working with matrices in r

Working With Matrices in R
Dr. T. N. Kavitha
Assistant Professor of Mathematics
SCSVMV

To create a vector in R
• > vec <- c( 1.9, -2.8, 5.6, 0, 3.4, -4.2 )
• Then the vector looks like:
• > vec
• [1] 1.9 -2.8 5.6 0.0 3.4 -4.2
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Dr T N Kavitha

To make a sequence of integers
• > vec <- -2:5
• Then the vector looks like:
• > vec
• [1] -2 -1 0 1 2 3 4 5
• > vec <- 5:-2
• Then we get:
• > vec
• [1] 5 4 3 2 1 0 -1 -2
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Dr T N Kavitha

To create the sequence function
• We have to give a starting value, and ending value, and an
increment value, e.g.,
• > vec <- seq( -3.1, 2.2, by=0.1 )
• Then we get:
• > vec
• [1] -3.1 -3.0 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1 -2.0 -1.9 -1.8 -1.7
• [16] -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2
• [31] -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
• [46] 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2
Note that R wraps the vector around to four rows, and at the beginning of each
row gives you the position in the vector of the first element of the row. We can
run a sequence backward, from a larger value to a smaller value, but we must
use a negative increment value.
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Dr T N Kavitha

To find the replicate function to a get a
vector whose elements are all the same
• > vec <- rep( 1, 8 )
• Then we get:
• > vec
• [1] 1 1 1 1 1 1 1 1
That is, we get 1 replicated 8 times.
• two or more vectors to make a larger vector, e.g.,
• > vec <- c( rep(2,4), 1:3, c(8,1,9) )
• Then we get
• > vec
• [1] 2 2 2 2 1 2 3 8 1 9
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Dr T N Kavitha

To know how many elements are in the
vector
• > length(vec)
If we want to reference an element in the vector,
we use square brackets, e.g.,
• > vec[5]
This will give us the fifth element of the vector vec.
• To reference a consecutive subset of the vector,
we use the colon operator within the square
brackets, e.g.,
• > vec[2:7]
• This will give us elements two through seven of
the vector vec.
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Dr T N Kavitha

To make a 2 x 3 matrix named M consisting
of the integers 1 through 6
• > M <- matrix( 1:6, nrow=2 )
• or this:
• > M <- matrix( 1:6, ncol=3 )
• We get:
• > M
• [,1] [,2] [,3]
• [1,] 1 3 5
• [2,] 2 4 6
• Note that R places the row numbers on the left
and the column numbers on top. Also note that R
filled in the matrix column-by-column.
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Dr T N Kavitha

If we prefer to fill in the matrix row-by-row,
we must activate the byrow setting, e.g.,
• > M <- matrix( 1:6, ncol=3 )
• Then we get:
• > M
• [,1] [,2] [,3]
• [1,] 1 2 3
• [2,] 4 5 6
In place of the vector 1:6 you would place
any vector containing the desired elements of
the matrix.
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Dr T N Kavitha

To obtain the transpose of a matrix
• > t(M)
• This gives us the transpose of the matrix
M created above:
• > t(M)
[,1] [,2]
[1,] 1 4
[2,] 2 5
[3,] 3 6
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Dr T N Kavitha

To add or subtract two matrices (or vectors)
which have the same number of rows and
columns
• we use the plus and minus symbols, e.g.,
• > A + B
• > A - B
• To multiply two matrices with compatible dimensions
• > A %*% B
If we just use the multiplication operator *, R will multiply
the corresponding elements of the two matrices,
provided they have the same dimensions.
• To multiply two vectors to get their scalar (inner)
product, we use the same operator, e.g.,
• > a %*% b
• R will transpose a for us rather than giving us an error
message.
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Dr T N Kavitha

To create the identity matrix for a desired
dimension
• > I <- diag(5)
• This gives us the 5 x 5 identity matrix:
• > I
[,1] [,2] [,3] [,4] [,5]
[1,] 1 0 0 0 0
[2,] 0 1 0 0 0
[3,] 0 0 1 0 0
[4,] 0 0 0 1 0
[5,] 0 0 0 0 1
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Dr T N Kavitha

To find the determinant of a square matrix N
• To find the determinant of a square matrix
N, use the determinant function, e.g.,
• > det( N )
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Dr T N Kavitha

To obtain the inverse N-1 of an invertible
square matrix N
• > solve( N )
• If the matrix is singular (not invertible), or almost
singular, we get an error message.
• A <- as.matrix(cbind(c(3,0),c(1,2)))
• A
• A1 <- matrix.inverse(A)
• A1
• solve(A)
• A %*% A1
• [,1] [,2]
• ## [1,] 1 0
## [2,] 0 1
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Dr T N Kavitha

>mp <-
matrix(c(0,1/4,1/4,3/4,0,1/4,1/4,3/4,1/2),3,
3,byrow=T)
> mp
>eigen(mp)
> $vectors
Øsolve(eigen(mp)$vectors)
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Dr T N Kavitha

TUTORIAL
• https://www.geeksforgeeks.org/find-
eigenvalues-and-eigenvectors-of-a-matrix-
in-r-programming-eigen-function/
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Dr T N Kavitha

working with matrices in r

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