Getting wrong return on Rand()

BlitzMax Forums/BlitzMax Programming/Getting wrong return on Rand()

Chroma

(Posted 2005) [#1]

Something's up with the Rand function. I'm trying to get a random number from -1 to 1 and it's only returning 0s and 1s. Try this:

For a=1 to 1000
     Print Rand(-1,1)
Next

In order to get -1s I'm having to do Rand(-2,1) which is throwing off my probabilities (which I don't want).

tonyg

(Posted 2005) [#2]

You're saying integers between -1 and 1 which is 0 and 1.
To include -1 you need rand(-2,1)
<Edit> Why is it throwing off your probabilities?
<edit> In fact, this might be caused by the 'double' precision issue.

Azathoth

(Posted 2005) [#3]

Rand() is for integers.

tonyg

(Posted 2005) [#4]

but uses doubles.

Matt McFarland

(Posted 2005) [#5]

Do you want a random float? If so try RndFloat() which returns a number from 0.0 to 1.0

tonyg

(Posted 2005) [#6]

Because RAND returns a double but the function is declared to return an int, values such as -0.98 are set
to 0.
It is possible to get -1 returned but not 1/3 of the time.
Setting rand(-2,1) will return values such as -1.9 set to
-1.
<edit> B3D doesn't have this problem.

Chroma

(Posted 2005) [#7]

Matt...Rnd() is for floats. I'm looking for an integer hence I'm using Rand().

When I say throwing off my probabilities I mean I'll get 0 more than -1 or 1. As you said there's not an even 1/3 chance to get each number by using Rand(-2,1) because anything in the -1 range is coming up a zero.

It would actually be 1 1/4 of the time, 0 1/2 of the time, and -1 1/4 of the time.

Hope this gets fixed soon.

tonyg

(Posted 2005) [#8]

What numbers will you be dealing with?
Could you create your own function which adds 100 to min/max?

Function small_rand:Int(Minvalue,maxvalue)
   startvalue=minvalue+100
   endvalue=maxvalue+100
   mynum=Rand(startvalue,endvalue)
   Return mynum-100
End Function
For x = 1 To 100
  Print small_rand(-1,1)
Next

Beaker

(Posted 2005) [#9]

For a=1 To 1000
DebugLog Rand(0,2)-1
Next

Floyd

(Posted 2005) [#10]

The Rand(0,2) - 1 trick is good enough for this particular problem.

The trouble is that Rand() should round down, i.e. toward -Infinity.
It actually rounds toward zero, which is how BlitzMax converts floating point values to integer.

Here is an alternate version of Rand() which uses Floor() to enforce the correct rounding.
I haven't seriously tested this, but it seems like it should work.

Function Rand2( min_value,max_value=1 )
	Local delta!=Double(max_value)-Double(min_value)
	If delta>0 Return Floor( RndDouble()*(1+delta) ) + min_value
	Return Floor ( RndDouble()*(1-delta) ) + max_value
End Function

Chroma

(Posted 2005) [#11]

Beaker for the win!

Fabian.

(Posted 2005) [#12]

Hope this gets fixed soon.

Me too!