Дана квадратная матрица размера NxM. Рассчитать след матрицы

(defun matrix-main (w &optional (n 0))
  (if (null w) 0 (+ (nth n (car w)) (matrix-main (cdr w) (1+ n)))))
 
> (setf a '((1 0 0) (0 1 0) (0 0 1)))
((1 0 0) (0 1 0) (0 0 1))
> (matrix-main a)
3
> (setf a '((1 0 0 0) (0 1 0 0) (0 0 1 0) (0 0 0 1)))
((1 0 0 0) (0 1 0 0) (0 0 1 0) (0 0 0 1))
> (matrix-main a)
4

(defun trace-matr (matr)
  (iter (for r in matr) (for k upfrom 0) (summing (nth k r))))
 
==> TRACE-MATR
 
(trace-matr '((1 2 3) (4 5 6) (7 8 9)))
 
==> 15

(defun trace-matr (matr)
  (apply '+ (mapcar 'nth (range 0 (- (length matr) 1)) matr)))
 
==> TRACE-MATR
 
(trace-matr '((1 2 3) (4 5 6) (7 8 9)))
 
==> 15

(defun matrix-main (w)
  (loop for a in w 
        for b upfrom 0
        sum (nth b a)))
 
> (matrix-main '((1 0 0) (0 1 0) (0 0 1)))
3
> (matrix-main '((1 0 0 0) (0 1 0 0) (0 0 1 0) (0 0 0 1)))
4

(defun matrix-main (w)
  (loop with b = -1 for a in w do
        (incf b)        
        sum (nth b a)))
 
> (matrix-main '((1 0 0) (0 1 0) (0 0 1)))
3
> (matrix-main '((1 0 0 0) (0 1 0 0) (0 0 1 0) (0 0 0 1)))
4

(defun matrix-main (w)
  (loop with b = -1 for a in w
        sum (nth (incf b) a)))
 
> (matrix-main '((1 0 0) (0 1 0) (0 0 1)))
3
> (matrix-main '((1 0 0 0) (0 1 0 0) (0 0 1 0) (0 0 0 1)))
4

(defun matrix-main (w &aux (n -1))
  (reduce #'+ w :key #'(lambda (a) (nth (incf n) a))))
 
> (matrix-main '((1 0 0) (0 1 0) (0 0 1)))
3
> (matrix-main '((1 0 0 0) (0 1 0 0) (0 0 1 0) (0 0 0 1)))
4