[WIP] Adding vertex names when a named and unnamed graph are combined?

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)
 

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)