Integer overflow and underflow in Blockchain / Solidity - Mini Project: Build & Apply

Integer Overflow and Underflow in Blockchain

📖 Scenario: Imagine you are building a simple blockchain smart contract that manages token balances for users. In blockchain, numbers have limits, and if you add or subtract too much, the number can wrap around unexpectedly. This is called integer overflow or underflow, and it can cause serious problems.

🎯 Goal: You will create a small program that simulates token balances and shows how overflow and underflow happen. Then, you will add a safety check to prevent these errors.

📋 What You'll Learn

Create a dictionary called balances with exact entries: 'Alice': 100, 'Bob': 50

Create a variable called max_uint and set it to 255 (simulate 8-bit unsigned integer max)

Write code to add 200 tokens to Alice's balance using modular arithmetic to simulate overflow

Print the final balances dictionary exactly as it is after the addition

💡 Why This Matters

🌍 Real World

Smart contracts on blockchains use fixed-size integers. Overflow or underflow can cause wrong balances or security bugs.

💼 Career

Blockchain developers must understand integer limits and how to prevent overflow bugs to build secure contracts.

1

Create initial balances

Create a dictionary called balances with these exact entries: 'Alice': 100 and 'Bob': 50.

Blockchain / Solidity

# Create the balances dictionary with Alice and Bob
# Your code here

Need a hint?

Use curly braces to create a dictionary with keys 'Alice' and 'Bob' and their token amounts.

2

Set maximum integer value

Create a variable called max_uint and set it to 255 to simulate an 8-bit unsigned integer maximum value.

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 50}
# Set max_uint to 255
# Your code here

Need a hint?

Just assign 255 to the variable max_uint.

3

Simulate overflow by adding tokens

Add 200 tokens to Alice's balance using modular arithmetic with max_uint to simulate overflow. Update balances['Alice'] accordingly.

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 50}
max_uint = 255
# Add 200 tokens to Alice's balance with overflow simulation
# Your code here

Need a hint?

Use the modulo operator % with max_uint + 1 to wrap the value around.

4

Print the final balances

Print the balances dictionary to show the result after the overflow addition.

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 50}
max_uint = 255
balances['Alice'] = (balances['Alice'] + 200) % (max_uint + 1)
# Print the balances dictionary
# Your code here

Need a hint?

Use print(balances) to display the dictionary.