Making a character jump in XNA/C# (Basic Concept Included)

In the last tutorial, I briefly went over implementing gravity in games. By the end of the tutorial, we are going to program a character jumping when a key is pressed. I included 'Basic Concept Included' in the title because it doesn't use any XNA/C# specific stuff, just some simple math.

Jump?

Well, I'll first start off by telling you want happens in a 'jump'. First, upward force is applied and the character moves up. Somewhere down the road, gravity acts on it and it moves down.
I’m going to define all the variables in one go:

/*GraphicsDeviceManager, SpriteBatch, Texture2D and Vector2 may be found only in XNA. They are just used for drawing objects and defining locations.*/
GraphicsDeviceManager graphics; 
SpriteBatch spriteBatch;
Texture2D charr;
Vector2 charPos;

bool jumping; //Is the character jumping?
float startY, jumpspeed = 0; //startY to tell us //where it lands, jumpspeed to see how fast it jumps

Now, to the Initialize() function, provided by XNA:

charr = Content.Load<Texture2D>("ow"); //Load image
charPos = new Vector2(230, 450);//Char loc, X/Y
startY = charPos.Y;//Starting position
jumping = false;//Init jumping to false
jumpspeed = 0;//Default no speed

And the Update(), where the following is updated every frame:

//Init keyboard
KeyboardState keyState = Keyboard.GetState();

if (jumping)
{
    charPos.Y += jumpspeed;//Making it go up
    jumpspeed += 1;//Some math (explained later)
        if (charPos.Y >= startY)
        //If it's farther than ground
        {
            charPos.Y = startY;//Then set it on
               jumping = false;
        }
    }

else
{
    if (keyState.IsKeyDown(Keys.Space)) 
    {
        jumping = true;
        jumpspeed = -14;//Give it upward thrust
    }
}

We can now draw it with spriteBatch:

spriteBatch.Begin();
spriteBatch.Draw(charr, charPos, Color.White);
spriteBatch.End();

Explanation

We update charPos’s Y axis by jumpspeed. Since jumpspeed is set to –14 when space is pressed, this makes it go up (or down, depends on the game engine/platform you are using). Now, the magic- by adding 1 to jumpspeed every frame after, it will turn positive after a while and move down instead!
The value of jumpspeed will look like this:

-14,-13,-12….0,1,2,3….


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