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lisp

lisp list with even position elements

I am trying to get a list from the list given as parameter that has only elements at even positions.

So if I have '(a b c d e f) I should get '(a c e). I also want to do it iteratively with do.

(defun pozpar (lst)
  (do ((l lst (cdr l))
       (y 0 (+ y 1))
       (x '() (cond ((eql 0 (mod y 2))
                     (cons (car l) x)))))
      ((null l) x)))
POZPAR
> (pozpar '(a b c d e f))
NIL

I don't see what's wrong with the code and why the list has only nil.

Solution

  • You missed the "else" clause of cond.

    Try this

    * (defun pozpar(lst)
    (do ((l lst (cdr l))
         (y 0 (+ y 1))
         (x '() (cond ((eql 0 (mod y 2)) (cons (car l) x))
                      (t x))))
        ((null l) x)))

* (reverse (pozpar '(a b c d e f)))
(A C E)