ERC-20 implementation in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of the transfer function call?

Consider this simplified ERC-20 transfer function snippet:

function transfer(address to, uint256 amount) public returns (bool) {
    require(balances[msg.sender] >= amount, "Insufficient balance");
    balances[msg.sender] -= amount;
    balances[to] += amount;
    emit Transfer(msg.sender, to, amount);
    return true;
}

If balances[msg.sender] is 100 and amount is 50, what does transfer return?

Blockchain / Solidity

function transfer(address to, uint256 amount) public returns (bool) {
    require(balances[msg.sender] >= amount, "Insufficient balance");
    balances[msg.sender] -= amount;
    balances[to] += amount;
    emit Transfer(msg.sender, to, amount);
    return true;
}

A Returns the new balance of the sender

B false

C Reverts with 'Insufficient balance'

D true

💡 Hint

Think about what the function returns after successfully transferring tokens.

The transfer function returns true after successfully transferring tokens. It reverts only if the sender has insufficient balance.

🧠 Conceptual

intermediate

Which event must an ERC-20 token emit on transfers?

In ERC-20 token contracts, which event is required to be emitted when tokens are transferred?

A Approval

B Mint

C Transfer

D Burn

💡 Hint

Think about the event that signals token movement between addresses.

The Transfer event must be emitted on every token transfer, including zero value transfers.

🔧 Debug

advanced

What error does this approve function cause?

Examine this 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 error will this code cause when compiled?

Blockchain / Solidity

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

A SyntaxError: Missing semicolon after return statement

B No error, compiles successfully

C RuntimeError: Approval event not declared

D TypeError: allowances is not defined

💡 Hint

Check the end of the return statement carefully.

The return true line is missing a semicolon, causing a syntax error.

📝 Syntax

advanced

Which option correctly declares the balances mapping?

In an ERC-20 contract, how do you correctly declare a mapping to store token balances?

A mapping(address => uint) balances() public;

B mapping(address => uint) public balances;

C mapping(address, uint) balances;

D mapping(address => uint) balances;

💡 Hint

Remember the correct syntax for mappings and visibility keywords.

The correct syntax uses mapping(address => uint) public balances; to declare a public mapping.

🚀 Application

expert

What is the total supply after minting 1000 tokens?

Given this snippet of an ERC-20 token contract:

uint256 public totalSupply;

function mint(address to, uint256 amount) public {
    totalSupply += amount;
    balances[to] += amount;
    emit Transfer(address(0), to, amount);
}

If totalSupply was initially 5000, what is its value after calling mint(0x123..., 1000)?

A 6000

B 1000

C 5000

D 0

💡 Hint

Think about how minting affects total supply.

Minting adds the minted amount to the total supply, so 5000 + 1000 = 6000.

Practice

1. What is the main purpose of the balanceOf function in an ERC-20 token contract?

easy

A. To approve another address to spend tokens

B. To transfer tokens from one address to another

C. To check the token balance of a specific address

D. To mint new tokens to an address

Solution

1. Step 1: Understand the role of balanceOf

   The balanceOf function returns how many tokens a given address owns.

2. Step 2: Compare with other functions

   Functions like transfer move tokens, approve allows spending, and minting creates tokens. These are different from checking balance.

3. Final Answer:

   To check the token balance of a specific address -> Option C

4. Quick Check:

   balanceOf = check balance [OK]

Hint: balanceOf always returns token count for an address [OK]

Common Mistakes:

• Confusing balanceOf with transfer function
• Thinking approve checks balances
• Mixing minting with balance checking

2. Which of the following is the correct Solidity function signature for transferring tokens in an ERC-20 contract?

easy

A. function transfer(address to, uint256 amount) public returns (bool);

B. function transfer(uint256 amount, address to) public returns (bool);

C. function transfer(address to) public returns (uint256);

D. function transfer(address from, address to, uint256 amount) public;

Solution

1. Step 1: Recall ERC-20 transfer signature

   The standard transfer function takes the recipient address first, then the amount, and returns a boolean.

2. Step 2: Check each option

   function transfer(address to, uint256 amount) public returns (bool); matches the standard: transfer(address to, uint256 amount) public returns (bool). Others have wrong parameter order, missing return, or extra parameters.

3. Final Answer:

   function transfer(address to, uint256 amount) public returns (bool); -> Option A

4. Quick Check:

   transfer(to, amount) returns bool [OK]

Hint: transfer always has (address to, uint256 amount) and returns bool [OK]

Common Mistakes:

• Swapping parameter order
• Omitting return type
• Adding extra parameters not in standard

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

mapping(address => uint256) balances;

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

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

medium

A. true

B. Returns the remaining balance

C. Throws an error

D. false

Solution

1. Step 1: Check balance condition

   The function checks if balances[msg.sender] is at least amount. Here, 50 < 60, so condition fails.

2. Step 2: Determine function return

   Since condition fails, the function skips token transfer and returns false.

3. Final Answer:

   false -> Option D

4. Quick Check:

   Insufficient balance returns false [OK]

Hint: Transfer returns false if sender lacks enough tokens [OK]

Common Mistakes:

• Assuming transfer throws error on insufficient balance
• Thinking it returns true anyway
• Confusing return value with balance amount

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

mapping(address => mapping(address => uint256)) allowances;

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

medium

A. No bug, function is correct

B. Should emit an Approval event after setting allowance

C. Should check if amount is less than balance

D. Should decrease allowance instead of setting it

Solution

1. Step 1: Review ERC-20 approve requirements

   The standard requires emitting an Approval event after changing allowance for transparency.

2. Step 2: Check function code

   The function sets allowance but does not emit the Approval event, missing a key part of the standard.

3. Final Answer:

   Should emit an Approval event after setting allowance -> Option B

4. Quick Check:

   approve must emit Approval event [OK]

Hint: approve must emit Approval event to follow standard [OK]

Common Mistakes:

• Ignoring event emission
• Checking balance in approve (not required)
• Decreasing allowance instead of setting

5. You want to add a feature to your ERC-20 token that prevents transfers when the contract is paused. Which is the best way to implement this without breaking the ERC-20 standard?

hard

A. Add a paused boolean and require it false in transfer and transferFrom functions

B. Remove the transfer function and replace it with a new function that checks pause

C. Add a new function pauseTransfer that users must call before transferring

D. Modify the balanceOf function to return zero when paused

Solution

1. Step 1: Understand pause feature integration

   To pause transfers, the contract needs a state variable like paused and checks in transfer functions.

2. Step 2: Check ERC-20 compliance

   Removing or renaming transfer breaks the standard. Changing balanceOf output breaks expected behavior. Adding a separate function for pause is not standard.

3. Final Answer:

   Add a paused boolean and require it false in transfer and transferFrom functions -> Option A

4. Quick Check:

   Pause by require(!paused) in transfer functions [OK]

Hint: Use a paused flag and require it false in transfer methods [OK]

Common Mistakes:

• Removing or renaming standard functions
• Changing balanceOf behavior
• Adding extra steps users must call