Event declaration 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. What will be the output in the transaction logs after calling setValue(42)?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    event ValueChanged(address indexed author, uint oldValue, uint newValue);
    uint public value;

    function setValue(uint _value) public {
        uint old = value;
        value = _value;
        emit ValueChanged(msg.sender, old, _value);
    }
}

A An event ValueChanged is logged with the caller's address, oldValue as 0, and newValue as 42

B An event ValueChanged is logged with the caller's address, oldValue as 42, and newValue as 0

C No event is logged because events require explicit subscription

D A compilation error occurs because events cannot have indexed parameters

💡 Hint

Remember that emit triggers the event and oldValue is the previous stored value.

The event ValueChanged is emitted with the caller's address, the old value (initially 0), and the new value (42). Indexed parameters are allowed and help filtering logs.

🧠 Conceptual

intermediate

Which statement about Solidity event declaration is correct?

Choose the correct statement about declaring events in Solidity.

A Events cannot have parameters marked as indexed

B Events are only used for internal contract state changes and not visible externally

C Events are stored permanently on-chain and can be accessed by smart contracts

D Events can have up to three indexed parameters to allow filtering in logs

💡 Hint

Think about how events help external tools filter logs.

Events can have up to three parameters marked as indexed to enable efficient filtering by external applications. Events are not accessible by contracts and are stored in transaction logs.

🔧 Debug

advanced

Why does this Solidity event declaration cause a compilation error?

Examine the following event declaration. Why does it fail to compile?

Blockchain / Solidity

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

A The event name cannot be <code>Transfer</code> as it is reserved

B The <code>indexed</code> keyword cannot be applied to the entire event declaration

C The event must have at least one indexed parameter

D The event parameters must be declared as <code>memory</code> or <code>calldata</code>

💡 Hint

Check where the indexed keyword is allowed in event declarations.

The indexed keyword can only be applied to individual parameters, not to the entire event declaration. The syntax here is invalid.

❓ Predict Output

advanced

What is the output of this event emission with multiple indexed parameters?

Given this Solidity contract, what will be the content of the event log after calling recordAction(7, 3)?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Logger {
    event ActionRecorded(address indexed user, uint indexed actionId, uint value);

    function recordAction(uint actionId, uint value) public {
        emit ActionRecorded(msg.sender, actionId, value);
    }
}

A The event log contains user address, actionId as 7, and value as 3 with user and actionId indexed

B The event log contains user address, actionId as 3, and value as 7 with no indexed parameters

C The event log contains only the value 3 because indexed parameters are not stored

D The event log is empty because events with multiple indexed parameters are not allowed

💡 Hint

Remember that indexed parameters appear in topics and non-indexed in data.

The event ActionRecorded logs the caller's address and actionId as indexed parameters, and value as non-indexed data. The values match the function call arguments.

🚀 Application

expert

How many indexed parameters can this Solidity event legally have?

Consider this event declaration:

event DataEvent(uint indexed id, address indexed sender, string message, uint indexed timestamp);

How many parameters are indexed, and is this declaration valid?

A Two parameters are indexed; Solidity limits indexed parameters to two per event

B Four parameters are indexed; all parameters can be indexed in events

C Three parameters are indexed; this is valid because Solidity allows up to three indexed parameters per event

D This declaration is invalid because string parameters cannot be indexed

💡 Hint

Recall the maximum number of indexed parameters allowed and restrictions on data types.

Solidity allows up to three indexed parameters per event. The string parameter cannot be indexed, but the other three parameters are indexed correctly. This declaration is valid.