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