Indexed parameters in Blockchain / Solidity - Mini Project: Build & Apply

Understanding Indexed Parameters in Blockchain Events

📖 Scenario: You are building a simple smart contract on the Ethereum blockchain. This contract will emit events when users send messages. You want to learn how to use indexed parameters in events to make it easier to search for specific data in the blockchain logs.

🎯 Goal: Create a Solidity contract that defines an event with indexed parameters, emits this event when a message is sent, and then prints the event data (simulated in code comments) to understand how indexed parameters work.

📋 What You'll Learn

Define an event called MessageSent with two indexed parameters: address sender and string message

Create a function called sendMessage that emits the MessageSent event

Use exactly the variable names sender and message in the event

Emit the event with msg.sender as sender and a string parameter as message

💡 Why This Matters

🌍 Real World

Blockchain developers use indexed parameters in events to quickly find specific transactions or actions in large blockchain data.

💼 Career

Understanding indexed parameters is essential for blockchain developers working on smart contracts and decentralized applications that require efficient event filtering.

1

Create the basic contract and event

Write a Solidity contract named MessageContract. Inside it, define an event called MessageSent with two indexed parameters: address indexed sender and string indexed message.

Blockchain / Solidity

contract MessageContract {
    // Define the event with indexed parameters here
    // Your code here
}

Need a hint?

Use the event keyword and add indexed before the parameters you want to index.

2

Add the sendMessage function

Inside the MessageContract, add a public function called sendMessage that takes a string memory message parameter. This function should emit the MessageSent event with msg.sender as sender and the message parameter.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract MessageContract {
    event MessageSent(address indexed sender, string indexed message);

    // Add the sendMessage function below
    // Your code here
}

Need a hint?

Use the emit keyword to send the event with the correct parameters.

3

Explain the role of indexed parameters

Add a comment inside the contract explaining that indexed parameters allow filtering events by those parameters when searching blockchain logs.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract MessageContract {
    event MessageSent(address indexed sender, string indexed message);

    // Explain indexed parameters here
    // Your code here

    function sendMessage(string memory message) public {
        emit MessageSent(msg.sender, message);
    }
}

Need a hint?

Write a simple comment starting with // inside the contract.

4

Simulate event output

Write a comment below the contract showing an example output of the MessageSent event when sendMessage is called by address 0x123... with message "Hello". Show the indexed parameters clearly.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract MessageContract {
    // Indexed parameters allow filtering events by these parameters when searching blockchain logs
    event MessageSent(address indexed sender, string indexed message);

    function sendMessage(string memory message) public {
        emit MessageSent(msg.sender, message);
    }
}

// Example event output:
// Your code here

Need a hint?

Write the example output as a comment exactly as shown.