R can legitimately be called both a programming language and a statistical package. Many books address both the programming and statistical components of R, but invariably the discussion of statistical topics is more detailed than the discussion of programming capabilities. As a supplement, I’ve started the list of links below. Each of these sources deals specifically and almost exclusively with the the programming aspects of R: objects, arrays, loops and conditional statements, custom functions, debugging, and so on. I’ll add to this list as I become aware of other sites. Please feel free to suggest additional sites in the comments.
- Programming in R
- Programming in R II (same title, but different content)
- R language definition (html)
- R language definition (pdf)
- Writing R Extensions
- R Internals
- R Inferno
For a list of complementary sites that focus on statistical principles and research methods (several deal specifically with R) read this article.