XPost: comp.lang.scheme

Ken Tilton wrote:

Mark Tarver wrote:

On 21 Oct, 10:15, Dan Kruchinin <just.asg...@gmail.com> wrote:

On Oct 21, 1:07 pm, Dan Kruchinin <just.asg...@gmail.com> wrote:

may be it's quite stupid question, but i don't know how to get more >>>beautiful solution than i have.
so the question is:
i have a list always containing 2 integer elements. each element can
be negative, positive or zero.
i need to get a list containing 3 lists using the following strategy: >>>if an element from source list is negative, it should be included to >>>the first sublist,
if it is positive, it should be placed to the third sublist
and if it is zero, it should be placed to the second sublist.
(note#1: each element should be abs'ed before including)
(note#2: i need side-effect safe solution)

for example i have a list from 2 elements: (setf l '(-2 35))
i should get the following list containing 3 sublists:
'((2) nil (35))
or if l is '(-2 -3)
the result list should be
'((2 3) nil nil)
or if l is '(0 5):
'(nil (0) (5))
and so on.

uff, sorry, i made an error in the description
if the element is negative it should be included to the *third*
sublist
and if it is positive it should be included to the *first* sublist.
so the examples will be:
src: '(-2 35)
res: '((35) nil (2))
--
src: '(-2 -3)
res: '(nil nil (2 3))
--
src :'(0 5)
res: ((5) (0) nil)- Hide quoted text -

- Show quoted text -

Dan,

You need to learn about accumulators and help functions.
Probably somebody here can recommend an introductory book.

In Qi

(define seperate
Ns -> (s-help Ns [] [] []))

Pos Zero Neg are accumulators

(define s-help
[] Pos Zero Neg -> [Pos Zero Neg]
[P | Ns] Pos Zero Neg -> (s-help Ns [P | Pos] Zero Neg) where (> P
0)
[0 | Ns] Pos Zero Neg -> (s-help Ns Pos [0 | Zero] Neg)
[N | Ns] Pos Zero Neg -> (s-help Ns Pos Zero [N | Neg]))

or in Lisp

(defun seperate (Ns) (s-help Ns () () ()))

(defun s-help (Ns Pos Zero Neg)
(cond ((null Ns) (list Pos Zero Neg))
((> (car Ns) 0) (s-help (cdr Ns) (cons (car Ns) Pos) Zero
Neg))
((zerop (car Ns)) (s-help (cdr Ns) Pos (cons 0 Zero) Neg))
(t (s-help (cdr Ns) Pos Zero (cons (car Ns) Neg)))))

I'm typing this wondering if its a homework question - but you've had
a bash yourself so a bit of help is ok.

The "ugly" solution was supplied as part of the question, with the instructions to find something more elegant.

Fortunately you answered in Qi, I am hard at work on a solution using
Cells, and I wonder where is the F# answer?

Gauche Scheme

(use gauche.collection)
(use srfi-1)

(define (group nums)
(let1 res (group-collection nums :key (cut compare <> 0))
(map! abs (last res))
res))

(group '(0 2 4 6 8 -3 -5 -7))
===>
((0) (2 4 6 8) (3 5 7))

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