William James
9/30/2015 8:15:00 AM
Giorgos Keramidas wrote:
> > say there's a list of even length
> > (a b c d e f ......)
> >
> > how do i turn this into
> > ((a b) (c d) (e f)....)
> >
> > simply how do i couple the elements?
>
> A bit of experimentation with (loop) seems easy:
>
> CL-USER(5): (loop for (a b) on '(1 2 3 4)
> collect (list a b))
> ((1 2) (2 3) (3 4) (4 NIL))
>
> Almost there. We only need the pairs starting from 'even' zero-based
> indexes. This is fairly straightforward to write using loop again:
>
> CL-USER(6): (loop for (a b) on '(1 2 3 4)
> for index from 0
> when (evenp index)
> collect (list a b))
> ((1 2) (3 4))
Gauche Scheme:
(use srfi-1 :only (unfold))
(unfold null? (cut take <> 2) cddr (iota 8))
===>
((0 1) (2 3) (4 5) (6 7))
Another way:
(let f ((xs (iota 8)))
(if (null? xs) () (cons (take xs 2) (f (cddr xs)))))
The easy way:
(slices (iota 8) 2)
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