Compute the intersection of all combinations of a list of vectors

overlap.R

## Compute the intersection of all combinations of the
## elements in the list of vectors l. Might be useful
## for generating Venn/Euler diagrams.
## There might be better ways to do this!
overlap <- function(l) {
  results <- list()
  
  # combinations of m elements of list l
  for (m in seq(along=l)) {
    
    # generate and iterate through combinations of length m
    for (indices in combn(seq(length(l)), m, simplify=FALSE)) {
      
      # make name by concatenating the names of the elements
      # of l that we're intersecting
      name <- do.call(paste, c(as.list(names(l)[indices]), sep="_"))

      # adding the init=l[indices[1]] parameter helps with the case
      # where we're only dealing with one set, i==1 and length(indices)==1,
      # and we want only unique items in that set.
      # Reduce(intersect, list(c(1,2,3,3))) => c(1,2,3,3)
      # Reduce(intersect, list(c(1,2,3,3)), init=l[[indices[1]]]) => c(1,2,3)
      results[[name]] <- Reduce(intersect, l[indices], init=l[[indices[1]]])
    }
  }
  results
}

# test it out!
overlap( list(foo=c('a','b','c','d','e','e'), bar=c('a','c','e','f','g'), bat=c('a','b','c','d','g')))