I was trying to implement a simple binary search using a purely functional approach, and after much hacking, googling and wikibooking, came up with this in Clojure.
(defn binarysearch
([lst n]
(binarysearch lst 0(dec (count lst)) n))
([lst lb ub n]
(if(> lb ub)-1; this is the case where no element is found
(let[mid (quot (+ lb ub)2)
mth (nth lst mid)]
(cond
; mid > n, so search lower
(> mth n)(recur lst lb (dec mid) n)
; mid < n, search upper
(< mth n)(recur lst (inc mid) ub n)
([lst n]
(binarysearch lst 0(dec (count lst)) n))
([lst lb ub n]
(if(> lb ub)-1; this is the case where no element is found
(let[mid (quot (+ lb ub)2)
mth (nth lst mid)]
(cond
; mid > n, so search lower
(> mth n)(recur lst lb (dec mid) n)
; mid < n, search upper
(< mth n)(recur lst (inc mid) ub n)
