Fix all the same problems that we had with SparseMatrix iterators.

git-svn-id: https://svn.dealii.org/trunk@8921 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/lac/include/lac/sparsity_pattern.h b/deal.II/lac/include/lac/sparsity_pattern.h
index ba4a8cf5fc88685756e086d274794963554ba5e8..fc8fb0f6c6d87e6a51258b191447ce80a2ded19a 100644 (file)
--- a/deal.II/lac/include/lac/sparsity_pattern.h
+++ b/deal.II/lac/include/lac/sparsity_pattern.h
@@ -592,12 +592,27 @@ class SparsityPattern : public Subscriptor
                                     /**
                                      * STL-like iterator with the first entry
                                      * of row <tt>r</tt>.
+                                     *
+                                     * Note that if the given row is empty,
+                                     * i.e. does not contain any nonzero
+                                     * entries, then the iterator returned by
+                                     * this function equals
+                                     * <tt>end(r)</tt>. Note also that the
+                                     * iterator may not be dereferencable in
+                                     * that case.
                                      */
     iterator begin (const unsigned int r) const;
 
                                     /**
                                      * Final iterator of row
                                      * <tt>r</tt>.
+                                     *
+                                     * Note that the end iterator is not
+                                     * necessarily dereferencable. This is in
+                                     * particular the case if the row after
+                                     * the one for which this is the end
+                                     * iterator is empty, or if it is the end
+                                     * iterator for the last row of a matrix.
                                      */
     iterator end (const unsigned int r) const;
     
@@ -1633,7 +1648,16 @@ inline
 SparsityPattern::iterator
 SparsityPattern::begin () const
 {
-  return iterator(this, 0, 0);
+                                   // search for the first line with a nonzero
+                                   // number of entries
+  for (unsigned int r=0; r<n_rows(); ++r)
+    if (row_length(r) > 0)
+      return iterator(this, r, 0);
+
+                                   // alright, this matrix is completely
+                                   // empty. that's strange but ok. simply
+                                   // return the end() iterator
+  return end();
 }
 
 
@@ -1651,7 +1675,11 @@ SparsityPattern::iterator
 SparsityPattern::begin (const unsigned int r) const
 {
   Assert (r<n_rows(), ExcIndexRange(r,0,n_rows()));
-  return iterator(this, r, 0);
+
+  if (row_length(r) > 0)
+    return iterator(this, r, 0);
+  else
+    return end (r);
 }