On Fri, 21 Jan 2005, Guillermo Fernandez Castellanos wrote:
> I'm trying to take a list and find all the unique combinations of that
> list.
>
> I mean:
> if I enter (1,2,3,4,5) and I watn combinations of 3, I want to find:
> (1,2,3) but then not (2,1,3), (3,1,2),...
> (1,2,4)
> (1,2,5)
> (2,3,5)
> (3,4,5)
Hi Guillermo,
There is a clean recursive way to define this. I'll try to sketch out how
one can go about deriving the function you're thinking of.
Let's say that we have a list L,
[1, 2, 3]
and we want to create all combinations of elements in that list. Let's
call the function that does this "createComb()". How do we calculate
createComb([1, 2, 3])?
We can just start writing it out, but let's try a slightly different
approach. Imagine for the moment that we can construct createComb([2,
3]):
createComb([2, 3]) -> [[2, 3], [2], [3]]
We know, just from doing things by hand, that createComb([1, 2, 3]) of the
whole list will look like:
createComb([1, 2, 3) -> [[1, 2, 3], [1, 2], [1, 3],
[2, 3], [2], [3]]
If we compare createComb([1, 2, 3]) and createComb([2, 3]), we might be
able to see that they're actually very similar to each other.
Does this make sense so far? Please feel free to ask questions about
this.
Good luck!
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