How do I find the index of an item in a vector?

Question

Any ideas what ???? should be? Is there a built in? What would be the best way to accomplish this task?

(def v [\"one\" \"two\" \"three\" \"two         


        

Answer 1

Stuart Halloway has given a really nice answer in this post http://www.mail-archive.com/clojure@googlegroups.com/msg34159.html.

(use '[clojure.contrib.seq :only (positions)])
(def v ["one" "two" "three" "two"])
(positions #{"two"} v) ; -> (1 3)

If you wish to grab the first value just use first on the result.

(first (positions #{"two"} v)) ; -> 1

EDIT: Because clojure.contrib.seq has vanished I updated my answer with an example of a simple implementation:

(defn positions
  [pred coll]
  (keep-indexed (fn [idx x]
                  (when (pred x)
                    idx))
                coll))