DIAG DIAG Diagonal Matrix Construction/Extraction

Usage

The diag function is used to either construct a 
diagonal matrix from a vector, or return the diagonal
elements of a matrix as a vector.  The general syntax
for its use is

  y = diag(x,n)

If x is a matrix, then y returns the n-th 
diagonal.  If n is omitted, it is assumed to be
zero.  Conversely, if x is a vector, then y
is a matrix with x set to the n-th diagonal.