Objective-C: Remove duplicates efficiently from large data structures

Question

I have an object containing an array of NSNumbers (indexes) and an array of NSDictionaries (indexesTitles) corresponding to indexes, containing some info. I have to call a method for each object.index and associate object.indexTitles to the returning results, saving them into a single array. At the end of it, I want to remove indexes duplicates, preserving the associated indextTitles in an efficient way, because I'm working with large arrays.

NSMutableArray *resultArray = [NSMutableArray array];
NSMutableArray *titlesArray = [NSMutableArray array];
for(NSNumber *index in object.indexes)
{
    NSArray *resultsIndexArray = [self methodThatReturnsAnArray];
    NSString *indexTitleDictionary = [object.indexesTitles objectAtIndex:i];
    for(NSNumber *resultId in resultsIndexArray)
    {
        [titlesArray addObject:indexDictionary];
        [resultArray addObject:resultId];
    }
    i++;
}
[fullResultsArray addObject:titlesArray];
[fullResultsArray addObject:resultArray];

I've found that the most efficient way to remove duplicates is using an
NSOrderedSet like this:

NSOrderedSet *orderedSet = [NSOrderedSet orderedSetWithArray:resultArray];
resultArray = [orderedSet.array mutableCopy];

How can I remove the corresponding entries in titlesArray? how can I preserve the association? I've also tried to use a NSDictionary like {resultId, titleDictionary} and storing them into an array, but I haven't found a efficient way to remove dictionaries with the same result, they are all too slow.

Any suggestion?

Use NSMutableSet instead of NSMutableArray to eliminate duplicates. — iphonic
– iphonic, Commented Aug 27, 2015 at 9:39

CRD · Accepted Answer · 2015-08-27 10:03:08Z

2

It is not completely clear to me what your problem is, maybe this will help:

A good way to remove duplicates is not to add them in the first place, replace:

for(NSNumber *resultId in resultsIndexArray)
{
    [titlesArray addObject:indexDictionary];
    [resultArray addObject:resultId];
}

with:

for(NSNumber *resultId in resultsIndexArray)
{
    // only add if resultId not already in resultArray
    if( ![resultArray containsObject:resultId] )
    {
        [titlesArray addObject:indexDictionary];
        [resultArray addObject:resultId];
    }
}

The containsObject: call requires a linear search, if your data set is large you might wish to change resultArray to an NSMutableSet and titlesArray to an NSMutableDictionary mapping from resultId to indexDictionary values.

HTH

answered Aug 27, 2015 at 10:03

CRD

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2 Comments

laucel Over a year ago

Thanks @CRD, I am trying your second solution, the problem is the in the NSMutableDictionary like {resultId, indexDictionary} I'm getting the last values of indexdictionary, I need the first occurrence instead, how can I manage that?

laucel Over a year ago

Solved, I've just added if(![titlesDictionary objectForKey:resultId]), then set object for the key.

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