Randomly Select sample from a cell array in MATLAB

Question

I have a cell array in MATLAB as follow, the first column is a list of user ID:

A = { 'U2', 'T13', 'A52';  
      'U2', 'T15', 'A52';  
      'U2', 'T18', 'A52';  
      'U2', 'T17', 'A995'; 
      'U4', 'T18', 'A53';  
      'U4', 'T13', 'A64';  
      'U4', 'T18', 'A64';
      ....
     }

I also have a cell array B contains the unique ID for user as follow:

B = {'U2', 'U4'}

My goal is try to randomly select two samples for each user. Assume each user at least have two samples in B.

One example is the C as follow:

C = { 'U2', 'T13', 'A52';  
      'U2', 'T18', 'A52';   
      'U4', 'T13', 'A64';  
      'U4', 'T18', 'A64';
        ...
     }

How to generate those sample?

Answer 1

The following code should produce what you are looking for:

A = {
  'U2', 'T13', 'A52';  
  'U2', 'T15', 'A52';  
  'U2', 'T18', 'A52';  
  'U2', 'T17', 'A995'; 
  'U4', 'T18', 'A53';  
  'U4', 'T13', 'A64';  
  'U4', 'T18', 'A64';
  'U7', 'T14', 'A44';  
  'U7', 'T14', 'A27';  
  'U7', 'T18', 'A27';  
  'U7', 'T13', 'A341';  
  'U7', 'T11', 'A111';
  'U8', 'T17', 'A39';  
  'U8', 'T15', 'A58'
};

% Find the unique user identifiers...
B = unique(A(:,1));
B_len = numel(B);

% Preallocate a cell array to store the results...
R = cell(B_len*2,size(A,2));
R_off = 1;

% Iterate over the unique user identifiers...
for i = 1:B_len

    % Pick all the entries of A belonging to the current user identifier...
    D = A(ismember(A(:,1),B(i)),:);

    % Pick two random non-repeating entries and add them to the results...
    idx = datasample(1:size(D,1),2,'Replace',false);
    R([R_off (R_off+1)],:) = D(idx,:); 

    % Properly increase the offset to the results array...
    R_off = R_off + 2;

end

Here is one of the possible outcomes for the code snippet above:

>> disp(R)

    'U2'    'T13'    'A52' 
    'U2'    'T18'    'A52' 
    'U4'    'T13'    'A64' 
    'U4'    'T18'    'A64' 
    'U7'    'T14'    'A44' 
    'U7'    'T13'    'A341'
    'U8'    'T17'    'A39' 
    'U8'    'T15'    'A58' 

For more information about the functions I used, refer to the following pages of the official Matlab documentation:

• datasample
• ismember
• unique