how to avoid an optimization warning in data.table

Question

I have the following code:

> dt <- data.table(a=c(rep(3,5),rep(4,5)),b=1:10,c=11:20,d=21:30,key=\"a\")
> dt
    a  b  c  d
 1: 3  1 11 21
 2: 3  2 1         


        

Answer 1

One way I could think of is to assign count by reference:

dt.out <- dt[, lapply(.SD,sum), by = a]
dt.out[, count := dt[, .N, by=a][, N]]
# alternatively: count := table(dt$a)

#    a  b  c   d count
# 1: 3 15 65 115     5
# 2: 4 40 90 140     5

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Edit 1: I still think it's just message and not a warning. But if you still want to avoid that, just do:

dt.out[, count := as.numeric(dt[, .N, by=a][, N])]

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Edit 2: Very interesting. Doing the equivalent of multiple := assignment does not produce the same message.

dt.out[, `:=`(count = dt[, .N, by=a][, N])]
# Detected that j uses these columns: a 
# Finding groups (bysameorder=TRUE) ... done in 0.001secs. bysameorder=TRUE and o__ is length 0
# Detected that j uses these columns:  
# Optimization is on but j left unchanged as '.N'
# Starting dogroups ... done dogroups in 0 secs
# Detected that j uses these columns: N 
# Assigning to all 2 rows
# Direct plonk of unnamed RHS, no copy.