multiply element of a row vector (A) by corresponding row of matrix (B) and sum

Question

Isma 2015-9-3

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/239427-multiply-element-of-a-row-vector-a-by-corresponding-row-of-matrix-b-and-sum

评论： Matt J 2015-9-3

采纳的回答： Star Strider

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Hi

I am currently looking for an assistance on the way to perform such calculation:

A=[0 1 2 3];B=[1 2;3 4;5 6;7 8];
        0*1 + 0*2
     C= 1*3 + 1*4
        2*5 + 2*6
        3*7 + 3*8
   ==> C=[0;7;22;45]

I had a look at

bsxfun(@times,,)

but it can't be useful here (unfortunately).

Best,

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Answer 1

Star Strider 2015-9-3

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https://ww2.mathworks.cn/matlabcentral/answers/239427-multiply-element-of-a-row-vector-a-by-corresponding-row-of-matrix-b-and-sum#answer_191219

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This works:

C = sum(bsxfun(@times, A', B),2);

To get bsxfun to work here, you have to transpose ‘A’ so that the two arguments have a dimension in common. Then sum across the columns (dimension 2).

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Isma 2015-9-3

Thanks a lot for the hint regarding the transpose of A. Cheers.

Star Strider 2015-9-3

My pleasure.

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Answer 2

Matt J 2015-9-3

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/239427-multiply-element-of-a-row-vector-a-by-corresponding-row-of-matrix-b-and-sum#answer_191244

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C=A(:).*sum(B,2);

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Isma 2015-9-3

Thanks a lot too. It's indeed interesting to know alternative solutions. Cheers.

Matt J 2015-9-3

I had a look at bsxfun(@times,,) but it can't be useful here (unfortunately).

It's actually fortunate that you can avoid bsxfun. If B is an M x N matrix, then using bsxfun will require M*N multiplications, whereas with what I propose, you only do M multiplications.

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