dolysis
/
benchmark
mirror of https://github.com/google/benchmark.git
3
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

Optimize by using nth_element instead of partial_sort to find the median. (#565)

This commit is contained in:
Fred Tingaud 2018-04-09 14:40:58 +02:00 committed by Dominic Hamon
parent 2844167ff9
commit 50ffc781b1
1 changed files with 12 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

21
src/statistics.cc
View File

@ -36,16 +36,19 @@ double StatisticsMean(const std::vector<double>& v) {
double StatisticsMedian(const std::vector<double>& v) {
if (v.size() < 3) return StatisticsMean(v);
std::vector<double> partial;
// we need roundDown(count/2)+1 slots
partial.resize(1 + (v.size() / 2));
std::partial_sort_copy(v.begin(), v.end(), partial.begin(), partial.end());
// did we have odd number of samples?
// if yes, then the last element of partially-sorted vector is the median
// it no, then the average of the last two elements is the median
std::vector<double> copy(v);
auto center = copy.begin() + v.size() / 2;
std::nth_element(copy.begin(), center, copy.end());
// did we have an odd number of samples?
// if yes, then center is the median
// it no, then we are looking for the average between center and the value before
if(v.size() % 2 == 1)
return partial.back();
return (partial[partial.size() - 2] + partial[partial.size() - 1]) / 2.0;
return *center;
auto center2 = copy.begin() + v.size() / 2 - 1;
std::nth_element(copy.begin(), center2, copy.end());
return (*center + *center2) / 2.0;
}
// Return the sum of the squares of this sample set