How can I extract unique pairs of matrix in reverse order?

Hi.

Here is my simple example.

1 1 2

2 1 4

3 2 1

4 2 3

5 3 2

6 4 1

Suppose that column 2 and column 3 represent ID of agents, and column 1 represents unique pair ID.

Note that row 1 and row 3 are basically same and they have different pair ID.

I want to make the following matrix.

1 1 2 3

2 1 4 6

3 2 1 1

4 2 3 5

5 3 2 4

6 4 1 2

That is, I want to allow for the fact that the pairs in revesre order are the same.

Thank you.

Answer 1

[Siwon Ryu]

I think you can use join after converting your array into tables.

First, find unique pairs of the combination of the first three columns. This procedure prevent the changes in total number of rows after joining.

And then, right-join (Because ID table only includes distinct values of two IDs, both inner and outer join would work) the unique ID table to the original table on two keys: column2 and 3 in the reverse order. See this:

A = [1 1 2; 2,1,4;3,2,1;4,2,3;5,3,2;6,4,1]

% Find unique pairs of Indices
A_ID = unique(A(:,1:3),'rows')

% Convert to Tables
T_ID = array2table(A_ID)
T_ID.Properties.VariableNames(1:3) = {'ID','ID1','ID2'}

T_ID_tg = array2table(A(:,[1,3,2]))
T_ID_tg.Properties.VariableNames(1:3) = {'ID','ID1','ID2'}

% Join
T = join(T_ID_tg, T_ID,'Keys',{'ID1','ID2'})

% Convert to array
A_result = T{:,:}

Answer 2

[Akira Agata]

How about the following way?

A = [...
  1     1      2;...
  2     1      4;...
  3     2      1;...
  4     2      3;...
  5     3      2;...
  6     4      1];

B = A(:,[3 2]);

[~, loc] = ismember(A(:,2:3),B,'rows');

A = [A, loc];

>> A

A =

1 1 2 3

2 1 4 6

3 2 1 1

4 2 3 5

5 3 2 4

6 4 1 2

[Daeeun Bae]

Great. Thank you!

Answer 3

[Cris LaPierre]

If you use a table, you can use join.

A = [   1     1      2
   2     1      4
   3     2      1
   4     2      3
   5     3      2
   6     4      1];
ID = A(:,1);
Agent1 = A(:,2);
Agent2 = A(:,3);
TA = table(ID,Agent1,Agent2);
TB = table(ID,Agent2,Agent1);

% Join tables
joinedData = outerjoin(TA,TB,"Type","left","LeftKeys",["Agent1","Agent2"],...
    "RightKeys",["Agent2","Agent1"],"MergeKeys",true)

joinedData = 6×4 table
    ID_TA    Agent1    Agent2    ID_TB
    _____    ______    ______    _____

      1        1         2         3  
      2        1         4         6  
      3        2         1         1  
      4        2         3         5  
      5        3         2         4  
      6        4         1         2  

[Daeeun Bae]

Thank you!