feat: error when adding named vertices to non-empty unnamed graphs

R/operators.R

@@ -1161,6 +1161,14 @@ path <- function(...) {
       e2[named]
     )
 
+    # Abort if adding named vertices to a non-empty unnamed graph
+    # Allow adding to graphs with no edges (like make_empty_graph(n))
+    if (!is.null(nn) && !is_named(e1) && ecount(e1) > 0) {
+      cli::cli_abort(
+        "Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have {.code NA} names."
+      )
+    }
+
     # When adding vertices via +, all unnamed arguments are interpreted as vertex names of the new vertices.
     res <- add_vertices(e1, nv = vctrs::vec_size_common(!!!e2), attr = e2)
   } else if ("igraph.path" %in% class(e2)) {
@@ -1188,6 +1196,13 @@ path <- function(...) {
     res <- add_vertices(e1, e2)
   } else if (is.character(e2)) {
     ## Adding named vertices
+    # Abort if adding named vertices to a non-empty unnamed graph
+    # Allow adding to graphs with no edges (like make_empty_graph(n))
+    if (!is_named(e1) && ecount(e1) > 0) {
+      cli::cli_abort(
+        "Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have {.code NA} names."
+      )
+    }
     res <- add_vertices(e1, length(e2), name = e2)
   } else {
     cli::cli_abort("Cannot add {.obj_type_friendly {type}} to igraph graph.")

tests/testthat/_snaps/operators.md

@@ -18,3 +18,19 @@
 
     Duplicate attribute names in `vertices()`: "foo" and "bar".
 
+# adding named vertices to non-empty unnamed graphs errors
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+

tests/testthat/test-operators.R

@@ -249,7 +249,9 @@ test_that("vertices() works", {
   expect_s3_class(V(g_none), "igraph.vs")
   expect_null(V(g_none)$name)
 
-  expect_snapshot_error(make_empty_graph(1) + vertices("a", "b", foo = 5:7))
+  expect_snapshot_error(
+    make_empty_graph(1) + vertices("a", "b", foo = 5:7)
+  )
 })
 
 test_that("vertices() errors on duplicate attribute names", {
@@ -274,6 +276,43 @@ test_that("vertices() errors on duplicate attribute names", {
   )
 })
 
+test_that("adding named vertices to non-empty unnamed graphs errors", {
+  # Test with vertex() function
+  expect_snapshot_error(
+    make_ring(10) + vertex(1)
+  )
+
+  expect_snapshot_error(
+    make_ring(10) + vertex("a")
+  )
+
+  expect_snapshot_error(
+    make_ring(10) + vertices("a", "b")
+  )
+
+  # Test with character vector
+  expect_snapshot_error(
+    make_ring(10) + c("a", "b")
+  )
+
+  # No error when adding to named graph
+  g <- make_ring(10)
+  V(g)$name <- letters[1:10]
+  expect_no_error(g + vertex("k"))
+  expect_no_error(g + c("x", "y"))
+
+  # No error when adding unnamed vertices
+  expect_no_error(make_ring(10) + vertex(foo = 5))
+  expect_no_error(make_ring(10) + vertices(foo = 1:3))
+  expect_no_error(make_ring(10) + 5)
+
+  # No error when adding to empty graph
+  expect_no_error(make_empty_graph() + vertex("a"))
+  expect_no_error(make_empty_graph() + c("a", "b"))
+  expect_no_error(make_empty_graph() + vertices("a", "b"))
+})
+
+
 test_that("infix operators work", {
   g <- make_ring(10)
   V(g)$name <- letters[1:10]