c++ - 如何使用vector和struct？

Question

我需要对字符串中的字母进行计数，按计数和 cout 结果对它们进行排序。为此，我尝试使用 vector 和 struct。这是我的部分代码，但它不起作用，因为我不知道如何实现某些东西:

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

using namespace std;

struct int_pair{
     int key;
     int value;
};  

bool sort_by_value(int_pair left, int_pair right){
    return left.value < right.value;
}

int main() {    
    string characters = "aasa asdfs dfh f ukjyhkh k wse f sdf sdfsdf";    
    vector<int_pair> most_frequent;     

    for (string::size_type i = 0; i <= characters.length(); i++) {
        int int_char = (int)characters[i];
        most_frequent[int_char]++; <-- I want to do something like this, but it's not working 
    }

    sort(most_frequent.begin(), most_frequent.end(), sort_by_value);

    for (vector<int_pair>::iterator it = most_frequent.begin(); it != most_frequent.end(); ++it) <-- is this call correct?
        cout << " " << it->key << ":" << it->value << endl;    

    return 0;
}

在这段代码中，我有 2 个部分不知道如何处理:

most_frequent[int_char]++; <-- I want to do something like this, but it's not working 

和

for (vector<int_pair>::iterator it = most_frequent.begin(); it != most_frequent.end(); ++it) <-- is this call correct?

也许您可以在此代码中看到任何其他错误和潜在问题。

Answer 1

我会使用 std::map 来确定每个字母的频率，然后将其复制到多重映射中，同时反转键和值以使它们按顺序排列。

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

template<class T, class U>
std::pair<U,T> flip_pair(const std::pair<T,U>& p) {
  return std::make_pair(p.second,p.first);
}

int main(){
  std::string characters = "zxcvopqiuweriuzxchajksdui";
  std::map<char,int> freq;
  std::multimap<int,char> rev_freq;

  // Calculate the frequency of each letter.
  for(char c: characters){
    freq[c]++;
  }

  // Copy the results into a multimap with the key and value flipped 
  std::transform(std::begin(freq), std::end(freq), 
                 std::inserter(rev_freq, rev_freq.begin()), 
                 flip_pair<char,int>);

  // Print out the results in order.
  for(std::pair<int,char> p : rev_freq){
    std::cout << p.first << ": " << p.second << std::endl;
  }
};