Varying behavior in print based on value of i

[rsaporta]

When assigning :=, output is given if i has no matches, whereas output is suppressed if i does have a match.

eg:

set.seed(1)
DT <- data.table(A=1:3, Val=rnorm(3))
DT[ A < 3, Val := 0]  # Some rows matched, no output printed
DT[ A > 3, Val := 0]  # Zero rows matched, output was printed
#     A       Val
#  1: 1  0.000000
#  2: 2  0.000000
#  3: 3 -0.835629

related to #1122

---

It is worth noting that this could be useful when working interactively. EG, if the user is expecting to modify the DT and output is issued it serves as an indirect/subtle warning that there was no change to the table.

This being said, if we desire that behavior perhaps we should insert a check and proper warning.

[franknarf1]

Related, regarding similar warnings for failure of := to make changes: #354

[mattdowle]

Already fixed. Item 2 on the v1.9.5 bug fix list. Checked tests were added to tests/autoprint.R in 84766f4

2 DT[FALSE,LHS:=RHS] no longer prints either, #887. Thanks to Jureiss for reporting.