1.1 Creating matrices in R

Create a \(3 \times 4\) matrix, meaning 3 row and 4 columns, that is all 1s:

matrix(1, 3, 4)

     [,1] [,2] [,3] [,4]
[1,]    1    1    1    1
[2,]    1    1    1    1
[3,]    1    1    1    1

Create a \(3 \times 4\) matrix filled in with the numbers 1 to 12 by column (default) and by row:

matrix(1:12, 3, 4)

     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12

matrix(1:12, 3, 4, byrow = TRUE)

     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4
[2,]    5    6    7    8
[3,]    9   10   11   12

Create a matrix with one column:

matrix(1:4, ncol = 1)

     [,1]
[1,]    1
[2,]    2
[3,]    3
[4,]    4

Create a matrix with one row:

matrix(1:4, nrow = 1)

     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4

Check the dimensions of a matrix

A = matrix(1:6, 2, 3)
A

     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6

dim(A)

[1] 2 3

Get the number of rows in a matrix:

dim(A)[1]

[1] 2

nrow(A)

[1] 2

Create a 3D matrix (called array):

A = array(1:6, dim = c(2, 3, 2))
A

, , 1

     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6

, , 2

     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6

dim(A)

[1] 2 3 2

Check if an object is a matrix. A data frame is not a matrix. A vector is not a matrix.

A = matrix(1:4, 1, 4)
A

     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4

class(A)

[1] "matrix" "array" 

B = data.frame(A)
B

  X1 X2 X3 X4
1  1  2  3  4

class(B)

[1] "data.frame"

C = 1:4
C

[1] 1 2 3 4

class(C)

[1] "integer"