How to undo a stack when the struct contains a char ** variable? - c

Question

This code below is something like a candy crush game, where you pop similar letters in a cluster, then they'll disappear and you score. On main, I tried to play it one step ahead and then undo it, and it works since the score managed to undo (from 0 to some score then back to 0 again). However, I just can't figure out why the game board isn't updated?
typedef struct {
int rs;
int cs;
char **board;
int score;
} Instance;
with random letters (a, b ,c or d)
extern Game *create(int nrows, int ncols) {
if (nrows > MAX_ROWS || ncols > MAX_COLS) {
return NULL;
}
Game *b;
b->top = 0;
b->stack[b->top].rs = nrows;
b->stack[b->top].cs = ncols;
b->stack[b->top].board = malloc(sizeof(char *) * nrows);
for (int row = 0; row < nrows; row++) {
b->stack[b->top].board[row] = malloc(sizeof(char) * ncols);
}
srand(time(0));
for (int row = 0; row < nrows; row++) {
for (int column = 0; column < ncols; column++) {
int random = rand() % 4;
if (random == 0) {
b->stack[b->top].board[row][column] = A;
} else if (random == 1) {
b->stack[b->top].board[row][column] = B;
} else if (random == 2) {
b->stack[b->top].board[row][column] = C;
} else {
b->stack[b->top].board[row][column] = D;
}
}
}
return b;
}
// Display the current matrix
extern void display(Game *b) {
/** Prints top border **/
printf(" +-");
for (int top = 0; top < b->stack[b->top].cs; top++) {
printf("--");
}
printf("+\n");
/** Prints the board **/
for (int row = 0; row < b->stack[b->top].rs; row++) {
if (row < 10) {
printf("0%d | ", row);
} else {
printf("%d | ", row);
}
for (int column = 0; column < b->stack[b->top].cs; column++) {
printf("%c ", b->stack[b->top].board[row][column]);
}
printf("|\n");
}
/** Prints bottom border **/
printf(" +-");
for (int bot = 0; bot < b->stack[b->top].cs; bot++) {
printf("--");
}
printf("+\n");
/** Prints vertical column indices **/
printf(" ");
for (int tens = 0; tens < b->stack[b->top].cs; tens++) {
printf("%d ", tens/10);
}
printf("\n");
printf(" ");
int count = 0;
for (int ones = 0; ones < b->stack[b->top].cs; ones++) {
if (count > 9) {
count = 0;
}
printf("%d ", count);
count++;
}
}
extern int select(Game *b, int r, int c) {
char colour = b->stack[b->top].board[r][c];
int n = recursive_helper(b, r, c);
if (n == 1) {
b->stack[b->top].board[r][c] = colour;
return 0;
}
b->stack[b->top].score += n*(n-1);
return n;
}
int main() {
Game *b = create(5, 10);
display(b);
printf("\n");
printf("%d", bp_score(b));
printf("\n");
select(b, 2, 2);
display(b);
printf("\n");
printf("%d", bp_score(b));
printf("\n");
b->top--;
display(b);
printf("\n");
printf("%d", bp_score(b));
printf("\n");
}

Answer 1

As pointed in the comments, your create function doesn't instantiate a Game struct, you need b = malloc(sizeof(Game));
Another error is that your mallocs use sizeof(int*) and sizeof(int) instead of sizeof(char*) and sizeof(char).
Other than that, your problem comes from the fact that b->stack[b->top+1] = b->stack[b->top]; copies a structure, but the board is a pointer and points to the same object! You only have one char** pointer and a single board in memory.
Thus, when you do b->top-- while you do access another struct, it still points to the same fully updated board.
You need to create a function Instance CopyInstance(Instance src) that creates a new board in the new struct and then copies each cell from the source board in the new one. That way each Instance will point to a different board!