sort words by frequencies.

word_freq.cc

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

typedef std::unordered_map<std::string, int> WordCount;

struct Greater
{
  bool operator()(const std::pair<int, WordCount::iterator>& lhs,
                  const std::pair<int, WordCount::iterator>& rhs) const
  {
    return lhs.first > rhs.first;
  }
};

int main()
{
  WordCount counts;
  std::string word;
  while (std::cin >> word)
  {
    counts[word]++;
  }

  typedef std::vector<std::pair<int, WordCount::iterator> > FreqList;
  FreqList freq;
  freq.reserve(counts.size());
  for (WordCount::iterator it = counts.begin(); it != counts.end(); ++it)
  {
    freq.push_back(make_pair(it->second, it));
  }

  std::sort(freq.begin(), freq.end(), Greater());
  for (auto item : freq)
  {
    std::cout << item.first << '\t' << item.second->first << '\n';
  }
}