matrix_diagonal_indices {oeli}

Get indices of matrix diagonal

Description

This function returns the indices of the diagonal elements of a quadratic matrix.

Usage

matrix_diagonal_indices(n, triangular = NULL)

Arguments

n

An integer, the matrix dimension.

triangular

If NULL (default), all elements of the matrix are considered. If "lower" ("upper"), only the lower- (upper-) triangular matrix is considered.

Value

An integer vector.

Examples

# indices of diagonal elements
n <- 3
matrix(1:n^2, n, n)
matrix_diagonal_indices(n)

# indices of diagonal elements of lower-triangular matrix
L <- matrix(0, n, n)
L[lower.tri(L, diag=TRUE)] <- 1:((n * (n + 1)) / 2)
L
matrix_diagonal_indices(n, triangular = "lower")

# indices of diagonal elements of upper-triangular matrix
U <- matrix(0, n, n)
U[upper.tri(U, diag=TRUE)] <- 1:((n * (n + 1)) / 2)
U
matrix_diagonal_indices(n, triangular = "upper")