while

    ((wrong < MAX_WRONG) && (soFar != THE_WORD))
            {
                cout << "nnYou have " << (MAX_WRONG - wrong);
                cout << " incorrect guesses left.n";
                cout << "nYou've used the following letters:n" << used << endl;
                cout << "nSo far, the word is:n" << soFar << endl;

                char enter_guess(string);
                int check_guess(vector<string>words);
            }

#include "pch.h"
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <ctime>
#include <cctype>

using namespace std;
char guess;
int wrong = 0;
const string THE_WORD;
string soFar;
string used;
char enter_guess();
int check_guess(vector<string>words);
int main()
{

    const int MAX_WRONG = 8;

    vector<string> words;
    words.push_back("GUESS");
    words.push_back("HANGMAN");
    words.push_back("DIFFICULT");
    string answer;
    cout << "Welcome to Hangman.  Good luck!n";

    while ((answer != "No") || (answer != "no"))
    {


        while ((wrong < MAX_WRONG) && (soFar != THE_WORD))
        {
            cout << "nnYou have " << (MAX_WRONG - wrong);
            cout << " incorrect guesses left.n";
            cout << "nYou've used the following letters:n" << used << endl;
            cout << "nSo far, the word is:n" << soFar << endl;

            char enter_guess(string);
            int check_guess(vector<string>words);
        }


        if (wrong == MAX_WRONG)
            cout << "nYou've been hanged!";
        else
            cout << "nYou guessed it!";

        cout << "nThe word was " << THE_WORD << endl;


        cout << "nDo you want to play again?(Enter: Yes or No) " << endl;
        cin >> answer;
    }
    return 0;
}
char enter_guess()
{
    cout << "nnEnter your guess: ";
    cin >> guess;
    guess = toupper(guess);
    return guess;
}

int check_guess(vector<string>words)
{
    srand(static_cast<unsigned int>(time(0)));
    random_shuffle(words.begin(), words.end());
    const string THE_WORD = words[0];

    string soFar(THE_WORD.size(), '-');
    string used = "";
    while (used.find(guess) != string::npos)
    {
        cout << "nYou've already guessed " << guess << endl;
        cout << "Enter your guess: ";
        cin >> guess;
        guess = toupper(guess);
    }

    used += guess;

    if (THE_WORD.find(guess) != string::npos)
    {
        cout << "That's right! " << guess << " is in the word.n";

        // update soFar to include newly guessed letter
        for (unsigned int i = 0; i < THE_WORD.length(); ++i)
        {
            if (THE_WORD[i] == guess)
            {
                soFar[i] = guess;
            }
        }
    }
    else
    {
        cout << "Sorry, " << guess << " isn't in the word.n";
        ++wrong;
    }
    return wrong;
}

string used = "";
while (used.find(guess) != string::npos) {
 //...

#include "pch.h"
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <ctime>
#include <cctype>

using namespace std;

int main()
{

    const int MAX_WRONG = 8;

    vector<string> words;
    words.push_back("GUESS");
    words.push_back("HANGMAN");
    words.push_back("DIFFICULT");
    string answer;

    while ((answer!="No")||(answer!="no"))
    {
        srand(static_cast<unsigned int>(time(0)));
        random_shuffle(words.begin(), words.end());
        const string THE_WORD = words[0];
        int wrong = 0;
        string soFar(THE_WORD.size(), '-');
        string used = "";

        cout << "Welcome to Hangman.  Good luck!n";


        while ((wrong < MAX_WRONG) && (soFar != THE_WORD))
        {
            cout << "nnYou have " << (MAX_WRONG - wrong);
            cout << " incorrect guesses left.n";
            cout << "nYou've used the following letters:n" << used << endl;
            cout << "nSo far, the word is:n" << soFar << endl;

            char guess;
            cout << "nnEnter your guess: ";
            cin >> guess;
            guess = toupper(guess);
            while (used.find(guess) != string::npos)
            {
                cout << "nYou've already guessed " << guess << endl;
                cout << "Enter your guess: ";
                cin >> guess;
                guess = toupper(guess);
            }

            used += guess;

            if (THE_WORD.find(guess) != string::npos)
            {
                cout << "That's right! " << guess << " is in the word.n";

                // update soFar to include newly guessed letter
                for (unsigned int i = 0; i < THE_WORD.length(); ++i)
                {
                    if (THE_WORD[i] == guess)
                    {
                        soFar[i] = guess;
                    }
                }
            }
            else
            {
                cout << "Sorry, " << guess << " isn't in the word.n";
                ++wrong;
            }
        }


        if (wrong == MAX_WRONG)
            cout << "nYou've been hanged!";
        else
            cout << "nYou guessed it!";

        cout << "nThe word was " << THE_WORD << endl;


        cout << "nDo you want to play again?(Enter: Yes or No) " << endl;
        cin >> answer;
    }
    return 0;
}