How do you maintain track of the elements that really contain data if an array is only half filled?

How do you keep track of items with data in an array that is only partly filled?

How do you keep track of which entries in a partially filled array contain data?

How do you keep tabs on the elements with data in an array that is only partially filled?

How are items containing data in an array that is only partly filled to be tracked when the array is only partially populated?

void changeAll(int x[]) { x[0] = x[0] + 5; return; The first element in an array would be increased by 5 The array would be the same because it was passed by value x would be increased by 5 All the elements in the passed array would be incremented by 5

In what situations is it helpful to have data stored in an array with several dimensions?