7 Matrices and arrays

7.1 Matrices

Matrices are collections of numerics arranged in a two-dimensional rectangular layout

• the first argument is a vector of values
• the second specifies number of rows and columns
• R offers operators and functions for matrix algebra

a_matrix <- matrix(c(3, 5, 7, 4, 3, 1), c(3, 2))
a_matrix

##      [,1] [,2]
## [1,]    3    4
## [2,]    5    3
## [3,]    7    1

7.2 Arrays

Variables of the type array are higher-dimensional matrices.

• the first argument is a vector containing the values
• the second argument is a vector specifying the depth of each dimension

a3dim_array <- array(1:24, dim=c(4, 3, 2))

7.3 Arrays

a3dim_array

## , , 1
## 
##      [,1] [,2] [,3]
## [1,]    1    5    9
## [2,]    2    6   10
## [3,]    3    7   11
## [4,]    4    8   12
## 
## , , 2
## 
##      [,1] [,2] [,3]
## [1,]   13   17   21
## [2,]   14   18   22
## [3,]   15   19   23
## [4,]   16   20   24

7.4 Selection

Subsets of matrices (and arrays) can be selected as seen for vectors.

a_matrix[2, c(1, 2)]

## [1] 5 3

a3dim_array[c(1, 2), 2, 2]

## [1] 17 18

7.5 apply

apply applies another function to each level of a set dimension of an array

apply(a3dim_array, 3, min) # apply on third dimension

## [1]  1 13

apply(a3dim_array, 1, min) # apply on first dimension

## [1] 1 2 3 4

apply(a3dim_array, 2, min) # apply on second dimension

## [1] 1 5 9