R devel - May 1999 - Factor structures not preserved after dump/dput (PR#200)

Full_Name: Marek Ancukiewicz
Version: 0.64.0
OS: Linux (RedHat 6.0)
Submission from: (NULL) (132.183.12.87)


I've noticed that factor structures get recoded when the data is dumped
using either dump or dput and then restored with source or dget. This
occurs when the values taken by factors do not include 1. For example:

  a <-
factor(1:5,1:5,c('a','b','c','d','e'))
  b <- a[3:5]
  dput(b,'b.data')
  new.b <- dget('b.data')

Then b is not the same as new.b:
  > b
  [1] c d e
  Levels:  a b c d e 
  > new.b
  [1] a b c
  Levels:  a b c d e 

This seems to be a very serious bug. It can make one to 
mislabel treatments: a very emabarassing (and potentially 
disastrous) mistake. The reason for this bug seems to
lie in a way in which structure() treats factor structures.

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msa@biostat.mgh.harvard.edu writes:
>   a <-
factor(1:5,1:5,c('a','b','c','d','e'))
>   b <- a[3:5]
>   dput(b,'b.data')
>   new.b <- dget('b.data')
> 
> Then b is not the same as new.b:
>   > b
>   [1] c d e
>   Levels:  a b c d e 
>   > new.b
>   [1] a b c
>   Levels:  a b c d e 
> 
> This seems to be a very serious bug. It can make one to 
> mislabel treatments: a very emabarassing (and potentially 
> disastrous) mistake. The reason for this bug seems to
> lie in a way in which structure() treats factor structures.
Ouch! The fix seems to be to have structure() contain

        if (is.numeric(.Data) && any(names(attrib) ==
"levels"))
            .Data <- factor(.Data, levels = 1:max(1,.Data))

rather than just plain  factor(.Data) .  Will commit this in a moment.

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