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Hi. I have a vector named A, e.g.

A=[2 5 8 7 8 9 3 7 6 5 4 1]

I want to sort the members of vector A from maximum to minimum values.

Also I want to find the indices of equal members in the sorted version of vector A.

How can I do that?

Thanks for your help.

Kaushik Lakshminarasimhan
on 29 Nov 2017

A=[2 5 8 7 8 9 3 7 6 5 4 1];

Asorted = sort(A,'descend'); % sorted from max to min

[~,indx] = unique(Asorted);

equalpairs = [(setdiff(1:length(Asorted),indx)-1)' setdiff(1:length(Asorted),indx)']; % each row contains pair of indices with equal values

Image Analyst
on 30 Nov 2017

With the accepted solution, if A is

A =[22 25 28 27 28 29 23 27 26 25 22 22 24 21 22]

The accepted solution gives

equalpairs =

2 3

4 5

7 8

11 12

12 13

13 14

However if you use my solution:

A = [2 5 8 7 8 9 3 7 6 5 2 2 4 1 2] + 20

sortedA = sort(A, 'descend')

props = regionprops(sortedA, sortedA, 'MeanIntensity', 'PixelIdxList');

missingNumbers = isnan([props.MeanIntensity]);

props = props(~missingNumbers); % Get rid of nans that occur when there is a missing number.

% Done! Now print them out so we can see:

for k = 1 : length(props)

fprintf('The indexes for %d are ', props(k).MeanIntensity);

fprintf('%d, ', props(k).PixelIdxList);

fprintf('\n');

equalIndexes{k} = [props(k).PixelIdxList];

end

celldisp(equalIndexes)

You will get a cell array where each cell has the indexes for a certain number:

A =

22 25 28 27 28 29 23 27 26 25 22 22 24 21 22

sortedA =

29 28 28 27 27 26 25 25 24 23 22 22 22 22 21

The indexes for 21 are 15,

The indexes for 22 are 11, 12, 13, 14,

The indexes for 23 are 10,

The indexes for 24 are 9,

The indexes for 25 are 7, 8,

The indexes for 26 are 6,

The indexes for 27 are 4, 5,

The indexes for 28 are 2, 3,

The indexes for 29 are 1,

equalIndexes{1} =

15

equalIndexes{2} =

11

12

13

14

equalIndexes{3} =

10

equalIndexes{4} =

9

equalIndexes{5} =

7

8

equalIndexes{6} =

6

equalIndexes{7} =

4

5

equalIndexes{8} =

2

3

equalIndexes{9} =

1

I think having a complete list is probably more useful and convenient than having a list of pairs which doesn't even include all pairs. For example the accepted solution does not list (11, 14), (12,14), etc. as pairs.

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