Thanks for the help. I found it amazingly well worded, giving me proper insights to reach a solution without giving the solution proper, which I happen to believe to be one of the most effective ways to help one learn anything. Much appreciated. And indeed, I reworked the code using the four steps you have so kindly and eloquently elaborated, and it did make things easy.
The check expects all pass now, and I trust I have a working solution, which refrain to post here now not being sure whether I should. As a side note, I got to know that course because of a friend, and then I discovered the book. I intend to finish the course and then study the book as seriously as I possibly can. I took a look at some parts of it and figured out I lack some basic math background to help me along. For example, in part one, composing functions <http://www.ccs.neu.edu/home/matthias/HtDP2e/part_one.html#%28part._sec~3acomposing%29> I found this: Stop! Explain the minus sign before you proceed. > I am really not sure what I am supposed to explain there (probably because my poor math background, which I am slowly improving through Khan Academy) and am afraid of asking too many dumb questions here lest I annoy the list members and take people's time with things that perhaps I would be expected to know before reading/studying a book like HtDP/2e. On Friday, September 8, 2017 at 10:21:13 AM UTC-3, Matthias Felleisen wrote: > > > You may wish to read Chapter 16 in HtDP/2e: > > > http://www.ccs.neu.edu/home/matthias/HtDP2e/part_three.html#%28part._ch~3a3use%29 > > It teaches how to re-use an existing abstraction, which is > what you are having trouble with. > > To help you along, I have changed your code in a few places: > > (1) I equipped it with a signature for fold-elt, > without which you cannot reuse the function BY DESIGN > (2) I noted where you failed to develop a signature > (3) I added a total dummy definition for ‘c2’ > (4) I gave respectable names to the parameters of fold-elt > > These questions are challenging w/o the design recipe. > W/ the recipe, they are quite doable. Stick to it. > > > ;; [String Integer Y -> X] [X Y -> Y] Y Element -> X > ;; The abstract fold/reduce function for Element. > (define (fold-elt combine-trees combine-lists identity element) > (local [;; signature MISSING > (define (fn-for-element e) ; -> X > (combine-trees (elt-name e) ; String > (elt-data e) ; Integer > (fn-for-loe (elt-subs e)))) > > ;; signature MISSING > (define (fn-for-loe loe) ; -> Y > (cond [(empty? loe) identity] > [else > (combine-lists (fn-for-element (first loe)) > (fn-for-loe (rest loe)))]))] > (fn-for-element element))) > > ;; String Element -> Integer or false > ;; Search the given tree for an element with the given name, > ;; produce data if found; false otherwise > (check-expect (find "F3" F1) #f) > (check-expect (find "F3" F3) 3) > (check-expect (find "D4" D4) 0) > (check-expect (find "D6" D6) 0) > (check-expect (find "F3" D4) #f) > (check-expect (find "F1" D4) 1) > (check-expect (find "F2" D4) 2) > (check-expect (find "F1" D6) 1) > (check-expect (find "F3" D6) 3) > > (define (find n elt) > (local [;; signature missing > (define (c1 name data loe) > (if (string=? n name) data loe)) > ;; signature missing > (define c2 > 0)] > (fold-elt c1 c2 #f elt))) > > > > On Sep 8, 2017, at 7:33 AM, Fernando Basso <[email protected] > <javascript:>> wrote: > > I am doing an exercise from EDX course based on H2DP book and am having > trouble implementing a `find` function using the abstract function > fold-elt. The function should find the element and return its data. > > ----- > #lang htdp/isl > > (define-struct elt (name data subs)) > ;; Element is (make-elt String Integer ListOfElement) > ;; interp. An element in the file system, with name, and EITHER data or > subs. > ;; If data is 0, then subs is considered to be list of sub > elements. > ;; If data is not 0, then subs is ignored. > > ;; ListOfElement is one of: > ;; - empty > ;; - (cons Element ListOfElement) > ;; interp. A list of file system Elements > > (define F1 (make-elt "F1" 1 empty)) > (define F2 (make-elt "F2" 2 empty)) > (define F3 (make-elt "F3" 3 empty)) > (define D4 (make-elt "D4" 0 (list F1 F2))) > (define D5 (make-elt "D5" 0 (list F3))) > (define D6 (make-elt "D6" 0 (list D4 D5))) > > > ; D6 > ; / \ > ; / \ > ; D4 D5 > ; / \ | > ; / \ | > ; F1 F2 F3 > > ;; The abstract fold/reduce function for Element. > (define (fold-elt c1 c2 b e) > (local [(define (fn-for-element e) ; -> X > (c1 (elt-name e) ;String > (elt-data e) ;Integer > (fn-for-loe (elt-subs e)))) > > (define (fn-for-loe loe) ; -> Y > (cond [(empty? loe) b] > [else > (c2 (fn-for-element (first loe)) > (fn-for-loe (rest loe)))]))] > (fn-for-element e))) > > ;; String Element -> Integer or false > ;; Search the given tree for an element with the given name, > ;; produce data if found; false otherwise > (check-expect (find "F3" F1) #f) > (check-expect (find "F3" F3) 3) > (check-expect (find "D4" D4) 0) > (check-expect (find "D6" D6) 0) > (check-expect (find "F3" D4) #f) > (check-expect (find "F1" D4) 1) > (check-expect (find "F2" D4) 2) > (check-expect (find "F1" D6) 1) > (check-expect (find "F3" D6) 3) > > (define (find n elt) > (local [(define (c1 name data loe) > (if (string=? n name) data loe))] > (fold-elt c1 ??? #f elt))) > ----- > > So, what should I do in the place marked with "???"? > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] <javascript:>. > For more options, visit https://groups.google.com/d/optout. > > > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
