iPhone - getting unique values from NSArray object

Question

I have an NSArray formed with objects of a custom class. The class has 3 (city, state, zip) string properties. I would like to get all unique state values from the array.

I did read through the NSPredicate class but couldn't make much of how to use it in this case. The only examples I could find were for string operations.

Can someone please help me out?

score 136 · Answer 1 · edited Jan 28 '12 at 05:24

136

Take a look at keypaths. They are super powerful and I use them instead of NSPredicate classes most of the time. Here is how you would use them in your example...

NSArray *uniqueStates;
uniqueStates = [customObjects valueForKeyPath:@"@distinctUnionOfObjects.state"];

Note the use of valueForKeyPath instead of valueForKey.

Here is a more detailed/contrived example...

NSDictionary *arnold = [NSDictionary dictionaryWithObjectsAndKeys:@"arnold", @"name", @"california", @"state", nil];
NSDictionary *jimmy = [NSDictionary dictionaryWithObjectsAndKeys:@"jimmy", @"name", @"new york", @"state", nil];
NSDictionary *henry = [NSDictionary dictionaryWithObjectsAndKeys:@"henry", @"name", @"michigan", @"state", nil];
NSDictionary *woz = [NSDictionary dictionaryWithObjectsAndKeys:@"woz", @"name", @"california", @"state", nil];

NSArray *people = [NSArray arrayWithObjects:arnold, jimmy, henry, woz, nil];

NSLog(@"Unique States:\n %@", [people valueForKeyPath:@"@distinctUnionOfObjects.state"]);

// OUTPUT
// Unique States:
// "california",
// "michigan",
// "new york"

edited Jan 28 '12 at 05:24

sbonami

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answered Sep 17 '09 at 16:05

probablyCorey

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Caveat: valueForKeyPath:@"@distinctUnionOfObjects.PROPERTY" does not preserve sort order. – Robert Altman Dec 07 '11 at 22:09
thanks @RobertAltman, for pointing that out. I guess that saved me some debugging time – Hulvej Jul 22 '13 at 16:31
What if the object to key is an array and the value exists on first index of it? – Qamar Suleiman Jan 28 '14 at 21:44
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what If i want to return that dictionary not only the value? – Abhishek May 21 '14 at 06:59
this approach won't work if you have a custom class. It will work if your array object is a NSDictionary object. – Liangjun Feb 04 '16 at 18:03
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use `NSOrderedSet` instead of `Set` to preserve order – Preetam Jadakar Mar 21 '16 at 09:36

Dave DeLong · Accepted Answer · 2012-12-01T00:57:52.343

The totally simple one liner:

NSSet *uniqueStates = [NSSet setWithArray:[myArrayOfCustomObjects valueForKey:@"state"]];

The trick is the valueForKey: method of NSArray. That will iterate through your array (myArrayOfCustomObjects), call the -state method on each object, and build an array of the results. We then create an NSSet with the resulting array of states to remove duplicates.

Starting with iOS 5 and OS X 10.7, there's a new class that can do this as well: NSOrderedSet. The advantage of an ordered set is that it will remove any duplicates, but also maintain relative order.

NSArray *states = [myArrayOfCustomObjects valueForKey:@"state"];
NSOrderedSet *orderedSet = [NSOrderedSet orderedSetWithArray:states];
NSSet *uniqueStates = [orderedSet set];

While I also like the power of @distinctUnionOfObjects, and hence, the answer user probablyCorey gave, the benefit with this concise approach here in this solution is that ordering is preserved. — idStar, Jan 29 '16 at 18:56

iPhone - getting unique values from NSArray object

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