[R] FAQ and Answer: How to convert factors back to integers

[Peter Dalgaard BSA]

rossini at biostat.washington.edu (A.J. Rossini) writes:


> 	as.numeric(levels(hivplasma))[codes(hivplasma)]
                                      ^^^^^

Whoops. Make that as.integer(), codes() has some Splus-compatible
breakage built into it:

> f<-factor(1:10)
> f
 [1] 1  2  3  4  5  6  7  8  9  10
Levels:  1 2 3 4 5 6 7 8 9 10 
> as.numeric(levels(f))[codes(f)]
 [1]  1  3  4  5  6  7  8  9 10  2

It happens because 10 sorts between 1 and 2, alphabetically. Yes, it
is silly... 

If you're not badly squeezed for space, as.numeric(as.character(f))
works too, and is easier to remember.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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