[racket] Tabulate from build-list

 From: Matthias Felleisen (matthias at ccs.neu.edu) Date: Tue Jul 31 08:43:58 EDT 2012 Previous message: [racket] Tabulate from build-list Next message: [racket] Strange error with OpenGL in Windows XP Messages sorted by: [date] [thread] [subject] [author]

```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
> <matthias at 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
>>

```

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