removing offset terms from a formula

Question

R has a handy tool for manipulating formulas, update.formula(). This works nicely when you want to get something like "formula containing all terms in previous formula except x", e.g.

f1 <- z ~ a + b + c
(f2 <- update.formula(f1, . ~ . - c))
## z ~ a + b

However, this doesn't seem to work with offset terms:

f3 <- z ~ a + offset(b) 
update(f3, . ~ . - offset(b))
## z ~ a + offset(b)

I've dug down as far as terms.formula, which ?update.formula references:

[after substituting, ...] The result is then simplified via ‘terms.formula(simplify = TRUE)’.

terms.formula(z ~ a + offset(b) - offset(b), simplify=TRUE)
## z ~ a + offset(b)

(i.e., this doesn't seem to remove offset(b) ...)

I know I can hack up a solution either by using deparse() and text-processing, or by processing the formula recursively to remove the term I don't want, but these solutions are ugly and/or annoying to implement. Either enlightenment as to why this doesn't work, or a reasonably compact solution, would be great ...

Answer 1

1) Recursion Recursively descend through the formula replacing offset(...) with offset and then remove offset using update. No string manipulation is done and although it does require a number of lines of code it's still fairly short and does remove single and multiple offset terms.

If there are multiple offsets one can preserve some of them by setting preserve so, for example, if preserve = 2 then the second offset is preserved and any others are removed. The default is to preserve none, i.e. remove them all.

no.offset <- function(x, preserve = NULL) {
  k <- 0
  proc <- function(x) {
    if (length(x) == 1) return(x)
    if (x[[1]] == as.name("offset") && !((k<<-k+1) %in% preserve)) return(x[[1]])
    replace(x, -1, lapply(x[-1], proc))
  }
  update(proc(x), . ~ . - offset)
}

# tests

no.offset(z ~ a + offset(b))
## z ~ a

no.offset(z ~ a + offset(b) + offset(c))
## z ~ a

Note that if you don't need the preserve argument then the line initializing k can be omitted and the if simplified to:

if (x[[1]] == as.name("offset")) return(x[[1]])

2) terms this neither uses string manipulation directly nor recursion. First get the terms object, zap its offset attribute and fix it using fixFormulaObject which we extract out of the guts of terms.formula. This could be made a bit less brittle by copying the source code of fixFormulaObject into your source and removing the eval line below. preserve acts as in (1).

no.offset2 <- function(x, preserve = NULL) {
  tt <- terms(x)
  attr(tt, "offset") <- if (length(preserve)) attr(tt, "offset")[preserve]
  eval(body(terms.formula)[[2]]) # extract fixFormulaObject
  f <- fixFormulaObject(tt)
  environment(f) <- environment(x)
  f
}

# tests

no.offset2(z ~ a + offset(b))
## z ~ a

no.offset2(z ~ a + offset(b) + offset(c))
## z ~ a

Note that if you don't need the preserve argument then the line that zaps the offset attribute can be simplified to:

attr(tt, "offset") <- NULL