MATLAB: Find double repetitions in a (sorted) array.

double repetitionsstruct array

Given an array submitted in a form of struct field, containing integer numbers. For convenience, let's assume that the numbers are already sorted in ascending order:
>> s.x
  ans =
       2
  ans =
       2
  ans =
       5
  ans =
       5
  ans =
       5
  ans =
       8
  ans =
       8
Find indexes of elements, which occur exact 2 times:
ind =
       1     2     6     7

Best Answer

x= [2; 2; 5; 5; 5; 8; 8; 13; 13; 13; 13];
[~,~,g] = unique(x); % OR for last versions of MATLAB: g = findgroups(x)
c = accumarray(g,1:numel(x),[],@(x){x});
out = cell2mat(c(cellfun(@numel,c) == 2));
or
[a,~,g] = unique(x);
out = find(ismember(x,a(accumarray(g,1) == 2)));
or (FIXED)
out = reshape(strfind([1,diff(x(:)')~=0,1],[1 0 1]) + [0;1],[],1);
out = reshape(bsxfun(@plus,strfind([1,diff(x(:)')~=0,1],[1 0 1]),[0;1]),[],1); % for old MATLAB
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