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.