RealRandom number between 0 and x

Robot Freak

The RealRandom object is great, but it doesn't give random numbers between 0 and x (to be specified).

This doesn't work:

repeat x
  random += rr.random & 1

Because it gives numbers that are always near x/2.

This doesn't work:

x /= 2
repeat while x <> 0
  temp := rr.random & 1
  if(temp == 1)
    random += x
  x /= 2

Because of the divisions the random numbers don't reach x.

Any idea's how to get this working?

Kind regards,
Robot Freak

Post Edited (Robot Freak) : 10/28/2008 4:28:23 PM GMT

Leon

Convert it to a real number between 0 and 1 by dividing it by 2^32, then multiply by x and take the integer part.

Leon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Robot Freak

Hi Leon,

I tried this:

temp := rr.random                                                          'random 32 bits
temp := fmath.ffloat(temp)                                                 'confert to float
temp := fmath.fdiv(temp, fmath.ffloat(%11111111111111111111111111111111))  'divide by 2^32
temp := fmath.fmul(temp, fmath.ffloat(x))                                  'multiply by float x
temp := fmath.fround(temp)                                                 'convert to integer

Most of the times it returns zero, sometimes -649142272 or 605822208.

So it isn't working jet, do you know what is wrong in this code?

Thanks for your help,
Robot Freak

Post Edited (Robot Freak) : 10/28/2008 8:11:57 PM GMT

Leon

I think thar RealRandom returns a signed value, you need to use an unsigned number.

An easier way might be to use the technique used with the gcc compiler and use something like rand()%(N+1), where rand() returns a value between zero and RAND_MAX.

Leon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 10/28/2008 8:31:40 PM GMT

Beau Schwabe

Robot Freak,
Here is a slight modification to the "RealRandom Demo" that should do what you want...
I added "PUB Random(N)" where N is the maximum random number minus one returned.

CON
  _clkmode = xtal1 + pll16x
  _xinfreq = 5_000_000

OBJ
  text  : "tv_text"
  rr    : "RealRandom"
  
PUB start | i
  'start RealRandom
  rr.start
  'start terminal and show a random number
  text.start(12)
  text.str(string(10,16,11,6))
  text.dec(Random(10))

PUB Random(N)
    Result := rr.random & $FFFF  ' Limit Random number to a 16-Bit value (prevent overflow)
    Result := (Result * N)/$FFFF ' Limit Random number result from 0 to N-1 

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Robot Freak

Thanks Beau!!

mpark

Beau Schwabe (Parallax) said...

    Result := rr.random & $FFFF  ' Limit Random number to a 16-Bit value (prevent overflow)
    Result := (Result * N)/$FFFF ' Limit Random number result from 0 to N-1 

Shouldn't that last line be

  Result := (Result * N) >> 16 ' Limit Random number result from 0 to N-1 

Otherwise there's a 1 in 65536 chance that Result = N.

Beau Schwabe

mpark,

Ohh I See.... Ok, just leave it the way it is and the number produced is between 0 and N .... NOT 0 to N-1

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

mpark

But if you leave it the way it is, the distribution is nonuniform: most of the time you'll get numbers from 0 to N-1, and very very rarely you'll get N, almost certainly not what is desired.
Doing what I suggest gives a uniform distribution of the numbers from 0 to N-1 (assuming RealRandom is uniform, of course).

Beau Schwabe

mpark,

·You are correct

    Result := rr.random & $FFFF  ' Limit Random number to a 16-Bit value (prevent overflow)
    Result := (Result * N) >> 16 ' Limit Random number result from 0 to N-1 

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.