First of all, your code does not look like it is in order. You define C after you calculate Q? You do not use X in any calculation?
Regardless, the thing you want to do do not require for loop.
data =[1 2 3 4 5 6;2 3 5 8 9 7];
C = 0.5;
X = data(1,:);
Y = data(2,:);
Q = -C + Y;
Q will be a vector the size of Y and Q(n) represents the value when Y = data(2,n). If you have X somewhere in the calculation, you can add it as such;
Q = -C + X.*Y - X + X.^2;