MATLAB: Max and min values in an array

matrixmaximummaximum in a matrix

thank you! Found the solution! Thank you!

Best Answer

See if this does what you want , first we split into 16 separate rows each and then we conquer in the third dimension:

[m,n]=size(A); % where A is your matrix of size 256 X 40K
parts = 16;
AA = permute(reshape(A.',n,m/parts,[]),[2,1,3]);
Max = max(AA,[],[1,2]); % max(max(AA)) for versions <= 2016b
[r,~]=find(AA == Max) % r represents rows

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MATLAB: To find Maximum value and minimum value for each group of four rows for a 1576*1024 matrix

Hello,

Please try below soluition.

naMax = []; 
naMin = [];  
for i = 1:4:size(H1,1) % H1 is your original matrix
    naMax = [naMax, max(max(H1(i:i+3, :)))]; % append values

    naMin = [naMin, min(min(H1(i:i+3, :)))];% append values
end 
% You can transpose if necessary 
naMin'; 
naMax';

MATLAB: Max / Min of sparse matrices

EDIT There was a mistake in the markers line and the s= line, fixed now.

OK, here's an accelerated version. What made my previous answer slower was the S(I == i) line, this is pretty wasteful. Vectorisation sometimes hurts you rather than helping, this is one of those cases.

So, here goes. Again, I'll just focus on the rows and assume that it is possible for a row to be all zeros.

First, define a test matrix

A = sprand(50000, 60000, 1/20000);

Preallocate the rowMin and rowMax vectors

[m,n] = size(A);
rowMin = nan(m, 1);
rowMax = nan(m, 1);

Find all the NZ entries ;) and sort them by row index

[I,J,S] = find(A);
[I,K] = sort(I);
S = S(K);

Now we need to find where the index changes, diff is great for this. Just got to be careful with how you pad the left hand end. I also add in an extra entry to mark the final entry (saves unnecessary calls to end).

markers = [find([1; diff(I)]); numel(I)+1];

Pull out the identified rows, (end-1) makes sure that we don't get the final row twice.

iRows = I(markers(1:end-1));

The loop ought to be faster now:

for i = 1:numel(iRows)
  s = S(markers(i):(markers(i+1)-1));
  rowMin(iRows(i)) = min(s);
  rowMax(iRows(i)) = max(s);
end

This completes in 0.2 seconds on my laptop ...

Best Answer

Related Solutions

MATLAB: To find Maximum value and minimum value for each group of four rows for a 1576*1024 matrix

MATLAB: Max / Min of sparse matrices

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