R/transform_to_min_spanning_tree.R

In DiagrammeR: Graph/Network Visualization

#' Get a minimum spanning tree subgraph
#'
#' @description
#'
#' Get a minimum spanning tree subgraph for a connected graph of class
#' `dgr_graph`.
#'
#' @inheritParams render_graph
#'
#' @return A graph object of class `dgr_graph`.
#'
#' @examples
#' # Create a random graph using the
#' # `add_gnm_graph()` function
#' graph <-
#'   create_graph() %>%
#'   add_gnm_graph(
#'     n = 10,
#'     m = 15,
#'     set_seed = 23)
#'
#' # Obtain Jaccard similarity
#' # values for each pair of
#' # nodes as a square matrix
#' j_sim_matrix <-
#'   graph %>%
#'     get_jaccard_similarity()
#'
#' # Create a weighted, undirected
#' # graph from the resultant matrix
#' # (effectively treating that
#' # matrix as an adjacency matrix)
#' graph <-
#'   j_sim_matrix %>%
#'   from_adj_matrix(weighted = TRUE)
#'
#' # The graph in this case is a fully connected graph
#' # with loops, where jaccard similarity values are
#' # assigned as edge weights (edge attribute `weight`);
#' # The minimum spanning tree for this graph is the
#' # connected subgraph where the edges retained have
#' # the lowest similarity values possible
#' min_spanning_tree_graph <-
#'   graph %>%
#'   transform_to_min_spanning_tree() %>%
#'   copy_edge_attrs(
#'     edge_attr_from = weight,
#'     edge_attr_to = label) %>%
#'   set_edge_attrs(
#'     edge_attr = fontname,
#'     values = "Helvetica") %>%
#'   set_edge_attrs(
#'     edge_attr = color,
#'     values = "gray85") %>%
#'   rescale_edge_attrs(
#'     edge_attr_from = weight,
#'     to_lower_bound = 0.5,
#'     to_upper_bound = 4.0,
#'       edge_attr_to = penwidth)
#'
#' @export
transform_to_min_spanning_tree <- function(graph) {

  # Get the name of the function
  fcn_name <- get_calling_fcn()

  # Validation: Graph object is valid
  if (graph_object_valid(graph) == FALSE) {

    emit_error(
      fcn_name = fcn_name,
      reasons = "The graph object is not valid")
  }

  # Transform the graph to an igraph object
  igraph <- to_igraph(graph)

  # Get the minimum spanning tree
  igraph_mst <- igraph::mst(igraph)

  # Generate the graph object from an igraph graph
  from_igraph(igraph_mst)
}