R devel - Dec 1997 - R-alpha: two-sided to one-sided formula

At times we want to convert a two-sided formula to a one-sided
formula.  In S we can do this by dropping the second entry in the
formula.  In R that object no longer has a formula class.
 R> ttt <- score ~ age | Infant
 R> class(ttt)
 [1] "formula"
 R> length(ttt)
 [1] 3
 R> ttt[-2]
 [[1]]
 ~

 [[2]]
 age | Infant

 R> class(ttt[-2])
 NULL
 R> do.call("~", ttt[-(1:2)])
 ~age | Infant

In general it would not be a good idea to propagate the formula class
to subsets but it does make sense in this case.  We can get around it
by replacing ttt[-2] by do.call("~", ttt[-(1:2)]) I suppose.  Any
opinions on whether ttt[-2] should still be a formula?
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Douglas Bates <bates@stat.wisc.edu> writes:
> In general it would not be a good idea to propagate the formula class
> to subsets but it does make sense in this case.  We can get around it
> by replacing ttt[-2] by do.call("~", ttt[-(1:2)]) I suppose.  Any
> opinions on whether ttt[-2] should still be a formula?
Hum. tt[-i] is a well-defined formula iff (i != 1). Somehow, I dislike
the idea of properties depending on the value of a parameter, so I'm
inclined to say no. BTW,

eval(as.call(ttt[-2]))

also works. And one would *think* that formula(as.call(ttt[-2])) did
too, but typeof(as.call(...))==language and !=call (is this weird or
not?), so formula.default protests.

-- 
   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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