WTF Solidity: 7. Mapping

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Recently, I have been revisiting Solidity, consolidating the finer details, and writing "WTF Solidity" tutorials for newbies.

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In this section, we will introduce the hash table in Solidity: mapping type.

Mapping

With mapping type, people can query the corresponding Value by using a Key. For example, a person's wallet address can be queried by their id.

The format of declaring the mapping is mapping(_KeyType => _ValueType), where _KeyType and _ValueType are the variable types of Key and Value respectively. For example:

    mapping(uint => address) public idToAddress; // id maps to address
    mapping(address => address) public swapPair; // mapping of token pairs, from address to address

Rules of mapping

• Rule 1: The _KeyType should be selected among default types in solidity such as uint, address, etc. No custom struct can be used. However, _ValueType can be any custom types. The following example will throw an error, because _KeyType uses a custom struct:

      // define a struct
      struct Student{
          uint256 id;
          uint256 score;
      }
      mapping(Student => uint) public testVar;

• Rule 2: The storage location of the mapping must be storage: it can serve as the state variable or the storage variable inside function. But it can't be used in arguments or return results of public function.

• Rule 3: If the mapping is declared as public then Solidity will automatically create a getter function for you to query for the Value by the Key.

• Rule 4：The syntax of adding a key-value pair to a mapping is _Var[_Key] = _Value, where _Var is the name of the mapping variable, and _Key and _Value correspond to the new key-value pair. For example:

    function writeMap (uint _Key, address _Value) public {
        idToAddress[_Key] = _Value;
      }

Principle of mapping

• Principle 1: The mapping does not store any key information or length information.

• Principle 2: Mapping use keccak256(key) as offset to access value.

• Principle 3: Since Ethereum defines all unused space as 0, all key that are not assigned a value will have an initial value of 0.

Verify on Remix (use Mapping.sol as example)

• Deploy Mapping.sol

  

• Check initial value of map idToAddress.

  

• Write new key-value pair

  

Summary

In this section，we introduced the mapping type in Solidity. So far, we've learned all kinds of common variables.