diag

diagonal including or extracting

Calling Sequence

[y]=diag(vm, [k])

Arguments

:vm vector or matrix (full or sparse storage) : :k integer (default value 0) : :y vector or matrix :

Description

for vm a (row or column) n-vector diag(vm) returns a diagonal matrix with entries of vm along the main diagonal.

diag(vm,k) is a (n+abs(k))x(n+abs(k)) matrix with the entries of vm along the kth diagonal. k=0 is the main diagonal k>0 is for upper diagonals and k<0 for lower diagonals.

For a matrix vm, diag(vm,k) is the column vector made of entries of the kth diagonal of vm. diag(vm) is the main diagonal of vm. diag(diag(x)) is a diagonal matrix.

If vm is a sparse matrix diag(vm,k) returns a sparse matrix.

To construct a diagonal linear system, use sysdiag.

Note that eye(A).*A returns a diagonal matrix made with the diagonal entries of A. This is valid for any matrix (constant, polynomial, rational, state-space linear system,...).

Examples

diag([1,2])

A=[1,2;3,4];
diag(A)  // main diagonal
diag(A,1)

diag(`sparse`_(1:10))  // sparse diagonal matrix

// form a tridiagonal matrix of size 2*m+1
m=5;diag(-m:m) +  diag(`ones`_(2*m,1),1) +diag(`ones`_(2*m,1),-1)

See Also

• sysdiag block diagonal system connection
• sparse sparse matrix definition