Recursive Binary Search

chop-recur.lisp

(defun chop-1 (num tree)
  (labels ((recur (num tree start-index)
             (if (equalp 0 (length tree))  -1
                 (let* ((mid-point (truncate (/ (length tree) 2)))
                       (mid-num (elt tree mid-point)))
                   (cond
                     ((= (length tree) 0) -1)
                     ((= mid-num num) (+ mid-point start-index))
                     ((= (length tree) 1) -1)
                     ((> mid-num num) (recur num (subseq tree 0 mid-point) 0)) ;; check in first half of tree
                 (t (recur num (subseq tree mid-point) mid-point)))))))
  (recur num tree 0))) ;; check in second half