[R] x[x$a=="q",,drop=TRUE]

[Gabor Grothendieck]

On 6/1/05, Mikkel Grum <mi2kelgrum at yahoo.com> wrote:
> I'm trying to select a subset of a dataframe while
> dropping some factors. While the dataset gets smaller
> all Factor levels remain and I need to get rid of
> them.  Strangely enough, I am almost certain that the
> same code on the same data worked OK earlier today -
> and it is not the first time that I'm not able to
> replicate earlier results with this command (I know, I
> might just be going crazy). What am I doing wrong?
> 
> I'm working on Windows Server 2003, R 2.1.0
> (2005-04-18).
> 
> > str(spray)
> `data.frame':   370 obs. of  7 variables:
>  $ PD        : Factor w/ 8 levels
> "Botrytis","Downy",..: 2 2 2 2 4 2 2 5 5 5 ...
>  $ postSpmtsQ: num  1309 1309  384  384 1044 ...
>  $ ante62Q   : num  284 284 218 218 366 ...
>  $ ante08Q   : num  331 331 228 228 492 ...
>  $ ante29Q   : num   297  297 1067 1067 1034 ...
>  $ ante16Q   : num  0 0 0.2 0.2 0 0 0 6.7 0 31.5 ...
>  $ Trt       : Factor w/ 41 levels "Acrobat MZ WP",..:
> 27 5 27 5 36 27 5 24 24 24 ...
> > sprayS <- spray[spray$PD == "Spidermites", , drop =
> TRUE]
> > str(sprayS)
> `data.frame':   111 obs. of  7 variables:
>  $ PD        : Factor w/ 8 levels
> "Botrytis","Downy",..: 5 5 5 5 5 5 5 5 5 5 ...
>  $ postSpmtsQ: num  13395 31588 84254   136   619 ...
>  $ ante62Q   : num   1357 21187 21819   218   237 ...
>  $ ante08Q   : num    973 21740 25112   228   134 ...
>  $ ante29Q   : num    2103 106970  66676   1067    119
> ...
>  $ ante16Q   : num  6.7 0 31.5 0.2 0 0 0 0 14.3 0 ...
>  $ Trt       : Factor w/ 41 levels "Acrobat MZ WP",..:
> 24 24 24 24 24 24 24 24 24 24 ...
> > table(sprayS$Trt)
> 
>    Acrobat MZ WP           Agrifos      Apollo 50 SC
>          CALMAG
>                0                 0                13
>               0
>            DM-31    Dynamec 1.8 EC   Equation Pro DF
>       Evisect S
>                0                13                 0
>               0
>            Flint         Floramite           Impulse
>          Karate
>                0                15                 0
>               0
>      Karate zeon            Melody    Meltatox 40 EC
>             MKP
>                0                 0                 0
>               0
>         Molasses       Nembicidine     Nimrod 250 EC
>  Nissorun 10 EC
>                0                 0                 0
>              12
>           Oberon     Orthene 75 WP       Oscar 20 SC
>         Pegasus
>               15                 0                 9
>              26
>     Polar 50 WSG            Potfos          Proplant
>           Pyrus
>                0                 0                 0
>               0
> Ridomil MZ 63 5WP   Rovral aqua flo      Score 250 EC
>    Secure 36 SC
>                0                 0                 0
>               8
>      Sequestrone          Shavit f         Sporekill
>    Stroby 50 WG
>                0                 0                 0
>               0
>           Switch            Tracer          Trafos K
>        Vandozeb
>                0                 0                 0
>               0
>          Vitomex
>                0
> 
> cheers,
> Mikkel
> 

Your code says to drop dimensions whereas you want to drop factor 
levels (I think).

For example, using the iris data set from R:

ii <- subset(iris, Species == "setosa")   # subset out setosa only
ii$Species <- ii$Species[drop = TRUE]  # drop unused factors
levels(ii$Species) # check that unused factors are gone


iris1 <- subset(iris0, Species == "setosa")
iris1$Species <- iris1$Species[drop = TRUE]