From: Arnaud Giersch <arnaud.giersch@iut-bm.univ-fcomte.fr>
Date: Wed, 22 Dec 2010 08:23:09 +0000 (+0100)
Subject: Use numerical algorithms for stddev computation.
X-Git-Tag: v0.1~204
X-Git-Url: https://bilbo.iut-bm.univ-fcomte.fr/and/gitweb/loba.git/commitdiff_plain/7725e3d7f34e70c0ba01ddc4e1055e096e72c2c3?ds=inline

Use numerical algorithms for stddev computation.
---

diff --git a/main.cpp b/main.cpp
index 8a23727..907869e 100644
--- a/main.cpp
+++ b/main.cpp
@@ -1,5 +1,7 @@
+#include <algorithm>
 #include <cmath>
 #include <cstring>
+#include <tr1/functional>
 #include <iostream>
 #include <numeric>
 #include <stdexcept>
@@ -87,16 +89,19 @@ static void check_for_lost_load()
 
 static void compute_load_imbalance()
 {
+    using std::tr1::bind;
+    using std::tr1::placeholders::_1;
+
     unsigned n = loads.size();
     load_avg = std::accumulate(loads.begin(), loads.end(), 0.0) / n;
-    double variance = 0.0;
-    double epsilon = 0.0;
-    for (unsigned i = 0 ; i < n ; ++i) {
-        double diff = loads[i] - load_avg;
-        variance += diff * diff;
-        epsilon += diff;
-    }
-    variance = (variance - (epsilon * epsilon) / n) / n;
+
+    std::vector<double> diff(loads);
+    std::transform(diff.begin(), diff.end(), diff.begin(),
+                   bind(std::minus<double>(), _1, load_avg));
+    double epsilon = std::accumulate(diff.begin(), diff.end(), 0.0);
+    double square_sum = std::inner_product(diff.begin(), diff.end(),
+                                           diff.begin(), 0.0);
+    double variance = (square_sum - (epsilon * epsilon) / n) / n;
     load_stddev = sqrt(variance);
 }