Utilities for Formulas, Expressions, Calls, Assignments and Other R Objects

These utilities facilitate the programmatic manipulations of formulas,
expressions, calls, names, symbols and other R language objects. These objects
all share the same structure: a left-hand side (lhs), an operator (op) and
right-hand side (rhs). This packages provides methods for accessing and
modifying this structures as well as extracting and replacing names and
symbols from these objects.

It is easiest to understand through the [#Examples].

Manipulate parts of a formula/expression/call/assignment:

lhs: get/set the left-hand side of an object

rhs: get/set the right-hand side of an object

op: get/set the operator of an object

Get variables (names) in a formula/expression/call/assignment:

get.vars: like base::all.vars except all symbols (e.g. .) can be
interpolated from data, so that things like y ~ . can return the correct
values.

lhs.vars: get.vars on lhs of the object

rhs.vars: get.vars on rhs of the object

Get terms from formula or expression:

terms: get the terms from an object by interpolating special symbols
(e.g. .) using a data set data. Extends stats::terms()

split_terms: split (mathematical) terms into an expression vector without
interpolating special symbols (e.g. .)

Examples

Formula terms and operators can be accessed or changed.

## Get parts of formula form<-y~x
lhs(form) # y
lhs(form) <- quote(t)
form# t ~ x
rhs(form) # x
rhs(form) <-1form# t ~ 1## Get variables (names)form<-y~m*x+b
get.vars(form) # "y" "m" "x" "b"
lhs.vars(form) # "y"
rhs.vars(form) # "m" "x" "b"## Split terms
terms(form)
split_terms(form) # expression(Species ~ . - Sepal.Length)
split_terms( quote(a+(b+(c+d))), recursive=TRUE ) # expression(a, b, c, d)

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