GetEntities.cpp

Example demonstrating entity querying and connectivity access.This example demonstrates how to:

• Get all entities in the mesh database
• Access entity connectivity (vertex connectivity for elements)
• Query vertex adjacencies (elements connected to vertices)
• Iterate through entities and examine their properties

The program reads a mesh file and reports connectivity information for all non-vertex entities and adjacency information for vertices.

To run: ./GetEntities meshfile

/** @example GetEntities.cpp
 * @brief Example demonstrating entity querying and connectivity access.
 *
 * This example demonstrates how to:
 * - Get all entities in the mesh database
 * - Access entity connectivity (vertex connectivity for elements)
 * - Query vertex adjacencies (elements connected to vertices)
 * - Iterate through entities and examine their properties
 *
 * The program reads a mesh file and reports connectivity information
 * for all non-vertex entities and adjacency information for vertices.
 *
 * To run: ./GetEntities [meshfile]
 * (default: uses hex01.vtk in the current directory)
 */
 
#include "moab/Core.hpp"
#include "moab/Range.hpp"
#include "moab/CN.hpp"
#include <iostream>
#include <memory>
#include <string>
#include <string_view>
 
// Using declarations for cleaner code
using moab::Core;
using moab::EntityHandle;
using moab::ErrorCode;
using moab::Range;
 
// Constants
namespace
{
const char* const DEFAULT_MESH_FILE = "hex01.vtk";
const int DIMENSION                 = 3;  // For adjacency queries
}  // namespace
 
void print_usage( const char* program_name )
{
    std::cout << "Usage: " << program_name << " [meshfile]\n"
              << "  meshfile - Path to the mesh file (default: " << MESH_DIR << "/" << DEFAULT_MESH_FILE << ")\n";
}
 
int main( int argc, char** argv )
{
    try
    {
        // Parse command line arguments
        if( argc > 1 && ( std::string( argv[1] ) == "-h" || std::string( argv[1] ) == "--help" ) )
        {
            print_usage( argv[0] );
            return 0;
        }
 
        // Get mesh file path
        const std::string mesh_file =
            ( argc > 1 ) ? argv[1] : std::string( MESH_DIR ) + "/" + std::string( DEFAULT_MESH_FILE );
 
        // Initialize MOAB
        auto moab = std::make_unique< Core >();
        if( !moab )
        {
            std::cerr << "Error: Failed to create MOAB instance\n";
            return 1;
        }
 
        // Load the mesh
        MB_CHK_SET_ERR( moab->load_mesh( mesh_file.c_str() ), "Failed to load mesh file: " << mesh_file );
 
        // Get all entities in the mesh
        Range entities;
        MB_CHK_SET_ERR( moab->get_entities_by_handle( 0, entities ), "Failed to retrieve entities from mesh" );
 
        // Process each entity
        for( const auto& entity : entities )
        {
            const auto entity_type = moab->type_from_handle( entity );
            const auto entity_id   = moab->id_from_handle( entity );
 
            if( entity_type == moab::MBVERTEX )
            {
                // For vertices, get adjacent elements
                Range adjacent_elements;
                MB_CHK_SET_ERR( moab->get_adjacencies( &entity, 1, DIMENSION, false, adjacent_elements ),
                                "Failed to get adjacencies for vertex " << entity_id );
 
                std::cout << "Vertex " << entity_id << " is connected to " << adjacent_elements.size() << " elements\n";
                if( !adjacent_elements.empty() )
                {
                    std::cout << "  Adjacent elements: ";
                    for( const auto& elem : adjacent_elements )
                    {
                        std::cout << moab->id_from_handle( elem ) << " ";
                    }
                    std::cout << "\n";
                }
            }
            else if( entity_type < moab::MBENTITYSET )
            {
                // For elements, get connectivity
                const EntityHandle* connectivity = nullptr;
                int num_vertices                 = 0;
 
                MB_CHK_SET_ERR( moab->get_connectivity( entity, connectivity, num_vertices ),
                                "Failed to get connectivity for " << moab::CN::EntityTypeName( entity_type ) << " "
                                                                  << entity_id );
 
                std::cout << moab::CN::EntityTypeName( entity_type ) << " " << entity_id << " has " << num_vertices
                          << " vertices: ";
 
                for( int i = 0; i < num_vertices; ++i )
                {
                    std::cout << moab->id_from_handle( connectivity[i] );
                    if( i < num_vertices - 1 ) std::cout << ", ";
                }
                std::cout << "\n";
            }
        }
 
        return 0;
    }
    catch( const std::exception& e )
    {
        std::cerr << "Error: " << e.what() << "\n";
        return 1;
    }
    catch( ... )
    {
        std::cerr << "Error: Unknown exception occurred\n";
        return 1;
    }
}