Add another paragraph.

git-svn-id: https://svn.dealii.org/trunk@5721 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/examples/step-13/step-13.cc b/deal.II/examples/step-13/step-13.cc
index 35124c9449a3f03a5bab4b58da543cf333a60b47..5065240e72a33c89e4429b4f0c90a97cc95aec5a 100644 (file)
--- a/deal.II/examples/step-13/step-13.cc
+++ b/deal.II/examples/step-13/step-13.cc
@@ -439,7 +439,7 @@ namespace Evaluation
                                     // that were not so we could not
                                     // give a reasonable value of the
                                     // solution there and the rest of
-                                    // the computation were useless
+                                    // the computations were useless
                                     // anyway. So make sure through
                                     // the ``AssertThrow'' macro
                                     // already used in the step-9
@@ -469,11 +469,42 @@ namespace Evaluation
                                     // wrong.
     AssertThrow (evaluation_point_found,
                 ExcEvaluationPointNotFound(evaluation_point));
-
-                                    // If we are sure that we have
-                                    // found the evaluation point, we
-                                    // can add the results into the
-                                    // table of results:
+                                    // Note that we have used the
+                                    // ``Assert'' macro in other
+                                    // example programs as well. It
+                                    // differed from the
+                                    // ``AssertThrow'' macro used
+                                    // here in that it simply aborts
+                                    // the program, rather than
+                                    // throwing an exception, and
+                                    // that it did so only in debug
+                                    // mode. It was the right macro
+                                    // to use to check about the size
+                                    // of vectors passed as arguments
+                                    // to functions, and the like.
+                                    //
+                                    // However, here the situation is
+                                    // different: whether we find the
+                                    // evaluation point or not may
+                                    // change from refinement to
+                                    // refinement (for example, if
+                                    // the four cells around point
+                                    // are coarsened away, then the
+                                    // point may vanish after
+                                    // refinement and
+                                    // coarsening). This is something
+                                    // that cannot be predicted from
+                                    // a few number of runs of the
+                                    // program in debug mode, but
+                                    // should be checked always, also
+                                    // in production runs. Thus the
+                                    // use of the ``AssertThrow''
+                                    // macro here.
+    
+                                    // Now, if we are sure that we
+                                    // have found the evaluation
+                                    // point, we can add the results
+                                    // into the table of results:
     results_table.add_value ("DoFs", dof_handler.n_dofs());
     results_table.add_value ("u(x_0)", point_value);
   };