[R] Updating a formula w a portion of another formula

[Gabor Grothendieck]

Daniel Almirall <dalmiral <at> umich.edu> writes:

: 
: R-list,
: 
: Suppose I have
: 
:  oldfmla <- y ~ x
: 
: I would like to update it to  y ~ x + z  which I know I can get using
: 
:  newfmla <- update(oldfmla, ~ . + z)
: 
: However, what if I have
: 
: fmlatmp <- ~ z
: 
: Can I combine oldfmla and fmlatmp to get  y ~ x + z  some how?
: 
: Clearly,
: 
:  newfmla <- update(oldfmla, ~ . + fmlatmp)
: 
: will not work.
: 
: Thanks in advance,
: Danny


Assuming we have:

	old <- y ~ x
	tmp <- ~ z

Then type in this:

	template <- . ~ . + X
	template[[3]][[3]] <- tmp[[2]]
	update(old, template)

In the above we used the fact that formulas are
represented internally as trees whose parts
can be extracted using indexing.  After
running the above, try entering the following to
get a better idea of how formulas are represented:

	as.list(old)
	as.list(tmp)
	as.list(template)
	as.list(template[[3]])