新手求助~（关于初级的神经网络问题）

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各位大虾，我在网上看到的教程里的一个程序段，本来想要试试看的：
5 o/ D4 @! x  a4 e8 `5 J# ^6 \* K7 Q' ynet=newff([-1 2; 0 5],[3,1],{'tansig','purelin'},'traingd');


6 o1 e7 b7 k1 A( Xnet.trainParam.show = 50;
net.trainParam.lr = 0.05;
net.trainParam.epochs = 300;
0 s' v# l& A, n5 s2 Fnet.trainParam.goal = 1e-5;
p = [-1 -1 2 2;0 5 0 5];
t = [-1 -1 1 1];


+ @( E* U8 ]# L; d! Gnet=train(net,p,t);
TRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10
8 ]$ ?% T& f9 c' ]! H- Q
# y- [  D: x8 n. ~% G& L
3 y2 t' V0 I  lTRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
2 D/ o- C! ^. s; j2 [& {TRAINGD, Epoch 100/300, MSE 0.000435947/1e-05, Gradient 0.0161202/1e-10
6 W% q+ z* O9 _7 W9 zTRAINGD, Epoch 150/300, MSE 8.68462e-05/1e-05, Gradient 0.00769588/1e-10
TRAINGD, Epoch 200/300, MSE 1.45042e-05/1e-05, Gradient 0.00325667/1e-10
TRAINGD, Epoch 211/300, MSE 9.64816e-06/1e-05, Gradient 0.00266775/1e-10
) m2 k* l8 l$ }3 E" o- K/ L# zTRAINGD, Performance goal met.
& b# y: R8 x7 D7 s/ W  \' ca = sim(net,p)

' M) m; P% J) P% @6 R
7 v- a$ Q+ b. Dplot(p,t,p,a,'o')
6 u/ E3 N/ S! p' Q5 `
: R! w7 I; R6 }3 d! U
但是为什么老是报错：
??? Input argument "net" is undefined.