Clojure sub-sequence position in sequence

Question

Does Clojure provide any builtin way to find the position of a sub-sequence in a given sequence?

Answer 1

Clojure provides a builtin way for easy Java Interop.

(java.util.Collections/indexOfSubList '(a b c 5 6 :foo g h) '(5 6 :foo))
;=> 3

Answer 2

A sequence is an abstraction, not a concretion. Certain concretions that you can use through the sequence abstraction have a way to find the position of a subsequence (strings and java collections, for instance), but sequences in general don't, because the underlying concretion doesn't have to have an index.

What you can do however, is create a juxt of the element identity and an index function. Have a look at map-indexed.

Here's a naive implementation that will lazily find the position of (all) the subsequence(s) in a sequence. Just use first or take 1 to find only one:

(defn find-pos
  [sq sub]
  (->>
    (partition (count sub) 1 sq)
    (map-indexed vector)
    (filter #(= (second %) sub))
    (map first)))

=> (find-pos  [:a :b \c 5 6 :foo \g :h]
                [\c 5 6 :foo])
(2)

=> (find-pos  "the quick brown fox"
                (seq "quick"))
(4)

Take care that index-based algorithms generally aren't something you would do in a functional language. Unless there are good reasons you need the index in the final result, lavish use of index lookup is considered code smell.