The error occurs because arr is a one-dimensional array and is being treated as a two-dimensional array. The code should be changed to: int arr[2]; for (int i=0; i<2; i++){ arr[i] = 1; } Alternatively, if the code is supposed to be two-dimensional, it should be defined as such: int arr[2][2]; for (int i=0; i<2; i++){ arr[i][0] = 1; }