Add an option for a single subdomain ID for the entire GMG

refs idaholab#27646

framework/src/meshgenerators/GeneratedMeshGenerator.C

@@ -48,8 +48,10 @@ GeneratedMeshGenerator::validParams()
                              "The type of element from libMesh to "
                              "generate (default: linear element for "
                              "requested dimension)");
-  params.addParam<std::vector<SubdomainID>>("subdomain_ids",
-                                            "Subdomain IDs for each element, default to all zero");
+  params.addParam<std::vector<SubdomainID>>(
+      "subdomain_ids",
+      "Subdomain IDs for each element, default to all zero. If a single number is specified, that "
+      "subdomain id is used for all element.");
   params.addParam<SubdomainName>("subdomain_name",
                                  "If specified, single subdomain name for all elements");
 
@@ -189,17 +191,20 @@ GeneratedMeshGenerator::generate()
   if (_has_subdomain_ids)
   {
     auto & bids = getParam<std::vector<SubdomainID>>("subdomain_ids");
-    if (bids.size() != _nx * _ny * _nz)
+    if (bids.size() != _nx * _ny * _nz && bids.size() != 1)
       paramError("subdomain_ids",
-                 "Size must equal to the product of number of elements in all directions");
+                 "Size must equal to the product of number of elements in all directions, or one.");
     for (auto & elem : mesh->element_ptr_range())
     {
       const Point p = elem->vertex_average();
       unsigned int ix = std::floor((p(0) - _xmin) / (_xmax - _xmin) * _nx);
       unsigned int iy = std::floor((p(1) - _ymin) / (_ymax - _ymin) * _ny);
       unsigned int iz = std::floor((p(2) - _zmin) / (_zmax - _zmin) * _nz);
       unsigned int i = iz * _nx * _ny + iy * _nx + ix;
-      elem->subdomain_id() = bids[i];
+      if (bids.size() == 1)
+        elem->subdomain_id() = bids[0];
+      else
+        elem->subdomain_id() = bids[i];
     }
   }