Find common third on large data set

Question

I have a large dataframe like

df <- data.frame(group= c("a","a","b","b","b","c"),
             person = c("Tom","Jerry","Tom","Anna","Sam","Nic"), stringsAsFactors = FALSE)

df
    group person
1     a    Tom
2     a  Jerry
3     b    Tom
4     b   Anna
5     b    Sam
6     c    Nic

and would like to get as a result

df.output
  pers1 pers2 person_in_common
1  Anna Jerry              Tom
2 Jerry   Sam              Tom
3   Sam   Tom             Anna
4  Anna   Tom              Sam
6  Anna   Sam              Tom

The result dataframe gives basically a table with all pairs of persons who have another person in common. I found a way to do it in SQL but it takes an awfully long time so I wonder if there is a efficient way to do it in R

Answer 1

Here's one using igraph package. The basic idea is to create a graph and then extract two adjacent nodes for each node.

library(igraph)
X1 = split(df$person, df$group)
X2 = X1[lengths(X1) >= 2]
dat = data.frame(do.call(rbind, unlist(lapply(X2, function(x)
            combn(x, 2, sort, FALSE)), recursive = FALSE)))
g = graph.data.frame(dat, directed = FALSE)
mydf = data.frame(as.matrix(get.adjacency(g)))
mydf = mydf[colSums(mydf) > 1]
ANS = sapply(mydf, function(x) t(combn(row.names(mydf)[which(x == 1)], 2)))
do.call(rbind, lapply(names(ANS), function(nm) data.frame(ANS[[nm]], nm)))
#     X1   X2   nm
#1   Sam  Tom Anna
#2  Anna  Tom  Sam
#3 Jerry Anna  Tom
#4 Jerry  Sam  Tom
#5  Anna  Sam  Tom

OR

mynames = unique(do.call(c, X2))
do.call(rbind,
        lapply(mynames, function(x){
            L = V(g)$name[unlist(adjacent_vertices(graph = g, v = x))]
            if(length(L) >= 2){
                setNames(data.frame(t(combn(L, 2)), x), c("P1", "P2", "P3"))
            }else{
                setNames(data.frame(NA, NA, x), c("P1", "P2", "P3"))
            }
        }))
#     P1   P2    P3
#1 Jerry Anna   Tom
#2 Jerry  Sam   Tom
#3  Anna  Sam   Tom
#4  <NA> <NA> Jerry
#5   Sam  Tom  Anna
#6  Anna  Tom   Sam