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Mappings.sol

pragma solidity ^0.6.0;

contract Mappings {
    // Mappings are data structure like dictionary in C# or hash tables in C++
    // O(1)
    // mapping(key => value) myMapping;
    // One can make nested mappings as well
    // Mappings are like databases for blockchain and they are very useful
    
    mapping(uint => string) public names;
    mapping(uint => Book) public books;
    mapping(address => mapping(uint => Book)) public myBooks;
    
    struct Book {
        string title;
        string author;
    }
    
    constructor() public {
        names[1] = "Adam";
        names[2] = "Bruce";
        names[3] = "Carl";
    }
    
    function addBook(uint _id, string memory _title, string memory _author) public {
        books[_id] = Book(_title, _author);
    }
    
    function addMyBook(uint _id, string memory _title, string memory _author) public {
        myBooks[msg.sender][_id] = Book(_title, _author);
    }
}

https://ropsten.etherscan.io/address/0x1172fe29e1aeef398ddc5c399a7eea8797802b29#code