Small Basic – Random Numbers

Random numbers can be very useful in your Small Basic games.  They can be used to make the game play vary from one game to the next.

The Small Basic keyword to get a random number is Math.GetRandomNumber(maxNumber).  It will return a random integer between 1 and the input value maxNumber.

Here are a few ideas and tips:

Random Position and Speed

We can position sprites with random positions and velocities.

radius = 25
numSprite = 10
gw = GraphicsWindow.Width
gh = GraphicsWindow.Height
For i = 1 To numSprite
  sprite[i] = Shapes.AddEllipse(2*radius,2*radius)
  spritePosX[i] = radius+Math.GetRandomNumber(gw-2*radius)
  spritePosY[i] = radius+Math.GetRandomNumber(gh-2*radius)
  spriteVelX[i] = Math.GetRandomNumber(11)-6
  spriteVelY[i] = Math.GetRandomNumber(11)-6
While "True"
  For i = 1 To numSprite
    spritePosX[i] = spritePosX[i]+spriteVelX[i]
    spritePosY[i] = spritePosY[i]+spriteVelY[i]
    If (spritePosX[i] < radius) Then
      spritePosX[i] = radius
      spriteVelX[i] = -spriteVelX[i]
    ElseIf (spritePosX[i] > gw-radius) Then
      spritePosX[i] = gw-radius
      spriteVelX[i] = -spriteVelX[i]
    If (spritePosY[i] < radius) Then
      spritePosY[i] = radius
      spriteVelY[i] = -spriteVelY[i]
    ElseIf (spritePosY[i] > gh-radius) Then
      spritePosY[i] = gh-radius
      spriteVelY[i] = -spriteVelY[i]

Random Branching

We can randomly do different things in the code,  perhaps equivalent to rolling a dice.

dice = Math.GetRandomNumber(6)
If (dice = 1) Then
  TextWindow.WriteLine("Do something for a dice roll of 1")
ElseIf (dice = 2) Then
  TextWindow.WriteLine("Do something for a dice roll of 2")
ElseIf (dice = 3) Then
  TextWindow.WriteLine("Do something for a dice roll of 3")
ElseIf (dice = 4) Then
  TextWindow.WriteLine("Do something for a dice roll of 4")
ElseIf (dice = 5) Then
  TextWindow.WriteLine("Do something for a dice roll of 5")
ElseIf (dice = 6) Then
  TextWindow.WriteLine("Do something for a dice roll of 6")

We could also only do something with a probability, say 1 in 10.

If (Math.GetRandomNumber(10) = 1) Then
  TextWindow.WriteLine("This is a 1 in 10 event!!!")
  TextWindow.WriteLine("A 10% probability event didn't happen")

Ranges and Intervals

If we want random numbers over a different range (not 1 to maxNumber) or with smaller intervals, for example 0.2, then we can use the following formula.

rand = min + (Math.GetRandomNumber(1+(max-min)/interval)-1)*interval

Here is an example using this to create random numbers between -5 and 3, with intervals of 0.2.

min = -5
max = 3
interval = 0.2
For i = 1 To 1000
Sub Random
  rand = min + (Math.GetRandomNumber(1+(max-min)/interval)-1)*interval


Comments (6)
  1. Hey I wonder if we can do a community project around random numbers. This is cool, thanks for explaining this, LitDev!

  2. anonymouscommenter says:

    LitDev, I think there is a slight error in your formula.  It generates a random number from min+interval to max+1.

    You can see in the image that it generates 4.0 when the max is set at 3.

    If you remove the +1 from the formula then it gets rid of the max+1 error, but I'm not sure how to get it to generate the min as the lowest number.

  3. litdev says:

    Ozjerry – Excellent catch,

    Does this look right to you.

    Sub Random

     rand = min + (Math.GetRandomNumber(1+(max-min)/interval)-1)*interval


  4. OzJerry says:

    LitDev – Works perfectly.  I ran it through a 10,000 iterations at varying min/max values at it's perfect every time.

    I tried to fix it myself prior to posting, but just couldn't get it to hit both the min and max values.  I figured it was something to do with the fact that GetRandomNumber doesn't generate a zero, but I couldn't figure out the workaround.  So kudos to you for working it out.


  5. litdev says:

    I have corrected the original post – Thanks again Ozjerry.

  6. anonymouscommenter says:

    Retrieve either neg or pos (-1 or 1) in a single line for eg. direction up/down, right/left, slower/faster, back/forward etc.

    NegOrPos = 2* (Math.GetRandomNumber(2)-1.5)

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