MATLAB: Find first element in each row greater than a given element from another matrix.

matrix manipulation

Suppose I have 2 matrices of different dimensions. Say
A=[2 3 5 7; 
   1 4 8 9; 
   3 4 9 7]
and
B=[2;
   8;
  11]
then I want each element from each row of B compared with corresponding row of A and then get an output as follows:
C=[3;
   9;
   0]
i.e take 2 from B and look for the first element in first row of A which is greater than it. then take 8 from B and look for first element in second row of A which is greater than it. In third row since there was no element in A greater than 11, it should return zero. Is there a way I can do this without using loop? I would like to get this done through vectorization. Many thanks!

Best Answer

I tried to come up with a vectorised solution but could not.
My non-vectorised solution:
A=[2 3 5 7; 1 4 8 9; 3 4 9 7];
B=[2; 8; 11];
C = zeros(size(B));
for k1 = 1:size(B,1)
    T = min(A(k1,(find(A(k1,:) > B(k1)))));
    if ~isempty(T)
        C(k1) = T;
    end
end
The ‘C’ (result) vector is the output of the script.
It produces:
C =
       3
       9
       0
A vectorised solution is not always more efficient than a loop. This one is a bit more efficient than your posted solution (it eliminates one loop and vectorises the comparison step), so it will speed things up a bit.
Related Question