Finding Duplicate Values per Column
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Greetings, suppose Column A has these values - 7 18 27 42 65 49 54 65 78 82 87 98
Is there a way to compare the values (row by row) and search for duplicates? (I'm using Matlab R2010b)I don't want the duplicated values to be removed.
Thanks.
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Accepted Answer
Jan
on 22 Oct 2011
A = [7 18 27 42 65 49 54 65 78 82 87 98];
[n, bin] = histc(A, unique(A));
multiple = find(n > 1);
index = find(ismember(bin, multiple));
Now the values A(index) appear mutliple times.
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More Answers (4)
the cyclist
on 22 Oct 2011
Here's a slightly different way:
X = [1 2 3 4 5 5 5 1];
uniqueX = unique(X);
countOfX = hist(X,uniqueX);
indexToRepeatedValue = (countOfX~=1);
repeatedValues = uniqueX(indexToRepeatedValue)
numberOfAppearancesOfRepeatedValues = countOfX(indexToRepeatedValue)
4 Comments
Anurag Pujari
on 25 Mar 2016
Edited: Anurag Pujari
on 25 Mar 2016
Accurate. What an excellent piece of code.
Wesley Allen
on 9 Feb 2018
Edited: Wesley Allen
on 9 Feb 2018
Duplicate Finding with Tolerance
If you want to find duplicates with tolerances (e.g., for non-integers), I use the following:
A = [1.313;2.4;2.400000001;1.31299999999;2.25;2.25;2.25000000001;3.7];
TOL = 1e-5;
uniqueA = uniquetol(A,TOL);
duplicateBool = abs(repmat(A,size(uniqueA.'))-repmat(uniqueA.',size(A))) < max(abs(uniqueA))*TOL;
duplicateCount = sum(duplicateBool).';
Just like with the cyclist's answer, if you want to isolate only the values that have more than one instance:
iDuplicate = (duplicateCount ~= 1);
repeatedValues = uniqueA(iDuplicate);
numberOfAppearancesOfRepeatedValues = duplicateCount(iDuplicate);
repeatedBool = duplicateBool(:,iDuplicate);
Using the Results
The unique values are in uniqueA:
>> uniqueA
uniqueA =
1.3130
2.2500
2.4000
3.7000
The quantity of each unique value is in duplicateCount:
>> duplicateCount
duplicateCount =
2
3
2
1
To get the indices of A corresponding to the n-th unique value, uniqueA(n)
>> n = 2;
>> uniqueA(n)
ans =
2.2500
>> duplicateIndex = find(duplicateBool(:,n))
duplicateIndex =
5
6
7
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Fernando Meo
on 13 Aug 2018
Here is another answer (a one liner)
If AA is a 2D matrix and you wish to find the rows which have a duplicate values in its columns,
RowsWhichHaveDuplicates = find(arrayfun(@(i (~isequal(length(unique(AA(i,:))),size(AA,2))), [1:size(AA,1)]));
Example
AA = [6 7 11 6; 7 11 4 8; 11 15 1 10; 15 4 14 12;
18 13 18 8; 12 13 18 1; 3 14 6 18];
>> RowsWhichHaveDuplicates = RowsWhichHaveDuplicates = find(arrayfun(@(i) (~isequal(length(unique(AA(i,:))),size(AA,2))), [1:size(AA,1)]))
RowsWhichHaveDuplicates =
1 5
If your values are real, then a tolerance can be set by using the matlab "round" function to the decimal places you wish to use.
AA = round(rand(10)*10,1); % First decimal place
AA =
6.0000 2.0000 0.4000 6.7000 9.4000 0.6000 8.3000 3.1000 1.0000 3.0000
9.1000 7.5000 0.6000 6.0000 0.7000 3.1000 0.3000 4.2000 9.0000 3.7000
2.5000 8.9000 5.0000 3.4000 7.2000 6.6000 8.4000 9.3000 9.0000 7.6000
8.6000 1.0000 4.1000 4.0000 8.3000 4.6000 2.6000 0.6000 0.8000 3.1000
7.6000 5.2000 2.2000 3.9000 7.3000 0.2000 6.6000 8.2000 5.2000 9.6000
2.2000 6.0000 4.3000 7.0000 5.1000 6.9000 6.7000 6.4000 2.8000 2.1000
4.2000 9.8000 9.5000 1.4000 5.2000 4.1000 2.6000 8.2000 8.8000 7.3000
1.3000 6.7000 2.0000 3.8000 7.6000 5.7000 3.3000 3.3000 6.7000 2.5000
9.2000 8.5000 7.1000 2.2000 6.3000 9.9000 2.5000 9.5000 1.2000 8.9000
2.9000 1.7000 7.8000 4.1000 0.7000 8.6000 7.1000 9.1000 3.7000 7.1000
RowsWhichHaveDuplicates = find(arrayfun(@(i) (~isequal(length(unique(AA(i,:))),size(AA,2))), [1:size(AA,1)]))
RowsWhichHaveDuplicates =
5 8 10
Hope this helps
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