Re: Random Number Question

[Michael Black <et472@ncf.ca>]

On Mon, 5 May 2008, Bryan Parkoff wrote:

> Hello folks,
>
>    Do you know how random number work?  You use either Applesoft BASIC or
> Integer BASIC.  You use the function like RND().  Take a look at my example
> here.
>
> 10 PRINT INT( RND( 10 ) * 10)
> 20 GET A$
> 30 GOTO 10
>
>    Apple II has chosen the random number of the range from 0 to 9 and print
> each random number to the screen.  How do Apple II know to guess random
> number?  Take a look at monitor routine $FD1B.  $4E and $4F are the high
> byte of random and low byte of random.  It waits for the key to be pressed
> while random number continues to increment.  If key is pressed, random
> number stops to increment before it is printed to the screen.  Repeat the
> procedure to increment random number while it waits for a key.
>    Do you know the alternative implementation rather than keyin?  Do you
> think to implement the clock to replace keyin?  The random number stops to
> increment each one second while clock reads one second and another.  What do
> you think?
If it's just counting until a keypress, that's your randomness.  One of
the odd things about random is that it is indeed random.  There's no
pattern to it. Just because you may get similar numbers in a row doesn't
mean it's not random.

One method often used is to sample some random source, like white noise.
The white noise is random, the sampling is clocked at a regular period.

That keyboard method, assuming I'm reading you right, is the reverse,
the source is a clock that's always counting, but when you press keys
is a fairly random thing and that gives the randomness.

The two methods are basically the same thing.

But since a white noise source isn't something you normally have on
hand, a lot of times equipment will have a pseudo-random output.  That's
done by clocking a shift register and having feedback at the right points.
But unlike the previous methods, it is merely pseudo-random.  Start it
up, and the output will always be the same as the last time it was
started.  Make the shift register long enough, and it will take a long
time before the pattern repeats, and thus it often is "random enough".


  Michael