Remove all strings with duplicates in an NSArray

Question

I am trying to figure out how to implement this in Objective-C.

I want to remove the strings in an NSArray that have appear more than once in the array.

At the end I want to have an array that only has the unique lines in an array (meaning that not just the duplicates are deleted but also the original string that matches the duplicates.)

For example if you had the following array:

NSArray *array = [NSArray arrayWithObjects:@"bob", @"frank", @"sarah", @"sarah", @"fred", @"corey", @"corey", nil];

I would want the new array to look like this:

@"bob", @"frank", @"fred"

Community · Accepted Answer · 2017-05-23 11:53:19Z

24

Use an NSCountedSet:

NSCountedSet *countedSet = [NSCountedSet setWithArray:yourArray];
NSMutableArray *finalArray = [NSMutableArray arrayWithCapacity:[yourArray count]];

for(id obj in countedSet) {
   if([countedSet countForObject:obj] == 1) {
      [finalArray addObject:obj];
   }
}

@Caleb suggested adding a method to NSCountedSet called -objectsWithCount:,, which I've implemented here:

@interface NSCountedSet (JRCountedSetAdditions)

- (NSArray *) objectsWithCount:(NSUInteger) count;

@end

@implementation NSCountedSet (JRCountedSetAdditions) 

- (NSArray *) objectsWithCount:(NSUInteger) count {
   NSMutableArray *array = [NSMutableArray array];
   for(id obj in self) {
      if([self countForObject:obj] == count) {
        [array addObject:obj];
      }  
   }
   return [array copy];
}

@end

Once that's done, all you need is one line:

NSArray *finalArray = [[NSCountedSet setWithArray:yourArray] objectsWithCount:1];

By the way, this is type-agnostic, so this will work with any Objective-C object. :-)

edited May 23, 2017 at 11:53

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answered Jun 16, 2011 at 16:46

Jacob Relkin

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6 Comments

Itai Ferber Over a year ago

Why use an enumerator? Just use fast enumeration!

Caleb Over a year ago

One line: NSArray *finalArray = [[NSCountedSet setWithObjects:yourArray] allObjects];

Jacob Relkin Over a year ago

@Caleb: If that would work, I would've done it that way. But alas, it does not as -allObjects returns unique objects (i.e. one of the dupes), while the logic behind this is that we don't want the dupes either.

Jacob Relkin Over a year ago

@Caleb: So, with your suggestion, the result would be: @"frank", @"corey", @"fred", @"bob", @"sarah".

Caleb Over a year ago

@Jacob: Gotcha. I missed the requirement to remove the duplicated string as well as the duplicates. ;-) Given that, it'd be nice to take your for loop and put it in a -objectsWithCount: method on NSCountedSet.

|

REALFREE · Accepted Answer · 2011-08-11 06:14:45Z

10

One liner : uniqueArray = [[NSSet setWithArray:duplicateArray] allObjects]; if you don't care about the ordering :D

answered Aug 11, 2011 at 6:14

REALFREE

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2 Comments

Deepukjayan Over a year ago

I was here to answer that!! Happy to see my answer is already here +1!! - (NSArray*)removeDuplicates:(NSArray*)list { return [[NSSet setWithArray: list] allObjects]; }

Albert Renshaw Over a year ago

[WARNING], Update: No longer surprised haha, after a buggy afternoon I learned the hard way, this code is not guaranteed to retain the order of your array. Use [[NSOrderedSet ordereredSetWithArray:duplicateArray] array] instead.

Community · Accepted Answer · 2017-05-23 12:24:59Z

4

A slightly different approach from Jacob's:

NSArray *array = [NSArray arrayWithObjects:@"bob", @"frank", @"sarah", @"sarah", @"fred", @"corey", @"corey", nil];

NSCountedSet *namesSet = [[NSCountedSet alloc] initWithArray:array];
NSMutableArray *namesArray = [[NSMutableArray alloc] initWithCapacity:[array count]];

[namesSet enumerateObjectsUsingBlock:^(id obj, BOOL *stop){
    if ([namesSet countForObject:obj] == 1) {
        [namesArray addObject:obj];
    }
}];

And

NSLog(@"old: %@\nNew: %@", array, namesArray);

gives:

2011-06-16 18:10:32.783 SetTest[1756:903] old: (
    bob,
    frank,
    sarah,
    sarah,
    fred,
    corey,
    corey
)
New: (
    frank,
    fred,
    bob
)

Blocks are your friends! And since NSCountedSet is a subclass of NSSet you can use the block methods that are available there.

edited May 23, 2017 at 12:24

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answered Jun 16, 2011 at 17:12

Abizern

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7 Comments

Jacob Relkin Over a year ago

@Abizem: I'm sorry, how on earth is that different other than the usage of -enumerateObjectsUsingBlock:?

Jacob Relkin Over a year ago

@Abizem: And, technically speaking, it is more expensive to allocate a block on the stack than a scoped id reference.

Abizern Over a year ago

It's different from your original answer that used an enumerator, and I am trying to show that blocks could also be used in this situation, and with a bit of thread safe code thrown in, this could be done concurrently.

Jacob Relkin Over a year ago

@Abizem Looks like you have a couple of memory leaks there... ;)

Abizern Over a year ago

Absolutely. I ran it in -applicationDidFinishLaunching:! Also, I use GC ;)

|

Zulqarnain Mustafa · Accepted Answer · 2016-05-21 06:34:16Z

2

Here is the simplest approach to remove duplicate strings:

NSArray *array = [NSArray arrayWithObjects:@"bob", @"frank", @"sarah", @"sarah", @"fred", @"corey", @"corey", nil];
NSArray *distintStrings = [array valueForKeyPath:@"@distinctUnionOfObjects.self"];

answered May 21, 2016 at 6:34

Zulqarnain Mustafa

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