nub . concat ? Richard O'Keefe wrote:
On 14 Jun 2009, at 8:06 pm, Fernan Bolando wrote:
Hi all
If I have a number of list example list1 = [2,3] list2 = [1,2] list3 = [2,3,4] list4 = [1,2,3]
I want to create a list from the list above with n elements, non-repeating and each elements index represents 1 of the elements from the corresponding list so for the above input I would get.
a = [3,2,4,1]
I have been staring at this off and on all day, and I haven't the faintest idea what you want.
What is "n". What is it that doesn't repeat? How does the index of an element represent 1 element? Which list corresponds to what?
I'm beginning to suspect that what you want is a "choice" function: f [s1,...,sn] = [x1,...,xn] when each xi is an element of the corresponding si and no two xs are the same.
Instead of finding one answer, let's find them all.
all_choices :: Eq a => [[a]] -> [[a]] all_choices [] = [[]] all_choices (set:sets) = [x:xs | xs <- all_choices sets, x <- set, not(x `elem` xs)]
The test case
all_choices [[2,3], [1,2], [2,3,4], [1,2,3]]
has the answer
[[3,2,4,1], [3,1,4,2], [2,1,4,3]]
and you probably want to use it something like
case all_choices sets of [] -> there are no such choices (first_choice:_) -> first_choice is one such choice
For inputs like [[1,2],[2,1],[1]] there is of course no such choice function.
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