Intersect all possible combinations of list elements

combinations, combn, intersect, list, r

Solution

`combn` works with list structures as well, you just need a little `unlist`'ing of the result to use `intersect`...

# Get the combinations of names of list elements
nms <- combn( names(l) , 2 , FUN = paste0 , collapse = "" , simplify = FALSE )

# Make the combinations of list elements
ll <- combn( l , 2 , simplify = FALSE )

# Intersect the list elements
out <- lapply( ll , function(x) length( intersect( x[[1]] , x[[2]] ) ) )

# Output with names
setNames( out , nms )
#$AB
#[1] 2

#$AC
#[1] 2

#$AD
#[1] 0

#$BC
#[1] 1

#$BD
#[1] 0

#$CD
#[1] 1

Problem

I have a list of vectors: ``` > l <- list(A=c("one", "two", "three", "four"), B=c("one", "two"), C=c("two", "four", "five", "six"), D=c("six", "seven")) > l $A [1] "one" "two" "three" "four" $B [1] "one" "two" $C [1] "two" "four" "five" "six" $D [1] "six" "seven" ``` I would like to calculate the length of the overlap between all possible pairwise combinations of the list elements, i.e. (the format of the result doesn't matter): ``` AintB 2 AintC 2 AintD 0 BintC 1 BintD 0 CintD 1 ``` I know `combn(x, 2)` can be used to get a matrix of all possible pairwise combinations in a vector and that `length(intersect(a, b))` would give me the length of the overlap of two vectors, but I can't think of a way to put the two things together. Any help is much appreciated! Thanks.

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