Yet another one using -apply-: (define (same-parity x . xs) (cond ((null? xs) (list x)) ((equal? (even? x) (even? (car xs))) (cons x (apply same-parity xs))) (else (apply same-parity x (cdr xs)))))
Jos > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Martin DeMello > Sent: 20 July 2010 15:37 > To: PLT-Scheme Mailing List > Subject: [racket] sicp exercise 2.20 > > By way of a puzzle, I've been trying to solve SICP exercise 2.20 > ---------------------------- > Using the (define (f x . args)) notation, write a procedure > "same-parity" that takes one or more integers and returns a > list of all the arguments that have the same even-odd parity > as the first argument. For example, (same-parity 1 2 3 4 5 6 7) > (1 3 5 7) > (same-parity 2 3 4 5 6 7) > (2 4 6) > ---------------------------- > using "straight recursion", that is, without using any `let` > or `define` constructs. Still not managed to find the trick > that will tack on the first argument as the head of the list. > Anyone have a hint? > > martin > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
