[racket] ufo structure problem

From: Pierpaolo Bernardi (olopierpa at gmail.com)
Date: Mon Apr 2 03:25:51 EDT 2012

On Fri, Mar 30, 2012 at 16:56, Roelof Wobben <r.wobben at home.nl> wrote:
> Op 30-3-2012 12:17, Pierpaolo Bernardi schreef:
>> On Fri, Mar 30, 2012 at 11:53, Roelof Wobben<r.wobben at home.nl>  wrote:
>>> Op 30-3-2012 9:46, Pierpaolo Bernardi schreef:
>>>> Hello Roelof,
>>>> On Fri, Mar 30, 2012 at 09:32, Roelof Wobben<r.wobben at home.nl>    wrote:
>>>>> (define (show-ufo ufo)
>>>>>  (place-image BLU (ufo-loc u) MTS))
>>>>> But still I can't get the show-ufo work.
>>>> Check carefully the name of your variables!
>>>> Doesn't DrRacket give you an explicative error message in this case?
>>>> P.
>>> Oke, I will do that.
>>> Yes, The error that variable u is not known.
>> I didn't want to tease you.  I thought that you would see it if I
>> pointed you at it.  8^)
>> (define (show-ufo ufo)
>>    (place-image BLU (ufo-loc u) MTS))
>> See that the variables ufo and u should actually the same variable.
>> So, either you rename u to ufo, or rename ufo to u.
> Are you sure ?


> I thought that the position of the ufo can be find at (posn-y p) and
> (posn-x) because the postition is a struct of Posn.

You must pay more attention to small details. What is the p in (posn-y
p)?  Isn't an argument missing in (posn-x)?
What is the u in (ufo-loc u)  (that's the variable that DrRacket says
it doesn't know).


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