Why events communicate contract activity in Blockchain / Solidity - Test Your Understanding

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

BSend

CReceive

DNotify

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-standard event name that won't be recognized by tools.

The event name Transfer is the standard name used to communicate token transfers in contracts.

2fill in blank

medium

Complete the code to emit the Transfer event when tokens move.

Blockchain / Solidity

emit [1](msg.sender, recipient, amount);

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

ASend

BReceive

CNotify

DTransfer

Attempts:

3 left

💡 Hint

Common Mistakes

Using emit with an event name that was not declared.

The emit keyword triggers the Transfer event to notify listeners of the token movement.

3fill in blank

hard

Fix the error in the event declaration to index the 'to' address.

Blockchain / Solidity

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

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

Aindexed

Bpublic

Cpayable

Dmemory

Attempts:

3 left

💡 Hint

Common Mistakes

Using visibility keywords like 'public' or 'payable' in event parameters.

The indexed keyword allows filtering events by that parameter in logs.

4fill in blank

hard

Fill both blanks to complete the event emission with correct parameters.

Blockchain / Solidity

emit Transfer([1], [2], amount);

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

Amsg.sender

Brecipient

Cowner

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping sender and recipient addresses.

The msg.sender is the sender address, and recipient is the receiver address in the event.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps addresses to balances greater than zero.

Blockchain / Solidity

mapping(address => uint256) public balances;

function getActiveBalances() public view returns (address[] memory) {
    address[] memory active;
    uint count = 0;
    for (uint i = 0; i < [1]; i++) {
        address user = users[i];
        if (balances[user] [2] 0) {
            active[count] = user;
            count [3] 1;
        }
    }
    return active;
}

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

Ausers.length

B>

C+=

Dbalances.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect loop length or wrong comparison operator.

We loop through users.length, check if balance is greater than zero, and increment count with +=.

Practice

1. Why do smart contracts use events to communicate activity?

easy

A. To send messages about contract actions to external listeners

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

C. To increase the gas cost of contract execution

D. To prevent users from interacting with the contract

Solution

1. Step 1: Understand the purpose of events in smart contracts

   Events are designed to notify external systems about what happens inside the contract without storing data permanently on-chain.

2. Step 2: Identify the correct use of events

   Events send messages to external listeners, like user interfaces or monitoring tools, about contract actions.

3. Final Answer:

   To send messages about contract actions to external listeners -> Option A

4. Quick Check:

   Events notify external systems = C [OK]

Hint: Events notify outside world about contract actions [OK]

Common Mistakes:

• Thinking events store large data permanently
• Believing events increase gas cost unnecessarily
• Confusing events with access control mechanisms

2. Which of the following is the correct syntax to declare an event in Solidity?

easy

A. event Transfer function(address from, address to, uint amount);

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

C. event Transfer {address from, address to, uint amount};

D. event Transfer = (address from, address to, uint amount);

Solution

1. Step 1: Recall Solidity event declaration syntax

   Events are declared with the keyword 'event', followed by the event name and parameters in parentheses, ending with a semicolon.

2. Step 2: Identify the correct syntax among options

   event Transfer(address indexed from, address indexed to, uint amount); matches the correct Solidity syntax with 'indexed' keywords for filtering.

3. Final Answer:

   event Transfer(address indexed from, address indexed to, uint amount); -> Option B

4. Quick Check:

   Correct event syntax uses parentheses and semicolon = D [OK]

Hint: Events use 'event Name(params);' syntax in Solidity [OK]

Common Mistakes:

• Using '=' instead of parentheses
• Using curly braces instead of parentheses
• Adding 'function' keyword in event declaration

3. Given the Solidity event and function below, what will be emitted when transfer(0x123, 100) is called?

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

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

medium

A. No event emitted because emit keyword is missing

B. Transfer event with from = 0x123, to = caller address, amount = 100

C. Transfer event with from = caller address, to = 0x123, amount = 100

D. Transfer event with from = 0x0, to = 0x123, amount = 0

Solution

1. Step 1: Understand the function call and event emission

   The function emits a Transfer event with 'msg.sender' as 'from', 'to' as the passed address, and 'amount' as passed.

2. Step 2: Apply the call parameters

   Calling transfer(0x123, 100) sets 'to' to 0x123 and 'amount' to 100; 'msg.sender' is the caller's address.

3. Final Answer:

   Transfer event with from = caller address, to = 0x123, amount = 100 -> Option C

4. Quick Check:

   emit Transfer(msg.sender, to, amount) = A [OK]

Hint: emit uses caller as from, passed args as to and amount [OK]

Common Mistakes:

• Confusing 'from' and 'to' parameters
• Forgetting 'emit' keyword
• Assuming default zero addresses

4. Identify the error in the following Solidity code snippet that prevents the event from being emitted:

event Deposit(address indexed sender, uint amount);

function deposit(uint amount) public {
    Deposit(msg.sender, amount);
}

medium

A. Event parameters are in wrong order

B. Function must be marked as 'payable' to emit events

C. Event name should be lowercase

D. Missing 'emit' keyword before the event call

Solution

1. Step 1: Review event emission syntax in Solidity

   Since Solidity 0.4.21, events must be emitted using the 'emit' keyword.

2. Step 2: Check the code for missing 'emit'

   The code calls 'Deposit(msg.sender, amount);' without 'emit', so the event won't be emitted.

3. Final Answer:

   Missing 'emit' keyword before the event call -> Option D

4. Quick Check:

   Events require 'emit' keyword = B [OK]

Hint: Always use 'emit' before event calls in Solidity [OK]

Common Mistakes:

• Omitting 'emit' keyword
• Assuming event names are case-insensitive
• Thinking 'payable' is required for events

5. You want to track token transfers in your contract efficiently. Which approach best uses events to minimize gas costs and allow external monitoring?

hard

A. Emit an event with indexed sender and receiver addresses and the amount transferred

B. Store all transfer details in a public array inside the contract

C. Use a mapping to save transfer history and emit no events

D. Emit events without indexed parameters to save gas

Solution

1. Step 1: Understand gas cost and event indexing

   Indexed event parameters allow efficient filtering by external tools and cost less gas than storing data on-chain.

2. Step 2: Evaluate options for tracking transfers

   Emitting events with indexed sender and receiver is efficient and allows external monitoring without high storage costs.

3. Final Answer:

   Emit an event with indexed sender and receiver addresses and the amount transferred -> Option A

4. Quick Check:

   Indexed events minimize gas and enable monitoring = A [OK]

Hint: Use indexed event params for efficient tracking [OK]

Common Mistakes:

• Storing all data on-chain increases gas costs
• Not indexing event parameters reduces filtering ability
• Avoiding events leads to poor external monitoring