From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Tue, 27 Aug 2019 01:33:28 +0000 (-0600)
Subject: Simplify and clarify the use of std::complex in DataPostprocessor.
X-Git-Tag: v9.2.0-rc1~1168^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3f403829605f4ede11859538ba66de87fc00d6a5;p=dealii.git

Simplify and clarify the use of std::complex in DataPostprocessor.
---

diff --git a/examples/step-29/step-29.cc b/examples/step-29/step-29.cc
index 7448575008..2cbc7efc06 100644
--- a/examples/step-29/step-29.cc
+++ b/examples/step-29/step-29.cc
@@ -333,9 +333,15 @@ namespace Step29
            ExcDimensionMismatch(computed_quantities.size(),
                                 inputs.solution_values.size()));
 
-    // The computation itself is straightforward: We iterate over each entry
-    // in the output vector and compute $|u|$ from the corresponding values of
-    // $v$ and $w$:
+    // The computation itself is straightforward: We iterate over each
+    // entry in the output vector and compute $|u|$ from the
+    // corresponding values of $v$ and $w$. We do this by creating a
+    // complex number $u$ and then calling `std::abs()` on the
+    // result. (One may be tempted to call `std::norm()`, but in a
+    // historical quirk, the C++ committee decided that `std::norm()`
+    // should return the <i>square</i> of the absolute value --
+    // thereby not satisfying the properties mathematicians require of
+    // something called a "norm".)
     for (unsigned int i = 0; i < computed_quantities.size(); i++)
       {
         Assert(computed_quantities[i].size() == 1,
@@ -343,9 +349,10 @@ namespace Step29
         Assert(inputs.solution_values[i].size() == 2,
                ExcDimensionMismatch(inputs.solution_values[i].size(), 2));
 
-        computed_quantities[i](0) = std::sqrt(
-          inputs.solution_values[i](0) * inputs.solution_values[i](0) +
-          inputs.solution_values[i](1) * inputs.solution_values[i](1));
+        const std::complex<double> u(inputs.solution_values[i](0),
+                                     inputs.solution_values[i](1));
+
+        computed_quantities[i](0) = std::abs(u);
       }
   }