blocks|key|547368|text|当集合中项的顺序不重要时，集合可以提供更好的性能来查找集合中的项。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|547369|原因是set使用散列值来查找项(如字典)，而数组必须遍历其全部内容才能找到特定的对象。|547370|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

When the order of the items in the collection is not important, sets offer better performance for finding items in the collection.

The reason is that a set uses hash values to find items (like a dictionary) while an array has to iterate over its entire contents to find a particular object.

blocks|key|330598|text|数组用于按索引访问项。任何项都可以多次插入数组中。数组控制其元素的顺序。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|330599|set基本上只用于检查项目是否在集合中。这些项目没有顺序或索引的概念。一个项目在一个集合中不能有两次。|330600|如果一个数组想要检查它是否包含一个元素，它必须检查它的所有项。Sets旨在使用更快的算法。|330601|你可以想象一个集合，就像一个没有值的字典。|330602|注意，array和set不是唯一的数据结构。还有其他的，例如队列，堆栈，堆，斐波那契堆。我建议你读一本关于算法和数据结构的书。|330603|有关详细信息，请参阅wikipedia。|offset|length|330604|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Set_%2528computer_science%2529^0|0|0|0|0|0|A|9|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|W|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|X|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|Y|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|Z|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|10|8|@]|9|@$L|11|M|12|1|13]]|A|$]]|$1|N|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|O|$P|$5|Q|R|S|A|$T|U]]]]

An array is used to access items by their index. Any item can be inserted into the array multiple times. Arrays mantain the order of their elements.

A set is used basically only to check if the item is in the collection or not. The items have no concept of order or indexing. You cannot have an item in a set twice.

If an array wants to check if it contains an element, it has to check all its items. Sets are designed to use faster algorithms.

You can imagine a set like a dictionary without values.

Note that array and set are not the only data structures. There are other, e.g. Queue, Stack, Heap, Fibonacci's Heap. I would recommend reading a book about algorithms and data structures.

See <a href="http://en.wikipedia.org/wiki/Set_%28computer_science%29">wikipedia</a> for more information.

blocks|key|330654|text|iOS+5%2B中提供了NSOrderedSet，因此主要区别在于您是否希望在数据结构中包含重复的对象。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|330655|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

NSOrderedSet is available in iOS 5+ so with that the main difference becomes whether you want duplicate objects in the data structure.

blocks|key|330744|text|主要的区别已经在其他答案中给出了。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|330745|我只想指出，由于集合和字典的实现方式(即使用散列)，人们应该小心不要使用可变对象作为键。|330746|如果一个键改变了，那么散列也会改变，指向散列表中不同的索引/存储桶。原始值不会被删除，而是会在枚举或询问结构的大小/计数时考虑在内。|330747|这可能会导致一些很难定位的bug。|330748|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

The main differences have already been given in other answers.

I'd just like to note that because of the way sets and dictionaries are implemented (i.e. using hashes),one should be careful not to use mutable objects for the keys.

If a key is mutated then the hash will (probably) change too, pointing to a different index/bucket in the hash table. The original value won't be deleted and will actually be taken into account when enumerating or asking the structure for its size/count.

This can lead to some really hard to locate bugs.

blocks|key|547586|text|为了增加一点，我有时会使用set从数组中删除重复项，比如:-|type|unstyled|depth|inlineStyleRanges|entityRanges|data|547587|NSMutableSet+*set=[[NSMutableSet+alloc]initWithArray:duplicateValueArray];+//+will+remove+all+the+duplicate+values|code-block|syntax|javascript|547588|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Just to add a bit of it i use set sometimes just to remove duplicates from array like :-

<pre><code>NSMutableSet *set=[[NSMutableSet alloc]initWithArray:duplicateValueArray]; // will remove all the duplicate values
</code></pre>

I have used NSSets many times in my apps, but I have never created one myself. 

When is it better to use an <a href="https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSSet_Class/" rel="noreferrer"><code>NSSet</code></a> as opposed to an <a href="https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSArray_Class/index.html#//apple_ref/doc/c_ref/NSArray" rel="noreferrer"><code>NSArray</code></a> and why?

When is it better to use an NSSet over an NSArray?

数据结构

我在我的应用程序中使用过NSSets很多次，但我自己从来没有创建过。什么时候使用比使用更好?为什么？

问在什么情况下，通过NSArray使用NSSet更好？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问在什么情况下，通过NSArray使用NSSet更好？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问在什么情况下，通过NSArray使用NSSet更好？
EN