interaction
Compute Factor Interactions
interaction
computes a factor which represents the interaction
of the given factors. The result of interaction
is always unordered.
 Keywords
 category
Usage
interaction(..., drop = FALSE, sep = ".", lex.order = FALSE)
Arguments
 ...
 the factors for which interaction is to be computed, or a single list giving those factors.
 drop
 if
drop
isTRUE
, unused factor levels are dropped from the result. The default is to retain all factor levels.  sep
 string to construct the new level labels by joining the constituent ones.
 lex.order
 logical indicating if the order of factor concatenation should be lexically ordered.
Value

A factor which represents the interaction of the given factors.
The levels are labelled as the levels of the individual factors joined
by
sep
which is .
by default.By default, when lex.order = FALSE
, the levels are ordered so
the level of the first factor varies fastest, then the second and so
on. This is the reverse of lexicographic ordering (which you can get
by lex.order = TRUE
), and differs from
:
. (It is done this way for compatibility with S.)
References
Chambers, J. M. and Hastie, T. J. (1992) Statistical Models in S. Wadsworth & Brooks/Cole.
See Also
factor
;
:
where f:g
is similar to
interaction(f, g, sep = ":")
when f
and g
are factors.
Examples
library(base)
a < gl(2, 4, 8)
b < gl(2, 2, 8, labels = c("ctrl", "treat"))
s < gl(2, 1, 8, labels = c("M", "F"))
interaction(a, b)
interaction(a, b, s, sep = ":")
stopifnot(identical(a:s,
interaction(a, s, sep = ":", lex.order = TRUE)),
identical(a:s:b,
interaction(a, s, b, sep = ":", lex.order = TRUE)))
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