update.formula is used to update model formulae.
This typically involves adding or dropping terms,
but updates can be more general.

Usage

# S3 method for formula
update(old, new, …)

Arguments

old

a model formula to be updated.

new

a formula giving a template which specifies how to update.

…

further arguments passed to or from other methods.

Value

The updated formula is returned. The environment of the result is
that of old.

Details

Either or both of old and new can be objects such as
length-one character vectors which can be coerced to a formula via
as.formula.

The function works by first identifying the left-hand side
and right-hand side of the old formula.
It then examines the new formula and substitutes
the lhs of the old formula for any occurrence
of ‘.’ on the left of new, and substitutes
the rhs of the old formula for any occurrence
of ‘.’ on the right of new. The result is then
simplified viaterms.formula(simplify = TRUE).