[Haskell-begin] Alternating sequence

[Bernie Pope]

On 21/07/2008, at 11:34 PM, Dirk Markert 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?

Hi Dirk,

I doubt this is "easier", but it is a nice opportunity to show off  
some cyclic programming:

    suffix = 5 : [ x + y | (x,y) <- zip suffix (cycle [2,4])]
    list = 2 : 3 : suffix

cycle is from the Prelude and is used above to build an "infinite"  
list of [2,4,2,4 ...]

Note that suffix is recursive. Lazy evaluation allows us to pull  
values out of it whilst building it at the same time. Obviously it is  
important that we start building the list before trying to pull any  
values out of it, hence the "5 :" at the start is important.

I may not have answered your question, but hopefully it is interesting  
nonetheless.

Cheers,
Bernie.