Indexed parameters in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of event with indexed parameters

Consider the following Solidity event and transaction. What will be the output when the event is emitted and filtered by the indexed parameter?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    event Transfer(address indexed from, address indexed to, uint amount);

    function send(address to, uint amount) public {
        emit Transfer(msg.sender, to, amount);
    }
}

A The event logs will not include any indexed parameters as topics.

B The event logs will include data for 'from', 'to', and 'amount' but no topics.

C The event logs will include only the 'amount' as a topic and 'from' and 'to' as data.

D The event logs will include topics for 'from' and 'to' addresses and data for 'amount'.

💡 Hint

Indexed parameters appear as topics in event logs.

In Solidity, parameters marked as 'indexed' in events are stored as topics in the event logs, allowing efficient filtering. Non-indexed parameters are stored in the data section.

❓ Predict Output

intermediate

Number of topics in event logs

Given this Solidity event, how many topics will be present in the event log when emitted?

Blockchain / Solidity

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

A 3 topics: one for the event signature and two for the indexed parameters.

B 2 topics: one for the event signature and one for all indexed parameters combined.

C 1 topic: only the event signature is a topic.

D 4 topics: one for the event signature and three for each parameter.

💡 Hint

Each indexed parameter adds one topic, plus the event signature topic.

Event logs always have the event signature as the first topic. Each indexed parameter adds one additional topic.

🔧 Debug

advanced

Why does filtering by a non-indexed parameter fail?

A developer tries to filter events by a non-indexed parameter but gets no results. Why does this happen?

Blockchain / Solidity

event Deposit(address indexed user, uint amount);

// Filtering by amount
filter = {amount: 100};

A Because the amount parameter is indexed but the filter syntax is wrong.

B Because non-indexed parameters are stored in data, not topics, and cannot be filtered by the Ethereum node.

C Because the event signature is missing from the filter.

D Because the event was not emitted properly.

💡 Hint

Filtering by indexed parameters uses topics, non-indexed parameters are in data and not filterable.

Ethereum nodes can only filter events by topics, which correspond to indexed parameters. Non-indexed parameters are stored in data and require manual filtering after retrieval.

📝 Syntax

advanced

Identify the syntax error in event declaration

Which option contains a syntax error in declaring an event with indexed parameters?

A event Log(address sender indexed, uint value);

B event Log(address indexed sender, uint indexed value);

C event Log(address indexed sender, uint value);

D event Log(address sender, uint indexed value);

💡 Hint

The 'indexed' keyword must come immediately after the type.

In Solidity, the 'indexed' keyword must appear immediately after the parameter type. Option A places 'indexed' after the parameter name, which is invalid syntax.

🚀 Application

expert

Max number of indexed parameters in an event

What is the maximum number of indexed parameters allowed in a Solidity event, and why?

A 2 indexed parameters, because only two topics are reserved for parameters.

B 4 indexed parameters, because there is no limit on indexed parameters.

C 3 indexed parameters, because the Ethereum log topics limit is 4 including the event signature.

D 5 indexed parameters, because the EVM supports up to 5 topics per event.

💡 Hint

The event signature occupies one topic, and the total topics per log is limited.

Ethereum event logs can have up to 4 topics: one for the event signature and up to 3 for indexed parameters. This is a protocol limit.