Combin2Rank

Example:

Combs=nchoosek(1:5,3)
Combs =
1 2 3
1 2 4
1 2 5
1 3 4
1 3 5
1 4 5
2 3 4
2 3 5
2 4 5
3 4 5
Combin2Rank(5,3,Combs)
ans =
1
2
3
4
5
6
7
8
9
10

ref: A Numbering System for Combinations, Gary D. Knott, Communications of the ACM, Jan. 1974, v.17, n.1