Random Number Generator in Machine Code

[g0blinish]

try rnd()mod 20

[andycadley]

Even RANDOMIZE TIME sometimes isn't good enough. Try this code to see what I mean:

Code Select Expand

10 MODE 2
20 FOR a=1 TO 23
30 RANDOMIZE TIME
40 FOR b=1 TO 25
50 PRINT USING("## ");INT(RND*100);
60 NEXT b
70 PRINT
80 NEXT a

After you obtain about five or six numbers, the numbers are no longer random! However, if you re-initialise the random number seed by using RANDOMIZE TIME:RANDOMIZE RND, the quality of the random numbers is much better. (This was something I learnt from a listing in Computing with the Amstrad. )

Embarrasing late to the party, but I'm actually surprised nobody commented on this previously. You aren't supposed to call RANDOMIZE repeatedly, because every time you do it resets the seed so you'll end up using the same small subset of the PRNG.

The "correct" way to do the above in BASIC would be more like:

Code Select Expand

10 MODE 2
20 RANDOMIZE TIME
30 FOR a=1 TO 23
40 FOR b=1 TO 25
50 PRINT USING("## ");INT(RND*100);
60 NEXT b
70 PRINT
80 NEXT a

which moves the seed generation outside of the loop and thus generates a much more random sequence.

Perhaps someone more wiki-knowledgable could correct the entry?

[Nich]

Embarrasing late to the party, but I'm actually surprised nobody commented on this previously. You aren't supposed to call RANDOMIZE repeatedly, because every time you do it resets the seed so you'll end up using the same small subset of the PRNG.

The "correct" way to do the above in BASIC would be more like:

Code Select Expand

10 MODE 2
20 RANDOMIZE TIME
30 FOR a=1 TO 23
40 FOR b=1 TO 25
50 PRINT USING("## ");INT(RND*100);
60 NEXT b
70 PRINT
80 NEXT a

which moves the seed generation outside of the loop and thus generates a much more random sequence.

Perhaps someone more wiki-knowledgable could correct the entry?

I think you haven't fully understood what I was trying to convey with my example program.

Most programs that use RANDOMIZE TIME will only use it once, as is the case in the BASIC program you've posted. However, try running that program, resetting it, then running it again, resetting it again, running it a third time, resetting it again... After about six or seven numbers, the same sequence of numbers is generated (93, 54, 6, 98, 14, 3, 18, 35...)! Hence there is a need to use RANDOMIZE TIME:RANDOMIZE RND to reinitialise the seed and prevent the same sequence of 'random' numbers being generated every time you run a program.

The attached screenshots demonstrate the problem more clearly.

[attach=2] [attach=3] [attach=4] [attach=5]