here is an example contract code for an NFT (non-fungible token) using the Solidity programming language for the Ethereum blockchain:

pragma solidity ^0.8.0;

contract NFT { // Events event Transfer(address indexed from, address indexed to, uint256 tokenId); event Approval(address indexed owner, address indexed approved, uint256 tokenId);

// Variables
mapping(uint256 => address) public tokenOwner;
mapping(uint256 => address) public tokenApproval;
mapping(address => mapping(uint256 => bool)) public ownedTokens;
mapping(address => mapping(uint256 => bool)) public approvedTokens;
address public owner;

// Functions
constructor() public {
    owner = msg.sender;
}

function mint(address to, uint256 tokenId) public {
    require(msg.sender == owner, "Only owner can mint new tokens.");
    require(!tokenOwner[tokenId], "Token with this ID already exists.");
    
    tokenOwner[tokenId] = to;
    ownedTokens[to][tokenId] = true;
    emit Transfer(address(0), to, tokenId);
}

function transfer(address to, uint256 tokenId) public {
    require(tokenOwner[tokenId] == msg.sender, "You do not own this token.");
    require(to != address(0), "Cannot transfer to address 0x0.");
    
    clearApproval(tokenId);
    tokenOwner[tokenId] = to;
    ownedTokens[msg.sender][tokenId] = false;
    ownedTokens[to][tokenId] = true;
    emit Transfer(msg.sender, to, tokenId);
}

function approve(address approved, uint256 tokenId) public {
    require(tokenOwner[tokenId] == msg.sender, "You do not own this token.");
    
    tokenApproval[tokenId] = approved;
    approvedTokens[msg.sender][tokenId] = true;
    emit Approval(msg.sender, approved, tokenId);
}

function clearApproval(uint256 tokenId) public {
    require(tokenOwner[tokenId] == msg.sender, "You do not own this token.");
    
    tokenApproval[tokenId] = address(0);
    approvedTokens[msg.sender][tokenId] = false;
}

function takeOwnership(uint256 tokenId) public {
    require(tokenApproval[tokenId] == msg.sender, "You are not approved to take this token.");
    
    clearApproval(tokenId);
    tokenOwner[tokenId] = msg.sender;
    ownedTokens[tokenApproval[tokenId]][tokenId] = false;
    ownedTokens[msg.sender][tokenId] = true;
    emit Transfer(tokenApproval[tokenId], msg.sender, tokenId);
}

}

This code creates a smart contract for an NFT that allows for minting new tokens, transferring ownership of tokens, approving other addresses to take ownership, and taking