[datatable-help] rbindlist and factors

[Alexandre Sieira]

I think I found an unexpected behavior with rbindlist when columns are factors:

> dt1 = data.table(a=as.factor(c("a", "a", "a")))

> dt1
   a
1: a
2: a
3: a
> str(dt1)
Classes ‘data.table’ and 'data.frame':	3 obs. of  1 variable:
 $ a: Factor w/ 1 level "a": 1 1 1
 - attr(*, ".internal.selfref")=<externalptr> 
> dt2 = data.table(a=as.factor(c("b", "b", "b")))
> dt2
   a
1: b
2: b
3: b
> str(dt2)
Classes ‘data.table’ and 'data.frame':	3 obs. of  1 variable:
 $ a: Factor w/ 1 level "b": 1 1 1
 - attr(*, ".internal.selfref")=<externalptr> 

If I rbind them, I get the expected value - a table with 6 rows, 3 of which have value "a" and 3 with value "b":


> rbind(dt1, dt2)
   a
1: a
2: a
3: a
4: b
5: b
6: b


So if I do rbindlist(list(dt1, dt2)), I would expect to get the exact same result, only faster. Unfortunately, that is not the case:


> rbindlist(list(dt1, dt2))
   a
1: a
2: a
3: a
4: a
5: a
6: a

> str(rbindlist(list(dt1, dt2)))
Classes ‘data.table’ and 'data.frame':	6 obs. of  1 variable:
 $ a: Factor w/ 1 level "a": 1 1 1 1 1 1
 - attr(*, ".internal.selfref")=<externalptr> 


This was executed with R 3.0.1 and data.table 1.8.8 on a Mac OS X 10.8.3.

Is this expected behavior? Am I missing something?



-- 
Alexandre Sieira
CISA, CISSP, ISO 27001 Lead Auditor

"The truth is rarely pure and never simple."
Oscar Wilde, The Importance of Being Earnest, 1895, Act I
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