parts of data frames: subset vs. [-c()]

From: "Stefan Th. Gries" <stgries_lists at arcor.de> writes:

I have a problem with splitting up a data frame called ReVerb: I would like to extract all cases where SYNTAX=="Ditrans" from ReVerb, store that in a file, and then generate ReVerb again without these cases and factor levels. My problem is probably obvious from the following lines of code:

ditrans<-which(SYNTAX=="Ditrans")
ReVerb1<-ReVerb[-c(ditrans),]; dim(ReVerb1)

[1] 91532    16
# ok, so the 92713-91532=1181 cases where SYNTAX=="Ditrans" have been removed, but ...

ReVerb1<-subset(ReVerb, SYNTAX!="Ditrans"); dim(ReVerb1)

[1] 91528    16
# ... so why don't I get 91532 again as the number of rows?
# Any ideas??

From: Peter Dalgaard <p.dalgaard at biostat.ku.dk>
The SYNTAX variable is not necessarily the same. Could you retry the first case with
ditrans <- which(ReVerb$SYNTAX=="Ditrans")
?

The results were the same as with 'ditrans<-which(SYNTAX=="Ditrans")'.

Otherwise, try doing a setdiff() on the rownames of the two discrepant results and see which are the four cases that differ.

This solved the issue: Using setdiff, I found that the cases that the second way with subset fails to include are NA's ... - I was not aware of how subset treats NA, sorry.

Thanks a lot,
STG
--
Stefan Th. Gries
----------------------------------------
Max Planck Inst. for Evol. Anthropology
http://people.freenet.de/Stefan_Th_Gries
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