<div class="gmail_quote">On Wed, Dec 22, 2010 at 2:04 PM, Robby Findler <<a href="mailto:robby@eecs.northwestern.edu">robby@eecs.northwestern.edu</a>> wrote: <blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"> 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: <a href="http://www.htdp.org/" target="_blank">http://www.htdp.org/</a> Robby <div><div></div><div class="h5"> On Tue, Dec 21, 2010 at 8:49 PM, Sayth Renshaw <<a href="mailto:flebber.crue@gmail.com">flebber.crue@gmail.com</a>> wrote: > > > On Wed, Dec 22, 2010 at 1:19 PM, Sayth Renshaw <<a href="mailto:flebber.crue@gmail.com">flebber.crue@gmail.com</a>> > wrote: >> >> >> On Wed, Dec 22, 2010 at 1:02 PM, Danny Yoo <<a href="mailto:dyoo@cs.wpi.edu">dyoo@cs.wpi.edu</a>> 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 > </div></div>> _________________________________________________ > For list-related administrative tasks: > <a href="http://lists.racket-lang.org/listinfo/users" target="_blank">http://lists.racket-lang.org/listinfo/users</a> > </blockquote></div> Indeed I know what you are referencing. <pre class="scheme">;; Contract: <code class="scheme">area-of-ring : number number <tt>-></tt> number</code> ;; Purpose: to compute the area of a ring whose radius is ;; <code class="scheme">outer</code> and whose hole has a radius of <code class="scheme">inner</code> ;; Example: <code class="scheme">(area-of-ring 5 3)</code> 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 </pre>