[Haskell-cafe] Re: Ultra-newbie Question

Maciej Piechotka uzytkownik2 at gmail.com
Sat Sep 18 14:15:30 EDT 2010

On Sat, 2010-09-18 at 03:51 -0400, Christopher Tauss wrote:
> Hello Haskell Community -
> I am a professional programmer with 11 years experience, yet I just do
> not seem to be able to get the hang of even simple things in Haskell.
> I am trying to write a function that takes a list and returns the last
> n elements.
> There may be a function which I can just call that does that, but I am
> trying to roll my own just to understand the concept.
> Let's call the function n_lastn and, given a list  [1,2,3,4,5], I
> would like 
> n_lastn 3 = [3,4,5]
> Seems like it would be something like:
> n_lastn:: [a]->Int->[a]
> n_lastn 1 (xs) = last(xs)
> n_lastn n (x:xs) = ????
> The issue is I do not see how you can store the last elements of the
> list.
> Thanks in advance.
> ctauss

I'll add my $0.03 - unless you are doing it to learn about lists rethink
your approach. Taking k elements from end of n-element list will be O(n)

For example with appropriate structures (like finger trees) it would
look like O(k) operation.

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