I'm pretty new to closure and I don't understand why I'm getting this error message at runtime. Here is my code:
(defn invert [lst]
(if (empty? lst)
()
(cons (invert-helper lst) (invert (rest lst)))))
(defn invert-helper [lst]
(list (nth lst 1) (first lst)))
This should fix the problem:
(defn invert-helper [lst]
(list (nth lst 1) (first lst)))
(defn invert [lst]
(if (empty? lst)
()
(cons (invert-helper lst) (invert (rest lst)))))
That is: the invert-helper function must be defined before its first use in invert.
(nth lst 1) where lst may have only one element - this will throw the exception you're seeing.
lst might have less than two elements. Just to be sure: don't forget that indexes in clojure start at 0, that is: the first element is (nth lst 0), the second is (nth lst 1) and so on.Another option, apart from defining all the functions before using them, may be declaring invert-helper before invert:
(declare invert-helper)
(defn invert [lst]
(if (empty? lst)
()
(cons (invert-helper lst) (invert (rest lst)))))
(defn invert-helper [lst]
(list (nth lst 1) (first lst)))
You're also calling (nth lst 1) where lst may have only one element - this will throw an exception.