Question

让

typedef pair<int, double> Element;

然后我有两个向量：

vector<Element> A, B;

这些向量按Element.first中的整数排序。我想得到第三个向量C，它是A和B的联合。这听起来像set_union，但在A[i].first == B[j].first时我需要不同的行为。 set_union只会选择要包含在C中的其中一个源元素，但我需要将结果“合并”两个元素。换句话说，就像这样：

C[k].first = A[i].first; // == B[j].first.  set_union does this
C[k].second = A[i].second + B[j].second; // set_union does NOT do this.

我很感兴趣，如果可以使用标准库（或类似Boost）。手动执行此操作的代码并不是特别复杂，但我不想重新发明轮子。

我能找到的唯一其他相关操作是merge。它也不会合并元素，并且会涉及另一个组合传递。

Answer 1

我认为std::merge与boost::function_output_iterator的使用非常简洁。

#include <algorithm>
#include <iostream>
#include <vector>

#include <boost/function_output_iterator.hpp>

/* Convenience type alias for our element. */
using Elem = std::pair<int, double>;

/* Convenience type alias for the container of our elements. */
using Elems = std::vector<Elem>;

/* Our appender that will be created with boost::function_output_iterator. */
class Appender {
  public:

  /* Cache the reference to our container. */
  Appender(Elems &elems) : elems_(elems) {}

  /* Conditionally modify or append elements. */
  void operator()(const Elem &elem) const {
    if (!elems_.empty() && elems_.back().first == elem.first) {
      elems_.back().second += elem.second;
      return;
    }  // if
    elems_.push_back(elem);
  }

  private:

  /* Reference to our container. */      
  Elems &elems_;

};  // Appender

int main() {
  // Sample data.
  Elems lhs {{1, 2.3}, {2, 3}, {5, 3.4}};
  Elems rhs {{1, 1.3}, {3, 5.5}, {4, 2.2}};
  Elems result;
  // Merge and use appender to append elements.
  std::merge(std::begin(lhs),
             std::end(lhs),
             std::begin(rhs),
             std::end(rhs),
             boost::make_function_output_iterator(Appender(result)));
  // Print result.
  for (const auto &elem : result) {
    std::cout << elem.first << ' ' << elem.second << std::endl;
  }  // for
}

打印：

请注意。 function_output_iterator建议使用Benjamin Lindley。

Answer 2

以下是使用独立通用算法merge_elements的实现：

#include <algorithm>
#include <utility>

template <typename LInput, typename RInput, typename Output>
Output merge_elements(LInput lbegin, LInput lend,
                      RInput rbegin, RInput rend,
                      Output out) {
    while(true) {
        if (lbegin == lend) {
            return std::copy(rbegin, rend, out);
        }

        if (rbegin == rend) {
            return std::copy(lbegin, lend, out);
        }

        if (lbegin->first < rbegin->first) {
            *out++ = *lbegin++;
        } else if (rbegin->first < lbegin->first) {
            *out++ = *rbegin++;
        } else {
            *out++ = std::make_pair(lbegin->first, lbegin->second + rbegin->second);
            ++lbegin;
            ++rbegin;
        }
    }
}

#include <iostream>
#include <iterator>
#include <vector>

/* Convenience type alias for our element. */
using Elem = std::pair<int, double>;

/* Convenience type alias for the container of our elements. */
using Elems = std::vector<Elem>;

int main() {
  // Sample data.
  Elems lhs {{1, 2.3}, {2, 3}, {5, 3.4}};
  Elems rhs {{1, 1.3}, {3, 5.5}, {4, 2.2}};
  Elems result;
  // Merge and use appender to append elements.
  merge_elements(std::begin(lhs),
                 std::end(lhs),
                 std::begin(rhs),
                 std::end(rhs),
                 std::back_inserter(result));
  // Print result.
  for (const auto &elem : result) {
    std::cout << elem.first << ' ' << elem.second << std::endl;
  }  // for
}

它不需要增强，但与mpark的增强解决方案几乎完全相同。有趣的是，这个算法非常通用，可以与std::map<int,double>以及std::vector<std::pair<int,double>>一起保持不变：

#include <iostream>
#include <iterator>
#include <map>

/* Convenience type alias for the container of our elements. */
using Elems = std::map<int, double>;

int main() {
  // Sample data.
  Elems lhs {{1, 2.3}, {2, 3}, {5, 3.4}};
  Elems rhs {{1, 1.3}, {3, 5.5}, {4, 2.2}};
  Elems result;
  // Merge and use appender to append elements.
  merge_elements(std::begin(lhs),
                 std::end(lhs),
                 std::begin(rhs),
                 std::end(rhs),
                 std::inserter(result, result.begin()));
  // Print result.
  for (const auto &elem : result) {
    std::cout << elem.first << ' ' << elem.second << std::endl;
  }  // for
}

如何结合两个排序的向量并组合重叠元素？

2 个答案: