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?

Answer 1

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" 

Answer 2

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.

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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];