Hi Matthias, Thanks for the feedback. Just a couple of questions before I give it another go if that's okay. I know there is something I need to learn from this exercise.
I am going through htdp first edition (at chapter 17 which refers to arrangements from chapter 12 which is why I revisited the exercise). Would you suggest continuing with this edition or moving to the draft second edition? Do you mean the contract for insert-everywhere/word should be symbol word -> list of words? And will the words returned be the same length? Ie am I aiming for the same result as the current definition? Thanks Sean. On Jul 5, 2012 9:15 PM, "Matthias Felleisen" <matth...@ccs.neu.edu> wrote: > > Sean, > > 1. I have re-arranged your program so that it becomes readable: > > -- the function that explains it all comes first > -- if you need a helper function, it is placed below > -- don't place functions at top that are called by functions further down > (roughly) > > -- make sure lines are at most 80 keystrokes wide > > 2. I have separated the part you created with a == line from the one > that is given. > > 3. I added two comments in place where it becomes obvious that you > didn't design according to the book but with an ad-hoc method. > > Overall, you discovered a design method that is introduced 2 chapters > later. You used it when it is unnecessary to do so. The program is > far more complicated (and thus slower) than needed. > > 3a. It is possible that you have seen the exercise on creating a one-line > editor (like Word or Notepad). At least the data structure suggests this > is possible. IF SO (and I don't know whether this is true), then you might > have designed by "copy example, change until it works". IF SO, let me > strongly discourage this approach or -- if you do follow it -- let me > encourage you to be aware of it and to attempt a systematic approach, > which tends to produce much better results. > > 4. PLEASE feel encouraged to try again. Read the first piece of > in-lined comment to just skip the whole split-word data structure. > Follow the design recipe and you will find a wonderful solution. > > -- Matthias > > > > > #lang racket > > (require test-engine/racket-tests) > > ;; Word -> [Listof Word] > (check-expect (arrangements '(a)) (list '(a))) > (check-expect (arrangements '(a b)) (list '(a b) '(b a))) > > (define (arrangements a-word) > (cond > [(empty? a-word) (cons empty empty)] > [else > (insert-everywhere/in-all-words (first a-word) (arrangements (rest > a-word)))])) > > ;; > ============================================================================= > > ; A split-word is a struct where left and right are list of words > (define-struct split-word (left right)) > > ;; insert-everywhere/in-all-words symbol list-of-words -> list of words > ;; insert the symbol in all possible positions in each word returning an > ;; expanded list of words > > (check-expect (insert-everywhere/in-all-words 'a (cons '(b) empty)) > (list (list 'a 'b) (list 'b 'a))) > > (check-expect (insert-everywhere/in-all-words 'd '((e r) (r e))) > '((d e r) (e d r) (e r d) (d r e) (r d e) (r e d))) > > (define (insert-everywhere/in-all-words s a-low) > (cond > [(empty? a-low) empty] > ;; MF: this is where it gets weird (from what you know): > ;; why do you introduce an intermediate data structure (split-word) > ;; to represent a word? It is *difficult* to justify this design with > ;; the material in the first three chapters. > ;; the function insert-everywhere/word should really just consume a > word > [else (append (insert-everywhere/word s (make-split-word '() (first > a-low))) > (insert-everywhere/in-all-words s (rest a-low)))])) > > ;;insert-everywhere/in-one-word symbol split-word -> low > ;; take a symbol and a split word and insert the symbol before and after > each > ;; letter in the right hand side of the split creating a list of new words > with > ;; the symbol progressing through. > > (check-expect (insert-everywhere/word 'a (make-split-word empty empty)) > (list (list 'a))) > (check-expect (insert-everywhere/word 'a (make-split-word '(b) empty)) > (list '(b a))) > (check-expect (insert-everywhere/word 'a (make-split-word empty '(b))) > (list '(a b) '(b a))) > (check-expect (insert-everywhere/word 'a (make-split-word empty '(b c))) > (list '(a b c) '(b a c) '(b c a))) > > (define (insert-everywhere/word s a-sw) > (cond > [(empty? (split-word-right a-sw)) (cons (insert-symbol s a-sw) empty)] > [else (cons (insert-symbol s a-sw) > ;; MF: this is where you go beyond what the design ideas > you > ;; have seen in chapters 1 thru 3, and it makes the design > ;; utterly inelegant. Specifically, you are NOT applying > ;; insert-everywhere/word to a *piece* of a-sw but to a > complete > ;; re-arrangement. > (insert-everywhere/word s (move-split a-sw)))])) > > ;; move-split split-word -> split-word > ;; return a new split word with the list on the left having the first of > ;; the right list appended to the end and the rest of the right > (define (move-split a-sw) > (cond > [(empty? (split-word-right a-sw)) a-sw] > [else (make-split-word > (append (split-word-left a-sw) (list (first (split-word-right > a-sw)))) > (rest (split-word-right a-sw)))])) > > (check-expect (move-split (make-split-word empty empty)) > (make-split-word empty empty)) > (check-expect (move-split (make-split-word '(a) empty)) > (make-split-word '(a) empty)) > (check-expect (move-split (make-split-word '(a) '(b))) > (make-split-word '(a b) empty)) > (check-expect (move-split (make-split-word '(a) '(b c))) > (make-split-word '(a b) '(c))) > (check-expect (move-split (make-split-word '(a b) '(c d))) > (make-split-word '(a b c) '(d))) > > ;; insert-symbol symbol split-word -> word > ;; Insert the sysmbo at the front of the left list and append the right if > left is empty > ;; Append to end of left if right list is empty > ;; Otherrwise insert between - returning one list > > (check-expect (insert-symbol 'a (make-split-word empty empty)) > '(a)) > (check-expect (insert-symbol 'a (make-split-word '(b) empty)) > '(b a)) > (check-expect (insert-symbol 'a (make-split-word empty '(b))) > '(a b)) > (check-expect (insert-symbol 'a (make-split-word '(b) '(c))) > '(b a c)) > > (define (insert-symbol s a-sw) > (cond > [(empty? (split-word-right a-sw)) (append (split-word-left a-sw) (cons > s empty))] > [(empty? (split-word-left a-sw)) (cons s (split-word-right a-sw))] > [else (append (split-word-left a-sw) (cons s (split-word-right > a-sw)))])) > >
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