generating each  
set of five numbers. At first glance, this might seem like a  
reasonable thing to do. But depending on how the seed is set, it could  
produce unwanted results. In the second script below, the seed is  
reset using the random of an incremented number. (4570422 incremented  
by 1 to avoid getting stuck with the same seed)

The two scripts  measure the number of repeated sequences that are  
generated by the two methods. The first method has yet to produce a  
repeated sequence here. But the second produces repeated sequences on  
almost every run. So by resetting the seed for "each run", the  
occurrence of a repeated sequence changes from a highly improbable  
event to a fairly safe bet.


Cheers
Dave


METHOD 1 (no resetting)
on mouseUp

    repeat 5000
       put empty into tVal
       repeat 5
          put random(1000) & "," after tVal
       end repeat
       add 1 to tValArray[tVal]
    end repeat

    put 0 into tCount

    repeat for each element tEl in tValArray
       if tEl > 1 then
          add 1 to tCount
       end if
    end repeat
    put tCount
end mouseUp

METHOD 2 (reset on every "set")
on mouseUp
    -----------------------
    put 4570422 into tSeedBase
    repeat 5000
       put empty into tVal2
       add 1 to tSeedBase
       set the randomSeed to random(tSeedBase)
       repeat 5
          put random(1000) & "," after tVal2
       end repeat
       add 1 to tValArray2[tVal2]
    end repeat
    ---------------------------

    put 0 into tCount2

    repeat for each element tEl in tValArray2
       if tEl > 1 then
          add 1 to tCount2
       end if
    end repeat

    put tCount2

end mouseUp