Function takes as input two nonempty matrices A and B and returns the product AB

Question

Sophie Culhane 2020-10-20

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https://ww2.mathworks.cn/matlabcentral/answers/619473-function-takes-as-input-two-nonempty-matrices-a-and-b-and-returns-the-product-ab

评论： Asad (Mehrzad) Khoddam 2020-10-20

My task is to write a MATLAB function that takes as input two nonempty matrices A and B and returns the product AB where the number of columns in A is the same as the number of rows in B. I have a small program written so far, however it is not fully developed. I am asking for assistance on where to go next with developing a program that is accurate. I apologize that there is not much in my program so far. Here is what I have so far:

function AB = ex2(A,B)

%

[NRows,NCols] = size(A);

[NRows,NCols] = size(B);

AB = zeros('NRows of A, NCols') % I am not sure what to put here

for row = 1:NRows

for col = 1:NCols

AB = A(row,col) * B(row,col);

end

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Answer 1

Asad (Mehrzad) Khoddam 2020-10-20

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function AB = ex2(A,B)
%
%
% we need rows and columns of both A and B
% ColsA should be equal to RowsB
[RowsA,ColsA] = size(A);
[RowsB,ColsB] = size(B);
AB = zeros(RowsA, ColsB);
for row = 1:RowsA
    for col = 1:ColsB
        AB(row,col) = sum(A(row,:)'.*B(:,col));
    end
end

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Sophie Culhane 2020-10-20

I am not able to use the sum command, is there another alternative that would work the same?

Asad (Mehrzad) Khoddam 2020-10-20

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Yes

function AB = ex2(A,B)
%
%
% we need rows and columns of both A and B
% ColsA should be equal to RowsB
[RowsA,ColsA] = size(A);
[RowsB,ColsB] = size(B);
AB = zeros(RowsA, ColsB);
for row = 1:RowsA
    for col = 1:ColsB
        s = 0;
        for k=1:colsA:
            s = s + A(row,k)*B(k,col);
        end
        AB(row,col) = s;
    end
end

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Answer 2

James Tursa 2020-10-20

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https://ww2.mathworks.cn/matlabcentral/answers/619473-function-takes-as-input-two-nonempty-matrices-a-and-b-and-returns-the-product-ab#answer_518618

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This is a matrix multiply, so you need to keep the rows and columns of the inputs separately. E.g.,

[NRowsA,NColsA] = size(A);
[NRowsB,NColsB] = size(B);

Then, the product will have dimensions NRowsA x NColsB. I.e.,

AB = zeros(NRowsA,NColsB);

For your result AB, you will be looping over the size of AB, so

for row = 1:NRowsA 
    for col = 1:NColsB
        % code goes here to figure out AB(row,col)
    end
end

You would need to write code to calculate the AB(row,col) element. You could do a dot product calculation here, or you could write another loop to interatively calculate it.