MATLAB: Selecting records from a matrix subject to a condition

matrix manipulation

My matrix(M) looks like the data below; how can I pull the records that have a third column greater than or equal to 20?

1   0.4   12
2   1.3   20
3   1.2   15
4   2.1   21

M(M(:,3)>=20, :)

Are you looking for something like this

M = [1  1  2   60;
    2  3  2   90;
    3  1  3  240;
    4  2  3 200;
    5  3  3 160];
result = splitapply(@sum, M(:,4), M(:,2))

Result

>> result
result =
   300
   200
   250

   M = a;
   M(M<b) = 0;

M is your answer.