Sort & Index matrix for highest values in row

Question

dan berkowitz 2017-10-16

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评论： dan berkowitz 2017-10-17

Hi, I have a 5 x 5 matrix, A.

How can I create a matrix that has 1's for the largest two values in each row, and all remaining elements are zero? (I created a sort index by sort(A,2,'descend)...but am lost...).

The output matrix is a 5 x 5 matrix with 1's for the largest two values in each row.

Any help would be appreciated. Thanks!

Dan

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Answer 1

Jan 2017-10-17

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x          = randi([0,20], 5, 5);
[~, index] = sort(x, 2, 'descend');                % Sorting index
result     = zeros(size(x));                       % Create output
result(sub2ind(size(x), 1:5, index(:, 1).')) = 1;  % Mark largest
result(sub2ind(size(x), 1:5, index(:, 2).')) = 1;  % Mark 2nd largest

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dan berkowitz 2017-10-17

thank you, jan! where would humanity be without you?

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Answer 2

Cedric 2017-10-17

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编辑：Cedric 2017-10-17

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Here is another way:

 >> A = rand(5, 5)
 A =
    0.9337    0.1518    0.6164    0.3074    0.4584
    0.9017    0.1290    0.8862    0.4706    0.3778
    0.6309    0.8430    0.3314    0.1133    0.3268
    0.8527    0.1393    0.8076    0.4477    0.7315
    0.4614    0.8630    0.6206    0.1753    0.5531
 >> idm = A == max(A, [], 2) ;
    A(idm) = NaN ;
    B = double( A == max(A, [], 2) | idm )
 B =
     1     0     1     0     0
     1     0     1     0     0
     1     1     0     0     0
     1     0     1     0     0
     0     1     1     0     0

If you have more than one max or more than one second max, you will get more than two 1 per row though. If you need to make a choice (e.g. pick the left-most occurrences), you will need to implement more logic/sorting. See below what I mean:

 >> A = randi(10, 5, 5)
 A =
     9     3     6     5    10
     9     8     9     1    10
     4     5     8     2     7
     1     9     9    10     9
     6     7     5     4     4
 >> idm = A == max(A, [], 2) ;
    A(idm) = NaN ;
    B = double( A == max(A, [], 2) | idm )
 B =
     1     0     0     0     1
     1     0     1     0     1
     0     0     1     0     1
     0     1     1     1     1
     1     1     0     0     0

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dan berkowitz 2017-10-17

thx. appreciate your help!

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Answer 3

Ahmed raafat 2017-10-17

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you mean you want only first high values in each row right??

y=zeros(size(A,1),size(A,2));
for i=1:size(A,1)
    x=sort(A(i,:),descend');
    y(i,1:2)=x(1:2);
end

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dan berkowitz 2017-10-17

No. I want the output to be a 5x5 matrix. And there should be '1' in each row where the largest two values are. All other entries in the matrix should be '0'.

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