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

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?

[KALYAN ACHARJYA]

Can you elaborate with small example?

Answer 1

[Thiago Henrique Gomes Lobato]

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

Answer 2

[Stephan]

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