Make it functional!

// Add necessary includes

#include <iostream>
#include <cstdlib>
#include <ctime>
#include <string>
using namespace std;

// This is a simple game of "Rock, Paper, Scissors"
// It is also my first project ever in C++.

// Declare functions
void playGame();
bool playAgain();
string letUserChoose();
int genRandNum();
string letComputerChoose(int randomNumber, string computerChoice);
void compareChoices(string userChoice, string computerChoice);

bool playAgain() {
    string stayOrGo;
    cout << "Type 'y' to play again, 'n' to quit.\n" << endl;
    cin >> stayOrGo;
    return stayOrGo == "y";
}

int genRandNum() {
    // Generate a pseudo-random number between 1 and 3
    int randomNumber;
    srand(time(0));
    randomNumber = rand() % 3 + 1;
    return randomNumber;
}

string letUserChoose() {
    string userChoice;
    // Let the user choose either rock, paper or scissors
    cout << "\n\nPlease use only lower-case letters!" << endl;
    cout << "\n\nDo you choose 'rock', 'paper' or 'scissors'?\n" << endl;
    cin >> userChoice;
    cout << "\nYou have chosen: " << userChoice << endl;
    return userChoice;
}

string letComputerChoose(int randomNumber, string computerChoice) {
    // 1 = Rock, 2 = Paper, 3 = Scissors, assign a choice to the computer.
    if (randomNumber == 1) {
        computerChoice = "rock";
    }
    else if (randomNumber == 2) {
        computerChoice = "paper";
    }
    else {
        computerChoice = "scissors";
    }
    cout << "\nThe computer chose: " << computerChoice << endl;
    return computerChoice;
}

void compareChoices(string userChoice, string computerChoice) {
    // Decide who wins by comparing the userChoice and computerChoice
    // Choose what happens if userChoice is rock
    if (userChoice == "rock") {
        if (computerChoice == "paper") {
            cout << "\nPaper beats rock, you lose!\n\n";
        }
        else if (computerChoice == "scissors") {
            cout << "\nRock beats scissors, you win!\n\n";
        }
        else {
            cout << "\nIt's a tie!\n\n";
        }
    }

    // Choose what happens if userChoice is paper
    if (userChoice == "paper") {
        if (computerChoice == "scissors") {
            cout << "\nScissors beats paper, you lose!\n\n";
        }
        else if (computerChoice == "rock") {
            cout << "\nPaper beats rock, you win!\n\n";
        }
        else {
            cout << "\nIt's a tie!\n\n";
        }
    }

    // Choose what happens if userChoice is scissors
    if (userChoice == "scissors") {
        if (computerChoice == "rock") {
            cout << "\nRock beats scissors, you lose!\n\n";
        }
        else if (computerChoice == "paper") {
            cout << "\nScissors beats paper, you win!\n\n";
        }
        else {
            cout << "\nIt's a tie!\n\n";
        }
    }
}

void playGame(string userChoice, int randomNumber, string computerChoice) {
    letUserChoose();
    genRandNum();
    letComputerChoose(randomNumber, computerChoice);
    compareChoices(userChoice, computerChoice);
}

int main(int nNumberofArgs, char* pszArgs[]) {
    // Initialize variables
    string userChoice = letUserChoose();
    int randomNumber = genRandNum();
    string computerChoice = letComputerChoose(randomNumber, computerChoice);

    // Main loop of the game
    do {
        playGame(userChoice, randomNumber, computerChoice);
    } while (playAgain());
    // Let the player know that he or she has chosen to exit the game
    cout << "\nYou have chosen to exit the game.\n\n";
    system("PAUSE");
    return 0;
}