Getting keys of Solidity mapping

Solidity is an object-oriented language designed for the Ethereum blockchain to facilitate smart contract development. It offers a wide range of data structure support, including HashMap.

HashMap

HashMap, sometimes referred to as a Dictionary, is a key-value-based data structure that maps values against unique/non-repeating keys. Additionally, it allows constant look-up time on average due to the nature of its implementation.

Storing keys of a solidity mapping

Unlike other programming languages (C++, JavaScript), Solidity does not provide a way to directly access the keys of a HashMap. However, we can explicitly maintain a dynamic array that keeps hold of all the keys and an additional HashMap to check whether the key exists.

pragma solidity ^0.5.0;

// Defining contract
contract Persons {

  // For storing keys
  uint[] _ids;

  // Main Map for storing data
  mapping (uint => string) _names;

  // Additional Map for checking if key exists
  mapping (uint => bool) _exists;  

  function setPerson(uint id, string memory name) public {
         
         // if key does not exist then insert in array
         if (!_exists[id]) {
             _ids.push(id);
             _exists[id] = true;
         }
        
        // insert/update main map
         _names[id] = name;
  }

  function getKeys() public view returns(uint[] memory){
    return _ids;
  }
}

Note: Array _ids type and the additional HashMap _exists key type should be same as the key type of the main HashMap _names.

Code explanation

Line 4: We define a contract.
Line 7: We declare a dynamic array to store the keys.
Line 10: We declare the main HashMap that will hold the significant data.
Line 13: We declare an additional HashMap to check if a value already exists against a particular key.
Line 15: We create a utility function to insert/update the main HashMap _names. The function checks if the key was previously present or not. If not inserted previously, it adds the key inside the array _ids and updates the additional HashMap _exists against that key. In the end, it simply adds or updates the value against the key supplied.
Line 27: We create a read-only/view function that returns the array _ids holding all the keys.

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