How to extract diagonal elements of multidimensional array ?
12 views (last 30 days)
Show older comments
shuang Yang
on 23 Mar 2023
Commented: Mattia Marsetti
on 24 Mar 2023
If I have a m-order n-dimensional tensor. How should I extract the diagonal elements ?
For example
% Generate a 3-order 4-dimensional tensor
rng('default')
A = rand(4,4,4)
The diagonal elements are A(1,1,1) = 0.8147, A(2,2,2) = 0.0357, A(3,3,3) = 0.7655 and A(4,4,4) = 0.6991.
I was hoping to have a tensor_diag function that takes a tensor A as an input parameter and returns a vector consisting of its diagonal elements.
3 Comments
Stephen23
on 23 Mar 2023
Edited: Stephen23
on 23 Mar 2023
@Mattia Marsetti: your code throws an error on the example array:
rng('default')
A = rand(4,4,4)
get_tensor(A)
function out=get_tensor(v)
size_v=size(v);
if sum(size_v == size_v(1))<numel(size_v)
error('the input vector is not a sqare matrix');
end
N = size_v(1);
out=zeros(N,1);
for i=1:N
str='v(';
for s=1:N
str=[str 'i,'];
end
str(end:end+1)=');';
out(i)=eval(str);
end
end
Note that you could easily replace the evil EVAL with a cell array and a comma-separated list.
Accepted Answer
the cyclist
on 23 Mar 2023
rng('default')
N = 4;
A = rand(N,N,N);
A(1:N^2+N+1:end)
5 Comments
the cyclist
on 23 Mar 2023
Ah, I read your question too quickly, and didn't make my solution general enough. Glad you found it.
More Answers (1)
Bruno Luong
on 23 Mar 2023
N = 7;
A = rand(N,N,N,N,N);
p=ndims(A);
N=length(A);
% Method 1: generalization of cyclist's answer
step = polyval(ones(1,p),N);
idx = 1:step:N^p;
A(idx)
% Method 2
c = repmat({1:N}, [1,p]);
idx = sub2ind(size(A), c{:});
A(idx)
0 Comments
See Also
Categories
Find more on Operating on Diagonal Matrices in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!