How can I get the upper triangular matrix without using triu()?

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Jessica Avellaneda 2022-2-24

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评论： Voss 2022-2-27

I was able to get the lower triangular matrix by using A(:, 1:end-1), A being my function + argumented matrix but I need to get the upper triangular matrix.

The matrix is Aug= a matrix 3x3, b= 1x3. (A = [Aug b])

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Jessica Avellaneda 2022-2-27

Yes, you are correct. However, I cannot use tril or triu for this assignment. is there any other way I can get lower and upper triangular matrices ?

Voss 2022-2-27

@Jessica Avellaneda Please see my answer, which does not use triu() except to show that the result is correct, which is a part you can omit.

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Answer 1

Voss 2022-2-25

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https://ww2.mathworks.cn/matlabcentral/answers/1658045-how-can-i-get-the-upper-triangular-matrix-without-using-triu#answer_904105

编辑：Voss 2022-2-27

在 MATLAB Online 中打开

You can set all elements below the main diagonal to zero, e.g., using a for loop:

% A matrix, pick any size you want:
A = randn(5,6);
disp(A);
   -0.2986   -0.0896    3.0956   -0.0877   -0.1211   -0.8411
   -1.2512   -0.7189   -0.2814    1.0478   -0.5774    1.0418
    0.3396    0.8059    1.3927    0.8229   -0.7386   -0.3184
    0.4703    0.3028    0.7384   -0.1581    0.4213    0.2998
   -0.4320    1.3279   -2.2420   -1.0304    0.4372    0.4579
% zero out the elements below the diagonal to make it upper-triangular:
A_ut = A;
for ii = 1:size(A,2)
    A_ut(ii+1:end,ii) = 0;
end
disp(A_ut);
   -0.2986   -0.0896    3.0956   -0.0877   -0.1211   -0.8411
         0   -0.7189   -0.2814    1.0478   -0.5774    1.0418
         0         0    1.3927    0.8229   -0.7386   -0.3184
         0         0         0   -0.1581    0.4213    0.2998
         0         0         0         0    0.4372    0.4579
% make sure this gives the same result as triu(A):
isequal(A_ut,triu(A))
ans = logical
   1