Description of Terms Objects
- The object itself is simply the formula supplied to the call of
terms.formula. The object has a number of attributes and they are used to construct the model frame:
factors A matrix of variables by terms showing which variables appear in which terms. The entries are 0 if the variable does not occur in the term, 1 if it does occur and should be coded by contrasts, and 2 if it occurs and should be coded via dummy variables for all levels (as when a lower-order term is missing). Note that variables in main effects always receive 1, even if the intercept is missing (in which case the first one should be coded with dummy variables). If there are no terms other than an intercept and offsets, this is
term.labels A character vector containing the labels for each of the terms in the model, except for offsets. Note that these are after possible re-ordering of terms.
Non-syntactic names will be quoted by backticks: this makes it easier to re-construct the formula from the term labels.
variables A call to
listof the variables in the model.
intercept Either 0, indicating no intercept is to be fit, or 1 indicating that an intercept is to be fit. order A vector of the same length as
term.labelsindicating the order of interaction for each term.
response The index of the variable (in variables) of the response (the left hand side of the formula). Zero, if there is no response. offset If the model contains
offsetterms there is an
offsetattribute indicating which variable(s) are offsets
specials If a
specialsargument was given to
terms.formulathere is a
specialsattribute, a pairlist of vectors (one for each specified special function) giving numeric indices of the arguments of the list returned as the
variablesattribute which contain these special functions.
dataClasses optional. A named character vector giving the classes (as given by
.MFclass) of the variables used in a fit.
predvars optional. An expression to help in computing predictions at new covariate values; see
- The object has class
These objects are different from those found in S. In particular
there is no
formula attribute: instead the object is itself a
formula. (Thus, the mode of a terms object is different.)
## use of specials (as used for gam() in packages mgcv and gam) (tf <- terms(y ~ x + x:z + s(x), specials = "s")) ## Note that the "factors" attribute has variables as row names ## and term labels as column names, both as character vectors. attr(tf, "specials") # index 's' variable(s) rownames(attr(tf, "factors"))[attr(tf, "specials")$s] ## we can keep the order by terms(y ~ x + x:z + s(x), specials = "s", keep.order = TRUE)