[R] question about subseting a dataframe

[Yasir Kaheil]

you need to redefine the factor on ex1$id.. cast it again as.factor to
redefine the levels.


Dipankar Basu wrote:
> 
> Hi!
> 
> I am using R version 2.7.0 and am working on a panel dataset read into R
> as
> a dataframe; I call it "ex". The variables in "ex" are: id  year  x
> 
> id: a character string which identifies the unit
> year: identifies the time period
> x: the variable of interest (which might contain NAs).
> 
> Here is an example:
>> id <- rep(c("A","B","C"),2)
>> year <- c(rep(1970,3),rep(1980,3))
>> x <- c(20,30,40,25,35,45)
>> ex <- data.frame(id=id,year=year,x=x)
>> ex
>  id year  x
> 1  A 1970 20
> 2  B 1970 30
> 3  C 1970 40
> 4  A 1980 25
> 5  B 1980 35
> 6  C 1980 45
> 
> I want to draw a subset of "ex" by selecting only the A and B units:
> 
>> ex1 <- subset(ex[which(ex$id=="A"|ex$id=="B"),])
> 
> Now I want to do some computations on x for each selected unit only:
> 
>> tapply(ex1$x, ex1$id, mean)
>   A    B    C
> 22.5 32.5   NA
> 
> But this gives me an NA value for the unit C, which I thought I had
> already
> left out. How do I ensure that the computation (in the last step) is
> limited
> to only the units I have selected in the first step?
> 
> Dipankar
> 
> 	[[alternative HTML version deleted]]
> 
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> 
> 


-----
Yasir H. Kaheil
Catchment Research Facility
The University of Western Ontario 

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