Re: Given any list, group n number of sublists into a single list

Doing people's homework are we?

On 5/17/2017 8:08 PM, Matthew Butterick wrote:
> `slice-at`
>
> http://docs.racket-lang.org/sugar/index.html?q=slice-at#%28def._%28%28lib._sugar%2Flist..rkt%29._slice-at%29%29 
> <http://docs.racket-lang.org/sugar/index.html?q=slice-at#%28def._%28%28lib._sugar/list..rkt%29._slice-at%29%29>
>
>> On May 17, 2017, at 3:56 PM, Don Green <[email protected] 
>> <mailto:[email protected]>> wrote:
>>
>> Racket code that could perform this list manipulation?
>> (every-n-lists-into-list <n-lists-to-group> '(<in-list>))
>> (every-n-lists-into-list 1 '((1) (2))) ;=> '(((1) (2)))
>> (every-n-lists-into-list 2 '((1) (2) (3) (4))) ;=> '(((1) (2)) ((3) (4)))
>> (every-n-lists-into-list 3 '((1) (2) (3) (4)) (5) (6))) ;=> '(((1) 
>> (2) (3)) ((4) (5) (6)))
>

Very nice!  Unfortunately, the existence of sugar was unknown to me 
until just now.   More evidence that one can spend a lifetime learning 
what batteries Racket already has included.


My 1st thought was to use split-at:

#lang racket
(define (group-items n items)

   ;; check # of items
   (unless (= 0 (modulo (length items) n))
     (error "not enough items"))

   ;; split and reassemble
   (let loop [
              (items  items)
              (result '())
             ]
     (if [null? items]
         (reverse result)
         (let-values [((head tail)(split-at items n))]
           (loop tail (cons head result)))
         ))
   )

George

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