On Mon, Jul 21, 2008 at 11:21 AM, Bernie Pope wrote:

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

'suffix' can be simplified a bit by using zipWith: suffix = 5 : zipWith (+) suffix (cycle [2,4]) Thanks, -- Felipe.

Hi Bernie, I like your much more. I tried also to find a solution using cycle but didn't succeed. Thank you, Dirk 2008/7/21 Bernie Pope :

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.

Hi Kurt, thank you for the detailed explanation. Indeed, I could follow your explanation on first reading :-) Dirk 2008/7/21 Kurt Hutchinson :

On Mon, Jul 21, 2008 at 9:34 AM, 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?

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