Can't get length of array? (not querying for property)

Question

mapping(uint256 => address) public userList;

function getUserCount() public constant returns(uint userCount) {
 return userList.length;
}

function newUser(address _address) internal returns(bool success) {
    userList.push(_address);
    return true;
  }

function getUser(uint256 _userId) public constant returns(address){
      return(userList[_userId]);
  }

throws error "member length can not be found" - can somebody explain to me why this doesn't work? i've looked at the storage patterns list and saw that they would add an additional array to have a user count - i was really trying to avoid that so that on high demand two users would not get the same id, by setting their id to length - was just planning to push the new users without setting an id manually

Answer 1

In the code above userList is not an array, it is a mapping. Mapping's do not have a length property because we can't know what the size of a mapping is.

If you convert mapping(uint256 => address) public userList; to address[] public userList; then it should work as expected.

Also, regarding your last point. It's fine for you to use userList.length to determine a user's ID, even if two users are added in the same block/at the same time, as the transactions are processed sequentially, so they will still get different ID's.