Index

Chapter 3: balanceOf & ownerOf

Great, let's dive into the ERC721 implementation!
We've gone ahead and copied the empty shell of all the functions you'll be implementing in this lesson.
In this chapter, we're going to implement the first two methods: balanceOf and ownerOf.

balanceOf

function balanceOf(address _owner) external view returns (uint256 _balance);

This function simply takes an address, and returns how many tokens that address owns.
In our case, our "tokens" are Zombies. Do you remember where in our DApp we stored how many zombies an owner has?

ownerOf

function ownerOf(uint256 _tokenId) external view returns (address _owner);

This function takes a token ID (in our case, a Zombie ID), and returns the address of the person who owns it.
Again, this is very straightforward for us to implement, since we already have a mapping in our DApp that stores this information. We can implement this function in one line, just a return statement.
Note: Remember, uint256 is equivalent to uint. We've been using uint in our code up until now, but we're using uint256 here because we copy/pasted from the spec.

Putting it to the Test

I'll leave it to you to figure out how to implement these 2 functions.
Each function should simply be 1 line of code, a return statement. Take a look at our code from previous lessons to see where we're storing this data. If you can't figure it out, you can hit the "show me the answer" button for some help.
1. Implement balanceOf to return the number of zombies _owner has.

2. Implement ownerOf to return the address of whoever owns the zombie with ID _tokenId.

zombieownership.sol

pragma solidity ^0.4.25;

import "./zombieattack.sol";
import "./erc721.sol";

contract ZombieOwnership is ZombieAttack, ERC721 {

  function balanceOf(address _owner) external view returns (uint256) {
    // 1. Return the number of zombies `_owner` has here
    return ownerZombieCount[_owner];
  }

  function ownerOf(uint256 _tokenId) external view returns (address) {
    // 2. Return the owner of `_tokenId` here
    return zombieToOwner[_tokenId];
  }

  function transferFrom(address _from, address _to, uint256 _tokenId) external payable {

  }

  function approve(address _approved, uint256 _tokenId) external payable {

  }
}