Comp 210 Lab 3: Non-empty Lists of Numbers

• Solution 1: The base case should describe all lists of length one, the shortest possible non-empty lists:

       ; A Non-empty-List-of-Numbers is one of
       ;   - (cons num empty)
       ;   - (cons num Non-empty-List-of-Number)
  

  In this case, the template looks as follows:

  (define (func-for-nelon a-nelon)
     (cond
        [(empty? (rest a-nelon)) ..(first a-nelon)..]
        [else                    ..(first a-nelon)..(func-for-nelon (rest a-nelon))..]))
  

• Solution 2: We could use the definition of (perhaps-empty) list-of-numbers, that we've already seen.

       ; A Non-empty-List-of-Numbers is
       ;   (cons num List-of-Numbers)
  

  Here, the template is different, since it must refer to the template for a regular (non-empty) List-of-Numbers. In particular, you don't make a recursive call; you think to yourself "Ah, a-nelon is a (perhaps-empty) list; i'd better call a function for such regular ol' lists":

  (define (f a-nelon)
     ...(first a-nelon)...(func-for-lon (rest a-nelon)...)
  

  To compute the average, what function(s) of general (perhaps-empty) lists do you want to call, as your helper?
The first choice is usually the better option, but you're welcome to try both to see why. (Be sure to follow the template, and not to do data-extraction on sub-pieces you've already extracted!)