[datatable-help] Merging problem when column in one DT is factor and in the other character

[Christoph Jäckel]

I have raised a bug report under, which can be found under #1922.

On Mon, Apr 2, 2012 at 2:17 AM, Matthew Dowle <mdowle at mdowle.plus.com>wrote:

>
> Thanks for example and data, very clear.
>
> Yes, problem looks to be factors with unused levels, when joined to a
> character column, as you suggested. Work arounds are to drop the unused
> levels or convert to character, as you found.
>
> A fix is a bit more involved and won't have time for a while. Please
> could you file a bug report so it doesn't get forgotten.
>
> Thanks.
>
> On Fri, 2012-03-30 at 16:51 +0100, Matthew Dowle wrote:
> > Hi. A quick read suggests it's not intended and that's a bug. Just
> convert
> > the columns to character for now, and it should work. Character columns
> > are now preferred going forward, so I'd be suggesting that anyway even if
> > it worked.
> >
> > > So in case this is expected behavior, should data.table
> > > give at least a warning that says something like "You join two
> > > data.tables whereby one keyed column is a factor, one is a character.
> > > That is probably not your intention. Convert the factor column to
> > > character or vice versa."?
> >
> > Yes. It should be converting to character (with a warning) in this case.
> > Thought that's what I coded and tested. Will investigate...
> >
> > > Hi together,
> > >
> > > here is the problem I needed dput for:
> http://www.fileuploadx.de/287440
> > > (sorry,
> > > I know that this filehoster is annoying because you have to wait until
> you
> > > can download the file; I hope you have a coffee machine close by ;-)
> > >
> > > In this attachment, I basically load in two data.tables DT1 and DT2
> that I
> > > want to join, i.e. DT2[DT1], according to the keyed columns
> "Company_Code"
> > > and "intDatum" in DT1 and "DSCD" and "intDatum" in DT2. However, while
> > > "DSCD" is formatted as a character-column, "Company_Code" is formatted
> as
> > > a
> > > factor-column. As you can see from the long structure-object, there are
> > > plenty of levels here (the actual data.tables are very small).
> > >
> > > Now, when I try to join those with DT2[DT1], I get:
> > >
> > >
> > >         DSCD intDatum MONTH MV SICClass
> > >  [1,] 997859      151  <NA> NA       44
> > >  [2,] 997859      152  <NA> NA       44
> > >  [3,] 998064      151  <NA> NA       15
> > >  [4,] 998064      152  <NA> NA       15
> > >  [5,] 142268      151  <NA> NA       53
> > >  [6,] 142268      152  <NA> NA       53
> > >  [7,] 142859      151  <NA> NA       56
> > >  [8,] 142859      152  <NA> NA       56
> > >  [9,] 143415      151  <NA> NA       63
> > > [10,] 143415      152  <NA> NA       63
> > > [11,] 307045      151  <NA> NA       15
> > > [12,] 307045      152  <NA> NA       15
> > >
> > >
> > >
> > > Basically, data.table finds no values for MV and MONTH for any DSCD
> > > and intDatum combination. However, as DT2[DSCD=="142268"] clearly
> > > shows, there are values for that DSCD:
> > >
> > >
> > >
> > >        DSCD      MONTH      MV intDatum
> > > [1,] 142268 1997-08-28 1901.12      151
> > > [2,] 142268 1997-09-28 1829.00      152
> > >
> > >
> > >
> > > Those, however, only show up in the join after i get rid of all the
> > > unused levels (equivalently, I can also transform the Company_Code to
> > > a character column):
> > >
> > >
> > > DT1[, Company_Code := factor(Company_Code)]
> > > DT2[DT1]
> > >
> > >
> > >         DSCD intDatum      MONTH      MV SICClass
> > >  [1,] 997859      151       <NA>      NA       44
> > >  [2,] 997859      152       <NA>      NA       44
> > >  [3,] 998064      151       <NA>      NA       15
> > >  [4,] 998064      152       <NA>      NA       15
> > >  [5,] 142268      151 1997-08-28 1901.12       53
> > >  [6,] 142268      152 1997-09-28 1829.00       53
> > >  [7,] 142859      151       <NA>      NA       56
> > >  [8,] 142859      152       <NA>      NA       56
> > >  [9,] 143415      151       <NA>      NA       63
> > > [10,] 143415      152       <NA>      NA       63
> > > [11,] 307045      151       <NA>      NA       15
> > > [12,] 307045      152       <NA>      NA       15
> > >
> > >
> > >
> > > I'm pretty sure this behaviour occurred only with version 1.8.0,
> > > probably because data.table coerced every key to factor before (see
> > > the NEWS to 1.8.0). So my question is: Is what happens here intended
> > > behavior? I'm honest with you: I'm working now for a while with R and
> > > factors are one of those things that I never got. I just don't see
> > > their use and every so often they cause me huge problems (as in this
> > > case). So I'm probably making something stupid here. The nasty thing
> > > about this issue here is that mostly, however, the joins just work as
> > > expected (believe me, I tried to produce a simple example with one
> > > column factor and one character that would reproduce this behavior,
> > > but no matter what I did, the joins afterwards always worked as
> > > expected). So in case this is expected behavior, should data.table
> > > give at least a warning that says something like "You join two
> > > data.tables whereby one keyed column is a factor, one is a character.
> > > That is probably not your intention. Convert the factor column to
> > > character or vice versa."?
> > >
> > >
> > >
> > > Thanks for your help!
> > >
> > >
> > >
> > > Christoph
> > > _______________________________________________
> > > datatable-help mailing list
> > > datatable-help at lists.r-forge.r-project.org
> > >
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
> >
> >
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> >
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>
>
>
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