[Haskell-cafe] How to do the "permutation and combination" thing?

[Victor Mateus Oliveira]

Hi,

Give a try to this library: http://hackage.haskell.org/package/permutation
You can construct the combinations with list of indices and then apply
it to your sets.

[]s
Victor

On Fri, Mar 12, 2010 at 5:16 AM, Ketil Malde <ketil at malde.org> wrote:
> Casey Hawthorne <caseyh at istar.ca> writes:
>
>>>  For example, I have this:
>>>list1 = [a, b, c]
>>>list2 = [d, e, f]
>>>list3 = [g, h, i]
>
>> Think in abstract terms what you want to accomplish.
>
> A bit more specifically, let's say the input is a list of lists, and you
> want to produce all combinations of drawing one element from each of the
> input lists¹:
>
>  perms :: [[a]] -> [[a]]
>
> You need to consider two cases, when the input is empty, and when the
> input contains at least one list of elements:
>
>  perms (l:ls) = ...
>  perms [] = ...
>
> The second case shouldn't be so hard.
>
> Now, if you pretend that 'perms' is already implemented, then you can
> use it to generate all permutations for the tail of the input list.  The
> first case boils down to combining the first input list with all
> permutations of the rest of the lists:
>
>  perms (l:ls) = ... l ... perms ls
>
> Does this help?
>
> -k
>
> ¹ Using tuples is harder to generalize for length, but nicer typewise,
> since you'd get something like 'perms :: ([a],[b],..[x]) -> [(a,b,..,x)]
> --
> If I haven't seen further, it is by standing in the footprints of giants
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