Ok .I think I got the difference now. For the search-sorted we can check for the first and then if the number is greater than the first then the number is not in the list only otherwise we can check for the first and do the recursion on the rest. This is how I have done it. ;; search : number list-of-numbers -> boolean (define (search n alon) (cond [(empty? alon) false] [else (cond [(> n (first alon)) false] [else (or (= (first alon) n) (search n (rest alon)))])])) (define num (cons 3 (cons 2 (cons 1 empty)))) (search -1 num) (search 1 empty) (search 10 num) (search 3 num) (search 2 num) Thanks Aditya