storing integers in an array stores a random value - c++

Question

I'm coding my own minesweeper game for fun in C++ (The language I'm most familiar with) and when I'm storing a constant in a 2d array, it sometimes ends up storing a random value.
Here's my code:
using namespace std;
Table::Table() {
tiles[16][16] = {0};
covers[16][16] ={1};
}//stores position of mines, covers, and values
//places mines on the board
void Table::placemines() {
int minecount=0;
int i = rand()%15;
int j = rand()%15;
while(minecount<40){
if (tiles[i][j] == 0) {
tiles[i][j] = -10;
minecount++;
} else {}
i = rand()%15;
j = rand()%15;
}
}
and my main.cpp to display the values
using namespace std;
int main() {
Table newtable = Table();
newtable.placemines(6, 7);
for (int j = 0; j < 16; j++) {
for (int i = 0; i < 16; i++) {
cout << newtable.tiles[i][j] << ' ';
}
cout << '\n';
}
}
and the output
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 -10 -10 -10 0 0 1 -10 0 0
0 0 0 0 0 0 0 -10 0 -10 -10 0 -10 0 0 0
0 0 0 0 0 -10 0 0 0 -10 -10 0 -10 0 0 0
-10 0 0 -10 -10 0 0 -10 -10 0 0 0 0 -10 0 0
-10 0 -10 0 0 -10 0 0 0 0 0 0 0 -10 0 0
0 0 0 0 0 0 0 0 -10 -10 0 1920169263 0 -10 0 0
0 0 -10 0 0 0 0 0 0 -10 -10 1651076143 0 0 0 0
0 0 0 0 0 0 0 0 -10 -10 0 1819894831 -10 0 0 0
0 0 0 0 0 0 0 0 0 0 0 100 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 -10 0 0 0 0
0 0 0 0 0 0 0 -10 0 -10 0 32 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
-10 0 0 -10 0 0 0 0 0 0 -10 2 0 0 0 0
-10 0 0 0 0 -10 0 0 0 -10 0 4 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 4 0 0 0 0
Can anyone tell what's going on? Thank you!

Answer 1

Too much code missing, no declaration, no placemines() code
You have bug here:
tiles[16][16] = {0};
This statement sets some element of array, with indices 16 and 16, to 0. If your array defined as type tiles[16][16] that means you write into the Abyss and cause UB (because last element of array is tiles[15][15]).
If you have to initialize an array, use std::fill http://en.cppreference.com/w/cpp/algorithm/fill . For zero-intialization this works:
Table::Table() : tiles{0} {
Another bug possible:
int i = rand()%15;
You have to use array's size to cover the range properly.
PS. Avoid using namespace std; in global scope, pleeeese.