Instead of worrying about implementing a computer player, I think your first step shoud be to offer support for a second human player.
First, you will need some array to represent the state of the game board. A multi-dimensional array of type char seems appropriate, in which each char can have one of three values:
'E' for empty'X' for first player'O' for second player
You could define the array like this:
char board[3][3];
Also, you will need an additional variable to keep track of whose turn it currently is. A variable of type bool with the name first_players_turn would probably be appropriate. When it is the first player's turn, this variable should be set to true, otherwise to false.
I also suggest that you create your own function print_board for printing the game board to the screen.
You will also need a function evaluate_game_state in order to determine the state of the game, which can be one of the following:
- The game is still undecided.
- The game is a draw.
- Player 1 has won.
- Player 2 has won.
To represent these 4 possible game states, I recommend that you use an enum, like this:
enum game_state
{
GAMESTATE_UNDECIDED,
GAMESTATE_DRAW,
GAMESTATE_WINNER_PLAYER1,
GAMESTATE_WINNER_PLAYER2
};
You can now declare the function evaluate_game_state like this:
enum game_state evaluate_game_state( char board[3][3] );
As long as the function evaluate_game_state returns GAMESTATE_UNDECIDED, you will have to prompt the user for input in a loop.
Here is my implementation of everything stated above, which I have tested a bit and seems to work, but I have not tested all corner cases. I took over the function get_int_from_user from this answer of mine from another question, and made a slight modification, so that it accepts variadic arguments, so that I can pass a string containing %d to that function and it treats it as a printf format specifier. Although your implementation of using printf and scanf would also work, that solution has the disadvantage that scanf does not perform proper input validation, so I used my function get_int_from_user instead.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <limits.h>
#include <errno.h>
#include <stdarg.h>
#include <stdbool.h>
enum game_state
{
GAMESTATE_UNDECIDED,
GAMESTATE_DRAW,
GAMESTATE_WINNER_PLAYER1,
GAMESTATE_WINNER_PLAYER2
};
void init_board( char board[3][3] );
void print_board( char board[3][3] );
enum game_state evaluate_game_state( char board[3][3] );
int get_int_from_user( const char *prompt, ... );
int main( void )
{
char board[3][3];
bool first_players_turn = true;
enum game_state gs;
init_board( board );
while ( ( gs = evaluate_game_state( board ) ) == GAMESTATE_UNDECIDED )
{
//prompt user for input until input is valid
for (;;)
{
int input;
print_board( board );
input = get_int_from_user(
"It is player %d's turn.\n"
"Please select a move (1-9): ",
first_players_turn ? 1 : 2
);
if ( 1 <= input && input <= 9 )
{
int x = ( input - 1 ) % 3;
int y = ( input - 1 ) / 3;
//make sure field is empty
if ( board[y][x] != 'E' )
{
printf( "That field is already occupied!\n\n" );
continue;
}
//perform the move
board[y][x] = first_players_turn ? 'X' : 'O';
first_players_turn = !first_players_turn;
break;
}
printf( "Input must be between 1 and 9!\n\n" );
}
}
//game is over
print_board( board );
switch ( gs )
{
case GAMESTATE_DRAW:
printf( "The game is a draw.\n\n" );
break;
case GAMESTATE_WINNER_PLAYER1:
printf( "Player 1 wins!\n\n" );
break;
case GAMESTATE_WINNER_PLAYER2:
printf( "Player 2 wins!\n\n" );
break;
}
}
void init_board( char board[3][3] )
{
for ( int i = 0; i < 3; i++ )
for ( int j = 0; j < 3; j++ )
board[i][j] = 'E';
}
void print_board( char board[3][3] )
{
printf( "\n\n" );
for ( int i = 0; i < 3; i++ )
{
for ( int j = 0; j < 3; j++ )
{
//add spacing if necessary
if ( j != 0 )
printf( " " );
if ( board[i][j] == 'E' )
//replace empty field with number of field
printf( "%d", (i*3) + (j+1) );
else
printf( "%c", board[i][j] );
}
//end the line
printf( "\n" );
}
printf( "\n" );
}
enum game_state evaluate_game_state( char board[3][3] )
{
//check for horizontal wins
for ( int i = 0; i < 3; i++ )
{
char possible_winner = board[i][0];
if ( possible_winner != 'E' )
{
if ( possible_winner == board[i][1] && possible_winner == board[i][2] )
{
return possible_winner == 'X' ? GAMESTATE_WINNER_PLAYER1 : GAMESTATE_WINNER_PLAYER2;
}
}
}
//check for vertical wins
for ( int i = 0; i < 3; i++ )
{
char possible_winner = board[0][i];
if ( possible_winner != 'E' )
{
if ( possible_winner == board[1][i] && possible_winner == board[2][i] )
{
return possible_winner == 'X' ? GAMESTATE_WINNER_PLAYER1 : GAMESTATE_WINNER_PLAYER2;
}
}
}
//check for diagonal-down win
{
char possible_winner = board[0][0];
if ( possible_winner != 'E' )
{
if ( possible_winner == board[1][1] && possible_winner == board[2][2] )
{
return possible_winner == 'X' ? GAMESTATE_WINNER_PLAYER1 : GAMESTATE_WINNER_PLAYER2;
}
}
}
//check for diagonal-up win
{
char possible_winner = board[2][0];
if ( possible_winner != 'E' )
{
if ( possible_winner == board[1][1] && possible_winner == board[0][2] )
{
return possible_winner == 'X' ? GAMESTATE_WINNER_PLAYER1 : GAMESTATE_WINNER_PLAYER2;
}
}
}
//check if board is full
for ( int i = 0; i < 3; i++ )
for ( int j = 0; j < 3; j++ )
if ( board[i][j] == 'E' )
return GAMESTATE_UNDECIDED;
return GAMESTATE_DRAW;
}
int get_int_from_user( const char *prompt, ... )
{
for (;;) //loop forever until user enters a valid number
{
va_list args;
char buffer[1024], *p;
long l;
//prompt user for input
va_start( args, prompt );
vprintf( prompt, args );
va_end( args );
//get one line of input from input stream
if ( fgets( buffer, sizeof buffer, stdin ) == NULL )
{
fprintf( stderr, "unrecoverable error reading from input\n" );
exit( EXIT_FAILURE );
}
//make sure that entire line was read in (i.e. that
//the buffer was not too small)
if ( strchr( buffer, '\n' ) == NULL && !feof( stdin ) )
{
int c;
printf( "line input was too long!\n" );
//discard remainder of line
do
{
c = getchar();
if ( c == EOF )
{
fprintf( stderr, "unrecoverable error reading from input\n" );
exit( EXIT_FAILURE );
}
} while ( c != '\n' );
continue;
}
//attempt to convert string to number
errno = 0;
l = strtol( buffer, &p, 10 );
if ( p == buffer )
{
printf( "error converting string to number\n" );
continue;
}
//make sure that number is representable as an "int"
if ( errno == ERANGE || l < INT_MIN || l > INT_MAX )
{
printf( "number out of range error\n" );
continue;
}
//make sure that remainder of line contains only whitespace,
//so that input such as "6sdfh4q" gets rejected
for ( ; *p != '\0'; p++ )
{
if ( !isspace( (unsigned char)*p ) )
{
printf( "unexpected input encountered!\n" );
//cannot use `continue` here, because that would go to
//the next iteration of the innermost loop, but we
//want to go to the next iteration of the outer loop
goto continue_outer_loop;
}
}
return l;
continue_outer_loop:
continue;
}
}
This program has the following behavior:
1 2 3
4 5 6
7 8 9
It is player 1's turn.
Please select a move (1-9): 5
1 2 3
4 X 6
7 8 9
It is player 2's turn.
Please select a move (1-9): 4
1 2 3
O X 6
7 8 9
It is player 1's turn.
Please select a move (1-9): 1
X 2 3
O X 6
7 8 9
It is player 2's turn.
Please select a move (1-9): 6
X 2 3
O X O
7 8 9
It is player 1's turn.
Please select a move (1-9): 9
X 2 3
O X O
7 8 X
Player 1 wins!
If you want to make a good computer opponent, then I suggest that you take a look at the Wikipedia article on Minimax.
