Diagonal Matrices
DESCRIPTION:
Either creates a diagonal matrix or extracts the diagonal elements of a matrix. The function diag has been made generic. In particular, there are diag methods in the Matrix library that differ (both in argument list, as well as in returned value) from the S-PLUS default.
USAGE: 
diag(x, ...)
diag.default(x = <<see below>>, nrow = <<see below>>,
             ncol=<<see below>>)
diag(x) <- value
OPTIONAL ARGUMENTS:
x:
matrix or vector. Missing values (NAs) are allowed.
nrow:
number of rows of output matrix.
ncol:
number of columns of output matrix.
value:
vector whose length is the minimum of the number of rows and the number of columns. Missing values (NAs) are allowed.
VALUE:
if x is a matrix, the vector of diagonal elements of x;

if x is a vector of length greater than one, a matrix with x on its diagonal and zeroes elsewhere.

if x is a vector of length 1 and both nrow and ncol are missing, the value is an x by x identity matrix. By default, the matrix is square with zeros off the diagonal, but it can be made rectangular by specifying nrow and ncol.

the diag(x) <- value form allows replacement of the values on the diagonal of a matrix.

DETAILS:
If x is missing, then nrow must be given and ncol equals nrow if it is also missing; in this case 1s are placed on the diagonal of the result.
SEE ALSO:
col , length , library(help = Matrix), matrix , Matrix , ncol .
EXAMPLES: 
amat <- matrix(c(12,15,6,10,2,9), nrow = 2)
diag(amat)    # extract diagonal elements ((1,1),(2,2),etc.)


diag(diag(amat))   # create a square matrix with
                   # diagonal of amat


diag(5)   # 5 by 5 identity matrix
diag(nrow = 5) # same thing


diag(as.matrix(5)) # 5


diag(x, nrow = 2*length(x))  # replicate x on the diagonal of a matrix


diag(x, nrow = length(x))  # put x on the diagonal of a matrix
  # works even if length(x) is 1 or x is a matrix


diag(x) <- diag(y)  # put diagonal of y into diagonal of x