Solid basic structure and array, private / public function, return value and modifier of function, event

Use structs and arrays

Create a new structure

struct Person {
  uint age;
  string name;
}
Person[] public people;

This is Person structure , Next, we create a new Person structure , Then add it to a file called people In the array .

//  Create a new Person:
Person evelyn = Person(16, "Evelyn");

//  Will create a new satoshi Added to the people Array :
people.push(evelyn);
// You can also take two steps at a time , Simpler with one line of code :
people.push(Person(16, "Evelyn"));

notes ：array.push() In the array The tail Add new elements , So the order of elements in the array is the order we add

private / Public function

Solidity The properties of defined functions are public by default . That means either side ( Or other contracts ) You can call the functions in your contract .

obviously , It's not always necessary , And such contracts are vulnerable . So it's a good programming habit to define your own functions as private , Make it public only when you need the outside world to call it .

Define a private function , Use the keyword... After the function name private that will do ：

uint[] numbers;
function _addToArray(uint _number) private {
  numbers.push(_number);
}

This means that only other functions in our contract can call this function , to numbers Array add new members .

Return value and modifier of function

To return a value from a function , As defined below ：

string greeting = "What's up dog";
function sayHello() public returns (string) {
  return greeting;
}

Solidity in , The definition of the function can contain the data type of the return value ( In this case string).
The above function has not actually changed Solidity State in , namely , It doesn't change any value or write anything .
Define the function as view, This means that it can only read data and cannot change data ,
Define the function as pure , Indicates that this function does not even access the data in the application , Its return value depends entirely on its input parameters

function sayHello() public view returns (string) {

function _multiply(uint a, uint b) private pure returns (uint) {
  return a * b;
}

Hash function Keccak256

Hash function keccak256, It's used SHA3 edition . A hash function basically converts a string into a 256 Bit 16 Hexadecimal Numbers . A small change in the string will cause a great change in the hash data .

//6e91ec6b618bb462a4a6ee5aa2cb0e9cf30f7a052bb467b0ba58b8748c00d2e5
keccak256("aaaab");
//b1f078126895a1424524de5321b339ab00408010b7cf0e6ed451514981e58aa9
keccak256("aaaac");

event

event It is a mechanism for contract and blockchain communication . Your front end application “ monitor ” Some events , And react .

//  Set up events here 
event IntegersAdded(uint x, uint y, uint result);

function add(uint _x, uint _y) public {
  uint result = _x + _y;
  // Triggering event , notice app
  IntegersAdded(_x, _y, result);
  return result;
}

Yours app The front end can listen to this event .JavaScript The implementation is as follows :

YourContract.IntegersAdded(function(error, result) {
  //  Do something 
})