[Robotgroup] Basic Stamp RANDOM

[Vern Graner]

Kevin Booth wrote:
> Is there anyway to get around the non-randomness of the RANDOM statement?
> Currently every time i use it i get the same result.
> 
> Please help =D

Everything you need to know by Jon Williams of Parallax/ Nuts & Volts/ 
EFX-TEK fame:

http://www.efx-tek.com/php/smf/index.php?topic=41.0

Excerpt from the above:

------------------- CLIP --------------------
One of the most compelling reasons to use a programmable controller like 
the Prop-1, Prop-2, or Prop-SX in your designs is a function of the 
respective languages called RANDOM.  To be fair, RANDOM isn't truly 
random, it is in fact pseudo-random.  What this means is that the 
algorithm used generates a seemingly random list of values each time 
it's called.  The result of the RANDOM function is wholly dependent on 
the seed value feed to it; same seed, same result.

That said, our props are often waiting for something to happen, perhaps 
a PIR or mat switch to detect a guest.  While waiting, we can 
continuously call RANDOM and thus end up with a truly random value; 
we're using random timing of the external event to drive the value. 
Here's what it looks like in code:

Main:
   RANDOM lottery
   IF PIR = No THEN Main

As you can see, the RANDOM function will be called until the PIR goes 
active, and that is a random event, hence we end up with a random value 
in lottery.

Just a note: When using RANDOM to drive LEDs to create a candle-flicker 
effect, you should stir the random value a few times before sending it 
to the LEDs.  The reason is that the algorithm used for RANDOM is called 
a Linear Feedback Shift Register, and while the decimal values look 
random, one can see the apparent linear motion of the bits when the 
output is connected to parallel LEDs.  The fix is easy:

Light_Candles:
   FOR idx = 1 TO 3
     RANDOM lottery
   NEXT
   Wicks = lottery
   PAUSE 25
   GOTO Light_Candles
------------------- /CLIP -------------------

There's a lot more discussion in the thread. :)

Vern

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