problem with tictactoe

[wowkid]

I have my whole program and its works fine in human v. human. however i am having trouble with human v. computer. I looked it up online but i can figure out how to do this, as the game goes on the player and the computer(picking a random space every turn) start putting pieces on the board. So basically say spots 3,6,8,9 are all taken- if i am using rand() to determine of comptuters placement of pieces how can i take out those 4 numbers(out of the rand()) so they put in a correct space? If only my own turn if i put in one of them it would day invalid entry- but how can i do it so the computer doesnt pick a space already occupied?

[hamsterman]

there is no way to stop rand() from returning some specific value. There are several ways I can think of.
1. the most simple one: repeat randomization until a free spot is found.

do spot = rand()%9;
while(board[spot] == ' ');
board[spot] = 'X';

2. a little smarter one: generate a number from 0 to 'number of free spots', and then find the one you chose.

static int num_spots = 9;//assuming computer starts first..

int spot = rand()%num_spots;
for(int i = 0, j = 0; i < 9; i++){
    if(j == spot){
        board[i] = 'X';
        break;
    }
    if(board[i] == ' ') j++;
}

num_spots-=2;//it would be better not to use a static variable, 
//but to recalculate num_spots every time. I'm just lazy.