Blockchain Developer's Guide

Mapping

Mapping is used for key-to-value mapping. Mappings can be seen as hash tables that are virtually initialized such that every possible key exists and is mapped to a default value. The default value is all zeros. The key is never stored in a mapping, only the keccak256 hash is used for value lookup. Mapping is defined just like any other variable type. Take a look at this code:

contract Gotham {    struct Batman {        string friends;        string foes;        int funds;        string fox;    }    mapping (address => Batman) Catwoman;   address[] public Batman_address;}

The preceding code example shows that Catwoman is being initialized as a mapping.