From: David Wells <drwells@email.unc.edu>
Date: Sun, 12 Nov 2023 13:27:48 +0000 (-0500)
Subject: SymmetricTensor: add a comment on indexing.
X-Git-Tag: relicensing~304^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ed194eb2a76874b07856dd32c46b013b887941aa;p=dealii.git

SymmetricTensor: add a comment on indexing.
---

diff --git a/include/deal.II/base/symmetric_tensor.h b/include/deal.II/base/symmetric_tensor.h
index 9ce3e53895..7a31df5539 100644
--- a/include/deal.II/base/symmetric_tensor.h
+++ b/include/deal.II/base/symmetric_tensor.h
@@ -2057,7 +2057,11 @@ namespace internal
                 std::min(indices[0], indices[1]),
                 std::max(indices[0], indices[1]));
 
-              for (unsigned int d = 0, c = 0; d < dim; ++d)
+              // Here (d, e) are the row and column of the symmetric matrix and
+              // 'dim + c' is the index into the Tensor<1, dim> actually used
+              // for storage.
+              unsigned int c = 0;
+              for (unsigned int d = 0; d < dim; ++d)
                 for (unsigned int e = d + 1; e < dim; ++e, ++c)
                   if ((sorted_indices[0] == d) && (sorted_indices[1] == e))
                     return dim + c;