fix: use vertex count instead of edge count for empty graph check

- Changed check from ecount(e1) > 0 to vcount(e1) > 0
- Empty graphs are now correctly defined as having 0 vertices, not 0 edges
- Updated tests to use make_empty_graph() instead of make_empty_graph(1)
- Added tests for make_empty_graph(1) to verify it now errors correctly

Co-authored-by: krlmlr <1741643+krlmlr@users.noreply.github.com>

Author: Copilot

R/operators.R

@@ -1162,8 +1162,8 @@ path <- function(...) {
     )
 
     # 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) {
+    # Empty graphs have zero vertices
+    if (!is.null(nn) && !is_named(e1) && vcount(e1) > 0) {
       cli::cli_abort(
         "Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have {.code NA} names."
       )
@@ -1197,8 +1197,8 @@ path <- function(...) {
   } 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) {
+    # Empty graphs have zero vertices
+    if (!is_named(e1) && vcount(e1) > 0) {
       cli::cli_abort(
         "Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have {.code NA} names."
       )

tests/testthat/_snaps/operators.md

@@ -2,6 +2,10 @@
 
     Can't recycle `name` (size 2) to match `foo` (size 3).
 
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
 # vertices() errors on duplicate attribute names
 
     Duplicate attribute name in `vertices()`: "name".
@@ -34,3 +38,11 @@
 
     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

@@ -214,27 +214,28 @@ test_that("t() is aliased to edge reversal for graphs", {
 })
 
 test_that("vertices() works", {
-  g_all_unnamed <- make_empty_graph(1) + vertices("a", "b")
+  # Test adding named vertices to empty graph (0 vertices)
+  g_all_unnamed <- make_empty_graph() + vertices("a", "b")
   expect_s3_class(V(g_all_unnamed), "igraph.vs")
-  expect_identical(V(g_all_unnamed)$name, c(NA, "a", "b"))
+  expect_identical(V(g_all_unnamed)$name, c("a", "b"))
 
-  g_mix_named_unnamed <- make_empty_graph(1) + vertices("a", "b", foo = 5)
+  g_mix_named_unnamed <- make_empty_graph() + vertices("a", "b", foo = 5)
   expect_s3_class(V(g_mix_named_unnamed), "igraph.vs")
-  expect_true(is.na(V(g_mix_named_unnamed)$name[1]))
-  expect_identical(V(g_mix_named_unnamed)$name[-1], c("a", "b"))
-  expect_equal(V(g_mix_named_unnamed)$foo, c(NA, 5, 5))
+  expect_identical(V(g_mix_named_unnamed)$name, c("a", "b"))
+  expect_equal(V(g_mix_named_unnamed)$foo, c(5, 5))
 
-  g_mix_bigger_attribute <- make_empty_graph(1) +
+  g_mix_bigger_attribute <- make_empty_graph() +
     vertices("a", "b", "c", foo = 5:7, bar = 8)
   expect_s3_class(V(g_mix_bigger_attribute), "igraph.vs")
-  expect_identical(V(g_mix_bigger_attribute)$name, c(NA, "a", "b", "c"))
-  expect_equal(V(g_mix_bigger_attribute)$foo, c(NA, 5, 6, 7))
-  expect_equal(V(g_mix_bigger_attribute)$bar, c(NA, 8, 8, 8))
+  expect_identical(V(g_mix_bigger_attribute)$name, c("a", "b", "c"))
+  expect_equal(V(g_mix_bigger_attribute)$foo, c(5, 6, 7))
+  expect_equal(V(g_mix_bigger_attribute)$bar, c(8, 8, 8))
 
-  g_one_unnamed <- make_empty_graph(1) + vertices(letters)
+  g_one_unnamed <- make_empty_graph() + vertices(letters)
   expect_s3_class(V(g_one_unnamed), "igraph.vs")
-  expect_identical(V(g_one_unnamed)$name, c(NA, letters))
+  expect_identical(V(g_one_unnamed)$name, letters)
 
+  # Test adding unnamed vertices (attributes only) - should work with any graph
   g_all_named <- make_empty_graph(1) + vertices(foo = 5:7)
   expect_s3_class(V(g_all_named), "igraph.vs")
   expect_null(V(g_all_named)$name)
@@ -249,8 +250,14 @@ test_that("vertices() works", {
   expect_s3_class(V(g_none), "igraph.vs")
   expect_null(V(g_none)$name)
 
+  # Test that adding named vertices to non-empty graph with mismatched attributes errors
   expect_snapshot_error(
-    make_empty_graph(1) + vertices("a", "b", foo = 5:7)
+    make_empty_graph() + vertices("a", "b", foo = 5:7)
+  )
+
+  # Test that adding named vertices to non-empty unnamed graph errors
+  expect_snapshot_error(
+    make_empty_graph(1) + vertices("a", "b")
   )
 })
 
@@ -306,10 +313,14 @@ test_that("adding named vertices to non-empty unnamed graphs errors", {
   expect_no_error(make_ring(10) + vertices(foo = 1:3))
   expect_no_error(make_ring(10) + 5)
 
-  # No error when adding to empty graph
+  # No error when adding to empty graph (0 vertices)
   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"))
+
+  # Error when adding to non-empty unnamed graph (even with 1 vertex)
+  expect_snapshot_error(make_empty_graph(1) + vertex("a"))
+  expect_snapshot_error(make_empty_graph(1) + c("a", "b"))
 })