Create matrix row element if row elements of two previous matrices are identical

Sorry for the title. I could not think of something better.

I have the following problem.

I have two four-column matrices build up like this:

Property | X | Y | Z

The two matrices have different sizes, since matrix 1 has a large amount of additional rows compared to matrix 2.

What I want to do is the following:

I need to create a third matrix that only features those rows (of the large matrix) that are identical in columns X, Y and Z to rows in matrix2(the property column is always different).

I tried an if-statement but it did not really work out due to my programming syntax. Has somebody a tip?

Thank you!

Example - I tried something like this: (in this case A is the larger matrix and I want its property column for X,Y,Z-positions that are identical to another matrix B.. I am terrible with the MatLab-syntax..:

if (A(:,2) == B(:,2) and (A(:,3) == B(:,3) and (A(:,4) == B(:,4))

newArray(:,1) = A(:,1);

end

I fear that in the way depicted in the code example it might cancel since A and B are of different size and it cannot really compare

Answer 1

[Guillaume]

Use either intersect (if there's no duplicate X|Y|Z triplet or if you just want one of these triplets) or ismember (if you do want those duplicates in the output. Both are two be used with the 'rows' option:

A = [1 1 2 3
     2 3 2 1
     3 4 5 6
     4 7 8 9
     5 1 2 3];

B = [-1 1 2 3
     -2 4 5 6
     -3 3 2 1];

%with intersect, row 4 of A is removed (not in B), row 5 is removed (triplet already present)
[~, idx] = intersect(A(:, [2 3 4]), B(:, [2 3 4]), 'rows');
C1 = A(idx, :)

%with ismember, row 4 of A is removed (not in B)
C2 = A(ismember(A(:, [2 3 4]), B(:, [2 3 4]), 'rows'), :)

[Arne]

works out fine thank you!

[Guillaume]

You would be indeed better off starting a new question, as most people won't look at activity on questions marked answers. In fact, I only spotted your new comment because it jumped to the top of my answer page. Had I answered another question in the meantime, I would have never noticed your comment.

Answer 2

[Thorsten]

In this example A is the large matrix and B the small one

A = round(10*rand(10, 3)); A = [A; A(1:2, :)]; 
B = A(1:2:end, :);
A = [rand(size(A, 1), 1) A];
B = [rand(size(B, 1), 1) B];

Create the new A

Anew = A(ismember(A(:, 2:4), B(:, 2:4), 'rows'), :);