[R] Revert a factor to its numeric values

[Peter Dalgaard]

gb at stat.umu.se (Göran Broström) writes:

> I'm trying a recommendation on the help page for 'factor':
> 
> > x <- c(1, 2, 1, 2)
> > x <- factor(x, labels = c("one", "two"))
> > x
> [1] one two one two
> Levels: one two
> > as.numeric(levels(x))[x]
> [1] NA NA NA NA
> Warning message:
> NAs introduced by coercion
>  
> Also,
> 
> > as.numeric(as.character(x))
> [1] NA NA NA NA
> Warning message:
> NAs introduced by coercion
> 
> What am I doing wrong? This is R-1.9.1, Linux (debian installation)

You appear to be assuming tha R knows how to make numbers from text
strings

as.numeric(c("jedan","dva","tri","ceteri"))

is not going to give you 1,2,3,4 either (not even in a Croatian
locale).

The suggestion on the help page works for numeric levels: 

>  x <- c(1, 2, 1, 2)
>  x <- factor(x, labels = c("5", "7"))
>  as.numeric(levels(x))[x]
[1] 5 7 5 7


> Another question: I have a factor with four levels, which I want 
> to collapse to two. How do I do it in the simplest possible way?

 
> x <- factor(c("jedan","dva","tri","ceteri"))
> x
[1] jedan  dva    tri    ceteri
Levels: ceteri dva jedan tri
> levels(x) <- c("even","even","odd","odd")
> x
[1] odd  even odd  even
Levels: even odd


-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
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