Huffman Coding Algorithm/Data Structures

Question

So we were given the task of writing a compression alg for a .txt of text/numbers (presumably through huffman coding since our professor was very vague)

I have all the lines as keys in a map with frequencies as their values. I'm a little sketchy on how to proceed from here since maps are organized in order by key not value Should I be using a different data structure (not a map) or would it be easy enough to just find the 2 smallest min values every time I wanted to add to the tree? Code below, any help would be awesome!

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <algorithm>
#include <vector>
#include <map>
using namespace std;

int main()
{
    vector <string> words;
    map <string, int> store;
    ifstream infile("file.txt");
    string text;
    while (getline(infile, text))
    {
        istringstream iss(text);
        string input;
        if (!(iss >> input))
            break;
        words.push_back(input);
    }
    int freq = 0;


    while (!words.empty())
    {

        string check = words[0];
        if(check == "") //make sure not reading a blank
        {
            words.erase(remove(words.begin(), words.end(), "")); //remove all blanks
            continue; //top of loop
        }
        check = words[0];
        freq = count(words.begin(), words.end(), check);//calculate frequency
        store.insert(pair<string, int>(check, freq)); //store words and frequency in map
        words.erase(remove(words.begin(), words.end(), check)); //erase that      value entirely from the vector
    }

    map<string, int>::iterator i;

    for(i = store.begin(); i != store.end(); ++i)
    {
        cout << "store[" << i ->first << "] = " << i->second << '\n';
    } 
    return 0;
}

Answer 1

To find the min value you can use a Priority Queue .

A Priority Queue is a data structure that can give you the min or max value from a set of elements. Finding or inserting in it costs O(log(n)) . So in this case, it might be a perfect choice.

C++ has its own built-in Priority Queue.

Here is a simple example of a priority_queue in C++ .

#include <bits/stdc++.h>
using namespace std;

int main()
{
    priority_queue <int> Q;

    Q.push(10);
    Q.push(7);
    Q.push(1);
    Q.push(-3);
    Q.push(4);

    while(!Q.empty()){ // run this loop until the priority_queue gets empty

        int top = Q.top();
        Q.pop();
        cout << top << ' ';

    }
    cout << endl;
    return 0;
}

Output

10, 7, 4, 1, -3

And as you can notice this is in ascending order. That is because:

By default Priority Queue gives the highest value.

So you can either overload the Priority Queue or a very clever trick can be storing the values by inverting their signs and after you pop them from the queue, you can invert the sign again.