Extract top 10 values from each row

Hello,

I have a matrix and wish to keep only the top 10 values in each row and replace all the other (bottom 90) values with zeros. Is there an efficient way to achieve this?

Answer 1

[Walter Roberson]

[sortvals, sortidx] = sort(A,'descend');
B = zeros(size(A),class(A));
for K = 1 : size(A,2)
  B(sortidx(1:10,K),K) = sortvals(1:10,K);
end

Yes, it could probably be done without a loop, using sub2ind(), but that would not necessarily be any faster, and would probably be less clear.

[Walter Roberson]

Adjusting for rows:

[sortvals, sortidx] = sort(A,2,'descend');

B = zeros(size(A),class(A));

for K = 1 : size(A,1)

B(K,sortidx(K,1:10)) = sortvals(K,1:10);

end

Answer 2

[Laura Proctor]

This code will keep the top ten rows and replaces everything from the 11th row on with a zero.

A = rand(100);
A(11:end,:) = 0;

[Fangjun Jiang]

Maybe need to sort first?

[Laura Proctor]

For the maximum values, you will need to sort:

A = sort(A,'descend');

A(11:end,:) = 0;

[Saurabh]

Oh, I meant 'top 10' as in the maximum 10 values.

Example, for simplicity, consider the top 2 values

A =

1 2 3 4

4 3 2 5

6 5 4 2

1 4 2 3

Then I want the output as

B =

0 0 3 4

4 0 0 5

6 5 4 2

0 4 0 3

Thus, the top two values of each row are kept and others are replaced with zeros.

[Saurabh]

I answered a bit late.

Laura: That method would work, but it destroys the element order. I wish to preserve the order as in above example.