C++ Project: Tic-Tac-Toe

What you will build

Tic-tac-toe is a 3 by 3 grid. This project stores nine cells in a vector<string>. Empty cells are a single space. Players place X and O. After each move you print the board and check rows, columns, and diagonals for a winner.

Moves are a fixed sequence in main so the winner is the same every run. That keeps the demo deterministic at /cpp/try. Use Try it in C++.

Nine cells

Index 0 is top-left. Index 2 is top-right. Index 8 is bottom-right. Printing uses three cells per line with bars between them. A helper builds an empty board so tests start from the same state.

#include <iostream>
#include <string>
#include <vector>
using namespace std;

vector<string> emptyBoard() {
  return vector<string>(9, " ");
}

void printBoard(const vector<string>& b) {
  cout << " " << b[0] << " | " << b[1] << " | " << b[2] << endl;
  cout << "---+---+---" << endl;
  cout << " " << b[3] << " | " << b[4] << " | " << b[5] << endl;
  cout << "---+---+---" << endl;
  cout << " " << b[6] << " | " << b[7] << " | " << b[8] << endl;
}

int main() {
  vector<string> board = emptyBoard();
  printBoard(board);
  return 0;
}

Place a mark

A legal move needs an index from 0 through 8 and an empty cell. Return false when the cell is taken or the index is out of range. Do not overwrite an existing mark.

#include <iostream>
#include <string>
#include <vector>
using namespace std;

bool place(vector<string>& b, int cell, const string& mark) {
  if (cell < 0 || cell > 8) {
    return false;
  }
  if (b[cell] != " ") {
    return false;
  }
  b[cell] = mark;
  return true;
}

int main() {
  vector<string> board(9, " ");
  cout << place(board, 0, "X") << endl;
  cout << place(board, 0, "O") << endl;
  cout << place(board, 9, "X") << endl;
  cout << "cell 0 is " << board[0] << endl;
  return 0;
}

First place succeeds and prints 1. Second place on the same cell fails and prints 0. Index 9 is invalid. Cell 0 still holds X.

Detect a winner

Eight lines can win: three rows, three columns, two diagonals. If all three cells in a line share a mark that is not a space, that mark wins. Return that string, or a space when there is no winner yet.

#include <iostream>
#include <string>
#include <vector>
using namespace std;

string winner(const vector<string>& b) {
  const int lines[8][3] = {
      {0, 1, 2}, {3, 4, 5}, {6, 7, 8},
      {0, 3, 6}, {1, 4, 7}, {2, 5, 8},
      {0, 4, 8}, {2, 4, 6}};
  for (int i = 0; i < 8; i++) {
    string a = b[lines[i][0]];
    string c = b[lines[i][1]];
    string d = b[lines[i][2]];
    if (a != " " && a == c && c == d) {
      return a;
    }
  }
  return " ";
}

int main() {
  vector<string> board(9, " ");
  board[0] = "X";
  board[1] = "X";
  board[2] = "X";
  cout << "Winner: [" << winner(board) << "]" << endl;
  return 0;
}

The top row is three X marks, so the function returns X.

Complete game, fixed moves

X starts. The sequence fills the top row for X while O takes the middle row’s first two cells. After each successful place, print the board and stop if someone has won. There is no keyboard input, so the transcript never changes.

#include <iostream>
#include <string>
#include <vector>
using namespace std;

void printBoard(const vector<string>& b) {
  cout << " " << b[0] << " | " << b[1] << " | " << b[2] << endl;
  cout << "---+---+---" << endl;
  cout << " " << b[3] << " | " << b[4] << " | " << b[5] << endl;
  cout << "---+---+---" << endl;
  cout << " " << b[6] << " | " << b[7] << " | " << b[8] << endl;
}

bool place(vector<string>& b, int cell, const string& mark) {
  if (cell < 0 || cell > 8 || b[cell] != " ") {
    return false;
  }
  b[cell] = mark;
  return true;
}

string winner(const vector<string>& b) {
  const int lines[8][3] = {
      {0, 1, 2}, {3, 4, 5}, {6, 7, 8},
      {0, 3, 6}, {1, 4, 7}, {2, 5, 8},
      {0, 4, 8}, {2, 4, 6}};
  for (int i = 0; i < 8; i++) {
    string a = b[lines[i][0]];
    string c = b[lines[i][1]];
    string d = b[lines[i][2]];
    if (a != " " && a == c && c == d) {
      return a;
    }
  }
  return " ";
}

int main() {
  vector<string> board(9, " ");
  int moves[5] = {0, 3, 1, 4, 2};
  string marks[5] = {"X", "O", "X", "O", "X"};

  for (int i = 0; i < 5; i++) {
    cout << marks[i] << " plays cell " << moves[i] << endl;
    if (!place(board, moves[i], marks[i])) {
      cout << "Illegal move" << endl;
      return 0;
    }
    printBoard(board);
    string w = winner(board);
    if (w != " ") {
      cout << w << " wins" << endl;
      return 0;
    }
    cout << endl;
  }
  cout << "No winner yet" << endl;
  return 0;
}

X takes cells 0, 1, and 2. After the fifth move the top row is full and the program printsX wins. Change the last X move to cell 8 and you should see No winner yet instead.

Common mistakes

• Using a 3 by 3 nested vector and then mixing up row and column. Nine cells and a printed grid keep the index rule in one place: row * 3 + col if you add coordinates later.
• Checking only rows. Columns and diagonals are how most first games are actually won.
• Treating three spaces as a win. The empty mark must be excluded or a blank board wins on the first row.
• Allowing a second mark in a filled cell. Always test b[cell] != " " before you assign.

Practice

1. After nine legal moves with no winner, print Draw.
2. Add a sequence where O wins on a diagonal (cells 2, 4, 6) and confirm the message.
3. Write a function that returns how many empty cells remain, and print it after each move.

Next project: a contact book keyed by name in a map.