levels() counter-intuitif? (PR#1693)

[Torsten Hothorn]

> Suppose I have a factor size with levels "small", "medium" and "large". 
> Then, when I subset this factor:
> >ss<-size[size!="medium"]
> to get at the extremes,
> >levels(ss)
> ....
> Levels:  large medium small

sure, because you didn't change the levels attribute of the factor object
ss. And there is no statement in the documentation that subsetting a
factor object touches the levels attribute. So why is this a bug? If I
remember correctly, there was a dicussion about this issue a few month ago
on this list ...

Torsten

> 
> The same happens with
> >subset( size, size!="medium")
> 
> I understand that the resulting factor inherits the possible levels from its 
> "parent", but I do not see in what that is useful. Intuitively, I would 
> expect that levels(ss) return "small" and "large". For subset(), there might 
> be an option to request inheritance of the number of levels.
> 
> RenE J.V. Bertin
> College de France/LPPA
> 11, place Marcelin Berthelot
> 75005 Paris, France
> 
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