# [plt-scheme] HtDP > Chapter 2 > Exercise 2.3.3.

 From: Matthias Felleisen (matthias at ccs.neu.edu) Date: Fri Jun 27 09:59:25 EDT 2008 Previous message: [plt-scheme] HtDP > Chapter 2 > Exercise 2.3.3. Next message: [plt-scheme] Re: HtDP > Chapter 2 > Exercise 2.3.3. Messages sorted by: [date] [thread] [subject] [author]

```So you're working on a problem that is mentioned *before* the idea of
program design has been introduced into the book.

1. It's supposed to convince you that programming properly isn't
easy, despite all the books on "programming idiots" and "dummies for
programming" you can find in local book stores or mother's blogs for
programming.

2. In the next edition, we won't be indirect. We'll just say what we
said for 1.

3. For now, I recommend the following:

-- forget Scheme
-- write down a table like this:

# of customers : 0 | 1 | 2 | 3 | 10 | 100 | 200 |  n
--------------------------------------------------------
total profit   :   |   |   |   |    |     |     |

Fill out all the slots for concrete numbers. Once you get the hang of
it, fill out the entry for n. Then -- and only then -- translate into
Scheme.

The problem has very little to do with programming, but is all about
learning to read problem statements. Holler if you have problems with
the table.

-- Matthias

On Jun 26, 2008, at 6:30 PM, Mr. Watson wrote:

> Hello,
>
> The problem says,
>
> "An old-style movie theater has a simple profit function. Each
> customer pays \$5 per ticket. Every performance costs the theater \$20,
> plus \$.50 per attendee. Develop the function total-profit. It consumes
> the number of attendees (of a show) and produces how much income the
> attendees produce."
>
> I understand the problem but I cannot get it to work (not calculate
> correctly) in DrScheme.
>
> Here is my working progress:
>
> (define(total-profit attendee)
>   (- 0
>    (- 20 0
>     (* 0.5 attendee)
>     (+ 0
>      (* 5 attendee)))))
>
> Any ideas on what I am doing wrong? I might be miscalculating so tell
> me if its right.
>
> Thank you,
> _________________________________________________