This is a version of the 21 game with error checking, a computer player, scoreboard, and the ability to play multiple times.

21game-improved.cpp

#include <stdio.h>
#include <stdlib.h>
#include <iostream>

using namespace std;

const char * welcometext =
"Welcome to the game of 21! \n\
Rules: \n\
First choose who goes first, the game starts with 21 sticks in\n\
a pile, the player forced to take the last stick loses. \n\
At each turn, you can remove up to 3 sticks.";

const char * player1Turn = "It is player 1's turn";
const char * computerTurn = "It the computer's turn";

const char * player1Win = "Player 1 wins!";
const char * computerWin = "The Computer wins!";

const char * sticksLeftPrefixText = "There are ";
const char * sticksLeftSuffixText = " sticks left";

const char * errorInput = "Invalid input! Please enter a numeric value";
const char * errorBounds = "Invalid input! Please enter a number [1-3]";

const char * getFirstPlayerError = "Please enter h or c";
const char * yesNoError = "Please enter y or n";

//Note that you will lose everytime if the computer goes first
int ai(int sticks, int lastmove){
	// +[1-3] = sticks % 4 == 0 ? 
	int toTake = 1;
	for (int i = 1; i < 4; ++i){
		if((lastmove + i) % 4 == 0){
			return i;
		}
	}
	return toTake;
}

int chooseFirstPlayer(){
	string input;
	cout << "Who will go first? [h]uman or [c]omputer:" << endl;
	do {
		getline(cin, input);	
		if(input.length() == 0){
			cout << getFirstPlayerError << endl;
			continue;
		}

		if(input[0] == 'h' || input[0] == 'H'){
			return 2;
		}
		if(input[0] == 'c' || input[0] == 'C'){
			return 1;
		}

		cout << getFirstPlayerError << endl;
	}while(1);
}

int humanTurn(){
	string input;
	int sticksToTake = 1;
	do{
		cout << "How many sticks will you take?: ";

		getline(cin, input);
		if(input.length() != 1){
			cout << "Please enter 1 numeric value" << endl;
			continue;
		}	

		int sticksToTake = input[0] - '0';
		if(!(0 < sticksToTake && sticksToTake <= 3)){
			cout << errorBounds << endl;
		}else{
			break;
		}
		
		
	}while(1);
	return sticksToTake;
}

int playAgain(){
	string input;
	cout << "Would you like to play again? [y]es or [n]o: " << endl;
	do {
		getline(cin, input);	
		if(input.length() == 0){
			cout << yesNoError << endl;
			continue;
		}

		if(input[0] == 'y' || input[0] == 'Y'){
			return true;
		}
		if(input[0] == 'n' || input[0] == 'N'){
			return false;
		}

		cout << yesNoError << endl;
	}while(1);
}

int main(){
	cout << welcometext << endl;

	int sticks = 21;
	int currentPlayer = 2;
	int continuePlaying = true;
	int userWantsToPlay = true;
	int sticksToTake = 0;
	int validInput = false;
	string input;
	int userScore = 0;
	int computerScore = 0;

	do{
		currentPlayer = chooseFirstPlayer();

		while(continuePlaying){		
			cout << "There " << (sticks == 1 ? "is " : "are ") << sticks << " stick" << (sticks == 1 ? " " : "s ") << "left" << endl;
			cout << (currentPlayer % 2 == 0 ? player1Turn : computerTurn) << endl;
			
			if(currentPlayer % 2 == 0){
				sticksToTake = humanTurn();
			}else{
				sticksToTake = ai(sticks, sticksToTake);
				cout << "Computer takes " << sticksToTake << " stick" << (sticksToTake == 1 ? "" : "s") << endl;
			}

			sticks = sticks - sticksToTake;				
			if(sticks <= 0){
				continuePlaying = false;
			}
			currentPlayer++;
		}

		cout << (currentPlayer % 2 == 0 ? player1Win : computerWin) << endl;
		if( currentPlayer % 2 == 0 ){
			userScore++;
		}else{
			computerScore++;
		}
		cout << "You've won " << userScore << " games to the computers " << computerScore << endl;

		if(userWantsToPlay = playAgain()){
			//Reset Game parameters
			sticks = 21;
			sticksToTake = 0;
			continuePlaying = true;
		}
		

	}while(userWantsToPlay);

	cout << "Thanks for playing!" << endl;
}