What You Will Be Able To Do

• Read and write basic Solidity
• Compile and deploy in Remix
• Call free reads and paid writes
• Guard a function with require
• Explain why deployed code is fixed

Tools: Solidity And Remix

• Solidity: language for the EVM
• Remix: editor and deployer at remix.ethereum.org
• Remix VM: simulated chain in the browser
• Sepolia: real test network via MetaMask

Anatomy Of A Contract

• Licence comment and pragma line first
• contract keyword names the contract
• State variables live in storage
• Functions read or change that state

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

contract Hello {
    string public greeting = "hello";
}

Types You Meet First

• uint256: whole numbers from zero upward
• address: a 20-byte account
• bool and string
• mapping(address => uint256): table of balances

Function Words

• external and public set who can call
• view reads state and is free from outside
• payable accepts ether
• msg.sender is the caller, msg.value the ether sent

Counter: The Code

• Constructor records the deployer as owner
• increment is open to everyone
• reset is restricted to the owner

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

contract Counter {
    uint256 public count;
    address public owner;
    event Incremented(address indexed by, uint256 newCount);
    constructor() { owner = msg.sender; }
    function increment() external { count += 1; emit Incremented(msg.sender, count); }
    function reset() external { require(msg.sender == owner, "only owner"); count = 0; }
}

Compile

• Compiler tab: choose 0.8.20 or higher
• Press Compile for a green check
• Read errors by line number
• Common first mistake: missing semicolon

Deploy On The Remix VM

• Environment: Remix VM
• Pick Counter, press Deploy
• Blue buttons read, orange buttons send transactions
• Terminal shows gas used and logs

Access Control With Require

• require checks a condition before continuing
• A false condition reverts every change
• msg.sender == owner is the simplest guard
• Missing guards cause real losses

function reset() external {
    require(msg.sender == owner, "only owner");
    count = 0;
}

TipJar: Handling Ether

• tip is payable and records each giver
• withdraw sends the balance to the owner
• A contract holds its own ether balance

contract TipJar {
    address public owner;
    mapping(address => uint256) public tips;
    constructor() { owner = msg.sender; }
    function tip() external payable {
        require(msg.value > 0, "send some ETH");
        tips[msg.sender] += msg.value; }
    function withdraw() external {
        require(msg.sender == owner, "only owner");
        (bool ok, ) = owner.call{value: address(this).balance}("");
        require(ok, "transfer failed"); }
}

Test TipJar In Remix

• Set Value to 1 ether, press tip
• Read tips for your address
• Try withdraw from a second account
• Withdraw as owner and watch balances

Deploy To Sepolia

• Environment: Injected Provider with MetaMask
• Confirm the wallet shows Sepolia
• Press Deploy and approve in the wallet
• No recipient: this is a contract creation

Read Your Contract On Etherscan

• Paste the contract address into sepolia.etherscan.io
• Find the creation transaction and gas used
• Call increment and spot the Incremented event
• Optionally verify the source code

Deployed Code Cannot Change

• A bug means a new contract at a new address
• Test on the Remix VM and testnets first
• Audits matter when contracts hold value
• Private storage is still readable by anyone

Recap

• Solidity contracts hold state and functions
• Views read free, transactions write and cost gas
• require and msg.sender guard access
• Remix VM first, then Sepolia
• Deployed code is permanent

Next Week: Tokens

• What a token is: a contract with a balance table
• The ERC-20 standard and its functions
• Minting, transferring and approving
• Deploying your own test token

Questions And Exercise

• Deploy Counter and TipJar on the Remix VM
• Deploy Counter on Sepolia
• Find your event on Etherscan
• Add and test a decrement function