Is there an easier way of representing conditions in C++?

Question

I wrote a basic tic-tac-toe game based on multidimensional arrays. g[3][3]. In my program I have about 9 conditions like the one I am about to show you:

if((g[0][0] == X && g[0][1] == X && g[0][2] == X) || (g[0][0] == O && g[0][1] == O && g[0][2] == O))

This is quite insane. I am probably doing something wrong but this is why I am addressing this question. Is there an easier way of representing long and complicated conditions like this? For example couldn't I somehow do:

if(grid.hasXes)

Answer 1

You're probably going about it the wrong way. There are only 3^9, or 19683 possible combinations, so you can convert your grid to an int, even on a 16 bit machine:

int
asInt( char const (&grid)[3][3] )
{
    int results = 0;
    for ( int i = 0; i != 3; ++ i ) {
        for ( int j = 0; j != 3; ++ j ) {
            results *= 3;
            switch ( grid[i][j] ) {
            case 'X':
                results += 1;
                break;

            case 'Y':
                results += 2;
                break;

            case ' ':
                break;

            default:
                assert(0);
            }
        }
    }
    return results;
}

Afterwards, you can use the int to index into a table indicating who won (if anyone). Alternatively, you can convert just one or the other player's position into a 9 bit int:

int
asInt( char const (&grid)[3][3], char who )
{
    int results = 0;
    for ( int i = 0; i != 3; ++ i ) {
        for ( int j = 0; j != 3; ++ j ) {
            results *= 2;
            if ( grid[i][j] == who ) {
                ++ results;
            }
        }
    }
    return results;
}

You can then use a simple linear search into a table, verifying that the necessary bits are set:

static int const wins[] =
{
    0007, 0070, 0700,       //  rows
    0111, 0222, 0444,       //  columns
    0124, 0421              //  diagonals
};

class Wins
{
    int myToMatch;
public:
    Wins( char const (&grid)[3][3], char who )
        : myToMatch( asInt( grid, who ) )
    {
    }
    bool operator()( int entry ) const
    {
        return (entry & myToMatch) == entry;
    }
};

Then:

if ( std::find_if( begin( wins ), end( wins ), Wins( grid, 'X' ) )
            != end( wins ) {
    //  X wins
else if ( std::find_if( begin( wins ), end( wins ), Wins( grid, 'O' ) )
            != end( wins ) {
    //  O wins
else
    //  play another turn.

You could even consider keeping the grid as two ints, one per player. The bit number for a position would be 3 * i + j, and to test if a move is legal:

bool
isLegal( int gridX, int gridY, int i, int j )
{
    return ((gridX | gridY) & (1 << (3 * i + j))) == 0;
}