X = diag(v,k)

以向量v的元素作为矩阵X的第k条对角线元素,当k=0时,v为X的主对角线；当k>0时,v为上方第k条对角线

几个例子：

当k> v=[1 2 3];

>> x=diag(v,-1)

x =

0 0 0 0

1 0 0 0

0 2 0 0

0 0 3 0

v=[1];

x=diag(v,0)

x=

1 0 0 0

0 1 0 0

0 0 1 0

0 0 0 1

>> clear

>> v(1)=1;

>> n=5;

>> v(2:n)=2;

>> x=diag(v,-1)

x =

0 0 0 0 0 0

1 0 0 0 0 0

0 2 0 0 0 0

0 0 2 0 0 0

0 0 0 2 0 0

0 0 0 0 2 0