Create a diagonal matrix with a for loop from a vector

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Nathan Clark
Nathan Clark 2023-4-1
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
end
end

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采纳的回答

Dyuman Joshi
Dyuman Joshi 2023-4-1
编辑：Dyuman Joshi 2023-4-1
Ran in:
(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
0.7725 0.4016 0.8767 0.9691 0.1817
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

更多回答（1 个）

Adam Danz
Adam Danz 2023-4-1
编辑：Adam Danz 2023-4-1
Ran in:
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

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