<div class="gmail_quote">On Wed, Dec 22, 2010 at 1:02 PM, Danny Yoo <<a href="mailto:dyoo@cs.wpi.edu">dyoo@cs.wpi.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;"> <div class="im">>> (define (netpay gross tax-rate) >> (-(gross)(* gross tax-rate))) >> >> So I expect the function to calculate as >> >> = (-(240)(* 240 0.15) >> = ( - 240 36) >> = 204 </div>Just to be more careful: when you're showing the calculation, make sure to include the use of the function: (netpay 240 0.15) <div class="im"> = (-(240)(* 240 0.15) = ( - 240 36) = 204 </div>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: <div class="im"> function call: expected a defined function name or a primitive operation name after an open parenthesis, but found a function argument name </div>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. </blockquote></div> 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?