C语言变量没有被识别被传递

#include 
#include 
#define N 9
void playGame(char arr[N]);//Play the game.
char is_win(char a);//Judge the winner.
int is_full(char arr[N]);//Judge if the players fill the chessboard.

//Play the game.
void playGame(char arr[N])
{
    int p;
    int q;
    int b;
    char a; 
    do
    {
        printf("%c %c %c\n",arr[0],arr[1],arr[2]);
        printf("%c %c %c\n",arr[3],arr[4],arr[5]);
        printf("%c %c %c\n",arr[6],arr[7],arr[8]);//Output the chessboard.
        printf("Player X, please enter the number in the display (1-9)\nwhere you wish your mark to be placed.\n");
        scanf("%d",&p);//Player X.
        q=p-1;
        arr[q]='X';
        b=is_full(arr);
        a=is_win(a); 
        if(a!=2) //  ‘ ’：No one wins.
            break;//Break if there is a winner.
        system("cls");//Clear the screem.
        
        printf("%c %c %c\n",arr[0],arr[1],arr[2]);
        printf("%c %c %c\n",arr[3],arr[4],arr[5]);
        printf("%c %c %c\n",arr[6],arr[7],arr[8]);
        printf("Player O, please enter the number in the display (1-9)\nwhere you wish your mark to be placed.\n");
        scanf("%d",&p);
        q=p-1;
        arr[q]='O';
        b=is_full(arr);
        a=is_win(a); 
        if(a!=2) //  ‘ ’：没人赢
            break;
        system("cls");
    
    }
    while(a!=2);
}
//Judge the winner.
char is_win(char a)
{
    char arr[N];
    //Judge and return rows.
    int b;
    b=is_full(arr);
    if(arr[0]==arr[1]&&arr[1]==arr[2])
        a=arr[0];
    else if(arr[3]==arr[4]&&arr[4]==arr[5])
        a=arr[3];
    else if(arr[6]==arr[7]&&arr[7]==arr[8])
        a=arr[6];
    //Judge and return columns.
    else if(arr[0]==arr[3]&&arr[3]==arr[6])
        a=arr[0];
    else if(arr[1]==arr[4]&&arr[4]==arr[7])
        a=arr[1];
    else if(arr[2]==arr[5]&&arr[5]==arr[8])
        a=arr[2];
    //Judge and return diagonals.
    else if(arr[0]==arr[4]&&arr[4]==arr[8])
        a=arr[0];
    else if(arr[2]==arr[4]&&arr[4]==arr[6])
        a=arr[2];
    else if(b==2)//Judge if it is draw if it is, return ‘q'.
        a=1;
    else 
        a=2;
    return a;
    
}
//Judge if the players fill the chessboard.
int is_full(char arr[N])
{
    int b;
    int i;
    for(i=0;i<9;i++)
    {
        if(arr[i]==1||arr[i]==2||arr[i]==3||arr[i]==4||arr[i]==5||arr[i]==6||arr[i]==7||arr[i]==8||arr[i]==9)//判断是否有空格
            b=1;//The chessboard is not full, b= 1.
        else
            b=2;//The chessboard is full.
    }
    return b;//The chessboard is full, return 0.
}

int main() 
{
      printf("Tic-Tac-Toe\n");
      char arr[N];
      char a;
      //initialize array prior to playing
      for ( int i = 0; i < 9; i++)
        arr[i] = (char) ( '1' + i );
      arr[9] = '\0';   
      playGame(arr);
    if(a==1)
          printf("The game ended in a draw.");//Judge the winner.
      else 
          printf("The winner is player %c.",a);//Judge the winner.
      
    return 0;
}

为什么a不被识别运行出来不能循环呢

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