[R] factor vector manipulation

[Gabor Grothendieck]

On 6/3/05, Weiwei Shi <helprhelp at gmail.com> wrote:
> Hi,
> I have one question on factor vector.
> I have 3 factor vectors:
> 
> a<-factor(c("1", "2", "3"))
> b<-factor(c("a", "b", "c"))
> c<-factor(c("b", "a", "c"))
> 
> what I want is like:
>  c x
> 1 b 2
> 2 a 1
> 3 c 3
> 
> which means, I use b as keys and vector a as values and I find values for c.
> 
> I used the following codes:
> x<-c()
> d<-data.frame(b,a)
> for (each in 1:length(c)){   # i don't know why each in c did not work
>  tmp<-d$a[d$b==c[each]]                                  # question
>  x<-append(x, levels(tmp)[as.integer(tmp)])
> }
> 
> data.frame(c,x)
> 
> If someone has a better way to do it, please help!!
> 
> Also, I don't understand why I have to use levels() since w/o it, i
> only added the index for levels in to x.
> 
> BTW, when b is like 60,000, the process is very slow. Should I use hash here?
> 
> Thanks,
> 
> Weiwei
> 
cbind(c, x = a[match(c,b)])