Event filtering in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter events by their name.

Blockchain / Solidity

const filteredEvents = events.filter(event => event.[1] === 'Transfer');

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

Atype

Btopic

CeventName

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong property name like 'type' or 'topic' which does not hold the event name.

The property name holds the event's name, so filtering by event.name === 'Transfer' selects only Transfer events.

2fill in blank

medium

Complete the code to filter events emitted by a specific address.

Blockchain / Solidity

const filteredEvents = events.filter(event => event.[1] === '0x1234abcd');

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

Asender

Bfrom

Caddress

Demitter

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'address' or 'emitter' which may not be the correct property in the event object.

The from property usually stores the address that emitted the event, so filtering by event.from === '0x1234abcd' selects events from that address.

3fill in blank

hard

Fix the error in the code to filter events with a value greater than 1000.

Blockchain / Solidity

const bigValueEvents = events.filter(event => event.value [1] 1000);

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

A>

B==

C<

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '>' causes syntax or logic errors.

Using '==' checks equality, not greater than.

To filter events where the value is greater than 1000, use the greater than operator >. Using = is an assignment, not a comparison.

4fill in blank

hard

Fill both blanks to filter events with a topic matching '0xabc' and a block number greater than 5000.

Blockchain / Solidity

const filtered = events.filter(event => event.[1] === '0xabc' && event.[2] > 5000);

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

Atopic

BblockNumber

Cblock

Dhash

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'block' instead of 'blockNumber' which may not exist.

Using 'hash' which is unrelated to topic or block number.

The topic property holds the event topic, and blockNumber holds the block number. Filtering by these selects events with the given topic and block number above 5000.

5fill in blank

hard

Fill all three blanks to create a dictionary of event counts by event name, filtering only events with value greater than 10.

Blockchain / Solidity

const eventCounts = events.reduce((acc, event) => {
  if (event.[1] > 10) {
    acc[event.[2]] = (acc[event.[3]] || 0) + 1;
  }
  return acc;
}, {});

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

Avalue

Bname

Dtopic

Attempts:

3 left

💡 Hint

Common Mistakes

Using different properties for counting keys causes incorrect counts.

Checking wrong property for value filtering.

We check if event.value is greater than 10, then count events by their name. The accumulator key uses event.name to count occurrences.

Practice

1.

What is the main purpose of event filtering in blockchain applications?

easy

A. To encrypt blockchain data

B. To create new blockchain events

C. To mine new blocks faster

D. To listen only to specific events that match certain criteria

Solution

1. Step 1: Understand event filtering concept

   Event filtering allows you to listen only to events that match specific parameters or conditions.

2. Step 2: Identify the main purpose

   This helps reduce unnecessary data processing and improves app performance by focusing on relevant events.

3. Final Answer:

   To listen only to specific events that match certain criteria -> Option D

4. Quick Check:

   Event filtering = selective listening [OK]

Hint: Event filtering means listening only to events you want [OK]

Common Mistakes:

• Confusing event filtering with event creation
• Thinking event filtering speeds up mining
• Believing event filtering encrypts data

2.

Which of the following is the correct syntax to filter events by a parameter named user in a blockchain event listener?

contract.events.Transfer({ filter: { ??? } })

easy

A. { user: '0x123...' }

B. { 'user' }

C. [ user ]

D. user = '0x123...'

Solution

1. Step 1: Understand filter syntax

   Filters use an object with key-value pairs where keys are parameter names and values are filter values.

2. Step 2: Match correct syntax

   The correct syntax is an object with the parameter name as key and the desired value, like { user: '0x123...' }.

3. Final Answer:

   { user: '0x123...' } -> Option A

4. Quick Check:

   Filter = object with key-value pairs [OK]

Hint: Filters use {paramName: value} object syntax [OK]

Common Mistakes:

• Using array or string instead of object for filters
• Writing filter as assignment instead of object
• Omitting quotes around string values

3.

Given the following code snippet, what events will be received?

contract.events.Transfer({ filter: { from: '0xabc...' }, fromBlock: 100, toBlock: 200 })

Assuming events with from address '0xabc...' occurred at blocks 90, 150, and 210.

medium

A. Only event at block 150

B. Events at blocks 90, 150, and 210

C. Events at blocks 90 and 150

D. No events

Solution

1. Step 1: Analyze filter parameters

   The filter listens for events where 'from' equals '0xabc...'. The block range is from 100 to 200.

2. Step 2: Check event blocks against filter

   Events at blocks 90 and 210 are outside the block range, so only the event at block 150 matches both filter and block range.

3. Final Answer:

   Only event at block 150 -> Option A

4. Quick Check:

   Filter + block range = event at block 150 only [OK]

Hint: Events must match filter and block range [OK]

Common Mistakes:

• Ignoring block range and including all matching events
• Including events outside the block range
• Confusing filter parameter with block number

4.

Identify the error in this event filter code snippet:

contract.events.Approval({ filter: { owner: 0xabc123 }, fromBlock: 'latest' })

medium

A. Filter object should be an array

B. The filter value 0xabc123 should be a string

C. fromBlock cannot be 'latest'

D. The event name 'Approval' is incorrect

Solution

1. Step 1: Check filter parameter types

   Filter values must be strings for addresses, but 0xabc123 is missing quotes, so it's treated as a variable or number.

2. Step 2: Validate other parts

   The event name and fromBlock usage are valid; fromBlock can be 'latest'. Filter should be an object, not an array.

3. Final Answer:

   The filter value 0xabc123 should be a string -> Option B

4. Quick Check:

   Address filter values must be strings [OK]

Hint: Address filters need quotes around them [OK]

Common Mistakes:

• Omitting quotes around address strings
• Thinking fromBlock 'latest' is invalid
• Using array instead of object for filter

5.

You want to listen to Transfer events where the value is greater than 1000 tokens. Which approach correctly filters these events?

// Assume event signature: Transfer(address from, address to, uint256 value)

hard

A. Use filter: { value: '>1000' }

B. Use filter: { value: val => val > 1000 }

C. Filter events after receiving all by checking value > 1000 in code

D. Set fromBlock and toBlock to limit event range

Solution

1. Step 1: Understand filter limitations

   Event filters only support exact matches or arrays of values, not comparison operators like greater than.

2. Step 2: Choose correct approach

   To filter by value > 1000, you must receive events first and then check the value in your code logic.

3. Final Answer:

   Filter events after receiving all by checking value > 1000 in code -> Option C

4. Quick Check:

   Filters do exact matches; use code for range checks [OK]

Hint: Filters match exact values; use code for greater than checks [OK]

Common Mistakes:

• Trying to use comparison operators in filter
• Assuming string '>1000' works as filter
• Confusing block range with value filtering