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];
}