Finding matching rows in matrix for multiple points without a loop

I have matrix

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

B=[4 5 6; 3 2 1]

test = [2 6]

I want to be able to list the index of A where there is an exact matching row, in the same order for each line in B. I can find this value for one row at a time, but i am looking for a way to do search A for each row of B, without a loop.

test=find(ismember(A,B(1,1:3),'rows'),1)

I may also want to change the values in A to "zeros"

Thank you.

Answer 1

[Stephen23]

This can be achieved using sortrows and sort together with some basic MATLAB indexing. First we define the test-data (I added one non-matching row to B):

>> A = [1,2,3;4,5,6;7,8,9;9,8,7;6,5,4;3,2,1];
>> B = [4,5,6;3,2,1;0,0,0];

Then we determine any matching rows using sortrows and a simple element-wise equality:

>> [C,D] = sortrows([A;B]);
>> E = all(C(1:end-1,:)==C(2:end,:),2);
>> F = D(E)
F = 
   6
   2
>> A(F,:)
ans =
   3     2     1
   4     5     6

But these are in the order that they occur in A. To get the order that they occur in B, we can do this:

>> [~,X] = sort(D([false;E]));
>> F(X)
ans =
   2
   6
>> A(F(X),:)
ans =
   4     5     6
   3     2     1