[SOLVED] a function in lisp called xtoy which returns a list from x to y

a function in lisp called xtoy which returns a list from x to y

I am very new to lisp. I am trying to write a function named x2y which takes 2 arguments x and y which are integers and return a list of integers which starts from x and ends at y

(defun xtoy (X Y)
  (cond ((> X Y) (list nil))
        ((= X Y) (list Y)))
        (T (append (list X) x2y(+ 1 X) Y)))))

Solution

Starting with abo-abo's version, you can simplify a lot:

1) get rid of (= X Y) and replace (list nil) by nil in (> X Y)

(defun xtoy (X Y)
  (cond ((> X Y) nil)
        (t (append (list X) (xtoy (+ 1 X) Y)))))

2) simplify cond to an ifstatement

(defun xtoy (X Y)
  (if (<= X Y)
    (append (list X) (xtoy (+ 1 X) Y))
    nil))

3) leave out the final nil, because that's what's implicitly returned when the condition doesn't match

(defun xtoy (X Y)
  (if (<= X Y)
    (append (list X) (xtoy (+ 1 X) Y))))

4) use cons instead of append

(defun xtoy (X Y)
  (if (<= X Y)
    (cons X (xtoy (+ 1 X) Y))))