Why DeFi reimagines finance in Blockchain / Solidity

Introduction

DeFi changes how money works by using computers to do banking without banks. It makes finance open and fair for everyone.

When you want to send money to someone without a bank in the middle.

When you want to borrow or lend money without filling out lots of forms.

When you want to trade or invest without paying big fees.

When you want to keep control of your money without trusting a company.

When you want to use money apps that work 24/7 without closing.

Syntax

Blockchain / Solidity

DeFi uses smart contracts on blockchains like Ethereum.

Example smart contract function:
function transfer(address to, uint amount) public {
  require(balance[msg.sender] >= amount);
  balance[msg.sender] -= amount;
  balance[to] += amount;
}

Smart contracts are like computer programs that run on the blockchain automatically.

They make sure rules are followed without needing a bank or middleman.

Examples

This function lets users add money to their account in the contract.

Blockchain / Solidity

function deposit() public payable {
  balance[msg.sender] += msg.value;
}

This function lets users borrow money if they have enough collateral.

Blockchain / Solidity

function borrow(uint amount) public {
  require(collateral[msg.sender] >= amount);
  loans[msg.sender] += amount;
  balance[msg.sender] += amount;
}

Sample Program

This simple DeFi contract lets users deposit money and send it to others without a bank.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleDeFi {
    mapping(address => uint) public balance;

    function deposit() public payable {
        balance[msg.sender] += msg.value;
    }

    function transfer(address to, uint amount) public {
        require(balance[msg.sender] >= amount, "Not enough balance");
        balance[msg.sender] -= amount;
        balance[to] += amount;
    }
}

Important Notes

DeFi uses code to replace banks and middlemen.

It works on blockchains, which are like shared computers everyone trusts.

Smart contracts run automatically and cannot be changed once live.

Summary

DeFi makes finance open and automatic using smart contracts.

It removes banks and lets people control their own money.

DeFi works 24/7 and is available to anyone with internet.

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