Dear Shuguang,
Here are two ways. Perhaps they are not efficient enough, but the work:


# Data
mydata=read.table(textConnection("
factorA factorB
   0       0
   0       0
   1       0
   0       1
   1       1"),header=TRUE)
closeAllConnections()

# Option 1
mydata$factorC=as.factor(
apply(mydata,1,function(x){
paste(x,sep="",collapse="")
}
))
levels(mydata$factorC)<-list("0"="00", "1"="10", "2"="01","3"="11")
mydata

# Option 2
# You'll need to read the data again to see how this option works
mydata$factorC<-apply(mydata,1,function(x){
ifelse(sum(x)==0,0,
ifelse(x[1]==1 & x[2]==0,1,
ifelse(x[1]==0 & x[2]==1,2,3)))
}
)

mydata


HTH,


Jorge



On Tue, Nov 4, 2008 at 10:29 AM, Shuguang Sun <shuguang@gmail.com> wrote:

> How to generate a new factor variable by two other factor variables?
>
> For example, if I have two factor variables, factorA and factorB,
> factorA factorB
>    0       0
>    0       0
>    1       0
>    0       1
>    1       1
>
> Is there a simple way to generate a new 4-levels factor variable as
>
> factorC factorA factorB
>    0      0       0
>    0      0       0
>    1      1       0
>    2      0       1
>    3      1       1
>
> --
> Shuguang Sun
> Fudan University, China
>
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