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');
# F- w9 c) t3 M- a/ [+ \$ Z
" ^' s% C- `; w3 O, T; M' y2 T
5 `% D" Y8 i  `# znet.trainParam.show = 50;
net.trainParam.lr = 0.05;
net.trainParam.epochs = 300;
4 O6 [$ `, \$ H; a( s2 r9 t5 o( [" rnet.trainParam.goal = 1e-5;
( R/ @3 X( G# D2 ip = [-1 -1 2 2;0 5 0 5];
t = [-1 -1 1 1];

5 F3 V& G" |7 I$ {9 J& ^8 e( m5 x
# W0 R3 h3 e- _, z7 }net=train(net,p,t);
  t# B2 e2 K+ @! Z( H7 [TRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10


TRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
TRAINGD, Epoch 100/300, MSE 0.000435947/1e-05, Gradient 0.0161202/1e-10
- \2 G1 d: V2 u) w8 g; Z8 T; PTRAINGD, Epoch 150/300, MSE 8.68462e-05/1e-05, Gradient 0.00769588/1e-10
1 a) y' h  X3 QTRAINGD, Epoch 200/300, MSE 1.45042e-05/1e-05, Gradient 0.00325667/1e-10
3 \- u3 R: y, l: l9 dTRAINGD, Epoch 211/300, MSE 9.64816e-06/1e-05, Gradient 0.00266775/1e-10
! C2 M- Y4 t: M: x! fTRAINGD, Performance goal met.
a = sim(net,p)
/ G9 x8 K9 x: k. T  y4 q/ ]

* l8 f, f+ ~& w: K) gplot(p,t,p,a,'o')


但是为什么老是报错：
??? Input argument "net" is undefined.