Staking mechanisms in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this staking reward calculation?

Consider a simple staking contract where rewards are calculated as 5% of the staked amount per epoch. What will be the total reward after 3 epochs if a user stakes 100 tokens?

Blockchain / Solidity

def calculate_rewards(staked_amount, epochs):
    reward_rate = 0.05
    total_reward = 0
    for _ in range(epochs):
        total_reward += staked_amount * reward_rate
    return total_reward

print(calculate_rewards(100, 3))

A 15

B 15.0

C 10.5

D 5

💡 Hint

Multiply the staked amount by the reward rate for each epoch and sum it up.

The reward per epoch is 5% of 100, which is 5 tokens. Over 3 epochs, total reward is 5 * 3 = 15.0 tokens.

🧠 Conceptual

intermediate

Which staking mechanism prevents double-spending by locking tokens?

In blockchain staking, which mechanism ensures tokens cannot be used for other transactions while staked?

A Delegation

B Slashing

C Token Locking

D Yield Farming

💡 Hint

Think about what stops tokens from moving during staking.

Token Locking locks tokens so they cannot be spent or transferred, preventing double-spending during staking.

🔧 Debug

advanced

Identify the error in this staking reward function

The function below is intended to calculate compounded staking rewards per epoch. What error will it cause when run?

Blockchain / Solidity

def compounded_rewards(staked_amount, epochs, rate):
    total = staked_amount
    for _ in range(epochs):
        total = total + total * rate
    return total

print(compounded_rewards(100, 3, 0.05))

A TypeError because of wrong operator

B Outputs 115.7625 but should be 115.7625

C SyntaxError due to missing colon

D Outputs 115.7625

💡 Hint

Check if the formula correctly compounds the reward.

The function correctly compounds the reward each epoch: 100 * (1.05)^3 = 115.7625.

❓ Predict Output

advanced

What is the output of this delegation mapping code?

This code maps delegators to validators and sums total delegated tokens per validator. What is the output?

Blockchain / Solidity

delegations = [
    {'delegator': 'Alice', 'validator': 'Val1', 'amount': 50},
    {'delegator': 'Bob', 'validator': 'Val2', 'amount': 30},
    {'delegator': 'Charlie', 'validator': 'Val1', 'amount': 20},
    {'delegator': 'Dave', 'validator': 'Val3', 'amount': 40}
]

validator_totals = {}
for d in delegations:
    validator_totals[d['validator']] = validator_totals.get(d['validator'], 0) + d['amount']

print(validator_totals)

A {'Val1': 70, 'Val2': 30, 'Val3': 40}

B {'Val1': 50, 'Val2': 30, 'Val3': 40}

C {'Val1': 20, 'Val2': 30, 'Val3': 40}

D {'Val1': 70, 'Val2': 40, 'Val3': 30}

💡 Hint

Sum amounts for each validator key.

Val1 has 50 + 20 = 70 tokens, Val2 has 30, Val3 has 40.

🧠 Conceptual

expert

Which staking mechanism best balances security and liquidity?

Among these staking mechanisms, which one allows users to earn rewards while maintaining token liquidity?

A Liquid Staking

B Cold Staking

C Slashing Mechanism

D Lock-up Period Staking

💡 Hint

Think about staking that lets you trade or use tokens while still earning rewards.

Liquid staking issues derivative tokens representing staked assets, allowing liquidity while securing the network.

Practice

1. What is the main purpose of staking tokens in a blockchain network?

easy

A. To create new tokens instantly

B. To transfer tokens to another user

C. To help secure the network and earn rewards

D. To delete tokens from the blockchain

Solution

1. Step 1: Understand staking concept

   Staking means locking tokens to support blockchain security.

2. Step 2: Identify staking benefits

   Users earn rewards for staking, helping network stability.

3. Final Answer:

   To help secure the network and earn rewards -> Option C

4. Quick Check:

   Staking = Security + Rewards [OK]

Hint: Staking locks tokens to secure network and gain rewards [OK]

Common Mistakes:

• Confusing staking with token transfer
• Thinking staking creates new tokens
• Believing staking deletes tokens

2. Which of the following is the correct way to represent staking tokens in a smart contract pseudocode?

easy

A. stakeAmount = userBalance - lockedTokens

B. lockedTokens = stakeAmount + userBalance

C. userBalance = stakeAmount + lockedTokens

D. lockedTokens = stakeAmount

Solution

1. Step 1: Understand staking variables

   Locked tokens represent the amount staked by user.

2. Step 2: Match correct assignment

   lockedTokens should equal stakeAmount to show tokens locked.

3. Final Answer:

   lockedTokens = stakeAmount -> Option D

4. Quick Check:

   Locked tokens = stake amount [OK]

Hint: Locked tokens equal the stake amount directly [OK]

Common Mistakes:

• Adding stakeAmount to userBalance incorrectly
• Subtracting lockedTokens from userBalance wrongly
• Mixing variable roles in assignment

3. Consider this pseudocode for calculating staking rewards:

reward = stakedAmount * rewardRate * stakingDuration
print(reward)

If stakedAmount = 100, rewardRate = 0.05, and stakingDuration = 10, what is the output?

medium

A. 50

B. 5

C. 0.5

D. 500

Solution

1. Step 1: Substitute values into formula

   reward = 100 * 0.05 * 10

2. Step 2: Calculate reward

   100 * 0.05 = 5; then 5 * 10 = 50

3. Final Answer:

   50 -> Option A

4. Quick Check:

   100 * 0.05 * 10 = 50 [OK]

Hint: Multiply all values stepwise: amount * rate * duration [OK]

Common Mistakes:

• Multiplying only two values
• Confusing rewardRate as 5 instead of 0.05
• Adding values instead of multiplying

4. The following pseudocode has an error. What is the problem?

function stakeTokens(userBalance, stakeAmount) {
  if (stakeAmount > userBalance) {
    return "Error: Not enough balance";
  }
  lockedTokens = stakeAmount;
  userBalance = userBalance - stakeAmount;
  return lockedTokens;
}

medium

A. lockedTokens is not declared before assignment

B. The function does not return userBalance

C. The if condition should be stakeAmount < userBalance

D. The subtraction should add stakeAmount instead

Solution

1. Step 1: Check variable declarations

   lockedTokens is assigned without declaration, causing error in strict languages.

2. Step 2: Understand variable scope

   lockedTokens should be declared (e.g., let or var) before use.

3. Final Answer:

   lockedTokens is not declared before assignment -> Option A

4. Quick Check:

   Undeclared variable causes error [OK]

Hint: Always declare variables before assigning [OK]

Common Mistakes:

• Ignoring variable declaration errors
• Misreading the if condition logic
• Thinking subtraction should be addition

5. You want to write a function that calculates total rewards for multiple users staking different amounts for different durations. Which approach correctly applies staking mechanisms?

hard

A. Add all staked amounts first, then multiply by rewardRate and total duration

B. Loop through each user, calculate reward = stakedAmount * rewardRate * duration, then sum all rewards

C. Calculate reward only for the user with the highest stake

D. Multiply rewardRate by duration only once, ignoring staked amounts

Solution

1. Step 1: Understand reward calculation per user

   Each user's reward depends on their own stake and duration.

2. Step 2: Sum individual rewards for total

   Calculate each reward separately, then add for total rewards.

3. Final Answer:

   Loop through each user, calculate reward = stakedAmount * rewardRate * duration, then sum all rewards -> Option B

4. Quick Check:

   Calculate per user, then sum [OK]

Hint: Calculate rewards individually, then add for total [OK]

Common Mistakes:

• Summing stakes before multiplying
• Ignoring individual durations
• Calculating reward for only one user