[R] Computting statistics on a matrix with 2 factor column

[Marc Mamin]

Hello,

I suppose this is a basic question but couldn't find a solution.:

I have a large matrix with let say 3 columns:

V1	V2	V3
a	x	2
a	x	4
a	y	8
b	z	16

and I want to compute some statistics based on 
the levels resulting form the combination of the two first columns

e.g.:

SUM->

V1	V2	V3
a	x	6
a	y	8
b	z	16


Thanks for your hints .

Marc