More doc and fix problems with begin(r)

git-svn-id: https://svn.dealii.org/trunk@8918 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/lac/include/lac/sparse_matrix.h b/deal.II/lac/include/lac/sparse_matrix.h
index 85aedb487708b4e58b39853e5dacadb9e8989a6c..2b1930bc259fc148f3e7fcad6678dd6153b2a7b7 100644 (file)
--- a/deal.II/lac/include/lac/sparse_matrix.h
+++ b/deal.II/lac/include/lac/sparse_matrix.h
@@ -1214,6 +1214,14 @@ class SparseMatrix : public virtual Subscriptor
                                      * STL-like iterator with the first entry
                                      * of row <tt>r</tt>. This is the version
                                      * for constant matrices.
+                                     *
+                                     * 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.
                                      */
     const_iterator begin (const unsigned int r) const;
 
@@ -1221,6 +1229,13 @@ class SparseMatrix : public virtual Subscriptor
                                      * Final iterator of row
                                      * <tt>r</tt>. This is the version for
                                      * constant matrices.
+                                     *
+                                     * 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.
                                      */
     const_iterator end (const unsigned int r) const;
     
@@ -1228,6 +1243,14 @@ class SparseMatrix : public virtual Subscriptor
                                      * STL-like iterator with the first entry
                                      * of row <tt>r</tt>. This is the version
                                      * for non-constant matrices.
+                                     *
+                                     * 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);
 
@@ -1235,6 +1258,13 @@ class SparseMatrix : public virtual Subscriptor
                                      * Final iterator of row
                                      * <tt>r</tt>. This is the version for
                                      * non-constant matrices.
+                                     *
+                                     * 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);
     
@@ -2096,7 +2126,11 @@ typename SparseMatrix<number>::const_iterator
 SparseMatrix<number>::begin (const unsigned int r) const
 {
   Assert (r<m(), ExcIndexRange(r,0,m()));
-  return const_iterator(this, r, 0);
+
+  if (cols->row_length(r) > 0)
+    return const_iterator(this, r, 0);
+  else
+    return end (r);
 }
 
 
@@ -2118,7 +2152,11 @@ typename SparseMatrix<number>::iterator
 SparseMatrix<number>::begin (const unsigned int r)
 {
   Assert (r<m(), ExcIndexRange(r,0,m()));
-  return iterator(this, r, 0);
+
+  if (cols->row_length(r) > 0)
+    return iterator(this, r, 0);
+  else
+    return end (r);
 }