hello
to make your code work, b should be initiallized with same dimensions as a (as a vector, not just one scalar)
also it's not clear when you write b=a(1,1) if you meant the first or last element (iteration) of vector b
p=1;
c=[1 0 4 8];
n=4;
j=1;
a=flip(c);
b=zeros(size(a));
b(1,4)=a(1,1);
for i=n-1:-1:1
b(1,i)=b(1,i+1)*p+a(1,i);
end
b
