From: Ramakrishnan Muthukrishnan Date: Tue, 3 Aug 2010 19:36:05 +0000 (+0530) Subject: solution to 2.39. Took me some experimentation at the repl to find the fold-right... X-Git-Url: https://git.rkrishnan.org/%5B/%5D%20/file/URI:LIT:krugkidfnzsc4/@@named=/frontends/nxhtml.html?a=commitdiff_plain;h=8ceb357a6bf0c522451d980bf764306a0868ce44;p=sicp.git solution to 2.39. Took me some experimentation at the repl to find the fold-right version of reverse. --- diff --git a/src/sicp/ex2_39.clj b/src/sicp/ex2_39.clj new file mode 100644 index 0000000..2a17b55 --- /dev/null +++ b/src/sicp/ex2_39.clj @@ -0,0 +1,33 @@ +(ns sicp.ex2_39 + (:use [clojure.test] + [sicp [ch2-2 :only (accumulate)] + [ex2_36 :only (accumulate-n)] + [ex2_38 :only (fold-right fold-left)]])) + +;; Complete the following definitions of reverse (exercise 2.18) in +;; terms of fold-right and fold-left from exercise 2.38: + +;; (define (reverse sequence) +;; (fold-right (lambda (x y) ) nil sequence)) +;; (define (reverse sequence) +;; (fold-left (lambda (x y) ) nil sequence)) +(defn reverse-with-foldr [sequence] + (fold-right (fn [x y] + (concat y (list x))) + nil + sequence)) + +(defn reverse-with-foldl [sequence] + (fold-left (fn [x y] + (cons y x)) + nil + sequence)) + +(deftest test-reverses + (let [s1 '(1 2 3 4 5) + s2 '(1 2 3 (4 5))] + (are [x y] [= x y] + (reverse-with-foldr s1) (reverse s1) + (reverse-with-foldl s1) (reverse s1) + (reverse-with-foldr s2) (reverse s2) + (reverse-with-foldl s2) (reverse s2))))