let's say I have an group contract where users can join and leave groups. When a user joins a group, his address will be added to members[groupId]. When he lefts, the members[groupId] will be set to 0x0. Let's say members is 2,000 values long. The problem is, that not all of these 2,000 members are active/in the group as some addresses in that array are 0x0 addresses.