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.

Answer 1

[NsArray reverseObjectEnumerator]

Answer 2

From here:

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

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

Answer 3

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.

• -[NSArray reverseObjectEmumerator]
• Create a copy of the array where the order of the elements is reversed and then iterate that array as you would 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.

Answer 4

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