Distributed ledger concept in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this simple blockchain ledger update?

Consider a blockchain ledger represented as a list of blocks. Each block contains a transaction amount. What will be the total balance after adding these blocks?

Blockchain / Solidity

ledger = [100, -20, 50]
total_balance = sum(ledger)
print(total_balance)

A 130

B 150

C 180

D 100

💡 Hint

Sum all the amounts in the ledger list.

The ledger has transactions 100, -20, and 50. Adding them gives 130.

🧠 Conceptual

intermediate

Which property ensures all participants have the same copy of the ledger?

In a distributed ledger system, what is the key property that guarantees all nodes share the same data?

A Smart contracts

B Encryption

C Mining

D Consensus

💡 Hint

Think about how nodes agree on the ledger state.

Consensus is the process that ensures all nodes agree on the ledger's current state.

🔧 Debug

advanced

What error does this blockchain block validation code raise?

Analyze the code below that validates a block's hash. What error will it raise when run?

Blockchain / Solidity

def validate_block(block):
    if block['hash'] == block['prev_hash']:
        return True
    else:
        return False

block = {'hash': 'abc123', 'prev_hash': 'def456'}
print(validate_block(block))

A KeyError because 'hash' key is missing

B SyntaxError due to missing colon after else

C TypeError because block is not a dict

D Returns True

💡 Hint

Check the syntax of the if-else statement.

The else statement is missing a colon, causing a SyntaxError.

📝 Syntax

advanced

Which option correctly creates a new block dictionary with a timestamp?

Choose the code snippet that correctly creates a block dictionary with keys 'index', 'timestamp', and 'data'.

A block = {'index': 1, 'timestamp': '2024-06-01', 'data': 'Transaction'}

B block = {'index': 1, 'timestamp' = '2024-06-01', 'data': 'Transaction'}

C block = {'index': 1; 'timestamp': '2024-06-01'; 'data': 'Transaction'}

D block = {'index': 1, 'timestamp': '2024-06-01' 'data': 'Transaction'}

💡 Hint

Remember the syntax for dictionary key-value pairs in Python.

Option A uses correct colon separators and commas between items.

🚀 Application

expert

What is the length of the blockchain after adding these blocks?

Given the initial blockchain and a function to add blocks, what is the length of the blockchain after adding two new blocks?

Blockchain / Solidity

blockchain = [{'index': 0, 'data': 'Genesis'}]

def add_block(chain, data):
    index = chain[-1]['index'] + 1
    chain.append({'index': index, 'data': data})

add_block(blockchain, 'Tx1')
add_block(blockchain, 'Tx2')
print(len(blockchain))

A 4

B 2

C 3

D 1

💡 Hint

Count the original block plus the added blocks.

The blockchain starts with 1 block and adds 2 more, total 3 blocks.