diag {base}R Documentation

Matrix Diagonals


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


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


x a matrix, vector or 1D array.
nrow, ncol Optional dimensions for the result.
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.


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 row names.
If x is a vector (or 1D array) of length two or more, then diag(x) returns a diagonal matrix whose diagonal is x.
If x is a vector of length one then diag(x) returns an identity matrix of order the nearest integer to x. The dimension of the returned matrix can be specified by nrow and ncol (the default is square).
The replacement form sets the diagonal of the matrix x to the given value(s).


Using diag(x) can have unexpected effects if x is a vector that could be of length one. Use diag(x, nrow = length(x)) for consistent behaviour.


Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

upper.tri, lower.tri, matrix.


diag(10,3,4) # guess what?
all(diag(1:3) == {m <- matrix(0,3,3); diag(m) <- 1:3; m})

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

[Package base version 2.5.0 Index]