Why does my rectangle move faster in one direction than in the opposite?

Question

I am using Slick2d to create a top-down game, however an rectangle than I am drawing to the display is moving faster to the left than to the right, and faster up than down.

I am updating frames like so:

public void update(GameContainer gameContainer, int delta) throws SlickException { 
    player = new Rectangle(playerX, playerY, 10, 20);

    if (gameContainer.getInput().isKeyDown(Input.KEY_LEFT)) {
        x -= delta * 0.1f;
    }
    if (gameContainer.getInput().isKeyDown(Input.KEY_RIGHT)) {
        x += delta * 0.1f;
    }
    if (gameContainer.getInput().isKeyDown(Input.KEY_UP)) {
        y -= delta * 0.1f;
    }
    if (gameContainer.getInput().isKeyDown(Input.KEY_DOWN)) {
        y += delta * 0.1f;
    }
}

I then simply draw the rectangle to the display. It is noteworthy that the rectangle class is aorg.newdawn.slick.geom.Rectangle.

Why are the speeds for the movement different, and how can I fix this?

EDIT: I have fixed the problem by slowing the movement to the left and up, but this does not explain why the problem occurs

Answer 1

This is most likely a rounding error. I'm guessing that x and y are ints.

Keeping track of the x and y values as floats, I expect will fix the issue.

Consider if the values are being floored:

Moving Left 10 - 1.6 = 8

Moving Right 10 + 1.6 = 11