What does ".N" means in data table in r?

Question

I have a data.table dt:

library(data.table)
dt = data.table(a=LETTERS[c(1,1:3)],b=4:7)

   a b
1: A 4
2: A 5
3: B 6
4: C 7

The result of dt[, .N, by=a] is

   a N
1: A 2
2: B 1
3: C 1

I know the by=a or by="a" means grouped by a column and the N column is the sum of duplicated times of a. However, I don't use nrow() but I get the result. The .N is not just the column name? I can't find the document by ??".N" in R. I tried to use .K, but it doesn't work. What does .N means?

Answer 1

Think of .N as a variable for the number of instances. For example:

dt <- data.table(a = LETTERS[c(1,1:3)], b = 4:7)

dt[.N] # returns the last row
#    a b
# 1: C 7

Your example returns a new variable with the number of rows per case:

dt[, new_var := .N, by = a]
dt
#    a b new_var
# 1: A 4       2 # 2 'A's
# 2: A 5       2
# 3: B 6       1 # 1 'B'
# 4: C 7       1 # 1 'C'

For a list of all special symbols of data.table, see also https://www.rdocumentation.org/packages/data.table/versions/1.10.0/topics/special-symbols