Event filtering in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity event filter code?

Consider the following Solidity code snippet that filters events by an indexed parameter:

pragma solidity ^0.8.0;

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

    function testFilter() public pure returns (string memory) {
        // Imagine filtering Transfer events where from == 0x123...
        // This is a conceptual example, not actual filtering code
        return "Filtered 3 events";
    }
}

What would be the output of testFilter() assuming it returns the count of filtered events?

A "Filtered 3 events"

B "Filtered 0 events"

C "Filtered all events"

D "Filtered 1 event"

💡 Hint

Look at the return statement inside testFilter().

The function testFilter() returns the string "Filtered 3 events" directly, so that is the output.

🧠 Conceptual

intermediate

Which indexed event parameter allows filtering by that parameter?

In Solidity events, which keyword allows a parameter to be used for filtering when listening to events?

A public

B indexed

C constant

D payable

💡 Hint

It is a keyword placed before the parameter name in the event declaration.

The indexed keyword marks event parameters that can be filtered on when listening to events.

❓ Predict Output

advanced

What error does this event filter code produce?

Given this JavaScript code using ethers.js to filter events:

const filter = contract.filters.Transfer(null, '0x0000000000000000000000000000000000000000');
const events = await contract.queryFilter(filter);
console.log(events.length);

What error will this code produce?

A No error, prints number of events

B SyntaxError: unexpected token

C ReferenceError: contract is not defined

D TypeError: invalid address

💡 Hint

Check if the address used is valid and the filter syntax is correct.

The address '0x000...000' is a valid zero address, and the filter syntax is correct, so no error occurs. The code prints the number of matching events.

❓ Predict Output

advanced

What is the output of this Solidity event filtering example?

Consider this Solidity event and filter example:

pragma solidity ^0.8.0;

contract Example {
    event Data(uint indexed id, string value);

    function emitEvents() public {
        emit Data(1, "one");
        emit Data(2, "two");
        emit Data(3, "three");
    }

    // Conceptual filter: get events where id == 2
    function filterEvents() public pure returns (string memory) {
        return "Data with id 2: two";
    }
}

What does filterEvents() return?

A "Data with id 3: three"

B "Data with id 1: one"

C "Data with id 2: two"

D "No matching data"

💡 Hint

Look at the return statement in filterEvents().

The function returns the string "Data with id 2: two" directly, representing the filtered event.

🧠 Conceptual

expert

Which option correctly describes event filtering in Ethereum logs?

Which statement correctly describes how event filtering works in Ethereum logs?

A Filtering requires decoding all contract storage to find events.

B Filtering scans all transaction input data to find matching events.

C Filtering is done by scanning block timestamps for event matches.

D Filtering uses indexed event parameters stored in topics to quickly find matching logs.

💡 Hint

Think about how Ethereum stores event data and how filters use topics.

Ethereum stores indexed event parameters in topics, which are used to filter logs efficiently. Non-indexed parameters are stored in data and cannot be filtered directly.