Differentiate between an unset value and 0 in an mapping

Question

How can I know if a key is unset?

The Solidity documenation on Mappings says:

Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros: a type’s default value.

If I'm mapping to an int, how can I differentiate between key with a value which is set to zero, vs key with an unset value which defaults to 0x0?

Answer 1

If I had to do this, I would use two mappings, one for the values and one to check whether if the address is used (keeping a flag - as @RobHitchens has suggested in a comment).

pragma solidity ^0.4.0;

contract Example {

    mapping(address => uint) public values;
    mapping(address => bool) public usedAddresses;


    function update(uint newVal) {

        values[msg.sender] = newVal;
        usedAddresses[msg.sender] = true;
    }

    function dosomethiingwithval(){

        if(usedAddresses[msg.sender]){ 

             //do whatever with values[msg.sender]
        }
    }

}