Creating a loop for every letter to decode a Cipher file

[marina_ch]

Hello, I am stuck on a part of my code, where I need to decode a file. I have searched and exhausted every forum I could find to see if other people have maybe had the same issue and solved it. However, I haven't found anything.
Here is what I have:

/* Marina Chernysheva
 CS 161 Assignment 6
 Sources: Instructor
 */


#include <iostream>
#include <string>
#include <cctype>
#include <cstring>
#include <fstream>
using namespace std;

char getMenuselect (char m);

const int ALPHA_LEN = 26;

// Prototypes
bool loadSubstitutions(char substitutions[]);
void decrypt(string fileName, char substitutions[]);
void encrypt(string fileName2, char substitutions[]);

int main()
{
    ifstream input;
    ifstream in;
    ofstream out;
    char menu;
    char file[80];
    string fileName;
    char substitutions[ALPHA_LEN];

    menu = 0;

    menu = getMenuselect(menu); // Asks user what function to use

    // Print your greeting and info about program.
    cout << "Welcome to the Substitution Cipher Program." << endl;

    if(menu == 'e'|| menu == 'E') // Encrypt file option
    {
        cout << "Enter file name: ";
        cin >> fileName;
        input.open(fileName);
        //encrypt (file);
        input.close();
        cout << "Encrypted text follows: " << fileName << endl;
        return main();
    }
    if (menu == 'd' || menu == 'D') // Decrypt file option
    {
        cout << "Enter file name: ";
        cin >> fileName;
        input.open(fileName);
        decrypt (file, substitutions);
        input.close();
        cout << "Decrypted text follows: " << fileName << endl;
        return main();

    }
    if (menu == 'q') // End program option
    {
        cout << "Program terminated!";
        cin.ignore(2);
        return 0;
    }
    cin.ignore(2);
    return 0;
}
char getMenuselect (char m) //Menu select function
{
    cout << "Select 'E' to encrypt a file or selec 'D' to decrypt a file or 'Q' to quit: ";
    cin >> m;
    switch (m)
    {
        case 'e':

        case 'E': cout << "You have selected to encrypt a file: " << endl;
            break;

        case 'd':

        case'D': cout << "You have selected to decrypt a file: " << endl;

        case 'q':

        case 'Q': cout << "You have selected to terminate the program." << endl;
            break;

        default: cout << "Invalid entry, please try again: ";

            return getMenuselect(m);

    }return (m);
}


void decrypt(string fileName, char substitutions[]) // funciton for decrypt
{
    ifstream inFile;
    int i = 0;
    int letter;
    for(i = 0; i < ALPHA_LEN; i++)
    {
        if(letter == substitutions[i])
        {
            cout << " " << substitutions[i] << endl;
        }
    }

    inFile.close();

    cout << "/n/n";
    //create an ifstream, and use fileName to open it.
    //loop through every letter in the file. maybe use while loop?
    //inside loop: Find index of the letter in substitutions
    //  loop through the substitutions, looking for the letter.
    //   for (int i = 0; i < ALPHA_LEN; i ++)
    //      if letter = substitutions[i]
    //          i is what we are looking for. stop when equal, take note of i.
    //  Find the regular letter at index (A is 0, B is 1, etc)
    //  first cast the letter to an int. then do the following:
    //  if letter is B, letter - 'A' = 1;
    // cast back to char, maybe add 'A';
    // send the decrypted letter to screen and to file.
}

void encrypt(string fileName2, char substitutions[]) // function for encrypt
{
    ifstream inFile;
    cout << "What file would you like to encrypt?: ";
    getline(cin, fileName2);
    cin >> fileName2;

    inFile.open(fileName2.c_str());

    cout << "What would you like to name the Cipher file?: ";
    string name;
    getline(cin, name);
    cin >> name;

    cout << "Encrypted text follows: ";
    cin >> substitutions;
    cout << "/n/n";

}


bool loadSubstitutions(char substitutions[])
{
    char letter;
    ifstream inFile;
    inFile.open("Substitutions.txt");
    if (!inFile.is_open())
    {
        cout << "Error, can't open substitutions file. Exiting." << endl;
        return false;
    }
    for (int i = 0; i < ALPHA_LEN; i++)
    {
        inFile.get(letter);
        substitutions[i] = letter;
    }
    inFile.close();
    return true;

}

line 115 - 126 I commented out for what I need to do. I just need some direction or link with an example to get an idea of how the loop should look like. Do I need to list the entire alphabet? Or just the letters in the file that needs to be decoded? Any help is appreciated!