[R] Odd behaviour with scale()

[Anon.]

Moi!

A student here has been getting a bit irritated with some side effects 
of scale() (OS is Windows XP, the behaviour occurs in R 2.0.0, but not 
1.7.1).  The problem is that she scales a variable in a data frame, then 
does a regression, and tries to get some predictions for some new data.  
However, at this point she gets an error (see the example below).  This 
seems to be because the scaled variable in the new data frame does not 
have the center and scale attributes, but the one in the old data frame 
does.

The work-around is to put the scaled variable intro a new data frame, 
which again won't have the attributes.  But it seems odd to me that 
whether a scale()'d variable has attributes depends on where it's 
placed.  I presume that this is because I'm not understanding something 
about the way R is working, rather than it being a bug.  Would anyone 
care to enlighten me?

 > Data1=data.frame(xx=1:10, yy=2.1:12)
 > Data1$xx=scale(Data1$xx)
 >
 > reg1=lm(yy~xx, data=Data1)
 > New=data.frame(xx=2:4)
 > b=predict(reg1, New, se.fit=T)
Error: variable 'xx' was fitted with nmatrix.1 but numeric was supplied
 >
 > New=data.frame(xx=scale(2:4, center=5.5, scale=3.02))
 > b=predict(reg1, New, se.fit=T)
Error: variable 'xx' was fitted with nmatrix.1 but numeric was supplied
 >

Bob

-- 
Bob O'Hara
Department of Mathematics and Statistics
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FIN-00014 University of Helsinki
Finland

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