prints complete table with data.table when update by reference

---
title: "copy this into a Rmd"
author: "Henk Harmsen"
date: "September 10, 2015"
output: html_document

---

```{r}
library(data.table)

data(mtcars)
df = data.table(mtcars)
```

this works, no intention to print/head table:

```{r}
df[, mpg2 := mpg * 2]
```

but after loading printr, update by reference prints out the whole table:

```{r}
library(printr)
df[, mpg3 := mpg2 / 2]
```

[yihui]

For some reason, the table is still printed even before printr is loaded when I tried your example. My session info:

> library(data.table)
data.table 1.9.4  For help type: ?data.table
*** NB: by=.EACHI is now explicit. See README to restore previous behaviour.
> library(knitr)
> sessionInfo()
R version 3.2.2 (2015-08-14)
Platform: x86_64-apple-darwin14.5.0 (64-bit)
Running under: OS X 10.10.5 (Yosemite)

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] knitr_1.11.1     data.table_1.9.4

loaded via a namespace (and not attached):
 [1] magrittr_1.5      plyr_1.8.3        rsconnect_0.4.1.4 htmltools_0.2.9  
 [5] tools_3.2.2       reshape2_1.4.1    yaml_2.1.13       Rcpp_0.12.1      
 [9] stringi_0.5-5     rmarkdown_0.8     stringr_1.0.0     digest_0.6.8     
[13] chron_2.3-47     

A similar question has been asked before: http://stackoverflow.com/q/15267018/559676

the stackoverflow post refers to knitr, apparently in combination with data.table 1.9.4.
the table print problem does not occur in knitr / data.table 1.9.5 [that's why you couldn't reproduce my output [sorry for not including sessionInfo()]].

however, now the problem seems to be the printr / data.table 1.9.5 combination. I have posted the issue at data.table:
Rdatatable/data.table#1326

[yihui]

Right. data.table 1.9.5 is not on CRAN, and you didn't mention your session info, so I missed it.

Similarly, I guess the data.table author will be confused if you don't point out where you got the printr package.

[m-dz]

Just to let you know, the issue is still open with data.table 1.9.6 (on CRAN).

---
title: "test"
output:
  html_document
---

```{r load data}
library(printr)
library(data.table)
mtcarsDT <- data.table(mtcars)
```

```{r chunk}
mtcarsDT[, gear := factor(gear)]
```

sessionInfo():

R version 3.2.5 (2016-04-14)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1

locale:
[1] LC_COLLATE=English_United Kingdom.1252
[2] LC_CTYPE=English_United Kingdom.1252
[3] LC_MONETARY=English_United Kingdom.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United Kingdom.1252

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

other attached packages:
[1] data.table_1.9.6 printr_0.0.5

loaded via a namespace (and not attached):
[1] magrittr_1.5 formatR_1.3 tools_3.2.5 htmltools_0.3.5
[5] yaml_2.1.13 Rcpp_0.12.4 stringi_1.0-1 rmarkdown_0.9.5
[9] knitr_1.12.3 stringr_1.0.0 digest_0.6.9 chron_2.3-47
[13] evaluate_0.8.3

[yihui]

Thanks for the report. I don't understand the data.table printing magic, and I guess we will need the help from Matt Dowle.

[m-dz]

It looks like something related to this commit in data.table.