Computing functions over multiple lists

Question

I need to produce a list containing the averages of a lists of lists where the number of sub-lists could vary. So given the input list:

((l1a l1b l1c) (l2a l2b l2c) (l3a l3b l3c)...)

the output would be:

(average(l1a l2a l3a) average(l1b l2b l3b) average(l1c l2c l3c)...).

I'm sure there's a really elegant way to do this in lisp but I don't know where to start. Any advice would be gratefully received.

It looks like you just need to map. I don't know CL, but I do know Clojure, which is another lisp. If your lists are bound to a symbol lists, you'd just do, in Clojure, (map average lists). I'm sure it's very similar in CL. — Carcigenicate
– Carcigenicate, Commented Nov 18, 2018 at 21:35
That's it! Thank you. I knew there would be an easy way but wasn't aware of the map function. In CL to do that using add for example you would simply use: (map 'list #'+ '(1 2 3) '(1 2 3) '(1 2 3)) — djpeebles
– djpeebles, Commented Nov 18, 2018 at 21:51
Np. And if you're just starting into functional programming, make sure you have a very good understanding of map and reduce/fold (I don't know what CL calls it). Those are bread-and-butter functions that you'll likely be using regularly. — Carcigenicate
– Carcigenicate, Commented Nov 18, 2018 at 22:01
Carcinegate is correct. You need to define an average function that takes a list as a parameter then pass it to mapcar . The syntax is different from Clojure - use (mapcar #'average list) where average is the function you define. The average function should use reduce to sum the list. — David Hodge
– David Hodge, Commented Nov 18, 2018 at 22:09
Got it - thank you. (defun average (lst) (/ (reduce #'+ lst) (length lst))) and then (mapcar #'average '((50 23 35) (60 12 34) (70 52 38))) — djpeebles
– djpeebles, Commented Nov 18, 2018 at 22:43

Rainer Joswig · Accepted Answer · 2018-11-18 22:56:35Z

6

CL-USER 27 > (let* ((list-of-lists '((1.0 2.0 3.0)
                                     (1.0 3.0 5.0)
                                     (1.0 4.0 6.0)
                                     (1.0 5.0 8.0)))
                    (length (length list-of-lists)))
               (mapcar (lambda (x)
                         (/ x length))
                       (reduce (lambda (l1 l2)
                                 (mapcar #'+ l1 l2))
                               list-of-lists)))
(1.0 3.5 5.5)

answered Nov 18, 2018 at 22:56

Rainer Joswig

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1 Comment

djpeebles Over a year ago

Yes, I realised it was wrong when I just tried to apply my solution to my problem. Many thanks for your solution Rainer.

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