objective-c 有没有一种简单的方法可以向后迭代 NSArray？

Is there an easy way to iterate over an NSArray backwards?

提问by Thanks

I've got an NSArrayand have to iterate over it in a special case backwards, so that I first look at the last element. It's for performance reasons: If the last one just makes no sense, all previous ones can be ignored. So I'd like to break the loop. But that won't work if I iterate forward from 0 to n. I need to go from n to 0. Maybe there is a method or function I don't know about, so I wouldn't have to re-invent the wheel here.

我有一个NSArray并且必须在特殊情况下向后迭代它，以便我首先查看最后一个元素。这是出于性能原因：如果最后一个没有意义，则可以忽略所有先前的。所以我想打破循环。但是，如果我从 0 向前迭代到 n，那将不起作用。我需要从 n 到 0。也许有一个我不知道的方法或函数，所以我不必在这里重新发明轮子。

回答by Sijmen Mulder

To add on the other answers, you can use -[NSArray reverseObjectEnumerator]in combination with the fast enumeration feature in Objective-C 2.0 (available in Leopard, iPhone):

要添加其他答案，您可以-[NSArray reverseObjectEnumerator]结合 Objective-C 2.0 中的快速枚举功能（在 Leopard、iPhone 中可用）：

for (id someObject in [myArray reverseObjectEnumerator])
{
    // print some info
    NSLog([someObject description]);
}

Source with some more info: http://cocoawithlove.com/2008/05/fast-enumeration-clarifications.html

更多信息来源：http: //cocoawithlove.com/2008/05/fast-enumeration-clarifications.html

回答by Mike Abdullah

Since this is for performace, you have a number of options and would be well advised to try them all to see which works best.

由于这是为了性能，因此您有多种选择，建议您全部尝试一下，看看哪种效果最好。

• [array enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:…]
• -[NSArray reverseObjectEnumerator]
• Create a reverse copy of the arrayand then iterate through that normally
• Use a standard C for loop and start and work backwards through the array.

• [array enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:…]
• -[NSArray reverseObjectEnumerator]
• 创建数组的反向副本，然后正常迭代
• 使用标准的 C for 循环并开始并通过数组向后工作。

More extreme methods (if performance is super-critical)

更极端的方法（如果性能非常关键）

• Read up on how Cocoa implements fast object enumeration and create your own equivalent in reverse.
• Use a C or C++ array.

• 阅读 Cocoa 如何实现快速对象枚举并反向创建您自己的等效项。
• 使用 C 或 C++ 数组。

There may be others. In which case, anyone feel free to add it.

可能还有其他人。在这种情况下，任何人都可以随意添加它。

回答by Naaff

From here:

从这里：

 NSEnumerator* myIterator = [myArray reverseObjectEnumerator];
 id anObject;

 while( anObject = [myIterator nextObject])
 {
     /* do something useful with anObject */
 }

回答by CiNN

[NsArray reverseObjectEnumerator]

回答by Vyacheslav Zubenko

for (int i = ((int)[array count] - 1); i > -1; i--) {
    NSLog(@"element: %@",array[i]);
}