# diag

## Purpose

Diagonal matrices and diagonals of a matrix.

## Synopsis

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

## Description

`X = diag(v,k)`, where `v` is a vector with `n` components, returns s a square matrix `X` of order `n+abs(k)` with the elements of `v` on the `k`-th diagonal. `k` = 0 is the main diagonal, `k` > 0 is above the main diagonal, and `k` < 0 is below the main diagonal.

`diag(v)` simply puts `v` on the main diagonal.

`v = diag(X,k)`, where `X` is a matrix, returns a column vector `v` formed from the elements of the `k`-th diagonal of `X`.

`diag(X)` is the main diagonal of `X`.

## Examples

`diag(diag(X))` is a diagonal matrix.

`sum(diag(X))` is the trace of `X`.

The statement

    diag(-m:m)+diag(ones(2*m,1),1)+diag(ones(2*m,1),-1)

produces a tridiagonal matrix of order `2*m+1`.

## See Also

    tril, triu

(c) Copyright 1994 by The MathWorks, Inc.