How to select the first two (or first nth) rows of a matrix when the first and second elements of the rows always shows the same values fro more than two rows?

I have a matrix A with 3 columns and multiple rows,

A =

a_1 b_1 7

a_1 b_1 8

a_1 b_1 4

a_2 b_2 2

a_2 b_2 5

a_2 b_2 3

a_2 b_2 2

a_3 b_3 9

a_3 b_3 8

a_3 b_3 12

a_3 b_3 25

a_3 b_3 10

... ... ...

I would like to make another matrix B that contains the first two rows of each couple of parameter (a_i, b_i) in A , such that

B =

a_1 b_1 7

a_1 b_1 8

a_2 b_2 2

a_2 b_2 5

a_3 b_3 9

a_3 b_3 8

... ... ...

Note that the value a_i and b_i ae not unique, but each couple (a_i, b_i) would be different from (a_j, b_j).

Thanks in advance!

Answer 1

[Matt J]

G=findgroups(A(:,1),A(:,2));

discard=cell2mat( splitapply(@(x){x(3:end,:)} , (1:numel(G))' ,G ) );

B=A;
B(discard,:)=[];

[Rodrigues Bitha]

Thankyou very much!