One way to make the array's active is to initialize the array to a given size, then initialize each cell to 0. If you don't do this, these cells are considered inactive, and will not work with functions like (Replace subset, etc.). If you are using a constant, you can enter the values you want the array to start with, highlight the array, then select edit > make values default. This way your array constant will start with the specified values all the time.