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

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 !

Answer 1

[David Hill]

If A will always be a 3x3, then

B=A([4 7 8]);

[Tuong Nguyen Minh]

No, A is a K by K matrix K = 3 is only a special case

[David Hill]

If the order does not matter:

B=[];
for k=1:length(A)-1
    B=[B,diag(A,k)'];
end

[David Hill]

Same order as you specified

B=[];
for k=1:length(A)-1
    B=[B,A(k,k+1:end)];
end

Answer 2

[Image Analyst]

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.