update.formula             package:base             R Documentation

Model Updating

Description:

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

Usage:

     update.formula(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.

Details:

     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
     occurence of "`.'" on the left of `new', and substitutes the rhs
     of the `old' formula for any occurence of "`.'" on the right of
     `new'.

Value:

     The updated formula is returned.

See Also:

     `terms', `model.matrix'.

Examples:

     update.formula(y ~ x,    ~ . + x2) #> y ~ x + x2
     update.formula(y ~ x, log(.) ~ . ) #> log(y) ~ x

