How can I index the edges of a matrix

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Anthony Womack 2019-1-30

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https://ww2.mathworks.cn/matlabcentral/answers/442351-how-can-i-index-the-edges-of-a-matrix

评论： Image Analyst 2019-1-30

My job is the identify only the edges of two matrixes of the same dimension and then take the sum of the product of those two matrixes. How can I avoid including the interna portion of said matrix? Example Matrix [.5, 4, 8, 1.2; 3, 0, 0, 5; 7.4, 0, 0, 3.5; 2, 1, 1.5, 4] where the zeroes are the portion to be ignored.

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Answer 1

Ollie A 2019-1-30

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编辑：Ollie A 2019-1-30

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I have interpreted your question to mean that for both matrices, you want the set all of the matrix, except the edge, to zero and then multiply the matrices and find the sum. To do this we can use indexing to select only the 'middle' of each matrix:

M1 = ones(n); % Your square matrices
M2 = ones(n);
M1(2:end-1,2:end-1) = 0; % Middle of matrices set to zero
M2(2:end-1,2:end-1) = 0;
sum(sum(M1.*M2)); 
% The first sum will sum along the first dimension of your product matrix
% giving you a 1xn vector.

I hope this is what you want to achieve!

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Anthony Womack 2019-1-30

That was perfect, thank you very much!

Image Analyst 2019-1-30

Anthony: then go ahead and accept the answer. It's the same code as what I would have suggested. If you don't want to destroy your original matrices, then make a copy of them first, and multiply the copies instead of the originals.

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