# how to fix Index exceeds matrix dimensions?

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sing lai on 23 Apr 2014
Commented: Niklas Nylén on 24 Apr 2014
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..

Niklas Nylén on 23 Apr 2014
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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Niklas Nylén on 24 Apr 2014