elseif r1<=r(i)<=r2
This will not do, what you expect. Matlab processes the expression from left to right. The first part is:
r1 <= r(i)
This is either true or false, which is converted to 1 or 0. Because r2 is 150, both values reply true for the second comparison:
(r1 <= r(i)) <= r2
You mean:
elseif r1 <= r(i) && r(i) <= r2
Now consider that you overwrite Sij in each iteration. So you get the last value as output only. Maxbe you want a vector instead? This will work easier without a loop:
decay0 = 1.0;
decay1 = 0.0;
decay2 = 2.22;
r0 = 50;
r1 = 50;
r2 = 150;
r = rrup;
S = nan(size(r));
index = (r < r0);
S(index) = (r0 ./ r) .^ decay0;
index = (r1 <= r & r <= r2);
S(index) = (r1 ./ r) .^ decay1;
index = (r > r2);
S(index) = (r2 ./ r) .^ decay2;
