find the multiple max values

I have a matrix

8 23 44 19

44 5 62 1

7 6 12 33

6 55 24 8

and I want to find 3 largest values(reserve duplicate matches),and then make the other be 0

just like

0 0 44 0

44 0 62 0

0 0 0 0

0 55 0 0

I have read some book ，but I still have no idea

help me plz

[Aditya Verma]

Hello, could you please specify what you have tried until now, and in which part you are facing problem.

[chung yen chang]

q=[6 7 3 2
5 3 6 4
7 7 5 3
7 9 10 5]

 [x,y]=sort(q(:))
 q(y(1:end-3))=0

in this case matrix can't preserve max three values

[chung yen chang]

the answer would be

q =

0 0 0 0

0 0 0 0

0 7 0 0

0 9 10 0

Answer 1

[madhan ravi]

MaX = maxk(matrix(:), 3);
Wanted = ismember(matrix, MaX) .* matrix

% for older versions
m = sort(matrix(:),'descend');
Wanted = ismember(matrix, m(1:3)) .* matrix

[Aditya Verma]

While this works for the example provided by OP. This would not work for all cases.

[madhan ravi]

Illustrate.

[Aditya Verma]

Consider the following:

matrix = [8 23 46 19;
          47 5 62 1 ;
          7 6 12 33 ;
          6 55 24 8];
[MaX, index] = maxk(matrix(:),4);
Wanted = zeros(size(matrix));
Wanted(index) = MaX

It gives the following output:

     0     0    46     0
    47     0    62     0
     0     0     0     0
     0    55     0     0

Which preserves four max values.

[madhan ravi]

Isn’t that what the OP is looking for? Change 4 to 3.

[Aditya Verma]

OP mentioned: "I want to find 3 largest values(reserve duplicate matches),and then make the other be 0". If I understand correctly, it means that OP wants to preserve max three values, but preserve the duplicates. So, if my elements in matrix in descinding order are: 9, 9, 9, 8, 8, 7, 7, 3, 2, 0, -1, -1. Then all the 9s, 8s and 7s should persist, and rest all should be 0.

In the example I provided above, it is preserving max 4 values.

[madhan ravi]

Yes, you’re right. Rectified.

[chung yen chang]

Yes AV, that is what I mean. Actually I have try the function "maxk" to use it before ,but this function can't run in 2015b so I try to seek another method

[madhan ravi]

Chung did you even see my answer with the second option even though I specifically mentioned for “older versions”. Come on!

[chung yen chang]

Madhan I apologize for that ,I misread .Actually this a great method

[madhan ravi]

In the command window just call the function

Matrix = randi(10,4); % an example
Wanted = Mx(Matrix)

% function definition
function Wanted = Mx(matrix) % save it as a separate file called Mx.m

... the Code
end

[Stephen23]

Note that this answer does not "find 3 largest values" as the question requested:

>> matrix = [4,4,4;4,4,4;3,2,1]
matrix =
   4   4   4
   4   4   4
   3   2   1
>> m = sort(matrix(:),'descend');
>> Wanted = ismember(matrix, m(1:3)) .* matrix
Wanted =
   4   4   4
   4   4   4
   0   0   0

The problem is caused by the use of sort. See my answer for the correct solution.

Answer 2

[Stephen23]

Where M is your matrix:

>> U = unique(M(:));
>> X = ismember(M,U(end-2:end));
>> M(~X) = 0
M =
    0    0   44    0
   44    0   62    0
    0    0    0    0
    0   55    0    0

[chung yen chang]

Thanks, bro this answer can work very well

[Stephen23]

"Thanks, bro this answer can work very well"

Note my answer actually gives the output that you asked for (unlike the answer that you accepted):

>> M = [4,4,4;4,4,4;3,2,1]
M =
   4   4   4
   4   4   4
   3   2   1
>> U = unique(M(:));
>> X = ismember(M,U(end-2:end));
>> M(~X) = 0
M =
   4   4   4
   4   4   4
   3   2   0

[madhan ravi]

However maxk(...) gives the right answer xD, but i do agree the loophole.