From cc6deead72a7f9727c911e534adc08bd82b203dd Mon Sep 17 00:00:00 2001
From: Wolfgang Bangerth <bangerth@math.tamu.edu>
Date: Sun, 15 Mar 2015 00:50:28 -0500
Subject: [PATCH] Improve error messages in DerivativeApproximation.

---
 .../deal.II/numerics/derivative_approximation.h | 17 ++++++++++++-----
 source/numerics/derivative_approximation.cc     |  4 ++--
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/include/deal.II/numerics/derivative_approximation.h b/include/deal.II/numerics/derivative_approximation.h
index fb729bb89a..9d3d33360d 100644
--- a/include/deal.II/numerics/derivative_approximation.h
+++ b/include/deal.II/numerics/derivative_approximation.h
@@ -25,7 +25,7 @@
 #include <deal.II/lac/vector.h>
 #include <deal.II/grid/filtered_iterator.h>
 #ifdef _MSC_VER
-#include <deal.II/dofs/dof_accessor.h>
+#  include <deal.II/dofs/dof_accessor.h>
 #endif
 #include <utility>
 
@@ -284,14 +284,21 @@ namespace DerivativeApproximation
   /**
    * Exception
    */
-  DeclException2 (ExcInvalidVectorLength,
+  DeclException2 (ExcVectorLengthVsNActiveCells,
                   int, int,
-                  << "Vector has length " << arg1 << ", but should have "
-                  << arg2);
+                  << "The output vector needs to have a size equal "
+                  "to the number of active cells of your triangulation "
+                  "but has length " << arg1 << "There are "
+                  << arg2 << " active cells in your triangulation.");
   /**
    * Exception
    */
-  DeclException0 (ExcInsufficientDirections);
+  DeclExceptionMsg (ExcInsufficientDirections,
+                    "We have encountered a cell on which the number of linearly "
+                    "independent directions that span the matrix Y (discussed "
+                    "in the documentation of the DerivativeApproximation "
+                    "class) is not equal to dim. The matrix Y then is "
+                    "rank deficient and can not be inverted.");
 }
 
 
diff --git a/source/numerics/derivative_approximation.cc b/source/numerics/derivative_approximation.cc
index 7c7653d18b..231f5d0ab6 100644
--- a/source/numerics/derivative_approximation.cc
+++ b/source/numerics/derivative_approximation.cc
@@ -974,8 +974,8 @@ namespace DerivativeApproximation
                             Vector<float>         &derivative_norm)
     {
       Assert (derivative_norm.size() == dof_handler.get_tria().n_active_cells(),
-              ExcInvalidVectorLength (derivative_norm.size(),
-                                      dof_handler.get_tria().n_active_cells()));
+              ExcVectorLengthVsNActiveCells (derivative_norm.size(),
+                                             dof_handler.get_tria().n_active_cells()));
       Assert (component < dof_handler.get_fe().n_components(),
               ExcIndexRange (component, 0, dof_handler.get_fe().n_components()));
 
-- 
2.39.5