<html> <head> <meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type"> </head> <body bgcolor="#FFFFFF" text="#000000"> <div class="moz-cite-prefix">I think I found the solution. I still have to design the render function. I have this : ; niet grafische constanten. (define lengte-werkblad 200) (define breedte-werkblad 1000) (define move-animal 3) (define move-gauge 0.1) ; grafische constanten (define kat  .) (define cham .) (define workspace (empty-scene breedte-werkblad lengte-werkblad)) (define gauge-omtrek (rectangle 1000 20 "outline" "black")) ; berekende constanten (define correctie-cat ( / (image-width kat)2)) (define ondergrens-cat ( - 0  correctie-cat)) (define bovengrens-cat ( + breedte-werkblad correctie-cat)) (define correctie-cham ( / (image-width cham)2)) (define ondergrens-cham ( - 0  correctie-cham)) (define bovengrens-cham ( + breedte-werkblad correctie-cham)) ; Te gebruiken structs ; Design a world program that works with both cats and chameleons: ; A VAnimal is either ; – a VCat ; – a VCham (define-struct Vcat (Xcat Hcat Richting)) ; Vcat = (make-editor Number Number) ; interp. (make-editor x h r) where x is the x-coordinate of the cat and h is the happiness of the cat and r the way the cat walks. ; make-editor Number Number  String -> Vcat ; Vcat-Xcat Editor -> Number ; Vcat-Hcat Editor -> Number ; Vcat-Richting -> String ; Vcat? Editor Any -> Boolean (define-struct Vcham (Xcham Hcham Richting)) ; Vcham = (make-editor Number Number) ; interp. (make-editor x h r) where x is the x-coordinate of the cham and h is the happiness of the cham and r the way the cham walks. ; make-editor Number Number String  -> Vcat ; Vcham-Xcham Editor -> Number ; Vcham-Hcham Editor -> Number ; Vcham-Richting -> String ; Vcham? Editor Any -> Boolean ; Vcham -> Vcham ; Function who makes the image move to the left or turn to the right (check-expect (links-of-draaien-cham (make-Vcham 20 100 "left")) (make-Vcham 17 99.9 "left")) (check-expect (links-of-draaien-cham (make-Vcham -39 100 "left")) (make-Vcham -39 99.9 "right")) (check-expect (links-of-draaien-cham (make-Vcham 1037 100 "left")) (make-Vcham 1034 99.9 "left")) (define (links-of-draaien-cham s) (if ( < (Vcham-Xcham Vcham) ondergrens-cham)    (make-Vcham  (Vcham-Xcham s)  (- (Vcham-Hcham s) move-gauge) "right")    (make-Vcham (- (Vcham-Xcham s) move-animal) (- (Vcham-Hcham s) move-gauge) "left"))) ; Vcat -> Vcat ; Function who makes the image move to the left or turn to the right (check-expect (links-of-draaien-cat (make-Vcat 20 100 "left")) (make-Vcat 17 99.9 "left")) (check-expect (links-of-draaien-cat (make-Vcat -39 100 "left")) (make-Vcat -39 99.9 "right")) (check-expect (links-of-draaien-cat (make-Vcat 1037 100 "left")) (make-Vcat 1034 99.9 "left")) (define (links-of-draaien-cat s) (if ( < (Vcat-Xcat Vcat) ondergrens-cat)    (make-Vcat  (Vcat-Xcat s)  (- (Vcat-Hcat s) move-gauge) "right")    (make-Vcat (- (Vcat-Xcat s) move-animal) (- (Vcat-Hcat s) move-gauge) "left")) ) ; Vcham -> Vcham ; Function who makes the image move to the right or turn to the left (check-expect (rechts-of-draaien-cham (make-Vcham 20 100 "right")) (make-Vcham 23 99.9 "right")) (check-expect (rechts-of-draaien-cham (make-Vcham -39 100 "right")) (make-Vcham -36 99.9 "right")) (check-expect (rechts-of-draaien-cham (make-Vcham 1040 100 "right")) (make-Vcham 1040 99.9 "left")) (define (rechts-of-draaien-cham s) (if ( > (Vcham-Xcham Vcham) bovengrens-cham)    (make-Vcham  (Vcham-Xcham s)  (- (Vcham-Hcham s) move-gauge) "left")    (make-Vcham (+ (Vcham-Xcham s) move-animal) (- (Vcham-Hcham s) move-gauge) "right")))     ; Vcat -> Vcat ; Function who makes the image move to the right or turn to the left (check-expect (rechts-of-draaien-cat (make-Vcat 20 100 "right")) (make-Vcat 23 99.9 "right")) (check-expect (rechts-of-draaien-cat (make-Vcat -39 100 "right")) (make-Vcat -36 99.9 "right")) (check-expect (rechts-of-draaien-cat (make-Vcat 1040 100 "right")) (make-Vcat 1040 99.9 "left")) (define (rechts-of-draaien-cat s) (if ( > (Vcat-Xcat Vcat) bovengrens-cat)    (make-Vcat  (Vcat-Xcat s)  (- (Vcat-Hcat s) move-gauge) "left")    (make-Vcat (+ (Vcat-Xcat s) move-animal) (- (Vcat-Hcat s) move-gauge) "right")) ) ; Vanimal -> Vanimal ; Function which change the world on clock ticks. ; On every tick the X-coordinate changes 3 pixels and the happiness decrease with 0.1.   (define (tock s)     (cond       [(Vcat? s)(if(equal? (Vcat-Richting s) "left") (links-of-draaien-cat Vcat) (rechts-of-draaien-cat Vcat))]       [(Vcham? s) (if(equal? (Vcham-Richting s) "left") (links-of-draaien-cham Vcham) (rechts-of-draaien-cat Vcham))] )) (define (render s)s) (define (main s)   (cond     [(Vcat? s) (big-bang s (check-with Vcat?) (on-tick tock) (on-draw render) )]     [(Vcham? s) (big-bang s (check-with Vcham?) (on-tick tock) (on-draw render) )]     )) (main (make-Vcat 12 100 "left")) Is this better ? Roelof Op 30-6-2012 9:03, Roelof Wobben schreef: </div> <blockquote cite="mid:4FEEA4CD.4010808@home.nl" type="cite"> <meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type"> <div class="moz-cite-prefix">Op 30-6-2012 1:38, Neil Van Dyke schreef: </div> <blockquote cite="mid:4FEE3C65.6070809@neilvandyke.org" type="cite">Stephen Bloch wrote at 06/29/2012 06:01 PM: <blockquote type="cite">Either you introduce this stuff much better than I do, or your students are much sharper. </blockquote> For the possible benefit of any students reading, I think someone say it, rather than leave it implied: Or the difference could be an isolated effect of, say, some subtle difference in how one concept was first introduced in their respective educations, not a reflection on the instruction or students overall. Aside: I think students are much the same everywhere, and most all students have potential to be good at this stuff.  But learning this stuff well requires a lot of work, and I think students generally do need to have sufficient sense that they're ``sharp,'' so that they stick with it and put in the necessary work.  Or, there is another category of person, who sees themself as slow but determined; that will also lead to learning the stuff, if the self image gets them to put in the necessary work.  (However, the ones who are self-assured and yet who don't do the learning work... they are doomed to be dim and to have few job options other than some kind of politician.) Neil V. ____________________  Racket Users list:  <a moz-do-not-send="true" class="moz-txt-link-freetext" href="http://lists.racket-lang.org/users">http://lists.racket-lang.org/users</a> </blockquote> Im rereading the chapter and I think I found a piece of the puzzle. In the chapter there is a mention of this : <table class="RktBlk" cellspacing="0"> <tbody> <tr> <td>; A <a moz-do-not-send="true" name="(tech._mix._sig)"></a>SIGS (short for “space invader game state”) is one of: </td> </tr> <tr> <td>; – (make-aim <a moz-do-not-send="true" class="techoutside" href="#%28tech._mix._ufo%29" pltdoc="x">UFO</a> <a moz-do-not-send="true" class="techoutside" href="#%28tech._mix._tank%29" pltdoc="x">Tank</a>)</td> </tr> <tr> <td>; – (make-fired <a moz-do-not-send="true" class="techoutside" href="#%28tech._mix._ufo%29" pltdoc="x">UFO</a> <a moz-do-not-send="true" class="techoutside" href="#%28tech._mix._tank%29" pltdoc="x">Tank</a> <a moz-do-not-send="true" class="techoutside" href="#%28tech._mix._missile%29" pltdoc="x">Missile</a>)</td> </tr> </tbody> </table> I can do the same for my problem. ; A Vanimal is one of : ; - (make-Vcat ( x happiness) ;   (make-Vcham ( x happiness) So Vanimal is not a struct but strictly a name. Am I on the right track. Roelof <fieldset class="mimeAttachmentHeader"></fieldset> <pre wrap="">____________________ Racket Users list: <a class="moz-txt-link-freetext" href="http://lists.racket-lang.org/users">http://lists.racket-lang.org/users</a> </pre> </blockquote> </body> </html>