In general you have to use a loop for that. However, in your simple case it is not necessary.
f(x) = 0.7^x * f(0)
where f0 is the inition condition.
So if you want a vector with the first n values of the function, use the following Matlab code:
f = 0.7.^(1:n) * f0
where f0 is the initial condition, a scalar.
