Re: [Haskell-cafe] How to take a minimum sub list that only contain certain number of elements of certain type?

2012/9/25 Ivan Lazar Miljenovic On 25 September 2012 16:51, Magicloud Magiclouds wrote:

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Hi, For example, I have an array [0..]. Now I want to take a sub list that starts from index 0, and only contain 4 odds, and is minimum result. The answer should be [0, 1, 2, 3, 4, 5, 6, 7].

What do you actually *mean*? When you say you "have an array", but actually display a *list*, do you really mean you have something fitting into Data.Array, or do you really mean you have a list? When you say "sub list" do you mean a *slice* (a contiguous chunk) or a *subsequence* (elements preserve their order but there may be gaps). Or looking at your example, do you mean a *prefix* (n `take` xs for some n)? When you say "odds" I presume you mean odd integer, although the even/odd concept applies to Gaussian integers too (m+ni is even if it is divisible by 1+i, which turns out to be equivalent to m+ni being even (odd) iff m and n have the same (different) parity). When you say "is minimum result", what does that mean? Does it mean the sum of the elements is minimal? (If you consider the list [1,3,5,7,-2,-4,-6,-8,-10,...] it is clear that a minimum result in that sense could be infinitely long.) Let's take just one interpretation: - the "array" is a list - whose elements are Integers - the result must be a prefix of the input - which contains four odd Integers - and is otherwise as short as possible. We'll generalise `take`: it will have an integer n, a predicate p, and a list xs. ptake :: Int -> (a -> Bool) -> [a] -> [a] ptake n p (x:xs) | n > 0 = x : ptake (if p x then n-1 else n) p xs ptake _ _ _ = [] answer :: [Integer] -> [Integer] answer xs = ptake 4 odd xs Now this is pretty trivial (it's _exactly_ `take` except for only counting elements where p is true), so that interpretation of the problem cannot be the right one.