feat: Add count_loops() to R interface (#2414)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: krlmlr <[email protected]>
Co-authored-by: Kirill Müller <[email protected]>

Author: Copilot

NAMESPACE

@@ -273,6 +273,7 @@ export(count.multiple)
 export(count_automorphisms)
 export(count_components)
 export(count_isomorphisms)
+export(count_loops)
 export(count_max_cliques)
 export(count_motifs)
 export(count_multiple)

R/structural-properties.R

@@ -2475,6 +2475,8 @@ girth <- function(graph, circle = TRUE) {
 #'
 #' `which_loop()` decides whether the edges of the graph are loop edges.
 #'
+#' `count_loops()` counts the total number of loop edges in the graph.
+#'
 #' `any_multiple()` decides whether the graph has any multiple edges.
 #'
 #' `which_multiple()` decides whether the edges of the graph are multiple
@@ -2496,6 +2498,7 @@ girth <- function(graph, circle = TRUE) {
 #'   all edges in the graph.
 #' @return `any_loop()` and `any_multiple()` return a logical scalar.
 #'   `which_loop()` and `which_multiple()` return a logical vector.
+#'   `count_loops()` returns a numeric scalar with the total number of loop edges.
 #'   `count_multiple()` returns a numeric vector.
 #' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
 #' @seealso [simplify()] to eliminate loop and multiple edges.
@@ -2508,6 +2511,7 @@ girth <- function(graph, circle = TRUE) {
 #' g <- make_graph(c(1, 1, 2, 2, 3, 3, 4, 5))
 #' any_loop(g)
 #' which_loop(g)
+#' count_loops(g)
 #'
 #' # Multiple edges
 #' g <- sample_pa(10, m = 3, algorithm = "bag")
@@ -2569,6 +2573,14 @@ any_loop <- function(graph) {
     graph = graph
   )
 }
+#' @rdname which_multiple
+#' @export
+#' @cdocs igraph_count_loops
+count_loops <- function(graph) {
+  count_loops_impl(
+    graph = graph
+  )
+}
 
 
 #' Breadth-first search

man/which_multiple.Rd

@@ -963,6 +963,31 @@ test_that("any_multiple(), count_multiple(), which_multiple() works", {
   )
 })
 
+test_that("any_loop(), which_loop(), count_loops() works", {
+  # Graph with loops
+  g <- make_graph(c(1, 1, 2, 2, 3, 3, 4, 5))
+  expect_true(any_loop(g))
+  expect_equal(which_loop(g), c(TRUE, TRUE, TRUE, FALSE))
+  expect_equal(count_loops(g), 3)
+
+  # Graph without loops
+  g2 <- make_graph(c(1, 2, 2, 3, 3, 4))
+  expect_false(any_loop(g2))
+  expect_equal(which_loop(g2), c(FALSE, FALSE, FALSE))
+  expect_equal(count_loops(g2), 0)
+
+  # Empty graph
+  g3 <- make_empty_graph(n = 5)
+  expect_false(any_loop(g3))
+  expect_equal(count_loops(g3), 0)
+
+  # Graph with only loops
+  g4 <- make_graph(c(1, 1, 2, 2, 3, 3))
+  expect_true(any_loop(g4))
+  expect_equal(which_loop(g4), c(TRUE, TRUE, TRUE))
+  expect_equal(count_loops(g4), 3)
+})
+
 test_that("edge_density works", {
   g <- sample_gnp(50, 4 / 50)
   gd <- edge_density(g)