Rust: Iterators look forwards and backwards in for loop

I’m trying to make a for loop more rust like. It currently looks like this:

for i in 0..length{
    b = d(i) / (d(i) + d(i+1));
    c = d(i) / (d(i-1) + d(i));

I was looking at using the next() with iterator but I can’t find a method to access the previous value.