MATLAB: Finding the first and the second repetetion of a value in a matrix

findfirstindexmatrixoccurencerepetitionsecond

Hello,
I have a matrix that contains 1,2,3,4 values repetitively I want to find the first and second occurences for all 1 2 3 and 4. My original matrix originally is more complex but to show you how it looks like here is a basic example:
1 *
1 *
2 *
3 *
2 *
4 *
3 *
4 *
1
1
4
3
2
Basically I want to find the ones marked with (*). Is there a basic way to perform this?
Thank you

Best Answer

x=[1  
   1 
   2 
   3 
   2 
   4 
   3 
   4 
   1
   1
   4
   3
   2];
X=unique(x);
n=numel(X);
N=2; % first two repetitions
R=cell(n,1);
for k = 1:n
idx=find(x==X(k));
if numel(idx)<N
R{k}=idx(1:N-1);  % if the numbers appears once then only the first instance is stored hence the cell array
else
R{k}=idx(1:N); % each cell represents the first two repetitions of each numbers 1, 2, 3 & 4 in ascending order
end
end
celldisp(R)
Gives:
R{1} =
 
     1
     2
 
R{2} =
 
     3
     5
 
R{3} =
 
     4
     7
 
R{4} =
 
     6
     8
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