MATLAB: How to find subgroup min or max

Question

group_id=[1 1 1 2 2 3 3 3]'; 
B=[ 5 6 7 4 1 2 2 3]';
output: C=[ 5 0 0 0 1 2 2 0]';

Answer 1

Use accumarray and bsxfun:

X = [1;1;1;2;2;3;3;3];
B = [5;6;7;4;1;2;2;3];
A = accumarray(X,B,[],@min);
Y = any(bsxfun(@eq,A,B.'));
C = zeros(size(B));
C(Y) = B(Y)

where the output is

C =
   5
   0
   0
   0
   1
   2
   2
   0