Blockchain & Smart Contracts test (Intermediate)

Sample questions

5 sample questions from the Blockchain & Smart Contracts test, with the answers. Samples are taken out of the live test, so you will not meet these when you sit it.

1. What happens if an attacker attempts to alter data in a block of a blockchain?

   1. The hash of the previous block is changed, but the rest of the blockchain remains unaffected.
   2. The hash of the altered block is changed, but the rest of the blockchain remains unaffected.
   3. The hash of the previous block is changed, and all subsequent block hashes are also changed.
   4. Only the hash of the altered block is changed, and the rest of the blockchain remains unaffected.

   Answer: C. The hash of the previous block is changed, and all subsequent block hashes are also changed.

   A blockchain is an append-only ledger where each block contains the hash of the previous block. If any data in a block is altered, its hash changes, which in turn changes the hash of the subsequent blocks. This ensures the immutability of the ledger and makes any tampering detectable.

2. You are debugging a bug in a Solidity smart contract where event logs are not being properly filtered by the client application. What is the correct approach to fix this issue?

   1. Use the `emit` keyword to log events and filter them using the `event` filter in the client application.
   2. Use `delegatecall` to log events and filter them using the `event` filter in the client application.
   3. Use `call` to log events and filter them using the `event` filter in the client application.
   4. Use `require` to trigger events and filter them using the `event` filter in the client application.

   Answer: A. Use the `emit` keyword to log events and filter them using the `event` filter in the client application.

   In Solidity, events are emitted using the `emit` keyword, and they are stored in the blockchain's transaction logs. To filter these events, the client application (e.g., a frontend dApp) can use the `event` filter provided by libraries such as Ethers.js or Web3.js. This allows developers to efficiently query and process events without scanning the entire blockchain. The other options are incorrect because they either describe unrelated processes (like using `require` for input validation) or incorrect methods for handling events (like using `call` or `delegatecall`).

3. Which of the following accurately describes a key characteristic of the EVM's execution environment?

   1. The EVM executes each transaction deterministically on every node.
   2. The EVM's execution environment is not consistent across nodes.
   3. The EVM does not execute transactions on all nodes.
   4. The EVM uses a random execution model for each node.

   Answer: A. The EVM executes each transaction deterministically on every node.

   The EVM is designed to ensure that all nodes execute transactions in the same way, maintaining consistency across the network. This deterministic execution is a key feature of the EVM, ensuring that all nodes agree on the state of the blockchain after each transaction. Other options may be incorrect because they either describe unrelated concepts or provide inaccurate information about the EVM's behavior.

4. You are developing a smart contract for a decentralized finance (DeFi) application that allows users to withdraw funds from their accounts. To prevent reentrancy attacks, which approach should you take?

   1. Use the checks-effects-interactions pattern and implement a reentrancy guard.
   2. Avoid using external calls altogether in the contract.
   3. Use a centralized access control mechanism to restrict external calls.
   4. Store all critical state changes in memory instead of storage.

   Answer: A. Use the checks-effects-interactions pattern and implement a reentrancy guard.

   The checks-effects-interactions pattern ensures that state changes are made before external calls, reducing the risk of reentrancy. A reentrancy guard (like OpenZeppelin's ReentrancyGuard) prevents recursive calls to the same function, which is essential for securing against reentrancy attacks. Other options do not directly address the issue of reentrancy.

5. A smart contract developer is experiencing high gas costs due to the use of an array to store user balances. Which of the following is the most effective way to optimize gas usage in this scenario?

   1. Use a library to handle the array operations
   2. Replace the array with a linked list data structure
   3. Increase the gas limit for the transaction
   4. Use a mapping instead of an array to store user balances

   Answer: D. Use a mapping instead of an array to store user balances

   Using a mapping instead of an array to store user balances is more gas-efficient because mappings in Solidity are optimized for direct lookups by key. Arrays require more gas for operations like pushing and popping elements, especially as the size of the array grows. The other options do not directly address gas optimization for this specific scenario.