The final four chapters cover two general programming techniques: finding and fixing bugs, and finding and fixing performance issues. Tools to measure and improve performance are particularly important because R is not a fast language. This is not an accident: R was purposely designed to make interactive data analysis easier for humans, not to make computers as fast as possible. While R is slow compared to other programming languages, for most purposes, it’s fast enough. These chapters help you handle the cases where R is no longer fast enough, either by improving the performance of your R code, or by switching to a language, C++, that is designed for performance.
Chapter 22 talks about debugging, because finding the root cause of error can be extremely frustrating. Fortunately R has some great tools for debugging, and when they’re coupled with a solid strategy, you should be able to find the root cause for most problems rapidly and relatively painlessly.
Chapter 23 focuses on measuring performance.
Chapter 24 then shows how to improve performance.