how to fix Index exceeds matrix dimensions?

Question

sing lai 2014-4-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/126835-how-to-fix-index-exceeds-matrix-dimensions

评论： Niklas Nylén 2014-4-24

 L = 0.8;c=1;T=0.5;
a = 0 :120;
 it0 = inline('0.2.*sin(x)','x');
M=80;N=50;
dx=L/M;dt=T/N;
for i = 1:M + 1, u(i,1) = it0(x(i));
end
r = c*(dt/dx); r1 = (r^2)/2; r2 = 2*(1 - (r^2));
u(2:M,2) = r1*u(1:M - 1,1) + (1 - (r^2))*u(2:M,1) + r1*u(3:M + 1,1);
for k = 3:N + 1
u(2:M,k) = (r^2)*u(1:M - 1,k - 1) + r2*u(2:M,k-1) + (r^2)*u(3:M + 1,k - 1)...
- u(2:M,k - 2); 
end
for n=1:N;
figure
plot(a,u(:,n));
end

I want plot waveform graph that u(x,t) versus x, but come out error [??? Index exceeds matrix dimensions.

Error in ==> osc at 8 end ]

Can anyone help me fix the error?thanks..

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Answer 1

Niklas Nylén 2014-4-23

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First, you should predefine the size of u instead of changing the size throughout your script.

Second, in the first for loop you have written

u(i,1) = it0(x(i));

But x is not defined, did you mean to use the variable a as input? Why do you even need a loop, the following should be fine (assuming u is predefined)

u(:,1) = it0(a);

When I run your code I do not get the "Index exceeds matrix dimensions" error but instead I get an error because the number of rows in u is not equal to the number of values in a. Because you did not predefine the size of u the size will be 81*81 and the vector a will have length 120.

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sing lai 2014-4-23

Thanks for helping, but since I'm new user to Matlab, i was not able to understand your meaning..actually I want to generate the graph as below,can you help me fix the error?thanks..