[racket] exercise problem

Op 22-2-2012 9:58, Roelof Wobben schreef:
> Thanks,
>
> I have now a answer to my question.
> I will debug my programm with check-expect and wil take a look at the 
> second edition.
>
> Roelof
>
>
>
> Op 21-2-2012 22:40, Stephen Bloch schreef:
>>
>> On Feb 21, 2012, at 3:18 PM, Roelof Wobben wrote:
>>
>>> Op 21-2-2012 20:51, Stephen Bloch schreef:
>>>> On Feb 21, 2012, at 2:43 PM, Roelof Wobben wrote:
>>>>
>>>>> But now I get this error message :
>>>>> unsaved editor>:27:23: function call: expected a function after the open parenthesis, but found a part in: (tarief1 amount)
>>>>>
>>>>> When I do (tarief 1 amount) instead of ((tarief amount))
>>>> Yes, that's where the syntax error is.  So you've fixed that.
>>>
>>> I fixed that?  I always thought that if I use another function that 
>>> I must be into ().
>>> So In my opinion one () for the then part and one () for the 
>>> function call.
>>
>> No, there is no pair of parentheses for the "then" part, only one for 
>> the function call.
>>
>> (cond [question answer]
>>            [question answer]
>>            ...
>>            [question answer])
>> where each question and each answer is an expression (usually a 
>> function call).
>>
>>>> 1) Have you written a complete set of test cases, with right answers, using "check-expect"?  Since there are four categories of input for this problem, you need at least four test cases, and you should probably have three more for the boundaries, a total of seven test cases.
>>>
>>> Nope, check-expect is not explained on chapter section 3 of the book 
>>> how to design programms. So I did not use that function.  I can make 
>>> test-cases but then in this form
>>>
>>> (= (/tarief 495/) 1,2375)
>>
>> That should work too, although it's not as convenient.  Make sure you 
>> put a bunch of those test cases in the Definitions pane, after the 
>> definition:
>>
>> (define (tarief blah blah)
>>    blah blah blah
>>    )
>>
>> (= (tarief 495) 1,2375)
>> (= (tarief 0) 0)
>> (= (tarief 100) whatever)
>> ...
>>
>> That will also solve the next problem:
>>
>>> 3) Have you used the Stepper to watch what your program is doing?
>>>
>>> Yes, when I do (payback 2600) on the prompt and press Step I only 
>>> see a message  All definitions have been sucessfully evaluated and 
>>> not other outcome so that did not help me any further.
>>
>> The Stepper only works on expressions in the Definitions pane, not 
>> the Interactions pane.  If you put your test cases in the Definitions 
>> pane as described above, the Stepper will help you find where things 
>> are going wrong.
>>
>> BTW, check-expect is not discussed in _How to Design Programs_ 
>> because it hadn't been invented yet when the book was written.  It's 
>> quite easy to use, though: replace the = sign in the above tests with 
>> check-expect, and when you hit "Run" you'll get a report of which of 
>> your test cases worked, the expected answer and the actual answer.
>>
>> You might consider working through the Second Edition of _How to 
>> Design Programs_ instead; see 
>> http://www.ccs.neu.edu/home/matthias/HtDP2e/ .
>>
>>
>>
>> Stephen Bloch
>> sbloch at adelphi.edu <mailto:sbloch at adelphi.edu>
>>
>

I found a working solution but I wonder if there is a better way to 
achieve the answer in the Beginning Student modus.

The solution I found is this one:


(define (tarief4 amount)
   (*(- amount 2500)0.01))


(define (tarief3 amount)
   (cond
     [ ( < amount 2500)(* (- amount 1500)0.0075)]
     [ else (  * .0075 1000)]))

(define (tarief2 amount)
   (cond
     [ ( < amount 1500)(* ( - 1000 amount) .0050) ]
     [ else ( * 1000 0.0050)]))


(define (tarief1 amount)
   (cond
     [ (< amount 500) (* 0.0025 amount)]
     [ else (* 0.0025 500)]
     ))


(define (payback amount)
   (cond
     [ (<= amount 500) (tarief1 amount)]
     [ (and ( <= amount 1500) (> amount 500)) (+ (tarief1 
amount)(tarief2 amount))]
     [ (and ( <= amount 2500) (> amount 1500)) (+ (tarief3 amount) 
(tarief2 amount) (tarief1 amount))]
     [ else (+ (tarief4 amount)(tarief3 amount)(tarief2 amount)(tarief1 
amount))]
     )
    )


Roelof Wobben

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