Indexed parameters in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are indexed parameters in blockchain event logs?

Indexed parameters are special event parameters that are stored in the blockchain's log topics, allowing efficient filtering and searching of events by these parameters.

beginner

How many parameters can be indexed in a single event in Ethereum?

Up to three parameters can be indexed in a single event in Ethereum smart contracts.

beginner

Why use indexed parameters in blockchain events?

Indexed parameters make it easier and faster to search for specific events by filtering logs based on these parameters without scanning all event data.

intermediate

Show an example of an event declaration with indexed parameters in Solidity.

event Transfer(address indexed from, address indexed to, uint256 value);

intermediate

What is the difference between indexed and non-indexed parameters in blockchain events?

Indexed parameters are stored in topics and can be filtered efficiently, while non-indexed parameters are stored in the event data and require full log scanning to access.

How many parameters can you mark as indexed in an Ethereum event?

A 1

B 3

C 5

D Unlimited

Ethereum allows up to three parameters to be indexed in an event for efficient filtering.

What is the main benefit of using indexed parameters in blockchain events?

A Increased contract storage

B Lower gas cost for transactions

C Automatic event execution

D Faster event filtering

Indexed parameters allow faster filtering of events by storing key data in topics.

In Solidity, how do you mark a parameter as indexed in an event?

A Add the keyword 'indexed' after the parameter type

B Add the keyword 'filter' after the parameter

C Use a special event type

D Indexed parameters are not supported

You add the keyword 'indexed' after the parameter type to mark it as indexed.

Which part of the event log stores indexed parameters?

A Topics

B Data

C Transaction input

D Block header

Indexed parameters are stored in the topics section of the event log.

Can non-indexed parameters be used to filter events efficiently?

A Only if the event is small

B Yes, all parameters can be filtered equally

C No, only indexed parameters can be filtered efficiently

D Only if the blockchain is private

Only indexed parameters are stored in topics and can be filtered efficiently.

Explain what indexed parameters are and why they are useful in blockchain event logs.

Think about how events are searched on the blockchain.

Describe how to declare an event with indexed parameters in Solidity and what happens when the event is emitted.

Recall Solidity syntax and event log structure.

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