Checks-Effects-Interactions pattern in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the sender has enough balance before proceeding.

Blockchain / Solidity

require(balances[msg.sender] >= [1], "Insufficient balance");

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

AtotalSupply

Bmsg.value

Cbalance

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

Using msg.value instead of amount, which is for payable functions.

Using balance or totalSupply which are not the correct variables here.

The require statement checks if the sender's balance is at least the amount they want to send.

2fill in blank

medium

Complete the code to update the sender's balance after the check.

Blockchain / Solidity

balances[msg.sender] [1]= amount;

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

A-

B*

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Adding the amount instead of subtracting.

Using multiplication or division operators incorrectly.

We subtract the amount from the sender's balance to reflect the transfer.

3fill in blank

hard

Fix the error in the interaction step to send Ether safely.

Blockchain / Solidity

(bool success, ) = payable(recipient).[1]{value: amount}("");
require(success, "Transfer failed.");

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

Asend

Bcall

Ctransfer

Ddelegatecall

Attempts:

3 left

💡 Hint

Common Mistakes

Using transfer which can fail with gas limits.

Using delegatecall which is for different purposes.

Using call is the recommended way to send Ether safely and handle failures.

4fill in blank

hard

Fill both blanks to complete the Checks-Effects-Interactions pattern safely.

Blockchain / Solidity

require(balances[msg.sender] >= [1], "Not enough balance");
balances[msg.sender] [2]= amount;

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

Aamount

B+

C-

Dmsg.value

Attempts:

3 left

💡 Hint

Common Mistakes

Using msg.value instead of amount in the check.

Adding instead of subtracting the amount.

First, check the sender has enough amount. Then subtract amount from their balance.

5fill in blank

hard

Fill all three blanks to complete the full Checks-Effects-Interactions pattern.

Blockchain / Solidity

require(balances[msg.sender] >= [1], "Insufficient funds");
balances[msg.sender] [2]= [1];
balances[recipient] [3]= [1];

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

Aamount

B-

C+

Dmsg.value

Attempts:

3 left

💡 Hint

Common Mistakes

Using msg.value instead of amount.

Mixing up addition and subtraction operators.

Check sender has enough amount, subtract it from sender, then add it to recipient.

Practice

1.

What is the main purpose of the Checks-Effects-Interactions pattern in smart contracts?

easy

A. To reduce the gas cost of transactions

B. To prevent reentrancy attacks by ordering operations safely

C. To increase the speed of contract execution

D. To allow contracts to call external APIs

Solution

1. Step 1: Understand the pattern's purpose

   The Checks-Effects-Interactions pattern is designed to prevent security issues by ordering operations carefully.

2. Step 2: Identify the main security benefit

   By checking conditions first, updating state second, and interacting last, it prevents reentrancy attacks.

3. Final Answer:

   To prevent reentrancy attacks by ordering operations safely -> Option B

4. Quick Check:

   Pattern purpose = Prevent reentrancy [OK]

Hint: Remember: check first, update second, interact last [OK]

Common Mistakes:

• Thinking it speeds up contract execution
• Believing it reduces gas costs
• Assuming it enables external API calls

2.

Which of the following Solidity code snippets correctly follows the Checks-Effects-Interactions pattern?

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
  balances[msg.sender] -= amount;
}

easy

A. Yes, it checks, then interacts, then effects

B. Yes, it updates state before checks

C. No, it interacts before updating state

D. No, it updates state before interaction

Solution

1. Step 1: Analyze the order of operations

   The code first checks the balance, then calls the external address, then updates the balance.

2. Step 2: Compare with pattern order

   The pattern requires checks, then effects (state update), then interactions. Here, interaction happens before effects.

3. Final Answer:

   No, it interacts before updating state -> Option C

4. Quick Check:

   Interaction before effect = Incorrect order [OK]

Hint: State update must come before external calls [OK]

Common Mistakes:

• Ignoring the order of state update and interaction
• Thinking checks alone are enough
• Confusing effects with interactions

3.

What will be the output of the following Solidity function if balances[msg.sender] is 100 and amount is 50?

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  balances[msg.sender] -= amount;
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
}

Assume the call succeeds.

medium

A. The balance becomes 50 and 50 ether is sent

B. The balance remains 100 and 50 ether is sent

C. The balance becomes 50 but no ether is sent

D. Transaction reverts due to incorrect order

Solution

1. Step 1: Check balance and update state

   The require passes since 100 >= 50, then balance is reduced to 50.

2. Step 2: Send ether and confirm success

   The call sends 50 ether and returns success, so require(success) passes.

3. Final Answer:

   The balance becomes 50 and 50 ether is sent -> Option A

4. Quick Check:

   Balance updated then ether sent = Correct [OK]

Hint: State updates before sending ether means balance changes [OK]

Common Mistakes:

• Thinking balance stays the same after withdrawal
• Assuming call fails without reason
• Confusing order causing revert

4.

Identify the bug in this Solidity function violating the Checks-Effects-Interactions pattern:

function withdraw(uint amount) public {
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
  require(balances[msg.sender] >= amount);
  balances[msg.sender] -= amount;
}

medium

A. It correctly follows the pattern

B. It updates balance before checking conditions

C. It does not check for call success

D. It calls external address before checking balance

Solution

1. Step 1: Review order of operations

   The function calls the external address first, then checks balance, then updates state.

2. Step 2: Compare with pattern rules

   Pattern requires checks first, then effects, then interactions. Here, interaction is first, violating the pattern.

3. Final Answer:

   It calls external address before checking balance -> Option D

4. Quick Check:

   Interaction before checks = Bug [OK]

Hint: Always check before calling external contracts [OK]

Common Mistakes:

• Ignoring the order of require and call
• Assuming call success check is missing
• Thinking balance update order is wrong

5.

You want to write a Solidity function that sends a reward to a user only if they have enough points. Which code snippet correctly applies the Checks-Effects-Interactions pattern?

function rewardUser(address user, uint points) public {
  // Option A
  require(userPoints[user] >= points);
  userPoints[user] -= points;
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);

  // Option B
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);
  require(userPoints[user] >= points);
  userPoints[user] -= points;

  // Option C
  userPoints[user] -= points;
  require(userPoints[user] >= 0);
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);

  // Option D
  require(userPoints[user] >= points);
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);
  userPoints[user] -= points;
}

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Analyze the order of operations

   The correct snippet first requires userPoints[user] >= points (checks), then subtracts points (effects), then sends ether via call (interactions).

2. Step 2: Identify violations in others

   The second snippet interacts before checks; the third updates state before checks; the fourth interacts before effects.

3. Final Answer:

   Option A -> Option A

4. Quick Check:

   Checks -> Effects -> Interactions = Correct snippet [OK]

Hint: Check first, update second, call last for safety [OK]

Common Mistakes:

• Calling external before checking conditions
• Updating state before checks
• Mixing order of effects and interactions