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 = ".")

### 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.

### 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`, by `.` by default.
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, 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, label = c("ctrl", "treat"))
    s <- gl(2, 1, 8, label = c("M", "F"))
    interaction(a, b)
    interaction(a, b, s, sep = ":")

[Package base version 2.4.1 Index]