MATLAB: Counting duplicate values in a column

Question

Good day,

I have a table with multiple columns that looks like

A=
     Num         Num_2
    -------------------
        2        a
        3        a
        1        a
        2        b
        6        a
        1        c
        7        c

I am trying to add a new column that shows the number of duplication of the values in column Num_2 just like this

ans=
     Num         Num_2       Column_3   
    ---------------------------------
        2        a              1
        3        a              2
        1        a              3
        2        b              1
        6        a              4
        1        c              1
        7        c              2   

Answer 1

I am not certain how robust this will be for a different problem,. however it works here with the example posted:

A = { 2        'a'
      3        'a'
      1        'a'
      2        'b'
      6        'a'
      1        'c'
      7        'c'};
[UAs,~,ic] = unique(A(:,2));                                    % Unique Indices Of ‘Num_2’
Tallyc = accumarray(ic, 1, [], @(x){cumsum(x)});                % Cumulative Sums Of Indices In ‘Num_2’
Idxc = accumarray(ic, (1:size(A,1)).', [], @(x){x});            % Their Respective Indices
Tallyv = cell2mat(Tallyc);                                      % Convert From Cell Arrays To Vectors

Idxv = cell2mat(Idxc);                                          % Convert From Cell Arrays To Vectors
T1 = cell2table(A);                                             % Create Table For ‘A’
T1 = [T1, table(Tallyv(Idxv))];                                 % Create Table For ‘Column_3’
T1.Properties.VariableNames = {'Num','Num_2','Column_3'}        % Result

producing:

T1 =
  7×3 table
    Num    Num_2    Column_3
    ___    _____    ________
     2     {'a'}       1    
     3     {'a'}       2    
     1     {'a'}       3    
     2     {'b'}       1    
     6     {'a'}       4    
     1     {'c'}       1    
     7     {'c'}       2 

.