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Optimize median calculation by using nth_element instead of partial_sort #565

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Merged
dmah42 merged 1 commit into from
Apr 9, 2018
Merged

Optimize median calculation by using nth_element instead of partial_sort #565

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21 changes: 12 additions & 9 deletions src/statistics.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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