MATLAB: Help with max function

max function

[Max,index]=max(array,[ ],2)….. Iam using this syntax to find the maximum value of every column but in case of two equal maximum values it returns the first one… how can I solve this?! Thanks

Best Answer

This code will return the row and column indices where the entry is equal to the column's maximum.

[rowIdx,colIdx] = find(A==max(A))

Related Solutions

MATLAB: Saddle points of a 2D matrix

Here is a simple approach. Note I define a saddle point as one that is either the largest in its column and smallest in its row or the smallest in its column and largest in its row. Maybe you only want to look for the second kind in which case you can modify the approach accordingly.

Also this code is quite inefficient. You could further optimize it by finding and saving the column maximums, column minimums, row maximums and row minimums before entering the loop.

You could also probably vectorize this further and not use a loop at all, but I think you wanted to see how the basic syntax would look.

% make a matrix to try algorithm on
% there are saddle points at 2,2 and 4,4
A = [10 12 7 3 12;
    3 10 6 2 8;
    12 24 17 6 10;
    15 21 10 8 12;
    1 18 22 4 15];
disp A
% get dimensions of the matrix
[numRows,numCols] = size(A);
% preallocate array to hold indices for saddle points, there can be at most two
indices = zeros(2,2);
% loop through rows and columns of matrix
numSaddle = 0; % counter
for iRow = 1:numRows
    for jCol = 1:numCols
        % check if it is the biggest element in its row and smallest
        % element in its column

        brsc = A(iRow,jCol) == max(A(iRow,:)) && A(iRow,jCol) == min(A(:,jCol));
        
        
        
        % check if is the smallest element in its row and biggest
        % element in its column
        srbc = A(iRow,jCol) == min(A(iRow,:)) && A(iRow,jCol)== max(A(:,jCol));
        
        if brsc || srbc
            numSaddle = numSaddle + 1;
            indices(numSaddle,:) = [iRow,jCol];
        end
        
    end
end
% delete off the unused entries in the indices array
indices = indices(1:numSaddle,:);
% display the result
disp(indices)
% display saddle points
for k = 1:numSaddle
    disp(A(indices(k,1),indices(k,2)))
end

MATLAB: How to find the element which is greater than or equal to its row and smaller or equal to its column in a matrix

Copying code from the internet is not always a good way to learn best-practice coding: Solving this problem using loops misses using MATLAB's excellent code vectorization abilities. It would be much neater and faster to use bsxfun instead, like this:

function idx = saddle(mat)
row_mx = bsxfun(@ge,mat,max(mat,[],2));
col_mn = bsxfun(@le,mat,min(mat,[],1));
[R,C] = find(row_mx & col_mn);
idx = [R,C];
end

Which gives this:

>> saddle(zeros(5,3))
ans =
   1     1
   2     1
   3     1
   4     1
   5     1
   1     2
   2     2
   3     2
   4     2
   5     2
   1     3
   2     3
   3     3
   4     3
   5     3

Best Answer

Related Solutions

MATLAB: Saddle points of a 2D matrix

MATLAB: How to find the element which is greater than or equal to its row and smaller or equal to its column in a matrix

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