Algorithm for unique objects from an array with sub-objects?

Question

I have a data class:

@interface Car : NSObject
{
  Engine *_engine;
}
//...

With the class:

@interface Engine : NSObject
{
  NSInteger _power;
  NSString *_name;
}
//...

And an array:

NSArray *myArray = [ // a bunch of Car objects with Engines

How would I make a new array that contains all unique items based on Engine 'power'?

For Example:

0 Car with Engine ( power = 20, name = "engine one" )

1 Car with Engine ( power = 45, name = "engine two" )

2 Car with Engine ( power = 20, name = "engine three" )

Final array would be car objects 0 and 1

Both data classes have accessors, e.g:

- (NSInteger)power;

Answer 1

NSArray *cars = ...;
NSMutableDictionary *carsByUniqueEnginePowers = [NSMutableDictionary dictionary];
for (Car *car in cars) {
    NSNumber *enginePower = [NSNumber numberWithInteger:car.engine.power];
    [carsByUniqueEnginePowers setObject:car forKey:enginePower];
}
NSArray *carsWithUniqueEnginePowers = [carsByUniqueEnginePowers allValues];

Will pick the last item per engine power.

To get the first one replace the line

[carsByUniqueEnginePowers setObject:car forKey:enginePower];

with this:

if (![carsByUniqueEnginePowers objectForKey:power]) {
    [carsByUniqueEnginePowers setObject:car forKey:enginePower];
}

Answer 2

Enumerate the array, building up a list of indices whose objects have previously-unseen power values.

Create a set to hold the already-seen powers; -[NSArray indexesOfObjectsPassingTest:] handles building the index list. Then ask the original array for its objectsAtIndexes:.

NSMutableSet * powerSet = [NSMutableSet set];
NSIndexSet * indexes;
indexes = [carArr indexesOfObjectsPassingTest:^BOOL(id car, NSUInteger idx, BOOL *stop) {
                  // valueForKey: will return an NSNumber instance for scalar values.        
                  NSNumber * powerNum = [[car engine] valueForKey:@"power"];
                  // This power is already in the set; pass over this car.
                  if( [powerSet containsObject:powerNum] ){
                      return NO;
                  }
                  // Add the new power to the set and include this car in the index set.        
                  else {
                      [powerSet addObject:powerNum];
                      return YES;
                  }
              }];
NSArray * uniqueEnginePowerCars = [carArr objectsAtIndexes:indexes];