##全探索##

search.scm

(define (binarysearch lst n)
  (let loop ((lst lst))
	(receive(left right)
	  (split-at lst (div (length lst) 2))
		(cond ((null? left)#f)
			  ((null? right)#f)
			  ((eq? (last left) n)#t)
			  ((eq? (car right) n)#t)
			  ((> (last left) n)(loop left))
			  ((< (car right) n)(loop right)))))))

##二分探索##

binarysearch.scm

(define (binarysearch lst n)
  (let loop ((lst lst))
	(receive(left right)
	  (split-at lst (div (length lst) 2))
		(cond ((null? left)#f)
			  ((null? right)#f)
			  ((eq? (last left) n)#t)
			  ((eq? (car right) n)#t)
			  ((> (last left) n)(loop left))
			  ((< (car right) n)(loop right))))))

##結果##
(1 3 5 11 12 13 17 22 25 28)から25を探索するとき
###全探索###

(1 3 5 11 12 13 17 22 25 28)
(3 5 11 12 13 17 22 25 28)
(5 11 12 13 17 22 25 28)
(11 12 13 17 22 25 28)
(12 13 17 22 25 28)
(13 17 22 25 28)
(17 22 25 28)
(22 25 28)
(25 28)

##二分探索##

(1 3 5 11 12):(13 17 22 25 28)
(13 17):(22 25 28)
(22):(25 28)

二分探索のほうが圧倒時に…はい