ERC-20 implementation 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 contract.

Blockchain / Solidity

uint256 public [1];

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

Aamount

Bbalance

Cowner

DtotalSupply

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names like balance or owner instead of totalSupply.

The totalSupply variable holds the total number of tokens in circulation in an ERC-20 contract.

2fill in blank

medium

Complete the code to define the balance mapping for token holders.

Blockchain / Solidity

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

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

Auint256

Bstring

Cbool

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using bool or address instead of uint256.

The balances mapping stores the token balance (as uint256) for each address.

3fill in blank

hard

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

Blockchain / Solidity

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

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

Afrom

Brecipient

Cto

Dspender

Attempts:

3 left

💡 Hint

Common Mistakes

Using from or spender instead of to.

The ERC-20 transfer function uses to as the parameter name for the recipient address.

4fill in blank

hard

Fill both blanks to complete the allowance mapping declaration.

Blockchain / Solidity

mapping([1] => mapping([2] => uint256)) private allowances;

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

Aaddress

Buint256

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using uint256 or bool as keys instead of address.

The allowances mapping maps an owner address to a spender address to the allowed token amount (uint256).

5fill in blank

hard

Fill all three blanks to complete the transfer function body that updates balances and emits Transfer event.

Blockchain / Solidity

require(balances[msg.sender] >= [1], "Insufficient balance");
balances[msg.sender] -= [2];
balances[[3]] += amount;
emit Transfer(msg.sender, [3], amount);
return true;

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

Aamount

Bto

Dfrom

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names like from or mixing up amount and to.

The function checks if the sender has enough amount, subtracts it, adds it to the recipient to, emits the Transfer event, and returns true.

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