[R] How to convert a factor to a numeric?

[Berton Gunter]

# If fact1 and fact2 are your factors, let prm be the permutation such that
# levels(fact2) corresponds to ("aligns to") levels(fact1)[prm] . In your
example, the permutation is
# apparently the identity, (1:3).

#Then

levels(fact2)[prm[fact1]]==fact2

## does what you want. I wouldn't be surprised if there are cleverer
solutions, though.


-- Bert Gunter
Genentech Non-Clinical Statistics
South San Francisco, CA
 
"The business of the statistician is to catalyze the scientific learning
process."  - George E. P. Box
 
 

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Chris 
> Bergstresser
> Sent: Tuesday, March 01, 2005 2:46 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] How to convert a factor to a numeric?
> 
> Hi all --
> 
>     I've got two columns, both of which correspond to three factor 
> levels (e.g., column1 is "a", "b", or "c"; column2 is "x", 
> "y", or "z"). 
>   I'd like to generate a third column, consisting on whether the two 
> factors are correctly aligned for a given case (in this example, "a" 
> corresponds to "x", "b" to "y", and "c" to "z").  For example:
> 
>     a   x   TRUE
>     a   y   FALSE
>     b   y   TRUE
>     c   z   TRUE
>     b   x   FALSE
> 
>     Several questions:
> 
>     The easiest way seemed to me to be comparing the numeric values 
> across columns, but the encodings are (a=1, b=2, c=3) and (x=1, y=3, 
> z=2).  Is there a way to change the underlying value 
> representing each 
> factor, so I could just run an equality on them?
>     Is there a simple way to check for correspondence without 
> recoding 
> the factors?
>     In the help for factor(), it says "In particular, 'as.numeric' 
> applied to a factor is meaningless, and may happen by 
> implicit coercion. 
>   To "revert" a factor 'f' to its original numeric values, 
> 'as.numeric(levels(f))[f]' is recommended and slightly more efficient 
> than 'as.numeric(as.character(f))'."  However, I get the following 
> results.  What's going on?
> 
>  > f = gl(3, 1, 6, labels=c("a", "b", "c"))
>  > f
> [1] a b c a b c
> Levels: a b c
>  > as.numeric(levels(f))[f]
> [1] NA NA NA NA NA NA
> Warning message:
> NAs introduced by coercion
>  > as.numeric(as.character(f))
> [1] NA NA NA NA NA NA
> Warning message:
> NAs introduced by coercion
>  > as.numeric(f)
> [1] 1 2 3 1 2 3
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
>