【CryptoZombies】lesson1 Chapter 10: More on Functions

Return Values

To return a value from a function, the declaration looks like this:

string greeting = "What's up dog";

function sayHello() public returns (string memory) {
 return greeting;
}

In Solidity, the function declaration contains the type of the return value (in this case string).

戻り値がある場合、引数の前にreturnsと記載する。

Function modifiers

So in this case we could declare it as a view function, meaning it's only viewing the data but not modifying it:

function sayHello() public view returns (string memory) {

Solidity also contains pure functions, which means you're not even accessing any data in the app. Consider the following:

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

This function doesn't even read from the state of the app — its return value depends only on its function parameters. So in this case we would declare the function as pure.

Note: It may be hard to remember when to mark functions as pure/view. Luckily the Solidity compiler is good about issuing warnings to let you know when you should use one of these modifiers.

test

We're going to want a helper function that generates a random DNA number from a string.

1.Create a private function called _generateRandomDna. It will take one parameter named _str (a string), and return a uint. Don't forget to set the data location of the _str parameter to memory.

2.This function will view some of our contract's variables but not modify them, so mark it as view.

3.The function body should be empty at this point — we'll fill it in later.

pragma solidity >=0.5.0 <0.6.0;

contract ZombieFactory {

   uint dnaDigits = 16;
   uint dnaModulus = 10 ** dnaDigits;

   struct Zombie {
       string name;
       uint dna;
   }

   Zombie[] public zombies;

   function _createZombie(string memory _name, uint _dna) private {
       zombies.push(Zombie(_name, _dna));
   }

   // start here
   function _generateRandomDna(string memory _str) private view returns (uint) {

   }

}

modifiers：修飾子

#1 Solidity Tutorial & Ethereum Blockchain Programming Course | CryptoZombies