fix: Use "weights" edge attribute in modularity() if available

Author: krlmlr

R/community.R

@@ -919,6 +919,9 @@ modularity.igraph <- function(
     cli::cli_abort("Membership is not a numerical vector")
   }
   membership <- as.numeric(membership)
+  if (is.null(weights) && "weight" %in% edge_attr_names(x)) {
+    weights <- E(x)$weight
+  }
   if (!is.null(weights) && any(!is.na(weights))) {
     weights <- as.numeric(weights)
   } else {

tests/testthat/test-community.R

@@ -574,14 +574,21 @@ test_that("modularity() handles NA weights correctly", {
   membership_vec <- membership(comm)
 
   # Test that modularity works with regular weights
-  E(g)$weight <- runif(ecount(g))
-  mod_with_weights <- modularity(g, membership_vec, weights = E(g)$weight)
+  gw <- g
+  E(gw)$weight <- runif(ecount(gw))
+  mod_with_weights <- modularity(gw, membership_vec)
   expect_true(is.numeric(mod_with_weights))
   expect_length(mod_with_weights, 1)
 
+  mod_with_implicit_weights <- modularity(gw, membership_vec, weights = NULL)
+  expect_equal(mod_with_implicit_weights, mod_with_weights)
+
+  mod_with_explicit_weights <- modularity(g, membership_vec, weights = E(gw)$weight)
+  expect_equal(mod_with_explicit_weights, mod_with_weights)
+
   # Test that modularity works when all weights are NA
   mod_with_all_na <- modularity(g, membership_vec, weights = rep(NA, ecount(g)))
-  mod_without_weights <- modularity(g, membership_vec, weights = NULL)
+  mod_without_weights <- modularity(g, membership_vec)
   expect_equal(mod_with_all_na, mod_without_weights)
 
   # Test that modularity works when some weights are NA and some are not
@@ -592,12 +599,6 @@ test_that("modularity() handles NA weights correctly", {
   expect_length(mod_with_mixed_na, 1)
 
   # Test edge case: empty weights vector treated as NA
-  mod_with_empty <- modularity(g, membership_vec, weights = numeric(0))
+  mod_with_empty <- modularity(gw, membership_vec, weights = numeric(0))
   expect_equal(mod_with_empty, mod_without_weights)
-
-  # Test that when all weights are NA, it's equivalent to unweighted
-  all_na_weights <- rep(NA_real_, ecount(g))
-  mod_all_na <- modularity(g, membership_vec, weights = all_na_weights)
-  mod_unweighted <- modularity(g, membership_vec)
-  expect_equal(mod_all_na, mod_unweighted)
 })