What is the most efficient way to sort an NSSet?
Solution 1
try using
[[mySet allObjects] sortedArrayUsingDescriptors:descriptors];
Edit: For iOS ≥ 4.0 and Mac OS X ≥ 10.6 you can directly use
[mySet sortedArrayUsingDescriptors:descriptors];
Solution 2
The "most efficient way" to sort a set of objects varies based on what you actually mean. The casual assumption (which the previous answers make) is a one-time sort of objects in a set. In this case, I'd say it's pretty much a toss-up between what @cobbal suggests and what you came up with — probably something like the following:
NSMutableArray* array = [NSMutableArray arrayWithCapacity:[set count]];
for (id anObject in set)
[array addObject:anObject];
[array sortUsingDescriptors:descriptors];
(I say it's a toss-up because @cobbal's approach creates two autoreleased arrays, so the memory footprint doubles. This is inconsequential for small sets of objects, but technically, neither approach is very efficient.)
However, if you're sorting the elements in the set more than once (and especially if it's a regular thing) this is definitely not an efficient approach. You could keep an NSMutableArray around and keep it synchronized with the NSSet, then call -sortUsingDescriptors: each time, but even if the array is already sorted it will still require N comparisons.
Cocoa by itself just doesn't provide an efficient approach for maintaining a collection in sorted order. Java has a TreeSet class which maintains the elements in sorted order whenever an object is inserted or removed, but Cocoa does not. It was precisely this problem that drove me to develop something similar for my own use.
As part of a data structures framework I inherited and revamped, I created a protocol and a few implementations for sorted sets. Any of the concrete subclasses will maintain a set of distinct objects in sorted order. There are still refinements to be made — the foremost being that it sorts based on the result of -compare: (which each object in the set must implement) and doesn't yet accept an NSSortDescriptor. (A workaround is to implement -compare: to compare the property of interest on the objects.)
One possible drawback is that these classes are (currently) not subclasses of NS(Mutable)Set, so if you must pass an NSSet, it won't be ordered. (The protocol does have a -set method which returns an NSSet, which is of course unordered.) I plan to rectify that soon, as I've done with the NSMutableDictionary subclasses in the framework. Feedback is definitely welcome. :-)
Solution 3
For iOS ≥ 5.0 and Mac OS X ≥ 10.7 you can directly use NSOrderedSet
Solution 4
NSSet is a collection of unordered objects. Looking at apple references Arrays are ordered collections.
Looking at NSArray there is a discussion with examples of sorting at http://developer.apple.com/documentation/Cocoa/Conceptual/Collections/Articles/sortingFilteringArrays ...
Example from the link:
NSInteger alphabeticSort(id string1, id string2, void *reverse)
{
if (*(BOOL *)reverse == YES) {
return [string2 localizedCaseInsensitiveCompare:string1];
}
return [string1 localizedCaseInsensitiveCompare:string2];
}
// assuming anArray is array of unsorted strings
NSArray *sortedArray;
// sort using a selector
sortedArray =
[anArray sortedArrayUsingSelector:@selector(localizedCaseInsensitiveCompare:)];
// sort using a function
BOOL reverseSort = NO;
sortedArray =
[anArray sortedArrayUsingFunction:alphabeticSort context:&reverseSort];
Boon
I manage a team that builds highly-innovative interactive apps and games for mobile and web. I love building things that reach a lot of people make people happy
Updated on July 05, 2022Comments
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Boon almost 2 years
What's the most efficient way to sort objects in an
NSSet
/NSMutableSet
based on a property of the objects in the set? Right now the way I am doing it is by iterating through each object, add them to aNSMutableArray
, and sort that array withNSSortDescriptor
. -
Quinn Taylor almost 15 yearsThis is not much different that the asker's suggestion, and probably roughly equivalent in speed since -allObjects returns an autoreleased NSArray, and -sortedArrayUsingDescriptors: returns a separate NSArray (both are immutable). The cost of allocating two arrays is not much less than enumerating all elements in a (moderate size) set, and requires twice as much space.
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Quinn Taylor almost 15 yearsWow, that particular Apple sample code is pretty terrible. Why are they using void*, NSInteger, and int where it's simpler to use a BOOL? Why would it return NSInteger instead of NSComparisonResult? I'm sure it's for compatibility with previous API decisions, but that's frickin' ugly! I suggest using a selector (method) rather than a function for sorting Cocoa collections — it's simpler and more elegant.
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Austin almost 14 yearsIt is good to note that sortedArrayUsingDescriptors: is a 10.6 only method. If you're targeting 10.5 or before you may want to try @QuinnTaylor's approach
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DonnaLea about 13 yearsNSSet also has a sortedArrayUsingDescriptors method, so you could skip the call to allObjects and simply call [mySet sortedArrayUsingDescriptors:descriptors];
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cobbal about 13 years@DonnaLea That does seem to be the best way to do it, but it didn't exist back when the question was asked. Updated answer, thanks.
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DiscDev about 11 yearsThis may be obvious, but note that sortedArrayUsingDescriptors takes an array of descriptors and not just a single descriptor. [mySet sortedArrayUsingDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES]]];
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Nico over 10 years@QuinnTaylor I just checked and, sure enough, the documentation for
sortedArrayUsingFunction:context:
says that the function is expected to take twoid
s and avoid *
and return anNSInteger
. In that, at least, the sample is correct. (They seem to have updated it, too—it sucks less now.) -
micnguyen almost 9 yearsAnd to add onto DiscDev's answer, remember the NSArray short @[], so you can do:
[mySet sortedArrayUsingDescriptors:@[[NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES]]];
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colincameron about 7 yearsThis doesn't address the question, where you've got an existing NSSet and want to sort it.
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Jageen over 5 yearsMore important NSOrderedSet does not mean sorted NSSet.