MEMBERA and a list of atoms LAT,
determine whether A occurs in LAT:
(DEFUN MEMBER (A LAT)
(COND
((NULL LAT) NIL)
((EQ A (CAR LAT)) T)
(T (MEMBER A (CDR LAT))) ) )
(Actually, MEMBER is already defined in LISP. Older LISP
systems will let you redefine built-in functions, and will use your
function in preference to the built-in version. The only Common LISP
system that I have used will let you redefine built-in functions, but
then gets horribly confused. In any case, redefining built-in
functions is generally a bad idea.)
Define a set to be a list of atoms, such
that no atom is ever repeated, and the order of atoms does not
matter. The following function computes the union of two sets,
that is, a third set containing any atom found in either or both of
the two given sets.UNION
(DEFUN UNION (SET1 SET2)
(COND
((NULL SET1) SET2)
((MEMBER (CAR SET1) SET2) (UNION (CDR SET1) SET2))
(T (CONS (CAR SET1) (UNION (CDR SET1) SET2))) ) ) )
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