Changes inside a function sometimes leak outside, sometimes not

Hello
After much work I was able to locate my problem to a small example:

library(data.table)

dat <- data.frame(col1=c(1,2), col2=c(3,4))

f1 <- function(d) { setDT(d); d[TRUE,   col2:=c(5,6)] }
f2 <- function(d) { setDT(d); d[col1<3, col2:=c(5,6)] }

f1(dat)
dat   # not changed:
#    col1 col2
# 1:    1    3
# 2:    2    4

f2(dat)
dat    # changed:
#    col1 col2
# 1:    1    5
# 2:    2    6

I read that data.table operations are by reference, but not sure what that means when a data.frame becomes a data.table inside a function. Are changes to this new born data.table supposed to apply to the external copy or not?
Either way I hope one of those can be done in all cases, otherwise it is very confusing.

Thank you very much for your good package!

# Output of sessionInfo()

sessionInfo()
R version 4.1.2 (2021-11-01)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 22000)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252 LC_CTYPE=English_United States.1252 LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C LC_TIME=English_United States.1252

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

other attached packages:
[1] data.table_1.14.2

loaded via a namespace (and not attached):
[1] compiler_4.1.2 tools_4.1.2

@OfekShilon

@OfekShilon could you PTAL? this looks similar to your recent contributions around functions using data.table operations on data.framez

@MichaelChirico

@MichaelChirico Thanks for noticing! Just verified that PR #4978 indeed solves this. It's not so recent - 9M+ old... Hopefully it can be merged now.

"PR" means there is a fix already on the way? Sorry I am new to github.

you can follow the link to see a fix. it's not yet merged -- Matt's OS bandwidth has been pretty limited of late.

In principle you could install from that branch to fix your issue.

generally I recommend using as.data.table() inside functions instead of setDT

Thank you, I will try to install the fixed version. In my real work the "d" is large so I was hoping not to copy it - but if I won't succeed in installing the fix that is what I will do. Anyway if you're already aware of this issue feel free to close this as a duplicate of #4978 . Thanks!

depending on your use case you might also try data.table:::shallow. the usual caution with private functions applies

@mattdowle

@mattdowle Any chance of merging #4978?

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Changes inside a function sometimes leak outside, sometimes not #5330

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Changes inside a function sometimes leak outside, sometimes not #5330

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MichaelChirico commented on Feb 12, 2022

OfekShilon commented on Feb 12, 2022

zigfried76 commented on Feb 12, 2022

MichaelChirico commented on Feb 12, 2022

zigfried76 commented on Feb 12, 2022

MichaelChirico commented on Feb 12, 2022

OfekShilon commented on Mar 15, 2022

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