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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8 P; v6 v. N0 r  w& R( k& |net.trainParam.show = 50;
net.trainParam.lr = 0.05;
( L# K7 {6 H3 i& S3 V4 A! f: vnet.trainParam.epochs = 300;
net.trainParam.goal = 1e-5;
$ U1 C6 A- V( Y6 B6 Z2 X  W* Cp = [-1 -1 2 2;0 5 0 5];
t = [-1 -1 1 1];
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net=train(net,p,t);
TRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10
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7 C6 f& v- c1 ^: VTRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
% J1 ]/ l2 K" i  o. j# eTRAINGD, 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
TRAINGD, Epoch 200/300, MSE 1.45042e-05/1e-05, Gradient 0.00325667/1e-10
. z* ]8 Y' c7 o# U  e: K, z# W4 y% j1 ?TRAINGD, Epoch 211/300, MSE 9.64816e-06/1e-05, Gradient 0.00266775/1e-10
: z% I, V! X8 h+ p/ y+ L5 M9 ETRAINGD, Performance goal met.
2 A# x/ n! D: {$ X4 U# Sa = sim(net,p)
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3 a5 J0 o, ~/ |6 e# Iplot(p,t,p,a,'o')
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但是为什么老是报错：
??? Input argument "net" is undefined.