You need to rethink your approach. You can generate the dice rolls in one vector, but the recursive nature of your method of calculating the coordinates will lead to very slow code. Apart from re-writing your code or lowering the loop count, there is not really a solution I can see.
And you are not using indices, so that pre-allocation does nothing apart from slowing down your code even more.
x_0(i)=(x_o(i-1)+x_a)/2;%don't forget the case where i==1
