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.
Is there an easy way to iterate over an NSArray backwards?
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5 Answers
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
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.
From here:
NSEnumerator* myIterator = [myArray reverseObjectEnumerator];
id anObject;
while( anObject = [myIterator nextObject])
{
/* do something useful with anObject */
}
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