Exercise 12.4.2 is notorius. When looking to (list 'a 'b 'c), you should exspect 6 distinct permutations. Jos
> -----Original Message----- > From: users-boun...@racket-lang.org > [mailto:users-boun...@racket-lang.org] On Behalf Of Sam Griff > Sent: 06 July 2010 20:00 > To: users@racket-lang.org > Subject: [racket] [htdp] Help with Exercise 12.4.2 > > Hello. Like many others working through HtDP I have hit the > wall at this exercise. I'm not sure what I have so far is in > line with the "proper" way to do this exercise and would > appreciate any help/suggestions. Here is the code I have so far: > > ;; A list of words is either > ;; 1. empty > ;; 2. (cons w low) > ;; where w is a word and low is a list of words > > ;; arrangements : word -> list-of-words ;; to create a list > of all rearrangements of the letters in a-word (define > (arrangements a-word) > (cond > [(empty? a-word) (cons empty empty)] > [else (insert-everywhere/in-all-words (first a-word) > (arrangements (rest a-word)))])) > > ;; Contract: > ;; insert-everywhere/in-single-word : symbol word -> > list-of-words ;; Purpose: > ;; to insert a symbol everywhere in a single word ;; Examples: > ;; (insert-everywhere/in-single-word 'a empty) should produce > (list 'a) ;; (insert-everywhere/in-single-word 'b (list 'a)) > should produce (list (list 'a 'b) (list 'b 'a)) ;; > (insert-everywhere/in-single-word 'c (list 'a 'b)) should > produce (list (list 'c 'a 'b) (list 'a 'c 'b) (list 'a 'b > 'c)) ;; Template: > ;; (define (insert-everywhere/in-single-word s w) > ;; (cond > ;; [(empty? w) ...] > ;; [else ... (first w) ... > (insert-everywhere/in-single-word s w) ...])) > (define (insert-everywhere/in-single-word s w) > (cond > [(empty? w) (list s)] > [else (list (list s (first w)) (cons (first w) > (insert-everywhere/in-single-word s (rest w))))])) > > ;; insert-everywhere/in-all-words : symbol list-of-words -> > list-of-words ;; to insert a symbol everywhere in a list of > words (define (insert-everywhere/in-all-words s low) > (cond > [(empty? low) empty] > [else (append (insert-everywhere/in-single-word s (first > low)) (insert-everywhere/in-all-words s (rest low)))])) > > > ;; Tests: > (define testword1 (list 'a)) > (define testword2 (list 'a 'b)) > (check-expect (insert-everywhere/in-single-word 'a empty) > (list 'a)) (check-expect (insert-everywhere/in-single-word 'b > testword1) (list (list 'b 'a) (list 'a 'b))) (check-expect > (insert-everywhere/in-single-word 'c testword2) (list (list > 'c 'a 'b) (list 'a 'c 'b) (list 'a 'b 'c))) > > > Thanks, > > Sam > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
