material

Exercises

Exercise 2.1. [*] DrScheme doesn’t provide the function line for drawing a

Use the language level
BS w/ Lists

line. Fortunately, this is not a problem because we have color-list→image, which consumes a list of colors and produces an image.

Develop the function make-line, which creates a ColorList that represents a horizontal line. The function consumes the width of the line (a natural number) and a color. It produces an appropriate list of colors.

Also, even if DrScheme did not provide rectangle, we could still draw rectangles. Develop the function make-rectangle. It consumes two natural numbers, width and height, and a color. Its result is a ColorList that represents the matching rectangle.

Hint: (append (list 1 2) (list 3 4 5)) produces (list 1 2 3 4 5).

After you have developed the functions, including automated tests, run them as follows:

( color-list→ image (make-line 100 (make-color 100 100 100)) 100 1)
or
( color-list→ image (make-rectangle 2 3 (make-color 0 0 0)) 2 3)

Keep in mind that these are not tests.

Exercise 2.2. [*] Copy and paste the program from section 2.5. Provide a definition

Add the teachpack
image.ss

for UFO (in the lecture code) so that it shows up as a flying green saucer.

Exercise 2.3. [difficult, but entertaining] Develop place-blocks. The program consumes a list of blocks:

Add the teachpack
image.ss

;; A BlockList is either: 
;; --- empty
;; --- (cons N BlockList)

Each number on a BlockList represents the height of a stack of blocks. The program produces an image, drawing a stack of blocks of the specified height starting in the lower, right corner of a 100 x 100 canvas.

Note: The program is a core piece of a Tetris-style game.

Hint 1: Use these definitions for your basic constants:

(define WIDTH 100)
(define HEIGHT 100)
(define CANVAS (empty-scene WIDTH HEIGHT))

(define BLOCK-WIDTH 10)
(define BLOCK
  (offset-image+
   (rectangle BLOCK-WIDTH  BLOCK-WIDTH 'red)
   0 0
   (outline-rect BLOCK-WIDTH  BLOCK-WIDTH 'black)))

Hint 2: Here is the image for (place-blocks (list 1 2)):

Produce the image by hand and then use it with image=? to test the program. The picture shows that the drawing starts at the right. The length of the list determines how far from the right a stack of blocks is drawn.

Exercise 2.4. [*] Use the conventional draw.ss teachpack to draw and erase green flying saucers from a canvas.

Add the teachpack
draw.ss

That is, develop the functions draw-ufo and clear-ufo for drawing and clearing a UFO-like shape from a canvas. Both functions consume a Posn, which represents the anchor position of the UFO (e.g., the center of the disk or the northwest corner of the rectangle).