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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net.trainParam.show = 50;
net.trainParam.lr = 0.05;
& J9 {1 \# {' I9 u7 wnet.trainParam.epochs = 300;
net.trainParam.goal = 1e-5;
p = [-1 -1 2 2;0 5 0 5];
% D9 a2 V9 [; v2 E/ X- U* kt = [-1 -1 1 1];
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% [% i0 X+ R# W: Tnet=train(net,p,t);
TRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10

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7 S+ D2 s1 }" U7 w" z( ]TRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
1 \+ I4 d  h" {7 P. [5 `$ ~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
3 p- x3 g5 _8 b) b8 Z  B5 mTRAINGD, 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
; J2 E" X4 z& d% ]5 A3 X' yTRAINGD, Performance goal met.
a = sim(net,p)
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$ i* ]' h# p. F$ w1 e: b0 k% H: aplot(p,t,p,a,'o')
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# J4 S" N4 |( R" r& t但是为什么老是报错：
??? Input argument "net" is undefined.