Unexpected result when creating a factor column with 'by=' present

[ben519]

For example,

library(data.table)

dt <- data.table(
  id = 1:9,
  grp = rep(1:3, each = 3),
  val = c("a","b","c",   "a","b","c",   "a","b","c")
)

dt[, valfactor1 := factor(val), by = grp]  # fine and dandy
dt[, valfactor2 := factor(val), by = id]   # breaks

dt
   id grp val valfactor1 valfactor2
1:  1   1   a          a          c
2:  2   1   b          b          c
3:  3   1   c          c          c
4:  4   2   a          a          c
5:  5   2   b          b          c
6:  6   2   c          c          c
7:  7   3   a          a          c
8:  8   3   b          b          c
9:  9   3   c          c          c

I suspect this has come up before, but I didn't see anything on it. Also, I'm not sure what a reasonable behavior would be if I was trying to assign factors by group with ordered levels. At the least, I think a warning in these cases would be helpful. Right now it silently does something strange.

Thanks!

sessionInfo()

R version 3.4.2 (2017-09-28)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS Sierra 10.12.6

Matrix products: default
BLAS: /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.4/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.10.5

loaded via a namespace (and not attached):
[1] compiler_3.4.2 tools_3.4.2    yaml_2.1.15

[franknarf1]

Seems related to #2199

[Henrik-P]

I assume this is the same underlying issue, but I thought I just as well could post it here for reference. I stumbled over it when I needed a factor version of .GRP for plotting purposes.

d <- data.table(x = rep(letters[c(3, 1, 2)], each = 2))
d[ , `:=`(
  g = .GRP,
  f = factor(.GRP)),
  by = x]

#    x g f
# 1: c 1 3
# 2: c 1 3
# 3: a 2 3
# 4: a 2 3
# 5: b 3 3
# 6: b 3 3