Writing a function to search for matches in lists

Question

I'm trying to make a function that will return true if it finds a match for an item in another list. But it seems like everything should work, but on a call (f1 '(2) '(1 2 3)) return nil

(defun f1 (x y)
 (cond 
  ((null y) x)
  ((equalp x (car y))
  (t (f1 x (cdr y))))))

Answer 1

Your code after I auto-format it in Emacs is as follows:

(defun f1 (x y)
  (cond
   ((equalp x(car y))
    (t(f1(x (cdr y)))))))

There is one clause in the cond, so the part that starts with (t ...) is in fact a call to function t (which does not exist). Also, you are calling f1 with (x (cdr y)) but x is not a function either.

You are also comparing x with (car y), but in your test you pass two lists as arguments:

(f1 '(2) '(1 2 3))

I think you should first write an auxiliary function belongs-to, which finds if some value exists in a list:

(defun belongs-to (value list)
  "Returns non-NIL if VALUE is found in LIST using EQUALP"
  (cond
    ;; empty list
    (... NIL)
    ;; general case - match head of list
    (... T)
    ;; general case - recurse on tail of list
    (T (belongs-to ... ...))))

Once you have this function, you can use it in f0:

(defun f0 (x y)
  (and x (or (belongs-to ... ...)
             (f0 ... y))))