Merge two maps, summing values for same keys in C++

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遇见更好的自我 2021-02-07 22:20

I have two std::map maps and wish to merge them into a third map like this: if the same key is found in both maps, create a pair in the third map wi

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死守一世寂寞 (楼主)

2021-02-07 22:47

An overly generic solution inspired by std::set_union. Unlike the first suggested answer, this should run in O(n) instead of O(n log n).

Edit: it's still O(n log n) because of insertions into the final map.

#include 
#include 
#include 
#include 

template
OutputIterT merge_apply(
    InputIterT1 first1, InputIterT1 last1,
    InputIterT2 first2, InputIterT2 last2,
    OutputIterT result, Comparator comp, Func func) {
  while (true)
  {
    if (first1 == last1) return std::copy(first2, last2, result);
    if (first2 == last2) return std::copy(first1, last1, result);

    if (comp(*first1, *first2) < 0) {
      *result = *first1;
      ++first1;
    } else if (comp(*first1, *first2) > 0) {
      *result = *first2;
      ++first2;
    } else { 
      *result = func(*first1, *first2);
      ++first1;
      ++first2; 
    }   
    ++result;
  }
}

template
int compare_first(T a, T b) {
  return a.first - b.first;
}

template
T sum_pairs(T a, T b) {
  return std::make_pair(a.first, a.second + b.second);
}

using namespace std;
int main(int argc, char **argv) {
  map a,b,c;

  a[1] = 10; 
  a[2] = 11; 

  b[2] = 100;
  b[3] = 101;

  merge_apply(a.begin(), a.end(), b.begin(), b.end(), inserter(c, c.begin()),
      compare_first >, sum_pairs >); 

  for (auto item : c)                                                                                                       
    cout << item.first << " " << item.second << endl;
}

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