[racket] Basic Racket Question

FWIW, if you were in my class, that solution would get few points. You
may have noticed people asking you about the design recipe in this
thread. That is a reference to this book that you might find useful:

  http://www.htdp.org/

Robby

On Tue, Dec 21, 2010 at 8:49 PM, Sayth Renshaw <flebber.crue at gmail.com> wrote:
>
>
> On Wed, Dec 22, 2010 at 1:19 PM, Sayth Renshaw <flebber.crue at gmail.com>
> wrote:
>>
>>
>> On Wed, Dec 22, 2010 at 1:02 PM, Danny Yoo <dyoo at cs.wpi.edu> wrote:
>>>
>>> >>  (define (netpay gross tax-rate)
>>> >>    (-(gross)(* gross tax-rate)))
>>> >>
>>> >> So I expect the function to calculate as
>>> >>
>>> >> = (-(240)(* 240 0.15)
>>> >> = ( - 240 36)
>>> >> = 204
>>>
>>>
>>> Just to be more careful: when you're showing the calculation, make
>>> sure to include the use of the function:
>>>
>>>     (netpay 240 0.15)
>>>     = (-(240)(* 240 0.15)
>>>     = ( - 240 36)
>>>     = 204
>>>
>>>
>>> There's a hitch on the first step in the calculation, and it has to do
>>> with the parens.  Unlike its use in traditional math notation, parens
>>> are significant in this language: that is, every use of paren has to
>>> mean something: it's not superfluous: if you have too many or too few,
>>> it changes the meaning of the program.
>>>
>>>
>>> So, within the larger term here:
>>>
>>>    (- (240) (* 240 0.15))
>>>
>>> the subterm
>>>
>>>    (240)
>>>
>>> means "call the function 240".  That may not be what you intend, but
>>> that what it means in this language.
>>>
>>>
>>> You can see this if you go back to what the error message is saying:
>>>
>>>   function call: expected a defined function name or a primitive
>>> operation name after an open parenthesis, but found a function
>>> argument name
>>>
>>> It's basically trying to point out this problem, that the use of
>>> "(gross)" within the expression
>>>
>>>   (-(gross)(* gross tax-rate))
>>>
>>> is trying to use gross as if it were a function, rather than the
>>> numeric argument to netpay.
>>
>> the subterm
>>
>>    (240)
>>
>> means "call the function 240".  That may not be what you intend, but
>> that what it means in this language.
>>
>> Awesome pickup, thank you. Can I ask how I could then specify gross a
>> single item in the formula and not as a function?
>
> Thank you everyone, I have solved it and thank you for picking me up on the
> small intricises.
>
> This is my solution though it works I do feel I "Cheated" somewhat but this
> is what I have done and tested working.
>
> (define tax-rate 0.15)
> (define pay-rate 12)
> (define (gross hours)
>   (* hours pay-rate))
>
> (define (netpay gross tax-rate)
>     (- gross 0 (* gross tax-rate)))
>
>
>> (netpay (gross 20)tax-rate)
> 204
>>
>
> Sayth
>
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