[datatable-help] ":=" with "by" reassignment/updation + adding new column leads to crash

[Matthew Dowle]

Hi, 

Yes I get that in latest dev too. Thanks for the nice example,
please file. 

Matthew 

On 12.05.2013 08:53, Arunkumar Srinivasan
wrote: 

> Hi, 
> I just discovered some weird R-session crash in
data.table. Here's an example to reproduce the crash. I did not find any
bug filed regarding this issue. Maybe others can verify this? Then I'll
file it as a bug. 
> The issue is this. Suppose you've a data.table with
two columns x and y as follows: 
> require(data.table) 
> DT <-
data.table(x = rep(1:2, c(3,2)), y = 6:10) 
> 
> x y 
> 1: 1 6 
> 2: 1 7

> 3: 1 8 
> 4: 2 9 
> 5: 2 10 
> Now you want to add a new column "z"
by reference grouped by "x". So, you'd do: 
> 
> DT[, `:=`(z = .GRP), by
= x] 
> 
> x y z 
> 1: 1 6 1 
> 2: 1 7 1 
> 3: 1 8 1 
> 4: 2 9 2 
> 5: 2
10 2 
> Now, for the sake of producing this error, assume that you
assigned "z" the wrong value and that you want to change it. But, also
you realised that you want to add another column "w" as well. So, you go
ahead and do (remember to do the previous step and then this one): 
>
DT[, `:=`(z = .N, w = 2), by = x] # R session crashes 
> Here, both R
and Rstudio session crashes with the traceback message: 
> 
> *** caught
segfault *** 
> address 0x0, cause 'memory not mapped' 
> Traceback: 
>
1: `[.data.table`(DT, , `:=`(z = .GRP, w = 2), by = x) 
> 2: DT[, `:=`(z
= .GRP, w = 2), by = x] 
> This on the other hand works as expected if
you assign both columns the first time. 
> 
> require(data.table) 
> DT
<- data.table(x = rep(1:2, c(3,2)), y = 6:10) 
> DT[, `:=`(z = .GRP, w =
2), by = x] # works fine 
> That is, if you assign by reference (:=)
with "by" and re-assign a variable while also creating another variable,
there seems to be a segfault. This error may not be limited to this
case, but that I've just tested this. 
> Here's my sessionInfo() from
before the crash: 
> 
> R version 3.0.0 (2013-04-03) 
> Platform:
x86_64-apple-darwin10.8.0 (64-bit) 
> 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_1.8.8 
>
loaded via a namespace (and not attached): 
> [1] tools_3.0.0 
> Best,

> Arun

 
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