# [plt-scheme] Re: HTDP Exercise 9.5.7

 From: S Brown (ontheheap at gmail.com) Date: Mon Apr 13 17:54:14 EDT 2009 Previous message: [plt-scheme] HTDP Exercise 9.5.7 Next message: [plt-scheme] Creating a suspended thread Messages sorted by: [date] [thread] [subject] [author]

```I apologize for posting complete code to a book exercise, it won't
happen again! That said, I appreciate the feedback from everyone and
I'm glad to see that I'm on the right track.

On Apr 13, 12:12 pm, S Brown <ontheh... at gmail.com> wrote:
> I'm working my way through HTDP and I have a question regarding
> Exercise 9.5.7. The exercise asks you to define the function average-
> price, which takes as it's input a list of prices, and gives as output
> the average of the prices in the list. I came up with  the following
> solution:
>
> <pre>
> (define (checked-average-price toy-list)
>   (cond
>     ((empty? toy-list) (error 'checked-average-price "expected a non-
> empty list"))
>     (else (average-price toy-list))))
>
> (define (average-price toy-list)
>   (cond
>     ((empty? toy-list) 0)
>     (else (/ (sum-list toy-list)
>              (num-items toy-list)))))
>
> (define (sum-list a-list)
>   (cond
>     ((empty? a-list) 0)
>     (else (+ (first a-list)
>              (sum-list (rest a-list))))))
>
> (define (num-items a-list)
>   (cond
>     ((empty? a-list) 0)
>     (else (+ 1 (num-items (rest a-list))))))
> </pre>
>
> I came to this solution after not being able to figure out how to keep
> track of the sum of the items and also the number of items at the same
> time. So my concern/question is, is my solution acceptable, or should
> I keep trying to figure out how to keep track of both values (sum and
> number of items) without resulting to helper-functions such as sum-
> list and num-items? Thanks.
> _________________________________________________