Why does my character move up forever when he jumps with this code?

Question

I've been doing some stupid stuff in Slick2D+LWJGL and I've run into a problem. I press the UP arrow and my character continues to go up until I let go :/ Here's the code:

if (inp.isKeyDown(Input.KEY_UP) && !jumping) {
    jumping = true;
    velY=-0.4f;
} else {
    jumping = false;
    velY=0.2f;
}

No idea what's happening :/ Help?

Answer 1

Presumably this code is called once every frame. What is likely happening is that while you hold down the up arrow, you are constantly toggling the state of jumping, which means you take the positive branch of the if every even frame and the negative branch every odd frame.

That also means you constantly toggle the velocity between -0.4 and 0.2. Every frame you presumably process that velocity and offset the player by it; however, because your upwards velocity exceeds your downwards velocity, you'll have a net upwards movement.

For example:

• On frame 0 you starting hold up. That sets your velocity to -0.4 (and jumping to true) and then you add that velocity to the player's position, giving you a delta in of Y of -0.4.

• The next frame you are still holding up, but jumping is now true, so you take the negative branch, setting velocity to 0.2 and jumping to false. When you apply velocity to position, you have a net delta of -0.4 + 0.2 = -0.2.

• Now, the third frame you are still holding up but jumping is now false, so you take the positive branch, set velocity to -0.4 and jumping to true. When you apply velocity to position you have a net delta of -0.2 - 0.4 = -0.6.

That's why your player always moves up.

To fix this, you could restructure the condition as follows:

if (inp.isKeyDown(Input.KEY_UP)) {
  if (!jumping) {
    jumping = true;
    velY=-0.4f;
  }
} else {
  jumping = false;
  velY=0.2f;
}

This means that if the up key is held, you start jumping if you have not already. If the up key is not held, you cancel the jump.