Matrix dimensions must agree when using .*

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Amy Phan 2017-5-21

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评论： Niladri Halder 2022-8-9

Hi all,

I've been trying to multiply two matrices together and I can not find where I went wrong. I'm trying to multiply the matrix and it's transpose together however I am getting the error, "Matrix dimensions must agree."

The two matrices contain the following information:

Name Size Bytes Class Attributes

D_tranpose   6x16384            98304  uint8  
D            16384x6            98304  uint8

Does anyone have any ideas? Thanks in advance

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Stephen23 2017-5-21

@Amy Phan: you need to learn about the differences between array and matrix operations:

https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

Niladri Halder 2022-8-9

nn2_p = nn2{nImage}.*mask_s;

Matrix dimensions must agree.

Please Help. Thank you.

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Answer 1

Walter Roberson 2017-5-21

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C = A.*B is element-by-element multiplication, for which unless one of A or B is a scalar, the result C(J,K) = A(J,K).*B(J,K) . For that to work, the matrices must be the same size.

Perhaps you want D_tranpose * D which would be algebraic matrix multiplication, and would give you a 6 x 6 result for those matrices.

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Amy Phan 2017-5-22

在 MATLAB Online 中打开

Thank you so much for your reply. When I try and compute D_tranpose * D I receive another error saying:

Error using  * 
MTIMES is not fully supported for integer classes. At least one input must be scalar.
To compute elementwise TIMES, use TIMES (.*) instead.

Walter Roberson 2017-5-22

在 MATLAB Online 中打开

double(D_tranpose) * double(D)

When you do algebraic matrix multiplication involving uint8 the result is quite likely to exceed the capacity of uint8, so you would not want to use * directly between uint8 even if it were permitted.

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