Correctly parse "formula" object in R

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How can I parse an R formula object (fo) correctly (i.e., not turning it into a string when parsing)?

Let's say I have the following:

## Creating a formula object
fo <- y ~ x1 + x2 | 0 + z1 + z2 + z3 + z4 + z5

class(fo)
##[1] "formula"

typeof(fo)
##[1] "language"

strsplit(fo, split='|', fixed=TRUE)
##Error in strsplit(fo, split = "|", fixed = TRUE) : non-character argument

Hopefully, I want to parse it into three atomic vectors:

  1. Dependent variable: c("y").
  2. Regressors: c("x1", "x2").
  3. Others: c("z1", "z2", "z3", "z4", "z5") (excluding the 0).
formula r parsing string
1 Answers

The tree structure of the formula breaks down as follows:

  • top level: ~( y, response)

Internally, this is a list-like object where the first element is the operator (~), the second element is the first argument, and the third element is the third argument.

So deparse(fo[[2]]) gets you "y"

  • next level (response): | (x1+x2, 0 + ...). Same general structure (first element is the operator |, second element is the first arg, third element is the second arg)

so fo[[c(3,2)]] gets x1+x2.

all.vars(fo[[c(3,2)]])

gets the variables to the left of the bar

all.vars(fo[[c(3,3)]])

gets the variables to the right of the bar

This gets considerably trickier if you want to extract terms rather than variables; for example all.vars(quote(log(x)) gets "x", not "log(x)"

user_1902770
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