Find maximum among the elements in the matrix lower than 80% of max element of entire matrix

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Ivan
Ivan 2020-10-14
评论： Fangjun Jiang 2020-10-14
Hi!
I have a matrix A(n,n). I determine the maximum element like S=max(A(:)).
Now, I would like to find the maximum element only among the elements of the matrix A which are lower than 0.8*S.
Could you suggest some solutions of that problem?
Thank you in advacne!

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采纳的回答

Sudhakar Shinde
Sudhakar Shinde 2020-10-14
编辑：Sudhakar Shinde 2020-10-14
%Try this:
Result = A(A<(0.8*S))

更多回答（1 个）

Ameer Hamza
Ameer Hamza 2020-10-14
编辑：Ameer Hamza 2020-10-14
A; % your matrix
S = max(A, [], 'all'); % same as: S=max(A(:))
S2 = max(A.*(A<0.8*S), [], 'all');
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Ameer Hamza
Ameer Hamza 2020-10-14
No, both will be equivalent, even if matrix have negative values. Infact, using multiplication is much faster
A = randn(10000); % both positive and negative values
S = max(A, [], 'all');
tic
S21 = max(A.*(A<0.8*S), [], 'all');
t1 = toc;
tic
S22 = max(A(A<0.8*S), [], 'all');
t2 = toc;
tf = isequal(S21, S22);
Result
>> t1
t1 =
0.3251
>> t2
t2 =
1.2465
>> tf
tf =
logical
1
Fangjun Jiang
Fangjun Jiang 2020-10-14
An easy counterexample:
%%
A=[1 -1]; % your matrix
S = max(A, [], 'all'); % same as: S=max(A(:))
S2 = max(A.*(A<0.8*S), [], 'all');
S1=max(A(A<0.8*S), [], 'all');
isequal(S1,S2)

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