Matrix Math Operations Using R

Matrix math operations using R are easy to conduct using basic math operators such as + - * and /. Generally speaking the matrix objects you perform math on must be the same dimensions (that is conformable) as each other but there are exceptions.

There are two kinds of matrix math:

• Math using a matrix simply as a series of values.
• Matrix math using array properties.

In the first case you can use R to carry out regular mathematical operations on a matrix (that is a 2-dimensional array). In the second case you use R to conduct “proper” matrix math.

This article is not intended to be a detailed treatise on matrix math; think of it more as a rough guide to possibilities. If matrix math is what you need then this might help point you in the right direction.

Here are some of the topics covered:

• Simple math
  • Identical dimensions
  • Matrix and single numeric
  • Matrix and multiple numeric
  • Other matrix operations
  • Non-conformable matrices
• Matrix math
  • Inner product
  • Outer product
  • Kronecker product
  • Diagonals
  • Eigenvalues and Eigenvectors
  • Singular Value Decomposition
  • Cholesky decomposition
  • QR decomposition
  • Determinant
  • Cross products
• Miscellaneous tools
  • Transposition
  • Upper/lower triangles
  • Row/column index

Simple math

Matrix math operations using R can be fairly simple. In the “simple” case you can use a matrix with basic math operators.

Identical dimensions

The “simplest” situation is where you have matrix objects that have the same dimensions.

x <- matrix(c(1,1, 3,0, 1,0), ncol = 3))
y <- matrix(c(0,7, 0,5, 5,0), ncol = 3)
x ; y

     [,1] [,2] [,3]
[1,]    1    3    1
[2,]    1    0    0

     [,1] [,2] [,3]
[1,]    0    0    5
[2,]    7    5    0

The regular math operators with deal with the calculations:

x + y
     [,1] [,2] [,3]
[1,]    1    3    6
[2,]    8    5    0

x * y
     [,1] [,2] [,3]
[1,]    0    0    5
[2,]    7    0    0

x / y
          [,1] [,2] [,3]
[1,]       Inf  Inf  0.2
[2,] 0.1428571    0  NaN

x - y
     [,1] [,2] [,3]
[1,]    1    3   -4
[2,]   -6   -5    0

In the preceding example 2 matrix objects were used but there is no reason why you cannot have more, as long as they all have the same dimensions.

Matrix and a single numeric

If you have a matrix you can also use the basic operators with a single value:

x * 4.5
     [,1] [,2] [,3]
[1,]  4.5 13.5  4.5
[2,]  4.5  0.0  0.0

y / 2
     [,1] [,2] [,3]
[1,]  0.0  0.0  2.5
[2,]  3.5  2.5  0.0

Matrix and multiple numeric

You can think of a matrix as a vector of values that happens to be split into rows and columns. This means that you can carry out math operations with a matrix and a vector of values, as long as the recursive process “fits” (i.e. is conformable).

In practical terms you can carry out a math operation between a matrix and a vector if the length of the matrix is divisible by the length of the vector exactly (i.e. producing an integer).

In the following example, the matrix has 6 elements and therefore any vector would have to contain 1, 2, or 3 elements. If your vector is too long it is truncated and you get a warning.

x
     [,1] [,2] [,3]
[1,]    1    3    1
[2,]    1    0    0

length(x)
[1] 6

x * 2:4
     [,1] [,2] [,3]
[1,]    2   12    3
[2,]    3    0    0

# Long vectors are truncated with a warning
x + 1:4
     [,1] [,2] [,3]
[1,]    2    6    2
[2,]    3    4    2

Warning message:
In x + 1:4 :
  longer object length is not a multiple of shorter object length

It can help to see your matrix as a vector:

c(x)
[1] 1 1 3 0 1 0

c(x) * 2:4
[1]  2  3 12  0  3  0

c(x) + 1:4
[1] 2 3 6 4 2 2

Warning message:
In c(x) + 1:4 :
  longer object length is not a multiple of shorter object length

Note that c(x) was used in the example but as.numeric(x) would also work.

Other matrix math operations

Many math functions that can operate on a vector will be able to work with a matrix. For example:

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

log(xx)
         [,1]      [,2]     [,3]
[1,] 0.000000 0.6931472 1.098612
[2,] 1.386294 1.6094379 1.791759

cos(xx)
           [,1]       [,2]       [,3]
[1,]  0.5403023 -0.4161468 -0.9899925
[2,] -0.6536436  0.2836622  0.9601703

sqrt(xx)
     [,1]     [,2]     [,3]
[1,]    1 1.414214 1.732051
[2,]    2 2.236068 2.449490

---

Non-conformable matrices

When you have two matrix objects of different dimensions you get an error. This example shows a single dimension matrix:

yy <- matrix(c(40,100), ncol = 1)
yy
     [,1]
[1,]   10
[2,]  100

x * yy
Error in x * yy : non-conformable arrays

In this case the matrix can be converted to a vector resulting in 2 elements. This is divisible by the number of elements in the first matrix and permits matrix math:

x * c(yy)
     [,1] [,2] [,3]
[1,]   10   30   10
[2,]  100    0    0

x + c(yy)
     [,1] [,2] [,3]
[1,]   11   13   11
[2,]  101  100  100

There are other solutions for this case:

xx * c(yy)
sweep(xx, 1, yy, `*`)
apply(xx, 2, function(x) x * yy)
xx * yy[,1]
xx * yy[row(xx),]

These solutions work for any of the math operators + - * and /.

---

Matrix Mathematics

Matrix math operations using R involve more than just simple math. In mathematics matrix math is a substantial topic. Using R you can carry out various matrix math computations. These include:

• Inner product %*%
• Outer product outer() and %o%
• Kronecker product kronecker() and %x%
• Matrix diagonals diag()
• Eigenvalues eigen()
• Singular Value Decomposition svd()
• Cholesky (Choleski) decomposition chol()
• QR decomposition qr()
• Determinant det()
• Cross-product crossprod() and tcrossprod()
• Transposition t()
• Upper/lower triangles upper.tri() and lower.tri()
• Row/column index row() and col()

Inner product

Standard matrix multiplication (or inner product) is a way to multiply two matrix objects. You use the %*% operator in Rto execute the math.

X %*% Y

In general if the number of columns of the first matrix equals the number of rows of the second matrix then you should be able to use the matrix-math operator %*% to multiply:

X <- matrix(c(2,7,4,8,5,9), ncol = 3)
Y <- matrix(c(1,3,4,6,1,2), ncol = 2)

X ; Y
     [,1] [,2] [,3]
[1,]    2    4    5
[2,]    7    8    9

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

X %*% Y
     [,1] [,2]
[1,]   34   26
[2,]   67   68

Outer product

The outer product is a matrix operation that takes a matrix (or vector or array) and applies a function to it, with a second matrix (or a vector or array) as an argument. There are two options in R:

outer(X, Y, FUN = "*", ...)
X %o% Y

The outer() command allows any maths function, with the default being *. Essentially X and Y are the main arguments (you can pass additional parameters too). The %o% operator (the letter o) is a convenience function for multiply (i.e. FUN = "*") so you end up specifying X multiplied by Y.

Use two vectors and the result is a matrix with dimensions determined by the length of the vectors:

outer(1:3, 1:3)
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    2    4    6
[3,]    3    6    9

You can use different function, and use Y as a parameter passed to FUN, in the following example Y specifies the base for the log:

m
     [,1] [,2]
[1,]    5    1
[2,]    1    3

outer(m, Y = 2, FUN = log)
, , 1

         [,1]     [,2]
[1,] 2.321928 0.000000
[2,] 0.000000 1.584963

Essentially what happens is that the second matrix is converted to a vector and this is “applied” to the first matrix using the function specified. Each element of the second matrix is applied in turn, and the result is an array with multiple dimensions. In the following example the first matrix is 2×2 and so is the second, so the result has dimensions of: 2, 2, 2, 2

A
     [,1] [,2]
[1,]    4   -1
[2,]    1    3

m
     [,1] [,2]
[1,]    5    1
[2,]    1    3

outer(A, m, FUN = "+")
, , 1, 1

     [,1] [,2]
[1,]    9    4
[2,]    6    8

, , 2, 1

     [,1] [,2]
[1,]    5    0
[2,]    2    4

, , 1, 2

     [,1] [,2]
[1,]    5    0
[2,]    2    4

, , 2, 2

     [,1] [,2]
[1,]    7    2
[2,]    4    6

# Get identical result using a vector
outer(A, c(5,1,1,3), FUN = "+")

Kronecker product

The Kronecker matrix product is similar to the outer product but the result is a single array with dimensions equal to dim(X) x dim(Y).

kronecker(X, Y, FUN = "*", ...)
X %x% Y

The kronecker() command allows any function to be used, whist the alias %x% is for multiply (i.e. FUN = "*").

m
     [,1] [,2]
[1,]    5    1
[2,]    1    3

m %x% 2
     [,1] [,2]
[1,]   10    2
[2,]    2    6

When your second array has >1 dimension, the result has dimensions equal to dim(X) x dim(Y).

kronecker(m, 2:3, FUN = "+")
     [,1] [,2]
[1,]    7    3
[2,]    8    4
[3,]    3    5
[4,]    4    6

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

yy
     [,1]
[1,]   10
[2,]  100

kronecker(xx, yy)
     [,1] [,2] [,3]
[1,]   10   20   30
[2,]  100  200  300
[3,]   40   50   60
[4,]  400  500  600

Diagonals

A matrix diagonal is more or less what it sounds like! Matrix diagonals are used in various branches of matrix math. In Ryou can use the diag() function to extract or create matrix diagonals.

diag(x = 1, nrow, ncol, names = TRUE)
diag(x) <- value

There are 4 main uses for diag():

1. To extract the diagonal from x (a matrix).
2. To make an identity matrix by specifying nrow.
3. To make an identity matrix by specifying x as a scalar.
4. To make a matrix with specified diagonals.

You can also assign diagonal values to an existing matrix.

Extract diagonals

To extract the diagonals from a matrix you simply specify the matrix:

C
     [,1] [,2] [,3]
[1,]    3    1   -5
[2,]    2    0    4
[3,]    1    6    3

diag(C)
[1] 3 0 3

If the matrix is not square you get the diagonals starting from the top-left:

E
     [,1] [,2] [,3]
[1,]   -3    2    0
[2,]    0    5   -1

diag(E)
[1] -3  5

Identity matrix from nrow

If you omit x and specify nrow you will return an identity matrix, that is one where the diagonals are 1 and the other elements are 0. You may also give ncol to create a non-square matrix (note that ncol may be larger or smaller than nrow):

diag(nrow = 4)
     [,1] [,2] [,3] [,4]
[1,]    1    0    0    0
[2,]    0    1    0    0
[3,]    0    0    1    0
[4,]    0    0    0    1

diag(nrow = 4, ncol = 3)
     [,1] [,2] [,3]
[1,]    1    0    0
[2,]    0    1    0
[3,]    0    0    1
[4,]    0    0    0

diag(nrow = 4, ncol = 6)
     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    1    0    0    0    0    0
[2,]    0    1    0    0    0    0
[3,]    0    0    1    0    0    0
[4,]    0    0    0    1    0    0

Identity matrix from scalar

If you specify x as a scalar (single value) you return an identity matrix with diagonals of 1 and with dimensions (nrow, ncol) set to the of the scalar:

diag(4)
     [,1] [,2] [,3] [,4]
[1,]    1    0    0    0
[2,]    0    1    0    0
[3,]    0    0    1    0
[4,]    0    0    0    1

Matrix from vector

If you specify x as a vector you will return a matrix where the diagonals are the values in x and the other elements are 0:

diag(x = c(3,4,2,6))
     [,1] [,2] [,3] [,4]
[1,]    3    0    0    0
[2,]    0    4    0    0
[3,]    0    0    2    0
[4,]    0    0    0    6

Set diagonals for existing matrix

You can set the diagonals for an existing matrix by assigning values:

mm
     [,1] [,2]
[1,]    8    1
[2,]    1    9

diag(mm) <- 100
mm
     [,1] [,2]
[1,]  100    1
[2,]    1  100

diag(mm) <- c(6,3)
mm
     [,1] [,2]
[1,]    6    1
[2,]    1    3

CC
     [,1] [,2] [,3]
[1,]    3    1   -5
[2,]    2    0    4
[3,]    1    6    3

diag(CC) <- c(99, 999)
Error in `diag<-`(`*tmp*`, value = c(99, 999)) :
  replacement diagonal has wrong length

Note:

• If you specify a single value it is used for all diagonals.
• If you specify the “correct” number of elements you replace the existing values.
• If you specify too few or too many elements you get an error.

Eigenvalues and eigenvectors

Spectral decomposition of a matrix produces eigenvalues and eigenvectors. These values are used often in linear algebra and various branches of mathematics.

The R function eigen() can compute the eigenvalues and eigenvectors of a (square) matrix.

eigen(x, symmetric, only.values = FALSE)

The eigen() function only works on square matrix objects.

The symmetric parameter allows you to specify TRUE to force the calculations to use only the lower triangle, when you have symmetrical matrix objects. If you leave this to the default then the function checks to see if the values are symmetric.

The only.values parameter can be set to TRUE to return only the eigenvalues.

m
     [,1] [,2]
[1,]    5    1
[2,]    1    3

eigen(m)
eigen() decomposition
$values
[1] 5.414214 2.585786

$vectors
           [,1]       [,2]
[1,] -0.9238795  0.3826834
[2,] -0.3826834 -0.9238795

If you have negative values you’ll return complex values:

M
     [,1] [,2]
[1,]    5   -2
[2,]    2    4

eigen(M)
eigen() decomposition
$values
[1] 4.5+1.936492i 4.5-1.936492i

$vectors
                     [,1]                 [,2]
[1,] 0.7071068+0.0000000i 0.7071068+0.0000000i
[2,] 0.1767767-0.6846532i 0.1767767+0.6846532i

Singular Value Decomposition

Singular value decomposition of a matrix is important in many statistical routines. SVD is a kind of factorisation of a matrix and is carried out in R with the svd() function:

svd(x, nu = min(n, p), nv = min(n, p))

In the svd() function:

• x is a matrix
• nu is the number of left singular vectors to compute
• nv is the number of right singular vectors to compute

The svd() function returns a result with several components:

• $d a vector of singular values in descending order.
• $u a matrix of left singular vectors.
• $v a matrix of right singular vectors.

C
     [,1] [,2] [,3]
[1,]    3    1   -5
[2,]    2    0    4
[3,]    1    6    3

svd(C)
$d
[1] 7.620874 5.810848 3.025941

$u
           [,1]       [,2]       [,3]
[1,] -0.4710376 -0.7804341 -0.4111522
[2,]  0.4517527  0.1869132 -0.8723434
[3,]  0.7576563 -0.5966457  0.2645201

$v
           [,1]       [,2]       [,3]
[1,] 0.03254861 -0.4412646 -0.8967866
[2,] 0.53470246 -0.7503738  0.3886290
[3,] 0.84441333  0.4921633 -0.2115217

You can use the nu and nv arguments to reduce (to 0 if you like) the number of left and/or right singular vectors.

VADeaths
      Rural Male Rural Female Urban Male Urban Female
50-54       11.7          8.7       15.4          8.4
55-59       18.1         11.7       24.3         13.6
60-64       26.9         20.3       37.0         19.3
65-69       41.0         30.9       54.6         35.1
70-74       66.0         54.3       71.1         50.0

svd(VADeaths, nu = 2, nv = 2)
$d
[1] 161.967972  10.037542   3.212076   1.194284

$u
           [,1]       [,2]
[1,] -0.1404491 -0.1476512
[2,] -0.2161352 -0.3623160
[3,] -0.3297674 -0.4373676
[4,] -0.5099065 -0.4819224
[5,] -0.7515374  0.6506817

$v
           [,1]       [,2]
[1,] -0.5243856  0.3123820
[2,] -0.4137210  0.6015805
[3,] -0.6229108 -0.7282861
[4,] -0.4072306  0.1005871

Cholesky decomposition

Compute the Choleski (Cholesky) factorization of a real symmetric positive-definite square matrix using the chol()function. It is a kind of optimisation that can be used in various mathematical problems.

chol(x, pivot = FALSE, tol = -1, ...)

In chol():

• x is a matrix, which should be positive and symmetric, (i.e. square).
• pivot can be set to TRUE in some cases where the matrix is semi-positive definite.
• tol is used to set a tolerance when pivot = TRUE.

How to tell if a matrix is positive definite, or positive semi-definite?

• A matrix is symmetric if nrow = ncol, in other words, square. In addition the transposed matrix must be the same as the original.
• If all the eigenvalues are positive (>0) the matrix is positive definite.
• If any eigenvalues are 0 and the others positive then the matrix is positive semi-definite.

You can check the eigenvalues with eigen(x).

m
     [,1] [,2]
[1,]    5    1
[2,]    1    3

# Check matrix is positive definite
eigen(m)$val
[1] 5.414214 2.585786

# Check that matrix is symmetric
identical(m, t(m))
[1] TRUE

chol(m)
         [,1]      [,2]
[1,] 2.236068 0.4472136
[2,] 0.000000 1.6733201

A
     [,1] [,2]
[1,]    4   -1
[2,]    1    3

# Check matrix (it is complex but is +ve definite)
eigen(A)$val
[1] 3.5+0.866025i 3.5-0.866025i

# Check if matrix is symmetric
identical(A, t(A))
[1] FALSE

# chol() will return a result as it does not check symmetry!

If you have an eigenvalue of 0 (with the others >0) then your matrix is positive semi-definite. You can perform Cholensky decomposition with chol() but need to use pivot = TRUE as an argument.

im <- matrix(c(1,-1, -1,1), ncol = 2)

im
     [,1] [,2]
[1,]    1   -1
[2,]   -1    1

# matrix is +ve semi-definite
eigen(im)$val
[1] 2 0

# Check symmetric
identical(im, t(im))
[1] TRUE

chol(im)
Error in chol.default(im) :
  the leading minor of order 2 is not positive definite

# Need to use pivot = TRUE
chol(im, pivot = TRUE)
     [,1] [,2]
[1,]    1   -1
[2,]    0    0
attr(,"pivot")
[1] 1 2
attr(,"rank")
[1] 1
Warning message:
In chol.default(im, pivot = TRUE) :
  the matrix is either rank-deficient or indefinite

Note that you still get a warning!

The eigenvalues need to be >0 but there is a tolerance:

mat <- matrix(c(4,12,-16, 12,37,-43, -16,-43,98), ncol = 3)
mat
     [,1] [,2] [,3]
[1,]    4   12  -16
[2,]   12   37  -43
[3,]  -16  -43   98

# Eigenvalues all >0 (just)
eigen(mat)$val
[1] 123.47723179  15.50396323   0.01880498

# Check symmetry
identical(mat, t(mat))
[1] TRUE

chol(mat)
     [,1] [,2] [,3]
[1,]    2    6   -8
[2,]    0    1    5
[3,]    0    0    3

a <- matrix(c(2,3,1,11, 3,9,3,30, 1,3,1,10, 11,30,10,101))
a
     [,1] [,2] [,3] [,4]
[1,]    2    3    1   11
[2,]    3    9    3   30
[3,]    1    3    1   10
[4,]   11   30   10  101

# Very small eigenvalue registers as effectively 0
eigen(a)$val
[1] 1.120990e+02 9.009892e-01 1.681988e-14 1.831868e-15

# Check symmetry
identical(a, t(a))
[1] TRUE

chol(a)
Error in chol.default(a) :
  the leading minor of order 3 is not positive definite

If you try to use pivot = TRUE on a matrix that is not positive (i.e. non-negative definite), you’ll get a result with a warning. However, the result will be meaningless!

# Negative definite i.e. not non-negative definite
b <- matrix(c(5,-5,-5,3), 2, 2)
b
     [,1] [,2]
[1,]    5   -5
[2,]   -5    3

# check symmetry -- okay
identical(b, t(b))
[1] TRUE

# negative eigenvalues, so no +ve
eigen(b)$val
[1]  9.09902 -1.09902

# pivot = TRUE runs but result is meaningless!
#.. not shown

QR Decomposition

QR decomposition is used in many statistical methods, especially in regression. The qr() function carries out the calculations.

qr(x, tol = 1e-07 , LAPACK = FALSE, ...)

In qr():

• x is a matrix to be decomposed.
• tol is a tolerance used for detecting linear dependencies (you probably won’t need to tinker with this).
• LAPACK is which QR algorithm to use, the default uses LINPACK.

The result of qr() is an object with various components:

C
     [,1] [,2] [,3]
[1,]    3    1   -5
[2,]    2    0    4
[3,]    1    6    3

qr(C)
$qr
           [,1]      [,2]     [,3]
[1,] -3.7416574 -2.405351 1.069045
[2,]  0.5345225  5.586975 2.787095
[3,]  0.2672612 -0.983516 6.410089

$rank
[1] 3

$qraux
[1] 1.801784 1.180821 6.410089

$pivot
[1] 1 2 3

attr(,"class")
[1] "qr"

QR decomposition is a potentially large topic, which there is not space for here! In addition to the qr() function there are various helper functions. See the help entry in qr() for more information about those.

Determinant

You can calculate the of a square matrix using the det() function. A square matrix is one where nrow = ncol but it doesn’t have to be symmetrical (where x = t(x)).

det(x)
determinant(x, logarithm = TRUE)

The det() function returns the determinant. The determinant() function returns the modulus of the determinant and the sign. The default for determinant() is to return the log of the determinant.

A
     [,1] [,2]
[1,]    4   -1
[2,]    1    3

# Matrix is square but no matter that it is not symmetric
identical(A, t(A))
[1] FALSE

det(A)
[1] 13

determinant(A)
$modulus
[1] 2.564949
attr(,"logarithm")
[1] TRUE

$sign
[1] 1

attr(,"class")
[1] "det"

determinant(A, logarithm = FALSE)
$modulus
[1] 13
attr(,"logarithm")
[1] FALSE

$sign
[1] 1

attr(,"class")
[1] "det"

Note that the logarithm argument cannot be abbreviated.

Cross-products

Cross products are where you multiply two matrix objects (inner product), but one is transposed. The R functions crossprod() and tcrossprod() can carry out the calculations.

crossprod(x, y = NULL)
tcrossprod(x, y = NULL)

You specify at least one matrix object x. Object y is optional, if not specified x is used. These functions are essentially equivalent to:

• t(x) %*% y for crossprod()
• x %*% t(y) for tcrossprod()

b
     [,1] [,2]
[1,]    5   -5
[2,]   -5    3

crossprod(b)
     [,1] [,2]
[1,]   50  -40
[2,]  -40   34

B
     [,1] [,2]
[1,]    1    3
[2,]   -2   -4
[3,]    2    0

D
     [,1]
[1,]    3
[2,]    2
[3,]   -2

crossprod(B, D)
     [,1]
[1,]   -5
[2,]    1

You get an error if the dimensions are “wrong”:

A
     [,1] [,2]
[1,]    4   -1
[2,]    1    3

D
     [,1]
[1,]    3
[2,]    2
[3,]   -2

crossprod(A, D)
Error in crossprod(A, D) : non-conformable arguments

If you specify a vector it will be “converted” to a one-column or one-row matrix:

tcrossprod(1:4)
     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4
[2,]    2    4    6    8
[3,]    3    6    9   12
[4,]    4    8   12   16

Miscellaneous tools

There are various other tools that you can use with matrix objects:

Transposition

Perhaps the simplest matrix operation is to transpose, that is switch the rows and columns. You can carry out matrixtransposition using the t() function.

t(x)

Example: transpose matrix objects

x
     [,1] [,2] [,3]
[1,]    1    3    1
[2,]    1    0    0

t(x)
     [,1] [,2]
[1,]    1    1
[2,]    3    0
[3,]    1    0

D
     [,1]
[1,]    3
[2,]    2
[3,]   -2

t(D)
     [,1] [,2] [,3]
[1,]    3    2   -2

Upper and Lower triangles

You can “see” the upper or lower triangles of a matrix (or any 2-D object) using upper.tri() and lower.tri()functions. These give you a logical matrix the same size as the original, i.e. TRUE or FALSE values.

upper.tri(x, diag = FALSE)
lower.tri(x, diag = FALSE)

You can optionally set diag = TRUE to return the diagonals as TRUE.

A
     [,1] [,2]
[1,]    4   -1
[2,]    1    3

upper.tri(A)
      [,1]  [,2]
[1,] FALSE  TRUE
[2,] FALSE FALSE

B
     [,1] [,2]
[1,]    1    3
[2,]   -2   -4
[3,]    2    0

lower.tri(B, diag = TRUE)
     [,1]  [,2]
[1,] TRUE FALSE
[2,] TRUE  TRUE
[3,] TRUE  TRUE

Row and Column index

You can produce a matrix of integer values based on their position in a matrix object (or similar 2-D object). The row() and col() functions operate over rows and columns respectively.

row(x, as.factor = FALSE)
.row(dim)

col(x, as.factor = FALSE)
.col(dim)

Essentially you provide a template object and the functions return a new object with the appropriate values.

C
     [,1] [,2] [,3]
[1,]    3    1   -5
[2,]    2    0    4
[3,]    1    6    3

row(C)
     [,1] [,2] [,3]
[1,]    1    1    1
[2,]    2    2    2
[3,]    3    3    3

If you specify the argument as.factor you can get the result stored as a factor rather than a regular numeric.

E
     [,1] [,2] [,3]
[1,]   -3    2    0
[2,]    0    5   -1

col(E, as.factor = TRUE)
     [,1] [,2] [,3]
[1,] 1    2    3
[2,] 1    2    3
Levels: 1 2 3

There are alternative versions .row() and .col(), which take a vector argument, giving the dimensions of the matrixfor which the indices are required.

.row(dim = c(3, 5))
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    1    1    1    1
[2,]    2    2    2    2    2
[3,]    3    3    3    3    3

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Summary

The topic of matrix math is extensive! This article is not intended to be an exhaustive thesis on the subject but more of an introduction to some of the R tools that you can use to carry out matrix math, and other operations.

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This article is partly in support of my book An Introduction to R see the publications page for more information.

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