Mapping

contract Mapping {
  // state mapping 定义
  mapping(address => uint) public ethBalances;
  mapping(address => mapping(address => uint)) coinBalances;
  
  function demo() external {
    //操作
    ethBalances[msg.sender] = 123; // 设置,修改
    ethBalances[msg.sender] += 234;
    
    uint bal = ethBalances[msg.sender]; // 取值
    uint zero = ethBalances[address(1)]; // 未设置返回默认值
    
    delete ethBalances[msg.sender]; // 删除
  }
}

可循环 Map 的实现

contract LoopMapping {
  //可循环 mapping 实现
  mapping(address => uint) public balances;
  mapping(address => bool) public exists;
  address[] public keys;
  
  function set(address _key, uint _val) external {
    balances[_key] = _val;
  
    if (!exists[_key]) {
      exists[_key] = true;
      address.push(_key);
    }
  }
  
  function getSize() external returns (uint) {
    return keys.length;
  }
  
  function getFirst() external returns (address) {
    if (getSize() == 0) {
      return address(0);
    }
    return keys[0];
  }
  
  function getLast() external returns (address) {
    if (getSize() == 0) {
      return address(0);
    }
    return keys[keys.length - 1];
  }
  
  function get(uint _index) external returns (address) {
    require(_index < getSize(), "index out of bound");
    return keys[_index];
  }
}