All m combinations of the first n integers taken p at a time
are computed and return as an p by m matrix. The columns
of the matrix are ordered so that adjacent columns differ
by only one element. This is just a reordered version of
combn in base R, but the ordering is useful for some
The n in n choose p
The p in n choose p
matrix of dimension p by
Implementation based on a Pascal algorithm of Limin Xiang
and Kazuo Ushijima (2001) translated to ratfor for R.
If you have rgl installed you might try
for a visual impression of how this works.
Limin Xiang and Kazuo Ushijima (2001) "On O(1) Time Algorithms for Combinatorial Generation," Computer Journal, 44(4), 292-302.
H <- combos(20,3)