Lesson 03 of 25

Your First Smart Contract

Write, compile, and deploy a SimpleStorage contract step-by-step.

Beginner

The SimpleStorage Contract

Let's build a contract that stores and retrieves a number. This covers the core lifecycle: write → compile → deploy → interact.

Solidity

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SimpleStorage {
    // State variable — stored permanently on the blockchain
    uint256 private storedNumber;

    // Write to storage (costs gas)
    function store(uint256 _number) public {
        storedNumber = _number;
    }

    // Read from storage (free — view function)
    function retrieve() public view returns (uint256) {
        return storedNumber;
    }
}

Understanding the Code

Let's break down each part:

// SPDX-License-Identifier: MIT — Required license comment. Use MIT for open-source.
pragma solidity ^0.8.0; — Accept compiler versions 0.8.0 and above (but not 0.9.0).
uint256 private storedNumber; — A state variable that persists on the blockchain.
public view returns (uint256) — view means the function reads but doesn't write state.

Deploying in Remix

Paste the contract into a new file in Remix
Go to the Solidity Compiler tab and click Compile
Go to Deploy & Run Transactions
Select JavaScript VM (London) as the environment
Click Deploy
Expand the deployed contract and call store(42)
Call retrieve() — you should see 42 returned

Adding a Constructor

Constructors run once at deployment time. Use them to set initial state.

Solidity

contract SimpleStorage {
    uint256 private storedNumber;
    address public owner;

    // Constructor runs once at deployment
    constructor(uint256 _initialValue) {
        storedNumber = _initialValue;
        owner = msg.sender; // the deploying address
    }

    function store(uint256 _number) public {
        storedNumber = _number;
    }

    function retrieve() public view returns (uint256) {
        return storedNumber;
    }
}

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