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

[Mikkel Grum]

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