嵌套循环的索引值达到500万-C

Question

I am programming a board game and I need to assign character values to a 2D array. To do this, I am using a nested for loop with the i as the row index and j as the column index. With a 4x4 dimension (n=4) The loop works fine until the second row. Using the debugger on codelite, I've noticed that the value of j does not increase from 0 to 1 like it should, but it increases to 5,560,570, disrupting the loop. I've also noticed that when using a dimension larger than 4, the program fails to display anything at all. Is this a memory error? I am stumped and have showed this to multiple other people as well.

int main(void){
int n;
char board[n][26];
printf("Enter the board dimension: ");
scanf("%d", &n);
for(int i = 0; i < n; i++){
    for(int j = 0; j < n; j++){
        if((i == (n/2)-1 && j == (n/2)-1) || (i == (n/2) && j == (n/2))){
            board[i][j] = 'W';
        }
        else if((i == (n/2) && j == (n/2)-1) || (i == (n/2)-1 && j == (n/2))){
            board[i][j] = 'B';
        }
        else{
            board[i][j] = 'U';
        }
    }
}

Answer 1

It appears that you are using n before you set it, in the declaration of board . Because this is undefined behavior, absolutely anything is permitted to happen; in this case, that is disrupting the value of other variables.

To fix this, you should probably wait until after initializing n in scanf to declare board , like so:

 int main(void) {
     int n;
     printf("Enter the board dimension: ");
     scanf("%d", &n);
     char board[n][26];
     ...
  }

As has been pointed out in the comments, this still will cause problems if n > 26 , and can be wasteful for n != 26 . Due to the way that arrays work in C, fixing that would probably require rethinking how the board is stored altogether.