Sorry, I forget to give you the chapter and the book.
Im talking about this chapter
2.5.2Programming with Structures Which can be found at this
url:
http://www.ccs.neu.edu/home/matthias/HtDP2e/htdp2e-part1.html#%28part._structure%29
Roelof
Op 30-3-2012 9:32, Roelof Wobben schreef:
Hello,
I try to make the ufo exercise work on this chapter :
So I have this :
; visual constants
(define MTS (empty-scene 100 100))
(define BLU (place-image (rectangle 25 25 "solid" "blue") 50 50 MTS))
(define-struct velocity (dx dy))
; A Velocity is a structure: (make-vel Number Number)
; interp. (make-vel d e) means that the object moves d steps
; along the horizontal and e steps along the vertical per tick
(define-struct ufo (loc vel))
; A UFO is a structure: (make-ufo Posn Velocity)
; interp. (make-ufo p v) is at location p
; moving at velocity v
(define v1 (make-velocity 8 -3))
(define v2 (make-velocity -5 -3))
(define p1 (make-posn 22 80))
(define p2 (make-posn 30 77))
(define u1 (make-ufo p1 v1))
(define u2 (make-ufo p1 v2))
(define u3 (make-ufo p2 v1))
(define u4 (make-ufo p2 v2))
; UFO -> UFO
; move the ufo u, i.e., compute its new location in one clock
; tick from now and leave the velocity as is
(define (ufo-move u)
(make-ufo
(posn+ (ufo-loc u) (ufo-vel u))
(ufo-vel u)))
(define (posn+ p v)
(make-posn (+ (posn-x p) (velocity-dx v))
(+ (posn-y p) (velocity-dy v))))
(define (show-ufo ufo)
(place-image BLU (ufo-loc u) MTS))
(define (game ufo)
(big-bang ufo (on-tick ufo move u) (on-draw show-ufo)))
But still I can't get the show-ufo work.
I my opinion ufo loc represent the position on the screen so the x and
y position.
Is this right ?
I find this a very confusing exercise and I hope someone can give me
tips how to solve this without given the answer to the pogramm.
Roelof
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