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：
& R; @' x/ ?( m% T8 Q0 dnet=newff([-1 2; 0 5],[3,1],{'tansig','purelin'},'traingd');


- W4 k$ J7 G3 h+ A6 D  V0 ]% Z, P% [net.trainParam.show = 50;
net.trainParam.lr = 0.05;
" W# c2 D1 q7 h. D6 E( Lnet.trainParam.epochs = 300;
7 n: f9 R7 b- m7 f6 p+ [5 vnet.trainParam.goal = 1e-5;
p = [-1 -1 2 2;0 5 0 5];
% @  ]- A/ Y4 \0 W  ]. ~2 b0 F3 U; E3 {t = [-1 -1 1 1];

) V& @0 ?& H# }6 v8 S. M, U2 S2 [
net=train(net,p,t);
( @  o; r2 j: @1 [7 H/ O; [. _TRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10

4 m$ s- {6 @& `. O& M
TRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
! Z. k. P7 j$ [% ITRAINGD, 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 P* K2 `. J2 v9 X6 xTRAINGD, 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.
a = sim(net,p)


plot(p,t,p,a,'o')


! q  U. r/ C/ \0 X  C$ L0 z但是为什么老是报错：
5 y1 M4 ], I4 ?- t5 [# e0 L# h??? Input argument "net" is undefined.