[racket] Basic Racket Question

Yep.

Robby

On Tue, Dec 21, 2010 at 11:11 PM, Sayth Renshaw <flebber.crue at gmail.com> wrote:
>
>
> On Wed, Dec 22, 2010 at 2:04 PM, Robby Findler <robby at eecs.northwestern.edu>
> wrote:
>>
>> 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
>> >
>> > _________________________________________________
>> >  For list-related administrative tasks:
>> >  http://lists.racket-lang.org/listinfo/users
>> >
>
> Indeed I know what you are referencing.
>
> ;; Contract: area-of-ring : number number  ->  number
>
>
> ;; Purpose: to compute the area of a ring whose radius is
> ;; outer and whose hole has a radius of inner
>
>
> ;; Example: (area-of-ring 5 3) should produce 50.24
>
> ;; Definition: [refines the header]
>
> (define (area-of-ring outer inner)
>   (- (area-of-disk outer)
>
>      (area-of-disk inner)))
>
> ;; Tests:
> (area-of-ring 5 3)
>
> ;; expected value
> 50.24
>
>