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Asked: 2026-06-02T22:50:11+00:00

I’ve recently started developing with XNA. I could say that I have more that

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I’ve recently started developing with XNA. I could say that I have more that basic programming language, but nothing so special.

As my first project with XNA. I would like to create a simple 2D platformer, but at the moment, I’m experimenting with simple movement.

I have 3 vectors (position, velocity, acceleration) and a simple integer (maximum velocity):

Vector2 pos = new Vector2(200, 200);
Vector2 vel = new Vector2(20, 20);
Vector2 acc = new Vector2(30, 30);
int maxVel = 50;

And then, I have some simple code that gets in the Update method of the XNA framework to make my sprite move:

// Horizontal Movement
if (KeybState.IsKeyDown(Keys.Right))
{
if (vel.X < maxVel)
vel.X += acc.X * curTime;
pos.X += vel.X * curTime;
}
if (!KeybState.IsKeyDown(Keys.Right) && vel.X > 20)
{
pos.X += vel.X * curTime;
vel.X -= 1;
}

So, when I press the Right arrow key, the sprite is starting to move. The velocity goes from 20, and with the help of acceleration and the elapsed game time, grows to maxVel (which is 50), then when the Right key is released, it loses velocity. It works perfectly.

But when I try to do the same thing for the Left Arrow key, I’m doing pos -= instead of pos+=, which decreases the pos.X value, but it uses a constant velocity (20) instead of increasing it till 50.

I think I’m not smart enough to think about why is this happening, so some help from better developers would be really appreciated.

Note that I do not want to look anywhere else, I want to make my version work as intended, so a straight answer is the best.

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1 Answer

  1. Editorial Team

    Your condition 

    if (!KeybState.IsKeyDown(Keys.Right) && vel.X > 20)

    vill be true every time the right arrow is not pressed (including when the left arrow is pressed) and the speed is higher than 20, so your code promptly decreases the speed back to 20

    You will need to check for whether no key (or at least no arrow key) is pressed to decrease the speed. The simplest way would be:

    if (!KeybState.IsKeyDown(Keys.Right) &&
    !KeybState.IsKeyDown(Keys.Left) &&
    !KeybState.IsKeyDown(Keys.Up) &&
    !KeybState.IsKeyDown(Keys.Down) && vel.X > 20)
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