CODEWITHSUNDEEP
common-lisp
user2161813
user2161813

Reputation: 23

What's wrong with my Lisp code?

I have some lisp code that I have to write multiple different ways. I've tried two different ways and I'm unsure of why they don't work. The function has to take a list and return only integers in that list. The first one has to take a map function and the second one has to use loop. This is in Clisp.

Here is my code. 

(defun posint1 (l)
  (mapcan #'(lambda (x)
              (and (integerp x)
                   (list l)))))

and 

(defun posint1 (l)
  (loop for x in (list l)
        when (integerp x)
        collect x)
  (format t "~A " x))))

Upvotes: 0

Views: 119

Answers (1)

Sylwester
Sylwester

Reputation: 48745

mapcan requires at least one list argument and you have supplied none in your first function (I can't use names since you call both the same)

The second function tried to format a variable, x, that does not exist in that scope. In loop x are the one element in (list l) which probably should just be l since making a one number list doesn't really need iteration. Perhaps you wanted something like:

(defun print-integers (list)
  (loop :for x :in list
        :when (integerp x)
        :collect x :into result
        :finally (format t "~A " result)))

;; or using the result as argument
(defun print-integers (list)
  (format t
          "~A "
          (loop :for x :in list
                :when (integerp x)
                :collect x)))

(print-integers '(-1 0.5 1/3 +inf.0 9)) ; ==> NIL (prints (-1 9)

Also notice that integerp works for whole numbers, also negative ones. The name hinted that you might want to use (and (integerp x) (>= x 0)) instead.

Upvotes: 2

Related Questions