Save New Variable to the Workspace for every iteration with a 'for' cycle

Question

kerozi 2016-5-23

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评论： kerozi 2016-5-24

Hello all,

I've been trying to save the Q_bar(3x3 matrix) answer seperately for every theta input but I am failing to save the Q_bar answer for each iteration. Because the only output is the final Q_bar matrix.

for theta = [0,45,60,45,45,60,45,0];
m = cos(theta*pi/180);
n = sin(theta*pi/180);
T = [m.^2 n.^2 2*m*n ; n.^2 m.^2 -2*m*n ; -m*n m*n m.^2-n.^2];
Tinv = inv(T);
Q_bar = Tinv*Q*T;
Q_bar = Q_bar(theta);
end

The final output should be Q_bar(0) = [3x3 matrix] , Q_bar(45) = [3x3 matrix], Q_bar(60) = [3x3 matrix] etc.

What can I do?

Thank You already

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kerozi 2016-5-23

在 MATLAB Online 中打开

I have thought with another for loop inside should give an answer but it does not.

theta = [0,45,60,45,45,60,45,0];
for i=1:8;
m = cos(theta(i)*pi/180);
n = sin(theta(i)*pi/180);
T = [m.^2 n.^2 2*m*n ; n.^2 m.^2 -2*m*n ; -m*n m*n m.^2-n.^2];
Tinv = inv(T);
Q_bar = Tinv*Q*T;
Q_bar = Q_bar(theta(i));
end

AND I FORGOT TO TELL THAT Q MATRIX IS ALSO 3x3 MATRIX.

Stephen23 2016-5-23

Don't try to create variables in a loop. This is slow, buggy, and not robust. Read this to know why:

http://www.mathworks.com/matlabcentral/answers/196952-how-to-create-new-variables-in-batches-with-strcat

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Answer 1

Stephen23 2016-5-23

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编辑：Stephen23 2016-5-23

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Exactly as Adam said in their comment, just use a 3D array like this:

Q = randi(9,3,3);
vec = [0,45,60,45,45,60,45,0];
out = NaN([size(Q),numel(vec)]);
for k = 1:numel(vec)
    theta = vec(k);
  m = cos(theta*pi/180);
  n = sin(theta*pi/180);
  T = [m.^2 n.^2 2*m*n ; n.^2 m.^2 -2*m*n ; -m*n m*n m.^2-n.^2];
  Tinv = inv(T);
  Q_bar = Tinv*Q*T;
  out(:,:,k) = Q_bar;
  %Q_bar = Q_bar(theta);
end

But note that you should use \ instead of inv.

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kerozi 2016-5-24

Thank You, as much as it seems an easy task, it was hard for me to comprehend the overall. But you made it happen. Thank You so much.

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Answer 2

Steven Lord 2016-5-23

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https://ww2.mathworks.cn/matlabcentral/answers/285510-save-new-variable-to-the-workspace-for-every-iteration-with-a-for-cycle#answer_223179

在 MATLAB Online 中打开

You're trying to create variables like Q_bar0, Q_bar45. etc.? DON'T DO THAT. That would make your code more difficult to read and probably slower. Read this Answer for an explanation.

Adam's suggestion is the recommended approach.

theta = 0:45:360;
M = zeros(3, 3, length(theta));
for k = 1:length(theta)
  T = theta(k);
  M(:, :, k) = [1 0 0; 0 cosd(T) -sind(T); 0 sind(T) cosd(T)];
end

The rotation matrix for the angle theta(N) [for N between 1 and length(theta)] is M(:, :, N).

And don't use inv, use the backslash operator \ instead.