CBDE Online Practice Questions and Answers

Smart Contracts can be written in:

A. Java, C++, Solidity and JavaScript, because the Ethereum Blockchain is completely language agnostic and cross compilers exist for every major language.

B. Solidity, Viper, LLL and Serpent, because those are high level languages that are compiled down to bytecode.

C. Solidity and JavaScript, because those are the official first implementations for Distributed applications and the Blockchain supports those languages fully.

To get most out of the blockchain, it is best:

A. to use it for the whole business logic. It's always best to have everything in once place.

B. to use it only for things which need the benefits of the blockchain.

Hashing Mining uses:

A. Keccack256 while internally to hash values it's easy to use the Dagger-Hashimoto to create a meaningful hash.

B. the Dagger-Hashimoto hashing while internally the EVM uses SHA256 which is an alias for Keccack256.

C. the Dagger-Hashimoto hashing while internally the EVM uses Keccack256 which is almost similar to SHA256, but has a different padding so produces different hashes.

GETH:

A. is the reference implementation of the Ethereum protocol and every other node implementation internally uses the closed-source from Geth.

B. is the library that is used for the blockchain node Go-Ethereum. It is also used by Parity is parts, because it's closed source.

C. is one of the many blockchain nodes that implement the Ethereum Protocol. It's open source and everyone can contribute.

You interact with a smart contract and see a gas usage of 50,000 gas with a gas cost of 15Gwei. How much Ether would you have to pay to the miner?

A. 750,000,000,000,000 Wei

B. 750,000,000,000 Wei

C. 750,000,000 Wei

D. A flat fee of 1 Ether

Files can be imported:

A. using relative and absolute paths, where the "." And the ".." depict that it's a relative path.

B. only via GitHub using the Repository and Username.

C. using the special requirefile(...) statement, which looks in a specific library path to import files.

View and Pure Functions: A. can only be accessed during calls.

B. can be accessed during transactions and calls.

Address.call.value():

A. sends the gas stipend of 2300 gas and returns a false on error.

B. sends all the gas along and cascades exceptions.

C. sends all the gas along and returns a false on error.

D. sends the gas stipend of 2300 gas and cascades exceptions.

All low-level functions on the address, so address.send(), address.call.valueQQ, address.callcode and address.delegatecall:

A. are interrupting execution on error, because they throw an exception.

B. continuing execution on error silently, which is the reason why they are so dangerous.

C. returning Booleans to indicate an error during execution.

D. .send() throws an exception, while the other functions are returning Booleans during execution to indicate an error.

When using assert to check invariants and it evaluates to false: A. all gas is consumed.

B. all remaining gas is returned.

It's not possible to use inheritance from multiple sources in Solidity.

A. True

B. False

Using truffle-contract over Web3.js:

A. is a must for every developer, because Web3.js changes so often.

B. is a convenient way because Web3.js is currently still in beta and truffle-contract can handle transactions with JavaScript-promises.

C. they are both completely different things. Truffle-Contract is a framework while Web3.js is a library.

To generate a random number:

A. it's good to use the block timestamp, as this is always different.

B. it's good to use the block hash as this is clearly always very different.

C. it's good to use the RANDAO smart contract.

D. it's not possible to have a random number in a deterministic environment such as the Ethereum blockchain.

When you do external calls to other smart contracts:

A. you should follow the checks-effects-interactions pattern and avoid state changes after the call.

B. you should follow the effects-checks-interactions pattern and avoid state changes before the call.

C. you should follow the checks-effects-interactions pattern, which is only necessary when you do calls to contracts where a direct contract call is not possible.

When you are programming a game like poker of battleships where you need to hide opponents values is:

A. with private state variables. This way nobody else other than the smart contract itself can see the information

B. with external contracts holding those values. This way we can make sure that the information flow is following a clear logic and nobody else can access this information.

C. You can't hide anything on the blockchain, because the information is public, just the call is private which means only other smart contracts would be limited in accessing that information.