[datatable-help] In 1.9.2, By with factor column do not work the same as in 1.8.10

[DERVIEUX Christophe]

Hi,

I have updated data.table package to 1.9.2 recently from 1.8.10 and I found errors on my previous code.

See reproductible example below:

On 1.8.10 :
DT<-data.table(X=factor(2006:2012),Y=rep(1:7,2))
DT[,Z:=paste(X,.N,sep=" - "),by=list(X)][]

X Y Z
1: 2006 1 2006 - 2
2: 2007 2 2007 - 2
3: 2008 3 2008 - 2
4: 2009 4 2009 - 2
5: 2010 5 2010 - 2
6: 2011 6 2011 - 2
7: 2012 7 2012 - 2
8: 2006 1 2006 - 2
9: 2007 2 2007 - 2
10: 2008 3 2008 - 2
11: 2009 4 2009 - 2
12: 2010 5 2010 - 2
13: 2011 6 2011 - 2
14: 2012 7 2012 - 2

In column Z, I get the level of the factor column X
pasted with count '.N' as expected

However, in the 1.9.2, with same code :
DT<-data.table(X=factor(2006:2012),Y=rep(1:7,2))
DT[,Z:=paste(X,.N,sep=" - "),by=list(X)][]

X Y Z
1: 2006 1 1 - 2
2: 2007 2 2 - 2
3: 2008 3 3 - 2
4: 2009 4 4 - 2
5: 2010 5 5 - 2
6: 2011 6 6 - 2
7: 2012 7 7 - 2
8: 2006 1 1 - 2
9: 2007 2 2 - 2
10: 2008 3 3 - 2
11: 2009 4 4 - 2
12: 2010 5 5 - 2
13: 2011 6 6 - 2
14: 2012 7 7 - 2

as results, I do not get levels of factor column X but the numeric values associated with the level.

is this working normally? Why has it changed? Is that a bug?

I use this kind of procedure to make labels for ggplot. All my previous code is not working anymore. It's kind of annoying.

Thanks

Christophe

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