Where have I made mistakes in this code

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Lukgaf 2018-3-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/389930-where-have-i-made-mistakes-in-this-code

评论： Walter Roberson 2018-3-22

%prob2.m
% u’’- 4u’ + 4u = exp(x)+ C;
% Subject to u(1) = 0; u(-1) = 0; C=(-4*exp)/(1+(exp)^2);
N = 40;
[D, x]=cheb(N); %%we call this function to get the Derivative matrix D
I = eye(N+1); % I is the identity matrix of size (N+1)
C=(-4*exp(x))/(1+(exp(x).^2)); %the value of the constant
A = D^2 - 4*D + 4*I; f = exp(x)+ C;
B = A(2:N,2:N); %to impose the boundary conditions
ff = f(2:N);
v = B\ff; %solve the BVP.
u = [0;v;0]; xx = -1:0.1:1;
u_exact = exp(xx)-(sinh(1)/sinh(2))*exp(2*xx)+ C/4;
plot(x,u); %plot the numerical solution
xlabel('x')
ylabel('u')
hold on
plot(xx,u_exact,'o'); % plot the exact solution
hold off
I have used the similar code to solve many questions, why is not working for this? Thanks. It shows this eror: Error 
in prob2 (line 11)
v = B\ff; %solve the BVP.

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Lukgaf 2018-3-22

在 MATLAB Online 中打开

>> prob2
Error using  \ 
Matrix dimensions must agree.
Error in prob2 (line 11)
v = B\ff; %solve the BVP.

Jan 2018-3-22

编辑：Jan 2018-3-22

@Lukgaf: This is the line, which causes the error. But what is the message, which explains what the problem is?

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采纳的回答

Walter Roberson 2018-3-22

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/389930-where-have-i-made-mistakes-in-this-code#answer_311311

在 MATLAB Online 中打开

v = B\ff.'; %solve the BVP.

You would get that error if xx is a vector or array and C is a vector or array of different size than xx (though starting in R2016b some size combinations can be meaningfully computed as if you had used bsxfun)

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