C++: How to Count the Frequency of the Elements in a Container

C++: コンテナ中の要素の出現頻度をカウントする

 

コード

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

template <class InputIterator>
std::vector<std::pair<typename InputIterator::value_type, int> >
frequency(InputIterator first, InputIterator last) {
  typedef typename InputIterator::value_type ValType;
  typedef std::map<ValType, int> MapType;
  typedef std::vector<std::pair<ValType, int> > VecType;

  MapType m;
  for (InputIterator it = first; it != last; ++it) {
    if (m.find(*it) == m.end()) {
      m.insert(typename MapType::value_type(*it, 1));
    } else {
      ++m[*it];
    }
  }

  return VecType(m.begin(), m.end());
}

// for utility
template <typename T>
std::ostream & operator<<(std::ostream & stream, std::vector<T> & v) {
  stream << "[";
  for (typeof(v.begin()) i = v.begin(); i != v.end(); ++i) {
    if (i != v.begin()) { stream << ", "; }
    stream << *i;
  }
  stream << "]";
  return stream;
}
template <typename T, typename U>
std::ostream & operator<<(std::ostream & stream, std::pair<T, U> & p) {
  stream << "(" << p.first << ", " << p.second << ")";
  return stream;
}
#define print(a) std::cout << a << std::endl
#define all(a) (a).begin(),(a).end()

// main
using namespace std;

int main(int argc, char **argv) {
  string apple = "apple";
  vector<pair<char, int> > a = frequency(all(apple));
  print(apple << ": " << a);

  int xs[] = { 1, 3, 5, 4, 3, 2 };
  vector<int> v(xs, xs + sizeof(xs) / sizeof(xs[0]));
  vector<pair<int, int> > b = frequency(all(v));
  print(v << ": " << b);
}

 

出力例

apple: [(a, 1), (e, 1), (l, 1), (p, 2)]
[1, 3, 5, 4, 3, 2]: [(1, 1), (2, 1), (3, 2), (4, 1), (5, 1)]

やはり、Scalaとかに比べるとストレスフルだなあ。