# R-alpha: two-sided to one-sided formula

**Douglas Bates
**
bates@stat.wisc.edu

*Tue, 2 Dec 1997 11:55:49 -0600 (CST)*

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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