diag

Extract a diagonal, or build a matrix from a diagonal.

Syntax

X = diag(v)
X = diag(v, k)
v = diag(A)
v = diag(A, k)

Description

Dual-purpose function:

• When the input is a vector v, returns a square matrix with v on the main diagonal (or on the k-th diagonal — positive k above, negative k below).
• When the input is a matrix A, returns the column vector of its main-diagonal entries (or its k-th diagonal).

Examples

diag([1 2 3])               % 3×3 with 1, 2, 3 on the diagonal
diag(magic(3))              % column vector [8; 5; 2]
diag([1 2 3], 1)            % 4×4 with 1, 2, 3 on the +1 superdiagonal

See also

• blkdiag — Block-diagonal matrix from input matrices.
• tril — Lower-triangular part of a matrix.
• triu — Upper-triangular part of a matrix.
• trace — Sum of diagonal elements.