如何获取data.table分组中当前组的长度？

Question

I know this can be achieved with other packages, but I'm trying to do it in data.table (as it seems to be the fastest for grouping).

library(data.table)
dt = data.table(a=c(1,2,2,3))
dt[,length(a),by=a]

results in

   a V1
1: 1  1
2: 2  1
3: 3  1

whereas

df = data.frame(a=c(1,2,2,3))
ddply(df,.(a),summarise,V1=length(a))

produces

  a V1
1 1  1
2 2  2
3 3  1

which is a more sensible results. Just wondering why data.table is not giving the same results, and how this can be achieved.

Answer 1

The data.table way to do this is to use special variable, .N , which keeps track of the number of rows in the current group. (Other special variables include .SD , .BY (in version 1.8.2) and .I and .GRP (available from version 1.8.3). All are documented in ?data.table ):

library(data.table)
dt = data.table(a=c(1,2,2,3))

dt[, .N, by = a]
#    a N
# 1: 1 1
# 2: 2 2
# 3: 3 1

To see why what you tried didn't work, run the following, checking the value of a and length(a) at each browser prompt:

dt[, browser(), by = a]