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

You may try

library(reshape2)
tbl <-  crossprod(table(df1[2:1]))
tbl[upper.tri(tbl, diag=TRUE)] <- 0
res <- subset(melt(tbl), value!=0)
colnames(res) <- c(paste0('pair',1:2), 'count')
row.names(res) <- NULL
res
#   pair1 pair2 count
#1    Joe  Dave     1
#2 Stuart  Dave     3
#3 Stuart   Joe     2

Or another option is

Subdf <- subset(merge(df1, df1, by.x='place',
               by.y='place'), id.x!=id.y)
Subdf[-1] <- t(apply(Subdf[-1], 1, sort))
aggregate(place~., unique(Subdf), FUN=length)
#  id.x   id.y place
#1 Dave    Joe     1
#2 Dave Stuart     3
#3  Joe Stuart     2