Updated section that demonstrates loop variable can be reassigned. Also added link to lesson on data structures instead of discussing them here.
Updated challenge to write function that prints natural numbers. Remove superflous examples of seq, similar to proposed changes to novice Python lessons.
Updated challenge to write function that performs exponentionation.
Removed exercise on reversing a string. Doing this with a vector in R would require more than what we taught them at this point, and even more if they were to properly do it pre-allocating the new vector. Furthermore, this challenge section already contains two substantial exercises.