Find index of an element matching a predicate in Clojure?

Question

With Clojure, how do I find the first index with a positive value in this vector [-1 0 3 7 9]?

I know you can get the first result of something rather elegantly with first and filter:

(first (filter pos? [-1 0 99 100 101]))

This code returns the value 99. The answer I want is the index which is 2.

Answer 1

Using keep-indexed you can get a sequence of indices for which a predicate is satisfied:

(defn indices [pred coll]
   (keep-indexed #(when (pred %2) %1) coll))

With this simple function you'll solve your problem with the expression

user=> (first (indices pos? [-1 0 99 100 101]))
2

Note that, due to the lazyness of keep-indexed (and indices), the entire sequence need not be realized so no extraneous calculations are performed.