[educationdata]

The second example should be written like this:

  diamondsDT[cut != "Fair", .(AvgPrice = mean(price),
                              MedianPrice = as.numeric(median(price)),
                              Count = .N), cut][order(-Count)]

There is no need to break it to 10 lines.

[cwyers]

I honestly find that less readable than Hadley's version, especially turning "by = cut" into "cut." This is where terseness really cuts into readability (and positional arguments is one of my least favorite features about R in terms of long-term readability of code).