When data is a time series, which has no input and one output, then
orders = [na nc]
z中的數據是來自iddata的返回值
所以應該是 z=iddata(y,x）%x是輸入y是輸出
如果z=iddata([],x)%只有出入沒有輸出
如果z=iddata(y,x)%即有輸入又有輸出
例如：我隨意定義
y=sin(0:0.1:40);
data=iddata(y');
m= armax(data,[2 3])
Discrete-time IDPOLY model: A(q)y(t) = C(q)e(t)
A(q) = 1 - 1.99 q^-1 + q^-2
C(q) = 1 - 0.2973 q^-1 + 0.08516 q^-2 + 0.179 q^-3
Estimated using ARMAX on data set data
Loss function 2.95475e-030 and FPE 3.0288e-030
Sampling interval: 1