1. Section 1.2: Drawing a UFO (continued)

2. Writing a program to draw UFOs Remember that the header includes: define program’s name(let’s call it UFO-draw) program’s variables (t, for the time) (define (ufo-draw t)... will fill in later …)

3. Writing a program to draw UFOs (define (ufo-draw t) (and (draw-solid-disk0......) (draw-solid-rect0......))) Since we drew a disk and a rectangle to draw the UFO, the function says that to draw a UFO first draw a disk and then a rectangle.

4. Writing a program to draw UFOs (define (ufo-draw t) (and (draw-solid-disk0...... 10 'green) (draw-solid-rect0...... 40 3 'green))) To keep the program relatively simple, we will assume that all UFO’s are green and exactly the same size.

5. Writing a program to draw UFOs Of course, the (…) tells us that the function is still incomplete. More precisely, the dots in draw-solid-disk0 (and draw-solid- rect0) say we don't know the X and Y coordinates of the UFO. Recall that the UFO’s location is expressed by X and Y. So if we know t, we can use the X and Y programs that we wrote in Section 1.1: (define (X t) (+ (* 10 t) 20)) [We used the + and * functions (define (Y t)to write X (and Y).] (+ (* 20 t) 60))

6. Writing a program to draw UFOs For draw-solid-disk0, the location is the center: (X, Y). (define (ufo-draw t) (and (draw-solid-disk0 (X t) (Y t) 10 'green) (draw-solid-rect0 … 40 3 'green)))

7. Writing a program to draw UFOs However, for draw-solid-rect0, the location is the top- left corner: (X – 20, Y). (define (ufo-draw t) (and (draw-solid-disk0 (X t) (Y t) 10 'green) (draw-solid-rect0 (- (X t) 20) (Y t) 40 3 'green))) [We used the draw-solid-disk0, draw-solid-rect0, X, and Y functions to write ufo-draw.]

8. Writing a program to erase UFOs Notice that the screen is (almost) white. To make it appear that we are erasing the UFO, we simply draw a white one. Let’s call this function erase-UFO. All we need to do is make 2 small changes to draw-UFO: -change the program’s name -change each of the colors to white

9. Writing a program to erase UFOs (define (ufo-draw t) (and (draw-solid-disk0 (X t) (Y t) 10 'green) (draw-solid-rect0 (- (X t) 20) (Y t) 40 3 'green))) BECOMES (define (ufo-erase t) (and (draw-solid-disk0 (X t) (Y t) 10 ‘white) (draw-solid-rect0 (- (X t) 20) (Y t) 40 3 ‘white)))

10. In summary… Using figure 1, we can now find a UFO’s location (using the X and Y functions. And, we can now draw and erase the UFO on the screen (using the UFO-draw and UFO-erase functions). Next time… In programming, one should not have two functions that both have similar code. We will see another way to write UFO-draw and UFO-erase. Then, finally on to the ANIMATION!