A minor tip : I use the hashtag #learning to annotate pieces of code which I can then revisit using a search (could be just grep '#learning' *.R | grep data.table, say) in case I'm stuck. This could work with any language, of course, but in the initial days, I found it very useful with R in particular given it's idiosyncrasies. (For the oldies on here, I grew up in the era of del.icio.us, so hashtag-ging code felt like a natural-but-novel idea :-) )

Examples:

#learning : a data.frame is a list. x = df with 10 rows, 21 columns, say. as.list(x) gives you a list with 21 elements, one per column

#learning : Above = getting a row and it's previous row using .I() in data.table