Extended Formulas: Multiple Responses and Multiple Regressor Parts
The new class Formula extends the base class
formula by allowing for multiple responses
and multiple parts of regressors.
Formula(object) ## S3 method for class 'Formula' formula(x, lhs = NULL, rhs = NULL, collapse = FALSE, update = FALSE, drop = TRUE, ...) as.Formula(x, ...) is.Formula(object)
object, x |
an object. For |
lhs, rhs |
indexes specifying which elements of the left- and
right-hand side, respectively, should be employed. |
collapse |
logical. Should multiple parts (if any) be collapsed
to a single part (essentially by replacing the |
update |
logical. Only used if |
drop |
logical. Should the |
... |
further arguments. |
Formula objects extend the basic formula objects.
These extensions include multi-part formulas such as
y ~ x1 + x2 | u1 + u2 + u3 | v1 + v2, multiple response
formulas y1 + y2 ~ x1 + x2 + x3, multi-part responses
such as y1 | y2 + y3 ~ x, and combinations of these.
The Formula creates a Formula object from a formula
which can have the | operator on the left- and/or right-hand
side (LHS and/or RHS). Essentially, it stores the original formula
along with attribute lists containing the decomposed parts for the LHS
and RHS, respectively.
The main motivation for providing the Formula class is to be
able to conveniently compute model frames and model matrices or extract
selected responses based on an extended formula language. This functionality
is provided by methods to the generics model.frame,
and model.matrix. For details and examples, see
their manual page: model.frame.Formula.
In addition to these workhorses, a few further methods and functions are provided.
By default, the formula() method switches back to the original
formula. Additionally, it allows selection of subsets of the
LHS and/or RHS (via lhs, and rhs) and collapsing
multiple parts on the LHS and/or RHS into a single part (via collapse).
is.Formula checks whether the argument inherits from the
Formula class.
as.Formula is a generic for coercing to Formula, the
default method first coerces to formula and then calls
Formula. The default and formula method also take an
optional env argument, specifying the environment of the resulting
Formula. In the latter case, this defaults to the environment
of the formula supplied.
Formula returns an object of class Formula
which inherits from formula. It is the original formula
plus two attributes "lhs" and "rhs" that contain the
parts of the decomposed left- and right-hand side, respectively.
Zeileis A, Croissant Y (2010). Extended Model Formulas in R: Multiple Parts and Multiple Responses. Journal of Statistical Software, 34(1), 1–13. doi: 10.18637/jss.v034.i01
## create a simple Formula with one response and two regressor parts f1 <- y ~ x1 + x2 | z1 + z2 + z3 F1 <- Formula(f1) class(F1) length(F1) ## switch back to original formula formula(F1) ## create formula with various transformations formula(F1, rhs = 1) formula(F1, collapse = TRUE) formula(F1, lhs = 0, rhs = 2) ## put it together from its parts as.Formula(y ~ x1 + x2, ~ z1 + z2 + z3) ## update the formula update(F1, . ~ . + I(x1^2) | . - z2 - z3) update(F1, . | y2 + y3 ~ .) # create a multi-response multi-part formula f2 <- y1 | y2 + y3 ~ x1 + I(x2^2) | 0 + log(x1) | x3 / x4 F2 <- Formula(f2) length(F2) ## obtain various subsets using standard indexing ## no lhs, first/seconde rhs formula(F2, lhs = 0, rhs = 1:2) formula(F2, lhs = 0, rhs = -3) formula(F2, lhs = 0, rhs = c(TRUE, TRUE, FALSE)) ## first lhs, third rhs formula(F2, lhs = c(TRUE, FALSE), rhs = 3)
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