terms.object
Description of Terms Objects
An object of class terms
holds information about a
model. Usually the model was specified in terms of a
formula
and that formula was used to determine the terms
object.
 Keywords
 models
Value

The object itself is simply the formula supplied to the call of
 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 lowerorder 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
numeric(0)
.  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 reordering of terms.Nonsyntactic names will be quoted by backticks: this makes it easier to reconstruct the formula from the term labels.
 variables
 A call to
list
of 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.labels
indicating 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
offset
terms there is anoffset
attribute indicating which variable(s) are offsets  specials
 If a
specials
argument was given toterms.formula
there is aspecials
attribute, a pairlist of vectors (one for each specified special function) giving numeric indices of the arguments of the list returned as thevariables
attribute 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
makepredictcall
. The object has class
terms.formula
. The object has a number of attributes
and they are used to construct the model frame:c("terms", "formula")
.
Note
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.)
Examples of the specials
argument can be seen in the
aov
and coxph
functions, the
latter from package \href{https://CRAN.Rproject.org/package=#1}{\pkg{#1}}survivalsurvival.
See Also
Examples
library(stats)
## 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)
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