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?

I use very complicate ways. use three nested for loops. Any faster way? — kevin2019
– kevin2019, Commented Apr 7, 2018 at 15:23

Tommaso Belluzzo · Accepted Answer · 2018-04-08 11:55:27Z

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:

Milad · Accepted Answer · 2018-04-07 19:33:31Z

1

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


userRep = [];
 for i = 1:size(A,1)
     for j = 1:size(B,2)
        if A{i,1} == B{j}
            userRep(end+1,:) = [j,i];
        end
     end
 end


 numberOfSamp = 2;
 samples = {};
 for i = 1:size(B,2)
     cellPos = userRep(userRep(:,1) == i,:);
     cellPos = cellPos(randi([1 size(cellPos,1)],[min(numberOfSamp,size(cellPos,1)),1]),:);
     for j = 1:size(cellPos,1)
        samples{end+1,1} = A{cellPos(j,2),1};
        samples{end,2} = A{cellPos(j,2),2};
        samples{end,3} = A{cellPos(j,2),3};
     end
end

samples

answered Apr 7, 2018 at 19:33

Milad

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1 Comment

Luis Mendo Over a year ago

If you use loops you should preallocate whenever possible

Luis Mendo · Accepted Answer · 2018-04-08 02:45:36Z

Let the input variables be defined as

A = { 'U2', 'T13', 'A52';  
      'U2', 'T15', 'A52';  
      'U2', 'T18', 'A52';  
      'U2', 'T17', 'A995'; 
      'U4', 'T18', 'A53';  
      'U4', 'T13', 'A64';  
      'U4', 'T18', 'A64';
     };                     % data
B = {'U2', 'U4'};           % unique identifiers
n = 2;                      % number of results per group

You can achieve what you want as follows:

Create a grouping variable, so each ID corresponds to an integer;
Pick n random values from the set of row indices corresponding to each group;
Use the set of all such indices to index into A.

Code:

[~, m] = ismember(A(:,1), B);                                  % step 1
s = accumarray(m, 1:size(A,1).', [], @(x){randsample(x, n)});  % step 2
C = A(vertcat(s{:}),:);                                        % step 3

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