In Ethereum how to check if in mapping, the key already exists?

Question

In this Proof of existence, integrity, and ownership contract of a file why files[fileHash].timestamp == 0 is used? why timestamp?

contract Proof

{
   struct FileDetails
   {
       uint timestamp;
       string owner;
   }

   mapping (string => FileDetails) files;
   event logFileAddedStatus(bool status, uint timestamp, string owner, string fileHash);


   //this is used to store the owner of file at the block timestamp
   function set(string owner, string fileHash)

   {

       //There is no proper way to check if a key already exists or not therefore we are checking for default value i.e., all bits are 0
       if(files[fileHash].timestamp == 0)

       {
           files[fileHash] = FileDetails(block.timestamp, owner);

           //we are triggering an event so that the frontend of our app knows that the file's existence and ownership details have been stored
           logFileAddedStatus(true, block.timestamp, owner, fileHash);
        }
else 
        {//this tells to the frontend that file's existence and ownership details couldn't be stored because the file's details had already been stored earlier
           logFileAddedStatus(false, block.timestamp, owner, fileHash);
        }           } 

   //this is used to get file information
   function get(string fileHash) returns (uint timestamp, string owner)
   {
       return (files[fileHash].timestamp, files[fileHash].owner);

   }
 }

Answer 1

There's no way to check directly if something exists in a mapping. In solidity, everything is set to it's "default value" until it's changed.

That means every integer starts as 0, every string starts a "", every array starts as []. Solidity has no concept of "null" like other languages.

A variable which is declared will have an initial default value whose byte-representation is all zeros. The “default values” of variables are the typical “zero-state” of whatever the type is. For example, the default value for a bool is false. The default value for the uint or int types is 0. For statically-sized arrays and bytes1 to bytes32, each individual element will be initialized to the default value corresponding to its type. Finally, for dynamically-sized arrays, bytes and string, the default value is an empty array or string.

So if you have a mapping to a struct, then every possible key points to the "default struct". In your case that will be a struct with a timestamp of 0 and a owner of "".

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.

So if you want to check if the mapping hasn't been set, you can either check if files[fileHash].timestamp == 0 or check files[fileHash].owner.length == 0.