10.Mapping

Maps的创建语法：mapping(keyType => valueType).

• The keyType can be any built-in value type, bytes, string, or any contract.
• valueType can be any type including another mapping or an array.

Mappings are not iterable.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.1;

contract Mapping {
    //mapping
    mapping(address => uint256) public balances;
    mapping(address => mapping(address => bool)) public isFriend;

    function examples() external {
        // set
        balances[msg.sender] = 123;
        // get
        uint256 bal = balances[msg.sender];
        uint256 bal2 = balances[address(1)];
        // update
        balances[msg.sender] += 456; //123+345
        // delete
        delete balances[msg.sender]; //0

        isFriend[msg.sender][address(this)] = true;
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.1;

contract Mapping {
    // Mapping from address to uint
    mapping(address => uint) public myMap;

    function get(address _addr) public view returns (uint) {
        // Mapping always returns a value.
        // If the value was never set, it will return the default value.
        return myMap[_addr];
    }

    function set(address _addr, uint _i) public {
        // Update the value at this address
        myMap[_addr] = _i;
    }

    function remove(address _addr) public {
        // Reset the value to the default value.
        delete myMap[_addr];
    }
}

Mapping in Solidity is not iterable unless you internally store all keys that were inserted.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.1;

contract IterableMapping {
    //mapping
    mapping(address => uint256) public balances;
    mapping(address => bool) public inserted;
    address[] public keys;

    function set(address _key, uint256 _val) external {
        balances[_key] = _val;
        if (!inserted[_key]) {
            inserted[_key] = true;
            keys.push(_key);
        }
    }

    function getSize() external view returns (uint256) {
        return keys.length;
    }

    function first() external view returns (uint256) {
        return balances[keys[0]];
    }

    function last() external view returns (uint256) {
        return balances[keys[keys.length - 1]];
    }

    function get(uint256 _i) external view returns (uint256) {
        return balances[keys[_i]];
    }
}