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
}

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
}

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);
    }
}

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

Hint

Write the example output as a comment exactly as shown.

Practice

1.

What is the main purpose of indexed parameters in blockchain events?

easy

A. To limit the size of event data

B. To encrypt event data for security

C. To tag event data for fast searching on the blockchain

D. To automatically execute functions on event trigger

Solution

1. Step 1: Understand the role of indexed parameters

   Indexed parameters are used to tag event data so it can be searched quickly on the blockchain.

2. Step 2: Compare with other options

   Encryption, size limits, or automatic execution are not related to indexed parameters.

3. Final Answer:

   To tag event data for fast searching on the blockchain -> Option C

4. Quick Check:

   Indexed parameters = fast search tags [OK]

Hint: Indexed means fast searchable tags in events [OK]

Common Mistakes:

• Thinking indexed encrypts data
• Confusing indexing with size limits
• Assuming indexed triggers functions

2.

Which of the following is the correct syntax to declare an indexed parameter in a Solidity event?

event Transfer(address ? from, address to, uint amount);

easy

A. indexed

B. index

C. indexing

D. indexedParam

Solution

1. Step 1: Recall Solidity event syntax

   The keyword to mark a parameter as indexed is exactly indexed.

2. Step 2: Check other options

   Other words like index, indexing, or indexedParam are invalid in Solidity event declarations.

3. Final Answer:

   indexed -> Option A

4. Quick Check:

   Use 'indexed' keyword for parameters [OK]

Hint: Use exact keyword 'indexed' for event parameters [OK]

Common Mistakes:

• Using 'index' instead of 'indexed'
• Adding extra words like 'indexedParam'
• Omitting the keyword entirely

3.

Given the Solidity event declaration:

event Approval(address indexed owner, address indexed spender, uint256 value);

Which parameters are indexed and can be filtered in event logs?

medium

A. owner and spender

B. owner and value

C. spender and value

D. owner, spender, and value

Solution

1. Step 1: Identify indexed parameters in the event

   The event declares owner and spender as indexed parameters.

2. Step 2: Understand filtering capability

   Only indexed parameters can be filtered in event logs, so owner and spender are filterable.

3. Final Answer:

   owner and spender -> Option A

4. Quick Check:

   Indexed parameters = owner, spender [OK]

Hint: Only parameters with 'indexed' keyword are filterable [OK]

Common Mistakes:

• Assuming all parameters are indexed
• Including non-indexed 'value' as filterable
• Confusing parameter order with indexing

4.

What is wrong with this Solidity event declaration?

event Deposit(address indexed from, uint indexed amount, uint indexed message, uint indexed timestamp);

medium

A. Indexed keyword is missing

B. Parameters must be public

C. Event name is invalid

D. More than 3 parameters are indexed

Solution

1. Step 1: Count indexed parameters

   The event has four indexed parameters: from, amount, message, and timestamp.

2. Step 2: Recall indexing limit

   Solidity allows a maximum of 3 indexed parameters per event. This declaration exceeds that limit.

3. Final Answer:

   More than 3 parameters are indexed -> Option D

4. Quick Check:

   4 indexed > max 3 [OK]

Hint: Max 3 indexed params per event [OK]

Common Mistakes:

• Thinking 4 indexed parameters are allowed
• Ignoring the 3 indexed parameter limit
• Confusing event name rules

5.

You want to create an event to track token transfers with these details: sender address, receiver address, amount, and a note message. You want to filter events by sender and receiver. Which event declaration is correct?

hard

A. event Transfer(address indexed sender, address indexed receiver, uint indexed amount, string indexed note);

B. event Transfer(address indexed sender, address indexed receiver, uint amount, string note);

C. event Transfer(address indexed sender, address receiver, uint indexed amount, string note);

D. event Transfer(address sender, address receiver, uint indexed amount, string indexed note);

Solution

1. Step 1: Identify which parameters need indexing

   Sender and receiver addresses must be indexed for filtering.

2. Step 2: Check indexing limits and types

   Only 3 parameters can be indexed. Amount can be indexed but note (string) cannot be indexed.

3. Step 3: Evaluate each option

   event Transfer(address indexed sender, address indexed receiver, uint amount, string note); indexes sender and receiver only, which is correct. event Transfer(address sender, address receiver, uint indexed amount, string indexed note); indexes amount and note incorrectly. event Transfer(address indexed sender, address receiver, uint indexed amount, string note); indexes amount but not receiver. event Transfer(address indexed sender, address indexed receiver, uint indexed amount, string indexed note); indexes 4 parameters which is invalid and string cannot be indexed.

4. Final Answer:

   event Transfer(address indexed sender, address indexed receiver, uint amount, string note); -> Option B

5. Quick Check:

   Index sender and receiver only; string can't be indexed [OK]

Hint: Index only filterable params; strings cannot be indexed [OK]

Common Mistakes:

• Indexing string parameters
• Indexing more than 3 parameters
• Not indexing all filterable parameters