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);

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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);

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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);

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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);

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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 +=.