Question 1

The difference between address.send() and address.transfer() is:

Accepted Answer

A)  .send returns a Boolean and .transfer throws an exception on error. Both just forward the gasstipend of 2300 gas and are considered safe against re-entrancy. 
B)  .send throws an exception and .transfer returns a Boolean on error. Both just forward the gasstipend of 2300 gas and considered safe against re-entrancy 
C)  .send returns a Boolean and .transfer throws an exception on error. .send is considered dangerous, because it sends all gas along, while .transfer only sends the gas stipend of 2300 gas along 
D)  .send and .transfer are both considered low-level functions which are dangerous, because they send all gas along. It's better to use address.call.value()() to control the gas-amount.

Question 2

Address.call.value():

Accepted Answer

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.

Question 3

Hashing Mining uses:

Accepted Answer

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.

Question 4

View and Pure Functions:

Accepted Answer

A)  can only be accessed during calls. 
B)  can be accessed during transactions and calls.

Question 5

Ethereum Nodes:

Accepted Answer

A)  must implement the Ethereum protocol and external access can only be done via the proprietary Ethereum Libraries like Web3.js. 
B)  must implement the Ethereum Protocol and a JSON-RPC to talk with clients. 
C)  must implement Web3.js to interact with Websites.

Question 6

What are Ethereum Nodes?

Accepted Answer

A)  Programs implementing the Ethereum Protocol to talk to each other and JSON-RPC interfaces to talk to the outside world. 
B)  A Java-Script library to compile and run Solidity Code. 
C)  A Framework for deploying and running smart contract in a decentralized way.

Question 7

Function and Variable Visibility:

Accepted Answer

A)  a function that is marked as internal cannot be called by other contracts, unless the function is used by a derived contract. Private Functions cannot be called by any other outside contract and public variables are generating automatically a getter function. 
B)  a function that is marked as external can never be called internally. Private functions can also be called by derived contracts using inheritance. Private variables are accessible also in derived contracts.

Question 8

Externally Owned Accounts (EoA):

Accepted Answer

A)  are changing their address every time a Transaction is sent because of the nonce. 
B)  are keeping their address, but on the blockchain a nonce is increased every time they send a transaction to avoid replay attacks.

Question 9

A version pragma is a great way to make it clear:

Accepted Answer

A)  for which compiler version a smart contract was developed for. It helps to avoid breaking changes. 
B)  for which blockchain a smart contract was developed for. It helps to avoid confusion with beta-customers. 
C)  for which blockchain node a smart contract was developed for. It helps to avoid mixing up different versions of go-ethereum.

Question 10

You need to use ____________ to get the address that initiated the transaction.

Accepted Answer

A)  Tx.origin 
B)  Msg.sender

Question 11

When a smart contract pays out money:

Accepted Answer

A)  it's good to use a push over a pull method. 
B)  it's good to use a push and a pull method to ensure that participants can get their money no matter the contract state. In addition to and pushing it should contain a withdraw method. 
C)  it's good to use only pull and no push method.

Question 12

Importing from GitHub:

Accepted Answer

A)  works across all compilers and platforms the same way. 
B)  is generally possible, but currently works only in Remix, but doesn't work in Truffle.

Question 13

Address.Call vs. Address.Delegatecall:

Accepted Answer

A)  Address.call() is used for calling other contracts using the scope of the called contract in terms of storage variables. Address.delegatecall() is used for libraries, which uses the storage variables of the contract who called. Libraries are a great way to re-use already existing code and delegatecall can make sure that no storage is used from the library, instead it looks like the code is directly copied into the calling contract. 
B)  Address.delegatecall() is used for calling other contracts using the scope of the called contract in terms of storage variables. Address.call() is used for libraries, which uses the storage variables of the contract who called. Libraries are a great way to re-use already existing code and call() can make sure that no storage is used from the library, instead it looks like the code is directly copied into the calling contract.

Question 14

Using truffle-contract over Web3.js:

Accepted Answer

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.

Question 15

To avoid issues during Ethereum platform upgrades:

Accepted Answer

A)  it's good to inform users about the updates via a newsletter. 
B)  it's good to have the ability to pause a contract in order to manage the money at risk. 
C)  Ethereum doesn't upgrade the platform. It's fixed and final.

Question 16

Sending one Ether is actually internally translated:

Accepted Answer

A)  to Wei, so it will send the equivalent of 10^18 Wei. 
B)  to Finney, so it will send the equivalent of 10^3 Finney. 
C)  to Szabo, so it will send the equivalent of 10^6 Szabo.

Question 17

Finish the sentence: The Library Web3.js is …:

Accepted Answer

A)  useful when developing distributed applications with HTML and JavaScript, because it already implements the abstraction of the JSON-RPC interface of Ethereum Nodes. 
B)  necessary when developing distributed applications with HTML and JavaScript, because the proprietary JSON-RPC interface of Ethereum Nodes is a closed source.

Question 18

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

Accepted Answer

A) True 
 B)False

Question 19

When considering smart contracts and the blockchain it's good:

Accepted Answer

A)  to move all existing logic to the blockchain, so everything runs on the same system. This way it might be more complex, but easier to maintain. 
B)  to move only those parts to the blockchain that really need the blockchain. This way smart contracts can be easier to read, easier to test and are not so complex. 
C)  to move those parts to the blockchain that deal with Ether transfers. All other parts can remain in traditional database systems. This way only the value-transfer is on the blockchain.

Question 20

Loops in Solidity:

Accepted Answer

A)  are a great way to circumvent gas requirements, because a loop will only consume gas once. 
B)  are dangerous when used with data structures that grow, such as arrays or mapping, because it is hard to estimate the gas requirements. 
C)  should be avoided where possible, because of unknown side-effects on the gas requirements.

Exam 2: BTA Certified Blockchain Developer - Ethereum