variables in c changing value randomly - c

Question

I'm learning C at school, and as homework I have to write the tictactoe game. No problem with the "algorithm", but I do not understand why if I change the order of the variables declaration, the program output drastically changes or even the programme stops working. For example, if I swap line 12 with line 13, the element of the array coord change values at random points of the programme. Can someone explain me why this happen?
#include <stdio.h>
#define DIM 3
#define MAX 11
int main(void) {
char c;
int state = 0; //Variable for the switch case
int nC, nR; //Variables used to count how many X or O there are in the rows and columns of the grid
int i, j;
int coord[2] = {0, 0}; //Array for the coordinates
char grid[DIM][DIM]; //Grid 3x3
char player1[MAX] = "", player2[MAX] = ""; //Name of the players
printf("Player 1, insert your name (max 10 characters): ");
gets(player1);
fflush(stdin);
printf("Player 2, insert your name (max 10 characters): ");
gets(player2);
for (i = 0; i < DIM; i++) { //Inizialize the grid with '.'
for (j = 0; j < DIM; j++) {
grid[i][j] = '.';
printf("%3c", grid[i][j]);
if (j == 0 || j == 1) printf(" |");
}
if (i == 0 || i == 1) printf("\n- - - - - - - -\n");
}
do{
switch (state) {
case 0: //State 0: Player 1 is asked for the coordinates corresponding to the position where you want to insert the X symbol
printf("\n%s your turn: ", player1);
scanf("%d %d", &coord[1], &coord[2]);
if (grid[coord[1] - 1][coord[2] - 1] == '.' && grid[coord[1] - 1][coord[2] - 1] != 'O') { //Check that the selected coordinates are free. Otherwise it prints an error message
grid[coord[1] - 1][coord[2] - 1] = 'X';
c = 'X';
state = 2;
}
else{
state = 0;
printf("Invalid coordinates!\n");
}
break;
case 1: //State 1: Player 2 is asked for the coordinates corresponding to the position where you want to insert the O symbol
printf("\n%s your turn: ", player2);
scanf("%d %d", &coord[1], &coord[2]);
if (grid[coord[1] - 1][coord[2] - 1] == '.' && grid[coord[1] - 1][coord[2] - 1] != 'X') { //Check that the selected coordinates are free. Otherwise it prints an error message
grid[coord[1] - 1][coord[2] - 1] = 'O';
c = 'O';
state = 2;
}
else{
printf("Invalid coordinates!\n");
state = 1;
}
break;
case 2: //State 2: Check if there a right combination of X or O
printf("\n");
for (i = 0; i < DIM; i++) {
for (j = 0; j < DIM; j++) {
printf("%3c", grid[i][j]);
if(j == 0 || j == 1) printf(" |");
}
if (i == 0 || i == 1) printf("\n- - - - - - - -\n");
}
nC = 0;
nR = 0;
i = coord[1] - 1;
for (j = 0; j < DIM; j++) {
if(grid[i][j] != c){
break;
}
else{
nR++;
}
}
j = coord[2] - 1;
for (i = 0; i < DIM; i++) {
if (grid[i][j] != c) {
break;
}
else{
nC++;
}
}
if (nC == 3 || nR == 3) state = 3;
else if (c == 'X') state = 1;
else state = 0;
break;
case 3:
if (c == 'X') printf("\n%s IS THE WINNER!\n", player1);
else printf("\n%s IS THE WINNER!\n", player2);
return 0;
break;
}
} while (1);
}

Answer 1

In C, array indices for an array with n elements run from 0 to n−1.
int coord[2] = {0, 0}; defines coord to have two elements, so their indices are 0 and 1.
Throughout the code, coord[1] and coord[2] are used. coord[2] is outside the defined array, so the behavior of the program is not defined by the C standard.