# [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 (λ (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
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