From 939e8ddd70844f8c0539e1d297ec6aa3b43ae702 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Thu, 10 Jul 2025 08:49:49 +0000
Subject: [PATCH 1/2] Initial plan


From 19f69ed770f5dc196f0c8e2bf5572ab360056bc9 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Thu, 10 Jul 2025 13:39:19 +0000
Subject: [PATCH 2/2] Fix disjoint_union to handle mixed named/unnamed graphs

- Add logic to detect when some graphs have vertex names and others don't
- Generate generic vertex names (V1, V2, etc.) for unnamed graphs
- Respect existing V-pattern names by continuing the sequence
- Add comprehensive tests for the new functionality
- Fixes issue #1946

Co-authored-by: krlmlr <1741643+krlmlr@users.noreply.github.com>
---
 R/operators.R                   | 24 ++++++++++++++++++++
 tests/testthat/test-operators.R | 40 +++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+)

diff --git a/R/operators.R b/R/operators.R
index 8722413f9a1..d1fcca95136 100644
--- a/R/operators.R
+++ b/R/operators.R
@@ -228,6 +228,30 @@ disjoint_union <- function(...) {
   )
   lapply(graphs, ensure_igraph)
 
+  ## Handle mixed named/unnamed graphs by generating generic names
+  have_names <- sapply(graphs, is_named)
+  if (sum(have_names) > 0 && sum(have_names) < length(graphs)) {
+    # Some graphs have names, others don't - generate names for unnamed graphs
+    existing_names <- unlist(lapply(graphs[have_names], function(g) V(g)$name))
+    
+    # Create a counter for generating new names (check if named graph already has generic names)
+    prefix <- "V"
+    name_counter <- if (any(grepl(paste0("^", prefix, "[0-9]+$"), existing_names))) {
+      max(as.integer(gsub(paste0("^", prefix, "([0-9]+)$"), "\\1", existing_names)), na.rm = TRUE) + 1
+    } else {
+      1
+    }
+    
+    for (i in seq_along(graphs)) {
+      if (!have_names[i]) {
+        n <- vcount(graphs[[i]])
+        num_id <- seq(name_counter, name_counter + n - 1)
+        V(graphs[[i]])$name <- paste0(prefix, num_id)
+        name_counter <- name_counter + n
+      }
+    }
+  }
+
   on.exit(.Call(R_igraph_finalizer))
   res <- .Call(R_igraph_disjoint_union, graphs)
 
diff --git a/tests/testthat/test-operators.R b/tests/testthat/test-operators.R
index afa804d5f16..5a1499ed435 100644
--- a/tests/testthat/test-operators.R
+++ b/tests/testthat/test-operators.R
@@ -345,6 +345,46 @@ test_that("disjoint union gives warning for non-unique vertex names", {
   )
 })
 
+test_that("disjoint union handles mixed named/unnamed graphs", {
+  # Test named + unnamed graphs
+  g1 <- make_ring(4)
+  g2 <- make_ring(3)
+  V(g1)$name <- c("A", "B", "C", "D")
+
+  result <- disjoint_union(g1, g2)
+  
+  expect_equal(V(result)$name, c("A", "B", "C", "D", "V1", "V2", "V3"))
+  expect_equal(vcount(result), 7)
+  expect_equal(ecount(result), 7)
+  
+  # Test with existing V-pattern names
+  g3 <- make_ring(3)
+  g4 <- make_ring(2)
+  V(g3)$name <- c("V1", "V2", "V3")
+  
+  result2 <- disjoint_union(g3, g4)
+  expect_equal(V(result2)$name, c("V1", "V2", "V3", "V4", "V5"))
+  
+  # Test unnamed + named graphs (should work the same)
+  result3 <- disjoint_union(g2, g1)
+  expect_equal(V(result3)$name, c("V1", "V2", "V3", "A", "B", "C", "D"))
+  
+  # Test all unnamed graphs (should not add names)
+  g5 <- make_ring(2)
+  g6 <- make_ring(2)
+  result4 <- disjoint_union(g5, g6)
+  expect_true(is.null(V(result4)$name) || all(is.na(V(result4)$name)))
+  
+  # Test all named graphs (should work as before)
+  g7 <- make_ring(2)
+  g8 <- make_ring(2)
+  V(g7)$name <- c("X", "Y")
+  V(g8)$name <- c("P", "Q")
+  
+  result5 <- disjoint_union(g7, g8)
+  expect_equal(V(result5)$name, c("X", "Y", "P", "Q"))
+})
+
 
 test_that("union of unnamed graphs works", {
   g1 <- make_ring(10)