Error in multipying a 3 by 3 matrix with a 3 by 1

Question

Joshua Naah 2021-8-11

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/896497-error-in-multipying-a-3-by-3-matrix-with-a-3-by-1

回答： Steven Lord 2024-2-16

Xa = app.previewTABLE.Data(:,1);

Ya = app.previewTABLE.Data(:,2);

Za = app.previewTABLE.Data(:,3);

B = [DifX;DifY;DifZ];

C = [Xa;Ya;Za]; %% 3x1 matrix

Ri = [Sci Rz -Ry;-Rz Sci Rx;Ry -Rx Sci]; %%3x3 matrix

R = C * Ri

%%% This is the error am getting for trying to multiply a 3x3 matrix by a 3x1 (C * Ri).

Error using *

Incorrect dimensions for matrix multiplication. Check that the number of columns in the first matrix matches the number of rows in the second matrix. To perform elementwise multiplication, use '.*'.

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Answer 1

Alan Stevens 2021-8-11

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https://ww2.mathworks.cn/matlabcentral/answers/896497-error-in-multipying-a-3-by-3-matrix-with-a-3-by-1#answer_765037

Check that the number of columns in the first matrix matches the number of rows in the second matrix.

Ri has 3 columns, C has 3 rows, so Ri*C will work, but with C having 1 column and R having 3 rows, C*Ri will not!

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Joshua Naah 2021-8-11

Yh I just tried that and it didn't work. I should mention that am working in app designer, so when I first got the error I tried the same code in the command window and it worked just fine so am a bit surprised getting an error with the same code in app designer.

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Answer 2

Steven Lord 2024-2-16

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/896497-error-in-multipying-a-3-by-3-matrix-with-a-3-by-1#answer_1410438

在 MATLAB Online 中打开

Xa = app.previewTABLE.Data(:,1);
Ya = app.previewTABLE.Data(:,2);
Za = app.previewTABLE.Data(:,3);
C = [Xa;Ya;Za]; %% 3x1 matrix

Is this a 3-by-1 matrix? What size is app.previewTABLE.Data? If it is an m-by-n array, each of Xa, Ya, and Za will have m rows and C will be (3*m)-by-1 (unless app.previewTABLE.Data is a table array whose variables themselves have multiple columns.) That's only 3-by-1 if m is 1 (meaning app.previewTABLE.Data is a row vector.) From the name I suspect that it's not a row vector and so C isn't 3-by-1.

Ri = [Sci Rz -Ry;-Rz Sci Rx;Ry -Rx Sci]; %%3x3 matrix

You didn't show the definition of any of the variables you use here, so it's impossible to say that Ri is a 3-by-3 matrix. Let's say, for example, that each of Sci, Rz, Ry, and Rz were 2-by-2 matrices. This concatenation would be defined and would result in a 6-by-6 matrix not a 3-by-3.