Adding an element to a set: `addelt'

This abstraction adds an element to a list if the element is not yet contained in the list thus preserving the essential characteristic of sets.

(define addelt 
   (lambda(x s)
      (if (memberp x s)
         s
         (cons x s))))

(define l1 (cons one (cons two (cons three 
                                        (cons four 
                                               nil)))))
;
(ldisp! l1)
                             --> 1
                                 2
                                 3
                                 4
;
(ldisp! (addelt three l1)) ;; ineffective because already a member
                             --> 1
                                 2
                                 3
                                 4
;
(ldisp! (addelt five l1))  ;; effective because not yet a member
                             --> 5
                                 1
                                 2
                                 3
                                 4
;

Georg P. Loczewski 2004-03-05

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