Introduction
In the previous post, we learnt to create matrices. In this post, we will learn to:
- combining matrices
- index/subset matrices
- dissolve matrices
Append Data
In this section, we will learn how to append data to a matrix. There are two functions that can be used for this purpose:
rbind()cbind()
cbind will append a new column to the matrix while rbind will append a new row.
Append Row/Column
# 3 x 3 matrix
mat <- matrix(data = 1:9, nrow = 3)
mat## [,1] [,2] [,3]
## [1,] 1 4 7
## [2,] 2 5 8
## [3,] 3 6 9# numeric vector
v <- c(10, 11, 12)
v## [1] 10 11 12# append row
rbind(mat, v)## [,1] [,2] [,3]
## 1 4 7
## 2 5 8
## 3 6 9
## v 10 11 12# append column
cbind(mat, v)## v
## [1,] 1 4 7 10
## [2,] 2 5 8 11
## [3,] 3 6 9 12Combine Matrices
When you use rbind to combine two matrices, the number of columns must match and in case of cbind, the number of rows must match.
Append Row/Column
# 3 x 3 matrix
mat1 <- matrix(data = 1:9, nrow = 3)
mat2 <- matrix(data = sample(9), nrow = 3)
# append rows
rbind(mat1, mat2)## [,1] [,2] [,3]
## [1,] 1 4 7
## [2,] 2 5 8
## [3,] 3 6 9
## [4,] 2 4 7
## [5,] 5 3 8
## [6,] 9 6 1# append columns
cbind(mat1, mat2)## [,1] [,2] [,3] [,4] [,5] [,6]
## [1,] 1 4 7 2 4 7
## [2,] 2 5 8 5 3 8
## [3,] 3 6 9 9 6 1Subset Matrices
In this section, we will learn to subset matrices. The [] operator can be used to subset matrices just like vectors but since matrices are two dimensional, we need to specify both the row number and the column number. Below are a few examples:
# 3 x 4 matrix
mat <- matrix(data = 1:12, nrow =3)
mat## [,1] [,2] [,3] [,4]
## [1,] 1 4 7 10
## [2,] 2 5 8 11
## [3,] 3 6 9 12# extract element from first row, first column
mat[1, 1]## [1] 1# extract all rows of first column
mat[, 1]## [1] 1 2 3# extract all columns of first row
mat[1,]## [1] 1 4 7 10# extract 2nd and 3rd row of first column
mat[c(2, 3), 1]## [1] 2 3# extract 2nd and 3rd column of first row
mat[1, c(2, 3)]## [1] 4 7# extract 2nd and 3rd row of first and third column
mat[c(2, 3), c(1, 3)]## [,1] [,2]
## [1,] 2 8
## [2,] 3 9Using Row & Column Names
In an earlier section, we learnt how to name the rows and columns of a matrix. Let us see how these names can be used to subset matrices.
# row names
row_names <- c('row_1', 'row_2', 'row_3')
# column names
col_names <- c('col_1', 'col_2', 'col_3')
# matrix with row and column names
mat <- matrix(data = 1:9, nrow = 3, dimnames = list(row_names, col_names))
# extract elements from first row/columns
mat['row_1', 'col_1']## [1] 1# extract all rows of first column
mat[, 'col_1']## row_1 row_2 row_3
## 1 2 3# extract all columns of first row
mat['row_1',]## col_1 col_2 col_3
## 1 4 7Using Logical Expressions
We can use logical expressions to subset matrices.
# 3 x 4 matrix
mat <- matrix(data = 1:12, nrow =3)
mat## [,1] [,2] [,3] [,4]
## [1,] 1 4 7 10
## [2,] 2 5 8 11
## [3,] 3 6 9 12# elements greater than 4
mat > 4## [,1] [,2] [,3] [,4]
## [1,] FALSE FALSE TRUE TRUE
## [2,] FALSE TRUE TRUE TRUE
## [3,] FALSE TRUE TRUE TRUE# extract elements greater than 4
mat[mat > 4]## [1] 5 6 7 8 9 10 11 12Dissolve Matrices
Till now we have learnt how to coerce a vector into matrix. Now let us learn how to coerce a matrix into a vector using:
c()as.vector()
# 3 x 3 matrix
mat <- matrix(data = 1:9, nrow =3)
mat## [,1] [,2] [,3]
## [1,] 1 4 7
## [2,] 2 5 8
## [3,] 3 6 9# using c()
c(mat)## [1] 1 2 3 4 5 6 7 8 9# using as.vector()
as.vector(mat)## [1] 1 2 3 4 5 6 7 8 9