Balance checking in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ethereum balance check?

Given the following Solidity function that returns the balance of an address in wei, what will be the output if the address has 3 ether?

Blockchain / Solidity

function getBalance(address addr) public view returns (uint) {
    return addr.balance;
}

// Assume 1 ether = 10**18 wei

A 0

B 3

C 3000000000000000

D 3000000000000000000

💡 Hint

Remember that balances are returned in wei, the smallest unit.

The balance property returns the amount in wei. Since 1 ether = 10^18 wei, 3 ether = 3 * 10^18 wei = 3000000000000000000 wei.

❓ Predict Output

intermediate

What error occurs when checking balance of an uninitialized address?

Consider this Solidity snippet: address addr; uint bal = addr.balance; What happens when this code runs?

A Runtime error: invalid address

B Compilation error: uninitialized address

C Returns 0

D Returns random value

💡 Hint

Default value of address type is zero address.

Uninitialized address variables default to the zero address (0x000...0), which has zero balance, so it returns 0.

❓ Predict Output

advanced

What is the output of this Web3.js balance check?

Given this JavaScript code using Web3.js to get balance in ether, what is logged if the account has 5 ether? const balanceWei = await web3.eth.getBalance(account); const balanceEth = web3.utils.fromWei(balanceWei, 'ether'); console.log(balanceEth);

A "5"

B "5000000000000000000"

C 5

D undefined

💡 Hint

fromWei returns a string representation.

fromWei converts wei to ether and returns a string, so console.log prints "5" as a string.

🧠 Conceptual

advanced

Which option correctly explains why balance checking is gas-free in Solidity?

Why does reading the balance of an address using addr.balance not consume gas in Solidity?

A Because balance is a state variable stored in contract storage

B Because balance is a global property accessed via EVM opcode that does not modify state

C Because balance is cached locally in the contract

D Because balance is computed by the contract at runtime

💡 Hint

Think about what operations cost gas in Ethereum.

Reading balance uses the EVM BALANCE opcode which only reads state and does not modify it, so it costs no gas when called externally.

❓ Predict Output

expert

What is the value of 'balance' after this Solidity code runs?

Consider this Solidity code snippet: address payable user = payable(address(0x123)); uint balance = user.balance; Assuming the address 0x123 has 0.1 ether, what is the value of balance?

A 100000000000000000

B 0.1

C 0

D Compilation error

💡 Hint

Balance is returned in wei, not ether.

The balance property returns the amount in wei. 0.1 ether equals 0.1 * 10^18 = 100000000000000000 wei.