I've tried several versions of this Clojure routine but it always results in a null pointer. How can I eliminate the errors?
(defn loopthru [n] (
(if-not (empty? n) (
(println (first n))
(loopthru (rest n))
))
))
(loopthru [1 2 3 4 5 6])
Thanks, Greg
As everyone pointed out, you have double parentheses around your if-not macro which is not correct. (Double parentheses are rarely correct in Clojure, unlike Scheme.) But there is also another problem in your if special form. There should be a do special form which evaluates the s-expressions in order.
(defn loopthru [n]
(if-not (empty? n)
(do (println (first n))
(loopthru (rest n)))))
A couple of other things. Use when / when-not in cases where you do not have an else block in your if statement. In fact, using when-not in this case eliminates the need for do since there is no ambiguity in the s-expressions with respect to the conditional. And I have to mention the obligatory comment that recursion in this case will chew up stack space so use recur instead
(defn loopthru [n]
(when-not (empty? n)
(println (first n))
(recur (rest n))))
(defn loopthru [col]
(doseq [n col] (println n)))
(defn loopthru [f col]
(doseq [item col] (f item)))
The pair of parentheses around the body of the function should be omitted. Then it will work.
(defn loopthru [n] (if-not (empty? n) (println (first n)) (loopthru (rest n))))