averaging

[Warren Wilkinson]

Now here is a working 'reduce' example:

Code: Select all

(defun average (lst)
  (/ (reduce #'+ lst :key #'(lambda (x) (first (last x))))
     (length lst)))

The :key argument tells 'reduce' to which element from the lists inside 'lst' the '+' function shall be applied. Because 'last' returns the last element as a list, the element itself must be extracted with 'first'...

Isn't #'(lambda (x) (first (last x))) the same as #'second in this case?

Code: Select all

(defun avg (lst) (/ (reduce #'+ lst :key #'second) (length lst)))

[edgar-rft]

Isn't #'(lambda (x) (first (last x))) the same as #'second in this case?

Yes, it is.

I'm used to search rather huge trees, so seeing something at the end of a list automatically makes me write (first (last x))...

The #'second solution in this case is clearly better, because no lambda construct is needed at all.

[Warren Wilkinson]

Yeah, I use that too --- Its often inconvenient that last returns a list rather than an item. Maybe I'll just define a utility function

Code: Select all

(defun last-item (list) (car (last list)))

and be done with it.