MATLAB: How to apply condition on multiple columns

Question

I have a large data set that is unequal in every way so to get things done i have to apply condition on the element of the specific columns. E.g

1 1 234
1 1 234
1 1 180
1 2 345
1 2 674
.
.
.
8  1  453
8  1  678
8  2  478
8  2  345
8  2  345
.
.
.
31 21 456
31 22 452
31 22 345
31 23 670
31 23 567
31 23 456

what i want the programme to do is that check the value in column 1 if it equals 1 the check the value in column 2 if it equals 1 then take the mean of all the values in column 3 corresponding to column 2. similarly i want it to repeat for all the values in column 2 for 1st then all the other numbers in column 1 till last number in this case which is 31.

Answer 1

A = [1 1 234
1 1 234
1 1 180
1 2 345
1 2 674
8  1  453
8  1  678
8  2  478
8  2  345
8  2  345
31 21 456
31 22 452
31 22 345
31 23 670
31 23 567
31 23 456] ;
B = [1 1] ;
[idx,val] = ismember(A(:,1:2),B,'rows') ;
% get mean
iwant = sum(A(idx,3))/nnz(idx)