映射如何可靠地工作，并类似于其他流行语言中的另一个概念进行映射

Question

谁能解释它的mapping工作原理以及为什么使用它？像数组一样是项目的集合。我没有扎实的经验，我才刚刚开始。我在solidity官方文档页面上找到了此代码。

pragma solidity ^0.4.11;

Contract CrowdFunding {
// Defines a new type with two fields.
struct Funder {
    address addr;
    uint amount;
}

struct Campaign {
    address beneficiary;
    uint fundingGoal;
    uint numFunders;
    uint amount;
    mapping (uint => Funder) funders;
}

uint numCampaigns;
mapping (uint => Campaign) campaigns;

function newCampaign(address beneficiary, uint goal) returns (uint campaignID) {
    campaignID = numCampaigns++; // campaignID is return variable
    // Creates new struct and saves in storage. We leave out the mapping type.
    campaigns[campaignID] = Campaign(beneficiary, goal, 0, 0);
}

function contribute(uint campaignID) payable {
    Campaign storage c = campaigns[campaignID];
    // Creates a new temporary memory struct, initialised with the given values
    // and copies it over to storage.
    // Note that you can also use Funder(msg.sender, msg.value) to initialise.
    c.funders[c.numFunders++] = Funder({addr: msg.sender, amount: msg.value});
    c.amount += msg.value;
}

function checkGoalReached(uint campaignID) returns (bool reached) {
    Campaign storage c = campaigns[campaignID];
    if (c.amount < c.fundingGoal)
        return false;
    uint amount = c.amount;
    c.amount = 0;
    c.beneficiary.transfer(amount);
    return true;
}
}

Answer 1

基本上，映射等效于其他编程语言中的字典或地图。这是键值存储。

在标准数组中，它是一个索引查找，例如，如果数组中有10个元素，则索引为0-9，并且它看起来像是一个整数数组：

[0] 555
[1] 123
...
[8] 22
[9] 5

或在代码中：

uint[] _ints;

function getter(uint _idx) returns(uint) {
    return _ints[_idx];
}

所有键的顺序都是基于其添加到数组的顺序。

映射的工作原理略有不同，描述它的最简单方法是使用关键查找。因此，如果这是地址到整数的映射，则它将类似于：

[0x000000000000000A] 555
[0x0000000000000004] 123
....
[0x0000000000000006] 22
[0x0000000000000003] 6

或在代码中

mapping(address => uint) _map;

function getter(address _addr) returns(uint) {
    return _map[_addr];
}

因此，基本上，您是用对象而不是整数来引用值。键也不必按顺序排列。