Is there a function to return the number of groups?

[Mullefa]

Wasn't sure if there was. Currently I'm using this function:

n_groups <- function(tbl) {
  g <- groups(tbl)

  if (is.null(g)) {
    tbl_nm <- deparse(substitute(tbl))
    warning(tbl_nm, " is not grouped", immediate. = TRUE)
    0
  } else {
    g <- unlist(lapply(g, as.character))
    nrow(unique(tbl[, g, drop = FALSE]))
  }
}

[hadley]

Could you please give me an example of where you need this?