R: Compute Factor Interactions

interaction {base} | R Documentation

## Compute Factor Interactions

### Description

`interaction` computes a factor which represents the interaction of the given factors. The result of `interaction` is always unordered.

### 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` is `TRUE`, 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

    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)))

[Package base version 4.0.5 Index]