feat: New largest_component() returns the largest connected component

NAMESPACE

@@ -574,6 +574,7 @@ export(laplacian_matrix)
 export(largest.cliques)
 export(largest.independent.vertex.sets)
 export(largest_cliques)
+export(largest_component)
 export(largest_ivs)
 export(largest_weighted_cliques)
 export(last_cit)

R/components.R

@@ -200,3 +200,20 @@ bridges <- bridges_impl
 #' @family components
 #' @export
 biconnected_components <- biconnected_components_impl
+
+
+#' @rdname components
+#' @family components
+#' @export
+largest_component <- function(graph, mode = c("weak", "strong")) {
+  if (!is_igraph(graph)) {
+    stop("Not a graph object")
+  }
+
+  comps <- components(graph, mode = mode)
+
+  lcc_id <- which.max(comps$csize)
+  vids <- V(graph)[comps$membership == lcc_id]
+
+  induced_subgraph(graph, vids)
+}

R/structural.properties.R

@@ -1846,6 +1846,11 @@ dfs <- function(graph, root, mode = c("out", "in", "all", "total"),
 #' `component_distribution()` creates a histogram for the maximal connected
 #' component sizes.
 #'
+#' `largest_component()` returns the largest connected component of a graph. For
+#' directed graphs, optionally the largest weakly or strongly connected component.
+#' In case of a tie, the first component by vertex ID order is returned. Vertex
+#' IDs from the original graph are not retained in the returned graph.
+#'
 #' The weakly connected components are found by a simple breadth-first search.
 #' The strongly connected components are implemented by two consecutive
 #' depth-first searches.
@@ -1870,6 +1875,8 @@ dfs <- function(graph, root, mode = c("out", "in", "all", "total"),
 #'   frequencies. The length of the vector is the size of the largest component
 #'   plus one. Note that (for currently unknown reasons) the first element of the
 #'   vector is the number of clusters of size zero, so this is always zero.
+#'
+#'   For `largest_component()` the largest connected component of the graph.
 #' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
 #' @seealso [decompose()], [subcomponent()], [groups()]
 #' @family structural.properties
@@ -1880,7 +1887,7 @@ dfs <- function(graph, root, mode = c("out", "in", "all", "total"),
 #' g <- sample_gnp(20, 1 / 20)
 #' clu <- components(g)
 #' groups(clu)
-#'
+#' largest_component(g)
 components <- function(graph, mode = c("weak", "strong")) {
   # Argument checks
   ensure_igraph(graph)

tests/testthat/test-components.R

@@ -0,0 +1,68 @@
+test_that("count_components counts correctly", {
+    g <- make_star(20, "undirected")
+    h <- make_ring(10)
+
+    G <- disjoint_union(g, h)
+
+    expect_that(count_components(G), equals(2L))
+})
+
+test_that("a null graph has zero components", {
+    g <- make_empty_graph(0)
+
+    expect_that(count_components(g), equals(0L))
+})
+
+test_that("component_distribution finds correct distribution", {
+    g <- graph_from_literal(
+        A,
+        B - C,
+        D - E - F,
+        G - H
+    )
+
+    ref <- c(0.00, 0.25, 0.50, 0.25)
+
+    expect_that(component_distribution(g), equals(ref))
+})
+
+test_that("largest component is actually the largest", {
+    g <- make_star(20, "undirected")
+    h <- make_ring(10)
+
+    G <- disjoint_union(g, h)
+
+    expect_true(isomorphic(largest_component(G), g))
+})
+
+test_that("largest strongly and weakly components are correct", {
+    g <- graph_from_literal(
+        A -+ B,
+        B -+ C,
+        C -+ A,
+        C -+ D,
+        E
+    )
+
+    strongly <- graph_from_literal(
+        A -+ B,
+        B -+ C,
+        C -+ A
+    )
+    weakly <- graph_from_literal(
+        A -+ B,
+        B -+ C,
+        C -+ A,
+        C -+ D
+    )
+
+    expect_true(isomorphic(largest_component(g, "weak"), weakly))
+    expect_true(isomorphic(largest_component(g, "strong"), strongly))
+})
+
+test_that("the largest component of a null graph is a valid null graph", {
+    nullgraph <- make_empty_graph(0)
+
+    expect_true(isomorphic(largest_component(make_empty_graph(0)), nullgraph))
+})
+