[Haskell-begin] Alternating sequence

[Dirk Markert]

Hi Kurt,

thank you for the detailed explanation. Indeed, I could follow your
explanation on first reading :-)

Dirk

2008/7/21 Kurt Hutchinson <kelanslists at gmail.com>:

> On Mon, Jul 21, 2008 at 9:34 AM, Dirk Markert <dirk.markert at gmail.com>
> wrote:
> > I am trying to generate the following list:
> > 2, 3, 5 -- and then alternating (+2) resp (+4) -- 7, 11, 13, 17, 19, 23
> >
> > I came up with the following solution
> > 2:3:unfoldr (\(a,b) -> Just (a,(a+b, if b == 2 then 4 else 2))) (5,2)
> >
> > Are there easier ways to generate the desired list?
>
>
> So you've got the beginnings of an infinite list, and a rule to modify
> that to generate new elements. How about turning that rule into a list
> of its own, and then combining them?
>
>  rule = cycle [ 2, 4 ]  -- this will give an infinite list of 2's and 4's
>
> Combining two lists is usually done with 'zip'. But in this case, we
> don't just want tuples, we want the sum of each pair. You can combine
> lists with a function by using 'zipWith'. Start at the point where the
> rule kicks in.
>
>  rest = 5 : zipWith (+) rest rule
>
> Now just tack on your first elements.
>
>  list = 2 : 3 : rest
>
> Here it is all in one place:
>
>  list = 2 : 3 : rest
>    where
>    rule = cycle [ 2, 4 ]
>    rest = 5 : zipWith (+) rest rule
>
> Kurt
>
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