A useful practice, in my experience, is to implement R code samples in some other language, like C/D. Implementing lower level math functions for oneself can also be fun instead of relying on a library, depending on one's ultimate goals/interests.
For example, I would assume that creating a mean function using numbers and operators would be language-agnostic.
- C: You will create the mean function
It's the difference between using a calculator and knowing how to multiply two 5 digit numbers with pen and paper.
You might say, "Oh but we can do that in R". Well, if you start doing that one function at a time, very soon you find yourself with R grinding to a halt.