Finding Duplicate Values per Column

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RDG
RDG 2011-10-22
回答： Fernando Meo 2018-8-13
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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采纳的回答

Jan
Jan 2011-10-22
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.

更多回答（4 个）

the cyclist
the cyclist 2011-10-22
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 个评论
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Jan
Jan 2012-11-19
@Yowh: It is unlikely, that Harold listens to comments after 8 months.
Anurag Pujari
Anurag Pujari 2016-3-25
编辑：Anurag Pujari 2016-3-25
Accurate. What an excellent piece of code.

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Hannes Greim
Hannes Greim 2013-1-19
you can use "tabulate" for cell arrays.
  1 个评论
Clemens
Clemens 2014-3-19
编辑：Clemens 2014-3-19
Working only for versions after 2012 with the intro of tables.

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Wesley Allen
Wesley Allen 2018-2-9
编辑：Wesley Allen 2018-2-9
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
Fernando Meo
Fernando Meo 2018-8-13
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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