[R] Is this an artifact of using "which"?
Peter Dalgaard
P.Dalgaard at biostat.ku.dk
Mon Apr 14 15:03:15 CEST 2008
Richard.Cotton at hsl.gov.uk wrote:
>> I used "which" to obtain a subset of values from my data.frame.
>> however, I find that there is a "trace" of the values I have removed.
>> Any suggestions would be greatly appreciate.
>>
>> Below is my data:
>>
>> d <- data.frame( val = 1:10,
>> group = sample(LETTERS[1:5], 10, repl=TRUE) )
>>
>> >d
>> val group
>> 1 1 B
>> 2 2 E
>> 3 3 B
>> 4 4 C
>> 5 5 A
>> 6 6 B
>> 7 7 A
>> 8 8 E
>> 9 9 E
>> 10 10 A
>>
>> ## selecting everything that is not group "A"
>> d<-d[which(d$group !="A"),]
>>
>> > d
>> val group
>> 1 1 B
>> 2 2 E
>> 3 3 B
>> 4 4 C
>> 6 6 B
>> 8 8 E
>> 9 9 E
>>
>> > levels(d$group)
>> [1] "A" "B" "C" "E"
>>
>
> The (imho) unintuitive behaviour is to do with the subsetting function
> [.factor, not which. There are a couple of workarounds:
>
In that case, your intuition needs readjustment....
There are other systems which (de facto) drop unused levels by default,
and it is a real pain to work around, especially for subgroup analyses.
E.g. there is no way to get PROC FREQ in SAS to include a count of zero,
and barplots of ratings fro 0 to 10 lose columns "randomly" in SPSS
(this _can_ be worked around, though).
Anyways, it is illogical: There's no reason that a tabulation of gender
distribution for (say) tenured CS professors should suddenly pretend
that the female gender does not exist!
> 1. Call factor to recreate the levels, and get rid of "A"
> factor(d$group)
>
> 2. Redefine [.factor; see dropUnusedLevels in the Hmisc package.
>
> Regards,
> Richie.
>
> Mathematical Sciences Unit
> HSL
>
>
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