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

Question

I've got an NSArray and 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.

"How can I reverse a NSArray in Objective-C?" is a completely different question. — Nestor, May 30 '14 at 11:44
Swift: http://stackoverflow.com/questions/24508592/how-to-iterate-for-loop-in-reverse-order-in-swift — Suragch, Aug 11 '16 at 17:53

score 199 · Accepted Answer · answered May 10 '09 at 00:16

199

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):

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

answered May 10 '09 at 00:16

Sijmen Mulder

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Is this considered fast enumeration, and is this faster than a regular c for loop that starts from `[myArray count]-1`? How do you iterate over an `NSString` backwards? – ma11hew28 Apr 17 '11 at 03:28
I don’t have any links here right now but there is a video somewhere (could’ve been a WWDC) where an Apple engineer explains how fast enumeration works and why it is fast. It outperforms both index-based NSArray enumeration and classic Foundation enumerators and, depending on the scenario, by a lot. – Sijmen Mulder Apr 20 '11 at 00:37
I have to run a reverse for loop on a dictionary, how to do so? – Ashwin G May 22 '14 at 05:05
4

@AshwinG Your question doesn't make sense, as `NSDictionary`s don't have an 'order'. You can enumerate them in any real order, unless you first order the keys/values. – mattsven Feb 24 '16 at 14:03

score 17 · Answer 2 · edited May 23 '17 at 12:10

17

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.

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

More extreme methods (if performance is super-critical)

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

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

edited May 23 '17 at 12:10

Community

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answered May 09 '09 at 23:45

Mike Abdullah

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I would say that the enumerateObjectsWithOptions:NSEnumerationReverse is the best answer, especially if you need to know what index you are on. Thanks!!! – Vincil Bishop Jul 17 '14 at 01:35

score 12 · Answer 3 · answered May 09 '09 at 22:24

12

From here:

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

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

answered May 09 '09 at 22:24

Naaff

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score 2 · Answer 4 · answered Mar 05 '18 at 08:03

2

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

answered Mar 05 '18 at 08:03

Vyacheslav Zubenko

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This does not detect changes to a mutable array instance during enumeration (elements, count). Otherwise probably a little faster than creating an NSEnumerator object. – osxdirk May 04 '20 at 09:12

score 2 · Answer 5 · answered May 09 '09 at 22:21

2

[NsArray reverseObjectEnumerator]

answered May 09 '09 at 22:21

CiNN

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

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