Front-running awareness in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Detecting front-running in a transaction log

Given a list of blockchain transactions with timestamps and prices, what is the output of the code that detects if a transaction was front-run?

Blockchain / Solidity

transactions = [
    {'tx_id': 'a1', 'timestamp': 100, 'price': 10},
    {'tx_id': 'b2', 'timestamp': 101, 'price': 12},
    {'tx_id': 'c3', 'timestamp': 102, 'price': 11}
]

front_run_detected = False
for i in range(1, len(transactions)):
    if transactions[i]['price'] < transactions[i-1]['price']:
        front_run_detected = True
        break
print(front_run_detected)

A True

B False

C SyntaxError

D IndexError

💡 Hint

Look for a price drop after a higher price transaction.

The code checks if any transaction has a lower price than the previous one, indicating a possible front-run. Here, price drops from 12 to 11, so it prints True.

🧠 Conceptual

intermediate

Understanding front-running impact on transaction ordering

Which statement best describes the impact of front-running on blockchain transaction ordering?

A Front-running removes transactions from the blockchain permanently.

B Front-running delays all transactions equally to maintain fairness.

C Front-running changes the order of transactions to benefit the attacker financially.

D Front-running encrypts transactions to hide their content.

💡 Hint

Think about how attackers gain advantage by reordering.

Front-running involves reordering transactions so the attacker profits by acting before others.

🔧 Debug

advanced

Identify the error causing incorrect front-running detection

Why does the following code fail to detect front-running correctly?

Blockchain / Solidity

txs = [{'id': 'x1', 'price': 5}, {'id': 'x2', 'price': 7}, {'id': 'x3', 'price': 6}]

front_run = False
for i in range(len(txs)):
    if txs[i]['price'] < txs[i-1]['price']:
        front_run = True
        break
print(front_run)

A The list txs is empty, so the loop never runs.

B The price comparison uses '<' instead of '>' causing logic error.

C The variable front_run is never updated inside the loop.

D The loop starts at 0, so txs[i-1] is invalid on first iteration causing wrong comparison.

💡 Hint

Check the index used in the loop and how it accesses previous elements.

The loop starts at 0, so txs[i-1] accesses txs[-1] (last element) on first iteration, causing incorrect logic.

📝 Syntax

advanced

Identify the syntax error in front-running detection code

Which option contains the syntax error preventing the code from running?

Blockchain / Solidity

def detect_front_run(txs):
    for i in range(1, len(txs))
        if txs[i]['price'] < txs[i-1]['price']:
            return True
    return False

A Missing colon ':' after for loop declaration.

B Missing parentheses in function call range len(txs).

C Incorrect indentation of return statements.

D Using single quotes instead of double quotes in dictionary keys.

💡 Hint

Look carefully at the for loop syntax.

Python requires a colon ':' at the end of for loop declarations.

🚀 Application

expert

Calculate number of front-run opportunities in transaction batch

Given a batch of transactions with prices, how many front-run opportunities exist where a transaction is immediately followed by a lower price?

Blockchain / Solidity

transactions = [
    {'tx_id': 't1', 'price': 20},
    {'tx_id': 't2', 'price': 25},
    {'tx_id': 't3', 'price': 22},
    {'tx_id': 't4', 'price': 30},
    {'tx_id': 't5', 'price': 28}
]

count = 0
for i in range(1, len(transactions)):
    if transactions[i]['price'] < transactions[i-1]['price']:
        count += 1
print(count)

A 3

B 2

C 1

D 0

💡 Hint

Count how many times price drops compared to previous transaction.

Price drops occur from 25 to 22 and from 30 to 28, so count is 2.