diag

R Base 3.6.2

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

Syntax

diag(x = 1, nrow, ncol, names = TRUE)
diag(x) &lt;- value

Arguments

Parameter	Description
`x`	a matrix, vector or 1D array, or missing.
`nrow, ncol`	optional dimensions for the result when x is not a matrix.
`names`	(when x is a matrix) logical indicating if the resulting vector, the diagonal of x, should inherit names from dimnames(x) if available.
`value`	either a single value or a vector of length equal to that of the current diagonal. Should be of a mode which can be coerced to that of x.

Return Value

If x is a matrix then diag(x) returns the diagonal of x. The resulting vector will have names if the matrix x has matching column and rownames. The replacement form sets the diagonal of the matrix x to the given value(s). In all other cases the value is a diagonal matrix with nrow rows and ncol columns (if ncol is not given the matrix is square). Here nrow is taken from the argument if specified, otherwise inferred from x: if that is a vector (or 1D array) of length two or more, then its length

Details

diag has four distinct usages: x is a matrix, when it extracts the diagonal. x is missing and nrow is specified, it returns an identity matrix. x is a scalar (length-one vector) and the only argument, it returns a square identity matrix of size given by the scalar. x is a ‘numeric’ (complex, numeric, integer, logical, or raw) vector, either of length at least 2 or there were further arguments. This returns a matrix with the given diagonal and zero off-diagonal entries. It is an error to specify nrow or ncol in the first case.

Examples

Example

# NOT RUN {
dim(diag(3))
diag(10, 3, 4) # guess what?
all(diag(1:3) == {m <- matrix(0,3,3); diag(m) <- 1:3; m})

## other "numeric"-like diagonal matrices :
diag(c(1i,2i))    # complex
diag(TRUE, 3)     # logical
diag(as.raw(1:3)) # raw
(D2 <- diag(2:1, 4)); typeof(D2) # "integer"

require(stats)
## diag(<var-cov-matrix>) = variances
diag(var(M <- cbind(X = 1:5, Y = rnorm(5))))
#-> vector with names "X" and "Y"
rownames(M) <- c(colnames(M), rep("", 3))
M; diag(M) #  named as well
diag(M, names = FALSE) # w/o names
# }