Could mapping data structure return the number of items it points?

Question

For example following code piece from https://ethereum.stackexchange.com/a/10763/4575 . I have added new lines that is followed by //*.

pragma solidity ^0.4.2;
contract test {    
  struct my_struct {
    int a;
    int my_length; //*to keep track of the length.
  }

  mapping (address=>my_struct[]) Map;

  function fill_map(my_struct struct1,my_struct struct2) internal  {    
    Map[msg.sender].push(struct1);
    Map[msg.sender][0].my_length = Map[msg.sender][0].my_length + 1;//*

    Map[msg.sender].push(struct2);   
    Map[msg.sender][0].my_length = Map[msg.sender][0].my_length + 1;//*

    Map[msg.sender].push(struct3);//*  
    Map[msg.sender][0].my_length = Map[msg.sender][0].my_length + 1;//*

    delete Map[id][2]; //*struct3 will be removed.
    Map[msg.sender][0].my_length = Map[msg.sender][0].my_length - 1;//*
  }
}

I can obtain data as follows: Map[id][index].a; index is an integer number. In this example it can be 0 (for struct1) or 1 (for struct2).

As we know Java could return the number of object inside a list:

String[] array = new String[10];
int size = array.length;

Is there anything similar as: Map[id].length ? or manually after each push or delete should I need to keep length?

On my solution Map[msg.sender][0].my_length will return the number of the pointed structs from Map[msg.sender].

Answer 1

there is some mistakes (or i didn't understand well your question) in your previous code and i think also in your question.

in your case you have a map of arrays. so Map[key].length exists and return the size of the corresponding array to your key. so Map[msg.sender].length is correct but Map[msg.sender][0].length is not. because in this second case you are returning the first struct which doesn't have a length parameter but a my_length.

Answer 2

You can't get the number of mapped keys or get a list of keys that map to data.

You can maintain a list a keys in a separate array as you insert data into a map and then you can get the count indirectly by asking how long the index is.

If you use event emitters to report state changes, you can often proceed on the idea that the contract itself doesn't need the count or the list and the clients don't need further help from the contract.