antidiag: Matrix antidiagonals
In matricks: Useful Tricks for Matrix Manipulation

Extract or replace the antidiagonal of a matrix, or construct a antidiagonal matrix.

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antidiag(x = as.numeric(c(1)), nrow = NULL, ncol = NULL)

antidiag(x) <- value

`x`	matrix, vector or 1D array, or missing.
`nrow`	number of rows (optional; when x is not a matrix)
`ncol`	number of columns (optional; when x is not a matrix)
`value`	either a single value or a vector of length equal to that of the current antidiagonal. Should be of a mode which can be coerced to that of x.

# Extracting antidiag
antidiag(diag(3))
# Creating antidiagonal matrix
antidiag(7, 3, 3)
antidiag(1:5, 3, 3)
# Assigning antidiagonal
mat <- matrix(0, 3, 3)
antidiag(mat) <- c(3, 4, 5)
mat