Mapping is one of ways of storing organized data in solidity.
It's essentially a key-value store for storing and looking up data.
Mappings act as hash tables consiting of key-value pairs.
mapping(_KeyType => _ValueType) public mappingNameMappings are frequently used to associate unique ethereum network address with value types, e.g. a user's address to theirr corresponding level in a game:
mapping(address => uint) public userLevel;We can only use mappings for state-variables.
Accessing value types from a mapping with key types
To identify user's level in a game we can create a function taking an address as a parameter:
function currentLevel(address addr) public constant returns (uint) {
return userLevel[addr];
}A mapping can store many _KeyTypes to _ValueTypes. Eg. many users playing the games all at once and they can have their own corresponding userLevel.
Default value
Mappings don't have length by design. They also don't have a concept of a key or value being set.
When mappings are initialized every possible key exists in the mappings and are mapped to values whose byte-representations are all zeros.
In details it goes like this:
When userLevel is initialized it’s done in such a way that every possible Ethereum address exists in the mapping and is mapped to a corresponding level of 0.
It doesn't matter whether it’s a random Ethereum user’s MetaMask address who perhaps has never even heard of the game before, or maybe an arbitrary smart contract on the Ethereum blockchain doing something totally unrelated. Our mapping still maps them to a value whose byte-representation are all zeros.