Creating a matrix that is more time efficient

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Nicholas Ootani 2018-3-16

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https://ww2.mathworks.cn/matlabcentral/answers/388846-creating-a-matrix-that-is-more-time-efficient

评论： Nicholas Ootani 2018-3-17

采纳的回答： James Tursa

在 MATLAB Online 中打开

Hey guys,

My problem is as follows.

I want to create a matrix that has certain logic like so.

 % code
n = 50; % Just an arbitrary number
B = rand(1,n); 
A  = rand(1,n);
% The components of the vectors A and B are not necessary a number between 0 and 1. But a calculated number, but for the purpose of this problem I'd rather use this random generated vector.
for i = 1:n
     for j = 1:n
          if i == j
               C(i, j) = 5*B(1, i) + A(1,j);
          else
               C(i, j) = -B(1, i) + 3*A(1, j);
          end
     end
end

The code up above does the works, but it takes quite a long time, since it makes each component individually.

So my question is, is there a more efficient way to create this matrix?

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采纳的回答

James Tursa 2018-3-16

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https://ww2.mathworks.cn/matlabcentral/answers/388846-creating-a-matrix-that-is-more-time-efficient#answer_310416

编辑：James Tursa 2018-3-16

在 MATLAB Online 中打开

C = 3*A - B(:); % or C = bsxfun(@minus,3*A,B(:))
C(1:n+1:end) = 5*B + A;

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James Tursa 2018-3-17

在 MATLAB Online 中打开

The bsxfun( ) version:

N = 1:n;
% D = sin(A)./((cos(A)-cos(A(:))).^2).*(((1-(-1).^(N-N(:)))/(2*(n+1))));
cosA = cos(A);
sinA = sin(A);
costemp = bsxfun(@minus,cosA,cosA(:));
sincostemp = bsxfun(@rdivide,sinA,costemp.^2);
Ntemp = bsxfun(@minus,N,N(:));
D = sincostemp.*(((1-(-1).^Ntemp)/(2*(n+1))));
D(1:n+1:end) = -((n+1)./(4*sinA)+B);