Mutual changes in two NSMutableArray objects - objective-c

Question

Assume, your .h file looks like this:
#property (retain, nonatomic) NSMutableArray *songs;
- (NSMutableArray *)popularSongs;
- (void)make20PercentDiscountToPopularSongs
Your .m file looks like this:
- (NSMutableArray *)popularSongs
{
NSMutableArray *popularSongs = [NSMutableArray array];
for (Song *song in self.songs) {
if (song.isPopular) {
[popularSongs addObject:song];
}
}
return popularSongs;
}
- (void)make20PercentDiscountToPopularSongs
{
for (Song *song in self.popularSongs) {
song.price = song.price * 0.8;
}
}
The code above adds a 20% discount to popular songs. I recognise that there is a more simplistic way of doing this. You could have the "make20PercentDiscountToPopularSongs" and "popularSongs" function in a single function but let's assume that the code is written the way it is written above.
In the example above, will the line:
song.price = song.price * 0.8;
Actually, make any changes to the object:
#property (retain, nonatomic) NSMutableArray *songs;
Or not? Because, it seems as if the line will change the newly created popularSongs NSMutableArray rather than the songs NSMutableArray, am I right? This problem is bothering me for a lot of time now. I would like to make changes just to the original songs array. To me popularSongs works as if you are storing the pointers to the songs in the songs array.

Answer 1

It changes neither array. It changes some of the objects in the songs array. And as the same objects are in the popularSongs array returned then they are 'changed' too.
This:
[popularSongs addObject:song];
Adds the pointer to the current song into the array. It doesn't create a new copy of the song.

Answer 2

You can create a quick mutable array like this:
NSMutableArray *popularSongs = self.songs.mutableCopy;
Rather than iterating through all the songs to determine if they are popular, you could use a filter predicate at the time you create the array.
NSMutableArray *popularSongs = [[self.songs filteredArrayUsingPredicate:
[NSPredicate predicateWithFormat:#"isPopular = %#", #YES]] mutableCopy];
With this one-liner you can avoid your methods altogether ;-)- Not sure what you want, but if you want to alter the original songs, leave out the mutableCopy part.