[R] issue with levels of a factor after subsetting

[marcos carvajalino]

Thanks Phil, exactly what i was looking for!!

2009/10/26 Phil Spector <spector at stat.berkeley.edu>:
> Marcos -
>   Either refer to
>
> levels(factor(car4$Rio))
>
> or
>
> levels(car4$Rio[,drop=TRUE])
>
> to show only the levels that are present in the variable.
>
>                                        - Phil Spector
>                                         Statistical Computing Facility
>                                         Department of Statistics
>                                         UC Berkeley
>                                         spector at stat.berkeley.edu
>
> On Mon, 26 Oct 2009, marcos carvajalino wrote:
>
>> Hi
>>
>> Second question in a day, i'm beginnning to feel incompetent...
>>
>> This time i'm having a weird problem, i'm importing the next data base:
>>
>>> car<-read.csv2("Historicos.csv")
>>
>> 'data.frame':   1818 obs. of  6 variables:
>> $ Dpto  : Factor w/ 11 levels "ANTIOQUIA","ATLÁNTICO",..: 2 2 2 2 2 1
>> 1 1 1 5 ...
>> $ Rio   : Factor w/ 43 levels "Acandí","Anchicayá",..: 26 26 26 26 26
>> 4 4 4 4 39 ...
>> $ Var   : Factor w/ 13 levels "CAUDAL","CD",..: 1 1 1 1 1 1 1 1 1 1 ...
>> $ Valor : num  7150 7150 7121 7121 7121 ...
>> $ Año   : int  2002 2003 2004 2009 2005 2002 2003 2004 2005 2009 ...
>> $ Región: Factor w/ 2 levels "CARIBE","PACIFICO": 1 1 1 1 1 1 1 1 1 2 ...
>>
>> The variable "Rio" contents names of 43 rivers in Colombia, now my
>> boss wants me to show just 4 of them in a graph and the other 39 in
>> another, i subsetted them using the following code:
>>
>> #The first 4 Rivers
>>>
>>> car4<-car[car$Rio%in%c("Magdalena","Atrato","San Juan","Mira"),]
>>
>> #The other 39
>>>
>>> car5<-car[!car$Rio%in%c("Magdalena","Sinú","Atrato","San
>>> Juan","Mira","Micay",
>>
>> "Patia","Canal del Dique","Iscuandé","Guapi"),]
>>
>> And I plot the two graphs using:
>>
>>
>> xyplot(Valor~Año|Var,groups=Rio,data=car4[car4$Var%in%c("NT","PO4","HDD","CTE",
>>
>> "SST","OCT"),],layout=c(2,3),subscripts=T,scale=list(y=list(relation="free")),type="b")
>>
>>
>> xyplot(Valor~Año|Var,groups=Rio,data=car5[car5$Var%in%c("NT","PO4","HDD","CTE",
>>
>> "SST","OCT"),],layout=c(2,3),subscripts=T,scale=list(y=list(relation="free")),type="b")
>>
>> Until then everything was going smoothly, but i tried to add a custom
>> key using
>> key=list(corner=c(1,1),border=T,lines=T,text=list(levels(car4$Rio)))
>> and i was very suprised when instead of the expected 4 names of the
>> rivers i got the whole 43 in the legend.
>>
>> i thought it was my fault and i missed something in the key
>> instruction but when i checked the structure of the car4 data frame
>> (The one with just the selected 4 rivers) i found out this:
>>
>>> str(car4)
>>
>> 'data.frame':   230 obs. of  6 variables:
>> $ Dpto  : Factor w/ 11 levels "ANTIOQUIA","ATLÁNTICO",..: 2 2 2 2 2 1
>> 1 1 1 5 ...
>> $ Rio   : Factor w/ 43 levels "Acandí","Anchicayá",..: 26 26 26 26 26
>> 4 4 4 4 39 ...
>> $ Var   : Factor w/ 13 levels "CAUDAL","CD",..: 1 1 1 1 1 1 1 1 1 1 ...
>> $ Valor : num  7150 7150 7121 7121 7121 ...
>> $ Año   : num  2002 2003 2004 2009 2005 ...
>> $ Región: Factor w/ 2 levels "CARIBE","PACIFICO": 1 1 1 1 1 1 1 1 1 2 ...
>>
>> The new data frame (car4) keeped the factor levels of the old data
>> frame (car), how can i drop them from the new data frame and just keep
>> the 4 selected levels?
>>
>> Thanks by advanced...
>> --
>> Marcos Antonio Carvajalino Fernández
>> Estudiante de Ingeniería Ambiental y Sanitaria
>> Universidad del Magdalena, Colombia
>>
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>



-- 
Marcos Antonio Carvajalino Fernández
Estudiante de Ingeniería Ambiental y Sanitaria
Universidad del Magdalena, Colombia