sicp-2.59.lisp

(define (element-of-set? x set)
  (cond ((null? set) false)
        ((equal? x (car set)) true)
        (else (element-of-set? x (cdr set)))))


(define (adjoin-set x set)
  (if (element-of-set? x set)
      set
      (cons x set)))


(define (intersection-set set1 set2)
  (cond ((or (null? set1) (null? set2)) '())
        ((element-of-set? (car set1) set2)
         (cons (car set1)
               (intersection-set (cdr set1) set2)))
        (else (intersection-set (cdr set1) set2))))

(define (union-set set1 set2) 
  (cond ((null? set1) set2)
        ((null? set2) set1)
        (else
         (if (element-of-set? (car set1) set2)
            (union-set (cdr set1) set2)
            (union-set (cdr set1) (cons (car set1) set2))))
        ))
        
;; define variables for testing 
(define set1 '(1 2 3))
(define set2 '(2 3 4))

; adjoin
(adjoin-set 3 set1) ; '(1 2 3)
(adjoin-set 4 set1) ; '(4 1 2 3)

; intersection
(intersection-set set1 set2)  ; '(2 3)

; union
(union-set set1 '())  ; '(1 2 3)
(union-set set1 set2) ; '(1 2 3 4)