How to extract one specific group in dplyr

Question

Given a grouped tbl, can I extract one/few groups? Such function can be useful when prototyping code, e.g.:

mtcars %>%
  group_by(cyl) %>%
  select_first_n_groups(2) %>%
  do({'complicated expression'})

Surely, one can do an explicit filter before grouping, but that can be cumbersome.

Answer 1

With a bit of dplyr along with some nesting/unnesting (supported by tidyr package), you could establish a small helper to get the first (or any) group

first = function(x) x %>% nest %>% ungroup %>% slice(1) %>% unnest(data)
mtcars %>% group_by(cyl) %>% first()

By adjusting the slicing you could also extract the nth or any range of groups by index, but typically the first or the last is what most users want.

The name is inspired by functional APIs which all call it first (see stdlibs of i.e. kotlin, python, scala, java, spark).

Edit: Faster Version

A more scalable version (>50x faster on large datasets) that avoids nesting would be

first_group = function(x) x %>%
    select(group_cols()) %>%
    distinct %>%
    ungroup %>%
    slice(1) %>%
    { semi_join(x, .)}

A another positive side-effect of this improved version is that it fails if not grouping is present in x.