NSOrderedSet vs NSArray indexOfObjectPassingTest: - ios

Question

The NSOrderedSet Class Reference Overview says:
You can use ordered sets as an alternative to arrays when the order of elements is important and performance in testing whether an object is contained in the set is a consideration— testing for membership of an array is slower than testing for membership of a set.
Which methods are considered "testing for membership"? Just containsObject:? Or, will indexOfObjectPassingTest: also be faster?
I'm asking because if I just have the object's ID (from the server for example) and want to check if the ordered set contains an object with that ID, I'd use indexOfObjectPassingTest:. But, that method, since it tests every object in the collection, seems that it'd be just as slow as it is for an array. On the other hand, containsObject: seems that it'd be faster since it takes advantage of the NSObject methods hash & isEqual:. I could just create a probe object with the ID I have, and then use containsObject:. But then, if the ordered set already contains an object with that ID, I'll just discard of the probe object and update the properties on the object already in the ordered set. It seems like extra work to have to create a probe object first. In that case, is it even worth using an ordered set over an array?
Also, I would be sorting objects by their date, not their ID.
I'd use an NSMutableDictionary with object IDs mapping to objects, as St3fan suggested, but I also want to display the objects in a UITableView.

Answer 1

The best way to test this is by writing some small benchmarks. I don't know how many objects you are dealing with but if it is less then a few hundred then you probably won't notice much difference between containsObject:, indexOfObjectPassingText: or even just iterating over all objects manually.
It sounds like an NSMutableDictionary is more appropriate for your use case actually. Why don't you store your objects in a dictionary that is indexed by your object's ID? Then you can really quickly find them by ID and you can also iterate easily over them if needed.

Answer 2

You can override -isEqual: and -hash in you class. If you do, it'll work with NSOrderedSet's fast lookup. It can be as simple as:
- (BOOL)isEqual:(id)otherObject
{
return self.myID == otherObject.myID;
}
- (NSUInteger)hash
{
return self.myID;
}
Here's a full example:
#import <XCTest/XCTest.h>
#interface MyClass : NSObject
#property (nonatomic) NSInteger myID;
#property (nonatomic, strong) NSDate *date;
#end
#implementation MyClass
- (BOOL)isEqual:(MyClass*)otherObject
{
return self.myID == otherObject.myID;
}
- (NSUInteger)hash
{
return self.myID;
}
#end
#interface MyTests : XCTestCase
#end
#implementation MyTests
- (void)testExample
{
MyClass *obj1 = [[MyClass alloc] init];
obj1.myID = 1;
obj1.date = [NSDate dateWithTimeIntervalSince1970:20000];
MyClass *obj2 = [[MyClass alloc] init];
obj2.myID = 2;
obj2.date = [NSDate dateWithTimeIntervalSince1970:10000];
MyClass *obj3 = [[MyClass alloc] init];
obj3.myID = 1;
obj3.date = [NSDate dateWithTimeIntervalSince1970:30000];
MyClass *obj4 = [[MyClass alloc] init];
obj4.myID = 3;
obj4.date = [NSDate dateWithTimeIntervalSince1970:30000];
NSOrderedSet *set = [[NSOrderedSet alloc] initWithArray:#[obj1, obj2]];
XCTAssertEqualObjects(((MyClass *)[set firstObject]).date, obj1.date);
XCTAssertEqualObjects(((MyClass *)[set lastObject]).date, obj2.date);
XCTAssertTrue([set containsObject:obj1]);
XCTAssertTrue([set containsObject:obj3]);
XCTAssertFalse([set containsObject:obj4]);
}
#end