No realloc warning when adding many inside a function #1633

packageVersion("data.table")
# [1] ‘1.17.99’
x = data.table(a=1:3)
ncol = truelength(x)
ncol
# [1] 1025

f = function(DT) {
 for (i in seq_len(ncol + 100L)) {
   DT[,paste0("new",i):=1:3]
 }
}
f(x)            # I thought there was a warning here that := on i==ncol did a realloc
dim(x)
# [1]   3 1025       # wrong result

f = function(DT) {
  DT[, paste0("new", seq_len(ncol + 100L)) := 1:3]
  NULL
}
f(x)
dim(x)
# [1]   3 1025       # wrong result

f = function(DT) {
 for (i in seq_len(ncol + 100L)) {
   DT[,paste0("new",i):=1:3]
 }
 DT                            # needed in this case
}
x = f(x)                       # x= needed in this case
dim(x)
# [1]   3 1126                      # correct result

x = data.table(a = 1:3)
f = function(DT) {
  for (j in seq_len(ncol + 100L)) set(DT, NULL, paste0("new", j), 1:3)
}
f(x)
# Error in set(DT, NULL, paste0("new", j), 1:3) : 
#   Internal error in assign: input dt has not been allocated enough column slots. l=1025, tl=1025, adding 1. Please report to the data.table issues tracker.
# dim(x)