Balance checking in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is balance checking in blockchain?

Balance checking is the process of finding out how much cryptocurrency or tokens an address holds on the blockchain.

intermediate

Which blockchain data structure stores the balance of an address?

The balance is stored in the blockchain's state, often in an account or UTXO (Unspent Transaction Output) model depending on the blockchain type.

beginner

How do you check an Ethereum address balance using Web3.js?

Use the method web3.eth.getBalance(address) which returns the balance in wei (smallest unit).

beginner

Why is balance checking important before sending a transaction?

To ensure the sender has enough funds to cover the amount and transaction fees, preventing failed transactions.

intermediate

What is the difference between checking balance on-chain vs off-chain?

On-chain balance checking reads directly from the blockchain state, while off-chain may use cached or third-party services for faster access but less trust.

What does the balance of a blockchain address represent?

A The total fees paid by the address

B The amount of cryptocurrency or tokens held by that address

C The number of transactions made by the address

D The number of smart contracts deployed by the address

The balance shows how much cryptocurrency or tokens the address currently owns.

Which method is commonly used in Web3.js to check an Ethereum address balance?

A web3.eth.getCode()

B web3.eth.sendTransaction()

C web3.eth.getTransactionCount()

D web3.eth.getBalance(address)

web3.eth.getBalance(address) returns the balance of the address in wei.

Why should you check your balance before sending a transaction?

A To confirm you have enough funds to cover the amount and fees

B To see how many transactions you have sent

C To check the network speed

D To verify the smart contract code

Checking balance prevents failed transactions due to insufficient funds.

In a UTXO-based blockchain, what does balance checking involve?

A Summing all unspent transaction outputs linked to the address

B Reading the account balance directly from the state

C Counting the number of blocks mined

D Checking the smart contract storage

UTXO blockchains track balance by summing unspent outputs for an address.

What is a downside of off-chain balance checking?

A It is slower than on-chain checking

B It requires mining new blocks

C It may not be fully up-to-date or trustable

D It uses more gas fees

Off-chain data can be cached or from third parties, so it might not reflect the latest blockchain state.

Explain how you would check the balance of a blockchain address using a programming library.

Think about how Web3.js or similar libraries provide balance info.

Describe the difference between on-chain and off-chain balance checking and why each might be used.

Consider direct blockchain queries vs third-party services.

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