You have got it almost right but you have to have the while loop on top to ask the user untill non zero values are received. Inside the loop you can have an if-else statement to calculate your series or to throw an error message.
It's better to start with some initial values so,
a = 0; %zero first because while loop should be executed atleast once
r = 0;
n=10; %length of your sequence
v = zeros(1,n);
go to the loop now,
while a==0 || r==0
a=input('Enter the starting value: ');
r=input('Enter the ratio for the function: ');
if a~=0 && r~=0
v(1) = a; %go here only if they are non zero values
for k=2:n %loop to calculate your series
v(k) = r*v(k-1);
end
else %else throw an error message and loop will be continued
disp('error: please enter non zero values');
end
end
finally display the result with
disp(v)
