How to use vector and struct?

Question

I need to count letters from the string, sort them by count and cout results. For this purpose I'm trying to use vector and struct. Here is part of my code, but it's not working, because I don't know how to implement something:

#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;
}

At this code I have 2 parts that I don't know how to deal:

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

and

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

Maybe you can see any other mistakes and potential issues at this code.

Answer 1

I would use a std::map to determine the frequency of each letter, then copy that into a multimap while reversing the key and value to get them in order.

#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;
  }
};

Answer 2

This should do what you need:

most_frequent[int_char].key = int_char;
most_frequent[int_char].value++; 

Yes, it sets the key many times, even though it doesn't need to.