Index

Chapter 10: More on Functions

In this chapter, we're going to learn about Function return values, and function modifiers.

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) {
return greeting;
}

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

Function modifiers

The above function doesn't actually change state in Solidity — e.g. it doesn't change any values or write anything.
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) {

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.

Put it to the 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.

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 ^0.4.25;

contract ZombieFactory {

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

    struct Zombie {
        string name;
        uint dna;
    }

    Zombie[] public zombies;

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

    function _generateRandomDna(string _str) private view returns (uint) {

    }

}