main.cpp

#include <iostream>
using namespace std;
// width and height of the board
const int width = 7, height = 6;

//pieces
const char EMPTY = '_', X = 'X', O = 'O';

//board is represented with a 2-dimensional char array
char board[height][width];

// this function initializes the board to have all empty spots
void setup(){
  for (int i = 0; i < height; i++) {
    for (int j = 0; j < width; j++) {
      board[i][j] = EMPTY;
    }
  }
}

//this function prints the board in a user-friendly way
void printBoard(){
  for (int i = 0; i < height; i++) {
    cout << "|";
    for (int j = 0; j < width; j++) {
      cout << " " << board[i][j] << " " << "|";
    }
    cout << endl;
  }
  for (int i = 0; i < width; i++) {
    cout << "  " << i << " ";
  }
  cout << endl;
  cout << endl;
}

// this function allows a player to add a move to a column
void playMove(char piece, int column) {
  for (int i = 0; i < height; i++) {
    if (board[i][column] != EMPTY) {
      if (i == 0) {
        // if the TOP row of the column that the player chose is NOT EMPTY,
        // then the column is FULL and the user is not allowed to go there.
        cout << "cannot move there! Skipping your turn." << endl;
        return;
      } else {
        board[i-1][column] = piece;
        return;
      }
    } else {
      // if the bottom of the column is empty, then the player is putting the first piece there
      if (i == height-1) {
        board[i][column] = piece;
        return;
      }
    }
  }
}

//this function checks if the player has won after a move is made
bool checkWin(char player) {
  int win = 0;
  for (int i = 0; i < height; i++) {
    for (int j = 0; j < width; j++) {

      //checking for horizontal win
      win = 0;
      for (int k = 0; k < 4; k++) {
        if ((j+k) < width) {
          if (board[i][j+k] == player) {
            win++;
          }
        }
      }
      if (win == 4) {
        cout << "Player " << player << " won!" << endl;
        return true;
      }


      //checking for a vertical win
      win = 0;
      for (int k = 0; k < 4; k++) {
        if ((i+k) < height) {
          if (board[i+k][j] == player) {
            win++;
          }
        } 
      }
      if (win == 4) {
        cout << "Player " << player << " won!" << endl;
        return true;
      }


      // checking up and right
      win = 0;
      for (int k = 0; k < 4; k++) {
        if ((i+k) < height && (j+k) < width) {
          if (board[i+k][j+k] == player) {
            win++;
          }
        } 
      }
      if (win == 4) {
        cout << "Player " << player << " won!" << endl;
        return true;
      }

      // checking up and left
      win = 0;
      for (int k = 0; k < 4; k++) {
        if ((i+k) < height && (j-k) >= 0) {
          if (board[i+k][j-k] == player) {
            win++;
          }
        } 
      }
      if (win == 4) {
        cout << "Player " << player << " won!" << endl;
        return true;
      }

      // checking down and left
      win = 0;
      for (int k = 0; k < 4; k++) {
        if ((i-k) >= 0 && (j-k) >= 0) {
          if (board[i-k][j-k] == player) {
            win++;
          }
        } 
      }
      if (win == 4) {
        cout << "Player " << player << " won!" << endl;
        return true;
      }

      // checking down and right
      win = 0;
      for (int k = 0; k < 4; k++) {
        if ((i-k) >= 0 && (j+k) < width) {
          if (board[i-k][j+k] == player) {
            win++;
          }
        } 
      }
      if (win == 4) {
        cout << "Player " << player << " won!" << endl;
        return true;
      }

    }
  }

  int tieCount = 0;
  for (int i = 0; i < height; i++) {
    for (int j = 0; j < width; j++) {
      if (board[i][j] != EMPTY) {
        tieCount++;
      }
    }
  }
  if (tieCount == width*height) {
    cout << "Both players tied!" << endl;
    return true;
  }

  return false;
}

void playGame(){
  char player = X;
  while (!(checkWin(X) || checkWin(O))) {
    int column;
    cout << "Player " << player << ", enter a column: ";
    cin >> column;
    cout << endl;
    while(!cin) {
      // user didn't input a number
      cin.clear(); // reset failbit
      cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); //skip bad input
      // next, request user reinput
      cout << "Invalid input! Player " << player << ", enter a number: ";
      cin >> column;
      cout << endl;
    }
    if (column >= width) {
      cout << "Invalid column! Try again." << endl;
    } else {
      playMove(player, column);
      printBoard();
      if (player == X) {
        player = O;
      } else {
        player = X;
      }
    }
  }
}

int main() {
  // setup
  cout << "Welcome to Connect 4!" << endl;
  cout << "Hit ctrl+c at any time to quit the game." << endl;
  int replay = 0;
  do {
    setup();
    printBoard();
    playGame();
    cout << "Enter 0 to quit or 1 to play again: ";
    cin >> replay;
    cout << endl;
    while (!cin || (replay != 1 && replay != 0)) {
      // user didn't input a number
      cin.clear(); // reset failbit
      cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); //skip bad input
      // next, request user reinput
      cout << "Invalid input! Please enter either 1 or 0: ";
      cin >> replay;
      cout << endl;
    }
  } while (replay == 1);
}