Most efficient way to find out if an object is already in an NSMutableArray?

Question

I just want to know if an object is in an array or not.

So I can use:

- (BOOL)containsObject:(id)anObj

But it would send -isEqual to every object in the array. Bad when there are thousands of them.

And then there is:

- (NSUInteger)indexOfObjectIdenticalTo:(id)anObject

which seems to only compare the memory addresses. I think this is faster. But a bit nasty to use, of course. One would have to check for NSNotFound.

Is -indexOfObjectIdenticalTo really the best option?

score 2 · Answer 1 · answered Apr 28 '11 at 11:16

2

But a bit nasty to use

Why? It seems to me that

if ([array indexOfObjectIdenticalTo: foo] != NSNotFound)
{
    // do what you need
}

is not much more nasty than

if ([array containsObject: foo])
{
    // do what you need
}

Which one you choose depends on what equality semantics you use. You almost certainly want to use -containsObject: for arrays containing NSStrings or NSNumbers because -isEqual: gives the correct equality semantics.

Both methods, by the way are O(n) which is where the real performance problem is. If the idea of a linear search is a problem, consider a different data structure e.g. based on NSDictionary.

answered Apr 28 '11 at 11:16

JeremyP

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Like you can see yourself the second one is a lot shorter and has less instructions. So yes, comparing to NSNotFound is somewhat nasty :=) – Proud Member Apr 28 '11 at 11:40
+1 for the O(n) thing. Yes this would suck. Making use of the powerful hash function of dictionaries is a good idea. Thanks. – Proud Member Apr 28 '11 at 11:42

justin · Accepted Answer · 2011-04-28T11:38:56.333

2

if you really need this often, you can create an instance method by category:

@interface NSArray (MONStuff)
- (BOOL)mon_containsObject:(id)object;
@end


@implementation NSArray (MONStuff)

- (BOOL)mon_containsObject:(id)object {
  return NSNotFound != [self indexOfObjectIdenticalTo:arg];
}

@end

or these's also CFArrayContainsValue.

a simple function would also suffice.

edited Apr 28 '11 at 11:38

answered Apr 28 '11 at 11:20

justin

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A good simplification of the interface. However, i would name it mon_containsObjectIdenticalTo: to mirror the indexOfObject: indexOfObjectIdenticalTo: pair. – JeremyP Apr 28 '11 at 12:48

score 1 · Answer 3 · edited May 23 '17 at 11:47

1

As per your explaining and comparison indexOfObjectIdenticalTo seems me the first choice to use..

Here is one more SO post ..

indexOfObject vs. indexOfObjectIdenticalTo

edited May 23 '17 at 11:47

Community

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1

answered Apr 28 '11 at 11:10

Jhaliya - Praveen Sharma

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score 1 · Answer 4 · answered Apr 28 '11 at 11:13

If possible (for example if sorting order is irrelevant) you could use an NSDictionary instead, with your object as keys and values of [NSNull null]. Note that the objects get copied when used as keys ! Your objects would need to implement the - (NSUInteger)hash method.

Also see the excellent NSArray or NSSet, NSDictionary or NSMapTable analysis from Matt Gallagher.

Most efficient way to find out if an object is already in an NSMutableArray?

4 Answers4