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* Lesson 08: Distance function

** Getting set up

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[[file:images/lesson01-ticblank.png]]

You can either edit over your existing code, or start a new program.
If you want to start a new program, press *ESCAPE* until you get to
the main prompt:

Then type in =new lua=, hit *ENTER*, then =save GAMENAME.LUA=, hit
*ENTER*. (Replace "GAMENAME" with whatever you want.)

#+BEGIN_SRC terminal
  > new lua
  new cart has been created

  > save GAMENAME.lua
  cart GAMENAME.lua saved!
#+END_SRC

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--

** Create a sprite

In the Sprite Editor *F2*, create a sprite we are going to display
on the screen.

[[file:images/lesson-distance-sprite.png]]

** Creating the distance function

Depending on where you are in your math classes, you might have seen
the *distance formula*:

$$ d = \sqrt{ (x_2 - x_1)^2 + (y_2 - y_1)^2 } $$

This is how we can tell how far two /(x,y)/ coordinates are from
each other.

[[file:images/distance-coordinates.png]]

So in our program we're going to keep track of the coordinate of a
/sprite/ and of the /mouse/, figure out the distance, and print it
to the screen. Once we have the formula, we can add things to our
games like /how to check if the player has picked up a coin/, or
/how to check if the opponent has reached the finish line/.

** Editing the code

Press the *F1* key to go to the code editor. Erase the starter code.

We're going to start by putting the *Distance* function at the very
top of our program, /before/ the TIC() function:

#+BEGIN_SRC lua
  -- Functions
  function GetDistance(x1,y1,x2,y2)
    return math.sqrt( (x2-x1)^2 + (y2-y1)^2 )
  end

  -- Game setup

  -- Game loop
  function TIC()

  end
#+END_SRC

Next we want to create a position for the sprite we will draw to the
screen. Create two variables, such as:

#+BEGIN_SRC lua
  -- Game setup
  pointX = 240/2
  pointY = 136/2
#+END_SRC

Within the TIC() function, we'll get the mouse coordinates, then use
the distance formula to find out /how many pixels apart/ the mouse
is from the sprite.

Afterwards, we'll clear the screen, draw the sprite, and print out
text that shows the distance between the mouse and the sprite.

#+BEGIN_SRC lua
function TIC()
  mouseX, mouseY, button = mouse()
  dist = GetDistance(mouseX,mouseY,pointX,pointY)

  cls(6)
  spr(0, pointX-4, pointY-4)

  print("Distance: " .. dist, 5, 5)
end
#+END_SRC

** Testing

Now you can run the program and move the mouse around, it will show
the distance between the two points:

[[file:images/lesson-distance-animation.gif]]

** Save your work

Make sure to press the *CTRL+S* key, or press *ESC* to go to the
prompt and type =save= to save your work! If you want to access the
folder with your code, type =folder=.

If you have a USB flash drive, make sure to copy your file over so
you don't lose it when the computer resets!

** Back to the course page!

[[../index.org][⬅️ Once you're all done, go back to the course homepage!]]