conecct four

ive just started programming, and am struggling rather a lot, i am to make a connect four game which i have started but cannot get past this point,

could anyone help me with how am i suppose to get the computer to recognise that the cell is occupied and rather than say choose another cell i need it to automatically re place '_' with the '@' above it/

[code]
#include <iostream>

#include <cstdlib>

#include <ctime>

#include <cstdio>

#include <limits>

using namespace std;

using std::cout;

using std::cin;

using std::endl;

int main()

{

//char a[] = {'|','_','|','_','|','_','|','_','|','_','|','_','|','_','|'};

char a[] = {'|','_','|','_','|','_','|','_','|','_','|','_','|','_','|'};

char b[] = {'|','1','|','2','|','3','|','4','|','5','|','6','|','7','|'};

char board[6][7];

int i, j, k =0;

int row;

int column;

int game =1;

int r = rand() ;

for (i=0; i<6; i++)

for (j=0; j<7; j++)

board[i][j] = ' ';

srand ( time(NULL) );

cout<<"";

for( j = 0; j<15; j++)

cout <<b[j];

cout <<endl;

for ( i=0; i<6; i++)

{ //cout <<i+1;

for( j = 0; j<15; j++)

cout <<a[j];

cout <<endl;

}

while(game ==1)

{

cout << "your turn please enter which cell you would like" << endl;

cin >> column;

if(board[5][column-1] ==' '){

board[5][column-1] = '@';

}else if(board[5][column-1] == '@')

cout << "Cell Not Empty, Choose another cell" <<endl;

cout<<"";

for( j = 0; j<15; j++)

cout <<b[j];

cout <<endl;

for(i =0; i<6; i++){

cout<<'|';

for(j = 0; j<7; j++){

if(i < 5 && board[i][j] == '@' && board[i+1][j] == ' ') {board[i][j] =' '; board[i+1][j] ='@';}

if(board[i][j] =='@' ) cout<<board[i][j]<<'|';

else cout <<'_'<<'|';

}

cout <<endl;

}

k++;

if( k ==10){

game =0; // game over

}

}

//cout << "thank you, computer's turn" << endl;

// int r = rand() % 7;
//cout << (rand() % 6) << endl;
//cout << (rand() % 7) <<endl;

cout << "Game over !" <<endl;

return 0;

}