Solidity

Question 1

Variable scope

Answer 1

public -> Can be used internally & externally. Automatically creates a getter function
external -> Meant to be called ONLY from other contracts ( state variables cannot be external )
internal -> Can only be called from within the contract or from derived ones
private -> Can only be called from within the contract - not even from derived ones

Question 2

Pure function

Answer 2

int_a = 6

function square(uint 256 int_a) public pure {
       int_a * int_a
}

No cost to this functions; pure computation and not creating / changing any state variables. Not affecting the blockchain.

Question 3

‘memory’ keyword

Answer 3

=> Only stored during execution of the function call

string person;

function assignName(string memory _name) public {
      person = name
}

Question 4

mapping

Answer 4

A data structure -> hash table?

mapping(address => uint256) public addressToAmountFunded;

addressToAmountFunded[msg.sender] += msg.value;
// value associated with address increased every time. Not overwritten

Question 5

‘payable’ keyword

Answer 5

In this function eth/wei is being transferred.

Whether it’s withdrawn or deposited.

Question 6

‘msg’ keyword

Answer 6

In every contract. (Incoming call?)

msg. sender -> address from incoming function call [type address]
msg. value -> value associated with txn / call [type uint256]

Question 7

nodes cannot do API calls because…

Answer 7

Because blockchains and smart contracts are deterministic systems. Nodes might call APIs at different times and get different results, and then wouldn’t be able to verify the other results.

Nodes can also not agree on a random number together

Question 8

Oracle issue

Answer 8

An external oracle supplying data to the blockchain could represent a single point of failure - something which is completely avoided with blockchain otherwise

Question 9

Variable types

Answer 9

State Variables − Variables whose values are permanently stored in a contract storage.

Local Variables − Variables whose values are present till function is executing.

Global Variables − Special variables exists in the global namespace used to get information about the blockchain.

Question 10

Sender of the transaction [GV]

Answer 10

tx.origin (address payable)

Question 11

Gas price of the transaction [GV]

Answer 11

tx.gasprice (uint)

Question 12

Current block timestamp [GV]

Answer 12

now (uint)

Question 13

First four bytes of the calldata (function identifier) [GV]

Answer 13

msg.sig (bytes4)

Question 14

Complete calldata [GV]

Answer 14

msg.data (bytes calldata)

Question 15

Remaining gas [GV]

Answer 15

gasleft() returns (uint256)

Question 16

Current block timestamp as seconds since unix epoch [GV]

Answer 16

block.timestamp (uint)

Question 17

Current block number [GV]

Answer 17

block.number (uint)

Question 18

Current block gaslimit [GV]

Answer 18

block.gaslimit (uint)

Question 19

Current block difficulty [GV]

Answer 19

block.difficulty (uint)

Question 20

Current block miner’s address [GV]

Answer 20

block.coinbase (address payable)

Question 21

Hash of the given block - only works for 256 most recent, excluding current, blocks [GV]

Answer 21

blockhash(uint blockNumber) returns (bytes32)

Question 22

if (getConversionRate(msg.value) < minValueUSD) {
revert
}

=> Improve

Answer 22

require((getConversionRate(msg.value) < minValueUSD), “That ain’t enough dude!”);

2nd argument is error message

Question 23

‘this’ keyword

Answer 23

Refers to this contract.

E.g. address(this)
=> output address of the contract executing

Question 24

Globale variable: block
[list properties]

Answer 24

block.chainid => (uint): current chain id

block.number => (uint): current block number

block.timestamp => (uint): current block timestamp as seconds since unix epoch