ERC-20 fungible token standard in Blockchain / Solidity - Time & Space Complexity

Time Complexity: ERC-20 fungible token standard

O(1)

Understanding Time Complexity

When working with ERC-20 tokens, it's important to understand how the time to execute token operations changes as the number of users or transactions grows.

We want to know how the cost of common token actions scales with input size.

Scenario Under Consideration

Analyze the time complexity of the following ERC-20 transfer function.

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;
}

This function moves tokens from one user to another by updating balances and emitting an event.

Identify Repeating Operations

Look for loops or repeated steps inside the function.

• Primary operation: Access and update two entries in a mapping (balances).
• How many times: Exactly once per transfer call, no loops or recursion.

How Execution Grows With Input

The function always does the same fixed number of steps regardless of how many users or tokens exist.

Input Size (n)	Approx. Operations
10	5 steps
100	5 steps
1000	5 steps

Pattern observation: The number of operations stays the same no matter how many users or tokens there are.

Final Time Complexity

Time Complexity: O(1)

This means the transfer function takes the same amount of time no matter how many tokens or users exist.

Common Mistake

[X] Wrong: "The transfer time grows as more users hold tokens because balances get bigger."

[OK] Correct: Each transfer only updates two specific balances directly, so the number of users does not affect the time.

Interview Connect

Understanding that token transfers run in constant time helps you explain efficiency in blockchain operations clearly and confidently.

Self-Check

"What if the transfer function also iterated over all token holders to update some data? How would the time complexity change?"

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