Cannot do sum in lisp with do loop

Question

(defun suma (L)
  (setq var 0)
  (do 
      ((i 0 (+ i 1)))
      ((= i (length L)))
    (+ var (nth i L)))
  var)

Why does it always returns 0?

Shouldn't it return sum of list L?

Answer 1

+ does not modify its arguments, so, since you never modify var, its initial value of 0 is returned.

You need to replace (+ var (nth i L)) with (incf var (nth i L)), of, equivalently, (setq var (+ var (nth i L))).

See incf.

Note that you should bind var with let instead of making it global with setq.

Most importantly, note that your algorithm is quadratic in the length of the list argument (because nth scans your list every time from the start).

Here are some better implementations:

(defun sum-1 (l)
  (reduce #'+ l))

(defun sum-2 (l)
  (loop for x in l sum x))

(defun sum-3 (l)
  (let ((sum 0))
    (dolist (x l sum)
      (incf sum x))))

Here is a bad implementation:

(defun sum-4 (l)
  (apply #'+ l))

The problem with sum-4 is that it will fail if the length of the supplied list is larger than call-arguments-limit.