Why DeFi reimagines finance in Blockchain / Solidity - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a smart contract function that returns the contract's owner address.

Blockchain / Solidity

function getOwner() public view returns (address) {
    return [1];
}

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

Aowner

Bmsg.sender

Caddress(this)

Dtx.origin

Attempts:

3 left

💡 Hint

Common Mistakes

Returning msg.sender instead of owner

Returning address(this) which is the contract address

The function should return the stored owner address of the contract, not the caller or contract address.

2fill in blank

medium

Complete the code to emit an event when a token transfer happens.

Blockchain / Solidity

emit Transfer([1], to, amount);

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

Afrom

Bowner

Cmsg.sender

Drecipient

Attempts:

3 left

💡 Hint

Common Mistakes

Using recipient or to instead of from

Using msg.sender which may differ from from in some cases

The Transfer event requires the sender's address as the first argument, which is from.

3fill in blank

hard

Fix the error in the function that checks if the caller is the owner.

Blockchain / Solidity

require([1] == owner, "Not authorized");

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

Atx.origin

Bowner

Cmsg.sender

Daddress(this)

Attempts:

3 left

💡 Hint

Common Mistakes

Using tx.origin which can be spoofed

Using address(this) which is the contract address

The msg.sender is the address calling the function and should be checked against the owner.

4fill in blank

hard

Fill both blanks to create a mapping that tracks balances and a function to update them.

Blockchain / Solidity

mapping(address => [1]) public balances;

function updateBalance(address user, [2] amount) public {
    balances[user] = amount;
}

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

Auint256

Baddress

Cuint

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using bool instead of number

Using address as balance type

The balances mapping stores uint256 values, and the function parameter should be a uint256 amount.

5fill in blank

hard

Fill all three blanks to create a function that transfers tokens safely.

Blockchain / Solidity

function safeTransfer(address [1], address [2], uint [3]) public {
    require(balances[[1]] >= [3], "Insufficient balance");
    balances[[1]] -= [3];
    balances[[2]] += [3];
}

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

Afrom

Bto

Camount

Duser

Attempts:

3 left

💡 Hint

Common Mistakes

Using user instead of from or to

Mixing parameter order

The function parameters should be from (sender), to (receiver), and amount (tokens to transfer).

Practice

1. What is one main way DeFi changes traditional finance?

easy

A. By removing banks and middlemen

B. By requiring physical bank branches

C. By using paper money only

D. By limiting access to certain countries

Solution

1. Step 1: Understand DeFi's core feature

   DeFi uses technology to remove banks and middlemen from financial processes.

2. Step 2: Compare options to this feature

   Only By removing banks and middlemen matches this key idea; others contradict it.

3. Final Answer:

   By removing banks and middlemen -> Option A

4. Quick Check:

   DeFi removes middlemen = C [OK]

Hint: Think about who controls money in DeFi [OK]

Common Mistakes:

• Thinking DeFi needs physical banks
• Assuming DeFi limits users by location
• Confusing DeFi with cash-only systems

2. Which of the following is the correct way to describe a DeFi smart contract?

easy

A. A paper document stored in a vault

B. A physical contract signed by banks

C. A self-executing code on blockchain

D. A manual process requiring human approval

Solution

1. Step 1: Define smart contract in DeFi

   Smart contracts are computer programs that run automatically on blockchain.

2. Step 2: Match options to this definition

   Only self-executing code on blockchain matches this definition.

3. Final Answer:

   A self-executing code on blockchain -> Option C

4. Quick Check:

   Smart contract = code on blockchain [OK]

Hint: Smart contracts run automatically, no paper needed [OK]

Common Mistakes:

• Thinking smart contracts are physical papers
• Confusing manual approval with automation
• Assuming banks sign smart contracts

3. Consider this simple DeFi smart contract code snippet in Solidity:

contract SimpleBank {
    mapping(address => uint) balances;
    function deposit() public payable {
        balances[msg.sender] += msg.value;
    }
    function getBalance() public view returns (uint) {
        return balances[msg.sender];
    }
}

What will getBalance() return after a user sends 2 ether to deposit()?

medium

A. 2 ether in wei units

B. 2

C. Error: function not payable

D. 0

Solution

1. Step 1: Understand deposit function behavior

   The deposit function adds the sent ether (msg.value) to the sender's balance in wei (smallest ether unit).

2. Step 2: Understand getBalance return value

   getBalance returns the balance in wei, so sending 2 ether means balance is 2 * 10^18 wei.

3. Final Answer:

   2 ether in wei units -> Option A

4. Quick Check:

   Balance returned in wei units = D [OK]

Hint: Ether amounts are stored in wei (smallest unit) [OK]

Common Mistakes:

• Assuming balance returns ether directly
• Thinking deposit is not payable
• Confusing zero balance with deposit amount

4. This Solidity code snippet aims to let users withdraw their balance:

contract SimpleBank {
    mapping(address => uint) balances;
    function withdraw(uint amount) public {
        require(balances[msg.sender] >= amount);
        payable(msg.sender).transfer(amount);
        balances[msg.sender] -= amount;
    }
}

What is the main issue with this code?

medium

A. It does not check if amount is positive

B. It subtracts balance after transfer, risking reentrancy attack

C. It uses transfer instead of send

D. It lacks a deposit function

Solution

1. Step 1: Analyze withdrawal order

   The code sends ether before updating the balance, which can allow reentrancy attacks.

2. Step 2: Identify security best practice

   Best practice is to update balance before sending ether to prevent reentrancy.

3. Final Answer:

   It subtracts balance after transfer, risking reentrancy attack -> Option B

4. Quick Check:

   Transfer before update risks reentrancy = B [OK]

Hint: Update balance before sending funds to avoid attacks [OK]

Common Mistakes:

• Ignoring reentrancy risks
• Thinking transfer vs send is main issue
• Missing importance of deposit function here

5. You want to create a DeFi app that lets users stake tokens and earn rewards automatically. Which combination best reimagines finance using DeFi principles?

hard

A. Limit staking to only users with bank accounts

B. Require users to visit a bank to approve staking manually

C. Use paper contracts signed by a central authority

D. Use smart contracts to automate staking and rewards without intermediaries

Solution

1. Step 1: Identify DeFi principles

   DeFi automates finance tasks with smart contracts and removes middlemen.

2. Step 2: Match options to these principles

   Only the option using smart contracts to automate without intermediaries matches.

3. Final Answer:

   Use smart contracts to automate staking and rewards without intermediaries -> Option D

4. Quick Check:

   Automation + no middlemen = A [OK]

Hint: Choose automation and no middlemen for DeFi apps [OK]

Common Mistakes:

• Thinking manual bank visits fit DeFi
• Assuming paper contracts are needed
• Limiting access contradicts DeFi openness