Random values seem to be not really random?

Question

Trying to make a simple bacteria-killing game using WinForm in C#, but the bacteria (I am using Panel for the time being) doesn't seem to move around at random.

Specifically, the problem I am having is, the bacteria tries to move towards the upper left corner and move around there only. Ideally, the bacteria needs to move around the rectangle range evenly, but I am not sure how to achieve that. Look at the gif file below.

As you can see the red Panel moves around the upper left corner only. How can I get it to move everywhere evenly and randomly?

Here is my code:

private Panel _pnlBacteria;     //Panel representing a piece of bacteria private Random r = new Random();  //For randomly-generated values private int _prevX;        //Stores the previous X location private int _prevY;        //Stores the previous Y location  public Form1() {     InitializeComponent();     _pnlBacteria = new Panel();     /* Get more property assignments to this._pnlBacteria (omitted) */      //Bacteria's start position is also randomly selected     _prevX = r.Next(50, 300);     _prevY = r.Next(50, 500); }  //Timer runs every 100 seconds changing the location of the bacteria private void TmrMoveBacteria_Tick(object sender, EventArgs e) {     int x, y;     //Get random values for X and Y based on where the bacteria was previously     //and move randomly within ±10 range. Also it cannot go off the screen.     do      {                x = r.Next(_prevX - 10, _prevX + 10);         y = r.Next(_prevY - 10, _prevY + 10);     }      while ((y <= 0) || (y >= 500) || (x <= 0) || (x >= 300));      //Save the new location to be used in the next Tick round as previous values     _prevX = x;     _prevY = y;       //Apply the actual location change to the bacteria panel     _pnlBacteria.Top = y;     _pnlBacteria.Left = x; } 

I tried changing the +10 to +12, leaving -10 as it is, but now this only made the bacteria move to the bottom right corner only. I am at a loss. Can anyone please help?

Answer 1

If you read the documentation of Random.next(int,int) you'll find the the lower bound is inclusive and the upper bound is exclusive, that's why -10 and +11 works.