Iterating the hash map to form combinations

[Shruthi Babu]

I need to iterate a hashmap and form all possible combinations for example if there are five keys say 1, 2, 3, 4 , 5

I need to form all possible distinct combinations for example (1,2) , (1,3) , (1,2,3) , (2,3,4) etc
Can someone help me with an algorithm for this?

[marc weber]

What have you tried so far? Can you describe the process in English before trying to code it?

Hint: A HashMap's keySet() method returns a Set of keys.

[fred rosenberger]

Please dont' double post.

[Shruthi Babu]

I am trying to use the combination rule of "n!/r!(n-r)!" but here is the pbm according to this if I wanted all the combinations for a particular number of pairs eg all two pair values ... eg 1, 2, 3, 4 the outcome would be (1,2) , (2,3) , (3,4) , (1,3) (1,4) , (2,4)

but I need all possible pairs say all 2 pairs, 3 pairs , 4 pairs etc... is it a good way to wrap it with a loop and pass the "r" value for each loop or is there a way to figure this out. Any help is highly appreciated.

[fred rosenberger]

if the order doesn't matter (i.e. (1,2) is the same as (2,1)), it's pretty simple. count from 0 to 2^(number of elements) - 1. Then, use a bit mask to figure out which to include.

in other words, assuming 3 elements:

    
counter              binary representation      use these elements
0                    000                        none
1                    001                        1
2                    010                        2
3                    011                        1,2
4                    100                        3
5                    101                        1,3
6                    110                        2,3
7                    111                        1,2,3

[ May 22, 2007: Message edited by: Fred Rosenberger ]