Ethereum Smart Contract example written in Solidity

Contract.sol

contract MyContract {

  // Declare persistent variables
  uint256 public peopleCount = 0;
  address owner;

  mapping(uint => Person) public people;
  
  modifier onlyByOwner() {
    require(msg.sender == owner);
    return "_";
  }
  
  struct Person {
    uint _id;
    string _firstName;
    string _lastName;
  }
  
  // Each Smart Contract is a singleton, so once deployed the constructor runs only once. 
  constructor() {
    address = msg.sender;
  }
  
  // Public function which can be called on the Ethereum Virtual Machine
  function addPerson(string memory _firstName, string memory _lastName) public onlyByOwner {
    peopleCount += 1;
    people[peopleCount] = Person(peopleCount, _firstName, _lastName);
  }
  
}