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：
  \, Z* ^8 Y" |0 Xnet=newff([-1 2; 0 5],[3,1],{'tansig','purelin'},'traingd');

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net.trainParam.show = 50;
$ K# A, o  L$ T9 Vnet.trainParam.lr = 0.05;
net.trainParam.epochs = 300;
8 J3 Y- g0 F! t9 enet.trainParam.goal = 1e-5;
p = [-1 -1 2 2;0 5 0 5];
t = [-1 -1 1 1];
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net=train(net,p,t);
TRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10

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2 n% D& H9 y+ o4 n0 v  i- V* g2 iTRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
- f0 I* I) G) E1 z* h! o2 mTRAINGD, Epoch 100/300, MSE 0.000435947/1e-05, Gradient 0.0161202/1e-10
TRAINGD, Epoch 150/300, MSE 8.68462e-05/1e-05, Gradient 0.00769588/1e-10
5 L6 x: e; I  g. f: }! ATRAINGD, Epoch 200/300, MSE 1.45042e-05/1e-05, Gradient 0.00325667/1e-10
* E) r9 r% f4 T5 C+ |TRAINGD, Epoch 211/300, MSE 9.64816e-06/1e-05, Gradient 0.00266775/1e-10
( h6 t1 c9 e5 p& e6 J* z' j, z  HTRAINGD, Performance goal met.
4 U, h: v) e# ?2 La = sim(net,p)

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- j" ?2 m, F5 D# p1 dplot(p,t,p,a,'o')
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6 z! P* g/ ?/ L: K8 s, E6 g但是为什么老是报错：
8 c( Y" K& ~5 p6 I??? Input argument "net" is undefined.