Basic Matrices in R

> # create a 4x3 matrix 
> a=matrix(c(1,2,3,4,5,6,7,8,9,10,11,12), nrow=4)
> a
     [,1] [,2] [,3]
[1,]    1    5    9
[2,]    2    6   10
[3,]    3    7   11
[4,]    4    8   12
> #arrange the numbers by row
> a=matrix(c(1,2,3,4,5,6,7,8,9,10,11,12), nrow=4, byrow=T)
> a
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
[3,]    7    8    9
[4,]   10   11   12
> # Combine 2x2 matrix by row and column
> e = matrix(c(1,2,3,4),nrow=2)
> f = matrix(c(5,6,7,8),nrow=2)
> e
     [,1] [,2]
[1,]    1    3
[2,]    2    4
> f
     [,1] [,2]
[1,]    5    7
[2,]    6    8
> cbind(e,f)
     [,1] [,2] [,3] [,4]
[1,]    1    3    5    7
[2,]    2    4    6    8
> rbind(e,f)
     [,1] [,2]
[1,]    1    3
[2,]    2    4
[3,]    5    7
[4,]    6    8
> # Take a single digit subset from Matrix a Row 3 Column 2
> a[3,2]
[1] 8
> # Take a 2x3 subset of Matrix a, change the numbers and then combine back in to the original.
> a[c(3,4),c(1,2,3)]
     [,1] [,2] [,3]
[1,]    7    8    9
[2,]   10   11   12
> a[c(3,4),c(1,2,3)] = matrix(c(1,2,3,4,5,6),nrow=2)
> a
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
[3,]    1    3    5
[4,]    2    4    6

Created by Pretty R at inside-R.org

More here