update a lfe formula containing 0

[MatthieuStigler]

I have difficulties using update() on a formula containing any 0: trying multiple combinations, I end up always with 1-1`, which will throw an error. Any idea how to handle this?

library(Formula)
fo <-  y ~ x+x2| id + firm| 0 |firm

update(as.Formula(fo), . ~ .| firm| . |firm)
update(as.Formula(fo), . ~ .| firm| 0 |firm)
update(as.Formula(fo), . ~ .| firm| -1 + 0 |firm)
update(as.Formula(fo), . ~ .| firm| -1|firm)

This could be an issue with the underlying update() function:

update(~0, ~0)
~1 - 1

And actually from the terms.formula() function with simplify = TRUE.

terms.formula(~0, simplify = TRUE)
terms.formula(~0, simplify = FALSE)

But at that point... any workaround for that issue? Thanks!

[karldw]

@MatthieuStigler, it looks like felm has to do this internally (it's a tricky process)

lfe/R/felm.R

Lines 40 to 58 in 49bac46

	# we can't just dot-update the iv-part, update will only keep the instruments
	if(numrhs < 2) Form <- update(Form, . ~ . | 0 | 0 | 0 | 0, drop=FALSE)
	else if(numrhs < 3) Form <- update(Form, . ~ . | . | 0 | 0 | 0 , drop=FALSE)
	else if(numrhs < 4) {
	# build from parts
	Form <- as.Formula(do.call(substitute, list(L ~ R1 | R2 | R3 | 0 | 0,
	list(L=formula(Form,lhs=NULL,rhs=0)[[2]],
	R1=formula(Form,lhs=0,rhs=1)[[2]],
	R2=formula(Form,lhs=0,rhs=2)[[2]],
	R3=formula(Form,lhs=0,rhs=3)[[2]]))))
	} else if(numrhs < 5) {
	Form <- as.Formula(do.call(substitute, list(L ~ R1 | R2 | R3 | R4 | 0,
	list(L=formula(Form,lhs=NULL,rhs=0)[[2]],
	R1=formula(Form,lhs=0,rhs=1)[[2]],
	R2=formula(Form,lhs=0,rhs=2)[[2]],
	R3=formula(Form,lhs=0,rhs=3)[[2]],
	R4=formula(Form,lhs=0,rhs=4)[[2]]))))
	}