Sort NSArray of custom objects based on sorting of another NSArray of strings

Question

I have two NSArray objects that I would like to be sorted the same. One contains NSString objects, the other custom Attribute objects. Here is what my "key" NSArray looks like:

// The master order
NSArray *stringOrder = [NSArray arrayWithObjects:@"12", @"10", @"2", nil];

The NSArray with custom objects:

// The array of custom Attribute objects that I want sorted by the stringOrder array
NSMutableArray *items = [[NSMutableArray alloc] init];
Attribute *attribute = nil;

attribute = [[Attribute alloc] init];
attribute.assetID = @"10";
[items addObject:attribute];

attribute = [[Attribute alloc] init];
attribute.assetID = @"12";
[items addObject:attribute];

attribute = [[Attribute alloc] init];
attribute.assetID = @"2";
[items addObject:attribute];

So, what I would like to do is use the stringOrder array to determine the sorting of the items array of custom objects. How can I do this?

Answer 1

Hereby, I compare directly the index of obj1.assetID in stringOrder with the index of obj2.assetID in stringOrder (using Objective-C literals for @() to transform NSString => NSNumber)

[items sortUsingComparator:^NSComparisonResult(Attribute *obj1, Attribute *obj2) {
    return [@([stringOrder indexOfObject:obj1.assetID]) compare:@([stringOrder indexOfObject:obj2.assetID])]
}];

Or without ObjC literals :

[items sortUsingComparator:^NSComparisonResult(Attribute *obj1, Attribute *obj2) {
    return [[NSNumber numberWithInt:[stringOrder indexOfObject:obj1.assetID]] compare:[NSNumber numberWithInt:[stringOrder indexOfObject:obj2.assetID]]]
}];

Answer 2

While cwehrungs answer will get the job done, the performance is not great on relatively small arrays.

Here is another method for performing the same kind of sort that is a bit quicker (though still far from perfect):

NSMutableArray *sorted = [NSMutableArray array];

// pre-populate with objects
for (int i = 0; i < stringOrder.count; i++)
{
    [sorted addObject:[NSNull null]];
}
// place the items at the correct position
for (Attribute *a in items)
{
    NSUInteger idx = [stringOrder indexOfObject:a.assetID];
    if (idx != NSNotFound)
    {
        [sorted setObject:a atIndexedSubscript:idx];
    }
}
// finally remove all the unecesarry placeholders if one array was smaller
[sorted removeObject:[NSNull null]];

Comparison

Here are the results form running the two methods on an iPhone 5:

sortUsingComparator:

100  - 0.012 s
1000 - 1.116 s
2000 - 4.405 s
3000 - 9.028 s

prepopulated array

100 -  0.003 s
1000 - 0.236 s
2000 - 0.917 s
3000 - 2.063 s

Answer 3

There are a couple approaches you could take.

You could store your Attribute objects in an NSDictionary, with the keys being the strings in your stringOrder array. Then, you could get a sorted array of the keys and use that to populate whatever view you're using to display them:

NSArray* sortedKeys = [dict keysSortedByValueUsingComparator:^(id obj1, id obj2) {
    return [obj1 compareTo:obj2];
}

The other is that you make the sort order an intrinsic property of your Attribute object, so an array of Attributes can be sorted directly. I would only recommend taking this approach if the sort order is actually an intrinsic property of your Attributes object. If it isn't and you do this, you'll wind up storing presentation information where it doesn't belong.

Here's an example:

NSArray* sortedAttrs = [attributes sortedArrayUsingComparator:^(id obj1, id obj2) {
    // Perform comparison of Attribute's, ahem, attributes
}

Answer 4

Here is the solution that I came up with that works extremely well. Anyone see performance issues with this?

for (Attribute *a in items) {
    int index = [stringOrder indexOfObject:a.assetID];
    a.sortOrder = index;
}

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"sortOrder" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
NSArray *sortedArray = [items sortedArrayUsingDescriptors:sortDescriptors];

Answer 5

Parallel Processing:

Results (quad core):

 1. sortme:95    sortby:852345 sorted:95    time:0.052576
 2. sortme:54248 sortby:852345 sorted:54243 time:0.264660





-(NSArray *)sortArray:(NSArray *)sortme sortBy:(NSArray *)sortBy{

CFAbsoluteTime time = CFAbsoluteTimeGetCurrent();

NSSet *sortmeSet = [NSSet setWithArray:sortme];

NSMutableDictionary *sortDictionary = [NSMutableDictionary dictionary];
dispatch_queue_t sortDictionaryThread = dispatch_queue_create("my.sortDictionaryThread", DISPATCH_QUEUE_CONCURRENT);

[sortBy enumerateObjectsWithOptions:NSEnumerationConcurrent usingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {

    if ([sortmeSet containsObject:obj]){
        dispatch_barrier_async(sortDictionaryThread, ^{
            sortDictionary[obj] = @(idx);
        });
    }
}];


__block NSArray *sortedArray = nil;
dispatch_barrier_sync(sortDictionaryThread, ^{
    sortedArray = [sortDictionary keysSortedByValueUsingSelector:@selector(compare:)];
});

NSLog(@"sortme:%li sortby:%li sorted:%li time:%f",sortme.count,sortBy.count,sortedArray.count, CFAbsoluteTimeGetCurrent() - time);

return sortedArray;
}