I am learning Lisp and have a problem where I have to remove all the numbers from a list, which also has nested lists. For example:
(REMOVE-NUM '(2 A (3 B) C D))
will output:
(A (B) C D)
I am not allowed to use loop, if, or mapcan. I am to use basic operators and cond statements. This is my current code:
(defun REMOVE-NUM (L)
(cond
((null L) nil)
((numberp (car L)) (REMOVE-NUM (cdr L)))
(t (cons (car L) (REMOVE-NUM (cdr L))))
)
My current code can remove the numeric atoms without nested lists, but once nested lists are involved, it lists out the nested list, without actually processing it. I know that this is an issue with my logic behind the recursion, but I for some reason cannot see it. My instinct is to change the last line to:
(t (cons (REMOVE-NUM (car L) (REMOVE-NUM (cdr L))))
another recursive call in the last line, but doing so gives an error because it tries to take car of an atom and pass it as a list to my function which can only take lists. I thought of maybe changing the base cases, but I can't think of a way to do so since numberp has to be condition used. Where is my recursion going wrong/what is it missing?
Isn't it obvious? Separate atom (which could be unified with the null case actually) and the listp (which could then be simply t) cases. You'll need to accept that your input isn't necessarily a list if you want recursion all the way, or else you need to peek at the cdr and make sure it's listp before you recurse. Throw in a cons or two in your test case.
(REMOVE-NUM '(2 A (3 B) C (D . 7) (8 . E) F))
If you don't need to handle cons cells in your lists, simply
(defun REMOVE-NUM (L)
(cond
((atom L) L)
((numberp (car L)) (REMOVE-NUM (cdr L)))
(t (cons (REMOVE-NUM (car L))
(REMOVE-NUM (cdr L)) ))))