c++ - 如何使用 OpenMP 实现 argmax？

Question

我正在尝试使用 OpenMP 实现 argmax。如果简短的话，我有一个计算浮点值的函数:

double toOptimize(int val);

我可以得到最大化值的整数:

double best = 0;
#pragma omp parallel for reduction(max: best)
for(int i = 2 ; i < MAX ; ++i)
{
    double v = toOptimize(i);
    if(v > best) best = v;
}

现在，如何获取最大值对应的值i？

编辑:

我正在尝试这个，但想确保它是有效的:

double best_value = 0;
int best_arg = 0;
#pragma omp parallel
{
  double local_best = 0;
   int ba = 0;
#pragma omp for reduction(max: best_value)
  for(size_t n = 2 ; n <= MAX ; ++n)
  {
    double v = toOptimize(n);
    if(v > best_value)
    {
      best_value = v;
      local_best = v;
      bn = n;
    }
  }
#pragma omp barrier
#pragma omp critical
  {
    if(local_best == best_value)
      best_arg = bn;
  }
}

最后，我应该有 best_arg toOptimize 的 argmax。

Answer 1

您的解决方案完全符合标准。无论如何，如果您愿意添加一点语法糖，您可以尝试如下操作:

#include<iostream>

using namespace std;

double toOptimize(int arg) {
  return arg * (arg%100);
}

class MaximumEntryPair {
public:

  MaximumEntryPair(size_t index = 0, double value = 0.0) : index_(index), value_(value){}

  void update(size_t arg) {
    double v = toOptimize(arg);
    if( v > value_ ) {
      value_ = v;
      index_ = arg;
    }
  }

  bool operator<(const MaximumEntryPair& other) const {
    if( value_ < other.value_ ) return true;
    return false;
  }  

  size_t index_;
  double value_;
};



int main() {
  MaximumEntryPair best;
#pragma omp parallel 
  {
    MaximumEntryPair thread_local;
    #pragma omp for
    for(size_t ii = 0 ; ii < 1050 ; ++ii) {
      thread_local.update(ii);
    } // implicit barrier
#pragma omp critical
    {
      if ( best < thread_local ) best = thread_local;
    }

  } // implicit barries
  cout << "The maximum is " << best.value_ << " obtained at index " << best.index_ << std::endl;
  cout << "\t toOptimize(" << best.index_ << ") = " << toOptimize(best.index_) << std::endl;
  return 0;
}