The question doesn't entirely make sense. You say you want to record the value of i when i equals 4, 6, 9, 12. Taking the question literally, you could do this
k = 0; % initialise index into M
for i = 1:100
if ismember(i, [4 6 9 12]) % test whether i is at an interesting place
k = k + 1; % increment index
M(k) = i; % store the value of i in M
end
end
but if that was really what you wanted you'd just do
M = [4 6 9 12];
in the first place.
Perhaps you could clarify what exactly you need to do.
(By the way, some people regard i as a bad choice of name for a variable, because it is also a name for the square root of -1 in MATLAB.)
