I am learning CIS 198 2016 fall course and working on the homework 02. I finished part of it but saw compiling error "cannot assign to immutable field
I did some search but failed to solve it yet. You could find my code at https://play.rust-lang.org/?gist=12a7963f055b6cc5c8ae09c34b24e2c9&version=stable&backtrace=0
Any tips are appropriated.