Grouping rows on the first two columns

Hello,

I have a nx3 matrix for which I need to a) group cells which have matching values in the first and second columns, b) find the max value in the third column for each of these groups, b) remove the rows that contain the max values

e.g.

Matrix:
13  30  7
13  30  6
13  40  5
13  40  6
12  30  7
12  37  5

Group values on first two columns:

13 40 5
13 40 6

Find max value and remove row from matrix:

      13  30  7
      13  30  6
      13  40  5
      12  30  7
      12  37  5

This needs to be done for every group. I tried a for loop to iterate through each group, but I'm having trouble grouping rows on the first two columns. My code is currently:

for i =1:nr
  % group  on first two columns
   L = A(:,1)==A(i,1)& A(:,2)==A(i,2);
     A2=A(L,:);
  %find max value for each 
  A(i,3)=max(A2(:,3));
end

Does anyone know a better approach?

[KL]

which version are you using?

also, read this: https://www.mathworks.com/help/matlab/ref/findgroups.html

Answer 1

[Jos (10584)]

A = [13  30  7
13  30  6
13  40  5
13  40  6
12  30  7
12  37  5] ;

% engine
[B,~,gi] = unique(A(:,[1 2]),'rows','stable') % unique rows, with grouping index
B(:,3) = accumarray(gi(:), A(:,3), [], @max)

[Si Cla]

Thankyou! It works just how I wanted.