On Sep 18, 2010, at 12:51 AM, Christopher Tauss wrote:
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.
Note that keeping just the suffix is the same as dropping the prefix.
Consider your data:
A finite list.
The length of the suffix to keep.
An algebraic relationship between lengths of lists, suffixes, and
their prefixes:
length(prefix) + length(suffix) = legnth(list)
Putting all of this together:
n_tail n list = drop (prefix_length) list where
prefix_length = length list - n
You may be interested in how drop is carrying around some state:
drop n xs | n <= 0 = xs
drop _ [] = []
drop n (_:xs) = drop (n-1) xs
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