How can I read and output the contents of.txt file using C++?

Question

I have been assigned as a school project to check the the frequency of occurrence of 26 lowercase letters, and then they are encoded by Hoffman code. In this assignment, the basic requirement is (1)to read the given text file and open it on the terminal and (2)to output the number of occurrence of lowercase letters and their corresponding encoded Hoffman code.

I was able to accomplish the (2) task but whenever I am trying to output the text on the terminal, my code was not able to count the occurrence of the lowercase letters and output their respective Huffman code.

terminal showing only the text content

output without the inclusion of code for reading the text on terminal

Here is my code for the assignment:

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <bits/stdc++.h>

int ch;
FILE  *file;

using namespace std;

struct Node
{
    // data members of each node of Huffman tree - alphabet and its frequency
    char data;
    unsigned freq;

    // pointers to left and right childs
    Node *left, *right;

    // constructor to initialize the data members of the Huffman tree node
    Node(char d, unsigned f)
    {
        data = d;
        freq = f;
        left = NULL;
        right = NULL;
    }
};

struct compare
{
    // overloading () to compare which of left,right childs have higher frequency
    bool operator()(Node* l, Node* r)
    {
        return (l->freq > r->freq);
    }
};

// recursive function to display huffman codes
void display(struct Node* root, string str)
{
    // if tree is empty return
    if (!root)
        return;

    if (root->data != '$')
        cout << root->data << ": " << str << "\n";

    // recursively call left, right childs
    display(root->left, str + "0");
    display(root->right, str + "1");
}

// build huffman tree
void createHuffmanTree(char data[], int freq[], int size)
{
    struct Node *left, *right, *top;

    // Create a min heap using STL
    priority_queue<Node*, vector<Node*>, compare> minHeap;

    // push alphabets, frequency into minheap as leaf nodes
    for (int i = 0; i < size; ++i)
        minHeap.push(new Node(data[i], freq[i]));

    // repeat until heap size becomes one
    while (minHeap.size() != 1)
    {
        // Extract two least frequent items from minheap
        left = minHeap.top();
        minHeap.pop();

        right = minHeap.top();
        minHeap.pop();

        // Create new internal node with two least frequent items as child nodes
        // frequency of internal node is equal to sum of frequency of child nodes
        top = new Node('$', left->freq + right->freq);

        top->left = left;
        top->right = right;
        // push newly created internal node into minheap
        minHeap.push(top);
    }
    // building Huffman tree is done
    // display Huffman codes for alphabets
    cout << "The Huffman codes for lower case characters: " << endl;
    display(minHeap.top(), "");
}

// reading text file
void readTxt() {

    FILE * txt;
    char ch;

    if (txt != NULL)
    {
        // open file to read english article
        txt = fopen("D:\\data3.txt", "r");
        do
        {
            ch = fgetc(txt);
            putchar(ch);

        } while(ch != EOF);

        //fclose(txt);
    } else {
        cout << "There is no such .txt file in the system." << endl;
        exit(0);
    }
}

int main()
{
    int   charCount = 0;
    int freq[26] = {0};
    readTxt();

    // reads english article from file and counts frequency of each lowercase alphabet
    while (1)
    {
        ch = fgetc(file);
        if (ch == EOF)
            break;
        if (ch >= 'a' && ch <= 'z')
        {
            freq[ch - 'a']++;
            // count total number of lowercase alphabets in file
            charCount++;
        }
    }

    fclose(file);

    // display total number of characters in file
    cout << "Number of lowercase alphabets in file: " << charCount << endl;

    char arr[] = { 'a','b','c','d','e','f',
                   'g','h','i','j','k','l',
                   'm','n','o','p','q','r',
                   's','t','u','v','w','x',
                   'y','z' };

    // display frequency of lowercase characters
    cout << "Frequency of lowercase characters: " << endl;
    int freqCount = 0;
    for(int i = 0; i < 26; i++)
    {
        if(freq[i]!=0)
        {
            cout << arr[i] << " : " << freq[i] << endl;
            // count number of lowercase alphabets with frequency greater than zero
            freqCount++;
        }
    }

    char arr1[freqCount];
    int k = 0,      freq1[freqCount];

    // copy lowercase alphabets with frequency greater than zero into arr1, freq1
    for(int i = 0; i < 26; i++)
    {
        if(freq[i]!=0)
        {
            arr1[k] = arr[i];
            freq1[k++] = freq[i];
        }
    }

    // call method to create Huffman tree
    createHuffmanTree(arr1, freq1, freqCount);
    return 1;
}

It will be a great help if you guide me through my mistake.

TIA