Diag:Diagonal matrices and diagonals of a matrix
Syntax
X = diag(v,k)
X = diag(v)
v = diag(X,k)
v = diag(X)
Description
X = diag(v,k) when v is a vector of n components, returns a square matrix X of order n+abs(k), with the elements of v on the kth diagonal. k = 0 represents the main diagonal, k > 0 above the main diagonal, and k < 0 below the main diagonal.
X = diag(v) puts v on the main diagonal, same as above with k = 0.
v = diag(X,k) for matrix X, returns a column vector v formed from the elements of the kth diagonal of X.
v = diag(X) returns the main diagonal of X, same as above with k = 0.
diag([1 2 3],-2) % 此时k=-2，注意黑色的部分
ans =
0 0 0 0 0
0 0 0 0 0
1 0 0 0 0
0 2 0 0 0
0 0 3 0 0
diag([1 2 3],2) % 此时k=2，注意黑色的部分
ans =
0 0 1 0 0
0 0 0 2 0
0 0 0 0 3
0 0 0 0 0
0 0 0 0 0
diag([1 2 3]) % 此时k=0，直接返回对角矩阵
ans =
1 0 0
0 2 0
0 0 3
diag([1 2 3;4 5 6; 7 8 9 ]) %若输入矩阵，返回对角
ans =
1
5
9
Diag:Diagonal matrices and diagonals of a matrix
原文：http://www.cnblogs.com/qqhfeng/p/5110532.html