Question

I am trying to write a function that returns a list of atoms in a list,lets say i have a list which has both atoms and a list in it and when I run the function it should return a list of atoms inside..

For example:

(func '(2 34 5 (12) 7 (A B C) +))
 -> (2 34 7 +)

And I want to try if the arguments inside the result are true such that when I run:

(ATOM ( (func '(2 34 5 (12) 7 (A B C) +)) )
->T

Any ideas on how I can go about it? Books or references?

Answer 1

Using standard CL functions,

[3]> (remove-if-not #'atom '(1 2 (12) +))
(1 2 +)

[6]> (every #'atom (remove-if-not #'atom '(1 2 (12) +)))
T

If you want to write them yourself, you can fuse the #'atom in and make two specialized functions, say remove-atoms and every-is-atom. But “atom” is a name of a built-in function atom.

One way to write the first is

(defun remove-atoms (xs &aux (ys (list 1)))
  (let ((p ys))
    (dolist (x xs (cdr ys))
      (if (atom x) (setf (cdr p) (list x) p (cdr p))))))

This builds the result list in a top-down manner using destructive update, which doesn’t violate the spirit of functional programming here because it is used locally, as an implementation technique. This can be seen as a Common-LISP–specific translation of the general functional tail-recursive-modulo-cons code from the other answer.

And the second function:

(defun every-is-atom (xs)
  (dolist (x xs T)
    (if (not (atom x)) 
      (return-from every-is-atom NIL))))