问题描述
在稀疏数组比(常规)对象更好的情况下，您可能有什么编程用途?
What possible programming use could you have where a sparse array would be better than an (regular) object?
通过稀疏数组，我的意思是:
By sparse array I mean one where:
arr = []; //Initialize
arr[0] = 'W';
arr[1] = 'T';
arr[3] = 'F';
console.log(arr[0] !== undefined) //true
console.log(arr[1] !== undefined) //true
console.log(arr[2] === undefined) //true
console.log(arr[3] !== undefined) //true
或更正式:
An object, O, is said to be sparse if the following algorithm returns true:
1. Let len be the result of calling the [[Get]] internal method of O with argument
"length".
2. For each integer i in the range 0≤i<ToUint32(len)
a. Let elem be the result of calling the [[GetOwnProperty]] internal method of O
with argument ToString(i).
b. If elem is undefined, return true.
3. Return false.
此外，ECMA 262 5.1标准进一步将length定义为:
Moreover, the ECMA 262 5.1 Standard further defines length specifically as:
因此，尽管仅定义了三个元素，但上面的示例arr.length === 4.
So the example above, arr.length === 4 despite there only being three elements defined.
实际上，根据标准，任何大于3的Number对于arr都是有效的length，包括Math.PI.
In fact, according to the standard, any Number greater than 3 is a valid length for arr, including Math.PI.
因此，这是否意味着没有人可以使用:
Consequently, does this mean that no one should use:
for(var i=0; i<arr.length; i++)
//Cannot trust arr[i] exists
相反，使用它会更合适
for(key in arr)
//Always exists
我从来没有在野外遇到过故意的人，实际上我只是在读这里奇怪的问答时才开始考虑它，现在我有点不安.
I've never encountered an intentional one in the wild, and really only began thinking about it while reading an odd Q&A here, and now I'm a little unsettled.
我早就知道，没有一种巧妙的方法可以从Array中删除元素，但是现在我更加困惑为什么您会故意留下一个孔，更不用说定义长度可以大于最后定义的元素的任何数字.
I've long known that there's not a neat way to remove an element from an Array, but now I'm even more confused as to why you would intentionally leave a hole, let alone define a standard where length can be any number greater than the last defined element.
如果我想要随机键值对，则可以使用Object.如果我希望能够完全迭代，请使用Array.我想念什么吗?
If I wanted random key value pairs, I'd use an Object. If I want to be able to cleanly iterate, I use an Array. Am I missing something?
注意，我正在寻找一个特定的用例，或一类通用的用例，而不是对标准的引用或意见.我知道这是允许的，并且我已经有了意见. :)
Note, I'm looking for a specific use case, or a class of generalized use cases not a reference to the standards, or an opinion. I know it's allowed, and I already have an opinion. :)
要添加测试或查看一些我已经看到的Array标准以意外方式工作的测试，请查看此小提琴
To add a test or see some of the ones I've seen where the Array standard works in unexpected ways, check out this fiddle
很抱歉，如果这有点抽象.整夜都在想这件事.
Sorry if this is a bit abstract. Been thinking about it all night.
推荐答案
我在实际使用中遇到的稀疏数组的一个可能用例是热图.
One possible use-case for sparse arrays that I've come across in real usage is for a heat-map.
从您的地图开始是X×的空数组； Y元素.加载数据，然后通过增加相关坐标处的数组元素将其填充到地图中.
Start with your map being an empty array of X × Y elements. Load your data, and populate it into the map by incrementing the array elements at the relevant co-ords.
另一个类似的例子可能是战舰游戏，通过在适当的坐标处填充数组元素，将船只放置在一个空网格中.
Another similar example might be a battleship game, where boats are placed into an empty grid by populating the array elements at the appropriate co-ordinates.
这并不是说这是这样做的唯一方法，或者甚至是最好的方法-这两个示例都可以很容易地实现而无需使用稀疏数组-但问题是需要用例，所以您就可以了
That's not to say this is this only way to do this, or even the best way -- both examples can quite easily be achieved without using a sparse array -- but the question was asking for use cases, so there you go.
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