Other possibilities

(defun incr (n &optional inc)

;; if the value of inc was not specified,

;; or if it was specified but not a number,

;; then set it to 1.

(if (not (numberp inc)) (setq inc 1))

(+ n inc))

(defun incr (n &optional (inc 1 increment-p))

;; increment-p TRUE when argument was specified

;; by caller

(if (and increment-p (not (numberp inc)))

(print "non-numeric increment"))

(+ n inc))

Previous slide

Next slide

Back to first slide

View graphic version