group_by: Group by one or more variables
In YTLogos/dplyr: A Grammar of Data Manipulation

Description Usage Arguments Tbl types Scoped grouping Examples

Most data operations are usefully done on groups defined by variables. group_by() takes an existing tbl and converts it into a grouped tbl where operations are performed "by group". ungroup() removes grouping.

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group_by(.data, ..., add = FALSE)

ungroup(x, ...)

`.data`	a tbl
`...`	Variables to group by. All tbls accept variable names, some will also accept functions of variables. Duplicated groups will be silently dropped.
`add`	When `add = FALSE`, the default, `group_by()` will override existing groups. To instead add to the existing groups, use `add = TRUE`
`x`	A `tbl()`

group_by() is an S3 generic with methods for the three built-in tbls. See the help for the corresponding classes and their manip methods for more details:

data.frame: grouped_df
data.table: dtplyr::grouped_dt
SQLite: src_sqlite()
PostgreSQL: src_postgres()
MySQL: src_mysql()

The three scoped variants (group_by_all(), group_by_if() and group_by_at()) make it easy to group a dataset by a selection of variables.

by_cyl <- mtcars %>% group_by(cyl)

# grouping doesn't change how the data looks (apart from listing
# how it's grouped):
by_cyl

# It changes how it acts with the other dplyr verbs:
by_cyl %>% summarise(
  disp = mean(disp),
  hp = mean(hp)
)
by_cyl %>% filter(disp == max(disp))

# Each call to summarise() removes a layer of grouping
by_vs_am <- mtcars %>% group_by(vs, am)
by_vs <- by_vs_am %>% summarise(n = n())
by_vs
by_vs %>% summarise(n = sum(n))

# To removing grouping, use ungroup
by_vs %>%
  ungroup() %>%
  summarise(n = sum(n))

# You can group by expressions: this is just short-hand for
# a mutate/rename followed by a simple group_by
mtcars %>% group_by(vsam = vs + am)

# By default, group_by overrides existing grouping
by_cyl %>%
  group_by(vs, am) %>%
  group_vars()

# Use add = TRUE to instead append
by_cyl %>%
  group_by(vs, am, add = TRUE) %>%
  group_vars()