Question

I am trying to set each element of a vector equal to 3 by looping. I get:

java.lang.ClassCastException: clojure.lang.PersistentVector can
not be cast to java.lang.Number

This is the code.

(def w [1 2 3])
(defn update [index value]
  (assoc w index value))

(loop [i -1]
  (if (< (count w) i)
    w
    (recur (update (+ i 1) 3))))

Answer 1

You are mixing integer and vector type by passing (update …) to recur function. The loop statement should be look like (loop [i -1 w w] ..) and then you could collect your new vector into the local “w”. If you want to use recure statement this code can help you (I guess there are a lot of other options to change values of a vector):

(let [value 4
      w [1 2 3]]

  (loop [i  0
         w  w]
    (if (< i (count w))
      (recur (inc i) (assoc w i value))
      w)))