Create a diagonal matrix with a for loop from a vector

Question

Nathan Clark 2023-4-1

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1939279-create-a-diagonal-matrix-with-a-for-loop-from-a-vector

编辑： Adam Danz 2023-4-1

I want to use the ML vector to create a diagonal matrix of only the values of the ML vector on the diagonal and make a new ML matrix with zeros everywhere else and the values of the ML vector along the diagonal of the new ML matrix. Essentially I am trying to write the code for diag(ML).

ML = rand([5 1])

for j = 1:length(ML)

for i = 1:j

if i < j

ML(i,j) == 0

elseif i > j

ML(i,j) == 0

else

ML(i,j) = ML(j)

end

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Answer 1

Dyuman Joshi 2023-4-1

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https://ww2.mathworks.cn/matlabcentral/answers/1939279-create-a-diagonal-matrix-with-a-for-loop-from-a-vector#answer_1206459

编辑：Dyuman Joshi 2023-4-1

在 MATLAB Online 中打开

(If you are required to use a loop)

It is better to use another variable to get an output.

ML = rand([5 1])
ML = 5×1
7725
4016
8767
9691
1817

Pre-allocate output matrix according to the size.

%Using max() in case ML is a row vector
Mout = zeros(max(size(ML)))
Mout = 5×5
     0     0     0     0     0
     0     0     0     0     0
     0     0     0     0     0
     0     0     0     0     0
     0     0     0     0     0
%You can also obtain the result without double for loop
for j = 1:numel(ML)
    %Directly define the values
    Mout(j,j)=ML(j);
end
Mout
Mout = 5×5
    0.7725         0         0         0         0
         0    0.4016         0         0         0
         0         0    0.8767         0         0
         0         0         0    0.9691         0
         0         0         0         0    0.1817

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Answer 2

Adam Danz 2023-4-1

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1939279-create-a-diagonal-matrix-with-a-for-loop-from-a-vector#answer_1206474

编辑：Adam Danz 2023-4-1

在 MATLAB Online 中打开

Here are three way to define a diagonal in a square matrix of zeros that do not require a loop.

eye()

ML = rand([5 1])
ML = 5×1
    0.2650
    0.0656
    0.8188
    0.8268
    0.6532
Mout = eye(numel(ML)).*ML
Mout = 5×5
    0.2650         0         0         0         0
         0    0.0656         0         0         0
         0         0    0.8188         0         0
         0         0         0    0.8268         0
         0         0         0         0    0.6532

Indexing with sub2ind

n = numel(ML); 
Mout = zeros(n); 
ind = sub2ind([n,n],1:n,1:n);
Mout(ind) = ML
Mout = 5×5
    0.2650         0         0         0         0
         0    0.0656         0         0         0
         0         0    0.8188         0         0
         0         0         0    0.8268         0
         0         0         0         0    0.6532

Indexing without sub2ind

This shortcut works because we're working with a square matrix.

n = numel(ML); 
Mout = zeros(n); 
Mout(1:n+1:end) = ML
Mout = 5×5
    0.2650         0         0         0         0
         0    0.0656         0         0         0
         0         0    0.8188         0         0
         0         0         0    0.8268         0
         0         0         0         0    0.6532