From: Timo Heister <timo.heister@gmail.com>
Date: Wed, 26 Feb 2014 18:12:19 +0000 (+0000)
Subject: step-22: add comment
X-Git-Tag: v8.2.0-rc1~773
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f20cf91f914664c1403636bb5939a892620a1f06;p=dealii.git

step-22: add comment

git-svn-id: https://svn.dealii.org/trunk@32558 0785d39b-7218-0410-832d-ea1e28bc413d
---

diff --git a/deal.II/examples/step-22/step-22.cc b/deal.II/examples/step-22/step-22.cc
index e63bae370c..69fbc3b615 100644
--- a/deal.II/examples/step-22/step-22.cc
+++ b/deal.II/examples/step-22/step-22.cc
@@ -674,6 +674,18 @@ namespace Step22
 
                   }
 
+		// For the right-hand side we use the fact that the shape
+		// functions are only non-zero in one component (because our
+		// elements are primitive).  Instead of multiplying the tensor
+		// representing the dim+1 values of shape function i with the
+		// whole right-hand side vector, we only look at the only
+		// non-zero component. The Function
+		// FiniteElement::system_to_component_index(i) will return
+		// which component this shape function lives in (0=x velocity,
+		// 1=y velocity, 2=pressure in 2d), which we use to pick out
+		// the correct component of the right-hand side vector to
+		// multiply with.
+		
                 const unsigned int component_i =
                   fe.system_to_component_index(i).first;
                 local_rhs(i) += fe_values.shape_value(i,q) *