# Extract duplicate values from both 1st and 2nd column

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F Z on 9 Apr 2015
Edited: Stephen23 on 20 Apr 2015
I have a large matrix (120000x4096)to process. There are repetitions in the 1st and 2nd columns. If 2 rows have the same number in the 1st and the 2nd column, only one row is stored. However, if 2 rows have the same number in the 1st columns but different numbers in the 2nd one, both rows are stored. Then extract the other columns related to each set of rows. This can seen in the attached file
Any ideas to do it without a loop please?

Stephen23 on 9 Apr 2015
Edited: Stephen23 on 20 Apr 2015
Removing duplicate rows, using fully vectorized code, is fast and easy with unique and its 'rows' and 'stable' options:
>> a = [10,10,10;10,10,10;5,5,5;4,4,4]
a =
10 10 10
10 10 10
5 5 5
4 4 4
>> [~,x] = unique(a(:,1:2),'rows','stable');
>> a(x,:)
ans =
10 10 10
5 5 5
4 4 4
Note how indexing is used to select only the first two columns of a as an input to unique, thus the third column is ignored, as the original questions requests that only the first two columns should be compared.
We can also test it on your test data (although putting it in a PDF is a complete pain: please provide text data instead, either within the question itself, or uploaded as a text file):
>> A = [1,2,120; 1,2,120; 1,3,112; 3,5,100; 3,5,100; 3,6,113; 4,2,106; 4,2,106; 4,6,88; 6,11,0; 6,11,0; 6,12,97; 6,12,97]
A =
1 2 120
1 2 120
1 3 112
3 5 100
3 5 100
3 6 113
4 2 106
4 2 106
4 6 88
6 11 0
6 11 0
6 12 97
6 12 97
>> [~,X] = unique(A(:,1:2),'rows','stable');
>> A(X,:)
ans =
1 2 120
1 3 112
3 5 100
3 6 113
4 2 106
4 6 88
6 11 0
6 12 97
Which exactly matches your required output (did I mention that putting the test-case in a PDF is a pain? Please provide plain text instead!)
F Z on 15 Apr 2015
thank you a lot for your help!

Thomas Koelen on 9 Apr 2015
Edited: Thomas Koelen on 9 Apr 2015
a=[10 10 10; 10 10 10; 5 5 5; 4 4 4];
a =
10 10 10
10 10 10
5 5 5
4 4 4
Code:
a=[10 10 10; 10 10 10; 5 5 5; 4 4 4; 4 4 4];
sizea=size(a);
for i=1:sizea(1,1)
for i2=1:sizea(1,2)
sizea=size(a);
if a(i,i2)==a(i+1,i2)
a(i+1,:)=[];
if i==sizea(1,1)-1
return
end
end
end
end
a =
10 10 10
5 5 5
4 4 4
Where a is your scan matrix!
Thomas Koelen on 9 Apr 2015
Made the code a bit nicer since it was hard to read.