Indexed parameters in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an indexed parameter in an event.

Blockchain / Solidity

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

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

Aindexed

Bpublic

Cmemory

Dpayable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' or 'memory' instead of 'indexed' for event parameters.

Forgetting to add 'indexed' before the parameter name.

The keyword indexed marks a parameter as indexed, allowing it to be searched in event logs.

2fill in blank

medium

Complete the event declaration to index the 'to' address parameter.

Blockchain / Solidity

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

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

Apublic

Bpayable

Cmemory

Dindexed

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'memory' or 'payable' instead of 'indexed'.

Placing 'indexed' after the parameter name.

Marking the 'to' parameter as indexed allows filtering events by recipient address.

3fill in blank

hard

Fix the error in the event declaration by correctly indexing the 'owner' parameter.

Blockchain / Solidity

event Approval(address [1] owner, address spender, uint256 value);

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

Apublic

Bindexed

Cmemory

Dpayable

Attempts:

3 left

💡 Hint

Common Mistakes

Placing 'indexed' after the parameter name.

Using incorrect keywords like 'payable' or 'memory'.

The 'owner' parameter must be marked with indexed before its name to be searchable in logs.

4fill in blank

hard

Fill both blanks to declare an event with two indexed address parameters.

Blockchain / Solidity

event Transfer(address [1] from, address [2] to, uint256 value);

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

Aindexed

Bpayable

Cmemory

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Indexing only one parameter when two are needed.

Using wrong keywords like 'payable' or 'memory'.

Both 'from' and 'to' parameters are marked indexed to allow filtering by sender and recipient.

5fill in blank

hard

Fill all three blanks to declare an event with three indexed parameters: two addresses and one uint.

Blockchain / Solidity

event Approval(address [1] owner, address [2] spender, uint256 [3] value);

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

Aindexed

Bpayable

Cmemory

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Indexing fewer than three parameters.

Using incorrect keywords like 'payable' or 'memory'.

All three parameters are marked indexed to allow filtering by owner, spender, and value.

Practice

1.

What is the main purpose of indexed parameters in blockchain events?

easy

A. To limit the size of event data

B. To encrypt event data for security

C. To tag event data for fast searching on the blockchain

D. To automatically execute functions on event trigger

Solution

1. Step 1: Understand the role of indexed parameters

   Indexed parameters are used to tag event data so it can be searched quickly on the blockchain.

2. Step 2: Compare with other options

   Encryption, size limits, or automatic execution are not related to indexed parameters.

3. Final Answer:

   To tag event data for fast searching on the blockchain -> Option C

4. Quick Check:

   Indexed parameters = fast search tags [OK]

Hint: Indexed means fast searchable tags in events [OK]

Common Mistakes:

• Thinking indexed encrypts data
• Confusing indexing with size limits
• Assuming indexed triggers functions

2.

Which of the following is the correct syntax to declare an indexed parameter in a Solidity event?

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

easy

A. indexed

B. index

C. indexing

D. indexedParam

Solution

1. Step 1: Recall Solidity event syntax

   The keyword to mark a parameter as indexed is exactly indexed.

2. Step 2: Check other options

   Other words like index, indexing, or indexedParam are invalid in Solidity event declarations.

3. Final Answer:

   indexed -> Option A

4. Quick Check:

   Use 'indexed' keyword for parameters [OK]

Hint: Use exact keyword 'indexed' for event parameters [OK]

Common Mistakes:

• Using 'index' instead of 'indexed'
• Adding extra words like 'indexedParam'
• Omitting the keyword entirely

3.

Given the Solidity event declaration:

event Approval(address indexed owner, address indexed spender, uint256 value);

Which parameters are indexed and can be filtered in event logs?

medium

A. owner and spender

B. owner and value

C. spender and value

D. owner, spender, and value

Solution

1. Step 1: Identify indexed parameters in the event

   The event declares owner and spender as indexed parameters.

2. Step 2: Understand filtering capability

   Only indexed parameters can be filtered in event logs, so owner and spender are filterable.

3. Final Answer:

   owner and spender -> Option A

4. Quick Check:

   Indexed parameters = owner, spender [OK]

Hint: Only parameters with 'indexed' keyword are filterable [OK]

Common Mistakes:

• Assuming all parameters are indexed
• Including non-indexed 'value' as filterable
• Confusing parameter order with indexing

4.

What is wrong with this Solidity event declaration?

event Deposit(address indexed from, uint indexed amount, uint indexed message, uint indexed timestamp);

medium

A. Indexed keyword is missing

B. Parameters must be public

C. Event name is invalid

D. More than 3 parameters are indexed

Solution

1. Step 1: Count indexed parameters

   The event has four indexed parameters: from, amount, message, and timestamp.

2. Step 2: Recall indexing limit

   Solidity allows a maximum of 3 indexed parameters per event. This declaration exceeds that limit.

3. Final Answer:

   More than 3 parameters are indexed -> Option D

4. Quick Check:

   4 indexed > max 3 [OK]

Hint: Max 3 indexed params per event [OK]

Common Mistakes:

• Thinking 4 indexed parameters are allowed
• Ignoring the 3 indexed parameter limit
• Confusing event name rules

5.

You want to create an event to track token transfers with these details: sender address, receiver address, amount, and a note message. You want to filter events by sender and receiver. Which event declaration is correct?

hard

A. event Transfer(address indexed sender, address indexed receiver, uint indexed amount, string indexed note);

B. event Transfer(address indexed sender, address indexed receiver, uint amount, string note);

C. event Transfer(address indexed sender, address receiver, uint indexed amount, string note);

D. event Transfer(address sender, address receiver, uint indexed amount, string indexed note);

Solution

1. Step 1: Identify which parameters need indexing

   Sender and receiver addresses must be indexed for filtering.

2. Step 2: Check indexing limits and types

   Only 3 parameters can be indexed. Amount can be indexed but note (string) cannot be indexed.

3. Step 3: Evaluate each option

   event Transfer(address indexed sender, address indexed receiver, uint amount, string note); indexes sender and receiver only, which is correct. event Transfer(address sender, address receiver, uint indexed amount, string indexed note); indexes amount and note incorrectly. event Transfer(address indexed sender, address receiver, uint indexed amount, string note); indexes amount but not receiver. event Transfer(address indexed sender, address indexed receiver, uint indexed amount, string indexed note); indexes 4 parameters which is invalid and string cannot be indexed.

4. Final Answer:

   event Transfer(address indexed sender, address indexed receiver, uint amount, string note); -> Option B

5. Quick Check:

   Index sender and receiver only; string can't be indexed [OK]

Hint: Index only filterable params; strings cannot be indexed [OK]

Common Mistakes:

• Indexing string parameters
• Indexing more than 3 parameters
• Not indexing all filterable parameters