Emitting events in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an event named Transfer.

Blockchain / Solidity

event [1](address indexed from, address indexed to, uint256 value);

Drag options to blanks, or click blank then click option'

ATransfer

BLog

CSend

DNotify

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic name like Log or Notify instead of Transfer.

Forgetting to declare the event.

The event name is Transfer, which is commonly used to log token transfers.

2fill in blank

medium

Complete the code to emit the Transfer event when tokens are sent.

Blockchain / Solidity

emit Transfer([1], to, amount);

Drag options to blanks, or click blank then click option'

Amsg.sender

Baddress(this)

Cowner

Drecipient

Attempts:

3 left

💡 Hint

Common Mistakes

Using owner or recipient which may not be defined.

Using address(this) which refers to the contract itself.

msg.sender is the address sending the tokens, so it should be the from parameter in the event.

3fill in blank

hard

Fix the error in the event emission code to correctly emit the Approval event.

Blockchain / Solidity

emit Approval(owner, [1], amount);

Drag options to blanks, or click blank then click option'

Amsg.sender

Brecipient

Cspender

Dto

Attempts:

3 left

💡 Hint

Common Mistakes

Using recipient or to which are not correct for Approval.

Using msg.sender which may not be the spender.

The Approval event requires the spender address as the second argument.

4fill in blank

hard

Fill both blanks to declare and emit a custom event named Deposit with an indexed sender and amount.

Blockchain / Solidity

event [1](address indexed sender, uint256 amount);

function deposit() public payable {
    emit [2](msg.sender, msg.value);
}

Drag options to blanks, or click blank then click option'

ADeposit

BTransfer

CApproval

DWithdraw

Attempts:

3 left

💡 Hint

Common Mistakes

Using different event names in declaration and emission.

Using standard event names like Transfer instead of Deposit.

The event name is Deposit, so both declaration and emission use the same name.

5fill in blank

hard

Fill all three blanks to declare an event named OwnershipTransferred and emit it with previous and new owner addresses.

Blockchain / Solidity

event [1](address indexed [2], address indexed [3]);

function transferOwnership(address newOwner) public {
    address oldOwner = owner;
    owner = newOwner;
    emit OwnershipTransferred(oldOwner, newOwner);
}

Drag options to blanks, or click blank then click option'

AOwnershipTransferred

BpreviousOwner

CnewOwner

Downer

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names like owner instead of previousOwner.

Mismatching event name and emission.

The event is named OwnershipTransferred and the parameters are previousOwner and newOwner as indexed addresses.

Practice

1. What is the main purpose of emitting events in a blockchain smart contract?

easy

A. To send signals about important actions to external listeners

B. To store large amounts of data permanently on the blockchain

C. To execute complex calculations inside the contract

D. To create new tokens automatically

Solution

1. Step 1: Understand event purpose

   Events are used to notify external systems about contract actions without heavy storage.

2. Step 2: Compare options

   Only To send signals about important actions to external listeners describes sending signals to outside listeners, which matches event use.

3. Final Answer:

   To send signals about important actions to external listeners -> Option A

4. Quick Check:

   Events notify external listeners = D [OK]

Hint: Events notify outside world, not store big data [OK]

Common Mistakes:

• Thinking events store large data permanently
• Confusing events with token creation
• Believing events run complex calculations

2. Which of the following is the correct syntax to emit an event named Transfer with parameters from, to, and amount in Solidity?

easy

A. event Transfer(from, to, amount);

B. Transfer.emit(from, to, amount);

C. emit Transfer(from, to, amount);

D. emit.Transfer(from, to, amount);

Solution

1. Step 1: Recall Solidity event emission syntax

   In Solidity, events are emitted using the keyword emit followed by the event name and parameters in parentheses.

2. Step 2: Check each option

   emit Transfer(from, to, amount); matches the correct syntax: emit Transfer(from, to, amount);. Others misuse dot notation or keywords.

3. Final Answer:

   emit Transfer(from, to, amount); -> Option C

4. Quick Check:

   emit keyword + event name + params = C [OK]

Hint: Use 'emit EventName(params);' exactly [OK]

Common Mistakes:

• Using dot notation like emit.Transfer()
• Writing event declaration instead of emission
• Placing emit after event name

3. Given the Solidity code snippet:

event Deposit(address indexed sender, uint amount);

function deposit() public payable {
    emit Deposit(msg.sender, msg.value);
}

What happens when the deposit function is called with 1 ether?

medium

A. The function throws an error due to missing return

B. The contract balance decreases by 1 ether

C. No event is emitted because emit keyword is missing

D. An event Deposit is emitted with sender's address and amount 1 ether

Solution

1. Step 1: Analyze the deposit function

   The function emits the Deposit event with the caller's address and the amount sent (msg.value).

2. Step 2: Understand event emission effect

   Calling deposit with 1 ether triggers the event with sender and 1 ether amount; contract balance increases, no error.

3. Final Answer:

   An event Deposit is emitted with sender's address and amount 1 ether -> Option D

4. Quick Check:

   emit Deposit with sender and value = A [OK]

Hint: emit sends event with current call data [OK]

Common Mistakes:

• Thinking contract balance decreases on deposit
• Missing that emit keyword is present
• Expecting return value in void function

4. Identify the error in this Solidity code snippet:

event Approval(address owner, address spender, uint amount);

function approve(address spender, uint amount) public {
    Approval(msg.sender, spender, amount);
}

medium

A. Missing the emit keyword before the event call

B. Event parameters are in wrong order

C. Function must be declared as view

D. Event name should be lowercase

Solution

1. Step 1: Check event emission syntax

   In Solidity, events must be emitted using the emit keyword before calling the event.

2. Step 2: Identify the error in code

   The code calls Approval(...) without emit, causing a syntax error.

3. Final Answer:

   Missing the emit keyword before the event call -> Option A

4. Quick Check:

   emit keyword required before event call = A [OK]

Hint: Always prefix event calls with 'emit' [OK]

Common Mistakes:

• Omitting emit keyword
• Assuming event names are case-insensitive
• Thinking function needs view modifier

5. You want to track token transfers in your smart contract and filter them by sender address efficiently. Which event declaration and emission pattern is best to achieve this?

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

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

hard

A. Do not use indexed to save gas, filter by reading all events externally

B. Use indexed keyword on sender and receiver to enable filtering in event logs

C. Emit event without parameters to reduce complexity

D. Store all transfers in contract storage instead of events

Solution

1. Step 1: Understand indexed keyword usage

   Indexed parameters in events allow efficient filtering by those fields in external tools.

2. Step 2: Evaluate options for tracking transfers

   Use indexed keyword on sender and receiver to enable filtering in event logs uses indexed on sender and receiver, enabling fast filtering by sender address.

3. Final Answer:

   Use indexed keyword on sender and receiver to enable filtering in event logs -> Option B

4. Quick Check:

   indexed parameters enable filtering = B [OK]

Hint: Use indexed for filterable event parameters [OK]

Common Mistakes:

• Avoiding indexed to save gas but losing filtering ability
• Emitting events without parameters
• Storing all data on-chain instead of events