dplyr - filter by group size

Question

What is the best way to filter a data.frame to only get groups of say size 5?

So my data looks as follows:

require(dplyr)
n <- 1e5
x <- rnorm(n         


        

Answer 1

I generalised the function written by docendo discimus, to use it alongside existing dplyr functions:

#' inherit dplyr::filter
#' @param min minimal group size, use \code{min = NULL} to filter on maximal group size only
#' @param max maximal group size, use \code{max = NULL} to filter on minimal group size only
#' @export
#' @source Stack Overflow answer by docendo discimus, \url{https://stackoverflow.com/a/43110620/4575331}
filter_group_size <- function(.data, min = NULL, max = min) {
  g <- dplyr::group_size(.data)
  if (is.null(min) & is.null(max)) {
    stop('`min` and `max` cannot both be NULL.')
  }
  if (is.null(max)) {
    max <- base::max(g, na.rm = TRUE)
  }
  ind <- base::rep(g >= min & g <= max, g)
  .data[ind, ]
}

Let's check it for a minimal group size of 5:

dat2 %>%
  group_by(cat) %>%
  filter_group_size(5, NULL) %>%
  summarise(n = n()) %>%
  arrange(desc(n))

# # A tibble: 6,634 x 2
#      cat     n
#     
#  1    NA    19
#  2     1     5
#  3     2     5
#  4     6     5
#  5    15     5
#  6    17     5
#  7    21     5
#  8    27     5
#  9    33     5
# 10    37     5
# # ... with 6,624 more rows

Great, now check for the OP's question; a group size of exactly 5:

dat2 %>%
  group_by(cat) %>%
  filter_group_size(5) %>%
  summarise(n = n()) %>%
  pull(n) %>%
  unique()
# [1] 5

Hooray.