Pseudo aggregator.

Take a vector or list and return the first element (pseudo-aggregation or projection). If the argument length is zero or there are different items throw in an error.

psagg(x, ..., strict = TRUE)

Arguments

x	should be a vector or list of items.
...	force later arguments to be passed by name
strict	logical, should we check value uniqueness.

Value

x[[1]] (or throw if not all items are equal or this is an empty vector).

Details

This function is useful in some split by column situations as a safe and legible way to convert vectors to scalars.

Examples

d <- data.frame(
  group = c("a", "a", "b"),
  stringsAsFactors = FALSE)
dl <- lapply(
  split(d, d$group),
  function(di) {
    data.frame(
      # note: di$group is a possibly length>1 vector!
      # pseudo aggregate it to the value that is
      # constant for each group, confirming it is constant.
      group_label = psagg(di$group),
      group_count = nrow(di),
      stringsAsFactors = FALSE
    )
  })
do.call(rbind, dl)
#>   group_label group_count
#> a           a           2
#> b           b           1