Count occurence of values for every possible pair

Question

I have a list of ids and places where these ids have been. Now I want to find pairs of ids that have most places in common.

My data frame looks like this:

Answer 1

Or...

library(dplyr)

df1 %>%
  left_join(df1, by = "place") %>%
  filter(id.x < id.y) %>%
  group_by(id.x, id.y) %>%
  summarise(count = n())

EDIT: If IDs are factors operator < won't work. Conversion adds another line to the solution (credits to Steven Beaupré):

df1 %>%
  left_join(df1, by = "place") %>%
  mutate_each(funs(as.character(.))) %>%
  filter(id.x < id.y) %>%
  group_by(id.x, id.y) %>%
  summarise(count = n())