Question

I have an array which is for example of a length of 4 elemnts
no I want to know how to calculate the possible combinations when using 3 digets out of it.

eg:

3 6 2
3 6 8
6 2 8
8 3 2

my array is : 3 6 2 8

there are 4 possibilitys, but how can I count them programaticly ?

Answer 1

Well, it seems that what you are actually looking for is Choose(n,k), which is the number of ways to chose k elements out of n candidates.

The formula for it is basically Choose(n,k) = n! / (k! * (n-k)!)

The rational behind it is:

Number of ways to sort n items (n! possibilities) and then choose first k. After you have chosen you don’t care for the order of the first k items (k! possibilities) and the last n-k items ((n-k)! possibilities).

In your example, Choose(4,3) = 4!/(3!*1!) = 4, as expected.