update.formula {stats}  R Documentation 
update.formula
is used to update model formulae.
This typically involves adding or dropping terms,
but updates can be more general.
## S3 method for class 'formula'
update(old, new, forceEnv=TRUE, ...)
old 
a model formula to be updated. 
new 
a formula giving a template which specifies how to update. 
forceEnv 
logical specifying if the result will have an

... 
further arguments passed to or from other methods. 
Either or both of old
and new
can be objects such as
lengthone character vectors which can be coerced to a formula via
as.formula
.
The function works by first identifying the lefthand side
and righthand 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 via terms.formula(simplify = TRUE)
.
The updated formula is returned. The environment of the result is
that of old
.
update(y ~ x, ~ . + x2) #> y ~ x + x2
update(y ~ x, log(.) ~ . ) #> log(y) ~ x
update(. ~ u+v, res ~ . ) #> res ~ u + v