modify only the diagonal entries in a matrix?

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shawin 2018-7-9

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评论： Image Analyst 2021-6-21

采纳的回答： Jan

How can I Multiply the diagonal of a matrix by a number, for example; by 2?

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Answer 1

Jan 2018-7-9

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编辑：Jan 2018-7-9

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A     = rand(4,3);
s     = size(A);
index = 1:s(1)+1:s(1)*s(2);  % Indices of the main diagonal
A(index) = A(index) * 2

This touches the elements on the diagonal only.

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Alvin Chen 2021-6-21

@Image Analyst For loop would work for sure, but would it be more performable without using loop? Just extract the logicals by the positions of the indices in A.

Image Analyst 2021-6-21

在 MATLAB Online 中打开

I don't know what you mean by "performable". Sure there are other ways to perform this operation that may work, however I did not understand your explanation of extracting the logicals so I invite you to show your non-for loop way that doubles the values in the 4 diagonals in a non-square matrix, for example a 5 by 10 matrix.

m = ones(5, 10)

Please show us.

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Answer 2

Image Analyst 2019-2-26

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在 MATLAB Online 中打开

Here's a way you can do it using eye() to create indexes of the diagonal. I show two ways,

one to create another output matrix and leave the input matrix alone, and
another to change the input matrix in place.

% Create a logical mask for the diagonal elements.
d = logical(eye(size(m)))
% First way: Create an output matrix m2, retaining the original m.
m2 = m % Initialize with a copy of m
% Multiply diagonal elements of m2 (in place) by 2.
m2(d) = m(d) * 2  % Input values are replaced.
% Second way: Change/replace the values in original matrix.
% Multiply diagonal elements (in place) by 2.
m(d) = m(d) * 2  % Input values are replaced.