modify only the diagonal entries in a matrix?

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shawin
shawin 2018-7-9
评论： Image Analyst 2021-6-21
How can I Multiply the diagonal of a matrix by a number, for example; by 2?

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Jan
Jan 2018-7-9
编辑：Jan 2018-7-9
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
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
Image Analyst 2021-6-21
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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Image Analyst
Image Analyst 2019-2-26
Here's a way you can do it using eye() to create indexes of the diagonal. I show two ways,
  1. one to create another output matrix and leave the input matrix alone, and
  2. 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.
The two ways are the same except if you want a second matrix for the output you have to copy the input matrix first.
  2 个评论
Athikah Raihah
Athikah Raihah 2019-2-26
Thanks a lot! That is very clear, exactly what I need!
Jan
Jan 2019-2-26
At least in current Matlab versions:
d = eye(size(m), 'logical'); % faster than: logical(...)

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