r - Find duplicated elements with dplyr

Question

I tried using the code presented here to find ALL duplicated elements with dplyr like this:

library(dplyr)

mtcars %>%
mutate(cyl.dup = cyl[duplicated(cyl) | duplicated(cyl, from.last = TRUE)])

How can I convert code presented here to find ALL duplicated elements with dplyr? My code above just throws an error? Or even better, is there another function that will achieve this more succinctly than the convoluted x[duplicated(x) | duplicated(x, from.last = TRUE)]) approach?

Answer 1

I guess you could use filter for this purpose:

mtcars %>% 
  group_by(carb) %>% 
  filter(n()>1)

Small example (note that I added summarize() to prove that the resulting data set does not contain rows with duplicate 'carb'. I used 'carb' instead of 'cyl' because 'carb' has unique values whereas 'cyl' does not):

mtcars %>% group_by(carb) %>% summarize(n=n())
#Source: local data frame [6 x 2]
#
#  carb  n
#1    1  7
#2    2 10
#3    3  3
#4    4 10
#5    6  1
#6    8  1

mtcars %>% group_by(carb) %>% filter(n()>1) %>% summarize(n=n())
#Source: local data frame [4 x 2]
#
#  carb  n
#1    1  7
#2    2 10
#3    3  3
#4    4 10