Intersect all possible combinations of list elements

Question

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         


        

Answer 1

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