1#' Combine levels from two or more factors to create a new factor
2#'
3#' Computes a factor whose levels are all the combinations of the levels of the input factors.
4#'
5#' @param ...  <[`dynamic-dots`][rlang::dyn-dots]> Additional factors
6#'   or character vectors.
7#' @param sep A character string to separate the levels
8#' @param keep_empty If TRUE, keep combinations with no observations as levels
9#' @return The new factor
10#'
11#' @export
12#' @examples
13#' fruit <- factor(c("apple", "kiwi", "apple", "apple"))
14#' colour <- factor(c("green", "green", "red", "green"))
15#' eaten <- c("yes", "no", "yes", "no")
16#' fct_cross(fruit, colour)
17#' fct_cross(fruit, colour, eaten)
18#' fct_cross(fruit, colour, keep_empty = TRUE)
19fct_cross <- function(..., sep = ":", keep_empty = FALSE) {
20
21  flist <- list2(...)
22  if (length(flist) == 0) {
23    return(factor())
24  }
25
26  .data <- tibble::as_tibble(flist, .name_repair = "minimal")
27  .data <- lapply(.data, check_factor)
28
29  newf <- exec(paste, !!!.data, sep = sep)
30
31  old_levels <- lapply(.data, levels)
32  grid <- exec(expand.grid, old_levels)
33  new_levels <- exec(paste, !!!grid, sep = sep)
34
35  if (!keep_empty) {
36    new_levels <- intersect(new_levels, newf)
37  }
38  factor(newf, levels = new_levels)
39}
40