211 S '05 Assignments Set 1 Set 2 Set 3 Set 4 Set 5 Set 6 Set 7 Set 8 Set 9 Set 10 Set 11 Set 12 Set 13 Set 14

### Problem Set 3

Due date: 1/25 @ 4:30 pm

This problem set is about processing `structure`d forms of data.

HtDP Problems:

6.3.1 (1,3,5), 6.4.1 (1,3,5), 6.5.1 (1,3,5), 6.4.2

Here is a data definition for keeping track the date on an electronic calendar (which we know is a computer with a display attached to it):
``````
(define-struct calendar (year month))
;; Calendar = (make-calendar Number Number)
;; Constraints:
;; The first number is always positive for the current year
;; The second number is always between 1 and 12 for the current month.
;; Examples:
(make-calendar 2000 1) ; Jan 2000
(make-calendar 1979 2) ; Feb 1979
```
```
Develop the function `next`, which advances to the next month Keep in mind you may have to change `year` as well.

Modify the data definition so that `Calendar` also keeps track of days. Then modify `next`. The new constraint is that `day` must be greater than `0` and no greater than the number of days in the current month.

Optional: Take into account leap year!

Here is a data definition for a world that consists of two objects: a rock and a bird. The rock drops at a constant speed of 5 pixels per tick; the bird flies horizontally at the same rate:
``````
(define-struct world (bird rock))
;; A World is
;; --- (make-world Number Number)
;; Interpretation:
;; The first number is the x coordinate of the bird;
;; the second number is the y coordinate of the rock.
```
```

Develop the function `world-next`, which creates the next world from a given world.

Develop the function `world-draw`, which creates a 200 x 200 scene with `empty-scence` and `place-image` from the world. Create a first example by hand and use it to test the program. Assume that the bird is always 120 pixels above ground level, and that the rock is always 80 pixels from the left.

After the program is fully developed, use the world.ss teachpack to run it like this:

``````
(define (world-next w) ...)

(define (world-draw w) ...)

;; tests:

;; --- run program run:

;; World -> World
;; run for every tick of the clock
;; display the given world on the canvas
;;  and produce the next world
(define (tock w)
(update (world-draw w) produce (world-next w)))

(big-bang 200 200 .1 (make-world 1 1))

(on-tick-event tock)

```
```

Hint: Study the documentation for `image.ss` in the DrScheme help desk, especially the functions `image-inside?`, `overlay/xy`, and `find-image`. Note that overlaying one picture on another puts the images on top of each other at an imaginary, centered pin-hole. (If you like body piercing, think of this as picture piercing, right smack in the middle.)