:= gets printed
library(data.table)
cars <- data.table(cars)
cars[ , test := rnorm(50)]
## speed dist test
## 1: 4 2 0.42487857
## 2: 4 10 -1.54678396
## 3: 7 4 1.26573903
## 4: 7 22 -0.05582449
## 5: 8 16 -1.09113291
## 6: 9 10 0.11685771
## 7: 10 18 -0.56852832
## 8: 10 26 0.47101954
## 9: 10 34 0.64398565
## 10: 11 17 -1.17438003
## 11: 11 28 1.31262343
## 12: 12 14 0.15020831
## 13: 12 20 -0.43420778
## 14: 12 24 -0.38025850
## 15: 12 28 0.54639997
## 16: 13 26 -0.53175668
## 17: 13 34 0.68741143
## 18: 13 34 -0.48723929
## 19: 13 46 -0.50722729
## 20: 14 26 -0.68792485
## 21: 14 36 0.36397453
## 22: 14 60 1.14673547
## 23: 14 80 0.74616270
## 24: 15 20 0.24726562
## 25: 15 26 -1.90988763
## 26: 15 54 -0.34072266
## 27: 16 32 0.47935956
## 28: 16 40 2.06894708
## 29: 17 32 0.52231184
## 30: 17 40 -0.13955539
## 31: 17 50 0.08701245
## 32: 18 42 0.60517871
## 33: 18 56 -0.76668196
## 34: 18 76 1.55025526
## 35: 18 84 1.79224226
## 36: 19 36 -1.73026703
## 37: 19 46 1.70613838
## 38: 19 68 -1.49811376
## 39: 20 32 0.22731891
## 40: 20 48 -0.43305643
## 41: 20 52 -1.33829450
## 42: 20 56 0.19028345
## 43: 20 64 1.49504192
## 44: 22 66 1.44899183
## 45: 23 54 -0.13699308
## 46: 24 70 -1.48371385
## 47: 24 92 -0.01923215
## 48: 24 93 0.45491672
## 49: 24 120 1.15459892
## 50: 25 85 0.30398326
## speed dist test
:= does not print if <- s used
cars <- cars[ , test2 := rnorm(50)]
sessionInfo()
## R version 3.3.1 (2016-06-21)
## Platform: x86_64-apple-darwin15.0.0 (64-bit)
## Running under: OS X 10.12.1 (Sierra)
##
## 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_1.9.6
##
## loaded via a namespace (and not attached):
## [1] backports_1.0.4 magrittr_1.5 RevoUtils_10.0.1 rprojroot_1.1
## [5] tools_3.3.1 htmltools_0.3.5 yaml_2.1.14 Rcpp_0.12.8
## [9] stringi_1.1.2 rmarkdown_1.2 knitr_1.15.1 stringr_1.1.0
## [13] digest_0.6.10 chron_2.3-47 evaluate_0.10