ticTacToe.c

//
// C program that runs a tic tac toe game with 2 players 
//
#include <stdio.h>
#include <stdbool.h>

// Prints the game board 
void printBoard(int board[], int playerA, int playerB){
	for(int i = 0; i < 9; i++){
		// If the value at index i is 10, that means it is player A's spot 
		if(board[i] == playerA){
			printf(" A");
		} else if (board[i] == playerB){
		// If the value at index i is 11, that means it is player B's spot 
			printf(" B");
		} else {
			printf(" %d", i + 1);
		}
		// New line of board
		if(i % 3 == 2){
			printf("\n");
		}
	}
}

// Ensures that each position is not claimed twice 
int requestValidInput(int board[], int playerA, int playerB){
	int index;
	scanf("%d", &index);

	while(true){
		if(index > 0 && index < 10){
			// User input is one more than array index so we need to subtract it 
			index = index - 1;
			// If index is not 0, that means it is claimed
			if(board[index] != 0){
				printf("That position has already been played, please try again.\n");
				scanf("%d", &index);
			} else {
				return index;
			}	
			
		} else {
			// User entered invalid position 
			printf("Invalid input, please try again.\n");
			scanf("%d", &index);
		}
	}
}

// Functions checks every possibly win (3 in a row) for either player and returns the winner 
int checkForWinner(int board[], int playerA, int playerB){
	for(int i = 0; i < 3; i++){
		if(playerA == board[i + 0] && playerA == board[i + 3] && playerA == board[i + 6]){
			return playerA;
		}

		if(playerB == board[i + 0] && playerB == board[i + 3] && playerB == board[i + 6]){
			return playerB;
		}

		if(playerA == board[i * 3 + 0] && playerA == board[i * 3 + 1] && playerA == board[i * 3 + 2]){
			return playerA;
		}

		if(playerB == board[i * 3 + 0] && playerB == board[i * 3 + 1] && playerB == board[i * 3 + 2]){
			return playerB;
		}
	}

	if(playerA == board[0] && playerA == board[4] && playerA == board[8]){
		return playerA;
	}

	if(playerB == board[0] && playerB == board[4] && playerB == board[8]){
		return playerB;
	}
	
	if(playerA == board[2] && playerA == board[4] && playerA == board[6]){
		return playerA;
	}
	
	if (playerB == board[2] && playerB == board[4] && playerB == board[6]){
		return playerB;
	}
	return 0;
}

// Checks if every position has been filled
bool checkforStalemate(int board[]){
	for(int index = 0; index < 9; index++){
		// 0 means no one has claimed that position 
		if(board[index] == 0){
			return false;
		}	
	}
	return true;
}

int main(void){
	int board[9], i, validIndex, winner;
	const int playerA = 10;
	const int playerB = 11;
	char turn = 'A';

	bool gameOver = false;
	// Initial entire board to 0 
	for(i = 0; i < 9; i++){
		board[i] = 0;
	}
  
	while(gameOver == false){
		printf("The current state of the Tic-tac-toe Board:\n\n");
		printBoard(board, playerA, playerB);

		printf("\nIt is Player %c’s turn.\nPlease enter a valid position to play.\n", turn);
		validIndex = requestValidInput(board, playerA, playerB);
		if(turn == 'A'){
			board[validIndex] = playerA;
			turn = 'B';
		} else {
			board[validIndex] = playerB;
			turn = 'A';
		}
		winner = checkForWinner(board, playerA, playerB);
		if(winner != 0){
			gameOver = true;
		// Checks if every position has been filled, if it has that means someone has won 
		} else if(checkforStalemate(board))
			gameOver = true;
	}

	// Checks for winner and displays the final board 
	if(playerA == winner){
		printf("Player A wins!\n\n");
		printBoard(board, playerA, playerB);
	} else if(playerB == winner){
		printf("Player B wins!\n\n");
		printBoard(board, playerA, playerB);
	} else {
		printf("It’s a draw!\n\n");
		printBoard(board, playerA, playerB);
	}
	return 0;
}