> m<-matrix(c(1,4,6,7,0,0,1,1,0,0,3,2),3,4, byrow=TRUE)
> m
     [,1] [,2] [,3] [,4]
[1,]    1    4    6    7
[2,]    0    0    1    1
[3,]    0    0    3    2

> m[1,c(1,2)]

[1] 1 4

> m[1,c(3,4)]

[1] 6 7

> m[c(2,3),c(1,2)]

     [,1] [,2]
[1,]    0    0
[2,]    0    0

> m[c(2,3),c(3,4)]

     [,1] [,2]
[1,]    1    1
[2,]    3    2