ERC-20 fungible token standard in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the total supply variable in an ERC-20 token contract.

Blockchain / Solidity

uint256 public [1];

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

Aowner

Bbalance

CtotalSupply

Dallowance

Attempts:

3 left

💡 Hint

Common Mistakes

Using balance instead of totalSupply.

Using owner which is unrelated here.

The totalSupply variable holds the total number of tokens in circulation.

2fill in blank

medium

Complete the code to declare the transfer function signature in an ERC-20 token contract.

Blockchain / Solidity

function transfer(address [1], uint256 amount) public returns (bool);

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Ato

Brecipient

Cowner

Dspender

Attempts:

3 left

💡 Hint

Common Mistakes

Using spender which is for allowances.

Using owner which is the sender, not recipient.

The transfer function sends tokens to the to address.

3fill in blank

hard

Fix the error in the approve function signature to match the ERC-20 standard.

Blockchain / Solidity

function approve(address [1], uint256 amount) public returns (bool);

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

Aspender

Bcaller

Crecipient

Downer

Attempts:

3 left

💡 Hint

Common Mistakes

Using owner which is the token holder, not spender.

Using recipient which is not correct here.

The approve function sets allowance for the spender address.

4fill in blank

hard

Fill both blanks to complete the balanceOf function signature and return type.

Blockchain / Solidity

function balanceOf(address [1]) public view returns ([2]);

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

Aaccount

Buint256

Cbool

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using bool as return type instead of uint256.

Using address as return type which is incorrect.

The balanceOf function takes an account address and returns its token balance as a uint256.

5fill in blank

hard

Fill all three blanks to complete the transferFrom function signature and return type.

Blockchain / Solidity

function transferFrom(address [1], address [2], uint256 [3]) public returns (bool);

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

Asender

Brecipient

Camount

Dspender

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up spender with sender or recipient.

Using wrong parameter names that don't match the standard.

The transferFrom function moves amount tokens from sender to recipient.

Practice

1. What is the main purpose of the ERC-20 standard in Ethereum?

easy

A. To ensure tokens have a common set of functions for compatibility

B. To create unique non-fungible tokens

C. To define the Ethereum network consensus rules

D. To manage smart contract deployment costs

Solution

1. Step 1: Understand ERC-20 purpose

   ERC-20 defines a common interface for fungible tokens to work uniformly.

2. Step 2: Identify the correct purpose

   This standard allows wallets and apps to interact with tokens easily by using the same functions.

3. Final Answer:

   To ensure tokens have a common set of functions for compatibility -> Option A

4. Quick Check:

   ERC-20 = Common token interface [OK]

Hint: ERC-20 standardizes token functions for easy use [OK]

Common Mistakes:

• Confusing ERC-20 with NFT standards
• Thinking ERC-20 controls Ethereum network rules
• Assuming ERC-20 manages deployment costs

2. Which of the following is the correct function signature to check a token balance in ERC-20?

easy

A. function balanceOf(address owner) external view returns (uint256);

B. function getBalance(address owner) public returns (uint256);

C. function balance(address owner) external returns (bool);

D. function checkBalance(address owner) public view returns (bool);

Solution

1. Step 1: Recall ERC-20 balance function

   The standard defines balanceOf as a view function returning a uint256 balance.

2. Step 2: Match function signature

   function balanceOf(address owner) external view returns (uint256); matches the exact signature: external view returns (uint256).

3. Final Answer:

   function balanceOf(address owner) external view returns (uint256); -> Option A

4. Quick Check:

   balanceOf = correct signature [OK]

Hint: balanceOf returns uint256 and is a view function [OK]

Common Mistakes:

• Using wrong function name like getBalance
• Missing view keyword for read-only function
• Returning bool instead of uint256

3. Given this Solidity snippet from an ERC-20 token contract:

function transfer(address to, uint256 amount) public returns (bool) {
    if (balances[msg.sender] < amount) {
        return false;
    }
    balances[msg.sender] -= amount;
    balances[to] += amount;
    emit Transfer(msg.sender, to, amount);
    return true;
}

What will transfer return if the sender has 50 tokens and tries to send 60 tokens?

medium

A. true

B. false

C. It will throw an error

D. It will return zero

Solution

1. Step 1: Check sender balance condition

   The function checks if sender's balance is less than amount; if yes, returns false.

2. Step 2: Apply given values

   Sender has 50 tokens but tries to send 60, so condition is true and function returns false.

3. Final Answer:

   false -> Option B

4. Quick Check:

   Insufficient balance returns false [OK]

Hint: Transfer returns false if balance is insufficient [OK]

Common Mistakes:

• Assuming it throws error instead of returning false
• Thinking it returns true regardless
• Confusing return value with event emission

4. Identify the error in this ERC-20 approve function snippet:

function approve(address spender, uint256 amount) public returns (bool) {
    allowances[msg.sender][spender] = amount;
    emit Approval(msg.sender, spender, amount);
    return true;
}

What is the common security issue with this implementation?

medium

A. It does not emit the Approval event

B. It does not check if spender is zero address

C. It allows race condition by directly setting allowance

D. It returns false instead of true

Solution

1. Step 1: Understand approve race condition

   Directly setting allowance can cause race conditions if spender uses old and new allowance simultaneously.

2. Step 2: Identify missing mitigation

   Safe implementations require resetting allowance to zero before changing it to new value to avoid this issue.

3. Final Answer:

   It allows race condition by directly setting allowance -> Option C

4. Quick Check:

   approve race condition = direct allowance set [OK]

Hint: Always reset allowance to zero before changing it [OK]

Common Mistakes:

• Thinking missing zero address check is main issue
• Assuming Approval event is missing
• Believing return value is incorrect

5. You want to create an ERC-20 token that charges a 1% fee on every transfer and sends it to a fee collector address. Which modification to the transfer function correctly implements this?

hard

A. Send full amount to recipient, then send 1% from sender to collector

B. Add 1% to amount, send full amount to recipient, send fee separately

C. Send full amount to recipient and fee collector equally

D. Deduct 1% from amount, send fee to collector, send rest to recipient

Solution

1. Step 1: Calculate fee and net transfer

   Calculate 1% fee from amount, subtract it to get net amount to recipient.

2. Step 2: Transfer fee and net amount correctly

   Send fee to fee collector address and net amount to recipient to implement fee deduction.

3. Final Answer:

   Deduct 1% from amount, send fee to collector, send rest to recipient -> Option D

4. Quick Check:

   Fee deducted and sent correctly = Deduct 1% from amount, send fee to collector, send rest to recipient [OK]

Hint: Subtract fee from amount before sending tokens [OK]

Common Mistakes:

• Adding fee on top of amount instead of deducting
• Sending full amount to recipient and fee separately from sender
• Splitting amount equally between recipient and fee collector