Reason: The first two elements are zero because you assign the values from index three so matlab assigns the first two values as zeros because matlab index starts from one.
You don't need loop at all:
A=10.^(-(3:6));
But if you insist one then:
n=3:6;
A=zeros(1,numel(n)); % preallocate
for j = 1:numel(n)
A(j)=10.^(-n(j));
end
A
Gives:
A=[ 0.001, 0.0001, 0.00001, 0.000001]
