Hi Sam -- As Jos commented, this is a very tough exercise, but it looks like you're off to a good start.
The most obvious problem with your current solution lies with 'insert-everywhere/in-single-word.' My first suggestion is to check the function examples to make sure the values they expect are consistent with your data definitions. Dave On Tue, Jul 6, 2010 at 1:15 PM, Jos Koot <jos.k...@telefonica.net> wrote: > 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 >
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