Solidity遍历

　　实际上，映射对于存储地址的标记值非常有用。我们在许多合约中都看到了它们，它们通常以这种方式定义：

mapping (address => uint) public users;

　　由于映射是公共的，我们得到一个免费的getter，我们可以通过使用简单的方法获取myAddress的值

users(myAddress);

　　Solidity映射看起来可能类似于关联数组，但它不是，它没有索引，因此很难遍历所有地址。但它仍然可以通过其它方法遍历。

　　数组更容易管理：

address[] public addressIndices;
// start adding address in array
addressIndices.push(newAddress);
...
// We know the length of the array
uint arrayLength = addressIndices.length;
for (uint i=0; i<arrayLength; i++) {
  // do something
}

　　假设我们想要计算所有地址的总值，拥有一组地址确实很有帮助。

mapping (address => uint) public mappedUsers;
address[] public addressIndices;
// start adding address in array
addressIndices.push(newAddress);
// define mappedUsers as well
mappedUsers[newAddress] = someValue;
...
// We know the length of the array
uint arrayLength = addressIndices.length;
// totalValue auto init to 0
uint totalValue;
for (uint i=0; i<arrayLength; i++) {
  totalValue += mappedUsers[addressIndices[i]];
}

　　如果我们想要有效地删除数组怎么办？我们必须将数组的最后位置移动到已删除的位置。

uint indexToBeDeleted;
mapping (address => uint) public mappedUsers;
address[] public addressIndices;
uint arrayLength = addressIndices.length;
for (uint i=0; i<arrayLength; i++) {
  if (addressIndices[i] == addressToBeDeleted) {
    indexToBeDeleted = i;
    break;
  }
}
// if index to be deleted is not the last index, swap position.
if (indexToBeDeleted < arrayLength-1) {
  mappedUsers[indexToBeDeleted] = mappedUsers[arrayLength-1];
}
// we can now reduce the array length by 1
addressIndices--;

　　参考上面的代码，假设我们不希望for循环查找要删除的地址的索引，我们需要在结构中记录项的索引。如果我们想要做一个合适的CRUD，它会变得有点复杂。

　　完整代码的示例参考如下：

pragma solidity^0.4.17;

contract Test {
    
    struct structUser {
        uint value;
        uint index;
        bool exists;
    }

    mapping(address => structUser) public arrayStructs;
    
    address[] public addressIndexes;
    
    function addAddress(uint _val) public returns (bool){
        // if user exists, add _val
        if (arrayStructs[msg.sender].exists > true) {
            arrayStructs[msg.sender].value += _val;
        }
        else {
            // else its new user
            addressIndexes.push(msg.sender);
            arrayStructs[msg.sender].value = _val;
            arrayStructs[msg.sender].index = addressIndexes.length-1;
            arrayStructs[msg.sender].exists = true;
        }
        return true;
    }
    
    function deleteAddress() public returns (bool) {
        // if address exists
        if (arrayStructs[msg.sender].exists) {
            structUser memory deletedUser = arrayStructs[msg.sender];
            // if index is not the last entry
            if (deletedUser.index != addressIndexes.length-1) {
                // delete addressIndexes[deletedUser.index];
                // last strucUser
                address lastAddress = addressIndexes[addressIndexes.length-1];
                addressIndexes[deletedUser.index] = lastAddress;
                arrayStructs[lastAddress].index = deletedUser.index; 
            }
            delete arrayStructs[msg.sender];
            addressIndexes.length--;
            return true;
        }
    }
    
    function getAddresses() public view returns (address[]){
        return addressIndexes;    
    }
    
    function getTotalValue() public view returns (uint) {
        uint arrayLength = addressIndexes.length;
        uint total = 0;
        for (uint i=0; i<arrayLength; i++) {
            total += arrayStructs[addressIndexes[i]].value;
        }
        return total;
    }
    
    function getTotalUsers() public view returns (uint) {
        return addressIndexes.length;
    }
}

　　代码来源地址：https://github.com/bernardpeh/solidity-loop-addresses-demo/blob/master/loop-demo.sol

文章来源：https://medium.com/@blockchain101/looping-in-solidity-32c621e05c22

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