<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> <HTML> <HEAD> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-7"> <META NAME="Generator" CONTENT="MS Exchange Server version 6.5.7036.0"> <TITLE>RE: [racket] sicp exercise 2.20</TITLE> </HEAD> <BODY>  Argument x is in the list anyway, so write (cons x ...) (define (same-parity x . xs)  (cons x (my-filter (... even? odd?) xs))) (define (my-filter pred? lst)  (cond    (null? lst) ...)    (pred? lst) ...)    (else ...))) If you already know function filter, you wont need my-filter. E,g: (define (same-parity x . xs)  (cons x (filter (if (even? x) even? odd?) xs))) and so on, and so on. Jos > -----Original Message----- > From: users-bounces@racket-lang.org > [<A HREF="mailto:users-bounces@racket-lang.org">mailto:users-bounces@racket-lang.org</A>] On Behalf Of Matthias Felleisen > Sent: 20 July 2010 15:54 > To: Martin DeMello > Cc: Racket Users > Subject: Re: [racket] sicp exercise 2.20 > > > > Do you mean this? > > #lang racket > > (require rackunit) > > ;; Nat Nat *-> [Listof Nat] > ;; which of the > (define (same-parity x . xs) >    (cons x (filter (ë (o) (or (and (even? x) (even? o)) (and > (odd? x) (odd? o)))) xs))) > > (check-equal? (same-parity 1 2 3 4 5 6 7) '(1 3 5 7)) > (check-equal? (same-parity 2 3 4 5 6 7) '(2 4 6)) > > > > > On Jul 20, 2010, at 9:36 AM, Martin DeMello wrote: > > > 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: > >  <A HREF="http://lists.racket-lang.org/listinfo/users">http://lists.racket-lang.org/listinfo/users</A> > > _________________________________________________ >   For list-related administrative tasks: >   <A HREF="http://lists.racket-lang.org/listinfo/users">http://lists.racket-lang.org/listinfo/users</A> </BODY> </HTML>