Get the indices of the n largest elements in a matrix

Question

Suppose I have the following matrix:

01 02 03 06
03 05 07 02
13 10 11 12
32 01 08 03

And I want the indices of the top 5 elements (in this case, 32, 13, 12, 11, 10). What is the cleanest way to do this in MATLAB?

Answer 1

There are a couple ways you can do this depending on how you want to deal with repeated values. Here's a solution that finds indices for the 5 largest values (which could include repeated values) using sort:

[~, sortIndex] = sort(A(:), 'descend');  % Sort the values in descending order
maxIndex = sortIndex(1:5);  % Get a linear index into A of the 5 largest values

Here's a solution that finds the 5 largest unique values, then finds all elements equal to those values (which could be more than 5 if there are repeated values), using unique and ismember:

sortedValues = unique(A(:));          % Unique sorted values
maxValues = sortedValues(end-4:end);  % Get the 5 largest values
maxIndex = ismember(A, maxValues);    % Get a logical index of all values
                                      %   equal to the 5 largest values