How to extract the upper diagonal part of a matrix into a 1 dimensional vector ?

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Tuong Nguyen Minh
回答： Image Analyst 2020-6-6
Dear Matlab user community
I have a K by K matrix like this for example when K = 3
A =
1 2 3
4 5 6
7 8 9
A =
1 2 3
4 5 0
7 8 9
A =
1 0 3
4 5 0
7 8 9
I would like to extract the value of [2 3 6] , [2 3 0] and [0 3 0] in this order .
My attemp was terribly inefficient
function result = flat_alpha(A)
K = size(A,1);
result = [];
for ii = 1:K
for jj = 1:K
if ii<jj
result = [result A(ii,jj)];
end
end
end
end
Could you kindly help me solve this problem ?
Thank you very much for your enthusiasm !

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采纳的回答

David Hill
David Hill 2020-6-6
If A will always be a 3x3, then
B=A([4 7 8]);
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更多回答（1 个）

Image Analyst
Image Analyst 2020-6-6
Try triu():
% Set up
K = 3;
A = reshape(1:K^2, K, K)'
% Solution:
upperTriangle = triu(A, 1)
oneDVector = upperTriangle(upperTriangle~=0)
You get [2;3;6] as requested.
I'm not sure what your rules are for getting the second and third version of your A matrix.

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