Dependencies

Description

Create a smart contract:

• a struct (named: ColoredWallet) with two values (unit256):

  • numberOfRedCoins
  • numberOfBlueCoins

• a function for:

  • users can build their ColoredWallet
  • init values of redcoin and blue coin are 0 and 100

• a function for:

  • users can Burn one blue coin and receive two red coins
  • check enough blue coin existence

• a function for:

  • transfer bluecoin between ColoredWallet
  • check receiver ColoredWallet existence
  • each user only has access to their wallet

• Bounce question:

  • find out what's the modifier? solve above question with it.

smart contract link on Goerli Testnet Network:

I've deploy my smart contract on Goerli Testnet Network

contract: https://goerli.etherscan.io/address/0x9a0dfa10de1d40c50f313ca44b201be4d3f9abaf

contract creation: https://goerli.etherscan.io/tx/0x6d214c3e9e09fc386b0f5b2524da442c69ef8c5b5eaa128067fd3397ca471c1c

What is a pure function and when is it used? what's the difference between pure and view? take a simple example.

view indicates that the function will not alter the storage state in any way. But it allows you to "view" it or read it.

pure is even more strict, indicating that it will not even read the storage state.

A pure function is a function that given the same input, always returns the same output. But the state of the contract keeps changing as users interact with it. So if you pass a state variable as an argument to the function, since the state is changing, that function will not be pure. That's why pure functions cannot access to state.

"pure" functions are heavily used in mathematical libraries. For example SafeMath.sol

Also inside pure functions, you cannot

• use address(this).balance

• call other functions except for pure functions

if you call to view or pure functions externally, you do not pay a gas fee. However, they do cost gas if called internally by another function.

A simple example:

pragma solidity ^0.7.0;

contract viewSample {

    //view is specified and function reads data from block chain
    function getBlock() public view returns (uint){
        uint blocknumber = block.number;
        return blocknumber;
    }    
}

pragma solidity ^0.7.0;

contract pureSample {

   //pure is specified
   //this functions is only concerned about the variables and arithmetic in this function
   function getResult() public pure returns(uint sum){
      uint a = 5; 
      uint b = 7;
      sum = a + b; 
   }
}

Comparison of Pure and View

	DEFAULT(NOT SPECIFIED IN FUNCTION)	VIEW	PURE
Ideal For	Transactions that alter data on the block chain	Getter functions to view data on the block chain	Defined to the scope of the function and do not alter or view data on the block chain
Data Access	read / write	read	None
Transaction Type	Send	Call	Call