Create a matrix that stores rows that are not from a random sample

Asked by Elanakayon Annalingam on 10 Nov 2019 at 3:22

Latest activity Edited by Stephan on 10 Nov 2019 at 9:54

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I have a 1020x9 matrix and I took a random sample that is half the original size (510x9), I want to retrieve the other half. I think the best way to do this is to do matrix comparison of the orignal matrix and the random sample and store the rows they don't have in common into a new matrix. How can I accomplish this?

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KALYAN ACHARJYA on 10 Nov 2019 at 9:37

Can you elaborate with small example?

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Answer by Thiago Henrique Gomes Lobato on 10 Nov 2019 at 9:42

The function ismember may be what you need to efficiently solve your problem:

OriginalMatrix = randn(1020,9);
RandomMatrixIndex = randperm(1020);
IndexFirstMatrix  = RandomMatrixIndex(1:end/2);
IndexSecondMatrix  = RandomMatrixIndex(end/2+1:end);
FirstMatrix = OriginalMatrix(IndexFirstMatrix,:);
IndexFound = ismember(OriginalMatrix,FirstMatrix,'rows');
EqualIndex = find(IndexFound==1); % Index for your first matrix
FoundedIndexSecondMatrix = find(IndexFound==0); % Remaining index, those that you want
% Comparison to check that they are the same
Cmp1 = norm( sort(IndexFirstMatrix)-EqualIndex')
Cmp2 = norm( sort(IndexSecondMatrix)-FoundedIndexSecondMatrix')
Cmp1 =
     0
Cmp2 =
     0

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Answer by Stephan on 10 Nov 2019 at 9:52

Edited by Stephan on 10 Nov 2019 at 9:54

A way smarter is using the ismember function - here is an example - all you need ist the last line, the first two lines are just to get some numbers to illustrate:

matrix = randi(10,6,3)
sample = matrix(1:2:end,:)
result = matrix(~ismember(matrix,sample,'rows'),:)
matrix =
     2     9     6
     7     4     5
     5     7    10
     8     2     7
     8     1     7
    10     8     9
sample =
     2     9     6
     5     7    10
     8     1     7
result =
     7     4     5
     8     2     7
    10     8     9

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