Balance checking in Blockchain / Solidity - Mini Project: Build & Apply

Balance checking

📖 Scenario: You are building a simple blockchain wallet app. Users have balances stored in a dictionary. You want to check how much balance each user has.

🎯 Goal: Create a program that stores user balances, sets a minimum balance threshold, finds users with balances above that threshold, and prints their names and balances.

📋 What You'll Learn

Create a dictionary called balances with exact user names and balances

Create a variable called min_balance with the exact value 100

Use a dictionary comprehension called rich_users to select users with balances greater than min_balance

Print the rich_users dictionary exactly as shown

💡 Why This Matters

🌍 Real World

Blockchain wallets need to check user balances to show available funds or to decide if a user can make a transaction.

💼 Career

Understanding how to filter and display balances is important for blockchain developers working on wallet apps or financial dashboards.

1

DATA SETUP: Create the user balances dictionary

Create a dictionary called balances with these exact entries: 'Alice': 150, 'Bob': 90, 'Charlie': 120, 'Diana': 80

Blockchain / Solidity

# Create the balances dictionary with exact user balances
# Your code here

Hint

Use curly braces to create a dictionary with keys as user names and values as their balances.

2

CONFIGURATION: Set the minimum balance threshold

Create a variable called min_balance and set it to the integer 100

Blockchain / Solidity

balances = {'Alice': 150, 'Bob': 90, 'Charlie': 120, 'Diana': 80}
# Set the minimum balance threshold
# Your code here

Hint

Just assign the number 100 to the variable min_balance.

3

CORE LOGIC: Select users with balances above the threshold

Use a dictionary comprehension called rich_users to select users from balances whose balance is greater than min_balance

Blockchain / Solidity

balances = {'Alice': 150, 'Bob': 90, 'Charlie': 120, 'Diana': 80}
min_balance = 100
# Create rich_users dictionary with users having balance > min_balance
# Your code here

Hint

Use a dictionary comprehension with for user, bal in balances.items() and an if condition.

4

OUTPUT: Print the rich users dictionary

Write a print statement to display the rich_users dictionary

Blockchain / Solidity

balances = {'Alice': 150, 'Bob': 90, 'Charlie': 120, 'Diana': 80}
min_balance = 100
rich_users = {user: bal for user, bal in balances.items() if bal > min_balance}
# Print the rich_users dictionary
# Your code here

Hint

Use print(rich_users) to show the dictionary.

Practice

1.

What does balance checking in blockchain primarily tell you?

easy

A. The private key of a wallet

B. The transaction history of an address

C. How much cryptocurrency an address currently holds

D. The mining difficulty of the blockchain

Solution

1. Step 1: Understand the purpose of balance checking

   Balance checking is used to find out the amount of cryptocurrency held by a blockchain address.

2. Step 2: Differentiate from other blockchain data

   Transaction history, private keys, and mining difficulty are different concepts unrelated to balance checking.

3. Final Answer:

   How much cryptocurrency an address currently holds -> Option C

4. Quick Check:

   Balance checking = cryptocurrency amount [OK]

Hint: Balance checking shows funds held by an address [OK]

Common Mistakes:

• Confusing balance with transaction history
• Thinking balance reveals private keys
• Mixing balance with blockchain mining info

2.

Which of the following is the correct way to call a function getBalance for an address addr in a blockchain library?

?

easy

A. getBalance(addr)

B. getBalance = addr

C. getBalance.addr()

D. getBalance->addr()

Solution

1. Step 1: Identify correct function call syntax

   In most programming languages, calling a function with an argument uses parentheses: functionName(argument).

2. Step 2: Check each option

   getBalance(addr) uses correct syntax: getBalance(addr). Options A, C, and D use invalid or incorrect syntax.

3. Final Answer:

   getBalance(addr) -> Option A

4. Quick Check:

   Function call syntax = getBalance(addr) [OK]

Hint: Function calls use parentheses with arguments [OK]

Common Mistakes:

• Using assignment instead of function call
• Using dot notation incorrectly for function calls
• Using arrow notation which is not valid here

3.

Given this JavaScript code snippet using a blockchain API:

async function check() {
  const balance = await blockchainAPI.getBalance('0x123');
  console.log(balance);
}
check();

What will be printed if the address has 500 tokens?

medium

A. undefined

B. 500

C. Error: getBalance is not a function

D. null

Solution

1. Step 1: Understand async/await usage

   The function uses await to get the balance asynchronously, so balance will hold the returned value.

2. Step 2: Check what is logged

   console.log(balance) will print the balance value, which is 500 tokens for the given address.

3. Final Answer:

   500 -> Option B

4. Quick Check:

   Async balance fetch = 500 [OK]

Hint: Await returns the actual balance value [OK]

Common Mistakes:

• Ignoring async and expecting immediate value
• Assuming undefined if await is missing
• Confusing error messages with correct output

4.

Find the error in this Python code that tries to get a balance:

def get_balance(address):
    balance = blockchain.getBalance(address)
    return balance

print(get_balance)

medium

A. Using print instead of return

B. Incorrect function name getBalance

C. Missing import statement for blockchain

D. Missing parentheses when calling get_balance in print

Solution

1. Step 1: Check how get_balance is used in print

   The code prints get_balance without parentheses, so it prints the function object, not the result.

2. Step 2: Correct usage to call the function

   Adding parentheses and an argument like print(get_balance('0xabc')) calls the function and prints the balance.

3. Final Answer:

   Missing parentheses when calling get_balance in print -> Option D

4. Quick Check:

   Function call needs () [OK]

Hint: Call functions with () to get results [OK]

Common Mistakes:

• Printing function name instead of calling it
• Assuming function runs without parentheses
• Ignoring required function arguments

5.

You want to check balances of multiple addresses and store them in a dictionary in Python. Which code correctly does this?

addresses = ['0x1', '0x2', '0x3']
balances = {}
for addr in addresses:
    balances[addr] = blockchain.getBalance(addr)
print(balances)

hard

A. This code correctly stores balances for each address in a dictionary

B. This code will cause a syntax error due to missing colon

C. This code overwrites balances instead of storing all

D. This code does not call getBalance correctly

Solution

1. Step 1: Analyze the loop and dictionary usage

   The for loop iterates over each address and assigns the balance to balances[addr], storing all balances.

2. Step 2: Check syntax and function call

   Syntax is correct with colon and indentation; getBalance is called properly with each address.

3. Final Answer:

   This code correctly stores balances for each address in a dictionary -> Option A

4. Quick Check:

   Loop + dict assignment = correct balances [OK]

Hint: Use a loop to assign balances to dictionary keys [OK]

Common Mistakes:

• Forgetting colon after for statement
• Overwriting dictionary instead of adding keys
• Calling getBalance without argument