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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity event emission?

Consider the following Solidity contract snippet that emits an event. What will be the output when the triggerEvent function is called?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract EventTest {
    event NumberSet(uint indexed number);

    function triggerEvent() public {
        emit NumberSet(42);
    }
}

A A compilation error occurs because the event is not declared correctly.

B The function returns the number 42 as output.

C The event NumberSet with number 42 is emitted and logged on the blockchain.

D The event NumberSet is emitted but with number 0 instead of 42.

💡 Hint

Events in Solidity are used to log information on the blockchain and do not return values from functions.

The emit keyword triggers the event NumberSet with the value 42. This event is logged on the blockchain and can be listened to by external applications. The function itself does not return any value.

🧠 Conceptual

intermediate

Which statement about blockchain event testing is true?

When testing events emitted by smart contracts, which of the following is correct?

A Events automatically revert the transaction if the event data is incorrect.

B Events are returned as function return values and can be asserted directly.

C Events cannot be tested because they are not stored on the blockchain.

D Events can be tested by checking transaction logs after the transaction is mined.

💡 Hint

Think about how events are stored and accessed after a transaction.

Events are stored in the transaction logs on the blockchain. Testing frameworks can access these logs after the transaction is mined to verify that the correct events were emitted with expected data.

🔧 Debug

advanced

Why does this event test fail to detect the emitted event?

Given the following test code snippet for a Solidity contract event, why does the test fail to detect the event?

Blockchain / Solidity

const tx = await contract.triggerEvent();
const receipt = await tx.wait();
const event = receipt.events.find(e => e.event === 'NumberSet');
assert(event.args.number === 42);

A The event argument is accessed incorrectly; it should be <code>event.args[0]</code> instead of <code>event.args.number</code>.

B The transaction receipt does not contain events because <code>wait()</code> was not called.

C The contract does not emit any event named 'NumberSet' because the event is not declared.

D The event name is case-sensitive and 'NumberSet' does not match the emitted event name.

💡 Hint

Check the exact spelling and capitalization of the event name in the contract and test.

Event names in Solidity are case-sensitive. If the emitted event is named 'Numberset' but the test looks for 'NumberSet', the event will not be found, causing the test to fail.

📝 Syntax

advanced

Which option correctly declares and emits an event with two parameters in Solidity?

Choose the correct Solidity code snippet that declares an event with two parameters and emits it properly.

A

event DataSet(uint id, string value)
function setData() public {
  emit DataSet(1, "hello");
}

B

event DataSet(uint indexed id, string value);
function setData() public {
  emit DataSet(1, "hello");
}

C

event DataSet(uint id, string value);
function setData() public {
  DataSet(1, "hello");
}

D

event DataSet(uint indexed id, string value);
function setData() public {
  emit DataSet(1);
}

💡 Hint

Remember that events must be declared with a semicolon and emitted with the emit keyword including all parameters.

Option B correctly declares the event with a semicolon and emits it with both parameters using emit. Option B misses the semicolon after the event declaration. Option B does not use emit. Option B emits the event with missing parameters.

🚀 Application

expert

How many events are emitted and what are their values after this function call?

Given the following Solidity function, how many events are emitted and what are their values when multiEmit() is called?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract MultiEvent {
    event Log(uint indexed id, string message);

    function multiEmit() public {
        for (uint i = 1; i <= 3; i++) {
            emit Log(i, string(abi.encodePacked("Event", uint2str(i))));
        }
    }

    function uint2str(uint _i) internal pure returns (string memory str) {
        if (_i == 0) {
            return "0";
        }
        uint j = _i;
        uint len;
        while (j != 0) {
            len++;
            j /= 10;
        }
        bytes memory bstr = new bytes(len);
        uint k = len;
        while (_i != 0) {
            k = k-1;
            uint8 temp = (48 + uint8(_i - _i / 10 * 10));
            bstr[k] = bytes1(temp);
            _i /= 10;
        }
        str = string(bstr);
    }
}

A Three events are emitted with values: (1, "Event1"), (2, "Event2"), (3, "Event3").

B Three events are emitted but all have the same value: (3, "Event3").

C No events are emitted because string concatenation is invalid.

D One event is emitted with value: (3, "Event3").

💡 Hint

Look at the loop and how the event is emitted each iteration with the current index.

The function emits the Log event three times with the index from 1 to 3 and the message "Event" concatenated with the index number. The helper function uint2str converts the number to a string correctly.