Find the height of the least common ancestor of two nodes in a dendrogram in R (requires dendextend package)

lca_height.R

# install.packages("dendexted")

lca_height <- function(dend,
                       input_a,
                       input_b) {

  subtrees <- dendextend::partition_leaves(dend)

  find_ancestors <- function(input_labels) {
    which(sapply(subtrees, function(x) input_labels %in% x))
  }

  ancestors <- lapply(c(input_a, input_b), find_ancestors)
  # find largest common
  lca <- ancestors[[1]][max(which(ancestors[[1]] %in% ancestors[[2]]))]

  # which column represents "evolutionary" height
  index <-  which(sapply(as.data.frame(dendextend::get_nodes_xy(dend)), max) != attributes(dend)$members)[[1]]
  dendextend::get_nodes_xy(dend)[lca, index]
}