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

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!

Answer 1

[Sudhakar Shinde]

%Try this:
Result = A(A<(0.8*S))

[Ivan]

Thank you!!

[Sudhakar Shinde]

Welcome. Glad to help

Answer 2

[Ameer Hamza]

A; % your matrix
S = max(A, [], 'all'); % same as: S=max(A(:))
S2 = max(A.*(A<0.8*S), [], 'all');

[Fangjun Jiang]

I think it won't be correct if A has mixed value of negative and positive. Force the value to be zero is problematic. Better to use index

[Ameer Hamza]

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]

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)