# How to find set of 3 in a 122x4 matrix?

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John Hunt on 13 Oct 2017
Commented: John Hunt on 17 Oct 2017
I have a matrix that is 122x4. These have index numbers 1-36 with two only rows containing three points that are common between them. How can I find these pairs?
ex
A=[ 1 2 13 15;
1 2 15 35;
1 2 17 22;
1 2 22 36...]
(there is not a pattern throughout the matrix like there are in the first couple rows) I want some thing that can identify the pair of combos
combos = [1 2;
3 4]
where 1,2,3,and 4 are row numbers
If I can also print out the numbers they share that would be great too combos = [1 2 1 2 15;...etc

John Hunt on 17 Oct 2017
yeah that is what I am trying to get over, I am not using intersect like he does.
dpb on 17 Oct 2017
Say what!!!??? I have no klew what the above is intending to say.
Are the three values you're trying to match those in the first three columns only or is it three out of the four in the line that are to be found?
John Hunt on 17 Oct 2017
three out of the four need to match three out of four for the other rows

Guillaume on 17 Oct 2017
Edited: Guillaume on 17 Oct 2017
This is O(n^2) so is going to be slow for largish matrices. I haven't got the time to think of a cleverer algorithm:
pairs = zeros(0, 5);
for row1 = 1 : size(A, 1)
for row2 = row1+1 : size(A, 1)
commonnumbers = intersect(A(row1, :), A(row2, :));
if numel(commonnumbers ) == 3
pairs = [pairs; row1, row2, commonnumbers]; %#ok<AGROW>
break; %assumes that there is always only one other row that matches
end
end
end

#### 1 Comment

John Hunt on 17 Oct 2017
Thank you I will go through it to see if I can get it to do what I need