(define (average ....)) in Lisp

Question

I'm just playing around with scheme/lisp and was thinking about how I would right my own definition of average. I'm not sure how to do some things that I think are required though.

• define a procedure that takes an arbitrary number of arguments
• count those arguments
• pass the argument list to (+) to sum them together

Does someone have an example of defining average? I don't seem to know enough about LISP to form a web search that gets back the results I'm looking for.

Answer 1

The definition would be a very simple one-liner, but without spoiling it, you should look into:

• a "rest" argument -- this (define (foo . xs) ...xs...) defines foo as a function that takes any number of arguments and they're available as a list which will be the value of xs.

• length returns the length of a list.

• apply takes a function and a list of values and applies the function to these values.

When you get that, you can go for more:

• see the foldl function to avoid applying a list on a potentially very big list (this can matter in some implementations where the length of the argument list is limited, but it wouldn't make much difference in Racket).

• note that Racket has exact rationals, and you can use exact->inexact to make a more efficient floating-point version.

---

---

---

---

And the spoilers are:

• (define (average . ns) (/ (apply + ns) (length ns)))

• Make it require one argument: (define (average n . ns) (/ (apply + n ns) (add1 (length ns))))

• Use foldl: (define (average n . ns) (/ (foldl + 0 (cons n ns)) (add1 (length ns))))

• Make it use floating point: (define (average n . ns) (/ (foldl + 0.0 (cons n ns)) (add1 (length ns))))

Answer 2

In Common Lisp, it looks like you can do:

(defun average (&rest args)
  (when args
    (/ (apply #'+ args) (length args))))

although I have no idea if &rest is available on all implementations of Lisp. Reference here.

Putting that code into GNU CLISP results in:

[1]> (defun average (&rest args)
       (when args
         (/ (apply #'+ args) (length args))))
AVERAGE
[2]> (average 1 2 3 4 5 6)
7/2

which is 3.5 (correct).