We want to know the index of the vowels in a given word, for example, there are two vowels in the word 'super' (the second and fourth letters).

vowel-indices.clj

(def vowels #{\a \A \e \E \i \I \o \O \u \U \y \Y})

;; First approach
(defn vowel-indices [word]
	(let [matches (map vowels (seq word))]
    (into [] (remove nil? (map-indexed (fn [idx el] (if-not (nil? el) (+ idx 1))) matches)))))

;; Better approach
(defn vowel-indices [word]
  (keep-indexed #(when (vowels %2) (inc %1)) word))