Solidity

Synopsis

Traditional lotteries depend on intermediaries to sell tickets, hold the money and pick the winner, so players must trust that the draw is fair and the prize is paid. A smart contract can make the rules – who may buy, how many tickets exist, how the winner is chosen and how much is won – public and automatic.

In this project the deploying account is the BlockAdmin. Other users buy one ticket each (the admin may not buy) until the ticket limit is reached. The admin then declares the result: the contract computes a pseudo-random index, selects that ticket holder as the winner and records the total collected amount as the winning amount.

Contract Architecture

The Random Function

The contract picks a winner index with a hash of values that change from call to call:

uint winnerIndex = uint(keccak256(abi.encodePacked(msg.sender, block.timestamp, startvalue))) % userlimit;

• abi.encodePacked joins the caller’s address, the block timestamp and a constant into bytes.
• keccak256 hashes them into a 256-bit number.
• % userlimit reduces it to an index between 0 and 4.

Key Requirements (Concepts Used)

• Struct declared at file level and imported (Chapters 2, 13).
• Mapping and dynamic array (Chapters 10, 12).
• Functions with string parameters in memory (Chapters 9, 15).
• Several modifiers on one function (Chapter 22).
• Global variables and hashing: msg.sender, block.timestamp, keccak256, abi.encodePacked (Chapter 14).

How to Run in Remix

1. Create contracts/UserRecord.sol and contracts/LotteryContract.sol from the resource files.
2. Compile and deploy LotteryContract from the first account (BlockAdmin).
3. From five different accounts, call lotteryTicketPurchase with a name. Try buying twice, or buying from the admin account, to see the errors.
4. From the admin account call declareLotteryResult(), then read winner.

Limitations and Extension Exercises

Exercise solution (one possible approach – randomness still for learning only)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract LotteryV2 {
    address public immutable admin = msg.sender;
    uint public constant TICKET_PRICE = 0.01 ether;
    uint public constant MAX_PLAYERS = 5;

    address[] public players;
    mapping(address => bool) public hasTicket;
    address public winner;

    function buyTicket() external payable {
        require(msg.sender != admin, "Admin cannot play");
        require(msg.value == TICKET_PRICE, "Wrong ticket price");
        require(!hasTicket[msg.sender], "Already bought");
        require(players.length < MAX_PLAYERS, "Tickets over");
        hasTicket[msg.sender] = true;
        players.push(msg.sender);
    }

    function declareResult() external {
        require(msg.sender == admin, "Only admin");
        require(winner == address(0), "Already declared");
        require(players.length > 0, "No players");
        uint index = uint(keccak256(abi.encodePacked(block.prevrandao, block.timestamp, players.length)))
                     % players.length;              // only valid indexes
        winner = players[index];
    }

    function claimPrize() external {
        require(msg.sender == winner, "Not the winner");
        uint prize = address(this).balance;
        (bool ok, ) = payable(winner).call{value: prize}("");
        require(ok, "Payment failed");
    }
}

Key Takeaways

Video : Project 2 : Smart Contract for Lottery System

Resource Files

• UserRecord.sol File
• LotteryContract.sol File

Question / Answer (15 questions)

MCQ Test : Check Your Understanding (5 questions)

Select an option and press Check Answer, or answer all questions and press Submit Test to see your score. Each answer comes with an explanation.

Bibliography/References

The notes, questions and MCQs on this page are original explanations written for this course. The works listed below were consulted and are cited for attribution and further reading. Product names, logos and trademarks belong to their respective owners. Short quotations, where used, are marked and attributed.

1. Solidity Team. (n.d.). Units and Globally Available Variables. Solidity Documentation. https://docs.soliditylang.org/en/latest/units-and-global-variables.html
2. Solidity Team. (n.d.). Security Considerations. Solidity Documentation. https://docs.soliditylang.org/en/latest/security-considerations.html
3. Chainlink. (n.d.). Chainlink VRF (Verifiable Random Function) documentation. https://docs.chain.link/vrf
4. ethereum.org. (n.d.). Oracles. Ethereum Developer Documentation. https://ethereum.org/en/developers/docs/oracles/
5. Remix Project. (n.d.). Remix IDE Documentation. https://remix-ide.readthedocs.io/en/latest/
6. Modi, R. (2018). Solidity Programming Essentials: A beginner’s guide to build smart contracts for Ethereum and blockchain. Packt Publishing. ISBN 978-1-78883-138-3.