Struggling with array dot operator

Question

Deepayan Bhadra 2017-12-17

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/373412-struggling-with-array-dot-operator

评论： Deepayan Bhadra 2017-12-17

I've initialized

t = 0:0.1:20;
L = zeros(4,4,length(t));

I'm trying to assign values to L as follows:

t=0:0.1:20;
for i =1:length(t),
L(:,:,i) = D*diag([(t.^2+1).^2*(sin(t).^2), (t.^2+1).^2*(sin(2*t).^2), (t.^2+1).^2*(sin(3*t).^2), (t.^2+1).^2*(sin(4*t).^2), (t.^2+1).^2*(sin(5*t).^2)])*D';
end

D is a constant matrix. Essentially, there is a different L for each time instant. When I execute this, it gives

Error using  * 
Inner matrix dimensions must agree.

I'm tried so many combinations of the dot operators since this is a long operation.

Can someone help identify the problem?

Thanks a lot.

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Answer 1

Matt J 2017-12-17

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https://ww2.mathworks.cn/matlabcentral/answers/373412-struggling-with-array-dot-operator#answer_296664

编辑：Matt J 2017-12-17

在 MATLAB Online 中打开

Define the t-data before the for loop

    T=0:0.1:20;
    for i =1:length(T),
     t=T(i);
  L(:,:,i) = D*diag([(t.^2+1).*(t.^2+1).*(sin(t).^2) (t.^2+1).*(t.^2+1).*
(sin(2*t).^2) (t.^2+1).*(t.^2+1).*(sin(3*t).^2) (t.^2+1).*(t.^2+1).*(sin(4*t).^2) 
(t.^2+1).*(t.^2+1).*(sin(5*t).^2)])*D';
end

Also, it is a good ideas to use commas to explictly separate entries of a vector/matrix.

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Matt J 2017-12-17

编辑：Matt J 2017-12-17

在 MATLAB Online 中打开

This works for me, if D is 5x5,

    T=0:0.1:20;
    for i =1:length(T),
      t=T(i);
L(:,:,i) = D*diag([(t.^2+1).*(t.^2+1).*(sin(t).^2),...
                   (t.^2+1).*(t.^2+1).*(sin(2*t).^2),...
                   (t.^2+1).*(t.^2+1).*(sin(3*t).^2),...
                   (t.^2+1).*(t.^2+1).*(sin(4*t).^2),... 
                   (t.^2+1).*(t.^2+1).*(sin(5*t).^2)])*D';
   end