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：
1 g. S8 }/ ?0 f' }0 f; X8 Qnet=newff([-1 2; 0 5],[3,1],{'tansig','purelin'},'traingd');
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net.trainParam.show = 50;
net.trainParam.lr = 0.05;
net.trainParam.epochs = 300;
+ v/ R7 x1 i2 Q' q7 r" B% gnet.trainParam.goal = 1e-5;
p = [-1 -1 2 2;0 5 0 5];
t = [-1 -1 1 1];


net=train(net,p,t);
% x4 x$ u' L9 _& aTRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10

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TRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
3 D+ n7 t8 g8 M+ pTRAINGD, 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
3 q( n% i9 c: g4 D7 h  Q% DTRAINGD, Epoch 200/300, MSE 1.45042e-05/1e-05, Gradient 0.00325667/1e-10
8 s9 F, o) H: G" H' VTRAINGD, Epoch 211/300, MSE 9.64816e-06/1e-05, Gradient 0.00266775/1e-10
TRAINGD, Performance goal met.
a = sim(net,p)


plot(p,t,p,a,'o')
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但是为什么老是报错：
4 i* Z0 j- w$ q! w6 {& \2 i3 u7 ~??? Input argument "net" is undefined.