Dot product not working for 2d matrix

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Kartik Kartik
Kartik Kartik 2020-9-21
评论： VBBV 2020-9-22
I want dot product of two matrix of shape (1,2)(2,3) but when I try it using dot(A,B) it thows an error to make the dimension same.
I am new to matlb, started today. any help is appreciated.
Thanks,
Kartik

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VBBV
VBBV 2020-9-21
编辑：VBBV 2020-9-21
Matrices must be of same size to perform dot product. size_of A must be same as _size of B. You have A = 2x1 and B as 3x2. If A = 2x1 then B should also have 2x1 I.e. 2 rows and 1 column
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Walter Roberson
Walter Roberson 2020-9-22
Algebraic Matrix Multiplication ("inner product") of a m x n matrix A and an n x p matrix B is defined as
C(i,j) = dot(A(i,:), B(:,j).')
which is
C(i,j) = sum(A(i,:).*B(:,j).')
However, dot product of two matrices A, B is defined in MATLAB as
C(1,i) = dot(A(:,i), B(:,i))
which requires that the two arrays be the same size.
If you want to be able to do dot product of two matrices of different sizes, you are going to need to define how you want the output to be calculated.
For example if your 1 x 2 is A and your 2 x 3 is B, then do you want
dot( repmat(A.', 1, size(B,2)), B) %-> 1 x 3 result
dot( repmat(A, size(B,2), 1), B.') %-> 1 x 2 result
VBBV
VBBV 2020-9-22
@Kartik. You mentioned about the general rule for matrix multiplication. The other option is to make A and B of same size as shown in Walter comment using repmat.

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