Unnesting parentheses in Lisp

Question

I want to write code (using recursive function) to unnest the parentheses in a LIST in Lisp language. Example:

(unnest '(1 (2 (3)) (4 5))) ==> (1 2 3 4 5)

Thanks. Any help is appreciated!

Answer 1

(defun unnest (lst)
  (cond ((null? lst) '())
        ((not (list? lst)) (list lst))
        (t
         (append (unnest (car lst))
                 (unnest (cdr lst))))))

> (unnest '(1 (2 (3)) (4 5))) 
(1 2 3 4 5)

Basically the idea is as follows:

• if you have an empty list, then you obviously don't need to unnest it;
• if it's not a list, then it must be an atom, and therefore you return a list containing that atom;
• in the last condition, you have a list, so you basically say: the result of an unnested list is the unnested version of the first element appended to the unnested version of the rest of the list, and that's it, recursion takes care of the rest.

Hope it helps.