Some suggestions: 1. (+ (* -1 x) y) is (- y x) 2. f is in scope of the entire definition; there is no need to pass it along. 3. Once you fix 2, you see that g == (inverter n) == (lambda (x) (f (- n x)) 4. So why not replace all occurrences of g with the lambda expression?
Voilà, you have a one-line definition. Send it back when done. On Jul 31, 2012, at 8:12 AM, Sean Kemplay wrote: > Ok, not sure if I went down the same route as Matthias was alluding to > but here is my solution - > > (define (tabulate f n) > (local > ((define (inverter y f) (lambda (x) (f (+ (* x -1) y)))) > (define g (inverter n f))) > (build-list (+ n 1) g))) > > > (check-expect (tabulate (lambda (x) x) 3) '(3 2 1 0)) > > I used a curried function to 'invert' each number in place before the > function provided to build list is applied. > > Regards, > Sean > > On Mon, Jul 30, 2012 at 10:32 PM, Matthias Felleisen > <matth...@ccs.neu.edu> wrote: >> >> Here is how I suggest our freshmen to find this function: >> >> ;; tabulate : (x -> y) Nat -> (listof y) ;; <------- MF: not fix of >> signature >> ;; to tabulate f between n >> ;; and 0 (inclusive) in a list >> >> (check-expect (tabulate (lambda (x) x) 3) '(3 2 1 0)) >> >> (define (tabulate.v0 f n) >> (cond >> [(= n 0) (list (f 0))] >> [else >> (cons (f n) >> (tabulate f (sub1 n)))])) >> >> ;; Using build-list >> ;; build-list : N (N -> X) -> (listof X) >> ;; to construct (list (f 0) ... (f (- n 1))) >> >> (define (tabulate f n) >> (local (;; Nat -> Y ;; <-------------- MF: you know you want build-list, >> design (!) the 'loop' function now >> (define (g i) ... f n i ...)) ;; <---- this is the data that's >> available >> (build-list (+ n 1) g))) >> >> Also see '2e'. >> >> >> On Jul 30, 2012, at 4:10 PM, Sean Kemplay wrote: >> >>> Hello, >>> >>> I am looking at the exercise from htdp 1e on building the tabulate >>> function from build-list. >>> >>> Would I be on the right track that I need to create a function to >>> reverse the result of (build-list (+ n 1) f) to do this? >>> >>> the function this is to emulate - >>> >>> ;; tabulate : (x -> y) x -> (listof y) >>> ;; to tabulate f between n >>> ;; and 0 (inclusive) in a list >>> (define (tabulate f n) >>> (cond >>> [(= n 0) (list (f 0))] >>> [else >>> (cons (f n) >>> (tabulate f (sub1 n)))])) >>> >>> Using build-list >>> ;; build-list : N (N -> X) -> (listof X) >>> ;; to construct (list (f 0) ... (f (- n 1))) >>> (define (build-list n f) ...) >>> >>> Regards, >>> Sean >>> ____________________ >>> Racket Users list: >>> http://lists.racket-lang.org/users >> ____________________ Racket Users list: http://lists.racket-lang.org/users
