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：
net=newff([-1 2; 0 5],[3,1],{'tansig','purelin'},'traingd');
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! V$ F" d& p3 e5 N$ Mnet.trainParam.show = 50;
net.trainParam.lr = 0.05;
net.trainParam.epochs = 300;
7 @( {$ F% Q' m. B0 F; n) dnet.trainParam.goal = 1e-5;
7 W" T  N+ D/ p: _( F2 Xp = [-1 -1 2 2;0 5 0 5];
, L8 }7 R9 g% i+ ^6 lt = [-1 -1 1 1];


net=train(net,p,t);
0 ?# ~2 ]1 h0 I$ A2 u$ g+ _TRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10
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5 _  Q  t2 I( V' ]TRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
6 Q. b" A9 O2 k. z) ?/ e. J8 ~TRAINGD, 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
8 X! Z' _9 @& M, vTRAINGD, 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
TRAINGD, Performance goal met.
3 |% T2 o6 G4 J4 b1 l& j7 Ma = sim(net,p)
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plot(p,t,p,a,'o')


但是为什么老是报错：
6 @9 ^- y4 d0 z, J  K+ _! G; h??? Input argument "net" is undefined.