R Matrices

Matrices are the R-objects, in which the two-dimensional rectangular data set are arranged. A matrix can be built using the matrix() function.

Syntax:

matrix( data, nrow, ncol, byrow, dimensions)

Where, data: - it is the input vector will be the data elements of the matrix

nrow: -  number of rows to be created

ncol: -  number of columns to be created

byrow: - it is just a logical clue. If it is TRUE then the input vector elements are arranged by row.

dimname: - it is the names assigned to the rows and columns.

Example 1:

> Mat <- matrix(c(1:9), nrow = 3, ncol = 3 )
> Mat

Output:

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

Example 2:

> Mat <- matrix(c(1:9), nrow = 3, ncol = 3, byrow = TRUE)
> Mat

Output:

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

Example 3:

# Define the column and row names.
rnames = c("r1", "r2", "r3")
cnames = c("c1", "c2", "c3")
m <- matrix(c(1:9), nrow = 3, byrow = TRUE, dimnames = list(rnames, cnames))
print(m)

Output:

    c1 c2 c3
r1  1  2  3
r2  4  5  6
r3  7  8  9

Accessing elements of a Matrix

We can access the elements of a matrix by using the column and row index of the element.

Example:

# Create the matrix.
P <- matrix(c(1:12), nrow = 4, byrow = TRUE)
P
# Access the element at 3rd column and 1st row.
P[1,3]
# Access the element at 2nd column and 4th row.
P[4,2]
# Access only the  2nd row.
P[2,]
# Access only the 3rd column.
P[,3]

Output:

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

Modifying Element of a Matrix

To modify the elements of a matrix we just need to assign the value through the assignment operator in the index of the value.

Example:

# Create the matrix.
P <- matrix(c(1:12), nrow = 4, byrow = TRUE)
P
# Modify the element at 3rd column and 1st row.
P[1,2] <- 0
P

Output:

      [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
[3,]    7    8    9
[4,]   10   11   12
# Modify the element at 3rd column and 1st row.
P[1,2] <- 0
P
        [,1] [,2] [,3]
  [1,]    1    0    3
  [2,]    4    5    6
  [3,]    7    8    9
  [4,]   10   11   12

Adding Row or Column in the Matrix

We can add a row or column in the matrix using rbind() and cbind() function respectively.

Example:

# Create two 2x3 matrices.
m <- matrix(1:6, nrow = 2, ncol = 2)
print(m)
# add column
m <- cbind(m, c(7, 8))
print(m)
# add row
m <- rbind(m, c(9, 0, 1))
print(m)

Output:

     [,1] [,2]
[1,]    1    3
[2,]    2    4
      [,1] [,2] [,3]
[1,]    1    3    7
[2,]    2    4    8
       [,1] [,2] [,3]
[1,]    1    3    7
[2,]    2    4    8
[3,]    9    0    1

Changing Dimension of a Matrix

We can change the dimension of a matrix through the dim() function.

Example:

# Create two 2x3 matrix.
m <- matrix(1:6, nrow = 2, ncol = 3)
print(m)
# change to 3X2 matrix
dim(m) <- c(3,2)
print(m)

Output:

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

Transpose a Matrix

We can transpose a matrix in R with the function t().

Example:

# Create two 2x3 matrices.
m <- matrix(1:9, nrow = 3, ncol = 3)
m
# transpose of matrix m
t(m)

Output:  

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

Matrix Computations

In R, we can perform various mathematical operations on the matrices using the R operators. The result of the operation is also a matrix.

To perform the operation, the dimension should be the same for the matrices.

Matrix Addition and Subtraction

Example:

# Create two 2x3 matrices.
m1 <- matrix(c(11:16), nrow = 2)
print(m1)
m2 <- matrix(c(1:6), nrow = 2)
print(m2)
# Add the matrices.
s <- m1 + m2
cat("Result of addition","\n")
print(s)
# Subtract the matrices
s <- m1 - m2
cat("Result of subtraction","\n")
print(s)

Output:

      [,1] [,2] [,3]
[1,]   11   13   15
[2,]   12   14   16
       [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6
        [,1] [,2] [,3]
[1,]   12   16   20
[2,]   14   18   22
        [,1] [,2] [,3]
[1,]   10   10   10
[2,]   10   10   10

Matrix Division

Example:

# Create two 2x3 matrices.
m1 <- matrix(c(11:16), nrow = 2)
print(m1)
m2 <- matrix(c(1:6), nrow = 2)
print(m2)
# Divide the matrices
r <- m1 / m2
cat("Result of division","\n")
print(r)

Output:

      [,1] [,2] [,3]
[1,]   11   13   15
[2,]   12   14   16
      [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6
     
Result of division
        [,1]     [,2]     [,3]
[1,]   11 4.333333 3.000000
[2,]    6 3.500000 2.666667

Matrix Multiplication

Example:

A <- matrix(c(2,3,-2,1,2,2),3,2)
B <- matrix(c(2,-2,1,2,3,1),2,3)
C <- B %*% A
C

Output:

       [,1] [,2]
[1,]    1   10
[2,]    0    4

When you change the order of the multiplication the result will be different. It means AB is not equal to BA.

Example:

A <- matrix(c(2,3,-2,1,2,2),3,2)
B <- matrix(c(2,-2,1,2,3,1),2,3)
C <- A %*% B
C

Output:

      [,1] [,2] [,3]
[1,]    2    4    7
[2,]    2    7   11
[3,]   -8    2   -4
Reference: https://www.datamentor.io/r-programming/matrix/ https://www.tutorialspoint.com/r/r_matrices.htm



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